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The effect of the addition of Zr, Ni, and Mn and the elevated temperature thermal exposure on the mechanical performance (tensile and hardness properties) of Al-Si-Cu-Mg cast alloy were investigated for this research study. The evolution of the strengthening precipitates during the prolonged thermal exposure were also investigated using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) in order to correlate the variation in the mechanical properties to the nature of the strengthening precipitates. Varying the thermal exposure technique from static into dynamic had little effect on the tensile and hardness data obtained at room temperature. This permitted using the static exposure (stabilization) technique to simulate the behavior of the material under dynamic thermal exposure conditions as in the case of the automotive engine components. Coarsening of the strengthening precipitates following prolonged exposure at 250 °C had a deleterious effect on the tensile properties and hardness values. Noticeable reduction in the strength values, particularly the yield strength, and a remarkable increase in the ductility values were observed. The coarsening kinetics of the precipitates decayed with time, due to the continuously increased distance between the precipitates with increase in the exposure time, causing the observed deterioration in the mechanical performance after exposure at 250 °C up to the first 100 h. However, further thermal exposure up to 200 h did not result in further reduction in the strength and hardness values.

This study investigated the active strengthening precipitates existing in the structure of 354-type Al–Si–Cu–Mg cast alloys following the addition of Zr as a primary alloying element in addition to Ni and Mn in different amounts and combinations. The investigation comprised three main parts: the first part simulated the formation and dissolution of strengthening precipitates and their precursors using differential scanning calorimetric (DSC) heating analysis of as-quenched samples, the second part presented transmission electron microscopy (TEM) observations of the fine precipitates existing in the structure of the alloys studied, and the third part examined the mechanical performance (tensile properties and hardness values) of the alloys studied. Interpretation of the DSC heating data in accordance to the available literature showed that the alloys are strengthened primarily by θ-Al2Cu and S–Al2CuMg precipitates and their precursors. Furthermore, some of the copper content was consumed in forming Al–Ni–Cu particles instead of the θ-Al2Cu strengthening phase and its precursors and hence the Ni-free alloys contain higher fractions of the θ-Al2Cu phase and its precursors than the Ni-containing alloys. TEM investigations confirmed the results obtained from the DSC heating analysis regarding the combined strengthening effect of θ-Al2Cu and S–Al2CuMg precipitates and their precursors as well as the fraction of θ-Al2Cu phase in Ni-free and Ni-containing alloys. TEM observations revealed, also, that the alloys studied have a secondary strengthening effect which is exerted by precipitates in the form of Alx (Zr,Ti)Si that formed following the addition of Zr. The Mechanical performance showed to be highly dependent on the chemical composition which controls alloys structure and response to the applied heat treatment and hence the formation of the fine precipitates which are mainly responsible to the strengthening effect.