Welding characteristics of copper and its alloys:
(1) Difficult to fuse and easy to deform during welding.
The thermal conductivity of copper and its alloys is relatively high, and its thermal conductivity is 7 times higher than that of iron at 20 °C; 11 times larger than that of iron at 1000 °C. The heat conduction is too fast, and it is easy to form incomplete penetration, incomplete fusion, or even unable to weld. Preheating is required even if a heat source with concentrated heat (argon arc welding, plasma welding, etc.) is used. Copper has a large coefficient of linear expansion and a large shrinkage rate (the shrinkage rate is more than twice that of iron), and the thermal expansion and cold contraction are obvious. 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, a large internal stress will be generated.
(2) High crack sensitivity
Oxygen is a difficult impurity in copper to remove. It not only exists in copper in the process of smelting and rolling, but also the trace oxygen in the welding pool will form Cu2O with copper. If 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 hot brittleness.
In the process that the strength and plasticity of the material are greatly reduced by the high temperature of welding; under the action of the large internal stress of the high-speed cooling, solidification and shrinkage of the weld; in the restrained state of the deformation of the weldment, hot cracks are very likely to occur. In addition, copper and its alloys do not undergo phase transformation during the heating process, and the grain growth tendency of the weld and heat-affected zone is serious, which also aggravates the formation of hot cracks.
(3) High stomata sensitivity
The porosity tendency of copper and its alloys during fusion welding is much more severe than that of mild steel. This is because: ①When the weld is crystallized, the supersaturation degree of hydrogen dissolved in the metal is several times larger than that of steel; ②Cu2O and H2 or CO in the molten pool generate water vapor or CO2, bubbles; ③The thermal conductivity of copper It is more than 7 times that of steel, and at such a high cooling rate, it is difficult for H2, CO, and water vapor bubbles to escape completely, and pores are formed.
(4) The strength and plasticity of welded joints are reduced
Oxidation, evaporation and burning of alloy elements of copper and its alloys during fusion welding; infiltration of impurities; weakening of intergranular due to the presence of low-melting eutectic; Coarse grains in the affected zone reduce the strength, plasticity, corrosion resistance and electrical conductivity of the welded joint.
