Hans Engström

Theoretical analysis of photodiode monitoring of laser welding defects by imaging combined with modelling

On-line process monitoring of laser welding defects by detecting characteristic changes of the process emissions via photodiodes has high potential but, due to the complex process, lack of directly interpretable and thus controllable correlations. Deep analysis of the process by high speed imaging in combination with modelling enables one to discuss the correlations between the signal and the process, as was demonstrated for quasi-steady state conditions and for transient phenomena. Despite improved knowledge through the study, several uncertainties like the emissivity, the keyhole radiation characteristics and the temperature field need to be identified more accurately for a complete theoretical description of the signal causes.

Modelling of the impact of melt surface dyanmics on a photodiode monitoring signal in laser welding

Today merely a few monitoring systems for in-process detection of laser welding defects are commercially available. Despite a trend towards cameras, the industrially most powerful concept is still a photodiode with optical filter, measuring thermal emissions from the melt surface and from the plasma or laser beam reflections. The monitoring rule for each application is identified empirically through temporal correlations between dynamic signal changes and obtained welding defects. The mechanisms behind are widely not understood. Thus the method does not provide a systematic estimation of success for identifying a certain welding defect.The here presented research approach studies the context between welding defects, the physical mechanisms behind, particularly the dynamics of melt pool, plasma and temperature field, and the photodiode signal. Numerical simulation results of the thermal emissions from the weld pool and keyhole dynamics and their non-linear conversion into a voltage signal are presented. An essential outcome is the sensitivity of the sensor signal to certain sub-mechanisms of the motion for judging under which conditions they can be monitored. Various results are discussed for simplified hypothetic cases as well as for observed weld pool dynamics of practical relevance.Today merely a few monitoring systems for in-process detection of laser welding defects are commercially available. Despite a trend towards cameras, the industrially most powerful concept is still a photodiode with optical filter, measuring thermal emissions from the melt surface and from the plasma or laser beam reflections. The monitoring rule for each application is identified empirically through temporal correlations between dynamic signal changes and obtained welding defects. The mechanisms behind are widely not understood. Thus the method does not provide a systematic estimation of success for identifying a certain welding defect.The here presented research approach studies the context between welding defects, the physical mechanisms behind, particularly the dynamics of melt pool, plasma and temperature field, and the photodiode signal. Numerical simulation results of the thermal emissions from the weld pool and keyhole dynamics and their non-linear conversion into a voltage signal are presented. An...