Welding Copper and Copper Alloys: Part Three

요약:

This article describes welding procedures, precautions and post-welding treatment copper-tin alloys, phosphor bronzes, Gunmetals, aluminum and silicon bronzes, cupro-nickels, and nickel silver.

Copper-Tin Alloys, Phosphor Bronzes and Gunmetals

Copper-tin alloys generally contain between 1 and 10% tin and are available in the wrought and cast forms. These alloys are susceptible to hot cracking in the stressed condition. The use of high preheat temperatures, high heat input, and slow cooling rates should be avoided. Examples of specific applications include bridge bearings and expansion plates and fittings, fasteners, chemical hardware and textile machinery.

Wrought alloys contain up to 8% tin with a residual phosphorus content of up to 0.4%. Cast phosphor bronzes contain at least 10% tin with lead additions to promote free machining and pressure tightness.

Gunmetals are essentially zinc-containing tin bronzes, again often with the addition of lead. Gunmetals are rarely fusion welded because the presence of zinc and lead causes welding difficulties. There is, however, some call for the weld repair of gunmetal castings for which gas-shielded arc welding techniques have been developed.

For the limited number of applications where wrought phosphor bronzes require welding, satisfactory results have been obtained with phosphor bronze filler metals to BS 2901, but complete freedom from weld metal porosity is normally only attained using non-matching filler metals containing more powerful deoxidents, such as those developed for copper and the aluminum bronzes.

Aluminum bronzes

Aluminum bronzes contain from 3-15% aluminum with substantial additions of iron, nickel and manganese. Common applications for aluminum bronzes include pumps, valves, other water fittings and bearings for use in marine and other aggressive environments.

Welding affects the metallurgical structure of these alloys, for instance, an alloy containing 6-8% aluminum, 2-2.3% iron, which is commonly used in heat exchanger plants, may suffer from embrittlement at the root in multi-run welds with matching filler metal or in autogenous welding. This embrittlement is caused by decomposition of retained phase to brittle during the reheating of weld metal which has been rapidly cooled in the root run.

In common with many copper alloys, aluminum bronzes exhibit a drop in ductility in a temperature range particularly critical during welding, and this phenomenon can make it difficult to obtain successful welds in these alloys. It is partly because of the potential welding problems that interest has been aroused in aluminum-silicon bronze alloys containing approximately 6% aluminum and 2% silicon, which seem as a promising alternative both for parent metal and filler metal.

The series of copper-manganese-aluminum alloys, which are essentially casting alloys, also needs to be considered under the general aluminum bronze heading. These alloys contain up to about 9% aluminum, 12% manganese, with additions of iron and nickel, have good weldability and do not suffer from intermediate temperature brittleness as normal aluminum bronzes.

They do nevertheless require heat treatment after welding to restore the mechanical properties and corrosion resistance. High-quality welds in aluminum bronze are currently produced commercially only by the gas shielded arc welding processes. In TIG welding with argon shielding, a.c. working facilitates the removal of the refractory oxide films from the weld pool, but greater success is achieved using direct current electrode negative working in helium.

Copper-Silicon Alloys (Silicon Bronzes)

Available in both wrought and cast forms, silicon bronzes are industrially important due to their high strength, excellent corrosion resistance, and good weldability. The addition of silicon to copper increases tensile strength, hardness and work hardening rates.

Silicon bronzes have excellent mechanical properties, comparable to mild steel, have good fatigue and corrosion fatigue properties and a thermal diffusivity similar to that of mild steel. There is a tendency to hot shortness in the temperature range 800-950°C and, after welding, cooling through this range should be as rapid as possible. But, too rapid a cooling rate may result in the formation of a metastable phase which is somewhat brittle and which, under conditions of restraint, may cause weld metal cracking.

However, entirely satisfactory welds can be made in silicon bronze if welding conditions are suitably adjusted. In TIG welding with argon shielding, a.c. working facilitates the removal of the refractory oxide film from the weld pool, but at the expense of arc stability. Direct current electrode negative working is for this reason normally preferred, particularly if helium shielding can be used.

Copper-Nickel Alloys (Cupro-Nickels)

The cupronickel alloys containing 10-30% nickel have moderate strength provided by the nickel which also improves the oxidation and corrosion resistance of copper. These alloys have good hot and cold formability and are produced as flat products, pipe, rod, tube and forgings. Common applications include plates and tubes for evaporators, condensers and heat exchangers. Besides nickel, alloys under this heading can have iron or manganese added, primarily to improve corrosion and impingement resistance in certain environments.

Although susceptible to hot cracking where trace impurities such as lead, phosphorus and sulphur are present, the quality of present-day commercial alloys is such that cracking due to intergranular impurity films is not generally experienced. The alloys are particularly susceptible to oxygen and hydrogen contamination from the atmosphere, leading to weld metal porosity.

Precautions should be taken during welding to ensure that the shielding gas flow is sufficient to protect the weld area, and additional protection of the underside of the weld by inert gas may be desirable but not always essential. In all cases it is necessary to use filler metals which have been developed for the gas-shielded arc welding of cupro-nickels, namely, those containing titanium as the major deoxidant.

The TIG and MIG processes are extensively used and have proved highly successful for all welding applications for which they have been designed, including the production of high quality welds in pipe work for which the plasma arc process has proved satisfactory.

Copper-Nickel-Zinc Alloys (Nickel silver)

Nickel silver alloys contain zinc in the range of 17 to 27% along with 8 to 18% nickel. The addition of nickel makes these alloys silver in appearance and also increases their strength and corrosion resistance, although some are subject to dezincification and they can be susceptible to stress corrosion cracking. Specific applications include hardware, fasteners, optical and camera parts, etching stock and hollowware.

The nickel silvers, which are essentially brasses to which various additions of nickel have been added, with or without a lead addition, are seldom fusion welded, brazing being the preferred technique, but if welding is required the comments given on brasses apply.

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