7075-T6铝合金厚板FSW焊缝沿厚度方向上的显微组织演变规律

采用金相显微镜（OM）、扫描电镜（SEM）和背散射电子衍射（EBSD）技术表征7075铝合金厚板搅拌摩擦焊（FSW）焊核中心区的微观结构，研究沿焊缝厚度方向上的显微组织演变规律。结果表明，焊核中心区发生了明显的动态再结晶，由原始粗大的板条状组织转变成了细小的等轴晶；随着距焊缝上表面距离的增加，中心区等轴晶晶粒尺寸呈现逐渐增大的趋势。其中，中心区上部4 mm处的晶粒尺寸最小，仅为7.8 μm；底部19 mm处的晶粒尺寸最大，达18.6 μm。中心区动态再结晶分数随距离的增大而逐渐减小，从中心区顶部1 mm处最大值90.4%减小至底部19 mm处最小值57.5%。受动态再结晶影响，焊核中心区主要以大角度晶界（HAGB）分布为主，且中心区大角度晶界随距离的增加呈逐渐降低的趋势；但每一个典型位置处的大角度晶界分布呈现先增大后减小的趋势，且在45°左右时达到最大值；此外，中心区并没有强取向组织产生。同时，焊核中心区主要以细小、弥散的二次析出强化相η相为主，且二次析出强化相颗粒尺寸随距离的增加而呈逐渐增大的趋势，但二次析出强化相颗粒总体数量却呈相反的趋势变化。

Microstructure evolution rule along weld thickness direction of FSW 7075-T6 aluminum alloy thick plate

In this paper, the microstructure in the nugget zone of friction stir welding (FSW) 7075 aluminum alloy thick plate weld is characterized by Optic Microscope (OM), Scanning Electron Microscope (SEM) and Electron Back Scattered Diffraction (EBSD) technology, and the microstructure evolution rule along the thickness direction of thick FSW weld is investigated. The results show that, the dynamic recrystallization occurs in the nugget zone, making the original course lath-shaped structure change to the fine equiaxed grain. With the increasing distance to the weld top surface, the grain size of the equiaxed grain shows an increasing trend. The grain size at 4 mm above the weld top surface is the smallest, being only 7.8 μm. However, the size is the biggest at 19 mm, reaching 18.6 μm. Moreover, with the increase of the distance, the fraction of the dynamic recrystallization reduces from a maximum value of 90.4% at 1 mm to a minimum value of 57.5% at 19 mm. Meanwhile, a large number of High Angle Grain Boundaries (HAGBs) are mainly distributed in the nugget zone due to the effect of the dynamic recrystallization, and the HAGBs show a decreasing trend with the increase of the distance. But, the HAGBs in each one typical position increase firstly and then decrease, with the largest value of the HAGBs at 45°. Also, no strongly oriented structure is found in the nugget zone. At the same time, many fine and disperse reprecipitated phases are distributed in the nugget zone, and they are mainly η phase. The particle size of the reprecipitated phases increases with the increase of the distance, while their quantity shows an inverse trend.