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Effect of Substrate Bed Temperature on Solute Segregation and Mechanical Properties in Ti–6Al–4V Produced by Laser Powder Bed Fusion

Published version
Peer-reviewed

Repository DOI


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Authors

Pedrazzini, S 
Pek, ME 
Ackerman, AK 
Cheng, Q 
Ali, H 

Abstract

jats:titleAbstract</jats:title>jats:pTitanium alloys are particularly sensitive to temperature during additive manufacturing processes, due to their dual phase microstructure and sensitivity to oxygen uptake. In this paper, laser powder bed fusion (LPBF) was used in conjunction with a heated substrate bed at 100 °C, 570 °C and 770 °C to produce specimens of Ti–6Al–4V, to investigate the change in mechanical properties and segregation of alloying elements. An initial increase in ductility was observed when increasing the temperature from 100 °C to 570 °C, followed by a significant loss in ductility when samples were produced at 770 °C. A suite of multi-scale characterisation techniques revealed that the as-printed microstructure was drastically different across the range of temperatures. At 100 °C, jats:italicα</jats:italic> + jats:italicα</jats:italic>′ phases were identified. Deformation twinning was extensively observed in the a phase, with Al and V segregating at the twin interfaces. At 570 °C (the most ductile sample), jats:italicα</jats:italic>′, jats:italicα</jats:italic> and nano-particles of jats:italicβ</jats:italic> were observed, with networks of entangled dislocations showing V segregation. At 770 °C, no martensitic jats:italicα</jats:italic>′ was identified. The microstructure was an jats:italicα</jats:italic> + jats:italicβ</jats:italic> microstructure and an increased volume fraction of tangled dislocations with localised V segregation. Thermodynamic modelling based on the Gibbs-free energy of formation showed that the increased V concentration at dislocations was insufficient to locally nucleate jats:italicβ</jats:italic> phase. However, b-phase nucleation at grain boundaries (not dislocations) caused pinning of grain boundaries, impeding slip and leading to a reduction in ductility. It is likely that the increased O-content within specimens printed at increased temperatures also played a key role in high-temperature embrittlement. Building operations are therefore best performed below sub-transus temperatures, to encourage the growth of strengthening phases jats:italicvia</jats:italic> solute segregation, and the build atmosphere must be tightly controlled to reduce oxygen uptake within the samples.</jats:p>

Description

Keywords

4014 Manufacturing Engineering, 40 Engineering

Journal Title

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Conference Name

Journal ISSN

1073-5623
1543-1940

Volume Title

54

Publisher

Springer Science and Business Media LLC