铸造铝合金中的组织异质性对其疲劳性能的影响

Effects of microstructural heterogeneity on fatigue properties of cast aluminum alloys

铸造铝合金材料被广泛应用于发动机部件、结构材料、齿轮箱、底盘等部件,在使用过程中承受交变应力。准确预测铸铝材料的使用寿命对于部件的安全运转极为重要,因疲劳裂纹而导致的漏油等事故可能产生极为危险的后果。铸铝材料的使用寿命可以通过其铸造缩/气孔的尺寸分布或者尺寸最大的缩/气孔的直径结合裂纹扩展速率公式进行预测。缩孔的深度及相对位置对于应力集中具有重要影响。通常情况下,铸造铝合金的组织主要包括铝基体、共晶硅、富铁相及其他一些第二相颗粒。因这些第二相颗粒开裂而产生的微裂纹会导致铸铝材料疲劳寿命的降低。因此,如何通过显微组织合理预测铸铝材料的疲劳寿命对于铸铝材料的使用极为重要。本文回顾了以往研究中铸造缩/气孔和第二相颗粒等组织缺陷对Al-Si-Mg铸造铝合金疲劳寿命的影响。

Cast Al alloys are widely employed for engine components,structural parts,gear box,chassis,etc.and subjected to mechanical cyclic load during operation.The accurate fatigue life prediction of these alloys is essential for normal operation as fatigue cracks initiated during operation induce the lubrication oil leak and serious safety hazard.Microstructural heterogeneity,including shrinkage/gaspores and secondary phase particles,is the most detrimental factor that affects fatigue life of cast Al alloys.The approximate fatigue life cycles could be estimated based on the size distribution and locations of shrinkage pores/defects.The relationship between crack population and stress was reported by statistical distributions and the cumulative probability for cast Al alloys fail at a certain stress could be predicted by combination of Paris law and pore size distribution.Pore depth was found to dominate the stress field around the pore on the surface and the maximum stress increases sharply when the pore intercepted with the surface at its top.The microstructure of cast Al alloys usually is composed of primary Al dendrites,eutectic silicon,Fe-rich particles and other intermetallic particles are dependent upon alloy composition and heat treatment.The coalescence of microcracks initiated from the fractured secondary phases was clearly found and can accelerate the initiation and propagation of the fatigue cracks.A link between defect features and the fatigue strength needs to be established through a good understanding of the fatigue damage mechanisms associated with the microstructural features under specific loading conditions.This paper reviews the influences of shrinkage/gaspores and secondary phase particles,formed during casting process,on the fatigue life of Al-Si-Mg cast Al alloys.