Post on 19-Jan-2018
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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 strain hardening 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 recrystallization 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 working2. More intricate work geometries possible3. Need for annealing may be reduced or
eliminated Disadvantage
1. 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 because1. Strength coefficient (K) is substantially less than at
room temperature2. Strain hardening exponent (n) is zero (theoretically)3. Ductility is significantly increased
Advantages of Hot Working1. 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 rates2. Better material utilization3. Stronger threads and better fatigue resistance
(1) Start of cycle, (2) end of cycle
Thread Rolling