Strain rate sensitivity and mechanical anisotropy of selective laser melted 17-4 PH stainless steel

Abstract

Quasi-static and dynamic tensile testing was performed upon machined tensile specimens fabricated from bulk primitives produced by the consolidation of water atomized, 17-4 precipitation hardened stainless steel powder by layer-based, selective laser melting. Such mechanical evaluation was performed by a screw-driven uniaxial tension testing machine and a split-Hopkinson tensile bar apparatus. Strain rates evaluated include 10-3 , 10-1 and 10 3 s-1. Prior to tensile testing, specimens underwent additional thermal processing in accordance with industry standards. Evaluations of the solution heat treatment and peak-age conditions were made alongside similarly prepared, but traditionally processed specimens meeting the same material standard for chemistry (drawn rod). Tensile strength across all strain rates is higher for these selective laser melted (SLM) specimens as a result of microstructure refinement through rapid melt, solidification and cooling during processing. Ultimate tensile and yield strengths increase with increasing strain rate and show no preferential direction relative to the building direction. Elongation anisotropy is observed as a consequence of directional porosity stemming from pores limited to within individual layers. Specimens loaded normal to the SLM building plane commonly rupture at small elongation because of this mechanical fibering from the selective laser melting process.

Pitting corrosion of 17-4PH stainless steel manufactured by laser beam melting

“…In addition, LBM 17-4PH MSSs has a specific microstructure induced by the process, which leads to morphological texture through the building direction of the parts with a dendritic microstructure. All these particular microstructural features lead to a complex mechanical response with significant differences depending on the loading axis during the mechanical test and the building direction of the as-built parts [15,[19][20][21][22][23][24][25]. Heat treatments performed after the building step can also modify the microstructure; results showed that the solution annealing and ageing heat treatment commonly used for conventional 17-4PH MSS, e.g.…”

Laser Powder Bed Fusion of Precipitation-Hardened Martensitic Stainless Steels: A Review

“…Previous work on as-built PH SS AM components revealed a non-equilibrium microstructure, as well as strong differences in texture parallel and perpendicular to the build direction, due to the very high cooling rates [17,99,[104][105][106][107]115]. Some of the literature claimed that the as-built microstructure via L-PBF contains martensite and retained austenite (metastable phase at room temperature) [26,27,95,98,[119][120][121][122], completely different from that of a wrought 17-4 PH stainless steel, which is fully martensitic. However, Alnajjar et al [117] and Sun et al [123] respectively reported a fully δ-ferrite microstructure (based on Figure 2) and a dominantly ferrite microstructure ( Figure 3) with small grains at melt-pool boundaries comprising bcc martensitic laths and equiaxed fcc austenite grains.…”

Absorption of Nitrogen during Pulsed Wave L-PBF of 17-4 PH Steel

“…Although 17-4 PH is well suited for production by L-PBF [ 8 ], when produced by this method austenite phase fractions of over 50% have been measured in the as-built structure [ 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. This large amount of retained austenite leads to inconsistent heat treatment response and reduced strength compared to a traditionally processed martensitic structure [ 13 , 16 , 17 ]. The phenomenon of high retained austenite L-PBF 17-4 PH is well documented and can be primarily attributed to the absorption of austenite stabilizing nitrogen into the material from either the cover gas used in the process or the atomization gas used for powder production [ 10 , 12 ].…”