Deformation behavior of additively manufactured GP1 stainless steel

Abstract

In-situ neutron diffraction measurements were performed during heat-treating and uniaxial loading of additively manufactured (AM) GP1 material. Although the measured chemical composition of the GP1 powder falls within the composition specifications of 17-4 PH steel, a fully martensitic alloy in the wrought condition, the crystal structure of the as-built GP1 material is fully austenitic. Chemical analysis of the as-built material shows high oxygen and nitrogen content, which then significantly decreased after heat-treating in a vacuum furnace at 650 °C for one hour. Significant austenite-to-martensite phase transformation is observed during compressive and tensile loading of the as-built and heat-treated material with accompanied strengthening as martensite volume fraction increases. During loading, the initial average phase stress state in the martensite is hydrostatic compression independent of the loading direction. Preferred orientation transformation in austenite and applied load accommodation by variant selection in martensite are observed via measurements of the texture development. Introduction.Additive manufacturing (AM) is a rapidly developing processing pathway that produces components by selectively melting and solidifying feedstock to build a desired geometry, rather than subtractive machining from cast or wrought stock material [1,2]. A major difference between the two routes is the microstructure of the material in the final component [3]. Casting and thermo-mechanical processing to produce wrought stock material has been optimized over centuries and a wide variety of thermal and deformation processing options are commonly used to obtain a desired component microstructure, which defines performance. In contrast, there are more limited opportunities to optimize microstructures of AM materials after manufacture, primarily because components are built to near-net shape to minimize the amount of material and post-build machining needed. As a result, most bulk thermo-mechanical treatments, (e.g., rolling, forging, etc.) cannot be readily applied. Options for modifying AM component microstructures after manufacture are generally limited to surface deformation treatments (e.g., shot or laser peening) and bulk or surface thermal/chemical treatments [4]. AM processing parameters have perhaps the strongest influence on component microstructure, and significant research is being performed to establish both empirical connections between the input parameters and the achieved microstructure and properties, and physics-based models to guide AM process optimization [5].The difference in microstructures between AM and wrought components can be significant, similar to the changes observed within welded microstructures. In some cases, welding can lead to introduction of additional phases, such as the appearance of ferrite in austenitic stainless steels [6], and welding processes often result in substantial residual stresses in and around the welded region [7] due to large thermal gradients and rapid cooling. These fe...

Evolution of the Microstructure of Laser Powder Bed Fusion Ti-6Al-4V During Post-Build Heat Treatment

“…This, in turn, forces sacrifices in data quality that limits the amount of quantitative microstructural information (e.g., phase fraction, texture, internal stress) that can be gleaned from the diffraction data. [22,24,27,29] In contrast, relatively few in situ studies of the evolution of the metastable AM microstructure during post-build heat treatments are to be found in the literature [15,[30][31][32] despite the fact that the relatively slow kinetics during heat treatments often enable quantitative determination of relevant microstructural parameters. Hysteretic lattice parameter expansion was observed during in situ heat treatment of shaped metal deposited Ti64 using low energy (8 keV; penetration depth ~0.01 mm) X-rays.…”

Material-Specific Phenomena and Developing Higher Yield Process Parameters in Selective Laser Melting of 17-4 PH Stainless Steel

“…11, the phenomenon of having "double yield" in the stress-strain curve is thus attributed to the formation of strain-induced martensite and further strengthening during deformation. The number of studies showing this effect is very limited for additive manufacturing [29]. Moreover, the highest UTS/YS ratios found in literature for this material after SLM are about 2 whereas it is equal to almost 4 for this study emphasizing the importance of double yield phenomenon.…”

How Austenitic Is a Martensitic Steel Produced by Laser Powder Bed Fusion? A Cautionary Tale

“…The reported values for many alloys, however, show significant variations. An example for 17-4 PH SS, which is extracted from an incomplete set of existing data [7,[26][27][28][29][30][31][32][33], is shown in Figure 1.…”

“…The reported values for many alloys, ho ever, show significant variations. An example for 17-4 PH SS, which is extracted from incomplete set of existing data [7,[26][27][28][29][30][31][32][33], is shown in Figure 1. Figure 1 shows a wide range of the reported retained austenite fraction of as-built 4 PH, from ≈35% to nearly 100%.…”

“…Other techniques that were used to acquire these values include SEM (EBSD) and neutron diffraction (ND). While it is tempting to attribute this level of variation to the complexity intrinsic to metal additive manufacturing, it is worth noting that a number of these studies [28][29][30][31][32][33] used the same grade powders (GP1) from the same powder vendor (EOS) and the AM parts were fabricated on EOS laser-powder bed fusion systems using their specified GP1 parameter sets. While geometrical shape may affect local cooling rates and batch differences of the starting powders cannot be ruled out, a variation of phase fraction to this extent is unexpected and can make eventual part qualification and certification challenging.…”