EP1159097B1 - Method for forming metal parts by cold deformation - Google Patents

Abstract

The invention concerns a method for forming metal parts by cold deformation, consisting in: (i) mechanically depositing a metal zinc layer on the free surface of the blank of the part to be produced; and (ii) forming said part by plastic deformation.

Description

The present invention relates generally to the forming of metal parts by cold deformation.

Among the different cold forming processes, first mention metal extrusion or forging at cold, which corresponds to a forming process consisting in making flow the metallic mass under a compressive force between a punch and a die. We can thus obtain different parts of well defined geometric shape. This type of deformation requires vertical or horizontal presses including a or several workstations with or without transfers.

Another cold deformation technique, close to extrusion, is known as cold stamping. In this case, one or more stages of deformation are carried out in a single machine, usually horizontal machines comprising one or more stations. These workstations are usually fed by a metallic wire which is subjected to plastic deformation under forces generally more weak than in the case of the actual extrusion.

Finally, we can mention as an example of the cold forming, the wire drawing which constitutes in fact a step intermediate or preliminary preforming, from a spool of wire to obtain sections of smaller diameter, generally intended to supply a cold punching station. This type of deformation is mainly used upstream of the manufacture of screws and bolts.

This cold forming technology is applicable to a very large number of steels and alloys generally not ferrous. The operations are generally carried out at temperature ambient from plots, blanks or blanks that have undergone a specific preparation operation.

As an example of possible types of deformation at cold, we will mention crushing, preforming, spinning direct or reverse spinning, direct hollow or thread, lateral spinning, stretching, upsetting, calibration or ogivage.

Extrudable steels liable to be subjected to such cold deformations belong to different categories, in particular general-purpose non-alloy steels, but preferably special non-alloy steels for heat treatments, generally fine carbon steels, special alloy steels. for heat treatments, stainless steels or microalloyed steels. These the latter are suitable for cold forming without annealing and acquire by work hardening of the levels of high mechanical resistance all retaining an acceptable residual ductility.

One of the main difficulties to be solved in the part of this cold deformation technology for parts metal lies in the obligation to have recourse, before forming, to preliminary surface treatments involving the most of the time from successive operations more or less long and expensive, sometimes relatively difficult to put on implemented and whose effectiveness is not entirely satisfactory.

The quality of surface treatments, for example specific to extrusion, conditions the good result obtained at following deformation operations. The main purpose of these surface treatments before forming is of course to decrease the as much friction as possible in the tools.

These are precisely the forces involved in this type cold forming operations which constitute the obstacle major in the development of these extrusion techniques.

It is therefore essential to be able to decrease the forces of friction so as to avoid as much as possible the seizure of the part, reduce the effort required for extrusion, and minimize tool wear.

These mainly based preprocessing operations on the lubrication of plots or blanks, may have to be implemented between two deformation operations successive, whether or not the parts are subjected to annealing.

In the case of carbon steels or steels weakly allied, pretreatment first involves a alkaline degreasing, pickling with sulfuric acid in presence of an inhibitor which aims to limit the attack of the metal itself, then phosphating, and finally the lubrication proper.

The purpose of the phosphating operation is to form a first adhesion layer, generally porous, of phosphate zinc which is intended to receive the lubricant. The deposit of this lubricant, generally consisting of zinc stearate resulting from the reaction of reactive soaps with the zinc phosphate, is difficult to control in practice. It is indeed necessary to adapt the thickness of the layer of zinc stearate, depending on mechanical stress to which the parts to be deformed will be subjected. This adaptation is all the more difficult to master as it involves the control of a chemical reaction that develops into depth in the thickness of the applied layers and the kinetics spans several hours.

Consequently, the lubrication operation involves generally an immersion of the previously phosphated material in hot baths of reactive soaps.

However, this combination between the phosphate layer zinc and zinc stearate, may remain insufficient for - avoid contact between the metal part and the tool.

In cases where the zinc stearate layer does not give not satisfied, it is then necessary to have recourse to other more sophisticated lubricants that require additional deposits by immersing the parts or by spraying, not only on the parts, but also on the tools. Such operations require monitoring constant of the concentration of the lubrication solution, as well as the application temperature to obtain coatings which unfortunately are usually not enough regular.

In the state of the prior art, it was up to present considered essential to proceed with a step prior to phosphating to allow both good adhesion and- the formation of zinc stearate filling the lubricant function for the part to be deformed. In a certain number of applications, and mainly in the context of cold stamping, it is imperative, after forming the part, to dephosphate the latter before proceeding to treatment thermal to avoid any risk of phosphorus diffusion in steel. Such heat treatments, generally resulting in temperatures of the order of 850 to 900 ° C are essential and actually lead to changes in the microstructure of the formed parts. Such a disadvantage of the prior art linked to the obligation to have recourse to a phosphate removal before heat treatment, is particularly severe in the case of screw formation and bolts, where we observe problems of embrittlement of parts intended to be subjected to permanent efforts which Often cause fatigue breakdowns.

The object of the present invention is precisely to reduce, or even completely remove the disadvantages previously cited. The invention relates more particularly to a method of forming of metal parts by cold deformation, in a first mechanical deposition operation of a layer based on metallic zinc on the free surface of the part to be produced, this layer which may possibly contain and / or be coated with a layer of lubricant, to then form the said part by plastic deformation of the metal.

Such a cold forming process has made it possible to facilitate greatly the phenomena of plastic deformation by reducing the friction forces brought into play, and may even go until the number of intermediate stages is reduced during the forming process.

The method according to the invention makes it possible to get rid of all the disadvantages of using pretreatment by phosphating the blanks or pieces of metal. Finally, it It turned out that certain parts produced by such a process of forming involving a mechanical pre-deposition of a layer based on metallic zinc, used to obtain parts whose duration life in fatigue was improved.

Depending on the forming process used, it may be sufficient to deposit a single layer of metallic zinc on the metal patch on the free surface of the latter. Such a layer consisting of zinc or more generally of an alloy of zinc and iron, or even a mixture of particles of zinc and iron, can be applied in the context of the present invention in an amount included between 50 and 250 mg / dm ² of filler metal. For certain particular applications, we can even be satisfied with lower deposited quantities.

Such a layer may be sufficient to fill itself the lubrication function for forming operations cold during which the plastic deformation of the metal is obtained under relatively low forces.

The mechanical deposition of the layer based on metallic zinc is advantageously obtained by a shot peening operation using steel balls having at least one layer exterior comprising either pure zinc or an alloy based on zinc.

Such a mechanical deposition of a zinc-based layer metallic can also be ensured by an operation of shot blasting using a mixture of steel balls and balls constituted by a steel core and-presenting on the surface at minus an outer layer based on a zinc alloy or a outer layer of pure zinc.

Finally, this mechanical deposition of the zinc-based layer metallic can also be obtained by shot blasting using beads made essentially from an iron alloy, shot peening being carried out in the presence of zinc powder or of zinc semolina which is therefore applied under the effect shot blasting mechanics.

The term ball or microbead used in the context of the present invention to describe the blasting operations must be understood in a broad sense, that is to say to include all types of particles or microparticles to be projected at the parts surface.

The shot blasting machine used to achieve this first layer on plots or metal blanks to deform, can for example be carried out in accordance with schematic diagram illustrated in Figure 1 attached.

We observe in this figure that the machine has mainly a blasting chamber 10 which can for example have two projection turbines 12 between which scroll through the parts to be treated. The projection turbines 12 go so project the microbeads of iron alloy or alloy to zinc base on the surfaces of the parts to be treated, in the presence if necessary, a zinc powder or semolina. The part bottom of this blasting chamber 10 is equipped with a device 14 for recycling shot blasting balls. These marbles are then brought to a particle size separator 16 of so as to separate balls of diameter which have become too small. In this way, metallic dust is eliminated in particular 18 generated during the blasting operation. In the event that only uses balls coated with a base alloy zinc, after a particle size sorting, the latter are brought to a magnetic separator 20 which allows to sort between the steel balls covered with a zinc-based alloy and spent zinc steel balls, that is to say who lost a large part of this alloy to zinc base, the spent steel balls in zinc are collected at post 22. At the exit of this magnetic separator 20, there is further provided a device 24 for measuring the zinc content of shot blasting balls.

In response to this zinc content measurement, the tank 26 microbeads which is intended to power the turbines of projection 12 of the shot blasting device 10, will be or not refilled with new balls at 28, i.e. loaded or recharged with zinc. The reservoir 26 is further advantageously equipped with a level 30 control system.

It is thus thus possible to carry out, in a way continuous or discontinuous, mechanical deposition of the base layer of metallic zinc on the surface of the pieces or blanks to be formed.

A layer based on zinc and / or an alloy of zinc and iron or a mixture of zinc and iron at a rate of 50 to 250 mg / dm ² is thus deposited on the surface of the metal pieces or blanks. The nature of this layer is not compact, since it actually results from the aggregation of a multitude of zinc and / or iron particles, which therefore gives it a sort of microporous or aerated structure. For certain cold forming operations, this single layer may be sufficient to play an effective lubrication role before the actual forming.

For other applications for forming parts of more sophisticated shape and structure, it may prove necessary to apply a layer of lubricant to the layer of metallic zinc previously deposited. The lubricant is preferably applied in a liquid form, which allows it to soak up well in the base layer. In terms of. the quantity of lubricant applied, a kind of saturation of the previous layer based on metallic zinc will be observed, or a more complete coating in excess thickness of the latter. The amount of lubricant applied will therefore also vary depending on the nature and exact shape of the parts to be produced. In practice, it has been found that such an amount of lubricant can be effectively applied in an amount up to 300 mg / dm ² .

The application of the lubricant layer is carried out preferably in liquid form, both by spraying and by soaking. The lubricant can in particular be applied in the form of an aqueous suspension based on particles of graphite. It is however perfectly possible to consider the replacement of graphite by other lubricants such as molybdenum disulfide, teflon, or even aqueous solutions copolymers such as styrene copolymers and of maleic anhydride in ethoxylated alcohol medium.

Finally, it is also possible to use a aqueous polypropylene composition possibly added graphite powder, boron nitride, polytetrafluoroethylene, talcum powder, zinc stearate, and / or molybdenum disulfide.

Conventionally, the viscosity of these solutions, suspension or emulsion, will be adjusted in a manner known per se by adding. required amounts of emulsifier and / or thickening agent. Finally, it is also possible to add additives to these lubrication liquids additional protection for metal parts.

After application of the metallic zinc-based layer possibly followed by the application of the layer of lubricant, the metal blank is then subjected to the forming operation which will mainly be a cold forging, cold stamping or wire drawing.

In practice, it has been possible to carry out parts such as drive shafts and turbine, considerably reducing the phenomena of friction and deformation often observed in the prior art.

This type of parts of relatively complex shapes, have were made by cold forging using a press 8,000 kN.

It will also be recalled that- the cylindrical blanks used for the realization of these parts can be directly subjected to the first mechanical deposition operation of the layer based on metallic zinc by an operation of shot blasting, without the need to use operations for preparing said metallic blank, such as in the prior art.

As a variant, it should be observed that the method according to the invention can possibly combine the two operations of mechanical deposition of a zinc-based layer and application lubricant in a single step. This is how that mechanical deposition of zinc can be envisaged by shot peening based on an iron alloy presence of zinc powder or semolina, mixed directly with a lubricant in solid form also in powder form, for example PTFE or molybdenum disulfide.

To demonstrate the benefits of the process of the invention in comparison with a conventional pretreatment by phosphating, the test results will be indicated below simulation friction comparisons during which a steel sample 21 B3 undergoes plastic deformation located using a G30 tungsten carbide indenter. The conditions of this compression-translation test allow to simulate the drawing and the extrusion which are two operations classics largely representative of the forming of parts of metal by cold deformation.

The precise experimental conditions of this test are for example recalled in the work entitled: Vortragstexte des Symposiums, Neuere Entwicklungen in der Massivumbormung in Fellbach bei Stuttgart, am 19. Und 20. May 1999, unter der leitung von Prof. Dr.-Ing. Dr. hc Klaus Siegert, Institut für Umformtechnik der Universität Stuttgart, in Zusammenarbeit mit der Deutschen Gesellschaft für Materialkunde eV, 1999 by MAT-INFO Werkstoff-Informationsgesellschaft mbH Hamburger Allee 26, D-60486 Frankfurt.

Comparative tests relating to the coefficient of friction µ **

phosphating + soap, compared to a mechanical deposit of zinc + lubricant in the form of an aqueous suspension of graphite. Average of the friction coefficients calculated for friction lengths between 5 and 35 mm Wire drawing: RS * = 14.7% def. Plast. 0.18 contact pressure = 800 MPa Front wire drawing + extrusion: RS * = 14.7% def. Plast. 0.18 contact pressure = 800 MPa RS * = 31% def. Plast. 0.80 contact pressure = 1380 MPa phosphating
+
reactive soap µ = 0.062 µ = 0.10 Mechanical zinc deposition
+
lubricant in the form of an aqueous suspension of graphite µ = 0.054 µ = 0.085

Comparative tests relating to the coefficient of friction µ **

phosphating + saven + extrusion oil, compared to a mechanical deposit of zinc + lubricant in suspension form aqueous graphite + extrusion oil.

The oil used is an MHE 68 oil which meets the specifications of standard ISO 6743/7. Average of the friction coefficients calculated for friction lengths between 5 and 35 mm Front wire drawing + extrusion: RS = 14.7% def. Plast. 0.18 contact pressure = 800 MPa RS = 31% def. Plast. 0.80 contact pressure = 1380 MPa phosphating
+
soap. reagent
+
impact oil during extrusion µ = 0.12 Mechanical zinc deposition
+
lubricant in the form of an aqueous suspension of graphite
+
impact oil during extrusion µ = 0.082

Variations in the process according to the invention were made from modifications in the weight of the mechanical zinc deposition layer. These variations were made between 0 mg / dm ² and 200 mg / dm ² .

The results of the study show that for simple wire drawing operations, a zinc layer weight of 50 mg / dm ² seems to be sufficient in practice.

On the other hand, for chained drawing operations followed by a front extrusion, a layer weight of between 50 mg / dm ² and 100 mg / dm ² seems to represent an optimization of the process according to the invention.

Claims (12)

1. Process for forming metal workpieces by cold deformation, characterized in that it involves the operations of:

   i) mechanically depositing a layer based on metallic zinc on the free surface of the blank of the workpiece to be produced; and

   ii) forming the said workpiece by plastic deformation.
2. Process according to Claim 1, characterized by an additional application of a layer of lubricant on the layer based on metallic zinc applied previously and prior to the forming operation.
3. Process according to either of Claims 1 and 2, characterized in that the layer based on metallic zinc is mechanically deposited by blasting with shot having at least one outer layer comprising a zinc-based alloy.
4. Process according to either of Claims 1 and 2, characterized in that the layer based on metallic zinc is mechanically deposited by blasting with the aid of a mixture of shot made of an iron-based alloy and shot having at least one outer layer comprising a zinc-based alloy.
5. Process according to either of Claims 1 and 2, characterized in that the layer based on metallic zinc is mechanically deposited by blasting with shot based on an iron alloy in the presence of zinc powder.
6. Process according to one of Claims 1 to 5, characterized in that the layer of lubricant is applied in liquid form, particularly by application of a liquid suspension based on graphite particles.
7. Process according to one of Claims 1 to 5, characterized in that the layer of lubricant is applied in solid form, particularly in the form of molybdenum disulphide or of Teflon.
8. Process according to one of Claims 1 to 7, characterized in that the mechanically deposited coating forming a layer based on metallic zinc consists of zinc particles, a mixture of zinc particles and iron particles, or else particles of zinc-iron alloys, preferably in an amount of 50 to 250 mg/dm².
9. Process according to one of Claims 1 to 8, characterized in that the layer of lubricant applied may amount to 300 mg/dm².
10. Process according to one of Claims 1 to 9, characterized in that the cold deformation is a cold-pressing operation.
11. Process according to one of Claims 1 to 9, characterized in that the cold deformation is a cold-forging operation or a metal extrusion operation.
12. Process according to one of Claims 1 to 9, characterized in that the cold-deformation operation is a wire-drawing operation.

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