Effect of the elements Fe, Ni, and Cr on the surface quality under machining of hyper-eutectic aluminium-based Al-Si automotive alloys has been carried out as the elements improve the properties of this alloy. Machining is done on a horizontal type milling machine using a high-speed steel slab milling cutter in dry condition. Only the cutting speed varies throughout the experiment, while the machining feed and depth of cut remain fixed. The experimental results show that the addition of these alloying elements increases the roughness and hardness specially due to formation of Fe-rich intermetallic . However, the needle-like  has been refined with the addition of Cr, as seen by the microstructure. The SEM fractography shows a huge cleavage of the brittle  phase, which initiates the crack propagation for Fe added alloys. The downward force causes compressive stress exerted in down-milling operation, so the results depict higher hardness and better surface finish.  Besides, shorter chips are formed in down-milling than up-milling process, which rather causes the brittleness of the alloys. When the cutting speed is raised, the surface quality deteriorates due to high temperature, while the hardness improves initially due to formation of precipitates then decreases due to coarsening of precipitates.

A thorough investigation has been carried out on the Al-12Si-1Mg-1Cu-1Ni automotive alloy considering different properties, specially mechanical properties associated with true stress and true strain with Zr addition of trace amount. A commercially available piston is melted to produce the alloy, and trace amount of Zr is added to make another. The base alloy along with the Zr added alloy had been applied to homogenization, solution treatment, quenching, and ageing in order to get the age-hardening response. The alloys have been heat-treated at 25 ºC, 200 ºC, and 300 ºC, respectively, for four hours for attaining the under, peak and over-aged states, respectively. During ageing, Al2Cu and Mg2Si phases are formed in the aluminium matrix leading to peak-aged strength, which is reduced at over-aged state because of coarsening of precipitation and recrystallizing, shown by the tensile and hardness properties. When Zr is added to the alloy, Al3Zr phases appear while casting and heat-treatment, resisting the drop of strength at over-aged state. It is visible in the stress-strain diagram that at over-aged conditions, the alloy with trace Zr shows improved strength and ductility. In the micrographs of Zr added alloy, finer distributed grains are visible through the grain refinement of Zr, which also prevents recrystallization at over-aged conditions. The homogeneity of the grains as a result of the Zr addition's microstructural change was further confirmed by fractography. It is clear that adding Zr to such alloys does not greatly increase their strength, but it does restrict the declining of strength by preventing the production of thermally stable Al3Zr precipitates, which coarsens the resisting behavior of various intermetallics in the thermally damaged alloy.