SLM Parts Research Articles

Effects of heat treatment and hot isostatic pressing on microstructure and fatigue improvements in Ti-6Al-4V alloy fabricated by selective laser melting

SLM plays a critical role in in-space intelligent manufacturing and defense industries, while inherent hurdles due to internal defects in the SLM parts restrict practical reliability. Post-processing of heat treatment and hot isostatic pressing was comparatively applied to reveal the microstructure and fatigue performance. In this work, the resulted microstructure was revealed by XRD, SEM and TEM. Also, the stress-life curve of high cycle fatigue performance was obtained. This work provides a direct comparison for two post-processing techniques, contributing to more reliable application for SLM Ti alloy industrialization.

Quantification and Analysis of Residual Stresses in Braking Pedal Produced via Laser-Powder Bed Fusion Additive Manufacturing Technology.

This study focuses on the experimental verification of residual stress (RS) in a 3D-printed braking pedal using the Powder Bed Fusion (PBF) method with SS316L material. The RS was measured at two representative locations using the hole drilling method (HDM) and the dividing method, which are semi-destructive and destructive methods of RS measurement, respectively. The finite element method (FEM) was used with Ansys Workbench 2020R2 and Simufact Additive 2021 software to determine the magnitude of RS. The results provide insights into how RS is incorporated into metal 3D-printed components and the available tools for predicting RS. This information is essential for experts to improve the accuracy and functionality of SLM parts when post-subtractive or additive manufacturing processes are used. Overall, this study contributes to the advancement of knowledge on the effects of RS on 3D-printed metal components, which can inform future research and development in this area.

Powder bed monitoring via digital image analysis in additive manufacturing

Due to the nature of Selective Laser Melting process, the built parts suffer from high chances of defects formation. Powders quality have a significant impact on the final attributes of SLM-manufactured items. From a processing standpoint, it is critical to ensure proper powder distribution and compaction in each layer of the powder bed, which is impacted by particle size distribution, packing density, flowability, and sphericity of the powder particles. Layer-by-layer study of the process can provide better understanding of the effect of powder bed on the final part quality. Image-based processing technique could be used to examine the quality of parts fabricated by Selective Laser Melting through layerwise monitoring and to evaluate the results achieved by other techniques. In this paper, a not supervised methodology based on Digital Image Processing through the build-in machine camera is proposed. Since the limitation of the optical system in terms of resolution, positioning, lighting, field-of-view, many efforts were paid to the calibration and to the data processing. Its capability to individuate possible defects on SLM parts was evaluated by a Computer Tomography results verification.

Differences between 3-D printed and traditionally milled CoCr dental alloy from casted block in oral environment

The aim of this study was to compare corrosion resistance and ion release in CoCr dental alloys with identical chemical compositions produced using different technologies (SLM – selective laser melting and milling from cast blocks) and heat treatment (SLM parts only). The corrosion properties were tested in artificial saliva, artificial saliva with fluoride ions, and artificial saliva with added lactic acid to simulate inflammations. The study included electrochemical tests, microstructural investigation and ion release tests in the three different environments for a total of 42 days. The best performance of CoCr as a result of potentiodynamic polarization was observed in the artificial saliva with added lactic acid, which is in contradiction with the highest ion release measured from all the materials tested and electrochemical impedance spectroscopy that showed a deterioration of the passive layer in an acidic environment. Microstructural investigation revealed that different phases precipitated by heat treatment trigger increased release of Mo and W ions. The printing method does not raise critically ion release from CoCr alloy, while the state of the surface greatly impacts the extent of the ion release. This study demonstrates the importance of an interdisciplinary approach to the study of corrosion and biocompatibility in dental alloys.

Characterization of Surface and Bulk Features of SLM Parts

An experiment-analytical procedure based on the building of an object in severe atmosphere resistant steel by SLM is proposed. The complex shape was investigated with the sectioning and laboratory observation of the physical object. The study evidenced the need to get a variable layer thickness to follow double curvature complex shapes. In particular the key variable in the process is the melt bath dimension by which the metal powder assumes by solidification the required global geometry. It was observed that the bath detected mainly in terms of the area in the section tends to decrease when approaching the surface of the physical model where the complex geometry needs to be described. Relationships describing the bath area behaviour and correlations between surface roughness and internal bath dimensions were found and proposed in detail. The surface roughness is highly correlated with the bath area in the zones of the section approaching the surface.

Development of a Fringe Printer With 0.35 μm Pixel Pitch

The development of computational devices can provide computer-generated holograms (CGH) of over 100 Giga-pixel easily. The performance improvement of the graphical processing unit makes calculations faster with a normal personal computer. In contrast, the output device of CGH is not reported much. Since CGH is a fringe pattern, high resolution and fine pixel pitch are required for the output device. We have been developing a direct fringe printer, which consists of a laser, an X-Y stage, an SLM, and optical parts available on the market. Our previous report presented a liquid crystal panel, which was used for SLM that had full HD resolution and 7 μm pixel pitch. Since the Pixel pitch of the liquid crystal panel is not small enough for CGH, the optical setup works to demagnify the pixel pitch of the printed fringe pattern. To record high resolution CGH, the calculated fringe pattern is split and exposed in tiles by the X–Y stage. The results showed a 0.44 μm pixel pitch and over 100 Gpixel CGHs. However, to output more high-quality CGH, the development of pixel pitch and resolution is very important. In this paper, we review the optical system of a fringe printer that achieved an output of 0.35 μm pixel pitch CGH. We also investigate the performance of the new fringe printer.

Effect of printing direction and thickness on the mechanical behavior of SLM fabricated Hastelloy-X

An FFT based polycrystalline homogenization framework is used to predict the temperature-dependent response of SLM parts fabricated in Hastelloy-X, and to ascertain the origin of the differences in the mechanical response experimentally observed for different printing directions and part thicknesses. Several specimens are considered, built and loaded in different directions and having variable thicknesses. The crystal response is modeled using a crystal plasticity model. The alloy microstructure for each specimen is represented by a Representative Volume Element which reproduces the grain size, shape (aspect ratio), and orientation distributions experimentally measured.The resulting model is able to accurately predict the Young’s modulus and plastic stress–strain response of all different specimens in a wide temperature range. It is concluded that the differences in the mechanical response observed can be fully attributed to the changes induced by the SLM process in the polycrystalline microstructure, in particular the texture and grain aspect ratios. The accuracy of the model for all temperatures considered confirms that the polycrystalline microstructure was insensitive to temperature changes in the range considered, between 298 K and 1023 K.

A new hybrid electrochemical-mechanical process (PEMEC) for polishing complex and rough parts

A new polishing process for metallic parts has been developed by simultaneously coupling drag finishing and electrochemical polishing. This new hybrid process, called PEMEC, enables an improvement in the surface roughness within some minutes, and at the same time, preserves the shape of the edges. This hybrid process is based on a synergistic effect between abrasive and chemical mechanisms. It has confirmed its suitability to be applied on complex and rough surfaces produced by additive manufacturing SLM parts.

Comparative study of cell growth and cellular adhesion on Ti-6Al-4V surfaces made by Selective Laser Melting followed by different surface post processing steps

Selective laser melting is generally considered as to improve the design of medical implants, thus supporting medical treatment and maintaining mobility of invalid and older people. In particular, medical grade titanium alloys are in favour for spinal implants, as being nowadays manufactured by, e.g., milling. Selective laser melting offers the advantage of an adapted elasticity as to avoid stress shielding within the backbone by including complex lattice structures inside the individualized implant. For the integration into the backbone, surface properties, particularly surface roughness, are crucial with respect to biocompatibility and cell growth. Opposite to conventional milling, selective laser melting, however, results in an inferior surface roughness, leading to the necessity of downstream process steps.We report on cell growth and cellular adhesion of human primary fibroblasts on medical grade Ti-6Al-4V fabricated by selective laser melting followed by combinations of milling, hot isostatic pressing, chemical surface treatment and steam-sterilization to generate different surface conditions for cell growth. For example, cell growth is studied for varying milling path spacing on SLM parts exhibiting different surface roughness. Our results reveal good cell growth for milling path spacing lower than 0.18 mm as compared to higher milling path spacing and not milled surfaces. Cell fluorescence images and SEM images show that the cell growth is additionally hampered by the edges of the milling path. Conveniently, process failures such as pores originating from the selective laser melting process do not hamper the cell growth.

Modelling Electrochemical Polishing of Additive Manufactured Parts. Limits and Improvements of Secondary Distribution Model in Circulating Flow

Additive manufacturing allows to design and to realize parts with complex geometries, which were difficult, even impossible, to obtain by conventional means. Nevertheless, the final surface finish obtained at the end of the 3D printing process do not fit with technological specifications especially due to high roughness levels. Thus, an additional finishing step must be implemented in order to get the expected surface finish. Electrochemical polishing (ECP) is showing promising results for smoothing SLM parts with high complex shapes. ECP is an electrolytic process based on the anodic dissolution of the workpiece under constant current or potential [1]. As it is done for electroplating systems, it is nonetheless necessary to uniformize the distribution of current lines in the treatment cell, in order to make the dissolution as homogeneous as possible on the whole surface. Indeed, the concentration of the current lines on the edges causes a greater dissolution which is detrimental to the geometric integrity of the parts. The ability to provide a trustworthy simulation of the process by the help of an accurate modelling is a major issue to ensure its industrial deployment through the development of specific tools and the selection of relevant operating parameters. To this end, the use in a first approach of a secondary current distribution model gives interesting results in term of dissolution rates. However, this approach faces limitations and it is difficult to predict roughness evolution as a result of the specificity of electropolishing processes i.e. wide potential ranges and the presence of a viscous layer. Moreover, parts after treatment present heterogeneities depending to their position in the electrolyte flow circulation, which can be explained by the presence of turbulences in the vicinity of the impact zone. In order to understand and contribute to the global model improvement, mechanisms involved at a microscopic scale have been investigated with a special attention paid to the influence of hydrodynamic on mass and heat transfer exchanges. Microscopic behaviors are described, and their sensitivity to global parameters (temperature, liquid flow velocity..) are studied to help to take into account the impact of fluid dynamics and / or heat transfers on the viscous layer, which governs the polishing.[1] C. Rotty, A. Mandroyan, M-L. Doche, J-Y. Hihn, Surface & Coatings Technology vol.307 p125–135 (2016).[2] V. Urlea, V. Brailovski, Int. J. Adv. Manuf. Technol. vol.92 p4487–4499 (2017).

Improving surface quality in selective laser melting based tool making

In the last years, Additive Manufacturing, thanks to its capability of continuous improvements in performance and cost-efficiency, was able to partly replace and redefine well-established manufacturing processes. This research is based on the idea to achieve great cost and operational benefits especially in the field of tool making for injection molding by combining traditional and additive manufacturing in one process chain. Special attention is given to the surface quality in terms of surface roughness and its optimization directly in the Selective Laser Melting process. This article presents the possibility for a remelting process of the SLM parts as a way to optimize the surfaces of the produced parts. The influence of laser remelting on the surface roughness of the parts is analyzed while varying machine parameters like laser power and scan settings. Laser remelting with optimized parameter settings considerably improves the surface quality of SLM parts and is a great starting point for further post-processing techniques, which require a low initial value of surface roughness.

The Thermo-Mechanical Coupling Effect in Selective Laser Melting of Aluminum Alloy Powder.

In the selective laser melting process, metal powder melted by the laser heat source generates large instantaneous energy, resulting in transient high temperature and complex stress distribution. Different temperature gradients and anisotropy finally determine the microstructure after melting and affect the build quality and mechanical properties as a result. It is important to monitor and investigate the temperature and stress distribution evolution. Due to the difficulties in online monitoring, finite element methods (FEM) are used to simulate and predict the building process in real time. In this paper, a thermo-mechanical coupled FEM model is developed to predict the thermal behaviors of the melt pool by using Gaussian moving heat source. The model could simulate the shapes of the melt pool, distributions of temperature and stress under different process parameters through FEM. The influences of scanning speed, laser power, and spot diameter on the distribution of the melt pool temperature and stress are investigated in the SLM process of Al6063, which is widely applied in aerospace, transportation, construction and other fields due to its good corrosion resistance, sufficient strength and excellent process performance. Based on transient analysis, the relationships are identified among these process parameters and the melt pool morphology, distribution of temperature and thermal stress. It is shown that the maximum temperature at the center point of the scanning tracks will gradually increase with the increment of laser power under the effect of thermal accumulation and heat conduction, as the preceded scanning will preheat the subsequent scanning tracks. It is recommended that the parameters with optimized laser power (P = 175–200 W), scanning speed (v = 200–300 mm/s) and spot diameter (D = 0.1–0.15 mm) of aluminum alloy powder can produce a high building quality of the SLM parts under the pre-set conditions.

A Mathematical Dimensional Model for Predicting Bulk Density of Inconel 718 Parts Produced by Selective Laser Melting.

Featured Application Mathematical tool for tuning Selective Laser Melting process parameters for achieving highly dense components. In this work, dimensional analysis is used to develop a general mathematical model to predict bulk density of SLMed components taking volumetric energy density, scanning speed, powder’s thermal conductivity, specific heat capacity, and average grain diameter as independent variables. Strong relation between dependent and independent dimensionless products is observed. Inconel 718 samples were additively manufactured and a particular expression, in the form of a power-law polynomial, for its bulk density, in the working domain of the independent dimensionless product, was obtained. It is found that with longer laser exposure time, and lower scanning speed, better densification is attained. Likewise, volumetric energy density has a positive influence on bulk density. The negative effect of laser power in bulk density is attributed to improper process conditions leading to powder particle sublimation and ejection. A maximum error percentage between experimental and predicted bulk density of 3.7119% is achieved, which corroborates the accuracy of our proposed model. A general expression for determining the scanning speed, with respect to laser power, needed to achieve highly dense components, was derived. The model’s applicability was further validated considering SLMed samples produced by AlSi10Mg and Ti6Al4V alloys. This article elucidates how to tune relevant manufacturing parameters to produce highly dense SLM parts using mathematical expressions derived from Buckingham’s π- theorem.

Experimental and numerical analysis of fatigue cracks emanating from internal defects in Ti6Al4V SLM

In the present work a series of fatigue tests on Ti6Al4V SLM parts are analyzed via both SEM and confocal microscopy. On the one hand, fracture surfaces are studied, and a common pattern is found, formed by a series of different textures which show the complex crack front evolution from crack initiation in a particular internal defect to complete failure. On the other hand, fatigue strength is observed to highly depend on the defect where initiation takes place, so experimental observation of that critical entity is carried out. Both defect location within the specimen and shape are studied, considering the crack-like or blunt feature of every defect. Once experimental analysis is complete, numerical simulation is attempted. By making use of critical defect and residual stress measurements obtained experimentally, both fatigue strength and crack front evolution are estimated.

Microstructure and Mechanical Properties of Laser Welded 316L SLM Parts

This paper focuses on a study conducted on laser welding of printed 316L parts that were produced with a selective laser melting process. Commercial sheet material was used as a reference for the printed 316L parts. The effect of heat treatment on joint properties, and on what stage of the process it should be applied, was studied with metallography and mechanical testing. Optical microscopy was applied to analyze the microstructure of the base material and the weld seam. Tensile testing was applied for determining monotonic strength of different structures. The printed base material showed higher strength, but lower ductility in comparison to the commercial sheet material. In the welded condition, tensile properties were impaired by the welding, but for the commercial sheet material, no clear effects were seen. The difference was hypothesized to be caused by the higher strength mismatch in the printed joints. For the welded structures, the best strength-ductility combination was achieved by heat treating the parts both before and after the laser welding.

Microstructure and tribological property of selective laser melted Fe-Mn-Al-C alloy

To investigate the microstructure and tribological property, Fe-16Mn-10Al-1.5C alloy was fabricated via selective laser melting technology at different laser scanning speeds for the first time in this study. The results show that the microstructure of Fe-Mn-Al-C alloy is composed equiaxed grains and columnar dendrites. And this alloy is mainly composed of γ-Fe. This alloy has high microhardness and low coefficient of friction (COF). The microhardness of the SLM part increases with the decrease of the laser scanning speeds. The sample prepared at the highest speed not only has the lowest COF, but also the lowest wear rate amongst these SLM parts and its wear mechanism is mainly abrasive wear and adhesive wear. The wear mechanism of the samples V0.4 and V0.28 is not only abrasive wear but also adhesive wear that produces grooves, which generates a large number of grooves during the wear test.

The effect of SLM process parameters on density, hardness, tensile strength and surface quality of Ti-6Al-4V

In this paper, we printed Ti-6Al-4V SLM parts based on Taguchi design of experiment and related standards to measure and compare hardness with different mechanical properties that were obtained in our previous research such as density, strength, elongation, and average surface. Then the effect of process parameters comprising laser power, scan speed, hatch space, laser pattern angle coupling, along with heat treatment as a post-process, in relation to hardness was analysed. The relation of measured factors with each other was also studied and related mechanisms were discussed in depth. The original contribution in this paper is in producing a large and precise dataset and the comparison with mechanical properties. Another contribution is related to the analysis of process parameters in relation to hardness and explaining them by rheological phenomena. The results showed an interesting similarity between hardness and density which is highly related to the formation of the melting pool and porosities within the process.

The effect of laser focus shift on microstructural variation of Inconel 718 produced by selective laser melting

Selective laser melting is gaining interest in the aerospace industry as a candidate for manufacturing flight hardware. With increasing popularity comes the need for rigorous qualification standards and a thorough understanding of how processing parameters can affect part quality. One such parameter is the laser focal shift, which moves the building plane relative to the focal plane of the laser. Changing the focal shift alters the power density while keeping the laser parameters constant, a novel method for tailoring the microstructures of parts made by SLM. In this study, samples were manufactured at different focal positions and their microstructures were characterized. Samples built at the focal plane had a finer microstructure than samples built away from it, a result of higher power density and a smaller spot size. Furthermore, samples in the heat-treated condition produced at the shifted locations exhibited coarser grains aligned parallel to the build direction, while those at the focal plane had a finer and more equiaxed microstructure. Differences originating from the as-built condition that remain through post processing may influence the final mechanical performance of SLM parts. Utilizing the focus shift to produce these changes presents a unique opportunity for microstructural tailoring and improved mechanical performance.

Numerical modelling and experimental validation of thermal history of titanium alloys in laser beam melting

During selective laser melting processes parts will heat up with each layer depending on the geometry and surrounding powder material. This leads to process boundary conditions that are not certainly defined and can induce unstable melt pool sizes. These will have an influence on surface roughness and dimensional accuracy. One way to deal with this is an individual adaptation of process parameters, but without knowing the exact thermal boundary conditions in each layer one will not be able to adapt the parameters properly. In this paper a model for prediction of the macroscopic temperature history is presented and experimentally calibrated. A sample with characteristic features like overhanging’s was designed. These samples were produced by selective laser melting and simultaneously monitored by an infrared camera to calibrate the boundary conditions of a numerical model. This lays the foundation for part individual adaption of process parameters to improve the quality of SLM parts.

Comparison of Single Ti6Al4V Struts Made Using Selective Laser Melting and Electron Beam Melting Subject to Part Orientation

The use of additive manufacturing technologies to produce lightweight or functional structures is widespread. Especially Ti6Al4V plays an important role in this development field and parts are manufactured and analyzed with the aim to characterize the mechanical properties of open-porous structures and to generate scaffolds with properties specific to their intended application. An SLM and an EBM process were used respectively to fabricate the Ti6Al4V single struts. For mechanical characterization, uniaxial compression tests and hardness measurements were conducted. Furthermore, the struts were manufactured in different orientations for the determination of the mechanical properties. Roughness measurements and a microscopic characterization of the struts were also carried out. Some parts were characterized following heat treatment (hot isostatic pressing). A functional correlation was found between the compressive strength and the slenderness ratio (λ) as well as the equivalent diameter (d) and the height (L) of EBM and SLM parts. Hardness investigations revealed considerable differences related to the microstructure. An influence of heat treatment as well as of orientation could be determined. In this work, we demonstrate the influence of the fabrication quality of single struts, the roughness and the microstructure on mechanical properties as a function of orientation.