Molten Pool Shape Research Articles

Cross-Scale Simulation Research on the Macro/Microstructure of TC4 Alloy Wire Laser Additive Manufacturing

A cross-scale model of macro-micro coupling is established for the wire laser additive manufacturing process of the TC4 titanium alloy. The model reproduces the dynamic evolution process of the molten pool shape, reveals the temperature change law in the molten pool, and simulates the microstructure and morphology of different regions of the molten pool. Finally, the model is used to quantitatively analyze the effects of process parameters (laser power, scanning speed) on the growth morphology of dendrites during solidification. The research shows that with the increase in laser power and the decrease in scanning speed, the peak temperature of the molten pool increases rapidly, and the size of the molten pool increases gradually. When the laser scanning speed is greater than 5 mm/s, the molten pool length decreases significantly. After solidification, an asymmetrically distributed equiaxed grain structure is formed at the upper part of the molten pool, the bottom of the molten pool is made up of slender columnar crystals, and the columnar-to-equiaxed transition (CET) occurs in the middle of the molten pool. With the decrease in laser power and the increase in scanning speed, the growth rate of dendrites becomes faster, the arm spacing and the overall morphology of dendrites become smaller, and the arrangement of columnar crystals have a tighter microstructure.

The detrimental molten pool narrowing phenomenon in wire feed laser beam welding and its suppression by magnetohydrodynamic technique

Element transport in the wire feed laser beam welding (WFLBW) is directly determined by the molten pool shape and thermo-fluid flow therein. In this paper, an untypical molten pool profile, i.e., elongated at its top and bottom but narrowed at the middle, is found experimentally by real-time metal/glass observation and numerically by multi-physical modeling. Ex-situ X-ray fluorescence element mapping is used to characterize the element dilution status in the fusion zone. For the first time, the detrimental influence of the molten pool narrowing on the element transport is identified and discussed, combining the experimental and numerical results. A magnetohydrodynamic technique is utilized to suppress the narrowing, aiming at a more homogenous element distribution. It is found that due to the interaction of the two dominant circulations from the top and bottom regions of the molten pool, a low-temperature region is formed. It leads to an untypical growth of the mushy zone, narrowing the molten pool in the middle region. Its detrimental effect on material mixing is non-negligible considering the direct blocking effect on the downward flow and the premature solidification at the middle region. The Lorentz force from a transverse oscillating magnetic field can change the flow pattern into a single-circulation type. The downward transfer channel is widened, and its premature solidification is prevented because the low-temperature-gradient region is mitigated. This paper provides a supplementary reason regarding the common issue of insufficient material mixing during LBW, and a promising technique to optimize the process.

Numerical Analysis of Microstructure Anomalies during Laser Welding Nickel-Based Single-Crystal Superalloy Part I: Salient Aluminum Redistribution Melioration

Correlation between welding conditions, such as laser power, welding speed and welding configuration, microstructure anomalies, including columnar/equiaxed transition (CET) and stray grain formation, and metallurgical factors, such as aluminum redistribution and supersaturation near dendrite tip, is elucidated by thorough numerical analysis to explain prevalent phenomenon of insidious microstructure development and face the challenge of microstructure amelioration as well as solidification cracking resistance improvement during course of ternary single-crystal Nickel-Chromium-Aluminum superalloy melt-pool solidification for crack-free laser surface modification. Metallurgical factors of γ gamma phase microstructure development strongly depend on welding configuration, and heat input is not as important as welding configuration. Although melted-pool center region is susceptible to microstructure anomalies, (001)/[100] welding configuration possesses auspicious growth crystallography under which the bimodal profiles of solid aluminum concentration and liquid aluminum supersaturation ahead of dendrite tip are symmetrically distributed to potentially decrease solute redistribution and increase resistibility to solidification cracking. Dissimilarly, (001)/[110] welding configuration possesses insidious growth crystallography under which the profiles of solid aluminum concentration and liquid aluminum supersaturation ahead of dendrite tip are asymmetrically distributed to preferably facilitate alloying plentiful enrichment and differentiate problematical microstructure instability on half side of molten pool. In the bottom of molten pool, beneficial Al-barren [001] dendrite is kinetically driven by homologous single-crystal epitaxial growth without columnar/equiaxed transition. On the right side of molten pool, detrimental Al-rich [100] dendrite is spontaneously susceptible to stray grain formation with equiaxed morphology. Symmetrical microstructure development is more enrichment-resistant than asymmetrical microstructure development, which is crystallographically ascribable to favorable thermo-metallurgical factors, i.e. aluminum redistribution mitigation and supersaturation relief, and substantially reduce metallurgical degradation and microstructure failure. The faster welding speed and the lower laser power are used, the smaller aluminum concentration and supersaturation ahead of dendrite tip are kinetically incurred to suppress columnar/equiaxed transition and stray grain formation with a number of thermometallurgical factors contribution to considerable microstructure amelioration and increasingly improve solidification cracking resistance and vice versa. Shallow molten-pool shape with symmetrical growth crystallography efficiently advances anomalies-resistant microstructure development with diminution of partition driving forces for solute redistribution and supersaturation during nonequilibrium solidification instead of deep molten-pool shape with asymmetrical growth crystallography. The latter importantly aggravates stray grain formation. Useful combination of optimum solidification conditions and favorable growth crystallography predominantly minimizes diffusion-controlled solute deviation and microstructure anomalies for excellent single-crystal superalloy laser processing without cracking, and possesses tenable relationship between welding conditions, solute redistribution and microstructure anomalies. The mechanism of crystallography-aided concentration fluctuation and copious supersaturation behind prominent phenomenon of microstructure anomalies, where nucleation and growth of stray grain formation near dendrite tip are activated, is consequently proposed. The comparison between numerical analyses and experiment results are valid and satisfactory. Microstructure anomalies are predictable for proper understanding of multicomponent microstructure development in the interior of fusion zone. The theoretical methodology is reproducible as consequence of availability of thermodynamic and kinetic properties of Nickel-based or Iron-based single-crystal superalloys.

Prediction of Upper Surface Roughness in Laser Powder Bed Fusion

In this study, a physics-based analytical method was proposed for the prediction of upper surface roughness in laser powder bed fusion (LPBF). The temperature distribution and molten pool shape in the melting process were first predicted by an analytical thermal model. The cap area of the solidified molten pool was assumed to be half-elliptical. Based on this assumption and the principle of mass conservation, the cap height and the specific profile of the cap area were obtained. The transverse overlapping pattern of adjacent molten pools of upper layer was then obtained, with given hatch space. The analytical expression of the top surface profile was obtained after putting this overlapping pattern into a 2D coordinate system. The expression of surface roughness was then derived as an explicit function of the process parameters and material properties, based on the definition of surface roughness (Ra) in the sense of an arithmetic average. The predictions of surface roughness were then compared with experimental measurements of 316L stainless steel for validation and show acceptable agreement. In addition, the proposed model does not rely on numerical iterations, which ensures its low computational cost. Thus, the proposed analytical method can help understand the causes for roughness in LPBF and guide the optimization of process conditions to fabricate products with good quality. The sensitivity of surface roughness to process conditions was also investigated in this study.

An integrated analytical model for the forecasting of the molten pool dimensions in Selective Laser Melting

Selective laser melting (SLM) is one of the most promising processes in the Additive Manufacturing of metals because of the possibility to fabricate complex geometry parts with a wide range of materials. The molten pool dimensions are notoriously crucial for the production of high quality parts, in terms of mechanical properties and roughness, and to control the ‘balling’ phenomenon. In the past, several studies have been conducted to monitor temperature gradient and thermal history, stress and deformation field, balling occurrence, effect of volume shrinkage and the effect of process parameters on temperature evolution. Up to now, very few works are available in literature on the effects of the process parameters on the molten pool shape and dimensions: moreover, they also neglect the simultaneously effect of various physical factors. In this work, an integrated analytical model, consisting of three sub-models (thermal, optical and melting sub-models), with the purpose to forecast the molten pool dimensions in terms of width and depth, was developed. An experimental plan has been carried out, processing the 18 maraging 300 steel, to demonstrate the capability and the accuracy of the presented model. The obtained results demonstrate that the integrated analytical model led to optimal forecasting.

The Marangoni Convection Effect on Melt Pool Formation during Selective Laser Melting Process

In order to predict the effect of the Marangoni convection and the morphology of melted stainless steel powder, during the selective laser melting (SLM) process, a transient three-dimensional numerical model is developed at the mesoscale. The evolution of the temperature and velocity fields’ is then studied. The initial powder bed distribution is obtained by the discrete element method (DEM) calculation, and the temperature distribution and the molten pool shape deformation are calculated and analyzed by the Ansys-Fluent commercial code. The molten pool shape is obtained by considering the influence of Marangoni convection on the internal flow behavior. The recoil force was not considered in our calculation. As main results, a slight deviation between the position of the maximum temperature of the molten pool and the center of the laser spot is observed. The direction of the heat diffusion is more likely to be horizontal and the flow centrifugal, which causes the melt track to be wide. Finally, the Marangoni convection is the main driver of the flow.

Prediction of solidification microstructure of titanium aluminum intermetallic alloy by laser surface remelting

By controlling the processing parameters in the laser additive manufacturing (LAM) process, the mechanical properties of titanium aluminum alloy could be modified. However, the difficulty of optimizing the solidification microstructure is to build the relationship between the microstructure and processing parameters. To solve this problem, a numerical model of combined processing parameters with dendrite arm spacing was developed in this paper to predict the solidification microstructure and reveal the effects of laser processing parameters on dendrite arm spacing. A geometry factor (α) was first introduced in the prediction model to correct the simulated molten pool shape to improve prediction accuracy. Moreover, the nonequilibrium solidification of LAM due to fast cooling was taken into consideration in this study. Based on the model, primary dendrite arm spacing (PDAS) and secondary dendrite arm spacing (SDAS) in laser surface remelting of Ti-47Al-2Cr-2V alloy were investigated in combination with simulations and experimental observations. The experimental results indicate that the model calculated using average solidification conditions applies well in predicting the solidification microstructure of titanium aluminum alloy. The dendrite arm spacing slightly decreases at low scanning velocity but increases at high scanning velocity as the laser power increases. In addition, the scanning velocity plays a dominant role in the change in dendrite arm spacing when the scanning velocity is lower than 600 mm/min, and the impact of laser power on dendrite arm spacing gradually increases as the scanning velocity increases.

Numerical Simulation and Experimental Investigation on 2205 Duplex Stainless Steel K-TIG Welded Joint

In this paper, 8 mm thickness 2205 duplex stainless steel (DSS) plates were successfully welded using keyhole tungsten inert gas welding (K-TIG) welding, and numerical simulations were performed applying the finite element method. Three models of combined heat source were adopted to verify accuracy of experiment. The welding process under different welding speeds were simulated, and the temperature field, molten pool shape, and thermal cycle curve were calculated. The welding simulation results show that a combined model consisting of the ellipsoid heat source and the conical heat source is more suitable for K-TIG welding. The results of the microstructure analysis of the welded joint showed that when the welding speed was increased from 280 mm/min to 340 mm/min, the austenite content and the ferrite and austenite grain size decreased. The evolution laws of welded joint morphologies, microstructure and grain sizes under different welding speed conditions were consistent with the analysis results of simulated molten pool morphologies, temperature field distributions and thermal cycle curves. It is proved that this kind of simulation method can effectively simulate the K-TIG welding process and ensure the welding quality, which is a guide for industrial applications.

Effects of laser and GMA hybrid welding parameters on shape, residual stress and deformation of HSLA steel welds

Abstract As a high-efficiency and high-quality welding process, hybrid laser-MAG welding (HLMW) has significant potential of application in welding thick plates. In the present study, based on thermal elastic–plastic theory, a three-dimensional finite element model is developed to predict the weld shape characteristics, residual stress and distortion in HLMW for a butt joint of 12-mm-thick high strength steel plate. Metal active gas arc welding (MAG) heat input and laser energy are modeled as one double-ellipsoid body heat source and one cone body heat source with enhanced peak density along the central axis, respectively. The comparison between calculated molten pool shapes and those obtained by the experiment shows a good agreement. Then weld shape characteristic, residual stresses and distortions are calculated in four different welding process parameters. The results show that the increase of laser power and current can effectively increase the weld penetration width when the welding speed is fixed. At the top surfaces of weldment, the peak stress of high laser beam power is more significant than that of low laser beam power. A high compressive transverse stress of low laser beam power can be found at the welding zone and the surrounding heat affected zone. However, at the bottom surfaces of weldment, the peak stress of low laser beam power is larger than that of high laser beam power. The peak stress of low laser beam power is much larger than that of high laser beam power. A high compressive transverse stress of low laser beam power can be found at the welding zone. The vertical deformation in low laser beam power and low arc current welding has the lowest value.

Effects of Defocus Distance on Three-Beam Laser Internal Coaxial Wire Cladding

Three-beam laser internal coaxial wire feeding cladding is regarded as a promising additive manufacturing technique because it is highly efficient and controllable. In this study, the effects of the defocus distance on cladding using a three-beam laser with internal wire feeding are experimentally and numerically studied. A process map indicating the surface characteristics at different defocus distances with various parameter combinations was developed. The transmission characteristics including laser intensity, beam size, and laser spot distance of the three-beam laser at different defocus distances were analyzed using TracePro software. Based on the TracePro results as heat source, a three-dimensional transient finite element (FE) thermal model was formulated to predict the thermal field, temperature history and molten pool shape at different defocus distances. A molten pool with a flat bottom and low melting depth is generated when the defocus distance is − 2.5 mm, whereas when this distance is − 1.5 mm, a pool with a valley-shaped bond and high melting depth is formed. The simulated results of the temperature cycle and clad geometry are both validated and found to well agree with experimental measurements. The influence of the defocus distance on the microstructure and microhardness are discussed based on the temperature history and cooling rate. With the increase in the absolute defocus distance, the height and dilution of the clad decreased, whereas the width increased. In addition, the effects of defocus distance with various parameter combinations on clad geometry were explored using the formulated FE model.

Micro pool characteristics of 316L and the influence of sulfur during SLM

Selective Laser melting (SLM) is one of the important additive manufacturing (AM) methods, the process of which is very complex, involving multiple physical field coupling. In this paper, the finite volume method is used to simulate the SLM process, and the heat transport, liquid metal flow and solidification behavior in the molten pool are studied. The model has taken into account the latent heat of phase transition, Marangoni effect and surface-active elements. The time when the molten pool reaches a stable state is confirmed by the temperature of a probe point and the shape change of the main molten pool. Based on a successful SLM process parameter combination, the better process parameters are pursued according to the simulation results. The effect of the surface-active element sulfur on the shape of molten pool is explained from the perspective of the influence on the surface tension.

Selective Laser Melting of Maraging Steel Using Synchronized Three-Spot Scanning Strategies.

The selective laser melting (SLM) process, a kind of metal additive manufacturing method, can produce parts with complex geometries that cannot be easily manufactured using material removal processes. With increasing industrial applications, there are still issues such as part quality and productivity that need to be resolved. In this study, maraging steel parts fabricated by synchronized three-spot scanning strategies, i.e., lateral spatial (LS) and spatial inline (SiL), are firstly presented. The LS and SiL represent the three-spot offset direction is perpendicular and parallel to the scanning direction, respectively. A laboratory SLM machine equipped with a fiber laser and three-spot module is used to fabricate the maraging steel parts with two scanning strategies, i.e., LS and SiL. The influence of these scanning strategies on the surface roughness, relative density, hardness, molten pool shapes, and microstructures are investigated. The relative density (~99.02%) and surface hardness (~34.0 HRC) are experimentally found to be higher than the SiL by the LS scanning strategy.

Prediction of lack-of-fusion porosity in laser powder-bed fusion considering boundary conditions and sensitivity to laser power absorption

This paper proposes an analytical modeling method for the prediction of lack-of-fusion porosity of parts fabricated by laser powder-bed fusion (LPBF), with the consideration of boundary heat transfer and sensitivity to laser power absorption. The temperature distribution of the part was first predicted by an analytical thermal model, which consists of a linear heat source solution and a heat sink solution. The temperature increase due to laser power input was calculated by the point moving heat source solution. The temperature drop due to thermal conduction, convection, and radiation at part boundaries was calculated by the heat sink solution. The coefficient of laser power absorption was inversely obtained by comparing predicted molten pool widths with experimental measurements. The lack-of-fusion area was then calculated by plotting molten pool shapes of multi-tracks and multi-layers on a transverse cross-sectional area of the part. The powder bed porosity was calculated by an advancing front method with the consideration of powder size distribution and packing pattern. Finally, the lack-of-fusion porosity was obtained by multiplying the lack-of-fusion area with powder bed porosity. The predicted results were close to the measurements of Ti6Al4V in LPBF. The maximum deviation for porosity prediction is 5.91%. The presented model shows high computational efficiency without relying on iteration-based numerical method. The computational time for five consecutive layers is less than 100 s. The acceptable accuracy, short computational time, and the ability to consider complex boundary effects make the proposed method a good basis for future research and a promising tool for optimization of process parameters in LPBF of complex parts.

Investigation on arc plasma, droplet, and molten pool behaviours in compulsively constricted WAAM

In order to study the special constriction effect and physical process features during compulsively constricted WAAM (CC-WAAM), the dynamic behaviours of arc, droplets, and molten pool were visually investigated. Possible interactions inside the narrow space were discussed to explain the mechanism of the compulsive constriction on ejected plasma and droplets. Based on the captured images, the strong radiative emission indicates that the ejected plasma was at high temperature at least 6000 K. As the current increases higher-temperature plasma jets were expected, which would lead to a larger plasma volume in the absence of constriction. The relationship between current and droplet diameters was preliminarily established to enable prediction of the droplet size. An important feature of small-size droplets (as low as 0.89 mm) offered CC-WAAM a great potential to improve the precision of the additive manufacturing layers, although the minimum width of the deposited layer is much larger than the droplet diameter due to liquid spreading and accumulation. The droplets were found to have a slight impact on the molten pool behaviours, which produces stable molten pool shapes. Under the wide-range parameters, the deposited layers showed good appearances, which indicates the good adaptability of this novel technology.

Simulation and analysis of electron beam welding residual stress in thin-walled high-temperature alloy aeroengine structures

Electron beam welding (EBW) is widely used to connect thin-walled high-temperature alloy structures in aeroengines. However, the residual stress and deformation caused by the high-temperature gradients generated during welding would affect the rigidity, dimensional stability, and fatigue resistance of the welded structures. The study reported here used a model combining ellipsoidal and Gaussian rotating body heat sources to undertake a numerical simulation of the temperature and residual stress generated during the EBW process. The model was systematically refined by observing and measuring the molten pool morphology. The error rates for key dimensions determining the molten pool shape were less than 6%. With a microscope-based examination, the energy distribution characteristics were detected by the microstructure analysis of grain type and size in different regions to verify the viability of the heat source model. The residual stress of a butt welding was simulated by the proposed heat source model based on the full consideration of a full-loop thin-walled combustor casing structure and material properties. It was found that the average errors for longitudinal and transverse residual stresses of welded joints and their adjacent areas were 10% and 12%, respectively, by comparing with the experimental results. Simultaneously, the low cycle fatigue life of the welded combustor casing would be decreased by 32% considering the influence of welding residual stress. These conclusions can be used as a basis for studying the integrity of thin-walled welded structures in aeroengines.

Weld Reinforcement Analysis Based on Long-Term Prediction of Molten Pool Image in Additive Manufacturing

Wire Arc Additive Manufacturing (WAAM) has developed rapidly in recent years and has been widely used in industry. Cold Metal Transfer (CMT), a kind of Gas Metal Arc Welding (GMAW), is widely used in the modeling of thin parts. In the monitoring of CMT process, it is necessary to monitor the weld reinforcement of the deposited layer. It can provide effective alerting for welding and help to improve welding quality. In this paper, the weld reinforcement and the position of molten pool image on the deposited layer are determined by the molten pool visual acquisition system. According to the data of the same direction additive manufacturing experiment, the future molten pool image is predicted by the improved prediction network (PredNet). Meanwhile through a regression network (SERes), the predicted results are regressed to the accurate weld reinforcement information of the deposited layer in advance. The experimental results show that the monitoring system designed in this paper can predict the change of molten pool shape 140ms in advance and the average precision of the weld reinforcement regression is better than 0.3mm. This paper provides the necessary basis for the shape online monitoring and controlling in WAAM process.

Temperature Field Distribution for Laser Cladding on Axially Symmetrical Parts

Laser cladding surface modification technology is widely used to strengthen and repair the surface of axially symmetrical parts. Based on the Gauss heat source model and COMSOL Multiphysics finite element analysis software, the surface modification process characteristics of laser cladding on the rotating substrate were studied by numerical simulation. The transient temperature field distribution under different process parameters was solved. Laser cladding pool corresponding to numerical simulation parameters was photographed by infrared high-speed camera. The results show that the temperature distribution of the Gauss heat source is delayed due to the normal distribution of heat flow and the cumulative effect of heat. In addition, with the increase of the rotational speed of the substrate, a cyclic preheating temperature band will be formed on the surface. The shape of molten pool obtained by temperature field analysis is droplet shape, which is basically consistent with the actual morphology of laser cladding molten pool.

Dynamic Mesh-Based Approach for Simulation of an Electromagnetically Controlled Vibrating-Electrode Electroslag-Remelting Furnace

A transient fully coupled mathematical model using the magnetohydrodynamic multiphase method and based on a dynamic mesh-based approach has been developed to simulate the temperature field and molten pool shape in relation to the slag depth in an electromagnetically controlled vibrating-electrode electroslag-remelting furnace. The results show that changing the fill ratio significantly influences the velocity, temperature, and metal pool profile. When the slag depth is increased to 140 mm, the maximum velocity rises to 0.243 m/s, approximately 30% higher compared with the case of 100 mm slag depth; the hottest zone area (T > 2150 K) dramatically enlarges towards the slag/air interface and covers over half of the slag layer; the maximum and average temperature increment in the vertical vibration mode are 75 K and 47 K, respectively, being much higher compared with other modes; the deepest distance of two-phase region decreases from 0.039 m to 0.019 m.

Effect of process parameters on formability and surface quality of selective laser melted Al-Zn-Sc-Zr alloy from single track to block specimen

The effects of the laser power and the scanning speed on molten pool shape, liquid metal spreading characteristics, and surface quality of the Al-Zn-Sc-Zr alloy fabricated by selective laser melting (SLM) were studied. The results showed that at the highest laser power of 360 W and the lowest scanning speed of 400 mm/s, the width and penetration of single track reached the maximum values of 244 μm and 241 μm. The powders were fully melted, and the liquid metal had more time to spread out when the line energy input is 800 J/m. The hole and balling defects disappeared, and the surface quality of the block specimen was significantly improved. With a decrease in the line energy density, the width, penetration, and overlap ratio of the single track gradually decreased. The surface morphology became rough with less melting and spreading time of the liquid metals. When the line energy density decreased from 800 J/m to 160 J/m, the roughness of the top surface increased from 9.4 to 16.1, and the densities of the block specimen increased from 98.1% to 99.5%.

High accuracy Numerical simulation on 3D weld-pool shape of large parts

The shape of weld pool and the distribution function of heat source are the premise and foundation of all welding simulations, but many assumptions are given to them which lead to different or even contradictory results. So the model of three-dimensional weld pool should be established without predetermining the shape of weld pool, the shape of free surface and the distribution of heat flux density, moreover it is combined with the thermal model. Through decoupling, the fluid velocity field, the temperature contour and the law of temperature distribution of the weld pool are obtained, the shape of molten pool is also gotten out which is like a revolving body of the pen holder, and the density function of heat flux distribution is fitted out. The internal relationship between simulation results and weld fusion line are presented. Welding experiments of large parts are carried out under the same process conditions. It shows the weld fusion line and the shape of the surface of the weld are in good agreement with the simulation results. This has great practical significance for unifying simulation results, guiding production and improving quality of large welding parts.