Managing friction in cold forming

In cold forging, friction is not a side effect of the process; it is one of the main variables that set its limits. As a bolt head is upset in a single station, the surface of the workpiece grows several-fold, the pressure at the die–part interface exceeds the yield strength of the material and the temperature rises locally. Under these conditions what holds the interface together is not the hardness of the die but the thin lubricating film that keeps the two metals from touching. The moment the film tears, the story continues not with die wear but with adhesion — galling.

Cold forming lubrication is therefore not an oil-selection problem but a system-design problem: a porous carrier layer chemically bonded to the metal surface, and a shear surface sitting on top of it. The pairing of KİMSOL zinc phosphate and Kimkal drawing soap, which Kimfosan has produced since 1982, separates exactly these two functions. Below we look at why that separation is necessary, how the friction coefficient shows up in process outputs, and how the approach changes with the severity of deformation.

Surface expansion: what sets cold forging apart

In machining, or in a moderate bending operation, the contacting surfaces are largely fixed. In cold forming, by contrast, the surface area grows rapidly as the workpiece fills the die cavity. In a nut preform produced by backward extrusion, or in a flanged bolt head, every unit of the starting surface becomes several units of surface by the end of the operation.

What that means for lubrication is this: fresh, chemically active metal surface that is not covered by lubricant is continuously exposed during the process. The contact pressure between that fresh surface and the die is in the hundreds of MPa, and the deformation heat accumulating at the interface lowers the viscosity of the oil. A liquid oil cannot form a hydrodynamic film under these conditions; it is squeezed off the die surface and pushed to the side. For the film to survive, you need a structure that can expand with the expanding surface and be fed from a reservoir.

• Contact pressure exceeds the yield strength of the material; classical boundary lubrication models fall short.
• As the surface expands, fresh unlubricated metal is exposed; the film has to renew itself.
• Interface temperature rises locally; low-viscosity films tear easily.
• Die and part are from the same family (steel on steel); the tendency to adhere is high.

Carrier and lubricant: the division of labour between two layers

The phosphate + soap system solves this problem by separating the functions. In a KİMSOL zinc phosphate bath a conversion reaction takes place between the acidic solution and the cleaned steel surface, and a crystalline, porous zinc phosphate layer chemically bonded to the metal grows on it. That layer is not a good lubricant in itself; its function is to be a carrier. Its porous structure forms a reservoir for the lubricant, its crystalline structure gives the soap a surface to key into, and it physically separates direct steel-on-steel contact.

The second layer is the Kimkal drawing soap applied over the phosphate. With reactive soaps, the sodium soap reacts with the zinc in the phosphate layer to form zinc soap (a metal soap) on the surface; that layer is chemically keyed to the phosphate and has low shear strength. With neutral soaps, mechanical adhesion takes the place of a chemical bond and the soap seats itself physically in the pores of the phosphate. In both cases, shear during deformation takes place within the soap layer — not between die and part.

The phosphate does not lubricate, it holds the soap. The soap does not carry, it shears. The system works only if each does its own job.

This division of labour is the most commonly misunderstood aspect of the system. A soap film applied on its own, to steel without phosphate, is stripped off in the first station; phosphate left without soap cannot bring friction down to an acceptable level and may even behave abrasively. The line is therefore always laid out as degreasing → rinsing/activation → phosphate → soap → forming; none of the intermediate steps is optional.

What does the friction coefficient determine?

The friction coefficient at the interface shows up directly in three different process outputs, and they usually deteriorate together:

1. Forming force: as friction rises, the force needed to make the material flow in the die cavity grows. This eats press capacity and raises the stress in the punch and die body; on heavy forms it can leave the part incompletely filled.
2. Die life: rising contact pressure and interface temperature create first micro-adhesion and then material transfer (pick-up) on the die surface. Once pick-up starts, wear accelerates and the die surface degrades itself. Die fracture is often the last link in this chain.
3. Part surface quality: a torn film shows up on the part as scratches, drag marks, matt-and-bright blotching and dimensional deviation. On fasteners that will be plated, these marks carry through to the subsequent surface treatment.

In production the friction coefficient is not measured directly; press force trend, die change interval and part surface are used as indirect indicators. A gradual rise in press force on the same product is usually the first signal of deterioration in the lubrication system — not because the die has worn, but because the film has weakened.

Approach by severity of deformation

Coating weight selection is not a matter of a standard value; it depends on the severity of the work being done. Light head upsetting and multi-stage backward extrusion do not require the same film thickness. Nor is more coating a solution: a thick, brittle phosphate layer breaks up during deformation, dusts, and builds up in the die cavity and the soap bath. The table below summarises how that choice sits within KİMSOL's 3–15 g/m² range.

Film continuity on multi-station presses

On a multi-station cold forging press the film cannot be renewed after the first station. Whatever was put on the surface before the wire was fed in has to carry the part through all five or six stages. Film consumed at the first station is not available for the most demanding form at the last. The system is therefore sized for the heaviest station — not the average one.

• The coating must be homogeneous along the wire diameter and inside the bundle as well; in an immersion process this is determined by rack/basket layout and bath circulation.
• The finer and more even the crystal structure of the phosphate layer, the better the soap keys in and the better it survives between stations.
• Oil residue left on the surface, or inadequate rinsing, leaves islands where the phosphate has not keyed in; these are the first failure points in heavy stations.
• The waiting time before forming and the humidity of the storage environment affect the condition of the soap film.

Symptoms of inadequate lubrication, and alternatives

In the field, film failure announces itself with a fairly familiar pattern. Bright pick-up on the die and axial scratches on the part are the first signs of adhesion (galling). A gradual rise in press force for the same product and material, die life falling short of expectations, matt-and-bright blotchy surfaces on the part and, in the final stage, punch or die fracture are all continuations of the same chain. When these symptoms appear, the first place to look is not die material but degreasing efficiency, phosphate coating weight and the concentration of the soap bath.

Besides phosphate + soap, single-stage polymer-based dry film lubricants are also in use. These systems offer advantages in wastewater and in the number of process steps; however, in heavy cold forging operations where surface expansion is very high, their performance depends strongly on part geometry and material and they do not replace the phosphate–soap system in every application. The right approach is to evaluate alternatives part by part through testing rather than ideologically. For heavy deformation, phosphate + soap remains the most widely used reference system, and the reason is the carrier–lubricant division of labour described at the start of this article. For the selection criteria on the soap side, see drawing soap selection.