Parameters controlling the performance of AA319-type alloys: Part I. Tensile properties

Al–Si–Cu AA319 alloy was selected for study in the present work due to its wide use in many automotive applications. In addition to the variation in the secondary dendrite arm spacing (SDAS), porosity level and the type of heat treatment applied, the presence/addition of certain alloying elements or modifiers to these alloys is known to greatly affect their mechanical properties. An extensive study was carried out to investigate the solidification and dissolution behavior of the CuAl2 phase in various AA319-type alloys (containing Sr, Fe and P element additions) and its effect on the alloy performance under two different heat treatment conditions (T5 and T6). Part I of this article reports on the tensile properties of these alloys. The tensile test samples were prepared from blanks sectioned from end-chilled castings, corresponding to dendrite arm spacings ranging from 23 to 83 μm. It was found that strontium modification leads to a fibrous eutectic Si morphology, resulting in a great increase in the ultimate tensile strength (UTS) and ductility values compared to the unmodified alloys. On the other hand, Sr addition also causes segregation of the blocky CuAl2 phase, which results in reducing the tensile properties. However, dissolution of CuAl2 particles after T6 heat treatment can minimize this kind of negative effect. Iron has a detrimental effect on the UTS and ductility values due to its precipitation in the form of the brittle β-Al5FeSi plate-like phase. The addition of phosphorus has a negative effect on the tensile properties on account of (i) its solubility in CuAl2, and (ii) the formation of (Al,P)O2 oxide particles, where the latter act as nucleation sites for the formation of the CuAl2 phase.