Investigation of microstructure and corrosion resistance of silumin after nickel and copper addition

Abstract

Aluminium whose Silicon as its main alloying element, i.e. Silumin, is widely applied in engineering due to its novel properties. For particular application, e.g. as block engine of seawater vessel, Silumin’s properties must be enhanced, e.g. strength at elevated temperature, hardenability and corrosion resistance. For those purposes, Nickel and Copper is added to Silumin in this work. As a result, Copper addition by 4.1 %wt increases Silumin density by 13.9 % from 2.95 to 3.36 g/cm3, decreases grain size by 35.3 % from 39.4 to 25.5 μm, increases hardness by 178.8 % from 21.66 to 60.38 HR30T and increases the corrosion rate by 24.5 % from 0.354 to 0.441 mmpy. Meanwhile, addition of Nickel by 6.35 %wt increases Silumin density by 14.9 % from 2.95 to 3.39 g/cm3, decreases grain size by 29.7 % from 39.4 to 27.7 μm, increases hardness by 161.8 % from 21.66 to 56.71 HR30T and decreases the corrosion rate by 41.2 % from 0.354 to 0.2211 mmpy. To investigate the influence of Copper and Nickel addition on Silumin’s microstructure, micrograph is also taken by using an optical microscope and a scanning electron microscope. The micrograph shows dendrite presence only for the case of Copper added Silumin which deleterious for mechanical properties of Silumin. This dendrite is predicted due to slower cooling rate of the cast Silumin after Copper addition. In addition, the needle-like β-precipitate is sharper after Copper addition. This circumstance is responsible for higher hardness increase for the case of Copper addition compared to Nickel addition. The micrograph shows the existence of CuAl2 intermetallic phase. This intermetallic phase is nobler than Aluminium which is responsible for higher galvanic corrosion in Silumin after Copper addition. On the other hand, the dendrite cannot be observed in the case of Silumin after Nickel addition. The intermetallic phases after Nickel addition, i.e. AlxNiy contribute to the hardness increase. These phases also remain stable at elevated temperature which then then contributes to better mechanical properties of Silumin at the elevated temperature