effect of cerium and barium additions and superheating melt treatment on the morphology and hardness of al-mg2si-cu composite

Abstract

Aluminium-based alloy, reinforced with particulate Mg2Si phase has been widely accepted to replace Al-Si alloy due to its improved properties in producing engineering products especially for automotive and aerospace applications. However, in as-cast Al-based reinforced with Mg2Si composite, the particles formed are coarse with large skeleton shapes and eutectic Al-Mg2Si phase which are also present in flake-like form. These phases are known to have detrimental effect on the mechanical properties of the composite. The present research is therefore aimed to investigate the effect of elements addition and superheating melt treatment in order to modify the undesired structures and phases in Al-Mg2Si-Cu metal matrix composite. The elements addition were Ce (0.3-1.0 wt.%) and Ba (0.1-1.0 wt.%). Meanwhile, superheating above the melting temperature of Al-Mg2Si-Cu composite was carried out at three different temperatures (850°C, 900°C and 950°C) and three different holding times (15, 30 and 45 minutes) to further modify the microstructures. The samples were produced by melting commercial Al-Mg-Si ingot and pouring into a ceramic mould and the transformation temperatures were determined by computer aided cooling curve thermal analysis (CACCTA). The phase and microstructural changes were characterized using optical microscopy, field emission scanning electron microscopy (FESEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Hardness test (ASTM E92) was performed in order to investigate the effect of morphology modification on the hardness of the composite. Both approaches, namely, elements addition and melt superheating with varying parameters were found to refine not only Mg2SiP reinforcement particles but also eutectic Al-Mg2Si phase. Various morphologies of the phases were observed, particularly, coarse skeleton of Mg2SiP has been transformed to finer polygonal structure. Likewise, flake-like morphology of Mg2SiE has transformed to rod and fibrous-like form while the needle-like intermetallic ß has transformed to a phase. The optimum concentrations to achieve the adequate refinement effect were found to be 0.8wt% Ce and 0.2wt% Ba. While, the optimum parameter for the melt superheating was 950°C and underwent 15 minutes holding melt duration. The modified composite with addition of optimum concentration of Ce and Ba were observed to increase in hardness property from 61.32Hv to 74.3Hv and 67.95Hv for Ce and Ba, respectively. Whereas, for the composite modified by melt superheating, the hardness improved from 61.32Hv to 70.22Hv.