Laser Melting Deposition Research Articles

Forming Behavior of Additively Manufactured Al/Ti Material Compounds Produced by Cold Spraying

Cold spraying has great potential for additive manufacturing, especially of oxidation-sensitive metals, because the material is not melted and significantly higher deposition rates can be achieved than with conventional additive manufacturing processes such as selective laser melting or direct metal deposition. Titanium is regarded as a high-performance engineering material due to its unique combination of properties, including good corrosion resistance, biocompatibility and high strength at comparatively low density. However, due to its high price, it appears reasonable for many applications to use material compounds in which titanium is only used on the surface of the workpiece, while less expensive materials such as aluminum are used for the remaining volume. In the present work, cold sprayed pure titanium coatings were deposited on Al substrates and then formed to defined 3-dimensional final contours by die forging and rotary swaging. Different porosities were selectively set in order to evaluate their influence on the coating adhesion and cohesion in the forming process. Pre-consolidation of the coatings and the use of Al/Ti interlayers proved to be promising strategies.

Study of microstructure and mechanical properties and residual stresses of 24CrNiMo steel prepared by selective laser melting and laser melting deposition

Selective laser melting (SLM) and laser direct metal deposition (LMD) have shown promise in advanced manufacturing. However, limited research fabricating the same material using these methods, particularly regarding mechanical properties and residual stresses. In this study, 24CrNiMo blocks were prepared using SLM and LMD and subjected to stress relief annealing (SR). The microstructure, residual stresses, and mechanical properties were investigated using SEM, XRD, EBSD, mechanical testing, and μ-X360n residual stress analysis. The results showed that the microstructure of both blocks composed of α-Fe. However, due to different thermal histories, there were differences in grain size between the SLM and LMD specimens. SLM specimens exhibited higher dislocation density, resulting in increased tensile strength and yield strength, while the LMD samples had higher proportions of high-angle grain boundaries (HAGBs), providing enhanced plasticity. LMD-SR specimens exhibited stronger texture and higher content of HAGBs in the <111> direction, further improving plasticity. Both as-deposited blocks exhibited high residual stresses, ranging from 450 MPa to 580 MPa for SLM and 20 %–50 % of SLM values for LMD. After SR treatment, the strength of both specimens decreased due to a reduction in dislocation density, but the increased content of HAGBs improved plasticity. Additionally, the residual stresses in both specimens significantly decreased, transitioned from tensile stresses to compressive stresses, greatly enhancing their wear resistance. Compared to the as-deposited specimens, the SLM-SR and LMD-SR specimens exhibited a reduction in weight loss due to wear by 46.1 % and 75.2 %, respectively, indicating a significant improvement in wear resistance.

Effects of annealing treatment on microstructure and mechanical properties of laser melting-deposited VCoNi medium-entropy alloy

Additive manufacturing (AM) is a promising approach to producing high- or medium-entropy alloys (HEAs or MEAs) with excellent properties. However, some undesirable microstructural defects and unstable precipitated phases were formed owing to the repeated heating and cooling steps and the rapid solidification nature of the AM process. Annealing treatment after AM fabrication is often beneficial for adjusting the comprehensive mechanical properties and stabilizing the microstructures. In this study, a VCoNi MEA single wall sample was fabricated successfully via laser melting deposition (LMD), and the effects of post-annealing treatments on the microstructures and mechanical properties were explored. The grain size and σ-phase particle size increase and the σ-phase particles undergo fragmentation, spheroidization, and coarsening during annealing. As the annealing time increases, the tensile strain increases, but the tensile strength decreases. An excellent combination of strength and ductility can be obtained after annealing for 1.5 h (The yield strength, ultimate tensile strength, and tensile strain are 741.2 MPa, 1287.8 MPa, and 30.5 %, respectively). HDI hardening is the most important strengthening mechanism in this work. The decrease in tensile stress after the annealing treatment is mainly attributed to the decrease of heterogeneity of microstructure, and the increase in tensile strain after the annealing treatment is mainly attributed to the decrease of the pores and the spheroidization of the rod-like σ-phase particles. This work provides practical insight into adjusting the microstructure and comprehensive mechanical properties of HEAs and MEAs for AM.

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.

Mechanical and magnetic properties of nonequiatomic Fe33Co28Ni28Ta5Al6 high entropy alloy by laser melting deposition

Soft-magnetic high-entropy alloy (HEA) Fe33Co28Ni28Ta5Al6 were prepared by laser-melting deposition (LMD). The microstructures and mechanical as well as magnetic properties of the fabricated samples were investigated. All samples comprised face- and body-centered cubic phases, where the proportion of the latter increased when the laser power was increased from 800 to 1400 W. With the increasing laser power, the yield strengths of the samples decreased slightly, and the saturation magnetization (BS) of the samples remained at approximately 1.07 T, the coercivity increased from 1034 A/m to 4456 A/m. The study showed that high laser powers resulted in high stress levels and increased the blocking effect on the motion of the magnetic domain walls of the samples. This work would provide a path way for the rapid manufacturing of high-performance soft-magnetic HEAs.

Laser melting deposition of fine-grained 90W-7Ni-3Fe alloys using pre-sintered granulated powder

Reducing the grain sizes of tungsten (W) in tungsten heavy alloys (WHAs) is pivotal for achieving high strength. Traditional liquid phase sintering (LPS) often results in the growth of W grains, due to the long sintering time and slow cooling rates. To achieve a high cooling rate, a laser melting deposition (LMD) additive manufacturing technique was employed in this study to prepare 90W-7Ni-3Fe alloys with unique pre-sintered powder prepared using a spray granulation method. The pre-sintered granulated powder particles, each consisting of many fine elemental particles, exhibit a spherical shape with a size up to 100 μm, contributing to optimal flowability. Thin plate samples were prepared using laser powers ranging from 500 W to 900 W. The results show that with laser powers below 600 W, WHAs with fine W grain sizes of 5 μm can be successfully obtained, representing a record of the finest grain size ever reported for the laser additive manufactured WHAs. On the other hand, at 900 W, the W grain size increases significantly to 17 μm, comparable to that of traditional LPS samples. The volume fraction of W grains increases with increasing laser power due to the Ni and Fe evaporation, and at laser power exceeding 700 W, this fraction aligns with 95WNiFe alloys. The fine grains and/or the increased volume fractions of W grains contribute to a higher micro-hardness in these samples compared to LPS samples. Nonetheless, high porosities are also observed in these samples, which leads to a diminished micro-hardness compared to previous LMD 90W-7Ni-3Fe alloys prepared with mixed elemental powder. This study underscores the potential of the LMD method using pre-sintered powder to fabricate fine-grained WHAs, paving the way for the fabrication of high-strength and high-ductility WHAs using LMD.

Investigation of Microstructure and Mechanical Properties of the Repaired Precipitation-Strengthened Ni-Based Superalloy via Laser Melting Deposition

In this study, a typical γ′ phase precipitation-strengthened Ni-based superalloy DZ411 was repaired using an LMD-based repairing technique with an IN738LC superalloy, and crack-free samples were acquired. The mechanical properties and microstructure of different areas inside the repair sample were investigated, including the IN738LC deposit, the DZ411 substrate, and the interface between these two parts. The differences in mechanical properties between different areas were explained via analyzing fractography and KAM maps. It was found that the coarse carbides of the DZ411 substrate might lead to rapid cracking of grain boundaries, resulting in the worst mechanical properties of the DZ411 substrate. The IN738LC deposit demonstrated significantly superior mechanical properties in comparison to the DZ411 substrate. Its tensile strength exceeded that of the substrate by over 250 MPa, while its relative elongation after fracture was twice as great as that of the substrate. The excellent mechanical properties of the IN738LC deposit could be attributed to its fine microstructure, which resisted rapid cracking and generated a large number of GNDs during the plastic deformation process. For the interface between the deposit and substrate, although its hardness before the tensile test was low, it could also generate many GNDs during the plastic deformation process, hence exhibiting commendable mechanical properties. The research results show that using an LMD-based repairing technique with IN738LC superalloy to repair γ′ phase precipitation-strengthened Ni-based superalloy DZ411 is a feasible solution.

Effects of pulsed frequency on the microstructure characteristics and properties of Ti6Al4Vlaser melting deposition additive manufacturing parts

Laser melting deposition–produced Ti6Al4V additive manufacturingparts tend to manifest as coarse columnar grains. This paper optimises the laser pulse frequency to manage the microstructuralcharacteristics of Ti6Al4V parts and enhance their tensile properties and corrosion resistance. A fiber laser processing equipment with a laser power of 1200 W and a spot diameter of 2.0 mm was employed for all experiments, which provides pulsed lasers at 4, 40, 400, and 4000 Hz at a scanning speed of 8 mm/s and a duty cycle of 95 % for comparative research.The findings demonstrated that increasing the pulse frequency improved the deposition efficiency.An appropriate pulse frequency could promote the refinement and columnar-to-equiaxed transition (CET) of primary β grains.At 400 Hz, the Ti6Al4V-deposited parts yielded refined equiaxed grains.The average grain size in the middle of the sample was about 206.8 μm,and the microstructure mainly comprised fine α-laths in a lamellar structure, which yielded the best overall performance of the parts, with the ultimate tensile strength and elongation reaching 1083 MPa and 4.09 %, respectively.Therefore, improvingthe structural properties of partsby regulating the deposition process, which is conducive to the online regulation of the forming quality of laser deposition parts, is an effective and feasible approach.

Microstructure development and cavitation erosion resistance enhancement of additive manufactured Hastelloy C276 alloy coating on martensitic stainless–steel via directed energy deposition

In this study, a Hastelloy C276 coating was prepared on a martensitic stainless-steel substrate using laser melting deposition, aiming at obtaining a coating with high resistance to cavitation erosion. The coating is up to 600 μm thick, and the grain morphology of the coating exhibits a progression from bottom to top, transitioning from cellular to columnar and ultimately to equiaxed grains. Additionally, the grains grow at a specific angle to the substrate plane, following the direction of the heat source. A 200 μm thick compositional mixing zone is formed at the interface of the substrate and the coating, in which the coating elements and substrate elements mutually permeate and form a stable metallurgical bond. The microhardness of the coating is measured as 334.4 HV0.05, signifying a substantial improvement of 33.2 % compared to the substrate. The cavitation erosion process of the coating can be classified into three stages: incubation, accumulation, and stabilization. The material spalling damage caused by cavitation erosion progresses in a fatigued manner, and the coating demonstrates considerably enhanced resistance to cavitation erosion compared to the substrate.

Microstructure and mechanical properties of Fe50Mn30Co10Cr10 high-entropy alloy with Si addition prepared by laser melting deposition

Microstructure and mechanical properties of Fe50Mn30Co10Cr10 high-entropy alloy with Si addition prepared by laser melting deposition

Microstructure evolution and corrosion behavior of TC11 laser melt deposition additive components after post-heat treatment

The TC11 components were produced using a high-power, high-speed laser metal deposition (LMD) process. Subsequent post-heat treatment significantly improved the microstructures and corrosion resistance of the components. The results showed that the microstructure of the as-deposited component primarily comprised α Widmanstatten microstructure and mesh basket-weave microstructure. After post-heat treatment, the original interlayer zone disappeared in the deposition direction, and the microstructures were transformed from α’ to α phase. The aspect ratio of the α phase decreased, and the α grain boundary (αGB) became spheroidized. The tensile strength decreased but plasticity increased significantly. Overall, the corrosion resistance was improved in both directions, with greater improvements observed in the deposition direction. Phase ratio, grain size and KAM value are all factors affecting the corrosion resistance, but for the deposited direction, the significant improvement of the corrosion resistance is mainly due to the weakening of the charged galvanic corrosion effect between the non-interlayer zone and interlayer zone.

Forming characteristics analysis of variable-process alternated overlapping strategy based on contour description for laser melting deposition

To predict the geometric characteristics and improve the forming quality and macro-micro defects of multi-track overlapping cladding layer effectively, the experimental research of laser melting deposition single-track Co-based alloy on 42CrMo substrate was carried out. The influence trends of laser process parameters on the single-track cladding height and cladding width were analyzed, and the regression prediction models were established. A variable-process alternated overlapping strategy was proposed using the geometric functions to fit the single-track profile. The surface forming quality and macro-micro defect of different overlapping cladding layers were compared and evaluated. The results showed that the cladding height and cladding width regression models established by an improved optimization algorithm had an excellent prediction effect. The variable-process alternated overlapping cladding layer prepared by the parabolic function had a flat surface, and the fluctuation distance was 81.56 μm, which was significantly improved compared with other overlapping strategies. The surface flatness of the continuous overlapping cladding layer was the worst and the spheroidization defect was the most unsatisfied. The internal defects of the constant-process alternated overlapping cladding layer were the most terrible, with more non-fusion and cracks caused by oxides. The variable-process alternated overlapping strategy proposed in this study can effectively prepare a multi-track cladding layer with a smooth surface and dense interior, which provides essential theoretical guidance for the near-net forming of component surface deposition and repair.

Microstructure, Multi-Phase and Corrosion Resistance of Amorphous Phase Reinforced Multi-Layer Fabricated by Laser beam

Abstract The Fe-Cr-B-Si deposited layers were prepared on the titanium alloy by the laser melting deposition (LMD) or the laser cladding (LC) technology. The microstructure of the clad layer and the deposition bulk was characterized by the scanning electron microscopy, transmission electron microscopy and electron-backscattered diffraction. The single clad layer was primarily composed of the amorphous phase (APs), the fraction of APs decreased with increasing of the LMD layer thickness due to the heat accumulation, producing the crystalline phases. Parts of the nanocrystalline phases (NPs) were produced due to the characteristics of a laser-induced pool (LIP), producing the deposition bulk with the good metallurgy bond between the adjacent laser fabricated layers. The corrosion resistance of the deposition bulk was enhanced due to the production of the oxides and hydroxides, forming a passive film to enhance the corrosion resistance.

Microstructure and mechanical properties of in-situ B4C reinforced TC4 composites prepared by laser melting deposition

In order to satisfy the increasingly harsh service environments of modern aircrafts, the novel in-situ reinforced xB4C/TC4 (x = 0–1.5 wt%) composites were prepared by laser melting deposition (LMD) and their microstructure, mechanical properties were investigated. The dual reinforcement phases of TiC and TiB were formed after B4C addition. A more favorable microstructure of refined α-Ti grains and uniformly-distributed reinforcement phases was obtained after adding 0.5 wt% B4C. The 0.5B4C/TC4 composite exhibited a moderate anti-abrasion ability owing to its beneficial microstructure and relatively high microhardness. Furthermore, it possessed the highest strength due to the effects of grain strengthening and dispersion strengthening induced by the (TiC + TiB) phases. This work provides theoretical basis and technical guidance for the development of high-performance Ti matrix composites.

Improved corrosion resistance of laser melting deposited CoCrFeNi-series high-entropy alloys by Al addition

The microstructural evolution, pitting performance, and passive-film properties of AlxCoCrFeNi high-entropy alloys (HEAs) fabricated via laser melting deposition (LMD) were investigated using material characterization techniques and electrochemical measurements. The substrates of the LMD HEAs transform from a face-centred cubic (FCC) to a body-centred cubic (BCC) structure with increasing Al addition. The BCC-structured B2 and A2 phases precipitate in the HEA substrate when the Al content reaches 10 mol.%. Ternary-phase HEA with Al addition of 10 mol.% exhibits the best corrosion resistance, which is attributed to favourable modifications in the Cr2O3 content and passive-film thickness.

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.

Grain refinement of Ti6Al4V by incorporating in-situ TiB nanowhiskers in laser melting deposition

One of the known problems in laser melting deposition (LMD) of alloys is columnar grain morphology and large grain size. Adding reinforcements is generally suggested as a viable approach to tailor the microstructure and enhance mechanical properties. However, quantitative relationships between grain morphology and reinforcement content are sparsely available. In this study, TiB2 particles with different weight ratios were applied in the LMD of Ti6Al4V alloy to investigate its impact on grain refinement. It shows that the increasing TiB2 content enhances the equiaxed grain growth from epitaxial growth and reduces the grain size. A linear function between decreasing grain size and the inverse of the growth restriction factor originating from TiB2 content was obtained. This was attributed to the heterogenous nucleation of in-situ TiB nanowhiskers that promoted the refinement of α lath and prior β grain. The results shed new light on controlling grain morphology and predicting grain size by mixing ceramic-metal materials in LMD additive manufacturing.

Microstructure and tribological behavior of Al–12Si – Nano graphene composite fabricated by laser metal deposition process

To improve the wear resistance of the Al-based alloys and extend its application spectrum, graphene nano-platelets (GNPs) reinforced Al–12Si matrix composites were fabricated by laser melting deposition (LMD) technology. The microstructure, wear morphologies and surface roughness were characterized by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) setups and 3D confocal microscopy. The microhardness and high-speed reciprocating frictional properties both Al–12Si and GNPs/Al–12Si composites were evaluated. The addition of graphene significantly refines the microstructure and improves the wear resistance of the matrix. The formation of a relatively stable mechanically mixed layer (MML) reduces the contact area between the friction pair material and the wear surface, the wear rate and roughness of the Al–12Si – 0.5 wt% GNP reinforced composites decrease with the increase of sliding speed, and the wear mechanism changes from adhesive wear accompanied by abrasive wear to fatigue wear. Meanwhile, as the load increases, the wear rate and roughness increase. The wear mechanism of the composite changes from abrasive wear to a mixed mode (combination of both adhesive and abrasive). This research not only offers valuable information about the Al–Si-based composites but provides theoretical light on their industrial applications.

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.

Micro-deformation behavior of laser melting deposited multi-gradient TC4/TC11 alloy under tension–tension cyclic loading

The microstructure of additive manufacturing titanium alloy is closely related to its mechanical properties. In this study, a TC4/TC11 titanium alloy with 20 layers was prepared by laser melting deposition (LMD) technology. Microstructure analysis revealed a transformation of the columnar grains into equiaxed grains along the build direction (BD), accompanied by the presence of heat affected zone (HAZ) bands and pore defects. The defect families of specimens in different directions were characterized by X-ray micro-computed tomography (micro-CT). The porosity of BD specimens was 0.0021%, and the porosity of the transverse direction (TD) specimens was 0.0001%, indicating that the material was dense. Specimens with different directions were subjected to tension–tension cyclic loading, and their deformation was analyzed by digital image correlation (DIC). The results show that the presence of noticeable pore defects leads to crack initiation around these regions due to local strain concentration. Moreover, the dispersion and anisotropy of mechanical properties are attributed to the influence of porosity and microstructure deformation, the latter being the dominant factor. In addition, the deformation mechanism and failure mode of the microstructure in LMD TC4/TC11 are proposed in this paper. The deformation of the α laths of TD specimen is hindered by the columnar grain boundary, resulting in improved resistance against crack nucleation and propagation compared to the BD specimen.