Development of high temperature creep resistant aluminium based sand cast magnesium alloys

Yalınız, Emrah
The aim of this thesis is to develop alternative sand casting aluminum based magnesium alloys to substitute current sand castable helicopter transmission housing magnesium alloys such as ZE41A and EV31A. AZ91 magnesium alloy has also been preferred for helicopter transmission housing material. However, the lower creep resistance above 120 oC due to the presence of -(Mg17Al12) phase in the microstructure and showing varying mechanical properties in different section thicknesses limit the usage of AZ91 in the modern helicopter transmission housings. This thesis has been aimed to eliminate these disadvantages of AZ91 alloy by adding different alloying elements. The aim of alloying elements e.g. Si, Sr, Ca and Sn addition to AZ91 alloy is to form high temperature creep resistant phases in the microstructure and decrease casting defects such as interdendritic porosity in order to obtain desired mechanical properties. Although simultaneous addition of 0.7 % Si, 0.5 % Ca and 0.9 % Sn addition to AZ91 alloy decreases the grain size, porosity percentage of the alloy is higher than even that of AZ91 alloy, which causes reduction of tensile test results. The test results show simultaneous addition of 0.7 % Si, 0.4 % Sr, 0.6 % Sn elements together with 0.1 % Al-5Ti-1B to AZ91 alloy results in higher tensile test results in comparison to ZE41A and AZ91 magnesium alloys. The decrease in volume fraction of -(Mg17Al12) phase network was achieved with addition of Sr and Sn elements to obtain a high creep resistant magnesium alloy. The elements like Sr and Sn provide the modification of mechanically detrimental “Chinese script” morphology of Mg2Si to polygonal and square-like shape, which is beneficial both at room and elevated temperature properties. The test results indicate the structural integrity in variable casting thicknesses achieved after the alloying elements (0.7 % Si, 0.4 % Sr and 0.6 % Sn) and grain refiner (0.1 % Al-5Ti-1B) addition to AZ91 alloy.
Citation Formats
E. Yalınız, “Development of high temperature creep resistant aluminium based sand cast magnesium alloys,” M.S. - Master of Science, Middle East Technical University, 2018.