铝合金搅拌摩擦焊缝和母材疲劳裂纹扩展行为

对7075-T6和LC4CS两种铝合金搅拌摩擦焊缝的疲劳裂纹扩展(da/d N)行为进行实验研究,并与7075-T6母材疲劳性能进行对比分析。结果表明:沿焊缝中心、前进和后退边方向,7075-T6搅拌摩擦焊疲劳裂纹扩展速率da/d N均低于LC4CS的da/d N;但沿垂直于焊缝方向两者da/d N基本一致;同种材料焊缝中心da/d N为最低,其次是后退边和前进边,而沿垂直焊缝方向da/d N为最高;7075-T6焊缝中心da/d N与其母材比较,低载荷ΔK下焊缝中心的da/d N较低、但高ΔK下母材的da/d N较低;在前进边、后退边和母材区域疲劳裂纹断口形貌明显不同,在母材区域可以观察到典型的疲劳条纹特征;但沿焊缝中心(焊核)的疲劳裂纹断口,没有任何疲劳条纹特征;7075-T6搅拌摩擦焊缝比LC4CS焊缝具有更好的抗疲劳裂纹扩展阻力,在高ΔK载荷下搅拌摩擦焊核区细小等轴晶粒不能提高疲劳裂纹扩展阻力。

Performances of Fatigue Crack Growth for Aluminum Friction Stir Welds and Base Materials

The fatigue crack propagation performances for friction stir welded AA7075-T6 and LC4CS (a Chinese brand aluminum alloy similar to AA7075-T6)joints and their corresponding base materials have been investigated.It was found that the fatigue crack growth rates (da/dN)for AA7075-T6 welded joints along the weld center line,advanced and retracted side regions are all slower than that of LC4CS alloy,but the da/dN values along the perpendicular direction of weld for both alloys are the approximate same values.The da/dN along the weld center line is the lowest,the second is that of retracted and advanced side and the da/dN along the vertical direction of weld is the highest.The da/dN of AA7075-T6 FS welds is slower at the range of low load ΔK compared to base material,but for the range of high ΔK,the da/dN of AA7075-T6 base material is slower.The fracture surface of crack growth along the weld center line is obviously different from that of advanced and retracted side regions and base materials.The typical fatigue striations can be observed across the surface of fatigue crack stable growth for advanced and retracted side regions and base material,but not any fatigue striation can be found along the fatigue fracture surface of weld center line.The resistance of fatigue crack propagation for AA7075-T6 FS welds is better than that of LC4CS welds and the fine and equiaxed grains at nugget region have no good benefit to reduce the da/dN of AA7075-T6 and LC4CS FS welds.