薄壁异质铝合金MIG角焊缝热裂纹萌生机理研究

Research on the Mechanism of Hot Crack Initiation in Thin-walled Heterogeneous Aluminum Alloy MIG Fillet Welds

为研究焊接热裂纹萌生机理,以薄壁7003铝合金型材与6111铝合金板材MIG焊接所得角焊缝裂纹失效件为研究对象,采用渗透探伤、金相观察、扫描电镜、能谱分析、硬度分布等手段分析了焊接裂纹的萌生机制。研究结果表明:位于板材侧的焊接裂纹1、2均为液化裂纹,其长度分别为4235.2μm和1357.4μm,且裂纹2距焊缝边缘最近距离约为165μm;型材侧和板材侧的硬度分布的数值方差分别为7.66和76.89,板材侧波动更大且硬度最大差值达34.32 HV,表明板材受焊接热影响组织与性能恶化更为严重;能谱分析表明裂纹周围的杂质相为沿晶偏聚分布的硬脆AlFeSi低熔点共晶相,其熔化温度低于铝合金熔点且会导致晶界韧性降低,熔化后形成晶间液膜受焊接收缩应力影响易被撕裂导致沿晶液化裂纹的萌生。通过对比分析焊前、焊中、焊后三种状态,研究了晶粒尺寸、焊接应力、低熔点杂质相对铝合金焊接液化裂纹产生机制差异性的影响。

To study the mechanism of welding hot cracking,the failed specimens of thin-walled 7003 aluminum alloy profile and 6111 aluminum alloy sheet MIG fillet weld cracks were taken as the research object,and the welding crack mechanism was analyzed by means of penetrant testing,metallographic observation,scanning electron microscope,energy spectrum analysis,hardness distribution and so on.The research results show that:the welding cracks 1 and 2 located on the sheet side are both liquefaction cracks,with lengths of 4235.2μm and 1357.4μm respectively,and the crack 2 is about 165μm away from the nearest edge of the weld;the numerical variances of the hardness distribution on the profile side and the sheet side are 7.66 and 76.89 respectively,the sheet side fluctuates more and the maximum difference reaches 34.32HV,indicating that the sheet is more seriously deteriorated by the welding heat effect on the structure and performance;the energy spectrum analysis shows that the impurity phase around the crack is the brittle AlFeSi low melting point eutectic phase distributed along the grain boundary,which has a melting temperature lower than the melting point of aluminum alloy and will lead to the decrease of grain boundary toughness,and the formation of intergranular liquid film after melting is easily torn by the welding shrinkage stress,resulting in the initiation of intergranular liquefaction crack.By comparing and analyzing the three states of pre-welding,welding and post-welding,the effects of grain size,welding stress and low melting point impurity phase on the mechanism of aluminum alloy welding liquefaction crack were studied.