2.1 Selective laser melting and boriding

This study used 316L stainless steel powder to produce a rectangular prism sample (13 mm×9 mm×9 mm) with the SLM process, which was performed utilizing a Concept Laser MLAb R machine (trade name CL 20 ES) [43]. The average powder size for 316L SS alloy was 25 µm, ranging from 15 to 45 μm. The chemical composition of the 316L SS powder was given according to the supplier’s data (GE Additive) in Table 1. Also, the average chemical composition of 316L SS powder was obtained by inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Scientific iCAP 600 Series).

The laser power was 40 W, and the laser speed was 400 mm/s. The layer thickness and the scan rotation in the process were 25 µm and 90°, respectively. The schematic illustration of the printed sample was shown in Figure 1(a). After 316L SS was printed, the samples to be borided were obtained by cutting pieces from the as-built 316L cuboid. A schematic representation was given in Figure 1(b). The size of the borided samples was 9.0 mm×4.5 mm×2.5 mm. Before the boriding process, each rectangular sample was mechanically polished by SiC grinding paper using 180, 240, 320, 400, 600, 800 and 1000 grit for each surface, respectively and cleaned in an ultrasonic bath using alcohol for five minutes. Then, all of the polished and cleaned samples were dried. The surface roughness, R_a, of the sample was estimated by Ambios XP-2 surface profilometer as about 0.25 µm, which is in agreement with the literature [44]. The pack boriding was performed in a sealed stainless steel (304L) container filled with Ekabor-II powder, and the samples were embedded. The top layer of Ekabor-II powder was also covered with SiC powder to prevent oxidation. The boriding process was carried out in an electrical-resistant furnace at atmospheric pressure, and the process parameters were set up to be 850 ℃ (2, 4 and 6 h), 900 ℃ (2, 4 and 6 h), and 950 ℃ (2, 4 and 6 h). The container was cooled to room temperature in the furnace at the end of the process, and the samples were taken out.

Figure 1全屏查看

(a) Schematic view of SLM printed 316L SS and the scanning strategy; (b) Schematic representation of the samples to be borided (dashed lines represent the cutting directions)

2.2 Physico-chemical characterization

The cross sections of the as-built SLM 316L and the borided samples were prepared metallographically by grinding with SiC papers (320, 400, 600, 800, 1000 grit) and final polishing (alumina slurry). The etching was applied using Aqua regia (V(HCl):V(HNO₃)=3:1) solution. The microstructural observation was performed with an optical microscope (Olympus) and scanning electron microscopy (SEM) via Carl Zeiss 300 VP at 15 kV accelerating voltage. The energy dispersive spectroscopy (EDS) was also utilized for the elemental analysis. The thickness of the boride layer (FeB+Fe₂B) was measured using Image-J software by following the procedure given in the literature [36], and the results were an average of 30 measurements. X-ray diffraction (XRD) analysis was carried out utilizing Rigaku ULTIMA 3-Rint 2200/PC instrument to characterize the as-built and borided samples at 850, 900, 950 ℃ for 4 h. For the XRD measurement Cu-Kα radiation (1.54 Å) at 45 kV-40 mA over a range from 10° to 90° was used. The hardness of the boride layers and substrate was measured on the cross-sections by using microhardness tester (FutureTech FM-ARS 7000). The applied load was 50 g.

3.1 Microstructure of SLM 316L SS

The optical microscope (OM) and SEM images of the as-built SLM 316L SS were depicted in Figure 2. The microstructure images in Figures 2(a) and (b) corresponded to the xz plane, which was parallel to the building direction. SEM image of the as-built SLM 316L SS was shown in Figures 2(c)-(f). As seen in Figure 2(a), it can be observed that the microstructure is composed of columnar grains that exhibit a molten pool structure with semi-elliptical-shaped borders. This kind of microstructure is typical for the AM 316L SS [45, 46]. These molten pools formed and extended along the building direction owing to the temperature gradient.

Figure 2全屏查看

Microstructure of as-built AM 316L SS: (a, b) OM images of as-built AM 316L SS; (c-f) Representative SEM images of SLM 316L SS samples showing the cellular and columnar structures at different magnifications

Moreover, as seen in Figure 2(b), fine columnar microstructures in the regions close to the molten pool boundaries occurred, whereas cellular structures formed inside molten pools at high magnification images. In other words, the finer microstructure could be observed near the edges of the molten pool and coarser in the center. The formation of fine columnar microstructures near molten pool boundaries could be ascribed to a rapid cooling rate compared to the upper side of columnar grains [47, 48]. As seen in Figure 2(c), the molten pool boundaries, cellular structures, and columnar grains could be seen as a typical SEM microstructure of AM 316L SS [49, 50]. In addition, Figure 2(d) showed the magnified picture of the area inside the square in Figure 2(c). As seen in Figure 2(d), the columnar structures formed and extended perpendicular to the boundaries of the molten pool. The cellular type structure could be easily observed as well. Furthermore, some of the fine and cellular structures formed along with the molten pool boundary. Figure 2(e) showed the magnified picture of the area inside the square in Figure 2(d). The image at lower magnification was shown in Figure 2(f). According to this image, the sizes of the cellular structures were measured, and their diameters ranged between 750 and 900 μm.

In addition to SEM images of SLM 316L SS samples, the EDS analysis was also conducted, as demonstrated in Figure 3. The EDS spot analysis was performed on the SLM 316L SS samples as seen in the SEM image in Figure 3(a). The EDS analysis spots 1 and 3 were taken from the interior of the cellular structures, while EDS analysis spots 2 and 4 were taken from the border. Figures 3(b)-(g) exhibited the elemental composition obtained from EDS analysis. As seen in Figures 3(b)-(g), the border of the cellular structures was slightly enriched in Cr, Ni, Mo, Mn, and Si.

Figure 3全屏查看

(a) EDS analysis conducted to estimate the chemical composition at the cells’ border and at the cells’ interior; (b-g) Results of the EDS compositional analysis

3.2 X-ray diffraction

XRD analysis of SLM 316L SS was demonstrated in Figure 4. As seen in Figure 4, the as-built SLM 316L SS had three major peaks at near 43.44°, 50.57° and 74.49° which corresponded to γ-austenite phase [51]. After the boriding process at determined temperatures (850, 900 and 950 ℃) for 4 h, it can be observed that both FeB and Fe₂B layers formed on the surface of the as-built SLM 316L SS. In addition, the formation of CrB and Cr₂B borides phases was also detected. As can be seen in Figure 4, the characteristic peaks of Cr₂B and CrB generally overlap the peak position of iron borides [34, 52, 53]. Moreover, the intensity of FeB peaks increased as the boriding temperature increased. Moreover, the appearance of B₄C compound might be related to boriding environment. Our results were in good agreement with the literature studies [54, 55]. The presence of the phase was also approved by Rietveld analysis as seen in Figure S1. The application of the Rietveld analysis assisted to define the phases formed during boriding process. The existence of both FeB and Fe₂B phases indicated the formation of double layers owing to the diffusion of boron atoms into the material. Furthermore, the residual stress analysis of the borided samples was carried out by Rietveld refinement analysis based on XRD results. The micro strain values of the FeB layers were found to be 6.6×10^-4, 2.1×10^-3 and 3×10^-3 for the samples borided at 850, 900 and 950 ℃, respectively for 4 h. It can be concluded that the strain increased as temperature soared. This increment of strain between the layer might lead to high stress and should cause the peeling off or defragmentation of FeB layer as seen in SEM images.

Figure 4全屏查看

XRD pattern of as-built SLM 316L SS before and after the boriding process

3.3 SEM analysis of borided SLM 316L SS

The cross-section SEM images of the borided SLM 316L samples were illustrated in Figure 5. As seen in Figure 5, the morphology of the boride layer-transition matrix interface was smooth and flat because of the alloying element effect [38]. The printed samples generally consisted of 4 different zones. The first zone corresponded to the formation of the FeB layer, represented by the red line, while the second region, represented by the orange line, belonged to the Fe₂B layer. It was seen that the FeB and Fe₂B layers were in different contrasts. The third region, represented by the green line, and the fourth zone were associated with the transition matrix layer and substrate, respectively. It could be observed that the thickness of each boride layer increased as the boriding temperature and time increased up. There are theoretically two factors in growth reaction kinetics. These are reaction products diffused on the reaction interface (diffusion-controlled) and a chemical barrier for transportation of the substances reacting on the reaction interface (reaction controlled). Boride layer grows linearly as the treatment time increases in the case of reaction-controlled process; it grows proportional to the square root of the reaction time if the process is diffusional. The changes in the thickness of both FeB and Fe₂B layers versus boriding time to temperature were given in Figure 6. As seen in Figure 6, the thickness of the iron boride layers grew proportional to the square root of the reaction time. Generally, the thickness of the samples ranged from (1.8±0.3) μm to (27.7±2.2) μm. Furthermore, the defragmentation of the formed FeB layers could be observed depending on the temperature and duration of the boriding process. As the boriding temperature and time increased, the defragmentation of the FeB layer was very severe because of intrinsic properties [56]. The formation of the Fe₂B phase is more desirable than the formation of the FeB and Fe₂B bi-phase layer on the surface. Because the FeB phase is rich in boron, this phase contains 16.23% boron by weight. This is undesirable because the FeB phase is more brittle than other iron-boron phases [57]. In addition to that, the high temperature and long duration could cause residual stress between the formed boride layers. Therefore, residual stress might lead to fracturing and peeling off the FeB layers at high temperatures and long duration. Similar layer regions can be supported by previous studies [22, 35, 58]. The change of Vickers microhardness profile of a representative sample (900 ℃ for 4 h) versus distance from the surface was shown in Figure S2. While the hardness value of boride layer was obtained to be HV_0.05 1900 nearby surface. As it moved away from the surface, a noticeable decrease in the hardness value was observed and the hardness value was determined as 237 HV_0.05. This is equivalent to the hardness of the substrate sample. This trend showed that the boride layers were made of different diffusion zones as seen in Figure 5.

Figure 5全屏查看

Cross-section SEM micrographs showing the boride layer microstructures formed on the as-built SLM 316L SS at different boriding temperatures and durations

Figure 6全屏查看

(a) The measured thickness of the FeB layer as a function of time at different temperatures on the SLM produced 316L SS; (b) The measured thickness of the Fe₂B layer as a function of time at different temperatures on the SLM produced 316L SS; (c) The measured thickness of the FeB+Fe₂B layer as a function of time at different temperatures on the SLM produced 316L SS

In addition to SEM images of borided samples, the EDS analysis was also conducted to prove the existence of the formation of FeB and Fe₂B boride layers on the SLM 316L SS alloys in Figure 7. The sample borided at 850 ℃ for 6 h was chosen as an example image to demonstrate boride layers. The amount of boron element for both FeB and Fe₂B layers was 17.44 wt.% and 7.06 wt.%, respectively. These are very close to both theoretical weight ratios of FeB and Fe₂B structure, which indicated that the dual boride layers occurred on the surfaces.

Figure 7全屏查看

EDS analysis of sample borided at 850 ºC for 6 h

The thickness of the boride layer was measured based on the boriding time at different temperatures. This relationship was illustrated in Figure 6. The values of the boride layers were also provided in Figure 6. It could be said that the initial increase in the thickness of each layer was rapid, but as time progressed, the rate of increase tended to decline. This meant that the thickness of the boride layer changed parabolic with time at all temperatures, as seen in Figure 6.

3.4 Growth kinetics of boride layers on as-built SLM 316L SS

Boriding is a diffusional process. The migration of boron atoms into the material to form boride phases on the surfaces is controlled by the diffusion mechanism in the boriding process [59]. The thickness of the boride layer strongly depends on the boriding medium, the chemical composition of the substrate, temperature, and time [60]. The boron concentration profiles and diffusion zone in FeB/Fe₂B bilayers were illustrated in Figure 8. In Figure 8, states the adsorbed boron concentration on the surface which should be high enough to form FeB layer. (16.40 wt.%B) and (16.23 wt.%B) represent the upper and lower equilibrium boron concentration values in FeB layer, whereas (9 wt.%B) and (8.83 wt.%B) are the upper and lower boron concentrations in Fe₂B boride, respectively. The term C₀ (35× 10^-4 wt.% B) represents the solubility limit of boron within the substrate [36, 61, 62].

Figure 8全屏查看

A schematic depiction of boron concentration profile through (FeB/Fe₂B) bilayer growing on a substrate

Some assumptions are generally taken into account to simplify the kinetic model application. Some of these assumptions are listed as follows [60, 63]: 1) The boron atom’s diffusion governs the boride layers’ growth kinetic into FeB/Fe₂B phases; 2) The boron concentration profile is assumed to be linear throughout the FeB and Fe₂B layers; 3) The diffusion of boron atoms and the growth of boride layers are perpendicular to the sample surface; 4) The boride layer is thin compared to the sample thickness; 5) The diffusion coefficient of boron in each iron boride is independent of concentration, and the effect of Fe diffusion on layer growth is neglected; 6) The nucleation of iron borides starts after incubation time, and the incubation time is neglected; 7) The temperature is constant and uniform throughout the sample. The growth kinetics of the boride layers into FeB/Fe₂B phases is generally controlled by diffusion of boron element as mentioned above. Therefore, Fick’s second law has been used to predict quantitative growth kinetics of boride layers such as FeB and Fe₂B layers formed on the surface of Fe substrates. Fick’s second law defines the alterations in the concentration of atoms for time and diffusion position during the boriding process in the following equation [64, 65].

(1)

Equation (1) transforms to the general expression after solving it as given in the following equation [66].

(2)

where Ci(x, t) expresses the concentration at a certain thickness (x) depending on time (t) and temperature (T). Di describes the diffusion coefficient. Ai and Bi are constants.

According to Wagner’s approach [67], the thickness of FeB and Fe₂B layers can be expressed in the following equations concerning time (t).

(3)

(4)

where ξ and η are constants, and D₁ and D₂ are the boron diffusion coefficients in the FeB and Fe₂B layers, respectively. D₁ and D₂ could be expressed in the following equations.

(5)

(6)

where A₁ and A₂ are constants; Q₁ and Q₂ define the activation energy.

The curves of x²_FeB versus t and x²_Fe2B versus t should be plotted to assess the ξ and η values for the boriding process. For this purpose, the square of boride layer thickness vs boriding time curves were plotted and shown in Figure 9. As seen in Figures 9(a) and (b), the slopes of the straight lines of (x²_FeB-t) and (x²_Fe2B-t) are equal to 4ξ²D₁ and 4η2D₂. While the 4ξ²D₁ values of FeB layers ​​for 850, 900 and 950 ℃​were found to be 7.8×10^-16, 4.33×10^-15 and 7.243×10^-15 m²/s, respectively; the 4η²D₂ values ​​were found to 1.47×10^-15, 3.6×10^-15 and 6.04×10^-15 for the Fe₂B layer, respectively.

Figure 9全屏查看

(a) Square of the FeB layer thickness as a function of time at different temperatures on the SLM produced 316L SS; (b) Square of the Fe₂B layer thickness as a function of time at different temperatures on the SLM produced 316L SS

The initial and boundary conditions of the boron diffusion can be expressed to determine ξ and η values. The initial conditions of the diffusion problems were set up as follows:

Initial condition: (t=0), for x>0

(7)

The following equations also gave the boundary conditions.

(8)

The two parameters and are the boride incubation time.

(9)

Equations (2)-(4) were incorporated with the initial and boundary conditions in Eqs. (7)-(9). As a result, the variation in B concentration in the FeB and Fe₂B phases can be designated as:

(10)

(11)

(12)

The growth rate of the borided FeB and Fe₂B layers could be expressed as follows due to the mass balance equation at the (FeB/Fe₂B) and Fe₂B/substrate) interfaces [68].

(13)

(14)

While x=u, taking the derivation of Eqs. (10) and (12) respectively, Eqs. (15) and (16) were obtained.

(15)

(16)

While x=v, taking the derivation of Eq. (12), we obtained the following equation.

(17)

Replacing Eqs. (15)-(17) into Eqs. (13) and (14), and reorganizing give:

(18)

(19)

The ξ and η constants in the Eqs. (18) and (19) were solved numerically using a written computer program in MATLAB software. The values of the ξ and η constants were 0.228 and 2.739, respectively. The diffusion coefficient of boron element (, ) in FeB and Fe₂B boride layers were calculated using these ξ and η constants. After that, the lnK versus 1/T plots were utilized to calculate the Q values (K represents the diffusion coefficient of boron element , ). The slope of the straight line was equal to -Q/R. The Q values for the individual FeB layer, Fe₂B layer, and dual FeB+Fe₂B layer were determined to be 256.56, 161.61 and 209.014 kJ/mol, respectively. The extrapolation of the straight line to the x-intercept yielded the value of lnK₀ (K₀ represents the pre-exponential constants in diffusion equations of Eqs. (20) and (21)). The K₀ values were also found by using the lnK₀ values. The K₀ values for both individual FeB and Fe₂B layers were 6.56×10^-4, 1.7×10^-9, respectively. An Arrhenius equation expressed the diffusion coefficients of the boron element in each boride layer in the following equations.

(20)

(21)

An empirical equation can be obtained to predict the thickness of the boride layers with the help of kinetic diffusion theory. The empirical equation can be generated by using the substitution Eqs. (20) and (21) into Eqs. (3) and (4) after solving unknown values such as ξ, η and D for experimental results. After determining the unknown values, the empirical equation can be formed, as seen in Eq. (22), and can be used to estimate boride thickness according to initial conditions.

(22)

Within this framework, the comparison of the layer thickness estimated from the experiment and empirical results was given in Figure 10. The thickness values obtained from experimental results were close to the thickness values generated from empirical results.

Figure 10全屏查看

The comparison of boride layer thickness results between the experimental and calculated: (a) FeB layer; (b) Fe₂B layer; (c) Total (FeB+Fe₂B) layer

In Table 2, some literature findings were given to compare the results of this study and the literature. The thickness of the boride layers ranged from (1.8±0.3) μm to (27.7±2.2) μm in this study. For example, FERNÁNDEZ-VALDÉZ et al [22] determined that the thickness of the total iron boride layer changed between approximately 14 and 47 µm for 2 h and 6 h at 900 ℃, respectively, on the surface of 316L SS. CAMPOS-SILVA et al [38] found the thickness of the total iron boride layer to be ~8 μm at 850 ℃ for 0.5 h on the surface of 316L SS. When the thicknesses of the boride layers at the same time and temperature are compared, it could be observed that the thicknesses of the boride layers in the literature are generally larger than this study. The Q is a critical parameter that affects the rate of boride layer growth during boriding processes. It influences the diffusion kinetics of boron or other elements into the substrate material, which, in turn, determines the thickness of the boride layer formed. Lower activation energy promotes faster growth, resulting in thicker layers, while higher activation energy slows down the growth process, leading to thinner layers [49]. As seen in Table 2, the Q values in this study are generally higher than those in the literature. This could be attributed to the microstructure of the SLM 316L SS. The microstructure of SLM 316L SS can significantly influence the diffusion of boron element. The microstructural features that impact diffusion include grain boundaries, porosity, and the presence of defects. In additive manufacturing, the microstructure often consists of columnar, fine, and cellular grains with distinct grain boundaries because of the rapid cooling. These structures possess higher internal stresses, higher dislocation density, higher defects, and a higher number of cellular grain boundaries and cellular structures. Grain boundaries can be diffusion paths for elements, especially at elevated temperatures. Grain boundary diffusion can be faster compared to diffusion within the grains. The presence of grain boundaries can enhance the diffusion of elements like boron and nitrogen in SLM-printed 316L SS, as well as defects, cellular structure, and dislocation density [71, 72]. Even though the microstructure of SLM 316L SS favors the diffusion process, the results contradict the advantages of the microstructure of SLM 316L SS for the diffusion process in this study. This might be due to the segregation of alloying elements such as Cr, Ni, Mo, Mn, and Si in the cell border for SLM 316L SS. As mentioned above, the presence and formation of cellular microstructure can increase the diffusion of elements such as boron. Nevertheless, simultaneously, it accelerates the migration of atoms such as Cr, Ni, and Mo in the SLM 316L SS towards the grain boundaries. It causes intense segregation in the grain boundary regions due to the continuous remelting and solidification of stainless steel in the additive manufacturing method. The formation of boron compounds of the segregation of alloying elements might reduce and hinder the inward diffusion of boron [73-78].