Effect of Heat Treatment on the Microstructure and Mechanical Properties of Stainless Steel 316L Coatings Produced by Cold Spray for Biomedical Applications

Abstract In this study, the effects of heat treatment on the microstructure and mechanical properties of cold sprayed stainless steel 316L coatings using N2 and He as propellant gases were investigated. Powder and coating characterizations, including coating microhardness, coating porosity, and XRD phase analysis were performed. It was found that heat treatment reduced porosity, improved inter-particle bonding, and increased ductility. XRD results confirmed that no phase transformation occurred during deposition. Significant increase in UTS and ductility was observed for the annealed specimens obtained with nitrogen propellant, whereas little changes were observed for the helium propellant produced specimen

Additive Manufacturing of High-Performance 316L Stainless Steel Nanocomposites via Selective Laser Melting

Austenitic 316L stainless steel alloy is an attractive industrial material combining outstanding corrosion resistance, ductility, and biocompatibility, with promising structural applications and biomedical uses. However, 316L has low strength and wear resistance, limiting its high-performance applicability. Adding secondary hard nanoscale reinforcements to steel matrices, thereby forming steel-matrix nanocomposites (SMCs), can overcome these problems, improving the performance and thereby the applicability of 316L. However, SMC parts with complex-geometry cannot be easily achieved limiting its application. This can be avoided through additive manufacturing (AM) by generating layer-by-layer deposition using computer-aided design data. Expanding the range of AM-applicable materials is necessary to fulfill industrial demand. This dissertation presents the characteristics of new AM-processed high-performance 316L-matrix nanocomposites with nanoscale TiC or TiB2 reinforcements, addressing specific aspects of material design, process control and optimization, and physical metallurgy theory. The nanocomposites were prepared by high-energy ball-milling and consolidated by AM selective laser melting (SLM). Continuous and refined ring-like network structures were obtained with homogenously distributed reinforcements. Additional grain refinement occurred with reinforcement addition, attributed to nanoparticles acting as nuclei for heterogeneous nucleation. The influence of reinforcement content was first investigated; mechanical and tribological behaviors improved with increased reinforcement contents. The compressive yield strengths of composites with TiB2 or TiC reinforcements were approximately five or two times those of 316L respectively. Hot isostatic pressing post-treatment effectively eliminated major cracks and pores in SLM-fabricated components. The effects of the SLM processing parameters on the microstructure and mechanical performance were also investigated. Laser re-melting through double-scanning created higher-density SLM-processed parts with improved mechanical properties but longer production times. Certain scanning patterns minimized texture, creating near-isotropic structures. The energy density η crucially improved densification at the expense of increased grain size, causing mechanical behavior tradeoffs. It also influenced the size and dispersion state of TiC. In-situ SMCs were fabricated by SLM, an encouraging low-cost processing approach for high-performance parts. Interestingly, in-situ SMCs exhibited higher microhardness values in comparison to the ex-situ composites under fixed SLM processing conditions because of fine, uniform reinforcement distribution. The developed nanocomposites show promise as high-performance materials. Future research is suggested for strategic material developments

Thermal behavior of the molten pool, microstructural evolution, and tribological performance during selective laser melting of TiC/316L stainless steel nanocomposites: Experimental and simulation methods

Bulk-form nanocomposites of TiC-reinforced 316L stainless steel matrix were fabricated by selective laser melting (SLM), an emerging powder-bed additive manufacturing technique which allows the direct fabrication of usable end-products, using various volumetric laser energy densities (a). The microstructural features of the distribution and sizes of TiC nanoparticles, as well as the grain sizes and tribological performances of the SLMprocessed nanocomposite parts, were sensitive to the applied eta. Increasing eta enhanced the dispersion state of nanoscale TiC owing to intensified Marangoni flow and the corresponding capillary force, which prevented TiC aggregation and promoted a uniform dispersion of reinforcements in the solidified matrix. However, with increasing eta, the TiC particle size also increased, and some nanoparticles lost their initial nanostructure because of significant thermal accumulation within the molten pool. Increasing eta also caused increases in the grain sizes of the fabricated nanocomposite because of the decreasing cooling rate. A simulation model was developed to enhance understanding of the manufacturability of these new materials, as well as to predict the temperature evolution and thermal behaviors of the molten pool under various eta. The simulation modeled the effects of various eta values on the temperature distribution evolutions and the corresponding effects of Marangoni convection during the SLM process. The temperature distribution was significantly influenced by the applied a; the maximum temperature gradient within the molten pool was increased significantly with increased a. The simulation results validated the experimental results and the underlying physical mechanism of the molten pool. The microhardness of the SLM nanocomposite decreased sharply with increased grain size due to the lower cooling rate, but increased with further increases in eta because of the enhanced densification degree. Nanocomposites processed under the optimum condition of eta = 200 J/mm(3) showed the lowest wear rates accompanied by the formation of adherent and strain-hardened tribolayers on the worn surfaces of the nano composites, suggesting improved tribological performance

Effect of Laser Spot Size, Scanning Strategy, Scanning Speed, and Laser Power on Microstructure and Mechanical Behavior of 316L Stainless Steel Fabricated via Selective Laser Melting

Selective laser melting (SLM) is a promising additive manufacturing process for fabricating complex geometries of metallic parts. The SLM processing parameters can have a major effect on microstructure and mechanical behavior of the fabricated metallic parts. In this work, the effect of laser spot size, hatch spacing, energy density, scan strategy, scanning speed and laser power on the microstructure and mechanical behavior of SLM-processed 316L stainless steel samples has been studied. These samples processed with different processing parameters were characterized by performing microhardness, tensile tests, x-ray diffraction (XRD) analysis, Energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) analysis. The samples fabricated with a larger laser spot size exhibited higher tensile strength as well as higher microhardness values. A similar trend was observed for samples processed with higher laser power and hatch spacing. For the same energy density, higher laser power and lower scanning speed significantly enhance the mechanical properties of SLM processed samples compared to those fabricated with lower laser power and higher scanning speed. Therefore, it can be concluded that laser power has a more dominant role in governing the mechanical properties of SLM processed parts than scanning speed

Effect of Microstructure and Unit Cell’s Geometry on the Compressive Mechanical Response of Additively Manufactured Co-Cr-Mo Sheet I-WP Lattice

Co-Cr-Mo based sheet I-WP lattice was fabricated via laser powder bed fusion. The effect of microstructure and the I-WP shape on compressive mechanical response was investigated. Results of compression test showed that yield strength of the sheet I-WP was 176.3 MPa and that of bulk Co-Cr-Mo (reference material) was 810.4 MPa. By applying Gibson-Ashby analytical model, the yield strength of the lattice was reversely estimated from that of the bulk specimen. The calculated strength of the lattice obtained was 150.7 MPa. The shape of deformed lattice showed collective failure mode, and its microstructure showed that strain-induced martensitic transformation occurred in the overall lattice. The deformation behavior of additively manufactured sheet I-WP lattice was also discussed

Effect of Laser Mode and Power on the Tribological Behavior of Additively Manufactured Inconel 718 Alloy

The influence of laser modes and power on the tribological behavior of additively manufactured Inconel 718 alloy using the directed energy deposition (DED) process was investigated. The samples were fabricated with continuous wave (CW) and pulse wave (PW) laser modes using 700, 900, and 1100 W laser power. The samples exhibited high hardness (3–5 GPa) and modulus (150–200 GPa) which increases with the laser power for CW- and PW-fabricated samples, and this was associated with the increasing densification and hardening secondary phase. The coefficient of friction increases with laser power for the CW samples but decreases for the PW samples. The samples exhibited low wear rates ranging between 25 and 70 × 10−5 mm3/Nm. Pulse wave samples demonstrated better tribological performance compared to continuous wave at any laser power. The dominant wear mechanism is the three-body abrasive wear followed by localized and discrete adhesion wear mechanism.</p

Influence of Milling Time and Ball-to-Powder Ratio on Mechanical Behavior of FeMn30Cu5 Biodegradable Alloys Prepared by Mechanical Alloying and Hot-Forging

FeMn30Cu5 is a biodegradable and multi-component alloy that can be used to repair bone defects in load-bearing parts in the medical field. This work focuses on studying the influence of milling time and ball-to-powder ratio (BPR) on the mechanical behavior of FeMn30Cu5 alloys via mechanical alloying and hot-forging. Three different milling times (1, 5.5, and 10 h) and BPRs (5:1, 10:1, and 15:1) were used as the main independent variables. MA was performed at 300 rpm in ethanol; the synthesized powders were dried, hot-compacted at 550 MPa, and sintered under an inert atmosphere (1000 &deg;C, 15 min) using a medium-frequency induction furnace and hot-forging. The mechanical behavior in terms of Vickers hardness, compressive stress&ndash;strain curves, and percentage theoretical density was investigated. This experimental work revealed that both milling time and BPR significantly influenced the grain size reduction owing to variations in the severe plastic deformation and mechanical collisions produced by the milling medium. The hardness and ultimate strength of the FeMn30Cu5 alloy processed at 10 h and 15:1 BPR were 1788.17 &plusmn; 4.9 MPa, which was 1.5 times higher than those of the same alloy processed at 1 h and 5:1 BPR (1200.45 &plusmn; 6.5 MPa). Austenite iron (g-Fe), ferrite-iron (a-Fe), a-Mn, and a-Cu phases were observed in XRD and SEM images. The formed a-Mn and a-Cu overlapped with the g-Fe lattice because of the diffusion of Mn and Cu atoms during sintering and hot-forging. The incorporated 30 wt.% of Mn and 5 wt.% of Cu stabilize the austenite phase (good for MRI scans in medical applications), which contributed to promoting superior mechanical properties with milling time (10 h) and BPR (15:1) due to severe structural defects

Microstructural and thermal expansion behaviour of graphene reinforced 316L stainless steel matrix composite prepared via powder bed fusion additive manufacturing

316 L Stainless steel is widely used industrial material having application in broad range of domain including automotive, structural as well as biomedical. It is also used for several high temperature applications in various industries, permanent deformation due to thermal cycling and continuous high temperature exposure is one of the major concern in industries. Therefore, enticing thermo physical properties is very crucial for better efficiency and service life of components. In the present work, graphene reinforced 316 L metal matrix composite was prepared via powder bed fusion additive manufacturing technique in order to make it more stable material at elevated temperature. Powder bed fusion technique was used to melt graphene coated 316 L metal powder layer by layer to prevent agglomeration of graphene inside the matrix. To identify the chemical state of the elements in the prepared composite, synchrotron based X-ray photoelectron spectroscopy was performed. However, no carbide formation was observed inside the matrix and the prepared composite was purely austenitic. The obtained composite with 0.2 wt % graphene has coefficient of thermal expansion (CTE) equal to 19.3 × 10−6 K-1 which is about 6.2% lower than bare 316 L at 1100 °C. The value further decrease after annealing process and this much decrement in CTE can increase the application window of the material to the large extent

Influence of Milling Time and Ball-to-Powder Ratio on Mechanical Behavior of FeMn₃₀Cu₅ Biodegradable Alloys Prepared by Mechanical Alloying and Hot-Forging

FeMn30Cu5 is a biodegradable and multi-component alloy that can be used to repair bone defects in load-bearing parts in the medical field. This work focuses on studying the influence of milling time and ball-to-powder ratio (BPR) on the mechanical behavior of FeMn30Cu5 alloys via mechanical alloying and hot-forging. Three different milling times (1, 5.5, and 10 h) and BPRs (5:1, 10:1, and 15:1) were used as the main independent variables. MA was performed at 300 rpm in ethanol; the synthesized powders were dried, hot-compacted at 550 MPa, and sintered under an inert atmosphere (1000 °C, 15 min) using a medium-frequency induction furnace and hot-forging. The mechanical behavior in terms of Vickers hardness, compressive stress–strain curves, and percentage theoretical density was investigated. This experimental work revealed that both milling time and BPR significantly influenced the grain size reduction owing to variations in the severe plastic deformation and mechanical collisions produced by the milling medium. The hardness and ultimate strength of the FeMn30Cu5 alloy processed at 10 h and 15:1 BPR were 1788.17 ± 4.9 MPa, which was 1.5 times higher than those of the same alloy processed at 1 h and 5:1 BPR (1200.45 ± 6.5 MPa). Austenite iron (g-Fe), ferrite-iron (a-Fe), a-Mn, and a-Cu phases were observed in XRD and SEM images. The formed a-Mn and a-Cu overlapped with the g-Fe lattice because of the diffusion of Mn and Cu atoms during sintering and hot-forging. The incorporated 30 wt.% of Mn and 5 wt.% of Cu stabilize the austenite phase (good for MRI scans in medical applications), which contributed to promoting superior mechanical properties with milling time (10 h) and BPR (15:1) due to severe structural defects