Editing Forging (section)

====Impression-die forging====<!-- [[Closed-die forging]] redirects here -->
Impression-die forging is also called "closed-die forging". In impression-die forging, the metal is placed in a die resembling a mold, which is attached to an anvil. Usually, the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. Depending on the size and complexity of the part, the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities, forming what is referred to as "[[flash (manufacturing)|flash]]". The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die, so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging, the flash is removed.<ref name="Degarmo391"/><ref name="Degarmo394"/>

In commercial impression-die forging, the workpiece is usually moved through a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called an "edging", "fullering", or "bending" impression. The following cavities are called "blocking" cavities, in which the piece is working into a shape that more closely resembles the final product. These stages usually impart the workpiece with generous bends and large [[fillet (mechanics)|fillets]]. The final shape is forged in a "final" or "finisher" impression cavity. If there is only a short run of parts to be done, then it may be more economical for the die to lack a final impression cavity and instead machine the final features.<ref name="Degarmo392">Degarmo, p. 392</ref>

Impression-die forging has been improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging.<ref name="Degarmo393">Degarmo, p. 393</ref> One variation of impression-die forging is called "flashless forging", or "true closed-die forging". In this type of forging, the die cavities are completely closed, which keeps the workpiece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and workpiece placement.<ref name="Degarmo392"/>

There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging preform from liquid metal. The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed, then the part is quench hardened. Another variation follows the same process as outlined above, except the preform is produced by the spraying deposition of metal droplets into shaped collectors (similar to the [[Osprey process]]).<ref name="Degarmo393"/>

Closed-die forging has a high initial cost due to the creation of dies and required design work to make working die cavities. However, it has low recurring costs for each part, thus forgings become more economical with greater production volume. This is one of the major reasons closed-die forgings are often used in the automotive and tool industries. Another reason forgings are common in these industrial sectors is that forgings generally have about a 20 percent higher strength-to-weight ratio compared to cast or machined parts of the same material.<ref name="Degarmo392"/>

===== Design of impression-die forgings and tooling =====
Forging dies are usually made of [[alloy steel|high-alloy]] or [[tool steel]]. Dies must be impact- and wear-resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling. In order to produce a better, more economical die the following standards are maintained:<ref name="Degarmo393"/>
*The dies part along a single, flat plane whenever possible. If not, the parting plane follows the contour of the part.
*The parting surface is a plane through the center of the forging and not near an upper or lower edge.
*Adequate [[Draft (engineering)|draft]] is provided; usually at least 3° for aluminium and 5° to 7° for steel.
*Generous fillets and radii are used.
*Ribs are low and wide.
*The various sections are balanced to avoid extreme difference in metal flow.
*Full advantage is taken of fiber flow lines.
*Dimensional tolerances are not closer than necessary.

Barrelling occurs when, due to [[friction]] between the work piece and the [[die (manufacturing)|die]] or [[punch (tool)|punch]], the work piece bulges at its centre in such a way as to resemble a [[barrel]]. This leads to the central part of the work piece to come in contact with the sides of the [[die (manufacturing)|die]] sooner than if there were no friction present, creating a much greater increase in the pressure required for the punch to finish the forging.

The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. The dimensions across the parting plane are affected by the closure of the dies, and are therefore dependent on die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.<ref name="Degarmo394">Degarmo, p. 394</ref>
{| class="wikitable"
|+ Dimensional tolerances for impression-die forgings<ref name="Degarmo394"/>
|-
! Mass [kg (lb)]
! Minus tolerance [mm (in)]
! Plus tolerance [mm (in)]
|-
| 0.45 (1)
| 0.15 (0.006)
| 0.46 (0.018)
|-
| 0.91 (2)
| 0.20 (0.008)
| 0.61 (0.024)
|-
| 2.27 (5)
| 0.25 (0.010)
| 0.76 (0.030)
|-
| 4.54 (10)
| 0.28 (0.011)
| 0.84 (0.033)
|-
| 9.07 (20)
| 0.33 (0.013)
| 0.99 (0.039)
|-
| 22.68 (50)
| 0.48 (0.019)
| 1.45 (0.057)
|-
| 45.36 (100)
| 0.74 (0.029)
| 2.21 (0.087)
|}
A lubricant is used when forging to reduce friction and wear. It is also used as a [[thermal barrier]] to restrict heat transfer from the workpiece to the die. Finally, the lubricant acts as a parting compound to prevent the part from sticking in the dies.<ref name="Degarmo394"/>