ABSTRACT: There are few systematic studies on the weldability of copper and copper alloys. After a long-term study on the weldability of copper and copper alloys and consulting related materials, the classification and properties of copper and copper alloys are briefly introduced The weldability of copper and copper alloys, the weldability of steel and copper and copper alloys and the causes of defects (blowhole and crack) during welding are analyzed The welding technology of copper and copper alloy, steel and copper and copper alloy was discussed. The practice has proved that the welding method and process are suitable, the welding material is reasonable, and the defects which are easy to appear in the welding process can be avoided.

Key words: copper; Copper Alloy; Weldability; process Literature Identification Code: B

For a long time, brazing, gas welding, ARC welding, Inert Gas Metal Arc welding, submerged arc welding, diffusion welding and so on have been the main welding methods for copper and copper alloys. In recent years, with the development of welding technology, high energy heat sources such as electron beam, laser and Plasma Arc are used to weld, and good results have been obtained. In this paper, the weldability of copper and copper alloy, the problems in the process of welding and the solutions are described.

1. Types and properties of copper and copper alloys

1.1 copper is cubic crystal system, has more deformation slip system, has good deformation ability at room temperature and high temperature, the annealed copper can be compressed 85% ~ 95% without intermediate annealing without producing crack. However, pure copper exhibits "medium temperature Brittleness" at 500 ~ 600 °C. In the process of welding, it is easy to crack in this temperature range. According to research, "medium temperature Brittleness" and the nature of impurities, content, distribution, solid solubility and so on. Copper can be divided into oxygen-free copper and pure copper containing a small amount of oxygen. Pure copper has good conductivity and is often used as a conductive material. However, there exists a low melting point eutectic of Cu2o-cu, which is liable to crack during welding. Oxygen-free copper can be divided into oxygen-free copper and oxygen-free copper with P, MN deoxidization, because of its good weldability, commonly used in welding structures.

1.2 copper alloy copper alloy is divided into brass, bronze and white copper. 1.2.1 brass

Brass is a Cu-Zn alloy, according to the content of Zn can be divided into many kinds, in order to change the properties of brass, you can also add other elements, such as AL, NI, MN. Thus formed aluminum brass, nickel brass, manganese brass and so on. According to the phase diagram of Cu-Zn binary system, there are six phases t, U, V, W, X and Z in brass solid state. The solubility of Zn in copper increases with the decrease of temperature, and decreases with the decrease of temperature after the solid solubility reaches the maximum value at 456 °C. T solid solution has good plasticity, can be cold and hot processing, and has good welding properties.

1.2.2 bronze

Bronze is an alloy of Cu with Sn, AL, SI and so on. According to the composition can be divided into tin bronze, aluminum bronze, silicon bronze, etc. . Bronze has high wear resistance, mechanical properties and corrosion resistance, elastic and welding properties are very good, and the linear shrinkage coefficient is small. Bronze is widely used in castings and fabricated articles.

1.2.3 white copper

Cupronickel is a copper-nickel alloy that is silver or grayish white in color. White copper has heat and cold resistance, medium strength, high plasticity, can be cold and hot pressure processing, there are very good electrical properties, in addition to being used as a structural material, but also an important high resistance and thermocouple alloy. Therefore, cupronickel according to its use can be divided into structural cupronickel and electrical cupronickel. Structural cupronickel has good corrosion resistance, excellent mechanical properties and pressure processing properties, good weldability, mainly used to manufacture condensing tubes, evaporators, heat exchangers and various high-strength corrosion-resistant parts. In addition, cupronickel can also be added to other elements to form iron cupronickel, zinc cupronickel, aluminum cupronickel, manganese cupronickel and so on.

2. Analysis of weldability 

2.1 welding of copper and copper alloys

 2.1.1 copper

The content of copper used in general engineering is above 99.95% , the rest is impurity. The existence of impurity has great influence on the weldability of copper.

(1) Bi and Pb are the main impurities in copper, and they are not soluble in solid copper. A small amount of PB forms a eutectic structure with low melting point (Cu + Pb) , and the eutectic temperature is 326 °c. Bi and Cu also form a eutectic structure with low melting point (Cu + Bi) , the eutectic temperature is 270 °C, the final crystallization of these eutectic, concentrated on the grain boundary, will make copper brittle, in the process of welding cracks, therefore, should limit the content of Bi, PB in copper, BI & Lt; 0.002% , PB & LT; 0.005% .

(2) the melting point of P is 44 °C, the solubility of P in copper is 1.75% at 700 °C, but the solubility of P in copper is only 0.4% at 200 °C. It can obviously reduce the conductivity and thermal conductivity of copper, but it has a good effect on the mechanical properties of copper.

(3) s can dissolve in the molten copper, the existence of s reduces the melting point of copper and forms cu2s Brittle compound, which reduces the plasticity of copper. When s content is less than 0.1% , the copper will have the hot Brittleness, and the hot state will produce the crack when welding.

4) oxygen rarely dissolves in copper and forms CU2O with copper. According to the phase diagram of Cu-O2, oxygen precipitates as eutectic (Cu + Cu2O) and distributes on the grain boundary. The melting point of the EUTECTIC is 1066 °C. The eutectic solidifies more than copper and distributes on the grain boundary, which reduces the plasticity and corrosion resistance of copper and makes its weldability worse. Copper has a high thermal conductivity (than low carbon steel thermal conductivity of 8 times) , if the heating temperature is not high, even if long-term heating is not easy to make Cu melting. With the increase of temperature, the crystalline structure becomes coarser and the bonding ability decreases. The main defects of copper in welding are cracks and air holes. First, there are thermal cracks and hydrogen corrosion cracks. Because copper contains a certain amount of impurities, such As PB, Bi, P, As, which enlarge the solid-liquid interval of copper. Therefore, even if the impurity content is very small, it is also very easy to produce hot crack. The second is the corrosion of hydrogen cracks, because copper contains a certain amount of oxygen, welding process, hydrogen will be spread to the copper, the following reactions occur:

The H2O formed by U2O, H2, Cu and H2O aggregates at the grain boundary in the form of gas, causing hydrogen erosion cracks. Finally, the pores are formed by the following reactions :2H(a) O(a) H2O(g) (1)2H(a) H2(g) (2) where (a) is the atomic state and (g) is the gaseous state. The H2O generated by reaction (1) does not dissolve in copper. In the solidification process of copper, it is too late to escape and form pores. The H2 generated by reaction (2) also forms hydrogen pores. The main measures to prevent pores are to add a certain amount of deoxidizer to the welding material to improve the preheating temperature before welding and slow down the cooling rate of the molten pool, so that H2 and H2O can have time to escape. In addition, electromagnetic generating equipment can be added to the welding torch to make the electromagnetic effect on the molten pool and stir the molten pool to make the gas escape.

2.1.2Copper Alloy

The main difficulty in brass welding is the evaporation of Zn in Copper Alloy (Zn evaporates at 906 °c) . When soldering tin bronze, when the temperature rises, Sn is easy to evaporate or oxidize to Sno 2, it is difficult to remove in the weld metal, so the blowhole and inclusion are formed. In order to remove Al2O3, the reductant used in aluminum alloy welding can be used. Ni is the main component of white copper, the melting point of NI is 1450 °c, the boiling point of Ni is 3075 °C, and it has the property of no oxidation when heated to 700 ~ 800 °C, the evaporation of Cu and the destruction of o 2, S and C should be prevented. The most important problem in the welding of copper alloy is the crack. Like copper, copper alloys are prone to crack formation due to the precipitation of impurities at Grain Boundaries. In aluminum bronze, because of the low Al content, the T single-phase weld structure is formed, and the crack sensitivity is high, especially when multi-layer welding, the previous layer is easy to crack. If the content of AL is increased, the dual phase structure of t + U will be formed, which can restrain the appearance of crack, but if the content of AL is too high, the hard v 2 phase will be precipitated from the U phase, and the crack sensitivity will be increased. Therefore, the content of Al should be from 7% to 11% , the addition of Ni, Fe and MN can inhibit the precipitation of V2 hard phase. Because Cu and Ni are infinitely miscible, the weld microstructure is a large T single-phase solid solution. Due to the influence of impurity elements, the solid-liquid phase interval is enlarged, and the eutectic material with low melting point is precipitated at the grain boundary. Multi-layer welding is more sensitive to cracks, so excessive welding line energy should be avoided.

2.2 welding of steel and copper and copper alloys

The difference of melting point, thermal conductivity and linear expansion coefficient between Fe and Cu is large, which is disadvantageous to the welding of steel and copper. According to the phase diagram of the binary alloy of FE AND CU, Fe & Lt; 0.3% is phase, Fe & Lt; 0.2% is x phase, and other conditions are t + x phase. When steel is welded with copper or copper alloy, the main problems are easy to appear hot crack, iron dilution invasion crack and infiltration crack, but the tendency of porosity is small. (1) the hot crack is mainly caused by low melting point EUTECTIC, microstructure and welding stress. Both steel and copper contain impurities, which can form various eutectic and brittle compounds with low melting point during welding, and are easy to produce segregation. FES (melting point 1189 °c) , FEP (melting point 1050 °c) and FE + FeS EUTECTIC (eutectic temperature 985 °c) are easily formed in the welds of low carbon steel and copper and copper alloys. In addition, some low melting point eutectic in copper, these compounds and low melting point eutectic segregation in the grain boundary, seriously weakened the metal in high temperature inter-crystal Bonding Force, weld easy to produce hot crack. The Fe content in the weld also has a great influence on the hot crack. When the FE content is 0.2% ~ 1.1% , the weld metal presents a single-phase t structure, and the hot crack resistance decreases. When the FE content is increased, the weld becomes t + X dual-phase structure, and the hot crack resistance increases. When the FE content is 10% ~ 43% , the hot crack resistance is the best. (2) dilution of Fe into the weld and segregation of Fe into the grain boundary due to the dilution of steel, which is almost insoluble in solid state. The Fe 3C, a brittle compound with high c content, is formed when Fe and Fe are melted into the weld at the same time. The precipitation phase of Fe is hard and easy to segregation, which reduces the toughness and anti-crack ability of the weld. (3) when the Grain Boundary Penetration crack of copper is deposited on the surface of steel and copper alloy, the main reason of the penetration crack near the crack is the penetration and tensile stress of liquid copper. It is considered in reference [4] that, in the process of crystallization, there are often defects in the structure of the metal, so a micro-crack will be formed on the crystalline surface of the steel. At this time, the liquid copper is easy to infiltrate the micro-crack surface, and under the action of capillary effect, into the microscopic fissures. An additional pressure of 24.5 MPA can be produced on the micro-crack wall when copper or copper alloy liquid penetrates into the steel. Thus, a hot crack is produced near the crack zone under the combined action of tensile stress. It is considered in reference [5] that the penetration crack in high temperature brazing stainless steel with cu-based filler metal is due to the three-dimensional wetting spreading of Cu-Mn rich liquid phase on the brazing surface, the evaporation of MN and the diffusion of MN to the base metal volume play the role of "Scavenger" and improve the brazing surface, which create a good condition for wetting and spreading.

3. Welding procedure for copper and copper alloys

 3.1 welding of copper and copper alloys

 3.1.1 copper

In the welding process, the most important thing is to preheat, heat preservation, and use a faster welding speed, so that the weld metal can quickly reach the melting temperature, the grain will not grow too large. Especially when welding thick plate, the preheating temperature must reach 400 ~ 500 °C. In addition, some deoxidization reducing agent should be added in the welding process in order to remove impurities such as O2, H2, s in the weld. With inert Gas metal arc welding, different gases are used to achieve different depths of penetration, and the diatomic gas N2 is deeper than Ar, so the preheating temperature of the protective gas N2 can be lowered. In general, it is better to add a certain amount of SI to the welding wire. It is better to use laser, plasma arc and Electron beam welding. When deoxidized copper is welded by electron beam, the defects will appear in the first welding, and the weld bead will form well in the second welding.

3.1.2 copper alloys

(1) in order to reduce the evaporation of Zn, brass should be welded as quickly as possible, which requires more welding line energy. Therefore, it is better to preheat the brass before welding, the preheating temperature is 200 ~ 250 °C. As far as possible do not repair welding or welding in the reverse side, because the re-heating will make the brass of Zn increased evaporation, grain size, prone to cracks. 

(2) bronze is brittle when heated, so shock and vibration should be prevented when welding. In aluminum bronze welding, AL and O 2 are combined to form refractory AL 2O 3. In order to remove AL 2O 3, the reductant used in aluminum alloy welding can be used. When silicon bronze is welded, SI and o 2 are combined to form Sio 2 film, which can reduce the evaporation of other alloy components and has good weldability. 

(3) the fluidity of Cupronickel and Cupronickel is better, and the welding quality can be guaranteed by adding reducing agent and adopting faster welding speed. Because B30 and B10 are brittle at medium temperature, the weld with high restraint is easy to crack, so the small welding line energy should be used as far as possible to make the grain fine and improve the welding quality. 3.2 the welding of steel and copper and copper alloys the welding of steel and copper and copper alloys is mainly by submerged arc welding, brazing and inert Gas metal arc welding. When aluminum bronze is welded to steel, the thermal conductivity of aluminum bronze is small, so it needs to be preheated, usually at 100 ~ 200 °C. Copper-nickel alloys do not need to be preheated, because steel and copper-nickel alloys have similar thermal conductivity. Attention should be paid to the selection of the correct welding wire, used to prevent penetration cracks and iron dilution into the crack. Therefore, the general use of depth of penetration TIG welding. In engineering, surfacing B30 and B10 on the surface of steel is often used, generally TIG welding, strip submerged arc welding, plasma arc surfacing is the best, because the dilution rate can reach 2% . When welding, pure nickel or Monel is used as the transition layer, and then welding the working layer, the effect is better. Otherwise, the crack is easy to appear, especially the dilution of Fe has obvious effect on the crack. Therefore, the plasma arc surfacing with low dilution can be used without the transition layer, if the process is reasonable, the material is properly selected, the surfacing quality is very good. 

Source: Chinanews.com, by Ji Jie

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