Laser Beam Melting Research Articles

Additive manufacturing of copper alloys: influence of process parameters and alloying elements

ABSTRACTOwing to the physical properties of copper and its alloys it is challenging to achieve good surface quality and low porosity by the widely used laser-based additive manufacturing processes. This paper deals with the role of alloy composition, powder size and process parameters in additive manufacturing with laser beam melting machine (with power up to 100 W). Test parts were produced in pure copper and CuNiSi(Cr) alloys. The porosity was investigated as a function of different process parameters and powder size ranges. The effects of the alloy physical properties (reflectivity, thermal conductivity, melting range and surface tension) are discussed. Moreover, the effect of thermal treatment on the properties of CuNiSi parts was assessed in conventional two-step heat treatments.

Industrial 3D printing for modern machine and handling systems—Potential and solutions

Parts manufactured by laser beam melting (LBM) are about to break through prototype status and into industrial applications, especially in the medical technology and aviation industries. Machine and handling systems (MHSs) have not yet benefited from the numerous advantages of this disruptive manufacturing method on a broader scale, although conditions appear promising for implementation. Certification and approval processes often require less time and costs for MHSs than for other industry branches, which promotes rapid implementation of LBM. In the future, modern MHSs will perform more specialized, diverse, and complex tasks, even going beyond industrial applications and entering the private use market, e.g., through household assisting robots. In Sec. I, an insight into the requirements and challenges in the context of existing manufacturing and use restrictions of MHSs is given. Furthermore, Sec. I briefly introduces the LBM process and its benefits and restrictions. In Sec. II, the concepts of lightweight construction, functional integration, and a high degree of design freedom reveal potential for the design and redesign of novel product solutions for current and future applications. Exemplarily, a 6-axis robot is being examined with regard to industrial LBM production. Suitable subcomponents will be identified. Possible solutions for the previously mentioned fields are described in Sec. III. In order to determine achievable weight savings and to allow masses to be more dynamic, conventional designs are compared with developed topology-optimized components. Additionally, feature integration allows MHS manufacturing steps to be reduced. An integral design is being studied to minimize the number of machine parts used. Biomimetic approaches are used to reduce interference contours. This is of great importance for preventive personal protection in human–machine interaction (HMI) in order to expand the range of applications of modern MHSs. Furthermore, modal properties are specifically adapted. Lattice and hybrid LBM construction methods are also being considered. Accumulated in-depth knowledge of industrial LBM, lightweight construction, functional integration, design flexibility in mass customization, and also HMI topics for MHSs are summarized. An outlook is given on further applications and future approaches for industrialization.

Novel approach to optimized support structures in laser beam melting by combining process simulation with topology optimization

Additive manufacturing is gaining importance in different industries, being on the verge to broad industrial application. Especially in laser beam melting (LBM) of metals, support structures play a vital role in the successful production of parts, since they are responsible for supporting overhanging features and preventing warpage. Today, these support structures are often massive and lead to high postprocessing effort for removal and surface finishing. Existing structures do not meet the needs of the individual part, adding cost to the production of additive parts without even fulfilling all their respective tasks. To reduce the manufacturing and finishing effort in LBM, new ways of support structure application have to be found. One way to decrease the material consumption, and therefore the overall costs in terms of raw material and manufacturing effort, is to use topology optimization for the generation of support structures. This study presents an extension of the current approaches, which take into account the task of supporting overhanging features, by using a finite element analysis of the manufacturing process of LBM to assess the loads applied to the support structures by residual stresses during the manufacturing process. This is critical especially to the LBM of metals. A case study of a cantilever beam is carried out to investigate the general validity of the proposed procedure. First, a simulation of the manufacturing process of the cantilever as well as the respective support structures is conducted. Second, using the simulation's results as input, topology optimization of the support structures by applying the solid isotropic material with penalization method is executed. The result, resembling tree-like features, demonstrates the capabilities of the procedure and points out the possibility of using variable densities within one structure. Finally, critical needs in research to further develop the approach are pointed out.

Influence of Porosity in LBM Layers on the Quality of Laser Deep Alloying

Abstract Powder bed-based processes such as laser beam melting (LBM) are becoming increasingly relevant in industrial production environments. A novel approach is to use the LBM process for the targeted addition of alloy elements for the laser deep alloying process. In this two-step process, pre-deposited element layers are remelted and mixed into the base material using a high-power laser and beam modulation. However, the pre-deposition of master alloy by means of LBM may induce pores of varying intensity in the applied layers. The present work deals with the influence of such porosity in pre-deposited layers on the resulting microstructure of deep-alloyed micro-samples and thus on the quality of laser deep alloying. The goal was to investigate the suitability of the LBM process for the addition of alloy elements into the melt pool to maintain high throughput material development. For this purpose, an atomized stainless steel was applied in different layer thicknesses on an unalloyed steel. In addition, a different porosity was set in the layers. On average, only a few pores appeared in the microstructure after laser deep alloying. Instead, cracks occurred unsystematically in the microstructure. The experiments have shown that the porosity of pre-deposited layers has no influence on the formation of pores or cracks in the resulting microstructure after laser deep alloying. Thus, it was established that regarding pores in the layers, the laser deep alloying process is stable against fluctuations in the energy deposition in the LBM process.

Additive manufacturing of pure tungsten by means of selective laser beam melting with substrate preheating temperatures up to 1000 ∘C

Abstract The preferred plasma-facing material in present-day and future magnetic confinement thermonuclear fusion devices is tungsten. This material is mainly chosen because of its high threshold energy for sputtering by hydrogen isotopes as well as its low retention of tritium within the material. From an engineering point of view, however, tungsten is a challenging material to work with as it is an inherently hard and brittle metal. In this respect, established fabrication technologies for tungsten and tungsten based materials are a limiting factor directly affecting the design of plasma-facing components. Against this background, additive manufacturing technologies could prove very beneficial with regard to plasma-facing component applications as they offer flexibilities beyond the possibilities that conventional manufacturing methods offer. Within the present contribution, we report on recent results regarding the additive manufacturing of tungsten by means of powder-bed based selective laser beam melting. In more detail, investigations on pure tungsten manufactured by using elevated substrate preheating temperatures up to 1000 ∘C are described.

Uncertainty Evaluation of Pore Analysis for Additively Manufactured Parts using Cross Sections

Additively manufactured (AM) workpieces, which have been produced by means of laser beam melting (LBM), tend to show quality relevant pores and cracks on the inside. Quality assurance on these parts may be conducted using x-ray Computed Tomography (CT), as CT is able to spot pores and cracks in the entire volume of a specimen. However, the uncertainty of detection of such cavities in a CT measurement is unknown. To tackle this shortcoming, different authors have compared metallographic cross sections to CT images. None of these investigations have compared the cross section’s plane directly to the corresponding plane of the CT’s 3D voxel image, without using features of the CT image itself, though. This paper introduces a special artifact as reference standard, whose pyramid like form allows a direct comparison of a metallographic cross section and the corresponding plane in a CT scan. The shape of the test object allows to calculate the parameters of the plane equation of the metallographic cross section in a coordinate system fixed to the test object. The plane equation is used to identify and investigate the corresponding plane in the CT scan. By comparing this artificial cross section from the CT scan with the cross section from the metallography specimen, the CT’s performance for detecting pores is assessed. As a result, a general method to test any CT’s capability to detect pores in an AM part is given by using cross sections of test objects with the proposed pyramid like form.

3D Shape Analysis of Powder for Laser Beam Melting by Synchrotron X-ray CT

The quality of components made by laser beam melting (LBM) additive manufacturing is naturally influenced by the quality of the powder bed. A packing density <1 and porosity inside the powder particles lead to intrinsic voids in the powder bed. Since the packing density is determined by the particle size and shape distribution, the determination of these properties is of significant interest to assess the printing process. In this work, the size and shape distribution, the amount of the particle’s intrinsic porosity, as well as the packing density of micrometric powder used for LBM, have been investigated by means of synchrotron X-ray computed tomography (CT). Two different powder batches were investigated: Ti–6Al–4V produced by plasma atomization and stainless steel 316L produced by gas atomization. Plasma atomization particles were observed to be more spherical in terms of the mean anisotropy compared to particles produced by gas atomization. The two kinds of particles were comparable in size according to the equivalent diameter. The packing density was lower (i.e., the powder bed contained more voids in between particles) for the Ti–6Al–4V particles. The comparison of the tomographic results with laser diffraction, as another particle size measurement technique, proved to be in agreement.

Additive manufacturing of an oxide ceramic by laser beam melting—Comparison between finite element simulation and experimental results

Recent progress in the application of Laser Beam Melting (LBM) of oxide ceramics has shown promising results. However, a deeper understanding of the process is required to master and control the track development. In this approach numerical modeling could allow higher quality, of additive manufacturing for such materials, to be achieved. The validation of an earlier developed finite element model for LBM of ceramic materials has been established through a comparison with experimental results. The model solves heat and mass transfers whilst accounting for fluid flow due to surface tension and Marangoni convection, as well as tracking the material/gas boundary. The volumetric heat source parameters used in the simulations have been calibrated with an analytical model combined with original in-situ reflectance measurements. Numerical results show good agreement with measurements of melt pool dimensions and shapes. They also provide a coherent description of the evolution of the track morphology when varying the heat source parameters. Track irregularities have also been revealed by simulations at high scanning speed and the balling effect highlighted and explained through similar simulations.

Smart components by additive technologies

Smart components are key technologies for industry 4.0 as well as the Internet of Things. The application areas include e.g. systems for sustainable production, automotive, smart agriculture, and smart home. In order to integrate intelligence within a component, a sensor or actuator system typically needs to be embedded within the component itself during fabrication. Additive manufacturing has been extended to more and more industries and is now being used not only in toolmaking, but also in the series production of components. Geometric functionalization for conformal cooling is already state of the art in tool making. Despite the great potential of function-integrated parts, publications on sensor integration into parts fabricated by additive manufacturing are still rarely found. First approaches address multimaterial printing by using insulating and conductive materials as well as the integration of smart elements during the printing process. Laser beam melting (LBM) is an additive manufacturing technology for geometrically complex parts based on the layer wise and selective melting of metal powders. As proof-of-concept of the developed integration concept, based on LBM, the functionalization of tools and medical implants will be presented.

New approach to evaluate 3D laser printed parts in powder bed fusion-based additive manufacturing in-line within closed space

Additive manufacturing that allows layer by layer shaping of complex structures is of rapidly increasing interest in production technology. A particularly rapid prototyping technique of additive manufacturing is laser beam melting (LBM). This 3D printing method is based on a powder bed fusion technique, using a high-powered laser to melt and consolidate metallic powders. The process needs a tightly controlled atmosphere of inert gas, which requires a confined space of a building chamber. This and more process related factors like elevated temperatures, laser radiation or the resulting light intensity caused by the melting of metals, make a closed-loop quality control very ambitious. In this paper, we propose a new in-process approach for quality control with high precision metrology based on structured light. The precise layer by layer dimensional measurement of both the printed part and the powder deposition, allows for process assessment in- or off-line.

Additive manufacturing of soft magnetic permalloy from Fe and Ni powders: Control of magnetic anisotropy

Abstract The influence of the process parameters in Laser Beam Melting (LBM) on the element distribution and magnetic properties of permalloy ( Ni 78.5 Fe 21.5 ) is studied. Iron and nickel powders are mixed in the respective proportions to build twenty-five permalloy samples. The process parameters for each sample are varied to achieve different volume energy densities. An increase of the saturation magnetization MS up to 14% of the samples with respect to the initial powder blend is found. For a volume energy density of 428 J mm 3 we detect a stripe-like segregation of iron and nickel in the uppermost layer. In the volume a homogeneous element distribution is found. The segregation at the surface leads to a sizable uniaxial magnetic anisotropy. When using parameter combinations resulting in similar volume energy densities, we observe different surface morphologies depending on scan speed and laser power. The implications for creating tailored magnetic anisotropy directions in Fe-Ni soft magnets are discussed.

Fast fatigue characterization by infrared thermography for additive manufacturing

High cooling rates and strong temperature gradients occurring during additive manufacturing (AM) by laser beam melting (LBM) are known to affect mechanical performances of the produced parts, and particularly fatigue properties. Mechanical properties strongly depend on the process parameters such as laser power, scan speed and laser spot size. A fatigue analysis was performed in this study by using infrared thermography to assess heat sources associated with fatigue damage. In particular, mechanical dissipation was identified thanks to suitable acquisition parameters. A model for the relationship between mechanical dissipation and mechanical solicitation was proposed and applied to two promising steels in LBM AM field, namely maraging 18Ni300 and L40 tool steel.

In-Situ Phase Transition Analysis of Conventional and Laser Beam Melted AlSi10Mg and X5CrNiCuNb16-4 Alloys*

Abstract Although metallic materials processed by laser beam melting have significantly different microstructures compared to their conventional counterparts no adjusted heat treatment parameters have been published to date. Conventional heat treatment will therefore not always result in the desired mechanical properties. This project examines the effect of heat treatment on the properties of laser beam melted components produced with AlSi10Mg (EN AC-43000) precipitation hardening cast aluminium alloy and X5CrNiCuNb16-4 (1.4542 or 17-4 PH) martensitic precipitation hardening steel. Comparisons to conventionally manufactured material are made in parallel. The kinetics of phase transformation during heat treatment is analysed in-situ by means of differential scanning calorimetry (AlSi10Mg and X5CrNiCuNb16-4) and dilatometry (X5CrNiCuNb16-4). The results show considerable differences in phase transformation kinetics between laser beam melted and conventionally processed materials.

In situ measurement of part geometries in layer images from laser beam melting processes

Laser beam melting (LBM) enables production of three-dimensional parts from metallic powder with very high geometrical complexity and very good mechanical properties. In LBM, a thin layer of metallic powder is deposited onto the build platform and melted by a laser according to the desired part geometry. Until today, the potential of LBM for critical applications such as medical devices and aerospace has not been exploited due to the lack of build stability and quality management. We present an image analysis method, which segments part contours in high-resolution images of LBM-produced layers. Based on the reference contour from 2D slices of the 3D part model and edge-detection results, a graph model is built and segmented using Graph Cuts (min-cut max-flow algorithm). Our method is evaluated on 124 part contours from 5 build jobs with different part geometries. Iterative GrabCut segmentation on nonlinearly smoothed images achieves the best results with a median Jaccard distance of 0.035 (32 % improvement over the reference geometry masks) and a mean contour distance below 2.4 px (36.4 % improvement).

A Smoothed Particle Hydrodynamics Model for Laser Beam Melting of Ni-based Alloy 718

Laser Beam Melting (LBM) – also referred to as Selective Laser Melting (SLM) – is an Additive Manufacturing (AM) technology and pertains to the Powder Bed Fusion (PBF) processes. Within this work a numerical simulation model of the LBM process based on the meshless computational method Smoothed Particle Hydrodynamics (SPH) in a multi-phase and weakly-compressible formulation is presented. Its implementation utilizes the parallelization capabilities of Graphics Processing Units (GPU) for achieving a reduced computation time. Physical phenomena such as thermal conduction, melting and re-solidification, convection and effects related to surface tension such as thermocapillarity are considered. Furthermore, the influence of the recoil pressure induced by evaporating atoms from the melt pool surface is taken into account. The material properties of relevance (e.g. dynamic viscosity, thermal conductivity and absorptivity) for the investigated alloy Inconel® 718 (respectively Alloy 718) are modeled by means of temperature-dependent functions. The underlying material data originate from literature. The simulation results are compared with experimental findings for single melt tracks to evaluate the model validity with regard to the process parameters scanning velocity and laser power.

Fabrication, Experiments, and Analysis of an LBM Additive-Manufactured Flexure Parallel Mechanism.

Additive manufacturing technology has advantages for realizing complex monolithic structures, providing huge potential for developing advanced flexure mechanisms for precision manipulation. However, the characteristics of flexure hinges fabricated by laser beam melting (LBM) additive manufacturing (AM) are currently little known. In this paper, the fabrication and characterization of a flexure parallel mechanism through the LBM process are reported for the first time to demonstrate the development of this technique. The geometrical accuracy of the additive-manufactured flexure mechanism was evaluated by three-dimensional scanning. The stiffness characteristics of the flexure mechanism were investigated through finite element analysis and experimental tests. The effective hinge thickness was determined based on the parameters study of the flexure parallel mechanism. The presented results highlight the promising outlook of LBM flexure parts for developing novel nanomanipulation platforms, while additional attention is required for material properties and manufacturing errors.

Laser beam melting and heat-treatment of 1.2343 (AISI H11) tool steel – microstructure and mechanical properties

Laser Beam Melting (LBM) of metals is an innovative additive manufacturing technology for producing complexly shaped parts. However, the spectrum of available materials is yet limited and the qualification of further alloys is subject of ongoing research. Considering tooling applications e.g. for injection moulding low alloyed tool steels like 1.2343 (AISI H11) would be of particular interest. The feasibility of processing 1.2343 by LBM has already been shown. Besides the LBM-process itself, also a heat-treatment process has to be taken into account. Heat-treatment is necessary to reduce the process-inherent internal stress and to adjust the desired mechanical properties. Hence, an experimental study on the heat-treatment of laser beam molten specimens made from 1.2343 is conducted. The resulting microstructure is characterised by metallographic microsections and electron backscatter diffraction (EBSD). Additionally, hardness measurements and tensile tests give information about the mechanical properties in dependence of the build direction and the heat-treatment strategy. The ultimate tensile strength after annealing reached 2148 ± 16 MPa along with an elongation at break of 8.8 ± 1.1 %. The hardness of the LBM-generated material was determined to 737 ± 16 HV1 after hardening and to 585 ± 9 HV1 after annealing.

Transient development of residual stresses in laser beam melting – A neutron diffraction study

Additive manufacturing (AM) is increasingly used for the production of functional parts. In order to ensure product reliability in challenging load cases and environments, a valid knowledge of the residual stress state is crucial. Since typical, complex AM geometries necessitate simulative efforts for this prediction, suitable validation data are essential. This study presents results from neutron diffraction measurements on different stages of a build-up of a simple cuboid structure by laser beam melting. Strains are generally measured in three perpendicular directions, coinciding with the symmetry axes of the structure, so that the stresses in these directions can be calculated using Hooke's Law. The strain-free reference is obtained from measurements on small matchstick geometries cut from an analogously manufactured cuboid at the respective measurement spots. By providing quasi-transient data of the evolution of residual stresses in both the base plate and the part, simulation models can be investigated towards their structural validity. Lastly, at two positions, strain measurements were obtained from six directions in order to determine an estimation of the full strain tensor. Results indicate that the assumption of negligible shear strains may not be justifiable.

Laser Alloying Advantages by Dry Coating Metallic Powder Mixtures with SiOx Nanoparticles.

Up to now, minimizing segregation of free-flowing, microscale metal powder mixtures driven by different mass density is an open challenge. In this work, effects of particle size variation on homogeneity of Al-Cu mixtures, with a density ratio of 3.3, are examined. Dry coating Al particles with 0.3 wt% fumed silica SiOx nanoparticles significantly decreases interparticle attraction. This enlarges the range of free-flowing Al particle sizes to < 20 µm. Powder mixture homogeneity is examined optically in vibrated bulk powder and thinly spread layers. From various powder mixtures, solid samples are built layer by layer with the Additive Manufacturing (3D printing) technology Laser Beam Melting in metal powder bed (LBM). Chemical homogeneity of solids is evaluated via energy-dispersive X-ray spectroscopy, backscattered electron microscopy, metallographic analysis and tensile tests. Persistent homogeneity of Al-Cu powder mixtures and LBM solids is found only with particles < 20 µm dry coated with SiOx nanoparticles. Observed segregation phenomena are explained with a decrease in particle mobility at increasing local concentration and the decreasing effectiveness of mass in smaller particles. The main effects are based on geometry, so they are expected to be transferrable to other nanoparticles, alloying components and powder bed technologies, e.g., binder jetting.

Validation of Mechanical Layer Equivalent Method for simulation of residual stresses in additive manufactured components

A challenge in the additive manufacturing process of laser beam melting of metals is the formation of residual stresses, which can cause large part deformations, when the part is released, lower the application range concerning tensile loads and provoke cracks. Due to the complexity and the interaction of different process parameters, the use of experimental studies for investigation is tedious and costly. An effective approach to analyze the effects is the numerical process simulation based on the method of finite elements. In this paper, the recently developed method of Mechanical Layer Equivalent for fast calculations of process induced distortions in laser beam melting is used to analyze the residual stresses during laser beam melting. For this purpose, small test samples have been generated additively and residual stresses have been measured by X-ray diffraction and compared to simulated values. The results show that despite the simplifications the model inhibits reliable prediction of residual stresses can be achieved.