不同应力水平下铝铜合金搅拌摩擦焊接头的蠕变时效行为

Stress-level dependency of creep ageing behavior for friction stir welded Al-Cu alloy

蠕变时效成形(CAF)在航空航天领域有着广泛的应用。但是,蠕变时效过程中复杂的应力状态对搅拌摩擦焊(FSW)制造的超大型尺寸构件成形是一个巨大的挑战。为了更好地制定并优化这类构件的蠕变时效成形工艺,需要系统地研究应力对焊缝组织性能演变的作用规律。本文系统地研究了不同应力水平下厚板铝铜合金搅拌摩擦焊接头的蠕变行为和微观组织演变规律。结果表明,随着外加应力的增大,焊缝的蠕变行为变化较母材更敏感。当应力增加到接近初始焊缝的平均屈服强度时(140 MPa),焊缝的蠕变量与强度都得到了显著提高。同时,在厚度方向上焊缝的蠕变行为存在差异,这与焊核区内部的不均匀梯度组织有着密不可分的联系。研究结果显示,焊核区沿厚度方向从上到下晶粒尺寸逐渐减小,而局部应变和位错密度逐渐增加,这使得焊缝底层在蠕变时效过程中表现出较高的蠕变量和稳态蠕变速率。随着外加应力的提高,焊缝蠕变时效后各区域内析出相的尺寸逐渐减小。此外,在相同的应力水平下,蠕变时效后焊核区内部的析出相形貌和尺寸分布自上而下发生了变化。其中,焊核区的上层与中层分布着大量的细小θ"相,而较大尺寸的θ'相分布在焊核区底层。因此,可以通过寻求合适的预处理工艺来增加焊缝的初始位错密度,进而提高接头的蠕变性能。

Creep ageing forming(CAF)has been widely used in the aerospace engineering,but how to optimize the processing conditions,especially under complex stress state of the CAF process for large-size components produced by friction-stir welding is still a great challenge to now.In this work,the creep ageing behaviors and underlying microstructure evolution of a thick friction-stir welded Al-Cu alloy plate after CAF process under different stress levels are systematically investigated.The creep strain and the strength of the joint are both significantly increased when the stress is close to the average yield strength of the initial weld joint.The grain size reduces while the local strain and dislocation density increase from top to bottom of the NZ;hence,the bottom layer of the weld joint exhibits higher creep strain and steady-stage creep strain rate during the CAF process.The results reveal that the gradient microstructures sensitive to the stress level effectively govern the creep-ageing performance from the upper to the bottom layer in a thick friction stir welded Al-Cu alloy plate.Rationally increasing the initial dislocation density of the weld joint can both enhance the tensile properties and promote the creep deformation of the weld joint for CAF process.