1、 Raw material selection and processing

The main process of copper casting production is the selection and processing of raw materials, which directly affects the quality and performance of the final product.

Copper material selection: Choose the appropriate type of copper alloy based on the product's intended use, such as pure copper, brass (Cu Zn), bronze (Cu Sn), etc. Different alloy compositions can significantly affect the mechanical properties, corrosion resistance, and electrical and thermal conductivity of castings. For example, lead containing brass (C36000) has excellent cutting performance but poor conductivity, while phosphor bronze (C51000) has excellent elasticity and wear resistance.

Raw material purity: Ensure that the purity of copper materials meets the standard and the impurity content is controlled within the allowable range. Especially for castings with high conductivity requirements, the oxygen content (O) should be less than 0.02% to avoid the phenomenon of "hydrogen disease".

Pre treatment process: The surface of raw materials needs to be cleaned before use to remove pollutants such as oil and oxides. For the recycling of copper materials, strict sorting, impurity removal, and composition analysis must be carried out, and alloying adjustments may be necessary.

Ingredient calculation: Accurately calculate the proportion of various metal elements added, considering the burning loss rate during the melting process (Zn about 3-5%, Sn about 1.5-2%). Using intermediate alloys can improve the uniformity of the composition.

2、 Melting process control

Melting is the core process of copper casting production, and temperature control and atmosphere protection are particularly critical.

Melting temperature: The melting temperature of different types of copper alloys varies, generally controlled between 1100-1200 ℃. Brass (Cu Zn) is about 900-950 ℃, and tin bronze (Cu Sn) is about 1150-1200 ℃. Excessive temperature can lead to increased volatilization of zinc, while insufficient temperature can affect fluidity.

Smelting equipment: Choose induction furnace, crucible furnace or reflector furnace according to production scale. Medium frequency induction furnace has fast melting speed and uniform composition, suitable for high-quality requirements; Gas reflector furnaces are suitable for large-scale production but have a high tendency towards oxidation.

Smelting protection: Charcoal covering or nitrogen protection is used to reduce oxidation. When smelting brass, a small amount of phosphorus copper (0.03-0.06%) can be added for deoxidation. Avoid excessive agitation to prevent gas entrapment.

Composition testing: Sampling for spectral analysis during the later stage of melting to ensure that the composition meets the standard. Pay special attention to real-time monitoring of volatile elements (such as Zn) and compensate for their addition if necessary.

3、 Casting process details

The selection and implementation of casting processes directly affect the internal quality and surface accuracy of castings.

Mold preparation: Choose sand mold, metal mold, or investment casting according to the product shape and batch size. Before sand casting, it is necessary to check the strength (wet compressive strength 0.08-0.12MPa) and permeability of the molding sand; The metal type needs to be preheated to 150-300 ℃ and coated with graphite or zinc oxide.

Pouring system design: Reasonably design the sprue, runner, and riser system. The shrinkage rate of copper alloy is about 1-2%, and sufficient compensating risers (with a height generally 1.5 times the diameter of the hot spot) need to be set up. Thin walled components should be bottom injection type, while thick and large components should be top injection type.

Pouring control: The pouring temperature is generally 50-100 ℃ higher than the liquidus line, with brass at about 980-1050 ℃ and bronze at about 1100-1150 ℃. Adopting the principle of "low-temperature rapid pouring", the pouring time is controlled at 1/3-1/2 of the filling time of the mold cavity.

Solidification control: Copper alloys have a wide solidification range and are prone to shrinkage porosity. Thick walled areas can be cooled by placing cold iron for accelerated cooling, or using sequential solidification technology. Special attention should be paid to the phenomenon of anti segregation in tin bronze.

4、 Post processing and quality control

The processing steps after casting molding are crucial for the final performance.

Sand cleaning treatment: After vibration sand removal, shot blasting (steel shot diameter 0.2-0.8mm) or chemical cleaning (5-10% NaOH solution) is used to remove residual sand and oxide scale on the surface.

Heat treatment process: stress relief annealing (brass 400-500 ℃, insulation for 1-2 hours); homogenization annealing (bronze 700-750 ℃, 2-4 hours); for beryllium copper and other age hardening alloys, solution treatment (780-800 ℃ water quenching)+aging (300-350 ℃, 2-4 hours) is required.

Mechanical processing: Copper alloy tends to stick to cutting tools, and large rake angle cutting tools (15-20 °) should be used during cutting. High speed steel cutting tools have a linear speed of 60-120m/min, and hard alloy cutting tools can reach speeds of over 200m/min. Fully utilize cutting fluid cooling.

Surface treatment: Choose electroplating (nickel, chromium), chemical plating (silver), passivation (chromate) or coating according to the application. Conductive components often require tin or silver plating treatment.

Quality testing: including dimensional inspection (tolerance generally CT6-CT8 level), X-ray inspection (porosity ≤ 2% cross-section), penetrant testing (surface cracks), conductivity testing (pure copper ≥ 98% IACS), and mechanical property testing (such as H62 brass tensile strength ≥ 330MPa).

5、 Common Problems and Solutions

Pore defects: mainly due to high gas content in the melt or high moisture content in the molding sand. The solutions include strengthening the degassing of the melt (by introducing nitrogen or hexachloroethane), controlling the moisture content of the molding sand (4-6%), and improving the slag blocking ability of the casting system.

Shrinkage problem: Optimize the riser design, use insulation or heating risers, and control the pouring temperature not to be too high.

Segregation phenomenon: especially the reverse segregation of tin bronze, can be improved by accelerating the cooling rate and adding a small amount of nickel (0.5-1%) or phosphorus (0.02-0.05%).

Hot cracking tendency: Copper alloys have low high-temperature strength and are prone to hot cracking. Severe temperature changes should be avoided, and the mold should have good yielding properties. If necessary, a small amount of iron (0.3-0.6%) should be added to improve thermal strength.

The production of copper castings is a systematic project, and every link needs to be strictly controlled. With the advancement of technology, new processes such as vacuum melting and semi-solid forming are gradually being applied to the production of high-end copper castings, but the basic detail control in traditional processes is still fundamental to ensuring quality. Enterprises should establish a comprehensive quality control system based on product characteristics and continuously optimize process parameters in order to maintain an advantage in fierce market competition.