The effects of process variables on pulsed Nd:YAG laser spot welds: Part I. AISI 409 stainless steel

The weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of Nd:YAG laser welding parameters, including laser pulse time and power intensity, and material-dependent variables, such as absorptivity and thermophysical properties, on laser spot-weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and sur-face cratering, have been studied experimentally. The results of this work are reported in two parts. In Part I, the weldability of AISI 409 stainless steel by the pulse laser welding process is reported. In Part II, the weldability of A A 1100 aluminum under the same operating con-ditions is reported and compared to those of the stainless steel. When welding AISI 409 stainless steel, weld pool shapes were found to be influenced most by the power intensity of the laser beam and to a lesser extent by the pulse duration. Conduction mode welding, keyhole mode welding, and drilling were observed. Conduction mode welds were produced when power in-tensities between 0.7 and 4 GW/m² were used. The initial transient in weld pool development occurred in the first 4 ms of the laser pulse. Following this, steady-state conditions existed and conduction mode welds with aspect ratios (depth/width) of about 0.4 were produced. Keyhole mode welds were observed at power intensities greater than 4 GW/m². Penetration of these keyhole mode welds increased with increases in both power intensity and pulse time. The major weld defects observed in the stainless steel spot welds were cratering and large-occluded gas pores. Significant metal loss due to spatter was measured during the initial 2 ms of keyhole mode welds. With increasing power intensity, there was an increased propensity for occluded gas pores near the bottom of the keyhole mode welds. Formerly Graduate Student.     

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.     

Welding parameters and the grain structure of weld metal — A thermodynamic consideration

The grain structure of the weld metal can significantly affect its resistance to solidification cracking during welding and its mechanical properties after welding. An experimental study was conducted to investigate the effect of two basic welding parameters,i.e., the heat input and the welding speed, on the grain structure of aluminum-alloy welds. Gas-tungsten arc welding was performed under various heat inputs and welding speeds, with thermal measurements in the weld pool being carried out during welding and the amounts and nuclei of equiaxed grains in the resultant welds being examined using optical and electron microscopy. The experimentally measuredG/R ratios and the clearly revealed heterogeneous nuclei together demonstrated the thermodynamic effect of the heat input and welding speed on the weld metal grain structure.     

Modeling macro-and microstructures of Gas-Metal-Arc Welded HSLA-100 steel

Fluid flow and heat transfer during gas-metal-arc welding (GMAW) of HSLA-100 steel were studied using a transient, three-dimensional, turbulent heat transfer and fluid flow model. The temperature and velocity fields, cooling rates, and shape and size of the fusion and heat-affected zones (HAZs) were calculated. A continuous-cooling-transformation (CCT) diagram was computed to aid in the understanding of the observed weld metal microstructure. The computed results demonstrate that the dissipation of heat and momentum in the weld pool is significantly aided by turbulence, thus suggesting that previous modeling results based on laminar flow need to be re-examined. A comparison of the calculated fusion and HAZ geometries with their corresponding measured values showed good agreement. Furthermore, “finger” penetration, a unique geometric characteristic of gas-metal-arc weld pools, could be satisfactorily predicted from the model. The ability to predict these geometric variables and the agreement between the calculated and the measured cooling rates indicate the appropriateness of using a turbulence model for accurate calculations. The microstructure of the weld metal consisted mainly of acicular ferrite with small amounts of bainite. At high heat inputs, small amounts of allotriomorphic and Widmanstätten ferrite were also observed. The observed microstructures are consistent with those expected from the computed CCT diagram and the cooling rates. The results presented here demonstrate significant promise for understanding both macro-and microstructures of steel welds from the combination of the fundamental principles from both transport phenomena and phase transformation theory.     

The effects of process variables on pulsed Nd:YAG laser spot welds: Part II. AA 1100 aluminum and comparison to AISI 409 stainless steel

In this two-part article, the weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of laser pulse time and power density on laser spot weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and surface cratering, have been studied and explained qualitatively in relation to material-dependent variables such as absorptivity and thermophysical properties. The weldability of AISI 409 stainless steel was reported in Part I of this article. In the present article, the weldability of AA 1100 aluminum is reported and compared to that of AISI 409 stainless steel. Weld pool shapes in aluminum were found to be influenced by the mean power density of the laser beam and the laser pulse time. Both conduction-mode and keyhole-mode welding were observed in aluminum. Unlike stainless steel, however, drilling was not observed. Conduction-mode welds were produced in aluminum at power densities ranging from 3.2 to 10 GW/m². The power density required for melting aluminum was approximately 4.5 times greater than stainless steel. The initial transient in weld pool development in aluminum occurred within 2 ms, and the aspect ratios (depth/width) of the steady-state conduction-mode weld pools were approximately 0.2. These values are about half those observed in stainless steel. The transition from conduction- to keyhole-mode welding occurred in aluminum at a power density of about 10 GW/m², compared to about 4 GW/m² for stainless steel. Weld defects such as porosity and cratering were observed in both aluminum and stainless steel spot welds. In both materials, there was an increased propensity for large occluded vapor pores near the root of keyhole-mode welds with increasing power density. In aluminum, pores were observed close to the fusion boundary. These could be eliminated by surface milling and vacuum annealing the specimens, suggesting that such pores were due to hydrogen. Finally, excellent agreement was obtained between experimental data from both alloys and an existing analytical model for conduction-mode laser spot welding. Two nondimensional parameters, the Fourier number and a nondimensional incident heat flux parameter, were derived and shown to completely characterize weld pool development in conduction-mode welds made in both materials.     

Mathematical modeling of three-dimensional heat and fluid flow in a moving gas metal arc weld pool

Mathematical models capable of accurate prediction of the weld bead and weld pool geometry in gas metal arc (GMA) welding processes would be valuable for rapid development of welding procedures and empirical equations for control algorithms in automated welding applications. This article introduces a three-dimensional (3-D) model for heat and fluid flow in a moving GMA weld pool. The model takes the mass, momentum, and heat transfer of filler metal droplets into consideration and quantitatively analyzes their effects on the weld bead shape and weld pool geometry. The algorithm for calculating the weld reinforcement and weld pool surface deformation has been proved to be effective. Difficulties associated with the irregular shape of the weld bead and weld pool surface have been successfully overcome by adopting a boundary-fitted nonorthogonal coordinate system. It is found that the size and profile of the weld pool are strongly influenced by the volume of molten wire, impact of droplets, and heat content of droplets. Good agreement is demonstrated between predicted weld dimensions and experimently measured ones for bead-on-plate GMA welds on mild steel plate.     

The effect of alloy composition and welding conditions on columnar-equiaxed transitions in ferritic stainless steel gas-tungsten arc welds

The columnar-equiaxed transition (CET) was investigated in full penetration gas-tungsten arc (GTA) welds on ferritic stainless steel plates containing different amounts of minor elements, such as titanium and aluminum, for a range of welding conditions. In general, the fraction of equiaxed grains increased, and the size of the equiaxed grains decreased with increasing titanium contents above 0.18 wt pct. At a given level of titanium, the equiaxed fraction increased, and the size of the equiaxed grains decreased with increased aluminum content. The CET was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions, since these inclusions were observed at the origin of equiaxed dendrites in the grain refined welds. Titanium-rich cuboidal inclusions, in turn, were found to contain Al-Ca-Mg-rich inclusions at their centers, consistent with observations by previous investigators for other processes. The welding conditions, in particular, the welding speed, were observed to affect the occurrence of the CET. Increasing the welding speed from 3 to 8 mm/s increased the equiaxed fraction noticeably, but a further increase in speed to 14 mm/s had a smaller additional effect. A finite element model (FEM) of heat transfer was used to examine the role of the welding conditions on the local solidification conditions along the weld pool edge. The results are compared with existing models for the CET. Formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo     

CO2 laser beam welding of 6061-T6 aluminum alloy thin plate

Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO₂ laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening β″ (Mg₂Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.     

Computational modeling of stationary gastungsten-arc weld pools and comparison to stainless steel 304 experimental results

A systematic study was carried out to verify the predictions of a transient multidimensional computational model by comparing the numerical results with the results of an experimental study. The welding parameters were chosen such that the predictions of the model could be correlated with the results of an earlier experimental investigation of the weld pool surface temperatures during spot gas-tungsten-arc (GTA) welding of Type 304 stainless steel (SS). This study represents the first time that such a comprehensive attempt has been made to experimentally verify the predictions of a numerical study of weld pool fluid flow and heat flow. The computational model considers buoyancy and electromagnetic and surface tension forces in the solution of convective heat transfer in the weld pool. In addition, the model treats the weld pool surface as a truly deformable surface. Theoretical predictions of the weld pool surface temperature distributions, the cross-sectional weld pool size and shape, and the weld pool surface topology were compared with corresponding experimental measurements. Comparison of the theoretically predicted and the experimentally obtained surface temperature profiles indicated agreement within ±8 pct for the best theoretical models. The predicted surface profiles were found to agree within ±20 pct on dome height and ±8 pct on weld pool diameter for the best theoretical models. The predicted weld cross-sectional profiles were overlaid on macrographs of the actual weld cross sections, and they were found to agree very well for the best theoretical models.     

Geometry of laser spot welds from dimensionless numbers

Recent computer calculations of heat transfer and fluid flow in welding were intended to provide useful insight about weldment geometry for certain specific welding conditions and alloys joined. However, no generally applicable correlation for the joining of all materials under various welding conditions was sought in previous work. To address this difficulty, computer models of fluid flow and heat transfer were used for the prediction of weld pool geometry in materials with diverse properties, such as gallium, pure aluminum, aluminum alloy 5182, pure iron, steel, titanium, and sodium nitrate under various welding conditions. From the results, a generally applicable relationship was developed between Peclet (Pe) and Marangoni (Ma) numbers. For a given material, Ma and Pe increased with the increase in laser power and decrease in beam radius. For materials with high Prandtl number (Pr), such as sodium nitrate, the Pe and Ma were high, and heat was transported primarily by convection within the weld pool. The resulting welds were shallow and wide. For low Pr number materials, like aluminum, the Pe and Ma were low in most cases, and low Pe made the weld pool deep and narrow. The cross-sectional areas of stationary and low speed welds could be correlated with welding conditions and material properties using dimensionless numbers proposed in this article.     

Prediction of weld pool and reinforcement dimensions of GMA welds using a finite-element model

Mathematical models of the gas metal arc (GMA) welding process may be used to study the influence of various welding parameters on weld dimensions, to assist in the development of welding procedures, and to aid in the generation of process control algorithms for automated applications. In this work, a three-dimensional (3-D), steady-state thermal model of the GMA welding process has been formulated for a moving coordinate framework and solved using the finite-element method. The model includes temperature-dependent material properties, a new finite-element formulation for the inclusion of latent heat of fusion, a Gaussian distribution of heat flux from the arc, plus the effects of mass convection into the weld pool from the melted filler wire. The influence of weld pool convection on the pool shape was approximated using anisotropically enhanced thermal conductivity for the liquid phase. Weld bead width and reinforcement height were predicted using a unique iterative technique developed for this purpose. In this paper, the numerical model is shown to be capable of predicting GMA weld dimensions for individual welds, including those with finger penetration. Also, good agreement is demonstrated between predicted weld dimensions and experimentally derived relations that describe the effects of process variables and their influence on average weld dimensions for bead-onplate GMA welds on steel plate. E. PARDO, formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1,     

A Quantitative Model of Keyhole Instability Induced Porosity in Laser Welding of Titanium Alloy

Quantitative prediction of the porosity defects in deep penetration laser welding has generally been considered as a very challenging task. In this study, a quantitative model of porosity defects induced by keyhole instability in partial penetration CO₂ laser welding of a titanium alloy is proposed. The three-dimensional keyhole instability, weld pool dynamics, and pore formation are determined by direct numerical simulation, and the results are compared to prior experimental results. It is shown that the simulated keyhole depth fluctuations could represent the variation trends in the number and average size of pores for the studied process conditions. Moreover, it is found that it is possible to use the predicted keyhole depth fluctuations as a quantitative measure of the average size of porosity. The results also suggest that due to the shadowing effect of keyhole wall humps, the rapid cooling of the surface of the keyhole tip before keyhole collapse could lead to a substantial decrease in vapor pressure inside the keyhole tip, which is suggested to be the mechanism by which shielding gas enters into the porosity.     

A New Model for Keyhole Mode Laser Welding Using FLUENT

Evolution of the free surface at gas?Cliquid interface during keyhole mode welding is complex and its calculation is computationally expensive. Similarly, models based on only heat conduction without considering vapour cavity and liquid convection around it, are computationally efficient but are not effective in defining the weld pool shape especially for low conducting material like steel. In the present study a useful yet computationally efficient model has been presented for keyhole mode laser welding using commercial software FLUENT. Here instead of evolving the free surface of keyhole in a rigorous way, various possible steady keyhole shapes are assumed partially based on literature evidence and subsequently their dimensions are calculated by an overall heat balance. The estimated keyhole profile is then mapped into the thermo-fluid framework of FLUENT and steady computational fluid dynamics calculations is carried out around the keyhole that is considered rigid wall at boiling temperature. Next, an optimized keyhole shape is identified by comparing the predicted fusion lines with the experimental weld fusion lines reported in literature. Finally, using this optimized keyhole shape independent predictions are made for two materials of widely different thermal conductivities, like steel and aluminum, under different operating conditions. In all cases the results of the present simulation is found to in close agreement with experimental data and even better than the model predictions reported in literature. The present model emerges as a simple yet effective model for predicting the weld bead profile encompassing wide range of materials under different operating conditions.     

Computational modeling of laser welding of Cu-Ni dissimilar couple

A three-dimensional transient model to solve heat transfer, fluid flow, and species conservation during laser welding of dissimilar metals is presented. The model is based on a control volume formulation with an enthalpy-porosity technique to handle phase change and a mixture model to simulate mixing of molten metals. Weld pool development, solidified weld pool shape, and composition profiles are presented for both stationary as well as continuous laser welding in conduction mode. Salient features of a dissimilar Cu-Ni weld are summarized and thermal transport arguments are employed to successfully explain the observations. It is found that the weld pool shape becomes asymmetric even when the heat source is symmetrically applied on the two metals forming the couple. It is also observed that convection plays an important role in the development of weld pool shape and composition profiles. As the weld pool develops, the side melting first (nickel) is found to experience more convection and better mixing. Results from the case studies of computation are compared with corresponding experimental observations, showing good qualitative agreement between the two.     

The effect of quenching on the solidification structure and transformation behavior of stainless steel welds

Weld solidification structure of three different types of stainless steel,i.e., 310 austenitic, 309 and 304 semiaustenitic, and 430 ferritic, was investigated. Welds of each material were made without any quenching, with water quenching, and with liquid-tin quenching during welding. The weld micro-structure obtained was explained with the help of the pseudobinary phase diagrams for Fe-Cr-Ni and Fe-Cr-C systems. It was found that, due to the postsolidification 5 → γ phase transformation in 309 and 304 stainless steels and the rapid homogenization of microsegregation in 430 stainless steel, their weld solidification structure could not be observed unless quenched from the solidification range with liquid tin. Moreover, the formation of acicular austenite, and hence, martensite, at the grain boundaries of 430 stainless steel welds was eliminated completely when quenched with liquid tin. The weld solidification structure of 310 stainless steel, on the other hand, was essentially unaffected by quenching. Based upon the observations made, the weld microstructure of these stainless steels was summarized. The effect of cooling rate on the formation of primary austenite in 309 stainless steel welds was discussed. Finally, a simple method for determining the relationship between the secondary dendrite arm spacing and the solidification time, based on welding speeds and weld pool configurations, was suggested.     

Three-dimensional heat flow and solidification during the autogenous GTA welding of aluminum plates

A theoretical and experimental study of heat flow and solidification during the autogenous GTA welding of aluminum plates was carried out. The theoretical part of the study involves the development of a computer model which describes three-dimensional heat flow during welding. The model, though valid for any plate thickness, is particularly useful for moderately thick plates since both full- and partial-penetration welds can be considered. The experimental part of the study, on the other hand, involves the measurement of the thermal response of the workpiece during welding, and the examination of the configuration, grain structure, and subgrain structure of the fusion zone. The experimental results were compared with the calculated ones and the agreement was very good. With the help of the computer model, the effects of welding parameters on weld penetration in moderately thick plates were discussed. These parameters are the heat input per unit length of the weld, the thickness of the workpiece, the preheating of the workpiece, and the power-density distribution of the heat source. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA     

The influence of thermofluids phenomena in gas tungsten arc welds in high and low thermal conductivity metals

A comprehensive thermofluids numerical model of stationary gas tungsten arc (GTA) welding has been developed and used to examine the effects of thermofluids phenomena on the temperatures and flow velocities in the weld pool and their impact on the resultant weld dimensions for welds produced in high vs low thermal conductivity metals. A dynamic grid re-mapping technique was used to map a block of finite elements into the liquid weld pool to permit use of a k–ε turbulence model in the liquid. Simulations for low conductivity AISI 304 stainless steel show that good correlation with experimental data was only possible if the effects of fluid flow and turbulent mixing in the weld pool were modeled accurately. Conversely, simulations in higher conductivity metals, 6061 and 1100 aluminum, showed that while the average flow velocities and level of turbulence were higher, their effect on the final temperatures and weld pool dimensions were less significant.     

Grain Refinement of AZ31 Magnesium Alloy Weldments by AC Pulsing Technique

The current study has investigated the influence of alternating current pulsing on the structure and mechanical properties of AZ31 magnesium alloy gas tungsten arc (GTA) weldments. Autogenous full penetration bead-on-plate GTA welds were made under a variety of conditions including variable polarity (VP), variable polarity mixed (VPM), alternating current (AC), and alternating current pulsing (ACPC). AC pulsing resulted in significant refinement of weld metal when compared with the unpulsed conditions. AC pulsing leads to relatively finer and more equiaxed grain structure in GTA welds. In contrast, VP, VPM, and AC welding resulted in predominantly columnar grain structures. The reason for this grain refinement may be attributed to the periodic variations in temperature gradient and solidification rate associated with pulsing as well as weld pool oscillation observed in the ACPC welds. The observed grain refinement was shown to result in an appreciable increase in fusion zone hardness, tensile strength, and ductility.     

超低碳微合金X100管线钢CO2焊焊接接头的组织和性能

 采用CO2焊接方法焊接X100管线钢,分析了不同焊接工艺下焊接接头组织和性能的变化特征。随着焊接热输入的增加,焊接接头的屈服强度和抗拉强度降低,焊缝和热影响区处的冲击吸收功呈现先增大后减小的变化趋势,而焊缝组织均以针状铁素体（AF）为主。焊接热输入为1.17 kJ/mm时,粗晶区的显微组织主要是贝氏体铁素体（BF）,强韧匹配性最为优异;当热输入增加至1.91 kJ/mm时,粗晶区的组织除了BF外,还出现了粒状贝氏体（GB）,强韧水平明显降低。综合考虑,可将1.17 kJ/mm作为X100管线钢CO2焊接时的最佳热输入。