A356 alloys processed mostly with the low pressure die casting method are always known to contain casting pores and inhomogeneous microstructure. In this study, the effects of microstructure coarsening and casting pores on the tensile and high-cycle fatigue properties of A356-T6 alloy are comprehensively investigated. The deformation behavior and fracture mechanisms are systematically analyzed by combining tensile tests and crystal plasticity finite element method (CP-FEM) simulations with distinctively initial microstructure features (fine microstructure, coarse microstructure, and coarse microstructure with pores). The results show that microstructure coarsening is presented in the notably grown α-Al dendrites, which leads to the connection and thickening of eutectic regions accompanied by the aggregative distribution of eutectic Si particles. With the coarsening microstructure, microcrack is easier to initiate in those thick eutectic regions, due to the higher stress concentration caused by the aggregated eutectic silicon. Particularly, the dominant crack propagation path changes from the trans-dendrite for fine microstructure to along eutectic regions with coarse microstructure. In addition, it confirms that the casting pores have larger detrimental effects on the tensile and fatigue properties than the microstructure coarsening in the aspects of crack initiation. Stress concentration is prone to be induced at the local edge of pore at the initial loading stage, resulting in the micro plastic deformation and lower fatigue properties. With the rapid strain accumulation in later stage, the microcracks can initiate prematurely at the local edge of pore, and thus lead to obvious decrease in tensile elongation. Moreover, a modified fatigue model considering the effects of casting pores and the microstructure difference is proposed to describe the fatigue performance of A356-T6 alloy.

众所周知，主要采用低压压铸法加工的 A356 合金含有铸件气孔和不均匀的微观结构。本研究全面研究了显微组织粗化和铸件气孔对A356-T6合金拉伸和高周疲劳性能的影响。通过将拉伸试验和晶体塑性有限元法 (CP-FEM) 模拟与独特的初始显微组织特征（细显微组织、粗显微组织和带有孔的粗显微组织）相结合，系统地分析了变形行为和断裂机制。结果表明，显着生长的α-Al枝晶出现显微组织粗化，导致共晶区的连接和增厚，伴随着共晶Si颗粒的聚集分布。随着微观结构的粗化，由于聚集的共晶硅引起的更高的应力集中，微裂纹更容易在那些厚的共晶区域引发。特别是，主要的裂纹扩展路径从细微观结构的反枝晶沿具有粗微观结构的共晶区域。此外，它证实了铸件气孔对拉伸和疲劳性能的不利影响大于微观组织粗化在裂纹萌生方面的不利影响。在初始加载阶段，在孔隙的局部边缘容易产生应力集中，导致微塑性变形和疲劳性能降低。随着后期应变的快速积累，微裂纹可以在孔隙的局部边缘过早萌生，从而导致拉伸伸长率明显下降。此外，提出了一种考虑铸件气孔和显微组织差异的修正疲劳模型来描述A356-T6合金的疲劳性能。