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- Additive Manufacturing (5)
- Laser beam welding (4)
- Hybrid laser-arc welding (3)
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The contact between train wheels and rail tracks is known to induce material degradation in the form of wear, and rolling contact fatigue in the railhead. Rails with a pearlitic microstructure have proven to provide the best wear resistance under severe wheel-rail interaction in heavy haul applications. High speed laser cladding, a state-of-the-art surface engineering technique, is a promising solution to repair damaged railheads. However, without appropriate preheating or processing strategies, the utilized steel grades lead to martensite formation and cracking during deposition welding.
In this study, laser cladding of low-alloy steel at very high speeds was investigated, without preheating the railheads. Process speeds of up to 27 m/min and laser power of 2 kW are used. The clad, heat affected zone and base material are examined for cracks and martensite formation by hardness tests and metallographic inspections. A methodology for process optimization is presented and the specimens are characterized for suitability. Within the resulting narrow HAZ, the hardness could be significantly reduced.
One of the challenges of the high-power hybrid laser welding of thick steels is the sensitivity of the process of the process to manufacturing tolerances. This usually leads to a time-consuming preparation of the welding edges, such as milling. The study deals with the influence of the edge quality of milled and plasma-cut steel made of S355J2 with a wall thickness of 20 mm on the laser hybrid welded seam quality. Furthermore, the gap bridgeability and the tolerances towards edge misalignment was investigated. An AC magnet was used as backing support to prevent sagging and positioned under the workpiece, to generate an upwards directed electromagnetic pressure. The profiles of the edges and the gap on the top and root side were measured using a digital camera. Single-pass laser hybrid welds of plasma-cut edges could be welded using a laser beam power of just 13.7 kW. A gap bridgeability up to 2 mm and misalignment of edges up to 2 mm could be achieved successful. Additionally, the independence of the cutting side and the welding side was shown, so that samples were welded to the opposite side to their cutting. For evaluation of internal defects or irregularities, X-ray images were carried out. Charpy impact strength tests were performed to determine the toughness of the welds.
In order to reduce weight of vehicles, the interest in multi-material-design has been growing within the last few years. For vehicles the combination of steel and aluminium alloys offers the most promising compromise between weight, strength and formability. Thermal joining of these dissimilar materials is still a challenge to overcome. A possible approach is a new joining technology, whereby a combination of laser beam welding and contactless induced electromagnetic forces are used to displace the generated melt of one joining partner into a notch of the other. This paper presents the working principle and shows numerical analyses to improve the understanding of this joining process. The simulations help to calculate the thermal development of the joining partners, which is important for the formation of intermetallic phases. Furthermore, the calculation of the time required for a complete displacement is possible. The numerical results are validated by experimental results.
Solidification cracking is still a particular problem in laser beam welding, especially in the welding of thick-walled plates. In this study, the influence of weld pool geometry on solidification cracking in partial penetration welding of thick plates is a subject of discussion. For this purpose, a special experimental setup of steel and quartz glass in butt configuration and lateral with high speed camera was used to capture the weld pool shape. Additionally, laser beam welding experiments were carried out to compare the crack positions and the cross section with the high-speed camera observations. The results showed a bulge in the weld pool root separated from the upper region by a nick area. This leads to the fact that three different longitudinal lengths with different solidification areas are taking place. This temporal sequence of solidification strongly promotes the solidification cracks in the weld root.
The study deals with the influence of the heat input on the thermal cycles and Charpy impact toughness for hybrid laser-arc welding of 25 mm thick structural steel S355J2 using a 20-kW high-power laser in combination with an electromagnetic weld pool support. The main focus is on the change of the mechanical properties over the entire seam thickness. The cooling times were measured using a pyrometer in combination with an optical fibre in three different locations near to fusion lines corresponding to different heights of the seam. Also, Charpy impact specimens were taken from different parts of the weld joint corresponding to the different heights. The influence of the heat input was investigated for 1.8 kJ mm-1 and 3.2 kJ mm-1. Despite the observed decreased values of both t8/5-cooling time and the Charpy impact toughness in the root part of the seam, the required values could be reached in dependance on applied heat input.
It is already known that the laser beam welding (LBW) or hybrid laser-arc welding (HLAW) processes are sensitive to manufacturing tolerances such as gaps and misalignment of the edges, especially at welding of thick-walled steels due to its narrow beam diameter. Therefore, the joining parts preferably have to be milled. The study deals with the influence of the edge quality, the gap and the misalignment of edges on the weld seam quality of hybrid laser-arc welded 20 mm thick structural steel plates which were prepared by laser and plasma-cutting. Single-pass welds were conducted in butt-joint configuration. An AC magnet was used as a contactless backing. It was positioned under the workpiece during the welding process to prevent sagging. The profile of the edges and the gap between the workpieces were measured before welding by a profile scanner or a digital camera, respectively. With a laser beam power of just 13.7 kW, the single-pass welds could be performed. A gap bridgeability up to 1 mm at laser-cut and 2 mm at plasma-cut samples could be reached respectively. Furthermore, a misalignment of the edges up to 2 mm could be welded in a single-pass. The new findings may eliminate the need for cost and time-consuming preparation of the edges.
-Elektromagnetische Schmelzbadstütze wurde beim
Laserhybridschweißen von Dickblechen erfolgreich zur
Unterdrückung der Tropfenbildung eingesetzt
-Auch bei „schlechteren“ Kantenqualitäten konnten einlagige
Schweißnähte mit hoher Qualität erzielt werden
-Spaltüberbrückbarkeit bis zu 2 mm (bei plasmageschnittenen
Proben) konnte gewährleistet werden
-Homogenere Verteilung des Zusatzwerkstoffes wurde erzielt
Because of its excellent corrosion resistance, high tensile strength and
high ductility, duplex stainless steel 2205 offers many areas of application. Though laser beam welding accompanied by high cooling rates, duplex steels tend to perform higher ferrite contents in weld metal as the base metal, which leads to a reduction of ductility and corrosion resistance of the weld joint. To overcome this problem, a solution, based on buttering the plate edges by laser metal deposition (LMD) with material containing higher Ni concentrations prior to laser welding was suggested.
In this context different process parameters for LMD and different mixtures of duplex and nickel powder, were investigated. In a second step the possibility of welding those edges defect free while achieving balanced austenite-ferrite ratio was verified with metallographic analysis, Electron Backscatter Diffraction (EBSD) and impact testing according to Charpy. The improved corrosion resistance was observed with ASTM G48 standard test method.
One of the challenges of the high-power hybrid laser welding of thick steels is the sensitivity of the process of the process to manufacturing tolerances. This usually leads to a time-consuming preparation of the welding edges, such as milling. The study deals with the influence of the edge quality of milled and plasma-cut steel made of S355J2 with a wall thickness of 20 mm on the laser hybrid welded seam quality. Furthermore, the gap bridgeability and the tolerances towards edge misalignment was investigated. An AC magnet was used as backing support to prevent sagging and positioned under the workpiece, to generate an upwards directed electromagnetic pressure. The profiles of the edges and the gap on the top and root side were measured using a digital camera. Single-pass laser hybrid welds of plasma-cut edges could be welded using a laser beam power of just 13.7 kW. A gap bridgeability up to 2 mm and misalignment of edges up to 2 mm could be achieved successful. Additionally, the independence of the cutting side and the welding side was shown, so that samples were welded to the opposite side to their cutting. For evaluation of internal defects or irregularities, X-ray images were carried out. Charpy impact strength tests were performed to determine the toughness of the welds.
Mathematical analysis of the bulging effect in high power laser beam welding of thick steel sheets
(2021)
The present work is devoted to the mathematical analysis of the bulging effect in high-power laser beam welding of thick steel sheets. The numerical results are based upon experimental results from previous studies, revealing the relationships between the bulging effect, the hot cracking formation, and the distribution of alloying elements in the weld pool. The widening of the molten pool in its middle area is observed for both complete and partial penetration welding of 8 mm - 15 mm thick structural steel sheets. The weld pool shape is extracted from the simulations to evaluate the extent of the necking and bulging of the solidification isotherm and their influence on the hot cracking formation and the mixing behavior of the weld pool. Relying on an earlier numerical study utilizing a fixed keyhole, simulation models considering a dynamic keyhole are developed thereto. The link between the bulge and the studied phenomena is found to be significant.