HOT ISOSTATIC PRESSING OF ALUMINUM-SILICON CASTINGS (POROSITY, MICROSTRUCTURE)

The commercial Al-Si casting alloys, containing 7.12 and 10.71 wt % Si, were examined. Samples of the as-cast alloys were Hot Isostatically Pressed (HIPed) at 68.95 MPa and various combinations of temperature (500-560(DEGREES)C) and time (15-20 minutes). To examine the effect of the elimination of porosity during HIPing, tensile properties of the HIPed castings were compared with those of the as-cast materials. Optical microscopy, scanning electron microscopy, and transmission electron microscopy were utilized to characterize the microstructures of these alloys in the as-cast, heat treated, and HIPed states.; HIPing led to the virtual elimination of porosity. This was particularly evident for the castings which had been HIPed for at least 60 minutes. It was established that any temperature between 500 and 560(DEGREES)C is suitable for HIPing these castings. In a few instances, porosity was retained in the HIPed castings. This was attributed to either short HIPing times or the retained pores being surface connected. After HIPing, there was a very significant improvement in the ductility of the castings.; In the as-cast, heat treated, or HIPed states, the main phases in these castings are the primary and secondary silicon-rich particles. The heat treated castings also contained substantial amounts of the iron-containing particles such as Al(,12)Fe(,3)Si. During HIPing, only the primary silicon-rich particles coarsened and spheroidized. The dislocation density in the HIPed castings was only modestly greater than that in the as-cast and heat treated materials.