Hot versus Cold and Warm Forming and in Between

The Ugine-Sejournet process (Sejournet, 1955; 1966) is based both on the use of a lubricant in a viscous condition at extrusion temperature and on a separation between the lubrication of the chamber wall and that of the die. A steel billet is heated to the extrusion temperature and then rolled in a powder of glass. The glass melts and forms a thin film, 0.5 to 0.75 mm (20 to 30 mil) thick, of viscous material coating the lateral surface of the billet and separating it from the chamber wall. The relevant coefficient of friction is thus so reduced that the force required for the extrusion is practically constant throughout the extrusion, whatever the length of the billet and with the exception of a starting point.
On the other hand, a thick solid glass pad, 6 to 18 mm (0.25 to 0.75 in) thick, rests on the entry face of the die, which, for this purpose, is at least partly flat. The front face of the billet (Fig. 28) shapes this glass pad into a longitudinal contour corresponding to the metal flow and, at the same time, melts a thin layer of glass, which will drift along with the outflowing metal and will lubricate its contact with the die. This melting will continue during the whole extrusion and ensure a continuous supply of viscous lubricant between die and extruded product. There is no metal dead zone, and a shear effect occurs in the viscous lubricant. Note that the actual film thickness of the lubricant is exaggerated in Fig. 28.

found for any specific temperature range. The thickness of the glass layer on the finished product is of the order of 1 mil, and after cooling it is easily removed. Initially devised for steel, the Ugine-Sejournet process has been extended to practically all metals and alloys that have a deformation temperature either above that of steel or limited to a narrow range.
Present-day trends in metal forming tend toward the replacement of hot forming and other manufacturing processes by cold forming. Some of the advantages are stronger products, better dimensional precision, surface finish, and savings in material waste. In the extrusion (and other forming operations) of steel, cold forming became feasible with the introduction of phosphate coating, a development that complements (and competes with) the Ugine-Sejournet process, When the steel surface is coated, the spongy phosphate coat absorbs the lubricating liquid, which thus becomes highly effective in reducing friction and wear.
One may say that both developments, the Ugine-Sejournet process and phosphate lubrication, are breakthroughs that solved friction and wear problems. Without these solutions, the metal forming of steel would not be where it is today.
When forming is conducted at temperatures above room temperature but below the recrystallization temperature, it is called warm forming. Today, many forming processes are performed warm to achieve a proper balance between required forces, ductility during processing, and final product properties. During warm forming of most steels, a specific range of temperatures (where the steel hardens by precipitation hardening) should be avoided. With today’s sophisticated equipment for the control of temperature and its distribution, the choice of the working temperature may be more precisely followed to ensure optimal production, as typically demonstrated by precision closed-die flashless forming.
Hot forging is usually done with high-alloy tools that can withstand elevated temperature. Tool-life considerations require as short a time of contact as possible between the tool and the workpiece; thus mechanical presses that do not dwell at the bottom of the stroke are recommended for hot forging. Hydraulic presses are commonly used for cold forging, especially of large components; recently they have been replaced for smaller parts by the faster mechanical presses. So today, the choice of mechanical versus hydraulic press may be decided, not only by the temperature of forging, but also by part size and production volume.
Forming in the mashy state is an emerging technology. Observing the tensile properties of metals, especially those of metal alloys, it is noted that the gradual drop in strength with increase in temperature undergoes a discontinuity in slope at the solidus line (see Fig. 29). The solidus line represents the temperature at which a solid metal alloy starts the transformation into the liquid state. This transformation proceeds gradually on heating, so that larger and larger portions of the specimen become liquid with increasing temperature until the liquidus temperature is reached, at which point the entire specimen melts. The way alloys liquefy is unique. First, drops of liquid nucleate
at the grain boundaries. When the temperature rises only slightly above the solidus temperature and only a small percentage of the workpiece is liquid, the entire network of grain boundaries is already liquid and each individual grain floats in liquid. This condition accounts for the drastic change in strength at this temperature and explains the different behaviors of metals during forming in the mashy state (Kiuchi et al., 1979a; 1979b). [See PHASE TRANSFORMATIONS, CRYSTALLOGRAPHIC ASPECTS.]
Plastic deformations in the mashy state occur mainly through solid grains sliding along the liquid grain boundaries. During compacting, the grains themselves may simply be rearranged relative to one another like sand or powder. The viscosity of the liquid metal and the thickness of the liquid layer determine the strength of the workpiece. Furthermore, the rate of shear within the liquid layer [see Eq. (13) and also Avitzur, 1979] determines the resistance to flow; the higher the rate, the higher is the resistance. The dashed lines in Fig. 29 indicate higher strengths for higher shear rates. In actual forming, these higher shear rates are caused by several factors. For example, they can be brought about by higher forming speeds, as with higher ram speeds in extrusion and forging. In the forging of disks, for a constant ram speed, the thinner the disk, the higher are the shear rate in the liquid and the resistance to flow.
In the application of forming in the mashy state, precautions must be taken to prevent squeezing of the liquid outward through the surface. For example, in extrusion, the billet is usually heated to the mashy state and placed in a preheated chamber. When the extrusion takes place, the extrudate may heat up due to deformation and friction, and thicker layers of the grain boundaries may then melt. Preventing
the liquid grain boundaries from squeezing out is of utmost importance and can be achieved by cooling the product at the exit. Closed-die forging is advantageous for forming in the mashy state because the confinement of the workpiece in the die automatically safeguards against liquid escape.
In general it is observed that forming in the mashy state at a higher temperature and liquid phase results in lower required pressures but with products of inferior properties (i.e., lower tensile strength combined with cast dendritic structure of lower ductility). Comparisons between forging from the melt and forging in the mashy state are made in Table II.
Forging from the melt is a process that competes with forming in the mashy state. Forging from the melt is accomplished by pouring molten metal into a mold, which serves as a die, and applying ram pressure while freezing progresses, (Ramati et al. 1978). As the molten metal gradually solidifies, its volume decreases and the ram force must be applied continuously. The surface of the workpiece freezes first. A drastic volume drop is associated with the phase transformation on freezing from the liquid to solid phase. This volume change due to phase transformation is vastly greater than the shrinking that occurs with the temperature drop in the solid state. Thus, as the interior freezes and shrinks, the ram advances into the workpiece and (depending on the skin temperature) causes the skin, which is already