ABSTRACT: the analysis of the weldability of red copper shows that red copper is easy to produce crack, incomplete penetration and gas hole, and red copper brazing can avoid these problems. Keywords: Red Copper; Brazing; fusion welding; flame brazing process.

1.Foreword

Because of the properties of copper pipe, it can be used in many environments, such as the manufacture of heat exchanger (condenser) , the low-temperature pipe in oxygen-making equipment; The lubrication system, oil pressure system and so on convey the liquid under pressure; due to the good corrosion resistance and hard texture of the copper pipe, it has also become the first choice of modern contractors in the commodity residential water pipe, heating, refrigeration pipe installation, in the past, galvanized steel pipe used in residence is easy to be corroded, the tap water will turn yellow and the flow of water will be small and so on. Other materials lose strength rapidly at high temperatures, causing potential hazards when used in hot water pipes, while copper has a melting point of up to 1083 °C, making it safe to use in hot water systems. Due to the wide application of red copper pipe, more and more attention has been paid to the welding technology and quality of red copper pipe. This paper mainly introduces the flame brazing technology of red copper pipe.

1. The problem of Red Copper Fusion welding

The welding methods that can be used for Red Copper Welding are electrode arc welding, gas welding, inert Gas metal arc welding welding and so on. Due to the thermal physical properties of Red Copper itself, it is easy to produce the following problems when welding.

1.1 although the melting point (1083 °c) of unpenetrated copper is much lower than that of steel, because of its high thermal conductivity, a great deal of heat is lost during welding, so it is difficult to melt and easy to form incomplete penetration.

1.2 large welding stress and deformation are easy to occur in order to ensure the penetration, large heat input and sufficient preheating are needed. Due to the large coefficient of linear expansion of red copper, large welding deformation is easy to occur. In order to prevent deformation, it is necessary to increase the rigidity of the structure, which will inevitably produce greater welding stress in the workpiece.

1.3 although there is no temperature range where solid and liquid coexist, the welding crack of red copper is easy to form because of the high welding stress and the large heat input, because of the existence of lead, bismuth and other impurities, the EUTECTIC with low melting point will be formed between the crystals, which will lead to the formation of hot crack.

1.4 easy to form porosity due to the high thermal conductivity of copper, the crystallization rate of the weld metal is relatively fast. The hydrogen gas porosity will be formed in the weld metal at high temperature when a large amount of hydrogen is not released in time. In addition, copper is oxidized at high temperatures to form Cu2o, which reacts with hydrogen dissolved in Liquid Copper: CU2O + 2h = 2Cu + H2o because water vapor does not dissolve in copper, the bath crystallizes quickly, when Water Vapor does not escape in time, reactive pores will be formed, so oxygen-containing copper stomatal sensitivity than oxygen-free copper. Therefore, red copper welding, easy to produce incomplete welding, deformation, cracks and porosity and other problems.

2. Brazing brazing is the use of a metal material with a lower melting point than the base metal as the filler metal, heating the solder and the filler metal to a temperature above the melting point of the filler metal but below the melting point of the base metal, wetting the base metal with a liquid filler metal, filling the joint clearance and diffusing with the base metal to realize the welding method of connecting the welding piece. Compared with the fusion welding, the base metal does not melt during the Brazing, which reduces the influence of the high thermal conductivity of the base metal on the welding quality, prevents the formation of the incomplete penetration, reduces the tendency of producing the gas hole and the crack, and guarantees the weld performance. In the welding process, the base metal does not melt, so the structure and properties of the base metal almost do not change, which can ensure the original service performance of the base metal. 

3. BRAZING process of Red Copper tube 3.1 selection of Brazing filler metal BCu60Zn Sn-r (wire 221) in the shape of wire or Rod (3.0ー5.0 mm) , chemical composition as shown in Table 1. The melting temperature of Silk 221 is 890 ~ 905 °C.

3.2 The selected flux is FB101. The composition of the flux is shown in Table 2. Its main components are boric acid and potassium fluoborate, and its melting temperature is 550 ~ 850 °C.

3.3 connector type connector type insert type, as shown in figure 1.

3.4 clean and correct the edge of the pipe mouth before welding and remove burrs. The pipe mouth should not have cracks, cracks or other defects. Before Assembly, the copper pipe should be inserted into the surface of the joint part and the surface of the joint part should be cleaned, the oxide should be removed by steel wire brush or sand paper, and the oil should be removed by acetone and other organic solvents.

3.5 spot welding

After Assembly, it is fixed by positioning welding, and the number, size and height of the positioning welds are shown in table 3. The order of location welding is symmetrical and evenly distributed along the circumference. In the process of Brazing, the location weld should be melted into the weld.

3.6 welding

The brazing shall be carried out in a prone position as far as possible to ensure continuous welding. Select a suitable welding torch according to the size of the copper pipe joint. Brazing flame should be neutral flame, because oxidation flame oxygen content is high, will make copper oxidation, and cause cracks; carbonization flame contains free-form hydrogen, will lead to pores. When heating the pipe, the flame should be perpendicular to the heating surface, and the welded copper pipe joint should be heated evenly. The base metal should be heated as quickly as possible. When the temperature is between 650 °C and 750 °C, the filler metal is fed into the bottom of the filler metal, and the molten filler metal is filled into the gap. When all the solder melt should immediately stop heating, avoid copper tube joint temperature is too high, heating time is too long. In the process of Brazing, the brazing seam and filler metal should be under the protection of flame. The braze seam is cooled naturally after welding, and the workpiece should not be moved before the braze seam solidifies. As the FB101 flux used contains fluoride, which has a certain corrosive effect, and the presence of welding slag will hinder the inspection of the brazing seam, therefore, after the brazing seam is cooled, hot water or wet cloth should be used to wipe the brazing seam and the surrounding area to remove the welding slag, ensure the corrosion resistance of the joint and the reliability of post-weld inspection.

4. brazing quality

After welding, the quality of the brazing seam is checked. The surface of the weld is free from defects such as cracks, blowholes and non-fusion. The Appearance Of the Weld is beautiful. The tensile strength of the brazed joint is not lower than that of the base metal. 

5. Conclusion flame brazing of Red Copper Tube can effectively prevent the defects such as Porosity, crack and incomplete penetration. Satisfactory welding quality can be obtained by using the combination of BCu60Zn Sn-r solder and FB101 flux.

 Source: Chinanews.com, by Zhao Liling