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WELDING RESEARCH -S223WELDING JOURNAL ABSTRACT. In recent years, laser beam welding using two laser beams, or dual- beam lase

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Content	WELDING RESEARCH -S223WELDING JOURNAL ABSTRACT. In recent years, laser beam welding using two laser beams, or dual- beam laser welding, has become an emerging welding technique. Previous studies demonstrated use of dual-beam laser processing can delay humping onset to higher speeds and slow down cooling rates. In this study, a detailed investigation was performed to quantify the benefits of dual-beam laser processing and to under- stand the mechanism for improving weld quality. A 6-kW CO2laser beam was split into two equal-power beams and the dual beams were located in tandem (one beam follows another) during welding. Experi- mental results indicated the dual-beam laser could significantly improve weld quality. For steel, surface quality was im- proved with fewer surface defects such as undercut, surface roughness, spatter, and underfill. Weld hardness and centerline cracking susceptibility were also reduced. In aluminum, quality improvements were in the form of smooth weld surfaces and fewer weld defects such as porosity, sur- face holes, and undercut. A high-speed camera investigation of welding vapor plumes above a workpiece showed plume height and size changed dramatically in conventional single-beam laser welding and the average fluctuation frequency was 1.2 kHz for steel. As the plume fluctuation was associated with keyhole instability, unstable vapor plume indicated the process was unstable and would result in poor welds. The vapor plumes in dual- beam laser welding were found to fluctu- ate at a certain frequency range, but the plume size changed only slightly during welding. The stabilized process con- tributed to improved weld quality in dual- beam laser welding. Introduction Laser welding has been widely used in the automotive, aerospace, electronic, and heavy manufacturing industries to join a variety of materials. In the automo- tive industry, high-power lasers are used to weld many components such as trans- missions, mufflers, catalytic converters, exhaust systems, and tailor-welded blanks. It was reported about 70 million tailor- welded blanks were produced in 2000, a number predicted to be 95 million in 2001 (Ref. 1). However, a number of defects, such as porosity, surface holes, irregular beads, undercuts, humping, and solidification cracking, are often found in laser welds. Industrial laser users are always looking for economical methods to improve weld quality and relax the strict fitup require- ment for workpieces. A welding technique that combines two high-energy beam sources (either electron beams or laser beams), called “dual-beam welding,” has been investigated in recent years. Initial experimental studies showed the dual- beam process offered several advantages over the conventional single-beam process. An early electron beam (EB) welding experiment performed by Arata et al., who used dual electron beams dur- ing welding, demonstrated a trailing beam impinging on a molten pool could increase the welding speed at which humping oc- curred up to 50% (Ref. 2). In dual-beam laser processing, the dual beams can be arranged either side by side (Fig. 1A) or in tandem — Fig. 1B. Conrad Banas (Ref. 3) used a bendable mirror to split a laser beam into two beams that were then arranged side by side during welding to in- crease the fitup tolerance of workpieces — Fig. 1A. A study on using side-by-side laser beams for improved fitup tolerance has been reported in welding tailored blanks (Ref. 4). The rule of thumb is that the air gap between two workpieces should be less than 10% of the sheet thick- ness for butt joints and 25% for lap joints in conventional single-beam laser welding. Use of the side-by-side dual-beam lasers could substantially increase the fitup tol- erance in welding tailored blanks (Ref. 4). Dual laser beams arranged in tandem (Fig. 1B) have been reported to provide benefits over conventional single-beam laser welding such as improved weld qual- ity (Refs. 5–11). The current study focused on the tandem dual-beam laser welding process and its impact on weld quality. In this paper, unless specified, dual-beam laser welding means two laser beams are arranged in tandem, as shown in Fig. 1B. One of the possible benefits of using the dual-beam laser was to decrease cool- ing rates in laser welding of high-carbon steel (Refs. 5, 6). It was said cooling time between 800 and 500°C could be extended from 3.8 up to 7 s by enlarging the distance between the two beams (interbeam spac- ing), where two 5-kW CO2 lasers were combined (Ref. 5). A dual-beam laser welding experiment on AISI 4140 steels was performed by Liu and Kannatey- Asibu in which the leading laser beam was focused on the surface of a workpiece and the trailing beam was defocused on the weld bead at an interbeam space of 10 mm (Ref. 6). The dual-beam process resulted in lower cooling rates, reduced hardness, Dual Beam Laser Welding An experiment in which a high-power CO2laser beam was split into two equal- power beams that were then used as a welding heat source indicated the dual- beam laser could significantly improve weld quality BY J. XIE KEYWORDS Laser Welding Beam Splitting Dual Beam Weld Quality Defect Steel Aluminum Vapor Plume Fluctuation Keyhole Instability JIANXIEis currently with St. Jude Medical, Syl- mar, Calif. This work was completed when he was with Edison Welding Institute, Columbus, Ohio. Paper presented at the 80th Annual AWS Con- vention, April 11–15, 1999, in St. Louis, Mo.
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