The effect of superheating temperature, solidification rate and soaking treatment on the formation of constituent phases and structure of Al-7175 aerospace alloy

Abstract

Prealloyed materials of Al-7175 alloy were melted under controlled (vacuum) and normal atmospheres and cast into a permanent cast iron mould (PMC) at various temperatures. Al-7175 alloy ribbons were produced by melt spinning (RSP). Specimens of both groups of materials were then solution treated in the temperature range of 400 degrees C to 535 degrees C for various times. RSP ribbons were also subjected to in-situ annealing operation (hot stage) in the TEM. The microstructural analyses were conducted on PMC ingots, RSP materials and soaked samples to establish and evaluate the influence of superheating temperature, rapid solidification and annealing conditions on the constituent phases and substructure. It was found that casting at 645 degrees C and 670 degrees C resulted in the formation of Al23CuFe4, Al7Cu2Fe, Mg2Si and a eutectic phase of AlMg-CuZn, In the case of the ingot poured at 950 degrees C, similar phases occurred except Mg2Si. The microstructure of RSP ribbons is composed of supersaturated alpha-Al solid solution. Annealing of PMC samples at 465 degrees C led to the precipitation of a Cr intermetallic in the form of Al18Mg3Cr2 which did not undergo any change in size and shape on raising the treatment temperature. The eutectic phase was dissolved at temperatures above 515 degrees C at which the microstructural characteristics of the ingots such as grain size, morphology and dendritic cell spacing were largely uneffected. In view of considering precipitation and dissolution characteristics of secondary phases, similar observations were made in the cases of RSP ribbons which were either annealed isothermally over the temperature range of 400 degrees C to 535 degrees C or in-situ annealed in TEM.