The most common defect in die castings is porosity.

Pore characteristics. Has a smooth surface with a circular or elliptical shape. The manifestation can be on the surface of the casting, or in subcutaneous pinholes, or inside the casting.

(1) Gas source

1) Gas evolution from alloy liquid - a related to raw materials, b related to melting process

2) Gas entrapment during die-casting process - a is related to die-casting process parameters, b is related to mold structure

3) The decomposition of release agent produces gas - a is related to the characteristics of the coating itself, b is related to the spraying process

(2) Analysis of gases generated during raw materials and smelting processes

The gas in aluminum liquid is mainly hydrogen, accounting for about 85% of the total gas.

The higher the melting temperature, the higher the solubility of hydrogen in liquid aluminum, but its solubility in solid aluminum is very low. Therefore, during the solidification process, hydrogen precipitates to form pores.

Source of Hydrogen:

1) Water vapor in the atmosphere and liquid metal absorb hydrogen from moist air.

2) The raw material itself contains hydrogen, the surface of the alloy ingot is damp, the recycled material is dirty, and there is oil pollution.

3) Tools and fluxes are damp.

(3) Gas analysis generated during the die-casting process

Due to the fact that the pressure chamber, pouring system, and mold cavity are all connected to the atmosphere, and the metal liquid is filled at high pressure and speed, if an orderly and smooth flow state cannot be achieved, the metal liquid will generate vortices and roll in the gas.

The development of die-casting process needs to consider the following issues:

1) Can the molten metal flow cleanly and smoothly in the pouring system without separation and eddy currents.

2) Are there any sharp corners or dead zones?

3) Is there any change in cross-sectional area of the pouring system?

4) Are the positions of the exhaust groove and overflow groove correct? Is it big enough? Will it be blocked? Can the gas be effectively and smoothly discharged?

The application of computer simulation of filling process is to analyze the above phenomena and make judgments to select reasonable process parameters.

(4) Analysis of Gas Generated by Coatings

Coating performance: High gas generation has a direct impact on the porosity of castings.

Spray coating process: Excessive use can result in high gas evaporation, excessive lubricant on the punch, or burning, all of which are sources of gas.

(5) Solution to porosity in die-casting parts

First, analyze the cause of the pores, and then take corresponding measures.

1) Dry and clean alloy material.

2) Control the melting temperature to avoid overheating and perform degassing treatment.

3) Reasonably select die-casting process parameters, especially injection speed. Adjust the starting point for high-speed switching.

4) Sequential filling is beneficial for gas discharge from the mold cavity, and the sprue and runner have sufficient length (>50mm) to facilitate smooth flow of the alloy liquid and provide opportunities for gas discharge. The thickness and direction of the gate can be changed, and overflow and exhaust grooves can be set at the location where air holes are formed. The total cross-sectional area of the overflow product cannot be less than 60% of the total cross-sectional area of the sprue, otherwise the slag discharge effect will be poor.

5) Choose high-performance coatings and control the spraying amount.