中文摘要：

采用搅拌摩擦焊对5 mm厚2024/7075异种高强铝合金进行上下板交换位置的搭接焊试验，分析接头的缺陷特征和拉伸性能。结果表明：材料位置和焊接速度对缺陷和拉伸性能有较大影响。当2024铝合金为上板时，接头中存在大面积孔洞缺陷，焊缝表面粗糙，尤其在300 mm/min焊速下存在较严重的起皮缺陷；钩状缺陷为搭接接头的典型缺陷，低焊速（50和150 mm/min）下钩状缺陷向上扩展距离较大，7075铝合金为上板时高焊速（225和300 mm/min）下后退侧钩状缺陷水平向焊核区扩展距离较大；在低焊速下，7075铝合金为上板的接头强度较高；在高焊速下，2024铝合金为上板的接头强度较高；钩状缺陷严重减小接头的有效板厚和有效搭接宽度，是接头强度降低的主要因素，优化搅拌摩擦焊搭接工艺必须同时增大有效板厚和有效搭接宽度。

英文摘要：

Lap joints of 5 mm-thick dissimilar 2024/7075 aluminum alloys were fabricated by friction stir welding （FSW） with changing the top and bottom sheet material position. The defect features and tensile properties of the joints were compared and analyzed. The results show that the defects and tensile properties are significantly affected by material position and welding speed. When 2024 alloy is placed as the top sheet, extensive void defects are found in the joints, the weld surface is coarse and severe peeling defect appears at 300 mm/min especially. As a typical defect in the lap joints, hook extends a large distance upwards at low welding speeds （50,150 mm/min）. At high welding speeds （225, 300 mm/min）, the retreating side hook propagates largely into the stir zone horizontally when 7075 alloy acts as the top sheet. At low welding speeds, the joint strength is higher when 7075 alloy acts as the top sheet. While the joint shows higher strength at high welding speeds when 2024 alloy is placed as the top sheet. The hook defect reduces the effective sheet thickness （EST） and effective lap width （ELW） of joints severely, which is the main factor reducing the joint strength. So, the EST as well as the ELW should be increased simultaneously to optimize the friction stir lap welding process.