Abstract
The microstructure, tensile properties and fracture behavior of the Al-6Si-0.5Mg and Al-6Si-0.5Mg-2Ni cast alloys were investigated and the results showed that the Mg2Si, Al3Ni and Si hard particles produced by precipitation hardening have great effects. The homogenized (24hr at 500 o C) and solutionized (2hr at 540 o C) cast alloys were subjected to ageing treatment from room temperature to 300 o C. The yield strength and fracture strength of the alloys increased with the ageing temperature and maximum at peakaged condition (1hr at 225 o C) where as ductility and impact toughness decreased with ageing temperature reaching the minimum at the peakaged condition. The properties of Ni content and Ni free Al-6Si-0.5Mg alloys were found to exhibit marked differences depending on the intermetallic phases and the heat treatment conditions. SEM-analysis revealed the existence of Al3Ni-phases and the microstructure of broken tensile specimens show both the particle fracture and interface de-bonding affect the fracture behavior of the alloys.
Highlights
Al-Si based multi-component foundry alloys provide several advantageous characteristics such as good castability, high corrosion and wear resistance as well as high thermal conductivity and adequate strength at elevated temperatures [1]
Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS) of selected sample was observed in a Scanning Electron Microscope
The graph indicates that the yield strength of Ni containing alloy (Al-6Si-0.5Mg-2Ni) higher than Ni free alloy (Al-6Si-0.5Mg) all over the ageing conditions except aged at 200oC
Summary
Al-Si based multi-component foundry alloys provide several advantageous characteristics such as good castability, high corrosion and wear resistance as well as high thermal conductivity and adequate strength at elevated temperatures [1]. When a heat-treatable A356 aluminum alloy undergoes precipitation hardening (age-hardening), fine precipitants will dissolve completely in saturated matrix to achieve high strength in material [3]. The supersaturated solid solution undergoes aging treatment; a large amount of small and uniformly distributed precipitants will appear the mechanical properties of the material can be improved. Uniform and delicate Mg2Si precipitants can be obtained in the matrix of a heat-treatable Al-Si-Mg casting part with T6 treatment (which involves solution treatment, quenching and artificial aging), leading to improvements in its tensile strength, elongation and toughness [4, 5]. In previous work Ni was identified to significantly enhance the high-temperature performance of Al-Si foundry alloys, though just to a certain level, depending on the fraction of eutectic phase in the alloy. The objective of this research work was to study the effects of Ni on tensile properties of Al-6Si-0.5Mg cast alloy under various ageing conditions
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