摘要
热裂纹是紫铜厚大结构件GTAW时的主要缺陷之一,严重影响和制约紫铜构件的使用。本文首先在普洛霍洛夫热裂纹理论基础上完善了热裂纹形成的判据。建立了基于刚性拘束热裂纹试验的紫铜厚板GTAW接热力耦合有限元模型,从而对热裂纹形成的原因进行了分析及对采用不同合金焊材时焊缝金属的热裂纹倾向进行了预测。最后研究了气体保护焊时采用不同合金焊材对热裂倾向的影响。
首先通过紫铜厚板GTAW刚性拘束热裂纹有限元模型对紫铜焊接热裂纹的产生原因进行了分析。其原因是HS201焊缝金属在BTR内的抗拉强度小于焊接过程中母材对焊缝金属的拉伸应力,并且焊接时焊缝金属所发生的内部变形率将大于HS201焊缝金属在BTR内的延性,从而使焊缝金属的热裂倾向较大。当裂纹形成后,随着裂纹的扩展,由于毛细作用共晶液相向裂纹尖端聚集,在1066℃以下时在α-Cu晶界上形成固态(Cu2O+Cu)共晶组织。但是由于共晶组织抗变形能力较差,在两侧变形的α-Cu晶粒拉伸作用下被拉断,裂纹沿固态共晶组织内部继续扩展,最终形成焊接热裂纹。
通过紫铜GTAW刚性拘束热裂纹有限元模型对在焊材中添加Ti后的铜钛焊缝金属的热裂倾向进行了预测。研究表明,在焊材中添加脱氧元素Ti后,由于Ti对焊缝金属的强化作用,焊缝金属在BTR范围内的抗拉强度有所提高,大于在不预热焊接条件下母材对焊缝焊缝金属的拉伸应力,使得在焊接中焊缝不易变形,内部变形率小于焊缝金属的延性,因此热裂倾向显著降低。提出了Ti抑制熔池氧化的物理模型。研究了不同Ti含量的添加对热裂倾向的影响规律。Ti含量为2%时生成弥散分布的点状β-TiCu4包晶组织,从而有效抑制热裂纹的出现。当Ti含量增加到3%以上时,在α-Cu晶界上形成了连续分布的点划线状的TiCu2和β-TiCu4低熔共晶组织,使得焊缝金属的热裂倾向又有所提高。
为了在抑制热裂纹的基础上同时保证焊接接头的导电导热性能,提出了在焊材中添加元素Al的方法。首先通过紫铜GTAW刚性拘束热裂纹有限元模型对在焊材中添加Al后的铜铝焊缝金属的热裂倾向进行了预测。研究发现虽然Cu6Al焊缝金属在BTR内的抗拉强度与HS201相比没有得到提高,但是由于Al抑制了由于氧化而形成的裂纹源,使得焊缝金属的BTR延性得到改善,从而使焊接时焊缝金属的变形并未超过材料本身的延性,因此热裂倾向显著降低。研究了不同Al含量的添加对焊接热裂纹的影响规律。当焊材中Al含量超过7.4wt%,如S214和S215焊缝金属在凝固过程中在1036℃左右会形成(α+β)的低熔共晶组织,这种低熔共晶组织在晶界上形成液态薄膜,从而增大了焊缝金属热裂纹敏感倾向。当焊材中Al降到7.4wt%以下时,如采用Cu6Al焊材,焊缝均由α-Cu(Al)组织组成。焊缝金属凝固时由于固液区间较窄,且凝固过程无低熔共晶组织生成,因此焊缝金属的热裂纹向明显降低。
提出了利用焊材中的合金元素P来抑制紫铜厚板焊接热裂纹的方法。采用Cu-P合金进行低温GTAW能够抑制热裂纹的主要原因:一是脱氧元素P元素存在,可以抑制熔池的氧化,从而抑制由于氧化引起的裂纹源;二是降低母材进入熔池的温度,使得熔池凝固时避开了紫铜的BTR区间。建立了变温条件下,Cu-P和Cu-Ag合金与母材Cu反应的物理模型。模型揭示了母材铜在Cu-Ag、Cu-P液态合金中的反应速度是实现低温GTAW的主要原因。通过对物理模型的推导,得到Cu在Cu-Ag、Cu-P合金中的反应速度常数,并存在着如下关系:kCu-P (T)=10kCu-Ag (T)。说明P元素是实现低温GTAW工艺必不可少的元素。通过对焊缝的微观组织和力学性能测试可知,P元素的添加增大了焊缝硬度、降低韧性,影响焊件的应用。Ag元素的添加可以提高焊缝的韧性,降低硬度。因此采用Cu-P-Ag系钎料既实现了溶解钎焊工艺又可以获得较好性能的焊缝金属及接头。
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.
引文
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