7-19. COPPER WELDING
a. General. Copper and copper-base alloys have specific properties which make them widely used. Their high electrical conductivity makes them widely used in the electrical industries, and corrosion resistance of certain alloys makes them very useful in the process industries. Copper alloys are also widely used for friction or bearing applications. Copper can be welded satisfactorily with either bare or coated electrodes. The oxygen free copper can be welded with more uniform results than the oxygen bearing copper, which tends to become brittle when welded. Due to the high thermal conductivity of copper, the welding currents are higher than those required for steel, and preheating of the base metal is necessary. Copper shares some of the characteristics of aluminum, but is weldable. Attention should be given to its properties that make the welding of copper and copper alloys different from the welding of carbon steels. Copper alloys possess properties that require special attention when welding. These are:
(1) High thermal conductivity.
(2) High thermal expansion coefficient.
(3) Relatively low melting point.
(4) Hot short or brittle at elevated temperatures.
(5) Very fluid molten metal.
(6) High electrical conductivity.
(7) Strength due to cold working.
Copper has the highest thermal conductivity of all commercial metals, and the comments made concerning thermal conductivity of aluminum apply to copper, to an even greater degree.
Copper has a relatively high coefficient of thermal expansion, approximately 50 percent higher than carbon steel, but lower than aluminum.
The melting point of the different copper alloys varies over a relatively wide ranger but is at least 1000°F (538°C) lower than carbon steel. Some of the copper alloys are hot short. This means that they become brittle at high temperatures, because some of the alloying elements form oxides and other compounds at the grain boundaries, embrittling the material.
Copper does not exhibit heat colors like steel, and when it melts it is relatively fluid. This is essentially the result of the high preheat normally used for heavier sections. Copper has the highest electrical conductivity of any of the commercial metals. This is a definite problem in the resistance welding processes.
All of the copper alloys derive their strength from cold working. The heat of welding will anneal the copper in the heat-affected area adjacent to the weld, and reduce the strength provided by cold working. This must be considered when welding high-strength joints.
There are three basic groups of copper designations. The first is the oxygen-free type which has a copper analysis of 99.95 percent or higher. The second subgroup are the tough pitch coppers which have a copper composition of 99.88 percent or higher and some high copper alloys which have 96.00 percent or more copper.
The oxygen-free high-conductivity copper contains no oxygen and is not subject to grain boundary migration. Adequate gas coverage should he used to avoid oxygen of the air caning into contact with the molten metal. Welds should be made as quickly as possible, since too much heat or slow welding can contribute to oxidation. The deoxidized coppers are preferred because of their freedom from embrittlement by hydrogen. Hydrogen embrittlement occurs when copper oxide is exposed to a reducing gas at high temperature. The hydrogen reduces the copper oxide to copper and water vapor. The entrapped high temperature water vapor or steam can create sufficient pressure to cause cracking. In common with all copper welding, preheat should be used and can run from 250 to 1000°F (121 to 538°C), depending on the mass involved.
The tough pitch electrolytic copper is difficult to weld because of the presence of copper oxide within the material. During welding, the copper oxide will migrate to the grain boundaries at high temperatures, which reduces ductility and tensile strength. The gas-shielded processes are recommended since the welding area is more localized and the copper oxide is less able to migrate in appreciable quantities.
The third copper subgroup is the high-copper alloys which may contain deoxidizers such as phosphorus. The copper silicon filler wires are used with this material. The preheat temperatures needed to make the weld quickly apply to all three grades.
c. Gas Metal-Arc (MIG) Welding (GMAW).
(1) The gas metal arc welding process is used for welding thicker materials. It is faster, has a higher deposition rate, and usually results in less distortion. It can produce high-quality welds in all positions. It uses direct current, electrode positive. The CV type power source is recommended.
(2) Metal-arc welding of copper differs from steel welding as indicated below:
(a) Greater root openings are required.
(b) Tight joints should be avoided in light sections.
(c) Larger groove angles are required, particularly in heavy sections, in order to avoid excessive undercutting, slag inclusions, and porosity. More frequent tack welds should be used.
(d) Higher preheat and interpass temperatures are required (800°F (427°C) for copper, 700°F (371°C) for beryllium copper).
(e) Higher currents are required for a given size electrode or plate thickness.
(3) Most copper and copper alloy coated electrodes are designed for use with reverse (electrode positive) polarity. Electrodes for use with alternating currents are available.
(4) Peening is used to reduce stresses in the joints. Flat-nosed tools are used for this purpose. Numerous moderate blows should be used, because vigorous blows could cause crystallizations or other defects in the joint.
d. Gas Tungsten-Arc (TIG) Welding (GTAW).
Never use a flux containing fluoride when welding copper or copper alloys.
(1) Copper can be successfully welded by the gas tungsten-arc welding process. The weldability of each copper alloy group by this process depends upon the alloying elements used. For this reason, no one set of welding conditions will cover all groups.
(2) Direct current straight polarity is generally used for welding most copper alloys. However, high frequency alternating current or direct current reverse polarity is used for beryllium copper or copper alloy sheets less than 0.05 in. (0.13 cm) thick.
(3) For some copper alloys, a flux is recommended. However, a flux containing fluoride should never be used since the arc will vaporize the fluoride and irritate the lungs of the operator.
e. Carbon-Arc Welding.
(1) This process for copper welding is most satisfactory for oxygen-free copper, although it can be used for welding oxygen-bearing copper up to 3/8 in. (9.5 mm) in thickness. The root opening for thinner material should be 3/16 in. (4.8 mm), and 3/8 in. (9.5 mm) for heavier material. The electrode should be graphite type carbon, sharpened to a long tapered point at least equal to the size of the welding rod. Phosphor bronze welding rods are used most frequently in this process.
(2) The arc should be sharp and directed entirely on the weld metal, even at the start. If possible, all carbon-arc welding should be done in the flat welding position or on a moderate slope.No tags for this post.