焊前固溶处理对铸态AZ91镁合金搅拌摩擦焊接接头组织和性能的影响

Effect of pre-solution treatment on microstructure and properties of friction stir welded joint of as-cast AZ91 magnesium alloy

对AZ91镁合金进行了直接搅拌摩擦焊接及焊前预固溶搅拌摩擦焊接实验。采用扫描电镜(SEM)、透射电镜(TEM)、电子探针(EPMA)和拉伸实验等分析了焊接接头的微观结构和力学性能。结果表明:直接搅拌摩擦焊接后接头搅拌区的微观结构由于晶粒细化和粗大第二相的破碎、溶解及再析出而得到改善,然而大量粗大的β相在焊缝后退侧过渡区连续性聚集,使得该β相与基体之间的界面成为裂纹萌生源。直接搅拌摩擦焊接头的拉伸强度及断后伸长率分为136.68 MPa和2.34%,对铸态AZ91镁合金在400℃固溶12 h后,其焊接接头的拉伸强度及断后伸长率分别为184.81 MPa和6.79%。尽管焊前预固溶搅拌摩擦焊接接头的搅拌区平均晶粒尺寸较直接焊接接头的稍大,但是微观结构较直接焊接更加均匀。说明预固溶处理能够显著改善焊缝区域第二相的分布,使镁合金变形协调性增加,显著提高了焊件的强度和塑性。

The direct friction stir welding and pre-solution friction stir welding of AZ91 magnesium alloy were carried out.Microstructure and mechanical properties of the welded joint were analyzed by means of scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron probe microanalysis(EPMA)and tensile test.The results show that the microstructure of the stir zone of direct friction stir welded joint is improved due to the grain refinement and the crushing,dissolution and re-precipitation of the coarse second phase.However,a large number of coarseβphase continuously gather in the transition zone at the retreated side of the weld,making the interface between theβphase and the matrix easy to become the source of crack initiation.The tensile strength and elongation of the direct friction stir welded joint are 136.68 MPa and 2.34%,respectively.After the as-cast AZ91 magnesium alloy is solution treatment at 400℃for 12 h,the tensile strength and elongation of the welded joint are 184.81 MPa and 6.79%,respectively.Although the average grain size in the stir zone of pre-solution friction stir welded joint is slightly larger than that of the direct friction stir welded joint,the microstructure is more uniform than that of the direct friction stir welded joint.It shows that the pre-solution treatment can significantly improve the distribution of the second phase in the weld area,increase the deformation compatibility of magnesium alloy,and significantly improve the strength and plasticity of the weldment.