**2. State-of-the-Art**

The Collaborative Research Center (SFB) 1153 tailored forming aims to develop and optimize a novel process chain for the manufacturing of hybrid solid components. This is done by thermomechanically joining semi-finished products and subsequent forming to produce components with locally adapted properties. By combining di fferent materials within one component, it is possible to reduce the component weight by the local use of lightweight materials or to reduce cost by combining low-cost base materials with high-quality alloys at certain functional surfaces. Among others, the mechanical properties of a hybrid cylindrical roller thrust bearing are of interest. Thus, the low-alloy steel AISI 1022M (material number 1.0460) was used as base material, cladded by the steel AISI 52100 (1.3505) and produced using the tailored forming technology to manufacture hybrid bearing washers. This process chain is depicted schematically in Figure 1. In the first step, the high strength steel was cladded onto the base materials using plasma transferred arc powder deposition (PTA) welding (Figure 1a). Subsequently, the hybrid workpiece was formed into the semi-finished product with a near net-shaped contour by means of forging (Figure 1b). This thermomechanical treatment improved the quality of the interface zones, as shown for deposition welded workpieces in [1,2]. In the following, a heat treatment by means of quenching and tempering, as well as a subsequent machining (Figure 1c), were carried out to produce the axial bearings as depicted in Figure 1d. In the load direction, the two hybrid discs on which the rolling element was running featured a material transition, with the high strength material serving as the raceway.

**Figure 1.** Process chain for manufacturing a hybrid cylindrical roller thrust bearing washer by tailored forming: (**a**) plasma-transferred arc deposition welding; (**b**) upsetting; (**c**) heat treatment by quenching, tempering, and subsequent mechanical processing; (**d**) bearing assembly for analysis in a test bench.

In the following, the term "hybrid bearing" is used for multi-material bearing components manufactured by means of tailored forming. Here, the application of two di fferent materials with a material transition in the load direction is characteristic. In contrast, the term "hybrid" in the context of rolling bearings is usually employed for bearings in which di fferent materials are combined for the bearing rings and rolling elements [3]. Nitrided rolling bearing steel is often used for the bearing rings and a high-strength ceramic, such as silicon nitride, for the rolling elements [4]. These bearings are suited for high performance applications and are complex regarding their proper evaluation [5].

Regarding iron-based cladded components and their fatigue behavior under complex loads due to rolling contacts, few previous works exist. Koehler et al. welded ASTM F75 powder (American Society for Testing and Materials standard for cobalt-28 chromium-6 molybdenum alloy; material number 2.4979) via deposition welding on AISI 4140 (American Iron and Steel Institute standard; 1.7225) and detected a reduced fatigue strength in 4-point bending tests compared to the mono-material, which could be explained by tensile residual stresses caused by the cladding process [6]. In further research by Koehler et al., fatigue tests were carried out on crankshafts of the same material combination repaired by deposition welding [7]. The specimen showed a reduced tensile strength and yield stress compared to newly manufactured crankshafts, resulting in a reduced fatigue life. Alam et al. cladded ASTM F75 on ASTM A572 (1.0045) and observed pores on the surfaces of cylindrical and square bars [8]. It was found that surface cracks were initiated due to these pores, with a negative e ffect on service life. Pores or defects below the surface were not investigated. In Chew et al., a cladding of 1 mm of ASTM F75 was applied to AISI 4340 (1.6511). Cross-section examination of the specimen showed that no macro defects such as pores, lack of fusion, or cracking appeared [9]. By further surface grinding processes of the specimen, a maximum fatigue life of 95% compared to the monolithic substrate could be achieved. At the location of direction change in the course of the welding track, early fatigue cracks were initiated due to the higher heat input and the local microstructure evolution. The positive influence of compressive residual stresses on the fatigue life of radial bearings made of AISI 52100 was investigated in [10]. In contrast to the aforementioned, tests were carried out on real components under a high Hertzian contact pressure of 2.5 GPa. By hard-turning and subsequent deep rolling, an improvement in service life was achieved in comparison to ground hybrid bearings.

Blohm et al. [1] used a subsequent forming process for shafts from AISI 1022M cladded with AISI 5140 (1.7035) by PTA welding. Due to this forming process, a defect-free interface zone could be obtained. Using the example of a hot-formed specimen manufactured by welding, Mildebrath et al. [2,11] demonstrated that the cladding completely recrystallized during the subsequent forming process and that disadvantageous microstructures present after welding were transformed. Behrens et al. [12] researched hybrid forging billets that were welded out of alloy steel AISI 5140 and carbon steel AISI 1022M. They employed a test matrix that utilized the variations of process parameters to influence the geometry and microstructure of the materials' joining zone. Pape et al. [13] investigated the manufacturability of multimaterial components by combining the high-alloy steel AISI HNV3 (1.4718) with the base substrate of ASTM A283 (1.0038) in a single component. The materials were joined by laser metal deposition by wire according to the stressed zones of the component and examined. The aim was to ensure an e fficient use of resources. Behrens et al. [14] welded bearing washers of AISI 1022M with the alloyed steel AISI 5140. They researched the grain refinement depending on the degree of forming and investigated the wear resistance of the specimen. Stanford and Jain [15] found that pores in the loaded material areas had the greatest influence on the service life of the components if they had not been closed or reduced in size by a subsequent forming process. In particular, they emphasized the positive influence of the forming process on fatigue life.

#### **3. Aim and Research Objectives**

As has been shown in the State-of-the-Art Processes section, the deposition welding of a low-alloyed steel can result in a reduction in its fatigue strength. The build-up welded surfaces of low-alloyed steels do not ye<sup>t</sup> have su fficient fatigue resistance to rolling bearing loads. No references are known for the welding and subsequent forming of rolling bearing steel. The aim of this study was therefore to develop a process route for the application of rolling bearing steel by means of tailored forming and to investigate the mechanical properties of the components produced in this way. For this purpose, the following questions have to be answered:


#### **4. Materials and Methods**

In this study, classical bearing steel was welded on a base material by means of plasma-powder-transferred arc welding (PTA) and later used as bearing raceway of a cylindrical roller thrust bearing type 81212, see Figure 2. A washer with an outer diameter of 95 mm was produced, see Figure 2a. The washers' thickness was 7.5 mm. It had a material transition in axial direction (Figure 2b), which was the direction of loading. The base material of the washer consisted of the unalloyed and heat-resistant weldable steel AISI 1022M, which is mainly used in valve construction. The cladding material for the PTA consisted of the rolling bearing steel AISI 52100, which had a CEV > 1 and was considered to be non-weldable [16]. In order to weld the material in spite of this, the material was atomized to utilize it for PTA. The material AISI 52100 has a high wear resistance and toughness, which is why components such as rolling bearings are made of it. The chemical compositions of the materials are shown in Table 1. The chemical compositions were measured with a spark analyzer LMX07 (Spectro Analytical Instruments GmbH, Kleve, Germany).

**Figure 2.** Tailored forming bearing: (**a**) finished specimen as roller bearing raceway; (**b**) sectional view; (**c**) cylindrical roller thrust bearing type 81212.


**Table 1.** Chemical composition of AISI 1022M and AISI 52100 determined by optical emission spectrometry in wt.%.

After PTA welding, the component was formed to improve the material properties. By means of detailed analytical methods, the processes necessary for production were investigated step by step. The components that were produced, as described in Figure 1, later served as rolling bearings and will be characterized in the following with regard to their material properties. Subsequently, fatigue tests were carried out on a rolling bearing test rig, which served as a baseline for further investigations. The roller and cage assembly was taken from conventional bearings, in order to assemble a cylindrical roller thrust bearing consisting of two discs, see Figure 2c.

#### *4.1. Plasma Transferred Arc Deposition Welding*

PTA is a thermal process for applying wear and corrosion resistant layers on surfaces of metallic materials. During the PTA welding process, a tungsten electrode created a plasma arc with high energy density, which melted the surface of the base material. At the same time, the cladding material was inserted into the arc by a powder stream and was molten. During solidification, a substance-to-substance bond between the cladding and base material was created. The welding process was shielded by argon gas. The advantages of this process were a low dilution rate, a small heat a ffected zone, and deposition rates up to 10 kg/h. Further benefits were a high degree of automation and reproducibility [17].

The welding process was carried out on a six-axis REIS RV industrial robot (KUKA AG, Augsburg, Germany), where two additional axes were realized by a turn and tilt table. The welding torch was a Kennametal Stellite HPM 302 (Kennametal Stellite, Pittsburgh, PA, USA), which was water-cooled. The equipment is shown in Figure 3a. To obtain the optimum grain size with a diameter of a minimum 63 μm to maximum 200 μm, the powder was filtered with sieving units (Retsch, Haan, Germany). This corresponds to the current industrial standard for additional materials in powder form that are used for PTA welding [18]. Powders with the standard grain size were used because of a regular melting behavior in the arc as well as a good transportability in the gas flow of the transport gas. Prior to the welding process, the surface of the discs was cleaned with acetone (E-Coll, Wuppertal, Germany). The substrate was at room temperature at the start of welding and was not preheated. Welding without preheating normally makes a welding process more di fficult, especially when high carbon equivalent steels are processed. However, by choosing a slow welding speed of 0.12 m/min, the resulting high heat input enabled the discs to be heated up to suitable joining temperatures in a short time by the welding process itself. This improved the weldability of the steel alloys used. The disc was placed on the additional axis, which was aligned parallel to the ground. Due to the spiral-shaped seam geometry (Figure 3a), the beginning and the end of the seam were not located in the area where the rolling contact will later take place. Therefore, any defects that occur in the beginning or the end of the weld, e.g., because of too low or too high temperatures, were not relevant for the later function of the components. During the welding process, the torch oscillated with a frequency of 2.0 Hz and amplitude of 4 mm at an angle of 90◦ to the welding direction. The oscillation increased the dynamics of the weld pool and allowed the degassing of the melt, preventing pores. The welding process took about 10 min and 30 s, whereby the disc heated up to 650 ◦C. To keep the dilution between the base material and cladding metal nearly constant, the welding current was dynamically adapted during the welding process (see Figure 3b). Starting with a current of 180 A, the current was gradually reduced to 130 A. A suitable current curve was empirically determined in preliminary tests by temperature measurements accompanying the welding process. A short working distance of 10 mm was selected. This allows a very precise control of the seam geometry. This was set with a seam-overlap of 1.5 mm. Due to the short working distance, the plasma gas was set as low as possible at 1.5 min−<sup>1</sup> to avoid spattering of the melt pool. The remaining gas values for transport and protective gas correspond to standard values. An overview of the general welding parameters is given in Table 2.

**Figure 3.** (**a**) Experimental setup of the welding process; (**b**) adjustment of current intensity.


