Pre-placed Powder Research Articles

Additive manufacturing of tantalum-zirconium alloy coating for corrosion and wear application by laser directed energy deposition on Ti6Al4V

In this work, a preplaced TaZr alloy powder layer (Ta:Zr = 7:3 in wt%) is coated on a Ti6Al4V substrate by laser-based directed energy deposition. Crack-free, smooth TaZr alloy coating is acquired, and the content of elements (Ta, Zr, Ti, Al, V) distributes in a gradient. In this coating, the bcc α-Ta grains with a rectangular shape and a size of 10–20 nm surrounded by the TiZr phase, the Al3Zr4 phase mainly gathers in grain boundary. The corrosion resistance of the coating and substrate is tested via electrochemical measurement and the coating exhibits a better resistance in 0.5 mol/L H2SO4 solution. The average microhardness of the coating is around 600 HV, which is ~1.7 times than that of the substrate. The wear test results show the mass loss of the coating is approximately 60 times lower than the substrate at room temperature. It is believed that this coating could shed light on not only the protection of Ti-based alloy, but also the Fe-based and Ni-based components with the functionally graded transition layers.

Laser surface alloying applied on Ti-3Mo and Ti-10Nb sintered parts

Laser surface treatments can be applied to sintered parts aiming to close porosity and produce properties gradients. In this work, the Ti-10Nb and Ti-3Mo alloys were produced by powder metallurgy and modified by laser surface alloying with preplaced Nb and Mo powders. The laser power was varied from 150 to 450 W, while other laser parameters were kept constant. Substrates and surface-modified layers were characterized by scanning electron microscopy, X-ray diffraction, and nanoindentation. Both sintered alloys presented α+β microstructure, and the pore-free surface layers were composed of α″ and α′ immerse in a β phase matrix. Higher laser powers produced deeper fusion zones, and, consequently, a higher dilution of the alloying element. The higher the laser power, the deeper the fusion zone, and the higher the alloying dilution. Mo preplaced powder allowed the attainment of thicker and more diluted surface layers under the same heat input. The surface-modified layers presented higher hardness than the substrate. The Ti-Mo surface layers' elastic modulus was slightly higher than that of the sintered Ti-3Mo substrate, and the Ti-Nb surface layers' elastic modulus was lower than that of the sintered Ti-10Nb substrate. The Ti-Nb surface layer processed at 300 W presented the most extensive presence of the β phase and the lowest elastic modulus.

Dissimilar unbeveled butt joints of AA6061 to S235 structural steel by means of standard single beam fiber laser welding-brazing

Abstract The design of parts involving the combination of different materials has gained much attention in recent years as a strategy to achieve weight reduction. Joining of dissimilar materials has become a necessity to produce such parts. However, obtaining dissimilar welds showing high mechanical performance usually takes complex, expensive and time-consuming approaches. In this study, an affordable welding approach is proposed to join 6061 aluminum alloy to S235 structural steel, based on a single laser beam, on unbeveled square butt joints, and on the combination of welding wire and preplaced Al-Si powder. The relation between welding parameters and imperfections formation is studied by using a combined parameters analysis, and thorough microstructural and mechanical characterization are performed. The application of AlSi5 wire in combination with AlSi12 powder led to crack-free welds showing homogeneous 3 ± 1 μm-thick intermetallic compound (IMC) layer, composed of Fe2Al5, Fe4Al13 and Fe4Al17.5Si1.5 intermetallics. Transverse tensile tests showed that ultimate tensile strength of joints was as high as 169 MPa, which is above the minimum value established by international standards on AA6061 welds and comparable to the values obtained by other authors’ dissimilar welds by using more complex and expensive welding approaches. The IMC layer showed 11.2 ± 0.7 GPa hardness and 257 ± 24 GPa modulus of elasticity. Both properties are much higher than the base materials ones, and are in agreement with the mechanical behavior seen under tensile testing.

Laser Beam Welding of Feconicrmn High-Entropy Alloys with Preplaced Powders

In this paper, as-annealed FeCoNiCrMn plates were laser-welded with preplaced FeCoNiCrMn and FeCoNiCrAl powders, respectively. The grains in the fusion zone of the weld with FeCoNiCrMn powder have a reduced aspect ratio compared to those without preplaced powders and the weld with FeCoNiCrAl powder presents relative equiaxed grains. The yield strength of each weld has been remarkably enhanced when referring to the base alloy, and the ultimate tensile strength of each weld with preplaced powder exceeds 80% of that of the base and the maximum reaches 88.5% when referring to the weld with preplaced FeCoNiCrMn powder. Cleavage fractography was observed in the welds. The finding of this work will service the engineering practices of high-entropy alloys.

Geometrical assessment and mechanical characterization of single-line Inconel 825 layer fabricated on AISI 304 steel by TIG cladding method

In the course of the deposition of Inconel 825 layer on AISI304 stainless steel by tungsten inert gas (TIG) cladding route, the geometry of the single line clad tracks for different processing conditions was evaluated and correlated with the employed processing current and pre-placed powder thickness. The analysis revealed an enhancement in the clad track width, clad height along with the dilution ratio, and the clad angle for increasing the TIG current. Nevertheless, no specific effect of pre-placed powder thickness was recorded. A detailed EDS analysis at the interface of the clad layer revealed an enhancement in the dilution of Inconel 825 with the substrate material in the clad zone and vice-versa in the substrate region for increasing the processing current. The micro-hardness value recorded at the cross-section of the clad track shows a consistent improvement (300–350 HV) as compared to the substrate material (220 HV). The sliding wear test of the samples extracted from the clad tracks performed against harden-die steel plate shows almost negligible height loss (6 µm) as compared to the uncoated steel specimen (125 µm) under similar test condition.

Development of process maps based on molten pool thermal history during laser cladding of Inconel 718/TiC metal matrix composite coatings

In the present study, laser cladding of Inconel 718/TiC metal matrix composite coating on SS 304 was carried out using a 2 kW Yb-Fiber laser by preplaced and blown powder method. The molten pool thermal history during the deposition process was monitored using an IR pyrometer and correlated with the evolution of microstructure. The effect of various process parameters, viz. laser power ‘P’ (400 W – 1200 W), scanning speed ‘V’ (200 mm/min to 1400 mm/min) and powder feeding rate (8 g/min – 20 g/min) on the molten pool lifetime and cooling rate is investigated. Molten pool lifetime increased and cooling rate decreased with the decrease of scanning speed and increase of laser power. However, scanning speed had dominant effect on the molten pool lifetime and cooling rate compared to laser power. Moderate molten pool lifetime resulted in the formation of shell-core structure between Inconel 718 matrix and TiC particles. The longer molten pool lifetime (>0.45 s) resulted in complete decomposition of TiC particles rendering brittleness to the coating. The feasibility of detecting TiC particle decomposition in Inconel 718 matrix from the recorded thermal cycles based on multiple solidification shelfs is discussed. At the end, a process map based on a combined process parameter, PD/V2 (spot diameter D) and molten pool lifetime (τ) was developed. An optimum range of molten pool lifetime i.e. 0.25 s < τ ≤ 0.45 s was found to result in a good quality of coating with the formation of shell-core structure both in case of preplaced and blown powder laser cladding method.

Spattering effects during selective laser melting

Additive manufacturing using preplaced powder became a widely used processing method during the last few years to build complex and precise structures. The laser beam is often used as a heat source to selectively melt the powder and create the structure layer by layer. However, during the processing, imperfections can occur. The spattering, accumulation of powder particles, and denudation effects lead to a redistribution of the powder in the powder bed, a loss of powder, and agglomerations. These effects can alter the processing of the subsequent tracks and layers. On the one hand, the track dimensions can vary by varied amounts of available powder. On the other hand, the process can become unstable due to different powder surface geometries, agglomerates, and the resulting variation of energy absorption. Therefore, this work aims to explain spattering effects during powder-bed processing, which can be used for modification of the processing parameters for spatter prevention. High-speed-imaging was used to record the powder and spatter behavior during the laser powder-bed processing from different angles. It was observed that different kinds of spattering can occur. Melt pool spattering occurs due to melt instabilities, while powder particles seem to be accelerated by vapor induced gas movement, pressure waves, and the gas flow induced by the Bernoulli-effect.

Effect of overlapping condition on large area NiTi layer deposited on Ti-6Al-4V alloy by TIG cladding technique

A large area coating of NiTi was fabricated on Ti-6Al-4V alloy by overlapping of single line clad tracks obtained by TIG arc scanning on the preplaced NiTi powder layer. The overlapping percentage was varied by shifting the successive clad track to a specific distance, and its effect on the clad morphology and the hardness value measured at different locations, i.e., the center of individual clad track and interface between two consecutive tracks were investigated. The wear characteristic of the NiTi clad layer produced at different overlapping conditions was also analyzed through a sliding wear test executed by a ball-on-disc arrangement. The results indicated that for higher overlapping percentage employed during the TIG cladding process, owing to pre and post-heating of the successive clad track resulted in harder phases of NiXTiY, and improved the hardness value and the sliding wear resistance of the clad layer.

Effect of energy density on laser powder bed fusion built single tracks and thin wall structures with 100 µm preplaced powder layer thickness

Laser Powder Bed Fusion (LPBF) is one of the advanced manufacturing technologies used for fabricating near net shaped components directly from CAD model data by selectively melting pre-placed layer of powder in layer by layer fashion. LPBF process is widely researched with layer thickness up to 60 µm and is now commercially deployed for many metallic materials. However, very limited literature is available in public domain for LPBF with layer thickness >60 µm as the process in this window has many challenges in geometry control and reproducibility due to inherent process instability. However, higher layer thickness with larger beam diameter can bring better productivity and shorter built time with limited compromise on minimum feature size. The present work focuses on a systematic parametric study on single track and thin wall fabrication using LPBF at layer thickness of 100 µm by varying laser power (150–450 W) and scan speed (0.02–0.08 m/s) using SS 316L powder. For the range of parameters under investigation, process window yielding stable tracks (regular and uniform) is obtained for energy density between 87.5 and 140 J/mm3. An analytical model for predicting the width of the track and a regression model for the depth of re-melted zone in the substrate subsurface and track area during single track fabrication is developed in terms of energy density. The average difference in predicted and experimental values for width and area of the track are 3.18% and 7.61%, respectively within the process window. Width of thin walls built at the same parameters is measured and the variation between width of thin wall and track is estimated in terms of energy density. The width of thin walls fabricated are observed to be larger than that of single track built at the same combinations of process parameters primarily due to preheating effect. For the range of parameters under investigation, the highest values of width of thin wall and its difference from corresponding width of track is observed at 112.5 J/mm3 in the process window. The study paves a way in understanding the effect of higher layer thickness on the geometry of LPBF built components.

Ultrafine eutectic coatings from Fe-Nb-B powder using laser cladding

In this paper, a laser cladding process was proposed to obtain ultrafine eutectic Fe-Nb-B alloy coatings on AISI 1020 steel substrate in which the power and scanning speed were defined. The laser cladding process can be used as a rapid solidification processing route to produce single laser tracks and coatings (overlapped tracks) with pre-placed powders deposited onto a mild steel substrate in order to obtain a minimal dilution between the coating/substrate. This research aims to produce a coating of Fe74.25Nb8.25B17.5 (at.%) alloy over the AISI 1020 steel substrate by laser cladding using a pre-placed powder method to investigate the parameters on coating production, dilution, microstructure, and properties. The final goal is to find an appropriate processing parameter range to obtain coatings with high hardness and possibly good wear resistance. The microstructure and behavior of powder, cladding layers and coatings were examined by X-ray Diffractometry, Differential Scanning Calorimetry, Scanning Electron Microscopy and Transmission Electron Microscopy of selected samples prepared with a Dual Beam microscope using a Focused Ion Beam. For the processing conditions used, the coatings showed larger exothermic crystallization peaks. The results of microhardness from coatings showed values of hardness over 700 HV.

Simulation and experimental investigations on the effect of Marangoni convection on thermal field during laser cladding process

Laser cladding process consolidates powder or wire materials into a dense metal part with the assistance of laser energy source. Simulation of temperature distribution and fluid flow of the laser cladding process was carried out using the finite volume method (FVM). The influence of the Marangoni convection on the dimensions of the melt pool was investigated. A double peak phenomenon of flow velocity on the surface of the molten pool was induced by the Marangoni convection. The heat and mass transfer in the molten pool caused by the Marangoni effect was measured by the Peclet number. Temperature gradient (G) and solidification rate (S) were calculated to predict the morphology and size of the solidification microstructures. During the solidification process of the molten pool, the cooling rate (G×S) declines from the top region of the cladding layer, and the solidification rate of the cladding layer gradually decreases from the top to the bottom. The corresponding microstructure is in a good agreement with the experimental results. The metallurgy bonding between the substrate and the cladding layer is not very well when the simulated molten pool depth is smaller than the thickness of the pre-placed powder bed.

AMORPHOUS/NANOCRYSTALLIZATION OF TiN-TiB2 REINFORCED LASER CLAD COMPOSITES

Laser cladding (LC) of the Stellite6-BN-Cu-Y2O3 pre-placed powders on the Ti-6Al-4V alloy substrate formed the bulk TiN and needle-like Ti-B compounds reinforced composites, which improved the wear resistance of the substrate surface. The results showed that lots of the amorphous-nanocrystalline phases were produced in such composites; the proper content of Cu was used to refine microstructures, also a large number of the nanocrystals can be formed; the elements such as Co, Ni and Fe in Stellite6 favored the amorphous phase to be formed. The addition of Y2O3 improved the fluidity of the liquid metal, also retarding the growth of the dendrite, leading the fine microstructures to be formed, improving the property of the LC composites.

MICROSTRUCTURES AND WEAR RESISTANCE OF THE ARGON ARC CLAD NICKEL-BASED COMPOSITES ON TITANIUM ALLOY

The composites were obtained by the argon-arc cladding (AAC) of the Deloro22-Si3N4-Fe pre-placed powders on a TA1 titanium alloy substrate, which improved the wear resistance of the substrate. Such composites were investigated by means of the scanning electron microscope (SEM), the microscope and the high resolution transmission electron microscope (HRTEM). The results indicated that the amorphous phases were produced in such AAC composites, increasing the wear resistance. With addition of Y2O3, lots of the micro/nanoscale particles were formed, which further improved the wear resistance of such AAC composites.

Design of wear and corrosion resistant FeNi-graphite composites by laser cladding

Friction and wear can cause severe surface damage of machine elements and result in their complete failure. Of the several ways to limit/control friction and wear, a measure to counter the potential damage is to apply a layer of a material with more desirable surface properties, which also provides the flexibility to use cost-efficient bulk materials. In systems under dry sliding conditions, such as railway switches, replacing the components cost considerable time and money. In the current study, laser cladding, a well-known powerful process for repair engineering, was used to fabricate new innovative low friction materials for dry sliding contacts that are exposed to corrosive attacks. The successful implementation of graphite as a solid lubricant in dry sliding conditions is well-known. A homogenous coating with an austenitic FeNi-based alloy containing different percentages of graphite was applied by means of laser cladding using preplaced powder method. The effect of graphite content and its lateral distribution on the friction and wear properties is investigated using a modified ASTM G65 test rig in two-body sliding configuration. Tribological results showed that increasing graphite content in the clads significantly reduces wear, but the level of the coefficient of friction appears to be determined mainly by the composition of the matrix. The wear resistance can be further increased by maximising the inter-particle distances. On the whole, clads of all compositions had superior wear resistance compared to reference materials such as stainless steel XNi22 and cast iron GJL250. The clads were further classified in terms of corrosion resistance against synthetic sea water in a modified potentiodynamic setup. The resistance to wear and corrosion improve with graphite content.

MICROSTRUCTURE PERFORMANCE AND SYNTHESIS OF THE ARGON-ARC CLAD COBALT BASE COMPOSITE COATING

Composite coating was obtained by argon-arc clad (AAC) of the Stellite20-Si3N4 pre-placed mixed powders on a TA7 titanium alloy substrate, which increased the micro-hardness of the substrate surface significantly; then this produced coating was re-melted by an argon arc, forming a hard layer on the coating surface. The AAC and re-melted coatings were investigated using a scanning electron microscope and high-resolution transmission electron microscope, indicating that amorphous and nanocrystalline phases were produced in the coating surface after an argon-arc re-melting process, which increased the wear resistance of such coatings significantly.

The influence of the pre-placed powder layers on the morphology, microscopic characteristics and microhardness of Ti-6Al-4V/WC MMC coatings during laser cladding

Ti-6Al-4V/WC metal matrix composite (MMC) coatings provide an adequate solution for high wear resistance application of Ti-6Al-4V. Laser cladding process, which has been extensively researched as an advanced repairing and strengthening technology, is considered to be an important candidate to combine the ductile metal matrix and high surface properties. In this study, the powder-presetting laser cladding technology was employed to fabricate WC particles reinforced MMC coatings applying pre-placed pure WC powder layers and Ti-6Al-4V/WC powder layers, respectively. These experiments were performed to investigate the influence of the pre-placed powder layers on the Ti-6Al-4V/WC MMC coatings. The morphology, microscopic characteristics and microhardness of the coatings were scrutinized using optical microscopy, X-ray diffractometer, scanning electron microscopy and Vickers hardness tester. The experimental results show that the coatings fabricated from pre-placed Ti-6Al-4V/WC powder are enhanced more obviously in comparison with pure WC powder, which is related to the more uniform distribution of WC, smooth morphology of coatings and homogeneous distribution of microhardness.

Track geometry variations in selective laser melting processes

Selective laser melting processes are widely used for many industrial applications using a laser beam to melt preplaced powder material layer by layer to create technical parts. The building process of those structures requires remelting of adjacent tracks and layers in order to avoid cavities and achieve the joining of the new track to the previous track and layer. In order to achieve a sufficient overlap and minimize cavities, usually conservative processing parameters are chosen. A higher energy and powder usage efficiency would be achieved if knowing about the formation process of the single tracks and their geometrical dimensions depending on the available powder. In this work, it is shown that the cross-sectional track geometry significantly varies within one layer. A simple model is developed describing the influence of the available powder for each track within one layer. Depending on the hatch distance, different variation patterns are observed and modeled showing that the track variations are inherent phenomena of the process. It can be concluded that the variations of powder availability can cause the geometric variations of the tracks.

Laser cladding of nanoparticle TiC ceramic powder: Effects of process parameters on the quality characteristics of the coatings and its prediction model

In laser cladding process, the quality characteristics of the coatings are directly affected by laser cladding process parameters. In this work, in order to investigate the effects of the process parameters on the quality characteristics of the ceramic coatings on Ti6Al4V substrates, which were prepared by laser cladding using nanoparticle TiC powder as pre-placed material, five single factors (laser power, scanning speed, pre-placed powder thickness, laser spot diameter and multi-track overlapping ratio) and four interactions related to laser power were selected as influencing factors, and then multi-track cladding experiments with L27(313) orthogonal array designed by Taguchi method were carried out to obtain the geometric characteristics (coating thickness, coating width and height difference) and mechanical property (micro-hardness) of the coatings. Based on experimental results, the influence extents of all factors on quality characteristics were ranked by signal-to-noise (S/N) ratio, and the significances of all factors on quality characteristics were evaluated by using P-value from the analysis of variance (ANOVA). Then the effects of the significance factors on quality characteristic were analyzed in detail, and the optimal process parameters were selected. Finally, a prediction model based on support vector machine (SVM) was developed for the quality characteristics of the cladding coatings. The results showed that the pre-placed powder thickness, laser spot diameter and laser power were the most crucial process parameters. The prediction model based on SVM method can correctly describe the relations between cladding process parameters (input variables) and quality characteristics of coatings (output characteristics), with the correlation coefficient CC > 0.94. This research work gives a guideline for the selection of appropriate process parameters for laser cladding of high-quality coatings using nanoparticle TiC ceramic powder as starting material.

Powder particle movement during Powder Bed Fusion

Powder Bed Fusion is a widely used technique to produce complex parts with different materials. In principle, a pre-placed powder layer is locally melted by a laser beam and thereby fused to the previous tracks and layers. This technique offers high flexibility at fast processing speeds. High-speed laser scanning enables, on the one hand, the fast processing but induces heat, forces and pressure in and around the processing zone on the other hand. The behavior of the single particles on the powder bed around the processing zone is hard to observe and therefore not sufficiently investigated. High-speed-imaging was used in this work to track the movement of the powder particles of the powder bed during Powder Bed Fusion in order to observe and explain their behaviour. It could be observed that powder particles move towards the melt pool affecting a large area around the melt pool, which changes the powder distribution of the powder bed. This indicates that a strong gas flow is constantly present during processing, which is thought to be due to the metal vapour induced by laser evaporation but can be also induced when no vapour is present due to the temperature and pressure increase around the processing zone.

Laser Cladding In-Situ Ti(C,N) Particles Reinforced Ni-Based Composite Coatings Modified with CeO2 Nanoparticles

To improve the wear resistance of titanium alloy parts used in the engineering applications, in-situ formed Ti(C,N) particles reinforcing Ni-based composite coatings are fabricated on Ti6Al4V alloys by the laser cladding technique using Ni60, C, TiN, and small amounts of CeO2 nanoparticles mixed powders as the pre-placed materials. Firstly, the formation mechanism of Ti(C,N) particles as a reinforced phase in the coating is investigated. Then, the influences of CeO2 nanoparticles on microstructures and wear resistance of the coatings are analyzed. It is indicated that the large Ti(C,N) particles form around TiN particles, and the small Ti(C,N) particles form through independent nucleation. CeO2 nanoparticles play important roles in increasing the nucleation rate and improving the precipitation of Ti(C,N), hence the microstructures and wear resistance of the coatings are apparently improved after adding CeO2 nanoparticles. It is observed that the 1 wt % content of CeO2 additive in the pre-placed powders is the best choice for the wear resistance of the coatings.