Laser Melting Deposition Process Research Articles

Printed layers height calibration curve and porosity in laser melting deposition of Ti6Al4V combining experiments, mathematical modelling and deep neural network

PurposePorosity is a commonly analyzed defect in the laser-based additive manufacturing processes owing to the enormous thermal gradient caused by repeated melting and solidification. Currently, the porosity estimation is limited to powder bed fusion. The porosity estimation needs to be explored in the laser melting deposition (LMD) process, particularly analytical models that provide cost- and time-effective solutions compared to finite element analysis. For this purpose, this study aims to formulate two mathematical models for deposited layer dimensions and corresponding porosity in the LMD process.Design/methodology/approachIn this study, analytical models have been proposed. Initially, deposited layer dimensions, including layer height, width and depth, were calculated based on the operating parameters. These outputs were introduced in the second model to estimate the part porosity. The models were validated with experimental data for Ti6Al4V depositions on Ti6Al4V substrate. A calibration curve (CC) was also developed for Ti6Al4V material and characterized using X-ray computed tomography. The models were also validated with the experimental results adopted from literature. The validated models were linked with the deep neural network (DNN) for its training and testing using a total of 6,703 computations with 1,500 iterations. Here, laser power, laser scanning speed and powder feeding rate were selected inputs, whereas porosity was set as an output.FindingsThe computations indicate that owing to the simultaneous inclusion of powder particulates, the powder elements use a substantial percentage of the laser beam energy for their melting, resulting in laser beam energy attenuation and reducing thermal value at the substrate. The primary operating parameters are directly correlated with the number of layers and total height in CC. Through X-ray computed tomography analyses, the number of layers showed a straightforward correlation with mean sphericity, while a converse relation was identified with the number, mean volume and mean diameter of pores. DNN and analytical models showed 2%–3% and 7%–9% mean absolute deviations, respectively, compared to the experimental results.Originality/valueThis research provides a unique solution for LMD porosity estimation by linking the developed analytical computational models with artificial neural networking. The presented framework predicts the porosity in the LMD-ed parts efficiently.

Thermal-fluid behavior and microstructure morphology during laser melting deposition of TiC/Ti6Al4V functionally graded materials

Laser melting deposition (LMD) is an additive manufacturing technology that involves complex thermodynamic and kinetic characteristics in the molten pool. A perspective on the thermal-fluid dynamic behavior in the molten pool is essential for investigating the microstructure morphology and solidification characterization. In this study, a three-dimensional transient thermal-fluid coupling model is proposed for describing thermal-fluid dynamic behavior within the molten pool during laser melting deposition of TiC/Ti6Al4V functionally graded materials. The simulated temperature distribution and deposition layer morphology agree well with the experimental results. The simulation results indicate that the heat accumulation phenomenon generated during the LMD process can exacerbate the penetration effect of the molten pool. The solidified deposition layer will be remelted to form a large cross-layer molten pool, and the molten metal in the remelted zone will move from the lower layer to the upper layer driven by the Marangoni convection. Furthermore, the unmelted TiC particles and the in-situ generated TiC particles can decrease the free energy of the molten pool, induce heterogeneous nucleation, and promote the growth of equiaxed crystals.

Microstructure distribution characteristics of LMD-WAAM hybrid manufacturing Ti-6Al-4V alloy

Laser melting deposition (LMD) and Wire arc additive manufacturing (WAAM) are representative metal additive manufacturing (AM) technologies. To explore the feasibility of using the LMD-WAAM hybrid process to fabricate large and complex components, a preliminary investigation of Ti6Al4V was conducted. In this study, a WAAMed sample was used as a substrate for the LMD process at different laser powers. The effects of laser power on the grain evolution and mechanical properties of the Ti6Al4V alloy were studied. The results indicated that different thermal histories formed a graded microstructure from the LMD to the WAAM zones. The Electron backscatter diffraction texture intensity of the α phase gradually increased from the remelting zone (RZ) to the top-LMD zone and from the WAAM zone to the heat-affected zone. In the LMD zone, the length of the acicular α decreased and the lamellar α content increased with increasing laser power. Ductile fracture characteristics were apparent with few cleavage fractures and pore defects, which led to different fracture locations. Further, the microhardness of the interface zone exhibited a peak because the microstructure of the RZ was fine, leading to a higher microhardness.

Cracking inhibition and strengthening of FeCrAlY alloy through addition of TiC nanoparticles during laser melting deposition

FeCrAlY alloy is one ideal candidate for nuclear fuel cladding. Laser melting deposition (LMD) provides a new method for its preparation. To solve the cracking problem as well as to strengthen the alloy, TiC nanoparticles were added during the LMD process. The results show that 4 wt% TiC addition can fully suppress crackings by prompting columnar to equiaxed grain transition. When TiC addition is 6 wt%, the alloy has the finest average grain size of 17 μm, which is nearly 13 times smaller than that without TiC addition. TiC addition also prompts the escape of gas in the molten pool, thus reduces the porosity of the alloys. Without TiC addition, the alloy contains Y-rich heterogeneous structured particles, including Y–O, Y–O–Al–Cr, and Y–O–Al–Cr–Fe particles. After adding TiC, other Y–O–Al–Cr, Y–Al, and C–O–Cr–Fe particles can also be observed. TiC particles mainly have flower-like/granule and plate-like shapes, which correspond to primary and eutectic particles, respectively. With 8 wt% TiC addition, the alloy has good mechanical properties with yield strength of 488 ± 25 MPa, ultimate tensile strength of 749 ± 47 MPa, and an elongation of 15 ± 2%, which are close to those prepared by traditional thermo-mechanically deformation processes.

The microstructure and mechanical properties of 316L austenitic stainless steel prepared by forge and laser melting deposition

In this study, the 316L austenitic stainless steel blocks were prepared by forge and laser melting deposition (LMD). To clarify the effect of deposition strategy on microstructure and mechanical properties of 316L samples, three different deposition strategies (30°, 67° and 90°) are selected. The results showed that the molten pool of LMDed sample consists of a large number of coarse columnar grains and a few irregular equiaxed grains, which contains plentiful cellular and fiber sub-structures with an average size of ∼4 μm. The LMD process can significantly improve the comprehensive mechanical properties perpendicular to the build direction of 316L austenitic stainless steel, which was mainly attributed to sub-structural strengthening, but also slightly due to the local misorientation and texture distribution. The yield strength and ultimate tensile strength of R90 sample increased to 484.0 MPa and 627.6 MPa from 305.3 MPa to 601.4 MPa of as-forged 316L, and its elongation has a negligible reduction. The sub-structures produced during solidification have the same fine grain strengthening effect as the high-angle grain boundary, and the yield strength and average size of sub-structures conform to the Hall-Petch relationship. The deterioration of the plastic deformation capacity of the R67 sample is attributed to the larger thermal stress and texture distribution with a certain hard orientation. The plastic deformation of as-forged sample is mainly coordinated by multi-system slip, while twinning and slip are activated together during deformation of LMDed 316L austenitic stainless steel. The grain boundary map indicated that the strain location preferentially appears along cell wall of sub-structures, which promotes the formation of sub-grain boundaries during deformation of LMDed samples.

Novel Assessment Methodology for Laser Metal Deposition of New Metallic Alloys.

Metal additive manufacturing technologies are gaining great interest. However, the existing metallic alloys are generally formulated for conventional manufacturing processes. Thus, it is necessary to adapt their chemical composition or develop new alloys for the manufacturing conditions of additive manufacturing processes. The main method for manufacturing metal powder is gas atomization, but it is very expensive with long manufacturing times. Therefore, it is necessary to develop alloy validation methods that simplify the development process of new alloys. This paper deals with a methodology based on thermodynamic heat transfer equations, simulation, and powderless tests. This novel methodology enabled the determination of the optimal conditions for the laser melting deposition process of the commercial AA7075 alloy with a reduced number of experimental tests with powder, reducing the difficulties inherent to powder processing. The developed process was divided into two stages. In the first stage, the heating of the substrate was studied. In the second stage, the depositions of single tracks were validated with the parameters extrapolated from the previous stage. Hence, it was possible to manufacture single tracks free of cracks with an adequate aspect ratio.

Beneficial effects of deep cryogenic treatment on mechanical properties of additively manufactured high entropy alloy: cyclic vs single cryogenic cooling

• Cyclic deep cryogenic treatment was utilized to homogenize the residual stress and tune the microstructure of a high entropy alloy fabricated by laser melting deposition, in order to improve its overall mechanical properties. • Cyclic deep cryogenic treatment can induce an increased compressive residual stress and introduce various types of reinforcing defect microstructures into a high entropy alloy, including dense dislocations, intersecting twins, transformed HCP phase and nanograins, thus effectively overcoming the strength-ductility trade-off. • Compared to single-step DCT, cyclic DCT can reduce the residual stress gradients in the LMD-built sample, relieving or removing the tensile residual stress at the top surface. • Transformation to HCP phase can occur during the cyclic DCT process due to the cyclic reverting of the thermal stress to a high level on each cryogenic cooling, which promotes twinning on secondary habit planes and more frequent faulting. Additively manufactured (AM) metallic materials commonly possess substantial tensile surface residual stress, which is detrimental to the load-bearing service behavior. Recently, we demonstrated that deep cryogenic treatment (DCT) is an effective method for improving the tensile properties of CoCrFeMnNi high-entropy alloy (HEA) samples fabricated by laser melting deposition (LMD), by introducing high compressive residual stress and deformation microstructures without destroying the AM shape. However, carrying out the DCT in a single-step mode does not improve the residual stress gradients inherent from the LMD process, which are undesirable as the mechanical properties will not be homogeneous within the sample. In this work, we show that carrying out the DCT in a cyclic mode with repeated cryogenic cooling and reheating can significantly homogenize the residual stress in LMD-fabricated CoCrFeMnNi HEA, and improve tensile strength and ductility, compared with single-step DCT of the same cryogenic soaking duration. Under cyclic DCT, the thermal stress is re-elevated to a high value at each cryogenic cooling step, leading to the formation of denser and more intersecting reinforcing crystalline defects and hcp phase transformation, compared to single-step DCT of the same total cryogenic soaking duration in which the thermal stress relaxes towards a low value over time. The enhancement of defect formation in the cyclic mode of DCT also leads to more uniform residual stress distribution in the sample after the DCT. The results here provide important insights on optimizing DCT processes for post-fabrication improvement of mechanical properties of AM metallic net shapes.

316L/Ti6Al4V bimetallic structure with Ni interlayer fabricated by laser melting deposition

The 316L/Ti6Al4V bimetallic structure (BS) was fabricated by laser melting deposition (LMD) via introducing a Ni interlayer. The results showed that the addition of Ni interlayer was effective in preventing the formation of brittle Ti-Fe intermetallic compounds. Good metallurgical bonds of 316L/Ni and Ni/Ti6Al4V were obtained, and a certain amount of reactions occurred between the Ni interlayer and Ti6Al4V part. The reaction layer consisting of TiNi3, TiNi and Ti2Ni was formed in the Ni/Ti6Al4V interface, and the thicknesses of the TiNi3, TiNi and Ti2Ni layers were about 20 μm, 70 μm, and 60 μm, respectively. The average ultimate tensile strength of the 316L/Ti6Al4V BS with Ni interlayer was 181.6 MPa, while the 316L/Ti6Al4V BS without Ni interlayer fractured during the LMD process.

Grain-based morphological simulation via fractal theory with experimental verification and corresponding optical properties in laser melting deposition additive manufacturing: A demystified approach

Laser melting deposition (LMD) stands for a challenging and promising 3D printing technology to fabricate near-net-shape parts. In this study, a 3D modified fractal model (3DMFM) has been developed by considering the surface texture orientation in the technical manufactured surfaces. Fractal parameters, including fractal dimensions and periodic lengths in the 3DMFM, have been solved for the grain-based morphologies developed by LMD. In combination with statistical parameters, these fractal parameters were applied in 3DMFM for analytical simulations. Single-track depositions of AISI 304 stainless steel powder on AISI 304 stainless steel substrate were subjected to scanning electron microscopy investigations for experimental validation of the proposed model. A customized investigational setup, having a He-Ne laser system (632.8 nm and 7.75 mW), was designed to identify the optical properties of each deposited layer. Three types of grains were identified: round quasi-continuous (RQC), elongated lath-shaped (ELS), and a combination of the two. The RQC grains were transformed into ELS grains with an increment in laser scanning speed. The RQC and ELS combination was converted into RQC grains with an increase in the powder feeding rate. RQC changed into ELS grains whenever the laser power was elevated. An increase in skewness and a decrease in kurtosis were observed whenever laser scanning speed or powder feeding rate was increased. A laser power amplification also determined a rise of skewness and a reduction in the kurtosis values. A rise in either laser scanning speed or powder feeding rate lowered the surface roughness, while an inverse correlation was observed between laser power and surface roughness. An increase in laser scanning speed or laser power increased the fractal dimensions (Dx and Dy). However, the powder feeding rate was inversely correlated with Dx and Dy. In the case of periodic lengths (Lx and Ly), the scanning speed, powder feeding rate, and laser power exhibited an inverse behavior with Dx and Dy. Larger magnitudes of Dx or Dy were resulted due to an increased optical reflection or a decreased optical absorption, while the opposite results were attained in the case of periodic lengths inclination (Lx and Ly). The developed fractal model can predict and select the best LMD process parameters in terms of quality control and time/cost-effectiveness.

Temperature variation and mass transport simulations of invar alloy during continuous-wave laser melting deposition

Continuous-wave laser melting deposition (LMD) offers several advantages, such as higher cooling rate, finer microstructures and improved mechanical properties over traditional processing technology. The fluid flow in the molten pool affects the solute distribution and the microstructure morphology when invar alloy mould is repaired with laser melting deposition. However, the effects of laser on the molten pool motion, thermal field and microstructure of invar alloy are not distinct. In this work, a three-dimensional model utilizing the volume of fluid method was developed to simulate the transient molten pool motion, heat transfer and fluid flow for the laser melting deposition process. The interaction among powder, laser and molten pool in LMD process was comprehensively considered in the model. The laser melting deposition with Nano-WC powder particles as a dispersoid was investigated. The simulated molten pool geometry was compared with experimental results. Moreover, temperature variation, fluid flow and their influence on the microstructure were analyzed. It is concluded that a significant Marangoni flow is observed with the highest velocity in the middle of the molten pool but lower in the surface and bottom of the molten pool. The lower velocity of fluid flow results in an element accumulation in the bottom of the molten pool. This study provides a referenced value for revealing the flow mechanism of molten pool in the LMD process of invar alloy.

Nanosized SiC particle reinforced Ti6Al4V matrix composites manufactured by laser melting deposition

In this work, in-situ (TiC+Ti5Si3)/Ti6Al4V matrix composites (TMCs) with enhanced hardness and wear performance were manufactured by laser melting deposition (LMD) via the addition of nanosized SiC powder. The microstructures of single tracks were investigated to determine the optimal parameters for LMD. Besides, the influence of the SiC content on the microstructural evolutions and mechanical properties was explored. Results demonstrate that the in-situ reaction occurs between SiC and titanium during the LMD process. In-situ TiC and Ti5Si3 were distributed uniformly at grain boundaries and titanium matrix with a full-columnar grains for 0.5 wt.% and 1.0 wt.% SiC addition. The morphology of the composites consists of equiaxed grains with a three-dimensional quasi-continuous network structure when the content of SiC increases to 1.5 wt.% and 3.0 wt.%. The hardness of the composites is 321.1, 342.3, 347.8, and 442.1 Hv with the increasing formation of reinforcements. Meanwhile, the average coefficient of friction decreases from 0.39 to 0.32 with the wear mechanism changing from abrasion to adhesive wear.

Microstructure and strengthening mechanisms of tantalum prepared using laser melting deposition

Laser melting deposition (LMD) additive manufacturing process has been attractive to the fabrications of pure tantalum due to the advantage of fast near net shaping of complex shaped components. In the present study, the LMD process has been applied to the fabrication of tantalum using two kinds of powders with different interstitial contents. The microstructure, chemistry, and mechanical properties of the LMD samples have been investigated to understand the strengthening mechanisms. The results show that the tantalum samples have near full dense, and large columnar grains with the growth direction slightly deviated from the deposition direction. The LMD tantalum can have high hardness of up to 300 HV, mainly due to the solid solution strengthening of nitrogen and oxygen interstitials. With relative low interstitial contents of about 400 ppm, the LMD tantalum can have ductile properties, showing ultimate tensile strength above 400 MPa, and elongations of about 14–16%, which are comparable to the properties of cold-worked tantalum. The present work suggests the LMD tantalum could have potential usages in the industry fields.

Laser Melting Deposition Additive Manufacturing of Ti6Al4V Biomedical Alloy: Mesoscopic In-Situ Flow Field Mapping via Computational Fluid Dynamics and Analytical Modelling with Empirical Testing.

Laser melting deposition (LMD) has recently gained attention from the industrial sectors due to producing near-net-shape parts and repairing worn-out components. However, LMD remained unexplored concerning the melt pool dynamics and fluid flow analysis. In this study, computational fluid dynamics (CFD) and analytical models have been developed. The concepts of the volume of fluid and discrete element modeling were used for computational fluid dynamics (CFD) simulations. Furthermore, a simplified mathematical model was devised for single-layer deposition with a laser beam attenuation ratio inherent to the LMD process. Both models were validated with the experimental results of Ti6Al4V alloy single track depositions on Ti6Al4V substrate. A close correlation has been found between experiments and modelling with a few deviations. In addition, a mechanism for tracking the melt flow and involved forces was devised. It was simulated that the LMD involves conduction-mode melt flow only due to the coaxial addition of powder particles. In front of the laser beam, the melt pool showed a clockwise vortex, while at the back of the laser spot location, it adopted an anti-clockwise vortex. During printing, a few partially melted particles tried to enter into the molten pool, causing splashing within the melt material. The melting regime, mushy area (solid + liquid mixture) and solidified region were determined after layer deposition. This research gives an in-depth insight into the melt flow dynamics in the context of LMD printing.

Efficient distortion predictions of high-performance steel alloy parts fabricated by pragmatic deposition strategies in laser melting deposition

For the prediction of residual distortions of high-performance steel alloy parts induced by laser melting deposition (LMD), two efficient simulation methods are developed, which are the improved thermal-mechanical coupled method and the multiscale method. In the improved thermal-mechanical coupled method, the detailed loading process of the moving laser heat source is omitted, and the deposited coarse blocks are used to divide the LMD parts, which can improve the efficiency of mechanism exploration for the LMD process. Based on the eight-layer line deposition model, the length range of the deposited coarse blocks that can improve the computational efficiency under the premise of accuracy is explored, which is 5–8 mm. The distortion of the substrate after printing two-layer multipath part by pragmatic deposition strategies is accurately predicted by this method, which is consistent with the experimental measurement. The deviation between the predicted maximum distortion and the actual value is only 0.8%. Furthermore, a multiscale method based on the inherent strain method is also adopted. The two simulation methods are used to predict the distortion of the substrate after depositing pragmatic square frame structure. The deviations of the maximum distortion predicted by the improved thermal-mechanical coupled method and the multiscale method are 5.2% and 4.3%, respectively. The time-consuming of the multiscale method is only 2.8% of the former. For the direct prediction of distortion results of large-sized LMD parts in engineering, the advantages of the multiscale method are significant.

Effect of heat accumulation on the microstructure of Invar alloy manufactured by multi-layer multi-pass laser melting deposition

Invar alloy has an important application in the manufacture of composite forming mould. The laser melting deposition (LMD) technology was utilized to fabricate multi-layer multi-pass deposited samples in this study. The effect of heat accumulation on the microstructure of deposition layer (DL) was investigated through experimental and simulation method. A finite element (FE) model was established to study the temperature field distribution in LMD process. Microstructure of DL was investigated by optical microscope (OM), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). The thermal cycle curves were extracted for analysis. The accumulation of heat during the deposition process would make the higher location DL reach a higher peak temperature. It could be seen from the observation of the transverse section of the deposited samples that the microstructure consisted of elongated cellular crystals and regular cellular crystals. At the bottom of the DL were columnar crystals grown epitaxially along the maximum temperature gradient and gradually transitioned to equiaxed crystals at the top of the DL. EDS detection results showed that there was no element aggregation phenomenon in the DL. The grains in the remelted zone were coarsened due to suffering a state of overheating.

Efficient Simulation Methods for Thermal‐Mechanical Coupled Analysis and Rapid Residual Distortion Prediction of Laser Melting Deposition Process

Herein, an improved thermal‐mechanical coupled analysis is used to increase the calculation efficiency of the detailed laser melting deposition (LMD) process. By optimizing the loading process of the laser heat source and lumping several deposited layers into an equivalent numerical layer, the efficiency is significantly improved. The calculation time is saved by 85% when lumping six layers. Compared with the experimental data, the minimum deviation of predicted maximum distortion reaches 3.2% when lumping four layers. A multiscale method is proposed to predict the distortion result of the large‐sized part efficiently. In this method, the optimal heights of equivalent numerical layers are explored. The comparisons with the experimental result show that the optimal heights of equivalent numerical layers are 1 and 1.5 mm in view of accuracy and efficiency. Based on the multiscale method, the residual distortion of the substrate after depositing a large‐sized eight‐layer line deposition structure is accurately predicted. The deviations of the maximum distortion predicted by the improved thermal‐mechanical coupled simulation and the multiscale method are 3.26% and 6.12%, respectively. Moreover, the calculation time of the latter is 98.8% less than that of the former.

Microstructure and mechanical properties of TiN particles strengthened 316L steel prepared by laser melting deposition process

Large columnar grains of 316L stainless steel prepared by laser melting deposition (LMD) additive manufacturing process lead to low yield strength and anisotropic mechanical properties, therefore limit its usage. In this study, different contents of nanoscale TiN particles (0 wt%, 2 wt%, 4 wt% and 6 wt%) have been added into the LMD process of 316L steel to promote heterogeneous nucleations of grains, and to improve the strength. The results show that the TiN particles additions can refine the grain sizes significantly, and prompt columnar to equiaxed transitions of grains. The grain sizes can be refined more than 20 times after adding 2 wt% TiN particles. The strength of the 316L-based composites increases with the increase of TiN addition, however, at the expense of ductility. With 2 wt% TiN particle addition, the 316L-based composite can have balanced mechanical properties with high yield strength of 323–355 MPa, high tensile strength of 629–640 MPa, and good elongations of 26–30%. The strength improvements are quantitatively analyzed considering the Hall-Petch strengthening, Orowan strengthening, thermal mismatch strengthening, and load transfer strengthening. The calculations agree well with the experimental values, and reveal that the grain refinement strengthening is the main strengthening mechanism. The present study provides an important basis for fabricating high strength 316L-based composites using the LMD process. • TiN strengthened 316L-based composites are fabricated by laser melting deposition. • Remarkable grain refinement and strength improvement are obtained. • The strength mechanisms of the 316L-based composites are quantitatively analyzed.

Efficient Distortion Prediction of Laser Melting Deposited Industrial-scale Parts Using Modified Inherent Strain Method

As a result of the large temperature gradients, residual distortion becomes a major technical challenge for laser powder-based Laser Melting Deposition (LMD) process, since excessive distortion may cause building failure, cracks and loss in structural integrity. In this paper, the theory of a modified inherent strain method is firstly developed. On the basic of this method, the prediction results of a small-scale model are compared with those obtained from the traditional thermo-mechanical coupled analysis. The deviation of the maximum distortion predicted by the modified inherent strain method is 7.72%, and the average deviation of the distortion of the substrate’s free end is 8.73%, demonstrating a good accuracy (< 10% error) of the modified method for predicting the residual distortion of the LMD components. Compared to a small loss of accuracy, the calculation time is significantly shortened by the modified inherent strain method. After verification, the distortion prediction of industrial-scale camshaft model during LMD process is realized by using the proposed method. It is found that the maximum distortion is 3.18mm, located at both sides of the camshaft. After the support is removed, the two ends of the camshaft are warped upward, while the position of the maximum distortion doesn’t change, but the value increased to 4.62mm. Moreover, the whole analysis takes only 3300 seconds, which is extremely significant for the distortion prediction for industrial-scale parts fabricated by LMD.

The relationship between thermo-mechanical history, microstructure and mechanical properties in additively manufactured CoCrFeMnNi high entropy alloy

Here, a single-track CoCrFeMnNi high entropy alloy (HEA) was successfully fabricated by laser melting deposition (LMD). Combining the experimental observations and numerical simulation, the microstructure and mechanical properties of the as-deposited parts were systematically studied from the perspective of thermo-mechanical history experienced during the LMD process. The strengthening mechanisms of the LMDed CoCrFeMnNi HEA parts were clarified. The frictional stress strengthening, grain boundary strengthening and dislocation strengthening contributed the whole yield strength of the parts. Dislocation strengthening dominated the strengthening mechanism. It was expected that the establishment of the relationship between thermo-mechanical history, microstructure and mechanical properties of the LMDed CoCrFeMnNi HEA could shed more insights into achieving HEA parts with the desired microstructure and high performance.

Columnar to equiaxed transition in additively manufactured CoCrFeMnNi high entropy alloy

Here, a single-track CoCrFeMnNi high entropy alloy（HEA） was prepared by laser melting deposition. The microstructural evolution of columnar to equiaxed transition was experimentally observed across the molten pool. The temperature gradient and solidification rate were theoretically calculated by a finite element model via numerical simulation. Combining experimental observations and numerical simulations, a solidification map was established to quantitatively reveal the underlying correlation between solidification parameters and microstructural morphology. A decrease in the ratio of gradient temperature to solidification rate corresponding to an increasing constitutional undercooling induced columnar to equation transition during the laser melting deposition process. It was expected that the fundamental understanding of microstructural prediction of single-track as-deposited HEA parts via the solidification map would have implications on the production of bulk components with the desired microstructure and high performance.