Zhang Hao, Chen Shujin, Zhang Yuye, Chen Xinyi, Li Zhipeng, Yang Zhidong
School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China.
Materials (Basel). 2021 Oct 12;14(20):6012. doi: 10.3390/ma14206012.
The butt joint of an Al alloy ultrathin plate with a thickness of 0.5 mm is realized by a high rotational-speed friction-stir welding process. It overcomes the welding difficulty that the ultrathin plate is often torn, and it cannot be formed by conventional friction-stir welding. The results show that the weld surface is well-formed at a high-rotational speed (more than 8000 rpm), and there are no obvious defects in each area of the joint section. The nugget zone (NZ) is a recovery recrystallization structure dominated by large-angle grain boundaries, with a grain size of about 4.9 μm. During grain growth, the texture is randomly and uniformly distributed, and the strength is balanced. The microhardness of the NZ increases significantly with the increase in rotational speed, and the fluctuation range of hardness value is small. The NZ β-MgSi is finer and significantly less than the base metal (BM). The heat dissipation of the thin plate is fast, so a Cu plate is used as the backing plate to slow down the steep temperature-drop process in the weld area. Compared with a low rotational speed, the precipitation amount of brittle phase Al-Cu-Mg-Cr and Al-Fe-Si-Mn is significantly reduced, which is conducive to improving the mechanical properties of the joint. At a high rotational speed, 12,000 rpm, the best tensile strength of the joint is 220 MPa, which is about 76% of the BM (290 MPa), and the highest elongation is 9.3%, which is about 77.5% of the BM (12%). The fracture mode of the joint is a typical plastic fracture.
采用高转速搅拌摩擦焊工艺实现了厚度为0.5mm的铝合金超薄板对接。该工艺克服了超薄板在常规搅拌摩擦焊时易撕裂、无法成型的焊接难题。结果表明，在高转速（大于8000转/分钟）下焊缝表面成型良好，接头截面各区域无明显缺陷。焊核区为以大角度晶界为主的回复再结晶组织，晶粒尺寸约为4.9μm。在晶粒长大过程中，织构随机均匀分布，强度均衡。焊核区的显微硬度随转速增加显著提高，硬度值波动范围小。焊核区的β-MgSi相更细小，且明显少于母材。薄板散热快，因此采用铜板作为垫板以减缓焊缝区域急剧的降温过程。与低转速相比，脆性相Al-Cu-Mg-Cr和Al-Fe-Si-Mn的析出量显著减少，有利于提高接头的力学性能。在12000转/分钟的高转速下，接头的最佳抗拉强度为220MPa，约为母材（290MPa）的76%，最高伸长率为9.3%，约为母材（12%）的77.5%。接头的断裂模式为典型的塑性断裂。
