A356铸造铝合金的疲劳裂纹萌生及短裂纹扩展研究

Study on fatigue crack initiation and propagation of cast aluminum alloy A356

研究了A356-T6铸造铝合金的缺口疲劳裂纹萌生与早期扩展行为及机制。结果表明,热等静压试样的疲劳抗力优于非热等静压试样。对于钝缺口试样,疲劳裂纹萌生于缺口根部附近的多个平面,最终哪个裂纹源扩展成主裂纹取决于局部微观组织。对于缺口几何形状不同的热等静压和非热等静压疲劳试样,在疲劳过程中,不管是在高应力状态下,还是在低应力状态下,都出现了铝基体的循环塑性变形和共晶硅粒子断裂导致疲劳裂纹萌生。对于非热等静压试样,铸造缩孔在构件的疲劳过程中起着重要作用,但即使缺口根部存在较大尺度的铸造缩孔,导致了疲劳裂纹萌生,但也同时观察到疲劳裂纹从共晶硅粒子、金属间化合物、铝基体的滑移带和铁基金属间化合物等处萌生。对于脆性的A356铸造铝合金可采用修正的断裂力学参量ΔKn、局部应力范围Δσ或局部应变幅Δε/2作为控制参量来表征疲劳裂纹萌生行为,而缺口有效应力强度因子范围ΔKneff和ΔJs参量可用来表征缺口场中短裂纹扩展行为。

Fatigue crack initiation and short crack growth behavior from notches of cast aluminum alloy A356 was investigated. The results show that the fatigue properties of the alloy processed by hot isostatic pressing was improved. Fatigue cracks initiated from multi-planes at nearby notch tip for blunt notch specimens ,which crack can grow to main crack depends on the local microstructure. It was found that the fatigue crack initiation was associated with cyclic plastic deformation of α-Al matrix and fracture of eutectic silicon particles for both sharply and bluntly notched specimens. Although the shrinkage cavity plays an important role for fatigue crack initiation in the non-HIPed alloy sample and the fatigue crack nucleated from the larger pores at notch tip,initiation of fatigue cracks from eutectic silicon particles,slip band in α-Al matrix and intermetallics was also observed. The modified stress intensity factor △Kn, local stress range △σ or local strain range △ε/2 at notch tip can be used as controlling parameters to characterize fatigue crack initiation behavior from notches ,and △Kneff and△Js are proper parameters to characterize short crack growth behavior in notch field for cast aluminum alloy A356.