EP1646735B1 - Method for processing surfaces of aluminium alloy sheets and strips - Google Patents

Abstract

Surface treatment of a strip, a sheet or a component in an aluminum alloy comprises: (a) preparation of the surface with the aid of a plasma atmosphere; (b) treatment with the aid of a bath containing between 1 and 10 % by wt of at least one salt of at least one of the metals Si, Ti, Zr, Ce, Co, Mn, Mo or V, in order to form a conversion layer on the strip, sheet or component.

Description

Field of the invention

The invention relates to the field of surface treatments of aluminum alloy sheets and strips, as well as parts stamped from these sheets, more particularly to type 6xxx or 5xxx alloys according to the designation of the Aluminum Association, intended for in particular to the manufacture of motor vehicle body parts.

State of the art

Aluminum is increasingly used in the automotive industry to reduce the weight of vehicles and thus fuel consumption and the discharge of pollutants and greenhouse gases. The sheets are used in particular for the manufacture of body skin parts, in particular the openings. This type of application requires a set of properties, sometimes antagonistic, in terms of mechanical strength, corrosion resistance, formability; with an acceptable cost for mass production.

These requirements led, in Europe, to the choice of Al-Mg-Si alloys, that is to say the alloys of the 6000 series, for the skin, and alloys Al-Mg of the 5000 series, for the reinforcements or linings. There are also surface condition requirements, which are related to the method of assembly used.

For the mechanical assembly, there is no particular requirement on the surface quality, except only a suitable state of cleanliness. Depending on the type, welding operations sometimes require a clean, ie degreased, surface in order to reduce the porosities and cracks in the welds. This is however less critical in the case of laser welding. The response of the surface is then determined by the value of the contact resistance measured in Europe according to the DVS 2929 standard.

For structural bonding in aircraft construction, surface pre-treatment is usually used before bonding, usually chromic and phosphoric anodizing. In other fields of application such as packaging or building, chromium-based chemical conversions are used. Although still often used, these conversions may disappear for environmental reasons for fear of the presence of hexavalent chromium.

More recent treatments use elements such as silicon, titanium or zirconium to replace chromium. Such treatments are described, for example, in patents US 5514211 (Alcan ) US 5879437 (Alcan ) US 6167609 (Alcoa ) and EP 0646187 (Boeing ).

For automotive structural parts, the need for a surface preparation suitable for assembly operations, such as bonding and spot welding, may be necessary. Performing these pre-treatments is time consuming and expensive. Indeed the formation of the surface layer requires a series of manipulations of different baths with a number of tanks that may be greater than 8. Thus a standard treatment line consists of 2 alkaline degreasing baths, followed by 2 baths of rinsing, an acid neutralization bath, a specific treatment bath, followed by 2 rinsing baths and a drying step. Most of these baths are sometimes heated up to 60 ° C, which is energy intensive.

The invention therefore proposes to carry out a pre-treatment on aluminum alloy strips or sheets adapted to the requirements of the automobile industry, while minimizing the handling operations of the strip or sheet. In particular, it aims to provide ready-to-assemble sheets for car body parts, with high performance for the adhesion of adhesives and adhesives used in automotive and spot welding, as well as a stability in the time of the quality of surface.

Object of the invention

The invention relates to a method for surface treatment of a strip, a sheet or a part stamped from an aluminum alloy sheet or strip, consisting of a surface preparation using an atmospheric plasma, and a chemical conversion treatment using at least one of Si, Ti, Zr, Ce, Co, Mn, Mo or V to form the conversion layer.

The conversion treatment can be carried out using a bath containing between 1 and 10% by weight of at least one salt of at least one of the elements Si, Ti, Zr, Ce, Co, Mn, Mo or V, and in this case the process preferably comprises, at the end of the treatment, a spin with a roller. It can be carried out by immersion in the bath, by spraying the bath on the strip, the sheet or the workpiece, or by coating the bath with a roller, according to a "no rinse" technique.

The conversion treatment can also be done using an atmospheric plasma in which the plasmagenic gas comprises a compound of at least one of the elements Si, Al, Ti, Zr, Ce, Co, Mn, Mo or V The element of the compound added to the plasma gas is preferably silicon.

Description of figures

• The figure 1 shows the results of the bonding tests on 5754 samples in state O and 6016 in the T4 state treated according to the process of the invention with two different baths compared to reference samples.
• The figure 2 shows results of the same type obtained on samples treated according to the invention with a plasma conversion.

Description of the invention

The invention is based on the finding made by the applicant that, when the chemical conversion treatment is preceded by a preparation, for example a degreasing, using an atmospheric plasma, this treatment can be considerably simplified compared with to the treatments of the prior art made for the same purpose, and that one could be satisfied with a fast treatment, for example of the "no-rinse" type using a conversion bath with spinning with a roller, but also a conversion treatment performed, too, using an atmospheric plasma.

Atmospheric plasma techniques have developed significantly in recent years and many applications have been proposed, especially in the treatment of metals. For example, the patent application WO 02/39791 (APIT Corp.) discloses a method and apparatus for atmospheric plasma treatment of a conductive surface, and in one of the examples mentions the cleaning of an aluminum foil from the rolling grease residues.

A treatment of this type has, surprisingly, proved more favorable than the usual chemical degreasing treatments for the implementation of the subsequent chemical conversion, the plasma performing both the degreasing and the modification of the natural oxide. present on the surface of aluminum. In addition, it has been found that the atmospheric plasma can also be used for the formation of the conversion layer itself, provided that the compound containing the decomposition of the desired element for the conversion layer is added to the plasma-forming gas.

By combining the degreasing and conversion stages, the use of an atmospheric plasma leads to a significant time saving and considerably reduces the constraints related to the treatment of rejects.

Finally, it allows processing speeds compatible with the running speeds of the aluminum alloy strips at the exit of the rolling lines. It is thus possible to reach without difficulty speeds of the order of 5 m / min to 600 m / min.

In a first embodiment of the process according to the invention, the chemical conversion treatment is preferably carried out using a solution containing metal elements such as Si, Ti, Zr, Ce, Co, Mn, Mo, V or combinations of these elements, for example a Ti / Zr product, which can chemically react with the metal surface to form a more stable oxide layer than the natural oxide. It has been found that this operation can be carried out although the strip, the sheet or the piece remains in contact with the liquid only for a very short time. In the case of tapes, this allows on-line processing compatible with the production speeds of these tapes.

It is preferable to exclude chromium-containing reagents to avoid the possible formation of products containing hexavalent chromium. The additives in the treatment baths are at a very low concentration, less than 10%, and preferably between 1 and 5%. Similarly, the aggressiveness of the bath in terms of acidity is limited by using baths of pH between 3 and 11.

The oxide formed combines both the aluminum and the element present in the bath. Many bath compositions are available on the market, such as containing salts of titanium, zirconium, cerium, cobalt, manganese, vanadium or silicon compounds.

After treatment in contact with the bath, the strip, the sheet or the workpiece is preferably dewatered using a roller according to the so-called "no-rinse" technique known to those skilled in the art, this technique being particularly suitable for continuous treatment of tapes.

The layers formed can be controlled by weight gain, X-ray fluorescence or ESCA analysis, the latter two techniques giving information on the constituents of the layer and, in addition, for ESCA, on the chemical bonds in which the elements are involved. .

The oxide thickness is very small, in the range 5 to 50 nm. ESCA analysis can give an estimate of the oxide layer if its thickness is less than about 6 nm and if the surface contamination is low. Indeed, most often, the surface is covered with a layer of carbon contamination that disturbs the measurement. To gain access to a more precise measurement, transmission electron microscopy can be used after sample preparation by microtomy. This technique makes it possible to calibrate the measurements made by ESCA.

The measurement of the contact resistance can also be used. With the method according to the invention, this resistance is less than 20 or 15 μΩ, which is compatible with the requirements of the automotive industry.

In a second embodiment of the invention, the conversion layer is obtained by a new passage in an atmospheric plasma, the plasmagenic gas, for example air, argon or a rare gas plus oxygen mixture. The plasmagene gas is enriched with a compound which decomposes the metal element of the Si, Al, Ti, Zr, Ce, Co, Mn, Mo or V group which it is desired to include in the conversion layer. One of the most efficient elements is silicon, which leads to an SiO _x type conversion layer where x is close to 2. Silicon can come for example from the decomposition of an organic compound containing silicon or silicon and oxygen, such as tetra-ethyl-disiloxane, tetra-methyl-disiloxane, hexa-methyl-disiloxane or hexamethyldisilazane, mixed with the argon used for the plasmagen mixture.

The oxide layer obtained by this embodiment has a uniform thickness layer 10 to 30 nm on which is deposited a set of aggregates nanobeads more or less interconnected, with an excess thickness may exceed 200 nm.

It can be assumed that this structure of the oxide layer comes from its formation in two successive stages. Firstly, the growth of a uniform and continuous barrier layer, where silicon combines with oxygen, and possibly other elements present on the surface, to form an amorphous deposit, then the growth of silica nanobeads forming aggregates, all the more important that the number of passages (equivalent to a longer transit time of the surface in front of the plasma) is higher. These aggregates contribute, by ensuring mechanical anchoring, to improve the adhesion of the base oxide layer in case of bonding.

The method according to the invention gives results as good as a conventional treatment comprising the passage in degreasing baths, pickling and rinsing, which leads to a shorter treatment time and reduced cost. This is even more pronounced when using a "no rinse" conversion or a plasma conversion, which avoids the passage into a rinsing bath. Finally, the use of compounds without chromium makes it possible to better respect the environment and to simplify the treatment of the effluents.

examples Example 1

Samples of aluminum alloy sheets AA5754 in the 0 (annealed) state of thickness 1 mm, and of AA6016 alloy in the T4 state with a thickness of 1.2 mm were prepared. The samples were degreased by atmospheric plasma treatment with an apparatus from Plasma Treat GmbH, with the operating parameters shown in Table 1: Table 1 Frequency of work 16 - 20 kHz Working voltage 5 kV Plasma power 1000W Plasma generator FG1001 minimum High voltage transformer HTR1001 Width of treatment 5 mm per nozzle and up to 120 mm per rotation of 2 nozzles Nozzle rotation speed 2000 rpm Processing speed 5 m / min Distance nozzle - surface 15 and 20 mm Compressed air 5-7 bar filtered deoiled 20 l / min (1.2 Nm ³ / h)

The plasma treatment is performed with several passes in front of the torch to accumulate energy on the metal avoiding excessive heating that could lead to an early fusion.

After plasma treatment, the ESCA analysis shows a clear decrease of the carbon layer, which goes from 40-50% of surface carbon to 25-30%. This value may still appear high, but it is probably related to the fact that samples are analyzed after exposure to the air. The oxide layer passes from its side by a value of between 3 and 5 nm to a value of between 6 and 8 nm depending on the alloy.

The ESCA analysis also indicates a magnesium enrichment of the surface oxide, magnesium oxide accounting for nearly one-third of the surface oxide, but paradoxically this magnesium level does not seem to hinder adhesion, contrary to this which is generally accepted.

1. A) Gardobond ® X4591 de Chemetall, à base de sels de titane et de zirconium.
2. B) Alodine ® 2040 de Henkel à base de sels de titane
3. C) Dynasylan ® Glymo (3-glucidyl-oxy-trimethoxy-silane) de Degussa

The samples were then immersed for 5 seconds in a treatment tank containing the bath and then dewatered manually using a roller, wiped after each operation. The following products were used for the bath:

1. A) Gardobond ® X4591 from Chemetall, based on titanium and zirconium salts.
2. B) Alodine ® 2040 from Henkel based on titanium salts
3. C) Dynasylan ® Glymo (3-glucidyl-oxy-trimethoxy-silane) from Degussa

The ESCA analysis shows that the 3 products lead to conversion layers that are practically identical to those obtained in conventional conversion. Product C shows a slightly higher surface carbon content, which can be attributed to the maintenance, in the silicon oxide, of carbon chains of the precursor.

Trick tests using the wedge cleaving test according to EN 30354, slightly adapted to meet the requirements of EN 30354, were carried out with treated samples 150 mm long and regreased with Quaker DC 1 55/45 dry lubricant. used for alloys intended for the automobile bodywork: the corner is half-pressed to avoid dissipate the energy too quickly, and the test is laminated on a test tube of the same format in alloy 2017 in the T4 state to stiffen the whole. The climatic chamber is aged at 50 ° C. and at a relative humidity of 100% at durations of 1, 5, 24, 48 and 96 h, respectively. The propagation of the crack is observed in the binocular on both sides after having left each time the test pieces 1 h at room temperature. We deduce an average propagation in groups of 3 test pieces.

The figure 1 shows the crack propagation for the conversion layers made according to the invention with a bath containing the products A and C, as well as for reference samples degreased with solvent Viapred from SID (product D). It is found that the samples treated according to the invention have, whatever the alloy used, a better behavior than the reference treatment, and are therefore suitable for bonding.

Example 2

Samples of aluminum alloy sheets AA5182 in the 0 (annealed) state of thickness 1 mm, and of AA6016 alloy in the T4 state with a thickness of 1.2 mm were prepared. The samples were degreased by atmospheric plasma treatment using a device such as that described in the patent application. WO 02/39791 and using as reactive gas hexamethyldisilazane.

• dégraissage : on effectue plusieurs passages devant la torche pour accumuler de l'énergie sur le métal en évitant un échauffement excessif qui pourrait conduire à une modification structurale du métal ou à début de fusion.
• dépôt d'une couche de composé d'oxyde de silicium SiOx de stoechiométrie proche de 2.

Plasma treatment is performed with two steps:

• degreasing: several passes are made in front of the torch to accumulate energy on the metal avoiding excessive heating which could lead to a structural modification of the metal or early melting.
• depositing a layer of SiOx silicon oxide compound of stoichiometry close to 2.

After plasma treatment, the ESCA analysis, whose results are shown in Table 2, clearly shows the presence of this layer of silicon oxide. Its thickness depends on the treatment conditions. Thicknesses of 100 to 300 nm could thus be deposited by the atmospheric plasma technique. This layer masks the other elements present at the extreme surface of the metal, but for the small thicknesses one can still detect elements such as A1 or Mg. Table 2 Sample and treatment C1s O1s MgKLL AI2p Si2p m σ m σ m σ m σ m σ 5182 O Si02 # 1 17,94 2.95 57.37 2.04 0.76 0.10 0.34 0.17 23.58 0.94 5182 O SiO2 # 2 15,26 2.04 59.63 1.63 0.66 0.29 0.56 0.34 23.88 0.52 5182 O SiO2 # 3 10.50 0.75 63.64 1.03 0.50 0.32 0.49 0.20 24.87 0.46 6016 SiO2 # 1 10.81 3.11 63.51 2.46 0.23 0.10 0.87 0.54 24.58 0.96 6016 SiO2 # 2 10.00 0.49 63.93 0.50 0.36 0.05 0.89 0.35 24.82 0.32 6016 SiO2 # 3 2.20 60.76 1.64 0.50 0.07 1.53 0.27 23,29 0.78 5182 H22 SiO2 # 1 16,20 2.06 59,00 1.03 0.86 0.24 0.86 0.28 23,07 1.30 5182 H22 SiO2 # 2 32.38 11.33 48.50 8.12 1.41 0.31 0.71 0.22 17,00 3.76 5182 H22 SiO 2 # 3 * 53.27 9.75 33.48 7.07 5.82 1.42 6.45 2.25 0.98 0.70

The table gives the atomic percentages of the elements on the surface of the samples.

Sample 5182-H22 SiO2 # 3 shows different values from the other samples. The carbon content is important while it shows practically no silica on the surface. This sample was analyzed on the untreated side, which confirms the effect of stripping and treatment. Other changes in carbon content can be attributed to contamination when handling treated plates. However, the detection of Al and Mg elements in a substantially larger amount may indicate that the thickness is slightly lower.

Trick tests using the wedge cleaving test according to EN 30354 were performed with treated 150 mm length samples, bare or re-greased with Quaker's DC 1 55/45 or Ferrocoat® 6130 lubricants. , slightly adapted to be used for the alloys intended for the automobile bodywork: the corner is half-pressed in order not to dissipate the energy too quickly, and the specimen is laminated on a specimen of the same alloy format 2017 in the state T4 to stiffen the whole. The climatic chamber is aged at 50 ° C. and at a relative humidity of 100% at durations of 1, 5, 24, 48 and 96 h, respectively. The propagation of the crack is observed in the binocular on both sides after having left each time the test pieces 1 h at room temperature. We deduce an average propagation in groups of 3 test pieces.

The figure 2 shows the crack propagation for atmospheric plasma deposition carried out on alloys 6016 and 5182 and used with or without lubricant, as well as for reference samples converted chemically according to methods used by some car manufacturers. It is found that the treated samples have, whatever the alloy used, a better behavior than the reference treatments. Bonding in the absence of lubricant, which is performed immediately after treatment, gives a slightly better result. Likewise, the 5182 alloy in the O state behaves slightly better than in the H22 state. The bonding operations for the lubricant-coated plates were carried out after storage in the lubricated state for a period of more than 1 ½ months in a normal laboratory atmosphere. This shows the robustness of the atmospheric plasma treatment which considerably improves the surface properties of the metal. This quality of the surface is also demonstrated through the observation of the fracture facies of the cleavage test. Unlike other treatments for which there is sometimes an adhesive rupture (RA), ie at the interface surface oxide - adhesive, here in all cases there are cohesive breaks (RC), ie intervening in the adhesive or close to its surface (superficial cohesive rupture).

Table 3 shows the failure mode of the joints bonded during the cleavage test. Table 3 Cases tested Δ96-0 origin 5 h 48 h 96 h End 6016 SiO2 # 1 No lube 3.3 RC RC RC RC RC 6016 SiO2 # 1 DC3 2.7 RC RC RC RC RC 6016 SiO2 # 2 DC3 4.1 RC RC RC RC RC 6016 SiO2 # 3 DC3 4.5 RC RC RC RC RC 5182 O SiO2 # 1 No lube 2.7 RC RC RC RC RC 5182 O SiO2 # 1 6130 2.9 RC RC80 RC80 RC80 RC 5182 O SiO2 # 2 6130 3.6 RC RC90 RC85 RC85 RC 5182 O SiO2 # 3 6130 3.7 RC RC RC RC RC 5182 H22 SiO2 # 1 6130 * 4.3 RC RC RC RC RC 5182 H22 SiO2 # 2 6130 * 4.8 RC RC RC RC RC 5182 H22 SiO2 # 3 6130 * 5.1 RC RC95 RC RC RC 6016 Alodine. 2040 DC1 14.1 RC RA RA RA RC 6016 Alodine. 2840 DC1 4.2 RC RC RA RA RC 6016 DR100 Gardobond 4591 DC1 7.1 RC RC RA RA RC 6016 DR100 Gardobond 4700 DC1 8.3 RC RC RA RA RC 6016 Degr. solv. DC1 14.5 RC RA75 RA RA RC95 6016 Lube DC1 17.8 RC RA55 RA RA RC95

Claims (12)

1. Process for the surface treatment of a strip, a sheet or a stamped part of a sheet or a strip made of an aluminium alloy consisting of a surface preparation using an atmospheric plasma and a chemical conversion treatment using at least one of the elements Si, Ti, Zr, Ce, Co, Mn, Mo or V to form the conversion layer on the strip, the sheet or the part.
2. Process according to claim 1, characterised in that the aluminium alloy is an alloy in the 5000 series or 6000 series.
3. Process according to one of claims 1 or 2, characterised in that the conversion treatment is done using at least one salt of one of at least the said elements.
4. Process according to claim 3, characterised in that the conversion treatment is done by immersion in the bath.
5. Process according to claim 3, characterised in that the conversion treatment is done by atomisation of the bath on the strip, the sheet or the part.
6. Process according to claim 3, characterised in that the conversion treatment is done by coating the strip, the sheet or the part with the bath.
7. Process according to one of claims 3 to 6, characterised in that the treatment bath has a pH of between 3 and 11.
8. Process according to one of claims 1 or 2, characterised in that the conversion treatment is done using an atmospheric plasma using a plasmagenic gas including a compound of at least one of the elements Si, Al, Ti, Zr, Ce, Co, Mn, Mo or V.
9. Process according to one of claims 1 to 8, characterised in that the treatment rate is from 5 m/min to 600 m/min.
10. Process according to one of claims 1 to 8, characterised in that the conversion layer has a thickness of between 5 and 300 nm.
11. Use of sheets or strips made using the process according to one of claims 1 to 10, for manufacturing glued or spot welded parts.
12. Use of sheets or strips made using the process according to one of claims 1 to 10 for manufacturing automobile bodywork parts.

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