Håkon Westengen

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 Cast Magnesium Alloys for High Temperature Applications

The development of creep resistant alloys for automotive drive train components has proven to be metallurgically challenging. This paper discusses the principles of high temperature alloy development, featuring metallurgical, microstructural and processing aspects of some alloys, relative to their high temperature performance. The creep resistant alloys within the Mg-Al base system obtain their creep resistance by a relatively low content of Al, and addition of elements that form stable intermetallic phases within the grain mantle. Various third elements affect the high temperature performance differently. The results demonstrate that rare earth elements (RE) show a remarkable potential as the third element(s).

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.

The Role of Rare Earth Elements in Structure and Property Control of Magnesium Die Casting Alloys

The performance of magnesium die cast parts is governed by the microstructure and by the distribution of structural features which occur as a result of the chemical composition and processing history of the alloy. The elevated temperature properties, especially mechanical strength under creep conditions, are primarily determined by the grain structure, the elements in solid solution and the effectiveness of second phase particles in stabilizing the grain boundaries. The current emphases in alloy development focus on the utilization of elements with low solubility in the solid state, leading to the formation of stable precipitates during solidification. Such elements include the rare earths, as well as silicon, strontium and calcium. A detailed analysis of the various microstructural features and attributes is given for a new family of rare earth-containing alloys. The optimization of alloy composition is addressed in terms of blending advantageous microstructural characteristics with phase equilibria considerations.

Characterization of the Squeeze Cast M6-Alloy AS41 with Al 2 O 3 Fibers

The magnesium alloy AS41 reinforced with Al2O3 have been through extensive microstructure investigations, mechanical and physical testing. A reinforcement of 20% fibers raises the ultimate tensile strength (UTS) by 40% at roomtemperature (RT) and 100% at 250 °C. The measured elastic modulus is dependent on the test method and the test parameters selected. Reinforced AS41 has a steady state creep rate in an order of magnitude lower than unreinforced AS41 at 250 °C and 25 MPa load. Thermal expansion is measured to be 18 ×10−6 °C−1 for reinforced material. The thermal conductivity of the composite is in the range 50–55 W/m°C at RT increasing to above 60 W/m°C at 500 °C as determined by a heat pulse method.