How to Weld Bronze
Welding bronze is a comparatively easier process than welding brass because tin does not have the tendency to separate from copper in the molten state during welding; whereas zinc does so in brass.
Bronzes are best welded with 100% argon shielding gas.
Aluminum bronzes (with Al content from 8 to 13.5%) enjoy good to excellent weldability by TIG, MIG, and SMAW processes. Oxy-acetylene welding is possible but not recommended because of aluminum oxide formation on the base metal during welding.
Aluminum bronzes containing relatively high percentages of aluminum, such as those specified for drawing and forming dies, require much greater preheat to avoid setting up severe stress differentials caused by the material’s low ductility and low thermal transfer rates.
Phosphor bronzes have a tendency toward hot shortness and formation of oxides when heated so widespread heating and slow cooling are likely to cause oxides and porosity in the weld metal.
Leaded phosphor bronzes, containing lead and zinc, can rarely be fusion welded without incurring excessive deposit porosity. Low-temperature soldering or brazing processes are better choices.
Silicon bronzes are commonly used for MIG and TIG brazing because of their aesthetically appealing gold finish, corrosion-resistant properties, and ability to weld dissimilar metals. Brazing means that the base metal is not melted during the welding process, but the filler metal is because it melts at a lower temperature. Brazing is not technically welding.
The low melting point of silicon bronze makes it a good choice to braze galvanized steel or thin sheet metal where warping can be avoided due to the lower heat input.
TIG Welding Bronze
TIG welding is commonly used to weld phosphor bronze for joining strips and other forms of wrought phosphor bronze up to about 12.5 mm thick and to make minor repairs in castings and rebuild worn surfaces.
Thoriated tungsten electrodes and filler rods with the following components are used for TIG welding phosphor bronze:
- 4 to 6% Sn, 0.10-0.35% P, balance Cu
- 7 to 9% Sn, 0.05-0.35% P, balance Cu
TIG Welding is used to weld aluminum bronze on up to 9 mm thick plates. It is used for the repair of castings also.
Zirconated/unalloyed tungsten electrodes and filler rods with the following composition are typically used for TIG welding aluminum bronze:
- 9-11% Al, 1.5% Fe, balance Cu
MIG Welding Bronze
MIG process is preferred for bronze when welding large fabrications and heavy sections because higher welding speeds and dense deposits can be obtained.
For welding phosphor bronzes with less than 8% Sn and with 8 to 10% Sn, filler wire of the following components are used respectively:
- 4 to 6% Sn, 0.10 to 0.35% P, balance copper
- 7 to 9% Sn, 0.05 to 0.35% P, balance copper
Filler wire to MIG welding aluminum bronze typically has the composition: 6-9 Al, others 0.5, Balance Cu
Welding can be done in all positions. Spray transfer is used for welding heavy sections in the downwards position. The short-circuiting and pulse arc processes can be used to weld in the vertical and overhead positions using electrodes of diameter up to 1.5 mm. Arc length should be tilted 20 to 30° in the direction of travel.
Stick Welding Bronze
Stick welding bronze is carried out, generally, using DCEP, however, AC electrodes can also be used. Covered electrodes of the same compositions as described in MIG welding are employed. A typical electrode composition for aluminum bronze is 10.25-11.75% Al, 3-4.25% Fe and the balance copper.
Weld deposits possess very good hot strength and hot ductility. To obtain fine grain structure in the weld deposit and maximum mechanical properties, stringer beads should be used. A short arc and stringer (or moderate weave) beads are recommended for groove welding.
A preheat and interpass temperature of 205°C for low aluminum grades in heavy sections and that of 370 to 427°C for higher aluminum grades (Rockwell B 90 or over) should be used to avoid cracking of the weld deposit.
Oxyfuel Welding Bronze
Bronzes are alloys of copper and tin, with small quantities of other metals added to give color or provide special mechanical properties. In bronzes having relatively large amounts of tin and lead, i.e. phosphor bronze or gunmetal, “boiling” starts when the metal has reached about a red heat.
This indicates that a neutral flame must be used for both preheating and welding. The flame adjustment may be varied to slightly reducing conditions until the oxide film almost disappears and the surface of the metal is left bright.
Alloys that have a large amount of lead (5 percent or more) may give trouble because of excessive formation of lead oxide; this difficulty can be overcome by abundant use of flux applied as a paste to the welding rod.
The reducing portion of the flame should be used for welding, i.e. the blue cone should be held approximately 1/2-2 in. clear of the molten metal.
Bronzes containing fairly high percentages of nickel are whitish in color, and are known as silver or nickel bronzes; these should be welded with a flame even more violently oxidizing than that previously described for brass. Manganese bronzes, however, require only a slightly oxidizing flame.
Make the flux paste, and paint it on the top and underside of the edges, and on the welding rod. Alternatively, flux can be applied by dipping both the plates and the welding rod in the water, and sprinkling them with flux before welding.
Preheat the metal to a dark red color (with the blowpipe) before commencing to weld. Hold the cone of the flame fairly close to the surface of the weld. Maintain the welding rod in the molten puddle, and do not lift the rod or blowpipe until the weld is finished. The flux should be applied to the rod before starting the weld; this avoids the necessity of removing the rod for further application of flux.
Confine the heat to the narrowest possible zone by placing carbon blocks, asbestos, firebricks or copper chill bars on each side of the weld, and both on the top and undersides.
It may be necessary to use a slightly larger flame if copper chill bars are employed, in order to counteract the extra heat absorption. On long seams, the welding rate should increase as the weld progresses. In order to obtain a weld that matches the color of the metal being welded, use a welding rod of the same metal (composition) along with a bronze welding flux.