**1. Introduction**

Many technical applications place partly contradictory demands on the construction material used. For example, the parts may need to be lightweight, low-cost, and at the same time, increase performance. Material selection then becomes a multi-criteria decision problem in which factors such as production, costs, or environmental effects are considered [1–3]. The goals of the engineer may be conflicting [4]. The combination of multiple material types in a single component has the potential to achieve several goals at once, for example load bearing capacity can be varied through local adaptation of material properties to reduce the overall amount of material required. Additive manufacturing, casting, and welding are currently used for this purpose [5–7].

In sheet metal forming, the use of welded sheet blanks, also known as tailored welded blanks, has been industrially established for over 20 years [8,9]. They are used in large quantities in the automotive industry for the production of body parts such as doors and side members [7]. However, the technology for this type of processing cannot directly be applied to a multi-material component with a three-dimensional geometry.

#### *1.1. Tailored Forming Approach*

Tailored forming involves deposition welding (see Section 2.1 and Section 2.2) and subsequent forming (see Section 2.3) to produce multi-material solid components [10]. An exemplary component of this process chain is shown in Figure 1 (left). Here, a multi-material shaft consisting of a cladding material (red) and a base material (blue) is shown. The deposition-welded part of the shaft (red) serves as a bearing race for a cylindrical roller bearing (CRB, yellow). Due to an external radial load on the CRB, high-contact normal stress exceeding 2 GPa [11,12] occurs in the contact between the rolling element and the shaft. According to the Hertz–Belyaev theory [13,14], the maximum equivalent stress occurs in a material volume below the stressed surface. When a loaded rolling element rolls over a point on the raceway surface, the maximum shear stress in the subsurface varies between 0 and τmax, see Figure 1, right. This cyclic loading of the material volume initiates and propagates fatigue cracks in the high-cycle regime (>>10<sup>6</sup> cycles), which is referred to as rolling contact fatigue (RCF) [15]. RCF eventually leads to material removal and, if a crack propagates to the surface and forms a chip/pitting, failure of the component. Lundberg and Palmgren [16] assumed the maximum orthogonal shear stress τO to be significant in causing fatigue failure. Other authors consider the von Mises–Hencky distortion energy theory [17] and the scalar von Mises stress to be better for predicting RCF failure, with the latter being directly proportional to the octahedral shear stress τoct. Figure 1, right, shows that the maximum shear stress occurs at depth *z* of approximately 0.5b to 0.8b. Here, RCF occurs in a highly localized volume of stressed material, so a high-strength material is required there. The remaining part of the component can be made of a less solid material with higher ductility and lower price.

**Figure 1.** Partial section of tailored forming shaft with mounted cylindrical roller bearing (**left**) and loaded material volume (**right**).

#### *1.2. Welding and Forming*

Deposition welding is particularly suitable as a joining step, as it creates a substance-to-substance bond between the cladding material and base material. As already mentioned, the maximum stress zone of a roller bearing is under the contact surface. Surface hardening processes such as nitriding [18] do not reach the necessary layer thicknesses to withstand the expected loads. Through deposition welding, it is possible to produce a cladding layer, which can be adjusted to the applied forces and stresses by alloy design and thickness selection. It is also possible to deposit many kinds of metallic alloys, sometimes even those considered non-weldable because of their high carbon equivalent values (CEV). Further advantages of deposition welding are a low dilution rate and a high degree of automation [19].

The chosen deposition welding procedures are laser metal deposition welding with wire (laser metal deposition welding (LMD-W)) and plasma powder transferred arc welding (PTA). The welding processes were chosen for their di fferent advantages. Laser metal deposition welding allows precise control of the seam geometry and produces low material dilution. PTA welding, on the other hand, allows significantly higher deposition rates but also introduces more heat into the components.

In laser metal-wire deposition welding, a wire is preheated using conductive heating and the laser radiation provides the energy necessary for melting the wire and the substrate material. The preheating of the wire reduces the required laser power, which leads to less heating of the substrate and thus to lower degrees of dilution between base material and applied material. Additional advantages of laser hot-wire processes are a better process stability and less sensitivity to tolerances in wire alignment. The low degrees of dilution ensure high quality of layers, so in most cases, the desired chemical or physical properties, such as corrosion resistance, wear resistance, and hardness, are achieved from the very first layer [20–22].

Plasma powder transferred arc welding is a thermal process for applying wear- and corrosionresistant layers on surfaces of metallic materials. During the PTA welding process, a tungsten electrode creates a plasma arc with high energy density, which melts the surface of the base material. At the same time, the cladding material in powder form is streamed into the arc and molten. During solidification, a substance-to-substance bond between the cladding material and the base material is created. The whole welding process is performed with argon as the shielding gas.

In order to further enhance material- and process-related advantages, a subsequent forming of the joined semi-finished product is necessary.

It is possible to simulate forming processes in addition to welding processes with FEM. The prediction of the resulting heat flows is of particular importance. The mapping of heat flows allows an estimation of the heat-a ffected zones and the resulting residual stresses in the material. This is important, because it allows for prediction of component distortion. An example of how this can be achieved is given by Lostado Lorza et al. [23]. Due to the production process of the components used in this work, a simulation of the welding influence is not necessary. As Blohm et al. [24] showed, a hot forming process after a welding process could completely eliminate the heat e ffects of welding on the steel structure. The reason for this is the complete re-austenitization of the microstructure with subsequent microstructure formation, which neutralizes previous residual stresses. As shown in Figure 2, this microstructure transformation can also be observed in the samples of this work. Figure 2a shows the microstructure of the base material C22.8 after welding. A classical Widmanstätten microstructure of long ferrite needles can be seen, and the impact of the heat from welding is obvious. The 100Cr6 is solidified in a pearlitic structure and is located in the darker region in the upper part of the picture. After cross-wedge rolling (CWR), the microstructure in the base material and in the coating material is much finer-grained than after welding and corresponds to a typical microstructure from forming (Figure 2b). In both microstructure states, a defined orientation of the grains or inhomogeneity cannot be identified, so for the simulation of the forming process, isotropic forming properties are assumed.

**Figure 2.** Microstructure of the joining zone after welding (**a**) and after cross-wedge rolling (CWR) (**b**).

#### *1.3. Bearing Fatigue Life of Tailored Forming Machine Elements*

Previous investigations [25,26] have shown that the performance of tailored forming machine elements, in particular their fatigue life, is dependent on the layer height of the cladding material. The cladding height *h* is defined as the radial distance between the rolling element/shaft contact point at one end, and the cladding layer/base material interface at the other, see Figure 1 (right).

Figure 3 shows the calculated bearing fatigue life for different radial loadings and cladding heights with test data for a monolithic specimen as reference [26]. Here, fatigue-life simulations were carried out for a radial loading on the tested CRB of *F*rad = 2 kN with a resulting Hertzian contact pressure of *p*max = 1.8 GPa. The calculated bearing life *L*50, where 50% of the specimens are expected to have failed, is *L*50 = 23.5 × 10<sup>6</sup> revolutions. This is within an error margin of 16% of the bearing fatigue life from the experimental studies. The basic trends of the calculated probability of survival are represented by the experimental values *L*10 and *L*63. A too-thin cladding layer reduces fatigue life of multi-material machine elements by a factor of 3. With a cladding height of *h* > 0.5 mm the difference in fatigue life compared to monolithic parts is within a 15% margin. These preliminary results show that a minimum cladding height in dependence of the load is necessary to achieve the same fatigue life as a monolithic component.

The approach for hybrid roller bearings is particularly interesting for large scales. Large-diameter bearings are currently manufactured from classic forging materials such as 42CrMo4, which are not specifically designed for roller bearing applications. By coating the running surfaces with a small amount of a high-performance material, it would be possible to increase the service life enormously or to use the bearings in much more corrosive environments.

**Figure 3.** Bearing fatigue-life simulation for different cladding layer thicknesses.

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

For the reasons given in the previous chapter, the production process for manufacturing tailored forming machine elements must meet the following conditions to be implementable in industrial applications:


The aim of this paper therefore is to achieve a precise adjustment of the cladding thickness *h* by controlling the production process, see Figure 4. For this purpose, first, two different joining processes—laser metal deposition welding and plasma-transferred arc welding—are presented in Sections 2.1 and 2.2, and the characteristics that particularly influence cladding thickness are empirically examined using welding test rigs. Subsequently, cross-wedge rolling as a forming process is described in Section 2.3, and the decisive variables that influence layer height are worked out. Extensive numerical analyses are performed, which are validated by experimental investigations on a test rig. The purpose of these cross-wedge rolling experiments is to determine the input geometry with regard to width and height of the cladding layer in advance. In this way, the fatigue-life requirements for the application can be reconciled with the target parameters for deposition welding.

*Metals* **2020**, *10*, 1336

PTA: Plasma Powder Transferred Arc Welding LMD-W: Laser Metal Deposition Welding with Wire FEA: Finite Element Analysis *h*: cladding thickness of welded material exp.: Experimental investigations

**Figure 4.** Methods for defining cladding material distribution.

Two different cladding materials are used for the investigation. The martensitic chrome silica steel X45CrSi9-3 is used as cladding material for the laser metal deposition by wire process. The cladding material for the PTA process consists of the roller bearing steel 100Cr6, which has a CEV > 1 and is therefore difficult to weld [27]. The material has a high resistance against cyclic loading. The 100Cr6 powder is filtered with a sieving unit. Only powder consisting of metal grains with a diameter between 63 μm and 200 μm is used for the welding [28]. This corresponds to the current industrial standard for additional materials in powder form that are used for welding. The basic material of the shafts consists of the unalloyed and heat-resistant steel C22.8, which is mainly used in valve construction and is considered to be easily to weld. The chemical composition of base material and cladding material are shown in Table 1.

**Table 1.** Chemical composition in wt.% of C22.8, X45CrSi9-3, and 100Cr6.


#### *2.1. Laser Metal Deposition Welding with Wire*

Hybrid semi-finished products are manufactured by deposition welding of the martensitic chrome silica steel X45CrSi9-3 onto a base cylinder made of the unalloyed steel C22.8. A coaxial laser hot-wire welding head is used to produce the hybrid components. Compared to lateral wire feeding, coaxial wire feeding has the advantage that the process is independent of the welding direction and is therefore suitable for the manufacturing of complex components [29,30].

Base cylinders with a diameter of 27 mm and 29 mm are used as substrate. The substrate is sandblasted and then cleaned with ethanol and kept at room temperature without any preheating.

For manufacturing of the hybrid semi-finished products, the base cylinder is placed in a rotary axis, which can be moved in the X–Y plane. By superimposing the rotational movement of the base cylinder and the movement of the rotational axis in the x-direction, spiral seams are applied to the base cylinder. The processing head does not move during welding. The welded layer is positioned in the middle of the base cylinder (see Figure 5).

**Figure 5.** Shaft with cladding applied by laser metal deposition with wire.

Hybrid semi-finished products with various geometries are produced to validate the simulation of the layer distribution during CWR. Cladding layers consisting of 6 or 11 adjacent seams with a width of 8 or 15 mm, respectively, are applied to these using the before-mentioned setup. The claddings are applied in one or two layers, whereby the two layers where welded unidirectionally, with a short break of 90–120 s between layers.

An overview of the welding parameters is shown in Table 2.



1Laser Metal Deposition Welding with Wire.

Examples of the seam geometry after deposition welding are shown in Figure 6. The seam height is approx. 1.2 mm for single-layer deposition and approx. 2.4 mm for double-layer deposition.

**Figure 6.** Examples of the seam geometry after deposition welding.

For each set of parameters, one shaft after welding and one shaft after CWR is cut, and the layer distributions are evaluated. The values for characteristics of the welded layer are used as input variable data for the simulation.

#### *2.2. Plasma Powder Transferred Arc Welding*

The PTA welding process is carried out by a six-axis REIS RV industrial robot (KUKA AG, Augsburg, Germany), where two additional axes are realized by a turn and tilt table. The welding torch is the Kennametal Stellite HPM 302 (Kennametal GmbH, Rosbach, Germany), which is water-cooled.

First, the bar blanks are turned and cut to size. The finished components have a length of 150 mm and a diameter of 27 mm. Prior to the welding process, the components are cleaned with acetone to ge<sup>t</sup> clean surfaces. The components are welded at room temperature. For the welding process, the shaft is fixed in the additional axis and aligned parallel to the ground. The welding head is in a horizontal welding position aligned with the shaft. During the welding process, the welding head only moves with a defined pendulum movement in the y-direction, while the shaft rotates 6 times around its own axis. The result is a spiral weld seam, which is 30 mm long in total (see Figure 7). The weld seam is welded a little longer than required, because the beginning and the end of the seam cannot be welded in an optimum way for technical reasons.

**Figure 7.** Welding seam after cladding by PTA.

To achieve an even application layer, it is important that the weld pool has enough time to solidify. Because of the small diameter and the rotation of the shaft, there is a risk that the weld pool does not cool down in time and drops to the ground due to gravity. Therefore, the torch is moved slightly off the center-point, which increases the arc length and gives the weld pool more time to cool down. In order to apply the weld seam as homogeneously as possible and to avoid pores, the welding torch oscillates over a short distance of 4.5 mm in the welding direction with a frequency of 1 Hz. This slightly increases the dynamics of the weld pool and allows degassing of the melt, which reduces the chance of pore formation. The whole welding process takes about 5 min, and the shaft reaches 700 ◦C. In order to counteract a strong heating of the cladding and base material, the temperatures are dynamically adjusted during the welding process. At the beginning, a current of 120 A is selected to generate a fast heating of the shaft. Further in the process, the current is gradually reduced in order to keep the dilution low and as constant as possible (see Figure 8). An overview of the general welding parameters is shown in Table 3.

**Figure 8.** Dilution during the welding process.

**Table 3.** Welding parameters for plasma powder transferred arc welding.

