Laser Direct Energy Research Articles

Influence of WC particle size and morphology on the performance of NiCu-based composite coatings deposited by laser direct energy deposition

NiCu alloys are commonly utilized due to their outstanding plasticity, toughness, corrosion resistance, and thermal conductivity. Nevertheless, their limited hardness and wear resistance have hindered their further development. In this study, 24CrNiMo alloy brake disc was coated with NiCu-based composite coatings containing different sizes and shapes of tungsten carbide, in order to investigate the effects of tungsten carbide particle characteristics on the microstructure, microhardness and wear resistance of Nickel‑copper composite coating. The results show that the microstructure of the coatings was minimally affected by the size and morphology of the WC particles, while non-spherical WC particles exhibited superior uniformity within the coatings. In terms of microhardness, smaller WC particles demonstrated a more consistent distribution. The ball-disk wear tests revealed that coatings with larger WC particles displayed better wear resistance in sizes, while those with non-spherical WC particles exhibited excellent resistance to wear in morphologies. Additionally, in abrasive wear tests, composite coatings containing spherical WC particles demonstrated superior wear resistance compared to those containing non-spherical WC particles. The diverse wear resistance observed in different wear experiments among the various WC coatings can be attributed primarily to differences in the distribution and preparation methods of the WC particles. Consequently, these findings hold significant promise for the practical application of NiCu-based composite coatings in industry.

Microstructure and Mechanical Properties of Laser Direct Energy Deposited Martensitic Stainless Steel 410.

The aim of this work is to study the phase transformations, microstructures, and mechanical properties of martensitic stainless steel (MSS) 410 deposits produced by laser powder-directed energy deposition (LP-DED) additive manufacturing. The LP-DED MSS 410 deposits underwent post-heat treatment, which included austenitizing at 980 °C for 3 h, followed by different tempering treatments at the temperatures of 250, 600, and 750 °C for 5 h, respectively. The analyses of phase transformations and microstructural evolutions of LP-DED MSS 410 were carried out using X-ray diffraction, SEM-EDS, and EBSD. Vickers hardness and tensile strength properties were also measured to analyze the effects of the different tempering heat treatments. It revealed that the as-built MSS 410 has very fine lath martensite, high hardness of about 480 HV1.0, and tensile strength of about 1280 MPa, but elongation was much lower than the post-heat-treated ones. Precipitations of chromium carbide (Cr23C6) were most commonly observed at the grain boundaries and the entire matrix at the tempering temperatures of 600 °C and 750 °C. In general, the tensile strength decreased from 1381 MPa to 688 MPa as tempering temperatures increased to 750 °C from 250 °C. Additionally, as the tempering temperature increased, the chromium carbide and tempered martensite structures became coarser.

Hierarchically heterogeneous microstructure and mechanical properties in laser-directed energy deposition of γ-TiAl alloy through intrinsic cyclic heat treatment

The complex thermal effects of laser direct energy deposition (L-DED) provide significant advantages for production of high-performance γ-TiAl alloy, whereas challenges still exist in microstructure tailoring and mechanical properties. This work focuses on the influence of intrinsic cyclic heat treatment (ICHT) on the evolution of microstructures and mechanical properties in different building height regions of an L-DED TiAl alloy thin-wall sample. The results indicate that ICHT induces discontinuous coarsening (DC) effects by consuming the lamellar interface energy of the initial lamellae, resulting in grain refinement. With the increase in building height, the thermal accumulation leads to an increase in the initial lamellar spacing, which weakens the DC effect. At the top, insufficient thermal cycles prevent fine grain growth and erode coarse grains, hindering overall grain refinement. Ultimately, the hierarchically heterogeneous microstructure forms in the L-DED TiAl alloy, including refinement of hierarchical lamellar colony structure decorated with fine (α2+γ) lamellar structure and (α2+6H-LPS) lamellar structure (6H-LPS refers to 6H long-period structure phase). Specifically, the bottom region exhibits the best combination of ultimate tensile strength (706 ± 33 MPa) and strain (0.68 ± 0.05 %). The high tensile strength can be attributed to the hindrance of dislocation movement by the fine lamellae and the 6H-LPS phase ,caused by the high cooling rate of L-DED, while the elongation is attributed to the reduction in crack propagation energy due to the presence of fine grains.

Multiscale mechanical performance evaluation of L-DED Ti6Al4V by novel ultrasonic burnishing (UB)

Laser-directed energy deposition (L-DED), as a novel technology to produce titanium (Ti) alloys (e.g., Ti6Al4V), has potential impactful applications. However, Ti alloys by L-DED is largely compromised by defects (porosity, cracks, etc.) at or near surfaces, which prevents strength-ductility synergy. In this study, a new method for Ti6Al4V sample preparation using ultrasonic burnishing coupled with heat treatment (UB/HT) after L-DED has been suggested. The results show that UB/HT has a more positive effect than mere UB to overcome turning's disadvantages, when the roughness, porosity, microstructures, and multiscale mechanical performance of L-DED Ti6Al4V are considered. UB/HT transforms residual tensile stress into compressive stress and causes severe plastic deformation (SPD) within a depth of approximately 10 μm on the surface layer. With this, UB/HT improves multiscale mechanical properties, e.g., hardness to 428 HV and strength to 976.7 MPa, and the related failure modes and fracture features induced by the UB/HT crack inhibition effect are confirmed with the fatigue results. With these observations, this research provides a good theoretical and practical reference for multi-scale mechanical analysis and understanding of L-DED Ti6Al4V, which expand the UB/HT applications.

Control of the grain structure and wear behavior of a Y2O3 nanoparticle dispersed Stellite 6 alloy fabricated by laser-directed energy deposition

A nuclear reactor is operated with load following that generates more wear than the base load; therefore, the wear resistance of the Stellite 6 alloy should be further improved. This study fabricated a wear-resistant oxide dispersion-strengthened (ODS) Co-based Stellite 6 alloy by varying the amount of Y2O3 nanoparticles (0, 0.6, and 1.5 wt%). The feedstocks for the ODS Stellite 6 alloy samples were prepared via mechanical mixing and radio frequency plasma spheroidization. The samples were then prepared by laser-directed energy deposition to obtain a finer grain structure with superior wear properties. In the ODS Stellite 6 alloy samples, the addition of nanoparticles changed the grain structure from columnar to equiaxed and refined the maximum grain size by 68.9 % compared with that of the Stellite 6 alloy sample. In addition, the nanoparticles were dispersed in the interdendritic region of the ODS Stellite 6 alloy samples. The nanoparticles formed a layer in front of dendritic grains and inhibited the movement of the grains and diffusion of solute atoms. As a result, the ODS Stellite 6 alloy samples had a small equiaxed grain structure. Furthermore, a high density of nanoparticles had an Orowan dispersion-strengthening effect. Therefore, the ODS Stellite 6 alloy samples exhibited a maximum increased Vickers hardness value of 15.9 % and maximum reduced specific wear rate of 58.2 % compared with those of the Stellite 6 alloy sample. Moreover, the ODS alloys exhibited superior wear behavior than Stellite 6 alloy because Y2O3 reinforcement prevents the ODS alloys from being easily abraded or plowed from counterpart material. This study indicates that the ODS Stellite 6 alloys with the optimal amount of Y2O3 nanoparticles dispersed control the grain structure and exhibit superior wear behavior compared with other reported Co-based alloys.

Tailoring microstructure and mechanical properties of U75V steel manufactured by laser direct energy deposition combining cobalt addition with preheating treatment

Laser direct energy deposition (LDED) exhibits great potential for application in rail repairing due to its low cost and high process automation. However, the martensite transformation accompanied by rapid solidification leads to the decrease in the mechanical properties and threatens the service safety. In this study, the reinforcement strategy of combining cobalt (Co) addition with preheating treatment was investigated systematically by combining thermodynamic calculations with experimental characterization to optimize the microstructure and mechanical properties of U75V steel by LDED. The results indicate that cobalt addition can promote the pearlite percentage by facilitating the leftward shift of the continuous cooling transformation (CCT) curve, and refining the interlamellar spacing. But the martensite start (Ms) temperature will also increase from 206.4 °C to 324.1 °C after 7 wt% Co addition. 350 °C preheating treatment on the substrate can also promote the pearlite transformation by elevating the valley temperature of thermal cycling over Ms, as well as the subsequent decrease in cooling rate, but it is not sufficient to prevent martensite formation. A synergistic reinforcement strategy of 7 wt% Co addition and 350 °C preheating was established, which can effectively suppress the occurrence of martensite in the bottom area of deposition layers and expand the pearlite range in the top area. The bonding strength between the substrate and deposition layers has been elevated by ∼20%, accompanied by the increase in elongation and wear resistance. A mixed mode of adhesive and abrasive wear has transformed into a mode dominated by adhesive wear after optimization.

Research of on-line monitoring technology and control strategy for laser-directed energy deposition: a review

Research of on-line monitoring technology and control strategy for laser-directed energy deposition: a review

Cr content dependent lattice distortion and solid solution strengthening in additively manufactured CoFeNiCrx complex concentrated alloys – a first principles approach

In the previous study, the authors reported that multicomponent CoFeNiCrx (x = 0, 12, 18, 24 at%) complex concentrated alloys fabricated through laser directed energy deposition experienced a substantial reduction in the saturation magnetization (Ms) and the Curie transition temperature (Tc) with the increase in Cr content. The present investigation additionally explores the effect of Cr concentration on the structural and strengthening behavior in laser directed energy deposition processed CoFeNiCrx alloy system. The novel approach adopted in the present work included the integration of experimental and computational efforts involving laser direct energy deposition (L-DED) based synthesis/fabrication of a series of new CoFeNiCrx alloys through in-situ compositional tuning which is often difficult using conventional techniques and prediction of mechanical properties of these compositionally tuned alloys using a computational methodology based on the density functional theory (DFT). X-ray diffraction and nanoindentation studies revealed that while there was a slight reduction in lattice parameters, the elastic modulus increased by approximately 30 % and the yield strength increased by 25 % with increasing Cr content. The density functional theory framework was employed to interpret the influence of Cr atoms on local lattice distortions and observed yield strength. The calculated local lattice distortion, in terms of first nearest neighbors, significantly increased with Cr concentration, which is attributed to the enlargement of Cr atomic radius in the alloyed multielement environment in comparison to its radius in the elemental form. This distortion at the local level caused solid solution strengthening. The theoretically predicted solid solution strengthening values using the Varvenne model are consistent with experimental results.

IoT-Based Data Mining Framework for Stability Assessment of the Laser-Directed Energy Deposition Process

Additive manufacturing processes are prone to production errors. Specifically, the unique physical conditions of Laser-Directed Energy Deposition (DED-L) lead to unexpected process anomalies resulting in subpar part quality. The resulting costs and lack of reproducibility are two major barriers hindering a broader adoption of this innovative technology. Combining sensor data with data from relevant steps before and after the production process can lead to an increased understanding of when and why these process anomalies occur. In the present study, an IoT-based data mining framework is presented to assess the stability of processing Ti6Al4V on an industrial-grade DED-L machine. The framework employs an edge-cloud computing methodology to collect data efficiently and securely from various steps in the part lifecycle. During manufacturing, multiple sensors are employed to monitor the essential process characteristics in situ. Mechanical properties of the 160 printed specimens were obtained using appropriate destructive testing. All data are stored on a central database and can be accessed via the web for data analytics. The results prove the successful implementation of the proposed IoT framework but also indicate a lack of process stability during manufacturing. The occurring part errors can only be partially correlated with anomalies in the in situ sensor data.

Effect of thermal exposure on microstructure and mechanical properties of Ti65 high-temperature titanium alloy deposited by laser direct energy deposition

Industrial design and new product development benefit from the adaptable processing approach offered by laser direct energy deposition (LDED). In this paper, a novel Ti65 alloy was successfully prepared by LDED and subjected to thermal exposure at 650 °C and 750 °C, respectively. The results showed that the as-deposited microstructure varies in different regions due to the cooling rate. Silicide precipitation was observed at the α/β interface, and a coherent α2 phase was generated in the matrix after thermal exposure at 650 °C. Two-scale silicides were observed at the α/β interface and inside the α laths after thermal exposure at 750 °C. The as-deposited ultimate tensile strength (UTS) was 1058 MPa and 690 MPa at room temperature and 650 °C, respectively. After thermal exposure at 650 °C, the room temperature UTS was 1135 MPa, and the elongation (EL) at 650 °C was 27.5 %. After thermal exposure at 750 °C, the room temperature EL was 11.05 %, and both UTS and EL at 650 °C showed a decrease. The quantitative analysis showed that silicide precipitation was primarily responsible for the strengthening mechanism of the specimens thermally exposed at 650 °C at room temperature. The results provide scientific data for the manufacture of Ti65 alloy with excellent mechanical properties, laying the foundation for its application in aerospace.

Effects of homogenization and deep cryogenic treatments on microstructure and mechanical property of D2 tool steel fabricated by laser direct energy deposition

The integration of laser direct energy deposition (L-DED) for manufacturing high-hardness D2 tool steel components presents a promising avenue for addressing the increasing complexity of product geometries demanded by modern industry. However, the inherent variability in microstructure and mechanical properties induced by L-DED across the build height necessitates the employment of post-processing techniques to achieve material homogeneity. This study explores the synergistic application of homogenization and deep cryogenic treatment (DCT) as a novel post-processing strategy aimed at mitigating microstructural inhomogeneities and enhancing the mechanical properties of L-DED-fabricated D2 tool steel. Initial analysis of the as-built samples revealed a microstructure characterized by a fine network of eutectic carbides enveloping an FCC matrix, which imposed constraints on martensite transformation during DCT. Homogenization was found to effectively alleviate these constraints, promoting compositional and microstructural uniformity and enabling a more consistent transformation to martensite across the samples upon subsequent DCT. The strategic combination of homogenization and DCT resulted in achieving uniformly distributed high hardness levels exceeding 750 HV throughout the entire sample, regardless of their initial positions within the build. This study demonstrates the potential of combining homogenization with DCT as an effective approach to enhance the structural integrity and mechanical performance of L-DED-fabricated D2 tool steel, offering valuable insights for the development of advanced manufacturing processes for alloyed materials.

High-performance functional coatings manufactured by integrated extremely high-speed-rate laser directed energy deposition with interlayer remelting

Extremely high-speed-rate laser directed energy deposition has attracted considerable attention for large-scale industrial component manufacturing owing to its outstanding fabrication efficiency. However, interlayer metallurgical defects and thickness fluctuation stacking caused by the previous non-uniform rough surface layer hinder the preparation of customized thicknesses of large-scale components with high performance. Herein, an integrated extremely high-speed-rate additive manufacturing technology, that is, extremely high-speed-rate laser-directed energy deposition accompanied by extremely high-speed-rate laser remelting, is proposed to eliminate porosity and reconstruct the microstructure of multilayer parts. The remelted specimens exhibited uniform roughness and ultrafine grains when defocusing amount was less than zero. The relatively lower temperature gradient G and morphology factor G/R in the remelting process led to more favorable subcooling, which further promoted more nucleation sites and contributed to grain refinement and columnar-to-equiaxed transition. A multilayer 316 L stainless steel material with an interlayer remelting treatment was further prepared, and a typical heterogeneous structure dominated by ultrafine equiaxed grains was obtained. The multilayer specimen characterized by such a special structure exhibited a higher yield strength of 546 MPa, along with a ductility of 49.1 %. This novel integrated manufacturing technology highlights a new strategy that can expand the extremely high-speed-rate additive manufacturing window and achieve simultaneous improvements in the manufacturing efficiency and performance of large-scale components.

Achieving fully equiaxed grains with minor Sc addition of a novel Al-Li alloy fabricated by laser direct energy deposition

In this study, a novel Al-Li alloy with minor Sc addition was fabricated by laser directed energy deposition (LDED), resulting in a fully equiaxed grain structure. The precipitation behavior of primary Al3Sc particles was studied based on transient nucleation theory, and the results showed that sufficient heterogeneous nuclei by primary Al3Sc particles could preferentially form at initial solidification stage and induce into equiaxed grains. Besides, a large number of acicular T1 (Al2CuLi) phases and spherical Al3Sc phases, as well as rod-shaped TB (Al7Cu4Li) phases and quasicrystalline T2 (Al6CuLi3) phases, were distributed in as-built alloy that served as primary strengthening phases by thermal cycling. However, the specific nucleation behavior of α-Al grains on primary Al3Sc particles and subsequent growth behavior during rapid solidification need to be further investigated.

316L Reinforced with Tungsten Carbide Particles by Laser-Directed Energy Deposition: Interface Microstructure and Friction-Wear Performance

316L Reinforced with Tungsten Carbide Particles by Laser-Directed Energy Deposition: Interface Microstructure and Friction-Wear Performance

Heterogenous phase evolution and mechanical response in additively manufactured low alloy martensitic steel processed via laser-directed energy deposition

This work investigates complex heterogeneity in microstructure and mechanical properties of a low alloy martensitic steel fabricated via laser-directed energy deposition (L-DED). A multi-scale characterization starting from macro-scale optical micrography, micro-scale X-ray diffraction and electron backscatter diffraction mapping to nano-scale transmission electron microscopy was adopted to fully comprehend the microstructural heterogeneity along the build direction. Notably, the top layers of the build demonstrated enhanced mechanical properties, exhibiting hardness of around 550 HV and an ultimate tensile strength of 2 GPa, which decreased progressively towards the base. Interestingly, resurgence in strength and hardness was observed for the layers located 10 mm above the substrate. The primary solidification cycle led to the formation of untempered martensite having high strength and hardness while subsequent reheat cycles led to the alteration of these martensites, enhancing ductility at the expense of strength. Auto-tempering and carbide (Fe-C based) precipitation due to prolonged thermal exposure contributed to the increased the strength in the lower layers. Additionally, the temperature profiles during the L-DED thermal cycles were modeled and correlated with the observed phase transformations, allowing for a detailed understanding of the structure–property relationships at different heights within the build. This study provides a fundamental insight into the microstructural heterogeneities and their implications on the strengthening mechanisms in L-DED fabricated low-alloy martensitic steel.

Microstructure, mechanical properties and corrosion resistance of direct energy deposited AlCu0.25CoCrFeNi2.1 high entropy alloy: Tailored by annealing heat treatment

In this study, a novel high entropy alloy AlCu0.25CoCrFeNi2.1 is additively manufactured by laser direct energy deposition process. The microstructure, mechanical properties and corrosion resistance of the prepared AlCu0.25CoCrFeNi2.1 high entropy alloy are investigated and tailored by annealing heat treatment. The results indicate the alloy exhibits a dual-phase microstructure with face-centered cubic (FCC) columnar dendrites and body-centered cubic (BCC) interdendritic structures. The annealing heat treatment facilitates the precipitation and coarsening of Cu-rich short rod-like B2 phase within the incipient FCC phase and the dissolution of incipient BCC phase, synergistically varying the proportion of FCC/BCC + B2 contents and tailoring the properties. The hardness of microstructure fluctuates under the coupling effect of precipitation strengthening and lattice distortion reduction and shows an overall downward trend with the increase of annealing time. The tensile properties are noteworthily improved within 6-h annealing and achieve a maximum ultimate tensile strength of 1156.4 MPa. After 8-h annealing, the ductility of the alloy obtains a maximum value of 30.3% with minor loss of ultimate tensile strength. The corrosion resistance is dramatically enhanced and obtains the best value after 2-h annealing, and then gradually deteriorates with the destroying of FCC matrix passive film phase by coarsened B2 phases. These results are of great importance in rapidly designing/manufacturing novel high entropy alloys by laser direct energy deposition process and tailoring microstructure, mechanical properties and corrosion resistance by annealing heat treatment.

Effect of heat treatment on phase transformation behavior and properties of Ni-Co-Mn-Al-Y shape memory alloy by laser direct energy deposition

Laser direct energy deposition (LDED) of Ni-Co-Mn-Al-Y magnetic shape memory alloys (MSMAs) with high elastocaloric properties has important application prospects in solid-state refrigeration. However, because the microstructure of laser non-equilibrium metallurgy does not have reversible martensitic transformation, further regulating the microstructure of deposited samples through scientific post-heat treatment to obtain better properties is still a top priority. In this work, Ni-Co-Mn-Al-Y alloy samples were prepared by an optimized LDED process, and then the deposited samples were heat-treated with different temperatures. The results showed that the sample subjected to heat treatment with 550 °C × 3.5 h + 1100 °C × 20 h had maximum entropy change of the phase transformation, compressive properties, and elastocaloric effect. The compressive fracture strength was up to 1327 MPa, the maximum compressive strain was up to 26.2%, and an adiabatic temperature change was up to −4.3 K under the condition of rapid unloading. Characterization of the microstructure changes of the alloy under 11 cycles of compression revealed that the deformation mechanism was mainly planar sliding of dislocations in the martensite and dislocation cutting the γ(Y-rich) phase. The alloy prepared by the optimizing LDED and heat treatment process had better cycle stability and preliminary refrigeration capacity.

AlNbTiVZr refractory high entropy alloy combining exceptional high-temperature performance and excellent ductility fabricated by laser direct energy deposition

In this study, a new refractory high entropy alloy (RHEA) AlNbTiVZr is fabricated by laser direct energy deposition (LDED), and an investigation is conducted to explore the hot deformation. The body-centered cubic (BCC) phase structure and a small number of close-packed hexagonal Laves intermetallic phases of RHEA are predicted by combining XRD with the calculation of empirical parameters. Compression tests are carried out at different temperatures, and the results show that no fracture occurs when the strain reaches 50% at any of the temperatures tested while exhibiting a high yield stress. The dynamic softening effect is enhanced as the temperature increases and the yield stress decreases significantly. A slight hardening trend is observed during the steady-state flow stage of the stress-strain curve. The softening of RHEA is mainly caused by dynamic reversion (DRV) and assisted dynamic recrystallization (DRX), where the atomic mobility at the grain boundaries increases with temperature, promoting the creation of recrystallized grains.

Effect of TiB2 on the microstructure and mechanical properties of TiAl alloy produced by laser direct energy deposition

A Ti-48Al-2Cr-2Nb alloy was prepared by laser directed energy deposition (DED). The microstructure of the DED-produced Ti-48Al-2Cr-2Nb alloy exhibited a coarse columnar structure along the building direction. Through a combination of experiment and numerical simulation, the temperature gradient and solidification rate of the single-track sample were calculated theoretically. The columnar to equiaxed transition (CET) behavior of molten pool solidification was quantitatively analyzed, and the microstructure transition diagram was established. The effect of TiB2 on the microstructure and mechanical properties of the TiAl alloy was investigated. The columnar changed to equiaxed grains in the vertical cross section by adding nanometer-sized TiB2 particles and the microstructure was significantly refined. The grain size was reduced from 48.5 μm to 4.9 μm. Some equiaxed γ grains were distributed around the banded TiB2. The compressive properties were improved from 628.4 MPa to 698.8 MPa by adding nanometer-sized TiB2 particles. Finally, the solidification behavior and the microstructure refinement mechanism caused by the addition of TiB2 particles were discussed.

Formation mechanism of AlNbTiVZr high entropy alloy/Ti multi-principal element alloy by laser direct energy deposition: Microstructure evaluation and mechanical properties

A novel refractory multi-principal element alloy with high strength and toughness matching characteristics was successfully prepared by mixing TA15 alloy with AlNbTiVZr high-entropy alloy. The effects of the composition ratio of the high-entropy alloy on the microstructure evolution and its influence on properties were investigated in this study. The tensile strength of the 70 wt % AlNbTiVZr HEA/Ti multi-principal element alloy is 1.18 times more than that of TA15 alloy, which has a value of 1068.0 ± 2.1 MPa. The increase in strength is mainly due to the production of equiaxed fine grains and the development of a robust and resilient body-centered cubic (BCC) phase in the alloy following the introduction of the high-entropy alloy. The attainment of the acceptable tensile strain is the result of the combined influence of the BCC matrix phase and the dispersed second-phase particles. The exceptional mechanical capabilities of the 70 wt % AlNbTiVZr/Ti multi-principal element alloy undeniably enhance its potential for use as a structural component in aerospace load-bearing constructions.