The Common Defects of Aluminum Die Castings

The Common Defects of Aluminum Die Casting Parts (Part One)

1. Blowholes
Features: First, the surfaces of the hole wall are relatively smooth with metallic luster. Second, blowholes are existing under the surfaces of the die casting parts individually or in groups. Third, the blowholes of lampblack are oil color.

Analysis of causes: First, the gas brought in will exist in the castings in the form of blowholes after the solidification of alloy melt when the liquid metal is casting. Second, there will be blowholes that are generated under the surface of the die casting part after the reaction of metal and the casting mold. Third, the slag inclusion of alloy melt or the gas attached to the oxide skin gets into alloy melt, which forms blowholes.

Prevention methods: First, avoid getting into air in the die casting process. Second, alloy melt should be filtered by the filter screen so that the slag inclusions, oxide skin and bubbles can be removed before entering the mold cavity. Third, change die casting materials or add some coating layers to avoid the reaction of alloy melt and the die casting mold. Fourth, clean up the parts allowing the repair welding and have the repair welding for the parts.

2. Pinholes
Features: First, they are evenly distributed across the whole fracture surfaces of the casting and they have the diameter less than 1mm. Second, the parts with the fast solidifying rate usually have small holes in a small quantity, while the parts with slow solidifying rate usually have big holes in a large quantity. Third, pinholes become circular holes in eutectic alloy and the pinholes become long-shaped holes during the solidifying interval. Fourth, they are black dots on the X-ray film, while they become discontinuous small milky pits on the fracture surfaces.

Analysis of causes: Hydrogen is the main gas which forms in the molten alloys, and it precipitates from the alloys during solidification, resulting in holes evenly distributing on fracture surfaces of die casts.

Prevention methods: First, thoroughly make alloy refine to degas when the alloy is in the liquid state. Second, accelerate the solidifying rate during solidification to prevent the fusible gases precipitating from alloy. Third, the die casting parts will solidify under the pressure, which will prevent the fusible gases precipitating from alloy. Fourth, the furnace burden, auxiliary materials and tools should be dried.

3. Porosity 
Features: First, the spongy loose structure will become, and the shrinkage cavity will happen when being in severe cases. Second, the surfaces of holes become coarse pits with large grains. Third, the fracture surfaces are gray or light yellow and they will become off-white, light yellow or black after heat treatment. Fourth, porosity usually happens on parts which solidify finally. Fifth, they are nebulous on the X-ray film, while they become dense little bright dots during the fluorescent inspection.

Analysis of causes: First, porosity forms because the alloy melt is not degassed completely. Second, there is not enough feeding for the last solidifying part. Third, the overheating, excessive moisture and poor discharge of the partial die casting mold can also cause porosity.

Prevention methods: First, keep the rational solidification sequence and feeding. Second, keep the furnace burden clean. Third, put the chill on the parts which have porosity. Fourth, clean up the parts allowing the repair welding and have the repair welding for the parts.