Melt Injection in HPDC Casting: Trapped Gas and Fatigue Strength

The issue of large gas bubbles trapped in high-pressure die casting (HPDC) technology is addressed. Die casters have long struggled with this issue, which often remains unresolved and results in significant scrap. Therefore it was decided to produce two batches of AlSi9Cu3(Fe) aluminium alloy castings, the first with the die designed by the foundry according to its established procedures, the second following principles that refer to the rules of Campbell and the NADCA Gating Manual. In the second batch, the area of the ingate has been increased and the flow rate decreased. This is generally contrary to normal foundry practices. The first batch show bubbles from trapped gas, sometimes large in some castings, which are therefore rejected. In the second batch, the porosity is much lower and no rejections occurred. Tensile and bending fatigue tests were carried out on accepted castings. Tensile properties were very similar for the two batches. Regarding fatigue tests, in the first batch many samples had inappropriate breaks that prevented their use in the staircase procedure. These problems did not occur in the second batch. After skimming off the scrap and samples with invalid breaks, the fatigue strength at 10 million cycles has a substantial statistical dispersion, typical in foundry components, very close for the two batches. It depends closely on the triggers of fatigue cracks. Eliminated the big trapped gas bubbles, fatigue strength depends on the initiation points, predominantly oxide film or cluster for both batches. This explains the very close fatigue resistance in both cases.