Aluminumdie casting alloy (fitting alloy) with a low iron and high magnesium content with high performance regarding corrosion resistance, chemical resistance and polishability. An overview of the chemical composition and properties of this alloy are given in Table 1.
The use of AlMg(Si) alloys in die casting is not new: The alloy Al Mg9 or AlMg9(Si) (EN 1706 AC-51200, Al Mg9, VDS no. 349) has been being used for decades already, particularly in the fittings industry. The die casting alloy contains up to 2.5% by weight of silicon which mainly serves the improvement of the casting properties. With lower silicon contents, the castability is reduced considerably. The alloy’s high magnesium excess leads to a strong saturation of the solid-solution; almost all of the Mg2Si formed exists in a eutectic. A heat treatment, e.g. artificial aging, is therefore not effective.
Disadvantages of this magnesium-rich alloy are the strong tendency to absorb hydrogen and high oxidation tendency, as well as the low to medium 0.2% yield strength (Table 2). Due to the long solidification interval and the spongy solidification morphology, the alloy must be grain refined and is therefore usually delivered in its refined state. The oxidation tendency is reduced by adding beryllium which is also present outside the factory in a suitable tolerance zone. However, for the smelter and foundryman, this alloy presents a challenge as it requires a lot of routine and careful handling. For higher circulating amounts, post-refinement must take place and, if necessary, the beryllium content must be corrected. Special purification salts and a rotary degassing process carried out with argon are necessary in order to remove enough of the oxides and the dissolved hydrogen.
Castings made of Al Mg9(Si) are characterized by their outstanding corrosion resistance. This alloy can also be cast in sand, whereby, however, more demanding requirements are set for the melt metallurgy and treatment and for the casting and solidification technique and the casting must undergo homogenization annealing. The homogenization is unnecessary for components produced in the die casting process. The machinability and polishability of this alloy are outstanding, as is the chemical resistance. Al Mg(Si) is therefore particularly suitable for the production of non-anodized fittings, small components with improved chemical resistance and household devices.