Achieving three-layered Al/Mg/Al sheet via combining porthole die co-extrusion and hot forging

Jianwei Tang,Liang Chen,Guoqun Zhao,Cunsheng Zhang,Lu Sun

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials(Ministry of Education),Shandong University,Jinan,Shandong 250061,

PR China

Received 14 December 2019;received in revised form 19 February 2020;accepted 23 February 2020 Available online 24 June 2020

Abstract Al/Mg/Al sheet with good bonding quality and mechanical properties was fabricated based on the proposed porthole die co-extrusion and forging(PCE-F)process.There were no voids,cracks or other defects on the Al/Mg interface.A continuous diffusion zone with two-sub-layer structure was formed across the Al/Mg interface,and its width increased with higher temperature or reduction ratio.The sub-layers formed at low and high temperature were identifie to be solid solutions and intermetallic compounds(IMCs)includingγ-Mg17Al12 andβ-Al3Mg.In Al layer,the welding zone mainly consisted of f ne equiaxed grains with several coarse elongated grains,while the majority of matrix zone is coarse elongated grains.The rolling textures were dominated in both welding and matrix zones.In Mg layer,the welding zone exhibited complete DRXed grain structure,while several unDRXed coarse grains were observed in the matrix zone.With the increasing temperature,the grain size of Al and Mg layer firstl decreased and then increased.High reduction ratio strongly refine the grain structure of Al layer,while slightly affected the Mg layer.The Al/Mg/Al sheet experienced stress-drops twice during the tensile test.The firs stress-drop was determined by the IMCs and microstructure of Mg layer,while the second stress-drop was closely related to the microstructure of Al layer.Al/Mg/Al sheet forged at the lowest temperature without the formation IMCs exhibited the highest stress for the firs stress-drop,and that forged under the highest reduction ratio with the smallest grain size in Al layer had the highest stress for the second stress-drop.? 2020 Published by Elsevier B.V.on behalf of Chongqing University.This is an open access article under the CC BY-NC-ND license.(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Al/Mg/Al;Porthole die extrusion;Forging;Microstructure;Mechanical properties.

1.Introduction

Bimetallic structural materials recently have attracted much attention,due to their excellent performances in mechanical and functional aspects compared with the mono-metallic materials[1,2].Till now,various bimetallic materials have been successfully fabricated,such as Al/steel[3],Al/Cu[4],Al/Ti[5],Cu/Ni[6]and Al/Mg[7,8].Among those bimetallic materials,Al/Mg composite provokes much interest in the industries of automobile and aerospace,since it can efficientl overcome the poor corrosion resistance of Mg alloys with the protection of Al alloys.Moreover,the advantages of low density and high specifi strength are remained.Hence,Al/Mg composite is regarded as a potential material to satisfy the growing demand for lightweight design.

The performance of Al/Mg composite is strongly dependent on the bonding quality of Al/Mg interface,which is a transition layer formed by element diffusion[2].The intermetallic compounds(IMCs)includingγ(Al3Mg2)andβ(Mg17Al12)phases likely to form on the Al/Mg interface.The formation and evolution mechanisms of IMCs have been investigated by many researchers.Nie et al.[1]studied the interfaces of Al/β,β/γandγ/Mg,and found that these interfaces exhibited a coherent relationship.Wang.et al.[9]developed a model to predict the growth kinetics of IMCs,and proposed that the formation of IMCs mainly depended on the bonding temperature and time.Venkateswaran et al.[10]carried out the tensile tests on Al-Mg weld joints,and found that the formation of thick IMCs was detrimental on the performance of Al/Mg composite due to the fact that IMCs were hard and brittle.However,the effects of IMCs on the failure mechanism of Al/Mg composite should be further studied.

Till now,various processing techniques have been developed to fabricate the Al/Mg composite,such as high pressure torsion[11],vacuum diffusion bonding[12],explosive welding[13],hot rolling[14],and hot extrusion[8].As is known,Mg alloys have poor formability due to the fewer-than-needed slip systems.Consequently,the hot extrusion exhibits a unique advantage on the plastic forming of Mg alloys,since the triaxial compressive stress can be provided to improve the plasticity.Moreover,compared with the other processing techniques,hot extrusion has the superiority of high efficien y and low cost.Hence,hot extrusion becomes an important method to produce the Al/Mg composite.The traditional extrusion to fabricate Al/Mg composite can be classifie into two categories,viz.,the co-extrusion of Al sleeve and cylindrical Mg core[8,15,16]and the co-extrusion of Al plate and Mg bar[17-19].The former one firstl put the cylindrical Mg core into the Al sleeve,and then the Al/Mg composite is extruded out from the die exit.The latter one firstl put the Al plate under the Mg bar,and then the Mg bar breaks through the Al plate to form the Mg profil cladded with Al alloy.However,for both categories,the billets should be carefully polished before extrusion,resulting in the inefficien y of the process.Furthermore,the impurity and oxidation on Al/Mg interface are difficul to remove,which seriously deteriorate the quality of Al/Mg composite.To avoid the aforementioned defects,a porthole die co-extrusion(PCE)method was proposed by our group to fabricate the Al/Mg/Al laminate[20].The preparation of billets for PCE process is simple,and there is no material waste.Importantly,the surfaces of billets can be remained inside the extrusion container,and the adverse effects of impurity and oxidation on bonding quality of Al/Mg interface are avoided.However,only semi-finishe products of Al/Mg/Al composites were obtained after the proposed PCE process,which required a further processing to realize the practical application.As is known,hot forging can improve the properties of Al and Mg alloys through grain refinemen and texture modificatio[21].Moreover,the element diffusion across the Al/Mg interface can also be intensifie during hot forging,which is beneficia for the improvement of bonding quality.Binotsch et al.[22]produced Al alloy coated Mg alloy by traditional co-extrusion and forging,and studied the interface behavior of Al/Mg composite under different load conditions.Mróz et al.[23]achieved a Mg/Al bimetallic handle by combining explosive welding and forging,and found that Al cladding layer greatly improved the corrosion resistance of the product.Hence,hot forging can be regarded as a reliable method to fabricate the finishe products of Al/Mg/Al composites.

In this study,a combination of PCE and forging(PCEF)process was developed to fabricate the Al/Mg/Al sheet.The hot forging was carried out under various temperatures and reduction ratio to investigate their effects on Al/Mg interface,microstructure of Al and Mg layers and mechanicalproperties of Al/Mg/Al sheet.The effects of IMCs on Al/Mg interface and mechanical properties were also analyzed.The results indicated that the Al/Mg/Al sheet with excellent bonding quality and mechanical properties were produced by the proposed PCE-F process.

Table 1Chemical compositions(wt.%)of the as-received 6063 Al and AZ91 Mg alloys.

Table 2Process parameters used in hot forging experiments.

2.Materials and methods

The commercial 6063 Al and AZ91 Mg alloys in as-cast state were used as raw materials,and the chemical compositions of them are listed in Table 1.The homogenization for 6063 Al and AZ91 Mg alloys was performed at 480°C for 12h and 420°C for 10h,respectively.Then,both billets were air cooled to the room temperature,and they were machined to have a cylindrical shape with the identical diameter of 20mm.The experimental setup and materials f ow behavior are schematically shown in Fig.1,where the length and thickness of Al/Mg/Al sheet corresponded to the extrusion direction(ED)and normal direction(ND),respectively.The process consists of PCE and hot forging.The detailed process parameters of PCE have been provided in our previous study[20],and a brief introduction is given as below.Two Al billets and one Mg billet were placed inside the container,and all of them were heated to 370 °C and held for 15min.Then,the PCE process was performed with a constant velocity of 0.1mm/s.Finally,the plate shaped Al/Mg/Al laminate with a cross-section of 12×7mm was extruded out.In case of hot forging,the laminate was firstl preheated to a certain temperature and held for 20min.Then,the laminate was forged to the preset thickness with a pressing velocity of 0.2mm/s.Finally,an Al/Mg/Al sheet was achieved by such PCE-F process.The hot forging experiments were carried out with different temperatures and reduction ratios to study their effects on the bonding quality and performance of Al/Mg/Al sheet.The detailed parameters of hot forging experiments are listed in Table 2.The Al/Mg/Al sheets forged at 250°C with a reduction ratio of 50%,at 300°C with a reduction ratio of 50%,at 350°C with a reduction ratio of 50%,and at 300°C with a reduction ratio of 71% were named as PCE-F1,PCE-F2,PCE-F3,and PCE-F4,respectively.The thicknesses of PCEF1,PCE-F2,PCE-F3 and PCE-F4 are around 3.5,3.5,3.5 and 2mm,respectively.Moreover,the thicknesses of Al and Mg layers in PCE-F1,PCE-F2,PCE-F3 are around 0.9 and 1.7mm,while those in PCE-F4 are around 0.5 and 1.0mm,respectively.

Fig.1.Schematic diagram of the designed PCE-F process and material fl w behavior.

The morphology and element distribution of the Al/Mg interface were firstl analyzed by an electron probe microanalyzer(EPMA).A further observation on Al/Mg interface was performed using transmission electron microscope(TEM),and the TEM sample was prepared by means of focused ion beam(FIB)technique.The grain morphology and micro-texture of Al layer were analyzed through electron back-scattered diffraction(EBSD).The microstructure and secondary phase of Mg layer were examined by scanning electron microscopy(SEM).The Al sample for EBSD analysis was electro-polished in the solution of 10ml perchloric acid and 90ml methanol at 30V for 10s,and the EBSD data was analyzed by Channel 5.The Mg sample for SEM analysis was mechanically polished and etched in the solution of 1g picric acid,2ml acetic acid,16ml alcohol and 2ml distilled water.The nano-indentation tests were conducted across Al/Mg interface using an Hysitron TI 980 TriboIndenter equipped with a triangular Berkovich diamond indenter.The indentation events were analyzed by displacementcontrolled method with a depth limit of 200nm.Then,the hardness value and load-displacement curves were obtained.The tensile tests were carried out at ambient temperature with a speed of 0.6mm/min,and the fracture surface was observed by SEM.Moreover,in order to analyze the fracture mechanism of Al/Mg/Al sheet,the fracture process during tensile test was recorded using a digital image correlation(DIC)system

3.Results

3.1.Microstructure and phase analysis of Al/Mg interface

Fig.2 shows the morphology and element distribution across the Al/Mg interface.It is observed that there is no void,crack or any other defect on Al/Mg interfaces for all samples,which indicates that a sound bonding was obtained by the proposed PCE-F process.Moreover,a continuous diffusion zone with two-sub-layer structure was observed on the Al/Mg interface.The sub-layers next to Al and Mg layer are abbreviated as sub-Al and sub-Mg,respectively,as indicated by the arrows in Fig.2.The width of sub-Al is always larger than that of sub-Mg.No matter increasing the forging temperature or reduction ratio,the width of sub-layers and diffusion zone increases.

The line scanning of element distribution was performed using EPMA along the yellow dashed line marked in Fig.2,and the results are plotted in Fig.3.As is seen,the concentration of Al element gradually decreases from 6063 Al side to AZ91 Mg side,while an opposite variation tendency is seen for Mg element.Such phenomenon should be attributed to the occurrence of element diffusion.As is seen from Fig.3(a),the continuous variation of element concentration indicates that IMCs were not formed,and the sub-Al and sub-Mg should be Al-based and Mg-based solid solution,as marked by white arrows in Fig.2(a).However,a smooth platform tends to appear in the transition zone of PCE-F2,which is an evidence of formation of IMCs.The results for PCE-F3 and PCE-F4 indicate that more IMCs were formed under higher forging temperature or reduction ratio.

3.2.Microstructure of Al layer

In order to investigate the effects of forging temperature and reduction ratio on the microstructure of Al layer,EBSD analysis was employed.As shown in Fig.4,the grains can be distinguished by thick black lines,which correspond to the high angle boundaries(HABs)with the misorientation angles higher than 15°.Moreover,the thin gray lines corresponding to the low angle boundaries(LABs)with the misorientation angles of 2°?15° exist inside the grains.For each sheet,the observed area is divided into two zones along ND based on the differences in microstructure,viz.,the welding zone and the matrix zone.It is observed that the welding zone mainly consists of large number of fin equiaxed grains and several coarse elongated grains.However,the majority of matrix zone is coarse elongated grains as well as few fin grains that preferentially distribute along the boundaries of elongated grains.Besides,lots of LABs are observed inside the elongated grains,which indicate the occurrence of dynamic recovery(DRV).The forging temperature and reduction ratio affect the average grain size in both welding and matrix zones,as plotted in Fig.5.For PCE-F1,the average grain size of the welding zone is 7.7μm,while that of the matrix zone is nearly twice as large as the welding zone.With the increase of forging temperature,the grain sizes in both welding and matrix zones firstl decrease and then increase,which is attributed to the competition between dynamic recrystallization(DRX)and grain growth.Compared PCE-F2 with PCE-F4,it is obvious that high reduction ratio promotes the grain refinement resulting in the decrease of grain size in both welding and matrix zones.In addition,with the increase of forging temperature or reduction ratio,the grain size difference between welding and matrix zones is reduced,indicating that the microstructure homogeneity is improved.

Fig.2.Morphology and element distribution across the Al/Mg interfaces of(a)PCE-F1,(b)PCE-F2,(c)PCE-F3,and(d)PCE-F4.

Fig.3.EPMA line scanning across the Al/Mg interfaces of(a)PCE-F1,(b)PCE-F2,(c)PCE-F3,and(d)PCE-F4.

Fig.4.Inverse pole f gure maps in Al layers of(a)PCE-F1,(b)PCE-F2,(c)PCE-F3,and(d)PCE-F4.

Fig.5.Average grain size in welding and matrix zones of Al layers.

The micro-texture in Al layer was analyzed based on the orientation distribution function(ODF)sections ofφ2=0°,45°and 65°.Fig.6 shows the ODF sections in welding zone of each sheet.As is seen,PCE-F1 has a strong Brass{011}＜211＞as well as some relative weak S{123}＜634＞components.PCE-F2 and PCE-F3 have strong Brass and some weak Copper{112}＜111＞and S.It is well known that the Brass,Copper and S components are belonged to the rolling textures.In this study,with the increase of forging temperature,the intensity of Brass and S frstly decreases and then increases,while an opposite tendency is observed for Copper.It means that there is a mutual transformation between these rolling textures during hot forging.However,PCE-F4 exhibits absolutely different components,including weak S,Goss and Cube,which should be related to the occurrence of DRX.Fig.7 shows the ODF sections in matrix zone.As is seen,the similar components are observed in PCE-F1,PCEF2 and PCE-F3,which are strong Brass,Copper and S.However,PCE-F4 contains the components of weak R-cube{001}＜110＞and S.

3.3.Microstructure of Mg layer

The grain structure and second phase in Mg layer were studied using SEM.According to our previous study[24],the distribution of temperature,strain,and strain rate varied significantl inside the die cavities during the porthole die extrusion,which consequently resulted in an inhomogeneous microstructure on the extruded profile The microstructure in welding zone of Mg layer is shown in Fig.8,in which the Al/Mg interface is marked by white circle.The coarse equiaxed grains as well as some fin grains can be observed in PCE-F1.PCE-F2 mainly consists of some fin equiaxed grains and several coarse grains,while PCE-F3 has relatively coarse and equiaxed grains.The microstructure of PCE-F4 is similar to that of PCE-F2.It is concluded that the grain size firstl decreases and then increases with the increase of forging temperature,and the reduction ratio has slight effects on the grain size.The distribution of second phase particles also strongly affects the mechanical properties of the profiles It has been reported that Mg17Al12was the main secondary phase in AZ91 Mg alloy,which is marked by yellow arrows[25].During PCE-F process,some coarse Mg17Al12particles were remained,while the others were broken into fin and dispersed particles.Besides,it is considered that some amount of fin particles was also formed due to the dynamic precipitation during the entire PCE-F process.Hence,two types of Mg17Al12particles are observed,including the coarse and fin particles.The number of particles is reduced in PCE-F3 forged at higher temperature,as shown in Fig.8(c).Moreover,the number of particles in PCE-F4 forged by higher reduction ratio is similar to that in PCE-F2.Fig.9 presents the microstructure in matrix zone,which consists of large number of fin DRXed grains and few unDRXed coarse grains.With the increase of forging temperature or reduction ratio,more DRXed grains are formed by consuming unDRXed grains.Moreover,the size of DRXed grains gradually increases with increasing forging temperature.In case of the particle distribution,the particle-rich region is in accordance with the DRXed area,while only few particles are observed in the unDRXed grains.The number of particles decreases with the increase of forging temperature or reduction ratio.

Fig.6.ODF sections in welding zones of Al layers of(a)PCE-F1,(b)PCE-F2,(c)PCE-F3,and(d)PCE-F4.

Fig.7.ODF sections in matrix zones of Al layers of(a)PCE-F1,(b)PCE-F2,(c)PCE-F3,and(d)PCE-F4.

3.4.Mechanical properties

The hardness of the Al,sub-Al,sub-Mg and Mg layers was measured by nano-indentation,and the results are plotted in Fig.10.As is seen,the hardness of two sub-layers is slightly higher than those of Al and Mg layers for PCE-F1.However,the hardness of sub-layers sharply increased to high levels for PCE-F2,PCE-F3 and PCE-F4,and the hardness of sub-Al is always higher than that of sub-Mg.Such difference also proves that IMCs were not formed on Al/Mg interface of PCE-F1,since the IMCs are regarded as hard and brittle phases with the hardness of two or three times higher than the matrix[26].

Fig.8.SEM images in welding zones of Mg layers of(a,b)PCE-F1,(c,d)PCE-F2,(e,f)PCE-F3,and(g,h)PCE-F4.

Fig.9.SEM images in matrix zones of Mg layers of(a)PCE-F1,(b)PCE-F2,(c)PCE-F3,and(d)PCE-F4.

The engineering stress-strain curves obtained from the tensile tests of Al/Mg/Al sheets are plotted in Fig.11.With the increase of strain,the stress frstly increases and then shows a two-time drop.The values of strain and stress corresponding to the frst and second drops were counted and shown in Fig.12.As is seen from Fig.12(a),the stress for the firs drop gradually decreases with the increase of forging temperature,while the corresponding strain shows an opposite tendency.Additionally,the relatively lower stress and strain values were observed for PCE-F4 forged at high reduction.In case of the second stress-drop,both the stress and strain exhibit an increasing tendency with the increase of forging temperature.However,PCE-F4 with high forging reduction ratio has the highest engineering stress for the second stress-drop and the lowest engineering strain.

Fig.10.Hardness values of the Al,sub-Al,sub-Mg and Mg layers for all profiles

Fig.11.Engineering stress-strain curves obtained from the tensile tests.

Fig.13 presents the fracture morphology of the Al/Mg/Al sheets.Based on the macroscopic images,the delamination between Al and Mg layers and the obvious necking of the Al layer can be observed in all sheets.For each sheet,three zones,viz.,Al/Mg interface(Z1),Al layer(Z2)and Mg layer(Z3),were further studied by high magnificatio observation.The degree of delamination for PCE-F1 is relatively lower compared with the other sheets,which should be attributed to the absence of IMCs layer.The degree of delamination becomes aggravated at higher forging temperature.Besides,compared PCE-F2 with PCE-F4,it is seen that higher reduction ratio can alleviate the delamination.The zone of Al layer is composed of some dimples and large area of fla facets,which is a mixed fracture mode with both ductile and brittle features.In the zone of Mg layer,some cracks,dimples,particles and cleavage planes are observed,which corresponds to the typical brittle and cleavage fractures.

4.Discussion

4.1.Al/Mg interfacial structure

As mentioned above,IMCs were not formed on the Al/Mg interface of PCE-F1.However,it was observed on the Al/Mg interfaces of PCE-F2,PCE-F3 and PCE-F4 due to the combined effects of preheating and hot forging.As is known,the bonding of dissimilar metals is closely related to the element diffusion.It has been reported that there are three mechanisms to improve the element diffusion in solid materials,viz.,mechanically induced atomic movement,pipe diffusion along dislocations,and diffusion attached to the vacancies migration[27].In this study,it is difficul for the moving dislocations to drag atoms during a relatively high strain rate deformation[28].Hence,the element diffusion is strongly affected by the pressure applied on Al/Mg interface and the migration of vacancies.The high forging temperature induces vacancies with high concentration and high migration mobility.Besides,the diffusion coefficien can also be enhanced by increasing temperature.Thus,high forging temperature accelerates the element diffusion,resulting in the increasing width of diffusion zone.On the other hand,when a high reduction is applied on PCE-F4,the high pressure promotes the diffusion of Al and Mg atoms.Moreover,the severe plastic deformation induces an obvious temperature rise,which means that PCE-F4 experienced higher temperature during forging than PCE-F2.As a result,PCE-F4 has a thicker diffusion zone than PCE-F2.

Fig.12.Engineering stress and strain values corresponding to the(a)firs stress-drop and(b)second stress-drop.

Fig.13.Fracture morphologies of(a)PCE-F1,(b)PCE-F2,(c)PCE-F3,and(d)PCE-F4.The rectangular boxes of Z1,Z2,and Z3 correspond to the Al/Mg interface,Al layer and Mg layer,respectively.

In order to identify the compositions of IMCs,PCE-F4 was selected for TEM analysis.Fig.14 shows the bright fiel TEM and the corresponding selected area electron diffraction(SAED)patterns.According to the SAED patterns,the sub-Al and sub-Mg can be identifie asβ(Al3Mg2)andγ(Mg17Al12)phases,respectively.Fig.15 shows the morphology of TEM sample prepared by FIB technique.As is seen,IMCs appear to be continuous rather than being broken after hot forging.Such kind of phenomenon has been reported by Binotsch et al.[22]that the forging could not cause the formation of broken IMCs,if the load direction was perpendicular to the Al/Mg interface.Moreover,the phase interfaces of Al/β,β/γandγ/Mg are obvious,and no void or crack exists on the interfacial area.Besides,βphase is much wider thanγphase,which is consistent with the EPMA results shown in Fig.2.According to the previous studies[29,30],βphase has a crystal lattice with large face-centered cubic unit cell(α0=2.82nm),whileγphase has a complex crystal structure composed of body-centered cubic unit cell(α0=1.054nm)[31].Moreover,it is commonly known that the crystal structures of Al and Mg alloys are face-centered cubic lattice and hexagonal close packed lattice,respectively.Thus,the similar crystal structure betweenβand Al promotes the growth ofβphase,while the different crystal structures betweenγand Mg impedes the growth ofγphase.Moreover,βphase has lower activation energy for inter-diffusion thanγphase[32].Therefore,these above two factors result in the width difference betweenβandγphases.

Fig.15.(a)TEM samples prepared using FIB,and the bright fiel TEM images for interfaces of(b)Al/β,(c)Mg/γandβ/γ.

4.2.Microstructure evolution in Al layer

During porthole die extrusion,the welding zone experiences higher strain and deformation temperature than the matrix zone due to the strong shearing stress attributed from the friction between metals and bridge[33].The subsequent hot forging also brings severer deformation on the welding zone.Hence,the microstructure evolution of welding zone in Al layer during PCE-F process can be interpreted as follows.Firstly,during the porthole die extrusion,the coarse equiaxed grains of the initial billet are firstl elongated along ED under the shearing and compression effects.Then,the elongated coarse grains are merged into strip-shaped coarse grains.With the accumulation of strain,large amount of LABs and fin equiaxed grains are formed due to the occurrence of DRV and DRX.Finally,most of the strip-shaped coarse grains transform into fin DRXed grains.The forging process further promotes the formation of fin DRXed grains,resulting in the microstructure composed of large number of fin equiaxed grains and several coarse elongated grains.In matrix zone,the microstructure evolution is similar to that in welding zone,while the degree of DRX is lower due to the lower temperature and strain.Consequently,the microstructure in matrix zone consists of more elongated grains as well as small number of fin grains.It was reported that high temperature was favorable for the occurrence of DRX in Al alloys[33].With the increase of forging temperature,the degree of DRX increases,resulting in the decrease of grain size in both welding and matrix zones of PCE-F2 forged at 300°C.However,high temperature is also beneficia for the grain growth.Thus,when the forging temperature is further increased to 350°C,the grain size in welding and matrix zones of PCEF3 increased to 8.2 and 10.6μm,respectively.Both the strain and temperature become higher with the increase of reduction ratio,which facilitates the occurrence of DRX and leads to the fines grain structure in PCE-F4.

It is discussed on Figs.6 and 7 that the rolling textures are the main components of Al layer.As reported in our recent study[20],the shear-typed textures were firstl formed during porthole die extrusion due to the strong shearing effects.Then,they gradually transformed into rolling textures due to the action of plane strain generated inside the welding chamber.Hence,the main textures of Al layer after PCE process were shear-typed components,and the fraction of rolling textures was low.During the subsequent hot forging,the shearing textures continue to turn into rolling textures until the shearing textures are completely consumed.Meanwhile,the rolling textures are gradually replaced by recrystallization textures with the proceeding of DRX.In the present study,the deformation temperature and strain are insufficien for the occurrence of complete DRX,leading to the fact that the rolling textures are still dominated in PCE-F1,PCE-F2 and PCE-F3.In case of PCE-F4,it has the highest degree of DRX,resulting in the formation of weak recrystallization textures and the sharp decrease of texture intensity.Compared the textures in welding and matrix zones,it is found that welding zone exhibits stronger Brass and weaker Copper and S components.This fact indicates that high shearing stress promotes the transformation from Copper and S to Brass.

4.3.Microstructure evolution in Mg layer

Fig.16.Load-displacement curves of Al,β,γand Mg layers for PCE-F4.

Compared with Al alloys,it is easier for Mg alloys to realize DRX due to the low stacking fault energy[34].The complete DRX occurred in welding zone of Mg layer during porthole die extrusion,resulting in an equiaxed grain structure.However,partial DRX occurred in matrix zone of Mg layer,and the matrix zone is composed of reserved coarse elongated grains and fin equiaxed grains[24].During the subsequent forging process,DRX takes place again.The coarse particles are able to promote the recrystallization nucleation due to the particle stimulated nucleation(PSN)effect,while the fin particles exert a pinning effect to retard the mobility of dislocations and grain boundaries[35].As aforementioned,increasing the temperature or reduction ratio can both improve the occurrence of DRX.Moreover,high temperature reduces the amount of particles.However,high reduction ratio promotes the rise of temperature and strain,and the latter factor is beneficia for the precipitates of particles due to the strain-induced dynamic precipitation(SIDF)[36].Because of these two contrary aspects,PCE-F2 and PCE-F4 have similar amount of secondary particles.PCE-F1 forged at 250°C has relatively coarse grains and some fin grains in welding zone.With the increasing temperature,those coarse grains are replaced by fin equiaxed grains,and the fin equiaxed grains also realize grain growth.Besides,the unDRXed area in matrix zone is decreased with the increase of temperature and reduction ratio.

4.4.Mechanical properties and fracture mechanism of Al/Mg/Al sheets

It has been discussed that the sub-layers of PCE-F1 next to Al and Mg layers are Al-based and Mg-based solid solutions,while those of PCE-F2,PCE-F3 and PCE-F4 are IMCs ofβandγphases.Hence,in PCE-F1,the hardness of sublayers is slightly higher than that of the matrix due to the solid solution strengthening effects.The formation of IMCs promotes to the sharply increasing hardness in sub-layers of PCE-F2,PCE-F3 and PCE-F4.Moreover,based on the previous studies[26,37],the hardness ofβphase is higher than that ofγphase.The load-displacement curves of Al,β,γand Mg layers obtained from the nano-indentation tests are plotted in Fig.16.The elastic modulus of Al,β,γand Mg are calculated as 80.5,65.8,64.3 and 60.7GPa,respectively.Besides,plenty of serrations are observed in the curves ofβandγphases during the loading stage,which is related to the discontinuous formation of micro cracks[38].

Fig.17.In-situ observation of the fracture of PCE-F2 during the tensile test.

Fig.17 shows the in-situ observation results about the fracture of PCE-F2.It is obvious that the Al/Mg/Al sheet fractures twice.The Mg layer firstl fractures,resulting in the firs stress-drop as shown in Fig.11.Then,the outer Al layer fractures,which is in accordance with the second stress-drop.As is known,the deformation of Al,Mg and ICMs follows the equal-stain model based on the lamination during tensile test[39].At the initial stage,Al,β,γand Mg layers experience elastic deformation and accommodate an identical strain.However,the stress levels in Al,β,γand Mg layers are different due to the variation in elastic modulus,as discussed above.Thus,an internal stress generates along the Al/Mg interface,and it accumulates with the increase of elastic strain.When the Al layer starts to yield during the tensile test,the Mg layer is still in elastic deformation stage.This fact causes a strong deformation incompatibility,which contributes to a sharp increase of the internal stress in Al/Mg interface.Thus,some microcracks tend to form and propagate in the hard and brittle IMCs layer.Mg alloy with HCP lattice has poor plastic deformability,and the grain boundaries can impede the movement of dislocations and realize the local accumulation of dislocations[40].As mentioned above,since the grain size of Mg layer is much smaller than that of Al layer,it is easier for Mg layer to facilitate the dislocation accumulation and lead to the strain localization.Hence,due to the stress localization,the cracks may propagate towards the Mg layer with the increase of strain,and the Mg layer breaks firstl.Then,the outer Al layer experiences necking and breaking with the increase of strain.

The frst stress-drop of Al/Mg/Al sheet is determined by the IMCs and the microstructure of Mg layer,while the second stress-drop is closely related to the microstructure of Al layer.Since IMCs are not formed on the Al/Mg interface of PCE-F1,the highest stress for the frst stress-drop is obtained.With the increase of temperature,the width of IMCs increases,and the stress is decreased for the firs stress-drop.Higher reduction ratio can not only make IMCs layer wider,but also refin the microstructure of Mg layer.Thus,PCE-F4 forged by a large reduction ratio exhibits the lowest stress and strain for the firs stress-drop.In case of the second stress-drop,fin grains can promote the tensile strength of Al layers.As discussed above,the grain size of Al layer firstl increases and then decreases with the increase of temperature,which causes the similar trend of the stress for the second stress-drop.The grain size of PCE-F4 forged by a larger reduction ratio is the smallest,resulting in its the highest stress for the second stress-drop.

5.Conclusion

Al/Mg/Al sheet with sound bonding quality and good tensile properties was produced based on the proposed PCE-F process.The effects of forging temperature and reduction ratio on the microstructure and mechanical properties were well studied.According to such investigation,the following conclusions were made.

(1)The Al/Mg interface without voids,cracks or other defects was achieved by PCE-F process.A diffusion zone with two-sub-layer structure was formed on the Al/Mg interface,and its width increased with the increase of forging temperature or reduction ratio.The sub-layers formed at low and high temperature were identifie to be solid solutions and IMCs(γandβ),respectively.

(2)The welding zone of Al layer mainly consisted of fin equiaxed grains with few coarse elongated grains,while the majority of matrix zone was coarse elongated grains.With the increase of forging temperature,the grain size firstl decreased and then increased,while high reduction ratio strongly refine grain structure.The rolling textures were dominated in both welding and matrix zones.

(3)The welding zone of Mg layer exhibited complete DRXed grain structure,while several unDRXed coarse grains were observed in the matrix zone.The grain size firstl decreased and then increased with the increase of temperature,while the forging reduction ratio exhibited slight effects on the grain size.

(4)Al/Mg/Al sheet experienced stress-drops twice during tensile test.The firs stress-drop was determined by the IMCs and microstructure of Mg layer,while the second stress-drop was closely related to the microstructure of Al layer.The strength of Al/Mg/Al sheet depends on the frst stress-drop.PCE-F1 forged at low temperature has no formation of IMCs,and it exhibits the highest strength among those Al/Mg/Al sheets.

Declaration of Competing Interest

None.

Acknowledgment

The authors would like to acknowledge the financia support from the National Natural Science Foundation of China(51875317),Key Research and Development Program of Shandong Province(2019GGX104087),and Natural Science Foundation of Shandong Province(ZR2019QEE030).