1. Introduction
Titanium and its alloys represent materials for various applications in mechanically stressed structures exposed to specific environmental conditions [
1]. At the same time, it has a low specific weight compared to steel [
2]. Typical areas of application of titanium alloys are, for example, aerospace or marine engineering [
3,
4]. The paramagnetic nature of these materials makes it particularly suitable for use in instrumentation applications such as electron microscopy [
5]. Another specific sector using titanium alloys is the field of body implants [
6,
7,
8]. An essential factor in applying titanium alloys over a wide temperature range, typically e.g., in the aerospace sector [
9], is the dilatation characteristics. As a result of heating or cooling by the external environment, the component changes its shape and especially its dimensional characteristics [
10]. For example, the declared value of the thermal expansion coefficient for Ti6Al4V alloy is approximately 8.5 × 10−
6 K
−1 [
11]. The quantity value represents the change in unit length dimension when heated or cooled by one temperature degree. Over a wide temperature range, a significant dimensional change may occur, and the proper function of the component itself or an assembly, including other constituents, may be compromised. The resulting thermal deformation and dimensional changes can lead to deformation stresses and force effects [
12]. Possible differences in thermal expansion characteristics in different directions, e.g., in orthotropic materials, may result in deformation in multiple directions due to different expansion characteristics [
13]. In comparison with steel, for example, the relevant titanium alloy shows approximately half the value of the expansion parameter. In the case of aluminum alloys, it is approximately one-third the value.
In comparison with polymeric materials, it is a quarter. Ti-based alloys are more expensive compared to steel materials. The alternative to Ti alloys is usually stainless steel, but these do not include the same specific attributes, notably low weight [
14]. A significant difference in mechanical characteristics is the modulus of elasticity, which is lower in titanium than steel; the components deform more quickly in the elastic deformation region. The consequence of the low modulus of elasticity is poorer machinability, mainly because of the easy deformation due to cutting forces during the machining process.
A progressive production technology applicable to the processing of titanium alloys is the additive manufacturing method of Laser Powder Bed Fusion of Metals (PBF-LB/M) [
15,
16]. The melting and bonding to the formed body takes place in layers [
17]. The tool path and how the material is joined create a layered structure. The influence of the laser scanning speed is an important factor in the resulting microstructure of the Ti6Al4V alloy [
18]. Phase transformation also occurs during PBF-LB/M processing; there are several studies in the literature documenting this [
19,
20,
21]. The morphology of the primary alpha phase is similar to the lamellar microstructure of the alpha phase in conventionally processed Ti6Al4V alloy [
22]. This fact is therefore an argument in favor of the use of this technology for commercial applications.
This paper presents the results of the effect of topological layered material structure on the dilatation behavior of Ti6Al4V alloy. Thermal expansion and related factors are considered. The aim is to determine the dilatation behavior depending on the respective printing topology and to derive general dilatation characteristics for 3D-printed titanium solids. An optodigital non-contact method is used to measure the deformation during temperature variation in the range from −70 to +60 °C. The obtained results can be applied to analytical and numerical computational procedures applicable during the design of components made by additive manufacturing from Ti6Al4V material and used in functional dimensional chains of thermally stressed assemblies.
2. Materials and Methods
Samples made using the additive PBF-LB/M method from Ti6Al4V material were used to measure dilatation and surface and structural characteristics. The parameters of the manufacturing method and the declared characteristics of the samples are given in
Table 1.
The geometrical parameters of the test sample bodies, including the markers for the optodigital strain sensing, are shown in
Figure 1. The print topologies, representing the layered structure concerning the reference geometry of the test body samples, are shown also in
Figure 1. A representation of the solids of the corresponding topology relative to the reference coordinate system of the production facility are shown in
Figure 1. The layered structure is characteristic of the PBF-LB/M additive manufacturing method, and the influence of the layers is the subject of the test results presented. The manufacturing method is chosen based on the declaration of the layout of the production equipment and the usual parameters recommended to achieve the optimal structural mechanical characteristics. Based on the results presented, producing the relevant components using the usual declared methods is assumed for application use. The shape and dimensions of the test samples are chosen to ensure a sufficient volume of material comparable to the expected volumes of the real components, for which the declared manufacturing method is applied. At the same time, their dimensions are adapted to the optimum parameters of the available measuring equipment. The aim is to set up the samples for measurement reliably and to capture the position of the reference features with an optodigital measuring device.
PBF-LB/M is an additive method based on the local sintering of powdered metal with a laser beam. The samples for testing the thermal behaviour, with the parameters shown in
Table 1 and
Figure 1 and
Figure 2, are produced on a Renishaw AM250 (Renishaw, Wotton-under-Edge, UK). The 3D printing parameters are shown in
Table 2. This standard 3D printing method is recommended for producing functional parts.
Technological characteristics for producing samples can be found in
Table 2. Using the PBF-LB/M method, they represent the commonly available production equipment parameters.
The data in
Table 2 represent the mean values of the parameters achieved by the available range, specific to the equipment used to produce the test samples. All samples are produced with the same parameters. The results of the broad temperature parameters are based on samples produced by a specific technology and any technical application. Samples produced by the PBF-LB/M additive method have shape, dimensional, and object characteristics adapted to the test method. The samples are clamped in the flat jaws of the test system so that degrees of freedom are taken in all directions, either by fixation or by measuring the relevant characteristic. The samples are not up-prepared for testing by any of the post-processing methods. The sampler recommends that the scanning speed be used to achieve acceptable productivity while maintaining the declared mechanical characteristics of the products. The positioning speed represents the auxiliary movements of the device during printing, repositioning the beam to the starting position to start the formation of each layer. The repositioning speed does not affect the product’s quality characteristics, but it does affect the productivity of the print.
The bill of materials declares the material structure of the supplier-prepared samples. For the practical application of the research results for real design solutions, it is assumed that the components are manufactured by the supplier using the declared method, or that the technological parameters can be requested from the supplier if the product is manufactured on different equipment. The presented research results are for reference. Since the manufacturing methods offer a wide range of parameters, variations cannot be excluded. The powder compositions for producing samples with mechanical properties according to
Table 1 and technological parameters according to
Table 2 are presented in
Table 3.
The measurements are performed on three variants of mutually perpendicular printing topologies. Topologies A, B, and C represent a 90° rotation of the layer direction within the identical printing plane lying in the technological plane XY of the production facility. Topology B represents the orientation of the print planes perpendicular to the Z-axis.
The temperature environment for testing and controlling the temperature progression is generated by a Eurotherm 3119 chamber (Instron, Norwood, MA, USA). Low temperatures are achieved using liquid nitrogen retained for temperature control in a Wesington PV-120 Dewar (Wesington, Houghton-le-Spring, GB). A combination of nitrogen cooling and electric heating controls and stabilizes the temperature at a specific value. The temperature generation in the chamber is performed within a control system, ensuring a uniform distribution of the temperature field around the sample and an appropriate temperature value, measured using a temperature sensor located in the flow environment of the temperature medium. A separate independent temperature sensor simultaneously performs temperature control in the immediate vicinity of the sample. The temperature progression is controlled in the temperature range 20 °C, −70 °C, +60 °C, 20 °C. Cooling and heating are controlled with a bidirectional gradient of ±5 °C·min
−1. The temperature waveform is graphically shown in the graph in
Figure 2.
The mechanical tests are performed on an Instron Electropuls 10000 (Instron, Norwood, MA, USA), which can generate combined loads in the direction of the piston axis and rotation around the piston axis. Torsional characteristics are not tested. Only the axial axis stabilizing force is used to set up the sample and create the test conditions. Consistent external conditions and test stability are achieved by generating a constant moderate compressive load of 1 kPa. The deformation measurement due to thermal loading is performed using an Instron video extensometer (Instron, Norwood, MA, USA). Based on capturing and evaluating the position of the geometric pattern in the form of a thick dot of a distinctive contrasting color, the change in the position of the geometric center of the pattern is evaluated. The extensometer allows simultaneous sensing of the position of points in two mutually perpendicular directions. The positions of the displaced points due to thermal deformation in two mutually perpendicular directions are recorded in the declared thermal points within one temperature curve. At the beginning of each measurement, the initial separation distances of the corresponding pairs of points are evaluated. The relative deformation concerning the initial positions of the points is then determined from the initial distances. The dilation experimental measurement parameters are listed in
Table 4.
The measurement is non-contact, and the extensometer is outside the temperature chamber. The principle of strain measurement is achieved via the video extensometer. The control of the measurement process and the control of the Instron Electropuls 10000 test rig is performed via the Instron Console V9 control software (Instron, Norwood, MA, USA). The temperature profile and test method are controlled through Bluehill Universal V3 software (Instron, Norwood, MA, USA). Calibration and control of strain values are performed via the Instron Video extensometer control software as part of Bluehill Universal V3 (Instron, Norwood, MA, USA). The accuracy and determination of measurement uncertainty are defined through statistical methods and calibration results of the used gauges. The sample standard deviation of the arithmetic mean of the measured values for each test set determines the type A uncertainty. The type B uncertainty is determined from the calibration sheet of the gauge used. The parameters given for the results represent the expanded uncertainty of the measurement with an expansion factor of 2, which, for a normal distribution, gives a coverage probability of about 95%.
Each test is performed on ten different samples of each topology, A, B, and C, to eliminate the possible factor of material changes at each declared temperature cycle.
X-ray Fluorescence (XRF) analysis was carried out on samples A, B, and C type structures using a Prospector 3 instrument (Elvatech, Kyiv, Ukraine) with an energy range of 0–10 keV. Each sample was measured three times, and the results were taken as the mean.
Samples were tested by X-ray diffraction (XRD) using settings reported in [
20] using Bruker D8 Advance instrument (Bruker, Fremont, CA, USA).
The surface mechanical properties of a 3D printed Ti6Al4V alloy were also investigated using a Hysitron TL 750 (Bruker, Fremont, CA, USA) nanoindenter with SPM capability with using the same settings as [
25]. The test method consisted of the trapezoidal load function shown in
Figure 3. Samples were tested before and after thermal-mechanical loading (TML) in two planes to the axis of each sample topology: A—perpendicular to the print axis and B—parallel to the print axis. Samples were tested in two planes to the axis of each sample topology: A—parallel to the print axis and B—perpendicular to the print axis.
Microstructure observation on individual samples was carried out using Videomicroscopes Hirox KH 7700 (Hirox, Limonest, FRA) with polarized light. A modified procedure based on [
26] was chosen for etching, i.e., using a modified Keller solution: HNO
3:HCl: CrO
3 (5:3:2) original form (leads to etching in the form of white lines on grain boundaries, without breaking the surface) 5:3:2 mL to 190 mL H
2O, reinforced form (leads to grain boundary etching) 50:30:20 mL to 100 mL H
2O. In both cases, repeated etching was performed in the 5–10 min range per batch. Rinsing was conducted in H
2O, and drying was sped up with ethanol. Samples were tested in two planes to the axis of each sample topology: A—parallel to the print axis and B—perpendicular to the print axis.
3. Results and Discussion
The microstructures of the samples before and after thermal-mechanical loading are shown in
Figure 4, where on the left are the samples before thermal-mechanical loading and on the right after loading [
27,
28,
29].
The microstructure of samples prior to TLM consists of α and β phases grains oriented according to the production process (3D printing). Mechanical properties obtained from the nanoindentation test clearly show the difference between parallel and perpendicular orientation, as seen in
Table 4. However, it is different for the post-TLM samples. Since the same etching method and agent were used to prepare microscopy samples, the reason must be in the material and treatment.
There are still α and β phases grains, but not many visible oriented structures are caused by the production process (3D printing). Mechanical properties obtained from nanoindentation are isotropic in the parallel and perpendicular orientation (compared to the samples pre-TLM, also in
Table 4).
Differences between structure and nanomechanical properties of samples pre- and post-TLM would be expected for PBF-LB/M treatment, which is more or less a classic powder metallurgy “sintering process“. The thermal cycling in TLM is only between −70 °C and 40 °C, and the structure should be compact, homogeneous, and mostly stable. Both the structure and mechanical properties of the material had changed. If such an observation is correct then the conclusion is that providing a relatively homogeneous microstructure at a suitable low temperature and pressure for TLM treatment is possible.
These results should be used as a starting point for additional research to prove the results and to further study mechanical behavior and microstructure as a function of time/number of cycles. It should be noted that the combination of thermal and mechanical stress is a source of defects in the structure [
27,
28,
29,
30]. What looks like defects in
Figure 4 post-TLM (b, d, and f sections) were not originally present on the cross-section surface during polishing. It is a result of the etching of the structure. Rather, these are spaces where etched-out grains had been. Since the etching attacks mainly the inter-crystalline and inter-granular interfaces, some of these interfaces have increased energy levels and thus lower chemical stability. Since the etchant does not create these energetic instabilities but renders them visible, the source of this particular behavior must be in the TLM process.
Elemental composition has been verified for all sample types and is the declared composition for alloys designated Ti6Al4V [
31]. The results are shown in
Table 5, and the comparison of individual measurements is shown in
Figure 5.
Figure 6a,b show the collected XRD patterns of measured samples before and after thermal-mechanical loading, respectively. The detected sharp Bragg’s peaks were found to belong to hexagonal closed-packet α/α′ Ti phases [
32,
33]. These phases substantially overlap due to their similar structures and lattice parameters, as already evidenced in [
34,
35]. The β-Ti phase, with its typical peak at around 39.5° [
32], was not detected in measured patterns. Looking closely at patterns collected before thermal-mechanical loading, the only difference seems to be in the intensity of the peak characterizing (002) crystal plane: only in sample C was this peak higher than the peak of the (100) crystal plane.
On the contrary, peaks of crystal planes (100) and (002) were detected to have similar intensities in samples A and C and only in sample B the peak of the (002) plane was detected to be higher than the peak of (100) plane after thermal-mechanical loading. In addition, a broad peak of low intensity appeared at around 37.8° (highlighted in grey rectangular in
Figure 6b) in samples after thermal-mechanical loading. All these findings suggest that the orientation of printing and thermal-mechanical loading may also affect the crystalline structure of Ti6Al4V alloy.
In terms of surface properties, the samples show different characteristics concerning the orientation of the print, with surface mechanics being an important parameter [
7]. The nanoindentation results for all samples are shown in
Table 6. The influence of thermal-mechanical loading was also evaluated. Samples in the parallel plane showed a higher hardness (H) around 5.7−6.3 GPa compared to the results of 4.3−5.0 GPa for the perpendicular section. Within orientation, the samples showed no variation and were comparable. A similar trend can be seen in the Young’s modulus (E) results. After thermal-mechanical loading, there was a decrease in hardness values, mainly for the parallel cut samples. Other data were comparable. A sample of the typical indentation applied to the material can be seen in
Figure 7.
The results of measuring the expansion characteristics for the individual print topologies indexed A, B, and C according to the declaration are given in
Table 7 and
Table 8.
Table 7 and
Table 8 show the results of axial and transverse deformations at the extreme temperature points, respectively, range. The thermal expansion coefficient for each topology and temperature waveform is determined using linear regression. From the results, the influence of the printing layers on the thermal expansion is evident, especially in the direction perpendicular to each layer.
The corresponding dilatation curves for the individual print topologies are shown in
Figure 8 and
Figure 9. The results for deformations in the axial and transverse directions are shown separately.
The presented results show the influence of the printing layers on the dilatation in the perpendicular direction. Along the press layers, the values in both perpendicular directions are close to the declared value of 8.5 × 10
−6 K
−1. The measured values corresponding to 8.6 × 10
−6 K
−1 agree with other declarations of the thermal expansion coefficient for Ti6Al4V. A significant finding is the thermal expansion coefficient in the direction perpendicular to the printing layers, where a reduction to a value corresponding to 7.6 × 10
−6 K
−1 occurs. This factor is particularly significant in the case of the design of an additively manufactured component subjected to temperature variations during its operation, where precise dimensions in specific directions are crucial, and their variation due to the wide temperature range may exceed tolerance limits, resulting in incorrect function, or completely disabling the intended function [
36,
37].
The similarity of the coefficient values for samples A and C and the dissimilarity of the coefficient value for sample B in the measured temperature ranges for axial and transverse dilatation can be observed. The anisotropy of the layered material structure is reflected here by slightly different values of the thermal expansion coefficient over the entire measured temperature spectrum. This factor is particularly evident when measured in the longitudinal direction, where sample B presents a layer perpendicular to the measured axis. The differences in dilation values measured in the direction perpendicular to the longitudinal axis are even more pronounced. A significantly lower Thermal Expansion value is found in the direction perpendicular to the printing layers, associatively confirming the printing layers’ influence on the dilatation characteristics.
The dilatation characteristics for this material are available only in general terms, usually for the basic variant of Ti6Al4V, regardless of the production method and technological processing. At the same time, an interval is given, usually (8.5–9.1) × 10
−6 K
−1, as listed, e.g., in [
23,
24]. In the temperature range, it directly impacts functionality. In particular, the variant of pattern B, representing layering in the longest direction of the body, shows relatively low values or significantly lower values of the declared intervals. Based on the results obtained, which are at or below the lower limit of the generally declared values, it is possible to consider the influence of the material processing method on the resistance in terms of thermal expansion, which is an advantage for the technical applicability of the material. In addition to aerospace applications, the issue of thermal expansion is a significant problem in the field of body implants. The likely influence of the processing method is also evident for other materials applied in additive manufacturing of PBF-LB/M, e.g., Al10SiMg, as discussed in, e.g., [
25]. Here, the values are lower for different print topologies, as in this text. Testing was performed using electromechanical sensors, thus eliminating any systematic error in the method. In addition to the direct effect on the mechanical behavior of the components over a wide temperature range, the values found are essential for developing manufacturing and assembly technical documentation of components for use in relevant industries using the appropriate additive method. A specific feature of additive methods is the relative ease with which free forms can be produced. The emerging approaches of minimum dimensioning can be used for complex components, combining freeform and exact shape features simultaneously, where most dimensions are contained within the relevant surface shape tolerance. The size of the tolerance field corresponds to the possibilities of the manufacturing methods. Mechanical temperature behavior is a functional attribute for using components over a wide temperature range, and the relevant tolerance must be considered and declared.
For the declaration of the relevant parameters, the effect of the manufacturing method on the thermal expansion can be based on a positive trend in terms of the effect on the overall surface tolerance concerning the three mutually perpendicular component bases, as shown, for example, in
Figure 10. This tolerance is universal and can also be considered in the relevant perpendicular directions to determine the relevant component of the tolerance of the exact dimensions.
The accuracy of the measurement is expressed by an expanded combined standard uncertainty consisting of two components. Type A uncertainty represents the standard deviation of the arithmetic mean. Type B uncertainty represents the metrological characteristics of the measuring equipment used, here an Instron video extensometer (Instron, Norwood, MA, USA) with the appropriate control software Instron Video extensometer control software as part of Bluehill Universal V3 (Instron, Norwood, MA, USA). The declared accuracy class of the measuring device is 0.5, representing 0.5% of the recorded value. For the relevant measuring range values, the value based on the accredited calibration performed represents 0.44% of the recorded value. The combined measurement uncertainty, including statistical evaluation of repeated measurements and metrological characteristics of the equipment, is expanded by a factor of 2 to cover a 95.4% probability interval of correct result. The accuracy of the measured, recorded, and evaluated values is always better than 1% of the evaluated record, with the most significant uncertainty being 0.98%.