materials

Article
Mechanical Properties of Selective Laser Sintering
Pure Titanium and Ti-6Al-4V, and Its Anisotropy
Yuu Harada 1 , Yoshiki Ishida 1,2 , Daisuke Miura 1 , Satoru Watanabe 1 , Harumi Aoki 1 ,
Taira Miyasaka 1 and Akikazu Shinya 1,3, *
1 Department of Dental Materials Science, School of Life Dentistry at Tokyo, The Nippon Dental University,
1-9-20 Fujimi, Chiyoda-ku, Tokyo 102-8159, Japan; y-harada_d2117005@tky.ndu.ac.jp (Y.H.);
yishida@tky.ndu.ac.jp (Y.I.); daisuke@tky.ndu.ac.jp (D.M.); s-watanabe_d2117004@tky.ndu.ac.jp (S.W.);
haruaoki@tky.ndu.ac.jp (H.A.); miyasaka@tky.ndu.ac.jp (T.M.)
2 Department of Life Science Dentistry, School of Life Dentistry at Tokyo, The Nippon Dental University,
1-9-20 Fujimi, Chiyoda-ku, Tokyo 102-8159, Japan
3 Department of Prosthetic Dentistry and Biomaterials Science, Institute of Dentistry, University of Turku,
Lemminkaisenkatu 2, 20520 Turku, Finland
* Correspondence: akishi@tky.ndu.ac.jp; Tel.: +81-3-3261-8663




Received: 19 October 2020; Accepted: 8 November 2020; Published: 11 November 2020


Abstract: Selective laser sintering (SLS) is being developed for dental applications. This study aimed
to investigate the properties of Ti-6Al-4V and pure titanium specimens fabricated using the SLS
process and compare them with casting specimens. Besides, the effect of the building direction
on the properties of the SLS specimens was also investigated. Specimens were prepared by SLS
using Ti-6Al-4V powder or pure titanium powder. Casting specimens were also prepared using
Ti-6Al-4V alloys and pure titanium. The mechanical properties (tensile strength and elongation),
physical properties (surface roughness, contact angle, and Vickers hardness); corrosion resistors (color
difference and corrosion), and surface properties (chemical composition and surface observation)
were examined. Both Ti-6Al-4V and pure titanium specimens produced using the SLS process had
comparable or superior properties compared with casting specimens. In comparing the building
directions, specimens fabricated horizontally to the printing platform showed the greatest tensile
strength, and the surface roughness scanned in the horizontal direction to the platform showed the
smallest. However, there was no significant effect on other properties. Thus, the SLS process with
Ti-6Al-4V powder and pure titanium powder has great performance for the fabrication of dental
prosthesis, and there is a possibility for it to take the place of conventional methods.

Keywords: additive manufacturing; selective laser sintering; Ti-6Al-4V; pure titanium; CAD/CAM

1. Introduction
In recent years, additive manufacturing (AM) has been widely used in various fields and has been
developed as a new molding technique [1]. This technology allows three-dimensional models to be
built by fusing the material layer by layer [2]. In dentistry, the milling method is commonly used to
fabricate prostheses in computer-aided design (CAD)/computer-aided manufacturing (CAM) systems;
however, it is challenging to manufacture complicated shape objects because of the limitations of the
milling tools, such as their size and applied angle [3]. In contrast, there are no limitations of the tools
in AM, allowing fabrication of objects with complicated shapes. Moreover, AM allows many models
to be manufactured at the same time. Therefore, in dentistry, AM might be a more efficient method
compared with milling.

Materials 2020, 13, 5081; doi:10.3390/ma13225081 www.mdpi.com/journal/materials

Materials 2020, 13, 5081 2 of 18

Several additive manufacturing processes are being applied in dentistry to manufacture casting
patterns, provisional restorations, surgical guides, or dental models [1,4–8]; however, it was difficult
to fabricate final restorations with metal using the AM process due to the limitation of materials for
additive manufacturing. Selective laser sintering (SLS) and selective laser melting (SLM), a powder bed
fusion process, are applied to manufacture metal dental prostheses, such as fixed partial dentures [9].
In these molding methods, a thin layer of metal powder is selectively sintered by a laser beam based
on 3D data. They have the advantage of being able to produce metal dental prostheses much faster
than the conventional casting method because they do not require wax-up and investment.
Recently, titanium materials have been developed as powders for the AM process. There are
several reports showing that frameworks and clasps of removable dentures and fixed dentures can
be produced by AM using titanium materials [10–16]. Moreover, Iseri et al. [17] reported increased
bonding strength of ceramics to titanium alloy specimens molded by AM compared to those of titanium
alloy specimens prepared by casting. However, the mechanical properties or resistance to corrosion of
AM-molded titanium specimens have not been clarified.
Therefore, this study aimed to clarify the properties of specimens molded by AM using titanium
alloy (Ti-6Al-4V) powder and pure titanium powder. In addition, we investigated the effect of the
building direction on the properties of the specimens. Finally, we compared the properties of the
AM specimens with those of specimens cast with titanium alloy and pure titanium to examine the
applicability of AM with titanium materials.

2. Materials and Methods

2.1. Specimen Preparation

The metal materials used in this study are listed in Table 1. Ti-6Al-4V specimen was molded
using Ti-6Al-4V alloy powder and SLS 3D printer (M2, CONCEPT LASER, Lichtenfels, Germany).
Using pure titanium (hereafter pure Ti) powder, specimens were molded using an SLS 3D printer
(EOS M280, EOS, Krailling, Germany). Dumbbell-shaped specimens (diameter of the parallel part:
2 × 35 mm (DU)) and block specimens (10 × 10 × 20 mm (BL)) were prepared with each material.

Table 1. Metals used in this study.

Aspect Molding
Product Name Constituent Figure Manufacturer
(Particle Size) Method
Rematitan CL Ti-6Al-4V powder 24 µm SLS Dentaurum (Ispringen, Germany)
Ti Alloy Gr.5 Ti-6Al-4V rod block casting Cosmo Metal (Gunma, Japan)
OSAKA Titanium Technologies
TILOP-45 Pure Ti powder 38–45 µm SLS
(Hyogo, Japan)
Titan 100 Pure Ti ingot block casting Shofu (Kyoto, Japan)

The DU was molded to set the angle formed by the building direction and major axis at 0◦ ,
45◦ , and 90◦ (Figure 1). For the molding of BL, a 10 × 10 mm plane horizontal to the building stage
and the building direction were designated as the XY plane and Z-axis, respectively. Accordingly,
a 10 × 20-mm plane is an XZ plane or YZ plane, but it is presented as a Z plane because they do not
have meaning regarding molding. For post-processing, sandblasting was performed both DU and BL
using a sandblaster (Cycle blaster Jr., Daiei Dental, Osaka, japan) with 400 µm glass beads (Sand-rough,
Daiei Dental, Osaka, Japan).
and 90° (Figure 1). For the molding of BL, a 10 × 10 mm plane horizontal to the building stage and
the building direction were designated as the XY plane and Z-axis, respectively. Accordingly, a 10 ×
20-mm plane is an XZ plane or YZ plane, but it is presented as a Z plane because they do not have
meaning regarding molding. For post-processing, sandblasting was performed both DU and BL
Materials
using a2020, 13, 5081
sandblaster (Cycle blaster Jr., Daiei Dental, Osaka, japan) with 400 µm glass beads3 of 18

(Sand-rough, Daiei Dental, Osaka, Japan).

Materials 2020, 13, x FOR PEER REVIEW 3 of 18

As comparative controls, specimens with the same size as above were prepared by casting
using the metals shown in Table 1. The DU casting pattern was prepared using an acrylic bar and
paraffin wax (Figure 2). The BL casting pattern was prepared as follows: equally sized resin blocks
were prepared using a 3D printer, an impression was taken using a silicone rubber impression
material (putty type: Silde fit putty type, Shofu, Kyoto, Japan, regular type: Dent silicone–V, Shofu,
Kyoto, Japan), and paraffin wax was poured into the impression.
Figure 1.
Figure 1. Building direction of SLS specimens.
These casting patterns were invested using a titanium-exclusive investing material (speed
titanium investment, controls,
As comparative Shofu, Kyoto, Japan).
specimens Casting
with wassize
the same prepared
as aboveusing a titanium-exclusive
were casting
prepared by casting using
machine (Ti Cascom, Denken, Kyoto, Japan). Ti-6Al-4V casting was performed
the metals shown in Table 1. The DU casting pattern was prepared using an acrylic bar and paraffin at a mold
temperature of 430 °C and casting temperature of 1540–1650 °C; pure Ti casting was
wax (Figure 2). The BL casting pattern was prepared as follows: equally sized resin blocks were performed in
an argon using
prepared atmosphere at a mold
a 3D printer, temperature
an impression was of 430using
taken °C and castingrubber
a silicone temperature of 1668
impression °C,
material
respectively. Post-processing was performed in the same manner as the specimens produced
(putty type: Silde fit putty type, Shofu, Kyoto, Japan, regular type: Dent silicone–V, Shofu, Kyoto, by SLS
process.
Japan), and paraffin wax was poured into the impression.

Figure 2.
Figure The pattern
2. The pattern of
of the
the cast
cast specimen.
specimen.

These casting patterns were invested using a titanium-exclusive investing material (speed titanium
Table 1. Metals used in this study.
investment, Shofu, Kyoto, Japan). Casting was prepared using a titanium-exclusive casting machine
(TiProduct
Cascom, Denken, Kyoto, Japan).
Constituent Figure
Aspect
Ti-6Al-4V Molding at a mold temperature of 430 ◦ C
casting was performed Manufacturer
name ◦ (Particle Size)
and casting temperature of 1540–1650 C; pure Ti casting was Method
performed in an argon atmosphere at a
mold temperature of 430 ◦ C and casting temperature of 1668 ◦ C, respectively.
Rematitan Dentaurum (Ispringen,was
Post-processing
Ti-6Al-4V powder 24 µm SLS
CL
performed in the same manner as the specimens produced by SLS process. Germany)
Ti Alloy Gr.5 Ti-6Al-4V rod block casting Cosmo Metal (Gunma, Japan)
2.2. Tensile Test OSAKA Titanium
TILOP-45 Pure Ti powder 38–45 µm SLS
In the tensile test, the tensile strength and the elongation to time of Technologies (Hyogo,
fracture (matching Japan)
method) of
Titan 100 Pure Ti ingot block casting Shofu (Kyoto,
the DU specimens were measured. This test was performed at a crosshead speed of 1.0 mm/min using Japan)
a universal testing machine (Autograph DCS-10T, Shimadzu, Kyoto, Japan). The SLS-0, SLS-45, SLS-90,
2.2.
andTensile
castingTest
specimens were subjected to this test, and the influence of the building direction of the
specimens
In the on the mechanical
tensile properties
test, the tensile was
strength investigated.
and the elongation to time of fracture (matching method)
of the DU specimens were measured. This test was performed at a crosshead speed of 1.0 mm/min
using a universal testing machine (Autograph DCS-10T, Shimadzu, Kyoto, Japan). The SLS-0,
SLS-45, SLS-90, and casting specimens were subjected to this test, and the influence of the building
direction of the specimens on the mechanical properties was investigated.

2.3. Physical Property Test

Materials 2020, 13, 5081 4 of 18

2.3. Physical Property Test

The surface roughness, contact angle to water, and Vickers hardness were measured in a physical
property test using BL specimens. Surface roughness was measured using a surface roughness
measuring instrument (SURFCOM, Tokyo Seimitsu, Tokyo, Japan). The contact angle was measured
with a contact angle meter (LSE-ME3, Nick, Saitama, Japan). The Vickers hardness was measured
with a Vickers hardness tester (AVK-15, Akashi, Tokyo, Japan) at a measurement load of 50 kgf and a
load holding time of 15 s. These measurements were performed on the surface condition as sintered.
Next, the specimen surface was polished to a mirror-like finish, and the same measurements were
performed on the surface after polishing. The properties of the specimen surface before (as sintered)
and after polishing were compared. These tests were performed on the Z plane (SLS-Z), XY plane
(SLS-XY), and the cast specimen, and the effect of the building direction of the specimen on the physical
properties was investigated.

2.4. Immersion Test

Two types of immersion tests were performed: a discoloration test (using sodium sulfide and
lactic acid solutions) and a corrosion test (using lactic acid solution). For these tests, the BL was cut
into a plate of 10 × 10 × 1 mm using a precision cutting machine (IsoMet 1000, BUEHLER, Lake Bluff,
IL, USA). The specimen cut vertical to the building stage (horizontal to the building direction) is
referred to as SLS-Z, and the specimen cut horizontal to the building stage (vertical to the building
direction) is referred to as SLS-XY. The two immersion tests were performed on SLS-Z, SLS-XY, and cast
specimens, and the effects of the building direction of the specimen on discoloration and corrosion
were investigated.
The plate specimen surface was polished with water-resistant abrasive paper (SiC abrasive paper,
Struers, Tokyo, Japan (#400 to #1200)). Subsequently, the color, surface area, and weight of the specimen
were measured (surface area and weight were measured only for the lactic acid immersed specimen).
Color was measured with a colorimeter (CR-221, Konica Minolta, Tokyo, Japan). The surface area was
measured with a digital microscope (VHX-2000, KEYENCE, Osaka, Japan). Weight was measured
with a precision balance (MC210S, Sartorius, Goettingen, Germany).
Sodium sulfide solution (50 mL, 0.1%) or lactic acid solution (10 g/L) was placed in a glass container
(capacity approximately 150 mL). One specimen was immersed in the solution so that it did not contact
the bottom of the container, and left at 37 ◦ C for three (sodium sulfide solution) or seven days (lactic
acid solution) according to ISO 10271:2011 [18]. After immersion, the same measurements as before
immersion (color, surface area, and weight (surface area and weight were measured only for lactic acid
immersed specimens)) were performed.
In the discoloration test, the color difference (∆E*ab) was calculated from chromaticity coordinates
of the specimen measured before and after immersion. To relate the color change measured by the
spectrophotometer to a clinical environment, the values of ∆E* ab were converted to National Bureau
of Standards units (NBS units) through the equation, NBS units = ∆E*ab × 0.92.
In the corrosion test, weight loss due to corrosion (lactic acid weight loss) was calculated from the
weight and surface area. Lactic acid weight loss was calculated using Formula (1):

Weight before immersion (µg)−Weight after immersion (µg)

 
Lactic acid weight loss µg/cm2 = Specimen surface area (cm2 )
(1)

2.5. Surface Property Test

Chemical composition analysis and observation of metallographic structures were performed
using plate specimens prepared in the same manner as the preparation of the specimens for the
immersion tests. Chemical composition analysis was performed by fluorescent X-ray analysis (XRF)
and X-ray diffraction (XRD). XRF was performed with a fluorescent X-ray analyzer (DELTA Professional,
OLYMPUS, Tokyo, Japan). XRD was performed using a desktop X-ray diffractometer (MiniFlex, Rigaku,
Materials 2020, 13, 5081 5 of 18

Tokyo, Japan). Surface structure was observed using a digital microscope (VHX-2000, KEYENCE,
Osaka, Japan). The observation was performed on three patterns: as sintered, after polishing, and after
etching (acidic solution (nitric acid:hydrofluoric acid:water = 4 1:5) was used for etching).

2.6. Test Repetitions and Statistical Processing

Each test was repeated six times from specimen preparation to measurement (n = 6). Two-way
ANOVA was performed to analyze the results of the tensile test using DU (factor A: metal type
(Ti-6Al-4V or pure Ti); factor B: building angle (SLS-0, SLS-45, SLS-90, or cast)). Three-way ANOVA was
performed to analyze the results of the physical property tests using BL (factor A: metal type (Ti-6Al-4V
or pure Ti); factor B: molding method (SLS-Z, SLS-XY, or cast); factor C: surface condition (as sintered or
after polishing)). Two-way ANOVA was performed to analyze the results of the immersion test (factor
A: metal type (Ti-6Al-4V or pure Ti); factor B: molding method (SLS-Z, SLS-XY, or cast)). One-way
ANOVA was performed on XRF for each detected element. From the ANOVA results, Tukey’s multiple
comparison was performed when there was a significant difference for the measurement item.

3. Results

3.1. Tensile Test

Table 2 shows the results of tensile test. A two-way ANOVA based on metal type (A) and building
angle (B) was performed on the tensile strength test results. There were highly significant differences
(p < 0.01) only in the main effects (A, B). Subsequent Tukey’s multiple comparison test showed there
was a highly significant difference (p < 0.01) in metal type (A) (Ti-6Al-4V: 1047.85 MPa; pure Ti:
458.30 MPa; 95% confidence interval: 15.55), but no significant difference (p > 0.05) in building angle
(B). Table 2 shows the results of Tukey’s multiple comparison for each metal type. For Ti-6Al-4V,
the tensile strength of the SLS-90 specimen was significantly larger (p < 0.05) than that of the SLS-0
specimen. The tensile strength of the specimens molded by SLS was significantly larger (p < 0.05)
than that of the cast specimens at any building angle. For pure Ti, the tensile strength of the SLS-90
specimen was significantly larger (p < 0.01) than that of the SLS-0 specimen. Comparing the tensile
strengths of the specimens molded by SLS with those of the cast specimens, the tensile strength of
SLS-45 and SLS-90 specimens was significantly larger (p < 0.01) than that of the cast specimen.

Table 2. Tensile strength and elongation.

Metal Angle Tensile Strength (MPa) Elongation (%)

0 1044.12 (43.36) A 10.90 (1.33) A
45 1061.89 (42.26) A,B 11.56 (1.14) A
Ti-6Al-4V
90 1118.17 (66.36) B 11.34 (1.47) A
Cast 967.21 (38.37) C 14.72 (1.67) B
0 430.51 (10.91) a,b 17.62 (0.74) a
45 493.64 (16.69) b,c 17.91 (0.09) a
Pure Ti
90 529.22 (25.57) c 17.82 (0.67) a
Cast 379.85 (31.85) a 18.86 (3.37) a
The same letter indicates a combination with no significant difference (p > 0.05). Standard deviations are given
in parentheses.

Two-way ANOVA was performed on the elongation results, and revealed highly significant
differences (p < 0.01) only in main effects (A, B). Subsequent Tukey’s multiple comparison test revealed
there was a highly significant difference (p < 0.01) in metal type (A) (Ti-6Al-4V: 12.13%; pure Ti:
18.03%; 95% confidence interval: 0.66), but no significant difference (p > 0.05) in building angle (B).
Table 2 shows the results of Tukey’s multiple comparison for each metal type. There was no significant
difference (p > 0.05) in elongation with regards to the building angle of SLS specimens. The elongation
of the specimens molded by SLS was significantly smaller (p < 0.05) than that of the cast specimen at
Materials 2020, 13, 5081 6 of 18

all building angles. For pure Ti, there was no significant difference (p > 0.05) between the molding
methods or among the building directions.

3.2. Physical Property Test

Table 3 shows the results of physical property test. A three-way ANOVA based on metal type
(A), molding method (B), and surface condition (C) was performed to analyze their effect on surface
roughness, contact angle, and Vickers hardness. Regarding surface roughness there were highly
significant differences (p < 0.01) in the main effects B, C, and in the interaction B × C. Figure 3 shows a
graph of B × C and the results of Tukey’s multiple comparison. As shown in Figure 3, the sintered
surface roughness was in the descending order of cast > SLS-Z > SLS-XY. After polishing, these values
were greatly reduced for all the molding methods, and there was no significant difference between the
molding methods or among the building directions (p > 0.05).

Table 3. Surface roughness, Contact angle, Vickers hardness, Lactic acid weight loss, and NBS in Na2 S
and lactic acid.
Surface Vickers Lactic Acid NBS Unit
Surface Contact
Metal Surface Roughness Hardness Weight Loss
Condition Angle (◦ ) Na2 S Lactic Acid
(µmRa) (HV) (µg/cm2 )
As 5.04 99.22 371.82 - - -
sintered (0.24) (6.37) (30.83)
SLS-Z
After 0.91 70.46 401.06 58.99 13.10 7.24
polishing (0.36) (6.63) (138.78) (39.83) (5.15) (4.45)
As 3.65 97.05 361.85 - - -
sintered (1.46) (8.72) (19.04)
Ti-6Al-4V SLS-XY
After 1.21 64.87 331.50 153.86 13.29 8.23
polishing (0.27) (8.61) (56.34) (121.42) (7.05) (3.31)
As 8.75 68.86 650.99 - - -
sintered (2.94) (7.40) (108.06)
Cast
After 0.63 63.53 394.42 201.05 11.52 3.39
polishing (0.41) (8.35) (17.09) (131.21) (2.82) (1.09)
As 5.91 50.81 200.33 - - -
sintered (1.08) (16.99) (23.23)
SLS-Z
After 0.79 69.94 193.82 65.94 17.70 0.89
polishing (0.51) (5.75) (5.43) (35.43) (1.03) (0.57)
As 3.40 52.49 206.15 - - -
sintered (0.73) (12.30) (12.64)
Pure Ti SLS-XY
After 1.43 63.99 196.21 59.12 18.86 0.59
polishing (0.50) (6.91) (6.29) (60.01) (1.23) (0.08)
As 7.71 69.49 278.51 - - -
sintered (2.92) (7.83) (49.71)
Cast
After 0.86 55.43 219.09 44.97 17.28 0.97
polishing (0.51) (11.34) (28.78) (25.01) (1.31) (0.24)
Standard deviations are given in parentheses.

Regarding contact angle, there were highly significant differences (p < 0.01) in A, C, A × B, A × C,
and A × B × C; and a significant difference (p < 0.05) in B. Figure 4 shows a graph of A × B × C and the
results of Tukey’s multiple comparison. As shown in Figure 4, for Ti-6Al-4V, the SLS-Z and SLS-XY
sintered contact angles were significantly larger (p <0.01) than that of the cast specimen. After polishing,
these values were similar to those of the cast specimen before polishing. For pure Ti, the SLS-Z sintered
contact angle was significantly smaller (p < 0.05) than that of the cast specimen. After polishing,
this value was similar to that of the Ti-6Al-4V cast specimen before polishing.
Materials 2020, 13, 5081 7 of 18
Materials2020,
Materials 2020,13,
13,xxFOR
FORPEER
PEERREVIEW
REVIEW 77of
of18
18

Figure3.
Figure
Figure 3.3.Surface
Surfaceroughness.
Surface roughness. The
roughness. Thesame
samelowercase
lowercaseletter
letterindicates
indicatesaacombination
combinationwith
combination withno
nosignificant
no significant
significant
difference(p
difference
difference (p>>>0.05).
(p 0.05).Error
Errorbar
barrepresents
represents95%
95%confidence
confidenceinterval.
interval.

Figure 4. Contact angle. The same lowercase letter indicates a combination with no significant
difference (p > 0.05). Error bar represents 95% confidence interval.
Figure 4.4. Contact
Figure Contact angle.
angle. The
The same
same lowercase
lowercase letter
letter indicates
indicates aa combination
combination with
with no
no significant
significant
difference(p
difference
Regarding (pVickers
>>0.05).
0.05).Error
Errorbar
barrepresents
hardness, represents 95%
95%
there was confidenceinterval.
confidence
a significant interval.
difference (p < 0.05) in A × C, and a highly
significant difference (p < 0.01) in all the other main effects and interactions. Figure 5 shows a graph
of A × B × C and the results of Tukey’s multiple comparison. As shown in Figure 5, for Ti-6Al-4V,
Materials 2020, 13, 5081 8 of 18

the sintered Vickers hardness of the SLS-Z and SLS-XY specimens was significantly lower (p < 0.01)
than that of the cast specimen. After polishing, the Vickers hardness of the cast specimen was similar to
those of the SLS-Z and SLS-XY specimens, and there was no significant difference between the molding
methods or among the building directions (p > 0.05). The Vickers hardness of pure Ti was significantly
smaller2020,
Materials than13,that of PEER
x FOR Ti-6Al-4V in both SLS and casting method (p < 0.05).
REVIEW 8 of 18

Figure 5.
Figure Vickers hardness.
5. Vickers hardness. The
The same
same lowercase
lowercase letter
letter indicates
indicates aa combination
combination with
with no
no significant
significant
difference (p > 0.05). Error bar represents 95% confidence interval.
difference (p > 0.05). Error bar represents 95% confidence interval.
3.3. Immersion Test
3.3. Immersion Test
Table 3 shows the results of immersion test.
Table
Two-way3 shows
ANOVA the results
basedof onimmersion
metal typetest.
(A) and molding method (B) was performed to assess
their effect on the color difference metal
Two-way ANOVA based on type (A)
(NBS unit) afterand molding in
immersion method (B) was performed
either sodium to assess
sulfide or lactic acid
their effect on the color difference (NBS unit) after immersion in either sodium
solution, and lactic acid weight loss. Regarding the color difference after immersion in sodium sulfide sulfide or lactic acid
solution, and lactic
solution, there acid weight
was a highly loss.difference
significant Regarding (p <the color
0.01) difference
in main effect A,after immersion
but there was noin sodium
significant
sulfide solution, there was a highly significant difference (p < 0.01) in main effect
difference (p > 0.05) in the molding method. The color difference of pure Ti was significantly increased A, but there was no
significant differencewith
(p < 0.01) compared (p that
> 0.05) in the molding
of Ti-6Al-4V (Ti-6Al-4V:method.
12.64; The
pure color difference
Ti: 17.95; of pure interval:
95% confidence Ti was
significantly
1.85). increased (p < 0.01) compared with that of Ti-6Al-4V (Ti-6Al-4V: 12.64; pure Ti: 17.95;
95% confidence interval: 1.85).
Regarding lactic acid weight loss, there was a highly significant difference (p < 0.01) in main effect
A, but there waslactic
Regarding acid weight
no significant loss, there
difference was ainhighly
(p > 0.05) molding significant
method. difference (p < weight
The lactic acid 0.01) inlossmainof
effect A, but there was no significant difference (p > 0.05) in molding method.
Ti-6Al-4V was significantly (p < 0.01) larger than that of pure Ti (Ti-6Al-4V: 137.96 µg/cm ; pure Ti: The lactic acid
2 weight
loss
56.68ofµg/cm
Ti-6Al-4V
2 ; 95%was significantly
confidence (p <38.82).
interval: 0.01) larger than that of pure Ti (Ti-6Al-4V: 137.96 µg/cm2;
pure Regarding
Ti: 56.68 µg/cm 2
the color ; 95% confidence
difference afterinterval:
immersion 38.82).
in lactic acid solution, there was a highly significant
difference (p < 0.01) in main effect A, and a significant in
Regarding the color difference after immersion lactic acid
difference (p <solution,
0.05) in thethere was a highly
interaction A × B.
significant difference (p < 0.01) in main effect A, and a significant difference
Figure 6 shows a graph of A × B and the results of Tukey’s multiple comparison. As shown in Figure (p < 0.05) in the6,
interaction A × B. Figure 6 shows a graph of A × B and the results of Tukey’s
for Ti-6Al-4V, the color differences of the SLS-Z and SLS-XY specimens were significantly (p < 0.05) multiple comparison.
As shown
larger thaninthat
Figure
of the6, forcastTi-6Al-4V,
specimen.the color
For puredifferences
Ti, there wasof thenoSLS-Z and SLS-XY
significant specimens
difference between were
the
significantly (p < 0.05) larger than that of the
molding methods or among the building directions (p > 0.05). cast specimen. For pure Ti, there was no significant
difference between the molding methods or among the building directions (p > 0.05).
Materials 2020, 13, 5081 9 of 18
Materials 2020, 13, x FOR PEER REVIEW 9 of 18

Figure 6.6.Color
Figure Colordifference withwith
difference lactic lactic
acid solution. The same
acid solution. lowercase
The letter indicates
same lowercase a combination
letter indicates a
with no significant difference (p > 0.05). Error bar represents 95% confidence interval.
combination with no significant difference (p > 0.05). Error bar represents 95% confidence interval.

3.4. Surface Property Test

3.4. Surface Property Test
Table 4 shows the results of the XRF. For Ti-6Al-4V, the titanium (Ti) fraction of the SLS-molded
Table 4 shows the results of the XRF. For Ti-6Al-4V, the titanium (Ti) fraction of the
specimens was higher than that of the cast specimen. In particular, the Ti fraction of SLS-Z was
SLS-molded specimens was higher than that of the cast specimen. In particular, the Ti fraction of
significantly (p < 0.05) higher than that of the cast specimen. The vanadium (V) fraction of the
SLS-Z was significantly (p < 0.05) higher than that of the cast specimen. The vanadium (V) fraction of
SLS-molded specimens was significantly (p < 0.05) lower than that of the cast specimen. The iron (Fe)
the SLS-molded specimens was significantly (p < 0.05) lower than that of the cast specimen. The iron
fraction was in descending order of SLS-XY > SLS-Z > cast. The aluminum (Al) and chromium (Cr)
(Fe) fraction was in descending order of SLS-XY > SLS-Z > cast. The aluminum (Al) and chromium
fractions did not significantly differ (p > 0.05) between specimens of the different molding methods.
(Cr) fractions did not significantly differ (p > 0.05) between specimens of the different molding
For pure Ti, the Ti fraction was in descending order of SLS-Z > SLS-XY > cast. The Fe fraction of SLS-Z
methods. For pure Ti, the Ti fraction was in descending order of SLS-Z > SLS-XY > cast. The Fe
specimens was significantly lower (p < 0.01) than that of the cast specimen. For the zirconium (Zr)
fraction of SLS-Z specimens was significantly lower (p < 0.01) than that of the cast specimen. For the
fraction, there was no significant difference between the molding methods or among the building
zirconium (Zr) fraction, there was no significant difference between the molding methods or among
directions (p > 0.05).
the building directions (p > 0.05).
Table 4. X-ray fluorescence analysis of elements (%).
Table 4. X-ray fluorescence analysis of elements (%).
Metal Surface Ti Al V Fe Cr Zr
Metal Surface Ti Al V Fe Cr Zr
89.74 6.00 4.06 0.19 0.00
Ti-6Al-4V SLS-Z 89.74 A 6.00 A 4.06 A 0.19 A 0.00 A -
Ti-6Al-4V SLS-Z (0.10) A (0.00) A (0.09) A (0.02) A (0.00) A -
(0.10) (0.00) (0.09) (0.02) (0.00)
89.65 6.00 4.10 0.23 0.03
Ti-6Al-4V SLS-XY 89.65 A,B
A, 6.00 A 4.10 A 0.23 B 0.03 A -
Ti-6Al-4V SLS-XY (0.15) (0.00) A (0.14) A (0.03) B (0.06) A -
(0.15) B (0.00) (0.14) (0.03) (0.06)
89.49 6.00 4.30 0.05 0.00 -
Ti-6Al-4V Cast 89.49 B 6.00 A 4.30 B 0.05 C 0.00 A
Ti-6Al-4V Cast (0.11) B (0.00) A (0.14) B (0.02) C (0.00) A -
(0.11) (0.00) (0.14) (0.02) (0.00)
99.99 - - 0.01 - 0.00
Pure Ti SLS-Z 99.99 a 0.01 a 0.00 a
Pure Ti SLS-Z (0.01) a - - (0.01) a - (0.00) a
(0.01) (0.01) (0.00)
99.97 - - 0.03 - 0.00
Pure Ti SLS-XY 99.97 a 0.03 a,b 0.00 a
Pure Ti SLS-XY (0.02) a - - (0.02) a,b - (0.00) a
(0.02) (0.02) (0.00)
99.94 - - 0.05 - 0.01
99.94 0.05 b a
(0.01)0.01
Pure Ti Cast b
Pure Ti Cast (0.02) b - - (0.01) b - a
(0.02) (0.01) (0.01)
The same letter indicates a combination with no significant difference (p > 0.05). Standard deviations are given in
The same letter
parentheses. indicates
Ti: titanium, Al:aaluminum,
combination with no significant
V: vanadium, Fe: iron, Cr:difference
chromium, (pZr:>zirconium.
0.05). Standard deviations
are given in parentheses. Ti: titanium, Al: aluminum, V: vanadium, Fe: iron, Cr: chromium, Zr:
zirconium.
Materials 2020, 13, x FOR PEER REVIEW 10 of 18

Figures 7 and 8 show the typical XRD spectra of the Ti-6Al-4V and pure Ti specimens,
respectively. The peak intensities of XRD differed in each spectrum. Regarding the diffraction
angles of the peaks, there were no differences in molding method for each metal.
Materials 2020, 13, 5081 10 of 18
Figures 9 and 10 show typical surface structure images of the Ti-6Al-4V and pure Ti specimens,
respectively. For Ti-6Al-4V, the molding method did not appear to affect the surface structure
Figures
pattern. For 7pure
and 8Ti,show
the the
as typical
sinteredXRD spectra
images of of theSLS
the Ti-6Al-4V and pure
specimens Ti specimens,
comprised a largerespectively.
block. In
The peak intensities of XRD differed in each spectrum. Regarding the diffraction angles
contrast, the cast specimen had a finer surface structure. After polishing and etching, they became of the peaks,
there were
smooth no differences
surfaces regardlessinofmolding method
the molding for each metal.
method.

Figure
Figure7.7.X-ray
X-raydiffraction
diffractionspectra
spectraofofthe
theSLS-Z,
SLS-Z,SLS-XY,
SLS-XY,and
andcast
castTi-6Al-4V
Ti-6Al-4V specimens.
specimens.
Materials 2020, 13, 5081 11 of 18
Materials 2020, 13, x FOR PEER REVIEW 11 of 18

X-raydiffraction
Figure8.8.X-ray
Figure diffractionspectra
spectraofofthe
theSLS-Z,
SLS-Z,SLS-XY,
SLS-XY,and
andcast
castpure
pureTi
Tispecimens.
specimens.

Figures 9 and 10 show typical surface structure images of the Ti-6Al-4V and pure Ti specimens,
respectively. For Ti-6Al-4V, the molding method did not appear to affect the surface structure pattern.
For pure Ti, the as sintered images of the SLS specimens comprised a large block. In contrast, the cast
specimen had a finer surface structure. After polishing and etching, they became smooth surfaces
regardless of the molding method.
Materials 2020, 13, 5081 12 of 18
Materials 2020, 13, x FOR PEER REVIEW 12 of 18
Materials 2020, 13, x FOR PEER REVIEW 12 of 18

Figure
Figure 9.
9. Digital
Digitalmicroscope
microscopeimages
imagesofofSLS-Z,
SLS-Z,SLS-XY,
SLS-XY, and
and cast
cast Ti-6Al-4V
Ti-6Al-4V specimens
specimens as
as sintered,
sintered,
Figure 9. Digital microscope images of SLS-Z, SLS-XY, and cast Ti-6Al-4V specimens as sintered,
after
after polishing,
polishing, and
and after
after etching
etching (×1000).
(×1000).
after polishing, and after etching (×1000).

Figure 10.
Figure Digital microscope
10. Digital microscopeimages
imagesof
ofthe
theSLS-Z,
SLS-Z,SLS-XY,
SLS-XY,and
andcast
castpure
pureTi
Ti specimens
specimens as
as sintered,
sintered,
Figure 10. Digital microscope images of the SLS-Z, SLS-XY, and cast pure Ti specimens as sintered,
after polishing,
after polishing, and
and after
after etching
etching (×1000).
(×1000).
after polishing, and after etching (×1000).
Materials 2020, 13, 5081 13 of 18

4. Discussion
In this study, several properties of the Ti-6Al-4V and pure Ti specimens produced using the SLS
process were evaluated and compared to that of the casting specimens. Consequently, it was found
that the specimens produced by SLS had comparable or superior properties compared with casting
specimens. Besides, the building direction affected the tensile strength and surface roughness, but did
not significantly affect other properties.
SLS is considered a promising AM method for manufacturing metal dental prostheses. To date,
Co-Cr alloy powder has been primarily used for SLS in dentistry. It is difficult to manufacture fine
titanium powders, and the production process to produce AM-qualified raw material were technically
complicated [19]. For these reasons, SLS molding using titanium alloy powder and pure Ti powder has
been delayed in practice. However, it has recently become possible to manufacture these powders for
SLS [20–22]; therefore, SLS molding using these metal powders is promising for dentistry. The purpose
of this study was to clarify the basic properties of molded objects manufactured by SLS using these
metal powders.

4.1. Tensile Test

The tensile strength of Ti-6Al-4V was more than twice that of pure Ti. Moreover, the tensile
strength values obtained for Ti-6Al-4V in this study satisfied those specified in JIS H4650 [23]. For pure
Ti, the tensile strength of the SLS-molded specimens was in agreement with that of pure Ti (type 2 and
type 3) specified in JIS H4650 [23]. The tensile strength of the cast specimen was in agreement with that
of pure Ti (type 1 and type 2) specified in JIS H4650 [23]. Overall, the tensile strength values obtained
in this study were similar to those previously reported for Ti-6Al-4V and pure Ti cast specimens [24,25].
The tensile strength of the SLS-90 specimen was greater than that of SLS-0 for all metal types. When the
SLS building direction is the same as the long axis of the specimen, as in SLS-0, the layers are molded
in the same direction as the direction of tensile stress. Therefore, tensile stress acts to separate the
layers. In contrast, since the stress is vertical to the building direction in SLS-90 specimens, increasing
their tensile strength. The tensile strengths of SLS-molded specimens were larger than those of the cast
specimens for all metal types tested. This result was likely because of a difference in the composition
of the metals used for SLS and casting. It is also possible the lack of casting defects in the SLS-molded
specimens contributed to this result.
The elongation of pure Ti was larger than that of Ti-6Al-4V. The elongation values obtained
in this study were close to those specified in JIS H4650 [23] (Ti-6Al-4V: 10% or more; pure Ti (type
3): 18% or more). These values are similar to those previously reported for Ti-6Al-4V and pure Ti
cast specimens [26,27]. The SLS building angle had no effect in Ti-6Al-4V specimens. In contrast to
tensile strength, the elongation values of SLS-molded specimens were smaller than those of the cast
specimen. This result could be because of the difference in composition of the metals used for SLS
and casting. In support of this, we found that the Fe fraction was low in the cast specimen, and this
slight difference in composition could have influenced elongation. In pure Ti, there was no significant
difference between the molding methods or among the building directions. Thus, the elongation of the
pure Ti specimens was not affected by either the molding method or the metal composition.

4.2. Physical Property Test

The metal type did not affect the sintered surface roughness. The surface roughness of the cast
specimen was the largest of all the metal types, and the surface roughness of the SLS-Z specimen was
larger than that of SLS-XY specimen for all metal types. Thus, the surface roughness scanning in the
building direction was larger than that scanning in the horizontal direction to a building platform.
This result was consistent with the observation that the SLS-90 specimen had a high tensile strength.
It is considered that this result was obtained because the SLS-Z specimen surface has building steps.
This result was compared the surface structure images. There was no clear difference between the
Materials 2020, 13, 5081 14 of 18

molding methods in the surface structure images of the Ti-6Al-4V specimens. However, the pure Ti
SLS-molded specimens had a surface structure comprising large blocks. This indicated a tendency
consistent with the surface roughness results. After polishing, there was no significant difference in
molding methods and metal type. This was consistent with the surface structure imaging results of the
specimens after polishing.
In Ti-6Al-4V SLS-molded specimens, the sintered contact angles were 90◦ or more, whereas that
of the cast specimen was approximately 69◦ . After polishing, the contact angles were similar to that
of the cast specimen before polishing, regardless of the molding method. Generally, the relationship
between surface roughness and contact angle is expressed by the Cassie–Baxter Formula (2) [28]:

cos θw = f (rcos θ + 1) − 1 (2)

In this formula, θ is the contact angle, r is the area ratio of rough surface to plane (r = 1), θw is the
contact angle on the rough surface, and f is the ratio of liquid droplets entering the concave surface
(if f = 1, the entire surface gets wet). Therefore, if f = 1, the larger the surface roughness, the larger
the apparent contact angle on a hydrophobic surface (θ > 90◦ ), and the smaller the apparent contact
angle on a hydrophilic surface (θ < 90◦ ). Therefore, for Ti-6Al-4V, the sintered contact angle of the
SLS-molded specimen was large because its surface roughness was large. However, although the
sintered surface roughness of the cast specimen was larger than that of the SLS-molded specimen,
the contact angle of the cast specimen was smaller than that of the SLS-molded specimen. This could be
because the f value is small; because the surface roughness of the cast specimen was larger; the concave
surface could not be filled with water droplets. As a result, the contact angle of the cast specimen
was small. For pure Ti, the surface roughness of the cast specimen was larger than that of the SLS-Z
specimen. However, before polishing, the contact angle of the cast specimen was larger than that
of the SLS-Z specimen. This could be because the effect of the f value was small. After polishing,
the contact angles were 55◦ –70◦ for both metals. There was no difference between the molding method
and metal type.
For Ti-6Al-4V, before polishing, the Vickers hardness of the cast specimen (650.99 HV) was larger
than that of the SLS-molded specimen. During casting, the metal reacted with the refractory and binder
materials of the investment material to form a hardened layer containing impurities on the surface
layer of the specimen. We consider that the Vickers hardness of the cast specimen was increased by
this hardened layer. Polishing removed the hardened layer; therefore, after polishing, the Vickers
hardness of the cast specimen was approximately 331–400 HV, which was hardness of the SLS-molded
specimen before and after polishing. No significant difference was not observed between the molding
methods or among the building directions. For pure Ti, before polishing, the Vickers hardness of the
cast specimen (278.51 HV) was larger than that of the SLS-molded specimens. However, after polishing,
it was 219.09 HV, which was not different from the values of the SLS-molded specimens before and
after polishing.

4.3. Immersion Test

The results of the color difference after immersion in sodium sulfide solution did not differ between
the molding methods. The value of Ti-6Al-4V was 12.64 (standard deviation: 5.04), which was slightly
smaller than, but within the standard deviation of, that of 15.2 previously reported for Ti-6Al-4V [29].
Furthermore, the previously reported value represents the distance in the Commission Internationale
de l’éclairage (CIE) chromaticity coordinates by ∆E*ab. Therefore, to convert it into the NBS unit used
in this study, it was multiplied by a coefficient corresponding to glossiness (0.92 [30]), giving a value
of 13.98, which was close to that obtained in this study. The value of pure Ti was 17.95 (standard
deviation: 1.32), which is close to that of 17.9 previously reported for pure Ti [29]. Converting it to
NBS units, it became 16.49. Nevertheless, considering the error, we considered that this value was
close to the value obtained in this study.
Materials 2020, 13, 5081 15 of 18

The molding method did not affect lactic acid weight loss. The value of Ti-6Al-4V was larger
than that of pure Ti (Ti-6Al-4V: 137.96 µg/cm2 ; pure Ti: 56.68 µg/cm2 ). According to a previous report,
when Ti-6Al-4V or pure Ti were immersed in lactic acid solution, the amount of Ti dissolved was 10
µg/cm2 and 23 µg/cm2 , respectively [31]. Therefore, the amount of Ti dissolved from pure Ti was larger
than that from Ti-6Al-4V. However, the corrosion resistance of pure Ti is generally superior to that of
Ti-6Al-4V. Therefore, it is not possible to discuss the lactic acid weight loss based on the amount of Ti
dissolved alone. Furthermore, the previous study examined the amount of Ti element dissolved in
immersion solution using inductively coupled plasma-atomic emission spectrometry [31]. Therefore,
considering the dissolution of Al and V, the values obtained in this study were reasonable as the value
of lactic acid weight loss, which was examined by weight change before and after immersion.
For Ti-6Al-4V, the color difference of the SLS-XY specimen after lactic acid immersion was larger
than that of the cast specimen. According to the results of the XRF on Ti-6Al-4V specimens, a larger Fe
fraction was found in SLS compared with that of casting specimens. It suggests that the difference in
composition caused the color difference. A previous study has reported a ∆E*ab color difference of
approximately 4 after Ti-6Al-4V immersion in lactic acid [31]. Converting this value into NBS units,
it was close to the value of the cast specimen obtained in this study. We consider that the values
of the SLS-molded specimens (SLS-Z: 7.24; SLS-XY: 8.23) were larger than this value because of the
difference in their composition, as described above. For pure Ti, the values of the color difference were
0.59–0.97. There was no difference between the molding methods or among the building directions.
Previous study reported a value of 4 for pure Ti [31], which was rather large. However, considering the
difference in corrosion resistance between Ti-6Al-4V and pure Ti, we consider that the values obtained
in this study are consistent and accurate.

4.4. Surface Property Test

For Ti-6Al-4V, the Ti fraction of the particles used for SLS was higher than that of the alloy used
for casting. Since the Fe fraction of the particles used for SLS was higher than that of the alloy used
for casting, there was a slight difference in mechanical strength; however, we consider that they have
essentially the same composition. For pure Ti, there was a slight difference in the Ti fraction between
the particles used for SLS and the metal used for casting; however, we consider that they had also have
the same composition.
There were differences in each spectrum for the peak intensities of XRD. Regarding the diffraction
angles of the peaks (2θ), there were no differences between the molding method for each alloy.
The results of XRD were obtained using the plate-shaped specimens, not from the powder specimen,
so the crystal orientation was not randomized. Therefore, the comparisons of the relative intensities of
each peak were considered to be meaningless, so in this study, we focused only on the comparison on
the positions of the peaks, namely the diffraction angles of the peaks (2θ). Consequently, it could be
considered that the spectra of SLS specimens were not affected by the molding directions or molding
methods in either Ti-6Al-4V or pure Ti. The diffraction angles of peaks obtained in this study were
close to those previously reported in the literature [32,33].
For Ti-6Al-4V, there was no obvious difference in the surface structures of specimens made
using the different molding methods. The pure Ti as sintered SLS specimen comprised a large block.
In contrast, the cast specimen had a finer surface structure. However, these differences were limited
to the surface layer. After polishing and etching, there were no differences in the surface structure
between the molding methods.
From the above, considering the mechanical, physical, chemical, and surface properties, Ti-6Al-4V
and pure Ti specimens molded by SLS had either the same or better properties than cast specimens
of Ti-6Al-4V and pure Ti. Only tensile strength and surface roughness were affected by the building
direction of these specimens. Therefore, it can be concluded that the SLS process with either Ti-6Al-4V
alloy powder or pure Ti powder is useful for clinical dentistry.
Materials 2020, 13, 5081 16 of 18

The main limitation of this study is that the environment of the oral cavity was not considered.
That should be more complex than the experimental environment, so the difference must affect these
properties. Further study is needed to clarify them.

5. Conclusions
In this study we compared the mechanical, physical, chemical, and surface properties of
SLS-molded specimens using Ti-6Al-4V alloy powder and pure Ti powder with those of cast specimens
of Ti-6Al-4V and pure Ti. The effect of the difference in the building direction of these specimens on
the properties was also investigated. As a result, the following conclusions were obtained:
(1) In both metals, the tensile strength of specimens with the same stress direction and building
direction was small. The values of the specimens molded by SLS (Ti-6Al-4V: 1074.73 MPa, Pure Ti:
484.46 MPa) were larger than those of the cast specimens (Ti-6Al-4V: 967.21 MPa, Pure Ti: 379.85 MPa)
for both metals.
(2) For Ti-6Al-4V, the elongation of the specimens molded by SLS (11.27%) was smaller than that
of the cast specimen (14.72%). In pure Ti, both values were equivalent.
(3) In both metals, the surface roughness of the surface of the building direction of SLS-molded
specimens (Ti-6Al-4V: 5.04 µmRa, Pure Ti: 5.91 µmRa) was larger than other building direction
(Ti-6Al-4V: 3.65 µmRa, Pure Ti: 3.40 µmRa). The value was smaller than that of the cast specimens
(Ti-6Al-4V: 8.75 µmRa, Pure Ti: 7.71 µmRa). After polishing, there was no difference in surface
roughness between the molding methods in both metals.
(4) For Ti-6Al-4V, the sintered contact angles of the SLS-molded specimens (98.14◦ ) were larger
than those of the cast specimen (68.86◦ ). In Ti-6Al-4V after polishing and pure Ti, there were no obvious
differences in the molding method.
(5) For Ti-6Al-4V, the sintered Vickers hardness of the SLS-molded specimens (366.84) was less than
that of the cast specimen (650.99). In Ti-6Al-4V after polishing and pure Ti, there were no differences in
the molding method.
(6) In both metals, there were no differences between the molding methods regarding color
difference after sodium sulfide immersion. The color difference of pure Ti was larger than that
of Ti-6Al-4V.
(7) In both metals, there were no differences between the molding methods regarding lactic acid
weight loss. Lactic acid weight loss in Ti-6Al-4V was larger than in pure Ti.
(8) For Ti-6Al-4V, the color difference of the SLS-molded specimens after lactic acid immersion was
greater than that of the cast specimen. In pure Ti, the values were small, and there was no difference
between the molding methods.
(9) In both metals, there were no obvious differences between the molding methods for XRF, XRD,
and surface structure.
From the above, it is concluded that SLS molding using either Ti-6Al-4V alloy powder or pure Ti
powder is an extremely excellent molding method for clinical dentistry.

Author Contributions: Our study was carried out with collaboration of all authors. T.M. and A.S. conceived this
study. H.A. prepared the materials and equipment for this study. Y.H. and S.W. performed the experiments. Y.H.,
Y.I. and D.M. analyzed the data. Y.H., T.M. and A.S. wrote the manuscript. All authors have read and agreed to
the published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.

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