Effect of Minor Addition of Ni and Zr on the High-Temperature Performance of Al-Si-Cu-Mg Cast Alloys

In this study, the results pertaining to the influence of various metallurgical parameters on the tensile properties of Al-Si-Cu-Mg 354-type casting alloys are presented. This is discussed with an emphasis on acquiring an insight regarding the mechanical limits of Al-Si-Cu-Mg 354-type alloys in the context of these parameters, and to obtain an understanding of the effects of the addition of nickel and/or zirconium on alloy 354 with respect to the mechanical properties obtained at high temperature (155 degrees C and 300 degrees C for holding times at temperature ranging from 10 to 100 h). Based on an analysis of the results obtained, the following conclusions may be drawn. The results show that additions of Zr and Zr + Ni increase the high-temperature tensile properties, in particular for the alloy containing 0.2 wt% Zr + 0.2 wt% Ni, which exhibits an increase of more than 30% in the tensile properties at 300 degrees C compared with the base 354 alloy. In other words, the addition of 0.4 wt% Ni + 0.4 wt% Zr to the 354 alloy is not sufficient to resist softening at 300 degrees C/100 h (cf. UTS: 52 MPa, YS: 45 MPa, %El: 23% with UTS: 361 MPa, YS: 343 MPa, %El 1.23%, observed after T6 treatment). The addition of 0.4 wt% Ni to alloy 354 leads to a decrease in the tensile properties, compared to the base alloy. This decrease may be attributed to a Ni-Cu reaction which could interfere with the formation of the Al2Cu strengthening precipitates, thereby affecting the age hardening process. As a consequence, percentage elongation increases with the testing temperature, from as-cast to T6-treated samples tested at high temperature (155 degrees C and 300 degrees C, respectively). In summary, the best high-temperature tensile properties are displayed by the alloy containing 0.2 wt% Zr + 0.2 wt% Ni.