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Increased requirements for crash energy management in automotive interiors have led to increased application of magnesium alloy AM50A. Successful integration of this new alloy with hot chamber diecasting process technology requires substantial adjustment and attention to processes and practices. This paper details the conversion of magnesium AZ91D steering column diecastings to high ductility structural alloy. Description is given of the changes made to foundry practices, casting parameters, process compliance monitoring, and hot end component management. The resulting improvements allow production of components comparable to the traditional alloy in manufacturing process demands while offering improved ductility and impact strength.

The persistent occurrence of microporosity defects in AZ91 castings has made it difficult to consistently produce sound parts. Earlier work established that dissolved hydrogen gas causes microporosity defects in AZ91; however, the exact role of hydrogen in the nucleation and growth of microporosity was not determined. In this paper, the behavoir of dissolved hydrogen gas in elemental magnesium, AZ91 alloy, and liquid binary Mg/Al alloys was studied. The results show that during the last stages of solidification, hydrogen gas is rejected from the Mg17Al12 intermetallic compound to assist in the nucleation and/or growth of microporosity.

The relationship between hydrogen gas and microporosity in magnesium alloy AZ91 was quantified, refuting the belief that hydrogen levels less than the maximum solid solubility are of no detriment. Sand castings were made from melts containing measured levels of hydrogen gas, and the amount of porosity was determined by density measurements. At concentrations below the maximum solid solubility, it was established that the amount of microporosity is directly proportional to the gas content. This supports the premise that dissolved hydrogen gas provides nucleation sites for microporosity, and that it is useful to remove all gas to achieve porosity-free cast parts.