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Laser-Assisted Bending

When sheets of high-strength (HS) and ultra-high-strength (UHS) steels are bent by a press brake the process suffers from large bending forces, considerable springback, and eventual cracks. Additionally, some unpredictable effects, such as lost contact to the punch, caused by strain hardening may occur producing a bend with erroneous radii. The strain hardening of the bending line may make further processes, such as forming or welding, more complex. One solution to these problems is to anneal the bending line with a laser in advance. Of course, it is also possible to utilise other types of heat sources, but the laser can offer the most precisely controlled heat treatment. The proper process parameters depend on the material, and it has been noticed that inadequate process parameters may harden the material instead of annealing. In this work some experiments on bending sheet metal samples of HS or UHS steel with previously laser-annealed bending lines have been carried out and the outcome analysed. The results show that the annealing produces better bending results compared to the conventional procedure. This includes lower springback, less hardening in the bending line and more precise geometry of the bend. It can be even suggested that proper annealing with strain hardening in bending will produce the original material structure. Obviously, more theoretical and experimental work is required to optimise the process parameters including the laser power and speed for each pair of material strength and thickness.

Identifying residual stresses in laser welds by fatigue crack growth acceleration measurement

During laser welding, residual stresses are thermally induced. They can have strong impact on the fatigue behavior and fatigue life. A standardized measurement method for the fatigue crack growth rate was expanded to identify residual stress along the cracking path. The second derivative of the measured crack opening and in turn the crack acceleration corresponded well with distinct acceleration maxima and minima and accordingly with tensile and compressive stress, as was basically proven by numerical simulation. The method is simple and extendable. It provides valuable information, as was demonstrated for various situations.

The Influence of the Shielding Gas to the Static and Dynamic Strength Properties of Laser Welded Workhardened Nitrogen Alloyed Austenitic Stainless Steel

As an interstitial atom, nitrogen strengthens the structure of austenitic stainless steel (ASS). It therefore has been used to increase the strength of ASS. On the other hand, work hardening of ASS is a common method to increase the strength of the sheet product. When a work-hardened structure is welded, the strength properties decreases at the melted zone and the heat-affected zone (HAZ) of the weld. The nitrogen content can also be reduced by the effect of the heat input of the weld. Because the width of the soft area of the HAZ depends on the energy input of the weld, the strength of the weld depends on energy input. Therefore, laser welding provides better strength to the welded structure. The role of the shielding gas is also significant. Argon shielding gas is inert, but nitrogen used as a shielding gas can strengthen the weld metal and HAZ microstructure. In this study, the effect of different shielding gases in the laser welding of AISI 201 LN TR type work-hardened ASS are tested and the results are reported. Both non-destructive material and destructive material tests are performed. According to the results of the tensile test, the use of nitrogen as a shielding gas strengthens the laser-welded structure. The results of the low-cycle fatigue test show that fatigue strength improves when nitrogen is used as the shielding gas.

Measuring the influence of laser welding on fatigue crack propagation in high strength steel

Laser welded products often have to be designed and optimized for fatigue load. The most common method to characterize the fatigue strength is periodic loading of a weld for different peak loads to identify an S-N-curve. To obtain information on how metallurgy influences fatigue cracking behaviour, three-point bending tests with a laser weld normal to the cracking direction were performed. The crack length was derived from measurement of the compliance of the specimen during fatigue loading in three point bending and the growth rate was obtained through differentiation. The weld surfaces were machined off to eliminate stress raisers. 8 mm thick high strength steel was laser welded, as bead-on-plate. When crossing the weld, a difference of the crack propagation rate was found when compared with the base material. Changes in the propagation rate are either caused by varying microstructure or by residual stress. Different steels and weld parameters were investigated.Laser welded products often have to be designed and optimized for fatigue load. The most common method to characterize the fatigue strength is periodic loading of a weld for different peak loads to identify an S-N-curve. To obtain information on how metallurgy influences fatigue cracking behaviour, three-point bending tests with a laser weld normal to the cracking direction were performed. The crack length was derived from measurement of the compliance of the specimen during fatigue loading in three point bending and the growth rate was obtained through differentiation. The weld surfaces were machined off to eliminate stress raisers. 8 mm thick high strength steel was laser welded, as bead-on-plate. When crossing the weld, a difference of the crack propagation rate was found when compared with the base material. Changes in the propagation rate are either caused by varying microstructure or by residual stress. Different steels and weld parameters were investigated.

The Low-Cycle Fatigue Strength of Laser-Welded Ultra-High-Strength Steel

The UUltra -high -strength (UHS) steels are used in booms, transport vechicles and other light weight structures. It is well -known that it is possible to achieve a strong weld statically, as the base material, by using laser welding as a weld method [1]. The design strength of the light weight structure is often rather high. In the case of booms and transport vechilevehicles, there can be very high dynamic forces in the structure. Therefore it is necessary to study how much fatigue stress the weld seam can resist and at the same time find the optimal welding parameters. The 4 mm bainitic-martensitic UHS steel was welded with laser without filler material to lasercut seam edges by using different weld parameters. Argon gas was blown by pipe onr coaxial nozzle near the key hole and through a 60 mm gas nozzle after the keyhole. Also, the root side of the weld was shielded with argon. The welds were tested by using the bending fatigue test. The test stresses were 800 MPa and 700 MPa. The fatigue strength results showed that with the laser welded seams, the number of cycles wereas about three times lower than with the base material. The fatigue strength was slightly better in welds which were welded with lower energy input. In the case of the weld seam which was welded with lowest energy input by using 300 mm optics, there was some incomplete penetration due to tooexcessively high surface roughness ofat the weld seam edges.

Yb:YAG Disc Laser Welding of Austenitic Stainless Steel Without Filler Material

The laser weldability of austenitic stainless steel (ASS) is good because of the material’s high absorptivity and favourable microstructure. There can be a slight possibility of solidification cracking at high welding speeds and low Crekv/Niekv ratios. Test welds were welded with a Yb:YAG disc laser. The test material was 3.2 mm EN 1.4404 2H C700 type stainless steel plate which was work hardened by cold rolling. The test materials were welded with different heat inputs ranging from 0.024 kJ/mm to 0.12 kJ/mm and with 300 mm and 200 mm focal lengths. The weld seams were square-groove welded as butt weld without filler material. The edges of the groove were made by mechanical or laser cutting. The hardness profiles from cross-sections of the welds were measured with a Vickers microhardness tester using 200 g weight. The mechanical properties were tested with tensile tests. The welds were classified with radiographic verification by an accredited laboratory. A number of the welds were fatigue tested with a bending fatigue tester. The mechanical properties (Rp 0.2%, Rm) of the laser welds were almost the same as in the base material except at the highest heat input. In the radiographic classification, the welds which were welded to the laser-cut edge were classified as class B (accepted). The other welds were classified as class D or C (rejected). The main reasons for the rejection of welds made on mechanically cut edges were lack of penetration or undercut of the weld. A problem with mechanically cut edges, and hence the welds, is that they can be non-square and bent edge. Fatigue tests and tensile tests gave no evidence of solidification cracking in the microstructure of the solidified parts of the welds.