Mechanical Properties of Austenitic Stainless Steel Made by Additive Manufacturing

Abstract

Using uniaxial tensile and hardness testing, we evaluated the variability and anisotropy of the mechanical properties of an austenitic stainless steel, UNS S17400, manufactured by an additive process, selective laser melting. Like wrought materials, the mechanical properties depend on the orientation introduced by the processing. The recommended stress-relief heat treatment increases the tensile strength, reduces the yield strength, and decreases the extent of the discontinuous yielding. The mechanical properties, assessed by hardness, are very uniform across the build plate, but the stress-relief heat treatment introduced a small non-uniformity that had no correlation to position on the build plate. Analysis of the mechanical property behavior resulted in four conclusions. (1) The within-build and build-to-build tensile properties of the UNS S17400 stainless steel are less repeatable than mature engineering structural alloys, but similar to other structural alloys made by additive manufacturing. (2) The anisotropy of the mechanical properties of the UNS S17400 material of this study is larger than that of mature structural alloys, but is similar to other structural alloys made by additive manufacturing. (3) The tensile mechanical properties of the UNS S17400 material fabricated by selective laser melting are very different from those of wrought, heat-treated 17-4PH stainless steel. (4) The large discontinuous yielding strain in all tests resulted from the formation and propagation of Lüders bands.

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“…The same standards have been referred for testing at room and elevated temperature; namely, tensile testing at 650°C has been conducted, being this the maximum operating temperature for IN718 AM-made parts under load, as per material data sheet [32]. Investigation on near-net-shape AM-made tensile specimens, with no effort to machine or grind the resulting surface, has been reported in the literature in case of flat specimens [22]. In this experimental campaign instead, an investigation on as-built parts is not feasible, given a mean arithmetic roughness ranging from 5 to 20 μm over as-built surfaces, depending on the sloping angle with the building direction.…”

“…As regarding LPBF, anisotropy has been discussed in the literature for stainless steel [21,22], titanium [23], and Ni-based alloys [24,25]; anisotropy has also been reported when AM is conducted by means of electron beam [26]. This issue is expected to be crucial for Ni-based superalloys which are highly anisotropic and are conveniently processed to the purpose of producing functionally graded components [27], indeed.…”

“…Depending on the base metal [21,22,24], normalized strength in LPBF has been found to range from 0.8 to 1.1. The position of building in the working area has even been suggested as possible additional reason of anisotropy [22], although this effect is thought to be negligible when focusing and deflecting are driven by means of an F-theta lenses providing uniform irradiance and approaching speed over the working surface [28].…”

“…Interestingly, higher strength compared to the casted counterpart has been reported for IN738LC parts, whilst less significant differences have been shown at elevated temperature [25]. An approach to measure the degree of AMinduced anisotropy as a function of the direction of building has been proposed in the literature [22]: the normalized yield or ultimate tensile strengths have been evaluated with respect to their counterpart as resulting from an in-plane (i.e., flatbuilt) specimen. Depending on the base metal [21,22,24], normalized strength in LPBF has been found to range from 0.8 to 1.1.…”

“…An approach to measure the degree of AMinduced anisotropy as a function of the direction of building has been proposed in the literature [22]: the normalized yield or ultimate tensile strengths have been evaluated with respect to their counterpart as resulting from an in-plane (i.e., flatbuilt) specimen. Depending on the base metal [21,22,24], normalized strength in LPBF has been found to range from 0.8 to 1.1. The position of building in the working area has even been suggested as possible additional reason of anisotropy [22], although this effect is thought to be negligible when focusing and deflecting are driven by means of an F-theta lenses providing uniform irradiance and approaching speed over the working surface [28].…”