Stian Sannes

Grain size distribution in a complex AM60 magnesium alloy die casting

Presolidified equiaxed dendritic crystals are observed in magnesium cold chamber high pressure die castings. Depending on the rate at which new crystals are formed and to what extent they survive in the shot sleeve, a mixture of liquid and crystals is injected into the die cavity resulting in floating crystals in the casting. Box shaped die castings of the AM60 magnesium alloy have been made with a cold chamber high pressure die casting machine. The resulting microstructure is generally observed to consist of (a) a fine grained structure or (b) a mixture of fine grains and coarse grains which is either centred or dispersed in the through thickness cross-section. The prevalence of structures is observed to vary with position in the casting. Close to the gate a coarse grained microstructure dominates, while fine grains dominate further from the gate. The volume fraction of floating crystals in the casting is shown to depend on the initial superheat of the melt. IJCMR/492

Optimizing the magnesium die casting process to achieve reliability in automotive applications

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 mechanical properties of the casting are also affected by the pre-solidification of metal in the shot sleeve. In the present paper the correlation between the thermal conditions in the process and the resulting microstructure and mechanical properties in the casting is discussed.

Die Casting of Magnesium Alloys - The Importance of Controlling Die Filling and Solidification

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 setup 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.

Numerical Modeling of the Structural Behavior of Thin-Walled Cast Magnesium Components Using a Through-Process Approach

A through-process methodology for numerical simulations of the structural behavior of thin-walled cast magnesium components is presented. The methodology consists of casting process simulations using MAGMAsoft, mapping of data from the process simulation onto a FE-mesh (shell elements) and numerical simulations using the explicit FE-code LS-DYNA. In this work, generic High Pressure Die Cast (HPDC) AM60 components have been studied using 3-point bending and 4-point bending tests. The experimental data are applied to obtain a validated methodology for finite element modeling of thin-walled cast components subjected to quasi-static loading. The cast magnesium alloy is modeled using a user-defined material model consisting of an elastic-plastic model based on a non-associated J2-flow theory and the Cockcroft-Latham fracture criterion. The fracture criterion is coupled with an element erosion algorithm available in LS-DYNA. The constitutive model and fracture criterion are calibrated both with data from material tests and data from the process simulation using MAGMAsoft.

Energy Absorption Capacity for HPDC Components

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.