Module 8 d1

Metal forming

Principle of the process

Structure

Process modeling

Defects, Pros and Cons

Design For Manufacturing (DFM)

Process variation

Module 8

Cold, warm, hot

2

Cold Working

Performed at room temperature or slightly above Many cold forming processes are used in mass

production operations Minimum or no machining usually required

– These operations are near net shape or net shape processes

Advantages of Cold Forming

Better accuracy, closer tolerances Better surface finish Strain hardening increases strength and hardness Grain flow during deformation can cause

desirable directional properties in product No heating of work required

Disadvantages of Cold Forming

Higher forces and power required for deformation Surfaces of starting work must be free of scale and dirt Decreased ductility and increased limit

the amount of forming that can be done

– In some cases, metal must be annealed before further deformation can be accomplished

– In other cases, metal is simply not ductile enough to be cold worked

Warm Working

Performed at temperatures above room temperature, but below temperature

Occurs in the range from 0.3Tm to 0.5Tm, where Tm = melting point (absolute temperature) for metal

Advantages & Disadvantages of Warm Working

Advantages1. Lower forces and power than in cold working

2. More intricate work geometries possible

3. Need for annealing may be reduced or eliminated

Disadvantage1. Work piece must be heated

Hot Working

Deformation at temperatures above the recrystallization temperature

Occurs in the range from 0.5Tm to 0.75Tm

Capability for substantial plastic deformation - far more than is possible with cold working or warm working. This is because

1. Strength coefficient (K) is substantially less than at room temperature

2. Strain hardening exponent (n) is zero (theoretically)

3. Ductility is significantly increased

Advantages of Hot Working

1. Work part shape can be significantly altered

2. Lower forces and power required

3. Metals that usually fracture in cold working can be hot formed

4. No strengthening of part occurs from work hardening – Advantageous in cases when part is to be

subsequently processed by cold forming

Disadvantages of Hot Working

1. Lower dimensional accuracy

2. Higher thermal energy needed to heat the work piece

3. Work surface oxidation (formation of scale)

4. Thus, poorer surface finish

5. Higher temperature decreases tool life of dies and rolls

Module 8 d11

Metal forming:

Rolling

Principle of the process

Structure

Process modeling

Defects, Pros and Cons

Design For Manufacturing (DFM)

Process variation

Rolling Mill Configurations

(a) Two-high, (b) three-high, (c) four-high

Rolling Mill Configurations

(d) Cluster mill, (e) tandem rolling mill

Thread Rolling

Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies

Important process for mass producing bolts and screws Performed by cold working in thread rolling machines Advantages over thread cutting (machining):

1. Higher production rates

2. Better material utilization

3. Stronger threads and better fatigue resistance

(1) Start of cycle, (2) end of cycle

Thread Rolling