Hardened austenite steel with columnar sub-grain structure formed by laser melting

140

This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 • C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 • C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.

Section: Resultssupporting

confidence: 92%

Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion

Krakhmalev

Fredriksson

Svensson

et al. 2018

140

“…This is consistent with various literature, in which the microhardness of AM 316L SS parts is typically higher than that of conventionally manufactured 316L SS parts [28,48]. Higher microhardness in AM 316L SS samples is attributed to the fine-grain microstructures obtained in the completed parts resulting in a higher dislocation density of austenite cells [49]. This makes slip motion along the grain boundaries difficult, thus increasing its strength and resistance to deformation.…”

Section: Microhardnesssupporting

confidence: 91%

Investigation on Porosity and Microhardness of 316L Stainless Steel Fabricated by Selective Laser Melting

Yusuf

Chen

Boardman

et al. 2017

146

This study investigates the porosity and microhardness of 316L stainless steel samples fabricated by selective laser melting (SLM). The porosity content was measured using the Archimedes method and the advanced X-ray computed tomography (XCT) scan. High densification level (≥99%) with a low average porosity content (~0.82%) were obtained from the Archimedes method. The highest porosity content in the XCT-scanned sample was~0.61. However, the pores in the SLM samples for both cases (optical microscopy and XCT) were not uniformly distributed. The higher average microhardness values in the SLM samples compared to the wrought manufactured counterpart are attributed to the fine microstructures from the localised melting and rapid solidification rate of the SLM process.

“…Whereas, the x-y plane shows mainly equiaxed cellular substructures. The formation of these cellular substructures occur due to the high cooling rates during the SLM process [15] that are ranging between 10 5 and 10 8 °C/s [32,33], depending on the material and processing parameters. However, the details of formation mechanisms of various morphological features resulting from the SLM process still require further research.…”

Section: Microstructure Of the As Slm-printed Specimensmentioning

confidence: 99%

Structure, Texture and Phases in 3D Printed IN718 Alloy Subjected to Homogenization and HIP Treatments

Mostafa

Rubio

Braïlovski

et al. 2017

193

3D printing results in anisotropy in the microstructure and mechanical properties. The focus of this study is to investigate the structure, texture and phase evolution of the as-printed and heat treated IN718 superalloy. Cylindrical specimens, printed by powder-bed additive manufacturing technique, were subjected to two post-treatments: homogenization (1100 • C, 1 h, furnace cooling) and hot isostatic pressing (HIP) (1160 • C, 100 MPa, 4 h, furnace cooling). The Selective laser melting (SLM) printed microstructure exhibited a columnar architecture, parallel to the building direction, due to the heat flow towards negative z-direction. Whereas, a unique structural morphology was observed in the x-y plane due to different cooling rates resulting from laser beam overlapping. Post-processing treatments reorganized the columnar structure of a strong {002} texture into fine columnar and/or equiaxed grains of random orientations. Equiaxed structure of about 150 µm average grain size, was achieved after homogenization and HIP treatments. Both δ-phase and MC-type brittle carbides, having rough morphologies, were formed at the grain boundaries. Delta-phase formed due to γ -phase dissolution in the γ matrix, while MC-type carbides nucleates grew by diffusion of solute atoms. The presence of (Nb 0.78 Ti 0.22 )C carbide phase, with an fcc structure having a lattice parameter a = 4.43 Å, was revealed using Energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) analysis. The solidification behavior of IN718 alloy was described to elucidate the evolution of different phases during selective laser melting and post-processing heat treatments of IN718.