Laser welding in a vacuum

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The laser welding in a vacuum or laser beam welding in vacuum (short lava or LasVak) is a method of modification of the laser beam welding . It combines vacuum technology, which is normally used in electron beam welding , with the established joining technology of laser beam welding. The process is usually used in a pressure range from 1 to 100 hPa and is characterized by a very high weld seam quality due to the avoidance of pores and weld spatter. The weld seams produced with LaVa welding are similar in design to electron beam weld seams.

history

The first attempts at LaVa welding were carried out in the 1980s with a CO 2 laser. It turned out that the welding speed can be reduced to an unusual order of magnitude with reduced pressure, whereby a significant increase in the welding depth is achieved. Furthermore, the plasma torch usually present in the atmosphere could be significantly reduced.

With the development of brilliant solid-state lasers such as B. Disk laser, the topic of LaVa welding was taken up again from 2009 at the Institute for Welding and Joining Technology at RWTH Aachen University .

The first tests with a 600 W singlemode fiber laser showed that electron beam welding and LaVa welding produce almost identical weld seam shapes and qualities with the same parameters.

Investigations by the Institute for Joining and Welding Technology at the TU Braunschweig also showed that the formation of spatter can be significantly reduced by the vacuum.

The potential of LaVa welding as an alternative to the electron beam was demonstrated in 2011 in tests with 12 kW beam power. A welding depth of 50 mm could be achieved on steel.

Basics

Comparison of laser welding in atmosphere and LaVa welding.

The “LaVa effect”, which causes an increase in the welding depth with constant laser beam power, a reduction in splashes and pores, and the avoidance of the steam flare, can be described by the following physical properties. While the melting point of metals is essentially independent of pressure (for iron there is theoretically a reduction in the melting point of 0.01 K in an absolute vacuum), the boiling point, on the other hand, is significantly reduced. At 10 −1 hPa the evaporation temperature of iron is reduced by more than 1300 K.

The lower evaporation temperature leads to a reduced temperature gradient between the wall of the steam capillary and the melt line , which results in a significantly reduced melt pool volume. Furthermore, due to the lower temperature, the vapor capillary wall is more stable than when welding in atmosphere, which also stabilizes the weld pool flows. In particular, the speed of the flow rising behind the vapor capillary is reduced. This reduction results in the reduced tendency to spatter and pore formation.

Advantages and disadvantages compared to electron beam welding

While electron beam systems require a pressure of approx. 10 −5 hPa or higher in the area of ​​beam generation , the laser beam is not generated in a vacuum. After focusing in the optics, the laser beam is coupled into the vacuum chamber through a protective glass transparent to the radiation, which is also used as a vacuum window. In order to avoid collisions of the electrons with air molecules, a pressure of 10 −3 - 10 −4 hPa must be present in the vacuum chamber itself during electron beam welding . With LaVa welding, however, a pressure of 10 0 - 10 2 hPa is sufficient . This reduces the evacuation times and improves the possible cycle time.

The cathode , the component of the electron beam generator from which the electrons emerge, is subject to continuous wear. Therefore it has to be changed at regular intervals. The service life of the cathode is largely dependent on the welding task. In the case of deep welds on aluminum, a replacement must take place after approx. 4 hours of welding time, while steel materials with low welding depths can have a service life of up to 100 hours. The failure of a cathode can only be foreseen to a limited extent, which in the worst case can lead to a total failure of the welding machine. This can cause significant economic damage, particularly with complex and cost-intensive individual components. When changing the cathode, there may still be deviations from the target installation position, which have a very negative effect on the welding result. In the laser beam generator or the optics of LaVa systems, however, there are no wearing parts that have a negative impact on the beam quality. This increases the reproducibility of the welding results in the medium term.

Since the electron beam consists of charged particles, it can be influenced by magnetic fields. This results in many options for targeted beam manipulation, which enables so-called multi-bath technology, for example. The deflectability by magnetic fields also poses problems if these are not set specifically. If, for example, magnetic component manipulation is used in the course of the production chain, the components must be demagnetized before electron beam welding. Furthermore, the so-called thermoelectric effect can occur when welding a combination of different materials . This creates magnetic fields that can unintentionally deflect the electron beam. Since the laser beam is not a particle beam, it is not influenced by magnetic fields. The use of magnetic manipulators and the thermoelectric effect are therefore not a problem.

When the electron beam hits metallic components, X-rays are generated; around 1% of the beam power is converted into X-rays. However, since the electron beam welding process takes place in a closed chamber, the X-rays are safely shielded. Depending on the performance of the welding machine, however, chamber thicknesses of 20-30 mm are required for shielding. In higher performance classes (from 60 kV acceleration voltage) a further protective layer made of lead is necessary for shielding. The laser beam, however, does not emit any X-rays. For this reason, chambers for LaVa welding can be manufactured cost-effectively from aluminum.

One of the advantages of electron beam technology over LaVa welding is the simple variation of the focusing length of the electron beam by changing the focus current. Since laser optics usually have rigid focussing lengths, an additional linear axis is required to change the focal point in the vacuum chamber.

Laser radiation of a defined wavelength has a material-dependent absorption rate. While with steel almost 100% of the beam power can be transferred into the workpiece during deep welding, the absorption rate with aluminum and copper is significantly lower. The electron beam, on the other hand, has an absorption rate that is independent of the material.

literature

  1. ^ Arata, Y .; Abe, N .; Oda, T .; Tsujii, N .: Fundamental phenomena during vacuum laser welding . In: Proceedings International Congress on the Application of Lasers and Electro-Optics ICALEO . San Francisco November 1985, p. 1-7 .
  2. ↑ Reisgen, Uwe; Olschok, Simon; Longerich, Stefan ;: Laser Beam Welding in Vacuum - A Process Variation in Comparison with Electron Beam Welding . In: Proceedings of 29th International Congress on Applications of Lasers and Electro-Optics ICALEO . Anaheim September 2010, p. 628-637 .
  3. ↑ Longerich, Stefan .: Investigation of laser beam welding under vacuum in comparison with electron beam welding . Shaker, Aachen 2011, ISBN 978-3-8440-0629-2 .
  4. ^ Dorsch, Friedhelm, SPIE (Society): High-power laser materials processing: lasers, beam delivery, diagnostics, and applications II: 5-7 February 2013, San Francisco, California, United States . Bellingham, Washington 2013, ISBN 978-0-8194-9372-9 .
  5. ↑ Reisgen, Uwe; Olschok, Simon; Jakobs, Stefan; Longerich, Stefan: High-performance laser beam welding of thick metal sheets in a fine vacuum - an alternative to EB welding . In: welding and cutting . tape 63 , no. 9 . DVS Media, Düsseldorf 2011, p. 522-527 .
  6. ↑ Ruf, Andreas .: Modeling of percussion drilling of metals with short- and ultra-short pulsed lasers . Utz, Munich 2004, ISBN 3-8316-0372-3 .
  7. ↑ honey, RE; Kramer, DA: Vapor Pressure Data for the Solid and Liquid Elements . In: RCA Review . tape 30 , no. 2 , 1969, p. 285-305 .
  8. ↑ Pang, S .; Hirano, K .; Fabbro, R .; Jiang, T .: Modeling Keyhole and weld poll dynamics of laser welding under variable ambient pressure . In: Proceedings of 31st International Congress on Applications of Lasers and Electro-Optics ICALEO . Anaheim September 2012, p. 685-694 .
  9. ↑ Börner, C .; Pries, H .; Dilger, K .: Influence of ambient pressure on spatter formation and weld seam properties during laser beam welding with the solid-state laser . In: DVS reports 284 . DVS Media, Düsseldorf 2012, ISBN 978-3-87155-298-4 , p. 85-93 .
  10. ↑ a b c d Shaker Verlag GmbH .: Laser beam welding in a vacuum Extension of the process limits for thick-walled sheet metal . 1st edition Herzogenrath 2015, ISBN 978-3-8440-4032-6 .
  11. ↑ a b c Schulze, Klaus-Rainer .: electron beam technologies . DVS Media, Düsseldorf 2011, ISBN 978-3-87155-225-0 .
  12. ↑ electron beam technology. By S. Schiller, U. Heisig and S. Panzer; Wissenschaftliche Verlagsgesellschaft Stuttgart 1977, 266 illustrations, 38 plates, approx. 400 pages, DM 68, - . In: Physics in Our Time . tape 10 , no. 1 , 1979, ISSN  1521-3943 , pp. 32–32 , doi : 10.1002 / piuz.19790100115 ( wiley.com [accessed December 18, 2018]).
  13. ↑ a b S. Hellberg and S. Böhm, S. Jakobs, S. Olschok and U. Reisgen: Influence of the fluctuations in cathode properties on the beam quality and the welding result in electron beam welding . In: DVS Congress proceedings . DVS Media, Düsseldorf September 2018, p. 295-300 .
  14. ↑ Schultz, Helmut, 1932-: Electron beam welding: Fundamentals, machines and applications . 3rd, completely revised and expanded edition. Düsseldorf 2018, ISBN 978-3-945023-85-3 .
  15. ↑ Mahamood, Rasheedat Modupe, 1973-: Laser metal deposition process of metals, alloys, and composite materials . Cham, Switzerland 2018, ISBN 978-3-319-64985-6 .