1. Field of the Invention
This invention pertains to the welding of coated materials and more particularly energy beam welding such as laser and electron beam welding of coated materials such as zinc and organic coated steel and other coated materials using an energy source with multiple, separated, energy beams.
2. Background of the Invention
Coated steels are used to make a variety of components for a number of industries. For example, zinc-coated steels are used for corrosion resistance for automobile bodies. Laser welding offers many advantages over conventional welding processes for joining zinc-coated steels such as one-sided access, less flange material (weight reduction), high welding speed, and improved structural stiffness. Consequently, there is major interest in developing reliable laser welding procedures to join coated steels in a lap-joint configuration. Unfortunately, the presence of the coating presents a major problem that limits the application of laser welding. The boiling point for most organic coatings is less than 400xc2x0 C. and it is about 900xc2x0 C. for zinc. Both are lower than the melting point of steel (about 1530xc2x0 C.). When coated steel is laser welded in a lap-joint configuration, the low boiling point of the coating at the interface of the two sheets results in vaporization of the coating material. If the two coated sheets are clamped firmly together with no gap, the coating vapor can only escape through the weld pool or keyhole. The venting of the coating vapor causes expulsion of the molten metal during welding and a portion of the coating vapor typically remains entrapped as porosity in the weld. The laser weld surface is full of holes due to the expulsion of molten base metal by zinc vapor during welding and weld porosity is observed in the cross section of the weld due to entrapped coating vapor.
Over the last 15 years, there have been numerous attempts to develop reliable laser welding techniques to join coated materials in a lap configuration. Most of these studies have focused on producing acceptable welds by introducing a gap between the two sheets. For example, one method by which this can be accomplished is by setting up pre-placed shims or spacers before welding. With a gap, the coating vapor can escape via the gap instead of the weld pool and the expulsion of molten metal is eliminated. Therefore, introducing the small gap can produce better-quality laser welds. The gap is usually in the range of 0.1 and 0.2 mm depending on a number of variables such as the type of coating, thickness of coating, and sheet material composition, laser variables and travel speed.
However, industry is continuously looking for methods of laser welding coated steel in a lap-joint configuration with no gap because it is difficult to maintain a controlled gap in production. Researchers around the world have been attempting to produce high quality laser lap welds with zero gap for at least 15 years with little if any success. For example, in U.S. Pat. No. 5,595,670 an oblong laser beam was suggested as a method for welding un-coated and coated steels in butt and la-joint configurations. The gapping issue was not addressed with respect to welding coated steel in the patent. Attempts by the present inventors to use the oblong beam to improve lap weld quality for zinc-coated steel with no gap resulted in non-acceptable welds. The welds typically were porous with entrapped coating material. A specified weld defect that occurred with the oblong beam was humping (the periodic accumulation of portions of the base material that project above the surface of the sheet material) which was especially severe at high welding speeds.
In order to overcome the various problems encountered with welding coated materials, it is an object of the present invention to provide a welding process that allows coated materials to be welded with zero gap between the materials during the welding process.
Another object of this invention is to improve the weld quality of coated materials.
Another object of this invention is to reduce the cost of welding coated materials.
A further object of the present invention is to reduce substantially the weld porosity in welds of coated materials especially when welded with no gap between the coated materials.
Another object of the present invention is to reduce substantially the expulsion and spatter of molten metal from the weld pool during the welding process of coated materials.
It is another object of the present invention to reduce substantially the amount of entrapped coating material found in the welds of coated sheet stock.
It is an object of the present invention to reduce substantially the incidence of weld humping, that is, the accumulation of small projecting mounds of metal at regular intervals along the weld, in the resulting welds of coated materials.
It is an object of the present invention to produce porosity free welds when welding coated materials with zero gap.
It is an object of the present invention to reduce weld hardness of coated material welds.
It is an object of the present invention to reduce centerline cracking susceptibility of coated material welds.
It is an object of the present invention to produce quality welds in coated materials that are free of blow holes.
It is an object of the present invention to produce quality welds in coated
materials that are free of weld irregularities.
It is an object of the present invention to produce quality welds in coated materials that are free of undercut.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail. It is contemplated that variations in procedures may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.
The above problems are solved and objects met by the present invention which features a method of welding coated materials comprising the steps of: a) placing at least two layers of material together with essentially no (zero) gap between the materials in the weld zone and with at least one of the layers being coated with a coating material in the weld zone; b) focusing an energy source with multiple, separate, energy beams on the weld zone of the materials to be joined; and c) moving the energy source with multiple, separate, energy beams along the materials in the direction of the common axis of the energy beams to weld the layers of material together.
The multiple, separate, energy beams, typically laser beams, can be formed by using two or more (multiple) energy beams from separate beam sources with the beams juxtaposed one to the next along a line in the direction of the weld line, or, alternatively, dividing a single beam into two or more beams and aligning the multiple, separated, energy beams along a line in the direction of the weld.
At a minimum, one of the sheets in a lap-joint configuration is coated with an organic or inorganic coating on at least one side at the interface of the two layers of material to be welded. For example, one or both sides of a steel sheet may be coated with zinc (galvanized) by hot-dipping, electro-galvanizing or galvannealing. At least one of the coated sheets is placed in contact with a second sheet in a lap weld configuration with no gap between the sheets, that is, the sheets are in contact with each other with a coating material on at least one of the sheets between the two layers.
When a conventional circular or elongated energy beam is used, lap welds in coated materials with zero gap between the surfaces during welding produce high levels of porosity due to 1) vaporization of the coating material that spews out the material to be welded as the coating violently boils off, 2) entrapment of gas/vapor bubbles of the coating in the weld, and 3) humping of the weld at higher weld speeds using an elongated beam. On the other hand, multiple separated energy beams from either multiple sources or single beam division according to the present invention, produce high quality (porosity free) lap welds in coated materials with zero gap. Multiple, separated, energy beams appear to extend the beam keyhole and increase the time and conditions available for vapor/gas to diffuse less violently through the weld.