**1. Introduction**

Additive manufacturing (AM) (according to ISO/ASTM 52900:2021) is a process used for the joining of materials based on 3D CAD data. In contrast to subtractive methods, this technology is characterized by building parts layer-by-layer. The inclusion of these data in the standardized definition is the main advantage of AM technology. Hence, it is possible to obtain complex geometries, such as internal conformal cooling channels and lattice structures, which are difficult to produce with the use of conventional manufacturing technologies. In the manufacturing of machine parts via AM, powder bed fusion (PBF) (ISO/ASTM 52900) [1] technologies are most commonly used. It is worth noting that in numerous studies, the most popular method has involved the use of a laser energy source. This method is referred to as laser-based powder bed fusion of metals (PBF-LB/M)—ISO/ASTM 52911-1—and is popularly called laser powder bed fusion (L-PBF). The use of PBF technology in the production of machine parts must be economically justified. Many different research articles related to PBF-LB/M have discussed this issue [2–5]. The mentioned works indicate that the purchase costs of machines and materials represent

**Citation:** Łuszczek, J.; Snie ´ zek, L.; ˙ Grzelak, K.; Kluczy ´nski, J.; Torzewski, J.; Szachogłuchowicz, I.; Wachowski, M.; Karpi ´nski, M. Processability of 21NiCrMo2 Steel Using the Laser Powder Bed Fusion: Selection of Process Parameters and Resulting Mechanical Properties. *Materials* **2022**, *15*, 8972. https://doi.org/10.3390/ ma15248972

Academic Editor: Gregory N. Haidemenopoulos

Received: 18 November 2022 Accepted: 13 December 2022 Published: 15 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the largest part of the financial expenditure in terms of production. On the one hand, Kamps et al. [6] pointed out that material expenses in the PBF-LB/M process constituted 32% of the overall cost. On the other hand, the American Gear Manufacturers Association (AGMA) [7] reported that the value was equal to 15%, dependent on the feedstock material type. In both cases, those costs are high and are included in the cost share of the powder in the overall process, as reported by Lindemann et al. [8]. The atomization process consumes most of the costs per 1 kg of powder. With the current balance of costs, the profitability of production using PBF-LB/M technology is achieved in unit production, in particular through the complex geometry of the parts produced. In order to expand the areas in which PBF-LB/M technology is applicable, it is necessary to develop new materials or implement those which are used in conventional processes, e.g., low-alloy steels.

One of the basic elements in the field of mechanical drive systems, which can be successfully produced via PBF-LB/M, is gears [9,10]. The loads acting on gears require the use of proper feedstock materials and postprocessing activities, which allow the proper strength properties to be obtained. Numerous studies have taken into account the production processes of power train components and the verification of their mechanical properties [11–14]. The main issue in these papers is the use of materials that are not typically dedicated to some exact solution. The most common commercially available metallic powders for PBF-LB/M are Ti6Al4V [14], AlSi10Mg [15], and the stainless steels 316 L [15], 420 [16], 17-4PH [13], and GP1 [17]. The number of steel grades that are intended for AM gears is limited [7]. One of the primary properties which allows the use of a given material in the PBF-LB/M process is its weldability (which is mostly dependent on its having a carbon amount below 0.3%). However, this feature can be modified by adding appropriate alloying elements. The most common conventionally used steels which have been implemented into PBF-LB/M processes are 16MnCr5 and 20MnCr5 [18–20], both of which are carburizing steels. According to the AGMA report [7], for this type of AM method, the following materials are in use: Pyrowear 53, Pyrowear 675, Ferrium C61, Ferrium C64, 100Cr6, M50 NIL, AISI 8620, and AISI 9310. Additionally, not all of the abovementioned materials are available in the commercial market [7]. Moreover, in the actual state of the art, some studies have been reported in which the authors used steels for quenching and tempering, i.e., 42CrMo4 [21,22], 30CrNiMo8 [23], 24CrNiMo [24], and HSLA-100 [25].

For the use of construction steels (including carburizing, quenching, and tempering) in the AM process, it is necessary to provide a properly prepared powder and the detailed development of process parameters. In the case of 16MnCr5 carburizing steel, Schmitt et al. [20,26] proved that it is possible to use AM for the production of gears without any internal cracks and imperfections and to maintain a high material density above 99.5% by means of an energy density above 100 J/mm<sup>3</sup> (for 99.94% it is necessary to use 108.3 J/mm3). With such high densities, the main defect was regular shape porosity, described as gas porosity. The authors highlighted that when using the proper energy density value with different combinations of process parameters (laser power, exposure velocity, and hatching distance) they observed changes in porosity and strength properties. The ultimate tensile strength (UTS) of the obtained parts was equal to 1050 MPa and had the same value as yield strength (YS), which indicates that there was no plastic deformation and thus also no work hardening. Before the heat treatment, the conventionally manufactured material was characterized by UTS = 715 MPa and YS = 591 MPa, which represent lower values than the AM material, at 32% and 43.7%, respectively. The heat treatment (stress relief annealing) conditions for AM parts were the same as those used for the conventionally made materials. This approach resulted in a drop in the mechanical properties (UTSATH = 730 MPa, YSATH = 658 MPa, hardness from 330 HV10 to 235 HV10); additionally, it led to the creation of a fine microstructure and the elimination of the layered structure of the material. Based on the authors' work [20,26], such changes indicate a gain in the recrystallization temperature. Additionally, microscopic investigation revealed structural heterogeneities as "white areas" after nital etching. Kamps et al. [27] and Scheitler et al. [28]

proved the existence of this kind of structural anomaly in additively manufactured steel parts. Kamps et al. [27] revealed that the chemical composition of the "white area" structure is similar to that of the base material. Neither of the mentioned authors described the formation mechanism of this structure. Yang and Sisson [19] additively manufactured samples with the use of 20MnCr5 carburizing steel, employing PBF-LB/M technology. However, neither production parameters nor porosity values were given. Their research focused on the influence of heat and chemical treatments on the hardness of AM parts. The measurements were conducted using two perpendicular cross-sections of the aforementioned samples. In the as-built parts, the hardness value was equal to 287 HV5 in the XY and YZ planes. At the same time, the measured value of the parent material was 189 HV5. The differences between the mentioned material types (the AM and parent material) were mostly caused by the different forms of microstructures. The parent material exhibited a ferritic-pearlitic structure, whereas the AM parts displayed a martensitic structure. The only parameters reported for the production of 20MnCr5 steel using the PBF-LB/M technique were provided in the work of Robbato et al. [18]—106 J/m m<sup>3</sup> (counted on base given process parameters). However, the authors of that study did not report the porosity value obtained. Based on the works related to 16MnCr5 steel, it can be concluded that the density values were similar in both cases.

In the case of quenching and tempering steel, Zumofen et al. [23] used 30CrNiMo8 in the PBF-LB/M process. The authors used the energy density at the level of 83 J/mm3, which allowed them to obtain a density at the level of 99.76%. The remaining voids in the material structure were characterized by a spherical shape, which is a typical indicator of gas porosity. There were no cracks in the structure of the material. The UTS and YS values of the printed material in the quenched and tempered state were higher than those in the as-built state and were comparable with those of the parent material after similar heat treatment. 42CrMo2 steel was used in the PBF-LB/M process in several works [21,22]. Damon et al. [21] achieved a porosity of 0.3% using an energy density of 85 J/mm3. No cracks were registered in the structure of the material. As in the case of 30CrNiMo8 steel, the authors obtained higher strength parameters (UTS and YS) of the steel after quenching and tempering treatment than in the as-built state. Moreover, for this material in the as-built condition, the microstructure was also characterized by an acicular, fine-grained martensitic structure.

Based on the literature review, it can be concluded that the number of low-alloy steels used in additive manufacturing is still meager. In the case of steel for carburizing, it is necessary to use a higher energy density (above 100 J/mm3) in order to obtain a density above 99.7% than in the case of steel for quenching and tempering (around 85 J/m m3). In both cases, the materials in the as-built state are characterized by higher hardness and strength than the parent material. This is due to their acicular, fine-grained martensitic, or martensitic-bainitic microstructure. The change in the properties of the additive manufacturing material depends on the heat treatment performed. The particular deficiencies in the broad description of the processability of steels using PBF-LB/M are related to carburizing steels. The literature lacks a description of steels that are alternatives to the widely used 16MnCr5 or 20MnCr5. In this study, 21NiCrMo2 steel was used in order to investigate its processability with the use of PBF-LB/M, as well as the mechanical properties obtained through this process. This will provide an answer as to whether 21NiCrMo2 steel can be used as an alternative to the typical carburizing steels used to date.

### **2. Materials and Methods**

The material taken into account in this research was the 21NiCrMo2 alloy. Such steel is traditionally used for the production of machinery equipment parts, i.e., gears, shafts, etc. Because of the limited availability of such material in the form of dedicated powder for PBF-LB/M processes, conventionally made 21NiCrMo2 steel bars were subjected to the gas atomization (GA) at the Institute of Non-Ferrous Metals (Gliwice, Poland). The LD Vacuum Technologies GmbH atomizer (Hanau, Germany) was used for powder production. The GA process parameters are shown in Table 1.

**Table 1.** Process parameters used in the GA.


The powder obtained during GA was sieved using a Retsch AS200 sieve shaker (Microtrac Retsch GmbH, Haan, Germany) which allowed us to separate the proper powder fraction for the PBF-LB/M process (20–63 μm). Most powder grains (indicated by red arrows) were characterized by spherical shapes with some satellites (indicated by green arrows), which is visible in Figure 1.

**Figure 1.** 21NiCrMo2 steel powder particles.

In further analyses, the material's chemical composition was investigated with the use of a scanning electron microscope (SEM) JEOL JEM-1230 (Jeol Ltd., Tokyo, Japan) equipped with an energy-dispersive spectroscopy (EDS) module. Spot measurements were made on the surfaces of the powder particles. Table 2 contains the EDS measurement results. Due to the fact that the measurements of the carbon content using the EDS method were flawed due to a large error, this value was not included in Table 2. Moreover, the authors did not have any other equipment at their disposal to conduct this type of research. During that process, there were not any material heterogeneities registered.

**Table 2.** The chemical composition of GA 21NiCrMo2 steel powder.


### *2.1. Laser Powder Bed Fusion (PBF-LB/M)*

SLM 125HL (SLM Solutions GmbH, Lubeck, Germany) was used for the AM samples. The device was equipped with a 400 W single Ytterbium-fiber-laser source (wavelength 1080 nm) and a maximum scanning velocity of 10 m/s. The maximum build volume is equal to 125 × 125 × 125 mm. The range of possible layer thicknesses was 20–75 μm. The substrate plate could be heated up to 200 ◦C, and the AM process was performed in an argon atmosphere (in which the amount of oxygen was lower than 0.3%).

The first stage of the study involved the development of process parameters based on the porosity analysis of the AM cubic samples. Regarding the limited availability of information on the process parameters for 21NiCrMo2 steel, the default settings for H13 tool steel were used as base parameters (laser power *PL* = 225 W, scanning velocity *vs* = 600 mm/s, hatch distance *dH* = 0.120 mm). To properly prepare for the parameter development stage, 57 different parameter groups were considered. The exact values of process parameters were tested in the following ranges: laser power *PL* ranging from 160 W to 240 W (not using the full laser power due to its tendency to generate cracks in the steel structure), scanning velocity vs ranging from 600 to 1100 mm/s, and hatching distance *dH* ranging from 0.070 mm to 0.120 mm. The layer thickness *lt* was kept at the same level and was equal to 0.03 mm. The platform heating was set at a value of 190 ◦C. As a representative parameter (dependent on *PL*, *vs*, *dH*, and *tL*) energy density *Ev* can be described by means of the following Equation (1):

$$E\_v = \frac{P\_L}{v\_s \cdot d\_H \cdot t\_L} \left[\frac{J}{m\pi r^3}\right] \tag{1}$$

AM samples for the porosity analysis had the form of cubes with an edge length equal to 10 mm (one sample for each parameter group). The porosity measurements were performed utilizing the Keyence VHX 7000 optical microscope (Keyence, Osaka, Japan) in both representative planes: XY(PρXY)—parallel to the substrate plate surface, and YZ (PρYZ)—perpendicular to the substrate plate surface (Figure 2). The samples were cut using wire electrical discharge machining (WEDM) and mounted in resin for further microscopical investigation. All samples were ground using abrasive papers with a gradation from 320 to 2400 and polished using 1 μm diamond paste. As a representative porosity value, the average value was taken from <sup>P</sup>ρXY and <sup>P</sup>ρYZ (three different measurements for each plane).

The maximum acceptable porosity value in the entire area of measurement in crosssections was equal to 0.3%. Additionally, to improve the development of process parameters, Design of Experiment (DOE) analysis using Statistica software 13.1 (TIBICO Software Inc., Palo Alto, Santa Clara, CA, USA) was used. For the description of mathematical porosity values, the quadratic area regression model was used. This selection was made because of the possibility of combining features of multinomial regression and fraction factorial regression models. Hence, it allowed the consideration of three independent variables and their mutual interaction. Adegok et al. [29] suggested such an approach in their work. The general form of the quadratic regression is shown in Equation (2):

$$y = \beta\_0 + \beta\_1 \mathbf{x}\_1 + \beta\_2 \mathbf{x}\_2 + \beta\_3 \mathbf{x}\_3 + \beta\_{11} \mathbf{x}\_1^2 + \beta\_{22} \mathbf{x}\_2^2 + \beta\_{33} \mathbf{x}\_3^2 + \beta\_{12} \mathbf{x}\_1 \mathbf{x}\_2 + \beta\_{13} \mathbf{x}\_1 \mathbf{x}\_3 + \beta\_{23} \mathbf{x}\_2 \mathbf{x}\_3 + \varepsilon \tag{2}$$

In Equation (2), y is a dependent variable (the estimated porosity value) and *x*1, *x*2, and *x*3 are independent variables, which can be described as follows:

*<sup>x</sup>*1—laser power;

*<sup>x</sup>*2—scanning velocity; and

*<sup>x</sup>*3—hatching distance.

**Figure 2.** (**a**) Flowchart of the adopted research methodology and (**b**) the orientation of samples orientaton in the substrate plate of SLM 125HL (Z—the direction of the layers' deposition).

βm and βmn (for m = 1, 2, 3; *n* = 1, 2, 3) are regression coefficients, and ε is the modeling residual or error. The values of the regression coefficients were calculated with the use of the method of least squares. The calculations were divided into two parts. The first stage was dedicated to the creation of the model, based on the first porosity measurements (based on two experiments) of 27 different samples. The second stage was based on the development of further parameter combinations using the 33 full factorial designs (three factors: laser power, exposure speed, and hatching distance were varied at three levels for each factor). Such an approach allowed us to supplement the statistical model with the obtained results.

Additionally, the R<sup>2</sup> and *p* values were calculated. The R<sup>2</sup> coefficient defines how the statistical model and its predictors describe the variability of a referred parameter. The *p* value is the cumulative probability of drawing a sample as extreme as or more extreme than the observed one, assuming that the null hypothesis is true. The *p* coefficient was estimated by constructing an analysis of variance (ANOVA) table, and this was related to statistical tests. Statistical significance was set at *p* < 0.05. The validation of the obtained PBF-LB/M process parameter groups with the statistical analysis results was possible via the experimental study of the microscopic observations and porosity measurements. As a result, five process parameter groups were chosen for further research (microstructure investigation, hardness testing, and tensile tests).

### *2.2. Microstructure and Tensile Analysis*

As-built PBF-LB/M samples and parent material parts were considered for the microstructural and tensile analyses. Because of the presence of high-temperature gradients, after the PBF-LB/M processing, all manufactured parts were subjected to stress relief annealing [30]. The lack of this operation caused deformation of samples during their separation from the substrate plate via WEDM. The heat treatment process was undertaken in a Nabertherm N11/H furnace (Nabertherm GmbH, Lilienthal, Germany). The conven-

tionally made material was examined after normalization. All the details of the annealing process are shown in Table 3 [31]. Temperature and time values were taken from the heat treatment of conventionally made 21NiCrMo2 steel.

**Table 3.** 21NiCrMo2 steel heat treatment conditions [31].


Dog-bone tensile samples were designed based on the ASTM E8 standard, and all the samples were oriented as shown in Figure 2b. This is the most favorable position for the specimens in terms of the strength of the additive manufacturing material when subjected to static tensile tests. Tensile tests were conducted on the INSTRON 8802 MTL (Instron, Norwood, MA, USA) testing machine in accordance with the PN-EN ISO 6892- 1:2010 standard. Using each selected group of parameters, 5 samples were produced and tested using a tensile test. In the case of hardness testing, all measurements were made by means of a Struers DuraScan 70 system (Struers GmbH, Kopenhagen, Denmark) in accordance with the PN-EN ISO 6507-1:2007 standard. Hardness tests were conducted on the same samples, which were dedicated to the investigation of porosity. To illustrate the methodology presented here, Figure 2a shows a flowchart that briefly summarizes Section 2.
