高强铝合金搅拌摩擦焊接头疲劳起裂机理及应力比的影响研究

Fatigue Cracking Mechanism of a High-Strength Aluminum AlloyFriction Stir Welded Joint and Influence of Stress Ratio

分别对7050-T7451铝合金母材及其搅拌摩擦焊接头进行高周疲劳试验,获得了应力比为0.1、0.3和0.5下母材与接头的S-N曲线,借助扫描电镜分析母材与接头断口区域的微观形貌,并进一步探究了粗大第2相对疲劳行为的影响.据此,Al_(23)CuFe_(4)、Al_(7)Cu_(2)Fe和Al_(2)Mg_(3)Zn_(3)是首选的接头裂纹起始点,而Mg_(2)Si是母体材料的主要断裂起始点,裂纹倾向于沿着密集的粗大第2相传播,对于较大的裂纹来说变得更加明显.根据实验观察,可以确定两种不同的裂纹形核机制分别为裂纹在粗大第2相的主体中形核和裂纹在第2相和基体的界面上出现.第1种形核机制主要出现在Mg_(2)Si中,第2种形核机制则以Al_(23)CuFe_(4)、Al_(7)Cu_(2)Fe和Al_(2)Mg_(3)Zn_(3)等白色第2相为主,又因为母材和焊缝的粗大第2相存在显著差异,所以,需要对不同相在焊缝和母材中的差异进行针对性分析,以此来确定接头与母材试样疲劳起裂机理的异同.结果表明,低应力比下,与基体连接紧密的白色第2相产生的应力集中低于母材的屈服强度,故与母材连接较弱的Mg_(2)Si相成为母材疲劳裂纹萌生的主要位置,而焊缝中由于热机械作用使得白色第2相粗化,其完整性相对母材中的白色第2相显著降低,进而成为裂纹萌生的主要位置.随应力比增加,白色第2相产生的应力集中增加,母材和焊缝中白色第2相成为裂纹萌生位置的数量逐渐增加.

High-cycle fatigue tests were carried out on a 7050-T7451 aluminum alloy base material and its friction stir welded joints.The S-N curves of the base material and the joint at stress ratios of 0.1,0.3,and 0.5 were obtained.The microscopic morphology of the base material and the fracture area of the joint was analyzed by scanning electron microscopy,and the influence of the coarse secondary phase on the fatigue behavior was further investigated.The results indicate that Al_(23)CuFe_(4),Al_(7)Cu_(2)Fe,and Al_(2)Mg_(3)Zn_(3)were the preferred joint crack initiation locations,whereas Mg_(2)Si was the major fracture initiation point of the base material.Furthermore,cracks tended to propagate along dense coarse secondary phases,and hence larger cracks were more pronounced.From experimental observations,two distinct crack nucleation mechanisms can be identified:cracks nucleate in the bulk of the coarse secondary phase and those emerge at the interface between the secondary phase and matrix.The first nucleation mechanism is mainly found in Mg_(2)Si,whereas the second nucleation mechanism is dominated by white secondary phases(e.g.,Al_(23)CuFe_(4),Al_(7)Cu_(2)Fe,and Al_(2)Mg_(3)Zn_(3)).The differences between the coarse secondary phases of the base material and the weld are significant.Therefore,a targeted analysis of these differences is required to determine the similarities and differences in the fatigue cracking mechanisms between the joint and base material specimens.The results indicate that,at low stress ratios,the stress concentration of the white secondary phase(tightly connected to the base material)is lower than the yield strength of the base material.Thus,the Mg_(2)Si phase(weakly connected to the base material)becomes the main location of the fatigue crack initiation in the base material.The white secondary phase in the weld is coarsened due to the thermomechanical and its stability is significantly reduced compared to the white secondary phase in the base material,making it the main location for crack initiation.As the stress ratio increases,the stress concentration in the white secondary phase increases,and the number of white secondary phases in the base material and the weld that become the location of crack initiation gradually increases.