The Need for a New Approach to Soldering in High Pressure Die Casting
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00504-4
Abstract Soldering remains one of the most common defects in high pressure die casting (HPDC). It is most simply defined as aluminum stuck to the die surface. At ejection, the stuck aluminum gouges the casting resulting in dimensional and leaking defects. For nearly 50 years, a thermodynamics and kinetics understanding of soldering has been accepted while explicitly assuming that other mechanisms are not important. It will be shown that this thermodynamics and kinetics modeling approach cannot adequately explain three common observations in die casting foundries. First, soldering occurs much faster than the measured intermetallic growth rate based on diffusion couple experiments. Second, the alloys that are industrially observed to
solder less form intermetallic phases quicker and dissolve the steel faster than alloys that solder more rapidly. Finally, the intermetallic layer is not strong enough to enable the buildup commonly observed on soldered dies. For these reasons, it is proposed that a new soldering modeling approach is required to gain a better understanding of the soldering mechanism and prevent the soldering defect in die casting.
Introduction
kinetics-limited approach to thermodynamic equilibrium. The obvious prediction is that increasing the die/casting interface temperature will increase the intermetallic growth rate.3 Observations agree with this prediction because soldering is worse in thicker (hotter) areas of the die casting. Direct metal impingement during filling also increases the temperature due to intense convective heat transfer, and these areas have more severe soldering. Thermodynamics and kinetics analysis would also predict that cooling the die would reduce soldering. This is supported by die lubricants efficacy at reducing soldering because it is the most effective way to cool the die surface. Higher thermal conductivity materials are also known to solder less severely due to the lower die temperatures.3,5
A common defect in aluminum high pressure die casting (HPDC) is the casting adhering to the die. When the stuck material builds up on localized areas of the die, it is called die soldering (Figure 1). It occurs most often in the hot areas of the die, but it can also form in cold areas.1,2 Internal water cooling combined with external lubricant cooling are the best tools for controlling solder, but they are only partially effective. The side effect of aggressive cooling is premature die failure from thermal shock. This increases die repair costs and production downtime. Controlling sticking and soldering without the aid of the die lubricant would represent a major advance in the die casting process. The true mechanism that causes soldering must be discovered and thoroughly investigated. To understand the mechanism of soldering, core pins have been studied after they have soldered beyond repair.3,4 The failed pins are often coated with a series of intermetallic phases. This suggested that the solde
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