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The weld quality and the possible defect formation are directly determined by the weld pool shape and the thermo-fluid dynamics therein. In this paper, an untypical weld pool profile, i.e., elongated at its top and bottom but narrowed at the middle, is found experimentally and numerically in the wire feed laser beam welding. The detrimental influence of the weld pool narrowing on the element transport is analyzed and discussed. A magnetohydrodynamic technique is utilized to suppress the narrowing, aiming at a more homogenous element distribution. It is found that a low-temperature region is formed in the middle of the weld pool due to the interaction of the two dominant circulations from the top and bottom regions. The weld pool is significantly narrowed due to the untypical growth of the mushy zone in the low-temperature region, which results in a direct blocking effect on the downward flow and the premature solidification in the middle region. The Lorentz force produced by a transverse oscillating magnetic field shows the potential to change the flow pattern into a single-circulation type and the low-temperature-gradient region is mitigated.
Therefore, the downward transfer channel is widened, and its premature solidification is prevented. The numerical results are well validated by experimental measurements of metal/glass observation and X-ray fluorescence element mapping.
In this contribution, we present a physically motivated heat source model for the numerical modeling of laser beam welding processes. Since the calibration of existing heat source models, such as the conic or Goldak model, is difficult, the representation of the heat source using so-called Lamé curves has been established, relying on prior Computational Fluid Dynamics (CFD) simulations.
Lamé curves, which describe the melting isotherm, are used in a subsequent finite-element (FE) simulation to define a moving Dirichlet boundary condition, which prescribes a constant temperature in the melt pool. As an alternative to this approach, we developed a physically motivated heat source model, which prescribes the heat input as a body load directly. The new model also relies on prior CFD simulations to identify the melting isotherm. We demonstrate numerical results of the new heat source model on boundary-value problems from the field of laser beam welding and compare it with the prior CFD simulation and the results of the Lamé curve model and experimental data.
In this study, a three-dimensional CFD-simulation model was developed to simulate the fluid flow in the weld pool. The CFD-model showed a bulging region in the middle of the depth, which is separated from the top surface and bottom surface by two narrowing regions. It can be concluded that the interaction of the movement of the laser source with the Marangoni vortex leads to a teardrop shape at the upper and bottom surface of the workpiece. Additionally, it shows that the bulging in the weld is a result of the backflows on the upper and lower sides due to the thermo-capillary-driven flows. The weld pool shape was used as a heat source in a two-dimensional thermomechanical model, which allows a highly accurate transformation of the weld pool dimensions obtained from the CFD model. This developed technique allows the consideration of physical aspects, which cannot be considered when using traditional heat sources. The mechanical model has shown that the chronological order of the solidification of the weld has a significant influence on the nature and distribution of the stresses in the weld. High tensile stress has been observed in the bulging region, i.e. in the susceptible region for solidification cracking, when compared to the other narrowing regions, which show compressive stress.
The present work deals with the development of a strategy for the prevention of end crater defects in high-power laser welding of thick-walled circumferential welds. A series of experiments were performed to understand the influence of the welding Parameters on the formation of end crater defects such as pores, cracks, root excess weld metal and shrinkage cavities in the overlap area. An abrupt switch-off of the laser power while closing the circumferential weld leads to a formation of a hole which passes through the whole welded material thickness. A laser power ramp-down causes solidification cracks which are initiated on the transition from full-penetration mode to partial penetration. Defocusing the laser beam led to promising results in terms of avoiding end crater defects. Cracks and pores in the overlap area could be effectively avoided by using defocusing techniques.
A strategy for avoiding of end crater imperfections was tested on flat specimens of steel grade S355J2 with a wall thickness of between 8 mm and 10 mm and then transferred on the 10 mm thick pipe sections made of high-strength pipeline steel API5L-X100Q.
Comparison between GTA and laser beam welding of 9%Ni steel for critical cryogenic applications
(2018)
IncomparisonwithGTAweldedjoints,highertensilestrengthcomparabletothatofthebasemetalwasobtained for laser beam welded joints due to fine martensitic microstructure. Impact fracture toughness values with much lower mismatching were obtained for laser beam welded joints due to similarity in the microstructures of its weld metal and HAZ. In this case, the lower impact fracture toughness obtained (1.37J/mm2) was much higher than that of the GTA welded joints (0.78J/mm2), which was very close to the specified minimum value (≥0.75J/mm2). In contrast to other research works, the overall tensile and impact properties are influenced not only by the fusion zone microstructure but also by the size of its hardened area as well as the degree of its mechanical mismatching, as a function of the welding process. A better combination of tensile strength and impact toughness of the concerned steel welded joints is assured by autogenous laser beam welding process.
Using a novel optical measurement technique together with the optical flow algorithm, a two-dimensional deformation analysis during welding was conducted. The presented technique is the first to provide a measurement of the full strain field locally in the immediate vicinity of the solidification front.
Additionally, the described procedure of the optical measurement allows the real material-dependent values of critical strain and strain rate characterizing the transition to hot cracking during laser welding processes to be determined. Furthermore, the above-mentioned technique is independet on the welding process, which means, it can be also used for arc welding processes. Dependency between the external strain rate and the critical local strain and strain rate has been observed. That is to say, the critical local strain and strain rate is increased with an increase of the strain rate.Moreover, this technique allows automatic identification of the cases that can be critical for the solidification crack formation by monitoring the state of strain on the crack-sensitive region within the mushy zone.
Experimental study and numerical simulation of hot crack formation for novel laser weldability test.
(2015)
Laser beam welding is a widely established manufacturing process in several industries. The solidification cracking seriously effecting the safety of welded joints could arise during the beam welding of stainless steels caused by high solidification rates. In this study the controlled tensile weldability test (CTW) was used to investigate the solidification cracking susceptibility the fully austenitic stainless steels CrMnNi (1.4376), CrNi (1.4301), CrNiMo (1.4404) and CrNiSi (1.4828) during laser beam welding. The test facility allows welding of specimens with simultaneous application of tensile load along or cross to the welding direction while the speed of tensile force application is either constant or increases linearly. The tensile force increment and/or the displacement are set by means of a CNC controller. Trials were conducted by varying the ultimate tensile strain and cross-head speed while keeping the welding parameters constant. By observing the crack-no crack behaviour and estimating the generated crack length for each trail using a new optimized experimental procedure the influence of the two important conditions (the strain and the strain rate) for the formation of solidification cracks can be investigated, the critical values of strain and strain rate that are responsible for solidification cracking formation have been determined. In the present study a three-dimensional FEM using the contact element technique was developed to simulate the solidification cracking during laser full penetration welding under external load conditions for the steel 1.4376 in order to get a better understanding of the mechanisms of hot crack initiation and the theoretical results were compared to the experimental ones. By comparing the resulting solidification crack with simulated crack, it is possible to determine the critical condition of solidification crack formation in the region where the strains and the strain rates cannot be measured due to the high temperature. The results show a good agreement between numerical calculation and experiment. It is proposed that the solidification cracking susceptibility may be predicted by FEM analysis by using the correct mechanical and thermo-physical constants of the materials.
Full penetration high power bead-on-plate laser beam welding tests of up to 20 mm thick 2205 duplex steel plates were performed in PA position. A contactless inductive electromagnetic (EM) weld pool support system was used to prevent gravity drop-out of the melt. Welding experiments with 15 mm thick plates were carried out using IPG fiber laser YLR 20000 and Yb:YAG thin disk laser TruDisk 16002. The laser power needed to achieve a full penetration was found to be 10.9 and 8.56 kW for welding velocity of 1.0 and 0.5 m min−1, respectively. Reference welds without weld pool support demonstrate excessive root sag. The optimal value of the alternating current(AC) power needed to completely compensate the sagging on the root side was found to be ≈1.6 kW for both values of the welding velocity. The same EM weld pool support system was used in welding tests with 20 mm thick plates. The laser beam power (TRUMPF Yb:YAG thin disk laser TruDisk 16002) needed to reach a full penetration for 0.5 m min−1 was found to be 13.9 kW. Full penetration welding without EM weld pool support is not possible—the surface tension cannot stop the gravity drop-out of the melt. The AC power needed to completely compensate the gravity was found to be 2 kW.
Full penetration high power bead-on-plate laser beam welding tests of up to 20 mm thick 2205 duplex steel plates were performed in PA position. A contactless inductive electromagnetic (EM) weld pool support system was used to prevent gravity drop-out of the melt. Welding experiments with 15 mm thick plates were carried out using IPG fiber laser YLR 20000 and Yb:YAG thin disk laser TruDisk 16002. The laser power needed to achieve a full penetration was found to be 10.9 and 8.56kW for welding velocity of 1.0 and 0.5 m min(-1), respectively. Reference welds without weld pool support demonstrate excessive root sag. The optimal value of the alternating current (AC) power needed to completely compensate the sagging on the root side was found to be approximate to 1.6 kW for both values of the welding velocity. The same EM weld pool support system was used in welding tests with 20 mm thick plates. The laser beam power (TRUMPF Yb:YAG thin disk laser TruDisk 16002) needed to reach a full penetration for 0.5 m min(-1) was found to be 13.9 kW. Full penetration welding without EM weld pool support is not possible-the surface tension cannot stop the gravity drop-out of the melt. The AC power needed to completely compensate the gravity was found to be 2 kW. (C) 2016 Laser Institute of America
The paper describes an experimental investigation of high power laser beam welding with an electromagnetic weld pool support for up to 20 mm thick plates made of duplex steel (AISI 2205) and mild steel (S235JR). The results of the welding tests show a successful application of this technology at ferromagnetic metals. Irregular sagging was suppressed successfully. An ac-power of less than 2 kW at oscillation frequencies between 800 Hz and 1.7 kHz is necessary for a full compasation of the hydrostatic pressure. Thus, it was demonstrated that the electromagnetic weld pool support is not only limited to non-ferromagnetic metals like austenitic steels. For future studies with duplex steel, the use of filler material has to take into account with regard to the balance of the mixed austenitic and ferritic phases.