材料期刊网

在2219-T8铝合金搅拌摩擦焊(FSW)接头焊核区(NZ)发现了局部液化现象。采用热处理和热模拟2种方法对比分析了局部液化区组织的特点,采用显微硬度计和原位拉伸实验研究了局部液化对接头力学性能的影响。结果表明,局部液化是由NZ中的Al₂Cu相与周围基体的共晶反应,即发生了组分液化所致。液化区为液相+α(Al)相的半固态组织,冷却时高熔点组元α(Al)相先析出,同时在搅拌挤压和材料流动的作用下发生固液分离,形成离异共晶。液化区的硬度低于NZ正常区域,是NZ微区试样断裂的起源,降低了NZ的抗拉强度和延伸率。不过,由于液化区域是局部的、极少量的存在于NZ中,其对接头拉伸性能的影响远小于整体均发生弱化的热机械影响(TMAZ)的影响。

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.