Defect detection during laser welding by laser speckle photometry

The modular design of steam turbines requires the welding of individual rotor discs to the finished single rotor. Conventional welding technology is very complex. Lack of fusion defects will occur after completion of welding and quality assurance is carried out by X-ray and ultra-sonic detection currently. Turbine rotors principally work at high temperature and therefore Ni superalloys are used to get those rotors manufactured. Fraunhofer institute IWS is currently developing a new welding technology based on laser-multi-pass-narrow-gap-welding (Laser-MPNG) including the implementation of a respective in-line quality assurance approach. For this approach, the nondestructive testing (NDT) i.a. Laser Speckle Photometry (LSP) was adapted to be applied in the narrow gaps of up to 200 mm in depth. LSP is an optical contactless, quick and quality-relevant NDT method, which will be used for characterization of material properties and defect detection, allowing process monitoring in many industrial fields. The LSP method is currently used at laboratory scale. For the transfer of LSP to an industrial level the LSP-setup and the evaluation algorithms need to be adjusted for in-line measurement during Laser-MPNG welding process. Additionally the results obtained by the LSP measurement was verified in the sense of a benchmark by simultaneously taking measurements with acoustic emission (AE) and ultrasonic technique (UT) and comparing the results with micrographs and visual inspection. This work presents the suitability of the LSP technique to monitor the Laser-MPNG welding and to detect defects less than 0.5 mm during the welding process.