Blog / Copper-Bronze
28 April 2020 | Copper-Bronze

WELDING COPPER ALLOYS

General

Copper alloys are used for a wide range of applications. They can be found in architectural de corations, household appliances, the electronics, marine, and chemical industries, etc. Such popularity is due to their excellent electrical and thermal conductivity, high corrosion resistance, easy formability and good tensile strength and fatigue limit. Other characteristics, such as their spark resistance and distinctive color put them in a special family of alloys.

Classification

Copper is often alloyed with several types of metals. The most common alloy elements are aluminum, nickel, silicon, tin and zinc. Other elements are also added in small amounts to improve other characteristics like corrosion resistance or machinability. The various alloys are divided into the following nine major groups :

-pure copper (at least 99.3% purity);

-high copper content alloys containing up to 5% alloy elements;

-copper-zinc alloys (red brasses containing from 5 to 20% Zn, and yellow brasses containing from 20 to 40% Zn);

-copper – tin alloys (phosphor bronzes) containing up to 10% tin and 0.2% phosphorous;

-copper – aluminum alloys (aluminum bronzes) containing up to 13% aluminum;

-copper – silicon alloys (silicon bronzes) containing up to 3% silicon;

-copper – nickel alloys (cupro-nickel) containing up to 30% nickel;

-copper – zinc-nickel alloys (nickel silver) containing up to 27% Zn and 18% Ni;

-special alloys containing some specific alloy elements to enhance a given property, like the machinability.

Copper alloys cannot be significantly hardened by quenching. It is essentially the amount of cold reduction applied to copper alloys that makes them available in a wide range of hardness and tensile strengths. The table on the next page shows the different copper alloys wrought designations.

Sodel

Physical properties of copper alloys

Electrical and thermal conductivity

Copper is slightly less electrically conductive than silver, but at equal volumes it exceeds the electrical and thermal conductivity of aluminum by 50%. The percent conductivity of copper according to the IACS means the ratio between the conductivity of a given metal and the conductivity of pure (electrolytic) copper. Using special manufacturing processes it is possible to obtain coppers having a conductivity greater than that of pure copper : 102% IACS. Electrical and thermal conductivity vary greatly when alloy elements are added and can even be found with values similar to steel.

Melting point

The melting point of copper is 1981°F (1083°C) and copper alloys have melting points that vary between 1616°F (880°C) and 2264°F (1240°C).

Coefficient of thermal expansion

Pure copper and its alloys have a coefficient of thermal expansion that is about 50% higher than steel. However distortion is about the same as steel since their melting point is about half that of steel.

Weldability of copper alloys

Surface preparation

To minimize welding defects, all traces of oil, grease, hydrocarbons and contaminants should be removed, as well as the oxide film coating on the alloy. This film can be removed by milling, by a stainless steel wire brush or by chemical stripping.

Groove angles should be about 30% wider than for steel to allow better penetration.

Preheating

Copper and its alloys have a high oxygen affinity and a tendency to form porosities during welding. Another problem with welding copper and some of its alloys is their high thermal conductivity. To counter heat losses through thermal diffusion and to limit porosities, it is often necessary to preheat the parts to be joined. Preheating stabilizes the welding arc by making it less erratic and also promotes good penetration and allows oxygen or other gasses produced to escape from the weld pool.

However, alloys that are hardened by cold working lose most of their tensile strength when heated above 750°F (400°C). In addition, several alloys, including copper – tin and copper – nickel, are subject to hot-cracking and must therefore be preheated with moderation.

Preheat temperatures vary greatly depending on the nature of the alloy, the thickness of the work piece and the amount of restraint. They can reach 1000°F (540°C) in some cases, but are usually between 400 and 700°F (200 and 370°C). Consult Sodel Technical Service for the appropriate choice of temperature for your application.

Before preheating, it is very important to place the work pieces on insulating refractory material to reduce heat losses through metal-to-metal contact.

Welding method

Copper should preferably be welded flat due to its high fluidity. Sometimes, when the joint opening is too wide, a ceramic, copper or graphite support plate can be used.

When welding with a coated electrode it is important to be sure not to advance too quickly or weave too much so that the weld pool’s gas shielding is not altered. Improper gas shielding could result in copper oxide contamination leading to a loss of ductility and decreased joint fatigue limit. An oscillatory motion is sometimes beneficial to minimize slag inclusions, but the movement should not exceed three times the diameter of the electrode. In addition, it is better to pause at each side to fill in any undercuts that may form.

For brazing with flux coated rods (Sodel 47FC, Sodel 66FC, Sodel 660FC) please refer to section 8, brazing and fluxes)

For cold-worked alloys and alloys sensitive to hot-cracking, it is a good idea to peen the bead with a rounded tool while it is hot to reduce shrinkage stresses and to restore part of the tensile strength lost in preheating or welding.

Several elements used in manufacturing copper alloys have low sublimation points (temperature at which an element goes directly from the solid to the gaseous state). To prevent the loss of such elements during welding, the arc should be directed into the weld pool.

Slag cleaning

Slag on the deposit can be removed with hand tools and a stainless steel wire brush. When doing multipass welding, it is better to remove all traces of slag from the weld before starting a new pass.

Post heating

It is usually not necessary to postheat copper and its alloys except to release residual stresses or provide better dimensional stability for the assembly.

Filler metal

Sodel 661 is designed to support welding of most copper alloys and several combinations of dissimilar metals, including joining a copper alloy with cast iron or steel. Please consult Sodel Technical Service to determine the operating parameters with respect to the metals to be joined.

The flux coated rods Sodel 47FC, Sodel 66FC and Sodel 660FC are used for brazing. Please refer to Brazing and fluxes section, for more information.

In general, when joining two different metals, it is best to direct the arc toward the more conductive metal. In some cases, the faces of the groove should be buttered before the two parts are welded together to reduce shrinkage stresses.

PRACTICAL TIPS FOR WELDING COPPER ALLOYS

1-Weld in flat position whenever possible because of copper’s high fluidity.

2-Using a support plate will help keep the pieces separate for better penetration.

3-Good cleaning before and during welding will help to produce a quality weld.

4-If there is arc blow while welding dissimilar metals, increase the preheat temperature.

5-If the piece is held in place by a vise or other metal device, preheating such devices will help to maintain the temperature of the work piece when preheating is required.

6-Heat losses during preheating can be prevented by using a non-flammable, insulating material such as Sodelfix and by placing the work pieces on insulating refractory material.

7-When welding dissimilar metals, use the minimum welding currents recommended whenever possible to minimize dilution.

8-To join cast iron to copper alloys with Sodel 661, use small diameter electrodes and make small, discontinuous beads as described in the cast iron article.