Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: cold forging, warm forging, or hot forging. For the latter two, the metal is heated, usually in a forge.
Forging is a very important manufacturing process. Forged part has high physical strength and good tough property. Forging process is used in auto industrial, machine industrial, and construction industrial parts.
Advantages and Disadvantages
Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics. Additionally, forgings can target a lower total cost when compared to a casting or fabrication. When you consider all the costs that are involved in a product’s lifecycle from procurement to lead time to rework, then factor in the costs of scrap, downtime and further quality issues, the long-term benefits of forgings can outweigh the short-term cost-savings that castings or fabrications might offer.
Some metals may be forged cold, but iron and steel are almost always hot forged. Hot forging prevents the work hardening that would result from cold forging, which would increase the difficulty of performing secondary machining operations on the piece. Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable. Alloys that are amenable to precipitation hardening, such as most aluminum alloys and titanium, can be hot forged, followed by hardening.
Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high-temperature furnace is required to heat ingots or billets. Owing to the size of the massive forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations use metal-forming dies, which must be precisely machined and carefully heat-treated to correctly shape the workpiece, as well as to withstand the tremendous forces involved.
Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting
Drop hammer forging machine
Drop forging is a forging process where a hammer is raised and then "dropped" onto the workpiece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the workpiece, while the latter does.
- Reduced chance of voids.
- Better fatigue resistance.
- Greater strength.
- Finer grain size.
- Improved micro structure.
- Continuous grain flow.
- Not very economical for short runs due to the high cost of die production.
- Drop forging presents a dangerous working environment.
Press forging machine
Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.
- Higher productivity than drop forging.
- Greater accuracy in terms of tolerances.
- More complicated shapes can be forged.
- Possibility of process automation through mechanism of blank feeding and forging removal.
- The operation is completed in a single squeezing action.
- The press forging, therefore, is suitable for mass production of nuts, bolts, rivets, screws, break levers, bearing races, valves, etc.
- The initial capital cost is higher compared with drop forging.
- The difficulty of descaling the blank is another short coming of this process.
- The process is economically suitable only when the equipment is efficiently utilized.
Forging Part Manufacturing Considerations
For forging part manufacture, the designer should consider the following: shape, parting line, draft angle, corner radii, fillet, wall thickness, tolerance and machining allowance.
- 分模線的位置應盡量避免在有深陷(Deep cavity)部位，因如在深陷部位有可能使鍛模破裂，或產生欠肉現象，使該部位成形困難。
由於在鍛件中如有銳利的型穴內外隅角，會使材料流動困難，產生流動穴模不足的現象(Flow through)現象，以致形成冷夾層(Cold shut)之缺陷，太小的外圓角(Corner)將使鍛模產生應力集中，同時因熱疲勞而變形，縮短鍛模壽命，有適當的內圓角及外圓角。