Copper Alloy Forging: Techniques & Tips

Common copper alloys include brass and bronze, etc., which have good thermal conductivity, a small forging temperature range, are prone to overheating during heating, and have low deformation resistance.

1. Key points of heating operation

When copper alloys are heated, their plasticity changes with the increase in temperature. When the temperature rises to 200-600°C, the plasticity is at its lowest, known as the brittle zone.

Starting from 650°C, the plasticity increases rapidly, and the strength decreases; but when it exceeds 850°C, the grains grow rapidly, and the grain boundaries are also damaged, leading to a decrease in plasticity.

The initial forging temperature of copper alloys is relatively low, generally between 800-900°C, while the final forging temperature is between 650-700°C. The forging temperature range of various copper alloys is shown in Table 7-4.

 Forging temperature range of copper alloys

1) Copper alloys have good thermal conductivity, and the scrap can be directly loaded at the highest furnace temperature (50~100°C higher than the starting forging temperature).

2) It is best to use an electric resistance furnace for heating copper alloys, but a coal-fired heating furnace can also be used. The heating operation is simple; by reducing the amount of coal and air, a so-called “gentle flame” can be maintained in the furnace, while also retaining heat.

3) When heating copper alloys, some copper scraps and impurities always fall on the bottom of the furnace, causing the furnace bottom to be coated with molten copper. When heating steel later, copper penetrates into the grain boundaries of the steel and diffuses, reducing the strength of the steel and forming thermal brittleness.

To avoid this phenomenon, a thin iron sheet should be placed on the furnace bottom when heating steel. To prevent the high-temperature flame from directly spraying onto the copper material and causing local melting, thin iron sheets should also cover the other three sides.

4) The surface quality of copper ingots is poor, and peeling and cutting off the riser are required before heating.

2. Characteristics of Copper Alloy Forging

Due to the lower initial forging temperature of copper alloys and a small forging temperature range (only 150-200°C), the following points should be noted during forging:

1) The tools used for forging (such as punches, dies, drip pans, mandrels, and upper and lower snaps, etc.) should be preheated to 200-250°C.

2) The hammering should be light and quick, and the material on the anvil should be frequently turned over to reduce the heat taken away by the anvil surface.

3) Strictly control the final forging temperature. There is a risk of fracture when completing or performing auxiliary work such as straightening below 600°C. When punching or reaming, if the temperature of the punch is too low, cracks may appear around the edge of the hole due to the temperature drop. When cutting off the excess material, it should also be done within the forging temperature range, otherwise, the cut surface will show a coarse-grained brittle fracture phenomenon.

4) Copper alloys generally have good plasticity, allowing for larger deformations. However, they have less toughness than steel and are prone to folding under hammering. To prevent the removal of folds from causing insufficient machining allowance, the allowance and material calculation should be appropriately increased.

5) When forging copper alloy dies with free forging, the radius of the round corners at the shape transition should be increased and smoothed.

6) The brittleness phenomenon in copper alloys is mainly due to the enrichment of impurities such as lead, antimony, and bismuth in the copper alloy.