Molten Pool Dimensions Research Articles

Finite element analysis of temperature and stress fields during selective laser melting process of Al–Mg–Sc–Zr alloy

A 3D finite element model was established to investigate the temperature and stress fields during the selective laser melting process of Al–Mg–Sc–Zr alloy. By considering the powder–solid transformation, temperature- dependent thermal properties, latent heat of phase transformations and molten pool convection, the effects of laser power, point distance and hatch spacing on the temperature distribution, molten pool dimensions and residual stress distribution were investigated. Then, the effects of laser power, point distance and hatch spacing on the microstructure, density and hardness of the alloy were studied by the experimental method. The results show that the molten pool size gradually increases as the laser power increases and the point distance and hatch spacing decrease. The residual stress mainly concentrates in the middle of the first scanning track and the beginning and end of each scanning track. Experimental results demonstrate the accuracy of the model. The density of the samples tends to increase and then decrease with increasing laser power and decreasing point distance and hatch spacing. The optimum process parameters are laser power of 325–375 W, point distance of 80–100 μm and hatch spacing of 80 μm.

Effect of sheet thickness on the fusion zone temperature distribution, melt pool dimensions, mechanical properties, and microstructure in laser welding of Ti6Al4V alloy

Generally, sheet thickness plays a significant role in the selection of appropriate process parameters in order to produce high quality weld joint in the laser welding process. The heat sink capacity and weld penetration are known as two criteria that are mainly influenced by sheet thickness. In this study, the effect of sheet thickness, welding speed, nozzle distance, and laser power were investigated in order to determine the temperature distribution near the melt pool, dimensions of molten pool through experimental and numerical analysis. The weld joint mechanical characterization was determined via elongation rate and tensile strength. The highest value of tensile strength is about 80% of the typical base metal and the elongation of the welded samples achieved about 40% of the base metal. The thinner sheets showed more sensitivity related to the elongation of the joint by increasing the welding speed. Also, the temperature rise with increasing laser power near the melt pool for the thinner sheet was about 200 °C in comparison to the 3 mm sheet, which is about 90 °C. The obtained simulation results for the maximum temperature discrepancy at near the melt pool was 12 °C and 4 °C for 1 and 3 mm thickness orderly, which depicts good agreement with the temperature experimental results.

Numerical investigation of temperature distribution and melt pool dimension during dissimilar laser welding of AISI 304 and pure copper

In this research, the numerical simulation of the heat transfer phenomenon in dissimilar laser welding was performed. Metals of stainless steel 304 (AISI 304) and pure copper in the moving state were exposed to laser energy, and an asymmetric thermal field was obtained. The cross-sectional area of the concentrated beam and its intensity distribution were considered circular and Gaussian, respectively. Physical properties of metals, depending on temperature and thermophysical changes in the material properties, were considered. Different temperature distributions were obtained by changing various parameters, such as focal length, welding speed, and power; the obtained results were then compared with the experimental ones. The results showed that the molten pool formation was asymmetric and deviated toward the AISI 304 sheet. The heat affected zone (HAZ) was also more toward the AISI 304 sheet. The increase of the molten pool dimensions was much more significant due to the enhancement in the average power, as compared to the other parameters, thus indicating that the average power was a key factor increasing the dimensions of the molten pool. The results of the numerical simulation were consistent with the experimental results, proving their correctness.

Comparative Assessment of Physics-Based Computational Models on the NIST Benchmark Study of Molten Pool Dimensions and Microstructure for Selective Laser Melting of Inconel 625

Comparative Assessment of Physics-Based Computational Models on the NIST Benchmark Study of Molten Pool Dimensions and Microstructure for Selective Laser Melting of Inconel 625

Numerical modelling of keyhole formation in selective laser melting of Ti6Al4V

In the selective laser melting process, the molten pool flow, particularly the keyhole phenomenon, plays an important role in the pore formation and then influences the mechanical properties. In this study, we develop a 3D numerical model to simulate the keyhole mode in SLM. The transient evolutions of the temperature field, fluid flow, molten pool geometry and energy absorption have all been reproduced. The simulated molten pool dimensions agree well with the experimental data, including the fluctuation. More thorough insights into the underlying physical mechanisms are achieved, including the correlation between the keyhole geometry and laser absorption, the influence of manufacturing parameters on the melting mode, and the fluid flow pattern under different melting modes. These understandings can provide guidance for the optimization of manufacturing process parameters to improve the product quality.

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.

Comparisons of 304 austenitic stainless steel manufactured by laser metal deposition and selective laser melting

Abstract Selective laser melting (SLM) and laser metal deposition (LMD) are the two methods of laser additive manufacturing (LAM), which are distinguished by the different powder feeding methods. In this paper, the temperature field (temperature distribution, cooling rate, temperature gradient and dimensions of molten pool), appearance and microstructure (grain shape and size, and phase composition) of the LMDed 304 austenitic stainless steel (SS304) specimen are compared with those of the SLMed SS304 specimen published in the previous literature. The results show that the LMDed SS304 specimen has the similar temperature field trend with the SLMed SS304 specimen. The differences for temperature field obtained by the finite element models (FEMs) are that LMD produces the higher temperature and dimensions of molten pool, but the lower cooling rate and temperature gradient. The appearances of the LMDed and SLMed specimens are compared. Since the molten pool during LMD is larger, the surface roughness of the LMDed SS304 specimen is higher than that of the SLMed SS304 specimen. Due to the different cooling rates, the LMDed SS304 specimen has the different microstructure from the SLMed SS304 specimen. The SLMed and LMDed specimens have the similar columnar grain shape, but the grain size of the LMDed specimen is larger. The microstructure of LMDed SS304 specimen is composed of two phases, which are austenite phase and small amount of ferrite phase. But the SLMed specimen has the full austenite phase. The differences between SLM and LMD are also studied in this paper.

Predictive Manufacturing: Subtractive and Additive

Manufacturing is the key to today’s industrial competitiveness, and it is broadly classified into two categories, subtractive and additive. In current study, the ability to predictively model manufacturing performance attributes in both categories is introduced. In subtractive manufacturing, modeling of laser-assisted and ultrasonic vibration-assisted milling are presented. In laser-assisted milling, the laser preheating temperature field is predicted, and the dynamic recrystallization as well as grain growth triggered under high temperature is considered, which enhances the accuracy of force and residual stress prediction. In ultrasonic vibration-assisted milling, the intermittent effect is considered through tool-workpiece separation criteria. And the force reduction in ultrasonic vibration-assisted milling is accurately predicted. In additive manufacturing, laser-assisted metal additive manufacturing is introduced. And the predictive modeling of temperature field in powder bed metal additive manufacturing is presented. The model considers heat transfer boundary including heat loss from convection and radiation at the part boundary. Through the comparison between measured and calculated molten pool dimensions, the model is proven to have high computational efficiency and high prediction accuracy.

Prediction of spatiotemporal variations of deposit profiles and inter-track voids during laser directed energy deposition

The spatiotemporal variations of the molten pool and deposit profiles during laser Directed Energy Deposition (DED) largely affect the formation of printing defects and the build quality. Quantitative assessment of the dependencies of molten pool characteristics on critical process variables is helpful to reveal the evolution of the depositing tracks. To this end, a novel 3D transient phenomenological model was developed in this work to explore the evolution of the temperature and velocity fields and the molten pool dimensions for both single-track and multi-track laser DED deposits. The influences of laser scanning speed, mass feed rate, laser power, Mass per Unit Length (MUL, g/mm) and Energy per Unit Mass (EUM, J/g) intensities on the deposit characteristics were comprehensively examined. The 3D profiles of the deposited tracks were sectioned, reconstructed and visualized. The computed deposit profiles showed that the contact angles of the single-tracks increased significantly with higher MUL intensity. In contrast, the contact angle decreased with higher EUM intensity. The formation of highly convex deposits was carefully examined in particular, and a prominent bulge was found near the beginning of the deposit with local contact angle larger than 100°. The simulation results showed that convex deposit profiles obtained at high MUL intensity further caused inter-track voids during multi-track deposition.

Analytical Thermal Modeling of Powder Bed Metal Additive Manufacturing Considering Powder Size Variation and Packing.

This work presents a computationally efficient predictive model based on solid heat transfer for temperature profiles in powder bed metal additive manufacturing (PBMAM) considering the heat transfer boundary condition and powder material properties. A point moving heat source model is used for the three-dimensional temperature prediction in an absolute coordinate. The heat loss from convection and radiation is calculated using a heat sink solution with a mathematically discretized boundary considering non-uniform temperatures and heat loss at the boundary. Powder material properties are calculated considering powder size statistical distribution and powder packing. The spatially uniform and temperature-independent material properties are employed in the temperature prediction. The presented model was tested in PBMAM of AlSi10Mg under different process conditions. The calculations of material properties are needed for AlSi10Mg because of the significant difference in thermal conductivity between powder form and solid bulk form. Close agreement is observed upon experimental validation on the molten pool dimensions.

A Closed-Form Solution for Temperature Profiles in Selective Laser Melting of Metal Additive Manufacturing

This paper presents a closed-form solution for the temperature prediction in selective laser melting (SLM). This solution is developed for the three-dimensional temperature prediction with consideration of heat input from a moving laser heat source, and heat loss from convection and radiation on the part top boundary. The consideration of heat transfer boundary condition and latent heat in the closed-form solution leads to an improvement on the understanding of thermal development and prediction accuracy in SLM, and thus the usefulness of the analytical model in the temperature prediction in real applications. A moving point heat source solution is used to calculate the temperature rise due to the heat input. A heat sink solution is used to calculate the temperature drop due to heat loss from convection and radiation on the part boundary. The heat sink solution is modified from a heat source solution with equivalent power due to heat loss from convection and radiation, and zero-moving velocity. The temperature solution is then constructed from the superposition of the linear heat source solution and linear heat sink solution. Latent heat is considered using a heat integration method. Ti-6Al-4V is chosen to test the presented model with the assumption of isotropic and homogeneous material. The predicted molten pool dimensions are compared to the documented values from the finite element method and experiments in the literature. The presented model has improved prediction accuracy and significantly higher computational efficiency compared to the finite element model.

Mesoscopic study of thermal behavior, fluid dynamics and surface morphology during selective laser melting of Ti-based composites

A mesoscopic simulation based on the randomly packed powder bed model was developed to study the thermal behaviors during selective laser melting (SLM) of Ti-based composites. Effects of processing parameters on the thermal behavior, fluid dynamics and surface morphology evolution within the molten pool were investigated. The obtained results revealed that the operating temperature, cooling rate and melt lifetime were highly enhanced as the laser power was increased. Meanwhile, the increased molten pool dimensions, the turbulent fluid flows, the improved escaping rate of the entrapped gas and the efficient rearrangement of reinforcing particles within the molten pool appeared at the application of the high laser power. At the optimized processing parameters, the peak of the operating temperature profile located in the laser and powder interaction area was apparently disappeared with the formation of the maximum temperature of 3300 K and, the mean operating temperature of the platform caused by the heat accumulation was as high as 1300 K. Moreover, the surface morphology of the molten pool predicted by the simulation showed a variation from continuous pores to fragments, then to the typical and regular liquid front, and finally to the turbulent liquid front and spatter and balling phenomenon as the laser power increased. At the laser power of 200 W and laser energy density of 140 J/m, the maximum velocity was located in the front and rear region and, the velocity vector located in the melt advanced front pointed to the rear region of the molten pool, indicating that the melt from the irradiation region would complete the efficient melt supplement and avoid the formation of residual pores and therefore, a good and flat surface with few spatters was obtained with the clear liquid front. The simulated surface morphology was found to be consistent with the experimental measurements.

Analytical modeling of part porosity in metal additive manufacturing

Porosity is a frequently observed defect in metal additive manufacturing due to the large thermal gradients caused by the repeated rapid melting and solidification. This work presents a physics-based model to predict the part porosity in powder bed metal additive manufacturing (PBMAM) per given process parameters, materials properties, powder size distribution. The molten pool dimensions were first calculated by a closed-form temperature solution considering the laser heat input and part boundary heat loss. The porosity evolution was calculated by subtracting the volume fraction of the preprocessed powder bed void from the postprocessed porosity. The volume fractions of powder and void in packed powder bed were calculated by advancing front approach (AFA) and image analysis considering powder size variation and statistical distribution. The porosity model was developed based on the correlation between molten pool dimensions and porosity evolution using regression analysis. Experimental validation of porosity values under different process conditions was included in PBMAM of Ti6Al4V. A close agreement was observed upon validation. Different from the previous statistical model, the presented model was developed based on the physical relationship between thermal behaviors porosity formation, which leads to an improved prediction accuracy with a small number of calibration data. Moreover, the short computational time of the closed-form temperature solution allows a fast porosity prediction for various processing window.

Experimental and numerical study of temperature field and molten pool dimensions in dissimilar thickness laser welding of Ti6Al4V alloy

Laser study is an important consideration in the present century with advances in laser technology. Titanium alloys are of great importance to the defense industry, aerospace and other industries due to its properties, including strength to weight. In this research, experimental and numerical study are investigated for laser welding on sheets of Ti6Al4V alloy with different thicknesses. Analysis of the temperature distribution around the molten pool and dimensions of the depth and width of the molten pool are performed by changing the parameters of laser such as focal length, speed of laser welding and power. The results show that the heat affected zone (HAZ) and molten pool is diverted to the thinner sheet. Also, by decreasing the focal length, the temperature of the workpiece and the dimensions of depth and width of the molten pool are increased. In addition, with enhancing the laser speed, the laser beam contact time with the workpiece surface reduces and the temperature decreases, resulting in a decrease in the dimensions of the depth and width of the molten pool. Eventually, as the power increases, the dimension of the melt pool increase and at both 180 W and 240 W powers, the thinner sheet experiences higher temperatures compared to the thicker sheet. In this study, the results of numerical simulation are matched with the experimental results and can be applied to obtain the temperature and geometry of the melt pool in other cases to reduce the cost and time.

Determination of Secondary Dendrite Arm Spacing for In-738LC Gas-Tungsten-Arc-Welds

Microstructure and defect development in the gas tungsten arc weld process is influenced by the solidification and melt-pool dynamics. Melt-pool geometrical parameters which depend mainly on heat input have profound influence on the dendrite growth velocity and growth pattern in the melt pool. Temperature magnitude and history during the process directly determine the molten pool dimensions and surface integrity. However, due to the transient nature and small size of the molten pool, the temperature gradient and the molten pool size are very challenging to measure and control. The proposed research aims to establish a methodology for characterizing direct energy deposited metals by linking processing variables to the resulting microstructure and subsequent material properties. Secondary Dendrite Arm Spacing (SDAS) optical metallographic measurements of equiaxed solidified IN-738LC gas tungsten arc welds were conducted to find a new expression that links the cooling rate that is imposed on the welding during solidification, and the resultant scale of the grain substructure.

Analytical modeling of in-process temperature in powder feed metal additive manufacturing considering heat transfer boundary condition

This paper presents a physics-based predictive model to estimate the in-process temperature in powder feed metal additive manufacturing (PFAM) based on the absolute coordinate with a stationary origin. Quasi-analytical solutions are developed without resorting to FEM or any iteration-based simulations. Heat transfer boundary condition, laser power absorption, scanning strategy, and latent heat are considered in the prediction of time-dependent thermal profiles. The temperature rise due to a moving laser heat source is predicted using a moving point heat source solution. The temperature drop due to convection and radiation at the part boundary is predicted by a heat sink solution, which is derived by modifying the heat source solution with an equivalent power for heat loss and zero velocity. The final temperature solution is constructed from the superposition of the heat source solution and the heat sink solution. Temperature profiles are predicted in multiple layers using the presented model in PFAM of Ti-6Al-4 V for a thin wall structure. Molten pool evolution is investigated with respect to laser travel distance from its starting point. The stabilized molten pool dimensions in multiple layers are obtained from predicted temperatures and agreed well with experimental measurements in the literature. In addition, the increasing molten pool dimensions were observed from predictions with increasing wall thickness, which confirms the finding reported in the literature. With benefits of high computational efficiency of the developed solution, consideration of heat transfer boundary conditions, and absolute coordinate, the presented model can be used for temperature analysis for a dimensional part in real applications.

Thermal Behavior During the Selective Laser Melting Process of Ti-6Al-4V Powder in the Point Exposure Scan Pattern

Currently, there are two main scan patterns, including the continuous exposure scan pattern and the point exposure scan pattern, during the selective laser melting (SLM) process. The point exposure scan pattern allows a three-dimensional (3-D) printer to build finer detail features as a static molten pool that is more stable than a dynamic one. However, there has been limited theoretical research on the thermal behavior characteristics during the process in the point exposure scan pattern. Therefore, in this study, the simulation of thermal behavior during SLM of Ti-6Al-4V powder in the point exposure scan pattern was performed. The temperature evolution behavior of different positions and the effects of exposure time on the temperature evolution behavior of different positions, temperature distributions, and dimensions of the molten pool were investigated. The results showed that the direct exposure position and unexposed position had significantly different temperature evolution behaviors under a given condition, and the changed exposure time had the greatest influence on the direct exposure position compared with unexposed position. Moreover, the thermal accumulation effect of a former exposure point on a later one decreased with increasing exposure time. In addition, with the increase of exposure time, the maximum temperature of the molten pool was enhanced and the surface morphology of the molten pool changed from an approximate ellipse to an approximate circle. Besides, the molten pool dimensions were found to increase with exposure time, which indicated that the exposure time played an important role in the stability of the molten pool and the metallurgical bonding in the process. Furthermore, the dimensions of the molten pool and metallurgical bonding in the cross-sectional view were obtained through experiments. Good agreement was obtained when comparing the simulated results with the experimental ones.

Numerical simulation of thermal and stress field of single track cladding in wide-beam laser cladding

This research established half of the finite element model to investigate the thermal and stress field evolution of single track cladding deposited by the wide-beam laser. The proposed FE model is validated by the molten pool temperature measured by a pyrometer. And the characteristic of the cladding shows good agreement with the temperature profile. Based on the temperature data, the molten pool length, width, and depth are calculated. Meanwhile, the effect of process parameters including laser power and scanning speed on the molten pool dimensions, temperature gradient G, cooling rate e, solidification rate R, and G/R are discussed. Three experiments under different process parameters are conducted to verify the simulation results. Furthermore, the thermal stress distribution of the cladding in different direction and different path are discussed. The results indicate that laser power and scanning speed have a significant influence on the thermal and stress field evolution.

Heat Source Modeling in Selective Laser Melting.

Selective laser melting (SLM) is an emerging additive manufacturing (AM) technology for metals. Intricate three-dimensional parts can be generated from the powder bed by selectively melting the desired location of the powders. The process is repeated for each layer until the part is built. The necessary heat is provided by a laser. Temperature magnitude and history during SLM directly determine the molten pool dimensions, thermal stress, residual stress, balling effect, and dimensional accuracy. Laser-matter interaction is a crucial physical phenomenon in the SLM process. In this paper, five different heat source models are introduced to predict the three-dimensional temperature field analytically. These models are known as steady state moving point heat source, transient moving point heat source, semi-elliptical moving heat source, double elliptical moving heat source, and uniform moving heat source. The analytical temperature model for all of the heat source models is solved using three-dimensional differential equations of heat conduction with different approaches. The steady state and transient moving heat source are solved using a separation of variables approach. However, the rest of the models are solved by employing Green’s functions. Due to the high temperature in the presence of the laser, the temperature gradient is usually high which has a substantial impact on thermal material properties. Consequently, the temperature field is predicted by considering the temperature sensitivity thermal material properties. Moreover, due to the repeated heating and cooling, the part usually undergoes several melting and solidification cycles, and this physical phenomenon is considered by modifying the heat capacity using latent heat of melting. Furthermore, the multi-layer aspect of the metal AM process is considered by incorporating the temperature history from the previous layer since the interaction of the layers have an impact on heat transfer mechanisms. The proposed temperature field models based on different heat source approaches are validated using experimental measurement of melt pool geometry from independent experimentations. A detailed explanation of the comparison of models is also provided. Moreover, the effect of process parameters on the balling effect is also discussed.

Analytical modeling of 3D temperature distribution in selective laser melting of Ti-6Al-4V considering part boundary conditions

Selective laser melting (SLM) is widely used in metallic additive manufacturing to create geometrically complex parts, in which temperature distribution directly affects thermal stress and residual stress states of the build. Finite element models were developed by researchers to predict temperature distribution of the build part with a finite volume, but they were computationally expensive. Analytical models were developed based on a semi-infinite medium assumption because a closed-form solution has not been provided to apply the boundary conditions. In this work, an original physics-based analytical model is presented to predict 3D temperature distribution in SLM with consideration of heat transfer boundary conditions so that the effects of build edges and geometries can be considered in the context of a closed-form solution. Heat input from a laser heat source is calculated using point moving heat source solution. Heat loss at part boundaries due to heat conduction, convection, and radiation is calculated by modifying the heat source solution with equivalent power loss and using zero moving velocity. The temperature distribution is obtained based upon linear heat source solution and linear heat loss solution using the superposition principle. Ti-6Al-4 V is chosen to demonstrate the capability of the presented model. Molten pool dimensions are determined by comparing predicted temperatures to material melting temperature, and then validated with documented values in references. Close agreements were observed upon validation. The presented analytical model predicts the 3D temperature distribution in SLM with boundary conditions solved by a closed-form solution for the first time.