Influence of solidification structure on austenite to martensite transformation in additively manufactured hot-work tool steels

Chou, Chia-Ying; Pettersson, Niklas; Durga, A.; Zhang, Fan; Oikonomou, Christos; Borgenstam, Annika; Odqvist, Joakim; Lindwall, Greta

doi:10.1016/j.actamat.2021.117044

Cited by 61 publications

(14 citation statements)

References 38 publications

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“…In the LPBF process with nonequilibrium rapid melting/solidification, it is common for solutes to be redistributed under the Marangoni effect (Figure 8b), resulting in elemental microsegregation. [ 40 ] However, an intermetallic brittle phase (Laves phase) could be formed in LPBF‐printed IN718 alloy due to elemental microsegregation during the solidification process, which could reduce the mechanical properties by the brittle fracture of the phase. [ 41 ] In addition, as the chemical formula of Laves phase is (Ni, Cr, Fe) 2 (Nb, Mo, Ti), the formation of Laves phase could consume elements such as Nb and Ti, reducing the number of strengthening phases (such as γ'‐Ni 3 (Ti, Al), γ″‐Ni 3 Nb) and weakening the solid‐solution strengthening effect of the IN718 alloy.…”

Section: Resultsmentioning

confidence: 99%

Effect of Scanning Strategies on the Microstructure and Mechanical Properties of Inconel 718 Alloy Fabricated by Laser Powder Bed Fusion

et al. 2022

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The scanning strategy is an essential factor that determines the thermal history and combination of melt tracks within parts in the laser powder bed fusion (LPBF) process, which can significantly influence the microstructure evolution, defect formation, and mechanical properties of parts. Herein, the effect of scanning strategies (i.e., Y‐scan, XY‐scan, Rot‐scan, and Island‐scan) on the microstructure and mechanical properties of Inconel 718 (IN718) parts manufactured by LPBF is investigated. The results show that different scanning strategies can lead to different morphologies of melt tracks, and the Rot‐scan is the most beneficial strategy for improving density of parts. Different scanning strategies change the direction of the temperature gradient within parts, resulting in different morphologies of grain growth. The tensile properties of samples built with four types of scanning strategies show superior mechanical properties compared with the cast. The ultimate tensile strength of samples manufactured by Y‐scan, XY‐scan, and Rot‐scan is equivalent to that of the wrought, while the ductility of them is higher than that of wrought. However, tensile properties of samples manufactured by island scanning are lowest due to defects (e.g., pores, spatters) at the overlap of islands.

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Section: Resultsmentioning

confidence: 99%

Effect of Scanning Strategies on the Microstructure and Mechanical Properties of Inconel 718 Alloy Fabricated by Laser Powder Bed Fusion

et al. 2022

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“…It is assumed, that the primary crystallization of δ-ferrite is not suppressed by the rapid cooling rates present in the PBF-LB/M process for the investigated tool steel [ 7 ]. Although it has to be mentioned, that Chou et al suggested a shift to a primary crystallization of fcc austenite for an H13 hot work tool steel based on calculations of austenite and ferrite dendrite tip temperatures at rapid cooling rates [ 39 ]. The performed Scheil–Gulliver solidification simulation is shown in Figure 13 , wherein the formed phases are plotted against the temperature.…”

Section: Resultsmentioning

confidence: 99%

Processability of a Hot Work Tool Steel Powder Mixture in Laser-Based Powder Bed Fusion

et al. 2022

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Powder bed fusion of metals using a laser beam system (PBF-LB/M) of highly complex and filigree parts made of tool steels is becoming more important for many industrial applications and scientific investigations. To achieve high density and sufficient chemical homogeneity, pre-alloyed gas-atomized spherical powder feedstock is used. For high-performance materials such as tool steels, the number of commercially available starting powders is limited due to the susceptibility to crack formation in carbon-bearing steels. Furthermore, scientific alloy development in combination with gas-atomization is a cost-intensive process which requires high experimental effort. To overcome these drawbacks, this investigation describes the adaption of a hot work tool steel for crack-free PBF-LB/M-fabrication without any preheating as well as an alternative alloying strategy which implies the individual admixing of low-cost aspherical elemental powders and ferroalloy particles with gas-atomized pure iron powder. It is shown that the PBF-LB/M-fabrication of this powder mixture is technically feasible, even though the partly irregular-shaped powder particles reduce the flowability and the laser reflectance compared to a gas-atomized reference powder. Moreover, some high-melting alloying ingredients of the admixed powder remain unmolten within the microstructure. To analyze the laser energy input in detail, the second part of the investigation focuses on the characterization of the individual laser light reflectance of the admixed alloy, the gas-atomized reference powder and the individual alloying elements and ferroalloys.

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“…Interestingly, this prediction is consistent with previous observations by Cheruvathur et al [28] in as-built 17-4, where they reported that Fe and Cr are enriched in the dendritic cores. Because these elements have a different solubility in the FCC austenite phase and BCC ferrite phase, this suggests that high temperature δ ferrite may remain upon solidification, through a solidification-velocity dependent competition between austenite and δ ferrite [53]. Recent synchrotron-based high-speed X-ray diffraction experiments that follow the solidification phase transformation sequence from melt to room temperature convincingly demonstrate that the high-temperature δ ferrite, once formed, can sustain to room temperature, albeit at a reduced phase fraction at room temperature than at solidus [54].…”

Section: Density (G/cmmentioning

confidence: 99%

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

Zhang

Stoudt

Hammadi

et al. 2021

Metals

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Accurate phase fraction analysis is an essential element of the microstructural characterization of alloys and often serves as a basis to quantify effects such as heat treatment or mechanical deformation. Additive manufacturing (AM) of metals, due to the intrinsic nonequilibrium solidification and spatial variability, creates additional challenges for the proper quantification of phase fraction. Such challenges are exacerbated when the alloy itself is prone to deformation-induced phase transformation. Using commonly available in-house X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) and less commonly used synchrotron-based high-energy X-ray diffraction, we characterized nitrogen-atomized 17-4 precipitation-hardening martensitic stainless steel, a class of AM alloy that has received broad attention within the AM research community. On the same build, our measurements recovered the entire range of reported values on the austenite phase fractions of as-built AM 17-4 in literature, from ≈100% martensite to ≈100% austenite. Aided by Calphad simulation, our experimental findings established that our as-built AM 17-4 is almost fully austenitic and that in-house XRD and EBSD measurements are subject to significant uncertainties created by the specimen’s surface finish. Hence, measurements made using these techniques must be understood in their correct context. Our results carry significant implications, not only to AM 17-4 but also to AM alloys that are susceptible to deformation-induced structure transformation and suggest that characterizations with less accessible but bulk sensitive techniques such as synchrotron-based high energy X-ray diffraction or neutron diffraction may be required for proper understanding of these materials.

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Influence of solidification structure on austenite to martensite transformation in additively manufactured hot-work tool steels

Cited by 61 publications

References 38 publications

Effect of Scanning Strategies on the Microstructure and Mechanical Properties of Inconel 718 Alloy Fabricated by Laser Powder Bed Fusion

Effect of Scanning Strategies on the Microstructure and Mechanical Properties of Inconel 718 Alloy Fabricated by Laser Powder Bed Fusion

Processability of a Hot Work Tool Steel Powder Mixture in Laser-Based Powder Bed Fusion

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

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