Dimensional Defects CONVENTIONAL DIE CASTING DEFECTS

162 DEFECTS IN HIGH PRESSURE CASTING PROCESSES cases, a component must be heat treated or welded. In such cases, blisters may form on or near the surface of a component. When metal is injected into the die, entrapped gases are often com- pressed into very small bubbles. When the temperature of the component is elevated during welding or heat treatment, com- pressed gases expand, forming blisters. For this reason, most con- ventional high pressure die castings do not undergo secondary thermal processes. As discussed earlier, cracks often occur after a component is manufactured. In many cases this includes secondary processing. Trimming is performed to remove flash as well as the runner sys- tem from a component. Trimming may cause cracks if the fixtur- ing does not support the component evenly or if trim punches are oversized. Machining operations often damage and crack com- ponents. Hydraulic clamps can easily overstress a component, causing cracks. Moreover, vibrations caused by cutting tool chatter during machining may induce cracks.

11.4 DEFECTS UNIQUE TO SQUEEZE CASTING AND

SEMI-SOLID METALWORKING When commercializing squeeze casting and semi-solid metal- working processes, component producers looked to conventional die casting to identify potential defects and control component quality. Several defects were expected, including cold shuts, cold flows, flash, drags, warping, and gas entrapment, to name a few. 1 Efforts were taken to avoid these defects by addressing processing methods, die design, and product design. Defect types, however, have surfaced unique to squeeze casting and semi-solid metal- working processes. Although squeeze casting and semi-solid metalworking have proven to be successful for many commercial applications, com- ponent producers have been reluctant to report defects for fear of giving these emerging processes a bad reputation. Nonetheless, these defects are easily classified and must be understood to avoid future problems. Contaminant veins and phase separation are the defect types presented in this discussion.