Failure by fatigue is a common problem associated with cast aluminum alloys due to defects like shrinkage porosities, non-metallic inclusions, etc. Friction stir processing (FSP) has recently emerged as an effective technique for local modification of microstructure. This study investigates the fatigue crack initiation and growth mechanisms in cast and FSPed A356 aluminum alloy. Two sets of parameters were used to friction stir the cast alloy resulting in the complete modification of the cast microstructure to a wrought microstructure. Both the FSPed microstructures exhibited severe abnormal grain growth (AGG) after heat treatment leading to a multimodal grain size distribution – the grain sizes ranging from a few microns to a few millimeters. One of the FSP conditions displayed an excellent improvement in fatigue life by an order of magnitude, while the other condition displayed an unexpectedly large scatter in fatigue lives. Detailed study of the fractured fatigue specimens by electron back scattered diffraction (EBSD) revealed that both, fatigue crack initiation and propagation, were intimately tied to the grain size as well as the grain misorientations in the microstructure.

由于诸如收缩孔隙，非金属夹杂物之类的缺陷，疲劳失效是与铸造铝合金相关的常见问题。摩擦搅拌加工（FSP）近来已成为一种用于局部改变微观组织的有效技术。本研究探讨了铸造和FSPed A356铝合金的疲劳裂纹萌生和生长机理。使用两组参数来摩擦搅拌铸造合金，从而将铸造显微组织完全改变为锻造显微组织。FSPed的两个微结构在热处理后均表现出严重的异常晶粒生长（AGG），从而导致多峰晶粒尺寸分布-晶粒尺寸范围从几微米到几毫米。FSP条件之一显示疲劳寿命显着提高了一个数量级，而其他情况则显示出疲劳寿命中出乎意料的大分散。通过电子反向散射衍射（EBSD）对断裂的疲劳试样进行的详细研究表明，疲劳裂纹的萌生和扩展都与晶粒尺寸以及微观结构中的晶粒取向错误密切相关。