چكيده به لاتين
Automobile and aerospace are one of the most important industries of recent times. One of the metallurgical solutions for making progress in these industries qualitatively and quantitatively is to use magnesium alloys instead of the widely used aluminum and steel alloys due to their specific strength compared with those of these two alloys. However, the disadvantage of manufacturing magnesium components by conventional casting routes is the high probability of the presence of casting defects. One of the novel methods to mitigate these defects is to use semi-solid processes. In addition, by the use of semi-solid processing, the capability of altering the microstructural morphology is achieved. Therefore, the physical and mechanical properties of magnesium alloys can be affected. In this research, semi-solid rheocasting method with applying shear tension was used to introduce a non-dendritic morphology in AZ91 magnesium alloy. With performing thermal analysis during the semi-solid processing, the relationship between temperature, shear tension and solid fraction was found. The metallographic results confirmed the development of a non-dendritic microstructure by using the rotational speeds of 180 rpm and 210 rpm. The accordance of the metallographic images with the results of the thermal analysis showed that the mechanism of development of a non-dendritic morphology is to create an effective convection fluid flow to increase the number and the lifetime of nuclei, the growth of the primary dendrite in multiple directions and decreasing the thermal and concentration gradients and remelting the roots of the dendritic secondary arms. The highest degree of spheroidity of non-dendritic grains (0.72) along with the highest surface density value of non-dendritic grains and the lowest grain size was obtained by applying the shear tension to the semi-solid mixture with the rotational speed of 180 rpm up to the temperature of 585 ⁰C. Under these conditions, the ultimate strength, the compressive yield strength and toughness increased by 25%, 33 %, and 49 %, respectively. The addition of 1 % Ca to AZ91 led to decreased values of the ultimate strength, yield strength, and toughness by 26 %, 57 %, and 48 %, respectively. However, the ductility decreased. With the addition of 1 % Ca concurrently with applying the semi-solid processing under optimal conditions resulted in enhanced values of ultimate strength, yield strength and toughness by 50 %, 88 %, and 85 %, respectively. The important result is the simultaneous improvement in strength and ductility which resulted in a significant increase in toughness. In fact, by applying the semi-solid RCP under optimal conditions on AZ91 and AZ91-1Ca, the sample under impact loading absorbed higher amounts of energy to fracture by 49 % and 85 %, respectively.
Keywords: Magnesium alloy, semi-solid processing, RCP, non-dendritic structure, strength, toughness, thermal analysis
