Study Of The Preparation Process And Performance Of Al-mg Composite Plates

In the present, magnesium alloys are promising structural materials because of their lightweight. They also possess many other advantages, such as low density, high specific strength and high specific rigidity, etc. However, poor deformation capacity at room temperature and poor corrosion resistance restrict their further development. Aluminium alloys have good corrosion resistence and deformation capacity. When aluminium alloys are used as cladding of magnesium alloys, the materials can keep the lightweight proper, and the corrosion resistance and deformation capacity can also be enhanced. Therefore, Al-Mg composite plates are a kind of materials which have wide applications.In this thesis, Al-Mg-Al composite plates were successfully prepared by roll bonding method. The rolling temperature of 350? and the reduction ratios of 30%, 40%, 50%, 60%were used to prepare composite plates. The reduction ratio of 50% was adopted eventually through comparing the microstructure and mechanical properties of composite plates which has been rolled with differert reduction ratios.In order to futher improve bonding capacity of composite plates and release residual stress during rolling, annealing was carried out on the composite plates. Low temperature slow annealing and high temperature rapid annealing were adopted to study the effect of annealing on microstructure and mechanical properties of composite plates.During annealing, intermetallic compounds(IMC) layer appears with the increase of annealing temperature and time, it consists of two layers. After the EDS and XRD analysis,the two IMC layers are Mg17Al12 phase near Mg plate and Al3Mg2 phase near A1 plate,respectively. According to diffusion rules, the diffusion model of composite plates isestablished, and the growth kinetics of IMC is obtained, Mg17Al12 phase: X2 = 0.9059e-110.1/RTtAl3Mg2 phase: X2 = 0.0676e-52.6/RTt.In the bonding process, the grain of magnesium alloys in the composite plates is elongated along rolling direction, and there exist many twins inside the grains. The yield strength and tensile strength of composite plates increase,however, the elongation decrease.From the microstructures of the composite plates which are annealed at differernt temperature and time, it is known that, only recovery occurs in the grain when the annealing temperature is 150?. The microstructure of magnesium alloys has no apparent change and no IMC layer appears with the annealing time increasing from 2 hours to 12 hours. The peel strength of composite plates increases gradually, the yield strength and tensile strength of composite plates decrease gradually, however the elongation increases gradually. The recrystallization occurs when the annealing temperature exceeds 200 ?, the number of twins decreases gradually and the number of recrystallized grains increases with the annealing time. The recrystallization time reduces and the speed of grain growth increases significantly with the increase of annealing temperature, the time that the formation of IMC requires decreases gradually. The peel strength, yield strength and tensile strength of composite plates decrease gradually, however the elongation increases gradually. When the annealing temperature is 350?,the recrystallization completes when the annealing time reaches 2 hours and the recrystallization grains growth phenomenon occurs.IMC layers influence the bonding property of composite plates directly, the composite plates will peel completely during tensile test when the thickness of IMC layer reaches a certain extent. The tensile process of the composite plate which is annealed at 350? for 2 hours is as follow, first, the elastic deformation occurs, then the composite plate yields when the stress reaches 106MPa and begins to deform plastically. IMC layers begin to break when the stress reaches 110MPa. First, transverse fracture occurs, then longitudinal fracture occur with the increase of stress. The IMC layers fracture completely when the stress reaches 160MPa, the Mg plate and A1 plate in the composite plates entirely separate. With the stress reaches 189MPa, Mg plate breaks. At the same time, the stress decreases rapidly, the cause of the decrease is stress relaxation phenomenon. Then stable plastic deformation continues in the Al plate, until the plate undergoes necking and fracture. According to the result of microhardness and EDS analysis of debonding interface, the peel of composite plates occurs in the Al3Mg2 phase, which is a brittle phase.Finally, from the result of evolution of hydrogen experiment, it is known that the corrosion resistance of the Al-Mg-Al plates is much better than the Mg plates.