4.9. Die Forging

The main disadvantages of forging process are:

- low output;

- low dimensional accuracy;

- difficulty in receiving forgings of intricate shapes.

Fig. 4.19. Hydraulic press: 1 – base; 2 – block head; 3 – traverse; 4 – column; 5 – rod; 6 – plunger;

7 – traverse; 8 – piston; 9 – raising cylinder; 10 – main cylinder; 11 - oil feed

a b

Fig. 4.20. Drawings of the gear (a) and the fording (b)

a b c d e

f g h i

Fig. 4.21. Technological process of manufacturing of a lever with a jaw: a – article; b – billet;

c, f, g - drawing; d, e, h - necking; i - bending and necking

To settle these problems die forging is used. There are two types of die forging: hot and cold ones. The hot die forging has a wide application in the machine industry. The cold die forging is used, mainly, for manufacturing the forgings of alloys which have high plasticity.

The essence of die forging is that a forging is manufactured by means of a metal die, which as a rule consists of two parts. When the parts are moved to meet together by a hammer or a press a billet is transformed into forging. The manufacturing die forging process is similar to a hammering one.

Two methods of hot die forging are distinguished (Fig.4.22):

- with flash (die is "open");

- without flash (die is "closed").

In the first case mass of a billet must be equal or exceed mass of the forging. When the billet will be squeezed-inside the die cavity, named impression; the surplusmetal will be squeezed into the ring flute, forming a flash. Then it is necessary to remove a flash by cutting.

In the second case mass of the billet must be equal to the mass of the forging. So we need to control the mass of the billet very carefully.

Fig. 4.22. Die forging process in open (a) and closed (b) dies: 1 – forging;

4, 6 – forging with flashes 2 and 3; 5 – billet for forging; 7, 8 – parts of the die; b: 1, 2 – parts of the die

Dies may have one impression for manufacturing the simple forging or several impressions for manufacturing the intricate forging. In the latter case the process consists of a sequence of die forging operations, first, in roughing impressions to draw or bend the billet and then in intermediate and finishing impressions to produce a finished forging (Fig. 4.23).

Finishing operations of die forging are as follows:

- flash cutting;

- film cleaning;

- dressing of forgings (correction of their shape) in hot or cold conditions by presses;

- heat treatment;

- cleaning from scale;

- calibration (coining) to receive more precise dimensions; calibration may be plane or volume one (Fig. 4.24).

Equipment for hot die forging includes hammers, crank presses, horizontal forging machines, hydraulic and screw presses, etc.

Fig. 4.23. Multi-impression die and operations of forging: 1 – fullering; 2 – drawing (extending);

3 – bending; 4 – blocking; 5 – finish fording

Fig. 4.24. Plane (a) and volume (b) calibration (operation of calibration undergo sizes indicated

on the sketches)

Hammers do not allow to obtain high dimensional accuracy of forgings because parts (halves) of dies are not interconnected and their replacement is very high.

The crank press (Fig 4.25) consists of electric motor 1, belt transmission 2-3, gear transmission 5-6, crank 8, plunger 10 and table 9 of variable height. Two parts of the die are fixed to the plunger and the table and slide along guiding columns. This ensures high dimensional accuracy of forgings.

Fig. 4.25. Cinematic scheme of a crank hot-forging press: 1-electric motor; 2 – pulley;

3 – fly-wheel; 4 – shaft; 5, 6 – gears; 7 – friction coupling; 8 – crank; 9 – table;

10 – plunger; 11 – rod; 12 – brake

Advantages of die forging process: high productivity (dozens and hundreds of forgings per hour); high accuracy.

Disadvantages:

- complex and expensive tools, which may be used only for definite articles;

- mass of work (forging), as a rule, are not more than 30 kg; for manufacturing of heavy forgings powerful equipment is required.

Cold die forging is mainly used for manufacturing the forgings of alloys with high plasticity (Al, Mg, Cu, sometimes, low carbon steel). There are three basic types of the process: cold forming, cold extrusion and cold upset forging.

Cold forming process is similar to hot die forging, but it is carried out at a room temperature.

Cold upset forging process is carried out by cold upsetting automatical machines, which are similar to horizontal forging machines, but have higher productivity (from 20 to 400 forgings per minute).

Cold die forging requires higher deforming forces. After it metal acquires strain hardening, but the method offers higher, than hot die forging, dimensional accuracy, surface quality and absence of scale losses.