This article introduces copper, brass and bronze and their properties pertinent to their weldability. In the latter stages of article, aspects related to welding of copper, brass and bronze have been discussed in some detail.
Copper
Copper has a reddish color, see the figure below. It has high electrical and thermal conductivity. Because of its’ excellent ability to conduct electricity, it is extensively used in making of electrical wires. Besides this, it also has excellent ductility and malleability. Copper and its’ alloys find wide application in our day-to-day lives. Welding techniques have been developed to obtain satisfactory welding of copper alloys.
Copper can be forged, cast, and cold worked. The weldability is also good for copper alloys. However, machinability of copper is not so good.
Due to its’ high ductility, it is difficult to do machining on copper, in spite of the fact that it is a soft metal. Alloying elements are added to improve the shortcomings of pure copper. For example, 1.5% to 2.75% beryllium is added to copper. The alloy thus formed – beryllium copper is ductile when soft. But it gains hardness and becomes amenable when hardened.
Likewise, in nickel-copper alloy, the amount of nickel varies from 10% to 30%. These alloys have a higher strength than pure copper. The strength increases with increasing percentage of nickel. These alloys also have good toughness, ductility, and moderate hardness.
These alloys also have good corrosion resistance in sea-water applications, and good corrosion fatigue resistance.
Nickel is also added to copper zinc to lighten the alloy’s color. The resulting alloy is called copper silver. Copper silvers are of two types: one, in which the combined percentage of copper and nickel is at least 65%. In the second type, the combined percentage of copper and nickel does not exceed 55 to 60%.
Uses And Limitations
As discussed above, copper find extensive usage in electrical industry due to its’ excellent electrical conductivity. It is also used as one of the elements in alloys such as brass, monel, and bronze. Typically, copper and its’ alloys are used in manufacture of statues, electrical wires, cartridge cases, sheet metal roofings, bushings, bearings, etc.
One of the limitations of copper alloys is that, the Electrolytic tough pitch copper, which is one of the main copper alloys, cannot be welded satisfactorily. Pure copper is not amenable to welding too, and is difficult to machine too due to high ductility.
Properties Of Copper
Pure copper is a non-magnetic material. It has a tensile strength of 32 ksi to 60 ksi (220 Mpa to 413 Mpa), and a hardness of 60 to 185 BHN (Brinell hardness number). The strength improves on addition of alloying elements, as does the hardness. The strength of copper alloys varies between 50 ksi to 90 ksi (344 MPa to 620 Mpa). The copper alloys have a hardness of 100 to 185 BHN (brinell hardness number).
Copper has a melting point of around 1985°F (1085°C), and a density of 8.96 gm/cm3 (559 lb/inch3). Noticeable, this melting point is substantially lesser than that of steel.
Copper gives off no sparks when held against a grinding wheel. It is red in color and becomes greenish when oxidized. When fractured, a smooth surface can be seen.
Due to its’ high thermal conductivity, it conducts heat at a rapid pace when subjected to torch test. Due to this reason, a larger amount of heat in comparison to steel is required to produce the same size of puddle.
Brass And Bronze
Brass is an alloy of copper and zinc. It contains about 60 to 68% of copper and 32 to 40% of zinc. Like copper, brass has a low melting point and high thermal conductivity. Several varieties of brass can be obtained by varying the proportion of copper and zinc. Alloying elements such as lead, manganese, tin are added to enhance machinability and weldability.
Bronze is an alloy of copper and tin. It also has small amounts of other alloying elements such as lead, zinc, manganese, nickel, etc. which improve its’ mechanical properties. Bronze has high tensile strength. It also has good corrosion resistance, and good weldability and machinability.
Like most other non-ferrous metals, brass and bronze give off no sparks in spark test. Due to high zinc content, brass gives off white fumes in torch test. If zinc is present as one of the alloying elements, bronze can also give off fumes in torch test. But these fumes are not as much as what we get when brass is tested.
Bronze has high amount of tin. Due to this, the fluidity of the metal is high. This can be observed during torch test on bronze.
Oxyacetylene Welding Of Copper
Copper has excellent thermal conductivity, meaning that it conducts heat applied by a welding torch to other parts of the work-piece very fast. Due to this reason, although its’ melting point is well below that of steel, a larger welding tip is required.
Another characteristic of copper is that is highly reactive. Copper oxide forms rapidly during welding if proper care is not taken. The formation of this oxide is not apparent to the welder. But the formation of oxides makes the weld brittle.
When copper is cold, it is quite ductile. However when it is hot it is brittle. So high input must not be used. Doing so can produce a fracture near the weld zone or a little away from it. The temperature of the adjoining base metal is only a little less than the weld zone due to high thermal conductivity.
In order to get a quality weld, cleanliness of the base metal surface is necessary. Some preheating should be done to prevent contraction cracks, or strain in the metal, that occurs owing to high thermal conductivity of the metal. The right type of oxyacetylene flame is important as well, to get a quality weld. The acetylene cone must not come into contact with the metal surface at any time.
Oxy-Acetylene Welding of Brass & Bronze
Brass and bronze are both alloys of copper, with zinc and tin as alloying elements. The alloying elements have melting point considerably lower than copper. So, proper care should be taken not to change the characteristics of the metal by burning out the alloys.
Like other non-ferrous metal, the acetylene cone must not touch the metal surface. The end of the cone should be a little above the metal surface.
Further reading: Would like to know some basics of oxyacetylene welding in general? There is an article on this website that explains that. Click here to access the same.
You said right that high input causes fracture in the weld area while welding copper. I once did that mistake and made the project total failure. Though I could fix that, it would be less strong and fragile, so I didn’t.
By the by, thanks for the information.