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This is an author produced version of a paper published in :
Materials Science and Engineering: A
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Paper:
Lavery, N., Cherry, J., Mehmood, S., Davies, H., Girling, B., Sacket, E., Brown, S. & Sienz, J. (2017). Effects of hot
isostatic pressing on the elastic modulus and tensile properties of 316L parts made by powder bed laser fusion.
Materials Science and Engineering: A
http://dx.doi.org/10.1016/j.msea.2017.03.100
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Author’s Accepted Manuscript
Effects of hot isostatic pressing on the elastic
modulus and tensile properties of 316L parts made
by powder bed laser fusion
N.P. Lavery, J. Cherry, S. Mehmood, H. Davies,
B. Girling, E. Sacket, S.G.R. Brown, J. Sienz
PII: S0921-5093(17)30415-X
DOI: http://dx.doi.org/10.1016/j.msea.2017.03.100
Reference: MSA34883
To appear in:
Materials Science & Engineering A
Received date: 27 September 2016
Revised date: 24 March 2017
Accepted date: 25 March 2017
Cite this article as: N.P. Lavery, J. Cherry, S. Mehmood, H. Davies, B. Girling,
E. Sacket, S.G.R. Brown and J. Sienz, Effects of hot isostatic pressing on the
elastic modulus and tensile properties of 316L parts made by powder bed laser
f u s i o n , Materials Science & Engineering A,
http://dx.doi.org/10.1016/j.msea.2017.03.100
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1
Effects of hot isostatic pressing on the elastic modulus and tensile properties of 316L parts
made by powder bed laser fusion
N. P. Lavery
A*
, J. Cherry, S. Mehmood, H. Davies
A
, B. Girling, E. Sacket, S.G.R. Brown
A
, J. Sienz
B
A
Materials Research Centre, College of Engineering, Swansea University Bay Campus, Fabian Way,
Swansea SA1 8EP, United Kingdom
B
Zienkiewicz Centre for Computational Engineering, College of Engineering, Swansea University Bay
Campus, Fabian Way, Swansea SA1 8EP, United Kingdom
*
Corresponding author, Tel: +44(0)1792 606873. N.P.Lavery@swansea.ac.uk
Abstract
The microstructure and mechanical properties of 316L steel have been examined for parts built by a
powder bed laser fusion process, which uses a laser to melt and build parts additively on a layer by
layer basis.
Relative density and porosity determined using various experimental techniques were correlated
against laser energy density. Based on porosity sizes, morphology and distributions, the porosity was
seen to transition between an irregular, highly directional porosity at the low laser energy density
and a smaller, more rounded and randomly distributed porosity at higher laser energy density,
thought to be caused by keyhole melting. In both cases, the porosity was reduced by hot isostatic
pressing (HIP).
High throughput ultrasound based measurements were used to calculate elasticity properties and
show that the lower porosities from builds with higher energy densities have higher elasticity moduli
in accordance with empirical relationships, and hot isostatic pressing improves the elasticity
properties to levels associated with wrought/rolled 316L. However, even with hot isostatic pressing
the best properties were obtained from samples with the lowest porosity in the as-built condition.
A finite element stress analysis based on the porosity microstructures was undertaken, to
understand the effect of pore size distributions and morphology on the Young’s modulus. Over 1-5%
porosity range angular porosity was found to reduce the Young’s modulus by 5% more than rounded
porosity. Experimentally measured Young’s moduli for samples treated by HIP were closer to the
rounded trends than the as-built samples, which were closer to angular trends.
Tensile tests on specimens produced at optimised machine parameters displayed a high degree of
anisotropy in the build direction and test variability for as-built parts, especially between vertical and
horizontal build directions. The as-built properties were generally found to have a higher yield stress,
but lower upper tensile strength and elongation than published data for wrought/hot-rolled plate
316L. The hot isostatically pressed parts showed a homogenisation of the properties across build
directions and properties much more akin to those of wrought/hot-rolled 316L, with an increase in
elongation and upper tensile strength, and a reduction in yield over the as-built samples.
Keywords: Powder Bed Laser Fusion, 316L steel, Porosity, Hot Isostatic Pressing, Tensile, Ultrasound
Measurements of Elasticity, Finite Element Analysis
2
1 Introduction
Additive Layer Manufacturing (ALM) based on the melting of pre-alloyed metal powders is a
processing route which is rapidly evolving from rapid prototyping with the capability of producing
functional net-shape parts with the strength characteristics of wrought parts [1]. It is ideally suited
to low-volume production, and can be cost-competitive or cheaper than CNC machining or processes
where the capital outlay for items such as dies are high [2]. However, as with all powder-based
processes, such as sintering [3], net-shape hot isostatic pressing [4], and powder compaction [5], as
well as other net-shape manufacturing methods such as casting [6], there is an inherent porosity
associated with the process.
The literature is rich in studies reporting on specific combinations of alloys, ALM techniques and
applications. Titanium alloys, such as Ti-6Al-4V are being examined for use as critical aerospace and
biomedical applications such as orthopaedic devices, and dental implants, and are understandably
receiving a large proportion of the effort.
Typically the material/process development cycle will start by studying the links between porosity
and tensile strength, as exemplified for powder bed fusion processes, of which selective laser
melting (SLM) generally referrers to processes specifically using optical based lasers, [7], wire-feed
processes [8] and electron beam processes, [1], [9]–[11]. Common conclusions from this type of
work are that anisotropic mechanical properties occur to a varying degree, and that there are also
various levels of porosity which have a detrimental effect on ductility, accompanied by high levels of
hardness and yield strength.
The effects on mechanical properties of surface finishing, heat treatments and hot isostatic pressing
are then examined, and for Ti-6Al-4V this is done for powder bed [7], [12] and for electron beam
[11]. Due to the relatively rough surfaces of additive processes, surface finishing such as polishing or
machining can improve mechanical properties, particularly fatigue strength [4], [11], [13].
For Ti-6Al-4V, heat treatments such as aging and annealing for stress reduction are found to have
relatively small effects on mechanical properties, slightly increasing ductility and reducing
anisotropy, with some reduction in the yield strength. Generally, more aggressive heat treatments
and hot isostatic pressing give a larger reduction in sometimes both yield and upper tensile strength,
and are accompanied by an increase in ductility and a reduction in build direction anisotropy, often
associated with the adequate closure of small porosity in the case of hot isostatic pressing. This is
also the case for nickel alloys such as Inconel 718 [14], [15], although in the case of this alloy the
ductility can be reduced with heat treatment as a consequence of an acicular δ-phase migrating to
grain boundaries. Nickel alloys have also been subject to studies with intended applications in
aerospace, concentrating on the microstructural characterisation and effects on mechanical strength
of parts build by the powder bed fusion process with Inconel 718, [16] and Nimonic 273, [17], both
examining the post-modification by heat treatment of the as-built part.
Heat treatments have significantly more effect on commonly used aluminium alloys such as AlSi10
[18]–[22] and AlSi12 [23], often intended for automotive and electronic applications, and much of
the current focus of powder bed based ALM research using aluminium has been on the Al-Si casting
alloys, such as AlSi10, [19], which although possibly easier to process than high strength aerospace
Al-alloy grades due to narrower freezing ranges, still pose significant challenges when compared to
3
steels and other higher melting point alloys. High strength aluminium alloys (5XXX and 7XXX-series)
are also being considered [24] for aerospace applications for powder bed ALM, and modified
compositions such as with higher scandium content,[25] are showing acceptable porosity and
promising strength and ductility characteristics. Porosity fractions of aluminium alloys can be
reduced to less than 0.5%, certainly comparable to casting routes with fewer inclusions and defects,
however, pore sizes tend to be larger than with other ALM alloys, with overall static strength tests
showing higher tensile and fatigue strength than cast materials, [22].
Fatigue strength requires longer term tests, which tend to come later in the material/process
development cycle, and fatigue studies have been reported for Ti-6Al-4V in [20], [26], [27], steels
[13], [28] and aluminium alloys [20], [22]. Generally the findings are that while heat treatments and
hot isostatic pressing can improve fatigue strength, that mostly these still be below 60-75% of an
equivalent wrought, annealed material.
Although much work has already been done on duplex steels such as 304 and 316L on a variety of
powder bed systems, [29], [20], [30]–[38], the published data covers a wide range of preparation
routes, machine settings and laser powers, different mechanical testing methodologies and various
post-process heat treatments. Unlike higher strength H13 and maraging steels such as 18Ni-300, [39]
which are used in injection moulding tools and dies, and aeroengine applications, the lower strength
316L is widely used but does not have any one single critical application possibly explaining the wide
range of research interests. However, this poses a difficulty in setting a baseline for the required
tensile properties of 316L, as demonstrated by the limited validation in the publications comparing
ALM with other processes such as hot rolling, wrought or casting.
As pointed out in [20], the proliferation and progress of additive processes means that the
mechanical characterisation even for standard alloys struggles to keep pace with the machine
developments. This is very evident in the case of 316L steel where even recent publications are
reporting tensile properties for samples built with the previous generation of powder bed systems,
with low laser powers (85-200W) and line speeds (down to 200 mm/s), and wider ranges of porosity
(1-3%). Laser powers of 500W and line speeds of 2-3000 mm/s are the norm in the current
generation of machines, resulting in lower levels of porosity (0.1-0.5%) expected across all alloys.
The work reported herein aims to add to the body of knowledge on 316L with an in-depth
characterisation of the material for a 200W laser machine, at line speeds in the 600-1000 mm/s
range. The claim to be in-depth is based on a thorough description of the measurement
methodologies (density, porosity and tensile properties), for both as-built and hot isostatically
pressed samples, to be a baseline for future researchers. It also introduces the use of ultrasound
testing which has seen a limited amount of use in ALM materials characterisation even though it is a
more rapid method of getting elasticity properties than through tensile testing. It is thought that
ultrasound techniques can contribute to process improvement, such as by reducing directional
variations across the build plate. Another objective of this work is in understanding how porosity
distributions and morphologies change with hot isostatic pressing, and, using FE analysis derive
empirical relationships for the Young’s modulus, [40]. These empirical relationships in conjunction
with the non-destructive and fast ultrasound testing, will lead to future improvements of
equipment, lasers and powders. Looking ahead, It may also be possible to use similar empirical
