How to Improve Magnesium Alloy Die Casting Quality

Structural parts for projector optical systems are usually made from die-cast magnesium or die-cast aluminium alloy to meet strength and heat-dissipation requirements. Although die-cast magnesium alloy outperforms die-cast aluminium alloy, the complexity of optical structural parts makes it harder to produce and pushes the rejection rate up. As magnesium die casting technology improves, magnesium alloy is steadily replacing aluminium alloy in these applications.

1. Product overview. A typical part has a wall thickness of 4.4 mm, an envelope of 91 × 93 × 60 mm and a net weight of 113 g, in material AZ91D. The mould carries one side slider in each of four directions. For functional reasons many areas originally had fillets that were too small, which is unfavourable for filling.

2. Scrap analysis. Statistics showed a rejection rate of 24%. A detailed analysis of 259,373 rejected parts found 250,378 under-cast parts (96.53%) and 8,995 short-filled parts (3.47%).

3. Causes of under-casting. (1) The alloy melt was severely oxidised and its fluidity had dropped. Because of the overflow and gating system, and because of rejected parts, more than 50% returns are typically generated. Magnesium requires very strict protection during melting; the plant had no compliant melting furnace and refining conditions were not met, so returns went straight into the holding furnace and the operator only skimmed the dross. Compared with refined ingots, returns are heavily oxidised and have poorer fluidity — the key factor behind under-casting.

(2) Mould temperature and pouring temperature were too low. Holding furnace temperature was set to 670 °C; measured with a portable pyrometer, the melt was 662 °C before charging, 661 °C thirty seconds after charging and 647 °C after three minutes — a significant drop. Heating the mould with hot oil from a mould-temperature controller is the ideal method because the mould then holds a stable temperature, which improves quality, extends mould life and stabilises the production rhythm. For magnesium die casting the mould is normally kept at 180–280 °C. With the controller set to 240 °C, an infrared pyrometer showed 170–180 °C on the fixed half and 190–200 °C on the moving half — the fixed half was too cold. Low mould and melt temperatures readily produce cold shuts and under-casting.

4. Countermeasures and verification. (1) Virgin ingot versus returns: because magnesium is chemically active and easily oxidised, and because refining and special degassing were not available, melt made from returns flows less freely. Running separate batches with all other parameters unchanged gave a 23.4% under-cast rate with returns and 19.0% with virgin ingot — virgin ingot clearly improves fluidity and reduces under-casting.

(2) Raising melt temperature: holding the melt at 690 °C with other parameters unchanged reduced the rejection rate to 16.2% with returns and 13.4% with virgin ingot.