Hybrid laser-arc welding has a good tolerance to poor fit up as compared to simple laser welding. For a butt joint, the joint fit-up variations can be reduced to two local properties: the gap width and the vertical edge mismatch. The impact of these two properties on the resulting weld quality has been studied systematically in this paper. The original edges as well as the resulting weld surface topography have been scanned in three dimensions in order to study trends. During hybrid welding, the melt flow and the electric arc were observed at the top surface by high speed imaging to analyze the complex fluid flow phenomena.

22MnB5/Usibor1500 is a hot-dip aluminized manganese-boron steel grade that is used withinthe car manufacturing industry and as lasers are increasingly used for welding of the car bodythe amount of research concerning laser welding of this material follows. For thicker materialthe laser hybrid arc welding process has grown to a well-known tool to increase productivity,but for thinner materials the advantages are less obvious. In the present research the melt pooland wire drop transfer was observed with a high speed camera. Differences between welding,with and without the coating were clearly visible and are described and discussed in the paper.The resulting welds were examined microscopically and chemically to unveil the compositionof the material. It is aimed to avoid possible brittle inter-metallic phases directly during theprocess, hence by skipping the usual additional manufacturing step of removing the AlSicoatingin advance.

That the final quality of a welded joint, using a laser hybrid arc process, is sensitive not only to the geometrical joint conditions, but also to the content of elements in the surface itself, is already known. However the description of the mechanisms forming undercut defects is still vague. In the following study two kinds of undercuts were identified. It is shown how the mill scales affects the melt motion and what mechanisms actuate the creation of the defects. The conclusion that surface oxides affect the severity of weld undercuts is straight forward but here also the mechanism is explained.

Two different kinds of undercut were identified when laser hybrid welding hot rolled HSLA-steel in either the as-rolled condition or with the top surface mill scale removed. The presence of mill scale on the steel surface was found to give a sharp angled undercut combined with a sharp oxide inclusion at the edge of the weld which would have the same mechanical effect as a crack in this position. Removal of the surface oxides before welding resulted in the elimination of the oxide inclusions and a more rounded undercut geometry indicative of superior mechanical properties, particularly fatigue life. The mechanisms of the formation of both types of undercut have been analysed by high speed photography and SEM.

The surface geometry after laser hybrid arc welding is an important quality criterion. What isoften called welding in a rough industrial environment can in many cases systematically bereduced to a few local conditions of the joint edge during welding, particularly depending onthe edge preparation technique. In the present study, beside the influence of surface oxidesstudied in separate publications, the conditions are reduced to the local gap width, verticalsurface position and edge mismatch. Gas cut edge conditions are compared to milling andlaser cutting. By scanning the steel edges of a 10 mm butt joint before and during hybridwelding and relating the systematic variation of the above three properties to the scannedtopology of the resulting weld, quantitative correlations are identified. By the aid of highspeed imaging, explanations for the trends can be found. Operating windows with controllabletrends can be quantitatively mapped and distinguished from instable regimes. Beside higherroughness, gas cut edges show a superimposed edge oscillation of longer periodicity, beingless visible but important. During welding, concave surface bending and gap widening alterthe initial conditions. Awareness and quantitative identification of the edge conditions isrecommended to control the welding process in a rough industrial environment.

High speed imaging has been used to analyze the sensitivity of the Hybrid laser welding process to variations in the laser-arcworkpiece geometry along the axis of the laser beam. The welding process was found to be stable within a certain range of workpiece positions. Outside of this range the process became unstable. If the workpiece was too close to the laser/arc combination, the two energy sources did not supplement each other sufficiently. If the workpiece was too far away the droplets from the arc interfered with the laser-keyhole interaction.

Compared to autonomous laser welding, the amount of parameters is higher for laser hybrid welding. Consequently, empirical optimisation of these parameters is a challenge. Handling and evaluation at a higher systematic level is desired in order to enhance the ability to build research on previous knowledge. Such new approach is studied on a case with 10 mm high strength steel sheets, fibre-laser hybrid welded. Aim of the paper is to provide a method of documenting and handling data, transferable to other disciplines, to continuously build knowledge, to simplify repetition of experiments and to facilitate the start-up phase of new trials. Starting from 30 experimental results depending on 23 parameters, by the Matrix Flow Chart a guideline has been developed that filters the information through combination, priorities and quality categorization. A chart resulted where five categories of poor quality are graphically related to a high quality category which can be achieved when following the guidelines for eight main arc- or laser-parameters. The chart is a guideline suitable for extension and for exploring the limits of its validity.

Process monitoring of laser welding defects by commercial systems based on photodiodes has high potential, but improved understanding is desirable for more systematic industrial use. The signal is generated from spectrally filtered emissions or reflections from the dynamic welding process. Improved knowledge on the cause of the signal was achieved by accompanying high speed imaging. Numerical evaluation of the images and subsequent modelling provided additional information to judge the potential and limits of commercial photodiode sensors. For welding with a 3 kW Nd:YAG-laser, a 15 kW fibre laser or a 14 kW CO2-laser, various joint cases were studied with respect to defects like craters, spatter, blowholes, underfill, root drop-out, lack of penetration, lack of fusion or pores. From the findings a more general theoretical description of the links between the welding process, the defect mechanism and the generated signal was initiated.