Experimental and numerical study of the effects of porosity on fatigue crack initiation of HPDC magnesium AM60B alloy

In this study, high-cycle fatigue tests were conducted on specimens machined from 50 sequentially cast instrument panels made from high-pressure die-cast (HPDC) AM60B magnesium alloy. The fatigue life data were described by a two-parameter Weibull model. SEM analyses on the fracture surfaces showed the initiation of the fatigue cracks occurred exclusively at casting pores close to the machined surfaces. The dependence of local maximum plastic shear strain range on casting pore features and loading conditions was studied quantitatively by finite element simulation including varying the pore size, geometry and spacing, proximity to the free surface, as well as loading ratios. A constitutive plasticity model, the classic Ohno-Wang's kinematic hardening rule, was employed to simulate the isothermal monotonic and cyclic behaviour of magnesium AM60B alloy under uniaxial loading. The simulation results illuminated the microstructure-property relations for fatigue crack incubation and the resultant scatter in fatigue life.