The effect of silicon content on the microstructure and creep behavior in die-cast magnesium AS alloys
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27/4/04
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The Effect of Silicon Content on the Microstructure and Creep Behavior in Die-Cast Magnesium AS Alloys M.S. DARGUSCH, A.L. BOWLES, K. PETTERSEN, P. BAKKE, and G.L. DUNLOP The effect of increasing levels of silicon on the microstructure and creep properties of high-pressure die-cast Mg-Al-Si (AS) alloys has been investigated. The morphology of the Mg2Si phase in die-cast AS alloys was found to be a function of the silicon content. The Mg2Si particles in castings with up to 1.14 wt pct Si have a Chinese script morphology. For AS21 alloys with silicon contents greater than 1.4 wt pct Si (greater than the ␣-Mg2Si binary eutectic point), some Mg2Si particles have a coarse “blocky” shape. Increasing the silicon content above the eutectic level results in an increase in the number of coarse faceted Mg2Si particles in the microstructure. Creep rates at 100 hours were found to decrease with increasing silicon content in AS-type alloys. The decrease in creep rate was most dramatic for silicon contents up to 1.1 wt pct. Further additions of silicon of up to 2.64 wt pct also resulted in significant decreases in creep rate.
I.
INTRODUCTION
THE Mg-Al-Si (AS) magnesium alloy series exhibit improved creep properties compared to the more widely used AZ (Mg-Al-Zn) and AM (Mg-Al-Mn) alloys.[1–7] This is commonly attributed to the lower aluminum content of AS21 and AS41 and the presence of stable Mg2Si particles.[2,5,8] The magnesium silicide Mg2Si is the only stable compound to form in the Mg-Si system. Importantly, Mg2Si is a high melting point phase and appears finely dispersed in high pressure die castings. The morphology of the Mg2Si phase has been found to be critical to the creep resistance.[5] AS21 is presently specified with a silicon content of 0.7 to 1.2 wt pct. The properties and general microstructure of the die-cast magnesium AS21 alloys have been reported by a number of authors.[2,3,5,8,9] Despite this, a detailed study on the effect of changes to the silicon level within this range has not been undertaken. This article aims to investigate the effect of increasing levels of silicon on the microstructure of high-pressure die-cast AS21 alloys. II.
EXPERIMENTAL
AS21 alloys were produced from pure magnesium ingots. The melt temperature as alloying commenced was 730 °C. Manganese was added in the form of a salt mix, aluminum as 99.7 pct purity ingots, and silicon in the form of Silgrain.* *SILGRAIN is a trademark of Elkem ASA, Oslo, Norway.
Casting was undertaken at a melt temperature of 700 °C. Additions of silicon were made to six different target levels
M.S. DARGUSCH and G.L. DUNLOP, Professor, are with the Department of Mining, Minerals and Materials Engineering, CRC for Cast Metals Manufacturing (CAST), The University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] A.L. BOWLES is with GKSS-Forschungszentrum Geesthacht GmbH, Geesthacht 21502, Germany. K. PETTERSEN and P. BAKKE are with Magnesium Materials Technology, Norsk Hydro ASA, N-
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