Search Results

A methodology that includes the processing history of the metal in the High Pressure Die Casting (HPDC) process in the simulations of the structural behavior of magnesium components has been established. In this methodology the results from the HPDC process simulations are used to modify the material model and the fracture criterion in the Finite Element Analysis (FEA). This paper focuses on the simulation of the HPDC process for thin-walled magnesium components. The close connection between the processing history and the mechanical properties of the casting mandates a careful analysis of the key factors influencing the final part performance. The definition of the boundary and initial conditions will strongly influence the ability to predict important features in the microstructure of the casting and consequently the final mechanical properties of the casting.

By selecting the right combination of alloy and processing method, a wide range of temperature exposed drive train parts can be made out of die cast magnesium, including engine blocks and automatic transmissions as probably the most demanding components. Successful new alloys for these purposes must fulfill a multitude of requirements to offer a viable solution, including mechanical properties, corrosion properties, die castability and recyclability. Therefore, selection of alloys must be based on the customers' requirements, at the same time as other factors are optimised. In this paper, results from the ongoing alloy development work by Hydro Magnesium are presented, focusing mainly on creep resistant alloys within the Mg-Al-RE system. High temperature tensile data, tensile creep-, stress relaxation- and bolt load retention results from a selection of AE alloys and reference alloys are presented.

High pressure die casting is characterized by rapid die filling and subsequent rapid cooling of the molten metal in the die. These characteristics are favourable for magnesium die casting alloys. The high cooling rate favours the formation of a fine dendrite and grain structure, which in turn leads to substantial hardening; this refinement also provides improved ductility. Since the cooling rate of the metal is highly dependent on both the process parameters and the geometry of the part, the three-dimensional flexibility associated with the latter factor means that the cooling rate cannot be uniform. This cooling rate difference in turn can lead to some variation in the mechanical properties between geometrically different portions of a die cast component. This variation is an inherent property of the material, in contrast to casting defects like microporosity, non-metallic inclusions, filling defects, and formation of hot cracks.

The long-term objective of this work is to develop design and modeling tools that allow the structural behavior of thin-walled cast components to be predicted when subjected to static and dynamic loads such as in crash situations. Here, the energy absorption potential of High Pressure Die Cast components made of magnesium alloys AM20, AM50, AM60, AZ91 and the aluminum alloy AlSi7Mg is investigated using a shear-bolt principle. For the AM60 alloy, single plates cast with different thickness have been tested in order to investigate the effect of plate thickness on the shear-bolt mechanism. It is found that this deformation principle gives an approximately constant mean force during the deformation process. The behavior seems to be very robust, especially for the magnesium alloys. A simple empirical model for prediction of the mean shearing force as a function of plate thickness and bolt diameter is proposed.

Permanent mould cast medallions and 3mm die cast test plates of 47 different AZ91 based alloys covering 3-129 ppm Ni, 7-2850 ppm Cu, 87-1740 ppm Si and 0-100 ppm Co were produced. Medallions and plates were subjected to 72 hours immersion in 5% NaCl solution at 25C and to the 10 day ASTM-B117 salt spray test. The results include: a) for AZ91, the corrosion rate values anticipated from salt spray testing of die cast test plates can be calculated from the results of immersion tests on permanent mould cast medallions; b) the effect of Co on corrosion of AZ91 is 35-75 times more detrimental than Cu and thus similar to that of Ni; and c) Si showed almost no effect on the corrosion rates in the alloys examined.

High pressure die casting is characterised by rapid die filling and subsequent rapid cooling and solidification of the metal in the die. These characteristics are favourable for the mechanical properties of magnesium die casting alloys. Since the filling pattern and the cooling rate of the metal is highly dependent on both process parameters and geometry of the part, there is a natural variation in mechanical properties. Variations in filling pattern can be caused by differences in the filling conditions set up by the gating system, pre-solidification in the shot sleeve and during filling as well as variations in the timing of the pressure intensification. In the present work the effects of solidification during filling are discussed with emphasis on the resulting microstructures and the correlation with mechanical properties.