Review of recent developments in manufacturing lightweight multi-metal gears

The transportation sector has continued to be subject to a broad range of constraints in recent years with severe market competition, stringent consumer demands on option customisation, sustainability and quality as well as legislative challenges. Moreover, all these challenges must be faced with a reduced development budget and competitive sales price. Manufacturers have aimed to undertake these challenges by forming partnerships including consolidating chassis and engine applications across model ranges [42], engaging in mergers and joint platform development [155]. Moreover, advances in alternative drive system technologies such as hybrid and all electric vehicles have gained increased acceptance as viable emission reduction systems [162] accompanied with numerous government incentives [157]. Whilst this avenue offers promising solutions to air quality in major cities, there remain significant technical challenges in terms of battery technology, energy storage density, rate of charge and cost of implementation [29].

To date, one of the most proven and effective methods to reduce emissions, which can be implemented across all vehicle propulsion methods including electric vehicles, is the reduction of vehicular weight, through the use of lightweight design technologies. Light structural designs involve the use of lightweight material substitution and novel structure design thus enabling significant savings. This has been effectively implemented in the automotive industry with the Range Rover models, having achieved a 400 kg weight reduction, largely through the substitution of steel by aluminium in recent models [69]. The impact of weight savings on fuel consumption is significant, with a 10% reduction in vehicle weight improving fuel efficiency by 6–8% [72].

Material substitution in non-structural components has been performed for decades. For example, cast iron and steel have been viably replaced with aluminium for engine blocks [129], for vehicle structural members [30] and aluminium or magnesium for gearbox casings [127]. In addition, materials such as carbon-fibres have been used as a replacement for skin panels and structures [2], although their labour intensive production and cost limits application to only the upper segment of vehicles. However, with the potential for lightweighting of simple structures being exhausted, industry and academia have begun to focus on the lightweighting of high performance mechanical components such as crankshafts, camshafts and gears. This is particularly challenging as these components are manufactured from high strength steels due to the high stress performance requirements. Industry has begun to address this challenge with projects such as the Lightweight Forging initiative formed by a range of forging and steelmaking companies [126].

In addition to the reduction of weight and emissions, light weighting offers the advantage of reduced rotational mass and inertia [160] as well as the reduction of noise and component wear [71, 164]. These benefits, combined with a rapidly expanding gear market, at an annualised growth of 5.6% from $221 billion in 2019 to $285 billion in 2024 [52] demonstrates the potential for lightweight gears.

There are a vast number of techniques applied to achieve weight reduction of gears. A review of the most commonly used techniques is presented in the following sections.

A review of the literature has demonstrated numerous avenues and processing routes in the forging of multi-metal gears. The majority of the studies involve the use of a traditional forging press, with key differences being in the production of the dissimilar metal blanks and associated heating methods to overcome the challenge of differing thermal properties of the metals.

Bi-metal components are commonly cold forged in order to avoid the complexity of controlling thermal parameters of dissimilar metals. However, for materials of low ductility, heating is required and involves the consideration of different melting temperatures, thermal expansion coefficients, heat transfer and microstructural changes. Due to the differing thermal properties such as expansion coefficient, steel-aluminium material pairs require the steel to be heated to a minimum temperature of 900 °C whereas aluminium must be at least 400 °C but not exceeding 500 °C to avoid melting [19]. Careful selection of temperatures can ensure that despite significantly differing expansion coefficients, with aluminium twice that of steel, the materials can expand and contract at a similar level thus avoiding excessive separation or substantial compressive stresses. The recommended thermal processing windows for aluminium-steel pairs are presented in Fig. 9.

According to Fig. 9, a steel ring profile heated to 1000 °C requires the temperature of the aluminium core to not exceed 435 °C, otherwise the aluminium will contract more than the steel resulting in a gap being formed after cooling. Conversely, if the temperature of the aluminium is lower than 435 °C, thermal contraction of the steel will be greater than the aluminium which could result in excessive hoop stresses and possible fracture of the ring.

In order to achieve the required temperature profiles, two methods to heat the metal preforms were mainly explored in the literature: chamber furnace and induction heating.

To this end, Politis [120] investigated the forging of workpieces, heated separately in individual chamfer furnaces and subsequently telescopically assembled in the die and forged in a single operation. Two chamber furnaces were utilised in the study with the aluminium core being heated to 400 °C and the steel ring to 950 °C. It was suggested that for a greater production run two conveyor furnaces could be used, each set to the target temperature for the selected alloys. The method was found to be effective for heating the workpieces in a laboratory setting although the heating time was in the order of minutes which would limit production efficiency. Moreover, in the study the workpieces were transported from the furnaces to the tool resulting in limited control of the temperature due to air cooling. It should be noted that disparities in heat transfer coefficient and thermal contraction may result in separation of the two materials leading to the presence of a gap at the interface as shown in the bi-metal gear forging study by Politis [120] and presented in Fig. 10.

The use of induction heating for aluminium—steel pairs for bi-metal forged bushings was investigated by Behrens et al. (17). Behrens and Kosch (19) developed a heating strategy to utilise induction heating for aluminium—steel combinations. In this work, the authors first combined an aluminium core with a steel ring and heated the preform simultaneously as shown in Fig. 11. It is stated that the presence of the air gap between the two metals is significantly influential on the heating parameters and can be beneficial as the air gap results in the heating of aluminium only by radiation rather than conduction, thus enabling heating control.

The use of induction heating enables additional control compared to furnace heating although with added complexity requiring the calibration of electromagnetic parameters such as electrical conductivity, permittivity and permeability in addition to thermal conductivity and heat transfer coefficient. Moreover, coil design and frequency is essential to ensure adequate heating. Therefore, the common approach taken by industry to optimise these parameters is trial and error. It should be noted that induction heating is significantly faster in achieving the target temperature than chamber furnace heating, with an aluminium sample temperature of 500 °C being achieved within 20 s [17,18] compared to several minutes for a chamber furnace.

Induction heating experiments on steel—aluminium and steel-copper pairs determined that it is the outer layer (steel) that determines the heating effect as opposed to the core material, with the copper and aluminium materials not responding to induction heating as significantly as the steel [131]. This is demonstrated in Fig. 12a.

In the work performed by Goldstein et al. [56] it was found that electromagnetic end effects could lead to overheating of parts of the workpiece resulting in localised melting. Therefore, it is possible that post-form operations, such as machining of the workpiece ends may be required to remove regions of unfavourable interface joint conditions. Moreover, the authors' found that aluminium, with its greater heat transfer coefficient, is more likely to exhibit uniform temperature after heating compared to steel. In the study, the authors' found that the temperature within the steel workpiece was not uniform even after 20 s of heating. It should be noted that temperature distribution studies were formed on workpieces with dimensions of 40 mm solid diameter, and therefore steel in the form of a thin exterior ring will exhibit a greater likelihood of temperature uniformity in a shorter time period.

The modelling of bi-metal forged components is challenging even at cold forging conditions due to the differing material properties, varying friction behaviour and large deformations. When incorporating hot forge properties, such as conduction, heat transfer coefficient, heat generation due to plastic deformation, pressure, and elastic deformation of tools, the processing power and simulation times required become inacceptable.

Politis et al. [118] modelled the cold forging of a bi-metal gear from model materials such as copper and lead. The material pairs were selected in order to avoid the need for elevated temperatures and enable lower forging loads in experimental work, thus removing a considerable number of variables from experimental and simulation work. The modelling of hot-forged bi-metal gears, from 8620 steel exterior and AA2014 aluminium interior materials, was performed by Wu et al. [169]. In this work, FE modelling of the process was conducted on Deform-3D. A comparison between the modelled and experimental results along cross sections of the bi-metal gear showed close comparison in the flow behaviour of the aluminium and steel materials. The simulation did include heat transfer parameters between the materials and tooling, however, the internal heat generation [23] with plastic deformation was not clearly stated with the study incorporating rigid rather than elastic tooling. It is expected that the inclusion of these variables is possible although with appreciable cost in computational efficiency.

The greatest challenge in bi-metal gear forging is the incorporation of the effect of the interlayer and intermetallic effects between the two base material pairs. Bambach et al. [7] proposed a finite element framework for a bond formed during plastic deformation processes with Khaledi et al. [78] proposing a mathematical model for bonding during large deformations. Finite element bond models have been applied to processes such as rolling [61], however, according to the authors’ knowledge, there is no available study modelling the entire bi-metal gear hot forging process with intermetallic effects. Studies to date have focused specifically on the interlayer through the bonding of dissimilar materials. For example, Behrens et al. [18] characterised the joint zone via tensile tests for EnAW-6082 to 20MnCr5 and C22 to 41Cr4 joint pairs. In the study, the authors friction-welded two cylindrical samples along the axial direction of which dog-bone tensile samples were extracted along the 0°, 20°, and 30° axis. However, in the forging of bi-metal gears, the varying geometry of the gear teeth, temperature variations and pressure profiles along the tooth root, flank, and tip, result in any experimental and numerical studies being focused on conditions found in specific regions of the bi-metal interface rather than the entire gear tooth profile.

To this end, Politis [120] proposed a simplified test rig and specimen geometries that were designed to replicate the interface conditions found in aluminium-steel gear forging along the tooth profile. The tests attempted to replicate bi-metal gear forging for the reviewed geometry in terms of the pressure, contact time, slip and temperature conditions at the tooth tip, flank and root simultaneously on a single conical workpiece, enabling subsequent tests of tensile and shear as shown in Fig. 22. The test rig also enabled the evaluation of interlayer materials to enhance bonding. As shown in the figure, the steel sample was cylindrical with a machined internal cone, with the aluminium sample having an inverted chamfer. The temperatures of 400 °C for the core, 950 °C for the ring, and 150 °C for the tooling were incorporated in the model similar to the gear forging trials. During forming, the radial movement of both materials was constrained, enabling the aluminium sample to flow into the cone of the steel sample, with a small contact band slipping on the steel surface towards the centre of the workpieces. As the load is increased, the locations marked as ‘Average’ and ‘Last Contact’ in Fig. 22a progressively roll out and contact onto the steel surface, with an increased strain and slip when compared with the ‘First Contact’ region. The overall effect is at the end of the deformation, there is a smooth variation in level of surface slip from the outside to the inside of the sample (excluding edge effects of the outermost diameter and central flat of the steel sample) which replicates the strain and slip values found in analogous regions during the gear forging.

As can be seen from Fig. 23a and b, the highly complex nature of the interface such as the contact pressure and temperature are reasonably represented. The proposed test rig was subsequently utilised for the evaluation of bonding interlayer materials, which enabled the elimination of costly equipment required for full scale gear forging trials.

In addition to the modelling of the bi-metal forging processes, several studies have been conducted in the literature with the focus on the mechanical performance of the gears. Karpat et al. [74] evaluated the effect of outer ring thickness on the stiffness of the gear for an aluminium-steel and CFRP-steel gear. For an aluminium core, it was found that a decreasing thickness of the high strength ring reduces the gear tooth stiffness, with a stiffness reduction of up to 27.5% compared to a solid gear. However, with a modest ring thickness and CFRP core, it is possible to achieve similar stiffness with a solid steel gear at 38% less weight. Yilmaz et al. [171] numerically investigated the static and dynamic behaviour of bimetallic spur gears using ANSYS. For the gear geometry selected, the authors found that ring thicknesses between 3.5–5 mm were optimum regarding weight reduction, and dynamic analysis showed that there is little difference between the solid and bimetallic gear designs. The variable that has a significant effect on the dynamic behaviour of the bimetallic spur gear is the addendum value. For example, when the addendum is increased by only 20% (from 1 to 1.2 m), the maximum dynamic factor reduces by approximately 50%. Moreover, the authors suggest that instead of modifying the stress profile within the gear by adjusting the steel ring thickness, it is possible to focus on adjusting gear variables such as tooth tip radius and pressure angle. In this way, bi-metallic gears can exhibit similar dynamic response and bending performance to solid steel gears whilst achieving up to 40% weight reduction. The effect on tooth stiffness for a range of steel ring thicknesses and a solid steel tooth are shown in Fig. 24.

As can be seen in Fig. 24, the tooth stiffness values decrease with decreasing ring thickness, which is expected due to the core material having a lower elastic modulus than the ring material. In the figure, the radius of 60 mm corresponds to the tooth tip whereas 53.25 mm corresponds to the base circle. In terms of thickness, the mean stiffness is reduced by 38%, 28%, 15% and 9% for ring thicknesses of 1, 2, 3.5 and 5 mm respectively.

In addition to the evaluation of gear tooth stiffness, Politis et al. [119] evaluated the stress distribution during contact of bi-metal gears. In this study, the stress distribution at the location of contact, and tooth roots were modelled for aluminium-steel gears of uniform steel ring thicknesses of 1 mm, 2 mm, 4 mm and 6 mm and compared to a solid steel gear. In addition, the study incorporated the flow behaviour of the steel ring, including the thinning of the tooth flank determined during forging. In the study, it was found that the thinning occurring during forging had a significant effect on the compressive and tensile fillet stresses. For the evaluated gear geometry, a uniform steel ring thickness of 6 mm presented a stress distribution almost identical (within 4%) to that of a solid steel tooth. However, when incorporating the true ring profile post forging, it was found that the stresses were 20% greater than in a solid steel gear (Fig. 25).

From these early works, it is found that lightweight bi-metal gears can be designed to exhibit comparable performance to traditional single material steel gears. However, it should be noted that these studies were numerical in nature and thus further investigation including dynamic experimental testing is required.

The present paper provides an overview of the literature related to the development of forged lightweight multi-metal gears, ranging from the initial workpiece design, workpiece heating methods, multi-metal workpiece production, multi-metal bonding and post forging analysis. A review of the literature has shown that each study has focused on a unique workpiece and gear geometry, thus resulting in a direct absolute comparison of the benefits of alternative methods being difficult, although it does provide indications as to the optimal processing route.

The production of effective multi-metal workpieces prior to forging is crucial for the success of forging. In the literature, this has been generally performed as either the production of a single workpiece combining two or more materials [51, 113, 150], or the assembly of two or more metals into the die prior to forging [118, 169]. For the production of a single multi-metal workpiece, significant processing prior to forging is required to optimise the workpiece integrity. For example, co-extruded inner core and outer sleeve materials require an extrusion tool arrangement which adds significant cost to small scale production lines [150]. Moreover, bi-metal casting [98], whilst being used for several decades is limited to the selection of metals with similar thermal properties. Welding is a simple to use and widely available technology, which requires care to ensure air gaps between the two materials do not occur throughout the contact interface. The use of magnetic pulse welding technologies has shown that an air gap free surface could be achieved between two materials [55]. An alternative method to the pre-joining of dissimilar metals, which involves the telescopic assembly of an outer high strength ring, followed by a lightweight cylindrical core material has been investigated. Through this method, the metals are joined during the forging process as a result of the high temperatures and pressures experienced. Whilst the bond between the two metals was found to not be consistent, due to the variation in pressure across the tooth root, flank and tip interfaces, this method has avoided time intensive and costly operations prior to forging.

Forging of dissimilar metals at elevated temperatures is challenging as dissimilar metals may have excessive differences in thermal properties and flow behaviour including poor ductility, vastly different thermal expansion coefficients and melting temperatures. Aluminium-steel pairs have been investigated in the literature showing the ideal processing window of steel being > 900 °C and aluminium between 400–500 °C, in order to avoid melting of the aluminium and to correlate the relative shrinkage due to differing thermal expansion coefficients. Behrens and Kosch [19] presented a processing window in order to correlate the differences in thermal behaviour and avoid the presence of an air gap after contraction of the materials post forging. Heating of preforms has been conducted in either individual furnaces each set to the required temperature of the metals, requiring the workpieces to be transported into the die and forged. Whilst a relatively simple method to perform, the transportation of workpieces from the furnace to the die can involve substantial temperature reductions of up to 50 °C resulting in a non-optimised forging [26]. Moreover, for gears requiring more than two metals, the use of several furnaces to heat individual workpieces becomes impractical. A promising solution is the use of induction heating coils to heat the workpiece whilst positioned within the die. Induction heating has been proven to heat telescopically assembled aluminium-steel pairs within 20 s, and moreover, the presence of the air gap between the steel ring and aluminium core is favourable due to the induction heating only heating the outer steel blank with radiation heating increasing the temperature of the aluminium. Through appropriate selection of coil design and frequency, a single coil arrangement positioned on the outer perimeter of the steel ring can simultaneously heat aluminium and steel to their target temperatures [19, 56].

Experimental gear forging studies have demonstrated that mechanical bonding on the macro and micro scale between dissimilar metals could be achieved during gear forging as a result of inherent forming of the gear teeth. The relative movement of an inner core material into the tooth profile restricts movement of the outer ring in both the rotational and axial direction, with axial restrictions forming as a result of friction between the punch and counterpunch against the workpiece. However, the greatest challenge to the integrity of a multi-metal gear is the metallurgical bond between the dissimilar pairs [4]. Forging conditions are favourable for diffusion bonding in forging as the pressure and temperature variables fulfil the requirements of such a bond. However, time under contact is an essential variable to the promotion of a strong metallurgical bond which is a competing requirement to the short production times required by industry [8]. Moreover, even thoroughly cleaned workpiece surfaces contain multiple contaminants of the order of several micrometres thick [75], with oxide quickly reforming post cleaning. It has been stated that an observable oxide layer can form on an aluminium workpiece within 15 s post cleaning at room temperature with times reducing at elevated temperature conditions [73].

Numerous methods to remove contaminant and passivation layers and promote diffusion bonding had been proposed in the literature. These include the use of a vacuum, macro and microscale deformation, the application of interlayer materials and protective coatings. The use of a vacuum, whilst effective, is impractical in a forging environment due to the size of the equipment involved [87]. For the application of a protective atmosphere, it may be possible to utilise an inert gas environment, such as argon gas, although it is recognised that there is significantly more oxygen present that could oxidise the workpiece surfaces. Macro and microscale deformation inherently occurs during the forging process. This is an effective diffusion bonding promoter as it disrupts the oxide layer that has a lower ductility than the parent materials, with strains of 40% being sufficient to break up this layer [158]. Whilst bonding is promoted, it should be noted that the oxide layer is not entirely dissolved or removed from the interface, but is simply broken up enabling base material to flow around it. Thus, these brittle areas of oxide may still potentially cause failure during long term operation of the gear. Interlayer materials have been proven to be highly effective at simultaneously encouraging bonding and also minimising galvanic corrosion [174] for materials of substantial corrosion potential such as aluminium-steel or magnesium-steel combinations. The application of interlayer materials has been performed as either a foil or an additively manufactured surface layer. According to the literature, nickel [66], copper [83] and even silver [10] are effective interlayers in bonding promotion.

Modelling of multi-metal forging is challenging due to the complexity of dissimilar properties and the interlayer surface. Finite element modelling of bi-metal forging processes has been conducted in the literature, however, the introduction of interlayer formation during such a forging process has not been performed according to the authors’ knowledge. The study of interlayer surfaces is typically subject to dedicated studies with Behrens et al. [18] modelling such a surface through the tensile testing of samples extracted through the bonding of EnAW-6082 to 20MnCr5 and C22 to 41Cr4 material pairs. The limitation of such studies is that during gear forging, pressure and temperature conditions along the tooth root, flank and tip profiles vary significantly resulting in any evaluation focusing on only a portion of the multi-metal tooth interface. Politis [120] designed a simplified test rig to model the pressure and temperature conditions along the tooth interface with preliminary analysis showing a promising representation of the conditions found during bi-metal gear forging. The literature has also numerically evaluated the potential performance of bi-metal gears through the evaluation of tooth stiffness [74], dynamic characteristics [171] and internal tooth stresses [119]. It was found that for the evaluated geometries, bi-metallic gears exhibit similar bending performance to solid steel gears with a 40% weight reduction, stiffness reduction between 9–38% depending on outer steel ring thickness and tooth stresses between 4–20% greater than solid steel. Therefore the literature concludes that substantial weight savings can be made with a minimised mechanical performance impact. However it should be stated that the evaluated studies were numerical in nature and experimental work is further needed to verify the multi-metal forged gear performance.

Modern modelling approaches such as the use of genetic algorithms and case based reasoning (CBR) systems are increasingly being applied for forging of single material components with promising potential to be extended to multi-metal production. Optimisation studies [138] have proven that knowledge based systems, once having defined appropriate boundary constraints, can rapidly converge to an optimised solution in a relatively short period of time. However, the user should take care in the selection of boundary condition constraints as optimisation systems may converge to local rather than global solutions. It is expected that such intelligent systems will enable rapid optimisation of preform design and processing parameter selection whilst avoiding the time intensive and repetitive unidirectional FE modelling and optimisation approach which is currently in use.

A review of the literature suggests that multi-metal gear forging is a promising solution to the reduction of gearbox assembly weight whilst maintaining performance. Numerous research works have focused on a range of material pairs including the achievement of cost reduction through the use of high strength steels at the tooth region with low carbon steel in the centre, or the reduction of weight through the replacement of low carbon steel with lightweight alternatives such as aluminium alloys. As research studies have focused on unique material pairs and geometries, it is difficult to directly compare all the methods to common benchmark values, although valuable lessons can be learnt regarding workpiece design, heating, forging and strengthening.

The co-extrusion of dissimilar metals is seen as the most rapid means of producing single workpiece bi-metal preforms, with the use of individual workpieces assembled in the die prior to forging as the lowest cost alternative due to the avoidance of manufacturing operations prior to forging. The advantage of pre-joining the multiple metals prior to forging is the more accurate control of the bond interface compared to bonding directly through forging. The effects of material compatibility are crucial to minimise the impact of galvanic corrosion and promote bonding, and interlayer materials are recommended to limit the adverse effects of these parameters. The thermal processing window is crucial to the success of a multi-metal gear as the use of inappropriate temperatures may lead to separation and the formation of an air gap due to differing thermal expansion coefficients, or result in cracking of the workpiece due to poor ductility. For aluminium-steel pairs it is recommended to heat steel to temperatures greater than 900 °C and aluminium between 400 to 500 °C. This temperature selection also enables the control of thermal contraction post-forging to produce a robust gear. Induction heating to attain such temperature control is superior to the use of furnace heating as induction systems could be placed in close proximity to the die thus minimising thermal loss during workpiece transfer. Moreover, the literature has stated that a single coil design could heat both metals to their target temperatures thus avoiding the use of multiple furnaces. However, the selection of any method, or combination of methods, to produce a multi-metal forged gear is highly geometry and material dependent and therefore practical constraints mean there is no single processing route that is ideal for the manufacture of all multi-metal forged gears. Expert systems are expected to aid users in the optimisation of a gear forging, although these are still in the early stages of development, particularly for multi-metal gear forging operations, and show enormous potential to optimise all aspects of the manufacturing process. These systems would enable users to upload their geometry and material requirements, and through an evaluation of prior case studies stored in a database, provide a suitable avenue to the most efficient processing route for the forging of multi-metal gears.