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Effect of heat treatment oncobalt‑chromium‑molybdenum alloy fabricatedby selective laser melting

Sing, Swee Leong; Yeong, Wai Yee

2018

Sing, S. L., & Yeong, W. Y. (2018). Effect of heat treatment oncobalt‑chromium‑molybdenum alloy fabricated by selective laser melting. Proceedings ofthe 3rd International Conference on Progress in Additive Manufacturing (Pro‑AM 2018),458‑463. doi:10.25341/D4NP43

https://hdl.handle.net/10356/88726

https://doi.org/10.25341/D4NP43

© 2018 Nanyang Technological University. Published by Nanyang Technological University,Singapore.

Downloaded on 01 Jun 2021 15:59:06 SGT

ABSTRACT: Cobalt-chromium-molybdenum (CoCrMo) alloys have excellent corrosion resistance, biocompatibility and strength. As such, they have been widely used in dental applications, such as removable partial dentures, metal frames, customized abutments, crowns and bridges in the anterior and posterior regions. The capability of selective laser melting (SLM), a powder bed fusion additive manufacturing process, to produce customizable products have led to extensive research of using this process for applications in the biomedical field. Several works have been reported on the production of CoCrMo alloys using SLM for various implants. However, there is still limited information on the effect of post process heat treatment on SLM CoCrMo alloys. In this paper, ASTM F75 CoCrMo alloy is fabricated using SLM and underwent heat treatment originally designed for casted ASTM F75 CoCrMo alloy. The mechanical properties of heat treated SLM produced CoCrMo parts are reported and benchmarked against their casted counterparts.

KEYWORDS: Additive manufacturing, 3D printing, Cobalt-chromium-molybdenum, Selective laser melting, Mechanical properties

INTRODUCTION

Cobalt-chromium-molybdenum (CoCrMo) alloys have excellent corrosion resistance, biocompatibility and strength (Qian et al., 2015). As such, they have been widely used in dental applications, such as removable partial dentures, metal frames, customized abutments, crowns and bridges in the anterior and posterior regions (Takaichi et al., 2013, Hedberg et al., 2014). Selective laser melting (SLM) is a powder bed fusion additive manufacturing, also commonly known as 3D printing, technique that uses a laser power source to fuse powder materials to form functional parts directly based on computer aided design (CAD) file. The details of the SLM process have been described in various works (Salmi and Atzeni, 2017, Sing et al., 2017).

The capability of SLM to produce customizable products has led to extensive research of using this process for applications in the biomedical field (Sing et al., 2017, Long et al., 2018, Sing et al., 2018). Several works have been reported on the production of CoCrMo alloys using SLM for various implants (Takaichi et al., 2013, Hedberg et al., 2014, Yager et al., 2015, Liverani et al., 2016, Song et al., 2016). A. Takaichi et al. investigated the mechanical properties and metal elution of CoCrMo alloy fabricated by SLM. It is reported that the as-built SLM parts have yield strength, ultimate tensile strength and elongation that were high than those of the as-cast alloy and satisfied the type 5 criteria in ISO 22764. Furthermore, the metal elution from the SLM built parts

EFFECT OF HEAT TREATMENT ON COBALT-CHROMIUM-MOLYBDENUM ALLOY FABRICATED BY SELECTIVE LASER

MELTING

SWEE LEONG SING Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang

Technological University, N3.1-B2C-03, 50 Nanyang Avenue Singapore 639798, Singapore

WAI YEE YEONG Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang

Technological University, N3.1-B2C-03, 50 Nanyang Avenue Singapore 639798, Singapore

458

Proc. Of the 3rd Intl. Conf. on Progress in Additive Manufacturing (Pro-AM 2018) Edited by Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng TorCopyright © 2018 by Nanyang Technological UniversityPublished by Nanyang Technological University ISSN: 2424-8967 :: https://doi.org/10.25341/D4NP43

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Table 1 SLM process parameters for CoCrMo Process parameters

Laser power (W) 360 Laser scan speed (mm/s) 500 Layer thickness (μm) 50 Hatch spacing (mm) 0.175

Heat Treatment

The heat treatment cycles were designed based on results obtained by previous research, which provided maximum improvement in mechanical properties of cast CoCrMo alloys (Montero-Ocampo et al., 1999, Bedolla-Gil and Hernandez-Rodriguez, 2013). The heat treatment cycles are summarized in Table 2.

Table 2 Designed heat treatment cycles for CoCrMo Heat treatment Duration (hours)

A B C Preheating (815 oC) 4 Solution treatment (1220 oC)

1 2 4

Mechanical Characterization

Tensile coupons, based on ASTM E8, were produced using wire-cut discharge machining (EDM, TROOP TP-50, J-Tech Machinery Trading Pte. Ltd., Singapore) from blocks fabricated by SLM. An Instron Static Tester Series 5569, 50 kN machine was used and a strain rate of 1 mm/min was applied on all tensile samples. Tensile test loading direction was perpendicular to the build direction of the samples. The microhardness test of the material was carried out using FM-300e Vickers microhardness tester (Future-Tech Corp., Japan) on the xy- and yz- planes (ISO/ASTM 52900:2015 Additive manufacturing – General principles – Terminology) where a load of 1 kg and a loading time of 15 s were used.

RESULTS AND DISCUSSIONS

The tensile properties of the specimens before and after heat treatment, in comparison with ASTM F75 requirements for cast CoCrMo alloys, are tabulated in Table 3.

Table 3 Tensile properties of SLM ASTM F75 CoCrMo Specimen Condition 0.2% Yield Stress (MPa) UTS (MPa)

SLM as-built 900 ± 21 1070 ± 24 A 451 ± 20 633 ± 81 B 415 ± 16 533 ± 39 C 410 ± 12 534 ± 7 ASTM F75 450 655

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