Forging and metal forming are manufacturing processes that shape solid metal by forcing it to deform without removing large amounts of material. Engineers use forming to make parts such as crankshafts, gears, aircraft fittings, rails, wire, and structural beams. These processes matter because they can create strong parts with less waste than machining and often faster production than casting.
The final strength depends on temperature, deformation amount, strain rate, die shape, and the metal microstructure.
Key Facts
- True strain in compression or tension can be estimated by ε = ln(Lf/L0).
- Engineering stress is σ = F/A0, where F is force and A0 is original cross-sectional area.
- Hot working is forming above the recrystallization temperature, so new grains can form during deformation.
- Cold working is forming below the recrystallization temperature, which increases strength but reduces ductility.
- Volume is approximately conserved in plastic metal forming, so A0L0 = AfLf for simple shapes.
- Percent reduction in area is %RA = ((A0 - Af)/A0) × 100%.
Vocabulary
- Forging
- Forging is a metal forming process in which compressive forces from hammers, presses, or dies shape a solid workpiece.
- Rolling
- Rolling is a forming process that reduces thickness or changes shape by passing metal between rotating rolls.
- Extrusion
- Extrusion is a process in which metal is pushed or pulled through a die opening to create a long part with a constant cross section.
- Drawing
- Drawing is a process that pulls metal through a die to reduce its diameter or thickness, commonly used for wire, tubes, and sheet.
- Grain flow
- Grain flow is the directional alignment of metal grains caused by deformation, which can improve strength along the load path.
Common Mistakes to Avoid
- Assuming formed parts are strong only because they are compressed is wrong because strength also depends on grain flow, work hardening, defects, temperature, and heat treatment.
- Treating hot working and cold working as temperature labels only is wrong because the key comparison is whether the process occurs above or below the metal's recrystallization temperature.
- Forgetting volume conservation in plastic forming is wrong because metal usually changes shape with little volume change, so a thinner section must become longer or wider.
- Using the final area in the engineering stress formula σ = F/A0 is wrong because engineering stress uses the original cross-sectional area, while true stress uses the instantaneous area.
Practice Questions
- 1 A cylindrical billet starts with length 80 mm and cross-sectional area 1200 mm^2. It is forged so its final cross-sectional area is 800 mm^2. Assuming constant volume, what is the final length?
- 2 A wire drawing process reduces a wire from 10 mm diameter to 8 mm diameter. Calculate the percent reduction in area.
- 3 A forged connecting rod has grain flow that follows the curved shape of the part, while a machined connecting rod cuts across the original grain structure. Explain why the forged part may resist fatigue failure better under repeated loading.