Interface And Fracture Behavior Of Mg/Al Laminated Composites Fabricated By Accumulative Roll Bonding

Accumulative roll bonding （ARB） was utilized to fabricate ultra-fine grained pureMg/AA1050laminated composites. The influence of thickness ratio of pure aluminumand pure magnesium sheet on the microstructure, Mg/Al interface and mechanicalproperties of Mg/Al laminated composites was investigated to determine the optimumthickness ratio of Mg and Al sheet in the laminated composites. The effect of annealingtemperature and annealing time on microstructure, Mg/Al interface and mechanicalproperties of Al/Mg/Al sandwich, ARB processed Mg/Al laminated composites wasstudied. Optical microscope, scanning electronic microscope （SEM） and X raydiffraction analysis （XRD） were employed to observe microstructure evolution and theMg/Al interface structure of ARBed sheets. The tensile test, interface shear test andhardness test were performed on the Mg/Al laminated composites to evaluate theirmechanical properties. The mechanism of microstructural evolution, strengtheningmechanism, interfacial bonding mechanism and fracture mechanism of the ARBedMg/Al composite sheets were duscussed.Mg/Al laminated composite sheet was processed by accumulative roll bonding（ARB） using different thickness ratio of pure aluminum and pure magnesium sheets atroom temperature. The Al/Mg/Al sandwich using0.4mm thick aluminum sheet exhibitsstraight Al/Mg interface, and localized necking took place only at the Al layer of thecentral region of the sheet. The bimodal grain size was observed in the Mg layer, largegrains were observed in the non-shear region, and small grains were observed in theshear zone. The microstructure was refined due to the large strain imposed on the sheetafter1ARB cycle. During the following ARB cycles, the homogeneity of themicrostructure was improved while the average grain size didn’t change obviously. Theunnoticeable refining effect during the subsequent ARB processing can be attributed todynamic recrystallization. Strengths of the Mg/AA1050laminated composites increasedgradually with the increasing of the ARB cycles and showed apparent anisotropy.Hardness of Mg layer and Al layer increased gradually with the increasing of the ARBcycles. The laminated composite sheets processed by ARB exhibit ductile fracturecharacteristics.Al/Mg/Al sandwich was annealed at different temperatures for the same time of60min. The average grain size of Mg Layer was refined after first ARB cycle, and then increased due to the recrystallization and grain growthduring annealing. After annealingattemperature of200℃for different time, the diffusion layer at the Mg/Al interfacebecame thicker with increasing annealing time, but no significant diffusion gradientsand intermetallic compounds was observed at the interface. After annealing at thetemperature of250℃for60min, the Mg/Al interface consisits of Mg-Al diffusionlayers, Al₃Mg₂and Mg17Al12interfacial reaction layers. The continuous Al₃Mg₂layerwas close to the Al layer, and the continuous Mg17Al12layer was close to the Mglayer.And the difussion layers were in the form of Al（Mg） solid solutions close to the Allayer and Mg（Al） solid solution close to the Mg layer. Therefore, the interfacisl regionwas composed of Al/Al（Mg）/Al₃Mg₂/Al₁₂Mg₁₇/Mg（Al）/Mg. Diffusion growth of theinterfacial layer can be described by the parabolic growth law. After annealing at200℃for60min, the Al/Mg/Al sandwich exhibited the highest tensile strengthand Mg/Alinterfacial shear strength, which is mainly resulted from the homogenousfine grainsandgood Mg-Al interfacial bonding.After annealing at250℃for60min, the2-cycle ARBed Mg/Al composite sheetexhibits thicker continuous interface reaction layer at the Mg-Al interface than the1cylce ARBed Mg/Al composite sheet. The interfacisl region was also composed ofAl/Al（Mg）/Al₃Mg₂/Al₁₂Mg₁₇/Mg（Al）/Mg. The increasing stored energy with increasingARB cycles induced rapid diffusion between Al atoms and Mg atomic through theMg-Al interface during annealing treatment, leading to the thicker diffusion layer at theMg-Al interface. The hardness of the Mg layer and Al layer of ARBed Mg/Al laminatedcomposites decreased dramatically after annealing at250℃for60min. The formation ofcoarse grain size and thick continuous brittle interfacial reaction layer of Al₃Mg₂andMg17Al12after annealing at250oC degraded the mechancial properties of the compositesheet.