INFLUENCE OF SEMI-SOLID CASTING PROCESS PARAMETERS ON MICROSTRUCTRAL CHARACTERISTICS OF MECHANICALLY STIRRED A356 ALUMINUM ALLOY

Abstract

The present investigation is focused on studying the microstructural changes in the A356 AI-Si -Mg alloy, such as the primary a-AI grain size, shape factor, the state of agglomeration between particles, and the levels at which some eutectic can be entrapped between solid agglomerates, when stirring the alloy using a mechanical impeller under different chosen conditions of stirring, namely: shearing rate, shearing duration, and fraction liquid. The morphological changes in the sodium-modified eutectic silicon, as well as microsegregation of silicon according to those different stirring conditions were also investigated. Evaluation of the porosity contents in the final obtained microstructures, according to the different selected shearing conditions was also investigated. It was found that the morphology of primary phase grains, at specified fraction liquid, depends on the shear rate/duration combination. Optimization of the processing parameters is very critical to achieve the desirable microstructural characteristics to ensure a high quality rheocast parts. Two empirical equations has been put forward for calculation the average a-Al particles and its shape factor. The average agglomeration sizes of the primary a-AI showed a direct increase in its values by increasing the shearing durations. An empirical formula has been put forwarded for calculation the volume fraction of the entrapped eutectic The morphology of the eutectic silicon in A356 alloy changes as a result of semi-solid processing combined with mechanical stirring. The shearing parameters have a significant influence on the morphology of eutectic Si. The shearing temperature has a noticeable effect on the eutectic Si morphologies, as fine-low spaced Si plates were obtained when the modified alloy was quenched from the higher temperature of 620 °C, in comparison to those obtained when stirring took place at lower shearing temperatures of 610 and 602 °C. At constant shearing intensity and liquid fraction for processing temperatures of 602 and 610 °(, increasing the shearing duration beyond I 0 minutes, caused the eutectic silicon plates to become coarser and retain their plate-like shape. A combination of high stirring temperature 620 °C, low shearing rate of 147S"1 , and 30 minutes stirring time results a

finer fibrous or rounded Si particles morphology.