Dimensional Errors Due to Overhanging Features in Laser Powder Bed Fusion Parts Made of Ti-6Al-4V

The rise in popularity of Additive Manufacturing technologies and their increased adoption for manufacturing have created a requirement for their fast development and maturity. However,there is still room for improvement when compared with conventional manufacturing in terms of the predictability, quality, and robustness. Statistical analysis has proven to be an excellent tool for developing process knowledge and optimizing different processes efficiently and effectively.This paper uses a novel method for printing overhanging features in Ti-6Al-4V metal parts, by varying process parameters only within the down-facing area, and establishes a methodology for predicting dimensional errors in flat 45◦down-facing surfaces. Using the process parameters laser power, scan speed, scan spacing, scan pattern, and layer thickness, a quadratic regression equation is developed and tested. An Analysis of variance (ANOVA) analysis concluded that, within the down-facing area,the laser power is the most significant process parameter, followed by the layer thickness and scans peed. Comparatively, the scanning pattern is determined to be insignificant, which is explained by the small down-facing area where the various scanning patterns play no role. This paper also discusses the interaction effects between parameters. Some thoughts on the next steps to be taken for further validation are discussed

Effect of Process Parameters on the Generated Surface Roughness of Down-Facing Surfaces in Selective Laser Melting

Down-facing surfaces are one of the most challenging features in metal parts produced by laser powder bed fusion (LPBF). A combination of reasons, primary of which are residual stresses and overheating cause these features to have the worst surface finish and dimensional accuracy of all LPBF surfaces. In order to examine this phenomenon, a Design of Experiments (DoE) study is conducted for three different inclination angles, namely 45°, 35° and 25° and for two different layer thicknesses of 60 µm and 90 µm. The results from the DoE are used to establish quadratic regression equations that can be used to predict the quality marks of surface roughness and the relative dimensional error.This fundamental investigation helps to explain the reasons for the major defects in down-facing surfaces of parts produced with Ti-6AL-4 V material, namely the dross formation and attempts to improve the predictability of quality within the region. Further to the establishment of the quadratic equations, a discussion is conducted on the thermomechanical processes involved in the mechanism of dross formation and explanations are given on the reasons behind the observed physical phenomena. The trend of the propagation of (Root Mean Square) RMS Surface roughness (Sq) and the relative dimensional error with respect to the Volumetric Energy Density (VED) is discussed in detail. The respective quadratic equations are then tested by a second round of validation prints, and the results confirm the feasibility of the developed quadratic models to accurately predict process outcomes especially when operating near the suggested optimal printing zones. The high roughness of low VED printing is attributed to the formation of ‘inverse mushroom’ structures, and the low roughness of high VED surface is attributed to the formation of large flat regions formed as adjacent meltpools that can fuse together at various locations

Down-facing surfaces in laser powder bed fusion of Ti6Al4V: Effect of dross formation on dimensional accuracy and surface texture

Down-facing surfaces are one of the most challenging features in metal parts produced by laser powder bed fusion (LPBF). A combination of reasons, primary of which are residual stresses and overheating cause these features to have the worst surface finish and dimensional accuracy of all LPBF surfaces. In order to examine this phenomenon, a Design of Experiments (DoE) study is conducted for three different inclination angles, namely 45°, 35° and 25° and for two different layer thicknesses of 60 µm and 90 µm. The results from the DoE are used to establish quadratic regression equations that can be used to predict the quality marks of surface roughness and the relative dimensional error.This fundamental investigation helps to explain the reasons for the major defects in down-facing surfaces of parts produced with Ti-6AL-4 V material, namely the dross formation and attempts to improve the predictability of quality within the region. Further to the establishment of the quadratic equations, a discussion is conducted on the thermomechanical processes involved in the mechanism of dross formation and explanations are given on the reasons behind the observed physical phenomena. The trend of the propagation of (Root Mean Square) RMS Surface roughness (Sq) and the relative dimensional error with respect to the Volumetric Energy Density (VED) is discussed in detail. The respective quadratic equations are then tested by a second round of validation prints, and the results confirm the feasibility of the developed quadratic models to accurately predict process outcomes especially when operating near the suggested optimal printing zones. The high roughness of low VED printing is attributed to the formation of ‘inverse mushroom’ structures, and the low roughness of high VED surface is attributed to the formation of large flat regions formed as adjacent meltpools that can fuse together at various locations

Elucidation of dross formation in laser powder bed fusion at down-facing surfaces : Phenomenon-oriented multiphysics simulation and experimental validation

Dross formation is a phenomenon that is observed while printing metallic components using Laser Powder Bed Fusion (L-PBF) and occurring primarily at down-facing surfaces that are unsupported and suffer inadequate heat removal. Naturally, dross formation causes dimensional inaccuracy, high surface roughness and also adversely affects the mechanical properties of printed components. Through simulation and experimentation, this study fundamentally elucidates the driving phenomenon behind dross formation. The simulation results, in terms of the degree of generated dross domain, well agree with the ones observed in the printed samples and the behaviour of the melt pool while moving from bulk material to the powder domain is clearly depicted in this study. The simulations show that due to the low thermal conductivity of loose powder and the inability to conduct heat away, the quasi steady state melt pool collapses while entering the powder domain and transitions to a keyhole-like melt mode which causes a pronounced drilling effect. This causes excessive melting known as dross that is seen both in the simulation and the experimental parts. This work also shows through simulation and experimentation the reasoning behind the production of larger and smaller dross domains while printing with high and low laser energy densities respectively. Additionally, through SEM imagery this study also explains the observed deep internal grooves and near-surface porosity that are present within this dross domain which can further affect mechanical properties such as density, fatigue strength etc

Genome-wide detection of predicted non-coding RNAs in Rhizobium etli expressed during free-living and host-associated growth using a high-resolution tiling array

Non-coding RNAs (ncRNAs) play a crucial role in the intricate regulation of bacterial gene expression, allowing bacteria to quickly adapt to changing environments. In the past few years, a growing number of regulatory RNA elements have been predicted by computational methods, mostly in well-studied gamma-proteobacteria but lately in several alpha-proteobacteria as well. Here, we have compared an extensive compilation of these non-coding RNA predictions to intergenic expression data of a whole-genome high-resolution tiling array in the soil-dwelling alpha-proteobacterium Rhizobium etli.Journal ArticleResearch Support, Non-U.S. Gov'tinfo:eu-repo/semantics/publishe

Microstructure and Texture of Metal Parts Produced by Selective Laser Melting (Microstructuur en textuur van metalen stukken geproduceerd via selectief laser smelten)

Selective laser melting (SLM) is an additive manufacturing technique by which structural geometrical complex parts an be created directly by selectively melting consecutive layers of powder. The high energy den sity applied by a laser beam results in high and directional thermal gra dients. In combination with the additive character of the process, this results in the formation of a unique microstructure in SLM parts which c an be altered by varying the process parameters or the constitution of t he alloy. In this work, the microstructure and texture of pure Ta, Ti-6Al-4V, AlSi 10Mg and maraging steel 18Ni(300) SLM parts are characterised by microst ructural analysis (LOM, SEM and EBSD) and X-ray diffraction. The influen ce of the unique microstructure on the mechanical properties are analyse d as well. The experimental work is supported by using a pragmatic model for SLM to calculate the temperature distribution and by using a model to estimate the plastic deformation behaviour based on the measured text ure. For most alloys, the high thermal gradients result in very fine submicro n-sized cellular-dendrites growing toward the centre of the melt pool al ong the easygrowth direction (AlSi10Mg and maraging steel 18Ni(300)). Du e to the partial remelting or previously consolidated layers, most grain s solidify epitaxially and grow across the layers. For pure metals (Ta) and Ti-6Al-4V, the solidification front remains stable, resulting in lar ge elongated grains more or less along the building direction. The competition between the epitaxial solidification and the orientation of the easy-growth of the parent grain with respect to the local heat f low direction is found to be the main determining factor to determine th e morphological as well as crystallographic texture during SLM.nrpages: 294status: publishe

The unique microstructure of metals produced by Selective Laser Melting

The unique microstructure of metals produced by Selective Laser Meltingstatus: publishe

Fine-structured aluminium products with controllable texture by Selective Laser Melting of pre-alloyed AlSi10Mg powder

This study shows that AlSi10Mg parts with an extremely fine microstructure and a controllable texture can be obtained through Selective Laser Melting (SLM). Selective Laser Melting creates complex functional products by selectively melting powder particles of a powder bed layer after layer using a high energy laser beam. The high energy density applied to the material and the additive character of the process results in a unique material structure. To investigate this material structure, cube-shaped SLM parts were made using different scanning strategies and investigated by microscopy, X-ray diffraction and electron backscattered diffraction. The experimental results show that the high thermal gradients occurring during SLM lead to a very fine microstructure with submicron sized cells. Consequently, the AlSi10Mg SLM products have a high hardness of 127 ± 3 Hv0.5 even without the application of a precipitation hardening treatment. Furthermore, due to the unique solidification conditions and the additive character of the process, a morphological and crystallographic texture is present in the SLM parts. Thanks to the knowledge gathered in this article on how this texture is formed and how it depends on the process parameters, this texture can be controlled. A strong fibrous texture can be altered into a weak cube texture along the building and scanning directions when a rotation of 90° of the scanning vectors within or between the layers is applied.status: publishe

Processing AlSi10Mg by Selective Laser Melting: Parameter optimization and material characterization

AlSi10Mg is a typical casting alloy which is, due to its high strength/density ratio and thermal properties, highly demanded in aerospace and automotive industries [1]. The alloy combination of aluminium, silicon and magnesium results in a significant increase in strength and hardness which might even reach 300 MPa and 100 HBS, respectively, by applying a proper heat treatment [2]. Selective Laser Melting (SLM) of AlSi10Mg, may be interesting to open new application areas such as heat sinks with complicated geometry [3], and therefore is taken under investigation in this study. The process optimization of SLM for this alloy is not straightforward due to high reflectivity and conductivity of the material. In this study, the main goal is to optimize the process parameters, namely scan speed, scan spacing and laser power, to achieve almost full density and good surface quality taking productivity as a key issue. A relative density up to 99% is achieved with an average roughness (Ra) of about 20 µm measured on horizontal top surfaces while the scanning productivity is about 4.4 mm3 /s. The reasons spherical and irregular porosity formed are investigated. Moreover, microstructural analysis of the SLM samples is conducted.peerreview_statement: The publishing and review policy for this title is described in its Aims & Scope. aims_and_scope_url: http://www.tandfonline.com/action/journalInformation?show=aimsScope&journalCode=ymst20status: publishe

Heat treatment of Ti6Al4V produced by Selective Laser Melting: Microstructure and mechanical properties

The present work shows that optimization of mechanical properties via heat treatment of parts produced by Selective Laser Melting (SLM) is profoundly different compared to conventionally processed Ti6Al4V. In order to obtain optimal mechanical properties, specific treatments are necessary due to the specific microstructure resulting from the SLM process. SLM is an additive manufacturing technique through which components are built by selectively melting powder layers with a focused laser beam. The process is characterized by short laser-powder interaction times and localized high heat input, which leads to steep thermal gradients, rapid solidification and fast cooling. In this research, the effect of several heat treatments on the microstructure and mechanical properties of Ti6Al4V processed by SLM is studied. A comparison is made with the effect of these treatments on hot forged and subsequently mill annealed Ti6Al4V with an original equiaxed microstructure. For SLM produced parts, the original martensite α' phase is converted to a lamellar mixture of α and β for heat treating temperatures below the β-transus (995°C), but features of the original microstructure are maintained. Treated above the β-transus, extensive grain growth occurs and large β grains are formed which transform to lamellar α+β upon cooling. Post treating at 850°C for two hours, followed by furnace cooling increased the ductility of SLM parts to 12.84 ± 1.36 %, compared to 7.36 ± 1.32 % for as-built parts.status: publishe