- 15.1 Introduction to Metal Forming
- 15.2 Forming Processes: Independent Variables
- 15.3 Dependent Variables
- 15.4 Relationship between Variables
- 15.5 Process Modeling
- 15.6 General Parameters
- 15.7 Friction
- 15.8 Temperature Concerns
Forming Parameters and Equipment
The forming categories are defined by three major process variables.
* Configuration of shaping device (open dies/mated dies)
* Material Temperature (hot or cold working)
* Method of applying force (forging, drawing, swaging)
Open dies, such as those used in open die forging, allow the component to spread laterally as the die is closed. The extent of the lateral flow of material is controlled only by the distance the die moves. (video)(video)(video)(video)
One piece shaped dies are those that only control the geometry on one side of the material. These are used primarily for sheet forming processes such as thrmoforming, metal spinning dies, stretch forming dies, etc.
Mated dies control both sides, and commonly the extent of lateral flow of a workpiece. These are common for coldforming and forging operations.
Rolls are another type of die used to control geometry of a rolling process (hot rolling, roll forming, sheet metal seaming, roll forging).
Material Temperature (hot or cold working)
To understand the differences between hot and cold forming, it is important to understand the phenomenon of work hardening. Work hardening occurs when a force deforms a material. It rearranges and elongates the grain structure, exhausting the mechanisms of plastic deformation (slip and twinning), and limiting the amount of plastic deformation that can occur with a given force. The material becomes harder and stronger, but ductility is decreased. Eventually fracture will occur as plastic deformation continues.
At about 50% of their melting point (on an absolute scale) thermal vibrations allow atoms to more freely move about and reorient themselves. The lattice structure of the atoms breaks down and reorganizes itself freely. This allows a great increase in the amount of plastic deformation that can occur. A process is classified as hot working if it takes place when the workpiece is above the recrystallization temperature.
Hot Working Advantages
- Hardness and ductility of metal is not changed.
- Porosity is eliminated.
- Structure is improved by reforming smaller crystals.
- Large shape changes are possible without ruptures.
- Smaller, faster acting machines.
- Impurities are broken up and distributed throughout material.
- Surfaces need not be clean and scale free.
Disadvantages of Hot Working
- Oxide scale produced on surface.
- Close dimensional tolerances are difficult.
- Carbon lost from surface layer (creating a weak surface)
Cold working takes place below the recrystallization temperature, and therefore is fundamentally different at the atomic level with effects up to the macroscopic level (video)
Advantages of Cold Working
- No heating required.
- Close dimensional tolerances possible.
- Surface finishes are better.
- Strength, hardness, and directional properties are improved.
Disadvantages of Cold Working
- Produces stresses that sometimes have to be removed by heat treating.
- Higher forces and heavier equipment needed.
- Metal must be clean and scale free.
- Straight Side Press
- Gap or C-Frame Press
- Eccentric Geared
- Rack and Pinion (Gear)
- Single Gear
- Double Gear
- Hydraulic (piston)
- Single-Action Press
- Double-Action Press
- Triple-Action Press
- Four-Slide Press (video)
- Inclinable Press (OBI)
- Adjustable Bed
- Horn Press
Hammers are distinct from presses. Hammers are classified as energy-restricted machines. That is, the deformation of the workpiece absorbs the kinetic energy from the hammer and brings the stroke to a stop. In contrast, a press is a stroke-restricted machine, and the deformation comes from the complete press cycle. A hammer does not need to come to its fully shut height before returning up, while presses typically cycle their full length with each stroke. Hammers can be driven or drop-type. (video)
Rolls can be used to bulk form (as in hot rolling of structural shapes) as well as to bend (roll form) sheet stock (as in roll forming gutters) or to bend material (3-roll rolling of sheet stock into a cylinder).
Draw benches are used for the cold reduction in cross-sectional area, as in wire or tube drawing. Material is pulled through a hardened die. (video)(wire drawing video)
Process variables that may affect the process are:
- Workpiece geometry (size variation)
- Workpiece initial condition (hardness)
- Changes in workpiece (work hardening)
- Workpiece surface condition (scale)
- Workpiece temperature
- Die design
- Die temperature
- Die surface condition
- Rate or speed of the forming process
Friction and Lubrication
Friction is the resistance to sliding action. The sliding of a workpiece over the faying surfaces of the tooling. It can result from simple abrasion of asperities, or adhesion of the work surface to the die surface. The hardness and surface finish of the die material is an important factor to consider when forming. Additionally, lubrication is often the key to successfully forming a workpiece.
If friction is wearing down the tool, accuracy, surface finish of the workpiece, and process capability will be affected.
Forming Lubricant Types
- Mineral Oils
- Natural Oils (fatty oils)
- Synthetic Oils
- Compound Oils
- Extreme-Pressures (EP) Oils
- Solutions (mixtures of water with other fluids or additives
- Glasses (liquids under process conditions).