Molten Pool Temperature Research Articles

Temperature Control of Quartz-Glass Melting Areas in Laser Additive Manufacturing.

Direct energy deposition is an additive technology that can quickly manufacture irregularly shaped quartz-glass devices. Based on this technology and coaxial laser/wire feeding, open-loop tests were conducted under different process parameters. A closed-loop temperature control system was designed and built for the molten pool temperature in quartz-glass additive manufacturing. It was based on a PID (proportional-integral-derivative) control algorithm for adjusting laser power. Changes in the macroscopic morphology, microstructure, and other qualities of the final additive result before and after the temperature control of the quartz glass were examined. Relative to constant laser powers of 120 W and 140 W, the temperature control of the multi-pass single-layer lateral additives produced dense surface microstructures of the additively produced quartz glass, and the molding quality was better.

New findings on reduced solidification brittle temperature range in epitaxially grown CMSX-4 superalloy welds via varestraint testing

This study examines the solidification brittle temperature range (BTR) of CMSX-4 superalloy welds. Based on the BTR evaluation results, we quantitatively suggest the minimum BTR value for the solidification crack-free welding of the alloy. For the BTR evaluation, a transverse-Varestraint test correlated with real-time visualization of the molten pool temperature was applied. The BTR was evaluated to be 28 K for the 10A-linear condition, 109 K for the 20A-linear, and 336 K for the 30A-linear. With the application of oscillation, the BTR expanded to 434 K (0.6Hz-oscillated). The wide variation in BTR according to the welding conditions was investigated through the characteristics of the crystal growth behavior at the solidification cracking regions. For the 10A-linear condition with the narrowest BTR of 28 K, the entire weld metal, including the solidification cracking region, exhibited perfectly epitaxial growth identical to the base metal's crystal orientation. As the BTR expanded, controversially, the solidification cracking region showed polycrystalline behavior. Consequently, the BTR in the CMSX-4 superalloy welds can be reduced, and weldability can be improved by enhancing the degree of single crystallization during welding fabrication. This improvement is attributed to the reduction of dendrite coalescence undercooling.

Effect of scanning strategy on the thermo-structural coupling field and cracking behavior during laser powder bed fusion of Ti48Al2Cr2Nb alloys

Laser powder bed fusion (LPBF) is characterized by complicated non-equilibrium processing characteristics, involving extremely high temperature gradient and cooling rate within the mesoscopic dynamic molten pool, which brings great challenges to the forming of metal materials with high crack sensitivity (such as γ-TiAl alloy). In this paper, based on the LPBF forming process of Ti48Al2Cr2Nb alloy, a corresponding multi-track and multi-layer thermal-structure sequential coupled finite element model was constructed, and the influence of four different scanning strategies on the temperature and stress evolution behavior in the powder bed forming region was quantitatively studied, and the crack initiation criterion was also established according to the relationship between nodal flow stress and equivalent stress. It was found that island scanning strategy could induce the higher molten pool temperature and attendant larger molten pool size compared with the non-island linear scanning strategy. Besides, island scanning strategy was conducive to relieving the thermal stress inside the division region but also inducing the stress concentration in the sub-island overlap region. Specially, the strip island scanning strategy exhibited the lowest residual stress and most homogeneous stress distribution. By the experimental measurements, the strip island scanning strategy contributed to the lowest crack density of 0.33mm/mm2.

Microstructure Control and Hot Cracking Prevention During Laser Additive Manufacturing of Cobalt-Based Superalloy

Hot cracking is a frequent and severe defect that occurs during laser additive manufacturing of superalloys. In this work, a pulsed-wave (PW) laser modulation process was employed to control the solidification microstructure and reduce the hot cracking susceptibility of laser additive manufactured cobalt-based superalloy. The effects of continuous-wave (CW) and PW laser processing modes on the dendrite morphology, element segregation, eutectic phase, and hot cracking of fabricated Co-based superalloys were investigated. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to characterize the microstructural characteristics of samples. A two-color pyrometer was used to measure the molten pool temperature variation under different laser processing modes. The results show that coarse columnar dendrites, chain-like eutectic carbides, and hot cracks were observed in the CW sample. In contrast, the fine equiaxed crystals, discrete eutectic carbides, and low-level residual stresses were obtained to avoid hot cracks, owing to the high cooling rate and the periodic melting and solidification of the molten pool under the PW laser processing mode. This work provides a new method for controlling solidification structure and hot cracking of laser additive manufactured Co-based superalloy.

Forming characteristic analysis in twin tungsten electrode – Wire electrode indirect arc based additive manufacturing

In this study, the forming characteristics of twin tungsten electrode – wire electrode indirect arc (TTWIA) based additive manufacturing were preliminarily investigated. Arc shape, droplet transfer behavior, molten pool temperature field, and molten pool flow state were obtained respectively. Results showed that the increase of arc current enhanced the coupling degree of TTWIA arc, and promoted the detachment of the droplet from the wire, which contributed to streaming spray transfer of the droplet. Besides, the temperature and fluidity of the molten pool both increased with the increase of the arc current due to the increasing temperature of the arc, enthalpy carried by the droplets, and driving forces acting on the molten pool. Moreover, the driving forces acting on the liquid metal in the deposited layer achieved equilibrium and a stable molten pool was acquired only when the arc current was selected appropriately, which resulted in a well forming quality of the deposited part with a lower surface roughness and a higher material utilization rate.

Effect of Welding Torch Mode on Heat Input, Microstructure, and Mechanical Performance of Inconel 625 via Wire‐Arc‐Directed Energy Deposition

Cold metal transfer‐based wire‐arc directed energy deposition offers high molding efficiency and the capability to produce large, complex structural components. However, the grain structure and mechanical performance of the deposited parts are significantly influenced by the thermal history during deposition. This study prepared Inconel 625 samples using two different torch modes (vertical and tilt). The effects of two torch modes and build heights on the molten pool temperature are investigated, and the solidification structure and mechanical performance of the parts at various locations are analyzed. Results show that the deposited samples primarily consisted of columnar dendrites growing epitaxially along the deposition direction, with Laves phase particles distributed in blocks and chains among the dendrites. Tilting the torch contributed to grain refinement and reduced Laves phase precipitates, resulting in samples with high strength. Additionally, the tensile performance of the deposited samples exhibits anisotropy.

Study on the influence of shielding effect for plasma cloud on multi-field coupling mechanism during laser cladding

Laser is a kind of electromagnetic wave. During laser cladding, metal vapor will be produced when the temperature reaches the material sublimation point, and plasma clouds will be produced after vapor atoms are ionized by laser. When the laser beam passes through the plasma cloud, it will be absorbed or refracted, resulting in the decrease of laser energy reaching the substrate surface. Plasma cloud shielding effect will directly affect the multi-field coupling behavior during laser cladding. It is significant to quantitatively reveal the influence mechanism of plasma cloud on multi-field coupling during laser cladding for improving the cladding layer quality. In the paper, a multi-field coupled numerical model for laser cladding of Fe60 with ASTM 1045 disk laser was established. The interaction between the waist beam of cladding powder jet, plasma cloud and laser beam was considered in the modeling. The influence of plasma cloud shielding effect on transient evolution for the temperature field, flow field, and stress field during laser cladding was compared and analyzed. The results show that the maximum temperature of molten pool is less than 2700 K, no plasma cloud will be produced during cladding. When the temperature is higher than 2700 K, the plasma cloud initiates. Under the same conditions, the molten pool temperature slightly decreases, the velocity decreases, and the stress increases slightly due to the shielding effect of the plasma cloud. And with the increase of laser power, the shielding effect of plasma cloud is more and more significant.

Insight into the keyhole behaviour and its role on residual stress formation in vacuum electron beam welding of TC4 titanium alloy

Electron beam welding (EBW) is the preferred technique for joining TC4 titanium alloy. Accurately characterizing the welding keyhole and residual stress distribution, along with analyzing their formation mechanisms, is of paramount importance for ensuring high reliability of the TC4 EBW structures. In this regard, a “thermal-fluid-metallurgical-mechanical” multi-field coupling numerical method was developed, which was then validated by the vital experiments. The evolution of the keyhole, molten pool temperature, phase volume fraction and residual stresses was then revealed. The influencing mechanism of keyhole on residual stress was explored comprehensively. The results show that metal vapor recoil pressure serves as the primary factor in keyhole formation, while the surface tension promotes keyhole closure. The β-phase in the weld zone undergoes a complete transformation into α′ acicular martensite. Moreover, the maximum longitudinal stress occurs at the weld center, while the transverse stress exhibits a substantial stress gradient along the thickness direction. Increasing the welding power raises the temperature of molten pool. The keyhole depth and width are also enlarged, accompanying by the increase of residual stress, which is expected to offer a theoretical foundation for managing the keyhole and residual stress generated during the EBW of TC4 titanium alloy.

Effect of auxiliary wire feeding on cold metal transfer directed energy deposition of stainless steel: A study on droplet transfer, formation characteristics, and microstructure

While traditional cold metal transfer direct energy deposition (T-CMT-DED) can be sped up by increasing the deposition current; this often negatively affects the deposited part's shape and microstructure due to increased heat input. To address this, we propose a novel approach: auxiliary wire feeding cold metal transfer direct energy deposition (AWF-CMT-DED). In this study, we analyzed the droplet transfer behavior, weld bead geometric characteristics, and stability of stainless steel printed utilizing both T-CMT-DED and AWF-CMT-DED. We observed that increasing the auxiliary wire feed speed led to a greater bead height while maintaining a constant bead width. When the auxiliary wire feed speed reached its upper limit for a given deposition current, the AWF-CMT-DED deposition rate was at least 1.46 times higher than that of T-CMT-DED at the same deposition current. Specifically, the AWF-CMT-DED mode, by reducing the molten pool's temperature gradient, facilitated the formation of fine grains, and the average grain size is 50.9 μm. Microstructural analysis indicated that the grain size decreased under the AWF-CMT-DED mode. In a word, this study illustrates that AWF-CMT-DED can achieve a higher deposition rate and lower heat input to the deposited layers, all while preserving high forming quality.

Evolutionary Mechanism of Solidification Behavior in the Melt Pool During Disk Laser Cladding with 316L Alloy

Laser cladding is an emerging environmentally friendly surface-strengthening technology. During the cladding process, the changes in molten pool temperature and velocity directly affect the solidification process and element distribution. The quantitative revelation of the directional solidification mechanism in the molten pool during the cladding process is crucial for enhancing the quality of the cladding layer. In this study, a multi-field coupling numerical model was developed to simulate the coating process of 316L powder on 45 steel matrices using a disk laser. The instantaneous evolution law of the temperature and flow fields was derived, providing input conditions for simulating microstructure evolution in the molten pool’s paste zone. The behavior characteristics of the molten pool were predicted through numerical simulation, and the microstructure evolution was simulated using the phase field method. The phase field model reveals that dendrite formation in the molten pool follows a sequence of plane crystal growth, cell crystal growth, and columnar crystal growth. The dendrites can undergo splitting to form algal structures under conditions of higher cooling rates and lower temperature gradients. The scanning speed of laser cladding (6 mm/s) has minimal impact on dendrite growth; instead, convection within the molten pool primarily influences dendrite growth and tilt and solute distribution.

Molecular dynamics research on the effect of selective laser melting parameters on porosity and properties of parts

Selective laser melting (SLM) is considered as a new production technology which has the potential to produce amorphous alloy parts. However, pores are often observed in parts prepared by SLM, which seriously affect the properties and life of the parts. In this work, an atomic model for SLM forming of Cu50Zr50 amorphous alloy is established. The effects of laser power and scanning speed on the porosity and mechanical properties of parts fabricate by SLM are researched using molecular dynamics methods. The results indicate that increasing laser power or reducing laser scanning speed could enhance the laser energy input of the molten pool and increase the molten pool temperature, which could improve the fluidity of the metal melt and prolong the existence time of the molten pool. The molten metal could fully fill the gaps between powders, which reduces the porosity and is beneficial to obtain dense parts. The tensile strength of samples formed by SLM is positively correlated with their porosity. Stress concentration occurs in the system with through holes and internal pores. Defects expand and interconnect rapidly under stress concentration, which leads to a decrease in strength and plasticity. The porosity could be reduced by adjusting the process parameters, thus improving the mechanical properties of the parts. This study provides a theoretical reference for a deeper understanding of the formation mechanism of SLM pores and the development of advanced alloys with high density.

Thermal behavior of coated powder during directed energy deposition (DED)

In powder-based additive manufacturing (AM), the quality of the feedstock material is critical for obtaining enhanced mechanical properties. Recently, the application of coated powders during directed energy deposition (DED) has been prompted by the goal of fabricating composite and functional materials in-situ. The complex temperature and momentum fields established during DED render direct experimental characterization of coated powder behavior challenging. To address this challenge, this study reports on the thermal behavior of coated powders during interactions with the molten pool by constructing three-dimensional heat transfer and phase distribution models using the finite elements method (FEM). Transient temperature and phase distributions were calculated for coated and uncoated stainless steel 316L and ZnAl powders under various particle size, coating thickness, molten pool temperature, and coating material conditions. Particle residence time values were extracted from the calculations, defined as time spent by the particle before a phase change. The results show large variations in particle residence time (85 μs to 2670 μs for stainless steel 316L particles, and 48 μs to infinity for ZnAl particles) as a function of the variables considered, especially the thermal diffusivity of the coating materials, thereby highlighting the potential value of coatings as an additional design parameter in DED. Significant increases in particle residence time for both stainless steel 316L and ZnAl particles were found when contact angle increases from 0° (submergence regime) to 180° (floating regime).

Numerical simulation and experimental research of molten pool flow field and temperature field by hollow-laser direct energy deposition

Hollow-laser Direct Energy Deposition (HL-DED) significantly surpasses traditional Laser Directed Energy Deposition (L-DED) methods, marked by its superior adjustability of energy density, exacting control of energy distribution, and versatility in creating cladding layers of varied geometries. This novel technique ensures a markedly uniform and stable melt path. In this paper, the HL-DED process was systematically analyzed by combining numerical simulation and experimental research. A novel three-dimensional transient computational fluid dynamics (CFD) simulation model is established based on the Flow 3D software to study the flow and heat and mass transfer behaviors. At the optimized processing parameters, the single-track and multi-track cladding layer processes were carried out respectively. Results show that the model can well predict and explain the flow of the molten pool and the morphology of the cladding layer in the HL-DED process. In the initial stage of single-track cladding, the molten pool exhibits a symmetrical bimodal distribution configuration characterized by the peak flow rate of 0.09 m/s, with temperatures being higher at the periphery and lower at the center. In the stabilization phase, the dynamics of the molten pool are primarily influenced by gas-powder momentum transfer, showing increased peak flow rates up to 0.26 m/s. Concurrently, the peripheral temperatures rise from 2001 K to 2122 K (0.06 % increase), but central temperatures ascend from 1400 K to 1800 K (0.29 % increase). The flow and temperature distribution patterns in multi-track cladding closely mirror those observed in single-track layers, maintaining a symmetrical bimodal temperature distribution with peak flow rates reaching 0.105 m/s. The temperature distribution evolves from a unimodal to a bimodal pattern, with the left peak approximately 100 K higher than the right, before reverting to a unimodal configuration. Simulation outcomes closely align with experimental findings, offering valuable insights for refining the experimental adjustment of the internal optical coaxial powder feeding process.

A Meshless Method of Radial Basis Function-Finite Difference Approach to 3-Dimensional Numerical Simulation on Selective Laser Melting Process

Selective laser melting (SLM) is a rapidly evolving technology that requires extensive knowledge and management for broader industrial adoption due to the complexity of phenomena involved. The selection of parameters and numerical analysis for the SLM process are both costly and time-consuming. In this paper, a three-dimensional radial basis function-finite difference (RBF-FD) meshless model is introduced to accurately and efficiently simulate the molten pool size and temperature distribution during the SLM process for austenitic stainless steel (AISI 316L). Two different volumetric moving heat source models were presented, namely the ray-tracing method heat source model and the double-ellipsoidal shape heat source model. The temperature-dependent material properties and phase change process were also considered, based on experiments and effective models. Results of the model for the molten pool size were validated with those of the literature. The proposed approach can be used to predict the effect of different laser power and scan speed on the molten pool size and temperature gradient along the depth direction. The result reveals that the depth of the molten pool is more sensitive to laser power than scan speed. Under the same scan speed, a 22% change in laser power (45 ± 10 W) affects the maximum temperature proportionally by about 9%. The developed algorithm is computationally efficient and suitable for industrial applications.

Multiscale Simulation of Laser-Based Direct Energy Deposition (DED-LB/M) Using Powder Feedstock for Surface Repair of Aluminum Alloy.

Laser-based direct energy deposition (DED-LB/M) has been a promising option for the surface repair of structural aluminum alloys due to the advantages it offers, including a small heat-affected zone, high forming accuracy, and adjustable deposition materials. However, the unequal powder particle size during powder-based DED-LB/M can cause unstable flow and an uneven material flow rate per unit of time, resulting in defects such as pores, uneven deposition layers, and cracks. This paper presents a multiscale, multiphysics numerical model to investigate the underlying mechanism during the powder-based DED-LB/M surface repair process. First, the worn surfaces of aluminum alloy components with different flaw shapes and sizes were characterized and modeled. The fluid flow of the molten pool during material deposition on the worn surfaces was then investigated using a model that coupled the mesoscale discrete element method (DEM) and the finite volume method (FVM). The effect of flaw size and powder supply quantity on the evolution of the molten pool temperature, morphology, and dynamics was evaluated. The rapid heat transfer and variation in thermal stress during the multilayer DED-LB/M process were further illustrated using a macroscale thermomechanical model. The maximum stress was observed and compared with the yield stress of the adopted material, and no relative sliding was observed between deposited layers and substrate components.

Thermal-mechanical evolution behaviors of metal matrix diamond composite by powder bed fusion-laser beam

The thermal damage of diamond and the residual stress have always been the critical issues to be resolved in the development of metal matrix diamond composites. In the presented study, FeCoCrNi high entropy alloys (HEAs)-diamond composites were fabricated by powder bed fusion-laser beam of metals (PBF-LB/M), using un-coated diamond and Ti–Ni coated diamond, respectively. Based on the numerical and experimental analysis, the effect of thermal-mechanical behavior on the microstructures and mechanical properties of the composites was systematically investigated. By establishing the temperature field of the PBF-LB molten pool, the defects, diamond graphitization and in-situ TiC formation were studied. The start-temperature for diamond graphitization in coated samples was 365 °C higher than that of un-coated samples, due to the diffusion barrier effect of TiC. Subsequently, by thermo-mechanical coupling, the property and value of the residual stress exhibited sudden change at the diamond/matrix interface. The TiC intermediate layer significantly relieved the stress concentration at interface, and effectively avoided cleavage fracture of the diamond and the initiation of interfacial cracks. At the moderate molten-pool temperature, the coated diamond composites exhibited optimal performance, as bending strength of 913 MPa, friction coefficient of 0.25 and worn mass loss of 0.67 mg. Finally, the quantitative relationship between PBF-LB molten-pool temperature and the relative density, graphitization degree, residual stress, retention(bending strength) and wear performance was established, to demonstrate the thermal-mechanical evolution of the HEAs-diamond composites by PBF-LB.

Effect of adaptive control of cooling rate on microstructure and mechanical properties of laser additively manufactured Inconel 718 thin walls

Ni-alloy Inconel 718 (IN718) is finding ever-increasing applications in various industries like aviation and aerospace. IN718 is a precipitation-strengthened alloy which derives its strength mainly from the intermetallic phases, γ” (Ni3Nb) along with the γ’ (Ni3(Al, Ti)) phase which can be precipitated using standard heat treatment schedules. However, the consummation of Nb in Laves phase formation during solidification tends to deteriorate the strength of the material. During laser additive manufacturing (LAM) by directed energy deposition (DED), the cooling rate variation along the build direction causes variations in the microstructure, properties and Laves phase distribution. The study aims to minimize the heterogeneities along the build direction by monitoring and controlling the cooling rate during DED of IN718. The feedback control system involves the use of two pyrometers in tandem with a leading pyrometer measuring the molten pool temperature and a trailing pyrometer measuring the temperature at a distance behind the molten pool. Two sets of thin walls of 120 layers were fabricated with one set being of constant process parameters without feedback control and the other set being of adaptive-controlled process parameters. The adaptive control was aimed at maintaining the cooling rates above a desired limit by changing the process parameters, namely the laser scan speed and the powder feed rate. The Laves phase morphology, which affects the strength of the deposited part deleteriously was finer and more discrete in the adaptive-controlled walls which led to their easier dissolution during heat treatment. This in turn helped in the precipitation of more amount of strengthening phases during heat treatment which led to better mechanical properties. The characterizations of the two sets of walls reflected the superior part quality of the adaptive-controlled wall in terms of homogeneity of microstructure, enhanced microhardness and tensile strength, thereby validating the importance of adaptive control of cooling rate in ensuring better quality in LAM parts.

Experimental and Thermal Stress Field Numerical Simulation Study on Laser Metal Deposition of Ti-48Al-2Cr-2Nb Alloy.

The experimental and numerical simulation analysis of a TiAl alloy by laser metal deposition technology is presented in this paper. The research examines the macroscopic morphology, microstructure, and mechanical properties of samples as laser power varies. It also delves into how the temperature field and residual stress evolve under different laser powers. The results reveal that the microstructure of samples is mainly composed of α2-Ti3Al phase and a γ-TiAl phase and that the details of the microstructure are significantly affected by laser power. As laser power increases, coarse lamellar structure content increases, corresponding to a decrease in α2 phase content. The deposited layer hardness ranges from 550 HV to 600 HV, and the average deposition layer hardness decreases with increased laser power. Simulation results predict the molten pool's size, temperature, and residual stresses. A significant increase in the molten pool size is observed when the laser power exceeds 1000 W, and the measured molten pool depths correspond closely to simulation predictions. However, significant tensile stresses are generated in the deposition layer due to high cooling rates, mainly in the x direction. Cracks are observed on the surface of the deposition layer at all laser powers.

Numerical analysis and process optimization of electron beam melting for superalloys

This study aims to determine the optimal parameters for the electron beam melting process by employing a modified transient three-dimensional thermal model. The validity of the thermal model is confirmed through experimental results. Moreover, the proposed method establishes mathematical relationships between the resulting molten pool temperature and process parameters. A broader range of process parameters for a new Ni–Co-based superalloy was achieved by investigating the effects of scanning velocities (10–100 mm/s), scanning radius (20–60 mm), and melting mass (1500–3100 g) on temperature at various electron beam power levels (10–30 kW). Regression models were established between scanning velocities-temperature, and scanning radius-temperature under different electron beam power levels. These models laid the foundation for real-time automatic control of molten pool temperature. Significantly, orthogonal experimental design and response surface method are employed to conduct a multi-objective optimization based on sixteen experiments and three parameters. The results of multi-objective optimization indicate that with a smelting mass of 1.5 kg, an electron beam power of 11 kW, a scanning radius of 60 mm, and a scanning velocity of 75 mm/s, the energy consumption is 34.95 kW h, and the theoretical mass loss of Ni element is 0.002 kg. The desirability is 91.2 %.

Study on improvement of weld defect in oscillating laser welding of aluminum alloy T-joints assisted by solder patch

During oscillating laser welding of aluminum alloy T-joints assisted by solder patch, it is found that the pore defect can be reduced and the high quality welded T-joints are obtained. To understand the effect of oscillating laser on the formation process of T-joint welds, a dynamic model for laser welding of aluminum alloy T-joints assisted by solder patch is developed in this paper to analyze the dynamic behaviors of keyhole and molten pool. Based on the dynamic model, the morphology characteristics of keyhole and the evolution processes of molten pool temperature and flow fields during conventional laser welding and oscillating laser welding are calculated and compared. The simulation results are found to be kept consistent with the experimental results. It is demonstrated that the keyhole is characterized with better morphology and higher stability and the degassing condition of molten pool is improved during oscillating laser welding compared with conventional laser welding. The molten pool temperature and flow fields are much more uniform during oscillating laser welding. The obtained results are efficient for suppressing weld defects and achieving high quality welding of aluminum alloy T-joints.