Haavard Gjestland

Characterisation of the surface films formed on molten magnesium in different protective atmospheres

Abstract Molten magnesium oxidises rapidly during casting and handling unless it is protected by an atmosphere that stabilises the surface. In this article, results from the analysis of magnesium melt surfaces exposed to SO 2 and different fluorine-containing atmospheres are reported. The microstructure of the surface films, formed during controlled exposure in laboratory scale experiments, have been characterised using X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). Both SO 2 and the fluorine-containing gases were found to protect the melt from burning and vaporisation in oxidising atmospheres. The protected surfaces generally had a shiny metallic appearance, but turned dull grey after extended exposure to high concentrations of fluorine containing gases. All the surface films initially consisted of small crystallites of MgO forming a thin continuous film. This film was found to contain some sulphur when the melt was protected by SO 2 , while fluorine was the only element detected in the oxide when SF 6 or other fluorine-containing gases were used for protection. With increasing exposure time, the films gradually grew thicker and the fluorine/oxygen-ratio of the films formed in fluorine-containing atmospheres increased. Finally, after long term exposure to fluorine containing atmospheres, the thermodynamically stable MgF 2 -phase was formed. In a N 2 atmosphere, SO 2 and SF 6 -additions did not protect the magnesium, indicating that a rapid initial formation of MgO is necessary to obtain protective films.

Stress-relaxation and creep behaviour of some rapidly solidified magnesium alloys

Abstract Application of rapid solidification to magnesium alloys generally increases the mechanical properties by a factor of 2–2.5 compared with conventional wrought magnesium alloys. This is mainly due to a smaller grain size. What is the creep behaviour of these high strength magnesium alloys? Planar flow cast magnesium alloys have been cast and extruded. The mechanical properties in tension and compression, the stress-relaxation in compression and the tensile creep properties have been tested at temperatures from RT to 150 °C. Traditionally one would expect poor creep resistance in a small grained material. In some rapidly solidified aluminium alloys, however, the creep resistance is good, due to fine particles pinning grain boundaries and impeding dislocation movements. In the magnesium alloys tested there is a big difference in the creep resistance depending on the microstructure of the alloys. The on-going study focuses on the optimal microstructure for good mechanical properties and creep resistance.

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

Friction Stir Welding of Magnesium Die Castings

Friction stir welding (FSW), being a solid-state process, is an attractive method for joining magnesium die castings. In this study, FSW of AZ91D and AM50A plates was performed both on the individual alloys and to join them together. The welds were sound and free from defects, except for small surface cracks in AM50A; a fine microstructure characterized the weld zones. The tensile properties of specimens transverse to the weld zone were measured, as were the corrosion properties. The tensile properties were somewhat lower than the base metal, with the largest percentage decrease found in the elongation of AM50A, perhaps due to the surface cracking. The corrosion resistance of the weld zone was relatively poor, most likely due to iron contamination from wearing of the tool. Further optimization of the FSW tool design and process parameters must take place to improve the reliability of FSW for magnesium die castings.

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.

Potential and limitations of rapidly solidified magnesium alloys

This paper will critically review the main achievements of a 3 year joint research program between Pechiney and Norak Hydro. Principles for alloy design were deduced from experiments which led to RS bulk products presenting a density of 1.8, a submicronic grain size and tensile strength as high as 600 MPa, with 2-5 % elongation to fracture. In NaCl solutions, the corrosion resistance of the alloys of the Mg-Al-Ca system is at least equal to that to AZ 91 E T6 conventional cast alloy. The corrosion rate of AZ 91 + 2% Ca products is 3 times slower than that of the conventional alloy. The damage tolerance properties (ductility, toughness and fatigue) seem to depend essentially upon the consolidation conditions. Finally, the presence of fine and stable dispersoids as well as of rare earths in the alloys increases the elevated temperature properties of the products.