Non-ferrous metals

Alloys and welding procedures

  There are two coppers that are mainly used for electrical transmission lines, tough pitch copper and oxygen-free high-conductivity copper. Arc welds in these materials are not very satisfactory; usually they contain defects and at best have undesirably high electrical resistance due to the use of deoxidized filler material, although in this respect boron-deoxidized wire produces welds of higher conductivity than other types. Cold pressure welding is applicable to rod and is used to make cold joints in electrical conductors. The process generally used for joining copper in electrical conductors is thermit welding, using a mixture of aluminium and copper oxide for the exothermic reaction. The mixture is held in a graphite crucible, with a steel disc covering the exit hole. It is ignited and in a very short time a pool of molten copper is formed, which melts the retaining disc and flows out into a mould surrounding the joint. The liquid copper has sufficient superheat to melt the joint faces and produce a sound weld. A collar of metal is left around the joint for strength and to ensure good electrical conductivity.

Phosphorus-deoxidized copper is the standard material for welded sheet and plate applications in copper. The copper–1.5% zinc alloy (cap copper) has been employed for domestic hot water tanks, which are welded by the gas tungsten arc process without filler wire. This material gives sound ductile joints when so welded. Silicon-bronze may also be welded by the inert-gas processes without special additions, and gives sound ductile joints of mechanical strength equal to the parent metal. Resistance welding (and of course soldering) finds substantial application in sheet metal products made from brass.

  Aluminium bronze is one of the more difficult materials to fabricate and weld owing to its susceptibility to hot cracking. The type most frequently specified is the single-phase Cu–7A1–2.5Fe alloy. Except for single-pass welds in thin material, the use of a matching filler material for welding this type of aluminium bronze is impracticable. Even if fissuring of the weld can be avoided, multi-pass joints may suffer embrittlement due to heat treatment of the weld deposit by subsequent runs. A duplex filler material containing about 10% aluminium is, however, virtually free from any tendency to crack. A composition that has been successfully used for inert gas welding is nominally 10% aluminium, 2.5 % iron and 5.5 % nickel. Duplex weld deposits may be subject to dealuminification in corrosive service, and this risk may be reduced by applying a single-phase capping run to the weld.

Although weld-metal cracking may be overcome by the proper choice of filler alloy, 93Cu–7Al plate material may sometimes crack during hot forming or welding. In welding, these cracks may extend for some distance away from the weld boundary. Intergranular cracking has also been observed close to the weld boundary. Such cracking is due to a deficiency of the plate material, the nature of which has not yet been explained.

  Cracking of the weld metal appears to be associated with phase constitution in a manner analogous to that which occurs in fully austenitic steel: the single-phase alloy is subject to cracking, while two-phase alloys are not. A further similarity is that both materials have a narrow freezing range, so that cracking due to a wide freezing range inherent in the constitution of the alloy (as, for example, in aluminium alloys) is not possible. If cracking occurs at high temperature, it is probably due to the formation of liquid intergranular films of low melting constituents. Alternatively, cracking may take place at lower temperatures owing to the formation of brittle intergranular constituents.

  Cupronickel is used mainly in sheet form for fabricated work, and the most suitable welding process is gas tungsten arc welding. Cracking is not a serious problem, and porosity is minimized by using a filler rod containing deoxidant. Cupronickel alloys vary in nickel content, typical compositions containing 5 %, 10 %, 20 % or 30 % nickel. If a matching filler is not available, the deoxidized 70/30 composition may be used. A suitable deoxidant is titanium.

  Silicon bronzes may exhibit hot shortness at temperatures between 800 and 950 °C, and under conditions of restraint this may result in cracking of fusion welds. In general, however, these alloys present few welding difficulties; they have relatively low thermal conductivity (54.4 W m− 1 K− 1) and are not subject to porosity in fusion welding. Silicon bronze may be welded using all the major welding processes. Generally, the speed of welding is high, and preheating rarely necessary, while the energy requirements for resistance welding are much lower than for other copper alloys: a reflection of their higher electrical resistance compared with copper.

  From J.F. Lancaster, in Metallurgy of Welding (Sixth Edition), 1999