EP0288853B1 - Process for the preparation of work pieces from titanium or titanium alloys - Google Patents

Description

The invention relates to a method for preparing workpieces made of titanium or titanium alloys for sliding operations and its use for preparing the workpieces for cold forming.

In the cold forming of metals, it is essential to create lubricating layers with the help of lubricants in order to prevent corrosion, i.e. avoid direct contact between workpiece and tool during cold forming. For example, in the case of cold forming of steel, oil-containing high-pressure lubricants are used at relatively low degrees of deformation, soaps or solid lubricants applied to phosphate or oxalate coatings at higher degrees of deformation.

Similar to steel, titanium or titanium alloys are also subjected to cold forming in various processes, such as tube drawing, wire drawing, cold extrusion, and cold rolling of sheet metal. In contrast to steel, however, titanium and titanium alloys tend to seize to a much greater extent, since their chemical resistance is greater and, accordingly, suitable lubricant carrier layers are difficult to apply. It is therefore the current state of the art to apply lubricating oil to incandescent scale or to a resin coating as a lubricant carrier in the pipe drawing. As far as conversion coatings are concerned, the investigations are limited to fluoride coatings.

When cold rolling sheet metal made of titanium or titanium alloys, large diameter rolls cannot be used because of the tendency to seize and because of the greater metal hardness. That is why Sendzimir rolling mills equipped with rolls of smaller diameters are used, such as for rolling stainless steel. Mineral oil-based lubricants are usually used as lubricants.

In connection with cold extrusion, numerous attempts have been made to determine suitable lubricating layers without, however, finding a solution suitable for practical use.

Apart from the fact that e.g. The lubrication layers that can be used for the pipe drawing are available with glow scale or a resin coating as the lubricant carrier, the next problem arises from the need to remove the remaining lubricating layer after the forming process. In the specific case, this is very labor-intensive.

The generation of lubricating layers with a lubricant carrier layer based on fluoride and subsequent soaping result in good lubricating properties, but the service life of the treatment baths for applying the fluoride coating is extremely short. It is therefore very difficult to ensure uniform coating formation over a long period of time. In the case of titanium alloys, in particular those with higher corrosion resistance, it is not possible to form a fluoride coating.

When cold rolling titanium or titanium alloys, even with a Sendzimir rolling mill, they are very easily scuffed, so that a cross-sectional reduction of only less than 15% per pass is possible. The throughput is correspondingly low. The generation of a thin oxide layer by heating the workpiece has not brought about any improvement in this regard. In this case, too, the application of a fluoride coating is advantageous, but the comments made above apply in the same way.

The object of the invention is to provide a method for preparing workpieces made of titanium or titanium alloys for sliding processes that does not have the known, in particular the aforementioned disadvantages, which in particular enables cold forming without gassing even with a large reduction in cross section and is of constant quality over a long period of time.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the workpieces are immersed in an aqueous phosphate solution containing acidic zinc phosphate and a zinc phosphate coating is produced on their surface by cathodic electrolysis.

The acidic zinc phosphate solution to be used in the process according to the invention contains primary zinc phosphate as the main constituent. The zinc content is in the range from 1 to 50 g / l, preferably in the range from 5 to 20 g / l. The phosphate content is usually 3 to 140 g per liter (calculated as PO₄), preferably 10 to 60 g per liter. The addition of ions other than zinc, such as calcium, manganese or iron, is possible and allows modified phosphate coatings to be obtained.

The phosphating solution usually contains oxidizing agents, which can be organic or inorganic. Examples include nitrate, nitrite, hydrogen peroxide, nitrobenzenesulfonate and para-nitrophenol compounds.

The pH of the phosphating solution is generally in the range from 0 to 5, preferably in the range from 1.5 to 3.5. He can e.g. be adjusted with the help of sodium hydroxide solution or sodium carbonate.

The phosphating solution is generally used at a temperature of 30 to 80 ° C, preferably 40 to 60 ° C. In cathodic electrolysis, the workpiece is switched as a cathode, while e.g. a zinc plate serves as an anode. Other suitable anode materials are graphite, platinum sheets, stainless steel sheets and the like.

During electrolysis, the conditions with regard to electrode spacing, current density and duration of treatment should be selected in such a way that the required coating properties are obtained. The electrode spacing is about 5 to 30 cm, the current density is 0.2 to 30 A / dm², preferably 0.5 to 5 A / dm², and the electrolysis time is 10 s to 5 min. An excessively high current density and an excessively long electrolysis time can lead to blackening of the coating produced or reduced adhesion of the coating.

The zinc phosphate coating which is obtained by the process according to the invention generally has a layer weight of 2 to 20 g / m². Known lubricants such as sodium soap from fatty acids, mineral oil and solid lubricants are applied to it.

According to a preferred embodiment of the invention, the workpieces are pretreated with an aqueous conditioning agent based on a colloidal titanium compound before immersion in the phosphating solution.

The conditioning agents known per se for the phosphating of metal surfaces serve as the aqueous conditioning solution. They generally contain 10 to 200 ppm titanium, 200 to 3000 ppm phosphate, 30 to 600 ppm pyrophosphate and have a pH of 7.5 to 9.5. Titanium sulfate, titanyl sulfate or titanium oxide, or phosphoric acid or alkali or ammonium phosphate or alkali or ammonium pyrophosphate can serve as sources for the individual components. The activation agent is produced in a simple manner by mixing the above-mentioned substances with water. Then the water is removed and the residue is homogeneously mixed with sodium carbonate and the like, so that a solid results, which, when in water with the above-mentioned quantities stated _ leads to the prescribed pH value. The task of the conditioning agent is to apply adherent, colloidal titanium compounds to the surfaces of the titanium or the titanium alloys, which improve the layer formation and the layer quality during the subsequent coating formation. As a result, lower titanium concentrations than those given above result in poorer coating formation. Higher titanium concentrations have no additional effect. Differing concentrations of phosphate and pyrophosphate have a similar effect.

As for the pH of the aqueous conditioning solution, lower values than those mentioned hinder the subsequent coating formation. The same is the case when the pH is higher.

After the treatment in the aqueous conditioning agent, the workpieces are immersed in the aqueous zinc phosphate solution for cathodic electrolysis. The conditions regarding the zinc phosphate solution and the cathodic electrolysis are the same as mentioned above. The lubricants known per se already mentioned are also used.

The following should be noted regarding the layer formation mechanism:

In the case of treating steel, a zinc phosphate coating can be applied to the workpiece by dipping it in an acidic one Zinc phosphate solution can be obtained in the simplest way, whereas in the case of the treatment of titanium or titanium alloys which have a dense oxide coating on the surface, the pickling reaction by phosphoric acid does not take place and consequently a phosphate coating is difficult to form.

$$\text{Me = Metall}$$

$$\text{3 Zn(H₂PO₄)₂ → Zn₃(PO₄)₂ + 4 H₃PO₄}$$

The reaction in acidic zinc phosphate solution can be expressed by the following relationships:
$$\text{Me + 2H⁺ → Me²⁺ + H₂ ↑}$$

$$\text{Me = metal}$$

$$\text{3 Zn (H₂PO₄) ₂ → Zn₃ (PO₄) ₂ + 4 H₃PO₄}$$

When reaction (1) takes place and, consequently, the pH increases in the immediate vicinity of the metal surface, reaction (2) takes place and tertiary metal phosphate separates out to form a coating. Without the reaction according to (1), the coating cannot be formed.

ab.In the case of the treatment of titanium or titanium alloys, reaction (1) does not take place and consequently no coating is formed. When using cathodic electrolysis, a reaction proceeds analogously to relationship (1)
$$\text{2H⁺ + 2e → H₂ ↑}$$

from.

This reaction also increases the pH in the vicinity of the metal surface, so that the reaction can take place according to the relationship (2). This means that titanium or titanium alloys can be coated with zinc phosphate without pickling. Furthermore, due to the presence of zinc ions in the treatment solution, a deposition of metallic zinc as a result of the cathodic electrolysis can be determined.

In the embodiment of the invention with conditioning of the workpieces in a solution containing a colloidal titanium compound, particles of titanium compound adhering to the metal surface are obtained, which play the role of a crystal nucleus and allow the zinc phosphate crystals to grow easily from the aqueous zinc phosphate solution. As a result of the large number of crystal nuclei, the phosphate coating formed is thin and fine-grained and has good adhesion. In contrast to this, the zinc phosphate deposition on cathodic areas of the metal surface is favored without a pretreatment by a conditioning agent. Then the initially deposited phosphate crystals play the role of a crystal seed for further layer formation. However, as a rule the coatings formed become thick and of a porous nature because of the small number of crystal nuclei.

In the case of the treatment of titanium or titanium alloys by cathodic electrolysis with the aid of zinc phosphate solutions, the pickling attack customary for steel does not take place. Therefore, almost no metal is released from the treated metal surface and transferred into the solution. This leads to the advantage that the Phosphating solution experiences practically no aging, nor does sludge occur, so that the phosphating solution can be monitored in the simplest way.

The zinc phosphate layers produced with the aid of the method according to the invention are aftertreated with a lubricant known per se, so that the workpieces have excellent properties for a subsequent sliding operation. The smear layer can be made uniform over a long period of time without e.g. comes to fretting during the subsequent cold forming.

The fact that no metal ions get into the solution from the treated metal surface due to the electrophoretic deposition of zinc phosphate has the advantage that the service life of the phosphating solution is very long and sludge formation is suppressed. This makes it easy to monitor the treatment solution.

The method according to the invention is intended for the treatment of all workpieces which are at least temporarily exposed to any sliding processes. These are in particular gear parts, bearings and the like, but also, for example, objects used for fastening, such as bolts, screw threads and the like. However, the greatest importance of the process lies in its application for the preparation of workpieces for cold forming.

The invention is explained in more detail and exemplified by the following examples and comparative examples.

example 1

Cleaned sheets of pure titanium (JIS grade 1) with the dimensions 100 × 50 × 0.8 mm were electrolytically phosphated with a phosphating solution of the following quality:

9.6 g / l zinc
36.3 g / l phosphoric acid (calculated as PO₄)
2 g / l nitric acid
0.5 g / l sulfuric acid
0.03 g / l nickel
pH approx. 3.0.

The electrolysis conditions were:

The phosphated sheets were then treated with a lubricant (Palube 235, a product of Nihon Parkerizing Co., Ltd., with sodium stearate as the main component). The concentration of the Lubricant was 70 g / l, its temperature 75 ° C and the treatment time 3 minutes.

Comparative Examples 1 to 3

Cleaned titanium sheets of the quality specified in Example 1 were treated as follows:

(Comparative Example 1)

The sheets were provided with the aid of 111 QD (product from Hangstafer), the main component of which is a rubber resin, with a plastic coating 10 μm thick, over which a lubricant containing an organic chlorine compound (J1, product from Hangstafer) was applied in an amount of 10 g / m² was applied.

(Comparative Example 2)

A lubricant containing an organic chlorine compound (J1, product from Hangstafer) was applied in an amount of 10 g / m 2 over the glow scale produced by treatment at 700 ° C. for 1 hour.

(Comparative Example 3)

With the aid of a fluoride-containing solution (Palmet 3851, product of Nihon Parkerizing Co., Ltd.), a fluoride coating was produced under the following conditions:

The palube 235 mentioned in Example 1, which was applied under the same conditions, served as the lubricant.

The four sheet series obtained in the above manner were tested with a Bauden test device (Toyo, Baldwin, model EFM-4). The test was carried out at room temperature under a load of 5 kg with a sliding amplitude of 10 mm and a sliding speed of 10 mm / sec.

The test was evaluated both by determining the number of sliding movements up to a friction coefficient of 0.25 (occurrence of contact marks) and by determining the friction coefficient.

The table shows that the titanium sheet treated according to Example 1 permits a considerably higher number of sliding movements until seizure occurs and also has a lower friction coefficient than the titanium sheet treated according to Comparative Examples 1 to 3.

With regard to comparative example 3, it should be noted that the sliding values are similarly good as in the case of the treatment by example 1, but that the treatment bath used is already processed after a throughput of 0.3 m 2 / l of solution such that the layers subsequently obtained are of significantly lower quality.

Example 2

Titanium sheets of the quality mentioned in Example 1, but with the dimensions 200 mm × 20 mm × 1.3 mm, were electrolytically phosphated in the phosphating solution and under the conditions according to Example 1. There was no after-treatment with lubricant.

Comparative Examples 4 to 6

The titanium sheet defined in Example 2 received the following treatment.

(Comparative Example 4)

No treatment at all.

(Comparative Example 5)

Heat to 300 ° C to form an oxide film 200 nm thick.

(Comparative Example 6)

Konzentration

24 g/l

Temperatur

60°C

Behandlungsdauer

2 min.

Treatment with a fluoride bath (Palmet 3851 from Nihon Parkerizing) with the conditions

concentration

24 g / l

temperature

60 ° C

Duration of treatment

2 min.

The sheets of the individual tests were rolled with the aid of a duofin sheet mill (testing device) using rolling oil (Finerol 704-3 from Nihon Parkerizing) at a concentration of 10% and 40 ° C.

The value Σ (% / T) was determined for each of the sheet series obtained according to Example 2 or Comparative Examples 4 to 6. It stands for the sum of the quotients from cross-section reduction (in%) per pass divided by rolling force (in t / mm²).

A comparison shows that the conventional method according to comparative experiment 4 is associated with very poor results.

Comparative example 6 leads to values that are close to that according to the present invention. However, due to the short life of the treatment baths, this method (fluoride method) has not been successful in practice.

Example 3

This example corresponds to Example 1 in terms of sheet quality, electrolytic phosphating and aftertreatment. It was only preceded by a conditioning treatment by immersing for 10 seconds in an aqueous conditioning agent containing 3 g / l of titanium compound (Prepalene Z from Nihon Parkerizing) at room temperature.

The test panels were subjected to the Bauden test explained in Example 1. Table 3 below illustrates that, as a result of the additional activation carried out, even better results are obtained than in the case of Example 1.

Claims (3)

1. Method for preparing work pieces made of titanium or titanium alloys for sliding purposes, characterized in that the work pieces are immersed into an aqueous acid zinc phosphate containing phosphatizing solution and in that a zinc phosphate layer is produced by cathodic electrolysis on the surfaces of the pork pieces.

2. Method according to claim 1, characterized in that the work pieces are pretreated before immersing into the phosphatizing solution with an aqueous conditioning medium based on a colloidal titanium compound.

3. Application of the method as described in claims 1 or 2 for preparing work pieces made of titanium or titanium alloys for cold working.