1. Introduction
Three-dimensional (3D) printing is a computer-controlled additive manufacturing process that creates 3D objects by continuous material deposition. The mechanical properties of raw materials evolve into mechanical properties of a 3D-printed component, including the yield strength, ultimate tensile strength, fatigue, ductility, and brittle behavior. Overall, the manufacturing process depends on the component’s geometry, the raw materials, the manufacturing parameters, and the post-treatment processes (e.g., thermal or thermomechanical processes). Traditional processes are usually either subtractive or additive manufacturing processes; however, traditional additive processes, such as foundry or molding, require tooling design, development, and validation. New additive processes in polymer printing, such as fused deposition modelling, eliminate the requirement for additional tooling stages because either only one material is added layer by layer or composite materials are developed in different ways, such as by adding reinforcements to the matrix. To improve the mechanical behavior of 3D-printed components, printing parameters, such as the velocity, bed temperature, extrusion temperature, and raster direction, have been examined because they can generate internal defects that can result in premature failure under quasistatic or dynamic loads [
1,
2,
3]. Travieso-Rodriguez et al. [
4] revealed the relationship between printing parameters and quasistatic response in terms of the stiffness and bending strength, but they did not define the parameters necessary to result in an improvement in fatigue strength.
There is still scope to improve 3D printing products, not only by evaluating the printing parameters, but also by adopting post-treatment processes to enhance the mechanical properties of the products [
5]. Furthermore, the product’s durability, in terms of fatigue life, should meet the requirements of dynamic loads and statistical parameters, and this should be validated in not just one sample; such validation should be included in lot production to reduce the scattered range found in all components made by considering the same parameters to improve the reliability. By combining printing parameters and optimized designs, the mechanical performance can be improved in a controlled manner [
6,
7]; however, the deposition process itself generates variability, and hence, there is scope to improve the mechanical performance [
8]. The additive metals and plastics are subjected to post-treatment processes, such as thermal treatments, to improve their properties [
9,
10,
11]; one parameter that changes is the hardness of the sample because of microstructural changes. However, the best improvement is observed in homogenization throughout the whole transverse section or as a function of the layer position similar to that observed in dual-phase materials. Homogenization in the microstructure enhances the fatigue strength of the material.
Heat treatments of reinforced plastics or polymer-based compounds show improvements in static and fatigue strengths [
9,
12]. Different heat treatments may influence the fracture behavior but have minimal effects on tensile strength [
13]. The change observed in the mechanical properties depends on the type of heat treatment, as seen in the case of metals. In polymers, thermal treatments can be normalized and annealed [
14].
To evaluate this proposal, the effect of heat treatments on hardness was analyzed by performing fatigue tests. The flexibility of additive manufacturing introduces variation among designs, which can generate diverse sets of printing variables. The inherent uncertainty in 3D printing technology, arising from the complicated interaction of heat transformation and dissipation, leads to more pronounced dispersion compared to conventional additive manufacturing methods, such as injection molding, casting, or subtractive processes. Since the material undergoes a phase change during the printing process, a temperature gradient emerges, giving rise to distinct structures across the cross-section. This gradient of hardness and properties contributes to a dispersion in the strength of the 3D-printed component. To achieve greater reliability, we heat-treated 3D samples at different temperatures, and the effects of these treatments were measured via hardness measurements and fatigue tests to evaluate the dynamic behavior of the printed product [
15]. Polymeric materials are highly likely to be used in 3D-printed components due to their potential for different applications in various industries, such as mobility (automotive and aircraft), medical, and manufacturing industries. Enhancing their behavior requires normalizing the material in the transverse direction across its thickness. This can be achieved through the application of a thermal treatment. Considering the component as a closed system, the process can be modeled by incorporating the conservation laws of mass, linear momentum, and energy.
where
is the density;
P is the pressure;
is the velocity vector;
is the viscous stress tensor; and the specific heat, thermal expansion coefficient, and the heat flux vector are expressed by
,
, and
, respectively.
Although the printing process has similarities to injection processes, one of the biggest differences between the two is that in the printing process, material is deposited directly from the nozzle onto the component. Such deposition affects the temperature gradient. This effect can be evaluated by a shift function (
) using the polymer parameters and the reference temperature (
), expressed by the Williams–Landel–Ferry relation [
16] as follows:
In this case, the proposed heat treatment is as follows: perform normalization of each one of the pieces, considering the result reported in previous works that the hardness on the bedside is greater than that on the printing side [
17]. The effect of heat treatment has been evaluated by hardness measurements and fatigue life [
18]. It is believed that the component’s position in the furnace influences the treatment process, and placing the printing side in contact with the furnace does not cause deformation by heat treatment. Thus, we can define the position in the process so that the side of the printing bed is not in direct contact with the furnace. This study aims to contribute to this growing area of research in the use of polylactic acid (PLA) printed components as structural components. This investigation contributes significantly through a comprehensive approach to printed PLA using FDM. The treatment involves increasing the printing bed temperature by 5 °C. The experimental results showed that the temperature influences the mechanical behaviors of the 3D-printed PLA materials. Hence, it is important to consider multiple printing times because this is the time required for the manufacturing process. The same printing time must be used for the heat treatment to achieve standardization in the component structure. When the oven time equals the printing time, the experimental results’ durability and dispersion are improved.
3. Results and Discussion
For designing a heat treatment method to improve fatigue strength, we considered a combination of temperature, time in the oven, and cooling, as previously described. Cooling was performed at room temperature (23 °C ± 2°). Heat treatment was carried out at 60 °C, 80 °C, and 120 °C for 1 h.
Table 3 summarizes the changes in fatigue life after heat treatment at different temperatures.
In the 60–120 °C range, the best approach of using 60 °C was evaluated for different oven times. The hardness at this temperature was 20 HV for oven times of 4, 8, 12, and 24 h. Note that there may be possible biases in these responses. Although the hardness was improved at temperatures above 60 °C, this resulted in a brittle component, as seen from the fatigue life evaluation shown in
Figure 5.
A comparison of the durability of components with and without treatments showed no enhancement in the behavior above 60 °C. Based on these evaluations, the oven temperature was updated to 55 °C, and the minimum and maximum oven times were 8 h and 168 h, respectively. It is believed that the internal homogenization of the component is not possible within a short time.
Figure 6 shows the duration of each treatment regarding fatigue under the conditions above. The graph shows that the longer the oven time, the higher the durability in some cases. The improvement in resistance to fatigue is noteworthy as it correlates with the duration of oven exposure, attributed to the alignment of polymer chains. This is a consequence of the printed components’ volumetric nature; it is necessary to extend the oven time. Therefore, short treatment durations only alter the edges of the component, leading to brittle failure, even more so than without any treatment.
Heat treatment enhances the fatigue strength not only in terms of durability but also in terms of scatter reduction. In cases where the results exhibit a positive trend in durability, a minimum of three components with identical characteristics undergo testing. Conversely, two components were tested when the trend indicated a decline in durability, leading to failure. By its inherent nature, the process of accumulated mechanical fatigue damage is a statistical phenomenon dependent on factors such as the design, the load, the material, and the manufacturing process. Design variations can originate from tolerances that allow for a deviation from the nominal value. Loads were then determined by a load spectrum involving all load cases and variables such as the frequency, load sequence, and environmental conditions. Materials can also exhibit variations in mechanical properties, even from the same supplier or within the same production batch. Moreover, manufacturing processes can improve components’ resistance dispersion for subtractive and additive methods. These parameters collectively contribute to an increased strength variation, leading to failure when the most critical load is applied to the component with the lowest strength, as shown in
Figure 6.
Hence, decreasing scatter in components exposed to cyclic loads is necessary to mitigate the risk of failure.
The first objective of this study was to improve fatigue strength. For this purpose, the mean value of cycles was evaluated at different oven times with the same printing time (8 h). The mean value of cycles improved 1.25-fold, but importantly, the scatter was reduced by 24.2%, increasing the hardness to 19 HV. The results are summarized in
Table 4; the treatment time is described as the oven time.
A temperature of 55 °C (
Figure 7) is optimal for the treatments at a time when the material has more life or resistance to fatigue. It is believed that this behavior provides greater durability to the material as it can withstand more load cycles of tension and compression. The material possesses ductility until it ruptures; on the other hand, test components with a shorter duration tend to be brittle.
During additive manufacturing processes, changes are generated in the properties of the used input material. These changes arise from the state transformation and the layer-by-layer material addition process. This process is based on thermo-mechanical processes and the temperature differences between the printing bed, the printing layer’s temperature, and the molten material being added. Internally, the material develops resistance based on the obtained structural characteristics, while externally, hardness is achieved. For materials produced with these differences, therefore, in this work, the aim is to define an approach that allows the implementation of a thermal treatment standardizing the structure of the component in its cross-section. Initially, the printing time is thought to correlate directly with the overall processing time. The printing time was initially considered to be directly related to the time required for processing. This assumption was based on the results obtained for an oven time of 16 h.
Interestingly, the durability decreased and scatter increased, but importantly, the hardness did not vary. To understand the effect of dispersion, two ranges were defined around this value of 16 h, differing from this value by 1 and 2 h. The same hardness was obtained for an oven time of 14 h, but the component’s durability did not improve without treatment, and a greater dispersion was observed. In 15 h, the fatigue resistance increased 1.30-fold, dispersion decreased by 15.1%, and hardness was reduced to 17 HV. Although at 17 h, the durability was increased 1.49-fold, the dispersion was only reduced by 7.4%; the hardness increased to 19 HV. While analyzing the duration, cycles alone may appear optimal; this approach yields the peak value. However, the integration of dispersion to enhance reliability does not consider of the global dispersion in load, material, design, and manufacturing processes. This could lead to premature failure with increased variability. Ultimately, the hardness was sustained at an oven time of 18 h, but the fatigue resistance decreased and dispersion increased.
These results show similarities with those reported by Shbanah et al. [
5], who evaluated the improvement in mechanical properties at 55 °C and 65 °C for 5 h. They reported the best mechanical behavior at 65 °C; however, they adopted a quasistatic evaluation and considered a bed platform temperature of 60 °C. Although the oven time was 5 h, the printing time was 4.5 h. More surprisingly, the oven time was correlated with the printing time. It is possible to define a relationship between the oven time and the same printing time, as shown in this work. Comparing the durability of untreated components with treated components, there is a reduction of 41.5% at 60 °C, 52% at 80 °C, and 78. 6% at 120 °C. The optimum parameters for heat treatment are a temperature of 55° and the same printing time (8 h). This increases the mean duration value at cyclic loads 1.25 times, reducing dispersion by 26.7%
To analyze the effect of the heat treatment on the standardization of the behavior of the component, an optical microscope was used. Three different materials were analyzed: a component without heat treatment, the part with the best durability, and a part with low durability.
Figure 8a shows the specimen without heat treatment. Ductile and brittle behavior can be observed in the same component. On the boundary, it has a lighter color due to ductile behavior. The reddish color in the middle of the cross-section of the component is a result of brittle behavior.
Figure 8b shows a specimen after heat treatment for 96 h at 55° with a zoom of 0.8×. Homogenization is shown as whitish bands, resulting in an improved fatigue strength. This component has a fatigue life two times longer than that without treatment. With a treatment of 60 °C for one hour, the structure generates brittleness, as is shown in
Figure 8c. The durability diminishes by 92.1%.
Figure 8d shows both failure mechanisms; the lower part of the image shows the brittleness and the upper part shows the ductile failure mechanism. In fragile zones, it is observed that failure did not cause any type of deformation, only detachment, i.e., the component separates. In the case of failure due to ductile behavior, plastic deformations and slower crack propagation occur until catastrophic failure occurs.
To analyze the behaviour, the component with the worst fatigue strength (
Figure 8c) was analyzed via SEM.
Figure 9a,b show the ductile behavior on the boundary generated by homogenization.
Figure 9c presents the failure in the internal void. This observation may support the hypothesis that the best durability is achieved by homogenizing the component structure along the cross-section.
The results of this study show that the fatigue strength of PLA 3D-printed materials can be improved, and the temperature of treatment increases with the temperature of the printing bed at 5 °C. The same printing time must be used to achieve standardization in the component structure. With very short durations, only the edges of the printed component are treated, generating a ductile structure, but within the cross-section, a fragile structure remains. Hence, it is important to state that heat treatments are necessary for additive manufacturing to achieve a homogenized microstructure, reduce scatter, and improve the reliability of the printed components.
The process of accumulated fatigue damage was analyzed by comparing the applied loads with the component strength. This resistance considers the dispersion parameters in design, the material, and the manufacturing processes. The probability of failure is also reduced by reducing the scatter of the component resistance.
Overall, these results indicate that it is possible to evaluate fatigue life strength enhancements as a quantitative value and by using the number of the cycles. Qualitatively, the best improvement was achieved in terms of the scatter. When printing polymers, predicting the dynamic response of fatigue life with a direct parameter such as hardness is impossible.