Magnesium research in Canada: Highlights of the last two decades

Frank Czerwinski

Natural Resources Canada, CanmetMATERIALS, Hamilton, ON, Canada

Abstract The highlights of major Canadian research projects of last two decades that aimed at advancing applications of magnesium and its alloys,being at the heart of lightweighting and ecological sustainability,are outlined.The research at universities,government laboratories,and other dedicated institutions,funded primarily through federal programs,was accompanied by strong activities of the industrial sector involved in designing and building the machinery for magnesium processing and production of components from magnesium alloys.The overall research directions matched the global trends of overcoming the key challenges that prevent magnesium alloys to play the major role in large-scale applications.Among industrially oriented projects the processing technologies in liquid,semisolid and solid states such as casting,twin roll casting,injection molding,rolling,extrusion,forging,and joining techniques were frequently pursued.Although the fundamental research aimed at understanding of a variety of magnesium behaviors and structural peculiarities up to nano and atomic levels,its essence spread around the inherently poor formability and high reactivity at room and elevated temperatures,including the ignition/flammability concerns.In recent years,a shift in research interests was observed and novel directions emerged such as magnesium air batteries,biodegradable alloys,additive manufacturing,and magnesium-rich high entropy alloys.The volume of data gathered in this report may constitute a base for specifically oriented assessments,analyses,and drawing conclusions.

Keywords: Magnesium;Magnesium alloys;Processing technologies;Canadian research projects.

1.Introduction

Magnesium offers a wealth of valuable properties that make it of interest for a diverse range of applications,including structural components of automotive and aerospace vehicles,consumer electronics,biomedical devices,hydrogen production,storage,electrodes in fuel cells or batteries,and anodic protection systems.Magnesium alloys are at the heart of lightweighting and ecological sustainability trends that reach through many industrial sectors associated not only with all forms of transportation,but more broadly with civil infrastructure,manufacturing,and clean energy technologies [1].Being at the center of the transport electrification strategy and due to growing concerns about the supply security of raw materials,magnesium is included into national critical mineral lists of Canada,USA,European Union,and other countries [2].The global magnesium market valued about USD 5 billion in 2022 is forecast to reach USD 6.4 billion in 2028 at a CAGR growth rate of 5.5 [3].Although engineering designers are aware of magnesium potential value,its use is still limited and an average content in cars for US automakers of 4.5–5.5 kg (10–12 lbs),achieved two decades ago,did not change significantly [4].

Over the last century,the primary magnesium industry in Canada has grown,peaking in the year 2000 with a volume of 80 thousand tonnes and primary smelters in Becancour,Quebec (Norsk Hydro),Asbestos,Quebec(Magnola),and Haley Station,Ontario (Timminco),placing Canada as the second largest global producer.After the geographic shift in magnesium market,when no primary magnesium was produced in Canada,it is reemerging with novel manufacturing techniques of a low-carbon footprint.The modern techniques are replacing the Pidgeon process invented in 1940s at National Research Council of Canada and dominating the present global output [5].The first magnesium ingots produced by Alliance Magnesium redefine the industry standards with a process that will allow to reduce the Greenhouse Gas (GHG) emitted by more than 22 tonnes per tonne of magnesium produced,compared to present commercial technologies [6].Another project by West High Yield Resources aims at bringing into production one of the world’s largest,greenest deposits of high-grade magnesium,Record Ridge,British Columbia,estimated at approximately 10.6 million tonnes of contained magnesium.The process will use green technology capable utilizing over 99% of mined ore and extract saleable products while the extraction stage minimizes CO2emissions [7,8].

Despite the global shift in magnesium production,the extensive scientific research at universities and government laboratories took place and was accompanied by R&D efforts at industrial sites.This report provides highlights of magnesium research activities in Canada over the last two decades,arranged along major technical subjects.To manage the number of supporting documents,multi-year university projects were referenced mainly by dissertation theses,since the related,often numerous publications are typically spread out around the thesis subject.The detailed research credits to researchers involved are listed in the thesis’s statement of contributions or acknowledgment sections.The report will help in identifying still existing challenges,subjects that require additional attention or specifying directions for future research.

2.Programs and funding of magnesium research

The CanmetMATERIALS laboratory of Natural Resources Canada with industrial size processing equipment and extensive material testing capabilities serves as development hub for new technologies to strengthen collaboration between universities and industry.The laboratory was paramount in supporting the magnesium research initiatives at both the national and international levels.

2.1.Government/academia/industry collaborations

The magnesium research in Canada was initially funded through two federal programs: the Canadian Lightweight Materials Research Initiative,and the Materials and Manufacturing Theme of the AUTO21 Network of Centers of Excellence.During 2009–2015,the magnesium research was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) through establishing the Magnesium Network,MagNET,that connected CanmetMATERIALS,industry and five Canadian universities: The University of British Columbia,école Polytechnique,McGill University,McMaster University and The University of Waterloo.The objective was to assist the North American automotive industry in reducing the carbon dioxide emissions,develop the knowledge necessary to produce magnesium components,to transfer this knowledge base to industry and to train the next generation of engineers and scientists.

To support twin roll casting activities the Magnesium Strip Technology Consortium was established,which included GM Canada,Novelis,Magnesium Elektron,McGill,McMaster,Ecole Polytechnique and Centre Integré de Fonderie et de Metallurgie (CIFM) with the objective to develop magnesium sheet products for the automotive industry.

The industrial-scale facility for magnesium processing using Thixomolding was set up in 2005 at the National Research Council of Canada’s Industrial Materials Institute (IMI-NRC)in Boucherville,Québec [9].The laboratory was equipped with 500 tonne Husky’s Thixomolding machine.The project was an initiative of the Consortium de recherche en fabrication industrielle à haute performance (advanced manufacturing research consortium) made up of researchers from a number of universities and colleges.In addition to local companies,the major project partners included école Polytechnique de Montréal,Industrial Materials Institute (IMI),Husky Injection Molding Systems Ltd,and Magna International Inc.

2.2.Magnesium Front End Research and Development(MFERD)

An important vehicle of international collaboration in 2007–2012 was the Magnesium Front End Research and Development project.The MFERD concept was a multi-task research effort involving Natural Resources Canada,China’s Ministry of Science and Technology,and the United States Department of Energy.CanmetMATERIALS of Natural Resources Canada (at that time CANMET Materials Technology Laboratory) was the Canadian coordinating organization for the MFERD project.On national level,the project involved three Canadian companies,the National Research Council of Canada,and five universities.Collaboration among government,academic,and private-sector partners was the principal reason that led to the success of the MFERD project.

The MFERD objective with total funding of USD 22 million for 2007–2012 was to develop the magnesium-intensive front end for an automobile to increase energy efficiency and reduce emissions,improve fuel economy and performance benefits in a multi-material automotive structure.The project aimed at novel magnesium automotive body applications and processes,beyond conventional die castings,including wrought components (sheet or extrusions) and high-integrity body castings.During phase I,a magnesium-intensive front end was designed,being 38 kg lighter (45%) than a typical front-end steel structure [10].

The MFERD initiated R&D in the following areas: crashworthiness,car noise,vibration and harshness (NVH),fatigue and durability [11],corrosion and surface finishing,extrusion forming,sheet forming,high-integrity body casting,as well as joining and assembly [12].Additionally,the MFERD project was linked to the Integrated Computational Materials Engineering (ICME) program that included the processing/structure/properties relations for various magnesium alloys and manufacturing processes,utilizing advanced computeraided engineering and modeling tools [13].

2.3.Light Metals Alliance (LMA)

Another international collaboration platform was covered by the Light Metals Alliance,initiated in 2001 between CanmetMATERIALS,Canada,CAST Co-operative Research Centre,Australia,and LKR Leichtmetallkompetenzzentrum Ranshofen,Austria [14].The LMA platform is still active and during last two decades the membership has grown to 11 members.During last two decades,there have been numerous scientific collaborations yielding in the exchanges of personnel for research and technology transfer,co-authored publications,and biennial Light Metal Technologies (LMT) conferences.The objective of the LMA concept was to enable better use of light metals in a broad range of real-world applications through knowledge exchange.The LMA research interests expand beyond magnesium and cover all aspects of the production of parts from light metal alloys and composites,and the assessment of their performance.

3.Solidification,casting

Casting is the dominant manufacturing process of magnesium components,representing about 98% of all structural applications of magnesium.This is due to magnesium unique solidification features,including high fluidity and low susceptibility to hydrogen porosity that make it a good candidate for casting operations.The academic research aimed at solidification control to prevent cracking and developing new casting alloys with improved performance.The collaborative projects with industry focused on applications of commercial and proprietary magnesium alloys to cast specific commercial components,mainly for automotive industry with challenges of their intricate shape and/or large size.

3.1.Improving properties of casting alloys

The study of ductility improvement of magnesium casting alloys for automotive structural applications considered the effects of rare earths Y and Er [15].Trace additions of Y to the Mg6Al (wt.%) alloy led to grain refinement and ductility improvement through the Al4MgY precipitates that co-nucleated along with Mg17Al12compounds,resulting in their refinement.The trace amounts of Y also caused refinement of the complex Mg-Al-Mn-Fe compounds and grain size reduction ofαMg.However,at high levels of Y the refinement effects were lost,leading to a decrease in mechanical properties.A similar effect on mechanical properties of the Mg6Al (wt.%)alloy was found after additions of 60–880 ppm of Er.Small additions of the order of 60 ppm Er caused the refinement of the Mg17Al12compounds by co-nucleated Al2Er phase.Similarly to Y,the refinement effects were lost at higher Er additions of 880 ppm.The research concluded that an improvement in ductility of the Mg17Al12phase was attributed to its refinement rather than the change in its intrinsic properties.

3.2.Hot tearing during solidification

Magnesium alloys exhibit a high susceptibility to hot tearing during casting.To investigate the influence of casting process parameters on the onset of hot tearing in the AZ91D and AE42 alloys,a novel method of casting stress measurements was implemented [16].The results revealed the importance of mold temperature during the nucleation of hot tears;when the cooling rate of the AZ91D casting reached about 15 °C/s for the mold temperature of 210 °C,the solidification proceeded sufficiently fast to prevent the long-range inter-dendritic feeding.During casting of the AE42 alloy,inter-dendritic feeding was hindered by AlxREyphases that blocked the liquid flow paths.Hence,to prevent cracking in the AE42 casting a very slow cooling rate below 8 °C/s was required.

Another control of hot tearing was developed through novel grain refiners,generated by the powder metallurgy process of spark plasma sintering [17].The refiners reduced the grain size of AZ91D by 75 to 85% along with improving the dispersion of secondary phases and eutectics.The spark plasma sintered refiners outperformed the common commercially available solutions and led to elimination of hot tearing in castings where this detrimental phenomenon was earlier observed.

3.3.High pressure die casting (HPDC)

High-pressure die casting with its manufacturing efficiency,net shape capabilities,low energy consumption,and competitive cost is particularly well suited for mass scale production,typical for automotive industry (Fig.1a and b).It is estimated that roughly half of the world production of light metal castings is obtained through this technology [4].

A number of projects investigated the casting process and the relationship between solidification conditions and microstructure of magnesium alloys [18–23].The experiments and computer simulations were employed to understand the microstructure development of magnesium alloys during HPDC and to correlate it with processing parameters.As a part of this research,the casting skin thickness was investigated in regions experiencing different solidification conditions[24].Based on relationship between the internal porosity and fracture strength of the diecast AM60B alloy it was concluded that the local area fraction of porosity was the primary factor in determining the tensile properties [25].

During study of the process-structure-property relationships in the high pressure diecast AM60 alloy a new method was developed that utilized knowledge of local cooling rates to predict the grain size and the casting skin thickness [26].The commercial casting simulation package,ProCASTTMwas found useful to determine the interfacial heat transfer coefficient between the cast alloy and die.Then,by combining it with the die temperature and filling time the technique allowed to determine the grain size at various locations of the casting.The quantitative analysis of casting defects such as total porosity,shrinkage porosity,gas porosity,and externally solidified grains (ESGs) was used to corelate their formation frequency with HPDC processing parameters (Fig.1c and d).The study determined that the area fraction of ESGs was influenced by the residency time in the shot sleeve and was controlled by the slow-stage piston velocity.At the same time,the percentage of gas porosity was affected by the fast-stage piston velocity.The latter stage of injection controlled the air entrapment and intensification pressure through the pressure balance at the void-metal interface.

Fig.1.High Pressure Die Casting of magnesium alloys: (a) CanmetMATERIALS pilot scale High Pressure Die Casting cell: 1200 ton two-platen Buhler Carat 105 L high pressure die casting machine with vacuum capability for light alloys and (b) Stotek 800 kg of magnesium furnace;(c) investigation of knit line impacts on the formation of porosity in AM60B alloy,top hat casting with six selected locations representing the two flow directions incorporating knit lines [26];(d) top hat casting with three selected locations from flow direction including knit line between left runner and middle runner [26];(e) Meridian’s methodology that incorporates an integrated computational materials engineering into design of die-cast component [29].

Recent developments in industrial casting of magnesium automotive components were summarized in Refs.[27,28].A methodology was developed to incorporate an integrated computational materials engineering approach into design of die-cast magnesium alloy component.It introduced the variability in local solidification conditions into computer-aided engineering predictions of component performance [29].The methodology allowed improving the material usage and optimizing the component weight reduction (Fig.1e).In general,a capability of microstructure prediction using computer modelling helps reducing the necessity of costly destructive testing.

4.Magnesium injection molding,Thixomolding

Manufacturing industry is continuously challenged to satisfy requirements of evolving markets.As a result,there is a search for novel technologies enabling large-scale production of high performance,net shape components.The magnesium injection molding is seen as the environmentally friendly alternative to die casting that must deal with large volumes of flammable molten metal in an open furnace,relying on protection by sulfur hexafluoride,SF6,one of the most harmful of atmospheric greenhouse gases.The original research was conducted with magnesium by Dow Chemical Company,Midland,MI,USA,starting in 1979 and after a decade the technology was acquired by established in 1990 Thixomat Inc.,Ann Arbor,MI,USA.Since 1992 it is marketed under the trade name of ThixomoldingR○[30].

4.1.Injection molding machinery

Husky Injection Molding Systems Ltd Bolton,Ontario,Canada,the builder of plastics injection molding machinery,entered the magnesium molding market in 1999 and after intensive development,built and supplied the commercial magnesium injection molding machines with a clamp force of 400,500,650 and 1000 tonnes globally until 2007 (Fig.2).When early designs assumed just a direct replacement of plastic pellets with magnesium chips [31] or granules [32],subsequent research revealed essential differences in machinery and processing requirements,imposed by metallic alloys and related numerous challenges [33].The presence of massive injection screw with non-return valve instead of a plunger is one of the key design differences between injection molding and die casting that expands processing options but adds an operational complexity.The machine design allowed introducing the hot runner technology [34,35].In this innovation,the multiply injection sites of the hot runner system,substantially shortened the flow distance of semisolid alloy in the mold cavity so it allowed to manufacture part with large dimensions.Also,the alloy consumption per component was reduced since the cold-sprue scrap,an equivalent to the biscuit in die casting,was eliminated.

Fig.2.Magnesium injection molding (ThixomoldingTM): (a) Husky HylectricTM injection molding system with a clamp force of 650 tonnes designed and manufactured in Bolton,Ontario,Canada;(b) injection unit with components indicated;(c) mechanically comminuted feedstock of AZ91D alloy;(d) feedstock within the hopper before feeding into machine barrel;(e) concept of 4-drop hot runner;(f) automotive magnesium component molded by direct injection using 4-drop hot runner [30].

4.2.Process theory and practice

The extensive research was conducted to develop the theory and practice of magnesium injection molding.The technique belongs to the family od semisolid processing with an advantage that the forming itself is free from handling the superheated liquid metal [36,37].The product microstructure with its key feature of globular unmelted phase that controls thixotropic properties in semisolid state is completely different than that after conventional casting [38,39].Also,the microstructure-property relationship is different,what is especially true for alloys with complex chemistry,as creep resistant grades [40].

Since the benefits of semisolid processing are associated with the unmelted solid phase it appears that the liquid content should be kept as low as possible.At the same time,however,a certain minimum volume of liquid is required to ensure processibility.The experiments demonstrated that the molding of magnesium alloys can be successfully conducted at ultra-high contents of the unmelted phase of the order of 75–85%,leading to an invention of semisolid extrusion molding [41–43].

Mixing of thixotropic slurries,formed during partial melting of particulate precursors constituted the novel technique of alloy generation [44,45].As next processing route,it was determined that the tight control of the alloy temperature within a narrow range around the liquidus level,generated structures with high integrity.Moreover,this novel processing route,called near-liquidus molding,diminished some disadvantages inherent to conventional casting,which utilized superheated melts,including intensity of casting defects[46–48].As another opportunity,injection molding allowed manufacturing of foamed components from magnesium alloys,particularly when utilizing the co-injection techniques[49,50].

A cluster of projects aimed at studies of rheological behavior of magnesium alloys in semisolid state [51] and developing numerical simulation tools for the prediction of mold filling by semisolid magnesium alloys [52].

4.3.Processing knowledge impact on machinery design

The key discovery of researching the magnesium injection molding process was that the dendrite-to-globule transformation during coarse particulate melting along its flow path within the barrel/screw channel was caused by strain induced melt activation (SIMA) [53,54] due to feedstock deformation,imposed at its manufacturing stage.According to earlier beliefs,the dendrite-to-globule transformation was triggered by the injection screw shearing during processing inside the injection machine barrel [55–57].

Although there is a difference of orders of magnitude in size,there is a microstructural similarity between bulk billets manufactured by solid-state deformation and coarse particulates produced by mechanical comminution.As a result,the process control parameters and the machine screw and barrel design could be optimized,focusing on other screw functions beyond its shear role.

Understanding the mechanism of dendrite-to-globule transformation in coarse metal particulates provided fundamentals for the alternative machinery concepts.The recent design by MAXImolding that was initiated in Canada represents a vertically oriented,semisolid metal alloy injection molding machine with a plunger instead of the injection screw [58].The essence of the design is that the generation of thixotropic slurry with spherical solid particles during melting the particulate feedstock takes place in the thermal mass reactor under the sole influence of heat.

5.Twin roll casting (TRC)

Twin roll casting has potentials to overcome barriers in magnesium formability and allow manufacturing the sheet or strip products directly from a molten state.The technology takes advantage of best features of both fundamental processes: casting and rolling since it combines them into a single-step operation.As a result,it reduces the manufacturing cycle time,energy consumptions,pollutant emissions and final cost,compared to traditional sheet production using a direct chill ingot casting.Although twin roll casting was commercialized for a strip production from ferrous and nonferrous alloys in the 1950s,its application to magnesium has proven difficult and still creates major challenges.

The industrial size,twin roll caster at CanmetMATERIALS operated from 2007 to 2022 and was the only system in North America,devoted to magnesium as a part of the pilot scale facility,containing foundry with melting furnaces,casting presses,and rolling mills.The goal was to develop technologies and processing strategies to produce low-cost magnesium alloy sheet (Fig.3a,b,and c).The twin roll caster was used in many collaborative projects with academia as described separately [59].

To understand and control the TRC process the mathematical modelling was employed.As a part of related activities,a 2D model for the AZ31 alloy was developed and validated based on the TRC hardware at CanmetMATERIALS;first a thermal-fluid model and after validation a thermal-fluid-stress model [60,61].The modeling revealed that increasing casting speed,casting thicker strips,and applying the higher heat transfer coefficient led to less uniform microstructure through thickness in terms of the secondary dendrite arm spacing.The higher casting speeds led to deeper sumps and higher exit temperatures as well as lower overall rolling loads and lower total strains experienced during TRC.Also,the larger roll diameters increased the surface normal stress and rolling loads but had little effect on the mushy zone thickness (Fig.3d).Increasing the set-back distance decreased the risk of centerline and inverse segregation formation with increasing the roll diameter causing reduction of the propensity to inverse segregation but having a minor effect on the center-line segregation formation.The modeling results were verified through experiments and key findings served as an input for TRC hardware modification to expand its processing capabilities.

6.Magnesium sheet development

The major challenge of industrial scale magnesium rolling is its inherently poor formability,especially at room temperature (Fig.4).To address the magnesium sheet technology perspectives there is a quest for new magnesium alloys with improved formability and for novel rolling techniques.The described above twin roll casting is considered as one of the approaches to development of magnesium sheet since it allows the major reduction of hot rolling steps through providing the relatively thin as-cast strip [62,63].During sheet development research,both the TRC strips and cast blocks were used as a raw rolling material.

6.1.Experiments with AZ31 alloy

Majority of research projects employed the commercial AZ31 alloy as the raw material for sheet production with some attempts to modify its composition.

During investigation of the hot rolling of AZ31 to 2 mm strip,post rolling annealing at 450 °C was found to significantly reduce the centerline segregation [64,65].In a study of inhomogeneous plastic deformation in the AZ31 sheet the large strain gradients were identified on the sheet surface parallel and perpendicular to the loading direction [66].In contrast,very little deformation occurred in the thickness direction that caused an abrupt fracture following the development of a premature but extensive diffuse neck.The study found that friction stir processing,used to refine the microstructure and enhance strength,modified the basal texture thus significantly improved the forming limit for in-plane strain path.

Fig.3.Twin roll casting of magnesium alloys: (a) twin roll caster at CanmetMATERIALS and (b) the processing line schematics;(c) surface view and cross section of the AZ31 strip 5 mm thick and 250 mm wide with individual zones and asymmetric microstructure of top and bottom columnar zones [59];(d)mathematical modeling of the twin roll casting process,model-predicted strip surface stress development (upper graph) and the corresponding liquid-solid profile (lower graph) for set-back distance SB=32.5 mm and casting speed v=1.0 m/min [60].

The study of mechanical response of 2 mm thick AZ31BO sheet at room temperature explored five sheet orientations and tension/compression experiments to measure the strain componentsviaa digital image correlation [67].The evolving asymmetry was revealed and the continuum-based material model,utilizing the Cazacu–Plunkett–Barlat (CPB)-type yield function,was proposed to fit the alloy behavior as a continuous function of plastic strain.A considerable improvement of the material behavior was achieved as the number of stress transformations used in the CPB yield surface formulation increased.The effect of strain rate-sensitivity on mechanical behavior of AZ31 was investigated under wide range of applied strain rates by using an improved self-consistent polycrystal plasticity model [68].

The role of alloy chemistry was researched by changing the aluminum and manganese content in AZ alloy series and hot rolling of six compositions containing 3,4,and 6 wt.%Al at two levels of manganese [69].The study revealed that increasing the Al content to 6 wt.% resulted in edge cracking during hot rolling.In contrast,additions of Mn compensated for the detrimental effect of Al.The deformation mechanisms involved dislocation creep dominating at high strain rates to grain boundary sliding dominating at low strain rates and were more influenced by Mn than by Al.Moreover,the effect of Al content on flow stress was evident at high strain rates but was negligible at low strain rates.

6.2.Testing the commercial ZEK100 alloy for sheet production

As a solution to improve deficiencies of AZ31,the commercial ZEK100 (1Zn,0.25Zr,0.2Nd,wt.%) alloy,containing small addition of rare earth neodymium was introduced into sheet manufacturing [70].Researching the ZEK100 alloy forming behavior and a possibility of further improving its performance was a subject of several projects.

To compare the warm forming behavior of the ZEK100 and AZ31 grades the Limiting Dome Height (LDH),and Cylindrical Cup Draw experiments were utilized at temperature up to 350 °C [71].The results confirmed the enhanced formability of ZEK100 over the entire temperature range,in particular between room and 150 °C.

Fig.4.Examples of poor formability of magnesium alloys: (a) internal cracking of AZ31 strip during twin roll casting;(b,c) edge cracking during rolling of ZEK100 at 350 °C with argon protection;(d) cracking with “alligator split” during rolling at various parameters of ZEK100 containing 0.47 wt.% Ca;(e,f)brittle intergranular fracture surface of Ca containing ZEK100.

To assess the role of Nd in ZEK100,a combination of FactSage calculations and measurements using several experimental techniques was explored [72].For Zn contents of 2 wt.% and 4 wt.%,an increase in Nd content in the range of 1–2 wt.% caused a reduction in liquidus temperature but an increase in solidus and eutectic temperatures.No obvious influence of Nd content on the secondary dendrite arm spacing was revealed for a wide range of cooling rates.In another study,the as-cast ZEK100 alloy was subjected to hot rolling at temperatures from 350 °C to 450 °C to reach the 1.5–1.7 mm thick sheet [73].The rolling temperature affected the sheet properties and an increase from 250 to 450 °C caused the reduction of tensile strength from 257 to 228 MPa.For the same rolling temperature change,the compressive strength reduced from 418 to 351 MPa.

6.3.Experimental alloys with yttrium

In addition to the commercial ZEK100 alloy,novel compositions were tested in rolling and compression.As alternatives,the single-phase solid solution alloys,Mg2.9Y and Mg2.9Zn(wt.%),and two-phase (αMg solid solution+intermetallic)Mg-Zn-Y alloys were selected using the CALPHAD (Calculation of Phase Diagram) method [74–77].Hot compression experiments revealed that the effect of Y solute and texture weakening were connected.The progress of dynamic recrystallization during deformation did not lead to texture weakening,regardless of the presence of Y.In contrast to Zn,a presence of Y in solid solution suppressed dynamic recrystallization,however,a strong basal texture with the same maximum intensity developed in the Mg2.9Y and Mg2.9Zn alloys after compression and rolling at 350 °C (Fig.5).

7.Magnesium forging

Forging is considered a promising candidate for industrial manufacturing of net shape components of magnesium alloys.

The improved mechanical properties of forgings result from a reduction of casting defects due to healing of pores,refinement of primary phases,refined and elongated grains in the direction of the metal flow during plastic deformation.The magnesium forging projects involved the process modelling and experiments with the forging stock in as-cast state and the pre-deformed,mainly extruded stock of commercial grades.The alloy properties after forging were subject of several studies with a focus on the role of process parameters.

7.1.Forging process modelling and tool design

The feasibility of forging of complex automotive component of magnesium alloys was researched through numerical simulation using the commercial finite element package DEFORM 3D [78].The process verification through multiple parametric sensitivity analysis of the AZ80 and ZK60 extruded stock was investigated to determine the effect of factors such as flash land,friction,temperature and ram speed on the forging load,and material flow.The research concluded that increasing the flash width increased the press load while a substantial decreasing the flash width resulted in under-fill in the die cavity.A similar role as flash land was played by friction.

A design of the hot forging tool was a subject of research to manufacture forgings of the control arm in a single-hit operation [79].The as-forged component exhibited a shape of a near-complete arm with excess flash,requiring subsequent trimming and finishing operations.The tool was validated using thermal and mechanical simulations under assumptions of a batch type,low-volume production rate at a maximum operational temperature of 400 °C and a loading capacity of 1600 tonnes.

Fig.5.Magnesium sheet development,hot rolling of Mg2.9Y and Mg2.9Zn alloys: EBSD map of the as-rolled Mg2.9Y alloy: (a) parent grain with basal orientation;(b) ND-TD orientation.low angle boundaries are shown in black,and the colored triangle indicates the orientation of the ND;(c) side views of the reduced zones displaying the RF vectors associated with low angle boundaries present in the EBSD maps (a) and (b),respectively,the two dashed white lines represent vectors that are inclined at 30° and 60° to the c-axis,and the white quarter circles identify the misorientation angles associated with the RF vectors;Flow curves of the Mg2.9Zn (e) and Mg2.9Y (f) alloys from tensile tests (room temperature,strain rate of 10–3 s-1) in three sheet directions (RD,45°,TD);alloy compositions are in wt.% [77].

A study of hot deformation behavior of cast and the extruded AZ80 stock destined for forging showed that multiple dynamic recrystallization mechanisms were simultaneously active in both starting stock,resulting in significant grain refinement [80].Since industrial forgings typically involve strains much higher than 1,the comparable final microstructure and texture were expected in industrial-scale forgings of the AZ80 alloy at 400 °C,irrespective of the starting material state.

7.2.Strength and fatigue of alloys after forging

The study of fatigue of the forged AZ80 alloy aimed at understanding its cyclic behavior and predicting the fatigue life.It concluded that the forging thermomechanical history of material caused texture strengthening and a rotation of the crystallographic cells to align with the loading direction during forging [81].Moreover,the biaxial fatigue response was dominated by the axial component and the non-proportional effect being detrimental to the fatigue life.For the extruded AZ80 stock,the optimal forging condition of temperature of 250 °C and a speed of 20 mm/sec favored the coldest temperature and fastest strain rate,which could produce forgings free of defects.The forged alloy exhibited comparatively longer fatigue life under strain-controlled cyclic loading [82,82,83](Fig.6).The merits and drawbacks of various fatigue damage parameters were investigated for predicting the fatigue behavior of die-forged AZ80 components and the strain energy density proved to be the most robust method of comparison[84].

For the AZ31 alloy,a significant increase in fatigue life was achieved after forging at 400 °C [85].Forging improved the fatigue lifeviaa combination of grain refinement and texture modification,resulting in higher strength and ductility.Based on an improvement in both the monotonic and cyclic properties as compared to the as-extruded stock,the optimum forging conditions pointed towards the lowest of temperatures utilized during research.

The open-die hot forging of the cast AZ31B alloy generated a refined microstructure with strong basal texture causing 143% increase in tensile yield and 23% increase in ultimate strength[86].The strength increase was attributed to the grain refinement and the formation of strong basal texture,which activated the non-basal slip on the prismatic and pyramidal slip systems instead of extension twins.

8.Extrusion

Fig.6.Forging of magnesium alloys: (A) CanmetMATERIALS forging presses,1500 ton triple action press (left) and 1200 ton single action squeeze casting press (right);(B) microstructural characterization of AZ80 extrusion forged at 250 °C and 20 mm/sec.Basal and prismatic pole figures are presented in locations 1 (a),location 15 (b) and location 8 (f) of the forging.Figure (c) and (e) show optical images of locations 1 and 8,respectively within the forging(view in the longitudinal direction).Figure (d) denotes the spatial variation of superficial hardness (30T) in the direction normal to the cross-section of the forging.FD denotes forging direction,TD: transverse direction,and LD: longitudinal direction;(C) axial stress strain response for as-extruded and extruded forged AZ80;solid lines indicate monotonic response and dotted symbols indicate the stabilized cyclic response;(B,C) are from Ref.[83].

Extrusion suits the structural components where fatigue strength is the design requirement.Therefore,the fatigue life assessment of extruded alloys was a frequent research topic.Although the major alloy utilized during extrusion research was the commercial AZ31,attempts of development of novel extrusion alloys were also entertained.

8.1.Fatigue life of extrusions

The study of multi axial behaviors under proportional and non-proportional loadings of wrought alloys from AM,AZ,and ZK family showed that the fatigue life predicted using the energy-life model was in good agreement with experimental findings [87].Fatigue testing of the AZ31B extrusion under monotonic and cyclic conditions with axial and torsional loading modes showed that the monotonic axial stress-strain behavior was a direction dependent due to different deformation mechanisms involved [88,89].A significant yield anisotropy and sigmoidal-type hardening were observed and twinning-detwinning deformation appeared to be the major cause of these behaviors.The overall conclusion was that the total energy as the sum of plastic and positive elastic strain energy densities correlated with the fatigue life for several wrought alloys under different loading conditions.

The study of cyclic deformation characteristics and low cycle fatigue behavior of the AM30 extruded alloy concluded that increasing the total strain amplitude caused increasing both the plastic strain amplitude and mean stress while decreasing the fatigue life[90].A significant difference between the tensile and compressive yield stress occurred,leading to asymmetric hysteresis loops at high strain amplitudes due to twinning in compression and subsequent de-twinning in tension.

In another project,the anisotropic fatigue and cyclic behavior of the AM30 extrusion was investigated by fully reversed strain-controlled tension-compression cyclic tests at strain amplitudes between 0.3% and 2.3%,along extrusion and transverse directions [91].Moreover,multiaxial fatigue characteristics of the AM30 extrusion were investigated through fully reversed strain-controlled cyclic experiments,including pure torsional and combined axial-torsional at 0,45 and 90° phase angle shifts.For the ME20 alloy with additions of 0.3 wt.%Ce the measurements of strain-controlled cyclic deformation characteristics and low-cycle fatigue life,revealed that while cyclic stabilization was barely achieved even at the lower strain amplitudes,cyclic softening was the predominant characteristic at most strain amplitudes [92,93].

Fig.7.Twinning and texture development in an extruded AM30 alloy during compressive deformation: (a) microstructure;(b) (0001) and (c) (1010) pole figures of extruded state;schematic illustration of the growth of {1012} extension twins in a grain: (d) formation of twins inside a grain by rotating basal planes at an angle of 86.3°,(e) twin growth,(f) coalescence of twins (or vanishing of twin boundaries) to continue twin growth [96].

8.2.Formability of extruded alloys

A cyclic plasticity model for AZ31B,based on a modified Armstrong-Fredrick nonlinear model that involved two sets of material constants for cyclic tension-compression and cyclic shear loading was proposed [94].The model was able to generate different hysteresis loops under axial,shear,and biaxial loading conditions.At the same time,the model could predict yield asymmetry and mean stress in cyclic tensioncompression loading and symmetric hysteresis in cyclic shear loading.

The research on twinning and texture development in the extruded AM30 alloy during compressive deformation aimed at evaluating the microstructure,mechanical behavior,and texture response in uniaxial compression[95].A special attention was paid to the effect of compressive strain amount,sample orientation,loading direction,compressive pre-strain,and annealing.During compression of the extruded AM30 alloy along the extrusion direction,three stages of twin growth with increasing strain were observed: (I) relatively slow and gradually accelerating growth;(II) steady stage with twins width increased linearly with increasing strain;(III)stage when twin growth became decelerated,exhibiting a plateau-like character[96] (Fig.7).

The study of microstructure effects on the formability of rolled and extruded AZ31 sheet revealed that the degree of dynamic recrystallization was the factor that controlled elongation [97].The digital image correlation analysis revealed that the local stresses developed during deformation satisfied the Considere’s criterion,although the global strains were lower than the theoretical predictions.For the AZ31 alloy,refining the grain structure was identified as the major benefit of equal channel angular pressing or extrusion at 200,250 and 300 °C.For the same AZ31 alloy,processing by equal channel angular pressing led to increased strength and ductility reduction,suggesting the larger effect by grain size than by modification of the crystallographic texture [98].

8.3.Improving properties of AZ31 extrusions by strontium additions

The conventional low-cost extrusion alloys based on the Mg-Al-Zn and Mg-Zn-Zr systems have many deficiencies,including poor formability.A number of activities were oriented towards overcoming this challenge through alloying with strontium.Since the 1970s,Sr is the preferred modification element of aluminum alloys.In contrast to rare earth elements,Sr forms second phases that are thermally stable due to its negligible solubility in Mg at elevated temperatures.Therefore,Mg-Sr second phases are expected to be more effective than rare earths in nucleating recrystallization and in potentially weakening the texture.

For example,small additions of Sr in the AZ31 alloy refined the Mg17Al12phase and increased its thermal stability during annealing [99].With higher concentrations of Sr,the volume fraction of the Mg17Al12phase decreased while the volume fraction of the Sr-rich (Al-Mg-Sr) interdendritic precipitates increased.At room temperature,the AZ31 alloy exhibited higher ductility and lower strength than Sr-containing alloys but at 200 °C the AZ31 alloy showed the higher flow stress.Another project explored Sr additions of about 2 wt.%to develop new extrusion alloys based on the Mg-Mn-Sr system [100].The research concluded that high Sr levels that increased the amount of second phases for particle stimulated nucleation (PSN) around Mg-Sr intermetallics during recrystallization would be beneficial unless the second phases do not reach the critical amount,which would be detrimental to the alloy ductility.

9.Alloy deformation mechanisms,recrystallization,texture

Fundamental understanding of the deformation behavior of magnesium,including single crystals,laboratory and commercial alloys was extensively studied with substantial number of projects [101,102].A recent review of emerging hot topics and research questions in wrought magnesium alloy development by an international panel with Canadian contribution,identified the crystallographic texture and its modificationviaalloying and thermomechanical processing as one of major challenges [103].

9.1.Hot deformation

An experimental program to assess deformation mechanisms of commercial extrusions of AM30 and AZ31,sheet of AZ31,and high pressure die castings of AM50 and AM60 was summarized in Ref.[104].Hot deformation of as-cast and rolled AZ31 was also investigated [105].A combination of unique biaxial experimental tests and plasticity simulations of biaxial crystal using a visco-plastic self-consistent formulation was conducted for the extruded and as-cast AZ80 alloy[106].The experiments involved tubular samples,loaded in axial tension or compression along the tube and with internal pressure to generate hoop stresses orthogonal to the axial direction.Although simulations generally matched well with the experiments,particularly for the cast material with weaker texture,some differences were observed for cases where basal slip and extension twinning were in close competition.

The dynamic mechanical behavior of cast AZ and AE alloys with different chemistries was investigated at strain rates between 800 and 1400 s-1to determine the effects of composition on the response to shock loading [107].The results showed that an increase in the aluminum content of the AZ alloys increased the volume fraction of the Mg17Al12and Al4Mn phases,strength and strain hardening but decreased the ductility and twinning fraction,particularly the extension twinning fraction.The role of Al content in hot deformation and fracture characteristics of Mg1Zn (wt.%) based alloys was also examined in a combination with varying Mn levels[108].The microscale plastic strain heterogeneity can arise in polycrystalline Mg and its alloys in a variety of different ways and digital imaging research showed a link between the heterogeneity and plasticity,damage,and ductility [109].

Hot deformation behavior of a superlight dual phase Mg9Li3AlY (wt%) alloy was investigated by developing constitutive equations and constructing processing maps with relationships established by predicting the materials constants based on true stress-strain curves [110].Moreover,the critical resolved shear stress for basal slip in magnesium alloys was determined using instrumented indentation.The good agreement was achieved between the analysis from spherical indenters with different radii and literature results from single crystal tests using Mg-Al alloys [111].During testing of the Mg1Zn0.5Nd (wt.%) alloy the indentation size effect showed a change in behavior.

The effect of precipitate characteristics on extension twinning was studied in a Mg6Zn (wt.%) alloy after extrusion at 370 °C.The findings revealed that the increment of twin system strengthening was attributed to the back stress generated by elastically deforming particles [112].An investigation of the influence of precipitate state and annealing temperature of 190 °C,350 °C and 400 °C on the recrystallization of a Mg2.8Nd (wt.%) alloy using electrical resistivity measurements and modeling helped understanding how Nd interacts during recrystallization and how it affects the final microstructure [113].A deformation of several alloys including AE42,AJ32,AX30,EZ33 and ZE10 at elevated temperatures up to 175 °C was investigated with utilizing the non-conventional techniques to assess their creep behavior [114].The results showed that the aluminum free magnesium alloys (i.e.,EZ33 and ZE10)had higher creep resistance compared to aluminum containing alloys (i.e.,AE42,AJ32 and AX30).As explanation,the nanoprecipitates uniformly dispersed throughout the matrix were credited for contributing to dispersion strengthening and the improved creep resistance.

9.2.Texture

Fig.8.Hot deformation of magnesium alloys: (a) equilibrium phase diagram and (b) thermomechanical processing procedures of micro-alloyed ZE-01,ZE-02,and ZE-05 alloys;microstructures and corresponding Intragranular misorientation analysis (IGMA) distribution of the (c) basal,(d) RD-split,(e) TD-split texture components of 550 °C-solutionized ZE-02 subjected to hot rolling and 3-minutes annealing [117].

Research of uniaxial compression testing over a wide range of strain rates covered the microstructure and texture evolution during high-speed rolling (HSR) of AZ31B and four Mg-Zn-Ce alloys[115].The results showed that with increasing strain rate the twin-induced dynamic recrystallization fraction increased at a constant temperature during the compression test over a wide range of strain rates.At the same time the contribution of continuous dynamic recrystallization decreased.The hot deformation behavior of the as-extruded ZK60 alloy was investigated using uniaxial hot compression tests over the temperature range of 300–450 °C to determine the influence of compression direction on recrystallization behavior and texture evolution [116].When a weak and close to random texture developed after deformation along the extrusion direction,the radial direction samples exhibited mainly texture strengthening.

The texture modification by alloying additions was pursued in a number of projects.For example,small neodymium additions of 0.2 wt.% led to the best combination of the weak TD-split texture and enhanced ductility [117].To reduce the content of Nd,calcium was introduced through Mg1Zn0.1Nd0.2Ca and Mg1Zn0.2Ca (wt.%) alloys.The alloys were designed based on the concept of promoting prismatic slip with the increased solute of alloying elements(Fig.8).Understanding differences between an influence of Ca and rare earths on dynamic recrystallization were studied using the Mg0.5Ca,Mg1Ca and Mg2Zn2Nd (wt.%) compositions[118].The research concluded that the Nd related texture component was generated by extruding the Mg0.5Ca (wt.%)alloy under negative strain rate sensitivity conditions and that the strain rate influenced the deformation and dynamic recrystallization behavior of the Mg2Zn2Nd (wt.%) alloy.

10.Thermodynamic modelling and phase diagrams

For metallic alloys,phase diagrams form visual representations of their state as a function of temperature,pressure,and concentrations of the constituents.The progress in CALPHAD allowed development of thermodynamic databases and creating phase diagrams using computer modelling and selective experimental verification.Using different modelling approaches,many binary,ternary and higher order magnesium systems were developed.

The thermodynamic database for the Mg-Cu-Ni-Y system was determined by combining the thermodynamic descriptions of the constituent binary and ternary sub-systems.Then,the modified quasi-chemical model was used to describe the liquid phase in this system,which accounts for the short-range ordering[119].In another study,the 450°C Mg-Mn-{Ce,Nd}and 300 °C Ce-Mg-Zn isothermal sections were established using diffusion couples and key binary alloys [120].The Mg-Nd-Zn isothermal section at 300 °C was created in the full composition range using diffusion couples and equilibrated key alloys [121].The solubility of Zn in Mg-Nd compounds was found to increase with the decrease of magnesium concentration.

Strontium has drawn much attention as Mg alloying addition for improving high-temperature mechanical properties.Phase equilibria in the Mg-Zn-Sr ternary system were investigated using two diffusion couples and 23 alloy samples.As a result,the isothermal phase diagram of the Mg-Zn-Sr ternary system,in the composition range 0-35 at% Sr,was developed at 300 °C [122] (Fig.9a).Thermodynamic modeling of the Mg-Al-Sb system with the constructed database was used to predict thermodynamic properties,binary phase diagrams of Al–Sb and Mg–Sb,and liquidus projections of the ternary Mg–Al–Sb system [123,124] (Fig.9b).Liquid phases were described by the Redlich–Kister polynomial model,whereas the high temperature modification of Mg3Sb2compound in the Mg–Sb system was described by the sublattice model.The Mg–Al–Sb system modelling predicted a closed ternary liquid miscibility gap,six ternary eutectics,two ternary peritectics,four saddle points and a critical point.

Fig.9.Thermodynamic modeling and phase diagrams of magnesium alloys: (a) isothermal section of the Mg–Zn–Sr ternary diagram at 300 °C [122];(b)projection of the liquidus surface of the Mg–Al–Sb system onto a ternary composition triangle [123].

Moreover,modelling of the Mg-Al-Bi and Mg-Al-Sb systems was conducted where four binaries: Mg-Bi,Mg-Sb,Al-Bi and Al-Sb were critically evaluated and optimized based on phase-equilibrium and thermodynamic data available [125].

11.Magnesium matrix composites

Research on magnesium composites was rather limited with several examples,covering manufacturing the composites through mechanical alloying,reactive infiltration,or the bimetallic assembly concept.Injection molding was also explored to manufacture composites,mainly with reactive SiC reinforcements.

Mechanical alloyingviaSPEXTMhigh energy milling was used to synthesize the Mg-Al-Zr nanocomposites,characterized by the dispersion of nanocrystals in an amorphous matrix [126].To promote the glass formability,zirconium was added and an improvement in thermal stability achieved was attributed to the presence of Al3Zr compound that prevented the grain growth through grain boundary pinning.The costeffective processing route ofin situreactive infiltration was used to fabricate the magnesium AZ91D matrix composites reinforced with a network of TiC–TiB2particulates [127].As compared with the unreinforced AZ91D alloy,the elastic modulus,flexural and compressive strengths of the composite were greatly improved at the cost of reduced ductility.

In addition to conventional metal matrix composites,bimetal composites were researched.In general,bimetal composites are made from two kinds of metals with different physical,chemical,and mechanical properties and become metallurgically bonded at the interface.The research involved the bimetal composite rods,composed of a softer AZ31 sleeve and a harder WE43 core.The composites were fabricatedviaa special process of combining the hot-pressing diffusion with co-extrusion [128].The gradient in composition and microstructure along with the superior interfacial bonding led to a higher compressive and tensile yield strength of AZ31/WE43 bimetal composite rods,compared to the AZ31 sleeve.

12.Welding and joining

Understanding the weldability of magnesium alloys is essential for their applications in automotive industry [129].As a thermomechanical processing with optimal welding parameters,friction stir welding positively affects the alloy microstructure through refining the grain size in the weldmentviasevere plastic deformation and recrystallization.Ultrasonic welding is a solid-state joining method that produces weld joints by localized high-frequency tangential vibration under moderate clamping pressure with a sonotrode,a tool that generates ultrasonic vibration and delivers energy to the workpiece.Both techniques were explored in projects that involved joining alloys of magnesium with magnesium and dissimilar joints of magnesium with aluminum.The particular research interests were generated by the latter one due to many challenges and opportunities it represents.

12.1.Magnesium to magnesium joining

During investigation of the friction stir welded AZ31 alloy,both the alloy strength and ductility decreased with a joint efficiency from 75% to 82%.The changes were caused by both the grain structure and texture modifications,which also weakened the strain rate dependence of tensile properties [130].The welding speed and rotational rate exhibited stronger effect on the yield stress than on the tensile strength.Further,with increasing tool rotational rate or decreasing welding speed more energy was supplied,leading to higher temperature of the stir zone and reducing the maximum intensities of [0002] and [01–10] poles.In conclusion,the higher weldment temperature caused more complete dynamic recrystallization and weaker texture (Fig.10).

Fig.10.Friction stir welding of the AZ31 alloy: (a) process schematics and cross section of the welded joint;(b) effect of welding speed and rotational rate on the maximum relative pole intensity at the center of stir zone in a joint made at welding speed of 20 mm/s and tool rotational rate of 1000 rpm and 2000 rpm [130].

Monotonic and fatigue behavior of the AZ31 alloy after friction stir spot welding were investigated as a part of the MFERD project [11].Fatigue tests were conducted in load control atR=0.1 at two different maximum loads: 1 kN and 3 kN.A good agreement was reported between the participating laboratories,regarding the number of cycles to failure.Differences in the failure modes were observed between the two loading conditions tested: when at the higher load,fatigue failure was caused by the interfacial fracture,at the lower load,fatigue cracks formed perpendicularly to the loading direction.

12.2.Dissimilar joints of magnesium to aluminum

The projects on welding of dissimilar metals covered the magnesium-aluminum combination.In particular,the feasibility of joining the ZEK100 Mg alloy to 5754 Al alloyviasolid-state ultrasonic spot welding [131] and ZEK100 to AA6022 Al alloy [132] was researched.

Dissimilar ultrasonic spot welding of the ZEK100 magnesium-to-aluminum alloy with a zinc interlayer was a part of a collaborative project with the objective to characterize both Mg/Zn and Al/Zn interfaces between the ZEK100 Mg alloy and the AA6022 aluminum alloy with a zinc interlayer [133].The study revealed that the joining mechanisms of mainly mechanical interlocking and metallurgical bonding that was enhanced by grain refinement at both interfaces,as caused by dynamic recrystallization during ultrasonic spot welding.The peak tensile lap shear strength of the Mg-Al dissimilar joints was attained at a welding energy of 1000 J and the failure occurred within the diffusion layer at the Mg/Zn interface.

Joining the ZEK100 alloy and AA6022 alloy with an Ag interlayerviaultrasonic spot welding created the diffusion layer at the Mg/Ag interface that consisted of two distinctive sub-layers: Mg3Ag intermetallic compound adjoining Ag foil,and Mg3Ag+Mg eutectic structure adjacent to Mg [134].Only a thin diffusion layer containing mainly Ag3Al was formed at the Al/Ag interface.The shear strength achieved with Ag interlayer was about 31% higher than that of the joint without interlayer.Interfacial failure occurred at all energy levels,with Ag foil remaining on both Mg and Al sides due to its intense interaction with Mg and Alviaaccelerated diffusion that took place during ultrasonic spot welding.

13.Corrosion and coatings

The limited corrosion resistance of magnesium alloys represents a major obstacle in expanding their applications.To overcome this challenge,several studies aimed at explaining the magnesium corrosion mechanisms and at design of protective measures through the alloy chemistry and surface treatments.The research contributed to progress in understanding the microscale corrosion of magnesium alloys,development of new testing methodologies,and protective treatments.

13.1.General corrosion in aqueous solutions and testing techniques

Overall characteristics of corrosion resistance of magnesium alloys with procedures for testing of general and localized corrosion were extensively researched [135,136].Detailed study of oxide formation on the AZ91 alloy surface revealed the role of alloy ingredients pointing that Mg oxidized preferentially while leaving the Al component relatively unchanged[137].New approaches in testing of galvanic and microgalvanic corrosion using scanning probe techniques were correlated with the component manufacturing method including sand,graphite and die casting [138].The quantification of local reaction kinetics helped to determine the corrosion layer thickness and its porosity through numerical predictive tools.

The corrosion behavior of the uncoated friction stir welded AZ31 joints for automotive applications was evaluated by ASTM B117 salt fog testing and electrochemical techniques including potentiodynamic polarization as a part of the MFERD project[139].The improved corrosion resistance was observed in the stir zone compared to the base material in the as-received condition.The stir zone showed,however,almost identical corrosion behavior as the base material after the asreceived surface was cleaned.The results emphasized the deteriorative effect of surface contamination and insignificant role of grain size on corrosion resistance of the AZ31 alloy.

13.2.Role of alloy chemistry and microstructure in corrosion

In a study of influence of microstructure expressed by the size and distribution of intermetallic phases on corrosion of the AM50 alloy in 1.6 wt.%NaCl solution,components made by sand,graphite and die casting were compared [140].The corrosion performance improved in the order sand cast

To simulate the cathodic environment of Al-Mn compounds during corrosion of magnesium alloys,the electrochemical behavior of two Al-Mn materials (Al-5.5 at% Mn and Al-13.5 at% Mn) was studied in 0.275 M NaCl and 0.138 M MgCl2solutions [141].The experiment confirmed that the appearance of corrosion product domes on the Al-Mn intermetallic particles during corrosion was an indication of their cathodic behavior.Moreover,the Al-Mn intermetallic particles were the efficient but unstable cathodes.The beneficial effect of yttrium on wear,corrosion in tap water and salt solution of as-cast Mg3Al (wt.%) and AZ31 alloys was shown to be associated with yttrium additions through the formation of harder second phases [142].When small amounts of Y improved the corrosion resistance,benefits diminished at its higher contents.In another study,micro-alloying of the AM60B alloy with As,well-known cathodic hydrogen gas evolution poison,shown to be beneficial in reducing corrosion in 3.5 wt.% NaCl (aq),but not to the extent that was reported for unalloyed Mg [143].However,consistently with thermodynamic predictions,corrosion resulted in a formation of the toxic AsH3arsine gas,in addition to H2.

13.3.Protective surface treatments, self-healing coatings

To improve the corrosion resistance of magnesium alloys several surface treatments were researched.The study of room temperature degradation of the ZX10 alloy in different corrosive media determined the influence of the solution pH on passivation mechanisms [144].While the acidic systems led to the highest rate of metal corrosion the lowest corrosion rate was observed in the alkaline systems.The MgO coatings produced by micro-arc oxidation provided the robust corrosion protection to magnesium substrates.In particular,for the ZK60 wrought alloy,MgO coatings improved the general corrosion resistance and stress corrosion behavior under constant load of 80 MPa in 3.5 wt.% NaCl solution [145].

Although the self-healing materials attracted the attention of researchers for decades,their application is limited,and a mechanical strength is often listed as the weak link of the concept.Research on acrylic materials and self-healing mechanisms revealed that an acrylic elastomer,VHB 4910,had an excellent self-healing ability accompanied by sufficient mechanical strength [146].Based on this idea,self-healing anticorrosion coatings on the AZ31 alloy were developed that comprised a cerium-based conversion layer,a graphene oxide layer,and a branched poly (ethylene imine) (PEI)/poly(acrylic acid) (PAA) multilayer [147].The graphene oxide acted as corrosion inhibitor and the PEI/PAA multilayers provided the self-healing ability to the coating system.

Fig.11.Corrosion of magnesium alloys: (a) progress of ECORR measured on a sand cast AM50 Mg alloy in 3 mM NaCl+ethylene glycol.EIS measurements were made just before and after the addition of 25 mL of water at～2h;(b) Nyquist plots,and (c),(d) Bode plots recorded just before (black) and after (red)water addition;Schematic illustration showing the microgalvanic coupling process of naturally corroding AM50 surface (e) and a chronopotentiometrically controlled surface (f) [140].

Fig.12.Corrosion fatigue and crack propagation model in magnesium alloys: (a) applied load history (stress intensity factor K);(b) Stress corrosion Cracking,SCC velocity vs K curve;(c) Fatigue Crack Growth,FCG rate in inert and corrosive environment vs ΔK curves.Crack growth stages: I -SCC does not occur,fatigue is the only contributor to CFCG;II -SCC starts joining FCG to contribute to the CFCG;III -SCC velocity is independent of stress intensity [149].

13.4.Corrosion fatigue of coated alloys

An accelerated environmentally assisted degradation under cyclic loading is a concern when considering the magnesium structural applications.The effect of coatings on magnesium substrate degradation under stress was a subject of several studies.

The cold spray coatings of pure aluminum,applied to the wrought AZ31B alloy were evaluated in term of their influence on the corrosion and corrosion fatigue [148].A comparison of corrosion-fatigue S-N curves of stress relieved before and after coating showed the fatigue life reduction of almost 90%,attributed to a low strength of the coating.The low strength of Al coating led to the early crack initiation during fatigue cycles,from which the electrolyte penetrated to the Mg substrate and caused the localized corrosion and failure.

To develop a general corrosion fatigue model several surface treatments were researched including a top polymer painting,Mn-P conversion coating,chromate conversion coating,and micro-arc oxide coatings [149].As substrates,extruded/forged AZ80 and ZK60 alloys,cast AZ31B alloy after variety of pre-treatments were employed.The salt spray chamber testing according to the ASTM B117 standard revealed that the micro-arc oxide coatings provided the best corrosion performance for the ZK60 alloy without a scribe while the chromate conversion coatings provided the best corrosion performance for the same alloy with a scribe.Assuming that the corrosion fatigue is an interaction process between the pure fatigue and stress corrosion cracking the model describing the corrosion fatigue crack growth was developed(Fig.12).

14.High temperature oxidation,ignition,and flammability

Although magnesium is not the heat resistant material,its reactivity with oxygen at increased temperatures is of great concern for processing in liquid and solid states.The high surface reactivity of magnesium at elevated temperatures represents a major drawback to synthesis of alloys,manufacturing raw feedstock for further processing,and end-use components(Fig.13).Its serious challenge to manufacturing is emphasized by the fact that the new wave of magnesium oxidation research was initiated in an industrial environment [150].

14.1.High temperature oxidation

Fig.13.Degradation of magnesium alloys during processing: (a,b) oxidation and cracking during extrusion,AZ31;(c) surface oxidation and edge cracking during rolling in air,AZ31;(d) edge burning during twin roll casting,AZ31;(e) burning of liquid alloy on die cast pump during service cleaning,AZ91.

The initial research that mainly focused on Mg-Al system,revealed three distinct stages of the oxidation of magnesium alloys where a formation of protective oxide was followed by an incubation period with a subsequent transient to non-protective oxidation,at a rate either constant or sharply increasing over time [151].During early-stage reactions,depending on temperature and time,the alloy experienced protective or non-protective oxidation with linear or accelerated oxide growth kinetics [152].During reaction in an oxidizing atmosphere,additions of beryllium delayed the transient from the protective to non-protective scale formation.In an inert atmosphere,as that one used during processing utilizing argon gas,increased beryllium contents reduced the magnesium evaporation rate.The finding is of practical importance since a concentration of magnesium vapor represents a danger of explosion.The overall research determined the oxide growth mechanism and factors affecting the oxidation kinetics of magnesium alloys,helping to design and oversee the industrial protective atmospheres (Fig.14a–f)

14.2.Ignition and flammability

The high affinity of magnesium for oxygen at increased temperatures is often seen through its ignition and flammability (Fig.14g and h).

The most frequent exposure of magnesium alloys to high temperatures takes place during primary operations of raw alloy melting and then during component manufacturing that typically starts from casting.The perceived easy ignition creates a detrimental safety feature when considering the aerospace applications of magnesium [153].To improve the oxidation and ignition resistance of magnesium,alloying and/or surface treatments with rare earths are frequently seen as the most effective but also expensive option.Understanding the so-called“reactive element effect”and opportunities it creates for controlling the high-temperature oxidation and ignition of magnesium alloys was extensively researched [154].

The development of ignition resistant magnesium alloys by additions of Sr and Nd,confirmed the positive effect of both elements [155,156].In the range of 0 to 6 wt.% Nd,the effect of Nd was composition dependent,and the ignition temperature increased from 640 °C for pure Mg to 770 °C for 0.5 wt.% Nd.For Nd-rich alloys,oxidation rate increased due to accelerated oxidation at the metal/oxide interface.Alloying with Sr caused the gradual increase in ignition temperature from 640 °C to 860 °C at 6 wt% Sr and a formation of dense SrO-containing scale delayed the ignition.

In the subsequent study,simultaneous alloying with Sr and Ca,instead of rare earths,was found to improve the formation of a compact oxide scale at high temperatures [157,158].In contrast to pure Mg,heating rate affected the ignition temperature of modified alloys,hindering the protection effectiveness at high heating rates.The simultaneous use of Sr and Ca resulted in an increase of the ignition temperature of approximately 110 °C,as compared to pure Mg [159].

15.Corrosion of materials in liquid magnesium alloys

Liquid metal corrosion is understood as a physical or physico-chemical process that follows the formation pattern of metallic alloys.In contrast to corrosion of metals in aqueous solutions,no transfer of electrons is involved.Magnesium,in addition to high affinity for oxygen,while in liquid state is also highly corrosive towards materials it contacts.

Fig.14.High temperature oxidation and ignition of magnesium alloys: (a) oxidation kinetics of a magnesium alloy showing protective and accelerated reactions;(b) formation of oxide nodules on AZ91D alloy [151];(c–f) morphological development of magnesium oxide showing uniform film,oxide ridges,nodular growth and loose structures [150];(g) ignition test and temperature-time plot for heating the ZE41 alloy (h).

The corrosive nature of liquid magnesium imposes challenges for machinery designers to select materials to contain,transfer or process molten,or semisolid magnesium during manufacturing operations.The materials available at present have limitations and the research continues to develop more durable but less expensive alloys capable of increasing the processing temperature and extending the machinery service lifetime.Understanding the reactivity of liquid magnesium with engineering materials to eliminate or to reduce the components degradation is paramount for manufacturers of magnesium processing equipment.

15.1.Material solutions for containing molten magnesium

The industrial equipment for magnesium processing,with components that are entirely or partly exposed to liquid metal,include crucibles of melting furnaces,elements of die casting and injection molding machinery such as shooting pots,plungers,dies,injection barrels,injection screws,check-valves,then parts of pumps,nozzles,and melt transfer systems [160].Although the primary requirement imposed on machinery materials specifies the chemical resistance to molten magnesium,there are also other essential properties needed,depending on service conditions.The equally critical properties require a sustainable level of strength and toughness,creep resistance,corrosion fatigue,erosive wear,as well as the resistance to oxidation in air at high temperatures typically above 600 °C on the opposite side that contacts air.Single alloys are often restricted by such complex service conditions and bimetal composite solution help meeting the design requirements (Fig.15).The extensive research of liquid magnesium corrosion was conducted by industrial laboratories as a part of the machinery design and testing but due to the proprietary nature the results remain unpublished.

16.Biomedical applications of magnesium alloys

Magnesium based materials are considered promising biodegradable metals for orthopedic bone implants as they exhibit similar density and elastic modulus to that of bone,biodegradability,and excellent osteogenic properties.Their use eliminates the limitations of currently used implant materials such as stress shielding and a need for the second surgery.However,the high corrosion rate caused by the lowest standard electrode potential of Mg (-2.37 V) results in the excessively fast degradation after implantation in the human body what may reduce the mechanical support before the bone heals [161].The interdisciplinary research of bio magnesium requires typically expertise in materials science,chemistry,and medicine.As a related subject,the laser-based additive manufacturing of magnesium alloys for bone tissue engineering is gaining attention [162].

16.1.Binary Mg-Sr and ternary Mg-Sr-Ca alloys

Investigation of new biodegradable magnesium alloys with improved biocorrosion,biocompatibility and mechanical properties for use in temporary cardiovascular stents focused on the Mg-Sr and Mg-Ca-Sr compositions[163,164].The biocorrosion evaluation in simulated body fluid showed the slowest corrosion rate by the Mg-0.5Sr (wt.%) alloy.The indirect cytotoxicity assays using human umbilical vascular endothelial cells(HUVECs)indicated that this alloy did not cause any inhibitory effect on the viability of the cells.The Mg-0.3Sr-0.3Ca (wt.%) alloy showed better thermal stability than the binary alloy and no thrombosis caused by the stent.The research concluded that the surface-active effect of Sr was a key advantage of materials for biodegradable implants (Fig.16).

Fig.15.Corrosion of materials in liquid magnesium alloys and industrial machinery solutions: (a) factors affecting machine components performance;(b–d)tool steel component after intermittent exposures to liquid magnesium alloy;(e) bimetal concept showing concentric tubes joined by a shrink fit method to handle liquid magnesium corrosion,wear (internal) and high temperature strength,and creep (external);(f-i) plasma-transferred arc,PTA,bimetal composite cladding of Inconel 718 with Stellite 12 alloy: (f,g) microstructure of surface clad of Stellite 12;(h) transient zone between base and clad;(i) Inconel 718 of the substrate;(c–i) are from Ref.[160].

The comparative study of biocompatibility involved four Mg-Sr based alloys including Mg0.5Sr,Mg0.3Sr0.3Ca,Mg0.3Sr0.3Ca0.1Zn,and Mg0.3Sr0.3Ca0.3Zn (wt.%).As a reference,pure magnesium,the WE43 alloy,and 316 stainless steel were used [165].The tests concluded that the zinc addition to the Mg-Ca-Sr alloy improved the corrosion resistance and developed alloys caused less hemolysis compared to the WE43 grade.Hemolysis results showed that Mg-Sr based alloys caused less hemolysis compared to WE43 and the hemolysis rate increased with the increase in alloy weight.

16.2.Coatings of hydroxyapatite (HA), dicalcium phosphate dehydrate (DCPD) and dicalcium phosphate dehydrate with or without stearic acid (DCPD±SA) on pure magnesium

A novel application of coated magnesium was researched to evaluatein vitrobiocompatibility and anti-proliferative properties of differently coated magnesium alloys in a primary culture of human Tenon’s capsule fibroblasts (HTCFs) [166].The coatings included Hydroxyapatite,Dicalcium phosphate dehydrate,and Dicalcium phosphate dehydrate with Stearic acid(DCPD+SA),The goal was to establish a proof of principle for further exploration of magnesium as a potential adjunct to glaucoma surgery with an anticipation that bio-degradable coated magnesium alloys will inhibit cellular proliferation and reduce myofibroblast activity in a primary culture of HTCFs.The study concluded that coatings were able to affect the corrosive properties of magnesium and no significant difference was found in metabolic activity or necrosis at different times during the logarithmic phase of HTCFs and in comparison to titanium the coated magnesium alloys attenuated the HTCFs proliferation.

16.3.Plasma enhanced chemical vapor deposition(PE-CVD) polymer and electrochemically formed polypyrrole (PPy) coatings on WE43 alloy

This research focused on investigating the possibility of using biocompatible plasma polymer coatings fabricated by PE-CVD under various deposition condition to control degradation rate of the biocompatible WE43 alloy [167].The study confirmed that a PE-CVD coating produced from 1,3-butadiene and ammonia (PPB:N) offered the best anticorrosive properties,and this coating was subsequently chosen for further optimization and characterization for its anticorrosive properties.It was found that the anti-corrosive properties of the PPB:N depend on both the deposition time and deposition pressure.

Improving anticorrosive properties of electrochemically formed polypyrrole coatings on the WE43 alloy by graphene oxide composites and forced passivation has application potentials [168].Further improvement of the corrosion protection properties of PPy coatings was researched through (i)incorporation of reduced graphene oxide (r GO) into the PPy polymer matrix to create r GO/PPy composite coating s (r GO/PPy WE43) on WE43,and (ii) forced passivation of WE43 after being coated with PPy.When compared to unmodified PPy coated WE43,the total corrosion resistance for r GO/PPy WE43 increased 82% and for passivated PPy WE43 it increased by 110%.

Fig.16.Biomedical applications of magnesium alloys: (A) challenges of utilizing magnesium alloy for industrial and biomedical applications [161];(B)optical and histology images of vascular tissue surrounding (a,b) Mg0.3Sr0.3Ca and (c,d) WE43 tubular stent samples implanted in right and left femoral artery for 5 weeks;schematic presentation of the Mg0.3Sr0.3Ca alloy biocorrosion interface in physiological environment: (e) galvanic corrosion and Mg dissolution;(f) formation of partially protective layer of Mg(OH)2;g) diffusion of Cl- ions and inward shift of corrosion interface,thus creating a dynamic scale formation/dissolution equilibrium with (f);(h) Sr-substituted HA formation;and (i) formation of a compact and homogenous layer of HA on the surface.The thickness of the layers is exaggerated to better visualize the mechanism [164].

17.Magnesium air batteries

To meet the requirements for high energy density storage systems,rechargeable batteries based on the “beyond lithium ion” technologies are investigated.Magnesium–air batteries that use magnesium metal as the negative electrode are promising candidates for next-generation electrochemical energy storage.They exhibit very high theoretical energy output,are biocompatible,can work in neutral electrolytes and are considered for implantable electronics[169,170](Fig.17).However,their development is severely hindered by sluggish solid-state diffusion and significant desolvation penalties of the divalent cation.In the search for high Mg mobility oxide positive electrodes,an analysis of the diffusion pathway and the topology of cation sites helped identifying the low-energy intermediate sites in addition to the requirement of Mg being found in a nonpreferred coordination environment [171].

The synthesis,structure,and electrochemistry of positive electrode materials,various aspects of magnesium air batteries were investigated [172].The study concluded that the magnesium battery is a promising candidate benefiting from the utilization of a Mg metal negative electrode,which offers high volumetric capacity (3833 mAh mL-1),low redox potential (-2.37 V vs Standard Hydrogen Electrode,SHE) nondendritic growth,low cost and safe handling in atmosphere.However,the discovery of the potential positive electrode materials beyond the seminal Mo6S8has been limited,mainly due to the sluggish mobility of a divalent Mg2+ion in solid frameworks.

The synthetic material solutions,potentially crucial in the discovery and design of novel Mg cathode materials were also considered with a prospect of CF-type NaV1.25Ti0.75O4[173,174].While the low migration barrier predicted by computation was partly based on the relative metastability of the theoretical CF-MgxV1.25Ti0.75O4lattice,the difficulty in stabilizing it also rendered the material synthetically nonaccessible,hindering this post-spinel application as an electrode material.

18.Magnesium for hydrogen storage

The storage problem is a continuous challenge in hydrogen economy that includes production,storage,safety,and utilization.The concept of storing hydrogen in a metal,known as a hydride,is not new and has been explored for decades [175].

Fig.17.Magnesium air batteries: (a) theoretical energy density and specific energy (including oxygen) of commonly researched metal-air batteries [169];(b)schematics of Mg2+ diffusion barriers [174];(c) activation barrier for Mg diffusion along hops 1 and 2 in the Mg2Mo3O8 structure,with the normalized path distance on the x axis;(d) a closer view of hop 1,where the numbered circles correspond to various intermediate sites along the hop as labeled in(c) intermediate tetrahedral site,which is edge-sharing with the stable tetrahedral site (green),is indicated in yellow;(e) alternate pathway for hop 1 that involves intermediate octahedral (dark blue) and tetrahedral (yellow) sites,which are face-sharing with the stable tetrahedral (green) and octahedral (orange)sites,respectively.The intermediate sites in (e) also share a face with the MoO6 octahedra (blue) [171].

Magnesium is an attractive material in this respect due to its reversible hydride formation,high theoretical gravimetric hydrogen capacity,natural abundance,and low cost.The reaction of Mg with hydrogen yields a stable hydride(MgH2)with a hydrogen gravimetric and volumetric capacity of 7.6 wt.%and 1.10 kg/L following reaction:

Although many promising solutions were developed the critical breakthrough in the hydrogen storage technology is still expected [176].A number of promising alloys and composites were researched as possible candidates for hydrogen storage.

18.1.Binary and ternary magnesium alloys

The binary alloys were used to establish a baseline case for the ternary Mg–Al–Ti,Mg–Fe–Ti and Mg–Al–Fe compositions [177].The ternary alloys were found to display remarkable sorption behavior: at 200 °C the films were capable of absorbing 4–6 wt.% hydrogen in seconds and desorbing in minutes.Furthermore,the sorption behavior was stable over cycling for the Mg–Al–Ti and Mg–Fe–Ti alloys.Even after 100 absorption/desorption cycles,no degradation in capacity or kinetics was observed.For Mg–Al–Fe,the properties were worse,compared to the other ternary combinations.

18.2.Magnesium-carbon composite

Fig.18.Magnesium for hydrogen storage: (A) schematic representation of current approaches to store hydrogen within metal hydrides [175];(B) Mg-AlTi multilayer nanocomposites: (a) TEM bright field and (b) dark field images with (c) corresponding SAD of 10/2 multilayer after 10 cycles;SAD shows a[121] zone axis of Mg and the bright field is taken using (010) reflection;(d) HRTEM image of intimate contact between Mg atomic planes and AlTi amorphous/nanocrystalline layers;(e,g) absorption and desorption behavior of 20/2 and (f,h) 34/2 multilayer samples [181,182].

A consideration of magnesium-carbon composite for hydrogen storage involved over 1000 hydrogen sorption cycles with a predetermined set of sampling points intended to capture the major morphological and performative states [178].The outcome revealed two stages of microstructural evolution:(i) rapid transformation of the as-milled material to form the activated structure,a branching ligamented structure formed by ‘c’-directional growth of the Mg phase on successive hydride decompositions with the activation process,being delineated into two distinct parts of functional and morphological activation and(ii)slow agglomeration and densification of the activated structureviasintering,leading to kinetic limitations which reduced the useful lifetime of the material.The material remained viable after 1000 cycles,retaining nearly 88%of its peak hydrogen capacity,which is suitable for many end-use applications.

Another study involved the nanocomposite consisting of MgH2covered by highly defective single-walled carbon nanotubes (SWCNTs) coupled to catalytic metal nanoparticles and mixed with amorphous carbon [179].The composite was synthesized by co-milling with unpurified SWCNTs as a method to promote hydrogenation/dehydrogenation cycling kinetic stability in nanocrystalline MgH2.An improved kinetic performance was revealed,both during initial postmilling desorption and during subsequent cycling.Activation energy analysis demonstrated that any catalytic effect due to the metallic nanoparticles was lost during cycling.Improved cycling performance is instead achieved because of the carbon allotropes,preventing MgH2particles agglomeration and sintering.

18.3.Sputtered magnesium aluminum and magnesium aluminum titanium alloys

To improve the magnesium hydrogen storage systems allowing faster operation at lower temperatures the multilayer nanocomposites of magnesium,aluminum,and titanium was examined [180].Although enhancements of kinetics were achieved,no thermodynamic changes were found,and hydrogen was stored predominantly as alpha magnesium hydride.

Another improvement in hydrogen sorption properties of magnesium was researched through sputtered multilayer Mgbased thin films,where Mg layers were confined by AlTi layers [181,182].The objective was to make use of high surface area of interface between Mg and AlTi to reduce the enthalpy of MgH2formation exploring the extremely high surface energy of nano-layers.Multilayer samples of different Mg thickness showed relatively long activation periods,compared to the conventional co-sputtered alloy thin films (Fig.18).It has been found that the cyclability performance of materials was strictly connected to the stability of microstructure and resistance of multilayer structure to grain growth.

19.Concluding remarks

This report outlines the major Canadian research projects of last two decades that aimed at advancing applications of magnesium not only in all forms of transportation but more broadly in clean energy technologies.Despite the global shift in the primary magnesium production that took place around year 2000,extensive and high-quality research was conducted at Canadian universities,government laboratories,and other dedicated institutions,funded primarily through federal programs.The overall research directions matched the global trends of overcoming the key challenges that prevent magnesium alloys to play the major role in large-scale industrial applications.Both the fundamental and application research was accompanied by strong activities of the industrial sector involved in designing and building the machinery for magnesium processing and production of components from magnesium alloys.In recent years,a shift in research interests was observed and novel directions emerged such as magnesium air batteries,additive manufacturing of biodegradable alloys,and magnesium-rich high entropy alloys.

The purpose of this report was to familiarize the scientists and engineers with subjects of major magnesium projects,links between them,and their general outputs.The report will help in identifying still existing challenges,subjects that require additional attention or directions for future research.Finally,the volume of data gathered in this report may constitute a base for specifically oriented assessments,analyses,and drawing conclusions.

Declaration of competing interest

The author declares that he has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

This research was funded by the Office of Energy Research and Development (OERD) of Government of Canada.