An adage within the Additive Manufacturing (AM) community is that-complexity is free‖. Complicated geometric features that normally drive manufacturing cost and limit design options are not typically problematic in AM. While geometric complexity is usually viewed from the perspective of part design, this advantage of AM also opens up new options in rapid, efficient material property evaluation and qualification. In the current work, an array of 100 miniature tensile bars are produced and tested for a comparable cost and in comparable time to a few conventional tensile bars. With this technique, it is possible to evaluate the stochastic nature of mechanical behavior. The current study focuses on stochastic yield strength, ultimate strength, and ductility as measured by strain at failure (elongation). However, this method can be used to capture the statistical nature of many mechanical properties including the full stress-strain constitutive response, elastic modulus, work hardening, and fracture toughness. Moreover, the technique could extend to strain-rate and temperature dependent behavior. As a proof of concept, the technique is demonstrated on a precipitation hardened stainless steel alloy, commonly known as 17-4PH, produced by two commercial AM vendors using a laser powder bed fusion process, also commonly known as selective laser melting. Using two different commercial powder bed platforms, the vendors produced material that exhibited slightly lower strength and markedly lower ductility compared to wrought sheet. Moreover, the properties were much less repeatable in the AM materials as analyzed in the context of a Weibull distribution, and the properties did not consistently meet minimum allowable requirements for the alloy as established by AMS. The diminished, stochastic properties were examined in the context of major contributing factors such as surface roughness and internal lack-of-fusion porosity. This high-throughput capability is expected to be useful for follow-on extensive parametric studies of factors that affect the statistical reliability of AM components.
The corrosion behavior of selective laser melted (SLM) 304L was investigated and compared to conventional wrought 304L in aqueous chloride and acidic solutions. Through immersed electrochemical testing and exposure in acidic solutions, the SLM 304L exhibited superior pitting resistance in the polished state compared to wrought 304L. However, the surface condition of the SLM material had a great impact on its corrosion resistance, with the grit-blasted condition exhibiting severely diminished pitting resistance. Local scale, capillary micro-electrochemical and scanning electrochemical microscopy investigations, identified porosity as a contributing factor to decreased corrosion resistance. Preferential corrosion attack was not observed to be related to the characteristic underlying cellular microstructure produced through SLM processing. This study highlights the effects of SLM microstructural features on corrosion resistance, specifically the substantial influence of surface finish on SLM corrosion behavior and the need for development and optimization of processing techniques to improve surface finish. Powder bed selective laser melting (SLM) has become a desirable and widely used technique for the additive manufacturing (AM) of metal parts. While a significant amount of research has been carried out on the mechanical properties of SLM materials, little is known regarding their corrosion behavior. Of the few corrosion studies that do exist, the primary focus has been on evaluating general corrosion resistance.1,2 A few recent investigations have examined the role of processing and microstructure on the material corrosion behavior in more detail.3,4 However, there is still a need for further investigation in order to develop a full understanding of the microstructural and morphological characteristics inherent to AM materials due to the unique processing conditions and their relative contributions to the materials' corrosion behavior. Ideally, a ranking of the deleterious or advantageous properties formed in AM materials with respect to corrosion should be established. This is necessary for a variety of alloy systems in engineering relevant solutions and environments to help inform and guide future processing parameters, build designs, and materials selection.The powder bed process along with extremely high cooling rates and temperature gradients during SLM processing create microstructures and part surfaces that differ greatly from their conventional thermo-mechanically processed counterparts. Several characteristics are expected or have known effects on corrosion behavior including surface finish, porosity, inclusions (MnS, oxides, etc.), and microstructures formed through non-equilibrium cooling conditions. For example, a strong relationship between surface roughness and corrosion susceptibility has been observed in which pit initiation decreases with decreasing surface roughness in chloride solutions.5-7 Porosity, formed by entrapped gas or lack of fusion of powder particles in the SLM build, can also reduce...
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