Weld Pool Formation Research Articles

Effect of the Heat Input on the Heat Affected Zone in the Austenitic Stainless Steel Welding by the GTAW Process-An Experimental and Computational Analysis

ABSTRACTArc welding processes such Gas Tungsten (GTAW), Gas Metal (GMAW) and Submerged Arc (SAW) are typically used in order to produce a weld joint in stainless steels (SS). However, welding thermal cycle generates a sensitization by formation of chromium carbides. In addition, the heat affected zone (HAZ) is also susceptible to sensitization and fracture of the weldment. Weld bead geometric parameters such depth penetration, fusion zone (FZ) width and size of HAZ are mainly determined by welding operation parameters. This research work studies the influence of welding current, welding speed and arc gap on the width and grain size in the HAZ produced by a single pass of autogenous GTAW process applied to a plate butt-welded joint of AISI 304 SS. The welded specimens were prepared for analysis by light optical (LOM) and scanning electron (SEM) microscopies to identify the interfaces between FZ-HAZ and base material as well as the grain growth in the HAZ. Adams equation for 2-D heat distribution was used to estimate theoretically the width of the HAZ. Furthermore, computational simulation which solved a convective-diffusion problem of the volumetric heat applied during the weld pool formation allowed to correlate the thermal gradient and the molten material flow of the FZ with the welding depth penetration, and width and grain size in the HAZ. The results demonstrated that the high heat input generates an important grain growth in the HAZ caused by low heat diffusion in the adjacent material to the fusion line. Welding speed was the main factor in the thermal gradient changes. Simulation results indicate that outward recirculating flow in the molten metal produced by surface tension forces is responsible for the shallow penetration of the autogenous GTAW process. Theoretical and computational estimations of the HAZ are in good agreement with the experimental results.

溶接熱源のモデル化とそれによる溶融池形成予測システムの開発

溶接熱源のモデル化とそれによる溶融池形成予測システムの開発

Understanding of humping formation and suppression mechanisms using the numerical simulation

Three-dimensional numerical models are established to investigate the convection in normal and high speed GMAW processes. A high speed photography system is used to capture the transient images of the weld pool. Based on the simulation and experimental results, the differences of weld pool formation, convection and stability are researched. The humping formation mechanism in high speed GMAW process, and humping inhibition mechanism in the twin wire GMAW process are also discussed. The results show that in normal speed GMAW process, a clockwise circulation and a backward fluid flow pattern exist in the weld pool behind the arc. While in high speed GMAW process, three main factors are responsible for the humping formation: the high momentum of the backward fluid flow, the large variation of the capillary pressure of the liquid channel in the welding direction, and the capillary instability. The first two factors impede the backfilling of molten metal, and make the liquid channel susceptible to premature solidification, the final factor makes the weld pool unstable and susceptible to collapse. In the twin wire GMAW process, these factors are suppressed, in order to obtain sound weld bead, the trailing wire current should not be larger than the leading wire current.

An investigation of heat transfer and fluid flow on laser micro-welding upon the thin stainless steel sheet (SUS304) using computational fluid dynamics (CFD)

A transient three-dimensional model is numerically developed using the method of computational fluid dynamics (CFD) to characterize some thermal phenomena and characterization of heat transfer and fluid flow in laser micro-welding by considering the heat source and the material interaction leads to rapid heating, melting and thermal cycles in the heating zone. The application of developed thermal models has demonstrated that the laser parameters, such as laser power, scanning velocity and spot diameter, have considerable effects on the peak temperature and resulted weld pool. The heat source model is consisted of surface heat source and adaptive volumetric heat source that could be well represented the real laser welding as the heat penetrates into the material. In the computation of melt dynamics, mass conservation, momentum and energy equations have been considered to compute the effects of melt flow and the thermo-fluid energy heat transfer. The simulation results have been compared with two sets of experimental research to predict the weld bead geometry and solidification pattern, which laser welds are made on thin stainless steel sheet (SUS304). The shape comparison describes those parameters relevant to any changes in the temperatures and melt dynamics are of great importance on the heat distribution and formation of weld pool during laser micro-welding process. The fair agreement between simulated and experimental results, demonstrates the reliability of the computed model.

Understanding of the weld pool convection in twin-wire GMAW process

A three-dimensional numerical model is established to study the temperature and fluid flow fields in the twin-wire gas metal arc welding (GMAW) process. The high-speed photography system is used to capture the images of the weld pool during the welding. Based on simulation and experimental results, the weld pool formation, convection, and stability in twin-wire GMAW process are investigated. Both “push-pull” and outward flow patterns exist in the twin-wire GMAW weld pool, which can contribute to decreasing the height of the bulge and increasing the width of the pool. The convection in the weld pool can proceed adequately, the arc force between the leading and trailing arcs is relatively balanced, surface tension normal force is uniform along the liquid channel, and the liquid channel is capillary stable, all of those contribute to the stability of the weld bead. The simulation results are in good agreement with those in the experiment.

A computational analysis of heat transfer and fluid flow in high-speed scanning of laser micro-welding

A transient three-dimensional model is numerically developed using computational fluid dynamics (CFD) method to understand some critical criteria such as temperature fields and melt pool formation by considering the heat source and the material interaction and the effect of laser welding parameters on laser micro-welding process. To gain more implicit insight of fluid dynamics, the issue of circulation of molten metal assisted by the surface tension, buoyancy and recoil pressure forces in the weld pool has been investigated Assuming that atmospheric and vaporised material pressures are balanced at the front of the laser beam. The governing equations from the Navier–Stokes for Newtonian fluid are prepared to estimate the melt flow that influences the rate of temperature distribution in a 3-D domain. The simulation results have been compared with two sets of experimental research to predict the weld bead geometry and solidification pattern which laser welds are made on thin stainless steel sheet (SUS304). The shape comparison describes those parameters relevant to any changes in the melt dynamics and temperatures are of great importance on the formation of weld pool and heat distribution during laser micro-welding. The fair agreement between simulated and experimental results, demonstrates the reliability of the computed model.

Welding of stainless steel using defocused laser beam

Type 321 stainless steel has a wide range of applications, including welded constructions. Common arc welding has disadvantages because of low speed and higher heat input that result in significant residual stresses and strains. The alternative to arc welding is laser welding that allows elimination of these negative effects. The results of research on welding of stainless steel with a defocused laser beam are presented and discussed in this paper. It was assumed that the use of defocused laser beam would increase volume of the weld pool that in turn would reduce requirements for preparation of edges and the gap between workpieces. Attention was mainly paid to metallurgical processes of the weld pool formation, depending on the welding conditions. Microhardness of the selected sample was measured, 2D-imaging, metallographic studies, quantitative evaluation of phase components of various zones of the weld pool were conducted. The obtained results prove that the use of the defocused beam allows obtaining quality welded joints, and this technique has also positive influence on the stability of the keyhole, and on the phase composition of the weld metal.

Gap bridging for two modes of laser arc hybrid welding

The topology of welds is an important quality criterion for which the mechanisms behind are only partially understood. The impact of gap width on the resulting weld shape produced by laser-arc hybrid welding was studied for two arc modes: pulsed arc and cold metal transfer. The weld bead and root topology were scanned and analysed. For widening gaps below a limiting width, top reinforcement turns into underfill. Above a certain gap width (0.8mm in this case), a bridge of melt is created across the weld zone. Although high speed imaging shows very different drop transfer and weld pool formation for the two arc modes, the bridging behaviour shows the same trend for both modes. This bridging effect is important from an industrial point of view because, when viewed from the top, it gives the misleading impression that a good weld has been produced. For even wider gaps the melt collapses into the gap. These different regimes are analysed and discussed for both modes.

Laser Overlap Welding of Zinc-coated Steel on Aluminum Alloy

Local reinforcement of aluminum with laser welded patches of zinc-coated steel can effectively contribute to crashworthiness, durability and weight reduction of car body. However, the weld between Zn-coated steel and aluminum is commonly susceptible to defects such as spatter, cavity and crack. The vaporization of Zn is commonly known as the main source of instability in the weld pool and cavity formation, especially in a lap joint configuration. Cracks are mainly due to the brittle intermetallic compounds growing at the weld interface of aluminum and steel. This study provides a review on the main metallurgical and mechanical concerns regarding laser overlap welding of Zn-coated steel on Al-alloy and the methods used by researchers to avoid the weld defects related to the vaporization of Zn and the poor metallurgical compatibility between steel and aluminum.

Numerical Simulation of Joining Process in a TIG Welding System Using Incompressible SPH Method

An SPH simulation of a welding process is performed for TIG welding, taking account the melting and solidification of the anode metal, the free surface movement of the liquid, and the four dominant flow-driving forces; gradient of surface tension (Marangoni effect), gas drag on the liquid surface, buoyancy, and electromagnetic force (Lorentz force). The process of welding two metal blocks is computed, including the dynamic formation of weld pool which joins the gap between the blocks. The difference of material’s surface tension dependency to temperature causes a significant difference in the weld pool penetration. These results show the usefulness of this particle method in arc welding simulations.

Effects of surface tension on predictions of weld pool formation in TIG welding

Effects of surface tension on predictions of weld pool formation in TIG welding

The Technology of Mig-Mag Welding with Low-Frequency Modulation of Arc Current

The given paper suggests a new approach towards implementation of arc current low frequency modulating method in MAG, MIG-welding. The process facilitates controlling heat and crystallization processes, regulates the time of the weld pool formation and crystallization. Theoretical study allowed formulating the main criteria of receiving strong permanent joints to produce structures with advanced reliability as it creates favorable conditions for producing a more balanced deposited metal structure and reducing the heat-affected zone. Regulating the time of weld pool formation and crystallization improves weld formation and increases labor productivity when welding sheet metals.

Effect of Intermetallic Compounds Content on Laser Welding Performance of Al-Fe Alloy Sheets

The present study investigated the laser welding performance of Al-Fe aluminum alloy sheets with different contents of intermetallic compounds. Under the same welding parameters, the alloy of higher intermetallic compounds content has wide and deep weld pools with uniform sizes. The alloy of lower intermetallic compounds content has narrow and shallow weld pools with nonuniform sizes. The higher content of intermetallic compounds results in higher laser absorptivity and lower thermal conductivity, and then increases the effective absorbed energy during welding, which is beneficial to the formation of wide and deep weld pools. The distribution uniformity of intermetallic compounds influences the size uniformity of weld pools. In the alloy with lower content of intermetallic compounds, the nonuniform distribution of intermeallic compounds results in the formation of abnormal weld pool, leading to the nonuniform size of the weld pools. In the alloy with higher content of intermetallic compounds, uniform distribution of intermetallic compounds make the size of weld pools more uniform.

非圧縮性 SPH法を用いた TIG溶接における溶融池形成シミュレーション

非圧縮性 SPH法を用いた TIG溶接における溶融池形成シミュレーション

Numerical simulation of the plasma–MIG process—interactions of the arcs, droplet detachment and weld pool formation

The plasma–metal inert gas (MIG) process is characterized by a variety of process parameters. Numerical simulation can be used to investigate the influence of these process parameters and thus helps to improve the properties of the weld. In this paper we discuss a procedure to describe the plasma–MIG process by coupling numerical models of the arc and the droplet detachment with a three-dimensional model for the plasma–MIG weld pool. Using the magnetohydrodynamic (MHD) arc model, the effects of process parameters on the arc pressure profile and the energy input profile on the workpiece can be analyzed. The volume-of-fluid (VoF)–MHD model of the droplet detachment describes the properties of the resulting droplet in terms of its size, temperature and speed. In the three-dimensional VoF model of the weld pool, the influences of the arc and the droplet are simplified by source terms in the mass, momentum and energy equations. Due to these simplifications in the physical complexity, the process and the seam shape can be described with reasonable computational effort.

Determination of enthalpy, temperature, surface tension and geometry of the material transfer in PGMAW for the system argon–iron

The metal transfer is a fundamental process in gas metal arc welding, which substantially determines the shape of the weld seam and strongly influences arc formation and stability. In this investigation the material transfer from the wire electrode (anode) to the workpiece (cathode) is analysed experimentally with high accuracy using various innovative diagnostic techniques for a pulsed gas metal arc welding (PGMAW) process. A high-speed two-colour pyrometer, a calorimeter, thermocouples, a stereo optical setup and a droplet oscillation technique are used to analyse a precisely defined PGMAW process. Thus, results obtained are verified by different measurement techniques and enable a comprehensive description of the material transfer procedure.The surface temperature of both electrodes as well as the droplet temperature, enthalpy and surface tension were determined. Furthermore, the geometry of the arc, wire, droplets and weld pool were extracted in three dimensions in order to describe the interaction between the material transfer and the formation of the weld seam. The experiments are performed using argon as shielding gas and pure iron as filler and base material to reduce complex chemical processes. It turned out that the wire feed rate has the biggest influence on droplet temperature and detachment. A correlation between weld pool formation and weld pool surface temperature gradient was observed, which is mainly a function of welding speed and wire feed rate. The experimental results obtained provide a detailed data pool for use in modelling.

Emissivity and temperature determination on steel above the melting point

The knowledge of the temperatures of materials above the melting point is of interest in the metal working industry, e.g., in welding to observe the weld pool formation. Therefore, procedures to determine the temperature also in the liquid phase are significant and necessary. In this paper, a spectroscopic method is presented to determine the surface temperature of liquid S235. A liquid metal-pool generated by a welding process was observed during its cool-down. The resulting temperature and emissivity distributions are presented. This procedure was also adapted to a controlled short arc process and the resulting data are shown.

Numerical analysis of arc plasma and weld pool formation by a tandem TIG arc

Multi-electrode welding processes are used in various industrial fields for higher productivity. These processes can use many combinations of welding methods, current values and polarities, and electrode alignments. However, this complicates welding phenomena. In this research, we focused on the tandem tungsten inert gas (TIG) arc, which is a relatively simple multi-electrode welding process. We performed a numerical investigation into the tandem TIG arc plasma and weld pool formation to clarify the phenomena involved. The weld spot of a tandem TIG weld is ellipsoidal in appearance and is deeper than that of a single-electrode TIG weld. Reduction in the shear stress of the plasma flow in the tandem TIG weld leads to a deeper penetration. Furthermore, the influence of the electrode alignment is calculated, and it is determined that the weld spot appearance significantly changes. The appearance is strongly related to the arc plasma shape. These numerical results show good agreement with the experimental results, which proves that our model has relatively high reliability.

Characterization of welding arc and weld pool formation in vacuum gas hollow tungsten arc welding

An experimental method is suggested to obtain the effective arc radii for various welding conditions in vacuum gas hollow tungsten arc welding. The irradiance distribution of welding arc next above the anode workpiece is obtained by applying Abel inversion algorithm to the CCD arc image, and then used to determine the distribution of arc heat flux, arc pressure and current density from the physical relations of arc irradiance, temperature and current density in gas tungsten arc welding. The resultant arc models are then adopted to simulate the gas hollow tungsten arc welding process for various gas flow rates.

Heat and mass transfer in laser dissimilar welding of stainless steel and nickel

Laser spot welding of stainless steel–nickel dissimilar couple has been studied experimentally and numerically. A three-dimensional heat and mass transfer model is used to simulate the welding process, based on the solution of the equations of mass, momentum, energy conservation and solute transport in weld pool. The calculated fusion zone geometry and element distributions are in good agreement with the corresponding experimental results. The role of fluid flow on temperature field and its evolution is analyzed by comparing two cases with and without considering convection. Temperature fields far away from the weld pool are quite similar, but exhibit large difference close to the heat source. During the early stage after formation of weld pool, the distribution of element Fe in weld pool is non-uniform, due to insufficient time for mixing. The speed for mass transport is the highest during the initial stage of weld pool formation and it decreases with time. Both heat and mass transport are significantly influenced by convection during laser spot welding of stainless steel and nickel.