Welding characteristics of copper and its alloys (1) Difficult to fuse and easy to deform during welding Copper and its alloys have high thermal conductivity, which is 7 times greater than iron at 20°C and 11 times greater than iron at 1000°C. If the heat conduction is too fast, it is easy to form incomplete penetration, incomplete fusion, or even impossible welding during welding. Even if a heat source with concentrated heat (argon arc welding, plasma welding, etc.) is used, preheating is required. Copper has a large linear expansion coefficient and shrinkage rate (the shrinkage rate is more than twice that of iron) and obvious thermal expansion and contraction. In addition, the heat conduction is fast, the heat affected zone of the weld is very wide, and the deformation after welding is large. If the deformation is blocked, large internal stress will be generated. (2) High crack sensitivity Oxygen is an impurity in copper that is difficult to remove. It not only exists in copper during the smelting and rolling process, but also trace oxygen in the welding molten pool will form Cu2O with copper. The melting point of the eutectic formed by it and α copper is 1064°C. If copper contains Pb, the eutectic formed by it and copper is 326°C.The low-melting eutectic weakens the intergranular bonding force, making the material have obvious hot brittleness. In the process of high welding temperature that greatly reduces the strength and plasticity of the material; under the action of large internal stress caused by high-speed cooling, solidification and shrinkage of the weld; in the restrained state where the deformation of the weld is blocked, hot cracks are very likely to occur. In addition, copper and its alloys do not undergo phase change during heating, and the grain growth tendency of the weld and heat-affected zone is serious, which also aggravates the formation of hot cracks. (3) High porosity sensitivity The porosity tendency of copper and its alloys during fusion welding is much more serious than that of low-carbon steel. This is because: When the weld is crystallized, the supersaturation of hydrogen originally dissolved in the metal is several times greater than that of steel; Cu2O in the molten pool reacts with H2 or CO to generate water vapor or CO2, bubbles; The thermal conductivity of copper is more than 7 times that of steel. With such a high cooling rate, H2, CO, and water vapor bubbles are difficult to escape completely, and pores are formed.(4) The strength and plasticity of the welded joint are reduced. The oxidation, evaporation and burning of alloying elements in copper and its alloys during the fusion welding process; the penetration of impurities; the weakening of the intergranular space by the low-melting eutectic; defects such as pores, slag inclusions and incomplete penetration, as well as coarse grains in the weld and heat-affected zone, reduce the strength, plasticity, corrosion resistance and conductivity of the welded joint. 2. Main measures to prevent welding defects and obtain high-quality welded joints 1) An important measure to prevent pores is to reduce the source of H and O2. Before welding, the cleaning of welding materials and workpieces should be strengthened. 2) Control the impurity content, treat the weld deterioration, add deoxidizers to the welding wire, etc. 3) Appropriately increase the welding heat input and cooperate with preheating and post-weld heat treatment when necessary.
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