Laser-hybrid welding
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Laser-hybrid welding is a new type of welding process that combines the principles of laser beam welding and arc welding.
[edit] 1. Introduction:
The combination of laser light and arc into an amalgamated welding process is known since the 1970s, but for a long time thereafter, no further research and development was undertaken. Recently, researchers have turned their attention to this topic again and attempted to unite the advantages of the arc with those of the laser in a hybrid weld process. Whereas in the early days, laser sources still had to prove their suitability for industrial use, nowadays they are standard technological equipment in many manufacturing enterprises. The combination of laser welding with another weld process is called hybrid welding process. This means that a laserbeam and an arc act simultaneously in one welding zone, they influence and support each other.
[edit] 2. Laser:
Laser welding not only requires high laser power but also a high quality beam to obtain the desired deep-weld effect. The resulting higher quality of beam can be exploited either to obtain a smaller focus diameter or a larger focal distance. For the projects that are currently underway, a lamp-pumped solid state laser with a laserbeam power of 4 kW is used. The laser light is transmitted via a 600μm glass fibre, in which the beginning and the end is watercooled. The laserbeam is projected onto the workpiece by a focussing module with a focal distance of 200 mm.
[edit] 3. LaserHybrid process:
For welding metallic workpieces, the Nd:YAG laserbeam is focussed to obtain intensities of more than 106W/cm2. When the laserbeam hits the surface of the material, this spot is heated up to vaporization temperature, and a vapor cavity is formed in the weld metal due to the escaping metal vapor. The extraordinary feature of the weld seam is its high depth-to-width ratio. The energy-flow density of the freely burning arc is slightly more than 104 W/cm2. Unlike a sequential configuration where two separate weld processes act in succession, hybrid welding may be viewed as a combination of both weld processes acting simultaneously in one and the same process zone. Depending on the kind of arc or laser process used, and depending on the process parameters, the processes will influence each other to a different extent and in different ways.
Thanks to the combination of the laser process and the arc process, there is also an increase in both weld penetration depth and welding speed (as compared to each single process). The metal vapor escaping from the vapor cavity acts upon the arc plasma. Absorption of the Nd:YAG laser radiation in the processing plasma remains negligible. Depending on the ratio of the two power inputs, the character of the overall process may be mainly determined either by the laser or by the arc.
Absorption of the laser radiation is substantially influenced by the temperature of the workpiece surface. Before the laser welding process can start, the initial reflectance must be overcome, especially on aluminium surfaces. This can be achieved by starting welding with a special start program. After the vaporisation 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 thus determined by the temperature dependent absorption and by the amount of energy lost by conduction into the rest of the workpiece. In LaserHybrid welding, vaporisation takes place not only from the surface of the workpiece but also from the filler wire, so that more metal vapor is available, which in turn facilitates the input of the laser radiation.
