Laser-based Powder Bed Research Articles

Heat treatments-induced wear resistance of Inconel 718 superalloy fabricated via Laser based Powder Bed Fusion

Laser based Powder Bed Fusion (LPBF) stands out among Additive Manufacturing (AM) techniques for its ability to fabricate intricate geometries with high precision. However, LPBF produced parts, particularly Inconel 718 (IN718) superalloys, demand further exploration in microstructural and mechanical properties aspect. This study explores the effect of heat treatments on LPBF fabricated Inconel 718. Three distinct heat treatments involving diverse solutionizing temperatures and ageing steps were applied to the LPBFed IN718 alloy. As-printed samples showed columnar dendritic microstructure. The heat treatments led to precipitation of strengthening γ′′ and γ′ phases. A substantial increase in microhardness was observed after heat treatments. Wear tests revealed as-printed specimens suffered higher wear loss (∼889 μm) and coefficient of friction (COF) (∼0.627) attributed to Laves phase and low hardness of the matrix. However, heat-treated specimens exhibited substantially lower wear loss (∼217 μm) and COF (∼0.342), with HT2 condition demonstrating superior wear resistance attributed to a homogeneous microstructure.

Orthogonal experimental method to investigate the effect of process parameters on the mechanical properties of thin-walled parts by PBF-LB/M

Lightweight thin-walled parts are widely used in the aviation and aerospace industries, and with the further increase in the complexity of their features, the traditional manufacturing process can no longer fully meet the high requirements of industrial component manufacturing. In this work, thin-walled parts are processed by Laser based powder bed fusion of metals (PBF-LB/M), and the effects of process parameters on residual stress, hardness, mechanical properties and microstructure of thin-walled parts are systematically investigated. The simulation results show that the maximum equivalent residual stresses are distributed in the combination of the solid and the substrate, and the minimum equivalent residual stresses are mainly distributed in the top two ends and the middle part of the solid, and the stress distribution is symmetrical. In addition, the maximum equivalent residual stress increases with the increase of laser power, and decreases with the increase of scanning spacing or scanning speed. The experimental results show that with the increase of laser energy density, the tensile strength and yield strength of thin-walled parts show a tendency of increasing first and then decreasing. Finally, high-quality thin-walled parts were successfully fabricated by the optimized process parameters, and their tensile and yield strengths were increased by 6.1% and 15.9%, respectively.

Heat treatments effects on Wear performance of Laser based Powder Bed Fusion fabricated Inconel 718 alloy

Among the AM processes Laser based Powder Bed Fusion (LPBF) technique offers precise and complex geometric fabrication. However, the microstructural and mechanical properties obtained from LPBF process requires further investigation, especially for IN718 superalloys. In this study, various heat treatments were applied to LPBFed Inconel 718 specimens to examine their effects on microstructure, microhardness and wear behavior. Three different heat treatments, each involving varied solutionizing and ageing steps were incorporated. As-printed specimens exhibited distinct fish scale structures with columnar dendrites. Heat treatments effectively dissolved the Laves phase and precipitated strengthening phases like γ′′ and γ′. Microhardness increased significantly after heat treatments, correlating with the formation of strengthening precipitates. Friction and wear tests showed as-printed specimens exhibited higher wear loss (922 ± 13 μm) and coefficient of friction (COF) (0.511 ± 0.07) due to the presence of Laves phase and softer matrix. Heat-treated specimens demonstrated significantly reduced wear loss (262 ± 5 μm) and COF (0.368 ± 0.01), with HT2 showing the best wear resistance attributed to a homogeneous microstructure. SEM analysis of worn surfaces confirmed abrasive and adhesive wear mechanisms in as-printed specimens, while heat-treated specimens exhibited reduced wear with smoother surfaces.

Surface treatment of additively manufactured stainless steel 316L lattice structures by plasma electrolytic polishing

Additive manufacturing facilitates the production of lattice structures for lightweight and biomedical applications. Typically, as-built structures exhibit rough surfaces, necessitating post-processing to meet functional requirements. Plasma electrolytic polishing (PEP) offers an approach for smoothing surfaces on complex and filigree parts in an environmentally friendly manner. Nonetheless, achieving uniform treatment depends on PEP process parameters and geometric properties of the workpiece.This study investigates capabilities of PEP for treating lattice structures made from 316L stainless steel. Test specimens consisting of face-centered cubic unit cells with an edge length of 3 mm were manufactured by laser-based powder bed fusion.The findings highlight the influence of developed PEP parameters on roughness and dimensional accuracy based on computed tomography scans. Pre-and-post-treatment-comparisons show that smooth, specular outer surfaces can be obtained.Moreover, smoothing of interior surfaces was demonstrated for a relative cell density of 11%. Increasing the cell density to 35% reduces the smoothing effect.

Fatigue strength assessment of additive manufactured components considering local quality and geometry using the theory of critical distances and 4R method

Laser based powder bed fusion (L-PBF) manufactured components are typically used in weight-critical fatigue loaded applications. Building orientation and parameters have strong influence on fatigue performance. Multiparametric method for fatigue performance analysis is applied on high-strength L-PBF specimens’ fatigue test results in this study. The fatigue strength analysis was conducted on unnotched and notched specimens with a consideration of local quality. The defect and surface profiles which defines local quality are characterized with the fractography. The results suggest that local quality influenced by building characteristics can be assessed when effective stresses are obtained by the Theory of Critical Distances (TCD) via finite element analysis. Fatigue strength assessment of notched components benefit from a consideration of local cyclic behaviour with the 4R method. Consequently, multiparametric method for fatigue strength assessment with single FAT class was proposed for AM-components.

Smart‐Alloying – Liquid in-situ re-alloying in additive manufacturing

Smart-Alloying enables local modification of the chemical composition of metallic components within additive manufacturing processes. In the example of laser based powder bed fusion (PBF-LB/M), this new technology uses suspensions to place alloying elements precisely at every requested point of the printed area. It allows a tailor-made change of the local microstructure and properties. High chromium-containing stainless steel and carbon as the alloying element serves as a demonstration example. Microprobe and EBSD analyses confirm a locally increasing carbon content, which leads to the formation of fine martensitic structures with chromium-rich carbides at the grain boundaries of the ferritic base material. The associated change in microstructure regarding grain size and phase composition results in modified mechanical properties. Indentation tests carried out show a corresponding locally increased indentation hardness. This decisive influence of even individual alloying elements paves a new way for functional grading of metallic components in additive manufacturing.

The effect of temperature and strain rate on the grain boundary sliding in a CM247 LC Ni-based superalloy processed with laser based powder bed fusion

The current work explores the meso-scale deformation behaviour of an additively manufactured CM247 LC at room and high temperatures. In particular, the study focuses on assessing grain boundary sliding (GBS), which can play a crucial role in the high-temperature deformation of superalloys. Specific samples were produced using the Laser Based Powder Bed Fusion technique (PBF-LB), heat treated and tested under monotonic compression in a Gleeble® system. Compression tests were carried out in a wide temperature range at two strain rates and the effect of testing parameters on GBS activity was studied. A thorough microstructural characterization of the PBF-LB material using EBSD and TEM revealed a γ/γ’ microstructure consisting of columnar grains decorated with Hf-rich MC carbides without any segregations of alloying elements. Qualitative and quantitative analysis of GBS was performed using FEG-SEM and AFM, and contribution of GBS into plastic deformation was estimated. It was demonstrated that GBS is activated at 760 °C. A direct correlation between the contribution of GBS into plastic deformation and testing temperature was found, while strain rate has the opposite effect. The highest GBS contribution (∼32%) was recorded at 1093 °C/10−3 s−1. Finally, intergranular microcracking at triple junctions and along grain boundaries was observed when the material was tested at the highest temperatures (871 °C and 1093 °C). The effect of the temperature and the strain rate on the GBS activity in the PBF-LB material is discussed.

Evaluation of influence of microstructural features of LPBF Ti-6Al-4 V on mechanical properties for an optimal strength and ductility

Recently, Additive Manufacturing (AM) has established its potential in the biomedical industry due to its capability to produce customized biomedical implants with complex geometries. This study involves the fabrication of Ti-6Al-4 V parts using Laser based Powder Bed Fusion (LPBF) additive manufacturing process. Due to rapid solidification, the as-built Ti-6Al-4 V parts exhibit an increased strength and reduced tensile elongation because of the formation of non-equilibrium acicular martensitic α’ structure. Thus, the present investigation aims at analysing the influence of post-heat-treatment on evolution of microstructure, mechanical properties and fracture- behaviour of LPBF Ti-6Al-4 V alloys in the α + β range. The phase transformation occurs through martensitic α’ decomposition into stable α- and β-phases upon heat-treatment and impacts the mechanical properties significantly. The heat-treated samples showed a gradual drop in strength (∼24% max drop in UTS vs as-built) and hardness (∼35% max drop vs as-built) with increase in β-fraction, whereas the ductility showed a complex behaviour because of the detrimental effect of GB-α precipitation and coarsening. Furthermore, the effect of microstructure on fracture modes was studied. It was observed that the thickness, nature and distribution of GB-α affects the mechanical properties and fracture behaviour. Moreover, this work provides insights towards understanding the structure-property correlation bringing forth a better tailored microstructure to obtain a beneficial combination of ductility and strength for orthopaedic implants.

Thermal rounding for shape modification of high-performance polyetherketoneketone and reinforced polyetherketoneketone-carbon fiber composite particles

This study presents shape transformation of anisotropic high-performance thermoplastic polyetherketoneketone (PEKK) and carbon fiber reinforced powder composite particles (HT-23) by thermal rounding. The shape transformation is achieved by (partial) melting of the high-temperature thermoplast microparticles. Three different process setups are presented, investigating the impact of the source of heat supply on the resulting shape modification: using a directly heated sheath gas flow, an indirect heat supply through the reactor wall and a combined approach. Regardless of the chosen setup, a modification of the particle shape was observable. The most advantageous shape transformation was observed in the indirect heating approach. In addition, the enhanced shape transformation yields an improved free flow behaviour of the powders, as quantified by ring-shear experiments. Reductions of the unconfined yield strengths of the powders for high consolidation stresses as high as 18 percent for PEKK and 30 percent for the HT-23 are achieved. Thereby, processability of the powder in laser based powder bed fusion is enhanced, extending the range of available (composite) polymer materials.

Development of microstructure and high hardness of Ti6Al4V alloy fabricated using laser beam powder bed fusion: A novel sub-transus heat treatment approach

The present work aims to study the effect of novel sub-transus heat treatment on the microstructure evolution and hardness of Ti6Al4V alloy fabricated using laser based powder bed fusion. The results revealed that the as-built microstructure is α′ martensite which decomposes to α + β during sub-transus heat treatment at 970 °C for different soaking times. The formation of a bi-modal structure with approximately 25% globular primary α and transformed β is observed. The development of globular primary alpha is attributed to thermal grooving and boundary splitting. The resultant coarsening of these α/β grains was associated with a time-temperature combination and orientation relation (OR) to 45° [100]. Moreover, precipitation of fine Ti3Al intermetallic (α2) in plate form is observed predominantly inside the primary α grains. Owing to the faster cooling rate post soaking time resulted in Ti3Al precipitates which is the conversion of α to α2 precipitates. The average micro-hardness of the specimen heat-treated for 12 h at 970 °C is found to be 45% higher in comparison to the as-built specimen. Whereas, the micro-hardness differs between the primary α and transformed β region due to the alloy element partitioning effect. The average micro-hardness of 592 HV is observed in primary α grains in the specimen heat-treated for 12 h at 970 °C. The prior results are associated with the formation of intermetallic nano-sized Ti3Al (α2).

Effect of Carbon Content on the Processability of Fe-C Alloys Produced by Laser Based Powder Bed Fusion

The present study examines the processability of Fe-C alloys, with carbon contents up to 1.1 wt%, when using laser based powder bed fusion (LB-PBF). Analysis of specimen cross-sections revealed that lack of fusion porosity was prominent in specimens produced at low volumetric energy density (VED), while keyhole porosity was prominent in specimens produced at high VED. The formation of porosity was also influenced by the carbon content, where increasing the carbon content reduced lack of fusion porosity, while simultaneously increasing the susceptibility to form keyhole porosity. These trends were related to an improved wettability, viscosity, and flow of the melt pool as well an increased melt pool depth as the carbon content increased. Cold cracking defects were also observed in Fe-C alloys that had an as-built hardness ≥425 HV. Reducing the carbon content below 0.75 wt% and increasing the VED, which improved the intrinsic heat treatment during LB-PBF, were found to be effective mitigation strategies to avoid cold cracking defects. Based upon these results, a process window for the Fe-C system was established that produces high density (>99.8%), defect-free specimens via LB-PBF without the requirement of build plate preheating.

Surface layer porosity and surface quality in dependency of contour scanning strategy of Ti6Al4V parts

Laser based powder bed fusion of metals (PBF-LB/M) grants the potential for the combination of modern design approaches like topology optimization with the production of near-net shape parts suitable for high performance application. But there is not only the need for high relative density of the parts but also a low surface layer porosity to achieve mechanical properties comparable or better than forging or casting. The surface layer porosity depends among other factors on scanning strategy, precisely the contour parameters, filler parameters and hatch offset. This work investigates the influence of the named parameters and energy input on the correlation of surface porosity and surface roughness of Ti6Al4V specimens. To correlate surface roughness and surface layer porosity with the used laser parameters micro computed tomography (µ-CT) measurements are used.

Influence of beam shape on spatter formation during PBF-LB/M of Ti6Al4V and tungsten powder

Laser based powder bed fusion of metals (PBF-LB/M) is an advanced manufacturing technology with high design freedom and a broad range of processable materials. A special advantage is the possibility of using local in-situ alloying. This in-situ modification needs local powder deposition and therefore an adjusted scanning strategy which decreases spatter formation and spatter range to impede feedstock contamination. Beam shaping is a valid option to reduce spatter formation by creating a less dynamic melt pool and vapor stream. This works aim is to investigate the influence of a ring-spot profile on the amount of generated spatters in comparison to a Gaussian beam by scanning over tungsten and Ti6Al4V powder. The amount of tungsten particles found in the molten material around the process zone via cross sections functions as indicator and shows the reduced amount of particles leaving the process zone using the beam shaped profile in comparison to a Gaussian beam.

Critical role of scan strategies on the development of microstructure, texture, and residual stresses during laser powder bed fusion additive manufacturing

Laser based powder bed fusion additive manufacturing offers the flexibility to incorporate standard and user-defined scan strategies in a layer or in between the layers for the customized fabrication of metallic components. In the present study, four different scan strategies and their impact on the development of microstructure, texture, and residual stresses in laser powder bed fusion additive manufacturing of a nickel-based superalloy Inconel 718 was investigated. Light microscopy, scanning electron microscopy combined with electron backscatter diffraction, and neutron diffraction were used as the characterization tools. Strong textures with epitaxially grown columnar grains were observed along the build direction for the two individual scan strategies. Patterns depicting the respective scan strategies were visible in the build plane, which dictated the microstructure development in the other planes. An alternating strategy combining the individual strategies in the successive layers and a 67° rotational strategy weakened the texture by forming finer microstructural features. Von Mises equivalent stress plots revealed lower stress values and gradients, which translates as lower distortions for the alternating and rotational strategies. Overall results confirmed the scope for manipulating the microstructure, texture, and residual stresses during laser powder bed fusion additive manufacturing by effectively controlling the scan strategies. • Laser powder bed fusion additive manufacturing (L-PBF AM) of a nickel-based superalloy Inconel 718 was performed. • Influence of four scan strategies X-, Y-, alternating (X-/Y-) and rotational (67º) was investigated. • X-/Y- strategies developed textured columnar grains and alternating/rotational strategies reduced the texture. • Fine microstructure with weak texture led to the least residual stress development in alternating/rotational strategies. • Scan strategies can be manipulated to control microstructure, texture and residual stresses during L-PBF AM.

Uniaxial static mechanical properties of regular, irregular and random additively manufactured cellular materials: Nominal vs. real geometry

The present work investigates cellular materials that can be potentially used as a replacement of solid implants owing to their mechanical properties and biocompatibility. More specifically, titanium alloy (Ti6Al4V) cellular materials of three different topologies were considered to study the effect of the degree of irregularity. Cubic regular, cubic irregular and trabecular structures were manufactured using Laser based powder bed fusion (LB-PBF) process. The LB-PBF process had an impact on the strut thickness of the samples. Samples were subjected to micro computed tomography to understand the geometrical deviations and to use the actual geometry for finite element analysis. Mechanical properties such as Young's modulus and strength were derived from compression and tensile testing. The results indicate that the Young's modulus was between 6 and 17 GPa, which were closer to the values of human cortical bone. The finite element analysis results showed a good correlation with the tensile test results as well. Furthermore, Gibson–Ashby model is used to study the effect of cell topology on the structural behavior. The model indicated that the misalignment of nodes of cubic regular structures to form irregular structure, transformed the stretching dominated behavior of cubic structures to bending dominated behavior like trabecular structures. Finally, the regular structures appeared to be much more prone to catastrophic failure than irregular and trabecular structures. Both visual observation of experimental testing and FE analysis explained this difference as result of different modes and zones of failures.

Degradation of AlSi10Mg powder during laser based powder bed fusion processing

Knowledge concerning powder degradation during additive manufacturing (AM) processing is essential to improve the reusability of the powder in AM and hence maximize feedstock powder reuse and economy of the process. AlSi10Mg powder degradation in Concept Laser XLINE 2000R machine over the total period of 30 months was analyzed in order to understand the extent and mechanism affecting powder aging. Thereby, detailed analysis of the powder morphology, microstructure and surface chemistry was performed by SEM, TEM and XPS. The results show an increase in volume fraction of heavily oxidized spatter particles up to 3% in 30 months. XPS analysis of the powder surface chemistry indicates that powder particles are covered by uniform oxide layer, formed by Mg- and Al-based oxides, average thickness of which increased from ~4 nm in case of the virgin powder up to about 38 nm in case of the reused for about 30 month powder, established by XPS. Analysis of the oxide characteristics were consistent with the observed oxygen content in the sampled powder. Columnar oxide scale formation on spatter particles was revealed as well, reaching up to 125 nm in thickness measured using STEM. Results of the XPS and STEM-EDX analysis of oxide composition are shown to be in agreement with the thermodynamic calculations confirming that oxide scale on sputter particles is formed by MgAl2O4 spinel and Al2O3 (corundum) oxides.

Integration of Simulation Driven DfAM and LCC Analysis for Decision Making in L-PBF

Laser based powder bed fusion (L-PBF) is used to manufacture parts layer by layer with the energy of laser beam. The use of L-PBF for building functional parts originates from the design freedom, flexibility, customizability, and energy efficiency of products applied in dynamic application fields such as aerospace and automotive. There are challenges and drawbacks that need to be defined and overcome before its adaptation next to rivaling traditional manufacturing methods. Factors such as high cost of L-PBF machines, metal powder, post-preprocessing, and low productivity may deter its acceptance as a mainstream manufacturing technique. Understanding the key cost drivers of L-PBF that influence productivity throughout the whole lifespan of products will facilitate the decision-making process. Functional and operational decisions can yield profitability and increase competitiveness among advanced manufacturing sectors. Identifying the relationships between the phases of the life cycle of products influences cost-effectiveness. The aim of the study is to investigate the life cycle cost (LCC) and the impact of design to it in additive manufacturing (AM) with L-PBF. The article provides a review of simulation driven design for additive manufacturing (simulation driven DfAM) and LCC for metallic L-PBF processes and examines the state of the art to outline the merits, demerits, design rules, and life cycle models of L-PBF. Practical case studies of L-PBF are discussed and analysis of the interrelating factors of the different life phases are presented. This study shows that simulation driven DfAM in the design phase increases the productivity throughout the whole production and life span of L-PBF parts. The LCC model covers the whole holistic lifecycle engineering of products and offers guidelines for decision making.

Additive manufacturing of heavy rare earth free high-coercivity permanent magnets

Laser based powder bed fusion is a promising manufacturing method that can be used for the fabrication of hard magnets such as NdFeB with nearly any given geometrical shape. However, the weak performance, e.g., low coercivity, of the 3D-printed magnets currently hinder their application. In this work, we demonstrated a proof-of-concept of powder bed additive manufacturing of heavy rare earth free NdFeB magnets with technologically attractive coercivity values. The 3D-printed NdFeB magnets exhibit the highest (up-to-date for the additively manufactured magnets without heavy rare earth metals) coercivity values reaching μ0Hc = 1.6 T. The magnets were synthesized using a mixture of the NdFeB-based and the low-melting eutectic alloy powders. The essential function of the eutectic alloy, along with binding of the NdFeB-based magnetic particles, is the significant improvement of their coercivity by the in-situ grain boundary (GB) infiltration. The fundamental understanding of the magnetization reversal processes in these 3D-printed magnets leads to the conclusion that the excellent performance of the additively manufactured hard magnets can be achieved through the delicate control of the intergrain exchange interaction between the grains of the Nd2Fe14B phase.

Correlation between real geometry and tensile mechanical behaviour for Ti6Al4V electron beam melted thin specimens

The Electron Beam Melting (EBM) is an Additive Layer Manufacturing (ALM) technique used to directly manufacture 3D functional parts from metal powder, selectively melted, layer by layer, by an electron beam according to a geometry defined by a CAD model. The EBM technology allows benefitting from countless advantages: material waste reduction, easy manufacturing of complex shapes, lead time reduction, etc; on the other hand the EBM process is typically associated with lower resolutions and higher surface roughness (Ra = 25–30 μm) compared to similar laser based powder bed metal processes.Therefore the surface morphology may be a critical issue for the structural integrity of components made in EBM and used in-service in their “as built” condition, i.e. with the characteristic surface released by the process.This study evaluates surface morphology and tensile properties of Ti6Al4V specimens of varying nominal thickness (1–5.0 mm), made by using EBM process with a layer thickness of 50 μm. The aim is therefore to investigate how the surface morphology and the tensile properties are affected by the nominal thickness of the component.

Detecting spattering phenomena by using high speed imaging in L-PBD of 316 L

High-speed imaging is nowadays a widely used monitoring system in laser based powder bed fusion (L-PBF), because it enables observation of the occurring process phenomena. The quality-price ratio of the high-speed cameras has significantly increased over the last few years, and therefore it is a more suitable method for monitoring. This study concentrates on preliminary understanding of the spattering phenomena based on theoretical background and experimental findings. A better understanding of the spattering phenomena provides a possibility to achieve better part quality more cost-efficiently and to optimize the quality control during the process. According to the results of this high-speed imaging study, spattering is in the key role of the build part quality in L-PBF. The spattering phenomena can be categorized into three different types: entrainment driven spattering, recoil pressure driven spattering and Marangoni effect. These types are linked to the melt pool modes and can be detected from different amounts and sizes of the spatters.