metals Article
Manufacturing of High-Performance Bi-Metal Bevel Gears by Combined Deposition Welding and Forging 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 2 3
*
Institute of Forming Technology and Machines, Leibniz Universität Hannover, 30823 Garbsen, Germany;
[email protected] (J.D.);
[email protected] (B.-A.B.) Institute of Material Science, Leibniz Universität Hannover, 30823 Garbsen, Germany;
[email protected] (M.M.);
[email protected] (T.H.) 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-arc welding (PTA) and subsequent forming. In the presented investigations, steel 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: die 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
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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 steels 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-slag-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 electrodes. 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 hardness of AISI 5115 steel coated with FeCrC-powder [4]. Motallebzadeh et al. implemented welding of a hypo-eutectic CoCrWC hard facing alloy 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 stainless steel 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.
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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 extrusion [10]. The study of Kong et al. focused on joining by means of forge 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 oxide 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 shielding gas 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 copper, aluminium and steel [17]. Frischkorn et al. examined the hot forging behaviour of further material combinations comprising steel, aluminium, titanium 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 Figure 1b. The corresponding semi-finished workpieces were designed in accordance with the load collective of the final parts (Figure 1a). 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
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the final bevel gear. Thus, the thickness of the welded layer includes also allowance required for the secondary processes (e.g., machining). Within a subsequent hot forging process, the deposition-welded workpieces with coaxial material arrangement were formed to hybrid bevel gears. The main Metals 2018, 8, x FOR PEER REVIEW 4 of 10 dimensions of the bi-metal workpieces and bevel gears are listed in Table 1. The workpieces were turned turnedafter afterthe thewelding weldingprocess processininorder ordertotoachieve achievethe therequired requiredworkpiece workpiecevolume. volume.The Theoutside outside diameter diameterofofthe thebevel bevelgear gearcorresponds correspondswith withthe thediameter diameterofofthe thecrown crowncircle. circle.
Figure1.1.(a)(a) Coaxial deposition-welded blank, (b)forged forgedhybrid hybridbevel bevelgear gear[16]. [16]. Figure Coaxial deposition-welded blank, (b) 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 Outside Diameter: Ø 30 mm OutsideCore Diameter: Ø 30 mm diameter: Ø 27 mm Core diameter: Ø 27 mm Height: 79 mm Height: 79 mm
Bi-Metal Bevel Gear Number of teeth: 15 Number of teeth: 15mm Outside Diameter: Ø 62 Outside Height: Diameter: Ø 62 30 mm 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 All which deposition processes welding commonly struggle with the problem of heat-affected zones, are created near using the welding seams due to the high temperatures of the liquid weld metal. zones, which are created near the welding seams due to the high temperatures of the liquid weld The welding heat acts as a short heat treatment on the component and has a negative influence on the metal. The welding heat acts as a short heat treatment on the component and has a negative influence microstructure of the material. As an example, this can result in increased brittleness and hardness onsothe of the material. an example, this can result in increased brittleness andby thatmicrostructure mechanical processing becomesAsmore difficult. However, if a welding process is followed hardness so that mechanical processing becomes more difficult. However, if a welding process is a hot forging process with a sufficient degree of forming and compressive stress states, most decisive followed by a hot process with a process sufficient degree of cracks forming and compressive stress states, defects caused byforging the previous welding (e.g., pores, and coarse microstructure) can be most decisive defects caused by the previous welding process (e.g., pores, cracks coarse eliminated. Within the studied gear component, a highly effective plastic strain of approx.and 2 within the microstructure) can be eliminated. Within the studied gear component, a highly effective plastic welded casing and prevailing compressive stress states, which a typically for closed die hot forging strain of approx. 2 within to theanwelded casing and prevailing compressive stress states, which a processes [21], contribute intensive dynamic recrystallization and pore healing of the coarse typically for closed die hot forging processes [21], contribute to an intensive dynamic recrystallization welding microstructure. The positive influence of forming has already been demonstrated in previous and pore on healing of blanks the coarse welding microstructure. The positive influence of forming has already studies hybrid in cross wedge rolling [22,23]. been demonstrated in previous studies on hybrid blanks in cross wedge rolling [22,23]. In the current study, the manufacturing of bi-metal semi-finished workpieces has been performed In the current study, the manufacturing of bi-metal semi-finished workpieces has system been by PTA deposition welding. The welding tests were conducted with an automated welding performed deposition welding. The welding were conducted consisting by of aPTA six-axis industrial robot Reis RV-16 (Reis tests Group Holding GmbH with & Co an KG,automated Obernburg welding system consisting of a six-axis industrial robot Reis RV-16 (Reis Group Holding GmbH & am Main, Germany), the PTA Torch Kennametal Stellite HPM 302 and the power supply Stellite Co KG, Obernburg Main,(Kennametal Germany), the PTA Torch Kennametal Stellite HPM 302 and thewelding power Starweld PTA 302am Control Inc., Belleville, ON, Canada) (Figure 2a). The PTA supply Stellite Starweld PTA 302 Control (Kennametal Inc., Belleville, ON, Canada) (Figure 2a). The process and its parameters are represented in Figure 2b and in Table 2, respectively. During the PTA welding process and its parameters are represented in Figure 2b and in Table 2, respectively. process, 41Cr4 powder is deposited on a cylindrical substrate made of C22.8. Due to the melting of During the process, 41Cr4 powder deposited on a cylindrical of C22.8. to the the coating layer accompanied byishigh temperature input, thesubstrate substratemade material heats Due up strongly melting of the coating layer accompanied by high temperature input, the substrate material heats up and the penetration of the filler material increases. In order to decrease this effect, the welding current strongly and the penetration of the filler material increases. In order to decrease this effect, the is reduced evenly from 150 ampere at the start, to 110 ampere until the end of the process is reached. welding reduced fromthe 150cylindrical ampere at specimen the start, to 110 ampere until the end During current weldingisunder theevenly PTA torch, rotates 15 times in total. Dueoftothe the process is reached. During welding under the PTA torch, the cylindrical specimen rotates 15 in rotational movement of the workpiece and the axial movement of the torch, the separatetimes welding 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.
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seams are deposited in a helical form which results in a wavy pattern of coating surface and interface 5 of 10 zone between the applied materials.
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HPM 302 Torch
Workpiece manipulator (a)
20 mm
Workpiece
(b)
Figure Figure 2. 2. (a) (a)Welding Welding equipment, equipment, (b) (b) plasma-transferred plasma-transferred arc process, 41Cr4 on C22.8. Table 2. 2. Process Process parameters parameters of of PTA PTA deposition Table deposition welding. welding. Parameter Value Parameter Value Shielding gas flow (Argon) 1010L/min Shielding gas flow (Argon) L/min Plasma gas flow (Argon) 1.5 L/min Plasma gas flow (Argon) 1.5 L/min Carrier gas flow (Argon) 6 L/min Carrier gas flow (Argon) L/min Welding velocity 1.76 mm/s Welding velocity 1.7 mm/s Working distance 12 mm Dynamic, Working Current distance 12150–110 mm A Voltage 25 V150–110 A Current Dynamic, Powder material 1.7035, atomised under argon atmosphere Voltage 25 V Particle size distribution in micron 50–150 µm (current industry standard) Powder material 1.7035, atomised under Powder feeding rate 10 g/minargon 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 represents the process limit for the current equipment. 3.3. Forming of stable Bi-Metal Bevel Gears
For the of manufacturing hybrid bevel gears, the forging tool system depicted in Figure 3b has 3.3. Forming Bi-Metal Bevel of Gears 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 screw 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.
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in the screw press Lasco SPR500 with a maximum capacity of 40 kJ (LASCO Umformtechnik GmbH, Coburg, (Figure 3a). The key parameters of the forging process are presented in Table 3. 10 Metals 2018,Germany) 8, x FOR PEER REVIEW 6 of Metals 2018, 8, x FOR PEER REVIEW
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Figure Figure 3. 3. (a) (a) Screw Screw press, press, (b) (b) forming forming tool tool system Figure 3. (a) Screw press, (b) forming tool system for for die die forging forging of of bi-metal bi-metal bevel bevel gears. gears. Table 3. 3. Process Process parameters parameters of of the the forging forging process. Table process. Table 3. Process parameters of the forging process. Parameter Parameter Parameter Workpiece temperature Workpiece temperature Workpiece temperature Forming tools temperature Forming tools tools temperature temperature Forming
Value Value
Value
◦C 1200 1200 °C 1200 °C ◦ 200 C 200 °C
200 °C
◦ C by Single Single bi-metal bi-metal workpieces workpieces were were heated heated up Single bi-metal workpieces were heated up to to the the forming forming temperature temperature of of approx. approx. 1200 1200 °C °C by means of of induction induction heating heating for for about about 50 50 s. s. The The chosen temperature range is is typical typical for for the the hot hot forging The chosen chosen temperature temperature range hot forging forging means of the achievement achievement of of high high formability, formability, appropriate of steel steel and and allows allows the the achievement of high formability, appropriate mould mould filling filling and and refinement refinement of of microstructure (e.g., by dynamic recrystallisation) in a single forming step [25]. In the case microstructure (e.g., by dynamic recrystallisation) in a single forming step [25]. In the case microstructure (e.g., by dynamic recrystallisation) in a single forming step [25]. In of theexamined case of of examined bi-metal workpieces, workpieces, only slight slight in differences in the the flow flow stresses andbehaviour in forming forming behaviour bi-metal workpieces, only slight differences the flow stresses and stresses in forming of behaviour both steels examined bi-metal only differences in and in of both steels (C22.8 and 41Cr4) are expected the Therefore, aa uniform (C22.8 41Cr4) are to be expected formingat Therefore, a uniform temperature of bothand steels (C22.8 and 41Cr4) are to toatbe bethe expected attemperature. the forming forming temperature. temperature. Therefore, uniform temperature distribution within the workpieces was used for the forging process. The distribution within the workpieces was used for the forging The warm workpieces were temperature distribution within the workpieces was used process. for the forging process. The warm warm ◦pre-heated workpieces were transported to the forging dies to 200 °C and subsequently formed transported to the forging dies pre-heated to 200 C and subsequently formed to the final geometry. workpieces were transported to the forging dies pre-heated to 200 °C and subsequently formed to to the final geometry. Figure shows bevel directly after the process. After forming Figure 4 shows a bevel gear4 thegear forging process. the forming stage, thethe bevel gears the final geometry. Figure 4directly shows aaafter bevel gear directly afterAfter the forging forging process. After the forming stage, the bevel bevel gears The wereforged cooledparts in sand. sand. The forged forged partsmould displayed complete mould filling without without were cooled in sand. displayed complete filling withoutmould any outward forging stage, the gears were cooled in The parts displayed complete filling any outward forging defects (e.g., folds), even in the crucial geared area. defects (e.g., folds), even in the crucial geared area. any outward forging defects (e.g., folds), even in the crucial geared area.
20 20 mm mm
Figure 4. 4. A A bevel gear after the forging process. process. Figure A bevel bevel gear gear after after the the forging
Characterization and 3.4. 3.4. Characterization Characterization and Microstructural Microstructural Analysis Analysis metallographic cuts cuts were were made For For the the analysis analysis of of the the manufactured manufactured components, components, metallographic metallographic cuts were made and and and on purpose, components subsequently prepared evaluated the basis of photographs. For this purpose, the components subsequently prepared and evaluated on the basis of photographs. For this purpose, the components were were cut cut using using aa wet wet cut-off cut-off machine machine Secotom-10 Secotom-10 (Struers (Struers GmbH, GmbH, Willich, Willich, Germany) Germany) and and embedded embedded performed cylindrical in transparent investment material. Subsequently, grinding was on in transparent investment material. Subsequently, grinding was performed on cylindrical grinding grinding machines machines Tegramin-30 Tegramin-30 (Struers (Struers GmbH, GmbH, Willich, Willich, Germany) Germany) with with grinding grinding wheels wheels of of grain grain sizes sizes H180 H180 to H2500. H2500. After After that, that, the the samples samples were were then then polished polished in in aa polishing polishing machine machine VibroMet VibroMet 22 (Buehler (Buehler to GmbH, GmbH, Esslingen Esslingen Am Am Neckar, Neckar, Germany). Germany). First, First, automatic automatic hardness hardness tests tests with with Q10 Q10 A+ A+ (Qness (Qness GmbH, GmbH, Golling, Austria) were performed on polished samples. For metallographic examinations, Golling, Austria) were performed on polished samples. For metallographic examinations, the the
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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 Metals 2018, 8, x FOR PEER REVIEW Germany). First, automatic hardness tests with Q10 A+ (Qness GmbH, 7 of 10 GmbH, Esslingen Am Neckar, Golling, Austria) were performed on polished samples. For metallographic examinations, the samples samples were additionally etched with two percent nitric A Leica DM4000 M (Leica Camera were additionally etched with two percent nitric acid. A Leicaacid. DM4000 M (Leica Camera AG, Wetzlar, AG, Wetzlar, was used as microscope. Germany) wasGermany) used as microscope.
Resultsand andDiscussion Discussion 4.4.Results Foraacharacterisation characterisationof ofthe the microstructure microstructureevolution, evolution, the the specimens specimens extracted extracted from from depositiondepositionFor welded and forged parts were metallographically investigated. The metallographic with welded and forged parts were metallographically investigated. The metallographic cuts withcuts resulting resulting and microstructures areinpresented Figures and 6. The micrographs show an macroandmacromicrostructures are presented Figures 5in and 6. The5micrographs show an appropriate appropriate material compound without any material separation, damage or microscopic cracks after material compound without any material separation, damage or microscopic cracks after deposition deposition welding as after forging. In the section longitudinal along the upsetting direction welding such as aftersuch forging. In the longitudinal alongsection the upsetting direction (shown with with red line), shape zone, of theresulting interfacefrom zone,the resulting the welding process, a(shown red line), theawavy shapethe of wavy the interface weldingfrom process, can be observed can be 5a). observed (Figure 5a).process, After the process, the wavy interface wavelike contour is (Figure After the forming theforming wavy interface contour is maintained in maintained a modified wavelike in 5b). a modified form (Figure 5b). Due tothickness upsetting, maximum thickness of the by welded form (Figure Due to upsetting, the maximum of the the welded layer was increased 76% layer was increased by 76% and the width of the welding seams was reduced by approx. 30% in and the width of the welding seams was reduced by approx. 30% in the middle of the tooth tip (Tablethe 4). middle of thewavy toothcontour tip (Table 4). The resulting contour the bonding zonesurface may lead a higher The resulting of the bonding zonewavy may lead to a of higher total joining areatobetween total joining and surface the materials the materials thusarea to abetween better bonding 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 After Deposition Welding After Machining After Forming Thickness of Welded Layer 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
Due plastic strains in theintooth welded at the toothatroot significantly Duetotohigher higher plastic strains the root tootharea, rootthearea, the layer welded layer theistooth root is thinner than at the tooth tip (Figure 6b, left). This can also be seen from the linear flow of elongated significantly thinner than at the tooth tip (Figure 6b, left). This can also be seen from the linear flow grains. Beforegrains. forming, theforming, welded layer showslayer a primarily microstructure with a certain of elongated Before the welded shows apearlitic primarily pearlitic microstructure with amount ferrite concentrated at the grain The core material exhibits so-called a certainofamount of ferrite concentrated at boundaries. the grain boundaries. The core(C22.8) material (C22.8) exhibits Widmannstaetten ferritic (WS-ferrite) patterns directly at the interface layer, induced by high cooling so-called Widmannstaetten ferritic (WS-ferrite) patterns directly at the interface layer, induced by rates the rates welding process (Figureprocess 6a, right). The ferritic-pearlitic structure in the cylinder is high after cooling after the welding (Figure 6a, right). The ferritic-pearlitic structurecore in the acylinder commoncore microstructure for the steel C22.8. After the subsequent thesubsequent resulting microstructure is a common microstructure for the steel C22.8.forming, After the forming, the became finer and more homogeneous as a result of recrystallisation processes and plastic strains at hot resulting microstructure became finer and more homogeneous as a result of recrystallisation ◦ forming temperatures (1200 C). As forming shown intemperatures Figure 6b (middle, right), completely consists of fine processes and plastic strains at hot (1200 °C). As itshown in Figure 6b (middle, ferrite pearlite grains. right),and it completely consists of fine ferrite and pearlite grains.
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Figure 6. Macro- and micrographs of the joining zone in cross section in a deposition-welded Figure 6. and micrographs of the zone in cross section in a deposition-welded workpiece Figure 6. MacroMacromicrographs of joining the workpiece (a) and and in a forged bevel gear (b).joining zone in cross section in a deposition-welded (a) and in a forged bevel gear (b). workpiece (a) and in a forged bevel gear (b).
In addition, profiles were were examined examined from from the the sample sample edge edge in in radial to the In addition, the the hardness hardness profiles radial direction 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 and forged bevel gears (Figure 7). The measurement middle in both, deposition-welded workpieces and forged bevel gears (Figure 7). The measurement middle in respectively both, deposition-welded workpieces and forged bevel (Figure The measurement lines are are marked with white arrows in Figure 6. gears In the case of7).the gear, the lines respectively marked with white arrows in Figure 6. In the case of the bevel gear,bevel the measuring lines are respectively marked with white arrows in Figure 6. In the case of the bevel gear, the measuring line was located at the middle of the tooth tip. The hardness was measured in Vickers HV line was located at the middle of the tooth tip. The hardness was measured in Vickers HV 0.1 with measuring line was located at the middle of the tooth tip. The hardness was measured in Vickers HV 0.1 with the WOLPERT Deutschland Aachen, Germany), hardness the WOLPERT TESTORTESTOR 930/250930/250 (Instron(Instron Deutschland Gmbh, Gmbh, Aachen, Germany), hardness tester. 0.1 withThe the hardness WOLPERT TESTOR 930/250 (Instron Deutschland Gmbh, Aachen, Germany), hardness tester. values differed from the deposition-welded layer (41Cr4) to the core The hardness values differed from the deposition-welded layer (41Cr4) to the core materialmaterial (C22.8). tester. The values from deposition-welded layerlayer (41Cr4) to forging, the corewhile material (C22.8). A hardness hardness drop ofdiffered approx. 21%the is in observed in the coating after A hardness drop of approx. 21% is observed the coating layer after forging, while the hardnessthe of (C22.8). A of hardness drop of approx. 21%change is observed in the coating layer after forging, while the hardness the core material does not significantly. This can be explained by structural the core material does not change significantly. This can be explained by structural transformation at hardness of the at core does not change significantly. This can be explained bythe structural transformation hotmaterial forming °C)rates andafter slower rates after forging hot forming temperatures (1200 ◦temperatures C) and slower(1200 cooling the cooling forging process compared with transformation at hot forming temperatures (1200 °C) and slower cooling rates after the forging process compared with the welding process. After welding, the component cools down faster due to the welding process. After welding, the component cools down faster due to self-quenching, which process compared withleads the welding process. After especially welding, the component cools down faster due to self-quenching, which to a higher hardness, in the outer area. leads to a higher hardness, especially in the outer area. self-quenching, which leads to a higher hardness, especially in the outer area. After forming After forming Joining Zone
Hardness HV 0.10.1 Hardness HV
Joining Zone 300 300 250 250 200 200 150 150 100 100 50 41Cr4 C22 50 41Cr4 C22 0 0 0 1 2 3 4 5 0 from 1 the 2 sample 3 4 edge, 5 mm Distance
Distance from the sample edge, mm (a) (a)
Hardness HV 0.10.1 Hardness HV
Before forming Before forming Joining Zone 300 300 250 250 200 200 150 150 100 100 50 50 0 0 0 0
Joining Zone
41Cr4 41Cr4
C22 C22
1 2 3 4 5 6 7 8 9 10 1Distance 2 3from 4 the5 sample 6 7edge, 8 mm 9 10 Distance from the sample edge, mm (b) (b)
Figure7.7.Hardness Hardnessprofiles profilesHV HV0.1 0.1in in aa deposition-welded deposition-welded workpiece Figure workpiece (a) (a) and andin inaaforged forgedbevel bevelgear 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], In comparison with previous studies implementation focused on the upsetting of forming bi-metal in pre-forms [13,18,19], the current study represents a successful of bi-metal context of 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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Behrens, B.-A.; Bouguecha, A.; Frischkorn, C.; Huskic, A.; Chugreeva, A. Process routes for die forging of hybrid bevel gears and bearing bushings. In Proceedings of the AIP Conference Proceedings, Dublin, Ireland, 26–28 April 2017. Doege, E.; Behrens, B.-A. Handbuch Umformtechnik; Springer: Berlin/Heidelberg, Germany, 2010. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
