The full penetration hole as a stochastic process: Controlling penetration depth in keyhole laser-welding processes

Research output: Contribution to journalResearch articleContributedpeer-review


  • Andreas Blug - , Fraunhofer Institute for Physical Measurement Techniques (Author)
  • Felix Abt - , University of Stuttgart (Author)
  • Leonardo Nicolosi - , TUD Dresden University of Technology (Author)
  • A. Heider - , University of Stuttgart (Author)
  • R. Weber - , University of Stuttgart (Author)
  • D. Carl - , Fraunhofer Institute for Physical Measurement Techniques (Author)
  • Heinrich Höfler - , Fraunhofer Institute for Physical Measurement Techniques (Author)
  • R. Tetzlaff - , Chair of Fundamentals of Electrical Engineering (Author)


Although laser-welding processes are frequently used in industrial production the quality control of these processes is not satisfactory yet. Until recently, the "full penetration hole" was presumed as an image feature which appears when the keyhole opens at the bottom of the work piece. Therefore it was used as an indicator for full penetration only. We used a novel camera based on "cellular neural networks" which enables measurements at frame rates up to 14 kHz. The results show that the occurrence of the full penetration hole can be described as a stochastic process. The probability to observe it increases near the full penetration state. In overlap joints, a very similar image feature appears when the penetration depth reaches the gap between the sheets. This stochastic process is exploited by a closed-loop system which controls penetration depth near the bottom of the work piece ("full penetration") or near the gap in overlap joints ("partial penetration"). It guides the welding process at the minimum laser power necessary for the required penetration depth. As a result, defects like spatters are reduced considerably and the penetration depth becomes independent of process drifts such as feeding rate or pollution on protection glasses.


Original languageEnglish
Pages (from-to)97-107
Number of pages11
JournalApplied Physics B: Lasers and Optics
Issue number1
Publication statusPublished - 5 Jul 2012

External IDs

ORCID /0000-0001-7436-0103/work/142240334