Development of Laser-Hybrid Welding
The method for combining laser light and a welding arc into an amalgamated welding process has been known since the 1970s, but for a long time thereafter, no further research and development was undertaken. Recently, however, researchers have again turned their attention to this topic and attempted to unite the advantages of the arc with those of the laser in a hybrid welding process. In the early days, the suitability of lasers for industrial use had to be proved; nowadays lasers are standard equipment in many manufacturing enterprises.

The combination of laser beam welding with another welding process is a "hybrid welding process," meaning a laser beam and an arc act simultaneously in one welding zone and influence and support each other.

For the projects currently underway at Volkswagen, a lamp-pumped solid-state laser with a laser beam power of 4 kW is used. The laser light is transmitted via a water-cooled, 600-mm glass fiber. The laser beam is projected onto the workpiece by a focusing module with a focal distance of 200/220 mm.

The temperature of the workpiece surface substantially influences absorption of the laser radiation. Before the laser welding process can start, the initial reflectance must be overcome, especially on aluminum surfaces. This can be achieved by starting welding with a special start program. After vaporization temperature has been reached, the vapor cavity is formed and nearly all radiation energy can be put into the workpiece. The energy required for this is determined by the temperature-dependent absorption and the amount of energy lost by conduction into the rest of the workpiece. In laser-hybrid welding, vaporization takes place not only from the surface of the workpiece but also from the welding wire, so more metal vapor is available, which in turn facilitates the input of laser radiation and prevents process dropout (Refs. 4-9).

Using the Process at Volkswagen
Volkswagen's strategy is to have the highest amount of laser weldments in the automotive industry. Figure 2 shows the Phaeton, previous called the Concept D1. In this car body, all doors are laser-hybrid welded. The company's requirements included a high degree of stiffness in the door structure. Without the laser-hybrid process, big, heavy aluminum cast materials would have been necessary. The geometrical tolerances had to be very small to achieve a perfect fit to the car body, resulting in low noise levels from the wind when driving. To achieve a door with that degree of stiffness, a good combination of sheet, cast, and extruded materials was necessary. In order to achieve a low weight, aluminum was the preferred and applied material because of its low density.

The Phaeton door shown in Fig. 3 consists of 7 gas metal arc, 11 laser, and 48 laser-hybrid weldments. The total length of welds on these doors is 4980 mm.

Figure 4 shows a laser-hybrid welded joint of a two aluminum cast material. The welding wire was AlSi12 with a diameter of 1.6 mm. The shielding gas was argon. With increasing laser power, higher welding speed is possible. Combining the laser beam with the arc results in a larger weld pool compared to the laser beam weld process alone, and welding of components with wider root openings becomes possible. The range of the welding speed is 1.2 to 4.8 m/min, but the process is optimized at 4.2 m/min.

In the automotive industry, there are many applications of overlap welding without joint preparation. At the moment, the state-of-the-art process for this welding job is laser beam welding with a cold welding wire, due to hot cracking of the AA 6xxx alloy. When the joint is welded with a welding wire, much of the laser energy will be lost in melting that welding wire.

Comparing Laser-Hybrid with Other Processes
Figure 5 represents the differences between laser-hybrid and laser welding on an overlap joint with a welding speed of 2.4 m/min. In the case of laser welding, there is no possibility of filling up the weld and undercut is produced. There is also only a little penetration into the base material. The weld bead width is very small and, therefore, a low tensile strength is expected. In the case of laser-hybrid welding, additional material is transported into the weld pool. The undercut is filled with wire from the gas metal arc welding (GMAW) process and a portion of laser energy is now saved. This saved laser energy can be used to increase the penetration into the base material and the weld bead width is larger than the material thickness, which is required for optimum mechanical properties.

The welding head can be applied for laser welding with and without welding wire, laser-hybrid welding, gas metal arc welding, and laser hot-wire brazing (especially for zinc-coated materials). In the case of laser hot-wire brazing, the wire is preheated with the same power source that can be used for laser-hybrid welding. There is only a difference in the software, not in the hardware configuration.

Advantages of Laser- Hybrid Welding
The merging of the arc and laser beam results in the following advantages of laser-hybrid welding over laser welding:

• Higher process stability
• Higher bridgeability
• Deeper penetration
• Lower capital investment costs because of savings in laser energy
• Greater ductility.

The advantages of laser-hybrid welding over GMAW are the following:

• Higher welding speeds
• Deeper penetration at higher welding speeds
• Lower thermal input
• Higher tensile strength
• Narrower weld joints.

The arc welding process is characterized by a low-cost energy source, good root opening bridgeability, and the facility for influencing the structure by adding filler metals. The laser beam process, on the other hand, allows large welding depth, high welding speed, low thermal load, and narrow weld joints. The laser beam produces a "deep-weld effect" in metallic materials over a certain beam density, which enables components with greater wall thickness to be welded- providing the laser power is sufficiently high. Laser-hybrid welding thus allows higher welding speeds, process stabilization due to the interaction between the arc and the laser beam, increased thermal efficiency, and greater workpiece tolerances. As the weld pool is smaller than in the GMAW process, there is less thermal input and a smaller heat-affected zone. This results in lesser weldment distortion, which reduces the amount of subsequent postweld straightening work. Where there are two separate weld pools, the subsequent thermal input from the arc means the laser beam welded area, especially in the case of steel, is given a postweld tempering treatment, spreading the hardness values more evenly across the joint. Figure 8 sums up the synergies of the combined (i.e., hybrid) process.

A laser beam apparatus costs approximately $120,000 per kilowatt of laser beam power. When utilization of the hybrid process makes it possible to use a 3-kW laser instead of one with 4 kW of beam power, investments of $120,000 are saved. However, costs of approximately $65,000 will be needed for the additional MSG equipment and welding head. Due to the higher welding speed, both fabrication time and welding costs can be reduced.

Further applications where the laser- hybrid welding process is suitable are power trains, vessels, axles, and car bodies.

Summary
Laser-hybrid welding is a new technology that offers synergies for wide fields of application in the automotive industry, especially where it is not possible or financially viable to achieve the component tolerances required for laser beam welding. The wider range of applications and the high capability of the combined process lead to enhanced competitiveness in terms of reduced investments, shorter fabrication times, lower manufacturing costs, and higher productivity.

The laser-hybrid process also offers a new approach to the welding of aluminum. However, a stable process has become possible relatively recently because of the higher available output powers of solid-state lasers. Many studies have examined the fundamentals of laser and arc hybrid welding processes. By "hybrid welding process," the combination of laser beam welding and the arc welding process is understood, with only one single process zone (plasma and melt). Research has shown by combining the two processes synergies can be achieved and the drawbacks of each separate process can be compensated for, resulting in enhanced welding possibilities, weldability, and reliability for many different materials and constructions. In particular, this has been demonstrated for aluminum alloys at Volkswagen on the Phaeton model. By choosing the current process parameters, it is possible to selectively influence weld properties such as geometry and structural constitution. The arc welding process increases the bridgeability by adding filler metal; it also determines weld joint width and reduces the amount of workpiece preparation needed. Moreover, the interactions between the processes lead to a substantial increase in efficiency. This combination process also requires considerably smaller investment costs compared to laser beam welding. New joint geometries are possible, especially fillet or butt joints, and it is not necessary to use a pressure wheel on the welding head, resulting in greater accessibility.

References
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