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Article Manufacturing of High-Performance Bi- Bevel Gears by Combined Deposition and

Anna Chugreeva 1,*, Maximilian Mildebrath 2, Julian Diefenbach 1, Alexander Barroi 3 , Marius Lammers 3, Jörg Hermsdorf 3, Thomas Hassel 2, Ludger Overmeyer 3 and Bernd-Arno Behrens 1

1 Institute of Technology and Machines, Leibniz Universität Hannover, 30823 Garbsen, Germany; [email protected] (J.D.); [email protected] (B.-A.B.) 2 Institute of Material Science, Leibniz Universität Hannover, 30823 Garbsen, Germany; [email protected] (M.M.); [email protected] (T.H.) 3 Laser Zentrum Hannover e.V., 30419 Hannover, Germany; [email protected] (A.B.); [email protected] (M.L.); [email protected] (J.H.); [email protected] (L.O.) * Correspondence: [email protected]; Tel.: +49-511-762-18280

 Received: 27 September 2018; Accepted: 26 October 2018; Published: 2 November 2018 

Abstract: The present paper describes a new method concerning the production of hybrid bevel gears using the Tailored Forming technology. The main idea of the Tailored Forming involves the creation of bi-metal workpieces using a joining process prior to the forming step and targeted treatment of the resulting joint by thermo-mechanical processing during the subsequent forming at elevated temperatures. This improves the mechanical and geometrical properties of the joining zone. The aim is to produce components with a hybrid material system, where the high-quality and expensive material is located in highly stressed areas only. When used appropriately, it is possible to reduce costs by using fewer high-performance materials than in a component made of a single material. There is also the opportunity to significantly increase performance by combining special load-tailored high-performance materials. The core of the technology consists in the material-locking coating of semi-finished parts by means of plasma-transferred- (PTA) and subsequent forming. In the presented investigations, cylinders made of C22.8 are first coated with the higher-quality heat-treatable steel 41Cr4 using PTA-welding and then hot-formed in a forging process. It could be shown that the applied coating can be formed successfully by hot forging processes without suffering any damage or defects and that the previous weld structure is completely transformed into a homogeneous forming-typical structure. Thus, negative thermal influences of the welding process on the microstructure are completely neutralized.

Keywords: forging; hot deformation; bi-metal bevel gear; tailored forming

1. Introduction Following the current trend towards engine downsizing and increasing output, technical components with improved performance and less weight are required. In this context, conventional materials commonly used in the established production processes approach their material-specific limits. In contrast, producing components from a combination of multiple materials could extend functionality and properties adjusted to specific applications and load conditions. Thus, the development of efficient process routes for the production of multi-material components has been gaining in importance in recent years. With this objective, the Collaborative Research Centre 1153 (CRC 1153) “Tailored Forming” investigates a novel process chain, which includes the joining of raw materials to a bi-metal workpiece in the first step, followed by the forming to the final geometry in a second step. Thus,

Metals 2018, 8, 898; doi:10.3390/met8110898 www.mdpi.com/journal/metals Metals 2018, 8, 898 2 of 11 the focus of the Tailored Forming technology is set on the targeted treatment of already existing joint by means of thermo-mechanical processing during forming. Using this concept, it is possible to improve microstructure, mechanical properties and the bonding quality in the joining zone. The current study is focused on the manufacturing of bi-metal bevel gears made of two different whereby material joining is performed by a deposition welding process. Subsequently, the semi-finished bi-metal workpieces are formed by die-forging. This investigation presents the analysis of the resulting microstructure and shape evaluation of the joining zone after the welding and forging process. The novelty of this work is that solid semi-finished products consisting of two materials, which are produced by plasma-transferred arc welding of steel, are undergoing a thermo-mechanical treatment within the subsequent forming step. Thus, high-performance hybrid components can be produced. The aim is to show that the previously existing weld structure can be transformed into a forming structure by the forming process, and therefore negative effects of the heat influence from the welding in the finished component are negligible.

2. Survey of the Current Literature Deposition welding represents an efficient method for manufacturing hybrid material parts. This technique allows for a local improvement of material properties by applying high-performance metallic coating materials to the substrate material. Depending on the energy source used for material melting, various types of deposition welding processes can be distinguished. Each process has advantages for certain applications. Methods that are well established in the industry nowadays to apply selective metal layers include metal inert gas welding, submerged arc deposition, plasma-transferred arc (PTA), laser deposition welding or electro--welding [1]. Plasma-transferred arc deposition welding is considered the most suitable process for this purpose because of its wide range of flexibility with regard to the alloys that can be processed, and it enables high deposition rates. The process is strongly recommended for applying high-performance coatings, as the majority of hard and wear resistant coating materials are only available in powder form. This is especially the case for heat-treatable steels. These steels usually have high carbon equivalents (CEV > 0.5) and are declared as difficult to weld or non-weldable according to current data sheets. Due to this fact, the welding process can be very challenging [2]. Plasma powder deposition welding uses a plasma arc as a heat source and metal powder as filler material. The energy input of the process can be precisely controlled, and enables high application rates. The welding process is similar to tungsten inert gas (TIG) welding, in which the arc burns between non-melting tungsten . Argon is used as shield gas. Compared to other processes, the geometric accuracy of PTA-welding is relatively imprecise. However, Jhavar et al. explain the development of a µ-PTA process for the cost-efficient and energy-efficient repair of small dies and moulds [3]. Ulutan et al. investigated the effects of PTA on the microstructure and of AISI 5115 steel coated with FeCrC-powder [4]. Motallebzadeh et al. implemented welding of a hypo-eutectic CoCrWC hard facing onto a steel substrate by PTA deposition welding and investigated the microstructure and high-temperature tribological performance [5]. Ferozhkhan et al. performed welding of Stellite 6 alloys on for wear resistance investigations [6]. Xinke Denk et al. welded a wear-resistant Fe-Mo alloy coating on an AISI 1045 steel substrate by plasma transferred arc (PTA) cladding, using pure Mo powders as the precursor material [7]. PTA-welding can also be used for in-situ alloying directly in the welding process. Youlu Yuan et al. successfully synthesized In-situ WC/Fe carbide coatings by PTA-welding and studied the wear properties [8]. The welding process can also be used for additive manufacturing. For example, Cheng Liu et al. analyzed the geometric stability and microstructure characteristics of metal additive manufactured parts using plasma transferred arc with WCp/NiBSi metal matrix composites. A macrohardness of HRC 65 ± 1 could be achieved [9]. Bulk metal forming offers high potential for the production of near-net-shape multi-material components with complex geometries, optimised grain flow and outstanding mechanical properties. Metals 2018, 8, 898 3 of 11

In previous studies, the manufacturing of multi-material compounds is commonly performed by processes combining forming and joining in a single stage. For example, Wohletz and Groche examined the joining of steel and aluminium raw parts (C15/AW 6082 T6) by means of combined forward and cup [10]. The study of Kong et al. focused on joining by means of welding of stainless steel (AISI 316L) and aluminium (6063) [11]. It was established that among the other process parameters, the forming temperature has the most decisive influence on the resulting quality and the tensile strength of the joint. The use of inhomogeneous temperature distribution between steel and aluminium raw parts for compound forging was investigated by Behrens et al. in order to achieve an approximately equal forming behaviour of both materials [12]. The achieved results were transferred to the forging of hybrid steel/aluminium gears (S235JR/EN AW 6082). Essa et al. studied the cold upsetting process of bi-metal billets consisting of a steel/steel combination (C15E/C45E) [13]. However, a steel/steel combination, especially in the case of a high-strength steel, can offer only a limited or moderate formability at room temperature. Joining by plastic deformation at high temperatures is challenging in terms of possible scale formation in the joining zone with its negative impact on the final bond strength, or in terms of cost-efficiency in the case of utilizing a atmosphere to prevent the oxidation. Thus, hot forming of previously welded blanks represents an interesting approach for steel/steel parts processing. Hereby, the main issues involve a sufficient deformation of the welded component, in order to refine the coarse welding microstructure as well as an appropriate forming process design to ensure the right position and thickness of the high-strength steel alloy after intensive material flow. In contrast to the joining by forming, few research works address the issue of the bulk metal forming of pre-joined multi-material workpieces. Klotz at al. deal with the isothermal forging of bi-metal gas turbine discs made of two different Ni-based superalloys from hot isostatically pressed billets [14]. Pfeiffer et al. implemented the co-extrusion of non-centric steel-reinforced aluminium profiles with subsequent forging [15]. Förster et al. carried out numerical and experimental investigations on a process chain consisting of hydrostatic co-extrusion of Al-Mg rods with aluminium on the external diameter and subsequent forging [16]. Domblesky et al. investigated hot compression tests in order to study the forgeability of friction-welded bi-metal pre-forms combining , aluminium and steel [17]. Frischkorn et al. examined the hot forging behaviour of further material combinations comprising steel, aluminium, and Inconel [18]. Wang et al. conducted numerical and experimental investigations on the hot upsetting of bi-metal billets produced by weld cladding (C15/316L) [19].

3. Materials and Methods

3.1. Bevel Gears and Semi-Finished Workpieces An application example is a bevel gear, where the different component regions are exposed to varying load conditions. Due to high loads on the contact area between two bevel gears, the tooth flanks are exposed to higher stresses than the rest of the component. Accordingly, high performance properties should be employed on the pitch surface and are not required in the centre. Therefore, a multi-material approach could be well appropriate for this application. In the considered bevel gear, a high-performance material is placed only in the critically loaded areas to counteract the high tribological stresses (e.g., high tensile steel 41Cr4). For the inner section, low-performance materials with a high toughness, ductility and breaking resistance are used (e.g., low-alloyed steel C22.8). The investigated bimetallic bevel gear is schematically illustrated in Figure1b. The corresponding semi-finished workpieces were designed in accordance with the load collective of the final parts (Figure1a). For the production of hybrid workpieces, a cylindrical base material was coated with a metallic layer by means of deposition welding. The thickness of the examined layer was chosen exemplarily with regard to the previous research on the manufacturing of cylindrical gear wheels by compound forging [20]. The crucial criterion was to ensure a full cover of high-stressed areas of Metals 2018, 8, 898 4 of 11

the final bevel gear. Thus, the thickness of the welded layer includes also allowance required for the secondary processes (e.g., ). Within a subsequent hot forging process, the deposition-welded Metalsworkpieces 2018, 8, x FOR with PEER coaxial REVIEW material arrangement were formed to hybrid bevel gears. The4 of main 10 dimensions of the bi-metal workpieces and bevel gears are listed in Table1. The workpieces were turnedturned after after the the welding welding process process in in order order to to achieve achieve the the required required workpiece workpiece volume. volume. The The outside outside diameterdiameter of of the the bevel bevel gear gear corresponds corresponds wi withth the the diameter diameter of of the the crown crown circle. circle.

FigureFigure 1. 1.(a()a Coaxial) Coaxial deposition-welded deposition-welded blank, blank, (b ()b forged) forged hybrid hybrid bevel bevel gear gear [16]. [16 ]. Table 1. Geometrical parameters of the bi-metal workpieces and bevel gears. Table 1. Geometrical parameters of the bi-metal workpieces and bevel gears. Deposition-Welded Workpiece Bi-Metal Bevel Gear Deposition-Welded Workpiece Bi-Metal Bevel Gear Outside Diameter: Ø 30 mm Number of teeth: 15 OutsideCore Diameter: diameter: Ø 30 Ø 27mm mm Outside Number Diameter: of teeth: Ø 6215 mm Core diameter:Height: Ø 7927 mmmm Outside Height:Diameter: 30 mm Ø 62 mm Height: 79 mm Height: 30 mm 3.2. Deposition Welding 3.2. Deposition Welding All deposition processes using welding commonly struggle with the problem of heat-affected zones,All whichdeposition are created processes near using the welding welding seams commonly due to the struggle high temperatures with the problem of the liquidof heat-affected weld metal. zones,The welding which are heat created acts as near a short the heat welding treatment seams on du thee componentto the high andtemperatures has a negative of the influence liquid weld on the metal.microstructure The welding of theheat material. acts as a short As an heat example, treatment this on can the result component in increased and has brittleness a negative and influence hardness onso the that microstructure mechanical processing of the material. becomes As more an exam difficult.ple, However,this can result if a welding in increased process brittleness is followed and by hardnessa hot forging so that process mechanical with a sufficientprocessing degree becomes of forming more difficult. and compressive However, stress if a welding states, most process decisive is followeddefects causedby a hot by forging the previous process welding with a processsufficient (e.g., degree pores, of cracksforming and and coarse compressive microstructure) stress states, can be mosteliminated. decisive Within defects the caused studied by gear the component, previous awe highlylding effective process plastic (e.g., strainpores, of cracks approx. and 2 within coarse the microstructure)welded casing can and be prevailing eliminated. compressive Within the stress stud states,ied gear which component, a typically a forhighly closed effective die hot plastic forging strainprocesses of approx. [21], contribute 2 within tothe an welded intensive casing dynamic and prevailing recrystallization compressive and pore stress healing states, of thewhich coarse a typicallywelding for microstructure. closed die hot The forging positive processes influence [21], of contribute forming has to an already intensive been dynamic demonstrated recrystallization in previous andstudies pore onhealing hybrid of blanksthe coarse in cross welding wedge microstructure. [22,23 The]. positive influence of forming has already been demonstratedIn the current in study, previous the manufacturing studies on hybrid of bi-metal blanks semi-finishedin cross wedge workpieces rolling [22,23]. has been performed by PTAIn the deposition current welding.study, the The manufacturing welding tests wereof bi-metal conducted semi-finished with an automated workpieces welding has systembeen performedconsisting by of aPTA six-axis deposition industrial welding. robot Reis The RV-16 weld (Reising tests Group were Holding conducted GmbH with & Co an KG, automated Obernburg weldingam Main, system Germany), consisting the of PTA a six-axis Torch Kennametalindustrial robot Stellite Reis HPM RV-16 302 (Reis and Group the power Holding supply GmbH Stellite & CoStarweld KG, Obernburg PTA 302 am Control Main, (Kennametal Germany), the Inc., PTA Belleville, Torch Kennametal ON, Canada) Stellite (Figure HPM2a). 302 The and PTA the welding power supplyprocess Stellite and itsStarweld parameters PTA 302 are Control represented (Kennametal in Figure Inc.,2b and Belleville, in Table ON,2, respectively.Canada) (Figure During 2a). The the PTAprocess, welding 41Cr4 process powder and is its deposited parameters on aare cylindrical represented substrate in Figure made 2b of and C22.8. in Table Due to2, therespectively. melting of Duringthe coating the process, layer accompanied 41Cr4 powder by is high deposited temperature on a cylindrical input, the substrate substrate made material of C22.8. heats Due up strongly to the meltingand the of penetration the coating of layer the filleraccompanied material increases.by high temperature In order to decreaseinput, the this substrate effect, thematerial welding heats current up stronglyis reduced and evenly the penetration from 150 ampere of the atfiller the start,material to 110 increases. ampere In until order the endto decrease of the process this effect, is reached. the weldingDuring current welding is underreduced the evenly PTA torch, from the150 cylindrical ampere at specimenthe start, to rotates 110 ampere 15 times until in total. the end Due of to the the processrotational is reached. movement During of thewelding workpiece under and the PTA the axial torch, movement the cylindrical of the specimen torch, the rotates separate 15 times welding in total. Due to the rotational movement of the workpiece and the axial movement of the torch, the separate welding seams are deposited in a helical form which results in a wavy pattern of coating surface and interface zone between the applied materials.

Metals 2018, 8, 898 5 of 11 seams are deposited in a helical form which results in a wavy pattern of coating surface and interface Metals 2018, 8, x FOR PEER REVIEW 5 of 10 zone between the applied materials.

Robot HPM 302 Torch

Workpiece manipulator Workpiece 20 mm

(a) (b)

FigureFigure 2. 2. ((aa)) Welding Welding equipment, equipment, ( (b)) plasma-transferred plasma-transferred arc process, 41Cr4 on C22.8.

TableTable 2. ProcessProcess parameters of PTA deposition welding.

ParameterParameter ValueValue ShieldingShielding gas flow gas flow (Argon) (Argon) 10 10 L/min L/min PlasmaPlasma gas flow gas flow (Argon) (Argon) 1.5 1.5 L/min L/min Carrier gas flow (Argon) 6 L/min Carrier gasWelding flow (Argon) velocity 1.7 6 mm/sL/min WeldingWorking velocity distance 1.712 mmmm/s Working Currentdistance Dynamic, 12 150–110 mm A CurrentVoltage Dynamic, 25 V 150–110 A Powder material 1.7035, atomised under argon atmosphere Voltage 25 V Particle size distribution in micron 50–150 µm (current industry standard) PowderPowder material feeding rate 1.7035, atomised 10 under g/min argon atmosphere Particle size distribution in micron 50–150 μm (current industry standard) Powder feeding rate 10 g/min In order to weld 41Cr4 without pores and cracks, the welding process was adjusted to a suitable parameter set. With the steel 41Cr4, there is a risk of cracking, especially if the material cools down too In order to weld 41Cr4 without pores and cracks, the welding process was adjusted to a suitable quickly. Therefore, a slow welding speed of 1.7 mm/s was selected. This leads to a strong heating of parameter set. With the steel 41Cr4, there is a risk of cracking, especially if the material cools down the base material, which prevents a self-quenching of the cladding material, especially at the beginning too quickly. Therefore, a slow welding speed of 1.7 mm/s was selected. This leads to a strong heating of the cladding process. Due to the pendulum movement with a frequency of 2 Hz carried out by of the base material, which prevents a self-quenching of the cladding material, especially at the the welding torch, a wide weld seam with a large material volume is welded on, which stores more beginning of the cladding process. Due to the pendulum movement with a frequency of 2 Hz carried heat than a seam produced without a pendulum movement. Due to the pendulum movement and out by the welding torch, a wide weld seam with a large material volume is welded on, which stores the slow welding speed, the molten pool solidifies very late and is thoroughly mixed. This leads to more heat than a seam produced without a pendulum movement. Due to the pendulum movement a very good degassing of the melt and prevents the inclusion of pores. If the plasma gas is set too and the slow welding speed, the molten pool solidifies very late and is thoroughly mixed. This leads high, it can penetrate the melt like a needle and gas-filled pores can remain during solidification. to a very good degassing of the melt and prevents the inclusion of pores. If the plasma gas is set too Therefore, the plasma gas flow has been additionally reduced to the technical limit of 1.5 L/h. With high, it can penetrate the melt like a needle and gas-filled pores can remain during solidification. a further reduction, the burner clogged due to too low gas velocities. This plasma gas velocity therefore Therefore, the plasma gas flow has been additionally reduced to the technical limit of 1.5 L/h. With a represents the stable process limit for the current equipment. further reduction, the burner clogged due to too low gas velocities. This plasma gas velocity therefore represents3.3. Forming the of stable Bi-Metal process Bevel Gearslimit for the current equipment.

3.3. FormingFor the of manufacturing Bi-Metal Bevel ofGears hybrid bevel gears, the forging tool system depicted in Figure3b has been developed [24]. It allows a forming of bi-metal semi-finished workpieces to the final geometry For the manufacturing of hybrid bevel gears, the forging tool system depicted in Figure 3b has in a single stage. The corresponding tool system is constructed modularly and consists in general of been developed [24]. It allows a forming of bi-metal semi-finished workpieces to the final geometry a lower die and a pre-stressed geared die which is installed in the upper tool in order to ensure smooth in a single stage. The corresponding tool system is constructed modularly and consists in general of removal of the forged gears. For the experimental forging tests, the forming tool system was integrated a lower die and a pre-stressed geared die which is installed in the upper tool in order to ensure smooth removal of the forged gears. For the experimental forging tests, the forming tool system was integrated in the press Lasco SPR500 with a maximum capacity of 40 kJ (LASCO Umformtechnik GmbH, Coburg, Germany) (Figure 3a). The key parameters of the forging process are presented in Table 3.

Metals 2018, 8, 898 6 of 11 in the screw press Lasco SPR500 with a maximum capacity of 40 kJ (LASCO Umformtechnik GmbH, Coburg, Germany) (Figure3a). The key parameters of the forging process are presented in Table3. Metals 2018, 8, x FOR PEER REVIEW 6 of 10

FigureFigure 3. 3. ((aa)) Screw Screw press, press, ( (b)) forming forming tool tool system system for for die die fo forgingrging of bi-metal bevel gears. Table 3. Process parameters of the forging process. Table 3. Process parameters of the forging process. Parameter Value Parameter Value ◦ WorkpieceWorkpiece temperature temperature 1200 1200C °C Forming tools temperature 200 ◦C Forming tools temperature 200 °C

◦ Single bi-metal workpieces were heated up to the forming temperature of approx. 1200 °CC by means of for about 50 s. The The chosen chosen temperature temperature range is typical for the hot hot forging forging of steel and allows the achievement achievement of of high high formab formability,ility, appropriate mould filling filling and refinement refinement of microstructure (e.g.,(e.g., by by dynamic dynamic recrystallisation) recrystallisation) in a in single a single forming forming step [25 step]. In [25]. the case In ofthe examined case of examinedbi-metal workpieces, bi-metal workpieces, only slight differencesonly slight indifferences the flowstresses in the flow and stresses in forming and behaviour in forming of behaviour both steels of(C22.8 both andsteels 41Cr4) (C22.8 are and to be41Cr4) expected are to at be the expect forminged at temperature. the forming temperature. Therefore, a uniformTherefore, temperature a uniform temperaturedistribution withindistribution the workpieces within the was workpieces used for thewas forging used process.for the forging The warm process. workpieces The warm were ◦ workpiecestransported were to the transported forging dies to pre-heated the forging to dies 200 pre-heatedC and subsequently to 200 °C formedand subsequently to the final formed geometry. to theFigure final4 shows geometry. a bevel Figure gear 4 directly shows aftera bevel the gear forging directly process. after After the forging the forming process. stage, After the the bevel forming gears stage,were cooled the bevel in sand.gears Thewere forged cooled parts in sand. displayed The forged complete parts mould displayed filling complete without mould any outward filling without forging anydefects outward (e.g., folds),forging even defects in the (e.g., crucial folds), geared even area.in the crucial geared area.

20 mm

Figure 4. AA bevel bevel gear gear after the forging process.

3.4. Characterization Characterization and Microstructural Analysis For the analysis of the manufactured components,components, metallographic cuts cuts were made and subsequently prepared and evaluated on the basis of photographs. For this this purpose, purpose, the the components components were cut using a wet cut-off machine Secotom-10 (Struers GmbH, Willich, Germany) and embedded in transparent investment material. Subsequently, Subsequently, grinding was performed on cylindrical grinding machines Tegramin-30 (Struers GmbH, Willich, Germany) with grinding wheels of grain sizes H180 to H2500. After that, the samples were then polished in a polishing machine VibroMet 2 (Buehler GmbH, Esslingen Am Neckar, Germany). First, automatic hardness tests with Q10 A+ (Qness GmbH, Golling, Austria) were performed on polished samples. For metallographic examinations, the

Metals 2018, 8, 898 7 of 11 machines Tegramin-30 (Struers GmbH, Willich, Germany) with grinding wheels of grain sizes H180 to H2500. After that, the samples were then polished in a polishing machine VibroMet 2 (Buehler GmbH,Metals 2018 Esslingen, 8, x FOR PEER AmNeckar, REVIEW Germany). First, automatic hardness tests with Q10 A+ (Qness GmbH,7 of 10 Golling, Austria) were performed on polished samples. For metallographic examinations, the samples weresamples additionally were additionally etched with etched two percent with two nitric percen acid.t Anitric Leica acid. DM4000 A Leica M(Leica DM4000 Camera M (Leica AG, Wetzlar,Camera Germany)AG, Wetzlar, was Germany) used as microscope. was used as microscope.

4.4. ResultsResults andand DiscussionDiscussion ForFor aa characterisationcharacterisation ofof thethe microstructuremicrostructure evolution,evolution, thethe specimensspecimens extractedextracted fromfrom deposition-deposition- weldedwelded and and forged forged parts parts were were metallographically metallographically investigated. investigated. The metallographic The metallographic cuts with cuts resulting with macro-resulting and macro- microstructures and microstructures are presented are inpresent Figuresed5 inand Figures6. The micrographs5 and 6. The showmicrographs an appropriate show an materialappropriate compound material without compound any without material any separation, material damage separation, or microscopic damage or microscopic cracks after cracks deposition after weldingdeposition such welding as after such forging. as after In the forging. longitudinal In the sectionlongitudinal along section the upsetting along the direction upsetting (shown direction with a(shown red line), with the a wavy red line), shape the of wavy the interface shape zone,of the resultinginterface fromzone, the resulting welding from process, the welding can be observed process, (Figurecan be5 a).observed After the (Figure forming 5a). process, After the the wavyforming interface process, contour the wavy is maintained interface wavelikecontour inis amaintained modified formwavelike (Figure in 5ab). modified Due to upsetting,form (Figure the maximum5b). Due to thickness upsetting, of thethe weldedmaximum layer thickness was increased of the bywelded 76% andlayer the was width increased of thewelding by 76% seamsand the was width reduced of the by welding approx. seams 30% in was the middlereduced of by the approx. tooth tip 30% (Table in 4the). Themiddle resulting of the wavy tooth contour tip (Table of the4). The bonding resulting zone wavy may leadcontour to a of higher the bonding total joining zone surface may lead area to between a higher thetotal materials joining andsurface thus area to a between better bonding the materials quality. and thus to a better bonding quality.

Figure 5. Macrographs of the joining zone in longitudinal section (a) in a deposition-welded workpiece Figure 5. Macrographs of the joining zone in longitudinal section (a) in a deposition-welded and (b) in a forged bevel gear at the middle of the tooth tip. workpiece and (b) in a forged bevel gear at the middle of the tooth tip. Table 4. Process parameters of the forging process. Table 4. Process parameters of the forging process. Thickness of Welded Layer After Deposition Welding After Machining After Forming Thickness of Welded Layer After Deposition Welding After Machining After Forming Maximal, dmax 3.96 2.96 5.20 Maximal, dmax 3.96 2.96 5.20 Minimal, dmin 2.40 1.40 2.70 Minimal, dmin 2.40 1.40 2.70

DueDue to to higher higher plastic plastic strains strains in the in tooth the roottooth area, root the area, welded the layerwelded at the layer tooth at root the is tooth significantly root is thinnersignificantly than atthinner the tooth than tip at (Figurethe tooth6b, tip left). (Figure This can6b, alsoleft). beThis seen can from alsothe be linearseen from flow the of elongatedlinear flow grains.of elongated Before grains. forming, Before the forming, welded layerthe welded shows laye a primarilyr shows a pearlitic primarily microstructure pearlitic microstructure with a certain with amounta certain of amount ferrite concentratedof ferrite concentrated at the grain at boundaries.the grain boundaries. The core materialThe core (C22.8) material exhibits (C22.8) so-called exhibits Widmannstaettenso-called Widmannstaetten ferritic (WS-ferrite) ferritic (WS-ferrite) patterns directly patterns at the directly interface at the layer, interface induced layer, by high induced cooling by rateshigh aftercooling the rates welding after process the welding (Figure process6a, right). (Figure The ferritic-pearlitic 6a, right). The structureferritic-pearlitic in the cylinderstructure core in the is acylinder common core microstructure is a common for themicrostructure steel C22.8. After for the the subsequentsteel C22.8. forming, After the the su resultingbsequent microstructure forming, the becameresulting finer microstructure and more homogeneous became finer as a resultand more of recrystallisation homogeneous processes as a result and plasticof recrystallisation strains at hot formingprocesses temperatures and plastic strains (1200 ◦ C).at hot As forming shown in temperat Figure6bures (middle, (1200 right),°C). As it shown completely in Figure consists 6b (middle, of fine ferriteright), and it completely pearlite grains. consists of fine ferrite and pearlite grains.

Metals 2018, 8, 898x FOR PEER REVIEW 88 of of 10 11 Metals 2018, 8, x FOR PEER REVIEW 8 of 10

Figure 6. Macro- and micrographs of the joining zone in cross section in a deposition-welded Figure 6. Macro- and micrographs of the joining zone in cross section in a deposition-welded workpiece Figureworkpiece 6. Macro- (a) and andin a forgedmicrographs bevel gearof the (b). joining zone in cross section in a deposition-welded workpiece(a) and in a(a forged) and in bevel a forged gear bevel (b). gear (b). In addition, the hardness profilesprofiles were examined from the sample edge in radial direction to the In addition, the hardness profiles were examined from the sample edge in radial direction to the middle in both, deposition-welded workpieces andand forged bevel gears (Figure7 7).). TheThe measurementmeasurement middle in both, deposition-welded workpieces and forged bevel gears (Figure 7). The measurement lines areare respectively respectively marked marked with with white white arrows arrows in Figure in Figure6. In the 6. caseIn the of thecase bevel of gear,the bevel the measuring gear, the lines are respectively marked with white arrows in Figure 6. In the case of the bevel gear, the linemeasuring was located line was at thelocated middle at the of themiddle tooth of tip.the Thetooth hardness tip. The hardness was measured was measured in Vickers in HV Vickers 0.1 with HV measuring line was located at the middle of the tooth tip. The hardness was measured in Vickers HV the0.1 with WOLPERT the WOLPERT TESTOR TESTOR 930/250 930/250 (Instron (Instron Deutschland Deutschland Gmbh, Gmbh, Aachen, Aachen, Germany), Germany), hardness hardness tester. 0.1 with the WOLPERT TESTOR 930/250 (Instron Deutschland Gmbh, Aachen, Germany), hardness Thetester. hardness The hardness values valu differedes differed from the from deposition-welded the deposition-welded layer (41Cr4) layer (41Cr4) to the core to the material core material (C22.8). tester. The hardness values differed from the deposition-welded layer (41Cr4) to the core material A(C22.8). hardness A hardness drop of approx. drop of 21% appr isox. observed 21% is inobserved the coating in the layer coatin afterg forging,layer after while forging, the hardness while the of (C22.8). A hardness drop of approx. 21% is observed in the coating layer after forging, while the thehardness core material of the core does material not change does significantly. not change This significantly. can be explained This can by structuralbe explained transformation by structural at hardness of the core material does not change significantly. This can be explained by structural hottransformation forming temperatures at hot forming (1200 ◦temperaturesC) and slower (120 cooling0 °C) rates and afterslower the cooling forging processrates after compared the forging with transformation at hot forming temperatures (1200 °C) and slower cooling rates after the forging theprocess welding compared process. with After the welding,welding process. the component After welding, cools down the component faster due tocools self-quenching, down faster due which to process compared with the welding process. After welding, the component cools down faster due to leadsself-quenching, to a higher which hardness, leads especially to a higher in ha therdness, outer area.especially in the outer area. self-quenching, which leads to a higher hardness, especially in the outer area.

Before forming After forming Before formingJoining Zone After formingJoining Zone 300 Joining Zone 300 Joining Zone 300 300 250 250 250 250 200 200 200 200 150 150 150 150 100 100 100 100 Hardness HV 0.1 Hardness HV 50 41Cr4 C22 0.1 Hardness HV 50 41Cr4 C22

Hardness HV 0.1 Hardness HV 50 0.1 Hardness HV 50 0 41Cr4 C22 0 41Cr4 C22 0 012345 0 012345678910 012345 012345678910 Distance from the sample edge, mm Distance from the sample edge, mm Distance from the sample edge, mm Distance from the sample edge, mm (a)(b) (a)(b)

FigureFigure 7. 7. HardnessHardness profiles profiles HV HV 0.1 0.1 in in a a deposition-welded deposition-welded workpiece ((aa)) andand inin a a forged forged bevel bevel gear gear (b ().b). Figure 7. Hardness profiles HV 0.1 in a deposition-welded workpiece (a) and in a forged bevel gear (b). In comparison with previous studies focused on the upsetting of bi-metal pre-forms [13,18,19], the currentIn comparison study represents with previous a successful studies implementafocused on thetion upsetting of bi-metal of formingbi-metal inpre-forms context of[13,18,19], complex the current study represents a successful implementation of bi-metal forming in context of complex

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In comparison with previous studies focused on the upsetting of bi-metal pre-forms [13,18,19], the current study represents a successful implementation of bi-metal forming in context of complex geared geometry in a single step. The forged parts demonstrate an appropriate mould filling; neither surface defects nor micro-cracks have been detected in the interface zone or in the deposited layer. Besides the morphology of the interface zone, the microstructural evaluation of the joining zone was taken into account, whereby a successful grain refinement was achieved.

5. Conclusions and Outlook The work presented in this paper describes the production of hybrid bevel gears by means of the process steps PTA-welding and subsequent hot forming. The steels C22.8 (1.0402) and 41Cr4 (1.7035) were used in the core and as coating, respectively. Results demonstrate that the composite material joined by welding withstands the forging process without defects and that the microstructure was completely recrystallised. The weld structure has been completely transformed into a homogeneous and fine-grained ferritic-pearlitic forming structure. Longer cooling times after forging led to a lower hardness of the coating than directly after welding. This enables better mechanical processing of the hybrid component, which had not been possible immediately after welding. These facts show that the production of hybrid solid components in high joining quality is possible. The technologies developed in this work, especially the possibility of pore-free welding of heat-treatable steels with high carbon equivalents, result in new competencies for the Institute of Materials Science and the Institute of Forming Technology and Machines at Leibniz Universität Hannover. The current limits for solid components are to be further expanded in the future. This can be particularly achieved by the application of novel materials, which make the process chain even more challenging. However, these new materials also show significantly more efficiency than those currently used. The welding process is to be further developed in order to be able to process rolling bearing steels such as 100Cr6, which are declared as non-weldable and are able to achieve the highest hardness and wear resistance. Material composites, whose individual materials differ more clearly in their yield stresses, will place higher demands on the design of forging processes. Especially for large components used in tribology, the cost of a workpiece, made entirely of high-performance material, would exceed the budget. In this area, the new hybrid concept is the first approach to almost uncompromisingly high performance that remains affordable.

Author Contributions: T.H. and M.M. designed the deposition welding process and manufactured the welded workpieces; T.H. is the supervisor of M.M. and of A4 subproject; B.-A.B., J.D. and A.C. designed the forging tool, performed the hybrid forging experiments and carried out the metallographic investigations and the micro hardness measurements; B.-A.B. is the supervisor of A.C. and of the B2 subproject; A.C., A.B. and M.M. wrote the paper. Funding: This research was funded by the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) within the framework of the Collaborative Research Centre (Sonderforschungsbereich SFB 1153, Projektnummer: 252662854, Teilprojekt A4, B2), which is promoted by the DFG. Acknowledgments: The results presented in this paper were obtained within the Collaborative Research Centre 1153 “Process chain to produce hybrid high-performance components by Tailored Forming” in subprojects A4 and B2. The authors would like to thank the German Research Foundation (DFG) for the financial and organisational support of this project. Conflicts of Interest: The authors declare no conflict of interest.

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