The Influence of Machining on the Failure of Die Casting Dies

As we all know, die-casting molds have a long manufacturing cycle and complex machining, involving processes such as turning, grinding, milling, drilling, planing, and electrical discharge machining (wire cutting, EDM).

Its processing quality, especially the processing quality of the surface, will significantly affect the fracture resistance, fatigue strength, thermal fatigue resistance, wear resistance, corrosion resistance, etc. of the mold in the mold manufacturing process and subsequent use process.

A slight error in the processing process may cause early failure of the mold. For example, cracking and hot cracking are common failure phenomena of aluminum alloy die-casting molds, which are caused by thermal fatigue, stress, low strength and surface roughness of the mold surface.

Ⅰ. The influence of die-casting mold cutting

During the machining process of the module, due to the destruction of the original balance of the workpiece matrix, stress will be generated. These internal stresses reduce the total strength of the module surface, resulting in insufficient thermal fatigue strength, resulting in cracks at the transition of corners and small radius arcs. Therefore, sharp corners should be avoided as much as possible. Sharp corners and machining tool marks can significantly cause stress concentration. Unless there are special requirements, sharp corners should be avoided, and the arc curvature radius of the corner should be fully enlarged, in order to avoid or reduce stress concentration.

For example, when the arc radius R is increased from 1mm to 5mm, the maximum internal stress is reduced by about 40%, which greatly improves the strength and toughness of the mold. When the module arc radius is increased from 2mm to 20mm, its impact toughness can be increased by 4 times.

In the same way, the surface roughness of the mold has a great influence on the life of the mold. If there are obvious knife marks, cracks, cuts, etc., these machining defects will cause stress concentration and become the root cause of cracks. Therefore, to ensure the surface roughness requirements of the die casting mold, the surface of the cavity should be ground and polished to remove defects such as knife marks.

If the drilling process of the cooling water hole is two-way drilling, misalignment occurs, which will cause early cracking at the place, resulting in water leakage.

The cutting allowance of the mold base is insufficient. The forged and annealed mold base, or the mold blank and core, are heated and quenched in an air furnace after processing. Generally, there is a certain thickness of decarburization layer, and the decarburization must be removed during cutting. All layers are removed. Otherwise, the residual decarburized layer will reduce the thermal fatigue resistance of the mold surface during the use of the mold, causing cracks in this part.

Ⅱ. Influence of die casting die grinding

After the heat treatment of the die-casting mold, the following problems may occur during grinding (including fitter and grinding wheel grinding):

1. Cracking or cracking occurs, which is due to the excessively hard grinding wheel, large feed amount, fast grinding speed, excessive grinding stress, insufficient cooling or improper coolant selection. Cracking occurs when the tensile stress generated by the frictional heat caused by the hard grinding wheel is greater than the fracture toughness of the material;

2. Grinding burn, surface softening. The softened surface is prone to cracking, erosion and pitting due to its low strength and insufficient thermal fatigue strength when in contact with molten die-cast metal.

3. Grinding stress. Grinding of the die-casting mold surface has grinding stress, which reduces the strength and thermal fatigue resistance of the mold surface, which will lead to cracks at the mold corners and small R arc transitions. A tempering treatment is carried out at a temperature of 20 to 30 ° C to achieve the purpose of stress relief.