Finite Element Modeling of Bolt Load Retention of Die-Cast Magnesium

The use of die cast magnesium for automobile transmission cases offers promise for reducing weight and improving fuel economy. However, the inferior creep resistance of magnesium alloys at high temperature is of concern since transmission cases are typically assembled and joined by pre-loaded bolts. The stress relaxation of the material could thus adversely impact the sealing of the joint. One means of assessing the structural integrity of magnesium transmission cases is modeling the bolted joint, the topic of this paper. The commercial finite element code, ABAQUS, was used to simulate a well characterized bolt joint sample. The geometry was simulated with axi-symmetric elements with the exact geometry of a M10 screw. Frictional contact between the male and female parts is modeled by using interface elements. Material creep is described by a time hardening power law whose parameters are fit to experimental creep test data. Numerical simulation correlates well with test measurements for magnesium alloys AZ91D. The finite element analysis shows that the contact pressure along the length of the screw is non-uniform. When temperature is initially increased, the load is primarily carried by upper part of the bolt, adjacent to the interface with the head. The contact pressure is more evenly distributed along the length of the screw when creep occurs. This finding leads to the possibility of an optimization of the bolt-load retention by judicious choice of the bolt and flange materials and geometry.