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Effect of the initial melt temperature on the microstructure of the sprayformed 2014 Al billet was studied with particular emphasis on the effect of superheating on the liquid fraction(FL) in the sprayed droplets. In order to determine the average liquid fraction within the sprayed droplets, the cooling rate and the flight time of droplets were calculated. The measured cooling rate of droplets was in the vicinity of ~104℃/sec and the flight time before impinging the preform ranges from 6 to 9 msec depending on the size of droplets. The obtained results from the experiments were in good agrement with those of theoretical models by P. Grant et al. and/or Lee et al. The liquid fraction of droplets did not vary significantly with the initial melt temperature such that FL was calculated to be 24% when Ti = 800℃ while FL = 19.5% when Ti = 720℃, resulting in considerable amount of porosity within the preform. Such a result indicates that the initial melt temperature control may not be effective for fabricating preforms with a minimal amount of porosity.
In order to manufacture large rod preforms of 2014 Al alloy with a good mechanical property by spray forming method, it was spray-formed at a droplet temperature of 715˚C, a droplet flight distance of 400㎜, and a spraying angle of 35˚. The rod preforms were extruded at 397˚C with the die temperature of 420˚C under the hot extrusion ratio 21:1 and T6 heat treatment was performed. The 2014 Al alloys cast by hot top process were also extruded and heat-treated at the same condition as a reference material. Microstructural observation and tensile test were carried out to investigate the effects of extrusion on microstructure and mechanical property of spray-formed Al alloy. Spray-formed Al alloys had many porosities due to inappropriate process conditions such as long droplet flight distance and low droplet temperature but have fine equiaxed grain. These porosities were reduced with decreasing in grain size by hot extrusion. Ultimate tensile strength and yield strength of spray formed-extruded 2014 Al alloy were inferior to those of the normal cast-extruded 2014 Al alloy, but elongations were superior. The control of porosity was important to get spray formed preform with a good mechanical property. (Received September 6, 1997)
A comprehensive methodology to consolidate the hypereutectic Al-25Si-X alloy via spray forming was investigated in an attempt to judge the feasibility of this alloy for the application to wear resistant components. Billets having desired shape and microstructures were fabricated using forming parameters obtained from numerical calculations. Prior to tube extrusion of the spray formed billets, effects of various extrusion conditions, such as extrusion ratio, die temperature, and die configuration, on microstructures of the billet were studied. Based on results obtained from the preliminary extrusion tests, the formed billets were then hot extruded into a tubular shape. Various material properties of the extruded billet were measured and compared with the other candidate materials for anti-wear applications.