Selective Laser Melting Of Ti6Al4V Research Articles

Manufacturability study in laser powder bed fusion of biomedical Ti alloys for orthopedic implants: an investigation of mechanical properties, process-induced porosity and surface roughness

PurposeSince the biomedical implants with an improved compressive strength, near bone elastic modulus, controlled porosity, and sufficient surface roughness, can assist in long term implantation. Therefore, the fine process tuning plays its crucial role to develop optimal settings to achieve these desired properties. This paper aims to find applications for fine process tuning in laser powder bed fusion of biomedical Ti alloys for load-bearing implants.Design/methodology/approachIn this work, the parametric porosity simulations were initially performed to simulate the process-induced porosity for selective laser-melted Ti6Al4V as per full factorial design. Continually, the experiments were performed to validate the simulation results and perform multiresponse optimization to fine-tune the processing parameters. Three levels of each control variable, namely, laser power – Pl (180, 190, 200) W, scanning speed – Vs (1500, 1600, 1700) mm/s and scan orientation – ϴ{1(0,0), 2(0,67°), 3(0,90°)} were used to investigate the processing performance. The measured properties from this study include compressive yield strength, elastic modulus, process-induced porosity and surface roughness. Finally, confirmatory experiments and comparisons with the already published works were also performed to validate the research results.FindingsThe results of porosity parametric simulation and experiments in selective laser melting of Ti6Al4V were found close to each other with overall porosity (less than 10%). The fine process tuning was resulted in optimal settings [Pl (200 W), Vs (1500 mm/s), ϴ (0,90°)], [Pl (200 W), Vs (1500 mm/s), ϴ (0,67°)], [Pl (200 W), Vs (1500 mm/s), ϴ (0,0)] and [Pl (200 W), Vs (1500 mm/s), ϴ (0,0)] with higher compressive strength (672.78 MPa), near cortical bone elastic modulus (12.932 GPa), process-induced porosity (0.751%) and minimum surface roughness (2.72 µm). The morphology of the selective laser melted (SLMed) surface indicated that the lack of fusion pores was prominent because of low laser energy density among the laser and powder bed. Confirmatory experimentation revealed that an overall percent improvement of around 15% was found between predicted and the experimental values.Originality/valueSince no significant works are available on the collaborative optimization and fine process tuning in laser powder bed fusion of biomedical Ti alloys for different load bearing implants. Therefore, this work involves the comprehensive investigation and multi-objective optimization to determine optimal parametric settings for better mechanical and physical properties. Another novel aspect is the parametric porosity simulation using Ansys Additive to assist in process parameters and their levels selection. As a result, selective laser melted Ti alloys at optimal settings may help in examining the possibility for manufacturing metallic implants for load-bearing applications.

Influence of the thermal-fluid behavior on the microstructure evolution during the process of selective laser melting of Ti6Al4V

To account for the microstructure evolution corresponding to the changed scanning speed, the thermal-fluid dynamic model of the meltpool during the selective laser melting (SLM) process of Ti6Al4V was established by numerical method to study the thermal characteristics and the melt flow behavior. Results showed that increasing the laser scanning speed would result in a lower peak temperature but a higher heating and cooling rate on the specimen. Both the meltpool size and its duration were reduced with the increased laser scanning velocity. Typically, a waved solid/liquid interface was observed at the bottom of the rear part of the meltpool as the time elapsed, especially for a larger scanning velocity. The melt flow velocity had a magnitude of hundreds of millimeters per second and showed almost a linear decrease with the increased scanning speed. Except for the change in cooling rate, the variation in flow velocities of the liquid metal consequent to different laser scanning speeds may be another possible reason for the observed microstructure change. The final result suggests that the scanning velocity must be carefully tailored to obtain the optimized combination of process parameters for industrial application, allowing for its adverse influence on the microstructure morphology and thermal stress/deformation caused by higher values.

Selective Laser Melting of Ti6Al4V: Effects of Heat Accumulation Phenomena due to Building Orientation

Titanium alloy Ti6Al4V is one of the most utilized alloys in the field of additive manufacturing due to the excellent combination of mechanical properties, density and good corrosion behavior. These characteristics make the use of this material particularly attractive for additively manufacturing components with complex geometry in sectors such as aeronautics and biomedical. Selective Laser Melting (SLM), by which a component is fabricated by selectively melting of stacked layers of powder using a laser beam, is the one of most promising additive manufacturing technologies for Ti6Al4V alloy. Although this technique offers numerous advantages, it has some critical issues related to the high thermal gradients, associated with the process, promoting the formation of a metastable martensitic microstructure resulting in high tensile strength but poor ductility of the produced parts. The formation of microstructural defects such as balling and porosity can occur together with the presence of residual stresses that may significantly affect the mechanical characteristics of the component. Specific process parameters and geometries can determine heat accumulation phenomena that result in a progressive decrease in thermal gradients between layers. These heat accumulation phenomena are influenced by the number of layers deposited, but also by the building orientation that, for a given geometry, determines a variation of the deposited surface for each layer.

Uncertainties Induced by Processing Parameter Variation in Selective Laser Melting of Ti6Al4V Revealed by In-Situ X-ray Imaging.

Selective laser melting (SLM) additive manufacturing (AM) exhibits uncertainties, where variations in build quality are present despite utilizing the same optimized processing parameters. In this work, we identify the sources of uncertainty in SLM process by in-situ characterization of SLM dynamics induced by small variations in processing parameters. We show that variations in the laser beam size, laser power, laser scan speed, and powder layer thickness result in significant variations in the depression zone, melt pool, and spatter behavior. On average, a small deviation of only ~5% from the optimized/reference laser processing parameter resulted in a ~10% or greater change in the depression zone and melt pool geometries. For spatter dynamics, small variation (10 , 11%) of the laser beam size could lead to over 40% change in the overall volume of the spatter generated. The responses of the SLM dynamics to small variations of processing parameters revealed in this work are useful for understanding the process uncertainties in the SLM process.

Influence of the Thermal-Fluid Behavior on the Microstructure Evolution During the Process of Selective Laser Melting of Ti6Al4V

Influence of the Thermal-Fluid Behavior on the Microstructure Evolution During the Process of Selective Laser Melting of Ti6Al4V

Selective Laser Melting of Ti6Al4V-2%Hydroxyapatite Composites: Manufacturing Behavior and Microstructure Evolution

In this work, selective laser melting of Ti6Al4V (Ti64) and 2 wt.% hydroxyapatite (HA) composites was performed with the purpose of osseointegration enhancement and biological fixation between implants and bone tissue. The microstructural evolution and mechanical properties were analyzed by using X-ray diffraction (XRD), optical microscopy (OM), a scanning electron microscope (SEM) equipped with (EDX) and (EBSD) systems, microhardness, nanoindentation, and tensile testing. The results showed that the Ti64-2%HA composite components exhibited complicated manufacturing behavior, which could be correlated with the decomposition of HA. The microstructure was found to mainly consist of α Ti with a small amount of HA distributed along grain boundaries. Furthermore, the interaction between Ti64 and HA leading to the formation of Ti3P, TixO, P, and CaTiO3 phases, resulted in poor tensile properties, as compared to pure Ti64 components. Conversely, the tensile properties of SLM Ti64-2%HA composite components were significantly higher than human bone reported previously in the literature.

Selective Laser Melted Titanium Alloy for Transgingival Components: Influence of Surface Condition on Fibroblast Cell Behavior

To mechanically characterize and assess the biological properties of Ti6Al4V surfaces obtained by Selective Laser Melting in order to determine whether this process is conceivable for producing of implant-supported prostheses and particularly trans-gingival components. As-built and polished surfaces were studied in comparison with components obtained by computer numerical control machining technology in order to consider whether the properties are in the same range as the conventional method currently used. Cylindrical specimens of Ti6Al4V (n = 6) were built with Selective Laser Melting for the characterization of mechanical properties according to ISO 22674 and discs (n = 12) were fabricated in the same conditions for cytotoxicity evaluation. Discs (n = 12) of Ti6Al4V were also obtained by computer numerical control machining as control. Half of the number of discs (n = 6) from each process were polished, to simulate the laboratory protocol for polishing of transmucosal components and half of the discs remained unaltered (as-built). Surface roughness measurements of disc specimens (as-built and polished) were compared with computer numerical control milling specimens (as-built and polished). Proliferation of human gingival fibroblasts on Ti6Al4V surfaces was also assessed for each condition. Viability and cell morphology were then evaluated qualitatively. Ra and Sa data were compared using Student t-test (α = 0.05) and metabolic activity data were compared using Kruskal-Wallis statistical test (α = 0.05). Selective Laser Melting specimens showed elongation at break greater than 2 % and 0.2% yield strength better than 500MPa which complied with ISO 22674 standards. Although Selective Laser Melting samples displayed significantly increased roughness on as-built surfaces compared to computer numerically-controlled milling samples (p<0.05), no statistically significant difference was observed after mechanical polishing (p = 0.279). Regarding metabolic activity, no statistical difference was observed between groups at day 3 (p>0.05) and fibroblasts showed a viability higher than 97 % on all discs. Cell shapes on polished samples suggested moderate adhesion compared to unpolished samples. With the manufacturing parameters selected in this study, Selective Laser Melting of Ti6Al4V appeared to be compatible with a prosthetic application type 4 according to ISO 22674. Surfaces obtained, followed by recommended post-processing provided components with equivalent biological properties compared to computer numerical control machining technology. This article is protected by copyright. All rights reserved.

Numerical modelling of keyhole formation in selective laser melting of Ti6Al4V

In the selective laser melting process, the molten pool flow, particularly the keyhole phenomenon, plays an important role in the pore formation and then influences the mechanical properties. In this study, we develop a 3D numerical model to simulate the keyhole mode in SLM. The transient evolutions of the temperature field, fluid flow, molten pool geometry and energy absorption have all been reproduced. The simulated molten pool dimensions agree well with the experimental data, including the fluctuation. More thorough insights into the underlying physical mechanisms are achieved, including the correlation between the keyhole geometry and laser absorption, the influence of manufacturing parameters on the melting mode, and the fluid flow pattern under different melting modes. These understandings can provide guidance for the optimization of manufacturing process parameters to improve the product quality.

Selective laser melting of Ti6Al4V: Effect of laser re-melting

Abstract Microstructural inhomogeneity in additively manufactured materials affects the material properties. The present study aims in minimizing such microstructural inhomogeneity in Ti6Al4V alloy fabricated using selective laser melting (SLM) from the gas atomized powder. A detailed and systematic study of the effect of remelting on the microstructure and mechanical properties of Ti6Al4V was undertaken. Acicular α′ martensite was present in all the samples (both in the as-built SLM and remelted) and the dimensions of the α′ phase change with the number of melting steps. The hardness of the as-built SLM sample increased and the material got homogenized with an increasing number of meltings. The ultimate tensile strength was higher in the double- and triple-melted samples while the ductility was lower than the single-melted sample. The present results clearly prove that the number of remeltings play a significant role in determining the microstructure (homogenization of the microstructure) and the mechanical properties of the SLM-built materials.

Topology optimization of a mandibular reconstruction plate and biomechanical validation

ObjectivesReconstruction plates, used to bridge segmental defects of the mandible after tumor resection or traumatic bone tissue loss, are subjected to repeated stresses of mastication. High stress concentrations in these plates can result in hardware failure. Topology optimization (TO) could reduce the peak stress by computing the most optimal material distribution in a patient-specific implant (PSI) used for mandibular reconstruction. The objective of this study was biomechanical validation of a TO-PSI. MethodsA computer-aided design (CAD) model with a segmental defect was created based on the geometry of a polyurethane mandible model. A standard-PSI was designed to bridge the defect. A TO-PSI was then designed with a maximum stress equal to the ultimate tensile stress of Ti6Al4V (930 MPa) during a loading condition of 378 N. Finite element analysis (FEA) was used to analyze stresses in both PSI designs during loading. The standard-PSI and TO-PSI designs were produced in triplicate by selective laser melting of Ti6Al4V, fixated to polyurethane mandible models with segmental defects identical to the CAD model, and subsequently subjected to continuous compression with a speed of 1 mm/min on a universal testing machine, while recording the load. Peak loads before failure in the TO-PSI group within a 30% range of the predicted peak load (378 N) were considered a successful biomechanical validation. ResultsFracture of the TO-PSI occurred at a median peak load of 334 N (range 304–336 N). These values are within the 30% range of the predicted peak load. Fracture of the mandible model in the standard-PSI group occurred at a median peak load of 1100 N (range 1010–1460 N). Failure locations during biomechanical testing of TO-PSI and standard-PSI samples corresponded to regions in the FEA where stresses exceeded the ultimate tensile strength of titanium and polyurethane, respectively. ConclusionThis study demonstrates a successful preliminary biomechanical validation of TO in the design process for mandibular reconstruction plates. Further work is needed to refine the finite element model, which is necessary to ultimately design TO-PSIs for clinical use.

Study of residual stress in selective laser melting of Ti6Al4V

Selective laser melting (SLM) produces a large amount of residual stress which limits its applicability. Despite the increasing number of studies to understand the relationship between the process parameters and the residual stress, the underlying complex mechanisms are yet to be understood. In this work, a 3D finite element model (FEM) is established to investigate the relationship between the process parameters and the residual stress during SLM. When the process parameters are in different ranges, the residual stress behavior is different. The complex relationship between the residual stress and the process parameters could be the reason for the contradictory parametric effects reported in the literature. The magnitude of the parametric effect on the residual stress is obtained. The influence of the scanning speed is stronger than that of the laser power, which is in turn stronger than that of the hatch spacing. These results can bring insights into the understanding of the residual stress during SLM.

Microstructural examination and corrosion behavior of selective laser melted and conventionally manufactured Ti6Al4V for dental applications

The selective laser melting of Ti6Al4V would induce definite changes in the microstructure that may affect its corrosion properties. Microstructural examination showed the formation of relatively thin beta (β) lamella in selective laser melted (SLM) Ti6Al4V compared to wrought Ti6Al4V. X-ray diffraction analysis (XRD) analysis confirmed the presence of alpha and beta phases in both SLM and wrought Ti6Al4V. However, the higher concentration of the β phase in SLM Ti6Al4V compared to wrought Ti6Al4V was evident in the microstructure. As candidate dental implant materials, the corrosion behavior of both SLM and wrought Ti6Al4V was assessed in artificial saliva (AS) and deionized water (DI) containing various species i.e. fluoride (F), calcium chloride (CaCl2) and lactic acid (LA). Electrochemical impedance spectroscopy and potentiodynamic polarization analysis was carried out to estimate the corrosion behavior of SLM and wrought Ti6Al4V at room temperature. SLM Ti6Al4V offered better corrosion resistance than wrought Ti6Al4V in all solutions at pH>6. However, wrought Ti6Al4V comparatively presented high corrosion resistance in AS + LA, DI+CaCl2 and DI+LA solutions (pH<6). The lower dissolution rate of SLM Ti6Al4V (at pH>6) was attributed to larger β content in the microstructure compared to wrought Ti6Al4V.

Development of processing strategies for multigraded selective laser melting of Ti6Al4V and IN718

In energy generation applications, Ti- and Ni-alloys are widely used for their complementary features, where Ti-alloys provide lightweight structures while Ni-alloys are adaptable to high temperature use. The combination of these alloys into a single component through additive manufacturing is highly desirable. This work explores the multi-material selective laser melting (SLM) of a Ti6Al4V-IN718 material system to produce multigraded specimens. An in-house developed multi-material SLM platform with double hopper and a mixing chamber was employed. A work frame based on studying process feasibility through premixed blends and assessing the processability of multigraded components is presented. Material characteristics, in terms of chemistry, microhardness and microstructure are investigated and supported by thermodynamic calculations. Defect-free grading was achieved until 20 wt% inclusion of IN718 in Ti6Al4V. The results were interpreted to reveal the processability limits of the metallurgically incompatible alloys as well as the defect formation mechanisms.

Thermodynamics of Molten Pool Predicted by Computational Fluid Dynamics in Selective Laser Melting of Ti6Al4V: Surface Morphology Evolution and Densification Behavior

Thermodynamics of Molten Pool Predicted by Computational Fluid Dynamics in Selective Laser Melting of Ti6Al4V: Surface Morphology Evolution and Densification Behavior

Correction to: Process parameter selection for selective laser melting of Ti6Al4V based on temperature distribution simulation and experimental sintering

The authors regret to inform that a missing reference [27] should be marked in the caption of Fig. 6(a) in the published paper.

Track-Scale Simulations of Selective Laser Melting to Investigate Development and Mitigation of Thermal Stresses

This work investigates the evolution of thermal stresses, dependence of residual stress on important process parameters, critical locations of high stresses and ways to minimize them by adopting the appropriate set of the parameters for selective laser melting of Ti6Al4V at track-scale. A fully coupled 3D thermo-mechanical model is developed. Elasto-plastic material and material properties considered are dependent on temperature. In the coupled modelling approach, the validation of thermal model carried out with the available experimental data. Detailed analysis of temperature field and resulting thermal stresses is then presented. During heating, compressive stress zone is observed in the neighbourhood of the melt pool and balancing tensile stress zone below it. As laser traverses forward, tensile stresses are generated in the solidified melt pool region and a balancing compressive stress zone underneath it is observed. It is quantified that with increase in laser heat source power and interaction time, the magnitude of residual stress increases, but pre-heating the substrate reduces the residual stress. The magnitude of residual stress is also decreased by adopting the alternate-scan strategy over the uni-directional scan-strategy. Apart from delineating the detailed quantitative analysis of residual stress, this work helps understand the evolution of thermal stress in the SLM process at the fundamental level, i.e. at track-scale which is the basic building block. This understanding is crucial to control the residual stresses at part scale. Volume reduction during the conversion of powder material to bulk liquid has been received less attention in the previous thermal models. Thus, its incorporation in the present thermal model makes the modelling approach more realistic, as the predictions of the thermal model act as important inputs to the coupled mechanical model that calculates stresses.

Effect of rescanning cycles on the characteristics of selective laser melting of Ti6Al4V

Rescanning has been introduced as a new method to improve the performance of selective laser melting (SLM) in recent years. However, many of the related works were done on the investigating of the rescanning parameters and only one rescanning cycle was used. In this work, the influences of rescanning cycle on the relative density, microstructure, residual stress and mechanical properties of SLMed Ti6Al4V alloy were investigated. The results show that as the rescanning cycles increase from 0 to 4, the relative density and residual stress of the samples increase at first and then decrease, the highest relative density and residual stress are found when the number of rescanning cycle is one. The number of rescanning cycle has little effect on the microstructure. The microstructures all consist of the martensitic α′ laths which grow inside the columnar prior β grains. The ultimate tensile strength (UTS), yield strength, micro-hardness and micro-strain of the samples all increase as the rescanning cycles increase from 0 to 3, but decrease when the samples are rescanned fourth times. This research provides an insight in the influence of rescanning cycles on characteristics of SLM process and can be used to optimize the process parameters of SLM.

A thermo-metallurgical-mechanical model for selective laser melting of Ti6Al4V

A thermo-metallurgical-mechanical coupling model is developed to predict temperature, solid-state phase and residual stress fields for the multi-track multi-layer selective laser melting process of Ti6Al4V. The model considers the solid-state phase transformation (SSPT) and powder-liquid-solid transition which includes melting, vaporization, solidification, shrinkage and cooling phenomena. The thermal analysis is based on the transient heat conduction problem with a volumetric heat source describing the laser absorption and scattering in the powder bed. The volume fraction evolution of metallurgical phases is determined by temperature history and used to obtain the volumetric change strain due to the SSPT. An elasto-plastic constitutive law considering the strains that are induced by thermal gradients and the SSPT is proposed to evaluate stress fields. Modelling results reveal that the consideration of the SSPT leads to the decrease of tensile residual stresses and increase of compressive residual stresses, and the residual stress component in the scanning direction is larger than the other two stress components.

New Approach for Verifying the Accuracy of X-ray Computed Tomography Measurements of Surface Topographies in Additively Manufactured Metal Parts

Surface topographies of metal additively manufactured components are inherently characterized by the presence of complex surface characteristics that are not accessible by contact or optical measuring techniques. Micro X-ray computed tomography is capable of measuring non-accessible surfaces and micro-scale surface features, including undercuts. In this work, an innovative approach for evaluating the accuracy and establishing the traceability of surface topography measurements obtained by X-ray computed tomography is presented. Reference samples produced by selective laser melting of Ti6Al4V were specifically designed in order to acquire reference cross-sectional surface profiles representing the actual morphology (including re-entrant features) using an imaging probing system. Surface topographies were measured on these samples by using three different techniques: X-ray computed tomography, confocal microscopy and focus variation. Moreover, the effect of different voxel dimensions on the accuracy of surface topography measurements performed by X-ray computed tomography was investigated. Results showed that X-ray computed tomography (at the highest tested resolution) can acquire surfaces and re-entrant features with small deviations with respect to the reference profiles; the deviations were quantified. On the contrary, focus variation and confocal microscopy can measure surfaces obtaining results that are close to the reference profiles only if such surfaces have no undercuts or inaccessible features.

Numerical and experimental analysis of the effect of volumetric energy absorption in powder layer on thermal-fluidic transport in selective laser melting of Ti6Al4V

A volumetric heat source is used in numerical modeling of selective laser melting (SLM) of Ti6Al4V powder. Single track and multi-track SLM simulations are performed by varying the two key process parameters-laser power and scan speed. The model is validated with the published experimental results for melt pool shape, size and temperature. The predictions are in good agreement with the experiments at low to medium energy density. The validated model is used for investigating the thermo-fluidic transport during SLM of Ti6Al4V and examining the dependence of the melt pool characteristics on the process parameters. As-solidified porosity is calculated numerically for the multi-track simulations and its formation is delineated with the transport phenomena. The predicted porosity compares reasonably well with the experimental values. Solidification parameters, such as temperature gradients and cooling rate are calculated at the instantaneous location of the solidification front and analyzed. This analysis suggests the formation of fully columnar grains of different sizes along the width and depth of the melt pool. Overall, the model provides a good description of thermo-fluidic transport in SLM of Ti6Al4V powder and the resulting temperature field, melt pool characteristics, as-solidified porosity and the expected grain structure. Based on the current analysis, an optimum processing window of 50–70 J mm−3 energy density is suggested for SLM of Ti6Al4V powder.