Effects of copper and nickel on the structure and properties of heat treated AI-6Si-0.5Mg alloy

Abstract

The purpose of this research was to investigate the influence of copper and nickel on Al-6Si- 0.5Mg alloy. Copper in the range of 0.5 - 4 wt%, individual nickel 2wt% and combined 2wt%Cu and 2wt% Ni have been added to Al-6Si-0.5Mg alloy by melt processing technique. The cast alloys were subjected to homogenization at 500oC for 24 hours. The homogenised alloys were solution treated for 2 hours at 540oC and quenched in ice-salt-water solution. The evolution of microstructure and mechanical properties during heat treatment was studied. The temperature ranges of interest were; solution treatment at 540°C, natural ageing at room temperature, and artificial ageing at 100-400°C. The changes in dendritic composition and eutectic morphology due to solution treatment were quantified by optical and scanning electron microscopic analysis. For the ageing investigations, characterisation of mechanical properties was done by hardness, resistivity and tensile testing. Artificial and isochronal ageing was conducted in an electric furnace. Isochronal ageing was conducted at 100, 150, 175, 200, 225, 250, 300, 350 and 400oC for 1 hour. For all alloys, the hardness increase was remarkable in the temperature range of 200-250oC and maximum hardness was found at ~225oC. The electrical resistivity of the alloys was found to decrease significantly at 225oC. The isothermal ageing was carried out at 150, 200, 225, 250 and 300oC for time ranging from 15 minutes to 360 minutes. For all alloys under investigation, significant improvement of hardness was achieved by ageing for ~60 minutes at 225oC. Optical Microscopes (OM) and Scanning Electron Microscopes (SEM) were used for the microstructural investigation. The morphology of the intermetallic phases were observed in the as-cast and the heat treated samples. The Cu and Mg-rich intermetallics completely dissolved but the Ni-rich intermetallics did not dissolve completely during solutionising. The eutectic Al- Si phases were modified after homogenization and solution treatment. The aged samples were tested for tensile properties. Tensile tests were performed at a constant strain rate of 10-3s-1 for all ageing conditions. In addition, the peakaged samples (1hr at 225oC) were tested at three different strain rates of 10-2s-1, 10-3 s-1 and 10-4s-1. Tensile strength was found to increase with ageing temperature, the maximum being attained at peak aged condition (1 hr at 225oC). The additions of Cu and/or Ni resulted in an increase in tensile strength and maximum strength was found for 2 wt% Cu addition. The strain rates affected the tensile properties