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Effect of the build orientation on the mechanical properties and fracture modes of SLM
Ti-6Al-4V
M. Simonelli
a
, Y.Y. Tse
a
, C. Tuck
b
a
Department of Materials, Loughborough University, Loughborough, LE11 3TU, United
Kingdom
b
Additive Manufacturing and 3D Printing Research Group, Faculty of Engineering, The
University of Nottingham, Nottingham, NG7 2RD, United Kingdom
Corresponding author: Marco Simonelli,
email. M.Simonelli@lboro.ac.uk,
Tel. 00447905512054
Abstract
Recent research on the additive manufacturing (AM) of Ti alloys has shown that the
mechanical properties of the parts are affected by the characteristic microstructure that
originates from the AM process. To understand the effect of the microstructure on the tensile
properties, selective laser melted (SLM) Ti-6Al-4V samples built in three different
orientations were tensile tested. The investigated samples were near fully dense, in two
distinct conditions, as-built and stress relieved respectively. It was found that the build
orientation affects the tensile properties, and in particular the ductility of the samples. The
mechanical anisotropy of the parts was discussed in relation to the crystallographic texture,
phase composition and the predominant fracture mechanisms. Fractography and electron
backscatter diffraction (EBSD) results indicate that the predominant fracture mechanism is
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intergranular fracture along the grain boundaries present and thus provide and explanation
for the typical fracture surface features observed in fracture AM Ti-6Al-4V.
1 Introduction
Recent studies have been shown that additive manufacturing (AM) technologies could
represent sustainable production routes for metals and, in particular, precious metals such as
titanium alloys [1, 2]. There are several advantages related to the production of titanium
components by AM. Firstly, components are produced in their near net shape and therefore
AM allows significant reductions in raw material consumption when compared to traditional
processing. Furthermore, additive manufactured parts require no or little machining before
being put in use, regardless of their shape complexity. Therefore, AM has shown to lower the
overall manufacturing costs and carbon emissions [3, 4]. Design freedom is the most
appealing aspect of AM, especially for the medical and aerospace areas where titanium alloys
have been extensively used [5, 6]. In particular, AM has shown the possibility to fabricate
patient specific medical devices (e.g. artificial joint replacements) [7-9] and parts with
optimised topology and lattice structures that could replace heavier counterparts currently
used in aircrafts [10-13].
With careful choice of process parameters, selective laser melting (SLM) of titanium alloys
have shown the possibility to fabricate near fully dense parts [14]. Parts with mechanical
properties comparable to those of the conventionally manufactured titanium alloys have also
been reported [15, 16]. As the process stability has shown to be crucial for the production of
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near fully dense products, most of the studies of CP-Ti and α/β Ti alloys manufactured by
SLM have focussed primarily on the densification mechanisms during the fabrication process
[17-20]. Indeed, it has been reported that by combining suitable scan strategy and high laser
energy density, it is possible to reduce significantly the volume fraction of defects (i.e. pores)
and thus produce near fully dense components [14, 17]. Microscopy studies have
demonstrated that due to the high cooling rates intrinsic to the SLM process, the
microstructure of the as-built components consists entirely of αʹ martensitic phase. The αʹ
grains are contained within elongated prior-β grains that grow epitaxially through successive
layer depositions [21-23]. It was observed that this microstructure might not be suitable for
most of the current applications of α/β Ti alloys as it is typically associated with high
strength but poor ductility and possess a certain degree of anisotropy [21, 22, 24]. In order to
achieve a better balance between strength and ductility, several SLM post-processing
treatments have been introduced, leading to mechanical properties that are much closer to
those of conventionally manufactured titanium alloys. However these researches have been
conducted on one build orientation only [16, 17].
Part quality and mechanical properties (in particular ductility) are however often inconsistent
when different SLM systems are used, thus, extensive process development and material
testing are still required for the potential establishment of SLM as an alternative
manufacturing route of titanium alloys [23-25]. In addition, limited work has been done to
show the correlation between the microstructure and the crystallographic texture with the
mechanical properties of SLM Ti-6Al-4V. It has been postulated that the residual stresses and
the martensitic microstructure of the as-built parts cause low ductility in the SLM Ti-6Al-4V
parts [15], whereas the defects are generally considered to be the main contributor to the
anisotropic behaviour of the components [17, 21]. Unfortunately, the contribution of the
elongated prior-β grains and the αʹ crystallographic texture to the fracture mechanisms in
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SLM Ti-6Al-4V has not yet been illustrated systematically. In addition, most of the available
research has focussed only on samples built perpendicular to the build direction and therefore
the effect of the building orientation on the mechanical properties remains unclear.
In this study, the mechanical properties of the as-built and stress relieved SLM Ti-6Al-4V
samples fabricated in three orthogonal orientations were discussed. By doing so, it was
possible to investigate the effect of the building orientation on the mechanical properties and
fracture mechanism of the components. This research work also aimed at studying how the
stress relieving process modifies the microstructure of SLM Ti-6Al-4V alloys and therefore
the tensile properties.
2 Materials and Methods
All the samples tested in this research were built using a Renishaw AM250. The starting
powder material is a plasma atomised Ti-6Al-4V provided by LPW Technologies Ltd. The
powders are spherical, fully dense and consist entirely of αʹ phase. The detailed
characterisation was reported elsewhere [26]. A series of experiments was conducted to
establish the process window that could lead to a production of near fully dense components.
The optimised process parameters and details on the fibre laser that were used in this research
are listed in Table 1. The Renishaw AM250 is equipped with a modulated 200W ytterbium
fibre laser. In the present research the laser had a spot size of approximately 150 μm. The
speed of the laser was controlled specifying the point distance, i.e. the distance between two
successive points in a straight line, and the exposure time, i.e. the duration of time during
which the laser dwells on each point. All the samples were built in a protective argon
atmosphere and on top of secondary supporting structures. The secondary supporting
structures were used for an easier detachment from the build platform once that the
production of the parts was completed. The supporting structures were generated
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automatically using the software Magics (Materialise, Belgium). The height of the supports
was 3 mm, while the nominal thickness of the individual teeth was 0.2 mm.
The laser scan strategy that was used to melt each layer of powder is reported in a related
study [27]. Each layer was scanned once. The laser scanned initially the edge area of each
cross section and then the inside area with parallel alternating scan vectors at an angle of 67°
to the previous deposited layer. The laser then scanned the boarder of each cross sections in
order to improve the surface roughness of the part and reduce the number of defects near the
surface of the component [5].
It is noteworthy that the tensile bars studied in this research were built directly in their near-
net shape and not cut from a block of SLM material as shown in Fig. 1. Two batches with 12
tensile bars were built in total. In accordance with the ASTM F2921, where the orientation of
the built part is described listing the axes of the AM machine that are parallel to the longest
and second longest dimensions of the part, each batch contained four tensile bars of vertical
zx-, edge xz- and flat xy-orientations (Fig. 1). Excluding the supporting structures, the vertical
zx-bars consisted of 2000 layers whereas the edge xz- and flat xy-oriented bars consisted of
200 and 60 layers respectively. The tensile bars where built with a gauge thickness, width and
length of 3mm, 6mm and 35 mm respectively. A batch of tensile bars was heat treated at
730°C for 2h in a N
2
protective atmosphere to relieve all the residual stresses that are reported
to occur in the SLM parts [23]. Once the stress relieving was completed, the tensile bars were
furnace cooled to room temperature at a cooling rate of 10°C/min (approximately ten times
faster than typical furnace cooling rate). The tensile bars were then mechanically polished
before the tensile test. This batch of tensile bars will be referred to as “stress relieved
condition” in the following sections. The tensile bars which were not stress relieved were also
mechanically polished to the same surface finish prior to the tensile test. These tensile bars
![Table 2 Surface Roughness Ra [µm]](/figures/table-2-surface-roughness-ra-um-y5wuuzi3.webp)
