Welding Copper-Base Alloys

Copper and copper-base alloys have specific properties which make them widely used. Their high electrical conductivity makes them widely used in the electrical industries and corrosion resistance of certain alloys makes them very useful in the process industries. Copper alloys are also widely used for friction or bearing applications.
Copper shares some of the characteristics of aluminum. Attention should be given to its properties that make the welding of copper and copper alloys different from the welding of carbon steels.

Copper alloys possess properties that require special attention when welding. These are:

High thermal conductivity.
High thermal expansion coefficient.
Relatively low melting point.
It is hot short, i.e., brittle at elevated temperatures.
The molten metal is very fluid.
It has high electrical conductivity.
It owes much of its strength to cold working.

Copper has the highest thermal conductivity of all commercial metals and the comments made concerning thermal conductivity of aluminum apply to copper, to an even greater degree.
Copper has a relatively high coefficient of thermal expansion, approximately 50% higher than carbon steel, but lower than aluminum. One of the problems associated with copper alloys is the fact that some of them, such as aluminum bronze, have a coefficient of expansion over 50% greater than that of copper. This creates problems when making generalized statements about the different copper-based alloys.

The melting point of the different copper alloys varies over a relatively wide range, but is at least 538oC lower than carbon steel. Some of the copper alloys are hot short. This means that they become brittle at high temperatures. This is because some of the alloying elements form oxides and other compounds at the grain boundaries, embrittling the material.

Copper does not exhibit heat colors like steel and when it melts it is relatively fluid. This is essentially the result of the high preheat normally used for heavier sections. Copper has the highest electrical conductivity of any of the commercial metals and this is a definite problem in the resistance welding processes.

All of the copper alloys derive their strength from cold working. The heat of welding will anneal the copper in the heat-affected area adjacent to the weld and reduce the strength provided by cold working. This must be considered when welding high-strength joints.

There is one other problem associated with the copper alloys that contain zinc. Zinc has a relatively low boiling temperature, and under the heat of an arc will tend to vaporize and escape from the weld. For this reason the arc processes are not recommended for the alloys containing zinc.

Welding Magnesium - Base Alloys
Magnesium is the lightest structural metal. It is approximately two-thirds as heavy as aluminum and one-fourth as heavy as steel. Magnesium alloys containing small amounts of aluminum, manganese, zinc, zirconium, etc., have strengths equaling that of mild steels. They can be rolled into plate, shapes, and strip.
Magnesium can be cast, forged, fabricated, and machined. As a structural metal it is used in aircraft. It is used by the materials-moving industry for parts of machinery and for hand-power tools due to its strength to weight ratio.

Magnesium can be welded by many of the arc and resistance welding processes, as well as by the oxy-fuel gas welding process, and it can be brazed. Magnesium possesses properties that make welding it different than the welding of steels. Many of these are the same as for aluminum. These are:

Magnesium oxide surface coating
High thermal conductivity
Relatively high thermal expansion coefficient
Relatively low melting temperature
The absence of color change as temperature approaches the melting point.
The normal metallurgical factors that apply to other metals apply to magnesium as well. Magnesium is a very active metal and the rate of oxidation increases as the temperature is increased. The melting point of magnesium is very close to that of aluminum, but the melting point of the oxide is very high. In view of this, the oxide coating must be removed.
Magnesium has high thermal heat conductivity and a high coefficient of thermal expansion. The thermal conductivity is not as high as aluminum but the coefficient of thermal expansion is very nearly the same. The absence of color change is not too important with respect to the arc welding processes.