Non-ferrous Metals | Hidden Keys to Welding of Copper and Copper Alloys

Influence of High Thermal Conductivity
Due to the high thermal conductivity, linear expansion coefficient, and shrinkage rate of copper and copper alloys, when welding them with parameters similar to those used for low-carbon steel of the same thickness, the base metal is difficult to melt, and the filler metal cannot fuse well with the base metal, resulting in incomplete penetration. Post-weld deformation is also severe, with poor appearance. Therefore, even if a high-power heat source is used for welding, pre-welding preheating or synchronous heating during welding is required. In addition, the thicker the base metal, the more severe the heat dissipation, and the harder it is to reach the melting temperature.

High Hot Cracking Tendency of Welded Joints
During welding, copper can form various low-melting-point eutectics with its impurities. Moreover, copper and copper alloys have no allotropic transformation during heating, and a large number of columnar grains form in copper welds. At the same time, the large linear expansion coefficient and shrinkage rate of copper and copper alloys increase the stress in welded joints, further enhancing the hot cracking tendency. Therefore, during fusion welding, the following measures are often taken: ① Strictly limit the impurity content in copper, especially oxygen; ② Add alloying elements such as silicon, manganese, and phosphorus through welding wires to enhance the deoxidation capacity of the weld; ③ Select welding wires that can form a dual-phase structure to refine the weld grains.

Porosity
The tendency of porosity formation during fusion welding is much more severe than in low-carbon steel. The formed porosity is almost distributed in all parts of the weld, mainly including diffusible porosity caused by dissolved hydrogen and reactive porosity caused by redox reactions. Therefore, to reduce or eliminate porosity in copper welds, the main measures are to reduce the sources of hydrogen and oxygen, and use preheating to extend the existence time of the molten pool to facilitate gas escape.

Changes in Joint Performance
During fusion welding, due to grain growth, incorporation of impurities and alloying elements, and oxidation and evaporation of useful alloying elements, the joint undergoes changes such as deteriorated plasticity, reduced electrical conductivity, decreased corrosion resistance, and grain coarsening. To improve joint performance, in addition to minimizing thermal effects and performing post-weld stress relief heat treatment, the main measures are to control impurity content, conduct modification treatment on the weld through alloys, and select materials according to the different requirements of different copper alloy joints.

Electrodes
Electrodes for shielded metal arc welding are divided into three categories: pure copper, bronze, and cupronickel (see Chapter 2). Due to the easy evaporation of zinc in brass, shielded metal arc welding is rarely used for brass. Electrodes are generally selected based on the composition of the base metal, using a core wire with a corresponding composition. For brass, bronze-core electrodes such as ECuSi and ECuSnB are usually selected. Pure copper electrodes (model ECu) have a low-hydrogen type coating and are used for welding deoxidized copper or oxygen-free copper structural parts. Electrodes must be strictly dried to remove moisture before use. Copper and copper alloy electrodes comply with GB/T3670—1995 "Copper and Copper Alloy Electrodes"; for specific selection.

Welding Wires and Fluxes for Submerged Arc Welding
Submerged arc welding is characterized by high arc thermal efficiency and good protection of the molten pool. The welding process for large and medium-thickness workpieces is basically the same as that for steel, but with unique measures in specific operations. High-manganese and high-silicon flux HJ431 can be used, but alloy element transition to the weld may occur; for workpieces with high joint performance requirements, HJ260 and HJ150 are preferred. Pure copper wires or bronze wires are used for welding pure copper and brass. Copper and copper alloy welding wires comply with GB/T 9460—2008 "Copper and Copper Alloy Welding Wires"; for specific selection.

Welding Wires and Shielding Gases for Inert Gas Shielded Welding
For welding thin and medium copper plates, gas shielded welding is gradually replacing gas welding and shielded metal arc welding. Non-consumable electrodes are generally thorium-tungsten electrodes. Inert gas shielded welding mainly adjusts the composition and performance of the weld through the welding wire to meet the requirements of the workpiece. Therefore, oxygen-free copper wires such as SCu1898 are generally selected for welding pure copper. For welding ordinary brass, tin bronze wires with oxygen-free copper and deoxidizers such as SCu5210 are used. For high-strength brass, silicon bronze wires or aluminum bronze wires with bronze and deoxidizers such as SCu6560 and SCu6100A are used. For cupronickel welding, cupronickel wires such as SCu7158 can be selected. For bronze welding, corresponding bronze wires are used. The principle for selecting wires is the same for both consumable and non-consumable inert gas shielded welding. For butt joints with straight bevels and thickness less than 3mm, no filler wire is needed; for thickness greater than 3mm, filler wire is usually required; and for thickness greater than 12mm, consumable inert gas shielded welding is preferred. Argon is usually used as the shielding gas; in special cases, such as welding pure copper or high-thermal-conductivity copper alloys where preheating is not allowed and a larger penetration depth is required, a mixture of 70% argon and 30% helium (by volume) can be used. The main advantage of the mixed gas is that it improves the wettability of the weld metal and enhances welding quality. Copper and copper alloy welding wires comply with GB/T9460-2008 "Copper and Copper Alloy Welding Wires"; for specific selection.