Optimisation of the mechanical properties in an investment cast aluminium alloy

2017-08-18T15:03:06Z (GMT) by Levy Siaminwe
The thesis reports an experimental investigation concerned with the optimisation of mechanical properties in investment cast LM25 aluminium alloy. The effects of melt hydrogen content, filter usage, grain refinement, eutectic silicon modification, heat treatment, pouring and shell preheat temperatures on the structure and tensile properties of LM25 investment cast tensile test specimens were studied. Four series of experiments were conducted to assess the effects of the parameters studied on the structure and tensile properties. The first series established the effect of melt hydrogen content, pouring temperature and shell preheat temperature on the casting porosity, pore morphology and tensile properties. The second series investigated the effect of using a ceramic foam filter on the tensile properties. Pouring temperature and shell preheat temperature were variable parameters in this part of the investigation. The objective of the third series of experiments was to investigate the effect of grain refinement and eutectic silicon modification on the structure and tensile properties. The interaction of these melt treatments with shell preheat temperature and filtration was also studied. In the final series of experiments the effect of heat treatment cycles on the samples produced in the third series of experiments was evaluated in terms of structure and tensile properties. The principal findings of the research were that: shell preheat temperature and hydrogen content are the most important process variables determining the total porosity content; shell temperature affects the structure and, hence, the tensile properties; grain refinement is enhanced as the titanium content increases to about 0.28% but the tensile properties remain unaffected; a modified eutectic Si structure is achieved with 0.01 - 0.02% Sr with optimum Sr addition, based on tensile properties, being 0.01%; and, as would be expected, heat treatment improves the tensile properties of investment cast LM25. On the basis of the inter-relationships between process variables, structural changes and tensile properties observed, an optimum processing route was proposed. Grain refined and modified specimens produced with low hydrogen content and ambient shell temperature had optimum tensile properties when fully heat treated to produce eutectic Si transformation.