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Upload organization:Japanest NIPPON   Upload date:2012/08/08

"Deep Penetration Welding in Low Vacuum Using High-Power Lasers" Professor Seiji Katayama, Joining and Welding Research Institute, Osaka University

Deep penetration welding is essential to the construction of large-scale structures such as automobiles and aircrafts. Electron beam welding (EBW) has been the primary method used to form these deep welds, but laser beam welding (LBW) is quickly emerging as an alternative, more cost-effective technique. Unlike EBW, LBW does not require high vacuum, allowing a reduction in cost, nor does it generate harmful X-rays, eliminating the need for heavy lead-containing shields. Also, stray magnetic fields can adversely affect EBW, but these are not a concern with LBW. 

The Katayama group conducts research aimed at the further development of LBW. Recently, the group succeeded in producing extremely deep welds in low vacuum (>0.1 kPa) with high-power, high-brightness fiber and disk lasers. A weld penetration depth of 73 mm was achieved on 80 mm-thick stainless steel plates, using welding parameters determined to be ideal by the group. 

Effects of Welding Parameters on Penetration Depth

The parameters of LBW in low vacuum include ambient pressure, defocused distance, laser power, and welding speed. The Katayama group measured the effects of each of these parameters on weld penetration depth. Type 304 austenitic stainless steel plates with a thickness of 80 mm were used as the sample material for the experiments.

The effect of ambient pressure between 0.1 and 100 kPa was observed while the laser power was maintained at 16 kW and defocused distance at 0. A series of measurements was taken at different welding speeds. The penetration depth was observed to increase as ambient pressure decreased, and the effect was more dramatic at lower welding speeds. The results also showed that penetration depth increased as welding speed decreased. A maximum penetration depth of 43 mm was obtained at 0.3 m/min with an ambient pressure of 0.1 kPa. 

Using negative defocused distances further increased the weld penetration depth. Defocused distances are measured away from the sample surface, and a negative value means that the laser is focused beneath the surface. The deepest welds were obtained with defocused distances between -20 and -40 mm. At 0.3 m/min, a maximum penetration depth of over 50 mm was obtained at a defocused distance of -30 mm. 

The weld penetration depth was even further increased by increasing the laser power. Two disk lasers were used in combination for a maximum power of 26 kW. With this high power, an ambient pressure of 0.1 kPa, a welding speed of 0.3 m/min, and a defocused distance of -40 mm, an extremely high penetration depth of 73 mm was achieved. 

The Advantages of Performing LBW in Low Vacuum

In order to understand why deeper welds are formed in low vacuum than under atmospheric pressure, the Katayama group studied the laser-induced plumes and the molten pool surfaces formed during the welding process under different ambient pressures. The laser-induced plume looked like a volatile pillar of fire and the molten pool surface like a stormy sea under atmospheric pressure, but both became calmer as pressure decreased. Plumes also became less intense in low vacuum, allowing the laser beam to remain well focused. A calm molten pool surface translates into an unobstructed keyhole inlet, allowing the focused laser beam to enter the keyhole effectively. This is thought ultimately to lead to an increased weld penetration depth in low vacuum. 

The Katayama group’s results have proven that LBW in low vacuum can produce welds of unprecedented penetration depth. The group is currently working on developing high-quality welding processes and investigating the laser weldability of various materials, including the welding of dissimilar materials. The techniques developed by the Katayama lab can be applied to many forms of machining and metalworking, and their studies will benefit a wide variety of industries, including the automotive, aerospace, watercraft, and heavy industries.

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