Impact of microstructural properties on the crack threshold of aluminium castings

This work focuses on the technological size effect of the cast aluminium alloy EN-AC 46200. Herewith, specimens inheriting specific local cooling rates and therefore varying local dendrite arm spacings are taken from crankcases with hot isostatic pressing (HIP) post-treatment. In order to evaluate the statistically distributed microstructural properties, the dendrite arm spacing λ₂ is probabilistically assessed. Thus, the λ₂ values follow the lognormal distribution, detached of local solidification times. Crack propagation tests are conducted at varying load ratios, such that the impact of the microstructure by means of λ₂ is evaluated. The statistically assessed investigations propose the intrinsic crack threshold to be independent of microstructural properties, in opposite to the long crack threshold which is significantly affected by varying λ₂ values due to enhanced crack closure effects. The evaluated cyclic R-curve results can be generalized into a microstructural independent manner. Moreover, the load ratio effect and its convolution with microstructural properties is evaluated based on the Newman approach. The linear extrapolation of the newly raised three-dimensional Newman approach, considering arbitrary λ₂ values as technological effect, is validated by literature data and the proposed stress intensity factor threshold course is revealed to be slightly conservative by about twelve percent on average.