| With the development of science and technology,and the improvement people s living standards,green and lightweight products are receiving more and more attention.Magnesium alloy is the lightest metal structural material,with many advantages such as high specific strength,high specific elastic,excellent damping performance,good dimensional stability,strong electromagnetic shielding capacity,abundant resources and recyclable reuse.However,its poor corrosion resistance and room temperature ductility greatly limits its widespread application.By comparison,as is well-known that aluminum alloys have some advantages such as good plastic and strong corrosion resistance.Therefore,it is necessary to fabricate the Mg/Al bimetallic composite plates for combining the advantages of the two alloys.On the one hand,the weight of aluminium alloy in the same volume can be reduced,and on the other hand,the poor corrosion resistance of magnesium alloy will be overcome.The two materials benefit from each other in terms of performance and produce a synergistic effect,which makes the composite plate better than the original material,thus rendering it more able to meet various demands imposed by use in most applications.In this paper,the Mg/Al composite plate was successfully prepared by solid-liquid cast-rolling method,and the AZ31B magnesium alloy preheated to 150 ?was used as the substrate.A356 was melted in a temperature range of 610 0C to 700 0C,and was coated as a cladding metal on the AZ31B substrate.The optimized process parameter was preferably selected within this casting temperature range.And the subsequent annealing process and rolling pass test were carried out on the optimized as-formed composite plate.The influence of A356 aluminum alloy cladding temperature,annealing process and rolling pass on the formation mechanism of micro structure and mechanical properties of the composite plate was studied by means of FEG-450 thermal field emission scanning electron microscope(SEM)with energy disperse spectroscopy(EDS)and electron backscatter diffraction(EBSD),EPMA-1600 electron probe(EPMA),Axio Scope A1 optical microscope(OM),D/max-2400 X ray diffractometer(XRD),HV-100 micro hardness tester,WDW-100D electronic universal material testing machine.It would provide theoretical guidance for the rational control of the micro structure of the Mg/Al composite plate and the formation of hard and brittle phases.At the same time,we explored the intrinsic relationship between the formation mechanism of microstructure and mechanical properties.When studied the influence of cladding temperature of A356,the results indicated that the interfacial transition zone of the roll-formed Mg/Al composite plate was divided into three regions:a Mg transition region(region I)consisting of ?-Mg and Mg17Al12 formed near the AZ31B side,an Al transition region(region ?)consisting of ?-Al and Al3Mg2 formed near the A356 side,and an intermediate region(region ?)composed of Mg17Al12,Mg2Si,and Al3Mg2,respectively,and the width of Mg transition region was significantly greater than that of Al transition region.The degree of inter-atomic diffusion increased with the increase of the A356 cladding temperature which also resulted in an exponential rise of the width of the interfacial transition zone,and the increment of type and volume fraction of the intermetallic compounds.It was found that the microhardness of the interfacial transition zone underwent a rapid increase compared with that of the bilateral alloys,which was due to the presence of hard and brittle phases,Al3Mg2(315HV)and Mg17Al12(275HV)in the interfacial transition zone.When the cladding temperature of A356 at 640?,the interface bonding was mainly dominated by the combination of atomic diffusion and reaction diffusion.The shear strength of the Mg/Al composite plate reached the maximum value of 108MPa.When the cladding temperature exceeded 640?,the volume fraction of the hard and brittle phases significantly increased and continuously distributed in the interfacial transition zone.At this point,the continuous hard and brittle phases played a role in internal cleavage,so the shear strength decreased rapidly.When studied the effects of the subsequent annealing process,it was found that there was no change in the phase type of the interfacial transition zone of the Mg/Al composite plate after annealing,yet the volume fraction of Mg-Al intermetallic compounds(Mg17Al12 and Al3Mg2)increased significantly.During the annealing process,the grains on the side of AZ31B underwent a process of recovery,recrystallization and grain growth.The new crystal nuclei were first formed in an area where the grain distortion was severe.After annealing at 250? for 180min,the AZ31B side consisted of isotropic fine equiaxial grains.Simultaneously,the recrystallization volume fraction of the AZ31B side reached 96%.As a result,a state of near-complete recrystallization was thus achieved.The volume fraction of LAGB decreased significantly.And most of the strain energy inside the grains was fully released,yet the thermodynamic and kinetic conditions required for the formation of intermetallic compounds had been met,so new intermetallic compounds begun to form.The width of the interfacial transition zone,annealing temperature,and time followed a diffusion kinetic model:(? X)2= 2.07 × 104exp(-60407/RT)(t-11.54exP21211/RT)And the width of region II was significantly greater than that of region I.This was mainly attributed to the diffusion activation energy Q of the intermetallic compound Al3Mg2 being less than that of Mg17Al12.The microhardness of the interfacial transition zone was obviously higher than that of AZ31B magnesium alloy and A356 aluminium alloy.It could also be observed that the average microhardness of the bilateral alloys after annealing were lower than those of the as-rolled specimens(80HV for the AZ31B magnesium alloy and 72HV for the A356 aluminium alloy),and decreased gradually with increased annealing temperature.This was mainly due to the fact that the annealing process greatly reduced the dislocation density in the grains,and the work hardening caused by the rolling process was substantially eliminated.As the annealing temperature increases,the shear strength first increased and then decreased.The shear strength reached a maximum value of 116MPa when the annealing temperature at 200 ?,both mechanical bonding and metallurgical bonding played the dominant role in the mode of interface bonding at this time.When investigated the influence of the rolling pass,it was found that as the rolling pass increased,the finer the crystal grains and the more obvious the preferred orientation.The amount of deformation gradually increased,and the stored distortion energy was also gradually increasing,and the thickness of the Mg/Al composite plate was getting thinner and thinner.After the first stage rolling(1st-3rd pass),the interfacial transition zone is narrowed and distributed in a zigzag pattern,and the inner crystal grains of the composite plate are changed from the initial thick sheet to the fibrous shape.At the 3rd pass(20%reduction rate per pass),the appearance of the composite plate was good and the interface bond strength was the best.After the second stage of rolling(4th-5th pass),the interfacial transition zone is widened again,and the crystal grains are changed from fibrous to fine particles dispersed in the matrix.After the 4th and 5th rolling passes,the amount of intermetallic compound in the interfacial transition zone was slightly increased compared to the previous conditions,which was mainly due to the formation of new intermetallic compounds when the Mg/Al composite plate was insulated at 400 C during the interval of each rolling pass.When a shear stress was applied to the interface thereof,stress concentrations were generated in the hard and brittle phases,which resulted in the phenomenon of brittle fracture along the brittle phase during the shear strength test.It could be seen that there had been no macroscopic plastic deformation on the fracture surface.There were several parallel and continuous cracks which were characterized by river pattern around the fractures near the AZ3 1B,where there were also numerous small facets.At the same time,several parallel but discontinuous tearing edges occupied the majority of fractures near A356.In combination with the crystal lattice type of the two alloys,it was showed that the cleavage fracture occured on the AZ31B side with quasi-cleavage fracture on the A356 side,both of which were brittle fractures. |
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