7075铝合金搅拌摩擦焊接头沉淀相析出行为

利用搅拌摩擦焊(friction stir welding,FSW)技术对厚度为6 mm的7075铝合金进行平板对焊,使用金相显微镜、显微维氏硬度仪对7075铝合金FSW接头进行微观组织观察和显微维氏硬度测试,使用X射线衍射仪(X-ray diffraction,XRD)对合金进行物相分析,使用透射电镜(transmission electron microscopy,TEM)对接头焊核区(welding nugget zone,WNZ)、热机影响区(thermal-mechanically affected zone,TMAZ)、热影响区(heat affected zone,HAZ)及母材(base metal,BM)中的沉淀相分布、形貌等进行观察,并对沉淀相的晶格条纹间距进行计算。研究结果表明,7075铝合金FSW接头的组织及显微维氏硬度分布极其不均匀;接头中沉淀相主要有棒状MgZn_2和椭圆状AlCuMg两种,沉淀相种类与形状不同,强化效果不同,AlCuMg强化效果好于MgZn_2;WNZ中沉淀相主要是AlCuMg,加之细晶强化,显微维氏硬度较高;相比WNZ,TMAZ中沉淀相AlCuMg数量少,MgZn_2相对增多,强化效果相对减弱,导致显微维氏硬度降低;HAZ中的MgZn_2相对更多,加工硬化和细晶强化效果减弱,HAZ与TMAZ交界处显微维氏硬度达到接头的最低值。

The phase precipitation behavior of friction stir welded 7075 aluminum alloy joint

The flat butt welding on 7075 aluminum alloy with a thickness of 6 mm was carried out using friction stir welding technology. The microstructure of FSW joints were observed by microscope and the micro vickers hardness was tested by micro vickers hardness tester. The X-ray diffractometer (XRD) was also introduced in phase analysis of 7075 aluminum alloy. The distribution and morphology of precipitated phases in the WNZ,TMAZ,HAZ and BM of the joint were observed using the transmission electron microscopy (TEM). The result was shown that the distribution of microstructure and micro vickers hardness of the 7075 aluminum alloy FSW joint was heterogeneous compared with the base metal. The precipitated phases in the joint were MgZn2 with rod shape and AlCuMg with elliptical shape. The strengthening effect of the AlCuMg was better than that of MgZn2. The strengthening mechanism of WNZ was mainly the precipitation strengthening by AlCuMg,as well as the fine grain strengthening. Therefore,the micro vickers hardness in this zone was pretty higher. The quantity of precipitated phases AlCuMg in the TMAZ was less and the MgZn2 was relatively higher compared with WNZ which leaded the decrease of micro vickers hardness. The quantity of precipitated phases MgZn2 in the HAZ was relatively higher. Micro vickers hardness on the junction of HAZ and TMAZ reached the lowest among the FSW joint due to the decreasing of strain hardening effect and fine grain strengthening.