Partially Melted Ti6Al4V Particles Increase Bacterial Adhesion and Inhibit Osteogenic Activity on 3D-printed Implants: An In Vitro Study.

Osteogenesis of 3D printed porous Ti6Al4V implants with different pore sizes.

Selective laser melting (SLM) is increasingly being used for the fabrication of cobalt chromium customised medical devices. The purpose of this study was to determine the effect of surface topographies from SLM parts on the in-vitro biological responses of mouse fibroblasts (L929). Fibroblast cells were cultured for 3 days on SLM and on conventionally cast Co-Cr-Mo surfaces and their proliferation, morphology, viability and cytotoxicity investigated. SLM processing parameters were used to manufacture discs with different microlevel surface topographies. Our analysis showed that the SLM implants met basic biocompatibility requirement on all surfaces. The effect of the SLM processing parameters was characterised by an improvement in cell viability with increasing surface roughness. The number of viable cells on all the SLM Co-Cr-Mo either equaled or was higher than those on the reference cast sample. A statistically significant difference (p < 0.05) was found between the cell viability on two SLM specimens (Ra = 29.25 and 20.62 μm) and reference cast (Ra = 4.32 μm). In comparison to a polymeric negative control, the absorbance measurement on Co-Cr-Mo surfaces was in the range of 60 to 80% of the control and the cytotoxicity was comparable to the cast sample. The surface roughness amplitude parameters were found to discriminate cell viability and cytotoxicity better, confirming that fibroblast cell growth on SLM surfaces was more influenced by the amplitude rather than by the organisation or morphology of the surface. It is concluded that SLM processing significantly affects surface topography which in turn influences the L929 fibroblast cellular response.

The use of selective laser melting (SLM) to fabricate 18Ni-300 maraging steel components for aerospace and mould making applications has attracted significant interest from both industry and academia. In particular, mould makers have utilised SLM to fabricate steel moulds with integrated conformal cooling channels, which provided enhanced cooling efficiency and reduced injection moulding cycle time compared to their conventional counterparts. However, its widespread application was hindered due to issues such as microstructural heterogeneity, mechanical anisotropy, and manufacturing defects. In the present thesis, the process-microstructure-properties relationships in SLM 18Ni-300 maraging steel were investigated to address these issues and facilitate the fabrication of additively manufactured steel moulds with mechanical properties comparable to their conventionally made counterparts. An extensive literature review was conducted to understand how SLM process parameters (or in-process parameters) influence microstructure evolution and mechanical properties in SLM 18Ni-300 maraging steel and four other steel mould materials (i.e. H13, P20, AISI 420 stainless steel, and S136). This facilitated a novel comparative analysis of laser-powder interactions, rapid solidification, and intrinsic heat treatment across the five steel mould materials, and addressed the lack of comparative analysis specific to this topic. Complex laser-powder interactions such as the scanning motion of the laser during SLM process resulted in mechanical anisotropy and manufacturing defects, which may be mitigated via statistical process optimisation. Rapid solidification resulted in microstructural heterogeneity (i.e. variations in grain size and crystallographic texture), which was subsequently altered during post-processing heat treatment. Intrinsic heat treatment resulted in in situ precipitation hardening in SLM 18Ni-300 maraging steel and martensite tempering in the other four steel mould materials. Experimental investigations were conducted to elucidate the process-microstructure-properties relationships in SLM 18Ni-300 maraging steel. Experiments on additively manufactured individual scan tracks and fully built samples revealed that increasing laser power (P), reducing the scanning speed (v), and reducing the hatch spacing (h) resulted in increased laser energy input (E_linear and E_volumetric) and reduced manufacturing defects. Following that, a combined statistical optimisation methodology featuring Taguchi’s methods and grey relational analysis (GRA) was implemented to determine optimal SLM in-process parameters for the multi-response optimisation of mechanical properties. Statistical analysis indicated both P and v had an approximately equal influence on the mechanical properties, while the influence of h was less significant. The mechanical properties of samples fabricated using the optimal SLM in-process parameters have high relative density (i.e. &gt; 99 %) and were comparable with their conventionally made counterparts. Macroscale plastic strain localisation phenomena including propagation of Lüders bands and necking leading to ductile fracture in the fabricated samples were captured using optical imaging-based digital image correlation (optical-DIC). The influence of microstructural heterogeneity (i.e. variations in grain size and crystallographic texture) on the microscale plastic strain localisation in SLM 18Ni-300 maraging steel was investigated using electron backscatter diffraction (EBSD), in situ uniaxial tensile experiments, and scanning electron microscope-based digital image correlation (SEM-DIC). Sub-micron sized speckle patterns were created via magnetron sputtering, which facilitated high-resolution strain measurements via SEM-DIC and addressed the lack of established speckle creation methodology for SLM 18Ni-300 maraging steel. Custom MATLAB scripts were employed to digitally align and overlay the EBSD and SEM-DIC datasets. This methodology facilitated a novel grain-to-grain comparison of the microscale plastic strain localisation in relation to the microstructural heterogeneity, provided grain-level insights into the role of internal misorientations on slip activity in individual grains, as well as addressed the lack of SEM-DIC investigations specific to SLM 18Ni-300 maraging steel. Experimental findings revealed that the microscale plastic strain localisation in SLM 18Ni-300 maraging steel was driven by the interplay between microstructural heterogeneity, Ni-based intermetallics, and the impediment of dislocation motion. The densely distributed equiaxed grains contributed to grain boundary strengthening in the as-built (AB) sample. After post-processing heat treatment, the precipitation of densely distributed Ni-based intermetallics contributed to strain hardening in the solution-aging treatment (SAT) sample. The main deformation mechanism was identified as dislocation slip. Slip preferentially occurred in grains with increased internal misorientation, and intersected regions that exhibited increased kernel average misorientation (KAM). Complex slip behaviour including discrete slip, diffuse slip, and possible cross-slip (or overlapping slip) was identified in the investigated grains of AB and SAT samples. The combined kinematics of the active slip systems were reflected in the in-plane deformation behaviour of the investigated grains. The findings of the present thesis provide fundamental insights into tailoring the microstructure of SLM 18Ni-300 maraging steel for optimised industrial performance. Finally, potential future research directions for SLM 18Ni-300 maraging steel were suggested, including numerical modelling of microstructure evolution and investigation of microscale plastic strain localisation under complex loading conditions such as multiaxial loading and fatigue.

The surface properties of titanium and its alloys are commonly modified by different techniques, including additive manufacturing (AM), to improve the osseointegration of dental implants. The aim of this study was to evaluate and compare the wettability, topography, chemistry, and structure of titanium-aluminum-vanadium (Ti-6Al-4 V) discs fabricated by selective laser melting (SLM) and subtractive manufacturing (conventional machining). Three different groups were evaluated: selective laser melting (SLM); conventional machining with H3 PO4 + NaOH surface treatment (CM + ST); and conventional machining without surface treatment (CM), including analysis of wettability and roughness, morphological and chemical analyses by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), and structural characterization analysis by computed tomography (micro-CT), and X-ray diffraction (XRD). The results showed that SLM surface had higher roughness (9.09 ± 1.94 Ra; 51.93 ± 11.59 Rz; 11.03 ± 1.95 Sa) and lower wettability (103.23° ± 13) than CM (0.06 ± 0.01 Ra; 0.42 ± 0.078 Rz; 0.07 ± 0.01 Sa) (76.95° ± 4.18) and CM + ST (0.17 ± 0.38 Ra; 0.88 ± 0.15 Rz; 0.18 ± 0.04 Sa) (18.55° ± 6.47) (p < 0.05). SEM images also proved the higher roughness of SLM surface, and CM + ST discs showed a topography resembling a sponge, characteristic of the nanometric treatment applied. EDX and XRD found no differences between the different surfaces, and micro-CT demonstrated the solid characteristic of the SLM disc. Compared with conventional machining, the SLM technique resulted in higher roughness and lower wettability. Meanwhile, the chemical properties and structure of the titanium alloy was not altered by the technique.

The aim of this study was to assess the osseointegration of two series of titanium (Ti) scaffolds with 0.8 and 1 mm cell size obtained by Selective Laser Melting (SLM) technique. One of the series had the Ti surface unmodified, while the other had the Ti surface coated with silicon-substituted nano-hydroxyapatite (nano-HapSi). The scaffolds were implanted in the femur bone defects of 6 White Californian male rabbits: three animals were implanted with 0.8 mm cell size scaffolds and three animals with 1 mm cell size scaffolds, respectively. The bone fragments and scaffolds harvested at 2, 4 and 6 months were histologically analyzed using conventional light microscopy (LM) and scanning electron microscopy (SEM) for the qualitative evaluation of the bone tissue formed in contact with the scaffold. Both LM and SEM images indicated a better osseointegration for nano-HapSi coated Ti scaffolds. LM revealed that the compact bone formed in the proximity of nano-HapSi-coated scaffolds was better organized than spongy bone associated with unmodified scaffolds. Moreover, Ti scaffolds with meshes of 0.8 mm showed higher osseointegration compared with 1 mm. SEM images at 6 months revealed that the bone developed not only in contact with the scaffolds, but also proliferated inside the meshes. Nano-HapSi-coated Ti implants with 0.8 mm meshes were completely covered and filled with new bone. Ti scaffolds osseointegration depended on the mesh size and the surface properties. Due to the biocompatibility and favorable osseointegration in bone defects, nano-HapSi-coated Ti scaffolds could be useful for anatomical reconstructions.

Additive manufacturing (AM) of hot-work tool steels such as H13 offers unique opportunities for producing complex, conformally cooled tools with reduced production time and material waste. In this study, five metal AM technologies—Fused Deposition Modeling and Sintering (FDMS, Desktop Metal Studio System and Zetamix), Binder Jetting (BJ), Laser Powder Bed Fusion (LPBF), and Directed Energy Deposition (DED)—were compared in terms of microstructure, porosity, and post-processing heat treatment response. The as-printed microstructures revealed distinct differences among the technologies: FDMS and BJ exhibited high porosity (6–9%), whereas LPBF and DED achieved near-full densification (<0.1%). Samples with sufficiently low porosity (BJ, LPBF, DED) were subjected to tempering and quenching treatments to evaluate hardness evolution and microstructural transformations. The satisfactory post-treatment hardness was observed in both tempered and quenched and tempered BJ samples, associated with secondary carbide precipitation, while LPBF and DED samples retained stable martensitic structures with hardness around 600 HV0.5. Microstructural analyses confirmed the dependence of phase morphology and carbide distribution on the thermal history intrinsic to each AM process. The study demonstrates that while FDMS and BJ are more accessible and cost-effective for low-density prototypes, LPBF and DED offer superior density and mechanical integrity suitable for functional tooling applications.

The microstructure, static, and fatigue mechanical properties of laser powder bed fused (LPBF) Ti-6Al-4V components subjected to three different post-processing treatments (PTs) are compared. The first treatment includes stress relief (SR) and beta-phase annealing (BA) heat treatments, the second one includes SR, beta-solution (BST) and over aging (OA) heat treatments, and the third procedure is a combination of hot isostatic pressing (HIP) and BST + OA. It was demonstrated that the three PTs led to the decomposition of α’ martensite inherited from the LPBF process and the formation of variable α + β structures. The SR + BA treatment forms a basket weave structure having an average α lamellae width of ~3 µm and surrounded by ~1 µm-sized zones of segregated β phase (4.6–5.2% β phase content) and globalized α phase (~10 µm in size) inside prior columnar β grains (~100 µm in width). The SR + BST + OA treatment forms semi-equiaxed α grains (~300 µm) containing colonies (~50 µm) of parallel-oriented α plates (~6 µm), and β phase (5.8–7.5%) in the interplate spacing. The HIP + BST + OA treatment leads to the formation of large grains (~500 µm) with both basket weave and colony (~40 µm) α structures containing α plates (1.1–4.2 µm) and β phase (5–7.1%). To compare the impact of these PTs on the mechanical properties of LPBF components, they were subjected to static and fatigue tensile testing at room temperature. The best combination of mechanical properties (yield strength ~920 MPa, ultimate strength ~1000 MPa, elongation to break ~22.5%, and fatigue strength ~600 MPa, 107 cycles) was obtained in the case of SR + BA specimens. These results demonstrate that an adequate thermal treatment, such as SR + BA, of the LPBF Ti64 components, could be a valuable and less expensive alternative to the established HIP + BST + OA treatment procedure when fatigue life is the main concern.

Surface roughness of Ti-6Al-4V parts obtained by SLM and EBM: Effect on the High Cycle Fatigue life

In this study, the influence of post-processing heat treatment on microstructure and mechanical properties of Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy fabricated by laser powder bed fusion (L-PBF) was investigated. The mechanical properties of the as-built and heat-treated samples with various temperatures (600–850 °C) were evaluated using a tensile test at room temperature. After heat treatments, both yield strength (YS) and ultimate tensile strength (UTS) gradually decreased, while the tensile elongation tended to increase as the heat treatment temperature increased. These variations were closely related to the microstructural evolution caused by heat treatment. Specifically, the decomposition of α′ martensite into the α + β lamellar structure and subsequent coarsening were promoted with increasing temperature, leading to stress relief and improved dislocation storage capability, which resulted in the variation in mechanical properties. Notably, although the mechanical strength was reduced after heat treatment with increasing temperatures, the lowest yield strength and ultimate tensile strength were measured as 1086.4 ± 16.5 and 1135.0 ± 15.0 MPa, respectively, which are comparable to or higher than those of conventionally processed Ti-6242. As a result, the post-processing heat treatment could be an effective approach to achieve desirable performance for targeted applications.

To address the limited biocompatibility of conventional removable dental prostheses (RDPs), new materials and manufacturing techniques are being actively investigated. This in vitro study aimed to evaluate and compare the biological response of human fibroblast (HGF) to cobalt-chrome (Co-Cr), titanium alloy (Ti), and polyetheretherketone (PEEK), fabricated via tree distinct methods: casting, selective laser melting (SLM) and machining. The surface chemical composition and the wettability of cast Co-Cr, SLM Co-Cr, SLM Ti, machined Ti, and PEEK were assessed. HGF were cultured in contact with material extracts, and all assays were conducted following this exposure. Ion release was assessed using inductively coupled plasma atomic emission spectroscopy (ICP-AES), and pH measurement. Surface roughness was subsequently measured on each sample. Cell viability was assessed using the Live/Dead staining, while metabolic activity was evaluated using the Alamar Blue assay. Cell morphology was observed by scanning electron microscopy (SEM) and confocal microscopy, while inflammatory profile was assessed using enzyme-linked immunosorbent assay (ELISA). Data were analyzed using one-way ANOVA and Tukey's test (p < 0.05 and 0.001). SLM Co-Cr exhibited higher Co and Cr surface content than cast Co-Cr, while SLM Ti showed a reduced titanium content and increased aluminum content relative to machined Ti. Among all tested materials, machined Ti demonstrated the highest surface wettability, evidenced by a significantly lower contact angle (p < 0.001). HGF exposed to extracts of machined Ti and PEEK exhibited greater cell viability and metabolic activity, comparable to the control group. In contrast, cast Co-Cr was associated with significantly increased IL-6 secretion, although TNF-α and IL-10 levels remained stable across tested groups. Machined titanium (Ti) and PEEK exhibited favorable in vitro biocompatibility, with high cell viability, enhanced cellular adhesion, and no significant cytotoxicity under our experimental conditions. These findings suggest that both materials may represent promising alternatives to CoCr alloys for the fabrication of removable partial dentures, although their clinical application requires further validation of their mechanical, physical, and long-term properties.

Hastelloy X (HX) is a Ni-based superalloy which is employed to produce gas turbine and gas-cooled reactor sectors due to its outstanding oxidation resistance and high tensile strength at high temperatures. This alloy can be processed by laser powder bed fusion (LPBF) fabricating complex geometries in a single step. However, post-processing thermal treatments must be applied to generate a suitable microstructure for high-temperature applications. The investigation reports the microstructure evolution of LPBF HX samples under specific post-processing treatments. A hot isostatic pressing (HIP) treatment can close the internal cracks and reduce the residual porosity (less than 0.1%). Moreover, the HIP-triggered recrystallization generated equiaxed grains, while the slow cooling rate generated a film of intergranular carbides (Mo-rich M6C and Cr-rich M23C6) and intragranular carbides (Mo-rich M6C carbides). Therefore, a solution annealing was performed to dissolve the film of carbides which may reduce the ductility. The post solution annealed material consisted of equiaxed grains with ASTM grain size number mainly 4.5-5.5 and inter/intragranular Mo-rich M6C carbides. The microstructure is highly comparable with solution annealed wrought HX alloy. Finally, after simulating short thermal exposure at 745 °C for 6 h, a significant formation of Cr-rich M23C6 carbides was observed strengthening the LPBF HX alloy.

This study investigates the microstructure and mechanical properties of 316L stainless steel (316LSS) samples fabricated by additive manufacturing (AM), in order to optimise the process for improving the properties of 316LSS parts built by laser powder bed fusion (LPBF). Accordingly, a substrate heating was performed on an SLM 280HL printer fitted with an experimental heating device. During the process, samples are built on a substrate heated up to 600 ∘C. The substrate temperature significantly influences the microstructural evolution and mechanical properties of manufactured parts. Concerning tensile properties, the ultimate tensile strength (UTS) and yield strength (YS) decrease as a function of the substrate temperature whereas the elongation (El) increases as a function of the substrate temperature. These results are similar to those obtained with a post-heat treatment performed on parts manufactured by forging or LPBF. The tensile and impact energies reach values higher than the minimum requirement for 316LSS manufactured by forging, according to the RCC-MRx code used for materials dedicated to French nuclear applications. When the substrate temperature is equal to 350 ∘C, the UTS, YS, El and impact energy reached 596 ± 5 MPa, 489 ± 3 MPa, 48 ± 3% and 123 ± 20 J, respectively. Finally, this study demonstrates that heating the substrate during the process is a promising solution to optimise the fabrication route by suppressing post-process heat treatment of 316LSS made by LPBF.

Platform heating is one of the effective strategies used in laser powder bed fusion (LPBF) to avoid cracking during manufacturing, especially when building relatively large-size components, as it removes significant process-induced residual strains. In this work, we propose a novel and simple method to spare the elaborate post-processing heat treatment typically needed for LPBF Al-Sc alloys without compromising the mechanical properties. We systematically investigated the effects of LPBF platform heating at 200 °C on the residual stress relief, microstructure, and mechanical performance of a high-strength Al-Mn-Sc alloy. The results reveal that LPBF platform heating at 200 °C is sufficient to largely relieve the process-induced residual stresses compared to parts built on an unheated 35 °C platform. Meanwhile, the platform heating triggered the dynamic precipitation of uniformly dispersed (1.5-2 nm) Sc-rich nano-clusters. Their formation in a high number density (1.75 × 1024 m-3) resulted in a ~20% improvement in tensile yield strength (522 MPa) compared to the build on the unheated platform, without sacrificing the ductility (up to 18%). The improved mechanical properties imply that platform heating at 200 °C can strengthen the LPBF-synthesised Sc-containing Al alloys via in situ aging, which is further justified by an in situ measurement study revealing that the developing temperatures in the LPBF part are within the aging temperature range of Al-Sc alloys. Without any post-LPBF treatments, these mechanical properties have proven better than those of most Al-Sc alloys through long-time post-LPBF heat treatment.

Characterization of as-built and heat-treated Ni-Mn-Ga magnetic shape memory alloy manufactured via laser powder bed fusion

Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h–hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150 °C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980 °C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications.