Bulk Deformation Processes: Cold Forging

BULK DEFORMATION PROCESSES

1. COLD FORGING

Cold forging is an impact forming process that plastically deforms a piece of raw material, under high compressive force, between a punch and a die within suitable equipment such as a machine press. Cold forging deforms metal while it is below its recrystallization point. Cold forging process takes place near room temperature, rather than at higher temperatures like warm and hot forging. It is done by placing the workpiece in-between two dies, and pounding the dies until the metal assumes their shape. Because of the friction created by the process, the temperature of the metal being forged can actually rise to 250° C or 482° F. The working principle and operation is similar to the Press forging explained in Chapter 3.3.9 on Page 3.12.

The process produces greater dimensional accuracy than hot forming, and does not produce scale. However, the plastic flow characteristics of the workpiece are not as good, so that higher forging pressures are required. Component size is generally limited to 25 kg or less. The majority of cold forgings weigh less than 5 kg.

Cold forging increases tensile strength some and yield strength substantially while reducing ductility. The most common metals in cold forging applications are usually standard or carbon alloy steels. Cold forging is typically a closed-die process.

Cold forging is generally preferred when the metal is already a soft metal, like aluminum. This process is usually less expensive than hot forging and the end product requires little, if any, finishing work. Sometimes, when metal is cold forged into a desired shape, it is heat treated after to remove residual surface stress.

Applications of cold forging:

1. Cold forging is being used in a wide variety of industries including fastener, automotive, pole-line hardware, truck-trailers, outboard engine controls, bicycle pedal cranks, constant velocity joints, universal joint crosses, and military projectile hardware.

2. Shapes generally have been limited to rotationally symmetrical and axisymmetric, including long shafts and struts.

3. It is used to weld titanium, zirconium and beryllium metals and its alloy.

Advantages of cold forging:

1. It is often more economical than other processes, as the final product doesn't require much finishing if any at all.

2. The dies used in cold forging last longer than in hot forging which reduces die cost. 

3. Yield and tensile strength of the cold forged metals increase greatly.

4. It is easier to impart directional properties.

5. Improved reproducibility is possible.

6. Increased dimensional control is achieved.

Limitations of cold forging:

1. The metal surfaces must be clean and free of scale before forging occurs.

2. The ductility of metal reduces.

3. Residual stresses may occur.

4. Heavier and more powerful equipment is needed.

5. Stronger tooling is required.

6. This process is limited to create certain shapes.