Effect of Mg on the ageing behaviour of Al-Si-Cu 319 type aluminium casting alloys

The present study was performed on primary A319.2 alloy to investigate the effect of magnesium addition as well as other melt treatment parameters such as Sr modification and grain refinement on the heat treatment behaviour of the alloy. The results show that increasing the Mg content in A319.2 up to 0.45% considerably enchances the alloy response to heat treatment in the T5 and T6 tempers, more particularly, the T6 temper. Modification of the high-Mg version of 319 alloy with Sr in amounts of similar to 350 ppm results in a marked amount of porosity formation which counteracts the beneficial effect of the modification, leading to a noticeable decline in the alloy strength. Grain refining the Sr-modified (A319.2 + 0.45% Mg) alloy produces sounder castings and, hence, an identical ageing response to that offered by unmodified high-Mg alloys. The properties, however, are more consistent. Addition of Mg (similar to 0.45%) leads to the precipitation of coarse particles of Al5Mg8Si6Cu2. Modification with Sr tends to cause severe segregation of both Cu-containing intermetallics, i.e., Al2Cu and Al5Mg8Si6Cu2 in areas away from the growing Al-Si eutectic regions. Thus, their dissolution rates are fairly sluggish upon solutionizing at 505 degrees C. Increasing the solutionizing temperature would lead to incipient melting of the phases and, hence, a catastrophic failure. Fracture of intermetallic phases in the interdendritic regions is mostly brittle, with the formation of microcracks at the Si, Cu, Fe-base intermetallics and aluminium interfaces. Fracture of the alpha-aluminium is always ductile. Hardening during ageing occurs by cooperative precipitation of Al2Cu and Mg2Si phase particles. (C) 1999 Kluwer Academic Publishers.