工具形状及工艺过程对搅拌摩擦增材成形及缺陷的影响

Influence of tool shape and process on formation and defects of friction stir additive manufactured build

采用2mm厚的2195-T8铝锂合金作为增材板条,利用5种不同形状的搅拌工具进行搅拌摩擦增材工艺实验。利用金相观察和硬度测试的分析方法,重点探讨搅拌工具形状与工艺过程对增材成形、界面缺陷及硬度分布的影响。结果表明:圆柱状和三角平面圆台状搅拌针下增材界面上下材料无明显混合,偏心圆柱状和三凹圆弧槽状搅拌针有利于增材界面上下材料混合及减小界面钩状缺陷;增材前进侧界面形成致密无缺陷冶金连接,而后退侧界面材料混合不充分,钩状缺陷易伸入焊核区,且弱连接缺陷起源于此。四层增材中,相邻两层焊接方向相反的增材工艺使除顶层增材外其他增材两侧钩状缺陷向焊核区外侧弯曲,弱连接缺陷得到改善;顶层增材后退侧钩状缺陷伸入焊核区。增材焊核区有明显软化现象,但不同增材工艺下焊核区硬度分布均匀,表明搅拌摩擦增材制造可获得性能均匀的增材;相比于单道焊接工艺,来回双道焊接工艺使单层增材焊核区进一步软化;四层增材中,越靠近顶部的增材,其焊核区平均硬度越大。

The friction stir additive manufacturing (FSAM) experiment was conducted with 2mm thick sheets of 2195-T8 Al-Li alloy by using five tools with various shapes. The influence of stir tool shape and additive process on the formation, the interface defects and the hardness distribution of friction stir additive manufactured build was mainly investigated by metallographic observation and hardness testing. The results show that there is no obvious mixing of the materials across the interface of the builds manufactured by the cylindrical pin and the conical pin with three flats. However, the Trivex TM pin and the pin with three concave arc grooves are good for mixing of the materials across the interface and reducing hooking defects. Dense and defect free metallurgical connection occurs at the interface on the advancing side of the nugget zone. However, the materials across the interface on the retreating side are not sufficiently mixed, hence the hooking defect on the retreating side is easy to extend into the nugget zone, and the weak-bonding defect is originated from the hooking defect on the retreating side. For the four-layer build, except for the top layer, the welding method that the welding directions of two adjacent layers are opposite can make the hooking defects on both sides of the other layers bend to the outside of the nugget zone and improve the weak-bonding defects. The hooking defect on the retreating side of the top layer extends into the nugget zone. There is an obvious softening phenomenon in the nugget zone of the builds. The distributions of the hardness of the nugget zones of the builds manufactured by different welding processes are uniform, which shows that the build with uniform properties can be obtained by friction stir additive manufacturing. Compared with the single pass welding method, the back and forth double passes welding method further softens the material of the nugget zone of the one-layer build. For the four-layer build, the closer the layer is to the top, the higher the average hardness of the nugget zone is.