超轻双相LZ91镁锂合金搅拌摩擦搭接焊接技术研究

Study on Friction Stir Lap Welding of Ultraligh Dual-phase LZ91 Mg-Li Alloy

以2 mm厚双相LZ91镁锂合金为研究对象,通过FSW技术对其进行搭接焊接,重点研究了焊接速度的变化对焊缝成形、接头中原始搭接界面迁移以及接头性能的影响。结果表明,采用普通的锥形搅拌针工具,在工具转速为2000 r/min和轴肩下压量0.2 mm下,当焊接速度为800和1200 mm/min时,均可以实现LZ91合金的搅拌摩擦搭接焊接(FSLW)。但是,当焊接速度过大(1600 mm/min)时,会在接头内部出现由于热输入不足而形成的隧道缺陷。对接头的微观组织分析发现,FSLW接头中的搅拌区由细小的等轴晶构成,前进侧(AS)中热机影响区(TMAZ)/热影响区(HAZ)中的白色α-Mg相晶粒尺寸略大于后退侧(RS);而灰色的β-Li相晶粒尺寸明显小于RS,且随着焊接速度的增加,热输入变小,搅拌区中晶粒细化的现象更加明显。此外,LZ91合金FSLW接头中存在轻微的界面迁移现象。RS中冷搭接缺陷由原始搭接界面一直延伸至搅拌区中,呈现先向上再向下变化的趋势;AS中的钩状缺陷迁移程度较轻,并且呈现轻微向下弯曲的特征,而且随着焊接速度的增加,界面迁移的程度也发生轻微的加重。另外,通过宽度为12 mm的FSLW接头的性能测试表明,在本实验条件下,将冷搭接作为受力侧的接头性能(小于1.9 kN)要低于将钩状缺陷作为受力侧的接头(高于2.1 kN),冷搭接缺陷对接头性能的不利影响要比钩状缺陷的影响更为恶劣。

2 mm thick dual-phase LZ91 Mg-Li alloy was used as a research object to friction stir lap welding(FSLW).The influence of welding speeds on the weld formation,the migration of the original lap interface in the joint and the joint performance was investigated emphatically.The results show that the FSLW of LZ91 alloys could be realized with ordinary conical stirring pin by a plunge depth of 0.2 mm under the rotational speed of 2000 r/min at the welding speeds of 800 mm/min and 1200 mm/min.However,when the welding speed is too high(1600 mm/min),the hole defects caused by insufficient heat input was formed in the joint.It was found that the stirring zone in the FSLW joint was composed of fine equiaxed crystals.The grain size of the whiteα-Mg phase in the thermo-mechanically affected zone(TMAZ)for the advancing side(AS)was larger than that of TMAZ for the treating side(RS),and the greyβ-Li phase in TMAZ for the RS was much larger than that of TMAZ for the AS.With increasing the welding speed,the heat input decreased and the grain refinement became more obvious.In addition,there was the slight interface migration in LZ91 alloy joints.Cold lap defects(CLD)in the RS extend from the original lap interface to the stirring zone,showing a morphology of firstly upward curve and then downward.The hook displayed a slight downward curve in AS.As the welding speed increased,the degree of interface migration increased slightly.In addition,the load of the joint with a width of 12 mm that the CLD served as the stress(less than 1.9 kN)is lower than that of the joint that the hook as the stress(more than 2.1 kN),so the CLD had a more adverse effect than that of hook in this study.