Numerical simulation of material flow in AA7075 during constrained friction processing

Lightweight aluminum alloys, such as AA7075, are desirable for applications in various industries, but their limited formability and workability often pose significant challenges. Constrained Friction Processing (CFP) has emerged as a promising technique to address these challenges by refining microstructures through the relative motion between two tools and the workpiece. The process involves axial extrusion, dual tool rotation, and constraint of the extrudate to control material flow and enhance microstructural refinement. CFP is particularly attractive for high-strength AA7075 as it imposes constrained material flow and increased shear deformation, overcoming the limited formability observed in existing conventional extrusion processes. This study investigates CFP using both experimental and numerical methods, establishing correlations between process conditions, material flow, microstructural evolution, and hardness for the aluminum alloy AA7075. The process was investigated at rotational speeds of 1000–1400~rpm, resulting in the highest measured peak temperature up to 445°C and refined grain sizes of 2–3µm. The results show that initial shear deformation and material flow under the rotating tools are redistributed during processing, leading to helical flow behavior in the extruded rod. The refined stir zone (SZ) exhibited increase in hardness, about 25% compared to the base material. The developed finite-element model demonstrates good agreement with experimental results with respect to the complete thermal cycle, spatial temperature distribution, and material flow patterns, providing insight into the microstructural evolution during CFP of aluminium alloys.以AA7075为代表的轻质铝合金在诸多工业领域中具备优异应用价值，但其有限的成形性与加工性能往往带来显著挑战。约束摩擦加工（Constrained Friction Processing，CFP）作为一种极具潜力的技术应运而生，其通过两套工具与工件间的相对运动实现微观组织细化，从而解决上述难题。该工艺包含轴向挤压、双工具旋转以及对挤出物的约束环节，以此调控材料流动并强化微观组织细化效果。对于高强度AA7075铝合金而言，CFP工艺尤为适用，因其可实现受限材料流动与强化剪切变形，克服了传统常规挤压工艺中存在的成形性局限。本研究采用实验与数值模拟相结合的方法对CFP工艺展开研究，建立了AA7075铝合金的工艺参数、材料流动、微观组织演变与硬度之间的关联关系。本次研究设置的旋转转速范围为1000~1400转每分钟，实测最高峰值温度可达445℃，晶粒细化尺寸可达2~3μm。研究结果表明，旋转工具作用下的初始剪切变形与材料流动在加工过程中发生重新分布，最终使挤出棒材呈现螺旋流动特征。经细化的搅拌区（stir zone，SZ）硬度较母材提升约25%。本研究构建的有限元模型在完整热循环、空间温度分布以及材料流动模式方面均与实验结果吻合良好，可为铝合金CFP工艺中的微观组织演变研究提供理论参考。