Effect of transition elements on micro-structure and mechanical properties of secondary Al-7si-mg cast aluminium alloy.

Abstract

A majority of aluminium scrap have differences in chemical composition. Consequently, this makes the quality of recycled cast aluminium alloys to be questionable due to difficulty in chemistry control. This subsquently affects the microstructure and the general mechanica l properties of these resulting products. Therefore, during aluminium recycling,it is vital to look for means of maximising the chemistry control of the alloy so as to improve the process efficiency. The aim of this project was to contribute towards efficient aluminium recycling through the development of a model recycle friendly alloy for cylinder applications utilising direct recycling of automotive aluminium wheels. Four alloys were developed as a result of four ingots (obtained from recycled automotive wheels) being alloyed with strontium, titanium, zirconium, vanadium and copper in different proportions. The effect of these elements individually and in various combinations on the formation of intermetallic compounds was evaluated using microstruc ture analysis (optical, and SEM) and mechanical tests (tensile test, and fatigue tests). The tensile tests at room temperature showed that alloy additions increased the ultimate tensile strength by as much as 31 % (from 218 MPa in alloy 356 to 285 Mpa in alloy 356+3.5Cu+X) while the fatigue strength increased by 37 % (from 71.35 MPa in alloy 356 to 98.05 MPa in alloy 356+0.5Cu+X). The alloying elements added formed compounds like (AlSi)xTiVZr, with rod/blocky morphology which precipitated into very fine precipitates after heat treatment that impended the movements of dislocations thereby increasing the strength of the alloys.