Powder-based Laser Metal Deposition Research Articles

Prediction of single track clad quality in laser metal deposition using dissimilar materials: Comparison of machine learning-based approaches

Powder-based laser metal deposition (LMD) offers a promising additive manufacturing process, given the large number of available materials for cladding or generative applications. In laser cladding of dissimilar materials, it is necessary to control the mixing of substrate and additive in the interaction zone to ensure safe metallurgical bonding while avoiding critical chemical compositions that lead to undesired phase precipitation. However, the generation of empirical data for LMD process development is very challenging and time-consuming. In this context, different machine learning models are examined to identify whether they can converge with a small amount of empirical data. In this work, the prediction accuracy of back propagation neural network (BPNN), long short-term memory (LSTM), and extreme gradient boosting (XGBoost) was compared using mean squared error (MSE) and mean absolute percentage error (MAPE). A hyperparameter optimization was performed for each model. The materials used are 316L as the substrate and VDM Alloy 780 as the additive. The dataset used consists of 40 empirically determined values. The input parameters are laser power, feed rate, and powder mass flow rate. The quality characteristics of height, width, dilution, Fe-amount, and seam contour are defined as outputs. As a result, the predictions were compared with retained validation data and described as MSE and MAPE to determine the prediction accuracy for the models. BPNN achieved a prediction accuracy of 0.0072 MSE and 4.37% MAPE and XGBoost of 0.0084 MSE and 6.34% MAPE. The most accurate prediction was achieved by LSTM with 0.0053 MSE and 3.75% MAPE.

Damage characterisation of tantalum ion source electrodes and reconditioning by wire- and powder-based laser metal deposition

The internal ion source of the Advanced Molecular Image Technologies (AMIT) superconducting cyclotron uses cathodes made of pure tantalum to generate high energy H− ion beams for the production of isotopes for positron emission tomography. During service, the cathodes are impacted by high-energy ions from the plasma. The resulting erosion creates craters that reduce the current density of the extracted beam. The cathodes eventually need to be replaced when the ion source can no longer be activated. This research explores the possibility of repairing the tantalum cathodes used in the AMIT cyclotron through laser metal deposition additive manufacturing. The damaged parts were first characterised by 3D imaging, scanning electron microscopy, and Vickers microhardness to understand the damage mechanisms occurring during service and quantify the extent of the damage. Different repair strategies were then tested employing both high-purity tantalum wire and powder feedstocks and the properties of the reconditioned electrodes were determined. The ability of laser metal deposition to restore the damaged cathodes for use in the AMIT cyclotron has been demonstrated.

Prediction of particle-laden pipe flows using deep neural network models

An accurate and fast prediction of particle-laden flow fields is of particular relevance for a wide variety of industrial applications. The motivation for this research is to evaluate the applicability of deep learning methods for providing statistical properties of the carrier and dispersed phases in a particle-laden vertical pipe flow. Deep neural network (DNN) models are trained for different dependent variables using 756 high-fidelity datasets acquired from point-particle large-eddy simulations for different values of Stokes number, St, bulk particle volume fraction, Φ¯v, and wall roughness, Δγ, for the range St=10−500, Φ¯v=5×10−5−10−3, and Δγ=1°−6°. The considered parameter space corresponds to the inertia-dominated regime and covers a large extent of the typical conditions in powder-based laser metal deposition. We find that the DNN models capture the nonlinear dynamics of the system and recreate the statistical properties of the particle-laden pipe flow. However, DNN predictions of the particle statistics are of higher accuracy compared to the fluid statistics, which is attributed to the highly non-monotonic dependence of the fluid statistics on the control parameters. Owing to significantly decreased time-to-solution, the trained DNN models are promising as surrogate models to expedite model development and design process of various industrial applications.

In situ monitoring of build height during powder-based laser metal deposition

A powder-based laser metal deposition (LMD) system can fabricate customised three-dimensional (3D) parts, layer by layer, based upon a computer-aided design (CAD) model. However, the deposition will not always feature the expected geometry due to excessive heat input and inconsistent powder flow. Due to the layer-by-layer nature of LMD, geometrical error in one layer is compounded in all following layers and may result in a build failure. Thus, it is critical to monitor online the track and layer height. This study developed an in situ monitoring system integrating a webcam and a narrow bandpass filter. The laser/powder defocus distance was extracted from the melt pool images, and the track/layer height was calculated from the laser/powder defocusing distance and preprogrammed layer spacing. The presented approach does not need additional illumination sources and is a nonintrusive online method. Therefore, it is a potential precursor to a feedback build height control system. It also can be used for measuring omnidirectional height, i.e. height in different build directions relative to the substrate, which has been tested by fabricating two thin-wall structures with customised shapes. These online-measured height data were successfully validated against dimensional measurements from an offline 3D scanner, thus demonstrating the online system’s potential utility in a feedback control system for ensuring acceptable part geometrical accuracy.

Laser Metal Deposition with Coaxial Wire Feeding for the Automated and Reliable Build-Up of Solid Metal Parts

Due to their outstanding characteristics, additive manufacturing processes are attracting increasing industrial interest. Among these processes, laser metal deposition (LMD) is an innovative technology for the production of metal components. In order to create three-dimensional parts, wire or powder is deposited layer-wise onto a substrate. When wire is used as feedstock, major drawbacks of the powder-based process, such as the low material usage, contamination of the process cell with metal powder, and health or safety issues, can be overcome or even avoided. In addition, recent developments in laser optics allow for a coaxial wire feeding in the center of an annular laser beam. This eliminates the strong directional dependence of the process when feeding the wire laterally. However, wire-based LMD is highly sensitive to process disturbances, which impedes its broader industrial application. Since it is necessary to completely melt the fed wire to achieve a stable process, self-regulating effects such as overspray in powder-based LMD are not present. In contrast to the widely investigated thin walls, the build-up of multi-track solid structures poses a particular challenge. Therefore, process strategies for producing such solid structures are presented in this paper. The experiments were carried out using a laser processing head that enables coaxial wire feeding (CoaxPrinter, Precitec). By systematically varying the lateral overlap between adjacent weld beads, it was shown that an optimum exists at which minimum surface waviness is achieved. Based on this, defect-free multi-layer solid components could be generated in a reproducible manner. During the process, the melt pool temperature was evaluated using a pyrometer. Furthermore, a microscopic examination of the resulting parts was conducted. The results obtained show the need for process monitoring and control, for which a novel and holistic approach has been developed.

Multi-physics based methodology for evaluating powder feeding quality for Laser Metal Deposition

Powder based Laser Metal Deposition (LMD) is an additive manufacturing technology that offers great advantages to the production of high added value products, like the possibility to manufacture in one whole piece parts with very complex geometries, or to repair existing ones. One of the critical elements of this technology is the powder feeding system, which, depending on its specific design, can provide powder flows that are more or less stable, thus heavily affecting the homogeneity of deposited tracks and, ultimately, of final part quality.In this work, we propose a methodology for assessing the fitness of a powder feeder for a LMD system. The assessment is based on deposition quality maps, which provide a reference for predicting the waviness of a deposited track in function of powder flow oscillation frequency and amplitude. Such maps are generated by performing multi-physics simulations of straight deposition tracks and evaluating their level of regularity. The simulation model is calibrated by a preliminary experimental campaign and can be used further on to assess the effects that any flow frequency-amplitude couple has on deposition quality, without the need to perform new deposition experiments for assessing a new powder feeder. The methodology is demonstrated by calibrating the simulation model for a commercial LMD machine loaded with Inconel 718 powders.

Effect of DED process parameters on distortion and residual stress state of additively manufactured Ti-6Al-4V components during machining

One of the major challenges in high-deposition rate Directed Energy Deposition processes is the resultant residual stresses generated during material deposition, often leading to distortion and poor material characteristics. Important part families suitable for DED process in aerospace sector are thin-wall components, characterized by a large base surface area with rib-like strengthening structures. Here, the substrate plate can be designed to be a part of the final component. The integration of substrate plate into final component results in possible deformation due to residual stress release during machining. This paper therefore investigates the effect of various powder-based Laser Metal Deposition process parameters and strategies on the residual stress state of the additively manufactured Ti-6Al-4V components and the resulting stress release during machining process. The analysis has been carried out during the machining process by including in-process strain measurements of the substrate. The embraced layer removal method allows the determination of machining zone specific stress release mapping, based on an analytical and FEM-model. Hence, the initial residual stress state of the builds was calculated, which revealed that although the heat treatment resolved most of the residual stresses, also in heat treated parts residues were identified depending on the part clamping during the treatment. Furthermore, the study revealed that the significant residual stresses are present in the layers close to the substrate.

Digitalization of directed energy deposition process through a multidirectional height monitoring sensor system

Industrialization of directed energy deposition (DED) process relies on the development and implementation of a quality assurance system that makes the process reliable and repetitive. One of the geometrical characteristics that needs constant monitoring and control during any DED process is the height of the deposited layer. In this paper, a multidirectional layer height monitoring system relying on the laser triangulation principle that has been developed for real-time direct measurement, in contrast to the majority of the sensor systems that derive layer height based on melt pool temperature, is presented. Existing commercial laser triangulation systems are unidirectional, which hinder the flexibility of the material handling system, like industrial robot, in most of the cases. An additional challenge lies on the integration with the deposition head due to its form, size, and extreme working conditions. Therefore, a novel configuration of laser triangulation sensors that enable multidirectionality and limit shadowing effect has been developed at Fraunhofer IAPT. In this paper, the development of such a sensor with a focus on the mechanical design has been presented. The sensor is designed for operation at high temperature and harsh environmental conditions close to the build zone. The developed sensor has been successfully tested for the powder-based laser metal deposition system. In-process layer height measurement and control are possible with this sensor, thereby enabling digitalization and quality assurance during the build process. 3D point cloud generated using this sensor can be used for the dimensional deviation measurement and also for downstream processes like machining. Such an in-process quality measurement and assurance system negates a postmeasurement step, thereby saving cost and time and ensuring quality.

Laser Metal Deposition of Inconel 718 Alloy and As-built Mechanical Properties Compared to Casting.

In the last years, powder-based Laser Metal Deposition (LMD) has been attracting attention as a disruptive Additive Manufacturing (AM) technique for both the fabrication and restoration of Inconel 718 components, enabling to overcome current limitations faced by conventional manufacturing processes in terms of manufacturing costs, tool wear, and lead time. Nevertheless, the uncertainty related to the final mechanical performance of the as-built LMD parts limits a wider adoption of such technology at industrial level. This research work focuses on the mechanical characterization of as-built Inconel 718 specimens through split Hopkinson tensile bar tests performed at different strain rate conditions. The influence of laser power on the final mechanical behavior of the as-built tensile samples is discussed and compared with the mechanical response of as-cast ones. The as-built specimens exhibit a high internal density (i.e., 99.92% and 99.90% for 300 W and 400 W, respectively) and a more ductile behavior compared to the as-cast ones for every evaluated strain rate condition. The strain hardening capacity of the as-built samples increases with the laser power involved in the LMD process, reaching an average Yield Strength of 703 MPa for specimens realized at 400 W and tested at 800/s.

Tensile behaviour of Inconel 718 alloys under extreme conditions of temperature and strain-rate

Nickel-based superalloys are widely used in critical applications where structural components are subjected to harsh operating conditions such as elevated temperatures and high strain-rate. These alloys are also among the most hard-to-cut materials. For this reason, some critical components with complex geometrical features along with critical dimensions cannot easily manufactured by conventional technologies. A rising disruptive Additive Manufacturing (AM) technique, namely powder-based Laser Metal Deposition (LMD), is able to overcome these limitations in terms of manufacturing costs, tool wear, as well as lead time. As a consequence, the mechanical response under harsh condition of additively manufactured Nickel-based superalloys has to be accurately understood in order to guarantee the reliability of the structural parts made with them. Presently very few researches were addressed to study the dynamic tensile behaviour of Inconel 718 produced by additive manufacturing under high strain-rate combined with elevated temperature. To overcome this lack, the coupled effect of strain rate (0.001 s−1, 250 s−1 and 800 s−1) and temperature (20°C, 350°C and 550°C) on the tensile properties of Inconel 718 alloys produced through cast and additive manufacturing technologies has been experimentally studied. The experiments were also addressed to investigate and compare the flow stress behaviour in function of strain rate and temperature considering the differences in terms of microstructure. A modest strain-rate sensitivity has been observed for both as-cast and as-built material, as well as a moderate decrease of the mechanical strengths has been highlighted for increasing temperatures. Finally, comparable mechanical behaviour has been observed between additively manufactured samples produced with a power laser of 400W and as-cast samples.

Defect detection in additive manufacturing via a toolpath overlaid melt-pool-temperature tomography

Additive manufacturing of metals has emerged as a potential technology for companies to create highly integrated and individualized products. In particular, powder-based laser metal deposition has advantages such as flexibility and multimaterial capabilities. It is possible to mix powders and create alloys inside the melt-pool during the build process. Consequently, purpose made material combinations with set or even varying thermal properties can be realized. Inherently, the process becomes increasingly challenging because of the great number of variables. Analyzation of the manufactured part ensures top quality and detects errors and defects. To accomplish this, specimens have to be x-rayed or ground and cut into microsections. In order to save time and keep the parts’ integrity, a new method uses temperature data from the process to determine irregularities. During the additive manufacturing process, a 680 W diode laser melts the substrate and the powder locally. The powder is composed of 42% nickel and 58% iron. A pyrometer samples the temperature of the molten pool at a spectral range from 1.45 to 1.85 μm. The recorded data are mapped onto the toolpath of the process head. A script converts the time dependent signal to spatially resolved temperature points. The feedrate and the laser status aid to synchronize the data throughout. As a result, the overlaid melt-pool temperature visualizes the process and creates a tomography for the produced part. Initial experiments show that errors and defects like porosities and cavities are identifiable inside the manufactured structure. Furthermore, correlations between the visualization and errors detected with microsections are possible. Overall, this technique is an addition to the repertoire of data visualization and quality control in additive manufacturing and can be transferred to other machines and laser processes.

Additive manufacturing of LMD nozzles for multi-material processing

Additive manufacturing of metals has become a leading technology for function-integrated and individualized components. Ongoing research leverages this technology from single-material parts to gradient compositions and multi-material combinations. Powder-based laser metal deposition is the preferred process for these novel applications because of the ability to add onto freeform surfaces and flexible material use. A crucial aspect of this powder-based process is material loss due to overspray. Furthermore, the shape and size of the powder stream must be altered depending on the intended use. For example, thin walls and fine details require a narrow powder focus whereas a larger focus can accelerate the manufacturing of volumes substantially. To address this challenge, novel 3D printable and replaceable nozzles are developed. Compared to common nozzle designs, this new approach enables quick changing of different sized nozzles and incorporates additively manufactured flow paths. Finally, the stream quality is visualized by Schlieren photography.

Process characterization in laser metal deposition of EN AC-43000

EN AC-43000 is a well-established cast material for the production of parts, which are used in the automotive and optical industries. Typical components like cylinder heads, control blocks for automatic transmissions, or housing are manufactured via die or sand casting. The casting process depends on a lot of influencing parameters and is difficult to control. Reject rates up to 10% are the consequence. Those parts are mostly characterized by defects like pores, blowholes, or surface deviations. Such defects restrict the functionality of the component, especially if they are located next to sealing surfaces or at mechanically or thermally high strained areas. In general, defect components are recycled by putting them back to the melt feeding of the casting process. An alternate approach in the presented paper is the repair of the defect components via powder-based laser metal deposition (LMD). EN AC-43000 represents a group of aluminum alloys that are characterized by a restricted weldability. Difficult material properties like the affinity to oxygen and the high thermal diffusivity cause porosity and high dilution. Welding experiments are performed using a disc laser with a maximum output power of 4 kW. The typical seam parameters, such as width, height, depth, and cross-sectional area, are measured in order to characterize the LMD process and to define process limits. Furthermore, the ways to optimize dilution and porosity are presented.

Phase field simulation of dendrite evolution during powder-based laser metal deposition

An algorithm for simulation of laser metal deposition (LMD) processes is developed, which implements the concept of multi-scale modeling of crystallization. The macro-level model takes into account the coupled dynamics of the free surface (melt-gas) and the energy and mass transfer of the multiphase mixture, which is applied on scales of the order of a few to tens of millimeters. The micro-level model describes the evolution of dendritic structures using the phase-field method (PFM) at scales of the order of a few to tens of micro-meters. Wherein, the resulting macroscopic temperature distribution in the system defines the boundary and initial conditions of the micro-model, which, in turn, control the growth patterns and evolution of the dendrites. In local sub-region, the evolution of the dendrite in an advantageous position was simulated, i.e. having favorable thermodynamic conditions of its growth. Before solving the micro-scale problem with the PFM, the 3D mesh conversion procedure to the new 2D mesh is performed in the sub-region of interest. To do this, we select the plane in which the local temperature gradient vector lies. The test dynamic profiles of the phase field and temperature in different spatial regions of the formed bead are calculated. Simulations used the values of physical quantities for iron. The results of calculations showed that the developed numerical model qualitatively describes the physics of crystallization and the formation of dendrites in the LMD process. The effect on the growth dynamics of dendritic structures of such PFM parameters as thermal noise and the anisotropy amplitude is analyzed. It is shown that thermal fluctuations strongly affect the branching of dendrites. With an increase in the fluctuation amplitude, the phase boundary becomes unstable with the formation of secondary and tertiary branches.

On verification of numerical hydrodynamic model of powder-based laser metal deposition process

The developed hydrodynamic model of laser cladding (LC) considering catchment efficiency is verified with experimental data for steel powder LC in a wide range of process parameters. The comparison of the main calculated output parameters (depth of the penetration, track width and height) with experimental data is held. The experimental track profile is compared with the results of the calculations. It is shown that the model works well even for unhealthy high dilution parameter sets and could be used for processing window search.

Overlap Ratio in Wire- and Powder-Based Laser Metal Deposition of H11 + Nb for Hot Forging Dies

AISI H11 tool steel is a complex tool steel alloy used to manufacture hot forging dies. These dies however have a limited life, which depends upon the working conditions, the tool design, the heat treatment, and the quality of tool steels. In this paper, a novel wire-and powder-based laser metal deposition (WP-LMD) process was investigated to deposit H11 wire and niobium (Nb) powder simultaneously and develop a coating on existing forging dies for enhancing their life. The main aim was the development of a novel WP-LMD process, and consequently a new H11 tool steel with improved toughness and hardness. The developed WP-LMD process was later implemented to build a multilayer block made of the modified H11 tool steel. The overlap ratio was optimized in both, horizontal and vertical, directions, and were found to be 30% and 20% respectively in order to achieve a fully dense coating and avoid pores and unmelted Nb particles. The potential of the WP-LMD can be used to fabricate an outer layer of the modified H11 tool steel with improved toughness and hardness, which ultimately enhances the life of hot forging.

Process characterization of powder based laser metal deposition on thin substrates

Powder-based laser metal deposition is a well-established generative manufacturing process in the field of tool and mould building to produce or repair components. Conventionally manufactured tools are often completely constructed of a high-alloyed, hardened-tempered, and expensive tool steel. An alternative way to manufacture tools and moulds is the combination of a cost-efficient, mild steel, and a functional coating. Such specific, locally adapted coating can be generated by laser metal deposition. The functional coating is just located in the area of the interaction zone of the tool. Consequently, costs and resources can be saved. Such coatings are created by positioning multiple individual weld tracks next to and on top of each other in an overlapping manner. It is therefore useful to characterize und optimize the individual weld track. Thermal processing methods, like a laser metal deposition, are always characterized by thermal distortion. The resistance against the thermal distortion of the work piece is decreasing with the reduction of the material thickness. Thus, there is a special process management for the laser based coating of thin-walled parts or tools with a small material thickness needed to reduce thermal distortion. The experimental approach in the present paper is to keep the energy and the mass per unit length constant by varying the laser power, the feed rate, and the powder mass flow. The typical seam parameters (such as width, height and depth, cross-sectional area, and angular distortion) are measured in order to characterize the cladding process, define process limits, and evaluate the process efficiency of an individual weld track. Ways to optimize dilution, angular distortion, and clad height are presented. After the characterization of individual weld tracks, optimized process parameters are deduced from the process window and used to create functional, two-dimensional coatings on thin substrates. Different scanning strategies are compared with each other to reduce processing time and thermal distortion.

Effect of niobium on microstructure-property relationship in H11 hot-work tool steel via wire- and powder-based laser metal deposition

This paper presents an effect of niobium (Nb) on the microstructure-property relationship in H11 hot-work tool steel. Various amounts of Nb powder were introduced into the molten pool of H11 wire through a novel wire- and powder-based laser metal deposition (WP-LMD) process. With the increase in Nb content up to 3.45 wt.%, the average grain size of H11 decreased from 14.3 µm to 2.83 µm, while the average Vickers microhardness decreased from 706 HV0.2 to 301 HV0.2. The critical amount of Nb was then found to be 1.73 wt.%, which produced a hard (average hardness of 606 HV0.2), and refined (average grain size of 3.01 µm) microstructure of H11. Such kind of microstructures can assure high toughness and high hardness; hence more suitable for hot-working industries such as die and mould, forging and cutting tools.

Effect of niobium on microstructure-property relationship in H11 hot-work tool steel via wire- and powder-based laser metal deposition

Effect of niobium on microstructure-property relationship in H11 hot-work tool steel via wire- and powder-based laser metal deposition

Influence of different Al-Cu substrates on the properties of laser metal deposited Al coatings

The application of coatings to structures is generally done in order to locally tailor properties. For this purpose, powder-based laser metal deposition (LMD) can be utilized. The resulting properties of the coatings are not only affected by the LMD process parameters, but also by the substrate material. In this regard, the chemical composition as well as its initial microstructure of the substrate has a significant influence. In the present study two Al-Cu alloy substrates, 2139 and 2198, are used for the LMD of pure Al powder. The resulting coatings possess clearly different properties, although the same process parameters are used.