Manufacturing Elements with Small Cross-Sections of 17-4 PH Steel (1.4542) with the Application of the DMLS Additive Manufacturing Method

The application of direct metal laser sintering renders it possible to manufacture models with complex geometries. However, there are certain limits to the application of this method connected with manufacturing thin-walled cuboidal elements, as well as cylinders and holes with small diameters. The principal objective of the research was to determine the accuracy of manufacturing geometries with small cross-sections and the possibility of application in heat exchangers that are radiators with radially arranged ribs. To that end, four specimens were designed and manufactured; their geometries of representations assumed for the purpose of research (analysis) changed dimensions within the following scope: 10–0.1 mm. The specimens to be applied in the research were manufactured with 17-4 PH stainless steel (1.4542) with the application of 3D-DMLS printing and an EOS M270 printer. The measurement of accuracy was performed with the application of an optical stereomicroscope (KERN OZL-466). In addition to that, research into the chemical composition of the material, as well as the size of spherical agglomerates, was conducted with the application of a scanning electron microscope. The analysis of the chemical composition was conducted as well (after the sintering process). The analysis of the results based on the values received by means of measurements of the manufactured geometries was divided into three parts. Based on this, it is possible to conclude that the representation of models manufactured with the application of DMLS was comparable with the assumptions, and that the deviations between a nominal dimension and that received in the course of the research were within the following scope: 0–0.1 mm. At the final stage of research and based on the received results, two heat exchangers were manufactured.


Introduction
Throughout recent years, it has been possible to observe a significant development in rapid prototyping methods, which is contributed to by an ever-greater interest of numerous industries such as the aviation industry [1][2][3][4][5][6][7][8][9][10]. In practice, DMLS (direct metal laser sintering) is an alternative to models manufactured with the application of casting processes. It consists of sandwich direct metal laser sintering with the application of a fiber ytterbium laser [11,12]. The material is taken from a dispenser and afterward placed in layers with the application of a drift fender in the working space (height of a layer is dependent upon the kind of material in a dispenser; possible range: 10-80 µm). Parts are manufactured directly based on a three-dimensional computer aided design (3D CAD) model (after appropriate data processing-recording in the "stl" format), owing to which The research was conducted to manufacture heat exchangers radiators with radially arranged ribs with the application of 3D printing. The radiators were designed in two variants presented in Figures 2 and 3. The principal assumptions in the course of designing a radiator were a heat exchange surface exceeding 0.1 m 2 , the number of ribs, their radial arrangement, and limits connected with the size of the working space of an EOS M270 printer [22].
In addition to that, in Figures 2 and 3, the location of the XY plane, as well as the direction of a drift fender movement (red arrow) in the working space of an additive device, is presented.

Materials
For manufacturing specimens with the application of DMLS, the 17-4 PH stainless steel (1.4542) of the EOS company (trade name: GP1) (EOS GMBH, Krailling, Germany) was applied as a powder in the form of spherical particles with a grain size between 20 and 80 µm. To verify the data provided by the producer and relevant to the chemical composition of a material (Table 2), as well as to the size of the spherical particles, a scanning electron microscope was applied to research the powder (Figure 4). In addition to that, the analysis of the chemical composition after the sintering process was conducted ( Table 1). The area of the microanalysis of chemical composition SEM is presented in Figure 5. In Table 3, the selected mechanical properties of the 17-4 PH stainless steel, applied in the course of manufacturing the analyzed research models, are collated. The presented data are available on the website of the producer.    Figure 5).

DMLS Part Manufacturing
Specimens to be applied in the research were manufactured of the powder of the high-chromium DMLS steel with the application of an EOSINT M270 printer (EOS GMBH, Krailling, Germany) and designed in the Autodesk Inventor Professional 2020 environment (Autodesk, Inc., Mill Valley, CA, USA), to be exported afterward to an STL surface format. The base size and the location of particular elements of the researched geometry were selected to render it possible to manufacture the details in question with the application of direct metal laser sintering [16,17,23]. For the purpose of analyzing the accuracy of the representation of the geometry of prototypes manufactured with the application of DMLS, four research models (marked as presented in Table 4) were applied. The rationale behind the designed geometry of the specimen is the time required for printing. Manufacturing a single heat exchanger requires approximately 80 h, which, in the case of the conducted research, would result in a significantly extended manufacturing time and in an increased consumption of the material. In Figure 6a-d, the dimensions of the specimen marked MB1 are presented. In the upper part of the specimen, there are cylinders with the following diameters: Ø10-Ø2 mm, with a step of 1 mm (from the left-cross-section A-A- Figure 6b), and below, there are holes with the following diameters: Ø1-Ø0.1 mm, with a step of 0.1 mm (from the left-cross-section B-B- Figure 6c). In turn, in the lower part of the object presented in cross-section C-C (Figure 6d), there are holes with the following diameters: Ø10-Ø2 mm, with a step of Ø1 mm. In Figure 7a,b, cylinders with the following diameters: Ø1-Ø0.1 mm, with a step of 0.1 mm, with the specimen marked MB2 from the left, are presented.    The models presented in Figures 6-9 were manufactured of the powder of the highchromium GP1 steel (EN 1.4572, 17-4 PH) with the application of an EOS M270 printer. Prior to starting the printer, "stl" model files were read in the Magics program of the Materialise company. In this program, the best possible location of the details in the virtual working space of a prototyping device was determined, and the supporting structures were defined. After verifying the correctness of the model representations, the program was applied to generate an "sli" file, containing, among others, information on the layer height (20 µm) and the location of the models on the working plane of the prototyping device, and also a "cli" file, with information on the supporting structure. Afterward, the file with information on the support was exported to an "sli" file.
In the course of pre-processing, a printing device was prepared: a laser was heated up, material residues were removed, steel powder was prepared for printing, and the work table was leveled. Afterward, the "sli" files were transferred to a program dedicated to the manufacturing process on an EOS M270 printer. This program renders it possible to determine such parameters of sintering as laser rapidity in the course of contour sintering, and also fillings, the height of the sintered layer, or laser power in the course of scanning the appropriate areas. The basic parameters of sintering are presented in Table 5. At the further stage (post-processing), the received research models underwent initial purification and were also removed from the printer work table with the application of a hack-saw. Only the model-supporting structures were subjected to grinding.
In Figures 10-17, the method of the measurement of cylindrical elements, and of the rectangular ones, manufactured with the application of DMLS, on which it is possible to observe the model structure (such as detail surface and the quality of workmanship), is presented. The holes, as well as the rectangular objects (Figures 14-17), were measured three times in the center and at both ends of the measured element.

Results and Discussion
In Table 6, the received results relevant to manufactured prototypes measured with the application of an optical microscope, together with the calculated values of arithmetic means of the measured parameters, as well as deviations between the nominal dimension and the received mean values, are collated.

Comparison of the Obtained Results
The results received in the course of the research were divided into three parts. The first included those measurements that were divided in terms of the researched parameter, and the objective of that was to demonstrate differences between the nominal dimension (that assumed in the course of designing the details) and that received in the course of research.
In Figures 18-21, the received mean values of the result measurements relevant to cylindrical elements, and the names of the researched parameters that match the nominal dimension assumed in the course of designing research models, are presented.     Comparing the values received in the course of the measurements of cylindrical holes with the dimension assumed in the course of designing details that are presented in Figure 6c,d, it is ascertained that they were comparable with the nominal dimensions of the researched parameters. All the holes within the considered range were manufactured in research details. The situation was the same in the case of cylinders (Figure 7a,b), where the measured values were also similar to the nominal dimension, wherein the following range of cylinders: Ø1-Ø0.1 mm, the accuracy of workmanship and reducing cylinder diameter was decreasing. It is also worth adding that all the cylinders having the assumed diameters were manufactured with the application of the analyzed DMLS; however, the height of the cylinders (namely, Ø0.3, Ø0.2, and Ø0.1 mm) was lower than those of the other ones, which resulted in it being impossible to calibrate a research device to fit their height, which is presented in Figure 22.     Comparing the values received in the course of the measurements of holes and rectangular objects, it is possible to observe that they are comparable with the nominal dimension for the researched parameter (Figures 8 and 9). It is also possible to observe that, for the holes having the following width: 0.1-1 mm (Figure 8c), results higher than the assumed sizes of the considered parameter were received. It is also worth mentioning that a nonsignificant deformation was observed only in the case of an object having a width of 0.1 mm, which is presented in Figure 27.

Comparison of the Difference between the Nominal and Measured Dimension
In the second part of the elaboration of the results of research into geometrical sizes, the values of deviations between the dimension assumed and that measured for the object models, as well as cylindrical and rectangular holes, were compared.
In Figures 28 and 29, the charts of deviations for rectangular models are presented.  Looking at the chart presented in Figure 28, it is possible to observe that the values of deviations in the case of rectangular objects with the following widths: 10-7 mm were negative; in turn, the values of deviations in the case of objects with the following widths: 3-0.1 mm (except for an object with the width of 1 mm) were positive. The situation was the opposite in the case of rectangular holes (Figure 29), whose values of deviations were positive (except for holes with a width of 2 mm).
In Figures 30 and 31, the values of deviations in the case of cylindrical elements (holes and cylinders) are presented.  In the cylinder model (Figure 30), in the course of the analysis of accuracy, it is possible to observe that, in the case of a cylinder with the dimension of Ø2 mm, the values of deviations were below 0.05 mm. In the case of cylinders with diameters of Ø1 mm and smaller, the values of deviations increased, unlike in the case of cylindrical holes (Figure 31), for which holes between Ø6 and Ø0.1 mm were associated with positive values within the following scope: 0.02-0.1 mm.

The Analysis of the Values of Means Deviations
In the third part of the elaboration of the measurement results, attention was focused upon the comparison of the largest and smallest absolute value, as well as the mean value of differences received as a result of research into rectangular and cylindrical models, and the mean values of deviations between the nominal dimension and that measured for spherical and angular models. The described collations are presented in Table 7. Based on the collation presented in Table 3, it is possible to observe that the greatest absolute difference between the nominal dimension and that measured in the case of cylindrical elements (both for the holes and objects) was ascertained in the case of cylinders within the following scope: Ø1-Ø0.1 mm, contrary to the smallest absolute difference, which was observed in the case of elements within the following scope: Ø10-Ø2 mm. In the case of the rectangular elements, it was impossible to observe such a correlation; however, in terms of rectangular holes, the greatest mean value of deviations between the nominal dimension and that measured for all the researched elements was observed.
In order to verify the received results, two radiators were referred to in the Introduction with appropriately shaped ribs with small cross-sections. In both of these cases, the ribs are arranged radially, which, in addition to anything else, may render manufacturing difficult (rake angle between a drift fender edge and a rib long edge ought not to be 0 [ • ]). In Figures 32 and 33, the radiators manufactured with the application of the DMLS methods are presented (variants 1 and 2) and designed in accordance with the assumptions determined at the beginning of this article, and also based on received research.

Conclusions
The knowledge of the mechanical properties of materials renders it possible to design a given construction in a way that ensures that the loads caused in the course of operating it do not result in damage. It also refers to the ability of the material to be deformed in the course of being shaped. As observed based on the data contained in Table 1, the producer determined in detail and accurately the mechanical properties of the considered materials of which they had informed, which may prove that this material has been researched well. The resistance of the analyzed material is high (up to 1 GPa). The highest value for the 17-4 PH steel amounts to 40.8 HRC, which proves that the material is highly hard [40,41].
The research presented in the article was intended to show the possibility of manufacturing thin-walled structural elements that have a fulcrum only at the base. Additionally, already at the modeling stage of a complex structure, the constructor must have basic knowledge related to the specifics of the DMLS process. At this stage, the direction of the incremental forming in the DMLS method was determined, and it followed the direction of the material being applying by the recoater. Subsequently, the cross-section at the point of contact of the knife with the model should be as small as possible. It is related to the force exerted by the recoater on the model, which was minimal. Otherwise, the structure may deform or collapse, and thus, the process of forming the geometry will not process properly. Therefore, the article aimed to carry out research on the manufacture of thin-walled elements used in the construction of radiators. They were conditioned by the necessity to make radiators with an increased exchange surface through thin-walled elements such as ribs arranged in a radial manner. Manufacturing such long elements for a predetermined height in a radial manner can cause technological problems because it is impossible to arrange the research model in such a way that all the ribs are positioned in the direction of the thrust line of the recoater, which is consistent with the direction indicated by the red arrow ( Figure 3).
The angle between the adjacent ribs of the radiator variant No. 2, which is presented in Figure 3, was almost zero. Thanks to the preliminary samples made with different wall thicknesses, it was possible to determine the minimum thickness of the rib and indicate the height so that the process was carried out correctly. The article presents two variants of radiator geometry (Figures 2 and 3) in order to confirm the validity of the research and the assumptions made. The article also takes a practical nature, indicating tendencies in the incremental formation of elements with small cross-sections. Thanks to the obtained results, it was also possible to determine whether the obtained geometries have an "on the plus" or "on the minus" tendency in relation to the nominal dimension determined during 3D-CAD geometry modelling. The obtained data were collected and presented in the form of graphs and tables. The experimentally determined research results for thin-walled models can be a kind of guide for the constructor during modelling geometries intended for additive manufacturing. Furthermore, in this article, variants No. 1 and No. 2 of the radiator serve as examples of a functional prototype confirming the design assumptions that were determined after conducting a dimensional analysis of the MB1-MB4 research samples.
Based on the analysis of geometrical parameters contained in Table 4, it is possible to ascertain that the accuracy of the representation of prototype models is not much different from the 3D CAD model. The 17-4 PH steel is a material having high levels of mechanical properties, which is directly reflected in high-dimensional-shape accuracy of the manufactured details, in which, in the case of cylindrical and rectangular elements, deviations between the nominal dimension and that received in the course of the research were within the following scope: 0-0.1 mm.
The accuracy of manufacturing cuboidal and cylindrical research models was consistent with the parameters specified by the material manufacturer. The manufacturing of models in the range from Ø0.1 to Ø0.3 mm was problematic primarily for cylindrical models, as presented in Figure 22. However, for cuboidal models, the only deformations occurred in the model with a width of 0.1 mm, as presented in Figure 27. In the case of manufacturing holes in the research cylindrical models, their accuracy was much higher than in the case of cylindrical solids, which can be seen in Figures 18-21, Figure 30, and Figure 31. The value of the deviation between the nominal dimension and the average value obtained during measuring elements for rectangular holes in the width range from 10 to 0.1 mm was comparable and ranged from 0.02 to 0.1 mm, with two exceptions, which have been presented in Figure 29. Taking into consideration both cylindrical and cuboidal solids, elements in the range from 7 to 2 mm were characterized by the highest accuracy, which have been presented in Figures 28 and 29. Comparing the research models, cuboidal models were characterized by higher accuracy compared to the cylindrical models.
Comparing the photographs of the models taken in the course of the research stage, it was possible to ascertain that the staircase effect in the case of models manufactured with the application of DMLS was not observed and that the model structure was uniform.