1. Introduction
Additive Manufacturing (AM) techniques allow the production of objects with complex geometry. Fabrication can be started straightforwardly by using a three-dimensional Computer Aided Design (CAD) model, without tools. The basic idea is to think of every object as consisting of thin layers, usually in the range of 0.03–0.05 mm. The part is built up by progressive addition of material, which enables unprecedented ease of manufacturing of extremely complex shapes, since the three-dimensional manufacturing issues are simplified to two-dimensional problems [
1,
2]. It becomes, thus, possible to produce parts with cavities and undercuts that, by conventional subtractive methods, would have been unfeasible or would have caused great manufacturing hurdles and costs. AM technologies were introduced in industry in the late 1980s to realize models and prototypes, but nowadays the advances in materials and technology are sufficient to make the production of end products of major interest [
3,
4]. The great potential and good evolution of techniques led to introduce AM in medicine, where the need for parts that are customized for each patient, with a high degree of personalization, allows full exploitation of the inherent benefits of additive processes [
5]. In particular, the Laser-based Powder Bed Fusion (L-PBF) process can be successfully used for the production of prostheses [
6], including for example long-span and cantilever metal-ceramic-fixed partial dentures for maxillary and mandibular prosthodontics [
7,
8]. Apart from restorations, also surgical guides aimed at operation planning can take advantage of the quick production by AM [
6,
9].
This paper focuses on the characterization of alloys developed on purpose for maxillo-facial surgery, in particular for oral implants. Implantology is a field under continuous innovation, where research efforts are concurrently dedicated to finding new materials, new components, new fabrication processes, with the aim to improve the duration, the aesthetics and the functionality of prostheses and thus ensuring a better quality of life for patients. An easy example is that of gold crowns that have been superseded by metal-ceramic or ceramic crowns [
10,
11,
12]. An increasing variety of metallic biomaterials is being developed, ranging from commercially pure Titanium [
13] and Titanium alloys, through stainless steels, to Cobalt–Chrome alloys. Promising results have recently been attained for innovative β-type Ti alloys with increased wear resistance and lower elastic modulus, so as to better match that of the human bone and prevent the stress shielding effect [
14]. Some of these novel alloys exhibit composite microstructures where β–Ti dendrites are surrounded by intermetallic phases so that efficient reinforcing phenomena are established [
15]. The promising outlook of these Titanium-based matrix composites for AM has been very recently reviewed [
16].
L-PBF process can be used in the construction of metal-ceramic fixed partial dentures (FPDs). Traditionally, the manufacture of the metallic part of FPDs involves a large series of manual operations performed by the dental technician, and the result is often strongly determined by his ability. For metal-prostheses fabrication, the adoption of L-PBF grants a much higher repeatability and predictability with respect to the manual process [
17]. Compared to other powder-based methods that require molds, PBF offers outstanding personalization capabilities, in extremely short times and with low costs. On the other hand, the L-PBF process is quite complex and many factors are involved in order to achieve good part quality [
18]. Despite the extremely diffused studies on this process, many efforts are still needed to better understand the relation between microstructure, processing, and properties for parts built by L-PBF [
19,
20,
21].
In L-PBF, at each step, a thin layer of metallic powder is evenly distributed onto the previous layer and a laser selectively scans the regions corresponding to the cross-section of the part. As a consequence, the powder melts and then consolidates into a solid slice. Inherent in the process are two types of possible anisotropy: a primary one, due to the superimposition of layers in the direction that is usually called Z; and a secondary one that may manifest as direction-dependence of properties even within the XY plane, that is to say parallel to the layers [
22]. The latter is usually ignored by machine- and material suppliers, and has been disregarded by scientific literature until now. Secondary anisotropy may be caused for example by the action of the recoater blade that spreads the powder in the bed, or by the inert gas flux that blows the melting slags away from the build area [
23]. Each of the two phenomena usually acts along either the X or the Y direction, depending on the specific machine architecture. The investigation of secondary anisotropy is markedly important if the intended application is the production of FPDs, because complete prostheses are not straight structures but develop along the maxillary/mandibular arch, hence they involve material properties in several directions of the XY plane. A robust design of the restoration requires a reliable knowledge of any direction-dependent feature.
This research tackles the mechanical properties and the microstructure of two L-PBF fabricated dental alloys, namely Co–Cr–Mo and Ti–6Al–4V, by proposing a statistically-based enquiry of secondary anisotropy.
2. Materials and Methods
Tensile specimens were produced by L-PBF using the two alloys Ti–6Al–4V (EOS GmbH, Krailling, Germany) and Co–Cr–Mo (EOS Cobalt Chrome MP1, EOS GmbH, Krailling, Germany).
The specimens were fabricated on the L-PBF machine EOSINT-M270, by using the following process parameters:
for Ti–6Al–4V: laser power 340 W, laser spot diameter 0.1 mm, layer thickness 30 µm, scan speed 1250 mm/s, hatch distance 0.12 mm, protective atmosphere (max 0.1% oxygen);
for Co–Cr–Mo: laser power 200 W, laser spot diameter 0.2 mm, layer thickness 20 µm, scan speed 7000 mm/s, hatch distance 0.3 mm, protective atmosphere (max 1.5% oxygen).
For both alloys, tensile specimens were built in three different orientations relative to the machine distinctive directions, all of the three parallel to the layers and perpendicular to the growth direction. The three groups, each of 6 specimens, are specified as follows:
“X” group: the axis of the specimens aligns to the direction which the recoater blade spreads the powder in the bed along;
“Y” group: the axis of the specimens aligns to the direction of the inert gas flux on the powder bed;
“XY” group: the axis of the specimens is angled 45° with respect that of X and Y groups.
The size and geometry for the tensile test specimens conformed to the prescription specified in standard ASTM E8M [
24]. Details are reported in
Figure 1.
Of the powders, the nominal physical/mechanical properties and chemical composition are listed in
Table 1 and
Table 2, respectively. The powders were characterized by means of laser granulometry (Malvern Mastersizer 3000, Malvern Panalytical Ltd., Malvern, UK) to assess their size distribution, according to ISO 13320 standard [
25].
The specimens were tested in the as built condition, without any heat treatment, so as to avoid any smoothing of the secondary anisotropy produced by the L-PBF process.
Before the tensile tests, the Archimedes principle was used to measure the density of all the samples (6 for each group), with an analytical electronic balance having a resolution of 0.1 mg (Pioneer® Plus PA124C, OHAUS GmbH, Greifensee, Switzerland). The residual porosity was then calculated by using the nominal density of each alloy.
Tensile tests were performed on a SCHENK HYDROPULS PSB testing machine (SCHENCK RoTec GmbH, Darmstadt, Germany) with a capacity of 250 kN, using a crosshead speed of 5 mm/min. Five samples were tested for each alloy and orientation, and one extra specimen of each group was used to measure hardness and to obtain the metallographic sections. The choice of the hardness scale was made according to ISO standard 4498 [
28]. Rockwell C was selected and performed following the specifications of standard ISO 6508 [
29], by repeating five measurements on each sample. Numerical results for hardness (HRC), tensile strength (UTS) and total extension at fracture (ε
b) were processed through statistical tools (Statistica 8, Statsoft, Hamburg, Germany): the
t-test with a level of significance of 0.05 was performed to investigate the presence of significant differences between the groups of specimens produced along different orientations.
After tensile tests, rupture surfaces were observed by using a scanning electron microscope, SEM (ESEM, Quanta FEI, Thermo Fisher Scientific, Eindhoven, The Netherlands), in order to investigate the failure mechanisms and the joining phenomena between the particles.
Metallographic sections of the samples were obtained and observed by an optical microscope (OM) (Eclipse LV150N, Nikon, Tokyo, Japan), to get a cross-check of residual porosity and compare the results with those obtained by the Archimedes method. A comparative assessment of the two methods is raising the interests of the scientific community [
30], growingly as the two techniques are more and more diffused in industry for the control of AM parts. Preparation of the metallographic sections consisted of micro-cutting, embedding in epoxy resin and polishing till a fine grinding. The final step was carried out with a plan cloth and 1 μm diamond suspension. Several micrographs were acquired through a CCD camera, made binary and analyzed through a software tool for image analysis to determine:
After OM observation, polished sections of Ti–6Al–4V underwent chemical etching with the Dix-Keller reactant (HF 2% vol, HCl 1.5% vol, HNO3 2.5% vol; water bal.); while metallographic sections of the Co–Cr–Mo alloy were subjected to electrochemical etching (HCl 0.1 M, 2 V, 2 min). Microstructures were observed on the etched samples by means of OM and SEM.
4. Conclusions
In view of the increasing interest in the use of additively manufactured parts for dental prostheses, the mechanical behavior and the microstructure of Ti–6Al–4V and Co–Cr–Mo parts, built by L-PBF, were investigated, with a specific focus on the evaluation of secondary anisotropy.
For both alloys, the measured hardness and strength were in good agreement with those reported in the datasheets; ductility was remarkably high and nearly full densification was measured. The observed microstructures, typical of the extreme cooling rates experienced by the materials during L-PBF processes, allow to account for the outstanding mechanical properties that were appraised in this study. Statistical analysis of the mechanical properties allowed to attest the substantial absence of secondary anisotropy and the result was confirmed by the observation of identical failure modes of the specimens produced in the different orientations.
On the whole, the results enable the conclusion that the two alloys considered here may achieve exceptionally high properties if manufactured by L-PBF, and that secondary anisotropy is negligible if not totally absent.