Forging is a metal forming process where workpieces are shaped by compressive forces usually applied through forming die, tool, or machine tool so that the final required shape is obtained. The material shape is normal bar stock or billet cut from bar stock.
As one of the oldest and most important metalworking operations, metal forging may be dated back at least to 4000 B.C. and was used to make jewelry, coins, and various implements by hammering metal with tools made of stones. Forging parts are now widely used in many industrial and commercial areas, including automotive, machinery, railway, aircraft, etc.
According to the condition, if the material needs heating or not, metal forging may be classified as cold forging and warm or hot forging. Cold forging is processed at room temperature, and warm or hot forging is processed with material heated. To increase material ductility and its formability, cold forging material needs to be annealing heat-treated to eliminate residual stress and refine the crystal size of the material.
As material grain flow or structure reshaped or controlled through a hammer tool, the forging parts have good strength and toughness and are very reliable for highly stressed and critical applications comparing with the same part that is made from process of machining or casting.
Forging Process Type
According to the condition if dedicated forging die is needed or not, metal forging may be classified as open-die forging and impression-die forging in general.
This is the simplest forging methods, also called smith forging. It is the traditional forging operation done openly or in general-purpose open dies by the village blacksmith or modern shop floor by manual hammering or power hammer (refer right figure as reference).
Normally the process involves heating the stock in the blacksmith’s hearth and then beating it over the anvil. To get the desired shape, the operator has to manipulate the component between the blow.
Open-die forging includes almost all of the forging operations, such as fullering, flattening, bending, upsetting and swaging, etc. Smith forging involves a lot of skill of the operator and also is more time-consuming. But since there have no special dies needed, it is more beneficial in the manufacture of small lots or in trial production.
This is the operation done in closed impression-dies by means of drop hammers to obtain the desired shape of workpiece, so it is often called as Drop forging. It is usually carried out at elevated temperature to increase workpieces ductility and lower the required forces. The die impressions are machined in the die cavity.
The shaping is done by the repeated hammering given to the material in the die cavity. The equipment used for delivering the blows are called drop hammers. The final desired shape in impression-die forging can’t not be obtained directly from the stock in a single pass. Depending on the shape of the component, and the desired grain flow direction, the material should be manipulated in a number of passes.
The various pass typically involved in drop forging have fullering, edging, bending, blocking, finishing and trimming. Refer figure of stages in drop forging of a lever.
If the shaping is done by a single continuous squeezing action through a hydraulic press, then the forging is called Press forging. Press forging dies are similar to drop forging with various impressions in die as the above figure showed. To provide necessary alignment of two die halves in press forging, die posts are usually attached to the bottom die so that the top die would slide only on the posts and thus keep aligned with the bottom die.
In the LED lighting industry, the blank material of pin fin heatsinks is normally produced by Press forging process because of heatsink mechanical structure limitation.
Machine forging is a variant drop forging in which the power source comes from a punching machine, not a hydraulic press. Sometimes it is simply called as upset forging as it involves only the upsetting operations at most conditions. Upsetting is a forging method to increase the cross-sectional area of the stock at the expense of its length. To achieve the upsetting, force is applied in a direction parallel to the long axis.
Though both open-die forging and press forgings are also done by machines, historically, only upset forging is referred to as machine forging. Because the forging process is done with material not heated, the machine forging is often called cold forging
Originally machine forging was developed for making bolt heads in a continuous fashion, but now there has a fairly large number of diverse uses of this process. Because of the beneficial grain flow obtained from upsetting, it is used for making gear blanks, shafts, axles, and post-tension wedge that is widely used in construction and mining industry.
Upsetting machines, called upsetters, are generally horizontal acting. The die of machine forging consists of two parts, one called stationary gripper die that is fixed to the machine frame, and the other, movable gripper die, which moves along with the die slide of the upsetter.
Similar to open die forging, it is not possible to get part final shape in a single pass. The die of machine forging consists of multi-cavities that is required for various operations, and the forging process is moved from one stage to the other in a proper sequence till the final forging is finished.
Machine forging is the most efficient forging process, but the cost of the machine and die is extremely high, and may only be compensated by high volume production.
Forging Workmanship and Quality Control
The workmanship of metal forging typically include the following steps:
- Material preparation. Prepare slug or billet through processes such as shearing, cropping, or sawing for hot forging(open-die forging and impression-die forging), cold drawn wire or rod to the right size for cold forging (Machine forging)
- Annealing drew wire or rod to increase material ductility and forgeability, and phosphate drew wire or rod to reduce material frication for cold forging
- Deburr or clean billet surface to prevent hot forging die from scratching or galling
- For hot forging, heating the workpieces in a suitable furnace and then, if necessary, descale it with a wire brush
- Lubricate or preheat the dies
- Forge workpiece in die according to the proper sequence
- Perform any additional process, such as heat treating, straightening to improve workpiece mechanical strength.
- If necessary, make any finishing operations, such as machining, grinding that is required to meet product design requirements.
- Quality out-going inspection on any internal or external visual, dimensional, and functional requirements
Though the metal forging process generally gives superior quality products compared to other manufacturing processes, there are some defects that are likely to come if proper care is not taken. The typical defects are as below:
- Unfilled sections: some sections of the die cavity not completely filled by the metal flow.
- Cold shut: small crack at the corner of forging.
- Scale pits: irregular depressions on the surface of forging.
- Die shift: misalignment of the two halves forging
- Flakes: internal ruptures
The cause of defects is varied from multi-factors, such as design, material, and manufacturing process, special attention to be the focus on forging radius or fillet, microstructure changes caused by phase transformations, temperature gradients throughout the workpieces during forging, etc.
Forging defects can cause fatigue failures, and may lead to such problems as corrosion and wear during the service life cycle.
Forging and Die Design Consideration
Forgeability relies on not only material properties, but also proper forging and die design.
Since forging is formed through die in general, then a properly designed parting plane is selected to depart two halves of forging. The choice of the parting plane greatly influences the cost of the die as well as the grain flow of forging. In any forging, the parting plane should be the largest cross-sectional area of the forging since it is easier to spread metal than to force it into deep pockets. A flat parting plane is more economical. Also, the parting plane should be chosen in such a way that an equal amount of material is located in each of two die halves so that no deep die cavities are required.
For similar castings, it is necessary to provide a draft on the forging surface to release die after forged. Internal surfaces require more drafts than external surfaces. During cooling, forging tends to shrink towards its center and as a result, the external surface is likely to be separated, whereas the internal surface tends to cling to the die more strongly.
Fillet and corner radius
Forging involves metal flow in the die cavity. When two or more surfaces meet, a corner is formed which restricts the flow of metal, therefore these need to be rounded off to improve the flow of metal. Fillets are for rounding off the internal surface intersection angle, whereas the corner is that of the external surface intersection angle.
In addition, other considerations, such as shrinkage allowance for hot forging and finish allowance, shall also be considered properly along with the design and quality requirements.
Die design and material
The design of forging dies requires considerable knowledge and experience regarding the shape and complexity of the workpiece, its ductility, its strength and sensitivity to the deformation rate and temperature, and its frictional characteristics. The most important rule in forging die design is the principle that the part will flow in the direction of least resistance. Thus, the workpiece intermediate shaping should be planned so that they properly fill the die cavities.
Most forging operations are carried out at elevated temperatures, so the common die materials are a tool and die steel containing chromium, nickel, molybdenum, and vanadium that has high strength and toughness at elevated temperatures, resistance to mechanical and thermal shock, and wear resistance to abrasive.
A wide variety of forging machine is available as below list:
- Hydraulic Press. It operates at a constant speed, and normally used in Press forging.
- Mechanic press. This type of machine has a crank-slide block power transition mechanism to translate the flywheel rotary motion into a reciprocating linear motion. The speed varies from a maximum at the center of the stroke to zero at the bottom of the stroke, thus it is stroke limited.
- Screw press. This type of presses derive their energy from a flywheel, and forging load is transmitted through a large vertical screw. The ram comes to a stop when the flywheel energy is dissipated. If the die does not close at the end of the cycle, the operation is repeated until the forging is completed. Screw presses are used for various open-die and close-die forging, and suitable particularly for small production quantities.
- Hammers. The machine derives its energy from the potential energy of the ram, and the ram’s downstroke is accelerated by steam, air or hydraulic pressure. To complete the forging, several successive blows usually are made in the same die. Hammers are available in a variety of designs and are the most versatile and the least expensive type for forging equipment.
- Multi-station cold heading machine. This machine, called upsetting machine or upsetters, is usually used in Machine forging. The die set consists of a die and a corresponding punch or heading tool.
In sum, the metal forging process has a very broad application in many different industry sectors. The neat shape manufacturing makes it having strong competence to get low-cost and high-quality parts. Each type of forging process has its proper application. Material, design structure, quality requirement, and usage volume have a major impact on the selection of forging process. The ultimate purpose is to produce quality parts at the lowest cost, manufacturers need to balance all related factors involved to achieve the target.
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