Control of temperature field, microstructure and mechanical properties of variable rotation speed refill friction stir spot welded high strength aluminum alloys
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摘要: 针对7B04-T74铝合金,采用可变转速回填式搅拌摩擦点焊(variable rotation speed-refill friction stir spot welding, V-RFSSW)新方法开展了数值仿真及试验研究. 结果表明,V-RFSSW温度场围绕搅拌头轴线呈圆形对称分布,焊点高温区域集中在搅拌套空腔内部. 与扎入阶段转速相同的常规回填式搅拌摩擦点焊(refill friction stir spot welding, RFSSW)相比,V-RFSSW新方法既可在扎入阶段使材料充分塑化,以保证焊点成形,同时,通过降低回填阶段搅拌头转速降低焊接峰值温度及高温停留时间,抑制组分液化的发生,避免了共晶相的生成. V-RFSSW与常规RFSSW接头显微硬度均呈“W”形分布,且扎入阶段转速相同的情况下V-RFSSW接头搅拌区平均硬度更高. 在拉剪载荷下两种接头均以“纽扣”形式发生断裂,其中V-RFSSW接头拉剪失效载荷为8835 N,高于RFSSW接头的8162 N.Abstract: For 7B04-T74 aluminum alloy, a new method of variable rotation speed refill friction stir spot welding (V-RFSSW) was used. Numerical simulation and experiments were conducted. The results showed that the V-RFSSW temperature field was symmetrically distributed around the axis of the welding tool, and the high temperature area was concentrated in the cavity of the sleeve. Compared with the conventional refill friction stir spot welding (RFSSW) with the same rotation speed during the plunging stage, the new V-RFSSW method can fully plasticize the materials during the plunging stage to ensure the weld formation. At the same time, by reducing the rotation speed of the welding tool during the refilling stage, the welding peak temperature and high temperature dwelling time can be reduced, the occurrence of component liquefaction can be restrained and the formation of eutectic phase can be avoided. The microhardness distribution of the V-RFSSW joint and conventional RFSSW joint was "W" shape The average hardness of V-RFSSW joint was higher at the condition of the rotation speed iduring the plunging stage was the same. Under the tensile shear load, the two kinds of joints fractured in the form of "button", in which the tensile shear failure load of V-RFSSW joint was 8 835 N, which was higher than that of RFSSW joint (8 162 N).
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图 3 不同时刻V-RFSSW接头上表面及横截面的温度分布
Figure 3. Temperature distributions of the top surface and cross-section of the joints at different time for V-RFSSW. (a) t = 1 s (the top surface); (b) t = 2.5 s (the top surface); (c) t = 4 s (the top surface); (d) t=1 s (cross-section); (e) t = 2.5 s (cross-section); (f) t = 4 s (cross-section)
图 6 RFSSW和V-RFSSW接头搅拌区微观组织
Figure 6. Microstructures in stir zones of RFSSW and V-RFSSW joints. (a) ω1 = ω2 = 1500 r/min; (b) ω1 = 1500 r/min, ω2 = 1000 r/min; (c) SEM magnification of black strip microstructure in Fig. 6a; (d) EDS results of eutectic phase
表 1 7B04-T74铝合金的化学成分和力学性能
Table 1 Chemical compositions and mechanical properties of 7B04-T74 aluminum alloy
化学成分(质量分数,%) 力学性能 Zn Mg Cu Mn Fe Cr Si Ni Ti Al 抗拉强度Rm/MPa 屈服强度ReL/MPa 断后伸长率A(%) 5.86 2.51 1.62 0.34 0.18 0.15 0.07 0.05 0.03 余量 550 492 10.0 表 2 工艺参数
Table 2 Welding parameters
序号 方法 扎入阶段转速
ω1 /(r·min−1)回填阶段转速
ω2 /(r·min−1)下扎深度
p /mm焊接时间
tw /s峰值温度
Tmax /℃高温停留时间
th /s1 RFSSW 1250 1250 2.5 5 475 0 2 RFSSW 1500 1500 2.5 5 522 1.6 3 V-RFSSW 1500 1000 2.5 5 493 0.4 -
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