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In this study, steel-glass experiments were conducted to observe the melt pool geometry using a high-speed camera. The high-speed recordings and optical flow analysis show that two main flows take place in form of vortices. The lower vortex drives the melt backwards from the front keyhole wall and thus causes an extension of the melt pool, which is called bulging. This bulging promotes solidification cracking by forming a closed area filled with melt and the accumulation of impurities in the final solidification phase, resulting in low-melting phases which are under tensile stress at the end of solidification.
Laser welding is a widely established manufacturing process in many industry sectors. Solidification cracking represents one of the most inadequately solved problems in welding and has major economic implications. The avoidance of hot crack forms for most fusion welding processes poses a key challenge for an important range of metallic construction materials and affects not only the manufacturers of welding equipment and material manufacturers, but also a large number of customers using welding technologies, as well as welding technical standardization and research. Solidification cracking susceptibility was examined with the help of the Controlled Tensile Weldability Test (CTW) developed by Federal Institute for Materials Research and Testing (BAM), Berlin. The test is based on the fact that hot crack formation depends on a critical strain that emerges within a critical temperature range, the so called brittle temperature range (BTR). Using this test and defined investigation programme a centreline solidification crack was generated. By controlling the applied strain during the laser beam welding process, it was possible to determine the critical strain and strain rate that led to solidification cracking formation. The hot cracking susceptibility of the tested stainless steels was qualified and quantified. The results demonstrate that the crack length increases with increasing applied strain. Furthermore, the strain rate has a significant influence on the formation of the solidification crack.
Laser welding is a widely established manufacturing process in many industry sectors. Solidification cracking represents one of the most inadequately solved problems in welding and has major economic implications. The avoidance of hot crack is for most fusion welding processes a key challenge for an important range of metallic construction materials and affects not only the manufacturers of welding equipment and material manufacturers, but also a large number of customers using welding technologies, as well as welding standardization and research. In this study a new investigation programme has been developed to qualify the hot cracking susceptibility of a variety of austenitic stainless steels. The results show the possibility of using this technique to determinate the critical values that occur with initiation of solidification cracking during laser beam welding
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.
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.