Laser Ultrasound as a tool for the characterisation of electromagnetic welded joints

Joining by Electromagnetic Forming (EMF), also called electromagnetic pulse joining or welding, is a promising innovative technology for e.g. the joining of dissimilar materials. EMF is a high-speed forming technology using pulsed magnetic fields for forming electrically-conductive tube or sheet metal workpieces without mechanical contact between tool and workpiece. Depending on the interaction with additional components, apart from mere shaping processes, also joining by EMF is possible (electromagnetic welding - EMW). In EMW, a metallic bonding is created. For the characterization of the properties of electromagnetic welded joints two different laser ultrasound setups were developed. The first of these, comprising a nanosecond Q-switched pulse laser (wavelength: 1064nm, 1ns pulse duration, <2mJ/pulse, <2kHz repetition rate) and a two-wave-mixing (TWM) interferometer using a cw laser of 532 nm wavelength in transmission configuration, is limited for the investigation of planar samples and requires a two-sided accessibility. With the second approach, using a fiberized nanosecond Q-switched pulse laser (wavelength: 532nm, 10ns pulse duration, energy of 30mJ), a pulsed detection laser (PDL, wavelength: 1064nm, 10ms pulse duration, <500 Watts peak power) and a TWM interferometer with GaAs photorefractive crystal, in reflection configuration, also tubular samples can be measured and a one-sided accessibility is sufficient. Both setups were used to investigate the quality of electromagnetic welded joints on sheet metal and tubular specimen, respectively. When evaluating the data, suitable strategies were developed for the respective configuration and the results were compared with destructive tests (metallographic micro sections, tensile tests,EL) and non-destructive characterisation methods like leak tests (for tubular samples). For the transmission scheme, the weld seam properties such as width, length and flyer thickness have been determined and based on them, a quality number was calculated. The result of the laser ultrasound measurements showed good agreement with the results of the destructive testing.