Direct Metal Deposition Process Research Articles

Reducing Oxidation during Direct Metal Deposition Process: Effects on Ti6Al4V Microstructure and Mechanical Properties

The production of materials with a high affinity for oxidation using the direct metal deposition (DMD) process requires an extended process examination that goes beyond the usual, purely energetic consideration, with the aim of providing sufficient energy to melt the substrate and the powder material supplied. This is because the DMD process does not allow any conclusions to be drawn as to whether it and its respective selected parameters result in an oxidation critical process. To assess this, a superposition of the temperature field with the existing spatial oxygen concentration is required. This work uses this approach to develop an oxidation model that reduces oxidation during the DMD process. In addition to the numerical model, an analytical model is derived, with which the temperature of a material element can be calculated analytically and the resulting boundary oxygen concentration calculated using Fick’s 2nd law. The model also takes into account two-stage oxidation kinetics for Ti alloys. The effect of too high a travel speed (with the same specific energy of the other experiments) is shown visually in the numerical calculation of the temperature field. However, if the process model is taken into account, the components do fulfil the specified requirements. Finally, the effect of oxidation on the microstructure, microhardness, ultimate strength, yield strength and elongation at failure of Ti6Al4V structures produced using DMD is also investigated, and further supports our conclusions regarding the effectiveness of the proposed model.

Comprehensive Distortion Analysis of a Laser Direct Metal Deposition (DMD)-Manufactured Large Prototype Made of Soft Martensitic Steel 1.4313

Additive manufacturing (AM) by using direct metal deposition (DMD) often causes erratic distortion patterns, especially on large parts. This study presents a systematic distortion analysis by employing numerical approaches using transient–thermal and structural simulations, experimental approaches using tomography, X-ray diffraction (XRD), and an analytical approach calculating the buckling distortion of a piston. The most essential geometrical features are thin walls situated between massive rings. An eigenvalue buckling analysis, a DMD process, and heat treatment simulation are presented. The eigenvalue buckling simulation shows that it is highly dependent on the mesh size. The computational effort of the DMD and heat treatment simulation was reduced through simplifications. Moreover, artificial imperfections were imposed in the heat treatment simulation, which moved the part into the buckling state inspired by the experiment. Although the numerical results of both simulations are successful, the eigenvalue and DMD simulation cannot be validated through tomography and XRD. This is because tomography is unable to measure small elastic strain fields, the simulated residual stresses were overestimated, and the part removal disturbed the residual stress equilibrium. Nevertheless, the heat treatment simulation can predict the distortion pattern caused by an inhomogeneous temperature field during ambient cooling in an oven. The massive piston skirt cools down and shrinks faster than the massive core. The reduced yield strength at elevated temperatures and critical buckling load leads to plastic deformation of the thin walls.

Experimental study of SS316L, Inconel 625, and SS316L-IN625 functionally graded material produced by direct laser metal deposition process

Three types of walls were produced by the direct laser metal deposition (DLMD) process; SS316L, Inconel 625 (IN625), and SS316L-Inconel 625 FGM. The thin walls were deposited on an AISI 4130 austenitic steel substrate in five layers with a length of 2 cm. The solidification behavior, secondary dendrite arm spacing (SADS) value, chemical composition, and hardness of samples were studied. Microstructures were evaluated using optical microscopy (OM) images, scanning electron microscopy (SEM), EDS Line, EDAX, and Map analyses. OM and SEM images showed an acceptable bonding between the layers in all samples. The main solidification was equiaxed dendritic and columnar dendritic in most parts. The growth of dendrites was perpendicular to the substrate. Some dendrites grew epitaxially at the interface of two sequential layers. The standard deviation of SDAS value for SS316L, IN625, and SS316L-IN625 FGM was 0.63, 0.71, and 0.73, respectively. The lowest value of SDAS was obtained in layer one, while the highest value was obtained in layer five of the thin walls. The average SDAS values for SS316L, IN625, and SS316L-IN625 FGM were 4.40, 4.53, and 4.76 µm, respectively. Therefore, the difference between the SDAS values was not significant. EDAX and Map analyses showed that the segregation of Nb and Mo to the dendritic boundary and the formation of eutectic, secondary phases, and brittle Laves phases have occurred. Additionally, the segregation of Nb and Mo to the dendritic boundary in the SS316L-IN625 FGM sample was higher than SS316L and IN625 samples. The microhardness values oscillated significantly and decreased by moving away from the substrate (in the build direction). The microhardness values of different points of the SS316L-IN625 FGM walls were in the range of 218–278 HV. EDS line scan, microstructure, and microhardness analyses indicated the successful fabrication of the SS316L-IN625 FGM by the DLMD process.

Research on gas pore formation and inhibition mechanism of high nitrogen steel during laser direct metal deposition

Nitrogen pore formation is an inherently tricky problem that limits the application of high nitrogen steel (HNS) in various fields. In the present study, a CA-LBM coupling model was proposed to understand the nitrogen pore formation mechanism during laser direct metal deposition (DMD) of HNS. Based on the proposed model and theoretical analysis, the effects of pulse frequency and shielding gas nitrogen content on the porosity were clarified systematically. Results showed that after the onset of solidification, the ferrite phase with extremely low nitrogen solubility precipitated first, inducing the continuous enrichment of nitrogen in the residual liquid phase, which in turn provided the prerequisite for nitrogen pore nucleation. The application of quasi-continuous-wave (QCW) laser mode accelerated the molten pool solidification, and shortened the nitrogen supersaturated region, allowing more nitrogen to be solid dissolved in the matrix. On this basis, the porosity was inhibited to a certain extent. Compared with the high-frequency pulse, the low-frequency pulse showed better porosity inhibition. In addition, following the classic nucleation theory, nitrogen segregation and nitrogen solubility synergistically dominated nitrogen pore formation. The nitrogen partial pressure, determined by the shielding gas nitrogen content, played a dual role in porosity defects. Elevated nitrogen partial pressure served to increase nitrogen solubility while promoting the nitrogen absorption process to intensify nitrogen segregation. However, due to the short duration of the molten pool, the nitrogen absorption process could not be fully processed. Nitrogen solubility enhancement was the key factor in inhibiting porosity during the DMD process.

The dynamic carbon footprint modeling for laser direct metal deposition based on processing states

The Laser Direct Metal Deposition (LDMD) process offers notable advantages and obtains wide application in aerospace, defence equipment and other industrial sectors. However, the characteristics of multiple-inputs and low-efficiency in the process will bring about high carbon footprint. Additionally, the carbon footprint fluctuates with processing states and equipment systems owing to the discontinuous process in LDMD involving the layer-by-layer deposition and the laser-powder coupling. This paper investigated dynamic carbon footprint based on the resource and energy consumption from the process-oriented standpoint in LDMD process, which quantified the processing time, power distribution and material consumption of each processing state for every sub-system. In this research, the discontinuous process was divided into effective processing state, rapid moving state and transformed delay state and the processing time of each state were estimated with process parameters. Besides, the operating state time and power distribution of five sub-systems were estimated combining the equipment triggering mechanism and the processing state time. The orthogonal experiment was carried out to deposit the 316L stainless steel powder into parts with the size of 15 mm*15 mm*10 mm under the given process parameter conditions (Vf ∈ [8,12] g/min, P ∈ [600,1000] w, Vs ∈ [500,900] mm/min). The results show that carbon footprint of deposited parts fluctuates in the range of 3624.68–15759.08 g under parameter combinations and the low carbon footprint parameter-interval is obtained. The average carbon footprint of effective processing state, the transformed delay state and the rapid moving state account for 55%, 40% and 5% respectively, while the carbon footprint of powder supply sub-system contributes to 68% among the five sub-systems. This research revealed and qualified the dynamicity of carbon footprint at the levels of processing states, process parameters and sub-systems, offering potential for carbon footprint reduction in advanced manufacturing processes.

Residual stress computation in direct metal deposition using integrated artificial neural networks and finite element analysis

Direct Metal Deposition (DMD) as a part of additive manufacturing processes, utilizes a laser heat source to deposit metallic material to construct components layer by layer, allowing for controlled manufacturing. DMD processes inherently induce residual stresses and distortions in the build part. While finite element (FE) analysis is one of the approaches to predict the residual stress, the high computational time required for the FE analysis makes it undesirable for the early stages of product development. This study extends a previously developed approach to more complex structures to evaluate the accuracy and computational efficiency of predicting residual stresses. For the present study, six different metallic structures with various complex shapes made from AISI 304 L were considered. Comparing the residual stress fields predicted by the integrated ANN-FE with thermomechanical FE analysis, the results demonstrated consistent distributions for all stress components across structures. Although localized regions exhibited higher errors, these areas coincided with low-stress zones, posing no critical concerns. Despite higher errors in some areas, most structures displayed prediction errors below 15%. Moreover, the analysis achieved a substantial computational efficiency enhancement of approximately 6 times for all structures analyzed.

Oxygen Uptake of Ti6Al4V during Direct Metal Deposition Process

The efficient fabrication of titanium components using laser direct metal deposition (DMD) is gaining significant importance in the aerospace and medical sectors. The DMD process must be appropriately designed to address the issue of oxidation, as titanium exhibits a high affinity for oxygen. The carrier gas flow and shield gas flow, which have been considered secondary factors so far, are shown to exert a substantial influence on the gas dynamics of the DMD process. By varying these parameters, it is possible to identify the influence of the gas volume flows on the oxidation behavior exhibited during the DMD process. To quantify the oxygen uptake in titanium structures during buildup, hot carrier gas extraction is employed. Experiments are conducted using both a three-jet and a coaxial nozzle to assess the influence of nozzle geometry. Additionally, the experiments are conducted within a shielding gas chamber to demonstrate the benefits of such a chamber in mitigating oxidation. Finally, the study reveals that by appropriately combining the parameters of carrier gas volume flow, shield gas volume, and travel speed, it is possible to fabricate titanium components, which fulfill the requirements regarding oxygen content of aerospace and medical applications even without the utilization of a shielding gas chamber.

Effect of laser direct metal deposition process on the microstructure and mechanical properties and temperature and stress fields of 24CrNiMo

Effect of laser direct metal deposition process on the microstructure and mechanical properties and temperature and stress fields of 24CrNiMo

Investigating the relationship between mechanical properties and residual stress in the laser cladding process of Inconel 625 superalloy

In the present study, the tensile strength, fracture surface, hardness, and amount of residual stress in Inconel 625 super alloy cladded with direct metal deposition (DLD) process in the states before and after stress relief was studied. Residual stresses on the cladding layer surface were determined via XRD method. According to results, the yield strength of Am sample increased by 10% compared to thecast sample (reference sample). Although the yield strength experiebced an increase, the ductility followed an opposite trend falling from 42.5% to 26%. According to residual stress test outcomes, tensile residual stress of 361 MPa in the additive-manufactured sample. After stress relaxation heat treatment and almost complete removal of residual stress, the ductility reached 52.5%, the ultimate strength was also improved by 17% from cast sample. Also, after stress relaxation, the hardness of the sample and its fluctuations are reduced.

Online monitoring of direct laser metal deposition process by means of infrared thermography

AbstractDirect laser metal deposition (LMD–DED) is an additive manufacturing (AM) process that is used to build up and repair high-quality metal components. It works by overlapping layers of powder material and melting them with a laser. To get a stable process without defects and to reach, at the same time, high mechanical properties, a robust assessment and control of the process parameters, and above all of their combination, is required. The ideal goal is to assure the online control, to stop or correct the process in case of unexpected anomalies. In this work, a robust online monitoring of the laser metal deposition (LMD–DED) process based on the use of infrared thermography was developed and proposed. After choosing the suitable process parameters, a customized design of experiments (DOE) was set, and the statistical analysis of different thermal features was carried out to develop the most robust models that correlate them with the input process parameters (laser power, scanning speed, and powder flow rate). The proposed procedure was based on the extraction of different thermal features from suited regions of interest (ROI), performing statistical analyses by means of analysis of variance (ANOVA) and building regression models to correlate the process parameters with the thermal behavior. The obtained results demonstrated the possibility to control the process by means of the chosen thermal features, independent of the position of the ROI. Moreover, the possibility to use the models to detect typical AM defects, and anomalies, online directly during the process, has been proved and verified by destructive macrographs carried out on the manufactured coupons.

Spatial Oxygen Distribution of the Direct Metal Deposition Process for Different Powder Nozzles

Additive manufacturing processes are among the most innovative manufacturing processes of this century. Powder-based direct metal deposition (DMD) is one of these processes. In the DMD process, local shielding takes place via the powder nozzle. The process is therefore critical for oxidation, especially for materials with an affinity for oxidation such as titanium, aluminum and their alloys. In order to study the oxidation behavior in more detail, the present gas dynamics must be further understood. Wirth and Wegener have made a first approach with their gas flow simulation. In this study, a measurement method for spatial oxygen concentration determination is presented. It can be shown that the spatial oxygen concentration follows the nozzle geometry. Furthermore, the coaxial nozzle is superior to the three-jet nozzle with respect to a low oxygen concentration from a carrier gas to shielding gas volume flow ratio of equal to or greater than 0.4. Finally, it can be shown that the use of a shielding gas chamber eliminates the optimization of the gas flow settings.

Functionally Graded Additive Manufacturing of Thin-Walled 316L Stainless Steel-Inconel 625 by Direct Laser Metal Deposition Process: Characterization and Evaluation

Direct Laser Metal Deposition (DLMD) is a state-of-the-art manufacturing technology used to fabricate 316L stainless steel/Inconel 625 functionally graded material (FGMs) in this research. For the practical application of these materials in the industry, the effects of process parameters on the geometric characteristics and surface roughness require more investigation. This FGM was additively manufactured in five layers by changing the 316L stainless steel/Inconel 625 ratio in each layer. The effects of laser power on geometric characteristics, height stability, and surface roughness were investigated. The microstructural analysis and microhardness profiles were studied. The results show that despite the high solidification rate, the segregation of alloying elements into dendritic areas occurred. It was also found that increasing the laser power will increase the height, width, height stability, and surface roughness of the gradient walls. The maximum width and height of the deposited layers were 1.615 and 6.42 mm, respectively, at the highest laser power (280 W). At the laser power of 220 W, the least surface roughness (Ra = 105 µm) and the best height stability (0.461 mm) will be obtained. The microhardness values will differ in various sections of the gradient walls in a range of 225–277 HV.

An investigation into the effect of scanning pattern and heat treatment on the mechanical properties of Inconel 718 in the direct metal deposition process

In this study, the additive manufacturing process of Inconel 718 superalloy on a homogenous forged substrate with three different patterns is investigated, followed by studying the samples’ mechanical properties and heat treatment effect on them. After additive manufacturing, the tensile properties of the samples were investigated by form of 50%-forging specimens and 50%-additively manufactured specimens. Results showed the best tensile properties are obtained in a sample made with a circular pattern. Also, the difference in the tensile properties of the sample made parallel to the tensile axis, and the sample created in the vertical direction indicate the anisotropic behavior of the additively manufactured samples. After additive manufacturing of samples with parallel, perpendicular, and circular patterns, the ultimate strength increased by 7, 6, and 11% compared to the control sample (forging). Despite the strength increase, the ductility of the specimens decreased by 28, 20, and 11% compared to forging specimen. In the 100%-additively manufactured sample, the ultimate strength increased by 6%, and the ductility decreased by 45%. After investigating the pattern for mechanical properties, annealing heat treatment at 1080 °C for 1 h and two aging processes at 620 and 720 °C for 4 and 8 h, respectively, were performed to improve the properties and create a strengthening phase. The ductility of the sample made with a 50% circular pattern and 50% forging sample increased by 51% and 36% by annealing compared to the raw state and the control sample, respectively. Moreover, the hardness level and ductility became uniform and increased after annealing.

Prediction of deposit characteristics based on the discrete coaxial nozzle during laser direct metal deposition

The variation in powder mass concentration distribution caused by the nozzle structure influence the deposit profiles and quality to a considerable extent in laser direct metal deposition (DMD) process. For symmetrical discrete nozzles, the powder mass concentration distribution was described, and the results were utilized in a molten pool model without powder feeding simulations. The evolution of the molten pool dimensions, and the temperature and velocity fields of single-track deposit were explored. The influences of specific energy (E) and mass per unit length (MUL) on the characteristics of single-track deposits were investigated. The correlation between the powder stream position and the concentration distribution was clearly expressed. The results indicated that the width/height ratio of single-track deposits decreased significantly, and the contact angle first increased and then decreased with higher specific energy intensity. Besides, the variation in the spatial position of the powder stream led to different mass concentration distributions, resulting in the tilt of deposit cross-sections towards the side with a higher mass concentration. The model exhibited a capacity for predicting the profiles of the deposits formed by symmetrical discrete nozzles.

Development of a calibration technique of a finite element model for calculating compensation of deformations from the action of residual stresses in additive manufacturing

In additive manufacturing, it is necessary to take into account and compensate for the warping of the part due to the action of residual stresses. One of the effective fast methods for calculating the magnitude of deformations in CAE-systems is mechanical finite element analysis, which does not require numerous iterations. To ensure the accuracy of calculations in CAE-systems, they must be calibrated on special samples. The paper proposes a method of such calibration on annular samples for the process of direct metal deposition (DMD), the result of which are the values of internal deformations necessary to calculate the deformations of parts during DMD. With the help of the Simufact Additive CAE system, a digital model of the DMD process was designed and the effect of residual stresses was simulated. Based on the obtained results of comparison with full-scale samples, the calculated parameters have been adjusted, which can be applied to optimize the geometry of the workpiece parts, taking into account the features of the DMD process.

Review on surface characteristics of components produced by direct metal deposition process

Direct metal deposition (DMD) is a metal additive manufacturing (AM) process that builds objects layer by layer. The surface properties of DMD components are discussed in this study. The fluctuation of surface attributes such as roughness, finish, texture, and so on as a function of operation parameters has been investigated for a number of materials. This research assists in identifying the optimal process parameters for the material chosen, such as material feed rate, gas flow rate, and laser power, in order to generate the best surface characteristics. The results show that wire feed deposition surpasses powder feed deposition. The laser power and scanning speed of the laser were found to be the most influential process parameters. The study results reveal that the optimum process parameter combinations are material specific and is the keyfactor for obtaining better products with reduced surface roughness and waviness. The microstructural study also explores the material specfic effect in processs parameter combinations. This research could be used to determine or predict the best process parameters for a wide range of industrial materials.

Influence of normalized enthalpy on inconel 718 morphology in direct metal deposition

Morphology of each layer contributes to the final shape of a component fabricated by direct metal deposition process, because of the layer-on-layer fabrication technique. Hence, it's a requisite to understand the layer morphology evolution and factors affecting it. The present work deposited single tracks of Inconel 718 at systematically varied process conditions, and investigated their effect on the track/deposit geometry. The various parameters considered for the study are laser power ( P), scan speed ( v), and powder feed rate ( ṁ). The measurements carried on the deposits cross-section using optical microscope are width ( w), height ( h), and depth of penetration ( b). Additionally, dilution ( D %), deposit area, fusion zone shape and area are measured. The change in layer morphology with input parameters either individually or coupled, is also investigated. The coupled parameters are derived from the simultaneous variation of two are more individual parameters, such as linear heat input, line mass, and normalized enthalpy. The results have shown a proper relationship between input parameters and output morphology. Coupled parameters are observed to be a more valid measure in predicting the deposition geometry, than individual parameters. The relations obtained are linear between width and normalized enthalpy, height and line mass respectively. The fusion zone shape is evolving from semicircular to a wavy (double parabolic) nature with increasing energy input. The microstructure observations resulted a scenario of rapid cooling, and dendritic grain morphology.

Multi-axis direct metal deposition process with effective regrouping strategy

Direct metal deposition (DMD) process has been regarded as a promising additive manufacturing technology. But this process is hard to fabricate structures with overhang features in a support-free manner because the machine equipped with a 3-axis platform could only fabricate structures in 2.5 dimensions instead of 3 dimensions. Therefore, the multi-axis DMD process has been developed to fabricate complicated metal structures without support combined with the motion of the platform. Decomposition operation of the input 3D model to generate several sub-volumes and the following regrouping operation of the sub-volumes are two key procedures in the multi-axis printing process. We adopt the decomposition strategy to obtain the horizontal deposition bases on the previous depth after rotation operations of the platform. However, the traditional regrouping strategy may result in excessively accumulated temperature fields at the connecting regions, especially for some hard-to-print materials like nickel-titanium alloy (NiTi). To solve these problems, the multi-axis DMD process with an improved regrouping strategy is developed in this paper. A novel sequence planning algorithm is proposed by minimizing the cost function to avoid excessively accumulated temperature fields. The cost function is directly related to the moving distance during the regrouping of sub-volumes at connecting regions. Numerical simulations with different interval time values are conducted to determine the cost function. Experiments are conducted to validate the effectiveness of our proposed multi-axis DMD algorithms. Three groups of “Y shape” samples are fabricated using different strategies for comparison and then various characterizations are carried out. Results show that compared with traditional decomposition and regrouping strategies, the samples fabricated with our proposed algorithms could obtain the pore-free morphology, higher microhardness value, and more homogeneous and fine microstructures. Therefore, our proposed strategies in this paper can significantly improve the characterization and microhardness of the fabricated structures during the multi-axis DMD process.

Covariance matrix adapted grey wolf optimizer tuned eXtreme gradient boost for bi-directional modelling of direct metal deposition process

The process of direct metal deposition for cladding has become extremely popular. Online condition monitoring of the process is necessary to obtain a desired clad geometry. The aforementioned fact has necessitated the development of accurate predictive models for establishing relationships between the input and the output parameters of the process in a bi-directional fashion. The present study attempts to build both forward and backward models of a direct deposition process of austenitic steel utilizing the extreme gradient boost technique. Inspired by the successful application of grey wolf optimization algorithm in solving real life problems, the authors further propose a novel modification of the same and use it to tune the parameters of the model with an aim to improve its performance. The improvement has been accomplished by changing the initial perspective prey location guided by the covariance matrix adaptation and recombination. The new optimization algorithm has been tested and compared with the original algorithm along with some of its variants and several other state-of-the-art meta-heuristics on 23 benchmark objective functions from literature and 5 from CEC 2017. The new optimizer has been further validated on some real engineering case studies. The developed hybrid models for bi-directional mappings using different optimizer turn-by-turn have been trained and tested using statistically augmented experimental data-set. The overall mean absolute percentage errors in testing forward mapping and backward mapping using the model with new optimizer have been found within 10%, which is quite satisfactory. The adequacy of the developed models has also been found acceptable.

Modulation of electrical properties in MoTe2 by XeF2-mediated surface oxidation.

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties. Their surface oxidation is of interest because their electrical properties can be easily modulated by an oxidized layer on top of them. Here, we demonstrate the XeF2-mediated surface oxidation of 2H-MoTe2 (alpha phase MoTe2). MoTe2 exposed to XeF2 gas forms a thin and uniform oxidized layer (∼2.5 nm-thick MoOx) on MoTe2 regardless of the exposure time (within ∼120 s) due to the passivation effect and simultaneous etching. We used the oxidized layer for contacts between the metal and MoTe2, which help reduce the contact resistance by overcoming the Fermi level pinning effect by the direct metal deposition process. The MoTe2 field-effect transistors (FETs) with a MoOx interlayer exhibited two orders of magnitude higher field-effect hole mobility of 6.31 cm2 V−1 s−1 with a high on/off current ratio of ∼105 than that of the MoTe2 device with conventional metal contacts (0.07 cm2 V−1 s−1). Our work shows a straightforward and effective method for forming a thin oxide layer for MoTe2 devices, applicable for 2D electronics.