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Compared to electron beam welding, the penetration depth of high power laser welding is smaller due to theattenuation effect of the plasma plume on laser power deposition. To date, numerous researches on the suppression oflaser plasma plume had been undertaken, such as pulse laser welding, weaving laser welding, change the shielding gascompositions, and apply the side-assistant gas and electromagnetic fields. However, the increase of the penetration depthand the improvement of the weld quality were inconspicuous. Compared with conventional laser welding, the weldingformation and quality was improved significantly while the laser welding was conducted under vacuum. In this work,the influences of ambient pressure on the laser welding penetration depth, surface formation and porosity defect weresummarized. The domestic and overseas research findings on mechanism of laser welding under vacuum were elaborated from the aspects of plasma plume, keyhole and molten pool behaviors. In addition, the applications of laser welding under vacuum in the industry were introduced. Finally, the problems of reported researches were analyzed and theprospects of the technology were discussed. The previous researches on the laser welding under vacuum indicated thatthe penetration depth of the weld seams increased sharply, the welding formation was improved and the porosity defectswere suppressed effectively. Critical vacuum degree enough for improving the weld penetration depth and quality wasdetected for aluminum alloy, titanium alloy, nickel-base alloy and steel. The laser welding characteristics under vacuumwas related to the plasma plume, keyhole and molten pool flow behaviors. The planet wheel carrier in the power stationhad been welded successfully by applying this technology. The laser welding under vacuum exhibits the wonderful application prospects to weld the thick plates in the shipbuilding, nuclear instrument and pressure vessel industries. In thefuture works, the authors suggest that the laser power deposition mechanism should be investigated systematically byconsidering the physical properties of the materials and the collision characteristics of the ions in the plasma plume. Besides, the heat and mass transfer characteristics, solidification behavior of the molten pool should be studied. In order toexpand the application fields of laser welding under vacuum, the low vacuum and local subatmospheric pressure laserwelding equipments should be developed towards higher adaptability and integration. Local subatmospheric pressurelaser welding equipments with the excellent pressure maintaining property should be developed. Moreover, the feasibility of laser welding with filler and laser hybrid welding under vacuum is of vital interest for the development this technology.
The penetration depth of laser welding underlow vacuum[7].
Vacuum equipment for high-power laser welding[9]. (a) Schematic diagram. (b) Photo image of the vacuum chamber.
Cross-sections of laser welds in stainless steel undervacuum (0.1 kPa)[10]. (a) Defocused distance of-40 mm. (b) Defocused distance of +20 mm.
The influences of ambient pressure on the penetrationdepth of laser welded aluminum alloys[12].
The plasma plume area as a function of vacuum degree[13].
Photo images of local subatmospheric equipment for laser welding[14]. (a) Overall image. (b) Structure image (1. local subatmospheric module; 2. conical orifice; 3. fixed module; 4. air knife; 5. optical lens).
The characteristics of probe laser through the plasma plume. (a) Schematic diagram of equipment[21]. (b) Change of spotpositions and refracted angles[9].
Flow behavior and formation of porosity defectunder atmospheric pressure [8].
Flow behaviors of molten pool during laser welding process under vacuum[8].
Longitudinal distribution of transient temperature during laser welding process (t=31.5 ms)[26]. (a) Under vacuum. (b) Underatmospheric pressure.
Cross-sections of laser welds[26]. (a) Under vacuum. (b) Under atmospheric pressure.
Laser welding equipment with vacuum chamber[29]. (a) Laser head. (b) Vacuum chamber. (c) Pumping station.
Laser welding equipment with laser beam in vacuum manufactured by PTR GmbH[30].
Laser welding of planet wheel carrier under vacuum[30]. (a) Schematic diagram of welding position. (b) Cross-section of weld seam.