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.
The microstructure of a lithium aluminosilicate (LAS) glass ceramic has been modified by ultra-short pulsed laser radiation during the crystallization process. Laser pulses with 10 ps pulse duration, 1064 nm wavelength and a repetition rate of 50 kHz have been focused inside LAS glass ceramic using a microscope objective with a NA of 0.4. Before laser treatment the LAS glass ceramic was already transformed to a primary crystallization stage by using a heat treatment at 540°C and 660°C. Caused by nonlinear absorption processes energy is transferred from the photons to the lattice and leads to local melting. The fast cooling of the melted volume due to heat conduction enables the formation of an amorphous phase. After a second heat treatment at 830°C the laser irradiated area shows a different microstructure compared to the untreated area. Influences of the modified microstructure on mechanical and optical properties have been studied. Potential applications of this process are proposed.
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.