| Due to the excellent physical properties such as low density, high specific strengthand stiffness, magnesium(Mg) alloys are very attractive in a variety of technicalapplication, especially in automotive and aircraft industries. As a hexagonalclose-packed(hcp) metal, Mg alloys exhibit poor ductility and cold workability owing totheir insufficient slip systems especially at room temperature, which restricts itswidespread application. We get complex magnesium alloy parts by casting methodusually, but it caused much problems such as inclusion and composition segregation ect.Welding processes play a critical role in the board application of magnesium alloys byjoining easily-cast segments into more complex parts if it is reliable. Recrystallization isimportant for magnesium alloy product processing because it can soften the metal、improve microstructure uniformity and control grain size. Researchs on recrystallizationin this paper is divided into two parts.First of all, we studied the effect of twin boundaries on static recrystallization ofcast magnesium alloy. As-cast AZ31magnesium alloy were compressed or forged by4%、8%and12%, and then it annealed at200℃or300℃for different time. Wecompared the effect of deformation mode and strian on twinning process and thedifference of recrystallization phenomenon under different conditions by, XRD andEBSD experimental method. The influence of different twin boundaris on magnesiumalloy static recrystallization and the grain nucleation and grow up mechanism werediscussed. The results show that twins are short and their orientations are disordered inforging samples, but twins in compression samples are long and narrow which have thesame orientation and exist with groups. These morphology differences are related to thedistinct rate of two deformation mode. We found {10-11}compression twin boundrywas much easy to nucleate than {10-12} tensile twin boundry. The number of{10-11-{10-12} double twins are related to deformation strain rather than deformationrate, and we found this twin variants is relative stable during annealing process which isdifferent from TDRX theory. We also put forward the "twin boundary bulge nucleation"theory to explains some special phenomenon of nucleation on twin boundary.Secondly, we discussed the influence of static recrystallization on microstructureand Mechanical properties of TIG welding joint which caused by welding heat in colddeformation magnesium alloy. As-extruded AZ31magnesium alloys were rolled by4%, 7%and10%after homogenization treatment, and then welded it using the sameparameter. We used optical microscope to observe the microstructure of different TIGwelding joints, and compared the mechanical properties by microhardness and tensiletest. The results show that: The average grains size in HAZ decreases with the increaseof initial cold rolling strain. The existence of defects after cold deformation can inhibitgrain growth in HAZ effectively, due to the fact that the energy (heat) is mostlyconsumed by recrystallization nuclei and new grain growth for rolled-weld samples.The strain-hardening in BM leads to a high value of microhardness and UST ofrolled-weld specimens, and the tendency of micro-hardness variation is in goodagreement with the change of grain size. The7%rolled-weld specimen reaches thehighest UTS (252MPa) and strength coefficient (87.6%) due to the noticeably refinedgrains in HAZ. However, the cold rolling strain should be lower than10%, otherwise,the negative effect such as micropores and stress concentration can be enlarged.Therefore, to refine grains in HAZ of weldments and improve joint performance theroom temperature deformation must be controlled within7%. |
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