Incremental Bulk Metal Forming

P. Groche1 (2), D. Fritsche1, E. A. Tekkaya2 (2), J. M. Allwood3 (2), G. Hirt4, R. Neugebauer5 (1) 1Institute for Production Engineering and Forming Machines, TU Darmstadt, Darmstadt, Germany

2Department of Manufacturing Engineering, Atilim University, Ankara, Turkey 3Institute for Manufacturing, University of Cambridge, Cambridge, United Kingdom

4Metal Forming Institute, RWTH Aachen, Aachen, Germany 5Fraunhofer Institute for Machine Tools and Forming Technology, Chemnitz, Germany

Abstract Incremental bulk forming is the oldest known technique in metal working. Many developments in this field have dramatically changed our society. Today incremental bulk forming processes are applied to quality products in small and large series production. Numerous advances have been realized. The motivation for using these processes is presented here. After a general definition of incremental processes and a classification of incremental bulk forming in particular, some innovative product and process examples are given that show the potential. Finally recent progress and challenges are illustrated in detail. This includes the development of new machinery for incremental bulk forming, advanced methods for process planning, occurrence of failure modes and the properties of finished products. Keywords: Forming, Metal, Incremental

1 INTRODUCTION: FROM CRAFT TO HIGH-TECH PRODUCTION

1.1 History of incremental bulk forming The history of incremental forming techniques goes back to the Neolithic Age. At this time man began to produce simple tools and ornaments from elementary metals like gold, silver and copper. Utilization of metals strongly changed the human society. The first upper classes were formed by the people who controlled winning and trading of metallic material and products. The oldest findings of processed copper are dated to 8000 BC and originate from Anatolia [1]. The practical value of these objects was not very high, because copper is rather soft and cutting edges are worn off quickly. The first metallic objects were mainly used for decorative or cultic purposes. When the processing was improved later tools became possible like the copper axe that the famous Neolithic man “Ötzi” carried [2]. In [3] E. G. and H.H. Thomsen haveshown that it is possible to manufacture gold or silver parts by the use of soft tools of the same material. So this fact could be an explanation why tools are only rarely found in archaeological sites as the material was probably remelted. The situation changed when bronze alloys were discovered by melting copper together with tin ores. In about 3300 BC early bronze tools were made that showed an improved durability. Many hundreds of reaping hooks and axes as well as several swords were found all over Europe [4]. But it is assumed that these swords were used primarily to express the owner’s splendour rather than fighting as they were very brittle. A famous artefact from the Bronze Age is the sky disc of Nebra that was probably used for astronomical purposes [4]. This piece of art is about 3600 years old and was initially made by casting bronze. The solidified material was hammered to

a thin disc afterwards as illustrated in Figure 1. TracesTraces of this incremental working can still be detected today as reported by an investigation with computerized tomography [5]. Another emerging application was bronze armour that was produced with similar techniques. Even after the processing of iron was established, in some regions armour was made from bronze until the early Middle Ages [6].

Figure 1: Cast and forged material, the sky disc from Bronze Age [4]. The first findings of worked iron are also from the ending of the Bronze Age. Very pure and formable iron can be found in nature mainly in the form of meteorites. These were the first resource used for ferrous material. Of course the availability of the material from this source was very limited.

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Figure 2: Timeline of incremental metal working. In about 1200 BC the Hittites developed the production of wrought iron. The smelting of the ores could only be realized below the melting temperature of iron at this time. A spongy mass results (sometimes called a bloom), consisting of a mix of wrought iron and slag. The sponge iron was repeatedly processed by hammering and folding to remove the slag mechanically. The resulting wrought iron with an acceptable purity grade was then formed to the desired shape. Finally the properties of the workpiece were improved by carburization and annealing. In this way a smooth transition to the Iron Age took place [7].

Figure 3: (a) Ancient hammer and anvil, (b) medieval blacksmith, (c) steam hammer from the time of the industrial revolution, sources: Gallery of Early Blacksmithing, North Carolina Archive.

The Egyptians, Greeks and Romans used hammers and anvils made from tempered iron. Several kilograms of this material represented a significant value at that time. Therefore the control of profitable ore deposits was often the cause of war. Besides the improvement of agricultural tools the development of weapons gained significance [6, 7].

Figure 4: (a) Modern technology for incremental forming with robots, (b) open die forging of large shafts, sources: IPA Stuttgart, Siempelkamp.

(b) (a)

(a)

(b) The production of iron was simplified with the invention of

the blast furnace in China. Ores could now be smelted above the melting point of iron omitting the mechanical removal of slag and impurities. Around 300 AD a process for the production of steel was used in India for the first time. Iron was heated together with charcoal and glass in a crucible. The glass floated on the melt and so it was hermetically sealed. Later Damascus steel was invented in Syria based on this technique. Several layers of hard and brittle steel on the one hand and soft and ductile steel on the other were piled up, hammered and folded. This process was repeated many times to produce work pieces that feature ductility and hardness at the same time [7]. In the Middle Ages the art of blacksmithing spread to the whole known world and was further improved. Since the 13th century water-powered tilt hammers were used in Europe. From about 1400 to 1600 the craft of armouring was at its height and some of the most beautiful pieces of art were manufactured by incremental techniques. During that time thin metallic parts were mainly made from hammering raw material that had the initial shape of a flat

(c)

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pancake. In the 15th century the first rolling mills were invented, but rolled sheet metal had only limited quality. During the industrial revolution in Britain some important discoveries were made. In the year 1709 Abraham Darby used coke instead of charcoal to produce iron for the first time. The steam hammer was invented in 1837 by James Nasmith. And in 1855 Henry Bessemer patented his Bessemer process for the refining of raw iron. Until the time of the industrial revolution most of the iron in the world was produced as wrought iron with the method invented by the Hittites nearly 3000 years before. But now for the first time in history it was possible to produce high quality raw material in great quantity and to process it to industrial products. New applications were possible like high rise buildings, bridges, the railway or large ocean liners. With the development of electrical and hydraulic drives the technology for mass production was finally available in the 20th century. As a result, the old craft of incremental metal working has largely fallen into oblivion with the exception of some special high-tech applications that are in current use [8, 9]. 1.2 Technical and economical advances Recent advances can in general be divided into process and into product related progress. Process related progress is based on the equipment and machinery as well as the process specific knowledge. Important improvements are the shortening of cycle times and an advanced output, an increase of reproducibility, the possibility to substitute manual operations by automation and the availability of high process forces. Process related progress is illustrated by the development of forging equipment for incremental forming in Figure 3 and 4.

Figure 5: (a) Rolling mill designed by Leonardo da Vinci in the 15th century, (b) pierce rolling process invented by Reinhard and Max Mannesmann in 1885, (c) modern ring rolling process, (d) pierce rolling process, sources: science and society, IBF Aachen, Mannesmann Archiv. The use of incremental rolling techniques did not start until the Renaissance. The required precision of a rolling mill and the need for process control that is more complex than that needed for hand driven forging had not been feasible until then. With rolling technology established

new incremental processes were developed. Leonardo da Vinci designed a water-powered rolling mill for the production of iron staves for cannons as shown in Figure 5. Flat rolling replaced the old technique of producing sheet metal by incremental hammering. This had significant economical advances. The demand for new products also affected the development of new processes. When there was a market for seamless tubes and rings, new rolling processes were developed for these products, like the ring rolling and the pierce rolling process shown also in Figure 5 [8, 10].

Figure 6: (a) Copper axes from Neolithic Age, (b) Iron Age tools, (c) Renaissance armor, (d) segment of Ariane booster, (e) hollow shaft, (f) clutch carrier and internal gear, sources: Carnet/MDC, Markham Museum, Hermann Historica, Aerospace technology, PtU Darm-stadt, Leico.

(a) (d) (b)

(e) (f) (c)

(b) (a)

(c) (d)

Figure 7: Weights of iron blooms excavated in Europe plotted against time [6]. The availability of metallic materials was significantly improved over the centuries. If the weight of iron blooms is plotted against time as shown in Figure 7, it can be seen that there was a peak in productivity during the Roman Empire. Since the end of the Middle Ages, the output weight has increased progressively. This is due to the fact that the incremental techniques, which are too laborious for the initial material production from metallic ores, were replaced by refining [6]. In the 19th and 20th

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centuries, in addition to copper alloys and ferrous materials, new metallic materials that could be shaped with incremental forming were used, e.g. aluminium, titanium, magnesium, cobalt or nickel alloys. Incremental processing of these materials has been investigated in recent decades and is now successfully applied in industry. In many cases incremental bulk forming can simplify the processing of hard-to-form materials because of its advantageous stress states. The product related progress compared to techniques used in the past should be considered with respect to the purpose of the manufactured part. The variety of producible shapes and materials in one-piece design has increased over the centuries. Process developments have also led to improved tolerances for finished parts, the ability to form very large or very small parts, enhanced and adjustable material properties and the integration of additional functions. However, at present, the effort of producing a part in a die-defined operation is lower compared to incremental manufacturing, while machining yields higher accuracy. For that reason the application of incremental processes has to be planned very carefully to use its advances in an effective way [11].

Economical Technological High flexibility Net-shape-products Low investment Low friction and wear + Low tooling costs Hardening and grain

structure High local tool loads Complex process control - Low productivity Experience driven technology

Table 1: Positive and negative aspects of the application of incremental bulk forming processes with respect to economical and technological criteria.

1.3 Motivation for application of incremental bulk forming processes

While man was obliged to use incremental processes in the past, this technology has now become uncommon. This is mainly due to the competing technologies that are available today. While in the past metallic products were individual items, they are manufactured in series nowadays. There are economical reasons to use incremental forming while there are also several technological advantages. In particular, the flexibility and the lower forces compared to die-defined forming are important as well as the properties and formability of the finished parts that will be presented in section 4.4. Competing technologies are mainly die-defined forming and machining. On the one hand manufacturing by die-defined forming processes is more efficient in many cases and on the other hand cutting operations allow for more complex shapes. However, there are several aspects that are capable to turn the application of incremental processes into an attractive option again. This is possible if the investment and tooling costs become the determining factor or if other processes are to be integrated. Incremental processes typically operate with simple tools and low forces, while a wide range of products can be manufactured with no or only minor changes to the tooling system. If the production needs to be flexible to the customers needs, the application of incremental processes can be very profitable. This is for example the case in the automotive industry that is using

incremental manufacturing increasingly [12]. Also the possibility of using incremental forming processes for the assembly of components is an emerging field. Another aspect is the production of parts at a very large scale in smallest series. Typical examples are shafts for power plants or large vessels or the hull of booster rockets for space flight. These parts have to withstand highest loadings when they are in use. Incremental production ensures hardening of the material and an optimized grain structure caused by the high number of forming cycles during manufacturing [13]. The low process forces result in low friction and wear, while local tool loads can be very high in small areas causing failures. Many incremental processes feature complex kinematics that have to be controlled. In some cases this can be a restriction together with the necessary experience in this technology. Finally there are several efforts to apply incremental forming to the production of net-shape parts, which is concurrent to machining operations. This approach is especially used for the production of drive system components [14, 15]. 2 DEFINITION AND CHARACTERIZATION The previous section has shown how incremental bulk forming processes have a long history and how commercial demand for flexible production has motivated recent interest in automating what were previously craft processes. In this section incremental bulk forming processes are defined in order to develop a means to classify the range of processes. Such classification is useful both for identifying similarities between different processes and for facilitating the search for novel, as yet untested, processes. The following section will use this classification to present current process developments.

2.1 General definition and features Within this paper, the following definition will be used: In an incremental bulk forming process, regions of the workpiece experience more than one loading and unloading cycle due to the action of one set of tools within one production stage. The definition has three key components. At any instance in time, an incremental process creates deformation within regions of the workpiece, not the whole – so extrusion and upsetting, for instance, are clearly not incremental. In strip and long product rolling, the deformation occurs only within one region of a product, but these are not incremental processes because each material particle is deformed only once. Processes that do not satisfy the definition above are called die-defined processes because the shape of the finished parts is totally defined by the geometry of the dies. In contrast, in incremental processes the shape is at least partially generated by the kinematics of the tools. Some regions of the workpiece will experience more than one loading and unloading cycle, as the tools of the process traverse the workpiece. In tandem rolling of strip, each material particle is deformed several times as it passes through different stands of the mill. However, the tandem mill is a series of separate processes. In contrast, in an incremental process multiple loading cycles occur due to the action of one set of tools within one production stage. The separation of incremental sheet and incremental bulk processes is somewhat problematic. There are many configurations of incremental sheet forming processes, as reviewed by Jeswiet et al. [16], all of which satisfy the above definition. However, although these processes are applied to sheet workpieces, they typically lead to a small deformation zone which varies throughout sheet thickness – so locally share many of the characteristics of

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bulk forming. This problem is particularly apparent in recent developments of micro-forming processes, several of which (for instance the extrusion of micro-billets [17]) use sheet metal as a blank but are in all other respects bulk forming processes. For the purposes of this paper, incremental bulk forming processes are those which apply to workpieces which are not, initially, thin in one dimension – ie which do not have the characteristics of sheet, tube or wire. In the introduction, and in section 3 to follow, a wide variety of incremental bulk forming processes is illustrated with diverse product geometries and process configurations. Despite this diversity, they share several key features:

Machine and tool design Incremental processes typically have relatively small simple tools, which experience reduced forces compared to their non-incremental equivalents, so machines can be lighter for equivalent stiffness. However, incremental processes must generally allow more degrees of freedom in moving the tool, so are more complex and require more complex control. The increased tool motion combined with longer cycle times in incremental processes may lead to increased tool wear. Machine design may include the controlled application of heat – globally or locally – to the workpiece.

Process mechanics Deformation is generally not steady-state in incremental processes, and the forming zone is usually embedded within elastic zones. Typically gradients of stress are higher than in conventional processes. Generally incremental processes experience continuous contact change between tool and workpiece through a long process cycle, so process modelling is arduous.

Process operation Incremental processes have slower cycle times than die-defined processes but generally have much faster setup times as they do not require manufacture of dedicated tooling. Control of incremental processes may combine an open-loop tool path planning algorithm with closed loop feedback control. An attraction of such feedback is that, for the first time in automated production, corrective forming can be achieved. However, feedback control depends on the availability of suitably fast sensors and process models for predictive control. Only few processes can be modelled with sufficient speed to allow this. Combined control of temperature and deformation suggests future possibilities in controlling product microstructure as well as geometry. The area of incremental bulk forming is expanding rapidly with new process designs, and the common features above indicate a shared set of challenges for process development. Many of these challenges relate to the difficulty of predicting the effect of the next tool move on the workpiece. The processes currently in commercial use or near to implementation are generally those which are most predictable.

2.2 Classification The problem of classifying metal forming processes has attracted several efforts in the past 50 years, but has yet to lead to a convincing solution due to the number of parameters that may be used. Table 2 provides a compact survey of the key parameters used in five classification schemes proposed by Thomsen et al. [18], Blazynski [19], Kudo [20], Lange [21], and Allwood and Utsunomiya [21]. The widely used German DIN scheme [23] is mainly based on the stress state in the workpiece.

Thomsen et al. [17]

• Approximate state of stress (squeezing, drawing, bending)

• Instantaneous zone of deformation (whole part or zone)

• State of deformation (nonsteady or steady)

Blazynski [18]

• Stress system (triaxial compression, biaxial compression, biaxial tension, biaxial tension + uniaxial compression, uniaxial tension + uniaxial compression, uniaxial tension + biaxial compression)

Kudo [19] • Initial workpiece geometry • Extent of deformation zone and

state of strain • State of stress (direct, auxiliary,

hydrostatic) • Deformation sequence

(intermittent, continuous, general) • Tool geometry • Means of power transmission to

workpiece • Others – working temperature and

speed Lange [20] • Material behaviour in the plastic

zone • Characteristics of the workpiece

before deformation • Boundary between tools and

workpiece (friction, lubrication and wear)

• Tool layout and materials • Surface reactions between

workpiece and surrounding area • Design of machine tool • Integration of metal forming

process into production system as a whole

Allwood and Utsunomiya [21]

• Initial workpiece geometry • Source of flexibility (local tool

movement, multiple passes, multiple actuators)

• High or low hydrostatic stress • Continuous or intermittent

deformation • Tool and workpiece interaction

(sliding, rolling, impact, non-contact, stationary)

Table 2: Parameters used to classify metal forming processes.

A useful classification scheme will help to identify similarities between processes and to inspire novel designs. The challenge of classifying incremental bulk forming processes is a subset of the generic problem addressed by the authors in table 2 and can be achieved by a subset of the table’s parameters. All five schemes make some use of the state of stress in the workpiece to distinguish processes. Within bulk forming operations, tension is not common and can only be applied to long products by grippers acting away from the deformation zone. Therefore a subset of Blazynski’s stress characterisation is used. The remaining four authors also agree on the use of time variation in deformation – in particular whether it is continuous or intermittent – to distinguish processes, and this is also useful for the present need. Thus the main axes used to characterise incremental bulk forming processes will be

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� Stress system (Uniaxial compression, Uniaxial compression + uniaxial tension, Biaxial compression, Biaxial compression + uniaxial tension, Triaxial compression)

� Deformation sequence (intermittent, continuous) The stress system is clearly a crude categorisation, and is applied at the centre of the deformation zone - with uniaxial, biaxial or triaxial used to distinguish stress states with one, two or three dominant components. In addition, workpiece geometry mentioned by the last three authors in table 3 provides a useful means to find similarities between processes. For instance, orbital forging and ring rolling are mechanically similar. Three initial workpiece shapes will be considered: � Billet (brick shape or plug shape) � Long product (bars or rods) � Rings Using this simple parameterisation of processes, table 3 presents a classification of all known incremental bulk forming processes. A selection of exemplar processes in table 3 will be discussed in more detail in section 3.

Deformation sequence Intermittent Continuous

B B Orbital forging, Grob process

L In-plane strip bending L

Uniaxial compres-sion R R Ring rolling

B unlikely B unlikely L L Helical rolling

Uniaxial compres-sion + Uniaxial tension

R unlikely R unlikely

B Rotary swaging B

L

Incremental forging, Rotary swaging, Flow forming, Cross rolling

L Thread rolling, Roto-Flo process [24]

Biaxial compres-sion

R R

Incremental ring rolling, Flexible wheel rolling

B unlikely B unlikely L L

Biaxial compres-sion + Uniaxial tension

R unlikely R unlikely

B Localised hammering B Roller

burnishing L L

Stre

ss s

yste

m

Triaxial compres-sion R R Gear rolling

Table 3: Classification of incremental bulk forming processes (B - billet, L - long product, R - ring). The classification scheme of table 3 indicates various process development opportunities as a number of the cells in the table are currently empty. This feature of classification schemes – identification of ‘missing processes’ – gives the possibility of developing a structured search for new processes. An early example of this approach, by Roth [25] attempted to classify all possible forging processes, and more recently Allwood

[26] has attempted to specify all possible designs of future ring rolling machines. While such approaches are always limited by the assumptions on which they are built, they offer an interesting opportunity for process innovation – particularly at this relatively early stage in the development of novel incremental processes. 3 INNOVATIVE PRODUCT AND PROCESS

EXAMPLES Some of the processes listed in table 3 are relatively mature as was shown in section 1, but the interest of this paper is in exploring the rapid growth in incremental bulk forming processes from innovative concept through to commercialisation. This section provides examples of various contrasting innovative processes at different stages of the journey to commercialisation.

3.1 Orbital forming Orbital forming is an example of an incremental process with a continuous deformation sequence and mainly uniaxial compressive stresses. This is a mature technology, related to ring rolling and the processes of tube nosing and flaring. Figure 8 illustrates a schematic of the process and several products, included flanged automotive parts.

Figure 8: Orbital forming, source: Timken. While in conventional cold forming the force is applied over the entire surface of the part, in orbital forging it is applied only on a small segment. The orbiting upper die rolls over the part. Therefore, friction is reduced substantially and the metal can flow much more easily in the radial direction (rolling friction instead of sliding friction). Compared to conventional cold forming, the orbital forging process offers the following advantages: � smaller presses (investment, space requirement) � smaller stresses in dies (tooling costs) � longer die life � reduction of noise and vibrations Orbital forging also offers the advantage that substantially higher forming ratios can be achieved. This can eliminate expensive progressive dies and reduce related set-up times to a fraction. These facts make orbital forging economical, particularly in medium and small batch production. In many cases, this process is the only economically feasible way to cold form small batches of parts that, otherwise, would have to be machined [27 - 35].

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3.2 Incremental ring rolling Ring rolling is in general a continuous process in a biaxial compressive stress state. The idea of flexible ring rolling dates back to work in the early 1980’s at Hitachi Corporation in Japan [36] aimed at flexible production of railway wheels. This idea was applied in Aachen [37] for axial rolling, and a wide-ranging exploration of radial incremental ring rolling [38] included industry studies. Figure 9 shows schematics of the incremental process – in both axial and radial configurations – in which a narrow mandrel can move with increased degrees of freedom to allow some flexibility. Bearing rings can be formed by cold incremental ring rolling for example. The figure also shows the cross-section of a part made from wax as a simulation of hot incremental ring rolling. The key challenge in incremental ring rolling is to cope with the more complex flow of material away from the narrow mandrel which tends to cause conicity defects in radial rolling and dishing in axial rolling. At present this limits the industrial applicability of the process to rather shallow profiles, but future process innovation in which the ring is more heavily constrained may increase the range of achievable profiles. A model machine has been built in Cambridge to explore this possibility [39].

Figure 9: Incremental ring rolling, source: IBF Aachen. A key outcome of the trials in cold incremental ring rolling was that despite the increased stress gradients of the incremental process, the residual stresses remaining in the ring after processing were no more severe than those from conventional ring rolling [40 - 47].

3.3 Incremental forging The design of an incremental forging process differs from that of the two processes described above in which flexibility is achieved through increased freedom in the motion of the tools. Instead in incremental forging processes, flexibility arises from increased control of workpiece placement in a conventional forge. The process that is based on intermittent deformation and

biaxial stresses was originally explored by Ferrera and Osman in Bath [48], and also by Kopp et al. in Aachen [49]. In both cases a robot was used to place a long product in a forge, and by repeated movement and deformation a controllable curvature was created. An alternative configuration in figure 10c [50] has a long flat strip fed into a forge with wedge-shaped dies, and by controlling the feed between strokes, a variable in-plane curvature of the strip can be created. A further variant in figure 10d [51] shows forging combined with twisting in the manufacture of turbine blades.

Figure 10: Incremental forging, source: IBF Aachen. For most variants of these processes, no commercial implementations of incremental forging have been

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reported, and precise geometric control remains difficult– robots are notoriously flexible, and forged parts are typically heavy. Despite the apparent flexibility of handling the workpiece with a robot, the more conservative approach of feeding the workpiece through the forge in a linear manner may be a safer route for initial implementation [52, 53].

3.4 Rotary swaging Rotary swaging is a process for changing the cross section of a tubular or solid bar stock. It is classified by an intermittent deformation and a biaxial effective stress state. Most applications involve the reduction of diameter and the tapering of round tubing or solid bars. Other shapes can be swaged as well, such as square or hexagonal cross-sections into round profiles or vice versa. Moreover, for some typical parts, e. g. drive shafts, rotary swaging already dominates cutting processes. Regarding the power train of an automobile, further parts (e. g. gear shafts) can be found which are suitable to be manufactured by this process as well. The drawback here is the shaft collar which represents a local increase of outside diameter. A new process to solve this

Figure 11: Rotary swaging, source: PtU Darmstadt.

problem was developed at the Institute for Production Engineering and Forming Machines at Darmstadt University of Technology. It is called Axial-Radial-Forming and represents a combination of rotary swaging and lateral extrusion [54]. The shaft collar here is formed by crushing the material into engraved die segments of a Rotary Swaging machine. The local material flow behaviour of the workpiece is affected by partial heating prior to the forming operation [55 - 57]. Another important application is the GFM Precision Rotary Forge process for the production of parts with high quality.

4 RECENT PROGRESS AND CHALLENGES

4.1 Machines Incremental processes typically use relatively small and simple tools, which experience reduced forces compared to their non-incremental equivalents, so machines can be lighter for equivalent stiffness. However, incremental processes must generally allow more degrees of freedom in moving the tool, so they are more complex and require a control that is more complex. Regarding the different press types, there are typical characteristics of each actuation (Table 4). Mechanical presses have the advantage of a high yield rate in relation to hydraulic presses. Mechanical presses are classified in presses with crank mechanism and cam mechanism, where presses with crank mechanism are used for higher process forces and presses with cam mechanism for processes with complex path time behaviour and high stroke rates. Regarding a flexibly adjustable path time and force time behaviour, mechanical presses possess substantial disadvantages. The processes are rigidly coupled to the kinematics of the drive components. High forces may call for hydraulic drive systems that are applicable in particular for slow movements for example the adjustment of pressure rolls as shown in Figure 12. In order to realise press ram movements optimally co-ordinated for different forming operations a number of modified drives on mechanical base were developed. In order to uncouple the press ram movement completely from mechanics and to achieve the highest possible flexibility, there are efforts to implement the press ram

Figure 12: Control strategy, source: Fraunhofer IWU.

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flexibility, there are efforts to implement the press ram movement by a direct conversion of electricity by means of linear motors [64]. This form of the press drive unites the advantages of mechanical and hydraulic presses and opens new possibilities regarding the process control and adjustment. Linear motor presses allow a direct press-ram-actuation and very high punching frequencies as well as high accelerations with an adaptable path time behaviour. By using forming units, which are driven about linear motors, complex metal parts can be manufactured with ideal path-time behaviour for the individual stages.

Press type Characteristics mechanicalpress

- high yield rate - coupled to the kinetic of the

drive components [59]

hydraulic press

- flexible press ram movement - high forces

[59-60]

servo motor press

- flexible press ram movement - tilting press ram- limited forces

[61]-

[64]

Table 4: Characteristics of different press types.

At the moment, the availability of servo- or torque motors with high torque outputs enables the upgrade and improvement of mechanical presses [58]. While press machines receive their requisite torque for generating the press-force by the RPM-loss of a flywheel, the modern servo-drive units can generate the required torque directly. This provides the opportunity to design mechanical presses with relatively high press capacity but without the need of energy accumulating flywheels. The main and innovating consequence of avoiding the mass of a flywheel is the possibility to control the ram in every state of movement. If the ram position is back coupled to a unit that controls the servo motor, a machine system which commonly is called a servo-press is generated. This type of forming machine is adequate for many incremental forming processes [57,62,63]. While the first generation of servo presses focuses on the variation of the stroke- and the forging- or stamping- movement enabled by the servo drive, the second generation gives the possibility to control the unavoidable tilting of the ram when operating the press with eccentric forces. Because tilting under these conditions is partly caused by the different elastic deformation of the driving gears it can never be avoided by the quality of the design alone.Compared to mechanical driven systems, the complexity of control increases significantly. In servo-press systems, it is necessary to avoid losses in the maximum number of strokes per minute, caused by the time of control. Thereby the maximum speed during production processes is only limited by the characteristics of the servo motor. During the mode of adjustment, the system "has to learn" the required velocity -profile and the resulting effects on forces and torsion moments. During the automatic mode, this stored profile is the virtual master drive for the control unit and thus makes possible the coupled interactions between several drive systems and axes (Figure 12). It is obvious, that the possibility of free adjustment of the axes instead of one axis creates the need for suitable human-machine-interfaces (HMI). One possibility is to use a joystick during the machine adjustment in the teach-in-mode. Another way is to build up a library of ram

movements from which the operator can select during the definition and programming of the ram movements. Similar to the effect of introducing CNC-main-axis into the equipment of lathes, which expands the possible field of application dramatically and turns the machine into a machining center, the new generations of linear and servo presses enable new forming centers. As demonstrated by the described new system, the integration of process steps, usually reserved to other specialized forming machines, is practicable with two additional controlled ram-axes. Thereby the main features of press-productivity, like stroke rate and energy efficiency, do not decrease.

4.2 Process planning The planning of innovative incremental bulk forming processes ideally requires extended process simulations supplying not only the material flow but also the product properties such as hardness distribution, residual stresses, microstructure etc. These simulations are based widely on finite element models that have to cope with the characteristics of incremental forming processes as described in the next section. The recent progress made in modelling incremental bulk forming processes as well as the current challenges will be described in the following sections.

Characteristics of incremental forming processes In incremental bulk forming the plastic deformation zone is imbedded in elastic material regions. The active plastic deformation zone is small compared to the overall domain. Furthermore, the active contact zone is small compared to the workpiece size. Also multiple passes are often applied. From the modelling point of view these characteristics lead to the following consequences: � The contact region is small compared to the overall

workpiece dimensions. Compared to classical bulk forming processes the relative contact zone is about two orders of magnitude smaller in incremental bulk forming processes.

� The contact region is continuously moving through the workpiece so that, ideally, a fine mesh discretization is required throughout the workpiece

� Since the overall plastic deformation is achieved in small steps, ideally many deformation steps are necessary.

These characteristics of incremental forming processes force the model to have, ideally, a large number of elements and large number of time steps [65]. Furthermore, the elastic neighbourhood of the deformation zone necessitates the use of elastic-plastic material laws. A further characteristic of incremental bulk forming is that deformation is principally not steady state. Therefore, steady-state solutions as applied in some classical bulk forming processes such as plane rolling are ideally not possible. Also two-dimensional simplifications are usually not possible and symmetry planes do not usually exist. Also the tool kinematics is often complicated and contact between tool and workpiece can be temporarily lost. Finally, since incremental bulk forming processes exhibit more than one loading and unloading cycle, regions with eventually varying strain paths occur so that simple monotonic constitutive material laws should ideally not be applied to these processes.Because of these characteristics of incremental bulk forming processes the thorough modelling of them requires computational times that extent to months on typical fast computers and data storage that reaches several hundreds of GBytes. For this reason, special

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modelling methods have been developed to enable feasible analysis of incremental bulk forming processes as described in the next section.

Models with high computational efficiency Models suggested in the literature to increase computational efficiency can be grouped into two basic categories: Models that can be applied to all incremental bulk forming processes (General Models) and models that can be applied only to specific processes (Special Models).

Method Processes analysed References Dynamic Explicit

Ring-rolling and pilgering [67 - 74]

Arbitrary Lagrangian-Eulerian (ALE)

Ring-rolling [76 - 81]

Hybrid Mesh Ring-rolling and multi-pass forging [82 - 89]

Multigrid Methods

Ring-rolling and incremental forging [90]

Super-Elements Orbital forging [91]

Self- Similarity

Ring-rolling, flow-forming and rotary swaging

[92, 93]

Table 5: Summary of general models. General methods Promising recent methods are summarized in Table 5. Dynamic explicit methods are extensively used for instance to model ring rolling [67 - 73] and pilgering processes [74] since they are robust (variable contact conditions are easier to handle) and computationally efficient. Dop [68] reports that the speed-up by dynamic explicit methods is about 22 as compared to static implicit methods. Pauskar [70] emphasizes that mass scaling is necessary to speed up the computations in dynamic explicit methods and that this must not be used aggressively. Also the problem of volume increase is addressed in this study. Finally, explicit methods are not able to compute residual stresses accurately while being computationally efficient at the same time. Figure 13 shows dynamic explicit finite element simulation results for the cold ring rolling process of a CV joint cage [70]. The product geometry is predicted with an accuracy of 0.5 mm, whereas the load prediction deviates by more than 25%. This may be caused by the mass scaling, material modelling and the volume increase.

Figure 13: Simulation of the cold ring rolling of a CV joint cage [70].

Arbitrary Lagrangian-Eulerian (ALE) formulations are appropriate to deal with the feature of incremental processes that the deformation zone is much smaller than the overall workpiece dimensions. The formulation allows the separation of the material and computational reference frames, so that, for instance, a fine mesh can be kept at the deformation region and a coarse mesh elsewhere [75]. The relative motion between the material and computational reference frames requires the inclusion of convective terms in the formulation. Typical studies on the application of ALE to the incremental forming process ring-rolling are given in [76 - 81]. Recent developments described in [81] report on various methods to improve the mapping of the computational mesh to the material mesh, improving accuracy in maintaining a constant workpiece volume.

Figure 14: Hybrid mesh approach: (a) Actually Rotating Mesh System, (b) Spatially Fixed Mesh System [85]. Hybrid mesh formulations differ from ALE in that two distinct meshes are constructed for the material and for computational purposes. The first study known to apply this idea is [82]. In this study an “Actually Rotating Mesh System (AMS)” and a “Spatially Fixed Mesh System (SMS)” is introduced (Figure 14). The AMS is the fine mesh containing all the data for the process such as velocities, equivalent strains, etc., whereas the SMS is the coarser mesh that is used to solve the field equations. In [83, 84], based on [82], an efficient dual mesh model has been developed that speeds up the computations by a factor of eight compared to conventional models. The SMS and the AMS are made the same in the deformation zone, so that interpolation is only necessary in the other regions and hence interpolation errors are reduced considerably. In [85] a commercial finite element code based on the hybrid mesh method for ring-rolling is evaluated.

Figure 15: Comparison of hybrid method with con-ventional method as applied to multi-pass forging [88]. In [86 - 89] a hybrid mesh method has been applied to ring-rolling and multi-pass forging (Figure 15). Here the AMS is named the geometry mesh and the SMS the

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simulation. The hybrid-method in this example is faster by 72% as compared to the conventional method. Also, in [88, 89] microstructural modelling is realized. In [88] the critical issues of the hybrid mesh method are pointed out: No remeshing is performed on the AMS mesh and in case of state variables that change their values drastically outside of the deformation zone, such as temperature, the accuracy of the hybrid mesh method is reduced.

Figure 16: Basic idea of multi-grid technique [90]. Multi-grid methods are iterative solution schemes for systems of equations. The basic idea is to solve the system of equations over a set of hierarchical grids to achieve efficient error reduction (Figure 16). In [90] an algebraic multi-grid method is introduced that is based on multi-grid components without any geometrical information. The method is applied to ring-rolling and incremental forging. The superelement technique has been applied in [91]. The method is based on the idea to reduce the degrees of freedom of all elements that are outside of the deformation zone to only six degrees of freedom representing the rigid-body motion. Naturally this pre-assumes only rigid-plastic deformations. The approach is applied to orbital forging (Figure 17) and the computational time has been reduced by a factor of about 12.

Figure 17: Superelement technique applied to orbital forging [91].

In [92, 93] the idea of self-similarity has been used to model various incremental bulk forming processes. The method utilizes the quasi-periodicity of solution sets in incremental bulk forming processes and describes this periodicity by self-similarity rules. An example of applications is given in Figure 18. Here a gear rolling process is modelled efficiently by the proposed method. Rotary swaging is another example of application [93].

Figure 18: Modelling of gear rolling processes using self-similarity [93]. Special methods Another group of efficient models are the special methods that are restricted to specific incremental forming processes. An overview to such methods is given in Table 6.

Method Specific Process References Pseudo Plane-Strain Ring-rolling [94 - 100]

Steady State Partial Models Ring-Rolling [101 - 104]

Transient Partial Models Ring-Rolling [105, 106]

Automatic Expansion of Domain Method

Radial Forging [107]

Linearization Method Orbital Forging [108]

Table 6: Summary of special models. The first known efficient models for simulating specific incremental bulk forming are given in [94 - 96]. Here a pseudo plane strain model has been applied to plain ring rolling. The key idea is the modelling of the radial sections of the ring with two-dimensional finite element models for which the normal (circumferential) strain rates are determined iteratively. Despite the extra iterations and multiple radial section computations, this approach provides the first efficient solution strategy since only two-

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dimensional computations are performed. Similar studies are given by [96 - 99] as described in [100, 101]. Another method for efficient analysis of particular incremental bulk forming processes has been presented in [102, 103]. Here, a portion of the workpiece around the forming zone in ring rolling is modelled. The slice is assumed to be not moving (steady state partial models). In [102] a plane-strain slice is modelled, whereas in [103, 104] three-dimensional segments are employed. An axisymmetrical model of this has been presented in [104, 105].

rigid surfaces

ring

mandrel

rigid surfaces

ring

mandrel Figure 19: Partial ring method with oscillating rigid boundary surfaces [105].

RigidSurfaces

ElasticElasticRegionRegion

ElasticElasticRegionRegion

Ring Ring SegmentSegment

t

RigidSurfaces

ElasticElasticRegionRegion

ElasticElasticRegionRegion

Ring Ring SegmentSegment

t

Figure 20: Partial ring method with oscillating elastic boundary surfaces [106]. Recently two transient partial models have been introduced in [105, 106] for ring rolling. In both cases a sector of the ring around the deformation zone is modelled that is, however, allowed to oscillate as in the actual applications. In Figure 19 the partial ring model with oscillating rigid end surfaces is shown. As the deformation proceeds the ring expands out of the rigid boundary wedge [105]. An improvement of this model allowing also warping of the cross-sections is given in Figure 20. Here, elastic boundary layers have been added to the model allowing out-of-plane deformations along the axes of the rolls [106]. In the same figure a time comparison is given for an Intel P4 HT 2.6 GHz, 1.5GB RAM hardware. The speed-up is about a factor of 20 as compared to a full ring model. Radial forging is a typical incremental bulk forming process that requires several tens to hundreds of forging steps with small plastic deformations and the simulation is hence as all incremental processes time intensive. An elegant method to reduce the computational time is the recently introduced automatic domain expansion method [107]. In this method the forging domain is expanded during the course of the forming process (Figure 21). For a rigid-plastic material law the boundary conditions at the expanded domains correspond to either fixed rigid bodies or to translating rigid bodies. This procedure enables the reduction of computational times by a factor of 3 to 4. In [108] a rigid-plastic material law with a linearization scheme is employed and a speed-up about 10 is achieved. The key idea is simply to solve the state equations using the strain-rates of the previous increment. By this just a linear set of equations is solved only once at every increment. This requires however to use small time steps. In addition, a decomposition of

velocities is realized into rigid-body and deformation portions in order to reduce the volume loss. Figure 22 shows the application of this to orbital forging.

Figure 21: Automatic domain expansion method for radial forging process [107].

Figure 22: Simulation of orbital forging process by the linearization method [108]. Current challenges Modelling of incremental bulk forming processes is still exposed to several challenges. One basic challenge is the appropriate modelling of the material constitutive behaviour. In many incremental forming processes a material element is exposed on one hand to cyclic loading and unloading states and on the other hand, the strain paths are changing from one cycle to another one. This is the most ambitious type of material behaviour and represents a real challenge for the accurate modelling of incremental forming processes. The effect of strain path dependency has been investigated by Sillekens et al. [109, 110]. In their studies the effect of tension-compression and torsion-tension loading sequences have been analysed. Significant effects on the flow stresses (changes of about 20%) have been found. In these studies only a single cycle has been considered. Similar results are reported by Wanheim et al. [111]. Other studies are reviewed in Bariani et al. [112]. The effect of cyclic loading combined with a change of strain path has been covered in a recent study of Meyer et al. [113]. Figure 23 shows for instance the effect of cyclic torsion of 42CrMo4 for large forward shear strain amplitudes and different portions of back deformations. With increasing back deformation the stress-strain curve deviates significantly from the monotonic curve. The accurate prediction of product properties is another challenge for modelling incremental bulk forming processes. The basic issue here is the fact that numerical models include various simplifications to increase their computational efficiency. Hence, rigid-plastic models neglect elastic deformations and are therefore not able to

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predict residual stresses and spring-back. Explicit models, on the other hand, show low accuracy in stress predictions due to the simplified element formulations (one-point-integration). So, they can predict residual stresses only with large errors. The yield strength distribution of incrementally formed bulk parts is also inaccurate in case of cyclic and non-monotonic loadings. Figure 24 shows comprehensive results by Tekkaya et al. [114] in which various numerical models are compared with each other and experimental results. The circumferential residual stresses in incremental ring rolling show a rather good agreement with the full finite element model and the so-called velocity-coupling segment model (VCM). Unfortunately, the stresses on the form roll side do not show such a good agreement and clearly expose the existing challenge.

Figure 23: Effect of strain path on stress-strain behaviour of 42CrMo4 during cyclic loading [113].

Figure 24: Prediction of residual stresses in incremental ring rolling [114], (a) principle of incremental ring rolling, (b) segment model, (c) comparison of measured circumferential residual stresses with finite element computations. A final challenge is the efficiency of the numerical models. Although smart modelling increases computational speed compared to full analysis modelling, by up to one or two orders of magnitude (usually at the cost of reduced

accuracy), this is still not enough. The analysis of complicated incremental models currently requires computational times up to a full day, which is not appropriate for iterative analysis of new processes. One solution to this dilemma is obviously applying parallel processing technologies. Quigley & Monaghan [114] applied a domain composition method based on a network of single and dual processor computers utilizing a commercial simulation software. In their study they modelled the spinning process. Figure 25 shows impressively how the computational time has been reduced almost by a factor of four with four single processor machines. However, for more than four processors, no significant improvement im computational time is noticed. Since eight domains are used for modelling, this stagnation of the speed-up obviously needs further investigation. Another approach for increasing computational efficiency is the application of meshless methods [116]. Yet, further developments are necessary to bring these methods to everyday application.

Figure 25: Speed-up of spinning simulation using parallel processing [115]. 4.3 Failure modes The observed failure modes of incremental bulk forming are not very different from the limits of forming processes in general. Limitations arise from tool respectively from workpiece failures.

Tool failure Workpiece failure Fracture Underfilling Buckling Wrinkling/Buckling

Surface disruption Crack formation Wear Scale formation

Table 7: Common failure modes of incremental bulk forming divided into tool failures and workpiece failures. In this context the influence of incremental processes on the limitations listed in table 7 is of interest. Due to the cyclic behaviour of the processes only a small area of the workpiece is in contact with the tools at one time. This causes high local tool loads with a cyclic recurrence that can lead to fatigue failures and finally to fracture of the tools. Typical examples where these tool failures can be observed are gear rolling processes [117 - 119]. A broken mandrel after flow forming of an internal gear is shown in Figure 26. Furthermore the localized contact can cause buckling of the tools if the geometry allows it. This is also shown in Figure 26 for the spin extrusion process. Local tool loadings and instabilities can be minimized by an

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optimized workpiece and tool geometry as well as an improved process control [120 - 123].

Figure 26: (a) Flow forming of internal gears, (b) resulting tool fracture, (c) spin extrusion and resulting buckling of the mandrel, sources: PtU Darmstadt, Fraunhofer IWU. The incremental processing has a strong influence particularly on the workpiece behaviour. One of the observed phenomena is an increased forming limit of the workpiece appearing in crack initiation at higher strain levels. The micro mechanical reason for this behaviour is not known for sure but it is supposed that it is mainly caused by the advantageous hydrostatic stress state of incremental processes. The localized contact of tools and workpiece results in a high superposed pressure state that may increase the forming limit compared to die-defined forming as shown in Figure 27 [124]. Additionally it is important to know the role of cyclic loading on the distribution of dislocations and to understand how strain softening and healing mechanisms can be used. The question of increasing the forming limits with these principles will further be investigated [125].

Figure 27: Increased forming limit of orbital forming compared to conventional upsetting, source: Timken. Due to the high number of forming steps an underfilling is rarely observed if compared to die-defined forming. Nevertheless an insufficient mould filling can occur if the forming forces are acting in the wrong direction or are too low. Examples are ring rolling of profiled rings [126 - 128] or radial axial forming [129 - 132] as given in Figure 28. In most cases this failure can be eliminated by a modified process control. For example a slight displacement of the workpiece during a ring rolling process causes additional forces that are acting in the direction of the profiled rolls. The radial axial forming in Figure 11 can be optimized by a double sided feed motion. If mould filling by the modified control is achieved an overloading of the tools

should be avoided. For this purpose an exact adjustment is necessary [133].

(a) (b) (c)

(a) (b)

Figure 28: (a) Underfilling of rolled rings for tapered roller bearings, (b) insufficient filling during radial axial forming with single sided feed motion, sources: Timken, PtU Darmstadt. Multiple impacts of the tools are able to destroy the surface of the workpiece in some cases. Typical failures are shingling with separation of surface layers or disruption of surface layers like the examples shown in Figure 29 [134]. This is often due to high repetitive shear stresses acting on the surface that can only be eliminated by optimized tribological systems. The observed phenomena are not understood well and require further investigation.

(a) (b) (c)

Figure 29: (a) Shingling on a rolled anchor, (b) surface disruption of a radial axial formed part, (c) destroyed surface of toothings produced by incremental forging, sources: Hilti, PtU Darmstadt. In Figure 30 the risk of instabilities like wrinkling or buckling is recognizable if incremental bulk forming processes are applied [135]. Sometimes this is more of a problem compared to concurring techniques since small defects can intensify themselves from step to step. On the other hand it is also possible that small defects are removed from the workpiece during the production.

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Generally the danger of defects is small due to the low process forces.

Figure 30: (a) Wrinkling of a rolled anchor, (b) buckling of a workpiece during axial forming, (c) material wrinkle in a radial axial formed part, sources: Hilti, PtU Darmstadt. Other defects that can occur in the tools or in the workpiece could be wear or scale formation in the case of hot forming. No methodical difference between incremental and die-defined forming concerning these defects has been identified to date.

4.4 Product properties The properties of products manufactured by incremental bulk forming can roughly be divided into technological, ecological and economical criteria. Focussing on technological features is useful here as these define other dependent characteristics. For instance, a light product with high durability may be resource-saving in many cases and a net-shape product that was incrementally formed may be cost-efficient compared to a machined product with the same geometry [136 - 138]. The technological properties mainly consist of the mechanical features as well as the micro and macro geometry. Mechanical characteristics are for example an increased strength that is caused by strain hardening. This is of course typical for all kinds of forming processes especially in cold forming, but in the case of incremental bulk forming very high local deformation can be reached. In Figure 31 this is shown for an internal gear that was produced by flow forming [139]. The tooth root, which is the area with the highest loading during operation of the gear, features also the highest strain hardening.

Figure 31: Hardness distribution of an internal gear manufactured by flow forming, source: PtU Darmstadt.

Another aspect that is able to improve the strength of incrementally produced parts is the advantageous grain structure. If the direction of the grain structure is visualized by etching as shown in Figure 32, no cut-off in the structure is observable [140, 141]. This is an advantage over machined parts because cuts in the grain structure can cause notch effects that will reduce the durability. If the part is processed by heat treatment subsequent to the forming these effects are changed again.

(a) (b) (c)

(a) (b) (c) (d)

Figure 32: (a) Grain structure orientation for flow splitting, (b) gear rolling, (c) radial axial forming, (d) rotary swaging, source: PtU Darmstadt. It is also possible to improve fatigue life by incremental forming. Residual stresses that remain in the part cause high compression that is superposed on any external loading [142]. The danger of fatigue and crack initiation can be reduced significantly in this way. Often this effect is specifically used by incremental processes like roller burnishing [143]. The workpiece layer near to the surface features a compressive stress state while the core material remains unchanged as shown in Figure 33.

Figure 33: Residual stresses in a cylindrical workpiece after roller burnishing in simulation and experiment [143]. The micro structure of incrementally produced parts is in general of high quality. There are little or no structure defects like cavities, segregation or coarse grains due to the repeated deformation of the material. This is similar to the properties of drop forged parts that are often described as having high quality for this reason [144 - 146]. Another micro structural aspect is the good surface that can be produced especially by cold forming. Today it is possible to realize net-shape parts for some applications that had to be manufactured by machining not long ago.

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In terms of the ISO system it is possible to generate surface tolerances of IT5 with incremental bulk forming. This is required for functional features with high precision like threads or teeth as shown in Figure 34. Several patented incremental bulk forming processes are used for the production of teeth based on different rolling strategies [147 - 152].

Figure 34: (a) Toothed parts produced by flow forming, (b) longitudinal planetary rolling, (c) cross rolling of spur gears with racks, (d) cross rolling of helical gears with racks, sources: PtU Darmstadt, Grob, Ex-Cell-O. The limitations for the designer of incrementally manufactured parts are less restrictive than with concurrent techniques [153 - 155]. As there is no die that represents the whole part geometry, partial shape features are possible. These features can be varied or repeated on the same workpiece without changing the tooling system. Furthermore stepped parts with variable cross sections or hollow structures are realizable [156 - 161]. The dimensions can vary from few millimetres to several meters. Typical drive system components for the automotive industry can be produced in almost the same way as huge structures. The largest parts that are processed by open die forging have a mass up to 100 tons [162]. The biggest rolled rings have a diameter of 8 meters and a height of 1.50 meters. Incremental bulk forming is nearly the only forming process that is used in the field of very large structures, for example for the aerospace, ship or plant building industry. In contrast, applications in the field of micro forming are not known yet.

4.5 Process Combination The combination and integration of incremental bulk metal forming processes offers a high potential for economical and environmental improvement. In general, incremental bulk forming processes are part of larger process chains. Two of numerous possible examples are the manufacturing of complex transmission parts such as clutch housings and metal wheels [164]. By starting with a forged preform the products are formed by flow forming as shown in Figure 35. “Combined Processes” or “hybrid” processes can be defined as: � the integrated combination of usually separated

manufacturing processes (e.g. forming/turning) � the integrated use of various physical mechanisms

(e.g. forming/joining)

� integrated machines which perform various operations in one location

Combination of separated manufacturing processes: Incremental bulk forming and joining A particularly good example for an integrated combination of usually separated manufacturing processes, which can be found in the field of rolling, is the combination of the hot ring rolling process with solid state joining. (a) (b)

(d) (c)

Figure 35: Production of clutch housings and wheels [163]. Kluge [164] described the rolling of a composite work piece made of duplex (inner ring) and structural steel. Starting from an assembled preform composed of two different rings the process result is a composite ring. To avoid high temperature oxidation between the contact zones of the inner and outer ring during heating, it is necessary to seal the upper and lower surface. By using the heat generated in the forming process a metallic compound is established (Figure 36).

Figure 36: Preform and rolled composite ring [165]. Another example of integrated combination of separated manufacturing processes and of integrated use of physical mechanisms is the research project [[166].66]

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The aim of this project at the Ruhr-Universität Bochum is to develop a production process for the manufacture of ring-shaped work pieces with a wear-resistant outerlayer by combination of radial-axial ring rolling withpowder technology (Figure 37).

Figure 37: Process-integrated powder coating [167]. The approach of compacting a metal powder coating while ring rolling offers an alternative to the HIP-process and at the same time overcome several of its restrictions, especially regarding component size, processing time and costs. To achieve this aim, the sequential production steps of compacting, forming and heat treatment are combined and integrated into the ring rolling process [167].

Integrated use of various physical mechanisms: Incremental bulk forming and heat treatment Parts such as air nozzles, rim rings, pressure cans and casings are more and more often manufactured of titanium and nickel-base alloys. But cold metal forming of these materials requires intermediate recrystallisation steps leading to interruptions during the processes. In the course of a research project a laser-assisted spinning technique was developed in which the thermal energy needed for shaping is provided by a laser beam synchronized with the forming process. Thus materials with challenging forming behaviour can be formed without re-clamping [168, 169]. Rotary swaging of hollow shafts as described in section 3 is an example of combined incremental forming and partial heating. The material properties have to be changed in a localized area of the workpiece in order to make the process work.

Machines to perform various operations in one location: Incremental bulk forming and joining/turning/cutting An integrated machine to perform various operations was described in [170]. The paper describes combined axial profile tube rolling (APRW/WE) and in particular the cold rolling process which is useful to manufacture small bearing rings (outer diameter up to 100 mm). This manufacturing procedure allows performing shaping and mechanical processing of all forms and surfaces as well as final cutting in one clamping. Apart from the advantages of cold ring rolling such as saving of materials, increase in life time and surface quality, Ficker et. al. described the process combination as better in terms of material utilisation, reduction in investment cost and reduced space requirements (Figure 38). With the support of Pittler Tornos Werkzeugmachinen GmbH Leipzig a prototype machine was developed and presented at the 1997 EMO-fair.

Economical usage of incremental bulk forming: Assembly of components Incremental forming operations can be utilized for the assembly of individual parts. It is possible to integrate several production steps in this way to realize a more economic manufacturing of technical devices. This was

proven for wheel hubs that are assembled together with bearing parts by orbital forging [172] as shown in Figure 39. The hubs are compact and light while they are less expensive and have fewer components compared to the conventional assembly. Incremental bulk forming is in particular qualified for this task as it requires high precision [173].

Figure 38: Various operations in one location [171].

Figure 39: Precision forming of wheel hubs by orbital forging, source: Timken. 5 SUMMARY Metal working with incremental techniques has been practiced for thousands of years, but mass production caused this craft to fall into oblivion in the 20th century. However, there are indications of a renaissance in this technology. Incremental forming features high flexibility of the machinery by kinematical shaping processes, low costs, reduced forces and the possibility of working with less formable materials. Today incremental bulk metal forming is applied in various industrial manufacturing processes. Because of the high product quality that can be achieved with this technology, it is used especially for high-end applications in the automotive and aerospace industry. Several new processes that have recently been developed show current existing interest in these techniques. But if incremental bulk forming should be

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used on a larger scale again in the near future, there are some challenges to be mastered. Complex kinematicsused on a larger scale again in the near future, there are some challenges to be mastered. Complex kinematics demand new machinery and drive concepts as well as fast and reliable controls. Existing limitations of the processes should be eliminated by an advanced design. Simulation methods that are used for this task have to be improved to allow efficient modeling and calculation of incremental processes. This is also true for the implementation of advanced material and tribological laws. As shown in this paper the production of complex parts by incremental bulk metal forming is advantageous in many cases. With the possibility of achieving net-shape quality, this approach features technological and economically interesting alternatives to other manufacturing processes like drop forging or machining for example. To tap the full potential, the traditional knowledge of the craftsmen has to be converted into new processes and equipment that will be designed for future requirements.

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