Abstract
Samples with a variety of concentration gradients were produced in vertical Bridgman configurations by directional solidification. This was achieved by generating strong convection in the melt in two different ways: first, natural thermosolutal convection was stimulated by setting the temperature gradient and the direction of solidification in the direction of gravity. With this configuration, the influence of solidification front velocity, temperature gradient and crucible diameter on the gradients of concentration and microstructure in aluminium-copper alloys were investigated. Second, forced convection was induced in the melt by induction heating. In this case, the stirring of the melt is more effective as compared to natural convection. Plane front solidification throughout the experiments and concentration profiles close to Scheil profiles (representing the maximum achievable gradient) were obtained. The graded concentration distribution leads to a gradient in the hardness. The hardness gradient can be enhanced and the hardness distribution along the sample can be modified by heat treatment. Precipitation hardened samples of a commerical 7475-aluminium alloy were used to investigate the fatigue behaviour of the graded material (crack closure and crack propagation).
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