Formation Mechanism And Suppression Of Hot Cracking In GTAW Of Copper

The hot cracking is one of the major defects in the fusion welding of copper structures components with large dimension, which always restricts the usage of copper. In general, the formation of hot cracking is considered as the combined results of the metallurgical factors and the mechanical factors during the solidification of the welds. From the metallurgical considerations, the liquid film when the welds metal crystallize will form as the origination of hot cracking, which is the internal conditions of hot cracking; from the mechanical considerations, the tensile strain of the welds which is in the brittle temperature range (BTR) is the external conditions of the hot cracking. In this paper, based on the characteristics of GTA Welding of copper, the formation causes of hot cracking is researched from the view of metallurgical aspect and mechanical aspect respectively, and three methods to suppress the hot cracking in copper welding is proposed from the metallurgical point of view.The formation causes of the hot cracking in GTA Welding of copper were studied. The mechanical reason of hot cracking formation is mainly shown in two aspects. On the one hand, the strength of HS201 weld metal in the BTR is less than the tensile stress during the welding process; on the other hand,Δεof weld metal will be greater than the ductility of HS201 weld metal in the BTR. The metallurgical reason of hot cracking formation is mainly due to the oxidation of molten pool in the welding process. The liquid film composed of low melting point of Cu2O and Cu will form at the grain boundary ofα-Cu, which will be the origination of hot cracking. With the expansion of the cracking, the liquid film will gather at the tip of the cracking under the function of capillary to form the solid eutectic during the solidification. As the less resistance to deformation, the eutectic will be pulled down under the tension of theα-Cu grains on both sides, which results in the crack expand in the inner of the eutectic organization, and ultimately, the hot cracking will be formed.Based on the above analysis, a suppress methods of hot cracking in copper welding is proposed from the metallurgical point of view. The method is achieved by adding element Ti into the welding material in order to inhibit the formation of Cu2O. Moreover, adding element Ti can strengthen the weld metal, which increases the strength of the welds material in the BTR. When the value of the strength to deformation is greater than the tensile stress of the weld metal in the conditions of without preheating andΔεof weld metal will be less than the ductility of HS201 weld metal, the welds will not easy to be deformed. The influence of different content of Ti addition on hot cracking is researched. Results show that in order to suppress the hot cracking, the amount of Ti must be precisely controlled. If the level exceeds 4wt%, a higher level of low-melting point eutectics (β-TiCu4 and TiCu2) will be formed in the weld. Welds made with ERCuTi-2 filler metal are mainly composed of the solid solution ofα-Cu (Ti) and the peritectic ofβ-TiCu4 in form of spots distributed among columnar dendritic grains, which effectively decreases the susceptibility of cracking. Welds made with ERCuTi-4 filler metal are composed of the solid solution ofα-Cu (Ti) and the eutectic ofβ-TiCu4 and TiCu2, observed as intermittent lines among the dendrites. The micro-cracks appear again among the dendrites ofα-Cu and the susceptibility of cracking is increased.In order to ensure the conductive thermal and conductivity of the welded joints, a suppress methods of hot cracking in copper welding is proposed by adding element Al into the welding material. It is found that the high-temperature ductility of Cu6Al weld metal in the high-temperature BTR is greater than the internal deformation rate of the welding process, although the strength of Cu6Al weld metal in the high-temperature BTR is similar to that of HS201 weld metal. So there are no mechanical conditions to make the hot cracking form. From the metallurgical point of view, the influence of different content of Al addition on hot cracking is researched. It is found that the content of Al addition has to do with the formation of hot cracking. When the level exceeds 7.4wt%, the liquid film composed of (α+β) eutectics will form at the grain boundary at the temperature of 1036℃during the solidification process when using S214 or S215 as weld metal. The eutectic formation will increase the susceptibility of hot cracking. When the lever is less than 7.4%, the welds structure is identified asα-Cu(Al) when using Cu6Al as welding material. As the narrow liquid-solid temperature range and no low melting eutectic structures formation during the solidification process, the hot cracking susceptibility of weld metal is decreased obviously.A weld brazing process has been achieve through adding element P and Ag into welding material to suppress the hot cracking. By adding the element P and Ag, the reaction temperature of base metal can be decreased to the range out of the BTR. Weld brazing process can achieve a large number of base metal dissolution at lower temperatures and obtain the welding joint similar to the GTA welding. The characteristics of weld brazing process determined the short reaction time and rapid temperature change in single thermal cycle, so the key factor influence the dissolving amount of base mental is the dissolution rate of Cu in liquid Cu-Ag and Cu-P alloys. The dissolution rate constant of Cu in liquid Cu-Ag and Cu-P alloys is determined, which is following the relations, kCu-P (T)=10 kCu-Ag (T). It can be conclude that the main reason that a large dissolving amount of Cu in of Cu-P alloy is the higher dissolution rate. The test results of microstructure and mechanical properties of the welds show that, the addition of element P will increase the hardness and decrease the ductility of the welds, which will impact the application of welding parts. The addition of element Ag can increase the ductility of the welds, so using Cu-P-Ag filler metal can both realize the weld brazing process and obtain the joints with better performance.