1. Introduction
Materials’ processing technology has been continuously developed to improve production efficiency and quality. Computer-aided manufacturing (CAM), one of the most interesting technologies available, offers convenience and cost reduction for manufacturing by using a computer-aided design (CAD) [
1]. In the past, most CAM technologies used only subtractive manufacturing that lost the cut material, such as computer numerical control (CNC), waterjet, and laser cutting. Nowadays, additive manufacturing (AM) or 3D printing is widely used for fabricating complex parts as an up-to-date CAM [
2]. AM technologies can help to reduce material costs and manufacturing time by additively fabricating layer by layer to form the shape as designed by the CAD file. Additionally, these technologies can produce parts with desired material behavior, such as optimized shape and density for specific material properties. With these advantages, AM technologies are widely used in many industries, such as medical, automotive, and aerospace [
3,
4,
5,
6,
7]. Particularly in aerospace industry, a lightweight design of the components for spacecraft was presented by [
4,
5]. The material behavior of the components manufactured by AM was studied by adjusting the print parameters. The development and implementation of AM in the aerospace industry were comprehensively reviewed by [
6,
7], in which the primary applications and associated commercial and technical benefits were summarized.
An AM part can be designed by adjusting its print parameters, such as the number of layers, layer height, wall thickness, infill density, infill pattern, pattern orientation, etc. Many researchers have studied the impacts of print parameters, resulting in the mechanical properties of AM parts using the tensile test [
8,
9,
10,
11,
12,
13]. For example, Hsueh et al. [
9] studied the effect of infill density on the mechanical behavior of polylactic acid specimens fabricated by means of fused deposition-molding technology. The study showed that the Young’s modulus is significantly impacted by infill density. In a study by [
10], the effect of infill density with a triangular infill pattern on mechanical properties of seven different plastic materials was investigated using the tensile test. The results showed that the yield strength and Young’s modulus are directly affected by infill density. As the infill density increased, the tensile yield strength and Young’s modulus increased. Their study also showed that, for some materials, the failure mode also depends on infill density. The effect of layer parameters on the mechanical properties of plastic AM parts made from acrylonitrile butadiene styrene was experimentally investigated by [
12]. Specimens were printed with two different print parameters, i.e., print directions and the number of layers, while the layer height was set as a constant value for all layers. The results show that the print direction impacts the strength of specimens in every number of layers considered. The strength and Young’s modulus became constant at twelve layers or more. The effects of print parameters, e.g., nozzle diameter, on metal AM parts made from 316L stainless steel were investigated by [
13]. It was found that a smaller nozzle diameter gives more geometrical accuracy, but that the mechanical performance decreases. The influence of print parameters on the flexural properties of metal AM parts was also studied by [
14], in which build orientation effects were investigated.
In recent years, metal additive manufacturing (MAM) has received more attention in many sectors for producing metal parts. A well-known MAM technique is powder bed fusion (PBF), which spreads powder to form a metal part fused selectively by a high-energy beam. Since PBF needs strict safety regulations and is expensive, extrusion-based MAM methods are considered an alternative for manufacturing more accessible and safer metal parts without dust and lasers. Recently, bound metal deposition (BMD), a new extrusion-based MAM technology based on material extrusion processes, in which material is selectively dispensed through a nozzle, has been offered by Desktop Metal. This technique can enable low-volume production and economical metal parts. Various materials can be fabricated using the BMD, such as stainless steel, copper, and titanium. The raw material is in the form of a rod comprising a mix of metal powder, wax, and binder. The metal and ceramic rods contained in extruders are fed and fused to form a metal part and ceramic support media, respectively, in the print process. Next, the binder is dissolved in the debinding process and is then densified in the sintering process. After metal particles fuse as a 3D object, the ceramic interface can be removed to obtain the final part. The AM parts fabricated using the BMD can be divided into three portions, i.e., infill structure, side wall, and top/bottom wall, as shown in
Figure 1.
Most AM technologies can print a part with an infill density to reduce the consumption of raw materials and make the part with a lightweight feature. An AM part with an infill pattern inside can be treated as a lattice structure consisting of a number of unit cells. Lattice structures have been widely used as infill structures of AM parts because their mechanical properties can be efficiently designed by adjusting their unit cells. The mechanical properties of lattice structures with various patterns have been studied by many researchers [
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25]. For example, isotropic in-plane elastic properties of lattice structures with triangular unit cells were confirmed by an analytical study by [
15] and an experimental study by [
16]. Methods to determine the effective properties of lattice structures have also been investigated; in a study by [
17], the effective in-plane properties were derived by an analytical method. Theerakittayakorn et al. [
18] proposed the exact forms of effective elastic properties of lattice structures with different patterns using exact curve fitting. In the same lattice structures studied, Sam et al. [
19] presented the derivation of closed-form effective elastic properties of the lattice structures using a generic symbolic finite element program. Effective in-plane material properties of 2D hexagonal lattices with and without considering the shear deformation of their struts were presented by [
22]. The numerical and experimental investigations of in-plane and bending material behaviors of 2D lattice structures with various unit-cell patterns were proposed by [
24,
25], respectively. In AM technologies using the BMD, a lattice structure with a triangular pattern was generally used as the infill pattern of AM parts. The mechanical properties of the AM parts can be optimized by modifying some significant print parameters related to the infill structure to achieve a light weight and sufficient strength for their applications, e.g., layer, density, and orientation.
Print parameters influencing the mechanical properties of MAM parts fabricated using the BMD include infill density, side wall thickness, top/bottom wall thickness, and pattern orientations. Among the print parameters, infill density is directly related to the raw material used and the performance of AM parts, as well as time and cost in the manufacturing process. Consequently, the relationship between mechanical properties and infill density plays a significant role in assessing the specific performance of different infill parts. To date, few studies have reported the mechanical properties of metal parts fabricated using BMD [
26,
27,
28]; in addition, no works emphasizing the influence of infill density on the mechanical properties are yet available. Bjørheim and Lopez [
26] experimentally investigated the mechanical properties of additively manufactured specimens of 17-4PH processed by BMD using a tensile test. Three printing orientations with raster directions were investigated to observe the changes in the mechanical properties of the specimens. They found that the specimens printed on a plane with a raster horizontal direction show the highest ultimate strength and elongation and behave as ductile materials. In contrast, the specimens printed in raster vertical directions almost act as brittle materials. The results show that the specimens with different print orientations yield anisotropic behavior. This behavior is also indicated in the prediction of fatigue behavior estimated from the tensile results. The mechanical properties of 316L stainless steel parts manufactured using BMD were studied by [
27]. Print parameters, which varied in their study, included build orientation, chamber temperature, and infill pattern. Build orientations consist of horizontal and vertical directions. Chamber temperatures were investigated at room temperature and at 50 °C. The infill patterns include a concentric pattern filled by wall thicknesses and a ±45° raster pattern layered by top/bottom thicknesses. They concluded that the specimens built in the horizontal direction have higher ultimate strength than those in the vertical direction. The ±45° raster pattern presented a slightly higher ultimate strength rating than the concentric pattern. In addition to the studies on the mechanical properties, Gao et al. [
29] reported on the energy efficiency and lifecycle of AM parts using the BMD.
Many metal materials can be used as raw materials in MAM technologies, especially stainless steel. In BMD technology, 17-4PH and 316L are basic print materials for AM parts. 17-4PH stainless steel is characterized by its strength, hardness, and corrosion resistance, making it suitable for various applications, e.g., tooling, molds, and production parts. 316L stainless steel is a fully austenitic stainless steel, characterized by its corrosion resistance and performance at high and low temperatures. It is often used in chemical processing, saltwater environments, and household or industrial fixtures. Consequently, 17-4PH and 316L are commonly used in many sectors, and their mechanical behavior has been widely investigated [
13,
14,
26,
27,
28,
30,
31]. In reality, materials are usually loaded with a cyclic load, leading to failure due to fatigue. Due to its ductility, 316L stainless steel is often used to carry a cyclic load. The mechanical properties of 316L stainless steel under cyclic loading have been studied by a group of researchers [
32,
33,
34,
35,
36,
37]. Some of them investigated the fatigue behavior of 316L stainless steel fabricated by MAM technologies. The fatigue behavior of MAM parts made from other materials was also explored [
38,
39,
40,
41,
42].
Since the BMD is a new and attractive technique for MAM technologies, the mechanical properties of productions fabricated using the BMD have not been studied sufficiently, especially on fatigue performance. This study intends to investigate the mechanical properties of MAM parts, i.e., 316L and 17-4PH stainless steel, fabricated using the BMD technique. The considered mechanical properties of the metal parts are elastic modulus, yield strength, ultimate strength, and fatigue limit. The metal specimens with different infill densities ranging from approximately 50% to 75% of the relative density are investigated. All samples are designed for a tensile test according to the ASTM E8/E8M standard [
43]. Some mechanical properties obtained from the tensile test are validated by analytical results from the literature [
17,
19]. Additionally, according to the ASTM E466 standard [
44], the fatigue test is performed to study the fatigue life of the metal parts. The ratio between the weights of each infill specimen and the full-solid counterpart is considered as the weight density of the specimens with different infill densities. The weight efficiency is defined as the relationship between the performance per weight density of an infill specimen and the performance per weight density of a full-solid specimen with the same dimensions. The weight efficiency can be used to assess the efficiency among the infill specimens with different densities.
3. Results and Discussion
In this study, a full-solid specimen was considered as a specimen with 100% density equivalent. The stress–strain relations of 316L and 17-4PH stainless steel full-solid specimens obtained from the tensile test are presented in
Figure 7. It can be seen that the 17-4PH specimens have more strength than the 316L specimens. The 316L specimens have ductile behavior, while the 17-4PH specimens have brittle behavior.
The average values of the mechanical properties of the 316L and 17-4PH full-solid specimens obtained from the tensile test are shown in
Table 3. The yield and ultimate strength of the 17-4PH specimens obviously show more values than the 316L specimens. In contrast, the elastic modulus of both 316L and 17-4PH specimens shows similar values. The average elongation value of the 316L specimens is greater than that of the 17-4PH specimens, i.e., 54.63% and 3.32% elongation, respectively.
According to a datasheet provided by Desktop Metal, the yield strength and ultimate strength of the 316L specimens are 165 MPa and 494 MPa, respectively. With the same test standard, the yield strength and ultimate strength experimentally obtained in this study are approximately 96.81% and 97.63% of those in the datasheet, respectively. Additionally, the yield strength and ultimate strength of the 17-4PH specimens are 660 MPa and 1042 MPa, respectively, while the yield strength and ultimate strength experimentally obtained in this study are approximately 98.98% and 77.72% of those in the datasheet, respectively. This discrepancy could have occurred due to various factors in the manufacturing processes. For this reason, an experimental study is vital for assessing the mechanical properties of the AM parts to reflect their actual behavior in different printing contexts.
The stress–strain curves of infill specimens made from 316L and 17-4PH stainless steel are shown in
Figure 8 and
Figure 9, respectively. The 316L infill specimens show lower strength and behave as a ductile material, while the 17-4PH infill specimens show more strength and behave as a brittle material. Note that the shift of the curves at a strain value of 2% in
Figure 8 is the effect of the test speed change. Meanwhile, the yield point of the 17-4PH specimens cannot be indicated since the specimens failed near the ultimate strength.
The stress–strain curve comparison of the infill and full-solid specimens made from 316L and 17-4PH stainless steel are illustrated in
Figure 10 and
Figure 11, respectively, to show the distinctness of material behaviors when the full-solid specimens have their weight reduced using an infill pattern.
Moreover, the mechanical properties of 316L and 17-4PH infill specimens obtained from the tensile test and computed from the analytical method described in
Section 2.4 are compared in
Table 4. It can be observed that the values of the elastic modulus of the 316L and 17-4PH infill specimens are similar in each relative density.
With the assumption mentioned in
Section 2.4, the analytical method can be used to accurately predict the mechanical properties if the lattice structures have a low relative density. In this study, the elastic modulus of the specimen with a lower relative density (50.48% and 50.68% relative densities) shows a good agreement between the results obtained from the analytical method and the experimental study. The discrepancy between the results obtained from both methods increases for the specimens with a higher relative density.
For the elastic modulus, since some dimensional properties of unit-cell struts are included in the formula of [
19], the results obtained from [
19] are slightly closer to the experimental results than those obtained from [
17]. The yield strength values obtained from the analytical method in the work by [
17] are lower than those obtained from the experiment. With the considered relative densities in this study, the wall portion of all infill specimens reaches the yield point before the lattice portion. Thus, Equation (7) was used to compute the yield strength for this case. The discrepancy between the results obtained from Equation (7) and the experiment also increases, caused by the discrepancy in the elastic modulus results. Since, in the rule-of-mixtures model, the yield strength is a function of the elastic modulus, the prediction of the yield strength yields an accurate result for the specimens with a low relative density.
The experimental results of the tensile test show that, when the relative density of the specimens increases, the mechanical properties rapidly increase. For example, when considering the relationship between the elastic modulus and the relative densities, the relation is non-linear for specimens with a higher relative density [
23].
The 316L specimens were used in the fatigue test because this material is widely used in many applications with excellent corrosion resistance and ductility. The ultimate tensile strength values of the full-solid and infill specimens in
Table 3 and
Table 4 were used to define the stress level of the specimen for each considered infill density, i.e., 16%, 20%, 24%, and 100% equivalent (full-solid). The geometry and dimensions of the samples in the fatigue test are the same as those in the tensile test. The fatigue results of eight to ten samples were collected for each infill density. The S–N data of the full-solid specimens and infill specimens plotted in both linear–log and log–log scales are shown in
Figure 12 and
Figure 13, respectively. The stress values between 30% and 90% of the ultimate tensile strength were used to set the maximum stresses for each level in the fatigue test.
It can be observed that the fatigue limits of the specimens with 16% and 20% infill densities show a slight difference, while that of the specimens with 24% infill density shows a considerably higher fatigue limit. It is implied that the relationship between the fatigue behavior and the relative density is also non-linear when the specimens have a higher relative density.
To estimate the lifetime of the specimens with different stress levels, a relationship between the maximum stress
and lifecycle
can be expressed as
where
and
are constant parameters obtained from curve fitting using linear regression, as shown in
Figure 12 and
Figure 13, in which the relationship between
and
is linear. The
and
parameters for each relative density of the infill specimens and the fatigue limit at 10
6 cycles are presented in
Table 5. In linear regression, the determination (
) is used to measure how well the data fit the regression model or the goodness of fit.
The fatigue test results show a good fitting, with close to 1. As expected, the specimens with a higher relative density show a greater fatigue limit and parameters, while yields similar constant values.
The weight efficiency, defined as the relationship between a considered mechanical property and the relative density, can be computed using Equation (8). The elastic modulus and ultimate tensile strength were the mechanical properties considered for calculating the weight efficiency, i.e.,
and
.
Table 6 shows the values of the weight efficiency with different relative densities.
The weight efficiency is used to assess the efficiency of the infill specimens with different densities by concerning the mechanical properties and weight of the specimens. In fact, the greater the relative density, the more weight efficiency of the specimens. Nevertheless, this behavior is not linear because the weight efficiency rapidly increases when an infill specimen is denser, closer to being a full-solid one. Thus, selecting an appropriate relative density for the infill specimens could be taken into account to minimize the weight. It can be seen from
Table 6 that the values of
of the 316L and 17-4PH infill specimens are similar, while the values of
are slightly different. The
and
show little difference between the 316L and 17-4PH specimens, with 16% and 20% infill densities. In contrast, the
and
of the specimens with 24% infill density show a noticeable increase. It is implied that, if the weight is a major consideration, the specimens with 16% and 20% infill densities yielding similar performance per weight can be used. However, when strength is the main requirement, the specimens with 24% infill density may be a better choice. Note that the reduced weight of the specimens with a lower relative density influences the time and costs in the manufacturing process.
4. Conclusions
This study investigated the mechanical properties of additively manufactured metal materials, including 316L and 17-4PH stainless steel, under static and cyclic loading in tensile mode. The full-solid and infill specimens were fabricated by MAM using the BMD technique. The infill density of 16%, 20%, and 24% was set for the infill specimens, corresponding to a relative density of 50% to 75%, approximately. The full-solid specimen considered to have a 100% relative density equivalent was investigated to obtain the properties of the base materials. The results showed that the 17-4PH full-solid specimens behave as a brittle material and have higher strength than the 316L full-solid specimens. In contrast, the 316L specimens have more elongation and ductility. For the infill specimens, as expected, a higher relative density produces stronger specimens with a higher elastic modulus. However, this behavior is not entirely linear. The relationship between the mechanical properties and the relative density is linear when the specimen has a low relative density and rapidly increases when the relative density approaches a full-solid.
The analytical methods described in
Section 2.4 were used to validate the experimental results in this study by comparing the values of an elastic modulus and yield strength of 316L and the values of the elastic modulus of 17-4PH infill specimens. Good agreement is observed between both methods for the specimens with a low relative density, while more discrepancy presents when the relative density increases. Since the formulas were derived using the Euler–Bernoulli beam theory, it cannot be used to predict the mechanical properties of the infill specimens with a high value of the ratio between
and
, namely a high relative density. However, this analytical procedure can be satisfactorily used in the initial evaluation to determine the mechanical properties of the infill specimens with a low relative density.
Regarding the cyclic test, 316L stainless steel was selected for the test specimens because of its ductility and widespread application. The fatigue test was performed with the maximum stress, ranging from 30% to 90% of the ultimate tensile strength of each specimen with different relative densities. The results showed that the fatigue behavior is similar to the tensile strength one; the infill specimens with a higher relative density have a higher fatigue strength for every stress level. The fatigue behavior is also a non-linear function of the relative density, especially when the specimens have a high relative density.
The specimens with 16% and 20% infill densities yield slightly different weight efficiency, while those with 24% infill density show an outstanding weight efficiency. As a result, in terms of weight efficiency, the AM parts with a low relative density cannot compete with those that have a relative density. If lightweight AM parts are required, their relative densities between 50% and 60% of the full-solid counterparts can be considered. In contrast, a high relative density close to the full-solid one is recommended if the strength of AM parts is needed. However, the dense parts affect the time and costs in the manufacturing process. Thus, a specific core design requirement is vital for making a decision.