Metal Powder Layer Research Articles

3D printing of shape memory Alloys for complex architectures of smart structures

Shape Memory Alloys (SMA) are materials used to design smart structures with intrinsic functional properties and improved efficiency. This is a key aspect of aerospace industry and makes SMA good candidates in this field. One of the most widespread SMA is the equiatomic NiTi alloy which, however, has the strong limitation of poor machinability, so only simple shapes can be obtained. Additive Manufacturing processes allow to overcome this limit and to design complex shapes. Compared to other metallic materials, the optimization of the process for NiTi alloy is complicated because, beside mechanical properties and presence of defects, considerable attention needs to be dedicated to the material functionality. The high temperatures involved in the additive process significantly affect the material properties due to possible evaporation of Ni and formation of precipitates that enable a shift of the phase transformation temperatures. This paper is focused on the optimization of the process parameters of the NiTi alloy printed through the Laser Powder Bed Fusion (L-PBF) to ensure optimal pseudo-elastic behaviour, which is essential for the design of structural dampers. This was accomplished starting from simple structures and then designing a damper that couples the pseudoelasticity of NiTi with load support capacity.The L-PBF is a powder-bed technique that selectively melts layers of micrometric metal powder. A pseudoelastic NiTi powder with 50.8 at. % of Ni content was selected and characterized through scanning electron microscope (SEM) and observations connected to an Energy Dispersive X-ray Spectroscopy (EDX) probe. After that, some cubic samples were manufactured, with the dimension of 3 × 3 × 15 mm3. A set of different laser powers and scanning speeds were used to find the set of process parameters that optimize the functional properties of the printed parts. Near fully dense specimens with density higher than 99.5 % were selected for further investigations. Differential scanning calorimetry (DSC) and mechanical tests were performed on as-built and heat-treated samples.Quasi-static mechanical tests were accomplished in compression mode, at different strains, up to 8 %. It was observed that the residual strain for cyclic loading at 4 % is lower than 1 %, so good recovery of the deformation was shown. Moreover, numerical analyses that mimic the pseudoelastic behaviour in compression tests were implemented.Finally, the best set of parameters was selected on the basis of the material's ability to recover deformations and its loss factor.

Prediction of Melt Pool Dimension, Porosity Generation and Thermal Behavior of Ti-6Al-4V during Powder Bed Fusion at Various Scan Speeds and Laser Powers

The additive manufacturing technology called laser powder bed fusion enables to manufacture complex parts based on the fusion of a metallic powder layer by layer. In laser powder bed fusion, the produced component quality relies significantly on the parameters of the process. In this study, the powder titanium alloy Ti-6Al-4V is employed for the purpose of predicting the melt pool dimensions. To manufacture a single bead, several combinations of scan speed and laser power are used. This research studies the influence of the scan speed and the laser power on the melt pool dimensions and on the thermal history of a specified layer of powder. The results reveal that the geometry of the melt pool is considerably responsive to the scan speed and the laser power. Furthermore, unfavorable effects such as porosity defects are analyzed in detail. Suggestions are presented to employ optimal settings to prevent these undesirable outcomes. To validate the numerical results, a comparison with experimental results from the literature is carried out. Our numerical analysis proves a satisfactory correlation with the experimental investigations. The beam power and the scanning speed effects on the average temperature of the desired layers are discussed as well.

Quantitative analysis of thin metal powder layers via transmission X-ray imaging and discrete element simulation: Blade-based spreading approaches

Spreading uniform and dense layers is of paramount importance to creating high-quality components using powder bed fusion additive manufacturing (PBF AM). Blade-like tools are often employed for spreading powder metal feedstocks, especially in laser powder bed fusion (LPBF) and electron beam melting (EBM), where powders are characterized by a D50V of 30 μm or greater. Along with variations in boundary conditions introduced by the layer-wise geometry and surface topography of the printed component, stochastic interactions between the spreading tool and powder result in spatial variations of layer quality that are still not well understood. Here, to study powder spreading under conditions representative of PBF AM, we employ a modular, mechanized apparatus to create powder layers from moderately and highly cohesive powders (nominally 15–45μm Ti–6Al–4V and 20–63μm Al–10Si–Mg, respectively) with a selection of blade-like spreading tools. Powder layer effective depth is spatially mapped using transmission X-ray imaging, and uniformity is quantified via a statistical approach. We first compare layer density, or the effective depth of powder layer, and show that blade geometries with a curved profile lead to increased material deposition. Second, this approach enables quantification of local fluctuations, or layer defect severity. For example, we observe that the primary benefit of a V-shaped rubber (compliant) blade, as compared to a 45° rigid blade, lies in enabling local deflection of the blade edge to eliminate streaking from large particles, while also increasing deposition (layer density). Additionally, we employ a custom DEM simulation to elucidate the opposing roles of particle density and surface energy with a pseudo-material approach, where the balance of inertial and cohesive forces determine macro-scale powder flowability. For each alloy density, selected to represent Ti–6Al–4V and Al–10Si–Mg, we find via simulations a critical surface energy beyond which layer density is greatly impaired when powder spreading is performed using a blade.

Quantitative analysis of thin metal powder layers via transmission X-ray imaging and discrete element simulation: Roller-based spreading approaches

A variety of tools can be used for spreading metal, ceramic, and polymer feedstocks in powder bed additive manufacturing (AM) methods. Rollers are often employed when spreading powders with limited flowability, as arises in powders comprising fine particle sizes or high surface energy materials. Here, we study roller-based powder spreading for powder bed AM using the unique combination of a purpose-built powder spreading testbed with a proven method for X-ray mapping of powder layer depth. We focus on the density and uniformity of nominally 100 μm thick layers of roller-spread Ti-6Al-4V and Al-10Si-Mg powders. Our results indicate that when rotation is too rapid, roller-applied forces including shear and medium fluid drag impede the creation of dense and uniform layers from powders of high innate flowability, or where inertial forces driven by particle density dominate cohesive forces. Roller counter-rotation augments the uniformity of cohesive powder layers, primarily though reducing the influence of particle clusters in the flowing powder, which are otherwise shown to cause deep, trench-like streaks. Companion discrete element method (DEM) simulations further contextualize the experiments through isolation of the effects of cohesion on layer attributes. Results suggest that roller motion parameters could apply a strategic level of additional shear force to the flowing powder, thereby mitigating the clumping behavior characteristic of highly cohesive feedstocks while maintaining high layer uniformity.

Experimental Study on the Effect of Process Parameters on the Build Time and Part Accuracy of Direct Metal Laser Sintering Components

3D printing is an emerging technology that creates parts straight from CAD models. Direct Metal Laser Sintering (DMLS) is a 3D printing method that is becoming increasingly popular in the aerospace, medical, and orthopedics sectors. These are usually focused on precise, long-lasting, and lightweight parts. DMLS is an Additive Manufacturing (AM) technique that employs a laser to sinter a selected area of a metallic powder layer by layer to produce the required metal components. The heating power of the laser was discovered to have a strong effect on phase formation. The major issue with this process is that high residual and large deformations are introduced to the components during manufacturing. This causes a change in the fatigue strength of a part and can even lead to cracks. The quality of the DMLS parts is affected by various process parameters. In this study, the design of experiments is used to investigate the consequences of process parameters used in the DMLS process to make metal parts. Process parameters such as laser power and scanning speed must be identified because they have the largest influence on the part's characteristics. (Build time, part accuracy). The change in the controlling parameters, or process parameters, in the DMLS method, has been found to affect material properties, according to a literature review. Thus, in the proposed work, three process parameters laser speed, scanning speed, and hatching distance are taken into account at two distinct levels. L4 orthogonal arrays are used in the studies. Experimental research is done on the manufacturing process, build time and component accuracy. Finally, the impact of each parameter on the quality aspects is discussed based on the experimental findings.

Optimization of Sandblasting to Improve the Surface Finish of 17-4PH Parts Manufactured by SLM Using Different Laser Scanning Strategies

Great advances have emerged in recent years around additive manufacturing techniques, with an increasing number of different materials (polymers, ceramics, metals). However, metal part manufacturing has always been one of the most demanded in engineering. That is due to its ability to create final functional parts with good mechanical properties. One of the most widely used technique is Selective Laser Melting (SLM). The SLM process uses a laser power source to selectively melt metal powder layer by layer. Typically, this manufacturing technique requires mechanical post-processing operations, not only to split the parts from the build-plate, but also to improve the mechanical properties and surface finish of parts or the dimensional accuracy of specific regions to ensure assembly and interchangeability. In particular, sandblasting is a method of mechanical abrasion cleaning commonly used and very useful for improving the surface topology of SLM printed parts. Besides, the laser scanning strategy used in this additive manufacturing process influences the surface quality of parts. Therefore, in this work, the sandblasting post-process has been optimized for surface roughness improving in parts printed using the most common laser scanning strategies (normal, hexagonal, concentric). The role that sandblasting pressure and time plays in the surface quality of parts, indispensable to optimize this SLM post-process, has been evaluated. Thus, surface roughness of different specimens subjected to different sandblasting parameters has been measured to optimize both values related to the laser scanning strategy used in SLM manufacturing. The material used is 17-4PH stainless steel, an alloy that presents an excellent combination of high strength and good corrosion resistance, high hardness, good thermal properties, as well as excellent mechanical properties at high temperatures. This precipitation-hardened steel has important applications in the aerospace sector, chemical and petrochemical industry, energy sector, surgical instruments, high wear components, and general metallurgy, among others.

Experimental Analysis and Spatial Component Impact of the Inert Cross Flow in Open-Architecture Laser Powder Bed Fusion

Laser-based powder bed fusion is an additive manufacturing process in which a high-power laser melts a thin layer of metal powder layer by layer to yield a three-dimensional object. An inert gas must remove process byproducts formed during laser processing to ensure a stable and consistent process. The process byproducts include a plasma plume and spatter particles. An NC sensor gantry is installed inside a bespoke open-architecture laser-based powder bed fusion system to experimentally characterize the gas velocity throughout the processing area. The flow maps are compared to manufactured samples, where the relative density and melt pools are analyzed, seeking a potential correlation between local gas flow conditions and the components. The results show a correlation between low gas flow velocities and increased porosity, leading to lower part quality. Local flow conditions across the build plate also directly impact components, highlighting the importance of optimizing the gas flow subsystem. The experimental flow analysis method enables optimization of the gas flow inlet geometry, and the data may be used to calibrate the computational modeling of the process.

Influence of parameters of molding by the selective laser melting method on product quality

Purpose. The development and application of additive technologies, which have a high rate of material utilization and significant production flexibility in the case of single and small-scale production, are relevant for solving various types of engineering problems. The final mechanical, technological and operational characteristics of products obtained by additive technologies are affected by a large number of initial parameters that can be varied in order to obtain the necessary properties of the material. The purpose of this work is to study the influence of the parameters of shape formation by the method of selective laser melting, in particular the scanning plan, on the quality of the material and the productivity of production. The methods. To achieve the goal, studies of the microstructure of products obtained by different scanning plans were carried out using optical microscopy, during which the shape of a single track, as well as the structure of the material in different directions of the laser beam, were analyzed. Findings. The effect of particle size, the mechanism of formation of a metal product during selective laser melting is analyzed. The influence of the scanning plan on the porosity, strength, and productivity of manufacturing by the method of selective laser melting is shown. The correct choice of technological parameters, such as the power and speed of the laser beam, the thickness of the metal powder layer, the scanning step between two adjacent single tracks, ensures the minimization of the number and size of defects (pores, cracks, shells) in the product material. The originality. The process of metal powder melting in the process of shaping products by the method of selective laser melting is analyzed. The scheme of formation of a single track is obtained in the case of choosing different scanning schemes. A list of technological parameters, the variation of which affects the properties and quality of the product, has been formed. Practical implementation. The use of additive technologies for the molding of products in single and small-batch production allows to reduce the time of production preparation and reduce the amount of waste. When choosing technological parameters, one should understand their dependence on the final properties of the finished product, because the method of selective laser melting allows to obtain products of different porosity, strength and other properties.

A framework for graph-base neural network using numerical simulation of metal powder bed fusion for correlating process parameters and defect generation

Powder bed fusion (PBF) is the most common technique used for metal additive manufacturing. This process involves consolidation of metal powder using a heat source such as laser or electron beam. During the formation of three-dimensional(3D) objects by sintering metal powders layer by layer, many different thermal phenomena occur that can create defects or anomalies on the final printed part. Similar to other additive manufacturing techniques, PBF has been in practice for decades, yet it is still going through research and development endeavors which is required to understand the physics behind this process. Defects and deformations highly impact the product quality and reliability of the overall manufacturing process; hence, it is essential that we understand the reason and mechanism of defect generation in PBF process and take appropriate measures to rectify them. In this paper, we have attempted to study the effect of processing parameters (scanning speed, laser power) on the generation of defects in PBF process using a graph-based artificial neural network that uses numerical simulation results as input or training data. Use of graph-based machine learning is novel in the area of manufacturing let alone additive manufacturing or powder bed fusion. The outcome of this study provides an opportunity to design a feedback controlled in-situ online monitoring system in powder bed fusion to reduce printing defects and optimize the manufacturing process.

Integrated Design and Manufacturing of Aviation Aluminum Alloy Support Based on SLM Technology

Laser selective melting (SLM) is a manufacturing process that uses laser to melt metal powder layer by layer to form parts, which can realize the integrated manufacturing of complex parts. In this paper, SLM technology and topology optimization technology are combined to carry out the integrated design of the aviation aluminum alloy support . After optimization, the weight of the support is reduced by 24%, the maximum displacement is reduced by 82%, and the maximum stress is reduced by 65%. The process simulation analysis of the whole SLM forming process is carried out by using Simufact Additive software. On this basis, AlSi10Mg powder is selected for laser selective melting of the support, The forming of the whole structure meets the expectation, is in good agreement with the process simulation results, and there is no visible cracking failure.

An experimental investigation into influences of build orientation and specimen thickness on quasi-static and dynamic mechanical responses of Selective Laser Melting 316L Stainless Steel

Additive Layer Manufacturing (ALM) processes like Selective Laser Melting (SLM) enable the conception of complex designs with a high precision and equal or enhanced mechanical properties compared to Conventionally Manufactured (CM) structures. Nevertheless, this process, which consists in melting metallic powders layer by layer with a laser beam, greatly influences the microstructure and therefore the mechanical properties. While some studies have considered the effects of the thickness and/or the building direction of 316L Stainless Steel (SS) specimens produced by SLM on the quasi-static mechanical behavior, the strain rate effect for crash or impact applications on these two parameters has not been fully investigated. To complete the actual knowledge, the present work proposes to analyze the mechanical behavior of 316L SS tensile specimens produced by SLM with different build orientations (0°, 45° and 90°) and thicknesses (0.5, 0.75, 1 mm) and submitted to dynamic loadings at various strain rates up to 103 s−1. In addition, the microstructure and the fracture surfaces are analyzed to give a more detailed comprehension of the mechanical tests. It results that the SLM 316L SS achieves better Yield Stress (YS), similar Ultimate Tensile Stress (UTS) and equal or lower failure strain compared to the CM material. This is mainly a result of microstructure refinement. Anisotropy is observed at the macroscopic level with higher tensile stress and lower failure strain for horizontal specimens, which is explained by the different shapes, orientation and size of the grains at the microscopic level. The mechanical properties greatly decrease as the thickness reduces from 1 to 0.5 mm, by 14% for the YS and 16% for the UTS for a quasi-static loading. A minimum thickness of 0.75 mm is advised to at least recover the mechanical properties of the CM 316L SS. A positive strain rate sensitivity, higher than the CM material, is observed for all configurations, with the exception of 0.5 mm thickness. For strain rates ranging from to 10−3 to 103 s−1, there is an increase of 20% of the UTS. The material anisotropy is not affected by the strain rate sensitivity whereas the latter increases with the thickness.

Flexible and highly dynamic beam shaping for Laser-Based Powder Bed Fusion of metals

In state-of-the-art laser-based powder bed fusion processes, a thin layer of metal powder is typically melted by means of a small, Gaussian-shaped laser spot, limiting the stability and productivity of the process. The application of alternative beam shapes is a current research topic to stabilize, accelerate and extend laser-based powder bed fusion of metals, e.g. by tailoring the microstructure. Static beam shapes and sizes limit the theoretically possible process and geometry freedom. To overcome those limitations, an approach for flexible and highly dynamic beam shaping is presented. The experimental setup is based on two perpendicularly oriented acousto-optic deflectors. A synchronized superposition of the ultrafast deflections (switching rates above 100 kHz) is used to generate sequentially compound quasi-static beam shapes. This quasi-static beam shaping is combined with a state-of-the-art beam deflection system and proved to be a viable solution for improved processing. Specimens with a relative density of more than 99.5 % could be manufactured by applying three selected beam shapes. The proof of concept demonstrates the potential of the setup for flexible, fundamental research on the influence of alternative beam profiles in laser-based powder bed fusion of metals.

Identification of a Spatio-Temporal Temperature Model for Laser Metal Deposition

Laser-based additive manufacturing enables the production of complex geometries via layer-wise cladding. Laser metal deposition (LMD) uses a scanning laser source to fuse in situ deposited metal powder layer by layer. However, due to the excessive number of influential factors in the physical transformation of the metal powder and the highly dynamic temperature fields caused by the melt pool dynamics and phase transitions, the quality and repeatability of parts built by this process is still challenging. In order to analyze and/or predict the spatially varying and time dependent thermal behavior in LMD, extensive work has been done to develop predictive models usually by using finite element method (FEM). From a control-oriented perspective, simulations based on these models are computationally too expensive and are thus not suitable for real-time control applications. In this contribution, a spatio-temporal input–output model based on the heat equation is proposed. In contrast to other works, the parameters of the model are directly estimated from measurements of the LMD process acquired with an infrared (IR) camera during processing specimens using AISI 316 L stainless steel. In order to deal with noisy data, system identification techniques are used taking different disturbing noise into account. By doing so, spatio-temporal models are developed, enabling the prediction of the thermal behavior by means of the radiance measured by the IR camera in the range of the considered processing parameters. Furthermore, in the considered modeling framework, the computational effort for thermal prediction is reduced compared to FEM, thus enabling the use in real-time control applications.

DISTINCTIVE FEATURES OF SLM TECHNOLOGY APPLICATION FOR MANUFACTURING OF LPRE COMPONENTS

In view of the growing competition in the modern market for rocket and space technology products, the issue of maximizing the cost reduction of the process of its production is urgent. In particular, the rocket engine is traditionally one of the costliest and technologically demanding rocket units, which basically reduces the problem of reducing the cost of rocket production to the development of new, more technologically advanced and less costly, approaches to manufacturing LPRE components. Thus, it is of increased interest to use a relatively young method for producing parts by layer-by-layer melting of thin layers of metal powder by exposing it to high-power laser radiation. This method is a part of the methods of additive technologies and is called SLM (Selective Laser Melting). In order to assess the influence of the main features of the production of components, the study of hydraulic channels manufactured with the additive SLM technology was carried out, and a load-bearing element of the fastening structure was manufactured which geometry was obtained by applying topological optimization methods. The aim of the work is to determine the main hydraulic characteristics of inner channels of typical LPRE’s elements, as well as the limits of the technology applicability in terms of liquid-propellant rocket engines. The possibility of manufacturing elements, including hydraulic paths, was investigated: regeneratively cooled cylinders, throat inserts of a liquid-propellant engine, as well as experimental designs of film cooling rings were adapted to be produced by means of SLM. The possibility of producing thrust frame, the shape of which was obtained by the method of topological optimization, was investigated. Samples of designs of typical hydraulic channels, as well as the constituent elements of the design of the rocket engine chambers, were manufactured. The main hydraulic characteristics of the typical hydraulic channels, as well as the distinctive features of their production using the method of additive technologies SLM, were determined. The thrust frame, which geometry was obtained by means of topology optimization, was successfully manufactured.

Sensitivity of laser powder bed fusion additive manufactured HAYNES230 to composition and print parameters

We developed a coupled CALPHAD and finite element-based computational model of the Laser Powder Bed Fusion (LPBF) process for HAYNES230, considering the feedstock composition and packing density. We further used this model to investigate the effect of variation in feedstock composition and print parameters on the quality of the final printed part. Sensitivity of the maximum reached temperature to variations in characteristics of the laser source is also studied considering a single-track laser scan on a layer of metal powder. We analyzed temperature evolution in the powder bed and melt pool geometry along the path of the laser. Our results indicate that the LPBF process of HAYNES230 alloy requires a powder layer thickness of ∼20 μm and laser spot size ∼30 μm radius compared to other alloys. It is essential to achieve sufficient melt pool depth necessary for cohesion with the substrate while avoiding large melt pool width that adversely affects the formation of cracks and residual stresses. We also revealed that reducing the laser power or increasing scan speed drastically reduces peak temperature while less susceptible to solute composition.

Estudio de fatiga a bajos ciclos de la aleación Ti-6Al-4V obtenida por fusión de haz de electrones y su extensión a altos ciclos

La fabricación aditiva de fusión de haz de electrones o EMB, es una técnica usada para fabricar piezas fundiendo polvo metálico capa por capa con un haz de electrones en condiciones de alto vacío, generando así una impresión en 3D que permite obtener configuraciones complejas muy difíciles de obtener con métodos convencionales de fabricación. En esta investigación se estudia la caracterización a la fatiga a bajos ciclos para muestras de aleación Ti-6Al-4V obtenida por EBM y su extensión a la fatiga de altos ciclos. Se utilizan probetas cilíndricas mecanizadas bajo la norma ASTM E8, luego estas fueron sometidas a ensayos de tracción, en una maquina universal MTS Bionix®. Los resultados permitieron establecer las condiciones estáticas: esfuerzo de fluencia de 1023 MPa, y un esfuerzo máximo de 1143 MPa, a su vez una deformación unitaria máxima del 13%. Los ensayos de fatiga fueron realizados a aproximadamente una relación de inversión de carga R=-1 bajo desplazamiento controlado. El rango de ciclos a la ruptura se llevó hasta 13335 ciclos. Estos resultados se graficaron bajo la curva deformación unitaria vs número de ciclos a la falla (Ɛ -Nf) usando dos versiones de la regla de Coffin-Manson. Estas dos reglas son reglas para aleaciones de Ti y Al y una de estas es exclusiva para aleaciones Ti-6Al-4V impresas por manufactura aditiva. Finalmente, datos de fatiga a altos ciclos, para la misma aleación y fabricada bajo el mismo proceso, se juntan con datos de esta investigación para ajustarlos a una regla tipo modelo de Manson-Coffin-Basquin, la cual arrojó como resultado εt= 0.0364N -0.148.

A lumped parametric analytical model for predicting molten pool temperature and clad geometry in pre-placed powder laser cladding

Pre-placed powder laser cladding is one of the simplest laser-based deposition techniques, having high material usage efficiency and capable to provide sound clad quality. However, the quality along with the geometrical aspects of the deposited layer depends on various energy transfer mechanisms controlled by process parameters. Energy transfer phenomena include the partial absorption of the laser radiation by metal powder layer, melting of the powder layer and transmission of energy to the substrate, along with the losses, like convective heat transfer to the atmosphere and conductive heat transfer to the bulk material. Theses phenomena all together defines the molten pool temperature, which needs to be analyzed to control the geometrical features of the clad and dilution by optimizing the process parameters. Therefore, a lumped parametric analytical model has been formulated and presented in this work for the pre-placed laser cladding process considering energy transfer and loss mechanisms along with the surface tension phenomena of the molten material to predict the molten pool temperature, clad geometry and substrate dilution. The developed model is validated using the experimental results of molten pool temperature of the clad layer, clad width and height as well as the percentage dilution. A continuous mode Yb-Fiber laser system with maximum available laser power of 220 W is used for the deposition of Inconel 625 powder on AISI 304 stainless steel substrate.

Threshold conditions for thermocapillary formation of a deep cavity in the additive process for selective laser melting of a metal powder layer

ABSTRACT The deep penetration mode at selective laser melting of a metal powder layer is determined by a special and intense hydrodynamic process in a thin molten layer at rapid heating of the metal by using a focused beam. This mode is also widely used in laser and electron beam welding. This indicates the similarity of hydrodynamic processes, which differ significantly in the parameters of the applied radiation (power, power density and focusing spot diameter). Threshold conditions of thermocapillary deep penetration (without evaporation) for various metals (Al, Cu, Fe and Ti) in a wide range of changes in beam parameters, including those used for selective laser melting of a powder layer, are investigated. The comparison of calculated and experimental values of the radiation parameters corresponding to the transition to the deep penetration mode for selective laser melting of the powder layer and laser welding is presented. The correlation of these values confirms the thermocapillary mechanism of the deep cavity formation and the similarity of hydrodynamic processes at welding metals by using a laser beam.

Novel Calibration Strategy for Validation of Finite Element Thermal Analysis of Selective Laser Melting Process Using Bayesian Optimization.

Selective laser melting (SLM) produces a near-net-shaped product by scanning a concentrated high-power laser beam over a thin layer of metal powder to melt and solidify it. During the SLM process, the material temperature cyclically and sharply rises and falls. Thermal analyses using the finite element method help to understand such a complex thermal history to affect the microstructure, material properties, and performance. This paper proposes a novel calibration strategy for the heat source model to validate the thermal analysis. First, in-situ temperature measurement by high-speed thermography was conducted for the absorptivity calibration. Then, the accurate simulation error was defined by processing the cross-sectional bead shape images by the experimental observations and simulations. In order to minimize the error, the optimal shape parameters of the heat source model were efficiently found by using Bayesian optimization. Bayesian optimization allowed us to find the optimal parameters with an error of less than 4% within 50 iterations of the thermal simulations. It demonstrated that our novel calibration strategy with Bayesian optimization can be effective to improve the accuracy of predicting the temperature field during the SLM process and to save the computational costs for the heat source model optimization.

Microstructural and thermal expansion behaviour of graphene reinforced 316L stainless steel matrix composite prepared via powder bed fusion additive manufacturing

316 L Stainless steel is widely used industrial material having application in broad range of domain including automotive, structural as well as biomedical. It is also used for several high temperature applications in various industries, permanent deformation due to thermal cycling and continuous high temperature exposure is one of the major concern in industries. Therefore, enticing thermo physical properties is very crucial for better efficiency and service life of components. In the present work, graphene reinforced 316 L metal matrix composite was prepared via powder bed fusion additive manufacturing technique in order to make it more stable material at elevated temperature. Powder bed fusion technique was used to melt graphene coated 316 L metal powder layer by layer to prevent agglomeration of graphene inside the matrix. To identify the chemical state of the elements in the prepared composite, synchrotron based X-ray photoelectron spectroscopy was performed. However, no carbide formation was observed inside the matrix and the prepared composite was purely austenitic. The obtained composite with 0.2 wt % graphene has coefficient of thermal expansion (CTE) equal to 19.3 × 10−6 K-1 which is about 6.2% lower than bare 316 L at 1100 °C. The value further decrease after annealing process and this much decrement in CTE can increase the application window of the material to the large extent.