WELDING OF COPPER AND ITS ALLOYS

PROPERTIES OF Cu AND ITS ALLOYS Excellent electrical and thermal conductivities Outstanding resistance to corrosion Good strength and fatigue resistance Ease of fabrication Metal-to-metal wear resistance Low-permeability properties Distinctive color

TYPES OF COPPER ALLOYS Coppers, which contain a minimum of 99.3% Cu High-copper alloys, which contain up to 5% alloying elements Copper-zinc alloys (brasses), which contain up to 40% Zn Copper-tin alloys (phosphor bronzes), which contain up to 10% Sn and 0.2% P

Copper-aluminum alloys (aluminum bronzes), which contain up to 10% Al Copper-silicon alloys (silicon bronzes), which contain up to 3% Si Copper-nickel alloys, which contain up to 30% Ni Copper-zinc-nickel alloys (nickel silvers), which contain up to 7% Zn and 18% Ni Special alloys, which contain alloying elements to enhance a specific property or characteristic, for example machinability

EFFECT OF ALLOYING ELELMENTS ON THE WELDABILITY OF COPPER1) ZINC Decreases the melting temperature, density , electrical and thermal conductivity and the modulus of elasticity. Increases the strength, hardness, ductility and coefficient of thermal expansion . Reduces the weldability of copper in relative proportion to the percent of zinc in the alloy. Zinc volatilization which results in white fumes of zinc oxide and weld metal porosity

2) TIN Increases the hot-crack susceptibility during welding when present in amounts from 1 to 10% During the welding tin may preferentially oxidize relative to copper. This results in oxide entrapment, which may reduce the strength of the weldment Far less volatile and toxic than zinc

3) ALUMINIUM Form tenacious oxides that must be removed prior to welding. The formation of these oxides must be prevented by shielding gas or by fluxing & use of the appropriate welding current

4) SILICON Has a beneficial effect on the weldability of copper because of its deoxidizing and fluxing actions. The addition of silicon to copper increases tensile strength, hardness ,work hardening rate and corrosion resistance.

5) NICKEL Imparts moderate strength to copper and improves the oxidation and corrosion resistance. Cu-Ni alloys have good hot and cold formability.

6) IRON AND MANGANESE Do not significantly affect the weldability of the alloys that contain them. 7) FREE MACHINING ADDITIVES( Pb,Se,Te,S) Improve machinability but affect the weldability of copper alloys by rendering the alloys hotcrack susceptible. The adverse effect begins ~ 0.05% of additive Lead-most harmful w.r.t hot-crack susceptibility.

WELDING PROCESSES USED GAS TUNGSTEN ARC WELDING GAS METAL ARC WELDING PLASMA ARC WELDING BRAZING BRAZE WELDING SHIELDED METAL ARC WELDING OTHERS(Resistance welding, laser beam welding)

GTAW Well suited because of its intense arc, which produces an extremely high temperature at the joint and a narrow heat-affected zone (HAZ). For copper sections up to 16 mm thick SHIELDING GAS : Ar- upto 1.6 mm ; Ar(25%) + He(75%) for > 1.6 mm Forehand welding is preferred for GTAW of Cu with stringer beads or narrow weave beads Usually EWTh-2 may be used. DC -ve or AC can be used

Metal Thickness (mm) 0.3-0.8 1.0-2.0 2.0-5.0 6.0 10.0 12.0 16.0

Shielding Gas

Electrode and welding current type Thoriated/DCThoriated/DCThoriated/DCThoriated/DCThoriated/DCThoriated/DCThoriated/DC-

Welding Rod Preheat Diameter Temp. (C) (mm) __ 1.6 2.4 - 3.2 3.2 3.2 3.2 3.2 __ __ 50 100 250 250 250

Welding Current (A) 15-60 40-170 100-300 250-375 300-375 350-420 400-475

Argon Argon Argon Argon Argon Argon Argon

General guidance data for GTAW of Copper and its alloys.

GMAWGMAW process can be used for joining nearly all combinations of weldable Cu and Cu alloys. 2 main considerations for GMAW of copper: 1) The composition of welding wire to be used as electrode. 2)The preheat temperature required.The gas mixture required will be largely determined by the thickness of the copper section to be welded

GMAW OF Cu 1) ERCu copper electrodes are recommended.2) Argon is generally used for thk. 6mm. 3) The helium-argon mixtures are used for welding of thicker sections. Recommended Shielding Gases for the GMA welding of Copper and Copper Alloys: - Welding Grade Argon. - Ar + 25% He - He + 25% Ar 4) Spray transfer mode is used generally for depositing stringer beads or narrow weave beads.

Metal Electrode Preheat Thickness Diameter Temp. (C) (mm) (mm) 1.6 3.0 6.0 6.0 10 12 16 + 0.9 1.2 1.2 1.6 1.6 1.6 1.6 75 75 75 100 250 250 250

Welding Current (A) 150 -200 150- 220 180- 250 160 -280 250-320 290-350 320-380

Voltage (volts) 21-26 22-28 22-28 28-30 28-30 29-32 29-32

Gas Flow Travel Rate Speed (l/min) (mm/min) 10-15 10-15 10-15 10-15 15-20 15-20 15-25 500 450 400 350 300 300 250

General guidance data for GMAW of Copper and its alloys.

GMAW of Copper Silicon Alloys: ERCuSi-A type welding consumables + Ar shielding. It is important to ensure the oxide layer is removed by wire brushing between passes. Preheat is unnecessary and interpass temperature should not exceed 100 C.

GMAW of Copper Tin Alloys (Phosphor Bronze): ERCuSn-A is used. However, ERCuSnA wires do not yield a fluid weld puddle so preheating may be necessary.

GMAW of Copper Tin Alloys (Phosphor Bronze) cont : These alloys have a wide solidification range which gives a coarse dendritic grain structure, therefore care must be taken during welding to prevent cracking of the weld metal. The weld pool should be kept small using stringer beads at high travel speed.

Copper Electrode chemical composition

Tensile strength of different electrodes

PAW The welding of copper and copper alloys using PAW is comparable to GTAW of these alloys. ERCuSn electrodes(4-6% Sn) could be used for joining phosphor bronzes. Plasma arc welding has two distinct advantages over GTAW: (1) The tungsten is concealed and entirely shielded, which greatly reduces contamination of the electrode, particularly for alloys with low-boiling-temperature constituents such as brasses, bronzes, phosphor bronzes, and aluminum bronzes (2) The constructed arc plume gives rise to higher arc energies while minimizing the growth of the HAZ

PAW is preferred over GTAW where contamination can restrict production efficiencies Plasma arc welding of coppers and copper alloys may be performed either autogenously or with filler metal. Automation and mechanization of this process is readily performed. The plasma keyhole mode is preferred for thicker sections in a vertical-up position.

BRAZING Brazing is used widely for the joining of copper and copper alloys, with the exception of aluminum bronzes containing greater than 10 percent aluminium and alloys containing greater than 3 percent lead. The principle of brazing is to join two metals by fusing with a filler metal. The filler metal must have a lower melting point than the base metals but greater than 450C. The filler metal is usually required to flow into a narrow gap between the part by capillary action.

To achieve an adequate bond during brazing, the following points should be considered: 1. The joint surfaces are clean and free of oxides etc.(contamination can cause poor wetting) 2. The provision of the correct joint gap for the particular brazing filler metal(0.03 0.08 mm) 3. The establishment of the correct heating pattern so that the filler metal flows up the thermal gradient into the joint. 4. Flux removal should be done. Brazing of copper is used extensively in the electrical manufacturing industry, and in the building mechanical services, heating, ventilation and air-conditioning fields.

BRAZE WELDING

Braze welding is a technique similar to fusion welding except with a filler metal of lower melting point than the parent metal The Braze welding process derives its strength from the tensile strength of the filler metal deposited as well as the actual bond strength developed between the filler metal and parent metal It allows joining dissimilar metals , minimizes heat distortion, and to reduce extensive pre-heating. It eliminates stored-up stresses that are often present in fusion welding CuZn electrodes can be used

GENERAL CONSIDERATIONS Choice of alloy: The alloy most suited to the job requirement depends on the strength required in the joint, resistance to corrosion, operating temperature and economics. Flame adjustment: Use slightly oxidizing flame. Flux: Copper and Brass Flux, mix to a paste with water and apply to both sides of joint. Rod can be coated with paste or heated and dipped in dry flux. Preheating: Preheating is recommended for heavy sections only.

Blowpipe and rod angles: Blowpipe tip to metal surface40 to 50. Filler rod to metal surface 40 to 50.

Joint Preparation: Typical joint designs are shown below:

SMAW SMAW is normally used for the maintenance and repair welding of Cu -Ni, Cu- Al and Cu - Si. Covered electrodes for SMAW of copper alloys are available in standard sizes ranging from 2.4 to 4.8 mm. Direct Current electrode positive (DC+) should be used with a stringer bead technique. Relatively large welding grooves are required for good joint penetration. ECuSn-C ,E Cu Si,ECuAl-A2 can be used to weld different Copper- alloys. Sections over 3.0mm require a preheat of 250C or greater.

FACTORS AFFECTING WELDING OF Cu ALLOYS Welding Position. Due to the highly fluid nature of copper and its alloys, the flat position is used whenever possible for welding. The horizontal position is used in some fillet welding of comer joints and T-joints. Precipitation- Hardenable Alloys. The most important precipitationhardening reactions are obtained with beryllium, chromium, boron, nickel, silicon, and zirconium. Care must be taken when welding precipitationhardenable copper alloys to avoid oxidation and incomplete fusion. Whenever possible, the components should be welded in the annealed condition, and then the weldment should be given a precipitation-hardening heat treatmentHot Cracking. Copper alloys, such as copper-tin and coppernickel, are susceptible to hot cracking at solidification temperatures. Severe shrinkage stresses produce inter dendritic separation during metal solidification. Hot cracking can be minimized by reducing restraint during welding, preheating to slow the cooling rate and reduce the magnitude of welding stresses, and reducing the size of the root opening and increasing the size of the root pass.

Thermal Conductivity. When welding commercial coppers and lightly alloyed copper materials with high thermal conductivities, the type of current and shielding gas must be selected to provide maximum heat input to the joint. This high heat input counteracts the rapid head dissipation away from the localized weld zone. The interpass temperature should be the same as for preheating. Copper alloys are not post-weld head treated as frequently as steels, but some alloys may require controlled cooling rates to minimize residual stresses and hot shortness. Porosity. Certain elements (for example, zinc, cadmium, and phosphorus) have low boiling points. Vaporization of these elements during welding may result in porosity. When welding copper alloys containing these elements, porosity can be minimized by higher weld speeds and a filler metal which is low in these elements. Surface Condition. Grease and oxide on work surfaces should be removed before welding. Wire brushing or bright dipping can be used. Miliscale on the surfaces of Al bronzes & Si bronzes is removed for a distance from the weld region of at least 13 mm, usually by mechanical means. Miliscale on Cu- Ni alloys must be removed by grinding or pickling; wire brushing is not effective. Grease, shop dirt, and similar contaminants on Cu- Ni alloys may cause embrittlement and should be removed before welding.

PROBLEMS IN WELDING Cu AND ITS ALLOYS Beryllium copper: Causes tenacious surface oxide that inhibits wetting and fusion during welding. Phosphorus bronzes: The wide HAZ and fast cooling rate may result in hot cracking since phosphorus bronzes are hot short susceptible. Aluminium bronzes: Aluminium oxide is formed, so there is problem with fluxing. Copper-nickel alloys: An oxidizing flame will form cuprous oxide that will dissolve in molten metal, reduce corrosion resistance, and causes embrittlement. Coppers: Difficult to seam weld. Projection welding is not recommended for coppers and for most brasses.

REFERENCES B.S Raghuwanshi; A Course In Workshop Technology, Manufacturing Processes, Chapter 24 Weldin and Allied Processes, Vol 1. ,pp 591 596 , Dhanpat Rai Publications, 2006 www.keytometals.com/Article29 www.brazing.com/techguide/procedures/copper_ welding