Elimination of porosity in bulk metallic glass castings using hot isostatic pressing

This study presents design and implementation of a systematic method to remove the pores in as-cast bulk metallic glass using hot isostatic pressing, without changing the amorphous structure of the samples. The supercooled liquid region of Zr44Cu40Al8Ag8 was characterized using differential scanning calorimetry and dynamic mechanical analysis. This enabled informed choice of the range of hot isostatic pressing process variables likely to result in successful reduction of the porosity in the glassy alloy. The operating pressure in hot isostatic press processing was relatively less influential than either the temperature or the dwell time in controlling the porosity. It was shown that the dwell time should be longer than the average relaxation time in the glass transition range. With the specific bulk amorphous alloy under study, the optimized temperature, pressure and dwell time are 475 °C, 50 MPa and 3 min, respectively. Excess dwell times will result in crystallization.