Forging

The aim in most forging operations is that of producing parts close to the finished shape.

Open-die forging uses simple tools in a programmed sequence of basic operations (upsetting, drawing out), mostly in the hot-working temperature range, and the products (ranging from the one-off products of the blacksmith to huge turbine rotors) usually require finishing by machining. Rotary forging and swaging on special-purpose machines produce parts of axial symmetry to much tighter tolerances (axles, gun barrels).

Hot impression-die forging (sometimes termed closed-die forging) shapes the part between two die halves thus productivity in this process is increased, albeit at the expense of higher die costs. Excess metal is allowed to escape in the form of flash. The forging is kept within safe limits making ensure that the die filling is complete. More complex shapes, thinner walls, and thinner webs may necessitate forging in a sequence of die cavities, such as in case of connecting rods and crankshafts. Die design in case of closed die forging calls for a thorough knowledge of the materials flow and is greatly aided by computer models and expert systems. At high width-to-thickness ratios friction sets a limit to the minimum web thickness (Figure 2) that decreases with effective lubrication. In true closed-die forging the material is trapped in the die cavity.

With dies heated to or close to hot forging temperature (isothermal or hot-die forging), cooling is prevented and thin walls and webs can be produced, provided the die material is stronger than the workpiece material at the temperatures and strain rates prevailing in the process. This is relatively easy for aluminum alloys (airframe parts); use of low press speeds help to keep stresses and forces low. Titanium alloys and superalloys can be forged in the super plastic state (jet-engine fan blades and turbine disks).

The sequence of operations can be carried out by moving the heated end of a bar through the die cavities in an upsetter, achieving high production rates. Mechanized transfer between cavities in conventional presses is also possible. In all impression-die forging operations, die design calls for considerable use of theoretical aspects of metal deformation and flow knowledge; the die cost in these operations can be high, but the product often has superior properties because material flow can be directed to give the best orientation of the structure relative to loading direction in the service of the part.

Cold forging is related to cold extrusion and, when a complex shape is to be formed in a single step, requires special lubricants, often with a conversion coating, as in making spark-plug bodies. Alternatively the shape is developed by moving the bar or slug through a sequence of cavities, using a liquid lubricant. Cold forging is often combined with cold extrusion. It is the preferred process for mass producing near-net-shape parts such as bolts, nuts, rivets, and many automotive and appliance components.