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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.
11.4 SQUEEZE CASTING AND SEMI-SOLID METALWORKING
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Contaminants
Direction of Metal Flow
Contaminant Buid-up on
Metal Front a
b
Figure 11.1 Contaminant veins form as a a clean planar fill front collects
contaminants and b the metal progresses through the die cavity.
11.4.1 Contaminant Veins
Contaminant veins are unique defects that result from the planar filling phenomenon of squeeze casting and semi-solid metalwork-
ing processes. To understand this defect, one may examine the method of its formation. When filling the die, the metal front
remains relatively intact Figure 11.1a and picks up contaminants such as die lubricants, die steel corrosion products, and other im-
purities, as illustrated in Figure 11.1b. This contamination is ex- acerbated by oxidation at the metal front. As filling of the die is
