Introduction: Pure copper and copper alloys for casting are an important category of nonferrous metals, which are widely used in practical production. Commonly used copper alloys can be divided into two major categories according to their composition, namely, cast brass and cast bronze. Whether it is sand casting or investment casting, smelting is a crucial link in casting production. The smelting of various copper alloys has similarities and differences. In many years of production practice, we have summarized the smelting process experience of pure copper and copper alloys (mainly bronze and brass) for your reference.

The smelting equipment used: medium frequency induction furnace, thermocouple, ladle, graphite crucible, etc.

1. Smelting of pure copper

(1) Preheat the crucible to a dark red color, and add a layer of dry charcoal or covering agent (63% borax+37% broken glass) with a thickness of about 30-50cm to the bottom of the crucible. Then add the leftover materials, waste blocks, and rods in turn, and finally add electrolytic copper.

(2) The added alloy elements can be preheated on the furnace platform. It is strictly prohibited to add cold materials into liquid metal. During the entire melting process, the furnace charge should be frequently moved to prevent bridging.

(3) Heating causes the alloy to completely melt. After the alloy is fully melted, when the temperature reaches 1200-1220 ℃, 0.3% - 0.4% of the weight of the alloy liquid is added to deoxidize with phosphorous copper. The following reactions occur between phosphorous and cuprous oxide: 5Cu2O+2P=P2O5+10Cu and Cu2O+P2O5=2CuPO3. The generated phosphorus pentoxide gas escapes from the alloy, and the copper phosphate can float on the liquid surface and be removed by skimming, thus achieving the purpose of deoxidation. Continuous stirring is required during the deoxidation process.

(4) Finally, the slag is removed from the furnace, and the pouring temperature of the alloy liquid is generally 1100-1200 ℃.

2. Smelting of brass

The copper based alloy with zinc as the main alloying element is brass, which is divided into two categories: ordinary brass and special brass. Common brass is a binary alloy composed of copper and zinc, mainly used for pressure processing. Adding other alloy elements (such as silicon, aluminum, manganese, lead, iron, nickel, etc.) to ordinary brass becomes special brass. Cast brass is mostly special brass.

(1) The chemical composition of copper alloys is required for alloy batching and metal charging. Due to the large range of changes in the main components, appropriate batching components should be selected based on their performance requirements during the batching calculation process. The chemical composition of the alloy shall comply with GB1176-87, and the composition of several commonly used brass smelting ingredients shall be determined according to Table 1.

The furnace charge shall be dry and clean, and any dirt or rust shall be cleaned by sand blowing.

(2) The proportion of furnace charge shall be in accordance with the general casting practice, and the proportion of new charge composition to the total weight of furnace charge shall be ≥ 30%, and the proportion of returned charge shall be ≤ 70%. However, in actual production, we consider that there are many copper alloy returns. When the mass percentage of the return charge is ≥ 90%, the melting quality is still very good. Chemical spectrum analysis shows that the composition of the casting is qualified. When there is a large amount of recycled material, it is necessary to consider whether the impurities in the alloy exceed the standard.

(3) Preparation before melting ① Preparation of metal charge: The return charge is a waste casting, riser, and remelting ingot of the same brand, and requires a clear chemical composition. Before entering the furnace, blow sand to remove surface dirt, and charge the furnace after preheating (the first batch of cold furnace melting can be preheated along with the furnace); After electrolytic copper is removed by sand blowing and preheated at 500-550 ℃ to remove moisture, Before charging (The first batch of cold furnace melting can be preheated along with the furnace; pure metal elements can be preheated at the furnace edge before entering the furnace. The maximum lump size of the metal furnace charge should not exceed 1/3 of the crucible diameter, and the length should not exceed 4/5 of the crucible depth. ② Preparation of crucibles and melting equipment and tools: The crucible should be free of cracks and other damage that affects safety before use, and the new crucible must undergo low-temperature and slow heating treatment to prevent cracks. The inner surface of the old crucible should be cleaned of slag; When using a new graphite crucible or changing the type of smelting alloy, the crucible should melt the alloy of the same brand series and wash the furnace before smelting; The mixing rod made of refractory material and graphite must be thoroughly cleaned of residual paint and rust, and coated with a layer of refractory material or brushed with paint before drying for use; The ingot mold must be thoroughly cleaned before use, and preheated to 100-150 ℃ for use after coating.

(4) Preparation of covering agent and flux ① The charcoal should be placed in a sealed oven and baked at not less than 800 ℃ for 4 hours. When it is ready for use, it should be prevented from absorbing moisture. ② The covering agent consists of 63% borax and 37% broken glass, and dry charcoal can also be used as the covering agent. Covering agents require drying and removal of impurities.

(5) Alloy melting process

① Preheat the crucible to a dark red color and add 20-40cm thick charcoal to its bottom.

② Add electrolytic copper, rapidly heat up and melt, then add the intermediate alloy in the order of high melting point first and then low melting point (if equipped), finally add back the furnace charge, and add charcoal at the same time to ensure that the alloy liquid level is not exposed to the air.

③ Generally, deoxidation is also required for smelting brass. After all copper is melted, phosphorus copper (calculated as 0.04% - 0.06% of the weight of copper liquid) is added to deoxidize when the temperature reaches 1150-1200 ℃. By comparing the practice of deoxidation and non deoxidation, the surface quality of the castings after deoxidation is better than that without deoxidation.

④ Add alloy elements according to the composition requirements of each alloy brand: add aluminum copper intermediate alloy at 1100 - 1120 ℃, and add pure zinc and pure aluminum in batches at 1100 - 1150 ℃ after power failure, and stir. When melting silicon brass, silicon should be added first, followed by zinc; When melting lead and yellow copper, zinc should be added first and then lead. The addition temperature of zinc should be controlled. If the temperature decreases after adding zinc, intermediate power can be supplied. When the temperature of the alloy liquid is higher than 1200 ℃, zinc addition is not allowed.

⑤ Remove slag from the furnace, adjust the alloy liquid to the temperature required by the casting process card, and then quickly discharge the furnace for pouring. The pouring temperature of an alloy is one of the factors that affect the performance of the casting. The general discharge temperature is ZCuZn38: 1100-1130 ℃; ZCuZn40Pb2：1080-1100℃； ZCuZn31Al2：1120-1140℃； ZCuZn16Si4：1100-1140℃。

⑥ When smelting two different grades of alloys and their chemical composition has an impact, furnace cleaning should be carried out in the middle. For example, use crucibles and tools that have melted aluminum bronze to melt tin bronze, and crucibles and tools must contain aluminum. Although aluminum is a qualified component in aluminum bronze, it is the most harmful element in tin bronze.

After deoxidation of common copper alloys, qualified castings can be obtained. However, for aluminum bronze, aluminum brass, silicon bronze, etc., it is easy to oxidize and generate high melting point oxides Al2O3 and SiO2, which can form slag inclusions in the casting, and can be removed only after refining. Common refining agents include: sodium chloride with a mass ratio of 60%+cryolite with a mass ratio of 40% or cryolite with a mass ratio of 20%+fluorite with a mass ratio of 60%+sodium fluoride with a mass ratio of 60%.

3. Smelting of bronze

Cast bronze can be divided into tin bronze and non tin bronze according to composition. Tin bronze is a copper based alloy with tin as the main alloying element, which has good wear resistance, corrosion resistance, good strength and plasticity. "Non tin bronze includes aluminum bronze, lead bronze, and silicon bronze, which contain different main elements. For example, aluminum bronze is a copper based alloy with aluminum as the main alloy element.".

(1) Requirements for alloy ingredients and metal furnace charges: The composition of several commonly used bronze alloy smelting ingredients is shown in Table 2.

Requirements: It is allowed to add 0.7% to 0.9% Ni and 0.3% to 0.4% Mn into ZCuAl10Fe3 alloy to improve the mechanical properties of the alloy.

(2) The proportion of new material to the total weight of furnace material shall be ≥ 30%, and the proportion of returned material shall be ≤ 70%.

(3) Preparation before melting The preparation before melting bronze is the same as that for melting brass. The charcoal should be placed in a sealed oven and baked at no less than 800 ℃ for 4 hours. When waiting for use, moisture absorption should be prevented. Rice straw ash should be ground to a powder, moisture removed, thoroughly dried, and moisture protected when ready for use. Covering agents require drying and removal of impurities.

(4) There are many types of cast bronze in the alloy melting process. Here, only a few typical alloys are discussed, and other copper alloys of the same type can be used for reference.

Melting of ZCuSn10Pb 1 and ZCuPb 10Sn10: ① Preheat the crucible to a dark red color, and add 20 cm to 40 cm thick charcoal to its bottom. ② Add electrolytic copper, rapidly heat up and melt it before adding back to the furnace, and add charcoal to ensure that the alloy liquid level is not exposed to the air. ③ After melting the return charge, add phosphorous copper (generally accounting for 0.5% of the charge weight, and all phosphorous copper used for melting phosphorous tin bronze can be added) Add zinc, tin, and lead in turn (according to ingredient composition). After the previous charge is completely melted, add the next one, and continuously stir the alloy liquid Adjust the temperature of the alloy liquid between 1100-1150 ℃. ⑥ After tapping for slag, add phosphorous copper (typically 0.1% of the weight of the furnace charge) for deoxidation, stir evenly, and sprinkle a layer of straw ash on the surface of the alloy liquid. Adjust the alloy liquid to the temperature required by the casting process card (typically 1130-1180 ℃), and then quickly tap the furnace for pouring.

Melting of ZCuAl10Fe3 and ZCuAl10Fe3Mn2: ① Do not use a crucible that has melted other alloy brands to melt these two alloys. ② Preheat the crucible to a dark red color, and add the prepared flux. (Flux composition: cryolite 20% (mass percentage), sodium fluoride 60%, calcium fluoride 20%) Add the low-carbon thin steel sheet preheated to about 200 ℃ and the returning material at the same time, melt it, stir the alloy, and raise the temperature to 1150-1180 ℃. ④ Add 0.3% of the alloy weight of phosphorous copper for deoxidation, and add additional flux. ⑤ Add pure aluminum and pure manganese (according to the ingredient) preheated to 200 ℃ in batches. Press each batch with a stirring bar to melt rapidly, and keep stirring to make their ingredients uniform. Finally, adjust the temperature of the alloy liquid between 1120-1220 ℃. ⑥ Cover the slag with straw ash, adjust the alloy temperature according to the casting process card (generally 1160-1200 ℃), and then quickly adjust the pouring out of the furnace.

4. Safety precautions

Safe production is a basic requirement in the casting industry. The smelting of pure copper and copper alloys must meet the following requirements:

(1) The operator should wear protective equipment, and the work site should be kept clean and free of water and debris.

(2) Before opening the furnace, check whether the equipment used is in good condition, and eliminate any unsafe factors in a timely manner.

(3) Before preheating, it is necessary to carefully inspect and confirm that there are no explosive or dangerous substances in the furnace charge.

(4) Melting and pouring tools, such as stirring rods, iron spoons, slag removal tools, etc., shall not come into contact with the alloy liquid without preheating.

(5) During pouring, the remaining alloy liquid shall be poured into the preheated ingot mold, and it is not allowed to be directly poured on the ground or poured back into the furnace.

5. Several issues needing attention in smelting

(1) The melting time should be controlled. The time from the beginning of charging to the end of melting (alloy discharge) is called the melting time. The length of the melting time not only affects the productivity, but also significantly affects the quality of the castings poured. As the melting time increases, the element burning rate of the alloy increases, and the opportunity for air absorption increases. Therefore, the melting process should be completed in the shortest possible time. "If allowed, try to increase the preheating temperature of the furnace charge. The operation should be compact and the action should be rapid.".

(2) The stirring rod used for melting should be some elements in carbon rod copper alloy, such as iron, lead, etc., which exist in the form of a mechanical mixture during melting. Other elements have a tendency to produce specific gravity segregation and delamination due to their different densities. Practice has proven that these elements can easily cause unqualified chemical composition and mechanical properties during the melting and pouring process. To overcome this phenomenon, it is necessary to use stirring, which is an indispensable part of melt casting. However, during temperature measurement and cooling, stirring is generally not required. The material composition of the mixture used is generally suitable for graphite. This is because if other stirring materials such as iron rods are used, the iron rods will melt during the stirring process, affecting the chemical composition of the alloy. At the same time, if the preheating temperature of the iron bar in the furnace is relatively high or the stirring time is long, the oxides on the iron bar will enter the alloy liquid and become impurities; If the preheating temperature of the iron bar is low, the alloy will adhere to the iron bar during stirring, which can be observed in production.

(3) The use of covering agent during smelting. For smelting copper alloys, the amount of covering agent is generally 0.8% - 1.2% of the weight of the furnace charge when using glass and borax, as the thickness of the covering layer should be kept at 10-15cm; When using charcoal, the dosage should be about 0.5% - 0.7% of the weight of the furnace charge, and the thickness of the covering layer should be kept at 25-35 cm. The removal time of the covering agent is generally carried out before pouring. If it is carried out too early, it will increase the oxidation and air absorption of the copper alloy. If charcoal is used as a covering agent and the slag blocking effect is good, it can also be used without removing the covering agent, allowing it to also play a slag blocking role during the pouring process, with a more ideal effect.

Source: Tongxinbao