1、 Reasons for the formation of pores and shrinkage defects

The common porosity and shrinkage defects in copper castings during production mainly stem from the following aspects:

Gas sources: gases absorbed by the molten metal during the melting process (mainly hydrogen and oxygen), as well as gases produced by the decomposition of moisture and organic matter in the mold.

Solidification characteristics: Copper alloys undergo significant volume shrinkage during solidification, especially pure copper and certain high copper alloys, with a shrinkage rate of up to 4-5%, which can easily lead to the formation of shrinkage cavities and porosity.

Improper craftsmanship: Unreasonable design of the pouring system, improper control of pouring temperature, and poor exhaust of the mold can all exacerbate these defects.

2、 Specific measures to avoid porosity defects

1. Raw material control

Choose high-purity electrolytic copper and high-quality alloy elements, and avoid using raw materials that are damp or oxidized. The surface of copper ingots should be clean and free of oxide scale, and the recycled materials should undergo strict degassing treatment before use.

2. Smelting process optimization

(1) Smelting environment control: Use covering agents (such as charcoal, borax, etc.) to protect the molten pool and reduce contact between the molten metal and air. Enterprises with conditions can use vacuum or inert gas protection for melting.

(2) Degassing refining: Add phosphorus copper deoxidation (P content controlled at 0.02-0.05%), use nitrogen or argon gas to rotate degassing, and if necessary, add rare earth elements to refine grains and purify the melt.

(3) Temperature control: Strictly control the melting temperature. Generally, the pouring temperature of copper alloys is controlled between 1100-1200 ℃ to avoid excessive temperature that may increase gas solubility.

3. Improvement of casting process

(1) Mold material selection: Use low gas release molding materials, resin sand needs to be fully cured, and the moisture content of green sand should be controlled below 4-5%.

(2) Exhaust system design: Reasonably set exhaust holes and exhaust risers to ensure smooth discharge of gas from the mold cavity. Complex castings may consider using exhaust needles or exhaust fins.

(3) Coating application: Spraying refractory coating on the surface of the mold can not only reduce the reaction between the molten metal and the mold, but also help to release gases.

3、 Technical solution to prevent shrinkage defects

1. Design of pouring system

(1) Principle of sequential solidification: By designing a reasonable sprue and riser, the casting can be solidified sequentially from the location away from the sprue towards the sprue direction. Set up shrinkage feeders in thick and large areas, and the volume of the feeders should be sufficiently large.

(2) Cold iron application: Place cold iron on the back of thick parts of castings to accelerate local cooling speed and eliminate hot spots. The material of the cold iron can be cast iron or copper, and the surface should be smooth.

(3) Pouring method: Adopting a bottom pouring system to avoid splashing metal liquid and trapping gas, while also facilitating shrinkage filling.

2. Process parameter control

(1) Pouring temperature: Try to reduce the pouring temperature and minimize liquid shrinkage while ensuring filling. It is advisable to pour copper alloys at a temperature 50-100 ℃ higher than the liquidus temperature.

(2) Pouring speed: Control the appropriate pouring speed. If it is too fast, turbulent flow may be generated and gas may be trapped, while if it is too slow, it is not conducive to shrinkage.

(3) Pressure holding time: After pouring, maintain sufficient pressure holding time to ensure that the riser metal liquid can fully replenish the casting.

3. Alloy composition adjustment

(1) Adding trace elements: Adding appropriate amounts of zinc, tin, and other elements can improve the fluidity of copper alloys, while adding nickel, silicon, and other elements can reduce the tendency for shrinkage.

(2) Grain refinement: Adding grain refining agents such as titanium and boron can disperse and loosen the refined grains, improving their density.

4、 Comprehensive quality control measures

Process monitoring: Spectral analysis is used to monitor the composition of the melt, and thermocouples are used to monitor the melting and pouring temperatures in real time.

Non destructive testing: After the casting is completed, X-ray or ultrasonic testing is carried out to promptly detect internal defects.

Process validation: Conduct process tests on new products and verify the rationality of the process through dissecting castings or industrial CT scans.

Personnel training: Strengthen the skill training of operators to ensure that each process is executed according to specifications.

Avoiding porosity and shrinkage defects in copper castings requires comprehensive management from raw materials, melting processes, mold design to process control. Enterprises should develop suitable process plans based on specific product characteristics and the above measures. At the same time, establishing a sound quality traceability system and continuously optimizing production processes are necessary to consistently produce high-quality copper castings. In actual production, multiple measures are often required to be used in combination, and a better combination of process parameters is determined through experimentation.