The welding technology of copper alloy
Copper and its alloys have unique physical and chemical properties, which are different from iron and aluminum. Because the welding members mainly use pure copper and brass (copper zinc alloy). This paper focuses on the analysis of this kind of copper fusion welding characteristics.
Welding characteristics of copper and its alloys
(1) It is difficult to fuse and easy to deform when welding
The thermal conductivity of copper and its alloy is higher, which is 7 times larger than that of iron at 20℃. It's 11 times bigger than iron at 1000℃. The heat conduction is too fast, the welding is easy to form not weld penetration, not fusion, and even can not be welded. Even if the heat concentration of the heat source (argon arc welding, plasma welding, etc.) also to preheat. Copper has a large linear expansion coefficient and shrinkage rate (the shrinkage rate is more than twice that of iron). In addition, the heat conduction is fast, the heat affected zone of the weld is very wide, and the deformation is large after welding. If the deformation is blocked, it will produce greater internal stress.
(2) High crack sensitivity
Oxygen is an impurity in copper that is difficult to remove. It not only exists in copper in the smelting and rolling process, but also the trace oxygen in the welding pool will form Cu2O with copper, and the co-melting point of it and α-copper is 1064℃. If the copper contains Pb, the eutectic formed with copper is 326℃. The low melting eutectic weakens the intergranular bonding force and makes the material have obvious thermal brittleness.
In the process of welding material strength and plasticity greatly reduced by high temperature; Under the action of large internal stress of high-speed cooling, solidification and shrinkage of weld seam; Hot cracks are easy to occur in the constrained state where the deformation of the weldment is blocked. In addition, there is no phase change in the heating process of copper and its alloy, and the grains in the weld and heat-affected zone tend to grow up seriously, which also aggravates the formation of thermal cracks.
(3) High stomatal sensitivity
The tendency of porosity of copper and its alloys in fusion welding is much more serious than that of mild steel. This is because: (1) when the weld crystallizes, the degree of supersaturation of hydrogen in the metal is several times larger than that of steel;② Cu2O and H2 or CO in the molten pool generate water vapor or CO2, bubbles;(3) The thermal conductivity of copper is more than 7 times that of steel. With such a high cooling rate, it is difficult for H2, CO, and steam bubbles to all escape and form pores.
(4) the strength and plasticity of the welded joint are reduced
Oxidation, evaporation and burning loss of alloying elements of copper and its alloys in the welding process; The infiltration of impurities; There is intergranular weakening in the low melting eutectic. Defects such as porosity, slag inclusion and non-penetration as well as coarse grains in the weld and heat-affected zone decrease the strength, plasticity, corrosion resistance and electrical conductivity of the welded joint.
Second, the main measures to prevent welding defects and obtain high quality welding joints
1) An important measure to prevent pores is to reduce the source of H, and O2. Cleaning of welding materials and workpieces should be strengthened before welding.
2) Control the content of impurities, treat the weld deterioration, add deoxidizer in the welding wire, etc.
3) Appropriately increase the welding heat input and with necessary preheating and post-welding heat treatment, etc.
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