GB2335202A

GB2335202A - Flame-spray process for the pre-treatment and coating of surfaces

Info

Publication number: GB2335202A
Application number: GB9905623A
Authority: GB
Inventors: Heinz Huber; Wolfgang Staufner
Original assignee: DaimlerChrysler AG
Current assignee: Daimler AG
Priority date: 1998-03-11
Filing date: 1999-03-11
Publication date: 1999-09-15
Anticipated expiration: 2019-03-11
Also published as: GB9905623D0; FR2775913A1; ITMI990451A1; IT1309589B1; GB2335202B; FR2775913B1; DE19810382C1

Abstract

A process for applying a protective layer to the surface of a component comprising a blasting process for activating the surface of the component and at least one spraying process for coating the component, wherein the blasting process and the spraying process are performed by the same apparatus. For preference, the surface to be coated is a light-metal (eg aluminium or magnesium) alloy, copper alloy, nickel alloy or steel alloy which is blasted with aluminium oxide or silicon carbide and then coated with aluminium.

Description

2335202 1 Flame-Spray Process for the Pre-treatment and Coating of

Surfaces The present invention relates to a high-velocity flame-spray process for the pretreatment and coating of surfaces of a component, in order to improve the resistance thereof.

In spraying processes of this type, for example WOF (high-velocity oxygen fuel) spraying, the surface of a component or workpiece to be treated is generally first of all blasted in a first step with a blasting material, which is formed for example from aluminium oxide (A1203) or silicon carbide (SiC). This step has been performed with blasting apparatus usually provided separately therefor, inter alia a blasting gun. By means of a suitable supply of air into this blasting gun a suction effect is produced in a storage container in which the blasting material is held available, as a result of which the blasting material is drawn out of the storage container and is accelerated in the blasting gun. Blasting the surface with the blasting material results in a so-called activation of the blasted surface. During this process, the said surface is roughened, thus resulting in an increase of the surface area thereof and thus an improved adhesion of layers to be applied subsequently. In addition, the blasting treatment has the effect of cleaning the surface to be treated of residues adhering thereto, for example oxide skins formed during the manufacture of the material to be treated. In known methods, after the surface has been blasted, a spraying process is performed in a second step, in which the surface to be treated is coated by spraying apparatus, again provided separately for this step.

The disadvantage of these methods is that, in order to perform each of the two method steps, apparatus provided separately therefor has to be used in each case. In this way, a blasting plant has to be provided in which the workpiece to be treated has first to be activated as described. For this purpose, separate chambers or cubicles for manual 2 activation of the surfaces to be treated are frequently required. For the second method step it is then necessary to provide thermal spraying cubicles into which the component has to be conveyed, so that the spraying process can be performed by means of spraying apparatus provided therefor in an appropriate manner.

As well as the relatively high cost of separate operating apparatus required as a whole for the two method steps and the high installation and maintenance costs linked thereto, this method also has the disadvantage that it is not suitable for applying an anticorrosion layer to highly corrosive surfaces. In future, the use of easily oxidizing materials, such as for example magnesium, aluminium-magnesium-titanium alloys or copper materials, will play an important role. These are particularly important in the construction of vehicle bodies. In known methods, however, there exists between the surface-activation step and the spraying process an interval during which an oxidation skin forms on the surface of easily oxidizing materials, and this oxidation skin in turn significantly reduces the adhesion capacity for a coating to be applied subsequently.

A further disadvantage of the previous method is that the blasting material used for activating the surface of the component is collected several times after it has been blasted onto the surface, in order to reduce the quantity of blasting material required as a whole. After a cleaning step performed in parallel, the blasting material is returned to the storage container and is used again in the further course of the blasting procedure. In this way, the sharpness of the edges of the particles of blasting material is reduced, so that when blasting procedures of relatively long duration occur the effectiveness of the blasting particles is reduced. With this method, however, in conjunction with the treatment of highly corrosive light-metal surfaces, the problem arises in particular that despite cleaning the blasting material already used slight impurities remain on the blasting material. These impurities arise inter alia as a result of the impact of the blasting material upon the covering of the cubicle or as a result on wear on the walls of the storage container or on pipes and lines. These components of the plant are formed substantially from steel. Despite the cleaning, therefore, a certain amount of iron dust remains on the blasting material on each run through, and is embedded in the surface of 3 the light-metal component. Even if these impurities are covered by the layer applied in the following spraying process, they form triggers for corrosion on the surface of the component.

An object of the invention is therefore to provide a high-velocity flamespray process for the pre-treatment and coating of surfaces, which will ensure the resistance of the surfaces to be treated.

Claims

The invention provides a process as claimed in Claim 1. The method according to the invention can be used generally for applying a protective layer, in order for example to provide corrosion protection or wear protection. The method according to the invention has the advantage that the same apparatus may be provided both for blasting the surface of the component and for coating it. In this way, in the first place the entire plant for performing the method is simplified, so that the costs of the plant in this respect are markedly reduced. Secondly, there is a clear reduction in the time expenditure and therefore in the manufacturing costs of the component, which is required for performing the method. When the blasting and spraying process is performed in a single method step, the advantage is achieved that considerably less blasting material is required than in the case of the known methods. The blasting apparatus in the case of the known method is operated with compressed air, so that the blasting particles have a speed of between 30 and 60 metres per second. In the case of the particle energy resulting therefrom a relatively large quantity of particles is thus required for the necessary surface activation. On the other hand, the acceleration of the blasting particles in the case of the method according to the invention is produced by combustion processes in the blasting and coating appliance which provide an amount of energy of the blasting particles which is greater by a factor of 10 than occurs in the case of the known methods. In this way, only relatively small quantities of blasting particles (in the order of magnitude of 20 g 4 per minute) are required in order to activate to the necessary degree the surface of the component to be treated. Because of the small quantities of blasting material which are used in the method according to the invention it is not necessary to use it again within the blasting process, so that the problem of corrosion of the component as a result of contaminated blasting material does not arise. In this connexion it is additionally advantageous in particular that the quantity of particles in the blasting and coating apparatus used in the method according to the invention can be metered considerably more precisely than in the case of the known apparatus. As a result of using the same apparatus for both the blasting and the spraying process there is the additional advantage that the respective surfaces treated by blasting and spraying may be made to correspond considerably more precisely. On the other hand, in the case of the known methods, a frequently significantly larger area is blasted by the blasting process than is subsequently sprayed, so that an edge area remains uncoated. When highly corrosive metallic components are treated, the blasted but unsprayed edge area results in a considerable degree of corrosion, so that the known methods are not suitable for treating highly corrosive metallic components. In contrast, by means of the method according to the invention in which the same apparatus is used for the blasting and the spraying process, a more precise adaptation of the surfaces treated in each case is possible, thus ensuring that no edge area remains which has not been sprayed and which will subsequently corrode. In addition, because of the improved adaptability of the surfaces treated in each case, the corrosion protection is thus considerably improved by the method according to the invention. Preferred embodiments of the method according to the invention will now be described. In the case of the method according to the invention the same apparatus is used as a blasting plant and a spraying plant for performing the blasting and spraying process. As a result, in order to treat the component, it is unnecessary to move components to be treated from a blasting chamber provided separately therefor, as in the prior art, into a separate spraying chamber. Instead, the component can be kept in the same chamber or cubicle for both the method steps. Between the two method steps it is necessary only to change over the connexion of the blasting and spraying apparatus from a storage container with the blasting material to the storage container with the spraying material. This can be carried out within a relatively short time, within which even highly corrosive metals do not start to corrode. In order to perform the blasting process, the blasting means used is preferably in the form of a powder which has a grain size in which the individual particles of the powder can no longer be melted by the blasting and spraying gun. This prevents melted blasting material from being deposited on the inner wall of the blasting apparatus. Since blasting particles of a minimum size are used for the blasting process, such particles preferably also having a melting point higher than in the case of the prior art, in which case these particles are not re-used, these particles also retain their manufactured shape, i.e. the necessary sharpness of their edges, after leaving the blasting apparatus. For these reasons an optimum activation of the surface of the component to be treated is achieved with the blasting process according to the invention, and the spraying process is performed immediately subsequently, so that the risk of corrosion of the component is kept to a minimum. What is essential is that the blasting particles are not melted even in part during the process, since their consistency might possibly otherwise be altered. Depending upon the case of application, however, the blasting particles need not have a shape with sharp edges, but may be shaped in a different manner, for example in a spherical shape. The treated surface is then activated by the relatively hard structure of the blasting particles. In an alternative method it is also possible for the blasting and spraying material to be applied simultaneously to the surface of the component to be treated. In this case the blasting and spraying material is removed simultaneously from the corresponding storage containers, so that the two storage containers are connected to the blasting and spraying apparatus simultaneously. 6 Particles of aluminium oxide (A1203) or silicon carbide (SiC) are used for example as the blasting material, In many applications for which the method according to the invention is suitable, aluminium is provided as the coating material. The proportion of the blasting material or abrasive material measured in relation to the blasting and spraying material used as a whole is preferably at least 5% and at most 50% by weight, in which case a proportion of from 10 to 30% by weight should be provided in most applications. The particle size of the blasting particles may be in the range of between 5 and 150 gm. Particular advantages are afforded by the method according to the invention when a corrosion protection is applied to frame members of vehicle bodies, as is described below. In the case of vehicles which are exposed to atmospheric conditions, moisture and water mixed with salts penetrate into the interior of bodies and consequently also to places at which different metallic materials are in contact and thus promote corrosion. In vehicles of the future, bodies are envisaged in which the outer sidn consists of a metal sheet, such as an aluminium sheet and in which the body frame consists of highly corrosive materials, such as for example magnesium or magnesium alloys. In the lower region of the body frame, namely on the inside of the body where this water is formed and this water is frequently held for a considerable time (bilge), metal sheets are usually attached at the corresponding side of the magnesium member so that the bilge does not come into direct contact with the highly corrosive frame member. lEtherto, however, it was possible to prevent corrosion only to an unsatisfactory degree in this way, since the metal sheets do not seal these places off from the bilge completely. With the method according to the invention, on the other hand, it is possible to apply an al i i coating directly onto the corresponding points of the magnesium frame with a reliable scaling, so that the bilge does not come into contact with the magnesium even over a long period. 7 Claims:

1 A process for applying a protective layer to the surface of a component to be treated comprising a blasting process for activating the surface of the component by means of a blasting material and at least one spraying process for coating the component, wherein the blasting process and the spraying process are performed by the same apparatus.

2. A process according to Claim 1, wherein the shape and consistency of the blasting material is essentially unchanged during the process.

3. A process according to Claim 1 or 2, wherein the temperature of the blasting material during the process is below the melting point thereof.

4. A process according to any one of the preceding claims, wherein the blasting and the spraying process are performed simultaneously in the same method step.

5. A process according to any one of the preceding claims, wherein a light-metal alloy, a copper alloy, a nickel alloy or a steel alloy is used as the material of the surface to be treated.

6. A process according to Claim 5, wherein an aluminium or magnesium alloy is used as the light-metal alloy.

7. A process according to any one of the preceding claims, wherein the proportion of the blasting material is at least 10% and at most 50% by volume.

8. A process according to any one of the preceding claims, wherein the particle size of the blasting material is from 5 to 150 gm.

9. A process according to any one of the preceding claims, wherein aluminium is 8 used as a coating material.

10. A process according to any one of the preceding claims, wherein at least 20OWC is set as the outlet temperature for the blasting and spraying material from the blasting and spraying apparatus.

11. A method of applying an aluminium coating to areas of a body frame member at risk of corrosion using the process as claimed in any one of the preceding claims.