Microstructure and Performances for Wear-Resistant Steel through the WAAM Technology

)is work describes the wire arc additive manufacturing (WAAM) approach used to fabricate parts from wear-resistant steel. )e microstructure, crystal structures, and mechanical properties of the resulting samples were thoroughly analyzed. )e wearresistant steel parts demonstrated good forming, no internal defects, good metallurgical bonding, and excellent wear resistance. )e metallographic analysis confirmed that the main phase was ferrite. )e microhardness of the sample along its cross section was uniform in both horizontal and vertical directions and equals to 464.7HV0.2 and 482.4 HV0.2, respectively. )e average values of tensile strength, elongation ratio, and room temperature Charpy shock were equal to 945.3MPa, 4.3%, and 5 J, respectively.


Introduction
WAAM, similar to 3D printing technology, uses the heat generated by the arc to provide a heat source during work to melt the metal wire. e melted metal wire will follow the set forming path to melt the metal slurry. On the substrate, the molten slurry is piled up layer by layer, and it is cooled and formed until it becomes a metal part. erefore, almost any complex shape of metal parts can be produced [1][2][3][4][5][6][7][8]. is method has several advantages over traditional manufacturing methods such as environmental friendliness, high material utilization, and molding efficiency, as well as low manufacturing costs [9][10][11][12][13][14]. 3D printing is especially popular for applications related to biomedicine, shipbuilding, aerospace, mold production, and automobile manufacturing. WAAM uses an electric arc as a heat source and filler wires as raw materials. It is suitable for manufacturing large parts [15][16][17][18][19][20][21] especially for agricultural machinery prone to corrosion and wear after prolonged usage. As a result, wearable parts such as pins need to be replaced frequently, which is not economically feasible.
Wear is one of the primary forms of parts' failure. It consumes significant energy and wastes raw materials. Approximately 30-50% of the mechanical energy is consumed by friction and wear. Statistics provided by the German Federal Ministry of Technology and Science assessed that wear and tear cause 5 billion euros of losses [22,23]. China lacks such statistical data. However, a report provided by the Chinese Machinery Department states that in 1974-1975, the annual steel consumption for auto part production was 230,000 tons, two-thirds of which were used to maintain and repair damages caused by wear. In addition, according to sparse statistical data from Chinese departments of electric power, building materials, metallurgy, coal mining, and agricultural machinery, spare parts , production annually requires over 1.5 million tons of steel. For example, the losses due to the wear of the middle groove of the scraper conveyor used in coal mines account for 100-200 million yuan per year [24][25][26][27][28][29][30][31][32][33]. us, these and all other economic losses caused by wear and tear of the mechanical equipment and the steel consumption are enormous. However, these damages could be prevented to a certain degree if the quality of the resistant steel (to make it more wear-resistant) is improved. For this purpose, scientists and engineers need to develop new high-performance wear-resistant steel and understand steel wear mechanisms, which will provide desired insights [34][35][36][37]. erefore, the purpose of this work was to compare wear-resistant material used as raw sources for the arc additive manufacturing of the parts, for which excellent wear resistance is required. We studied the structure and mechanical properties of the wear-resistant materials subjected to the WAAM. is paper provides data needed to advance and improve the fabrication of reliable agricultural machinery and equipment. e wear-resistant steel typically contains Si, Mn, Cr, Mo, V, W, Ni, Ti, B, Cu, and rare earth elements. In this paper, we focused on the wear-resistant steel containing high levels of C and Cr.

Experimental Procedures
We used the FILLARC DUR 606M 1.2 mm in diameter wire consisting of wear-resistant steel. Its chemical composition is shown in Table 1. e wire arc additive manufacturing (WAAM) equipment used the AIR10 and AIR-TP automated software (see Figure 1). e substrate was a 300 × 100 × 10 mm steel plate, polished prior to the WAAM using a grinding wheel to remove the surface layers to reduce the plate's deformation. e substrate was attached to the base steel plate. Our prior work established the single-layer single-pass and single-layer multipass parameters, which were then used in our present work to obtain the optimum weld bead shapes. e control over the forming accuracy during WAAM was performed by measuring the interlayer temperature using infrared measurements. e sample was 150 × 20 × 60 mm in size. e samples were printed under the 80% Ar/20% CO 2 atmosphere. e interlayer temperature did not rise above 150°C. e interlayer temperature is considered to be one of the most important parameters [38]. e voltage and current were maintained at 19 V and 80 A, respectively. However, the first layer current was 90 A. e sample printing speed was 5 mm/s. All WAAM parameters are given in Table 2.
After the main sample was produced, it was cut into 17 × 28 × 20 mm pieces for further hardness, metallographic X-ray analysis, and scanning electron microscopy (SEM) analysis. e tensile tests were performed on both sample sides parallel and perpendicular to the deposition direction (see Figure 2). e room temperature tensile tests were performed according to GB/T 228.1-2010 standard using UTM-6104 material testing machine. e displacement control was implemented during sample stretching, which was performed at a 0.005 min −1 strain rate. e impact tests used a pendulum impact testing machine (JB-W300A). ese measurements were performed at room temperature and for each data point. One sample piece was polished and then etched in ferric nitrate solution in alcohol for further microstructural observations performed according to the GB/T 13298-2015 Metal Microstructure Inspection Method using a 4XC microscope. X-ray diffraction (XRD) performed using the XD-3 instrument was used to determine the phase composition. e analysis parameters were CuK-α emission as an X-ray source, 36 V, 24 A, 2°/min scan speed, 0.02°step, and 10-90°range. e friction tests were performed using dry and ball-disk frictions. For this purpose, the test sample was placed into a three-jaw fixture, after which the friction ball (6 mm in diameter) was placed onto the sample surface. e sample weight was recorded before the 30 N friction force was applied. Other friction test parameters were 500 rpm spindle speed and 1800 s test time.
SEM was performed by the Zeiss EVO ® 18 instrument to analyze the sample morphologies after the impact, tensile fracture, and wear tests. For transmission electron microscopy (TEM), performed using the JEM-2100 instrument operated at 200 kV, a 0.5 mm sample slice was ground to ∼120 μm. ree disks were punched out of this piece and then ground to 50 μm.
For the double-spray electrolysis tests, performed at −20°C and 75 V using Gatan 691 ion thinner to treat the Ar for 0.5 h, we used 4% perchloric acid solution as the electrolyte. e Vickers microhardness was tested according to GB/T 4340.1-2009 "Metallic material" procedure performed by using the HV-1000Z tester at 200 g force. e starting point of the test was the upper section of the sample 2 mm away from the edge. Twelve points 1 mm apart from each other were tested.

Results and Discussion
3.1. Forming Characteristics. 3D printed wear-resistant steel sample is formed without collapsing (see Figure 3) and with small welding wire splash. e cross section analysis of the sample showed minimum defects and satisfactory metallurgical bonding (see Figure 4). ese observations further confirmed the excellent formability of the WAAM-printed wear-resistant steel. Figure 5 shows the metallographic data obtained for different sections (columnar crystal, remelting, and heat-affected zones) of the part fabricated in this work by WAAM. e crystals corresponded to the ferrite phase. e black zigzag features corresponded to the grain boundaries, while the inclusions which appeared as small black particles were metal carbides. e low-magnification analysis of the sample revealed no cracks, holes, and solid inclusions and only some fusions, incomplete penetration, poorly shaped grains, and other defects.

Microstructure and Crystal Structure.
us, the internal structure of the wear-resistant steel sample prepared by WAAM was very satisfactory.
XRD of the steel sample prepared by WAAM showed three strong peaks at 43°, 64°, and 80°, corresponding to the α-Fe phase (see Figure 6). No other phases were detected very likely because their contents were below 5%. e SAED confirmed that the crystalline phase in our sample was α-Fe (see Figure 7).

Mechanical Properties.
e microhardness distribution diagram shows that the hardness of the wear-resistant steel samples prepared by WAAM was uniform and in the 410-520 HV 0.2 range (see Figure 8). e average microhardness values in the transverse and longitudinal directions were equal to 464.7HV 0.2 and 482.4 HV 0.2 , respectively. ese values were remarkably close to each other. e microhardness fluctuations were mainly due to the structural variations of the sample; the sample cross section analysis revealed that it possessed multilayer and multipass structure, with the first layer possessing the weld bead structure. Additionally, this first layer was affected by the second one, which also possessed the weld bead structure. e original columnar crystal, remelted, and heat-affected regions were observed because of the thermal cycle of the same-layer weld bead. eir microhardness varied because these regions have different grain sizes and precipitate contents [39,40]. e mechanical properties of the samples prepared in this work by WAAM were excellent: the average tensile strength, elongation, and room temperature Charpy shock values were equal to 945.3 MPa, 4.3%, and 5 J, respectively (see Table 3).
Microscopical analysis of the tensile fracture of our steel sample revealed no apparent defects (see Figure 9(a)), which agrees with the experimental results shown in Table 3, especially for the samples with lower elongation values.     Figures 9(b)-9(d) show the morphologies of the tensile fracture of the fibrous, radiation, and shear lip zones, respectively. e fracture was flat and bright, which implies that it was a brittle fracture, the fracture surface of which is usually perpendicular to the tensile stress. e fracture is composed of shiny crystalline surfaces with slight fibrous irregularities. e ratio of the radiation area was larger (see Figure 9) than that of the fibre area. us, the material possessed poor toughness and was very brittle. Figure 10 shows the samples before and after the friction tests. Five sets of friction tests under different loads were performed. e wear conditions were observed, and calculations and analysis results (namely, the mass change and wear rate of the sample before and after the friction tests) are presented in Table 4. Figure 11 shows the relationship between the wear rate and the load. As the load was increased, the wear rate increased as well.    Advances in Materials Science and Engineering e relationship between friction coefficient and time (shown in Figure 12) revealed that as the load was increased, the friction coefficient and the wear rate also increased.

Wear Resistance.
SEM of the sample subjected to the friction tests showed numerous groove-shaped wear marks along a fixed direction (see Figure 13). e surface of the material also demonstrated some spalling pits formed due to the abrasive and adhesive wear.
Continuous long strip furrows appeared on the worn surface, which is typical for abrasive wear. e depth of these furrows corresponds to the severity of the wear. In our case, the worn surface was relatively smooth, with only minor cracks. As the load force was increased, the wear rate increased and was on the 10 −7 order of magnitude. us, the material possessed excellent wear resistance, especially at higher loads.

Conclusions
e wear-resistant steel samples were fabricated by wire arc additive manufacturing (WAAM). e resulting samples possessed excellent forming properties and no significant defects. e internal metallurgical bondings of the samples were very satisfactory.
e microhardness values in the transverse and longitudinal directions were very similar, with the average values equal to 464.7 HV 0.2 and 482.4 HV 0.2 , respectively. e samples also possessed excellent mechanical properties: their average tensile strength, elongation, and room temperature Charpy shock values were equal to 945.3 MPa, 4.3%, and 5 J, respectively. We also observed excellent friction resistance of these steel-resistant parts. e results obtained in this work deliver insights into the theoretical and experimental understanding of WAAM for fabricating wear-resistant steel parts for agricultural machinery.

Data Availability
e data used to support this study are available from the corresponding author upon request. Advances in Materials Science and Engineering 9