The main alloying element in the brasses is zinc (Zn). There are three families; brass with zinc content less than 20%, high zinc alloys with 30-45% zinc and the nickel-silvers that contain 20-45% zinc and 20% nickel. These alloys are available as wrought or cast products, the low Zn alloys being used generally for jewellery and coins, the higher Zn alloys in applications where increased mechanical strength is required such as plumbing products, pump casings and thin wall low pressure vessels. Nickel silver, as the name suggests, is a less expensive alternative to silver (Ag) and is used for jewellery, coinage and cutlery. On an historical note, the panels of the 1907 Rolls Royce Silver Ghost are made from nickel silver, hence the name.
With the exception of brasses containing lead (Pb) all the brasses are weldable, the low zinc alloys being the easiest. The main problem with welding the alloys is weld metal porosity caused by the zinc boiling off during melting. Zinc melts at 420°C and boils at 910°C so brazing using an oxy-acetylene torch and a copper-silver filler is a possible alternative to welding, being capable of providing joints with adequate mechanical properties and without the porosity problems. Boiling the zinc may also result in large amounts of zinc oxide in the welding fume and this can be a health and safety issue. Brasses may be welded using MMA, MIG or TIG. Filler metals are available although these are generally based on copper-silicon or copper-tin alloys due to the problems of transferring zinc across the welding arc. A typical MIG/TIG filler metal would be the 3% silicon alloy specified in EN ISO 24373 SCu 6560 (CuSi3Mn1). Successful welds can also be made using copper-tin alloys such as Cu-7%Sn and Cu-12%Sn. These can be obtained as both MIG/TIG wires and as MMA electrodes.
The Cu-Si filler metal flows easily and a 60° included angle weld preparation should give acceptable results. The Cu-Sn weld metal is more sluggish and an included angle of at least 70° is advisable. The shielding gas used for MIG or TIG welding of thin section components is high purity argon. In thicker sections, over 5mm thick, the addition of helium will greatly assist in providing sufficient heat for full fusion as will the use of pulsed welding current. Brass, like copper, has a high coefficient of thermal conductivity. TIG welding is generally limited to joint thickness of around 10mm, MIG being the preferred process for thicker sections. Preheating to between 100 and 300°C, depending upon section thickness can be helpful in reducing zinc loss, particularly in the high zinc alloys, by enabling a lower welding current to be used, resulting in less melting of the parent metal.
There is a potential problem in service of stress corrosion, known as season cracking, in mildly corrosive media such as ammonia or sea water due to the residual stresses from welding. This can be largely dealt with by annealing the welded item at 260-300°C.
The next group of alloys is the bronzes. These may be alloyed with tin, generally described as phosphor bronze, silicon or aluminium. Many of these alloys, like the brasses, are alloyed with lead to improve machinability. These leaded alloys are generally regarded as unweldable and specialist advice should be sought if the need arises.
Phosphor bronze alloys contain between 1 and 12% tin with a small amount (0.01-0.1%) of phosphorus (P) when this is used solely as a deoxidising agent. True phosphor-bronzes contain at least 0.1%P and as much as 1.0%P in some of the cast phosphor bronzes.
The alloys are corrosion resistant and have excellent wear characteristics so they are used for valves, bearings and machine parts. From a weldability point of view the main problem is that the alloys are sensitive to hot cracking and the lower P content alloys are also prone to form oxide films on the weld pool. High welding heat inputs, high preheat and slow cooling rates should therefore be avoided. MIG and TIG welding are the preferred welding processes with argon or helium-argon mixtures. MIG is more suitable than TIG for welding heavier section joints and positional welding is best achieved using pulsed current. Filler metals matching the composition of the parent metal, e.g. EN ISO 24373 CuSn6P, are available. Although MMA welding consumables are available the process is not widely used. A stringer bead welding technique is generally necessary and heavy sections require preheat and interpass temperatures of around 200°C.
Silicon bronzes are probably the easiest of all the bronzes to weld. They contain between 1.0 and 4.0% silicon with small amounts, less than 1.5% in total, of zinc, manganese and/or iron. They have good strength and excellent corrosion resistant properties and are frequently used for heat exchanger tubing, marine hardware and in chemical process plant applications.
Unlike many of the other copper alloys thermal conductivity is relatively low and this makes it possible to use high welding speeds and to dispense with preheat for the thicker joints. One undesirable characteristic, however, is that the silicon tends to form an oxide film on the weld pool surface that requires vigorous wire brushing of individual weld passes during multi-run welding. There is also a slight tendency to hot shortness at elevated temperatures. It is advisable to stress relieve or anneal components prior to welding and to cool rapidly through the 1000-850°C temperature range.
As with the other bronzes, MIG or TIG welding are the processes of choice using pure argon as the shield gas and consumables that match the parent metal composition, e.g. EN ISO 24373 CuSi3Mn1. Low thermal conductivity means that helium mixes are not necessary and the TIG process can be used for welding components up to 25mm thickness at welding currents of 300amps. However, it should be noted that the weld pool size should be restricted to provide a fast cooling rate.
The last alloy in this series is aluminium-bronze. This family of alloys have compositions between 3 and 15% of aluminium with additions of iron, manganese and nickel. The alloys with less than 8%Al are single phase; those with more than 9%Al are two phase and capable of being quench hardened to give a martensitic micro-structure. All the alloys have excellent corrosion resistance, particularly in marine environments, and are used for pump bodies, valves, bearings and ships propellers.
The characteristic that gives the alloy its corrosion resistance is the strong tenacious aluminium oxide film that forms on the surface. This causes problems of oxide film entrapment and lack of fusion during welding and must be removed. Scraping and wire brushing the surfaces before welding is necessary. With respect to the welding processes, IG and TIG are preferred. With MIG there is no problem in dispersing the oxide film, the DC+ve current breaking up and dispersing the film. DC-ve TIG welding does not provide this cleaning action and it is necessary to use AC-TIG. Inverter-based square wave TIG power sources will give the best control. Argon is the recommended shield gas although a helium/argon mixture may be useful when welding very thick section joints with the MIG process. MMA welding is possible although the fluxes required to remove the oxide film are very aggressive and may cause corrosion problems if not completely removed before the item enters service.
Aluminium bronzes with less than 8% aluminium are prone to hot cracking at temperatures around 700°C and care needs to be taken to reduce residual stresses as much as possible by ensuring accurate fit-up and minimal root gaps. Low heat input procedures should be used and interpass temperature limited to 150°C. These alloys do not require preheat. A filler metal with around 8 to 10% aluminium such as EN ISO 24373 CuAl10Fe1 or AWS A5.7 CuAl-A2 is the best choice as this composition is relatively resistant to hot cracking.
The two phase alloys, i.e. those with more than approximately 9%Al, have very high tensile strengths although the very highly alloyed suffer from a substantial loss of ductility. All the alloys are, however, readily weldable and relatively insensitive to hot cracking. Heat input control is therefore less important although a maximum interpass temperature of 250°C is recommended and a preheat of 150°C may be used when MIG welding thick section joints. AWS A5.7 ER CuAl-A2 (EN ISO 24373 CuAl10Fe1) or, for higher strength, ER CuAl-A3 (EN ISO 24373 CuAl11Fe3) are readily available MIG/TIG filler metals.
Post weld heat treatment is rarely necessary but can be of benefit if the welded item is to experience very corrosive conditions. In this case a stress relief operation at 300-350°C may be beneficial, although precise temperatures and times will depend upon the specific alloy composition, thickness etc. It is possible for the high aluminium duplex alloys to be quenched from 950°C and tempered at 650°C to restore full corrosion resistance but this is rarely done due to cost and distortion issues.
This article was written by Gene Mathers.