As we all know, the melting point of copper is 1084℃, but in actual operation, a higher furnace temperature is often required. For foundry engineers and plant managers, in addition to how to melt copper, it is also necessary to consider how to cope with the high sensitivity of copper to oxidation and hydrogen absorption, as well as energy saving during the melting process. This article will go beyond the basics and explain the relevant content of melting copper in depth for you.
What is The Process Of Melting Copper?
First of all, let me explain to you the specific steps of melting copper material. The following is the specific process.
1. Cleaning & Preheating
Firstly, before melting copper material, you need to realize that you cannot directly melt dirty and old copper. The first step you need to do is to remove oil, paint, or plastic skin from the surface of the copper material. And you also need to preheat the crucible to make sure there is no moisture in it before adding copper.
2. Heating & Fluxing
Heat the furnace to above 1085°C. When you find that the copper material starts to turn red and soften, you can sprinkle in flux, and the most commonly used one is borax. During this process, you will hear a noticeable change in sound, from crisp to dull, and you will also hear the hissing sound of the flux melting. As for the reason for doing this step, copper absorbs oxygen very easily, and severely oxidized copper liquid will become brittle after being poured out. So, you must use borax, which can form a glass-like film on the copper surface, isolating the air and preventing the oxidation of the copper liquid.
3. Reaching Pouring Temp
In this step, all you need to do is heat the copper liquid to 1150°C – 1200°C, which can provide sufficient fluidity to prevent premature solidification of the copper when it is poured out of the container.
4. Skimming the Slag
When the entire copper material is completely melted, there will be a layer of black impurities floating on the surface of the molten copper, which is slag. What you have to do at this time is to use a preheated graphite rod or slag removal spoon to gently skim off the scum on the surface and let the bright copper liquid below.
5. Pouring
Finally, you just have to decisively pour the molten copper into the preheated mold. It should be noted that your operations must be consistent to avoid interruption of flow, otherwise cold shut defects are likely to occur inside the casting parts.
Various Ways to Melt Copper
Then, after talking about specific steps of melting copper, let’s talk about the ways to melt copper. I believe this is also an issue that you are more concerned about. There are mainly the following types:
1. Resistance Heating
Also called resistance furnace. If you have studied electricity, you must all know that heat is generated when electric current passes through high-resistance materials. The resistance furnace uses this physical principle to radiate the generated heat to indirectly melt the copper, and indirectly melt the copper material.
The advantage of this method is that it can control the temperature extremely accurately, and the temperature fluctuation can be controlled within ±5°C.
2. Induction Heating
This process is also one of the current mainstream production methods. You need to place copper material in a coil with high-frequency alternating current. At this time, the magnetic field induces eddy currents inside the copper, utilizing the material’s own resistance to generate intense heat instantly.
The greatest advantage of induction heating is that the magnetic field automatically stirs the copper liquid, ensuring absolutely uniform distribution of components in the copper alloy.
3. Laser Heating
Laser heating uses a high-power laser beam as a light source. Through an optical system, it can be condensed into a small spot, then shines on the surface of the copper material to be heated. The most significant benefit of this method is that only the desired spot will melt. The heat-affected zone is extremely small.
4. Electron Beam Melting
The principle of EBM is to use an electron gun to emit high-energy electron beams to bombard copper materials in a high-vacuum environment, converting kinetic energy into thermal energy. Because it is carried out in a vacuum environment, this process can remove volatile impurities from copper and produce ultra-high purity copper.
5. Arc Melting
Arc melting uses the huge heat released by the arc generated between the electrode and the copper material to melt the copper metal. This process can effectively prevent material oxidation and remove impurities, and the ingot structure produced in this way is very dense.
6. Plasma Arc Melting
This method uses a plasma gun to ionize the gas into a high-temperature plasma jet (can reach more than 10,000 °C) to melt the copper. In addition, it is performed in a specific atmosphere (such as argon or helium) and can completely isolate the air.
7. Oxy-fuel Torch
The principle is to mix oxygen and gas (acetylene, propane or hydrogen) to generate high temperature flame. And you do not need any complex electrical equipment, you can pick up a gun to work.
Key Factors in the Copper Melting Process
Next, let’s focus on discussing what factors are worthy of your attention during the process of melting copper materials?
1. Atmosphere & Deoxidation
I believe that all of you have encountered the phenomenon that honeycomb pores or surface oxide scale occur after the castings are cooled. In fact, this is because cuprous oxide (Cu₂O) reacts with hydrogen to form water vapor, which in turn causes pores to occur.
Engineer Suggestion:
- You can cover the surface of the melt with calcined charcoal or graphite to create a reducing atmosphere.
- Add phosphor copper before pouring.
It should be noted that phosphorus will greatly reduce the conductivity of copper. If your product is a highly conductive electrical component, do not use phosphorus. As an alternative, you can use lithium or boron-based deoxidizers.
2. Melting Temperature & Superheat
Many operators will overheat in pursuit of copper fluidity. In fact, this can lead to a problem of severe aspiration of copper fluid. Although the melting point of copper is 1085°C, the solubility of liquid copper in hydrogen increases exponentially with increasing temperature. Consequently, even with the use of flux, porosity rates will significantly increase.
Engineer Suggestion:
You need to strictly control the temperature of the copper liquid. It is generally recommended that pouring temperatures are only 50°C – 100°C above the melting point. Once the target temperature is reached, the molten copper should be poured immediately to avoid prolonged heating at high temperatures.
3. Crucible Compatibility
In real work, crucibles explode or copper liquid will be contaminated with impurities. Clay graphite crucibles are cost-effective and have good thermal shock resistance. The silicon carbide crucible conducts heat very quickly and has stronger corrosion resistance, making it suitable for high-frequency induction furnaces.
Engineer Suggestion:
When you use an induction furnace, the resistivity of the crucible must match the operating frequency of the furnace, otherwise it will cause low heating efficiency or local overheating of the crucible.
4. Refractory Selection
Copper material suppliers often experience the need for frequent shutdowns for maintenance due to the shortened lifespan of furnace lining, which will undoubtedly increase costs. In fact, the reason for this problem is more obvious: refractory materials. Copper and its oxides are very chemically corrosive to refractory materials. After a period of use, it is easy to cause problems with the furnace.
Engineer Suggestion:
- For induction furnaces, I recommend using a high purity dry ramming compound.
- The sintering process must be strictly implemented to prevent molten copper from penetrating through the furnace lining.
5. Equipment ROI
It’s about the rate of return on equipment purchases. Many people only look at the price of equipment when purchasing and ignore the operating cost. You should be aware that there will be material loss when melting copper. The oxidation loss of copper in a gas furnace is typically 2-4%, while for induction furnaces, it can be controlled at less than 1%.
Engineer Suggestion:
If your annual output is more than 100 tonnes, the initial investment in an induction furnace is indeed large, but the copper cost savings of 2% can often make up for the equipment price difference in 12-18 months.
6. Safety & Complian
Finally, there’s the security issue that everyone talks about, but it’s really important. You also know that water expands 1,600 times in volume when it turns into steam. Even the moisture contained in the waste material can cause an explosion at the moment it is injected into the copper liquid, so preheating the raw material is a mandatory safety measure. Furthermore, because zinc will volatilize at 907°C, you must also be equipped with an OSHA/ISO standard LEV system when smelting brass to ensure operator safety.
In Conclusion
Through reading the entire article, I believe you have a thorough grasp of copper. Of course, as mentioned above, obtaining high-quality copper liquid is only the beginning, and converting these copper liquids into high-precision, porosity-free parts still requires precise mold design and professional machining technology. It just so happens that our DSR company provides excellent casting services and CNC machining services, which can solve various difficult problems for you. We are not only proficient in the copper alloy casting process but also equipped with advanced CNC machining workshops. If your project requires high-quality copper parts, then please send us your drawings. Our engineers will provide you with a complete manufacturing solution quote.
FAQ
Q1: How hot to melt copper?
A: The melting point of pure copper is 1085 ° C. However, the actual pouring temperature is more strictly controlled in the range of 1150 ° C to 1200 ° C.
Q2: How long does it take to melt copper?
A: The melting time is usually related to the power density of the heating source, the furnace type technology and the charge amount. So, if you use a high-frequency induction furnace, it usually takes 45 to 90 minutes to melt several hundred kilograms of copper in a furnace.
Q3: What’s the easiest way to melt copper?
A: The easiest way should be induction melting. It mainly uses electromagnetic fields to directly generate heat in the copper material to achieve rapid melting.
Q4: What reacts badly with copper?
A: From a metallurgical perspective, molten copper is most averse to reacting with hydrogen, oxygen, and iron.
Q5: What does bleach do to copper?
A: Bleach is an extremely strong oxidizing agent for copper. It can cause the copper surface to react violently, and then oxidize and change colour in a short time.
Q6: What does salt and vinegar do to copper?
A: The mixture of salt and vinegar is a strong cleaner of copper surface oxidation layer, and the principle is to dissolve copper oxide in an acidic solution.
