Copper and nickel are completely soluble in each other, giving rise to a range of alloys that includes both copper-nickel (Cu-Ni) and nickel-copper alloys, the latter alloys having been covered in the Job Knowledge articles numbers 107 and 108.
Although there is a wide range of alloys, only two are commercially significant. These are the 90/10 and 70/30 grades and the Table shows typical compositions and mechanical properties. Both grades have excellent corrosion resistance, particularly in sea water applications, and are used extensively in marine and offshore applications. The 70Cu/30Ni alloy is the stronger of the two with a yield strength in the annealed condition of ~150MPa compared with 120MPa of the lower nickel alloy. The 90Cu/10Ni grade however is probably the most used grade as it is less expensive than the higher nickel alloy.
The alloys are single phase and cannot be hardened by heat treatment. The only method of increasing tensile strength is by cold working which, when the metal is in the fully hard condition, can match that of good quality carbon steel. Work hardening, however, has implications with respect to welding in that there will be some strength loss in the HAZs. Fortunately this region is relatively narrow due to the low coefficient of thermal conductivity; approximately the same as steel. This narrow, low strength region can cause problems during welding procedure qualification testing of transverse bend coupons, most of the deformation being concentrated in the narrow area of strength loss. Bend testing is therefore generally carried out using a longitudinal bend specimen.
The other main alloying elements are manganese, around 1%, that is used as a deoxidant and desulphuriser, and up to 2% iron which is added to improve erosion resistance. Some of this iron, perhaps 1% or more, may be replaced with chromium to increase the strength. Niobium may also be added to castings to increase the strength and at the same time improve weldability
Due to a deficiency in deoxidants in the alloys, porosity is a problem and they cannot be welded autogenously. A highly deoxidised filler metal needs to be used although there is an exception to this rule; thin sheet containing substantial amounts of titanium. A very strong deoxidant, is now available and this is capable of being welded autogenously by TIG, plasma-TIG and the power beam processes without significant porosity problems
There are filler metals available that match both grades but it is generally 70Cu/30Ni filler that is used; AWS
A 5.6 ECuNi MMA electrodes and AWS A5.7 ERCuNi for TIG and MIG wires. The weld metal from these filler metals overmatches the strength of both grades in the annealed condition. Having a 0.2% proof of some 270MPa it has better handling characteristics than the 90Cu/10Ni filler and is noble with respect to the 90Cu/10Ni parent metal. The 90Cu/10Ni filler metals have a lower 0.2% proof of around 200MPa and should be used for welding 90Cu/10Ni alloys only.
The weld metal from both grades of filler metal is more sluggish than, say, carbon steel. Weld preparations therefore need to be more open to enable the welder to control and manipulate the weld pool. An included angle of 70 to 80O is recommended. Root face dimensions would typically be 0-1.5mm root face with a zero-1.5mm root gap.
As mentioned above, porosity when welding either grade can be a problem and to reduce the risk the filler metals contain substantial amounts (around 0.5%) titanium. Cleanliness of weld preparations and filler wires is also important, as is the use of high purity shielding gas. Weld preparations may need to have the tenacious oxide films removed by belt or disc sanding and should be thoroughly degreased with commercially available solvents. Stainless steel wire brushes and stainless steel wire wool are also useful.
This cleaning equipment must not be used on any other metals otherwise cross contamination will occur. Ideally the Cu/Ni fabrication area should also be physically separated from other fabrication areas to prevent dust from activities such as grinding settling on the cleaned weld preparations. One point worth noting is, if air powered tools are used for wire brushing or sanding, these may leave a film of moisture and/or oil on the surface (compressed air is seldom completely free of contaminants) and this may result in porosity and or cracking.
Depositing a pore-free root pass can be particularly difficult. Insufficient filler metal coupled with a large amount of dilution from the parent metal may result in unacceptable porosity. Copious amounts of filler metal and a larger than normal root gap (~2-3mm) will reduce porosity to acceptable levels.
Other causes of porosity may be associated with inadequate gas shielding. When TIG welding, use as large a diameter ceramic as possible, together with a gas lens. Arcs should be kept short; too long an arc length may permit atmospheric contamination.
Both the alloys are sensitive to hot cracking. As with the other nickel alloys the main culprit is sulphur but lead, phosphorus and carbon will also have and adverse effect. Cleanliness, as discussed above, is therefore crucial and all grease, oil, marker crayon, paint etc must be removed from the weld preparation and the adjacent areas before welding. To reduce further the risk of hot fissuring the interpass temperature should be limited to 150OC.
The alloys have high coefficients of thermal expansion and more extensive tack welding than would be required for a carbon steel is necessary to prevent excessive distortion and root gaps closing up during welding. Tacks should be wire brushed or ground to bright metal if they are to be incorporated in the completed weld.
TIG (GTAW) welding will give the best quality weld metal and a well shaped root bead. DC-ve current should be used. Pulsed current will give good control and a neat appearance when welding positionally.
As mentioned above, a large a ceramic shroud equipped with a gas lens is recommended to give the most effective gas shield and the arc length should be kept short; 3.5-4.5mm. Argon or argon with small amounts of hydrogen, (1- 5%) are the appropriate shield gases with the Ar/H mixtures providing higher heat input. Above about 6mm thickness, TIG welding is generally replaced by the higher deposition rate MIG process, although mechanised/automated systems such as orbital TIG are very cost effective. A root purge of argon is recommended when welding a TIG root run and the next couple of fill passes.
MIG (GMAW) welding is carried out using either pure argon or argon-helium mixtures; particularly useful on thicker sections. As with TIG, pulsed current will give better weld quality and appearance than dip transfer when welding out of the flat position. The filler wire is relatively soft and low friction liners are essential. Intermediate wire feeders may be required if the welding is taking place some distance from the wire drive unit. The filler wire pack should be opened at the last moment and should be adequately protected from contamination when installed in the wire feeder.
MMA (SMAW) welding electrodes are available, generally with a basic flux coating and designed to operate on DC+ve. Whilst these electrodes do not require baking before use, they may be dried at around 250OC if they have absorbed any moisture. Damp electrodes will result in weld metal porosity, as will a long arc. Weaving should be restricted to 3-4 times the electrode diameter.
Submerged arc welding (SAW) becomes cost effective over a thickness of about 12.5mm if the component can be manipulated to enable welding to take place in the flat position. Weld preparation would be similar to that used for MIG welding. MIG wires of up to 2.4mm in diameter may be used so welding currents need to be correspondingly low, 300-350amps. The choice of welding flux should be discussed with the consumable supplier as an incorrect choice can result in slag detachability problems.
Post weld heat treatment is not necessary but if dimensional stability is important the component may be stress relieved at 350-450OC.