Evaluation of physical and mechanical properties of AZ91D/SiC composites by two step stir casting process

*K.Ponpp

aDepartment of Mechanical Engineering,Indian Institute of Technology-Delhi,Delhi 110016,India

bDepartment of Mechanical Engineering,Kongu Engineering College,Perundurai 638052,India

Evaluation of physical and mechanical properties of AZ91D/SiC composites by two step stir casting process

S.Aravindana,*,P.V.Raoa,K.Ponappab

aDepartment of Mechanical Engineering,Indian Institute of Technology-Delhi,Delhi 110016,India

bDepartment of Mechanical Engineering,Kongu Engineering College,Perundurai 638052,India

Magnesium alloy(AZ91D)composites reinforced with silicon carbide particle with different volume percentage were fabricated by two step stir casting process.The effect of changes in particle size and volume fraction of SiC particles on physical and mechanical properties of composites were evaluated under as cast and heat treated(T6)conditions.The experimental results were compared with the standard theoretical models.The results reveal that the mechanical properties of composites increased with increasing SiC particles and decrease with increasing particle size.Distribution of particles and fractured surface were studied through SEM and the presence of elements is revealed by EDS study. Copyright 2015,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.

Magnesium alloy;Metal-matrix composites(MMCs);Mechanical properties;Casting;Fractography

1.Introduction

Weight reduction has always been an important objective for air,ground transportation and space vehicles.High strength steels,aluminum and polymers are already being used to reduce the weight signifcantly,but additional reduction can be achieved by utilizing low density magnesium and its alloys (2/3rd of aluminum and 1/4th that of steel)[1].The application of non metallic materials such as fber reinforced plastics is not possible in some areas,owing to limited properties like low electrical conductivity,under low and elevated temperatures,low impact characteristics and damage tolerance.Fiber reinforced plastics are rather costly materials to be applied in primary structural applications.

Pure magnesium is rarely used in structural applications due to its poor mechanical properties[2].Addition of aluminum and zinc in magnesium increases the strength and castability.Manganese has been introduced to increase the ductility.These alloys exhibit low &high temperature strength,low stiffness and relatively poor corrosion resistance[3].Magnesium alloy composites overcome the demerits of monolithic magnesium and magnesium alloys[4]. The addition of reinforcement particle to the magnesium alloys can signifcantly improve the stiffness and strength at room and elevated temperatures[5-7].Reinforcement should be stronger and stiffer than the matrix,so as to produce expected strengthening effect.The addition of SiCp to magnesium alloy(AZ91D)matrix is to increase the stiffness, specifc strength,dimensional stability,damping capacity and creep properties at both room and elevated temperature [8].SiC has better wettability and stability in the magnesium melt[9].

Magnesium alloy(AZ91D)matrix composites can be fabricated by different techniques such as powder metallurgy,squeeze casting,disintegrated melt deposition technique and spray deposition.Improper selection of the process/technique may lead to degradation of the properties of the composites. Magnesium alloy components are usually produced by high pressure die casting and gravity casting particularly sand and permanent mold casting.Heavy components are produced by sand casting and mostly operated at elevated temperature[10]. Compared to other manufacturing processes,stir casting is more suited because of the reduction in cost(1/3rd to 1/10th in mass production)[11].Moreover stir casting is easily adoptable and can be useful to manufacture the near net shape components by suitable foundry techniques[12].Structural defects such as porosity,particle cluster,interfacial reactions and oxide inclusions were arise in conventional casting technique.In order to overcome these type of pitfalls two step stir casting method was adopted to fabricate defect free castings [13].

In the past two decades good amount of research efforts and development are carried out on magnesium and magnesium alloy casting.However,only a very little amount of work is available on the development and characterization of magnesium alloy based composites by the two step stir casting process.In this work,attempts were made to fabricate silicon carbide reinforced composites through a two step stir casting process.The cast composites are then subjected to heat treatment.Physical and mechanical properties were studied for the as cast and heat treated composites.

2.Experimental procedure

Silicon carbide particles with an average size of 32 μm and 105 μm were used as a hard ceramic reinforcement.In order to fabricate magnesium alloy composites,pure magnesium blocks(99.95%purity)along with 10%aluminium,1%zinc and 0.4%manganese were heated inside a mild steel crucible, kept in a microprocessor controlled electric resistance furnace under inert(Argon)environment.The required amount of pre heated reinforcement particles were mixed in the magnesium alloy melt to get uniformly distributed reinforced composite. Specially designed stainless steel stirrer was used to stir the magnesium alloy slurry with a constant speed of 600 rpm for 15 min at 680°C and the melt temperature was brought down to 620°C.It was then heated rapidly to 700°C and then poured into to a die steel mold to cast a block having the dimensions of 150 mm x 50 mm x 60 mm.The cast magnesium alloy composites were subjected to solution hardening and artifcial aging(T6)heat treatment as per the ASTM standard B661-06.The detailed chemical composition of magnesium alloy is presented in Table 1.Fig.1 shows the macrograph of casting setup and cast magnesium alloy composite.2.1.Porosity measurement

The porosity of the processed composites in as cast and heat treated conditions was determined by using Archimedes principle.Theoretical density is calculated by rule of mixture. From the theoretical and actual densities,porosity of the composites can be estimated by using the formula[14].

2.2.Characterization studies

Microstructural characterization studies were conducted on the as cast and the heat treated composites in order to understand the distribution of hard phase reinforcement particle.The matrix/particle interface were examined using a scanning electron microscope(FEI QUANTA 3D FEG) coupled with energy dispersive spectroscopy(EDS).Fractured surfaces were also analyzed through SEM.The phase analyses were carried out by X-Ray diffraction studies.

2.3.Mechanical properties

Magnesium alloy composites with different size of SiC particles were evaluated in terms of their hardness and tensile properties.The effects of the addition of SiC particles and their size on the hardness of the matrix were evaluated by evaluating Vicker's micro and macro hardness testing with the indentation load of 50 gm and 2.5 kg respectively.According to ASTM E8M-03,sub size rectangular specimens were prepared and the tensile tests were carried out in INSTRON tensile testing machine at a strain rate of 1×10-4s-1.For all experiments,three replicates were carried out.Fig.2 shows the typical macrograph of tensile specimen.

3.Results and discussion

3.1.Micro structure

Fig.3 Shows the FESEM micrographs of two step stir cast magnesium alloy reinforced with 32 μm size SiC particle with different volume percentage.Micrographs of Mg/SiC composites,with 105 μm SiC particle reinforcement are shown in Fig.4.TheuniformdistributionofSiCpcouldbeobservedfrom the micrographs.No slag/oxide inclusions are observed in the cast magnesium alloy composites.To improve the particle distribution in the magnesium alloy matrix second step mixing is needed(to heat the slurry to a temperature above the liquidus and then stirring).Due to high surface tension and poor wettingbetween the particles and the molten melt,the reinforcement particles were foating on the surface though they have larger specifc density than the molten alloys.When the gas layers are broken(mechanical stirring)and the particles are wet,the particles will tend to sink to the bottom.

Table 1Chemical composition of AZ91D alloy.

Fig.1.Typical experimental setup&cast composite specimens.

Fig.2.Typical macrograph of tensile specimen.

3.2.X-ray diffraction studies

Fig.5(a-c)presents the XRD profles of pure magnesium and magnesium alloy(AZ91D)composite.The major high intensity peaks of magnesium and SiCp could be observed in magnesium alloy(AZ91D)composites.Weak peaks of Al12Mg17 and Mg2Si were also observed in magnesium alloy composite.The formation of intermetallic compounds in magnesium alloy composites can be attributed to its alloying elements and the presence of SiC particle.The involved reactions to form Mg2Si are given below.

It is known that silicon carbide is thermodynamically stable in pure magnesium[15].Hence formation of intermetallic (Mg2Si)is attributed to aluminum in the magnesium alloy matrix.This is more pronounced in the silicon free alloy systems.Silicon carbide reacts with aluminum to form aluminum carbide and as a consequence the available silicon content in the matrix increases.This promotes the formation of intermetallic component Mg2Si.The weaker inter metallic phase(Al12Mg17)resides at the grain boundary.

Higher temperature of magnesium alloy accelerates interfacial reactions;the degree ofinterfacial reactions can also change the microstructure of the magnesium alloy composite.To obtain composite materials with the desired microstructure,physical and mechanical properties,the interfacial reaction should be controlled through surface treatment of the reinforcement(pre heating),and proper control of casting parameters.

Fig.3.Typical FESEM micrographs showing the distribution of SiC(32)particles with AZ91D(a)5 vol%(b)10 vol%(c)15 vol%(d)20 vol%.

Fig.4.Typical FESEM micrographs showing the distribution of SiC(105)particles with AZ91D(a)5 vol%(b)10 vol%(c)15 vol%(d)20 vol%.

Fig.5.(a-c)Typical XRD patterns of pure magnesium and magnesium alloy(AZ91D)composite.

3.3.Elemental analysis

Typical EDAX profles of AZ91D/SiCp(32 μm)and AZ91D/SiCp(105 μm)are shown in Fig.6&Fig.7 respectively.The EDS profle of point of Fig.6 reveals the magnesium alloy matrix.Clear peaks of Mg,Al,Zn and Mn of magnesium alloy matrix was observed.EDAX profle at the particle reveals carbon and silicon.The EDAX profle taken at point3,interface of particle and magnesium alloy matrix reveals the presence of magnesium carbon and silicon.Similar observation is noted in Fig.7 also.

3.4.Porosity

Theoretical density was calculated by the rule of mixture. Actual density of the processed composite in as cast and heat treated condition was estimated using Archimedes principle. Table 2 presents the theoretical,measured density and porosity of the as cast and T6 heat treated magnesium alloy (AZ91D)composites with two different reinforcement particle sizes.

The porosity of the magnesium alloy composite is higher than the porosity of the cast magnesium alloy.Irrespective of the particle size,the porosity of the composite is observed to be increasing with the increase in the volume percentage of the reinforcement.Addition of silicon carbide particles in the semi solid state and proper control of stirring movement make the silicon carbide particles to disperse uniformly in the magnesium alloy melt.This leads to increase in viscosity and reduction in fuidity of the melt.The entrapment of gas bubbles leads to increased porosity with the increase in volume percentage of reinforcement[16].

3.5.Mechanical properties

The reduced strength and stiffness of the magnesium restricts its applications in the feld of automobile and aerospace industries.Magnesium alloy composites can surpass such pitfalls.The tensile properties,including ultimate tensile strength,yield strength,youngs modulus(stiffness)and ductility were measured prior to and after T6 heat-treated conditions.Interface between the matrix and the reinforcement phase plays a major role in determining their mechanical properties and physical properties.Wettability between the reinforcement and the matrix alloy is one of the critical factors in the casting[17].The microstructure of these materials depends on the wettability of the reinforcement by the molten matrix and the interaction of the components.

Fig.6.Typical EDAX profles of AZ91D/SiCp(32 μm).

Wettability can be enhanced by way of either decreasing the surface tension of the molten metal or by increasing the surface energy of the hard reinforcement particle.Compared to aluminum,molten magnesium has low surface tension (magnesium:0.599 N/m,aluminum:0.720 N/mm)[18].And hence the wettability of SiCp with magnesium alloy is better than the aluminum matrix.Surface energy of the hard reinforcement particle(SiC)can be enhanced by preheating before pouring into the molten magnesium alloy.The stirring during semi solid state helps not only to break the gas and oxide layers but also to spread the liquid metal on the surface of SiC particulates and thus good wettability is achieved in stir casting.Semi solid stirring is usually considered for increasing the apparent viscosity,which in turn inhibits the settling& foating of particulates.It also aids to separate the cluster of SiC particles.

3.6.Hardness

Fig.8 shows the hardness of magnesium alloy(AZ91D) composite under as cast and heat treated(T6)conditions with two different particle size.The developed magnesium alloy composites exhibited increased hardness with the increase in the volume percentage SiC reinforcement particle irrespective of the size.The observed increase in hardness of the composite is attributed to the interactive infuence of the presence of SiCp phase which restricts the localized matrix deformation during indentation and fner grain size of the composite. Reduction in hardness of the composite is observed with the increased size of the SiC particle.Compared to bigger size particle,smaller size reinforcement particle produce more dislocations in the composite.The presence of hard SiC particles not only increases the load bearing capacity but also restricts the deformation of the matrix by constraining the dislocation movement[19].

Due to solution hardening effect,the heat treated composite exhibits higher hardness,than the as cast composites.The response to the heat treatment in terms of increase in hardness is higher for magnesium alloy(AZ91D)composites.The increased hardness in magnesium alloy composite can be attributed to intermetallic components.

3.7.Yield strength

Yield strength of the composites is observed to be increasing with the increase in volume percentage of SiC particle in magnesium alloy(AZ91D)composite.It is well known that strengthening effect in the composite depends upon a)Load-bearing effectsdueto thepresence of reinforcements b)Orowan strengthening c)Hall-Petch effect due to grain size refnement and the generation of geometrically necessary dislocations to accommodate thermal andelastic modulus mismatch between the matrix and reinforcements.Orowan strengthening mechanism does not play afundamentalrole in micro particle-reinforced MMC, Moreover Orowan strengthening mechanism is applicable when the grain size is less than 5 micron level.

Fig.7.Typical EDAX profles of AZ91D/SiCp(105 μm).

Fig.9 shows the experimentally evaluated and predicted yield strength of magnesium alloy(AZ91D)composite.While heat treatment dislocations are produced due to thermal mismatch between ceramic particles and the metallic matrix which strengthen the material[21].The strength of the composite depends upon the interfacial bond between the matrix and the reinforcement.If the bonding between the matrix and the reinforcement is good enough,then the applied stress can be transferred from the soft magnesium alloy matrix to the hard SiC particle.The higher strength of SiC particle protects the relatively soft magnesium alloy matrix.With increase in the volume percent more load can be transferred to the reinforcement which also results in higher yield strength.

The fner grain structure in the composite could result from addition of SiCp.It is reported that the primary magnesium grain refnement results from the heterogeneous nucleation of primary magnesium phase on the surface of SiC particles and the restricted growth of magnesium crystals by SiC particles during solidifcation.Only limited amount of SiC particles can act as heterogeneous nucleation sites of primary magnesium and only these particles would be captured by growing magnesium crystals and fnally stay within the magnesium grains in the composite[14].The continuum shear log models for reinforcement with an aspect ratio,the yield strength of the magnesium alloy composite can be predicted by Ref.[20].

where σm is the yield stress of magnesium alloy and s is aspect ratio.

Table 2Porosity of magnesium alloy(AZ91D)composites.

Fig.8.Hardness of magnesium alloy(AZ91D)composite.

The strength of the solution hardening and artifcial aging T6 AZ91D/SiCp composite is better than as cast AZ91D/SiCp composite and unreinforced monolithic magnesium alloy (AZ91D).The strengthening effect of matrix(magnesium alloy)is due to dislocation and precipitation hardening.The strength of magnesium is highly sensitive to its grain size. Grain refnement due to T6 heat treatment contributes to the great strength at room temperature for magnesium alloy composite.The fne grain size in the composite results in more grain boundary area,thus the amount of material allowed to diffuse rapidly along the grain boundaries would be increasing the yield strength.

3.8.Tensile properties

Fig.9.Predicted and experimental yield strength of magnesium alloy composite.

Magnesium alloy(AZ91)exhibits higher ultimate tensile strength than magnesium alloy composite.The addition of any secondary hard phase actually can reduce tensile strength.Under an external tensile load,a strong internal stress can be developed inside a material,and localized damage may occur when the local stress is beyond the strength of the material.Fig.10 shows the variation of ultimate tensile strength with different volume fraction of (AZ91D/SiCp)composite.In AZ91D/SiCp the uniform distribution of the reinforcement particle bearing the stress distribution homogeneously,this delaying the formation of localized damage.The volume percentage of the SiCp increases the stress distribution to hard phase,which in turn increases the tensile strength.If the volume percentage of reinforcement reaches certain level the reaction in magnesium alloy with SiC particle cause the formation of transition layers at the component interfaces.These layers,although enabling a bond to be obtained the SiC and magnesium alloy, they are not tough enough to carry loads and thus they often contribute to lowering the tensile properties of composites. Work hardening takes place when the composite is strained. The strain mismatch between the matrix and the reinforcement usually generated a higher density of dislocations in the matrix around the reinforcement.Work hardening rate decreases with increasing particle size.

3.9.Youngs modulus

Stiffness of the composite can be directly measured through modulus of elasticity.As expected,addition of SiC particles to magnesium alloy leads to the improvement of modulus.The dispersion of the fne and hard reinforcement particles(SiC)in the matrix(magnesium alloy)drastically blocks the motion of the dislocations and strengthens the magnesium alloy composite.Fig.11 shows the experimental and predicted modulus of elasticity of magnesium alloy(AZ91D/SiCp)composite.

Fig.10.UTS of magnesium alloy(AZ91D/SiCp)composite.

Fig.11.Variation of modulus of elasticity with volume fraction of(AZ91D/ SiCp)composite.

Fig.12.Ductility of magnesium alloy(AZ91D/SiCp)composite.

The dominant factors in controlling the elastic modulus are the volume fraction,distribution,shape and type of the reinforcement particle.By Halpin Tsai equation theoretical modulus of elasticity of the magnesium alloy composite is calculated.Among the dominant factors,Halpin Tsai considers only the volume fraction and aspect ratio of the reinforcement particle. The difference in theoritical and experimental values is also attributed to the uncertainty in the appropriate value for the modulus of the particle reinforcement.

The Halpin-Tsai equation[21]

where

Em:elastic modulus of matrix material(AZ91:45 GPa), Ep: elastic modulus of hard reinforcement particle (SiC:400 GPa)Vp:volume fraction of reinforcement S:is the particle aspect ratio.Aspect ratio of 32 μm and 105 μm SiCp was taken as 1.4 and 1.6 respectively.

3.10.Ductility

Fig.12 shows the variation of ultimate tensile strength with different volume fraction of(AZ91D/SiCp)composite. The ductility of the magnesium alloy composite can be affected by reinforcement content and matrix alloy.The addition of SiCp reinforcement probably overstrained the lattice,and thus the alloys have no longer suffcient strain energy remaining to gain its ductility.The decrease in ductility can be attributed to the void nucleation in advance with increased amount of SiCp.The micro plasticity took place in the metal matrix composites due to the stress concentration of the matrix at the pores of the reinforcement and or sharp corners of the reinforcing particle.

Fig.13.SEM micrographs of the tensile fracture surface(a)AZ91/5%SiCp(32 μm)and(b)AZ91D/5%SiCp(105 μm).

Fig.14.SEM micrographs of the tensile fracture surface(a)AZ91/20%SiCp(32 μm)and(b)AZ91D/20%SiCp(105 μm).

3.11.Fracture behavior

Fig.13(a&b)presents the fractographs of magnesium alloy (AZ91/SiC)composites with lower percentage of 32 μm and 105 μm respectively.Small voids&dimples observed occasionally at the fractured surface contribute to the ductile mode of fracture.Fig.14(a&b)presents the fractographs of magnesium alloy(AZ91/SiC)composites with 32 μm and 105 μm. In the case of magnesium alloy composites,with higher percentage of SiCp,matrix cracks adjacent to the SiC particles and limited amount of material displacement are observed in the fractographs.

4.Conclusion

Uniform distribution of the reinforcement reveals that the AZ91D/SiCp composites were successfully fabricated by two step stir casting technique with various volume percentages with two different sizes.

The formation of intermetallic compounds(Al12Mg17& Mg2Si)in magnesium alloy composites is mainly depends upon its alloying elements and not because of SiC particle.

As cast and solution hardening and artifcial aging(T6) conditions,the physical and mechanical properties of the magnesium alloy reinforced with SiCp is mainly depends upon the volume percent of the hard phase particle.

The dispersion of the hard particle in the composites drastically blocks the dislocation motions.This increases the yield strength and the modulus of elasticity and reduces the ductility. Smaller size particles had better properties than bigger one.

AZ91D/SiCp composites,the alloy reinforced with the fner SiCp exhibits superior properties while the coarse SiCp exhibits better properties when compared with unreinforced magnesium alloy(AZ91).

Experimental results are compared with the standard theoretical models and it is follows the similar trend.

Acknowledgment

This work was supported by Department of Science and Technology,Government of India,under Grant No:RP02197.

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Received 8 October 2014;revised 27 December 2014;accepted 30 December 2014 Available online 22 March 2015

*Corresponding author.

E-mail address:aravindans@mech.iitd.ac.in(S.Aravindan).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

http://dx.doi.org/10.1016/j.jma.2014.12.008.

2213-9567/Copyright 2015,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.