Direct Laser Deposition Research Articles

In-Situ Reactions during Direct Laser Deposition of a Functionally Gradient Ti64-B Composite

In-Situ Reactions during Direct Laser Deposition of a Functionally Gradient Ti64-B Composite

Influence of Latent Heat of Fusion on the Melt Pool Shape and Size in the Direct Laser Deposition Process

The melt pool calculating method is presented based on the solution of the heat conduction problem in a three-dimensional formulation, taking into account the latent heat of fusion and the change in thermophysical properties with temperature. In this case, the phase transitions of melting and crystallization are accounted for using the source method. Considering the latent heat of fusion in the heat transfer process leads to melt pool elongation, as well as to a slight decrease in its width and depth. Depending on the mode, the melt pool elongation can be up to 22%. The penetration depth is reduced by about 5%. The deposition width does not change practically. The presented model was validated by comparing the experimentally determined melt pool shape and its dimensions with the corresponding theoretically calculated results. Experimental data were obtained as a result of coaxial video recording and the melt pool crystallization. The calculated form of the crystallization isotherm changes from a U-shape to a V-shape with an increase in the power and speed of the process, which coincides with the experimental data.

Study of the Machinability of an Inconel 625 Composite with Added NiTi-TiB2 Fabricated by Direct Laser Deposition

This work studies the process feasibility of milling a metal-matrix composite based on Inconel 625 with added NiTi-TiB2 fabricated by direct laser deposition. The composite is intended for manufacturing turbine blades and it has strength characteristics on par with those of Inconel 625. However, the addition of TiB2 has improved its heat and wear resistance. This material is new, and its machinability has not been studied. The new composite was milled with end mill cutters, and recommendations were worked out on the cutting speed, feed per tooth, cutter flank angle, as well as depth and width of milling. The wear of cutter teeth flank was more intense. After the flank wear land on the back surface of a tooth had reached 0.11–0.15 mm, there was a sharp increase in the forces applied which was followed by brittle fracture of the tooth. Milling at a speed of 25 m/min ensured 28 min of stable operation. However, afterwards the critical wear value of 0.11 mm was quickly approached at a cutting speed of 50 m/min, and critical wear followed after 14 min. Dependencies of the cutting forces vs. time for all the selected cutting speeds and throughout the entire testing time period have a tendency to increase, which indicates the influence of cutter wear on the cutting forces. It was found that the durability of the cutters increases with an increase in the milling width and a decrease in the milling depth.

Post-processing influence on additively produced Ti–6Al–4V alloy structure and mechanical properties

This article studies the formation of microstructure and mechanical properties of Ti–6Al–4V alloy manufactured by direct laser deposition (DLD) as a result of post-processing. Heat treatment, hot rolling and hot isostatic pressing (HIP) of additively produced material as well as different combinations of these methods were carried out as part of the investigation. It was determined that material density increased as an average of 0.05% as a result of post-processing. Specimen’s microstructure changed subtly upon post-processing, however, HIP of specimens caused widmanstatten structure decomposition and resulted in the most intense material strength deterioration. Post-processing including plastic deformation causes a decrease in material tensile strength at an average of 3% and an increase in elongation by almost 2 times.

Research on Cracked Conditions in Nickel Chrome Alloy Ni50Cr33W4.5Mo2.8TiAlNb, Obtained by Direct Laser Deposition

Nowadays, additive manufacturing (AM) is a powerful way to make complex-shaped components for airspace engineering from nickel-based superalloys. So, while nickel-based superalloys could easily be we L-DED in sheet-metal thicknesses, they suffered from strain-age cracking and solidification during AM or in the post-weld aging treatment. This is attributed to the fact that besides the limitation of γ′- phase forming elements (Al and Ti), as they form by AM very rapidly and reduce ductility, the majority of the superalloys contain carbide-forming elements such as Cr, Mo, and W. The precipitation of carbides, which is very effective in strengthening, develops cracks in the heat-affected zone (HAZ) during AM. The difference in isochoric heat capacities and the thermal expansion coefficient (TEC) at the phase boundary leads to the appearance of dangerous local destruction energy. If the area of the interfacial interface is sufficiently extended, then the accumulation of this energy reaches a level sufficient for a crack formation. We have proposed a crack initiation criterion (CIC) for assessing the dangerous level of fracture energy. The CIC was derived from an estimate of the local energy balance from the heat transfer equation for the two-phase area. Practical approbation of the criterion was carried out after L-DED of samples from Ni50Cr33W4.5Mo2.8TiAlNb (EP648) alloy powder with an increased carbon content based on the study of the chemical composition near the crack formed during solidification. Using the proposed criterion provides an opportunity to give the rank to carbide-forming elements according to the degree of their influence on the fracture energy. Thus, the release of aluminum carbide turned out to be 5.48 times more dangerous than the release of titanium carbide and more than 5 times more dangerous than the release of tungsten carbide and molybdenum.

Comparative study of the microstructure evolution of dual-phase Al-Co-Cr-Fe-Ni high-entropy alloy prepared by direct laser deposition and vacuum arc melting

• Dual-phase Al 0.7 CoCrFeNi HEAs were fabricated by DLD and VAM. • BCC phase in VAM Al 0.7 showed basket-weave-like structure by spinodal decomposition. • DLD sample contained a higher content of FCC phase than the VAM sample. • The texture in BCC phase in DLD sample deviated from 〈0 0 1〉 orientation. Dual-phase (FCC + BCC) Al 0.7 CoCrFeNi high-entropy alloys were successfully fabricated by direct laser deposition (DLD) and vacuum arc melting (VAM). It has been found that there is a significant difference between the microstructures produced by the two methods. A hypoeutectic structure (primary FCC phase + lamellar FCC-BCC eutectic microstructure) was observed in the DLD sample, while BCC phase in the VAM sample displayed basket-weave-like structures by spinodal decomposition, which can be attributed to the different thermal gradients and cooling rates for the two processes. A higher percentage of FCC phase was found in the DLD sample (63 %), compared to the VAM sample (55 %). In both samples, FCC phase was rich in Fe, Co and Cr, whereas Al and Ni were segregated in BCC phase. Furthermore, chaotic grain orientations were observed in FCC phase in both samples. A strong 〈0 0 1〉 texture along the direction of solidification was formed in BCC phase in the VAM sample, while in the DLD sample the texture showed a deviation from 〈0 0 1〉 orientation because of the complex nature of the temperature field.

Phase transformation mechanism and mechanical properties of Ti-45Al-8Nb alloy prepared by directed laser deposition

In this work, the Ti-45Al-8Nb alloy was fabricated by directed laser deposition. The phase transformation mechanism and mechanical properties were analyzed. The results show that different solidification paths were found. In the dendritic stem, the path is L → L + β → β → β + αβ → β + αβ + γ → α2 + γ + B2. At the periphery of the dendrite, the path is changed to L → L + β → αp → αp + γ → α2 + γ. The different solidification paths are due to elemental segregation, with Nb segregation in the dendritic stem leading to the formation of the B2 phase in α2 + γ lamellar. The decrease in Nb at the periphery of the dendrites leads to an L + β → αp peritectic reaction. The tensile strength and elongation are 365 ± 11 MPa and 0.36 ± 0.08%, respectively. Strength was higher than the methods of electromagnetic levitation cold crucible (EMLCC) and vacuum consumable arc melting (VCAM). The crack propagation model was analyzed, which was along the lamellar fracture and through the lamellar fracture.

Effect of the Mass Fraction of NiTi-TiB2 SHS-Particles on the Phase Composition, Structure, and Mechanical Properties of Inconel 625-NiTi-TiB2 Composites Produced by Direct Laser Deposition.

This paper studies the impact of the mass fraction of NiTi–TiB2 particles obtained by the method of self-propagating high-temperature synthesis (SHS) on the phase composition, structure, and mechanical properties of composites made by direct laser deposition from an Inconel 625–NiTiz–TiB2 powder mixture. Composites were obtained from a powder mixture with the mass fraction of particles at 5–10 wt%, and they consisted of an Inconel 625 metal matrix wherein ceramic inclusions of titanium diboride TiB2 were distributed. Increasing the mass fraction of SHS-produced NiTi particles from 30 to 95 wt% led to the emergence of a NiTi intermetallide phase in the matrix material as well as an increase in the average TiB2 particle size and formation of their agglomerates. In addition, an increase in the microhardness of the materials was observed. The graph of tensile strength of Inconel 625–NiTi–TiB2 samples has a parabolic shape with a maximum at 1000 MPa (when the mass fraction of SHS-produced NiTi–TiB2 particles is at 30 wt%). A further increase in the mass fraction of NiTi–TiB2 led to a decrease in the tensile strength down to 400 MPa. Here the deformation of samples decreases linearly as the ratio of composite particles in the initial mixture increases. From a comparative analysis of the results obtained, the optimal mass fraction of composite NiTi–TiB2 particles in the Inconel 625-NiTi–TiB2 powder mixture was found to be 5 wt%.

Additive manufacturing of micro/nano multiphase synergistically reinforced Ti-55Al-7.5Nb with a reticular boundary precipitate via direct laser deposition

Additive manufacturing, with features of micro-area metallurgy and rapid solidification, has become an effective way to construct complex components with high efficiency and production flexibility. Hereby, this study presents a micro/nano multiphase synergistically reinforced Ti-55Al-7.5Nb/Ti 2 AlN-Ti 5 Si 3 metal–matrix composite (TiAl MMC) synthesized via direct laser deposition. The as-prepared TiAl MMCs exhibit homogeneously equiaxed grains and no cracks, compared with the coarse columnar microstructure and macro/micro cracks of Ti-55Al-7.5Nb alloy. The grain size of the TiAl MMCs increases in the form of a parabola with increase of the laser power, and also increases from the bottom to the top along the building direction with fixed laser power. Precipitates of Ti 2 AlN and Ti 5 Si 3 can alleviate the texture index, and thus facilitate an in situ production of isotropic MMCs. Meanwhile, TiAl MMCs feature excellent compressive and high-temperature (900 °C) tensile strength, resulting from the precipitates of rod-like Ti 2 AlN dispersed inside the grains, and the micro/nano-Ti 5 Si 3 reticularly distributed at the grain boundaries (GBs). The micro/nano multiphase reinforcement could help guide the additive manufacturing of preparing reticular-boundary-reinforced cracks-free TiAl MMCs, which feature controllable microstructures and enhanced mechanical properties. • Novel TiAl MMCs, prepared by in situ additive manufacturing, exhibit even equiaxed grains but no Al segregation or cracks. • Precipitates, the same orientation relationship with matrix, tune TiAl MMCs microstructures that show lower texture index. • The grain size of prepared TiAl MMCs varies with laser powers was revealed by a combination of experiment and simulation.

Powder particle–wall collision-based design of the discrete axial nozzle-exit shape in direct laser deposition

To improve the efficiency of the direct laser deposition (DLD) of metal powders, a concentrated powder-stream distribution is required, which can be affected by the shape of the powder-delivery nozzle. In this study, a simplified, powder particle–wall collision-based 3D numerical model of the powder flow in the nozzle was used to simulate the influences of the nozzle-exit shape on the concentration of the powder stream distribution, characterized by its diameter. The nozzle-exit shape was parametrized by the exit-cone angle, length, and inner-surface roughness. Based on the simulation results, the nozzle-exit shapes of three exit-cone angles (0°, 3.5° and 7.2°), various lengths and surface-roughness values were designed. For the two larger particle sizes of 22 μm and 82 μm considered, the wall-collision-dominated regime and the influence of the nozzle-exit shape were experimentally confirmed. In particular, a significant decrease in the powder-stream diameter when increasing the divergent nozzle-exit cone angle or decreasing its surface roughness and the nonlinear influence of the cone length were shown. Using single-layer, powder-deposition experiments it was demonstrated that by modifying the design of the nozzle-exit shape, the powder-catchment efficiency was increased by 13% due to the increased nozzle-exit cone angle and by 19% due to the reduced surface roughness. • Particle-wall collision-based model of powder flow in nozzle-exit was formulated. • Conical exit influence on powder stream was shown numerically and experimentally. • Increase of divergent nozzle-exit cone angle decreases powder stream diameter. • The nozzle-exit cone length nonlinearly influences the powder stream diameter. • Reducing the nozzle-exit cone roughness decreases the powder stream diameter.

Microstructure evolution and strength-toughness matching mechanism of Fe-Cr-Ni gradient alloy steel prepared by direct laser deposition

A novel 70Cr8Ni2Y alloy steel powder was designed and prepared to meet the requirement for composition design and preparation of gradient alloys with strength-toughness matching the preparation of nuclear power camshafts by direct laser deposition (DLD). A three-layer gradient structure alloy sample with strength-toughness matching was successfully prepared by DLD using 70Cr8Ni2Y, 50Cr6Ni2Y and 12CrNi2Y alloy steel powder. The microstructure evolution and strength-toughness matching mechanism of the gradient sample were studied. The interfaces between the gradient layers were metallurgical bonding and had no defects such as cracks and pores. With the change of the alloy composition of the three gradient layers, the main microstructure of the sample changed from granular bainite to lath bainite, and to martensite in turn. The mechanism of strength-toughness matching was that the changes of the three alloy components made the strength of microstructure increase, and a metallurgical bond formed at the interfaces of the gradient layers. The gradient sample demonstrated strength-toughness matching of high-strength surface and good toughness core through adjusting of the gradient layers and change of microstructure. The high-strength 70Cr8Ni2Y surface layer ensured the high hardness and wear resistance of the gradient sample, and the 50Cr6Ni2Y gradient layer acted as an intermediate layer to support the surface and protect the bottom layer. The 12CrNi2Y gradient layer with good toughness played a role in delaying fracture and hindering crack propagation. The microhardness of the gradient sample showed a gradient distribution of 739-534-321 HV. The average tensile strength was 1076 MPa, the elongation was 1.6%, and the specific wear rate (SWR) of the sample surface was 9.5 × 10−5 mm3/(N·m). This paper can provide a useful reference for the preparation of nuclear power camshaft parts with gradient properties of good toughness of the core and high hardness and wear resistance of the surface by DLD.

Microstructure and properties of Ta-reinforced cobalt based composite coatings processed by direct laser deposition

Circulating water pump is an important component in coastal nuclear power plant, and is prone to suffering erosion and corrosion from the sea water. In the present study, Ta-reinforced cobalt-based composite coatings were prepared on the surface of a martensitic stainless steel by direct laser deposition technology. The effect of Ta on the phase composition, microstructure, micro-hardness, residual stress, corrosion resistance and erosion performance of the cobalt-based composite coatings was systematically investigated. The results indicated that the addition of Ta can lead to the formation of dispersed TaC ceramic particles in the coatings. In the meantime, TaC particles with flaky shape greatly changed the morphology of reticular eutectic carbides and residual stress distribution in the coatings. The potentiodynamic polarization testing and electrochemical impedance spectroscopy (EIS) measurements showed that the addition of Ta had no prominent influence on the corrosion resistance of cobalt-based coatings. Generally speaking, the coating with 30 wt% Ta addition exhibited the best comprehensive performance considering micro-hardness measurement, residual stress value, potentiodynamic polarization testing, and slurry erosion testing under two erosion angles. In situ synthesized TaC phases could effectively alleviate the impact of solid particles in the erosion process, and improve the erosion resistance of the cobalt-based composite coating.

Влияние технологических параметров на структуру металла изделий, полученных методом прямого лазерного выращивания из титанового порошка ВТ6

Direct laser deposition is an efficient approach to manufacturing or repairing a range of metal goods. The process involves creating a three-dimensional solid object layer by layer. The weld metal structure primarily affects mechanical properties of the deposited parts, making it important to establish the specifics of microstructure formation. We analysed the microstructure of VT6 alloy samples deposited on a substrate forged from the same VT6 titanium alloy. Different modes of deposition and subsequent heat treatment resulted in samples with differing structures. We established characteristic features of microstructure formation for all modes. We investigated the effect of heat treatment on the sample structure after laser welding in the initial state and after heat treatment. The results obtained may help reduce the cost of repairing and manufacturing titanium alloy parts for gas turbine engines.

Mechanical and microstructural characterisation of Inconel 625 - AISI 431 steel bulk produced by direct laser deposition

The direct laser deposition process successfully produced a bulk material by mixing 50% Inconel 625 powders (M625) with 50% AISI 431 steel powders (M42C). The properties of this new material, such as tensile strength and wear resistance, were evaluated. The microstructure was also analysed using scanning electron microscopy. Moreover, the formation of defects and second phases in the bulk material were investigated by applying a tomography analysis. M625-M42C bulk material shows tensile strength and abrasive wear behaviour similar to Inconel 625 alloy, suggesting a potential replacement material for the more expensive Inconel 625. This study is focused on an innovative material, which had not yet been produced as a bulk, allowing the evaluation of the mechanical and metallurgical characteristics promoted by this mixture of powders. In addition, this deposition methodology seems very interesting for cladding or repair objectives since the failure did not happen at the transition interface between deposited bulk and substrate.

Chemical, microstructure, and mechanical property of TiAl alloys produced by high-power direct laser deposition

• A gradient TiAl specimen whose chemical composition varied greatly along the building direction was fabricated by high-power direct laser deposition. • The microstructure transforms from basket-weave microstructure into acicular martensite microstructure as the height increases, and the mechanical properties also vary greatly when the height changes. • By characterizing the deformation behaviors under the indentations, it is concluded that the phase constitution is responsible for the variation of the mechanical properties. Additive manufactured metals sometimes exhibit extraordinary microstructures and mechanical properties due to the particular processes. In this paper, we focus on a novel gradient TiAl alloys fabricated by high-power direct laser deposition, whose chemical composition, microstructure, and mechanical property vary along the building direction. The results indicate that Al concentration dramatically decreases from 39.5 at.% to 30.1 at.% as the height increases from the bottom to the top. Meanwhile, microstructural characterization indicates that the specimen appears basket-weave microstructure at the bottom, then the α 2 and γ phase gradually decrease, and eventually it transforms into acicular martensite microstructure in the top region. The indentation analysis shows that the associated hardness increases as the height increases, while the plasticity reaches a minimum value in the middle region. The increasing amount of β o (ω) is considered to be responsible for the increasing hardness because of the strong precipitation strengthening effect. The high plasticity in the bottom and top regions results from the strong deformation behaviors of the γ and β o phases.

Thermal and Mechanical Variation Analysis on Multi-Layer Thin Wall during Continuous Laser Deposition, Continuous Pulsed Laser Deposition, and Interval Pulsed Laser Deposition.

Direct laser deposition (DLD) is widely used in precision manufacturing, but the process parameters (e.g., laser power, scanning patterns) easily lead to changes in dimensional accuracy and structural properties. Many methods have been proposed to analyze the principle of distortion and residual stress generation, but it is difficult to evaluate the involvement of temperature and stress in the process of rapid melting and solidification. In this paper, a three-dimensional finite element model is established based on thermal–mechanical relationships in multilayer DLD. Differences in temperature and residual stress between continuous laser deposition (CLD) and pulsed laser deposition (PLD) are compared with the numerical model. To validate the relationship, the temperature and residual stress values obtained by numerical simulation are compared with the values obtained by the HIOKI-LR8431 temperature logger and the Pulstec μ-X360s X-ray diffraction (XRD) instrument. The results indicate that the temperature and residual stress of the deposition part can be evaluated by the proposed simulation model. The proposed PLD process, which includes continuous pulsed laser deposition (CPLD) and interval pulsed laser deposition (IPLD), were found more effective to improve the homogeneity of temperature and residual stress than the CLD process. This study is expected to be useful in the distortion control and microstructure consistency of multilayer deposited parts.

Exploring Potentialities of Direct Laser Deposition: Thin-Walled Structures

In the context of Industry 4.0, the interest towards the additive manufacturing processes is growing due to their numerous advantages, such as the possibility to prototype, the reduction of waste material, the inferior time to market, ad so on. In particular, a promising technology is the Direct Laser Deposition, which uses a focused laser beam to melt powders as there are deposited. In opposition to the well-established powder-bed fusion technologies, there are still some issues related to this process. This work aims to solve one of them, exploring the potentialities of DLD in printing thin-wall structures. For this purpose, the influence of the adopted deposition strategy and of the layer thickness on the geometrical accuracy and mechanical properties has been investigated. The results have pointed out that the first variable strongly influences the workpiece. It is possible to deposit thin-wall structures with a ZigZag strategy and consider a layer thickness equal to 90% of the height of the single track, printed with the same process parameters.

Phase composition and fatigue crack growth behavior of Inconel 718 under additive manufacturing

AM samples both in as-deposited and heat-treated states have been subjected to microstructural and phase characterization. Mechanical testing has been performed to heat-treated samples with respect to the build directions. AM samples were made by Direct Laser Deposition under the three different levels of efficient energy from Inconel 718 powder. As-deposited samples contained dispersed particles of Fe2Ti Laves phase, NbC and NbNC as well as starting clusters of γ′ phase. Although the standard heat treatment at 980 °C was found to result in partial dissolution of Laves phase precipitation of δ phase occurred around it. Depletion in Nb of Ni-based matrix due to formation of NbC, NbNC and Laves phase in as-deposited states causes precipitation of γ′ phase as the main strengthening phase in heat-treated states. Fatigue crack propagation manners were analyzed with fractography. Fine columnar dendrite structure of samples tested in parallel build direction to the loading is much more resistant to fatigue crack propagation. Fatigue crack bypass dispersed Laves phase which is fixed in matrix by δ phase.

Nano-TiB whiskers reinforced Ti–6Al–4 V matrix composite fabricated by direct laser deposition: Microstructure and mechanical properties

Titanium matrix composites (TMCs) have attracted widespread research interest due to their excellent properties. Herein, TMC reinforced with quasi-continuous reticular nano-TiB whiskers (nano-TiBw) was fabricated via a direct laser deposition method. The network of TiB reinforcements was found to be equiaxial and dendritic, and the addition of boron was shown to be beneficial to the equiaxial transition and refinement of α grains. The interface between the nano-TiBw and Ti64 matrix was observed to be clean and flat. The microhardness of the TMC was 465.1 HV0.025, 31% higher than that of the Ti64 alloy. Compressive tests showed that the TMC exhibited high ultimate compressive strength and good plasticity due to the grain refinement of the matrix and load transfer effect of nano-TiBw. The lower friction coefficients, smaller wear tracks, and minor wear weight loss revealed the superior wear resistance of the TMC in comparison to that of the Ti64 alloy. The reticular TiBw was found to be responsible for reducing the sensitivity of the TMC to adhesive wear.

Carrying Gas Influence and Fabrication Parameters Impact in 3D Manufacturing of In Situ TiN-Ti Composites by Direct Laser Deposition

The difficulty of getting a correct distribution of the reinforcement in the metal matrix and the complexity for achieving a good-metallurgy matrix-reinforcement bonding has limited the development of additive manufacturing of metal matrix composites. This research proposes the use of a reactive atmosphere during the fabrication process to obtain titanium matrix composites reinforced with TiN. The relation between the carrying gas and the process parameters used with the presence of porous and defects, the microstructure, and microhardness has been obtained. Nitrogen was used as the carrying gas of the titanium powder. Under laser irradiation, the particles melt and react with nitrogen, resulting in the formation of a titanium matrix composite highly reinforced with TiN. The composite obtained had a microhardness increase between 50 and 100% in comparison with titanium samples fabricated in the same conditions in an argon atmosphere. Three reaction mechanisms have been proposed to take place in the microstructure, depending on the amount of nitrogen in the titanium particles, and its diffusion in them during the manufacture.Graphic abstract