Impact of Density Variations and Growth Direction in 3D-Printed Titanium Alloys on Surface Topography and Bonding Performance with Dental Resins

Abstract

Titanium-based dental prostheses are essential for prosthodontics and can now be 3D printed using powder bed fusion (PBF) technology with different densities by controlling the process parameters. This study aimed to assess the surface topography and bonding strength of dental resins made of 3D-printed titanium alloys with varying densities and growth directions. Three groups of titanium alloy (Ti6Al4V) specimens differentiated by density (low, medium, and high) were produced using laser-melting 3D printing technology (N = 8). Each group included specimen surfaces with vertical and horizontal growths. Vickers microhardness, surface profilometry, wettability, and shear bond strength (SBS) of the titanium samples were measured for all groups. Scanning electron microscopy (SEM) was performed. Statistical analyses were conducted using a two-way ANOVA and Fisher’s multiple test. Higher-density specimens exhibited greater microhardness (p < 0.05), and those with horizontal growth directions were harder (p < 0.05) than their vertical counterparts within the same density category. Additionally, low-density specimens in both growth directions had the highest surface roughness values (p < 0.05) compared to the other groups. The wettability values were similar (p > 0.05) among the groups in the vertical direction, but not in the horizontal direction (p < 0.05). However, the density type did not significantly (p > 0.05) influence the bonding strength of 3D-printed titanium. This study revealed significant variations in surface roughness, contact angle, and microhardness based on density and growth direction.

1. Introduction

Titanium alloys, such as Ti6Al4V, Ti64, and Ti Grade 5, play a crucial role in prosthetic dentistry and serve as essential materials for the fabrication of various dental implants and prostheses [1,2,3,4]. These alloys are considered an optimal option for dental prostheses because of their excellent mechanical strength, superior corrosion resistance, remarkable biocompatibility with human tissue, and osseointegration ability [1,3,5,6,7]. Additionally, the lightweight nature of titanium alloys ensures patient comfort and enhances the overall efficiency of dental treatments [4].

The use of titanium alloys in fabricating dental prostheses, such as removable and fixed dental prostheses, has historically been limited because of the challenges associated with conventional manufacturing methods, such as lost-wax and casting techniques [6,7]. These limitations arise from the high melting point of titanium alloys and their susceptibility to oxidation, necessitating the use of specialized furnaces and casting machines. The precision and complex geometry required for dental prostheses further complicate the manufacturing of metallic dental prostheses using traditional methods, often resulting in dimensional shrinkage, increased costs, and longer production times [8].

Recent advancements in digital manufacturing methods, such as computer-aided design/computer-aided manufacturing (CAD/CAM) technologies, and additive manufacturing techniques, such as 3D printing, have revolutionized the use of titanium materials in the production of dental prostheses, addressing the challenges associated with conventional methods [4,5,6]. These digital manufacturing methods enable the production of highly accurate and precise dental products, significantly improving the fit of prostheses and the overall treatment quality [8,9]. Additionally, digital methods can expedite the fabrication process, reduce labor, and save time, thereby lowering overall treatment costs [9,10]. A crucial advancement in digital manufacturing is the ability to create patient-specific structures with tailored mechanical and physical characteristics (e.g., density, porosity, and surface roughness) for each dental application [3,9,11].

Additive manufacturing technology, known as Powder Bed Fusion (PBF), involves the incremental building of an object in multiple layers through the localized melting and solidification of metallic powder by a laser or electron beam in accordance with CAD data [2,3,9,11]. The fabrication process of a metallic object using Powder Bed Fusion (PBF) technology begins by spreading a layer of metal powder (25–45 mm in particle size and 0–100 mm in layer thickness) onto a substrate using a counter-rotating roller. Following this, a high-power laser or electron beam targets specific regions outlined by the CAD data file, melting and/or fusing the powder particles in these areas while the surplus powder remains unfused [8,9,12]. PBS involves numerous methods, such as selective laser melting (SLM), laser sintering (LS), direct metal laser sintering (DMLS), and selective electron beam melting (SEBM) [8,9,12]. Each of these techniques differs in terms of the type of energy source or material used to selectively fuse powder particles [9].

The PBF method has recently gained attention in the medical and dentistry fields, presenting substantial potential for enhancing the quality and speed of treatment [3,8,9]. The quality of objects printed through PBF is influenced by various factors, including powder characteristics and features such as particle size, shape, distribution, and chemical composition [3,12]. Additionally, parameters such as laser/electron type, power, spot size, layer thickness, building angle, and scan speed can affect the properties of the processed object [3,12,13,14]. One main limitation of PBF is the surface quality, mainly because of the “stair step” effect caused by the layering process that raises surface roughness [3]. Another crucial aspect is the density of the printed objects along with the formation of porosity, which can affect their overall properties [3]. Therefore, it is essential to meticulously control process parameters such as heat power, layer thickness, scan speed, and hatch spacing to improve the overall quality of the produced object [3].

Titanium alloys are essential in dentistry, particularly for implants used in both roots and abutments [3,6,9,10]. They are also utilized in dental crowns and bridges, especially for posterior teeth [6,7,9]. Additionally, titanium alloys can be used in the frameworks of removable partial dentures and overdentures [6,7,9]. Orthodontic appliances and maxillofacial prostheses benefit from titanium alloys, which have excellent strength, flexibility, and lightweight properties, for effective force application [10]. However, these dental applications often involve the use of titanium alloys in combination with other materials, such as acrylic resins in dentures and orthodontic appliances, or dental ceramics in crowns and bridges. A notable challenge in dental prostheses containing both metallic and polymeric components is the risk of catastrophic mechanical failures at the interface between metals and acrylic resin materials owing to weak bonding [1,6,7]. Thus, establishing a strong bond between titanium alloys and other materials is essential for preventing mechanical failure [1,6,7].

Bonding between different material surfaces can be achieved using chemical or mechanical methods [1,7,15,16,17]. A common method to enhance the bond strength between titanium and acrylic resin is mechanical bonding. This is often achieved by introducing surface roughness using techniques such as sandblasting or air abrasion, where abrasive particles are propelled by compressed air to create a textured surface, thereby improving bond strength [1,7,18]. Sandblasting is a simple and affordable surface preparation method used in dentistry to enhance the wettability of bonded surfaces, resulting in a higher bond strength between the resin and metal alloys [18].

Many studies have investigated and optimized the mechanical and physical properties of titanium objects printed by PBF by controlling the processing parameters, with most efforts aimed at producing fully dense objects with minimal porosity [2,3,11,16]. Although producing fully dense titanium with eliminated porosity and improved surface quality can enhance the mechanical properties of the printed object, it might affect the bonding performance of the dental resin when fabricating dental prostheses. Furthermore, the orientation of the growth direction, whether parallel or vertical to the building platform, can affect the mechanical properties and surface quality of the 3D-printed objects [19]. Thus, variations in the densities and growth directions of 3D-printed titanium can influence the properties of alloys, subsequently affecting the bonding strength between titanium and other materials.

The objective of this study was to assess the surface topography and bonding strength of 3D-printed titanium alloys with varying densities and growth directions bonded to dental resins. The null hypothesis proposes that there is no statistically significant difference in the surface properties and bonding strength of dental resins among different densities and growth directions of 3D-printed titanium alloys.

2. Materials and Methods

2.1. Sample Preparation

Three distinct groups, distinguished by varying densities, were produced using 3D printing technology (N = 8). The specimens were cubic in shape with dimensions of 10 mm. Subsequently, two distinct specimen surfaces, according to the growth directions (vertical and horizontal), were investigated for each group (Figure 1). The specifications of the test specimens are listed in

Table 1. Type of the testing specimens.

Group	Density Type	Growth Direction Surface
L-V	Low	Vertical
M-V	Medium
H-V	High
L-H	Low	Horizontal
M-H	Medium
H-H	High

.

This study utilized titanium alloy Ti6Al4V ELI powder;

Table 2. Chemical composition of powders by percentage.

Powder Type	AL	V	Fe	O	N	C	H	Ti
T1	6.5	3.9	0.2	0.11	0.03	0.01	<0.01	Bal.

displays the nominal chemical analysis. The powder particle size distribution (PSD) was characterized by D10 at 20.6 µm, D50 at 32 µm, and D90 at 48.6 µm. The powder was gas-atomized and spherical in shape. The laser melting machine was an AM250 model from Renishaw, UK, with a build envelope volume of 250 mm × 250 mm × 300 mm. The machine was equipped with a 200 W laser, which had a minimum nominal diameter of 70 µm for the laser beam and a wavelength of 1070 nm. The build platform was pre-heated to 170 °C, and all samples were fabricated in an Argon atmosphere, maintaining an oxygen level below 0.1%.

The chosen samples were fabricated as part of a comprehensive investigation to assess the essential process parameters affecting the quality of the fabricated parts in a PBF system [20]. The samples were carefully selected for the current study to be representative samples for high, medium, and low relative densities. The densities of the samples were determined using Archimedes’ principle [20]. The relative densities of samples were 88.88%, 96.04%, and 99.96% for samples 1(L), 2(M), and 3(H), respectively.

Table 3. Process parameters.

Parameter	Sample
	1(L)	2(M)	3(H)
Layer thickness, LT—(µm)	30	50	70
Laser power, LP—(W)	90	200	90
Point distance, PD—(µm)	45	45	70
Exposure time, ET (µs)	100	150	70
Hatching distance, HD—(µm)	85	50	100

displays the process parameters for the samples, specifically for the selected samples in the current study.

The samples were then washed with Ultraclean SA solution and water (5:100 mL) for 15 min at 45 °C to eliminate any remaining powder.

The 3D-printed titanium (Ti) specimens from all groups underwent a polishing procedure using a polishing machine (EcoMet/AutoMet 250, Buehler, Lake Bluff, IL, USA) equipped with silicon carbide papers (600, 100, and 1500 grit). Subsequently, the samples were polished on two distinct polishing cloths (medium and fine cloths) using a polishing paste (Abraso-Starglanz; Bredent, Senden, Germany). Subsequently, the specimens were sonicated for 5 min in a sequential bath of distilled water and alcohol. Next, all groups underwent surface treatment through sandblasting using an identical machine (GOBI-2, Wassermann Dental-Maschinen GmbH, Hamburg, Germany) with aluminum oxide (Al₂O₃) particles with a size of 50 µm, blasting duration of 15 s, blasting angle of 45°, and blasting distance of 25 mm. To minimize procedural variation, a single operator conducted all the sandblasting procedures.

To prepare the test specimens for the shear bond test (SBS) between titanium and heat-cured resin, individualized molds were created for each specimen (Figure 2). The molds were crafted by producing a wax replica of the test specimen affixed to the titanium sample. A mixture of dental stone and plaster (1:1) was poured into the lower/upper part of the flask until the flask was filled to the top surface of the test specimen. After allowing the mixture to set for approximately 20 min and applying a separating medium to the plaster/stone surface, an additional dental stone or plaster was poured to complete the mold. Following the removal of the wax and cleaning of the mold, dental resin material (PMMA) (Heat Cure Denture Acrylic, Sledgehammer, Keystone Industries, Gibbstown NJ, USA) was mixed according to the manufacturer’s specified ratios and instructions and then carefully packed into the mold cavity. The resin was set in a curing bath under the recommended time and temperature conditions (90 min at 74 °C and 30 min at 100 °C). After the resin was fully cured, a deflasking procedure was executed by separating the upper and lower parts of the flask, removing the excess material, and trimming any irregularities on the surfaces (Figure 2).

2.2. Density Measurement

To confirm the density values for the different sample types, the density of the titanium specimens was determined using the displacement method. Four specimens (N = 4) from each group were measured, with multiple replicates performed for each titanium specimen to enhance the reliability of the measurements. A graduated cylinder was filled with 10 mL of distilled water, and each titanium specimen was gently submerged in the liquid. The precise volume of water displaced by each sample was then measured. The mass of each Ti sample was determined using a calibrated balance. Density was calculated using the following formula:

$$Density=Mass / Volume$$

(1)

2.3. Microhardness Measurement

A Vickers microhardness indenter (Nova 130; Innovatest Europe BV, Maastricht, The Netherlands) was used to assess the microhardness of each group under an indentation force of 500 g and a 10 s dwell time. The mean microhardness values (HV) were derived from three randomly selected specimens (N = 4) from each group, and each specimen underwent five indentations at different locations. A built-in camera captured accurate images at each indentation location for use in the evaluation of microhardness.

2.4. Surface Roughness Measurement

The surface roughness of the ti samples was evaluated across all groups using a non-contact optical profilometer (Contour GT, Bruker, Billerica, MA, USA). Three separate measurements were performed at different sites on the two different printing surfaces for each group. Four samples were measured from each group (N = 4) at a length of 90 m, speed indicated by ×2, and threshold of 4%. The measurement type was set as VSL. Next, the average surface roughness (Sa) values in micrometers (μm) were computed from 12 examinations.

2.5. Wettability Testing

The water contact angle, indicative of wettability, was measured for the titanium surfaces for each group using a camera-based optical tensiometer (Theta Lite, Dyne Technology, Staffordshire, UK). Four specimens (N = 4) from each group were selected, and five measurements were conducted at various locations on each specimen. The contact angle tests involved applying a 5.0 µL droplet of distilled water using a syringe tip to the tested surface fixed on a movable table. After 15 s, when the droplet was stabilized, the illuminations of the contact angle drops were captured by the camera from the opposite side and then connected to a computer and testing machine. The surface free energy was measured using the Owens–Wendt–Rabel–Kaelble (OWRK) methodology [21].

2.6. Shear Bond Test

The shear bond strength (SBS) of the samples (N = 8) was evaluated through a pull-off test conducted on a universal testing machine (Instron Corp., Canton, MA, USA) at a controlled speed of 1.0 mm/min until the resin detached from the surface of the titanium alloys (Figure 3). The maximum force (Fmax) was recorded and preserved for subsequent statistical analysis. SBS was calculated using the following equation:

$$SBS=Fmax / B$$

(2)

where Fmax is the applied load at failure in Newtons (N) and B is the bonding surface area of the specimen (mm²).

2.7. Failure Mode Evaluation

The surfaces of the fractured or de-bonded specimens for each sample from the SBS test were analyzed under a stereomicroscope (Nikon SM2-10, Tokyo, Japan) at a magnification of 15×. The observed fractured surfaces were classified into three distinct failure patterns: “cohesive failure”, occurring within the resin composite; “adhesive failure”, which appears at the interface of the resin composite and Ti surfaces; or “mixed failure”, indicating a combination of both “adhesive” and “cohesive” modes.

2.8. Surface Morphology Test

The surface morphologies of the sandblasted 3D-printed specimens were analyzed using scanning electron microscopy (SEM; JEOL, JSM 5900LV, Tokyo, Japan). One titanium specimen from each group was inspected at an acceleration voltage of 20 kV and magnification of 100×.

2.9. Statistical Analysis

The study’s sample size was determined using G*Power software (v.3.01; Kiel, Germany) based on a pilot study (N = 5) with an expected effect size of 0.52, an alpha of 0.05, and 80% power. The Shapiro–Wilk test was used to assess the normal distribution of the data, and the mean and standard deviation (SD) were calculated. Statistical differences between the study groups in surface roughness, contact angle measurements, and shear bond strength (SBSs) were assessed using one-way analysis of variance (ANOVA) and Tukey HSD tests for density and SBS tests, whereas a two-way ANOVA was used for the other tests. The Origin program (v.9.0; Origin Lab, Northampton, MA, USA) was used for statistical analyses, with the significance level set at α = 0.05.

3. Results

The results presented in

Table 4. Density measurement results (mean ± SD) for the 3D-printed titanium group.

Group	Density Type	Volume (cm3)	Mass (g)	Density (g/cm3)
L-V/H	Low	0.94 ± 0.63 a	3.45 ± 0.13 a	3.67 a
M-V/H	Medium	0.95 ± 0.86 b	4.03 ±0.15 b	4.24 b
H-V/H	High	0.97 ± 0.11 c	4.16 ± 0.57 b	4.29 b

Different lowercase letters indicate significant differences between the groups in the rows (p < 0.05).

show the density measurements of the 3D-printed titanium groups with different density types: low (L-V/H), medium (M-V/H), and high (H-V/H). The calculated density data indicate an increase from 3.67 g/cm³ in the low-density group to 4.24 g/cm³ in the medium-density group and 4.29 g/cm³ in the high-density group.

Table 5. Microhardness Vickers measurement (HVs) for different densities of the 3D-printed titanium group with two growth directions.

Group	Density Type	Growth Direction Surface	Microhardness Value (HV) (Mean ± SD)
L-V	Low	Vertical	375.3 ± 0.8 a
M-V	Medium		403.2 ± 2.3 b
H-V	High		464.8 ± 2.6 c
L-H	Low	Horizontal	373.0 ± 2.5 a
M-H	Medium		405.5 ± 0.6 b
H-H	High		486.8 ± 1.8 d

Different lowercase letters indicate significant differences between the groups in the rows (p < 0.05).

presents the microhardness values for the 3D-printed titanium samples with different densities and two growth directions. The table reveals that the microhardness values of the 3D-printed titanium specimens varied significantly based on their density and growth direction. Lower-density specimens generally showed reduced hardness, with the low-density (L-V and L-H) groups having the lowest values of 375.3 ± 0.8 and 373.0 ± 2.5 HV, respectively. The high-density specimens with the horizontal growth (H-H) group achieved the highest microhardness value of 486.8 ± 1.8 HV.

Figure 4 shows the surface roughness (Sa) values for different groups of 3D-printed titanium specimens with varying densities and growth directions. The L-V (low density—vertical growth) and L-H (low density—horizontal growth) groups (3.03 ± 0.41 and 2.78 ± 0.42, respectively) exhibit the highest surface roughness values (p < 0.05) compared to the other groups (M-V = 2.10 ± 0.34, M-H = 1.91 ± 0.29, H-V = 2.34 ± 0.30, and H-H = 1.75 ± 0.47, respectively). Figure 5 shows 3D pictograms of the surface roughness images of these groups.

Figure 6 illustrates the contact angle measurements for different groups of 3D-printed titanium specimens characterized by varying densities and growth directions. The L-V, M-V, H-V, and H-H groups (51.95 ± 4.07, 49.48 ± 8.63, 47.31 ± 1.85, and 47.31 ± 1.85, respectively) exhibit similar contact angle values, indicating no significant differences among these groups. The L-H group (63.08 ± 11.41) showed the highest contact angle value compared to the other groups (p < 0.05). The M-H group (55.86 ± 6.84) exhibited a contact angle value that was significantly different from the L-H and H-H groups but not from the L-V, M-V, and H-V groups. Similarly, the H-H group showed a contact angle value significantly different from those of the L-H and M-H groups but not from those of the L-V, M-V, and H-V groups. Pictograms of the contact angle formation on the 3D-printed titanium for different groups are shown in Figure 7.

The results presented in

Table 6. Shear bond strength (SBS) values and corresponding failure modes for 3D-printed titanium groups with vertical growth surfaces.

Group	Density Type	SBS (Mpa)	Failure Mode Analysis (%)
		(Mean ± SD)	AD	CO	MI
L-V	Low	7.08 ± 2.50 a	87.5 a	0	12.5 a
M-V	Medium	6.39 ± 2.33 a	75.0 b	0	25.0 b
H-V	High	6.47 ± 2.26 a	87.5 a	0	12.5 a

Different lowercase letters indicate significant differences between the groups (p < 0.05); AD: adhesive failure mode; CO: cohesive failure mode; MI: mixed failure mode.

summarize the shear bond strength (SBS) values and corresponding failure modes for 3D-printed titanium groups with vertical growth surfaces. The SBS mean values were 7.08 ± 2.50 MPa for the L-V group, 6.39 ± 2.33 MPa for the M-V group, and 6.47 ± 2.26 MPa for the H-V group. The SBS values did not show significant differences (p > 0.05) among the groups, indicating that density type does not have a significant effect on the SBS of 3D-printed titanium within the scope of this study. The failure modes for the L-V and HV groups were 87.5% adhesive and 12.5% mixed, with no cohesive failures, whereas they were 75.0% adhesive and 25.0% mixed, with no cohesive failures for the M-V group.

Figure 8 shows SEM micrographs of 3D-printed titanium with varying densities and growth directions bonded to the dental resins. Distinct microstructural patterns were observed in the SEM images. The L-V and L-H groups exhibited defects and porosities on the polished surfaces, while the M-V and M-H groups displayed some irregular morphological patterns. In contrast, the H-V and H-H groups presented smooth surfaces with fine patterns.

4. Discussion

This in vitro study aimed to assess the surface topography and bonding strength of 3D-printed titanium alloys with different densities and growth directions bonded to dental resins. Significant differences were observed in the surface topography among the different densities and growth directions of the 3D-printed titanium; however, no significant differences were observed in the bonding strength. Therefore, the null hypothesis was rejected.

The authors are unaware of any previous study that has investigated the surface roughness of 3D-printed titanium with varying densities and growth directions and its relation to bonding with heat-cured resin. The outcomes of this study are clinically relevant, as they demonstrate how controlling densities and growth direction surfaces in the 3D printing process can influence the physio-mechanical performance of 3D-printed metallic frameworks for dental and medical prostheses. Understanding these effects is crucial for optimizing the production of prosthetic components to ensure better durability, fit, and overall functionality in clinical applications [22]. In addition, the low density of lightweight alloys, high strength-to-weight ratios, low thermal conductivities, and high ductility of titanium enable its use in a wide range of dental applications for titanium restorations and removable prostheses [23].

The density measurements presented in Table 4 confirm the different densities of the three groups. Parameters such as layer thickness, laser/electron beam power, scan speed, hatch spacing, powder characteristics, build orientation, and post-processing techniques can significantly control the density of 3D-printed objects [9,20]. Different densities were achieved by adjusting the process parameters, as demonstrated in a previous study [20]. This study identified the optimal combination of parameters to produce fully or nearly fully dense parts and determined the minimum exposure time required for various powder types and layer thicknesses [20].

The microhardness values, as listed in Table 5, varied significantly across different densities and growth directions. Lower-density specimens (L-V and L-H groups) exhibited the lowest microhardness values (375.3 ± 0.8 and 373.0 ± 2.5 HV, respectively) than medium- and high-density groups. The highest microhardness value (486.8 ± 1.8 HV) was observed for the higher-density specimens, especially those with horizontal growth directions (H-H). This suggests that a higher density enhances the hardness of 3D-printed titanium, likely because of the reduced presence of pores and defects, leading to a more uniform and compact material structure (Figure 8) [20]. Additionally, horizontal growth directions also play a significant role, which can be attributed to layer-by-layer construction in 3D printing. Horizontal layers may result in more consistent bonding and fewer interlayer weaknesses, further enhancing the overall hardness [24]. This was also observed in the SEM images (Figure 8) of the H-H and H-V groups, where the H-V surface showed some defects, while the H-H surface was smooth and clear of any defects.

Surface roughness measurements (Figure 4) revealed that the low-density groups, L-V and L-H, exhibited the highest surface roughness values (3.03 ± 0.41 and 2.78 ± 0.42, respectively), significantly higher than the other groups. This increased roughness could result from the porosity of the low-density groups [24]. Additionally, the higher roughness may be attributed to the decreased hardness of the low-density group compared with that of the medium- and high-density groups [17]. Although the growth direction did not significantly affect the surface roughness values, the vertical growth direction tended to be rougher than the horizontal growth direction. This increased roughness in the vertical direction may be due to structural gaps between the layers [24].

The measurement of the contact angle serves as a critical means to characterize the surface properties. A lower contact angle signifies superior substrate wetting by adhesive agents, thereby facilitating stronger bonding [25,26]. This parameter plays a pivotal role in predicting the adhesive performance of the surfaces. Contact angle measurements (Figure 6) revealed that the L-H group exhibited the highest contact angle value (63.08 ± 11.41), indicative of a less hydrophilic surface compared to the other groups. This observation may be attributed to the increased surface roughness within the L-H group, because rougher surfaces typically yield higher contact angles [23]. Notably, the contact angles of the M-H and H-H groups differed significantly from each other, and the L-H group did not differ significantly from the L-V, M-V, and H-V groups. This suggests that the surface morphology and density collectively influence surface wettability.

The shear bond strength (SBS) values (Table 6) indicated no statistically significant differences among the low-density (L-V), medium-density (M-V), and high-density (H-V) groups, with mean SBS values of 7.08 ± 2.50 MPa, 6.39 ± 2.33 MPa, and 6.47 ± 2.26 MPa, respectively. These findings suggest that within the parameters of this study, the density type does not significantly influence the bonding strength of 3D-printed titanium. Despite the lack of statistical differences among the groups, the highest bond strength was observed in the L-V group, likely owing to its higher surface roughness [17,18]. Previous investigations into the adhesion of dental resins to titanium alloys have reported bond strength values ranging from 0.84 to 19 MPa, depending on the surface treatment applied [6,18,23,27]. The bond strengths observed in this study fell within this acceptable range, although further enhancements in bond strength are recommended [6,16,28].

The failure modes observed in this study predominantly consisted of adhesive failure. Specifically, the L-V and H-V groups exhibited 87.5% adhesive failure and 12.5% mixed failure. In comparison, the M-V group demonstrated 75.0% adhesive failures and 25.0% mixed failures, with no cohesive failures recorded across any group. This suggests that the bond between the resin and titanium substrate typically fails at the adhesive interface. Additionally, the resin fragments on the exterior of some specimens indicated that the adhesive bond was partially retained in some cases. The higher percentage of mixed failures in the M-V group may reflect variations in adhesive bond quality, potentially influenced by surface roughness and morphology.

The manufactured titanium specimens produced in this study demonstrated anisotropic characteristics, displaying variations in microhardness, surface roughness, and wettability depending on the density and growth direction. Despite these variations, the mechanical and physical properties investigated were consistently high, making them suitable for dental prosthetics and capable of withstanding masticatory forces in practical applications [29]. However, other factors, such as biocompatibility, cellular reactions, ion release, and bacterial interactions, may not be suitable for practical prosthodontic applications and require further study.

This study has several limitations, including that it was conducted in vitro using flat specimens, which may not accurately represent in vivo conditions. While anisotropic surface characteristics did not affect the bonding strength of heat-cured dental resin, they may affect bonding with dental cement, ceramic, or composite. Additionally, the study did not include cyclic loading and thermocycling treatments to assess the long-term durability of bond strength on these surfaces. Future studies should investigate the effects of various surface treatments and adhesive formulations on titanium surfaces to better understand their performance under realistic conditions.

5. Conclusions

In this laboratory investigation, the following conclusions were drawn:

• Significant variations in surface roughness, contact angle, and microhardness were observed based on density and growth direction.
• Density type and growth direction influenced microhardness, with higher-density specimens exhibiting greater microhardness compared to low-density specimens.
• Specimens with horizontal growth directions were harder than their vertical counterparts in the high-density category.
• Both density type and growth direction affected surface roughness and wettability, with low-density specimens exhibiting the highest surface roughness values compared to other groups.
• Density and growth direction did not significantly influence the bonding strength of the 3D-printed titanium to the heat-cured acrylic resin.