DROP FORGING Drop forging is a mass production technique which hammers the metal between two dies. Half of the die is attached to the hammer (upper section) and half to the anvil (lower section). The hot metal is placed in the lower half of the die and struck one on more time with the upper die. This forces the metal to flow in all directions, filling the die cavity. Excess metal squeezed out between the die faces is called flash or flashing . After the forging is completed the flash is cut off in another press with a trimming die .
A metal shaping process, the metal to be formed is first heated then shaped by forcing it into the contours of a die, this force can be in excess of 2000 tons. The drop forging process can be performed with the material at various temperatures; Hot Forging During hot forging the metals are heated to above their recrystallization temperature. The main benefit of this hot forging is that work hardening is prevented due to the recrystallization of the metal as it begins to cool.
Cold Forging Cold Forging is generally performed with metal at room temperature below the recrystallization temperature. Cold forging typically work hardens the metal TWO TYPES OF DROP FORGING OPEN DIE Drop forging requires the operator to position the work piece while it is impacted by the ram. The die attached to the ram is usually flat or of a simple contour, most of the shaping is achieved by the operator physically positioning the work piece before each stroke of the ram. There are also special dies which can be used to cut the metal, form holes or notches.
CLOSED DIE (Impression die) Drop forging comprises of a die on the anvil which resembles a mould, the ram which falls and strikes the top of the metal billet can also be equipped with a die. The heated metal billet is placed on the lower die while the ram drives down forcing the metal to fill the contours of the die blocks.
Process details Closed-die forging A heated blank is placed between 2 halves of a die
A single compressive stroke squeezes the blank into the die to form the part. In hammer or drop forging this happens by dropping the top of the mould from a height. An alternative is to squeeze the moulds together using hydraulic pressure.
Once the die halves have separated, the part can be ejected immediately using an ejector pin.
Advantages 1. Parts of sizes up to 25 tons can be produced with closed die forging 2. Closed die forging can produce near net shapes that will require only a small amount of finishing 3. Economic (profitable) for large runs of product
4. Forging provides superior mechanical properties over castings due to the internal grain structure formation in forged parts 5. Makes strong products. 6. Many metals are forged cold, but iron and its alloys are always forged hot.
Disadvantages 1. Not very economical for short runs due to the high cost of die production 2. The business set up cost of drop forging is very high, not only are the machines and furnaces costly but special building provisions must be in place to cope with the powerful vibrations caused by drop forging. A special foundation must be laid to deal with this environment 3. Drop forging presents a dangerous working environment
Materials and shapes 1. Any metal can be forged, provided the blank is hot enough (( 60% of the melting temperature). 2. Typical possible sizes for closed dies range from 10g to 10kg, depending on complexity. 3. The part is left with good surface and mechanical properties, although cold-forging can perform even better.
4. Complex parts can be formed using a series of forging dies with increasing levels of detail. 5. A draft (taper) angle has to be incorporated to allow easy removal of the part. 6. Any waste material squeezed between the die halves, called flash, is readily recycled.
Economics Production rate is limited by the insertion and removal of the blank, so some form of automation is often used. As a result, machines can cost £100,000+, but can produce many parts a minute (if small). As both the machines and the dedicated dies are costly, production runs in excess of 50,000 are often needed to produce small parts economically. Large parts can be produced economically at smaller batch sizes, because there is less competition.
MATERIALS USED The materials that are used most commonly in drop forging are: aluminium, copper, nickel, mild steel, stainless steel, and magnesium. Best Material: Mild Steel Worst Material: Magnesium