Study On Fabrication Of Al/Mg/Al Alloy Composite Plate And Regulation Of Interfacial Microstructure And Properties

Mg alloy has the advantages of low density,high specific strength,excellent electromagnetic shielding property,and shock absorption property.It also has the disadvantages of poor deformation and corrosion resistance.The problem of poor corrosion resistance of Mg alloy can be effectively solved by fabricating Al/Mg/Al composite plate.The difficulties in fabrication are that Mg alloys have poor plasticity at room temperature and need hot working to improve working plasticity.At the same time,the Al/Mg interface is easy to diffuse to form a brittle transition layer at high temperature.This seriously affects the mechanical properties of Al/Mg/Al composite plates,especially the interfacial bonding strength and deformation coordination ability.Therefore,interface regulation becomes the key and solution idea to fabricate composite plates with excellent comprehensive mechanical properties.This paper mainly explores the effect of fabrication process and parameters on the mechanical properties of Al/Mg/Al composite plates.The results of fabrication process optimization show that hot roll-bonding（HRB）/rapid cooling（RC）process is an effective means to fabricate composite plates with excellent comprehensive mechanical properties.HRB can realize the hot working required by Mg alloy,while RC can prevent the formation of brittle transition layer at Al/Mg high temperature interface,so as to obtain Al/Mg/Al composite plate with plastic interface characteristics.This effectively solves the problem of poor bending plasticity of Al/Mg/Al composite plate.The results of process parameter optimization show that the strong-strong matching principle should be applied to the selection of rolling base metal.Under this principle,S7Tx composite plate made of 7075 Al alloy has higher specific strength and basically conforms to the rule of averages（ROA）.The fluctuation of mechanical properties of the composite plate is the result of the combined action of the intrinsic factors of property inheritance from the rolling base metal and the extrinsic factors of the rolling process modification.The inherent characteristic of property inheritance leads to the arrangement law of S7Tx>S6Tx≈S5Tx>S1Tx of mechanical properties of composite plates,which follows the law of 7075>6061≈5052>1060 of base metal.The extrinsic characteristic of process modification makes low-temperature rolling（T3 process）,too short preheating time（T5 process）and too large reduction（T1 process）easy to cause internal damage of Mg layer.However,high temperature rolling（T7 process）,too long preheating time（T6 process）and too slow cooling after rolling（T8 process）are easy to lead to the growth of interface transition layer and reduce the interface bonding property.Therefore,the T4 process of HRB/RC can effectively fabricate the S7T4 composite plate with good comprehensive mechanical properties.The interfacial microstructure evolution of S7T4 composite plate was studied by OM,SEM,EDS,EBSD,FIB+TEM and other methods.It was found that the microstructure of Al layer evolved from original annealing state to rolling texture composed of relatively strong（001）<100>Cube texture and relatively weak（011）<100>Goss texture.Mg layer undergoes dynamically recrystallized to form a fine equiaxed crystal microstructure.This also explains the essential reason why the HRB/RC process improves the mechanical properties of the composite plate.On the one hand,it is attributed to the realization of the grain refinement treatment by"large plastic deformation（SPD）+recrystallization annealing（RAT）"for the Mg layer,On the other hand,it is attributed to regulating the thickness of the transition layer at the interface of the composite plate.The interface of the composite plate is composed of theβlayer composed of theβ-Al₃Mg₂ phase near the Al side and theγlayer composed of theγ-Al₁₂Mg₁₇ phase near the Mg side.Nanoindentation test showed that their hardness values were 3.30GPa and 2.98GPa,respectively.The interface properties were studied by annealing treatment.It was found that the critical annealing temperature of the composite plate was 250℃,and the thickness of the interface transition layer and the grain size of the Mg layer increased with the increase of annealing temperature and annealing time.The hardness of the sub-layer of the composite plate shows the distribution law of"Al hard and Mg soft"with"low in the middle and high on both sides",and does not change with the annealing temperature and annealing time,while the average hardness of the Al and Mg layers has different variation laws with the annealing temperature.The interfacial diffusion of Al/Mg/Al composite plates belongs to the Kirkendall effect-type interfacial reaction-diffusion mechanism.In the process of interfacial diffusion,reaction diffusion occurs and is accompanied by a phenomenon of the different interdiffusion rates where theβlayer is thicker than theγlayer.The Kirkendall effect-type interfacial reaction-diffusion needs to meet the two preconditions of energy fluctuation and composition fluctuation at the same time.In the former,significant interfacial diffusion can only occur when the annealing temperature reaches above the critical temperature of 250°C.In the latter,the phase transition nucleus can only occur when the Al/Mg atomic ratio of the local regions on both sides of the interface reaches approximately 1.5.And the growth of the interfacial transition layer conforms to a certain empirical growth law.The bonding mechanism of the composite plate is the interface phase transition bonding mechanism.The theoretical basis of the bonding mechanism is the Kirkendall effect-type interfacial reaction-diffusion mechanism of the composite plate.The interfacial phase transition bonding mechanism can be divided into five stages:mechanical bonding stage,interfacial diffusion stage,phase transition nucleation stage,new phase growth stage and phase transition bonding stage.Among them,the reaction-diffusion at the Al/Mg interface is the theoretical basis and key to realize the interfacial phase transition bonding.The energy fluctuation and composition fluctuation of the interface are the two prerequisites for the interfacial phase transition combination,and both are indispensable.The interfacial phase transition bonding theory is based on thin film theory,mechanical meshing theory and diffusion theory.After the interface is bonding,theβandγphases will grow to the matrix on both sides of the interface in the form of columnar crystals along the diffusion direction.And the original Al/Mg interface will be replaced by theβ/γinterface and shift to the Mg side.According to the interface failure nature,the critical thickness of the transition layer of Al/Mg/Al composite plate is 3μm。The composite plate with the thin transition layer of less than 3μm is the plastic failure mode with plastic shrinkage cracking at the interface,and the thick transition layer of more than 3μm is the brittle failure mode with multiple transverse and longitudinal cracks in the transition layer.The composite plate with thin transition layer has good comprehensive mechanical properties.The ultimate tensile strength and interface bonding strength reach 360MPa and 62MPa respectively,and there is no interface cracking by bending 180°to U-shape.Its Erichsen number IE is about 8mm and has good plastic forming property,which is obviously beneficial to the plastic processing of Al/Mg/Al composite plate.