Laser Beam Melting Research Articles

On-Machine Measuring Method for the Reconstruction of Additively Manufactured Near-Net Shaped Parts

Abstract Additive manufacturing brings along many chances like functional integration or light-weight construction. Because of distortion and poor surface roughness, however, machining is required to produce high quality parts out of metal. Depending on the position on the build tray, there is a variation of the deformation so that the exact part geometry after the additive manufacturing process is unknown. Hence, there is a big challenge for the mechanical post-processing. In this paper, a strategy to measure additively manufactured parts made by Laser Beam Melting directly in a milling machine using the touch trigger probe was developed. Therefore, a test specimen was machined, that includes a selection of the most relevant geometric features like holes, cones, different chamfers and a free-form surface. Subsequently, a number of measuring points was chosen for every feature. In order to select the positions of the measuring points, different algorithms from literature were evaluated. As a result, the Hammersley algorithm was determined as the most suitable algorithm for this use case. A special measuring test cycle was implemented in a Computer Aided Manufacturing (CAM) software and tested on a five-axis milling machine. The measuring results were edited in the CAM so that an accurate geometric model of the real part could be created. Based on this model, an adaption of the tool paths is possible, with the effect of a reduction of the machining time and an optimized utilization of the tool. The results were also compared with an optical strip light projection. It was shown, that the discrepancy between the tactile and the optical measuring strategy was minimal. This means that the tactile measurement results have a sufficient accuracy. In a second step, the developed measuring strategy was applied to turbine blades made by Laser Beam Melting. Again, the turbine blades were measured using the touch trigger probe in a five-axis milling machine and the results were validated.

Sustainability of industrial components using additive manufacturing and foam materials

In the industrial world, the topic of sustainability is gaining in importance, and it is the main issue in research and development regarding manufacturing. Looking for higher resource-efficiency and lightweight components results in a continuously increasing use of parts made by additive manufacturing (AM) and cellular materials. This paper will include highlights in laser beam melting and metal open-cell structures compared to conventional materials, considering different environmental and socioeconomic aspects. Examples will be presented in the area of machine tools and medical engineering. The main advantages of additive manufacturing comprise the compact, lightweight, and customized design of the components and the manufacturing of geometrically complex parts, reducing assembling efforts. One example of an open-cell structure was manufactured by investment casting.

A comparison of material models for the simulation of selective beam melting processes

The focus of this contribution lies on the comparison of different material models for the thermo-mechanical simulation of selective laser melting (SLM) processes. A Finite Element (FE) framework for the analysis of residual stresses and distortions in laser beam melting processes is introduced. The FE solver for the thermo-mechanical problem uses a staggered algorithm, is based on the open source finite element library deal.ii and makes use of heavy parallelization. The most important aspects to be investigated here are the mechanical material behavior and stress relaxation in the range of the melting temperature. Material models, which reset certain variables as stress and plastic strain to zero at a critical temperature are compared to a thermo-viscoplastic constitutive law. Simulations, related to the SLM process of Ti-6Al-4V, are performed to compare the residual stresses and plastic strains predicted by different models after melting and cooling. Furthermore, the validity of geometrically linearized material model is analyzed.

Infrared monitoring of modified hatching strategies for laser sintering of polymers

In laser sintering of polymers, the exposure strategy, consisting of hatching strategy, laser power and scan speed determines the final properties. In polymer processing, mainly the meander hatching strategy is applied, which leads to geometry-dependent part properties at supposedly identical processing parameters due to overlay effects of temperature fields. Within this work, different hatching strategies, originating from laser beam melting of metals are investigated to overcome the limitations of laser sintering of polymers. The processability is evaluated characterizing single- and multi-layer parts. A direct correlation between the predominant temperature fields resulting from different hatching strategies and the resulting part properties, such as surface topology or part height, could be evidenced. These results contribute to the understanding of the impact of hatching strategy on the laser sintering process and should be considered in the future development of geometry-invariant processing strategies.

Study on the origins of residual stresses in Ti-6Al-4V processed by additive manufacturing

In additive manufacturing processes using laser beam melting high thermal gradients are generated, inducing residual stresses within the parts that may lead to deformations and, in worst cases, cracks. One of the materials that is the most sensitive to residual stresses is the Ti-6Al-4V alloy. In the present study, we focus on the various parameters that may control the genesis and build-up of the residual stress states. Dwell time and thermal conductivity, both influencing the heat evacuation, were studied. Higher thermal evacuation was find to rises residual stresses within the part. Then, the reliability of the energy density as a comparison parameter was investigated. Samples with the same energy densities but different power and scanning speed were elaborated. Energy density was shown as a non-reliable parameter to compare different processed parts.

Particle shape and size analysis for metal powders used for additive manufacturing: Technique description and application to two gas-atomized and plasma-atomized Ti64 powders

The particle size and shape distributions of metal powders used in additive manufacturing powder bed fusion processes are of technological importance for the final built product. Current three-dimensional (3D) measurements of these distributions assume a spherical shape, while techniques that measure both size and shape are always two-dimensional (2D) measurements of particle projections. This paper describes a set of techniques using X-ray computed tomography, combined with various mathematical algorithms, to measure the 3D size, shape, and internal porosity of individual particles. Calibrated by a limited amount of visual examination of 3D images of individual particles, these techniques can classify powder particles as single near-spherical (SnS) particles, and non-spherical (NS) particles, which consist of either single highly non-spherical particles or multi-particles, where two or more smaller particles have been joined together. From this 3D data, other algorithms can generate 2D particle size and shape information to compare with the results of 2D measurement techniques. These techniques are applied to two metal powders composed of a specific alloy of titanium with aluminum and vanadium, denoted as Ti64, which is in common use as a powder for selective laser or electron beam melting powder bed additive manufacturing. One powder was made with a gas-atomization process, the other with a plasma-atomization process, both have been recycled, and both pass the specifications for additive manufacturing use. The powders differ in the fraction of NS particles and porous particles, in their size and shape distributions, and in average shape and size statistics. The SnS/NS classification enables one to show how these classes contribute to the overall particle size distributions, even for a single powder type, and is useful for comparing different sources of powder as well as studying how the size/shape distributions of a powder might change over multiple recycling events.

A comparison of the high-temperature oxidation behaviour of conventional wrought and laser beam melted Inconel 625

The microstructure and oxidation resistance of Laser Beam Melted (LBM) and Conventionally Manufactured (CM) Inconel 625 alloys were studied at 900 °C and 1050 °C. The microstructure of the LBM samples was cellular, with Nb and Mo segregations located at the cell walls. At 900 °C, the oxidation rate was similar for both materials but was clearly higher for the LBM material at 1050 °C. This high oxidation rate induced poor oxide scale compactness, void formation in the subsurface region and the formation of a high amount of Nb1.5Cr0.5O4 at the alloy-oxide interface.

Review: Materials Ecosystem for Additive Manufacturing Powder Bed Fusion Processes

Additive manufacturing technologies are revolutionizing modern component design across many industries, while leading to an evolution in materials science and engineering. Understanding and controlling the materials ecosystem in additive manufacturing is an essential factor for successful adoption. The relationships among materials chemistry, powder characteristics, processes and final part performance are key and crucial concepts in additive manufacturing technologies. Powder bed fusion (PBF) processes including laser and electron beam melting processes are fundamentally based on controlling the solid-to-liquid and liquid-to-solid phase transformations in each process layer. The powder characteristics, evolution of the microstructure through the additive manufacturing process and subsequent metallurgical post-processing are primarily responsible for material performance. A more comprehensive understanding of aspects such as powder characteristics, liquid- and solid-phase transformations, and the effects of repeated thermal cycling on metallurgical structure development will be required to effectively apply a design-for-additive approach. Numerical modeling and machine learning are among tools that can be used for developing such understanding. This article will provide a review and summary of the materials ecosystem for additive manufacturing powder bed fusion processes.

Modeling of mechanical behavior in additive manufacturing at part scale

Laser Beam Melting is actually capable of producing parts with reliable mechanical properties. However, efficient production still remains a challenge and high quality numerical simulation is required in order to understand the physical mechanisms involved. Consequently, a macroscopic numerical model at part scale is actually under development for understanding the relationship between different process and material parameters with the mechanical state of final parts such as distortion and residual stress. Classical finite element method is used to solve the coupled thermo-mechanical problem on the whole domain defined by the workpiece, the baseplate and the support structures. At this scale, powder packing is neglected as well as the hydrodynamics behavior within the melt pool. Homogeneous equivalent heat source is used and imposed until several layers below the current deposited layer. Elastoplastic constitutive material law with temperature dependent parameters has been developed.

Microstructure and high-temperature properties of Fe-Ti-Cr-Mo-B-C-Y2O3 laser cladding coating

Fe-based composite coatings were fabricated on 5CrNiMo die steel by laser beam melting a precursor mixture of ferrotitanium, ferrochromium, ferromolybdenum, B4C and Y2O3 powders. Microstructure and properties of the coatings were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS), resistance furnace and high-temperature tribometer. The results show that (Ti,Mo)C particles with flower-like and (Ti,Mo)B2 with block-like shapes are in situ formed during laser cladding. Volume faction of multiple ceramic particles increases with the increasing of Y2O3. The cumulative oxidation mass of the coating with 2 wt% Y2O3 is decreased by one-third than that of the coating without Y2O3. The oxidation layer of the coating with Y2O3 is getting smooth. Meanwhile, high temperature wear volume loss of the coating with 2 wt% Y2O3 is about 40% that of the coating without Y2O3. The coating with 2 wt% Y2O3 shows a smoother wear scar and few flat grooves are observed after high temperature wear test.

Radio‐Frequency Identification of Metal AM Parts

AbstractRadio frequency identification technology (RFID) is used extensively as a transponder in product identification and goods logistics. Additive manufacturing (AM) technologies such as metal‐processing laser beam melting (LBM) make it possible to interrupt and continue production processes at any time. Using this advantage, it is possible to interrupt the layer‐by‐layer building process at a specific height of the z axis in order to integrate components such as RFID tags. This allows metallic components to be uniquely marked with an invisible and firmly fixed tag during their manufacture. The challenge of this approach is to find suitable criteria for integrating RFIDs into metal, as metallic environments have a strong negative impact on the transmission range.

Laser beam melting of Cr3C2-NiCr

Due to the high temperature wear resistance, low friction coefficient and corrosion resistance, chromium carbides are widely used for coating applications and commonly processed through thermal spraying. In contrast to classical processing, additive manufacturing technologies such as Laser Beam Melting (LBM) offer significant geometrical flexibility in design. Relying on the use of this advantage for manufacturing innovative products, the present study focuses on the qualification of Cr3C2-NiCr for the LBM process. Experiments are carried out with composite thermal spray powder through variation of the main LBM process parameters and pre-heating temperatures up to 800 °C. Specimens are generated and evaluated with respect to material microstructure and mechanical properties. The results indicate that pre-heating can significantly improve the material quality. It prevents thermal cracking and substantially reduces porosity, in particular for low laser energy inputs. An impact of the LBM process parameters and pre-heating on the balance between the binder and the carbide phase is detected only for high laser energy inputs above 1000 J/mm3. Measurements of the micro-hardness of the laser molten material indicate a hardness close to the specifications of thermally sprayed material. A maximum transverse rupture strength around 600 MPa is achieved for specimens generated with pre-heating and medium laser energy inputs. For high laser energy inputs, secondary carbide and graphite phases occur inside the material microstructure.

Laser beam melting process based on complete-melting energy density for commercially pure titanium

Laser beam melting (LBM) based on the complete-melting energy density for commercially pure titanium (CP-Ti) was investigated. Process parameters were set corresponding to the thermodynamically calculated energy density for full melting of CP-Ti. From the calculated energy density, power of 50–250 W and scan speed of 371–1855 mm/s were derived with the fixed other conditions. Microstructural study on formation of pores was conducted to understand the change in part’s density. Fundamental study with wide range of LBM process parameters, particularly based on the complete-melting energy density, could confirm the effects like high density and porous structure.

Critical evaluation of the material characteristics and environmental potential of laser beam melting processes for the additive manufacturing of metallic components

Abstract Nowadays, the main impetus to apply additive manufacturing (AM) of metals is the high geometric flexibility of the processes and its ability to produce pilot or small batch series. In contrast, resource and energy intensities are often not considered as constraints, even though the turnout of additive manufacturing is high, at least compared to chip removing processes. The study at hand analyses the material characteristics and environmental impacts of a hose nozzle as an example of a commercial product of simple geometry. The production routes turning (conventional manufacturing) and laser beam melting (additive manufacturing) are compared to each other in terms of natural resource use, climate change potential and primary energy demand. It is found, that the product shows a lower demand for natural resources when produced via AM, but higher carbon emissions and energy demand when using a steel, that is mainly (80%) produced from high-alloyed steel scrap. However, different case studies during the sensitivity analyses showed that a number of factors highly influence the results: the steel source as well as the source of electricity play a major role in determining the environmental performance of the production routes. The authors also found that other production processes (here cold forging of tubes) might be an eco-friendly alternative to both routes, if feasible from an economic point of view. In regard to the material characteristics, experimental testing revealed that the material advantages of AM produced hose nozzles (in particular higher yield strength) are reduced after a solution heat treatment is applied to the as-produced material, in order to increase corrosion resistance. However, products that do not require this production step might benefit from the higher yield strength, as a lower wall thickness could be realised.

High-Temperature Oxidation Behavior of the Ti-6Al-4V Alloy Manufactured by Selective Laser Sintering

The high-temperature corrosion behavior of Ti-6Al-4V manufactured by selective laser sintering in air at 600°C for 400 h was studied. The corrosion products were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive spectrometry. The results show that the oxidation kinetics of Ti-6Al-4V manufactured by selective laser sintering followed a sub-parabolic law at 600°C. The oxidation resistance of the Ti-6Al-4V made by selective laser sintering was better than that of the Ti-6Al-4V made by rolling and laser beam melting. Oxidation products on Ti-6Al-4V were composed of multiple layers of TiO2 and Al2O3. Several intervals of Al2O3 layers enhanced the oxidation resistance of Ti-6Al-4V manufactured by selective laser sintering. The growth mechanism of the oxide scales is proposed.

Assessment of geometrical defects caused by thermal distortions in laser-beam-melting additive manufacturing: a simulation approach

Purpose The purpose of this paper is to develop a numerical approach inspired by Geometrical Product Specifications (GPS) standards for the assessment of geometrical defects appearing during Additive Manufacturing (AM) by Laser Beam Melting (LBM). Design/methodology/approach The study is based on finite element (FE) simulations of thermal distortions, then an assessment of flatness defects (warping induced by the high-residual stresses appearing during the manufacturing) from the deformed surfaces provided by simulation, and finally the correction of the calculated flatness defects from preliminary comparison between simulated and experimental data. Findings For an elementary geometrical feature (a wall), it was possible to identify the variation in the flatness defect as a function of the dimensions. For a complex geometry exhibiting a significant flatness defect, it was possible to improve the geometric quality using the numerical tool. Research limitations/implications To the best of the author’s knowledge, this work is the first attempt using a numerical approach inspired by GPS standards to identify variations in thermal distortions caused by LBM, which is an initial step toward optimization. This paper is mainly focused on flatness defect assessment, even though the approach is potentially applicable for all types of geometrical defects (shape, orientation or position defects). Practical implications The study opens prospects for the optimization of complex parts elaborated using LBM, based on the minimization of the geometric defects caused by thermal distortions. Social implications The prospects in terms of shape optimization will extend the potential to benefit from the new possibilities offered by LBM additive manufacturing. Originality/value Unlike the usual approach, the proposed methodology does not require any artifacts or comparisons with the computer-aided-design (CAD) model for geometrical distortion assessment. The present approach opens up the possibility of performing metrology from FE simulation results, which is particularly promising in the AM field.

Laser Beam Melting of Complexly Shaped Honeycomb Structures

Laser beam melting (LBM) offers the opportunity to manufacture highly complex structures and geometries and thus provides a big potential to produce lightweight parts. In previous research projects, a software tool has been developed that achieves the placement of hexagonal honeycombs (of any size and wall thickness) on free formed surfaces in a load-oriented manner and thus offers entirely new possibilities for designing lightweight components in CAD (e.g. [1–2]). 
 This work examines the production of metal hexagonal honeycombs from the material AlSi10Mg with the LBM-process. By adapting the exposure and process parameters, it was possible to manufacture overhanging structures with an overhang angel < 30° (relative to build platform) without support structures, while still achieving an acceptable surface roughness (in the context of this study: Ra = 45 µm). Conventional complex and time consuming post-processing steps can thus be avoided and a higher utilization of building space can be achieved. Furthermore, since the critical size for a lightweight structure is the minimum possible density, it was investigated to which minimum values the wall thicknesses of the hexagonal structures can be reduced using LBM. Apart from that, the stability of the manufactured honeycombs was analyzed in as-built condition and heat treated by pressure test and related to the honeycomb density. This has been used to compare additively manufactured honeycombs with conventionally manufactured ones.

Customized exposure strategies for manufacturing hybrid parts by combining laser beam melting and sheet metal forming

Subjects of this work are the mechanical, geometrical, and microstructural properties of hybrid parts from Ti6Al4V sheet metal bending and laser beam melting (LBM) in dependence of the extended LBM process parameters. In this context, the effects of different laser exposure strategies for the interface area to increase the reproducibility of the joint strength are investigated. Also, the effect of varying scan patterns and two different laser beam sources (Gaussian beam profile, spot size: 120 μm, maximum laser power: 400 W and irregular beam profile, spot size: 680 μm, maximum laser power: 1000 W) on the LBM process and the resulting distortion of the parts are examined. Furthermore, three heat-treatment temperatures at 450, 850, and 1050 °C were applied to the hybrid samples, resulting in variable microstructures and different mechanical properties for the sheet metal body and the LBM structure.

Level-set modelling of Laser Beam Melting process applied onto ceramic materials – Comparison with experimental results

Laser Beam Melting (LBM) processes benefit from significant progress in recent years. Currently, manufacturing of ceramic parts for applications at high temperature in aeronautical industries can be planned. However, understanding of defect formation is required in order to optimize manufacturing strategy. In this work, level-set modelling is proposed to simulate tracks development during LBM processes. Thermo-mechanical solution is performed in both powder and dense domains. Fluid flow is computed considering the surface tension and Marangoni forces. In addition mechanical resolution is achieved to investigate stress evolution in the rear part of the track. Applications are developed on alumina material. The influence of laser power, scanning velocity and physical properties are investigated and discussed. Validations of the heat source model are proposed by comparisons of melt pool dimensions and shapes with experimental measurements. A coherent evolution of the track morphology is shown when varying process parameters or material properties.

Thermo-mechanical simulation of track development in the Laser Beam Melting process - Effect of laser-metal interaction

RésuméInterest has recently emerged for the manufacture of aeronautical parts by Laser Beam Melting (LBM) additive process. This energy efficient process can for instance be used to build complex geometries, which cannot be made with traditional processes. However, complex phenomena occur during powder melting and track development : vaporisation phenomena influence laser-matter interaction by creating metal vapours that are responsible for the reduction of absorbed energy. The recoil pressure generated by the vaporisation counteracts the surface tension between the melt pool and the inert gas, also inducing liquid instabilities. The study of laser-matter interaction and induced phenomena can help understand the origin of defects such as porosities or cracks. In this approach, a level-set modelling of the LBM process at a mesoscopic scale is proposed to follow melt pool evolution and track development during build. A volume heat source model is used for laser/powder interaction considering the material absorption coefficient. A surface heat source is used to take into account the high laser energy absorption by dense metal alloys. An energy solver is coupled with thermodynamic database and pre-determined solidification path. Shrinkage during consolidation from powder to liquid and compact medium is modelled by a compressible Newtonian constitutive law. An automatic remeshing adaptation is also used to save time and avoid high computational cost. In the future, the computation of multiple beads or the build of a wall in a context of lattice structures will have to be considered.