Repair of copper boiler from the Flying Scotsman
Of all metals copper is the most ancient, having been first used to fabricate tools and weapons since about 3500 years BC. Welders and metallurgists can therefore claim to have a very long pedigree! Pure copper is soft, ductile and easily worked but can be strengthened only by cold working. It does not undergo phase changes so cannot be hardened by heat treatment as can a steel. This also applies to many of the copper alloys so that any application of heat will soften the cold worked alloy, resulting in a significant loss of strength in the heat affected zones.
Two additional characteristics of copper and some of its alloys are
- high thermal conductivity, meaning that preheat is required for many joints, even at quite modest thicknesses, and
- the high coefficient of thermal expansion, meaning that distortion can be an issue with root gaps rapidly closing during welding.
Alloying with a range of metals can be used to improve the mechanical properties and/or corrosion resistance. These alloys can be conveniently placed into nine separate groups as listed below. In addition to those listed there are several grades of free machining alloys containing lead (Pb) or selenium (Se). These free machining grades are hot-short and very sensitive to hot cracking. They are best avoided by the welder although they can be successfully joined by brazing or soldering.
- Pure copper with less than 0.7% residual elements
- High copper alloys with less than 5% alloying elements
- Copper alloys with up to 40% zinc (Zn) (brasses)
- Copper alloys with less than 10% tin (Sn) (bronzes)
- Copper alloys with less than 10% aluminium (Al) (aluminium bronzes often shortened to ally-bronze)
- Copper alloys with less than 3% silicon (Si) (silicon bronze)
- Copper alloys with less than 30% nickel (Ni) (cupro-nickel alloys)
- Copper alloys with less than 40%Zn and less than 18%Ni (nickel silvers)
- Copper alloys with less than 10%Sn and less than 4%Zn (red brass or gunmetal)
- Special alloys containing
- 0.1-1.5% cadmium (Cd)
- less than 2.7% beryllium (Be)
- 0.6-1.2% chromium (Cr)
- 0.1-0.2% zirconium (Zr).
This group of special alloys are capable of being precipitation hardened.
Copper alloys can be welded with most of the conventional welding processes although of the arc welding processes, gas shielded arc methods are the most common.
Pure copper alloys
There are three separate grades of pure copper: Oxygen-free copper with less than 0.02% oxygen; tough pitch copper that contains <0.1% of oxygen, present as copper oxide, and phosphorous (P) deoxidised copper with 0.05% P up to 0.05% arsenic (As). Oxygen-free copper has the highest electrical conductivity, P-deoxidised copper is the alloy most frequently used for pressure vessel and heat exchangers. Oxygen-free copper is the most readily weldable although porosity may be a problem if non-deoxidised filler metals are used.
The copper oxides in tough pitch copper can result in embrittlement of the heat affected zones due to oxide films forming on the grain boundaries. Weld metal porosity, even when using fully deoxidised filler metals, is also a major problem caused by the dissociation of the copper oxide, particulaly when hydrogen (H) is present.
Phosphorus deoxidised copper presents less of a porosity problem although weld metal porosity is still likely to be formed, particularly in autogenous welds. It is essential therefore that filler metals contain strong deoxidants, the commonest being silicon (Si) and manganese (Mn). Hydrogen control is also necessary so correctly baked low hydrogen electrodes are necessary when manual metal arc welding. Clean, grease-free wires and rods and high purity shield gases are required when TIG or MIG welding.
The two filler metals most often selected to weld the pure copper alloys are AWS A5.7 ERCu, the C7 of the now superceded BS 2901 Part 3 and ERCuSi-A, the old C9 of BS 2901. ERCu typically contains 0.4% of Si and Mn with 0.8% of Sn to aid fluidity; ERCuSi-A contains 1%Mn and 3%Si and is the preferred filler metal for tough pitch and P-deoxidised copper. BS 2901 Part 3 has been replaced by BS EN ISO 24373:2009 Welding consumables. Solid wires and rods for fusion welding of copper and copper alloys.
Shielding gases for welding are argon, helium and nitrogen or mixes of two or more of these. Pure argon may be used for TIG welding up to a thickness of some 2mm and for MIG welding up to approximately 5mm - above these thicknesses an argon-helium mixture will give better results with greater heat input and less risk of lack of fusion defects.
Nitrogen and argon-nitrogen gas mixes have been used in the past with some advantages being gained in terms of increased heat input from the high voltage nitrogen arc but such gases are not commercially available and argon-helium or helium shield gases are now the preferred choice. The high thermal conductivity of copper means that not only are high heat input shielding gases required as thickness increases, but preheat is necessary at section thicknesses exceeding 2mm. A very rough guide to recommended preheat and welding current levels is given in the table for TIG and MIG welding.
|Process ||Thickness (mm) ||Shielding Gas ||Preheat °C ||Welding Current (amps) |
|TIG || || || || |
| ||1.0 ||argon ||>10 ||20 - 60 |
| ||1.0 - 2.0 ||argon ||>10 ||50 - 160 |
| ||2.0 - 5.0 ||argon/75helium ||50 ||120 - 300 |
| ||6.0 - 10.0 ||argon/75helium ||100 - 200 ||250 - 375 |
| ||12.5 ||argon/75helium ||350 ||350 - 420 |
| ||15.0 ||argon/75helium ||400 - 450 ||400 - 470 |
|MIG || || || || |
| ||<5.0 ||argon ||10 - 100 ||175 - 240 |
| ||5.0 - 7.0 ||argon/75helium ||100 ||250 - 320 |
| ||10.0 - 12.5 ||argon/75helium ||200 - 300 ||300 - 400 |
| ||>16.0 ||argon/75helium ||350 - 450 ||350 - 600 |
When welding thick copper with preheats of over 250°C and welding currents of more than 350 amps then the health and safety of the welder and personnel working in the vicinity must be considered.
Lagging the item being welded with thermal blankets is essential as is the provision of adequate screening from the very powerful TIG or MIG arc. The welder should select a dense filter glass of at least shade 13 when using welding currents above 300 amps to reduce eye strain.
Typical butt weld preparations are:-
- up to 1.5mm thickness - square edge,no gap
- 1.5 to 3mm - square edge with 1.5mm gap
- 3 to 12mm single -V, included angle of 60° to 90°, feather edge and up to a 1.5mm gap
- 12mm to 25mm single V, included angle of 60 to 90°, 1.5 to 3mm root face, 1.5mm maximum gap
- Over 25mm thickness double V, included angles of 60 to 90°, 1.5 to 3mm root face, 1.5mm maximum gap
Carbon, stainless steel or ceramic tiles or tape can be used as temporary backing strips and are helpful in controlling root bead shape.
The Job Knowledge series is aimed at the welder and therefore tends to concentrate on the conventional arc welding processes. It is worth bearing in mind that electron beam and friction welding, including friction stir, have been used extensively and very successfully to weld thick section copper without the need for filler metals, high preheat temperatures and expensive shielding gases.
This article was written by Gene Mathers.
For more information, please contact us.