Mechanical performance of heat treated 319 alloys as a function of alloying and aging parameters

An investigation was carried out to determine the effect of Mg, Sr, and Ti additions and artificial T5 and T6 aging treatments on the tensile properties of 319 alloys, with the aim of adjusting these parameters to produce castings with suitable mechanical properties. A statistical DOE model was also developed for the analysis of the correlation between the properties of alloy and independent parameters (composition and aging conditions). The microstructure and fracture of the alloy were examined using scanning electron microscopy. The obtained results show that addition of Ti to alloys containing Mg and Sr produces sounder casting with finer grain size and improves the alloy strength. Excess amounts of Sr, however, deteriorate the tensile properties of alloys containing high Mg content by increasing the total amount of Mg–Sr intermetallic phase and porosity. Fine and dense precipitates with a smaller inter-particle spacing are formed at a lower aging temperature of 150 °C compared to coarse, less dense and more widely dispersed at higher temperature, i.e., 250 °C. The strength of 319 alloys increases with the Mg content and decreases with Sr and aging temperature and time. Increasing the Mg content up to 0.45% enhances the alloy response to heat treatment in the T5 and T6 tempers, more particularly, in the latter case. Fracture of intermetallic phases in the interdendritic regions is mostly brittle, with the formation of microcracks at the interface between Si, Cu, Fe-base intermetallic particles and aluminum. From the statistical analysis, the most effective parameters on the tensile properties were Mg content and aging temperature. The interaction between Mg content and aging temperature and/or aging time was the most influential in this respect.

► Alloying additions effect on tensile properties of 319 alloys.
► Aging parameters effect on the tensile properties and fracture surface.
► Factorial design evaluates the statistically significant alloying and aging conditions.