Thick Plate Welding Research Articles

Effects of processing parameters on underfill defects in deep penetration laser welding of thick plates

Underfill defect in welding affects not only the surface integrity but also the mechanical properties of workpiece. In this paper, the effects of the process parameters on underfill defects were investigated, and the deep penetration welding by a 10-kW fiber laser on 12-mm-thick stainless steel plates was used as the case study. The correlation of the processing parameters with the formation and elimination of underfill defects was analyzed and discussed. It was found that during the autogenous laser welding of thick plates, the direction of laser beam relative to gravity has a significant effect on the formation of underfill defects, and the preferable direction was an inclination angle of 60° with respect to the gravity. A negative defocus tended to obtain a full penetration weld with underfill and undercut defects. The higher the welding speed was, the lower the underfill depth was on the top surface. This phenomenon was observed on the full penetration welds at a defocus of − 10 mm. With a negative defocus, it became feasible to optimize the combination of the focal position and the welding speed to achieve a satisfactory appearance of welds. In addition, a bottom-shielding ambience helped the full penetration in the laser welding process positively.

High Power Fiber Laser Welding of Single Sided T-Joint on Shipbuilding Steel with Different Processing Setups

Laser welding of thick plates in production environments is one of the main applications of high power lasers; however, the process has certain limitations. The small spot size of the focused beam produces welds with high depth-to-width aspect ratio but at times fails to provide sufficient reinforcement in certain applications because of poor gap bridging ability. The results of welding shipbuilding steel AH36 with thickness of 8 mm as a single-sided T-joint using a 10 kW fiber laser are presented and discussed in this research paper. Three optical setups with process fibers of 200 µm, 300 µm and 600 µm core diameters were used to study the possibilities and limitations set by the beam delivery system. The main parameters studied were beam inclination angle, beam offset from the joint plane and focal point position. Full penetration joints were produced and the geometry of the welds was examined. It was found that process fibers with smaller core diameter produce deeper penetration but suffer from sensitivity to beam positioning deviation. Larger fibers are less sensitive and produce wider welds but have, in turn, lower penetration at equivalent power levels.

Melt flow and thermal transfer during magnetically supported laser beam welding of thick aluminum alloy plates

A multi-field coupled numerical model was proposed to calculate the weld pool dynamics during the full-penetration laser beam welding of 12mm thick aluminum alloy plates under the function of a longitudinal magnetic field. The model dealt with the melt flow, heat transfer, solid-liquid transition and magnetic diffusion phenomena in welding process. The Lorentz force directing from the keyhole to the liquid-solid interface was induced in magnetic field supported cases. The Marangoni convection near the weld pool surfaces was obviously weakened due to the electromagnetic braking effect, which was featured with decreased velocity and compressed vortex. The heat transfer condition was largely changed, resulting in declined weld pool dimensions. When the magnetic flux density applied was larger than 0.5T, the convective heat exchange at the middle height of weld pool became notable again due to the reversed melt flow direction. The liquid-solid lines with lower curvature were achieved. The reinforcement of Hartmann effect became limited. The model and the simulated weld pool morphologies were verified in experiment.

Process stability during fiber laser-arc hybrid welding of thick steel plates

Thick steel plates are frequently used in shipbuilding, pipelines and other related heavy industries, and are usually joined by arc welding. Deep penetration laser-arc hybrid welding could increase productivity but has not been thoroughly investigated, and is therefore usually limited to applications with medium thickness (5-15 mm) sections. A major concern is process stability, especially when using modern welding consumables such as metal-cored wire and advanced welding equipment. High speed imaging allows direct observation of the process so that process behavior and phenomena can be studied. In this paper, 45 mm thick high strength steel was welded (butt joint double-sided) using the fiber laser-MAG hybrid process utilizing a metal-cored wire without pre-heating. Process stability was monitored under a wide range of welding parameters. It was found that the technique can be used successfully to weld thick sections with appropriate quality when the parameters are optimized. When comparing conventional pulsed and the more advanced cold metal transfer pulse (CMT+P) arc modes, it was found that both can provide high quality welds. CMT+P arc mode can provide more stable droplet transfer over a limited range of travel speeds. At higher travel speeds, an unstable metal transfer mechanism was observed. Comparing leading arc and trailing arc arrangements, the leading arc configuration can provide higher quality welds and more stable processing at longer inter-distances between the heat sources.

09.04: Fatigue behaviour of the closed rib to deck and crossbeam joint in a newly designed orthotropic bridge deck

ABSTRACTOrthotropic decks are widely used in large span and movable steel bridges and the design is dominated by fatigue resistance. Closed rib to deck welded joints in the orthotropic decks are sensitive to fatigue loads and a number of fatigue cracks are reported in bridge inspections. For the design of new bridge decks, thicker deck plates are proposed for reducing stresses in fatigue sensitive locations like the conjunction of the deck plate, rib and crossbeam. Compared to the joints in span, the variation of stress at the crossbeam is larger due to the high stress concentration near the weld root. In this study, fatigue tests are carried out on a full‐scale orthotropic deck segment to study the initiation and propagation of fatigue cracks in the deck plate. Trapezoidal stiffeners are welded to a 20 mm thick deck plate and different welding methods are used in manufacturing the specimen. Various load levels are applied on the deck plate by hydraulic jacks. The strain gauges are attached both at the top and bottom of the deck plate in the vicinity of the expected crack paths. The strain history and distribution of strain ranges at the characteristic sections are analysed. An ultrasonic method, time‐of‐flight diffraction, is used to inspect the sizes of the cracks after testing. Comparison of the number of cycles that corresponds to a crack propagation is given in the hot spot S‐N curve figure. With a failure criterion of 10% strain range deviation, the fatigue strength scatter of the 20 mm thick deck plate is similar to that of the 12 mm deck.

Effect of multiple thermal cycles on metallurgical and mechanical properties during multi-pass gas metal arc welding of Al 5083 alloy

Multi-pass gas metal arc welding was carried out for Al 5083 alloy plate with thickness 30 mm. During multi-pass welding of thick plate, the former welding pass experienced multiple thermal cycles that came from subsequent welding passes. This work focused on the effect of thermal cycle on metallurgical characteristics and mechanical properties. Different from what had long been recognized that welding thermal cycle was harmful, the results indicated that multiple thermal cycles caused the partial transition from columnar grains to equiaxed grains, which finally contributed to the refinement of microstructure. Increased precipitated particles resulted in the improvement of strengthening effect. These beneficial effects of microstructure and precipitated phase led to the increase of micro-hardness and the improvements of tensile strength and ductility. As the progress of welding process, overall properties of the welded joint improved. Conclusion could be made that appropriate thermal cycles were beneficial during multi-pass welding of Al 5083 alloy.

Root welding of V-groove thick plate without backing plate by MAG-TIG double-arc welding

MAG-TIG (metal active gas-Tungsten inert gas) double-arc welding process is applied to make stable back beads in the first layer during one-side multilayer welding without backing plate. To explain the reason why stable back beads can be formed, a comparison was made between single MAG welding and MAG-TIG double-arc welding. Results indicated that during the optimized condition in MAG-TIG double-arc welding, about one-third heat energy of the heat source acts on the root face, and about two-third heat energy acts on the molten pool. The distribution of temperature and surface tension in the molten pool must be more balanced and reasonable, in order to obtain the ideal back weld bead formation. This process is a new welding process giving high quality and efficiency in root welding of thick plate.

Defects inhibition and process optimization for thick plates laser welding with filler wire

One of the challenges in laser welding of thick plates is the formation of lack of fusion defects. In this paper, it is found that the formation of such defects has relations with the surface profile of the weld joint. Experiment results indicate that the weld joints can form perfect fusion when the surface of solidified filling metals is concave. Then the author studied the relations between input welding parameters and single bead geometry, which can affect the surface shape of solidified metals, filling passes and defects formations during the multi-pass laser welding process. Further investigations and analysis show that P/Vf (the radio of laser power to wire feed rate) and Vf/Vw (the radio of wire feed rate to welding speed) can be used to quantify the input variables and impede the formation of lack of fusion defects. Verification results show that the optimized parameters for perfect fusion are: P/Vf >1.5 and Vf/Vw<6.

A study on heat flow and thermal distortion in terms of air-carbon-arc gouging of steel plates

In groove design, a proper shape should be chosen during the preparation for the welding of thick plates of steel structures to minimize the potential for any welding deformations. While the effect of welding on both sides of the plate in terms of the deformation potential is a foremost consideration during the groove-design process, the effect of gouging is not included even though it involves the usage of a high-heat arc input that may significantly influence the welding-deformation potential. In this study, the effect of the air-carbon-arc gouging process on the deformation of steel plates was investigated using a numerical finite element method analysis. For this purpose, the heat input from the gouging torch was first modeled using an analytical solution that is based on the Rosenthal equation. The model was then used as a heat input for the thermo-mechanical steel-plate-gouging model that is proposed in this study to obtain deformation estimations during the gouging process. A series of numerical analyses were carried out to predict the deformations according to changes in the plate thickness. The results of the analyses were found to be in sound agreement with those of the experiments that were performed to verify the simulations. The comparison revealed that the models that are proposed in this study are feasible ways of estimating the deformation caused by the gouging on steel plates.

万瓦级高功率激光焊接时金属蒸气与熔池耦合行为研究现状

高功率激光焊接技术是实现厚板连接的先进技术，在现代装备制造中具有广阔的应用前景。本文分别从试验研究和理论仿真研究综述了高功率激光深熔焊接厚板过程中激光与材料的相互作用特征、蒸气与熔池相互作用规律和机理。在高功率激光深熔焊接时，由于各种不稳定因素包括焊接参数的变化，引起焊接小孔内激光能量分布的变化，从而改变局部材料的气化率，使得小孔内气流状态发生改变，蒸气流体与熔池流体之间的力平衡关系发生改变，促使孔壁和熔池的波动。实现良好焊接效果和焊接质量的前提是必须保证焊接蒸气和熔池流动状态的稳定性，控制焊接小孔的动态平衡，从而实现稳定的焊接过程。 High power laser welding technology is an advanced technology to realize thick plate connection, and has broad application prospects in modern equipment manufacturing. In this paper, the in-teraction characteristics of laser and materials, the interaction law between vapor and molten pool and its mechanism during high power laser deep penetration welding of thick plate were summarized from experimental research and theoretical simulation. In high power laser welding, due to various unstable factors including the change of welding parameters, the energy distribution of laser beam was changed inside the keyhole, thus the local material gasification rate was changed. Then the flow state inside the keyhole was changed from equilibrium between the vapor fluid and the molten pool to the unequilibrium. Thus, the keyhole wall and the molten pool became to fluctuate. The premise of achieving good welding quality is to ensure the stability of the vapor flow and molten pool state, and control the dynamic balance of the welding hole.

Effects of laser focusing properties on weldability in high-power fiber laser welding of thick high-strength steel plate

The effect of various high-power laser-welding parameters on obtaining deep penetration welds without weld defects has been investigated. However, there are no studies on the effect of laser focusing. In this study, high-power fiber laser welding of a 12-mm-thick high-strength steel plate was performed by using two optics systems with different power density distributions and focus depths (2 or 4 mm) to investigate the effects of laser focusing properties on weldability. Full penetration welds without weld defects were obtained with the 4 mm focus depth optics system at low welding speeds of 25–50 mm/s. High-speed video and X-ray transmission images showed that the behavior of the molten pool on the top surface during laser welding was more stable for the 4 mm system than for the 2 mm system. The keyhole was stable with no large fluctuations, and no bubbles were formed in the keyhole. This result was attributed to keeping the power density within 50–120 kW/mm2 to maintain a stable keyhole shape during the welding. Consequently, it was concluded that the laser-focusing properties during laser welding of a thick steel plate affected the weldability strongly, and the optics system with long focus depth was particularly useful in producing sound welds.

Microstructure, texture and mechanical properties of commercial high-purity thick titanium plates jointed by electron beam welding

Electron beam welding (EBW) is a fusion joining process particularly suitable for the welding of thick titanium plates. In the present work, commercial high-purity titanium plates with a thickness of 30mm were welded by EBW. Color metallography and electron backscattered diffraction (EBSD) were used to investigate the microstructure and texture after welding. The results show that the grains become elongated or coarsened in the weld zone (including the fusion zone and the heat-affected zone). Meanwhile, as revealed by the (0001) pole figures, the texture of the weld zone is enhanced. Despite the grain coarsening, the welded plates exhibit higher yield strength than the base metal. A Schmid factor analysis revealed that this increase in yield strength could be attributed to texture strengthening. Specifically, the grains in the weld zone are re-oriented. For the weld zone, the Schmid factor is relatively small for prismatic slip, which is the most easily activated deformation mode for titanium at room temperature. Compared to the base metal, the Schmid factors for {101¯2} twinning and {112¯2} twinning are larger for the weld zone. The EBSD observations confirmed that a large number of {101¯2} twins and {112¯2} twins are formed in the weld zone during tension. However, the plastic strain caused by the twin formation is much smaller than that caused by the prismatic slip in the base metal. As a result, necking occurred in the base metal during the transverse tensile tests.

Study on the characteristics of the torque vibration and the detection of the gap

Bobbin tool friction stir welding (BTFSW) is a new solid-state welding technology that has been developed successfully in many joining applications, such as thick plate and hollow component welding. The existence of the gap can possibly cause an unstable welding, and the variety of the torque is closely related to the existence of the gap. Compared with the conventional friction stir welding (FSW), BTFSW has the different tool and welding process, especially some new feature of the periodic vibration signal arises when welding two sheets in the presence of gaps. Compared with vibration-accelerated signal, torque signal is capable of reflecting the vibration of the welding process. The aim of this study was to develop an effective algorithm to detect gaps in BTFSW operations. Experimental studies were conducted to determine the features of welding process parameters and the gap. Time-frequency (TF) analysis provides a display of subtle torque features, shows the corresponding duration of low frequency signal clearly. Followed by a low-pass filter, gauss wavelet transform (WT) algorithm would improve the detection and identification of the gap presence. Several experimental studies with a variety of process parameters and proposed algorithm was shown to be able to reliably detect the presence of gaps in BTFSW operations for traverse speeds below 200 mm/min and gap sizes above 0.3 mm. This gauss WT algorithm could become a useful noninvasive tool for welding quality assessment in practice.

Development of gap sensing system for narrow gap laser welding

Narrow gap welding with an oscillation laser beam is one of the effective processes for thick plate welding. To put this welding process into practical manufacturing, a groove-sensing system using image processing for narrow gap welding with an oscillation laser beam is used. This developed system uses still images of the weld zone taken by a coaxial CMOS camera. It can recognize the position of the groove wall by analysing the brightness distribution in the still image. It can then control the oscillation width and the laser-irradiated area by calculating the groove width and the groove centre position. Some narrow gap welding experiments were performed to evaluate the performance of the developed system. The results revealed that the developed system is effective for narrow gap welding with an oscillation laser beam. Using this system, the narrow gap groove can be welded even if the groove width has changed during the welding process.

Finite element analysis of deformation in early stage of multi-pass circumferential dissimilar welding of thick-walled pipes with narrow gap

Three-dimensional thermal elastic-plastic finite element analyses were conducted in order to reveal the mechanism of gap shrinkage in early stage of multi-pass narrow gap welding of thick-walled dissimilar pipes and thick plates. The gap shrinkage up to the 8th pass indicates that the shrinkage of plates becomes to be much larger than that of pipes with increasing the weld passes. In addition, through the decomposition of gap shrinkage to transverse shrinkage and angular distortion, it is found that the gap shrinkage of pipes is mainly governed by the transverse shrinkage, while the shrinkage of plates is influenced by both the transverse shrinkage and angular distortion. Moreover, the serial computational results with various groove shapes suggest that the transverse shrinkage of pipes almost linearly increases with increasing the weld pass, and its increment can be predicted by a linear approximation function obtained by the transverse shrinkage of plates regardless of the groove shape.

Friction stir welding of thick plates of Al–Mg–Sc alloy

A technology is developed for single-pass friction stir welding (FSW) of 11- and 35-mm-thick plates of Al–Mg–Sc alloys. The microstructural and mechanical heterogeneity of the welded joints is investigated. The welded joints obtained under the optimum welding conditions are free from macrodefects. The strength of the welded joint equals 98% of the strength of the parent metal, which is higher than the strength of fusion-welded joints. It is concluded that the FSW of thick plates of Al–Mg–Sc alloy can be used efficiently in practice.

Effect of surface oxidation layer on tensile strength of Cu-Ni alloy in friction stir welding

Friction stir welding (FSW) of thick Cu-Ni plate was successfully completed. The fracture position after tensile testing was located at the weld nugget zone (WNZ), where surface oxidation occurred. The oxidation morphologies on the surface of the base metal were analyzed by SEM, EPMA and XRD, with the oxide layer being obtained by simple and useful way to analyze the oxide products, namely, collecting oxide powders after immersing of the oxidized specimen into HNO3 solution. The results highlighted that an oxide layer of 30 μm thickness consists of a mixture of two phases, Cu2O and NiO, on the surface of the base metal. After FSW, the thickness of the oxide layer on the surface was decreased to approximately 5 μm, and broken oxide particles, which is NiO, penetrated into the WNZ by the rotating tool. NiO was preferentially formed at the surface after FSW because it has a lower Gibbs free energy value at 950 °C, which is the peak temperature measured during FSW. Oxide layer of Cu-Ni plate was clearly only removed by mechanical method grinding with 1200-grit SiC paper. The removal of oxide layer results in improved mechanical strength.

Finite element modeling of an alternating current electromagnetic weld pool support in full penetration laser beam welding of thick duplex stainless steel plates

An electromagnetic weld pool support system for 20 mm thick duplex stainless steel AISI 2205 was investigated numerically and compared to experiments. In our former publications, it was shown how an alternating current (AC) magnetic field below the process zone directed perpendicular to the welding direction can induce vertically directed Lorentz forces. These can counteract the gravitational forces and allow for a suppression of material drop-out for austenitic stainless steels and aluminum alloys. In this investigation, we additionally adopted a steady-state complex magnetic permeability model for the consideration of the magnetic hysteresis behavior due to the ferritic characteristics of the material. The model was calibrated against the Jiles–Atherton model. The material model was also successfully tested against an experimental configuration before welding with a 30 mm diameter cylinder of austenitic stainless steel surrounded by duplex stainless steel. Thereby, the effects of the Curie temperature on the magnetic characteristics in the vicinity of the later welding zone were simulated. The welding process was modeled with a three-dimensional turbulent steady-state model including heat transfer and fluid dynamics as well as the electromagnetic field equations. Main physical effects, the thermo-capillary (Marangoni) convection at the weld pool boundaries, the natural convection due to gravity as well as latent heat of solid–liquid phase transitions at the phase boundaries were accounted for in the model. The feedback of the electromagnetic forces on the weld pool was described in terms of the electromagnetic-induced pressure. The finite element software COMSOL Multiphysics 4.2 was used in this investigation. It is shown that the gravity drop-out associated with the welding of 20 mm thick duplex stainless steel plates due to the hydrostatic pressure can be prevented by the application of AC magnetic fields between around 70 and 90 mT. The corresponding oscillation frequencies were between 1 and 10 kHz and the electromagnetic AC powers were between 1 and 2.3 kW. In the experiments, values of the electromagnetic AC power between 1.6 and 2.4 kW at oscillation frequencies between 1.2 and 2.5 kHz were found to be optimal to avoid melt sagging or drop-out of melt in single pass full-penetration laser beam welding of 15 and 20 mm thick AISI 2205.

Comparison of processing window and porosity distribution in laser welding of 10 mm thick 30CrMnSiA ultrahigh strength between flat (1G) and horizontal (2G) positions

With the development of high brightness and high power laser, it is possible to weld thick sections using single pass autogenous laser welding. However, it often suffers from very narrow processing parameter windows when welding thick sections. Furthermore, porosity, back sagging, and undercut are the main defects for laser welding of thick section high strength steel plates. A comparison of flat welding position (1G) and horizontal welding position (2G) in laser welded 10 mm thick 30CrMnSiA high strength steel plates was investigated in this paper. The results show that the 1G position has a very narrow processing window, while the 2G position has a much wider processing window. The porosity in the 2G position laser welded joint distributed in the upper part of welds, while that in the 1G position laser welded joint was distributed along the weld center. Welded joints with acceptable porosity in the 2G position laser welding can be obtained for an X-shaped weld profile and wider weld bead. The peak longitudinal residual stresses of the 2G position laser welded 30CrMnSiA steel were located in the heat-affected zone (HAZ), and the lower residual stresses were presented in the weld center. The peak transverse residual stress was presented in the weld center with the lower stresses located in the two sides HAZ.

Full penetration laser beam welding of thick duplex steel plates with electromagnetic weld pool support

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.