通过拉伸以及三点弯曲实验, 研究了底部包Al层聚集现象对搅拌摩擦焊接头力学性能的影响. 利用OM, SEM和EDS分析了底部包Al层聚集现象降低接头力学性能的原因. 实验结果表明: 焊接前未去除包Al层的试样,其抗拉和抗弯强度要比焊前或者焊后去除包Al层的试样低. 焊接过程中,焊缝底部平铺的包Al层在搅拌头的作用下, 被流动的金属材料挤到了洋葱环的两侧, 并在底部产生聚集. 由于包Al层的力学性能较差,聚集的位置成为了接头中力学性能薄弱的区域. 在受到拉应力作用时,聚集的包Al层首先发生断裂, 形成接头中的微裂纹. 在拉应力的持续作用下进而扩展成宏观裂纹, 诱导接头过早断裂.
Friction stir welding (FSW) is increasingly used in joining the high strength aluminum alloys. Compared to traditional fusion welding methods, one of advantages of FSW is to obtain defect–free joints more easily. Besides the forms of FSW tools and welding parameters, certain characteristics of the base material can also affect the quality of welded joints. There is usually a protective layer on aluminum alloy surface to prevent corrosion, called Al clad. Generally the mechanical properties of Al clad are significantly lower than those of the base metal. In this paper, tensile and three–point bending tests were applied to investigate the influence of accumulated Al clad on mechanical properties of joints. The mechanism of reducing the mechanical properties of joints, caused by the accumulated Al clad was analyzed by OM, SEM and EDS. The results show that the mechanical properties of the welded joints are lower than those of the joint with which the bottom Al clad on the base material is removed before or after welding. The Al clad tiled on bottom of the base material is squeezed to both sides of the onion ring in the role of the FSW tool and gathered in the bottom after welding. Due to the poorer mechanical properties of Al clad than the base metal, the Al clad aggregation positions become the weak areas in the joints, leading to the formation of the micro cracks in the joints under tensile stress where the micro cracks continue extending to generate macro cracks under the continuous tensile stress. This is the main eason of joint breaking more easily under low strss.
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