BACKGROUND ART Laser welding is desirable because it can be performed without any weld splatter on a robot arm with multiple axes of movement so as to have a high degree of freedom, and the result welds have minimal thermal stresses. However, such laser welding with galvanized or zinc coated sheet metal pieces can result in problems because the zinc coating vaporizes at a much lower temperature than the melting point of steel it coats and thereby produces a gas that causes weld porosity.

More specifically, the zinc melts at a temperature of about 420°C and vaporizes at about 900°C, which is substantially below the 1545°C melting point of steel over which the zinc is coated.

To overcome the porosity problem with zinc coated sheet metal pieces during laser welding, United States Patent 5,183,991 Arai discloses the use of an intermediate layer of carbon graphite or a carbon-base surface absorber to suppress evaporation of the zinc and thereby decrease weld porosity. Such suppression of the evaporation of the zinc is thus achieved by a layer of carbon which remains between the welded pieces until the steel melts and the carbon then goes into solution in the steel and thereby changes its composition.

Other prior art patents noted during an investigation conducted in connection with the present invention include United States Patents: 3,969,604 Baarden; 4,642,446 Pennington ; 4,682,002 Delle Piane et al.; 4,684,779 Berlinger et al.; 4,745,257 Rito et al.; 4,916,284 Petrick; 5,142,119 Hillman et al.; 5,180,099 Okabayashi et al.; 5,183,991 Arai ; 5,187,346 Bilge et al.; 5,347,528 Haruta et al.;

5,371,337 Campbell et al.; 5,539,180 Mori et al.; 5,611,478 Asanasavest ; 5,630,269 Wasserbaech et al.; 5,650,077 Zinke; 5,749,511 Aebersold et al.; and 5,780,802 Gnann et al.

DISCLOSURE OF INVENTION An object of the present invention is to provide an improved method for laser weld sheet metal pieces at least one of which has a zinc coating.

In carrying out the above object, the laser welding method of the invention is performed by clamping the sheet metal pieces to each other with a spacing layer therebetween and then directing a laser beam through one of the sheet metal pieces, thence through the spacing layer, and thence through the other sheet metal piece to provide a weld between the sheet metal pieces without porosity that would be present without the spacing layer. The spacing layer is a material which liquifies, volatilizes, or decomposes into liquid or vaporous components at temperatures below the vaporization temperature of zinc so as to thereby provide a layer where the zinc gas can be absorbed during the welding process without the zinc gas causing porosity of the weld.

In the preferred practice of the laser welding method, the spacing layer has a thickness in the range of 0.002 of an inch to 20% of the thickness of the one sheet metal piece through which the laser beam is initially directed. More specifically, the spacing layer preferably has a thickness in the range of 0.002 to 0.005 of an inch.

The spacing layer is preferably a natural or synthetic wax or a polyolefin.

In performing the laser welding, the spacing layer may be initially applied to one of the pieces of sheet metal.

Although the laser welding can be performed with only one of the sheet metal pieces zinc coated, it has its greatest use in welding of a pair of galvanized sheet metal pieces to each other.

The objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS FIGURE 1 is a schematic partially sectional view showing how the laser welding method of this invention is performed.

FIGURE 2 is a top plan view taken along the direction of line 2-2 in Figure 1 to show a resultant weld.

FIGURE 3 is a top plan view similar to Figure 2 but showing the manner in which a spacing layer of the welded assembly can be localized rather than extending the entire extent of the sheet metal pieces that are welded.

FIGURE 4 is a sectional view similar to Figure 1 but shown prior to clamping to illustrate the manner in which the spacing layer is applied to one of the sheet metal pieces to be welded.

BEST MODE FOR CARRYING OUT THE INVENTION With reference to Figure 1 of the drawings, the laser welding method of the invention is illustrated as being performed to weld first and second sheet metal pieces 10 and 12 to each other. At least one of the sheet metal pieces, and actually both of the pieces as disclosed, is galvanized so as to have a zinc coating 14 on each of its surfaces. Between these zinc coatings, each sheet metal piece is steel 16 that is protected from corrosion by the zinc coatings.

With continuing reference to Figure 1, a schematically illustrated clamp 18 has clamping portions 20 that clamp the sheet metal pieces 10 and 12 to each other with a spacing layer 22 therebetween as is hereinafter more fully described. A laser source 24 directs a laser beam 26 through the sheet metal piece 10, thence through the spacing layer 22, and thence through the other sheet metal piece 12 to provide a weld 28 (Figure 2) between the sheet metal pieces without porosity that would be present without the spacing layer. The weld 28 can be a straight stitch weld as shown and can also be a spot weld or any other weld shape.

The laser source 24 may be a diffusion cooled CO2 laser that provides a continuous laser beam and a nozzle 30 supplies a cover gas for the welding.

The spacing layer 22 as discussed above allows the production of welds which do not exhibit porosity. In order to do so, the spacing layer 22 comprises a material which liquifies, volatilizes, or decomposes into liquid or various components at temperatures below the vaporization temperature of the zinc coating.

Thus, the spacing layer provides a location for the vaporized zinc coatings 14 at the interface of the two sheet metal pieces 10 and 12 to migrate outwardly from the weld as opposed to migrating into the molten weld and producing porosity.

For best results, the spacing layer has a thickness in the range of 0.002 of an inch to 20% of the thickness of the sheet metal piece 10 through which the laser beam 26 is initially directed. More specifically, this spacing layer has a thickness in the range of 0.002 to 0.005 of an inch.

The spacing layer 22 is preferably a solid or waxy material, or a highly viscous liquid which allows the spacing between the sheet metal pieces to be maintained under the weld clamping pressure. More specifically, the spacing layer 22 may be either a natural or synthetic wax. In addition, the spacing layer 22 may be an organic polymer, either natural or synthetic with a melting point below the vaporization temperature of zinc. Preferred organic polymers are thermoplastic polymers which are relatively inert. Examples include polyolefins such as polyethylene and polypropylene, particularly oligomeric polyolefins which may be characterized as polyolefin waxes. Copolymers such as ethylene vinyl acetate are

also suitable. Natural polymers and waxy materials are also preferred. Examples include beeswax, tallow, carnauba wax, montan wax, and paraffin wax.

Hydrogenated animal and vegetable oils are also useful, particularly when hydrogenated to a solid or near solid state.

Thermosetting polymers are also useful for the spacing layer 22, provided that they may liquify or volatilize below the vaporization temperature of zinc. Many lightly crosslinked thermoset polymers may have enough mobility at elevated temperatures to exhibit liquid-like behavior, and many will decompose into smaller fragments which are themselves liquids or gases.

As previously mentioned, preferred materials for the spacing layer 22 are the natural and synthetic wax like materials. Such materials are easily applied, either neat, for example in the melt, or as solutions or dispersions. A continuous film is much preferred. Further examples of waxes which are useful may be found in the Kirk-Othmer Concise Encyclopedia of Chemical Technology, Wiley- Interscience, N. Y., t1985, in"Waxes"at pages 1259-1260.

As illustrated in Figure 3, the spacing layer in addition to extending the entire extent of the engaged surfaces of the two sheet metal pieces 10 and 12 can also be localized around the area at which the weld 28 is to be made, or spacing spots on each side of the weld area can also be used. The extent of the localized spacing layer 22 must be sufficient so as to accommodate for the migration of the vaporized zinc gas during the welding by the laser beam as previously described.

The spacing layer 22 may be inserted between the two sheet metal pieces 10 and 12 in any suitable manner. However, as illustrated in Figure 4, it is believed to be easiest to apply the spacing layer 22 to one of the sheet metal pieces such as the lower sheet metal piece 12 shown prior to the clamping and the laser welding. This application of the spacing layer 22 can be performed in any suitable manner such as by melting for flow over the surface to the desired thickness or by spraying while hot, etc.

While the best mode for carrying out the invention has been described, other ways for practicing the invention will be apparent to those familiar with the art to which this invention relates as defined by the following claims.