Full Penetration Welds Research Articles

Numerical modelling and experimental validation of the effect of laser beam defocusing on process optimization during fiber laser welding of automotive press-hardened steels

• This work uses numerical modelling and experimental validation to study the effect of laser beam defocusing on the weld pool geomtery during the laser welding of 22MnB5 grade press-hardened steel. • The study presents the effect of beam defocusing on the weld pool geometry while holding other parameters constant, as the welding mode transitions from the full penetration mode when the beam is fully focused, to the partial penetration mode as the beam is defocused. • It has been shown through a detailed experimental study that by defocusing the laser beam, the welding process can be stabilized which allows the weld pool geometry to be controlled and leads to the elimination of defects such as severe concavity without needing to decrease the laser power or welding speed. • A numerical solution is presented that uses three-dimensional transient finite element analysis to calculate the temperature fields and predict the molten weld pool geometry in both the full and partial penetration welding modes to show how the change in beam defocusing distance impacts the shape and size of the weld. • The findings of this work have a high impact potential as the simple, yet highly accurate numerical model presented in this work allows for the proposed method of defocusing the laser beam to improve the stability of the laser welding process, so it can be readily used in streamlining the manufacturing of laser-welded blanks used in the automotive industry. Automated robotic laser welding is commonly used for the joining of thin-gauge sheet metals for automotive applications due to the precision and speed of the welding process. Without proper process optimization, high-speed laser welding for automotive applications is known to have defects such as porosity, weld concavity, and cyclic humping. Several welding parameters can be adjusted to optimize the laser welding process: decreasing the laser power and the welding speed, changing the intensity distribution of the laser on the surface of the substrate, or by changing the type and flow rate of shielding gas used during welding. However, decreasing the laser power and welding speed leads to a loss in productivity which can be costly in large-scale manufacturing endeavors. This study presents the effect of beam defocusing on the weld pool geometry while holding other parameters constant as the welding mode transitions from full penetration or open keyhole mode (OKM) when the beam is fully focused to partial penetration or closed keyhole mode (CKM) as the beam is defocused. It has been shown that by defocusing the laser beam and controlling the weld pool geometry, the welding process can be stabilized to eliminate defects such as severe concavity without decreasing the laser power or slowing down the welding process. A numerical solution is presented that uses three-dimensional (3D) transient finite element (FE) analysis to calculate the temperature fields and predict the molten weld pool geometry in both the full and partial penetration welding modes using a double conical-Gaussian (DCG) volumetric heat source derived from the classical 3D conical-Gaussian (CG) heat source. The model was calibrated using video imaging of the welding process and known experimental data, and it was validated using an extensive experimental study. The results of this work show that the model can be used to predict the molten weld pool geometry that stabilizes the liquid flow in the melt pool during high-speed laser welding whereby allowing faster speeds to be used in the manufacturing of laser-welded components.

Synchrotron X-ray Analysis of the Influence of the Magnesium Content on the Absorptance during Full-Penetration Laser Welding of Aluminum

Full-penetration laser beam welding is characterized by a weld seam whose depth equals the material thickness. It is associated with a stable capillary and is therefore widely used for welding of sheet metal components. The realization of lightweight concepts in car body production requires the application of high-strength aluminum alloys that contain magnesium as an alloying element, which significantly influences the evaporation temperature and pressure. This change of the evaporation processes influences the geometry of the capillary and therefore its absorptance. In order to quantify the influence of magnesium on the capillary, their geometries were captured by means of high-speed synchrotron X-ray imaging during the welding process of the aluminum alloys AA1050A (Al99.5), AA5754 (AlMg3) and AA6016 (AlSi1.2Mg0.4). The 3D-geometries of the capillaries were reconstructed from the intensity distribution in the recorded X-ray images and their absorptance of the incident laser beam was determined by the analysis of the reconstructed 3D-geometry with a raytracing algorithm. The results presented in this paper capture for the first time the influence of the magnesium content in high-strength aluminum alloys on the aspect ratio of the capillary, which explains the reduced absorptance in case of full-penetration laser beam welding of aluminum alloys with a high content of volatile elements. In order to improve the absorptance in full-penetration welding, these findings provide the information required for the deduction of new optimization approaches.

The Effects of HLAW Parameters for One Side T-Joints in 15 mm Thickness Naval Steel

The present contribution is the first research reporting full penetration HLAW joints in 15 mm thick EH36 steel butt T-welds with square grooves on 2F welding position by single-sided welding. The effects of welding parameters were investigated to increase the quality of the joints. Conditions leading to defect-free full penetration welds fulfilling naval regulations includes a laser power of 12.5 kW, a welding speed of 1.6 m/min and the vertical laser offset distance from the flange of 1 mm. Advanced characterization of selected welds included a microstructural identification by optical microscopy, SEM, and XRD, revealing the presence of acicular, polygonal and Widmanstätten ferrite, lath martensite, and some retained austenite at FZ. Hardness and microhardness mapping tests showed values of 155 HV at base metal and 200 to 380 HV at the fusion zone connecting the web to the flange.

Investigation of side plate connections in an S-CFST column frame under a column-loss scenario

The progressive collapse resistance of four full-scale special-shaped columns composed of concrete-filled steel tubes (S-CFST column)-steel beam assemblies was studied in this paper. The failure modes, deflection profiles, vertical resistance development mechanisms and internal force development at the assumed plastic hinge section under the influence of infilled concrete, various span-to-height ratios of the steel beam and different connection details were compared. The test results showed that the fracture failure of all the specimens first occurred near the full penetration weld of the bottom beam flange or the bottom cover plate at the end of the side plate. Fracture failure of the steel beam web or shear plate occurred through the bolt holes. The beam chord rotation ranged from 0.055 to 0.088. The concrete infilled steel tube improved the ductility of the assembly and resulted in the development of the catenary mechanism, while reducing the span-to-height ratio of the steel beam resulted in the delay of the initiation of the mixed mechanism stage. Additionally, the development of the catenary mechanism was more sufficient for the specimen with the concrete infilled steel tube than the specimen without the concrete infilled steel tube under the same vertical displacement. The results showed that the ACPSP connection can avoid premature fracture of the full penetration weld and improve its progressive collapse resistance. By reducing the span-to-height ratio of the steel beam and using the ACPSP connection, the beam-column assemblies experienced not only the flexural and mixed mechanism stages but also the catenary mechanism stage. The GSPCP connection was proposed to improve the stress concentration in the ACPSP connection. Through further analysis by the verified finite element model, it is evident that the GSPCP connection can further improve the progressive collapse resistance of the S-CFST column-steel beam joint assembly.

Fabrication and testing of small mock-ups for helium cooled ceramic reflector breeding blanket development

Korea has developed a helium cooled ceramic reflector test blanket module (TBM) for fusion reactors. An advanced reduced activation alloy has been developed as a structural material and various joining methods such as E-beam welding, tungsten inert gas (TIG) welding, laser welding, and hot isostatic pressing have been applied to establish its fabrication method and procedure. Based on the current design of the TBM and breeding blanket, TIG welding was selected as the main joining method, and the U-shaped first wall (FW) was adopted. For welding of the breeding blanket, a special joining technique without a permanent backing strip with simultaneous full penetration welding in accordance with the design rules of class N2RX of RCC-MRx is required for the pressure vessel. For the U-shaped FW, distortion of the drilled cooling hole during the bending should be limited. Three small mock-ups were fabricated in this study to investigate these issues. The joints and bending parts were visually observed after cutting, and the hydrostatic pressure test was performed for regions with low thickness. The results show that the proposed fabrication methods are applicable in the fabrication procedure development for the breeding blanket.

Melt flow regularity and hump formation process during laser deep penetration welding

In this study, the relationship between melt flow and hump formation was directly revealed during laser deep penetration welding of transparentmaterial. In partial-penetration welding, the melt flowed obliquely upwards along the bottom of the keyhole to the rear surface of the molten pool. And its flow rate increased first and then decreased with the increase of the welding speed. There was a one-to-one correspondence between the humps formed on the weld surface and the fluctuation of the inclination angle of the front keyhole wall (FKW) near the Brewster angle. The maximum eruption rate of the laser-induced vapor on the FKW exceeded 30 m/s. In full-penetration welding, the melt flow direction changed and humping was suppressed. The laser-induced vapor on the FKW surface can act on the rear keyhole wall, and this is the main reason for melt flow. When the inclination angle of the FKW is close to the Brewster angle, the amount of laser-induced evaporation on the FKW will increase to form a high speed jet of vapor which will drive melt flow in the molten pool behind the keyhole, and this is the main reason for hump formation on the rear surface of the molten pool.

Weld Metal Microstructure Prediction in Laser Beam Welding of Austenitic Stainless Steel

In the present work an approach to weld metal microstructure prediction is proposed, based on an analytical method that allows the evaluation of the thermal fields generated during the laser beam travel on thick plates. Reference is made to AISI 304L austenitic steel as a base material, with the aim to predict the molten zone microstructure and verify the best condition to avoid hot cracking formation, which is a typical issue in austenitic steel welding. The “keyhole” full penetration welding mode, characteristic of high-power laser beam, was simulated considering the phenomenological laws of conduction by the superimposition of a line thermal source along the whole thickness and two point sources located, respectively, on the surface and at the position of the beam focus inside the joint. This model was fitted on the basis of the fusion zone profile, which was experimentally detected on a weld seam obtained by means of a CO2 laser beam, in a single pass on two squared edged AISI 304L plates, that were butt-positioned. Then the model was applied to evaluate the thermal fields and cooling rates, the fusion zone composition and the solidification mode.

Novel measures for spatter prediction in laser welding of thin-gage zinc-coated steel

This paper studies full-penetration remote laser stitch welding of thin-gage zinc-coated steel sheets in a lap joint condition. The spatter was classified as either Type A or Type B based on their characteristics, corresponding to low and high linear energies, respectively. The Type A spatter was mainly affected by the zinc vapor outgassing through the keyhole openings, while Type B was more influenced by the vulnerability of the keyhole rear wall to the impact of the high-pressure zinc vapor jet originating from the faying interface. Two measures, Zout and Zp, are proposed to predict the occurrence of Type A and Type B spatter. A thermo-fluid numerical model was developed to investigate the physics in the remote laser welding of zinc-coated steels, such as molten pool dimensions, keyhole stability, and zinc coating evaporation and to calculate Zout and Zp in the welding process. The effects of welding parameters, such as laser power and feed speed, on spatter occurrence were studied numerically and verified against experimental observations. It was found that the laser linear energy could directly affect values of Zout and Zp. A medium linear energy, striking a balance between Zout and Zp, could generate welds with much less spatter compared to a relatively low or high linear energy.

Parameterized multipoint-line analytical modeling of a mobile heat source for thermal field prediction in laser beam welding

The thermal effects of the “keyhole” full penetration welding mode, which is characteristic of high-power CO2 laser, are simulated introducing a multipoint-line thermal source model based on heat conduction, parameterized by the distribution of the laser beam power, and the setting of the source system layout, and to be fitted on experimentally detected cross sections of the heat-affected or fusion zones of the weld. The proposed model allows to express the thermal field according to the moving reference system fixed on the overall heat source, and to analyze the temperature profiles that develop in some detection points fixed on the workpiece, as time varies during the welding process. By this way, it can be useful in evaluating the thermal effects that result from the variation of the main welding parameters (beam power, welding speed, plate thickness), with the aim of simplifying the selection of the optimal process conditions. As an application, reference is made to a cross section of a joint between plates of AISI 304 L austenitic steel. The fitting procedure allows to set the power distribution and layout parameters of the most suitable source combination (obtained by superposition of a line and two point sources). The value of the absorption coefficient also is assumed as fitting variable, so to overcome the complex problem of evaluating the beam power actually absorbed by the material on keyhole mode welding. Finally, the fitted model is applied to carry out a detailed thermal field analysis in the welded plates.

Single-Pass Full-Penetration Welding for Thick Stainless Steel Using High-Current GMAW

Single-Pass Full-Penetration Welding for Thick Stainless Steel Using High-Current GMAW

Flexural behavior of a novel high-strength RCFST column-to-column connection

Brittle fracture is one of the principal disincentives to limit the use of ultrahigh-strength steels in tall buildings. This paper proposes a novel column-to-column connection without full penetration welding in reinforced concrete-filled steel tubular composite columns. The paper describes an experimental investigation into the flexural behavior of the proposed RCFST column-to-column connection, in which the steel was manufactured using ultrahigh-strength steel grades H-SA700 and USD 685. Four specimens with varying configurations of reinforcing steel bars (separated type or gathered type) and column shapes (square or circular) were tested under four-point bending to evaluate the failure modes, flexural capacity, and deformation capacity. The results show that the gathered type of configuration of reinforcing steel bars can effectively improve the flexural capacities while having a negligible effect on the strain distribution of steel tubes or steel bars. Besides, the column type was found to significantly influence the strain distribution of the steel tube. The design formulae show accuracy and reliability and could be applied to assess the yield and ultimate strength of the proposed new connections.

The Effect of an External Magnetic Field on the Aspect Ratio and Heat Input of Gas-Metal-Arc-Welded AZ31B Alloy Weld Joints Using a Response Surface Methodology.

This study attempted to analyze and optimize the effect of an external magnetic field (EMF) on the aspect ratio and heat input for AZ31B weld joints that were welded using the gas metal arc welding (GMAW) process. The response surface methodology (RSM) was adopted for the critical analysis, and subsequently, mathematical models were developed based on the experimental results. It was observed that the EMF and its interaction with the wire feed rate significantly affected the aspect ratio and heat input, respectively. At 119 G (magnetic field), 700 mm/min (welding speed), 5.8 m/min feed rate, and 11.5 L/min (gas flow rate), the aspect ratio was 2.26, and the corresponding heat input factor (HIf) was 0.8 with almost full weld penetration.

Temperature Gradients at the Solidification Front of Deep Hybrid Laser Welds

Abstract Grain refinement and the avoidance of columnar solidification is a great challenge in the deep penetration laser welds of thick-section steels. Further knowledge about the heat distribution and the temperature gradients in laser welds is vital for future attempts on the grain refinement of such welds. In this study, a comparative investigation was carried out for full and partial penetration hybrid laser welding of structural steel. The transient temperature distribution and temperature gradients were calculated for the experiments using a simplified three-dimensional finite element analysis. A comparative analysis was presented to investigate the influence of penetration mode on the temperature gradient in the liquid at different weld depths. The results of the numerical analysis suggested that, for given welding process parameters, full penetration welds have lower temperature gradients at the solidification front, meaning that they potentially have a higher chance of grain refinement, compared with partial penetration welds.

An investigation of different parameters on the penetration depth and welding width of Ti-6Al-4V alloy by plasma arc welding

In this paper, the effect of plasma arc welding (PAW) process parameters on the mechanical properties and macro- and microstructure of Ti-6Al-4V alloy is discussed. These parameters including welding current, welding speed, and plasma gas orifice diameter have an effect on the weld bead depth and width. Macrograph results indicate that there is a certain range of welding electric current and welding speed in which full-penetration weld is obtained and weld beads are free from defect. Experimental results show that the specimen mechanical strength is favorable and comparable to those of the base material, but the sample elongation is low entirely. Moreover, the results indicate that using an optimum range of welding speed and current, which lead to decreased heat input, makes greater mechanical strength and better relative ductility. The fusion zone microstructure consists of martensite (α′), massive α (αm), acicular α, and retained β phase.

Formation mechanism of liquid metal embrittlement in laser lap welding of zinc-coated GEN3 steels

Liquid metal embrittlement (LME) was observed in the laser lap welds of zinc-coated GEN3 steels. When line energy input ranged from 54 kJ/m to 60 kJ/m, the LME crack initiated from the weld edge at the faying interface, propagated along the growth direction of columnar grains and extended upwards along the weld centerline, exhibiting an inversed Y-shape. Multiple Fe–Zn phases such as α-Fe (Zn), Γ-Fe3Zn10 and δ-FeZn10 were created by liquid zinc in grain boundaries reacting with the austenite matrix during the cooling stage of the laser welding process. Those hard and brittle Fe–Zn phases, especially Γ-Fe3Zn10, deteriorated the grain boundary cohesion and led to crack initiation. Simulation results indicate that a weld profile being deep with partial penetration will result in a high stress concentration at the notch root during the LME active temperature range. Therefore, full penetration welding is recommended to reduce the risk of LME by enabling zinc vapor to escape from both the top and bottom of the keyhole.

Multi-response mathematical modelling, optimization and prediction of weld bead geometry in gas tungsten constricted arc welding (GTCAW) of Inconel 718 alloy sheets for aero-engine components

The gas tungsten constricted arc welding (GTCAW) parameters namely Main Current, Delta Current, Delta Current Frequency and Welding Speed were optimized to obtain full penetration and optimum weld bead geometry using Response Surface Methodology (RSM) to join thin Inconel 718 alloy sheets (2 mm thick). Empirical relationships were formulated to predict the weld bead characteristics such as width of bead, depth of penetration, width of heat affected zone (HAZ) and area of fusion zone (FZ). The weld bead characteristics were predicted with good accuracy using developed empirical relationships. The direct and interaction effect of GTCAW parameters on weld bead geometry is discussed in this paper.

Simulation and Experimental Validation of EBW Studies in Austenitic Stainless Steel AISI-321

Electron beam welding of austenitic stainless steel AISI-321 has been studied with butt weld configuration. The numerical simulation is done in both thermal and structural analysis modes using ANSYS™ software for welding as well as tensile testing. Optimization of the process parameters such as the welding speed, power, beam offset and number of passes was carried out to obtain a sound weld joint using bead on trials and also through welding simulation to get a full penetration weld. Voltage of 60 kV with current of 90–95 mA was found to be optimum parameters for electron beam welding for the welding of a 4-mm-thick plate. The microstructure examination of various weld zones has been carried out. The tensile tests were carried out on the welded samples at room temperature and elevated temperatures (1173 K, 1223 K and 1273 K). It is observed that, with increasing strain rate, there is an increase in strength of the weldment at elevated temperatures indicating positive strain rate sensitivity. Further, a comparative study between the simulated and experimental results of room temperature tensile tests has been carried out. A good match in ambient temperature engineering stress–strain curves obtained through simulation and experimentation has been observed. The fractography analysis of the ambient temperature tensile tested specimens was carried out. It has been observed that the material exhibits necking and a cup cone fracture with fine dimples indicating a ductile fracture.

Evaluation of hot cracking susceptibility on laser welded aluminum alloy using coaxially arranged multiple-beam laser

Dual-beam laser welding using an adjustable ring mode (ARM) fiber laser was implemented to evaluate weld bead characteristics and hot cracking susceptibility. An ARM laser with dual beams having diameters of 175 and 450 μm was implemented in the focal position. By controlling the core laser power (700–1800 W) and ring laser power (0–1100 W), respectively, full penetration welds were achieved for a 1 mm-thick Al 6014-T4 alloy even at a high welding speed of 120 mm/s under a total laser power of 1800 W. High welding speed enabled a small molten area and a narrow equiaxed dendrite structure along the bead center, which reduced hot cracking susceptibility.

Study on mechanical and metallurgical properties of fiber laser welded Nb-1% Zr-0.1% C alloy

Laser welding of Nb-1% Zr-0.1% C was attempted in butt-welding configuration using top and bottom sided inert gas shielding. The precautionary measure during welding was to limit the reactivity of niobium alloy in ambient atmosphere. The ranges of input parameters, that is, laser power (P), welding speed (V) and beam diameter (D) for full penetration welding were attempted by carrying out bead-on-plate (BOP) experiments. The selection of the combination of process parameters was such that the formed weld area could be minimized without hampering the depth of penetration. Bead-geometry, hardness and tensile strength were quantified to study the influence of input process parameters during laser welding. Base metal had an average hardness of 108 VHN and the average hardness of fusion zone (FZ) was found to lie between 278 and 546 VHN. The steeper increment in microhardness value of the FZ could be due to the grain refinement, dissolution of precipitates and formation of brittle intermetallic phases of carbide and oxides, which were evident by the result of energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) phase analysis. The weld joint that failed in the weld zone exhibited the brittle failure, and ductile mode was achieved in the joint, where failure occurs at base metal. The range of elongation in laser welded joints varied in between 1.97 and 5.73 mm. The reduction of tensile strength and ductility of the joints could be due to marginal enhancement of microhardness and increment of brittle phase density in fusion zone, as were evident from XRD phase analysis. The main focus of the present work was intended towards the establishment of laser welding as an alternative technique for fabrication of reactive niobium alloy in ambient atmosphere.

Characterization of Metallurgical and Mechanical Properties of Thixowelded AISI D2 and AISI 304 Steels

In this study, AISI D2 tool steel was thixowelded with AISI 304 austenitic stainless steel by using a direct partial remelting technique. The joining parameters (temperature, soaking time, and contact area) have been investigated by studying their effects on the mechanical and metallurgical properties of the thixowelded joint. The joint interfacial microstructures were investigated by optical microscopy, scanning electron microscopy equipped with an EDX analyzer, and x-ray phase analysis was performed to identify the phase evolution. Mechanical characterization was determined by tensile and micro-hardness tests. Maximum tensile strength that was achieved is 635 MPa at 1300 °C, with a 5 mm insert diameter and 10 min holding time. This represents 80% of the D2 base metal strength. The joints were fractured on the D2 base metal side close to the diffusion zone characterized by brittle fracture. Consequently, the joints also achieve a full penetration weld with a smooth and uniform hardness profile.