Products are generally finished by applying a coating thereto. The functions of coatings differ, whereby they may be categorized as: modification of mechanical qualities, improvement of corrosion resistance, modification of sliding or sticking friction properties of a surface, formation of a reconditioning layer onto a surface and modification of other qualities such as the thermal or electrical conductivity of a surface. Frequently, surface coating serves to simultaneously modify more than one of the above-listed qualities. According to their manufacturing method, coatings applied to metallic surfaces are categorized as: electrochemical coatings, reaction coatings, thermally applied coatings, hot-melt dip coatings and diffusion coatings.

Thermal coating methods include conventional welding-on (MIG/MAG, TIG, powder-arc and stick welding) and the more recent physical gas-phase coating methods such as PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposi- tion), high-velocity flame spraying HVOF (High Velocity Oxy-Fuel) and plasma spraying. One further thermal coating method is laser coating wherein coating material, generally in powder form, is melted on the surface of the base material with the help of a laser beam. Alternatively, the coating material can also be applied as a wire, chips or even a plate. During coating, the laser beam melts a thin layer of the base material surface while the coating material is simultaneously fed into the melt, whereby a melted alloy is formed between the coating and the base material. During the coating process, the melt is protected by a shielding gas atmosphere. As the melt solidifies, a coating layer is formed onto the base material. Laser coating is described, e. g., in application publications EP 293945 and EP 176942.

A benefit of laser coating over other thermal coating techniques is that by virtue of the high energy intensity of the laser beam, the overall thermal load imposed on the workpiece being coated remains small and is applied in a controlled manner. Hereby, the need for preheating and thermal posttreament is reduced. Moreover, changes in the base material structure and properties, as well as deformations of the workpiece such as elongation and warping remain minimal. A strong metallurgical bond is formed between the coating and the base material. Laser coating is suited for most combinations of different coating additives with different base materials. The only precondition is the metallurgical compatibility of these two that can be improved by a proper choice of additives and modification of the coating material composition.

Laser coating is particularly applicable when the coating is required to have qualities that are distinctly different from the respective qualities of the base material.

Although laser coating is suited for most base materials, it has been very difficult, even impossible, in the prior art to coat copper and copper alloys using the laser coating method. This is because copper can efficiently reflect radiation imposed thereon, since a laser beam aimed at the surface of the workpiece to be coated is reflected aside and the surface will not heat up to a sufficiently high temperature.

Conventionally, copper or copper-alloy workpieces have been coated using spray methods or by dipping the workpiece into a melt or solution of the coating material.

It is an object of the present invention to an entirely novel kind of method for coating copper and copper alloys.

The goal of the invention is achieved by way of directing the laser beam at an oblique angle to the envelope of the surface being coated. Advantageously powder- form coating material having a metallic composition is applied to the working area of the laser beam on the surface to be coated simultaneously as the beam is active.

Further in the method according to the invention, the surface of a copper or copper alloy workpiece to be coated may be prepared dull to improve its radiation absorp- tion capability. When necessary, the surface to be coated can be machined to a desired roughness prior to coating in order to improve its radiation absorption qualities. When appropriate, the workpiece to be coated may also be preheated prior to commencing the coating process.

The invention offers significant benefits.

The method according to the invention makes it is possible to coat workpieces pro- duced from copper or a copper alloy using laser coater equipment; an operation which in the prior art has been considered complicated. Moreover, the method according to the invention generally disposes with any special arrangements inasmuch it can be carried out simply by adjusting the operating parameters of the coating apparatus to appropriate values.

In the following, the invention is described in more detail with reference to the ap- pended drawing illustrating a laser coating apparatus and a workpiece being coated.

Referring to the drawing, the C02 laser coating equipment shown therein comprises a coaxial laser gun 1, wherefrom a laser beam 2 is applied to a workpiece 3 of copper or a copper alloy. Herein, copper must be understood to include, not only pure copper, but also such alloyed copper compositions wherein the copper content is greater than 97.5 %. Respectively, copper alloys must be understood to includes compositions having copper alloyed with at least one alloying component whose content in the composition is at least 2.5 %. Copper alloys include, e. g.,: - copper-zinc alloys aka brasses - copper-tin alloys aka tin bronzes - copper-nickel-zinc alloys aka German silver/nickel silver - copper-nickel alloys aka nickel copper alloys - copper-aluminum alloys aka aluminum bronzes - special alloys of copper.

The workpiece 3 may have a rotation-symmetrical or planar shape, for instance.

Advantageously, the powder-form coating material is fed via the laser gun 1 with the help of a shielding gas to an area of the workpiece 3 heated by the laser beam 2. The coating material feed takes place in a coaxial manner in regard to the laser beam 2 so that the flow of the coating material with the shielding gas circumferentially encloses the laser beam 2. Carbon dioxide or argon can be used as the shielding gas. Owing to its high energy intensity, the laser beam 2 melts a thin layer of the surface of workpiece 3 as well as the coating material applied to the surface of the workpiece 3, whereby a melt joint is formed therebetween. The surface of the workpiece 3 to be coated is moved relative to the laser beam 2 and/or the laser beam is moved relative to the surface of the workpiece 3 to be coated. Rotation-symmetrical workpieces can be mounted on a lathe, for instance.

The scanning speed of the laser beam 2 over the surface of the workpiece 3 to be coated is advantageously 100 to 1000 mm/min, advantageously 230 to 270 mm/min.

As the heat imported by the laser beam is primarily absorbed by the workpiece 3, the melted coating material solidifies rapidly with the progress of the coating process.

The molten coating material is protected with the help of a shielding/carrier gas atmosphere. The width of a coating layer strip applied during a single sweep of the beam is determined by the location of the laser beam focus point relative to the surface to be coated. Typically, the width of a single strip of coating layer formed by moving the laser gun 1 and/or the surface to be coated is 2 to 3 mm, advantageously 2.4 to 2. 6 mm The coating material must be metallurgically compatible with the base material.

When necessary, the compatibility of these two components can be improved by using a different alloying component in the coating compound or by modifying the composition thereof. Generally, the coating material is selected from the group of compositions of high resistance to heat and/or wear. The qualities achievable through the use of a coating are dictated by the application, the base material, the coating material and the process parameters of the coating process. Typically, the thickness of a coating layer formed in a coating process according to the invention is 0.1 to 4 mm. When necessary, plural coating layers can be applied over one another.

In the method according to the invention, the center axis of the laser beam 2 is directed at an oblique angle to the envelope surface of the workpiece to be coated.

When applied at an oblique angle to the surface to be coated, the laser beam meeting the surface to be coated will undergo scattering instead of being reflected back to the laser gun 1. This arrangement prevents excessive heating of the laser gun 1. Typical- ly, the angle a between the center axis of the laser beam 2 and the normal of the envelope surface of the workpiece 3 being coated is advantageously 10° to 50°, most advantageously 30° to 38°. In the context of the present discussion, the term envelope surface must be understood as the ideal external surface of a workpiece wherefrom the actual envelope of the workpiece may differ, e. g., due to manufacturing tolerance variations, wear or other damages.

Prior to coating, the surface of the workpiece 3 to be coated is dulled into a matte condition. A matte surface is by definition substantially dull and incapable of reflec- tion thus making the surface to act as an efficient absorber of radiation. When made matte, the surface will absorb the radiation of laser beam 2 at a greater efficiency than a bright surface of the respective copper or copper-alloy workpiece. The workpiece surface may also be dulled by spraying thereon a material of high radiation-absorbance such as graphite.

Surface radiation absorbance can be enhanced by machining the surface to be coated to a coarser roughness prior to the dulling thereof. The surface may be roughened by milling to roughness Ra that advantageously is 2 to 3.5 um, most advantageously 2.5 to 3 , m.

When necessary, the workpiece 3 to be coated can be preheated prior to coating, whereupon the coating as well as the surface of the base material can be melted using a smaller output power of the laser beam. Using preheating, the surface of the workpiece 3 to be coated is advantageously elevated to a temperature of 200 to 500 °C, most advantageously to 250 to 350 °C.

The coating method according to the invention is particularly well suited for coating, e. g., copper casting molds. Copper has a high thermal conductivity that permits quick removal of heat released from a cast object or continuous section to the casting mold.

A problem hampering the use of copper herein arises from softness of copper that causes rapid wear of casting molds. The wear rate of molds can be reduced by coating the internal surface of the mold with a wear-resistant material. Next, an example is given on the parameters used in a coating process of a casting mold made from copper: - laser beam power approx. 6 kW - distance of laser gun nozzle tip from the surface being coated approx. 18 mm - distance of laser beam focus point relative to the laser gun nozzle tip approx.

38 mm - angle between laser beam center axis and the normal of the surface of workpiece being coated approx. 35° - velocity of laser beam sweep relative to velocity of surface of workpiece being coated approx. 250 mm/min In a test, the surface of a workpiece to be coated was machined by milling to a roughness Ra of approx. 2.6 m, whereupon the surface to be coated was cleaned with acetone. Next, graphite was sprayed onto the workpiece surface to improve radiation absorption. Additionally, the workpiece was preheated to about 300 °C prior to coating. The coating was applied to a surface area of 120 mm by 30 mm. The laser beam was moved laterally by about 2.5 mm at a time. The number of super- posed coating layers was two. The feed rate of the coating material powder was about 13 g/min. Carbon dioxide was used as the shielding/carrier gas. The coating material was Höganäs HMSP 153840 powder having the chemical composition of : - carbon, C: 0.05 % - silicon, Si: 3.0 % - boron, B: 2.2 % - iron, Fe: 0.4 % - nickel, Ni: as a residual component.

The invention may also have embodiments different from those described above. Instead of being applied via the laser gun 1, the coating material may also be applied to the surface of the workpiece 3 from a separate dispenser. Moreover, the coating material can be applied to the surface being coated prior to the start of the coating process.