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Content | WELDING RESEARCH -S225WELDING JOURNAL varied from 5 to 7 mm at various angles be- tween the two beams (Ref. 12). Since the previously mentioned results were ob- tained at different experimental condi- tions, it was not easy to conclude the im- pact of dual laser beams on weld morphology. In most reported experiments, the term “dual beam” means two laser beams were used during welding. However, the lasers used in these experiments changed dramatically from low-power CW/pulsed Nd:YAG lasers to high-power CW CO2lasers, and the setup of the two laser beams was different, such as interbeam spacing, angles between two beams, fo- cusing positions, and laser power ratios. Essentially, the dual-beam laser systems could be built by either combining two lasers with an angle between two beams (Refs. 5, 10–12) or splitting a laser beam into two parallel beams with an optical splitter (Ref. 6). Among the reported dual-beam systems, many were the combi- nation of two Nd:YAG lasers because of easy manipulation by using two focus heads with fiber optics (Refs. 10–12). It was possible to combine CO2lasers by using a special optic device as well (Ref. 5). The combined dual-beam laser systems were more flexible in changing interbeam spacing and the power ratio of dual laser beams. However, the split dual laser beams were almost parallel and had the same planes of polarization (coherent) as many lasers produced polarized beams. Based on the arrangements of the two laser beams, the dual-beam process can basically be divided into two types, angled and parallel. Most of the reported dual- beam systems were angled and a small in- terbeam spacing could easily be achieved in these systems. In the parallel dual- beam systems, spac- ing was usually large and they were often used for reducing cooling rates (Refs. 6, 7). In such systems with a large inter- beam spacing, two Nd:YAG laser heads could simply be put together by a common holding fixture (Refs. 7, 11, 12), or a transmissive beam splitter was inserted in a CO2laser beam path if the laser power was not high (<2 kW) (Ref. 6). The welding mechanisms and impact on laser welds are believed to be slightly different between the angled and parallel dual-beam processes and, also, the welding mechanism changed with variations in interbeam spacing in both the angled and parallel beam systems. Generally, there might be three types of welding mechanisms in parallel dual- beam laser welding, depending on inter- beam spacing, as shown in Fig. 2. The first type is the dual-beam process with large interbeam spacing in which one of the two beams creates a keyhole and the other acts as a heat source for heat treating the laser beam weld. The second is the two laser beams generate two keyholes in a com- mon weld pool and the mass flow pattern of the molten metals is changed. In Type 3 of the parallel dual-beam process, inter- beam spacing is small and the two beams interact with materials in a common key- hole. In the angled dual-beam process, the mechanism is also believed to be changed at various interbeam spacings similar to the parallel dual-beam process. When interbeam spacing in the paral- lel dual-beam process is large (Type 1), the leading beam usually acts as a welding heat source to create a keyhole on the workpiece, and the trailing beam is usually defocused or has a lower laser power to do heat treating on the laser weld. In this case, the cooling rate is reduced and this feature can benefit some crack-sensitive materials such as high-carbon or alloyed steels. Additionally, the amount of the bainitic structure is increased in the weld metal and heat-affected zone (HAZ), and improved toughness is expected for the welds. This benefit has been verified by a number of experiments and was well ana- lyzed by mathematical modeling (Refs. 5–9). As interbeam spacing is reduced to a certain degree, the welding mechanism switches to Type 2, in which two laser beams interact in a common weld pool but the dual laser beams create two separate keyholes, as shown in Fig. 2. In an early work on dual EB welding (Refs. 2, 16), the influence of interbeam spacing on mass flow of liquid metal in a weld pool and the formation of humping and undercut were discussed; the tested interbeam spaces were 4, 7, and 16 mm, respectively. It was found humping and irregular welds could only be prevented at the 7-mm space be- cause of the change in the flow direction of molten metal in the weld pool. How- ever, humping and some surface defects were present at the 4- and 16-mm spaces. In the experiment, two separate keyholes were generated by two electron beams in a common weld pool (Ref. 2). When in- terbeam spacing is reduced further to the Type 3 mechanism, the two laser beams are close enough to create one common keyhole in the weld pool. Few welding ex- periments have been reported for such a parallel dual-beam process with small spaces. In angled dual-beam laser welding, the welding mechanism could be slightly dif- ferent from that in parallel dual-beam laser welding. It was reported a funnel- shaped keyhole was produced by combin- ing two high-power CO2lasers at a 30-deg angle and 1- to 2-mm spaces (Refs. 5, 15). The keyhole created by angled dual beams was enlarged; thereby, the keyhole might not be easy to collapse. Therefore, the an- gled dual beams enhanced the keyhole stability and weld quality was improved (Refs. 13, 15). In this dual-beam CO2laser system, a special optical device was de- signed to combine the two high-power CO2lasers (Ref. 5). Although dual-beam laser processing Fig. 4 — Dual-beam laser power density distribution at focus position. Fig. 5 — Experimental setup for investigating vapor plumes during laser welding. |
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