The austenitic stainless steel 316L was additively manufactured using Selective Laser Melting (SLM). The corrosion characteristics of the additively manufactured (3D printed) specimens were investigated by both potentiodynamic and potentiostatic techniques. The production parameters were deliberately varied during SLM, to produce 316L specimens fabricated by different laser scan speed (in the range of 860-1160 mm/s) and laser power (in the range of 165-285 W). The fabrication parameters were found to influence the porosity of the resulting specimens. The pitting potentials, metastable pitting rates and repassivation potentials of the 3D printed specimens are presented herein as a function of the laser scan speed and laser power, and also discussed in the context of specimen porosity. The corrosion characteristics of the 3D printed 316L were also compared with wrought 316L, revealing higher pitting potentials and lower rates of metastable pitting for most SLM 316L specimens, the related concepts of which are discussed herein. Stainless steels are a critically important class of alloy in several industries.1 The corrosion resistance of stainless steel (SS) is attributed to the presence of alloyed chromium (> ∼11 wt%), enabling the formation of a chromium oxide (Cr 2 O 3 ) based passive film upon the metal surface.1-3 The addition of elements such as nickel, nitrogen, molybdenum, carbon, aluminum, copper, sulfur and selenium can modify the corrosion resistance, strength, ductility, machinability, and the phases present (and their stability) in stainless steels.1-3 The types of stainless steel are most conveniently categorized according to their microstructure, classified as: austenitic, martensitic, ferritic and austenoferritic (duplex). Such structures are realized by specific alloying additions and metallurgical processing.1-4 Additive manufacturing has recently been explored as a means to produce SS components in net shape, 5-7 circumventing the requirement of traditional manufacturing methods such as casting, rolling, welding, machining, forging, etc. Selective Laser Melting (SLM) is one such additive manufacturing method, which can produce dense products by laser processing of metal powders. In SLM, metal powder layers are successively fused in a layer-by-layer manner into the requisite 3D structure, employing a fiber laser. [8][9][10][11] The metallic component in essence is therefore 3D printed into the requisite shape, by additive manufacturing processes like SLM. The process takes place in chamber of well-controlled inert atmosphere (either nitrogen or argon). The primary parameters that govern the microstructure of a 3D printed specimen are the laser power and the laser scan speed, as they influence the thermal gradients and growth rate of the metal at the solid-liquid interface (in the melt pool). 12 The porosity that may develop in 3D printed specimens depends upon the heating and cooling rates of the melt pool. 13 The laser scan speed relates to the duration for which the laser beam is in contact with ...

Abstract:Functionally graded lattice structures produced by additive manufacturing are promising for bone tissue engineering. Spatial variations in their porosity are reported to vary the stiffness and make it comparable to cortical or trabecular bone. However, the interplay between the mechanical properties and biological response of functionally graded lattices is less clear. Here we show that by designing continuous gradient structures and studying their mechanical and biological properties simultaneously, orthopedic implant design can be improved and guidelines can be established. Our continuous gradient structures were generated by gradually changing the strut diameter of a body centered cubic (BCC) unit cell. This approach enables a smooth transition between unit cell layers and minimizes the effect of stress discontinuity within the scaffold. Scaffolds were fabricated using selective laser melting (SLM) and underwent mechanical and in vitro biological testing. Our results indicate that optimal gradient structures should possess small pores in their core (~900 µm) to increase their mechanical strength whilst large pores (~1100 µm) should be utilized in their outer surface to enhance cell penetration and proliferation. We suggest this approach could be widely used in the design of orthopedic implants to maximize both the mechanical and biological properties of the implant.

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