TEMPERATURE RISE OF RAPIDLY SOLIDIFIED DEPOSIT LAYERS OF AL-SI ALLOYS DURING LOW-PRESSURE PLASMA SPRAYING AND ITS EFFECT ON THEIR STRUCTURES AND MECHANICAL-PROPERTIES

Powders of Al-Si alloys containing about 8-24 wt.% Si are plasma sprayed onto a steel substrate in a low pressure chamber and rapidly solidified deposit layers are produced. When the substrate is strongly cooled during spraying, extremely fine silicon particles are dispersed in the aluminium matrix which is supersaturated with silicon. The deposit layer is separated from the substrate and then machined to tensile test pieces. Owing to the presence of pores and the weak adhesion between the flattened particles which compose the layer, the fracture strength of the layer is low despite the high hardness. When the temperature of the deposit layer is allowed to rise during spraying, the porosity decreases and the adhesion between the flattened particles is enhanced. The fracture strength is much higher than that of the conventionally cast alloys. Excessive temperature rise, however, coarsens the silicon particles and lowers the strength. A moderate temperature rise of the deposit layer is necessary for the improvement of its tensile properties.