The keyhole produced during deep penetration laser welding emits a plume of hot metal vapor and particles. The interaction between the plume and the incident laser beam results in beam scattering, absorption, and phase front deformation. The combination of scattering and absorption leads to a partial extinction of the laser beam, while the phase front deformation adversely effects the beam quality. In this study we present a measurement setup which allows for diagnostics of the beam characteristics after interaction with the plume. This is achieved by utilizing an additional measurement beam, which is coaxially aligned to the high-power laser beam used for welding. The experimental procedure presented here enables high-frequency measurements of the caustic changes and relative power losses of the measurement beam. The measurements obtained provide a quantification of the various interaction mechanisms between the laser beam and vapor plume. This knowledge is crucial to prevent weld defects, which result from the adverse effects of the vapor plume on the laser beam.
During welding the localized heat input results in high temperature gradients between the weld seam and the base material leading to residual stress. The residual stress is a result of two competing processes; thermal shrinkage of the material while cooling (resulting in tensile stress in the weld seam) and phase transformation induced volume expansion (resulting in compressive stress in the weld seam). Both processes superimpose to a resulting residual stress profile. To counteract the problems of residual stress and distortion, in the past few years low-transformation-temperature (LTT) materials have been successfully used as filler wire. Typically, LTT materials are highly alloyed Fe-based materials with levels of Cr and Ni that ensure that austenite transforms to martensite at reduced temperatures. This transformation is accompanied by large volumetric dilatation. The surrounding base material prevents this dilatation in the weld seam and compressive stress builds up while reducing residual stress and distortion. A way to use the LTT effect, other than using a LTT filler wire, is to combine dissimilar materials. By combining high alloy and low alloy materials a microstructure is formed in-situ that shows similar properties as a common LTT weld metal. The displacements after welding are always lower when using LTT filler material when compared to conventional wire, proving that LTT can be used to mitigate distortion during laser beam welding. In this paper the strain distribution by the use of digital image correlation is examined. The influence of dissimilar welding on the microstructure is considered and it is investigated whether the LTT effect can be reproduced with conventional filler wire.
Research and development carried out by the ISF Welding and Joining Institute of RWTH Aachen University has proven that combining high power laser and low vacuum atmosphere provides a welding performance and quality, which is comparable to electron beam welding. The developed welding machines are still using a beam forming which takes place outside the vacuum and the focusing laser beam has to be introduced to the vacuum via a suitable window. This inflexible design spoils much of the flexibility of modern laser welding. With the target to bring a compact, lightweight flying optics with flexible laser transport fibers into vacuum chambers, a high power fiber-fiber coupler has been adapted by II-VI HIGHYAG that includes a reliable vacuum interface. The vacuum-fiber-fiber coupler (V-FFC) is tested with up to 16 kW sustained laser power and the design is flexible in terms of a wide variety of laser fiber plug systems and vacuum flanges. All that is needed to implement the V-FFC towards an existing or planned vacuum chamber is an aperture of at least 100 mm (4 inch) diameter with any type of vacuum or pressure flange. The V-FFC has a state-of-the-art safety interface which allows for fast fiber breakage detection for both fibers (as supported by fibers) by electric wire breakage and short circuit detection. Moreover, the System also provides connectors for cooling and electric signals for the laser beam optics inside the vacuum. The V-FFC has all necessary adjustment options for coupling the laser radiation to the receiving fiber.
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