2219鋁合金攪拌摩擦焊中的局部液化現(xiàn)象及對接頭力學(xué)性能的影響

2219鋁合金攪拌摩擦焊中的局部液化現(xiàn)象及對接頭力學(xué)性能的影響Local Liquation Phenomenon and Its Effect on Mechanical Properties of Joint in Friction Stir Welded 2219 Al Alloy

在2219-T8鋁合金攪拌摩擦焊(FSW)接頭焊核區(qū)(NZ)發(fā)現(xiàn)了局部液化現(xiàn)象。采用熱處理和熱模擬2種方法對比分析了局部液化區(qū)組織的特點(diǎn),采用顯微硬度計和原位拉伸實驗研究了局部液化對接頭力學(xué)性能的影響。結(jié)果表明,局部液化是由NZ中的Al2Cu相與周圍基體的共晶反應(yīng),即發(fā)生了組分液化所致。液化區(qū)為液相+α(Al)相的半固態(tài)組織,冷卻時高熔點(diǎn)組元α(Al)相先析出,同時在攪拌擠壓和材料流動的作用下發(fā)生固液分離,形成離異共晶。液化區(qū)的硬度低于NZ正常區(qū)域,是NZ微區(qū)試樣斷裂的起源,降低了NZ的抗拉強(qiáng)度和延伸率。不過,由于液化區(qū)域是局部的、極少量的存在于NZ中,其對接頭拉伸性能的影響遠(yuǎn)小于整體均發(fā)生弱化的熱機(jī)械影響(TMAZ)的影響。

Al alloy 2219 (AA2219) exhibits excellent mechanical properties in a wide temperature range from -250 ℃ to 250 °C, indicating great potential for application in aerospace structures. Compared to fusion welding, friction stir welding (FSW) could significantly improve mechanical properties of the AA2219 joints. Since invented by the welding institute (TWI) of UK in 1991, FSW has been treated as a solid-state joining technique by the commercial companies, which has been in an agreement in most scientific researchers. However, recently a controversy that has been raised over the viewpoint that FSW is a strict solid-state process, and some observations of liquation have been reported, especially in the stir zone of friction stir spot welding (FSSW) joint. However, the phenomenon of liquation in FSW AA2219 joints has not been reported previously. Therefore, the aim of this work is to reveal the evidence of local liquation during FSW AA2219-T8 and its effect on mechanical properties of the joints. In this work, AA2219-T8 plates (8 mm thick) were friction stir welded at a welding speed of 180 mm/min and a rotation speed of 800 r/min using a welding tool with threaded pin. Heat treatment and thermal simulation experiments were carried out to contrast the characteristics of the local liquation regions. A Vickers microhardness testing machine and an in situ SEM imaging tensile test facility were employed to study the effect of local liquation on mechanical properties of the joints. The results showed that the microstructures in the local liquation regions were divorced eutectic, and its formation was related to the coupled thermal-mechanical interaction during the FSW process. In the FSW process, the local high temperature led to constitutional liquation. During the cooling period, the semisolid mixtures decomposed into α(Al) matrices and θ (Al2Cu) particles under stir and material flow actions. The liquation regions had a lower value of hardness than the normal regions in the nugget zone (NZ), making the liquidation region susceptible to cracks initiation and decreasing the ultimate tensile strength and elongation for a local liquation region contained NZ sample. However, the negative effect of local liquation regions on the mechanical properties of the FSW AA2219-T8 joint was less than that of the thermo-mechanically affected zone (TMAZ), since the local liquation regions were only localized and tiny fractions being in the NZ, whereas the TMAZ was whole softened.

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