Abstract:
A method for surface treatment of sheets and strips of aluminum alloy to form a chemically altered layer. The sheet or strip is the product of a manufacturing process including a thermal treatment step followed by cooling in a liquid in which a chemical conversion is carried out using the cooling liquid. The cooling liquid preferably contains between 1 and 10% by weight of at least one salt of one at least of the metals Si, Ti, Zr, Ce, Mn, Mo and V. The invention is particularly applicable to sheets and strips which need a controlled oxide surface for the production of chassis components for the automobile industry for spot gluing or welding.

Description:
DOMAIN OF THE INVENTION  
       [0001]     The invention relates to the domain of surface treatment of aluminium alloy sheets and strips, and more particularly to a type 6xxx or type 5xxx alloy according to the Aluminum Association, to be used particularly for manufacturing of car body parts.  
       STATE OF THE ART  
       [0002]     Aluminium is increasingly used in automobile construction to reduce the weight of vehicles and thus reduce fuel consumption and releases of pollutants and greenhouse effect gases. Sheets are used particularly for making body skin parts, particularly doors. This type of application requires a number of properties that are sometimes contradictory, for example:  
         [0003]     good formability for drawing and crimping operations,  
         [0004]     controlled yield stress in the delivery state of the sheet, to control spring back effect,  
         [0005]     high mechanical strength after baking the paints to obtain good dent resistance while minimizing the weight of the part,  
         [0006]     a surface quality adapted to assembly operations on a dry or greasy sheet,  
         [0007]     good resistance to corrosion, particularly filiform corrosion,  
         [0008]     good surface quality after shaping and painting,  
         [0009]     compatibility with requirements for recycling of manufacturing waste or recycled vehicles,  
         [0010]     an acceptable cost for large series production.  
         [0011]     In Europe, these requirements have led to the choice of Al—Mg—Si alloys, in other words alloys in the 6000 series, for the skin, and Al—Mg alloys in the 5000 series for stiffeners or inner panels. Requirements for the surface condition are related to the assembly method used.  
         [0012]     For the mechanical assembly, there is no particular requirement about the surface quality, except only for a suitable cleanliness condition. For crimped glued parts such as hoods, the bonding operation is usually done on greasy sheets without any deterioration to the resistance of the joints.  
         [0013]     Welding operations sometimes require a clean surface depending on the type, in other words a degreased surface in order to reduce porosities and cracks in the welds. However, this is less critical in the case of laser welding. The surface response is then determined by the value of the contact resistance measured in Europe according to standard DVS 2929.  
         [0014]     For structural bonding in aeronautical construction, a surface pre-treatment is usually applied before bonding, usually consisting of chromic and phosphoric anodising. Chemical conversions based on chromium are used in other application fields such as packaging or building. Although these conversions are frequently used, they could disappear for environmental reasons due to the presence of hexavalent chromium.  
         [0015]     More recent treatments use elements such as silicon, titanium or zirconium to replace chromium. For example, this type of treatment has been described in U.S. Pat. No. 5,514,211 (Alcan), U.S. Pat. No. 5,879,437 (Alcan), U.S. Pat. No. 6,167,609 (Alcoa) and EP patent 0646187 (Boeing).  
         [0016]     For automobile structure parts, there may be a need for a surface preparation adapted to assembly operations. These pre-treatments consume time and are expensive. The formation of the surface layer requires a complete series of manipulations of different baths, possibly requiring more than eight vats. Thus, a standard treatment line is composed of two alkaline degreasing baths, followed by 2 rinsing baths, an acid neutralisation bath, a specific treatment bath followed by two rinsing baths and a drying step. Most of these baths are sometimes heated up to 60° C., which consumes energy.  
         [0017]     Therefore, the invention proposes pre-treatment on aluminium alloy sheets or strips adapted to the requirements of the automobile construction, by minimising sheet or strip manipulation operations. In particular, its purpose is to provide sheets ready for assembly for automobile body parts, with high performances for bonding of glues and adhesives used in automobiles and for spot welding, and a stable surface quality in the long term.  
       PURPOSE OF THE INVENTION  
       [0018]     The purpose of this invention is a surface treatment process for an aluminium alloy sheet or strip to form a chemical conversion coating, the sheet or strip being derived from a manufacturing procedure comprising heat treatment followed by cooling in a liquid, in which chemical conversion is done using the cooling liquid, preferably without any other prior or post surface treatment. Preferably, the cooling liquid contains between 1 and 10% by weight of at least one salt of at least one of the metals Si, Ti, Zr, Ce, Mn, Mo or V.  
       DESCRIPTION OF THE INVENTION  
       [0019]     The process for manufacturing 6XXX alloy sheets, for example, comprises casting of a sheet, followed by scalping of this sheet if required, and homogenisation at a temperature of between 550 and 580° C. Hot rolling preferably takes place at an input temperature of more than 540° C. The hot rolled strip is then cold rolled down to the final thickness. The last cold rolling pass may be made with a textured cylinder, for example by electron beam treatment (EBT), electrical discharge treatment (EDT), or by laser beam which improves the formability and surface appearance of the part formed after painting.  
         [0020]     The solution heat treatment takes place at a temperature as close as possible to the alloy melting initiating temperature, while avoiding burning. The strip enters into a passage homogenisation furnace in which the temperature is more than 500° C. The solution heat-treated metal is immediately quenched by passing it into a tank containing cold water or it is sprayed with this cold water.  
         [0021]     The characteristic of the invention is particularly to do this quenching with a liquid that reacts with the surface to form an oxide layer with bonding and stability properties comparable to what can normally be obtained with long chemical conversion surface treatment operations. Thus, chemical pre-treatment and post-treatment operations normally used in chemical conversion can be entirely eliminated.  
         [0022]     Quenching is preferably done using a solution containing metallic elements such as Si, Ti, Zr, Ce, Mn, Mo, V or combinations of these elements, for example a Ti/Zr product that can react chemically with the metal surface to form a more stable oxide layer than natural oxide. It has been observed that this operation can take place although the band remains in contact with the liquid for a very short time, compatible for production rates.  
         [0023]     It is preferable to exclude the use of reagents containing chromium, to avoid the formation of products containing hexavalent chromium.  
         [0024]     The oxide formed combines both the aluminium and the element present in the bath. Various bath compositions are available on the market, such as those containing titanium, zirconium, cerium, cobalt, manganese, vanadium salts and siliceous compounds.  
         [0025]     Bringing the bath into contact with the hot metal could result in the lack of reaction, or a precipitation of compounds in the solution. This is not the case, and it is observed by carrying out an Electron Spectroscopy for Chemical Analysis (ESCA) or an X-ray analysis, that the main elements already present in the bath are present in the coating. These elements are combined to form a stable coating that will be used as a bonding base for adhesives on the treated metal. This oxide formed during the quenching operation appears more stable than the oxide that could be obtained by quenching the metal in a bath at ambient temperature. Furthermore, the temperature activates the chemical reaction and therefore reduces the contact time, which has the effect of increasing productivity. It might be thought that the first molecules of the bath that come into contact with the surface could be used to vaporise all other contaminants that were not eliminated by the solution heat treatment operation at high temperature in the furnace. It is found that this entrainment can eliminate low mass particles, which cannot be removed simply by passing in the furnace. During the same operation, the alloy is subjected to a boehmite reaction and the bath then reacts with the surface to form a new oxide which will grow to a certain thickness depending on the contact time and the product used.  
         [0026]     Additives into the quenching bath 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 with pH between 3 and 11.  
         [0027]     The same type of treatment may be done on alloy sheets and strips that have not been solution heat treated, but only heat treated at a significantly lower temperature, as is the case for alloys in the 5000 family treated at approximately 400° C. This temperature may be dropped to 250° C. or even lower, depending on the alloy type, without significantly harming the quality of the product formed. Admittedly, there is a risk of reducing the thermal degreasing effect in this case, but degreasing may be done by other means. Therefore, the invention may be applicable to all alloys subjected to a heat treatment followed by cooling in a liquid and requiring a controlled surface oxide.  
         [0028]     The layers formed may be controlled by X-rays or ESCA analysis, which provide information about constituents of the coating and also for ESCA, on chemical bonds in which the elements are involved.  
         [0029]     The oxide is very thin, within the range 5 to 50 nm. It can be measured by the contact resistance equal to less than 20 μΩ, and the values are thus compatible with the requirements of the automobile industry.  
         [0030]     Quenching may be followed by a heat treatment, for example pre-annealing intended to improve the hardening performances during baking of the paint. The strip is then re-wound after planing. Before forming, the sheet may be coated with a lubricant, particularly a dry lubricant, adapted to drawing, assembly, and surface treatment of the part to be made.  
         [0031]     The sheet is usually stored for a variable time period at this stage, which leads to natural ageing that increases the yield stress with time of 6xxx alloys.  
         [0032]     If storage conditions (temperature, humidity) are not controlled, there is often a hydration of the natural oxide. Although this reaction is thermodynamically reversible, it is sometimes necessary to assemble the parts with a high hydration rate, which is prejudicial to assembly operations and life of the assembled parts.  
         [0033]     The bondability of the surface may be tested by a bonding test, for example a cleavage test (called the Boeing test) according to standard ASTM D-3762 (or NF T76-114), well known for evaluating the suitability for structural bonding. After bonding, test pieces are aged in a humid atmosphere at a temperature of 50° C.  
         [0034]     The surface treatment process of aluminium alloy sheets and strips according to the invention is particularly suitable for the treatment of bodywork panels before assembly, particularly by bonding or spot welding. But it can also be applied in other domains that require a passivated surface of the sheet for greater stability in time and a controlled oxide layer. 
     
    
     EXAMPLES  
     Example 1  
       [0035]     A 1.2 mm thick sheet made of 6016 alloy was heated to a temperature of close to 550° C. and was then dipped in a bath containing 2.5% of the product supplied by the Chemetall company to make the passivation solution called Gardobond 4591®, 1 that contains titanium and zirconium as active bond elements. After quenching, two slabs of the sheet were cut out to make reinforced bonding test pieces according to the procedure described in standard NF T76-114. The 200×150 mm slabs were counter-bonded onto one face with slabs made of a 2017-T4 alloy treated by phosphoric anodising to reinforce the structure. The adhesive used for bonding is a single component epoxy glue supplied by the Dow Automotive company.  
         [0036]     After cutting the 125×12.5×1.2 mm test pieces, a wedge is inserted between the two bonded sheets, and the crack propagation induced by this operation is measured. The crack length at this instant T 0  is denoted l 0 . The test piece is then placed in a moist cataplasm type atmosphere at 50° C. After 96 h holding under these ageing conditions, the test pieces are taken out to measure a new crack length l t . For each test, three test pieces are made to give an average value of l. It is observed that the propagation variation Δl=l t −l 0  is 10 mm, which is very low, indicating that the behaviour of the bonded joint is very good. For non-performing assemblies, separation of the slabs by decohesion can occur in a very short time.  
         [0037]     The contact resistance on a sheet metal test piece treated by quenching as before, is measured according to standard DVS 2929. The average value of 10 measurements was made equal to 17.3 μΩ, which means that the behaviour of this treated material must be excellent when being spot-welded.  
       Example 2  
       [0038]     A 1.2 mm sheet made of 6016 alloy was heated to a temperature of close to 530° C. and was then immersed in a bath containing 2% of the Glymo DYNASYLAN® (3-glucidyl-oxy-trimethoxy-silane) product supplied by the Degussa company. After quenching, two slabs of the sheet were cut out to make reinforced bonding test pieces according to the test described in standard NF T76-114. The 200×150 mm slabs are counter-bonded onto one face with slabs made of a 2024 alloy in order to reinforce the structure. The adhesive used for bonding is a single-component epoxy glue XW1044-5 supplied by the Dow Automotive company, with 20 minutes baking at 180° C.  
         [0039]     After cutting the 125×12.5×1.2 mm test pieces, a wedge is inserted between the two glued slabs and the crack propagation induced by this operation is measured. The crack length at this instant T 0  is denoted l 0 . The test piece is then placed in a moist cataplasm type atmosphere at 50° C. After 96 h holding under these ageing conditions, the test pieces are taken out to measure the new crack length l t . It is observed that the propagation variation (average of 3 test pieces) Δl=l t −l 0  is 8 mm, which is very low indicating that the behaviour of the bonded joint is very good. As before, the contact resistance is measured according to standard DVS 2929, the average value of ten measurements being 17.3 μΩ, which means that the behaviour of this treated material must be excellent when being spot welded.  
       Example 3  
       [0040]     The test pieces are made using the same process, but using a bath containing the Ameo DYNASYLAN® (amimi-propyl-ethoxy-silane) product made by the Degussa company. The propagation value Δl is 13.9 mm. This value is close to previous values, which once again indicates good bonding behaviour.  
       Example 4  
       [0041]     The test pieces are made using the same process, but using a bath containing the Alodine 2040 product made by the Henkel company. After bonding, it is observed that the slabs separate immediately during the wedge placement operation, which shows poor bonding, caused by a catastrophic surface preparation. A rough and non-homogenous surface condition was observed for these test pieces. This product is not adapted to the treatment according to the invention.  
       Example 5  
       [0042]     Test pieces were made from slabs that had been subjected to alkaline degreasing, acid deoxidation, a classical Henkel Alodine 2040 titanium conversion with a contact time of 15 s, and drying, with rinsings between the different steps. A value of Δl equal to 14.1 mm is obtained which is slightly higher than the treatment according to the invention. This example illustrates classical in-line treatment based on quenching or sprinkling. This treatment is long and expensive and requires a large investment.  
       Example 6  
       [0043]     The treatment is identical to the previous case, except that a chromium free treatment based on titanium/zirconium is used, using the Gardobond 4591 ® product made by Chemetall. A value Δl equal to 9.1 mm is obtained, which is approximately equal to the value obtained with the treatment according to the invention.  
       Example 7  
       [0044]     The treatment is identical to the previous treatment, except that a chromium free bath based on Henkel Alodine 2840® titanium is used. A value Δl=4.2 mm is obtained, which is slightly better than the value obtained with the treatment according to the invention. However, this process is much longer and more expensive in terms of manipulations.  
       Example 8  
       [0045]     The treatment is identical to that described in the previous example, except that a Gardobond 4707® chromium free titanium based bath made by the Chemetall company is used. A value Δl equal to 8.4 mm is obtained, which is approximately equal to the value obtained with the treatment according to the invention.  
       Example 9  
       [0046]     During this treatment, sheets have been degreased by a solvent and then dipped in the Henkel 2040® Alodine bath. The contact time is identical to the contact time for examples 5, 6, 7 and 8, namely 15 s. A value Δl equal to 65.3 mm is obtained. The high crack propagation value indicates inefficient treatment. However, this treatment is less catastrophic than the treatment described in example 4.  
         [0047]     The observed behaviour may be related to the lack of the bath reaction when the test piece was not chemically degreased. In the case of a treatment according to the invention, degreasing is done by raising to a high temperature and the fact that the reaction is activated by the temperature, and residues are eliminated during contact of the hot sheet with the product.  
         [0048]     ESCA analyses carried out on test pieces treated according to the invention show that the carbon ratio is low, except for test pieces corresponding to the silicon bath, and that it is at the same level as for chemically degreased test pieces according to conventional pre-treatment lines. The thicknesses of the layers formed are greater than the thicknesses of layers formed conventionally. Silane compounds (examples 2 and 3) show a higher carbon ratio related to the fact that these materials contain organic chains. Layers formed from the treatment involving these silane products (examples 2 and 3) contain a large quantity of oxidised silicon, this material being known to improve the bond of glues, varnishes and paints.  
         [0049]     Note that chemical elements present in the bath are located in the oxide layer left by the treatment according to the invention. The bath according to example 1 contains titanium and zirconium, and layers formed from this bath contain them in equivalent proportions. The layer formed under the conditions of example 4 has a titanium content similar to the contents of products prepared by conventional pre-treatment techniques, but this layer did not give good results in the bonding test. Analyses made using transmission electron microscopy show that layer thicknesses obtained according to the method described in the invention are within the range 8 to 50 nm.  
         [0050]     The results of the various examples are given in table 1:  
                                     TABLE 1                               Propagation       Example   Bath reference   length Δl (mm)                                1   Gardobond 4591 TL   10       2   S Glymo TN   8       3   S Ameo TM   13.9       4   Alodine 2010 TK   immediate               separation       5   Alodine 2040 Ref. P   14.1       6   Gardobond 4591 Ref. Q   9.1       7   Alodine 2840 Ref. CR   4.2       8   Gardobond 4707 Ref. CS   8.4       9   Alodine 2040 Deg. T   65.3