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THERMAL BALANCING AND POWDER DIE LUBRICANT PROCESSES
Figure 6.1 Die cavity surface temperature over a single processing cycle.
tain such defects, and alternative strategies to combat these prob- lems for use with high integrity die casting processes.
6.2 HEAT CHECKING AND SOLDERING
Die surfaces are typically hardened tool steel with a martensitic microstructure. This hard wear-resistant surface is excellent for
use with high integrity die casting processes. However, martensite is also brittle. The aggressive thermal cycling inherent in high
integrity die casting processes often causes the surface to crack. Over time initial microcracks will develop into larger cracks, or
heat checks, penetrating into the die. This progression is shown in Figure 6.2. Once cracks form on the die surface, liquid metal
injected into the die will fill these voids. The result is checking and small veins on the surface of the casting.
In many cases, minor heat checking is not a problem. However, for components in which surface quality is a requirement, heat
checking is a costly defect often requiring the addition of second- ary finishing and polishing operations. Regardless of surface finish
requirements, heat checking is the dominant mode of failure for dies as product dimensional stability will eventually be affected.
6.3 THE EFFECTS OF HEAT CHECKING AND SOLDERING
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Transition Martensite
Bulk Die Steel Heat Check
Progression Initial Cracks
Figure 6.2 Die surface cross section illustrating heat-checking formation and
progression.
Soldering is another problem, which can occur when utilizing high integrity die casting processes. This phenomenon occurs
when a bond forms between a component and the die surface during solidification, often resulting in permanent damage to both
the component and the die surface. Soldering typically occurs at hot spots on the die surface. Examples of such locations include
small uncooled cores and points subjected to impinging metal flow. Areas around welded die repairs are also prone to soldering.
The repair process often transforms the resistant martensitic layer on the die face into a soft annealed surface Figure 6.3 These
annealed areas are much more susceptible to soldering.
6.3 CONTAINING THE EFFECTS OF HEAT
CHECKING AND SOLDERING
Several strategies are used by component producers to contain heat checking and soldering. In all cases, economics are affected.
Since many components produced using high integrity die cast- ing processes do not have stringent surface finish requirements,
low cost shot blasting may be used to remove heat checking as
104
THERMAL BALANCING AND POWDER DIE LUBRICANT PROCESSES Prone to Soldering
Martensite
Bulk Die Steel Transition
Transition to Bulk Die Steel
Heat-affected Zone AnnealedSoft
Martensite
Figure 6.3 Die surface cross section illustrating microstructural weaknesses in
a welded die surface.
well as flash. For components with stringent surface requirements, dies are often designed for easy maintenance. Die cavities are
often replaced when heat checking occurs. This leaves the major- ity of the die untouched. Even severe heat checking of the runner
system does not affect component functionality. The most costly solution to resolve heat checking is the complete replacement of
a die.
To combat soldering, many strategies may be used. In some cases, a slower cycle time may eliminate the problem, allowing
water lines to cool the die. More often, component producers look to liquid die surface lubricants. Longer spray times may be used
to apply additional lubricant across the entire die surface. This may overlubricate the die. An alternative is to spray additional
lubricant on select areas of the die prone to soldering. Component metal chemistry may also be adjusted. Additions of iron can re-
duce the soldering potential of an alloy. However, iron additions have a negative effect on mechanical properties. In many cases,
once soldering occurs, it cannot be totally eliminated. Re-
