NEAR-NET-SHAPE PROCESSES:NEAR-NET-SHAPE MANUFACTURING EXAMPLES IN ADVANCED PROCESS CHAINS

NEAR-NET-SHAPE MANUFACTURING EXAMPLES IN ADVANCED PROCESS CHAINS

Casting: Selected Examples

The main advantage of shaping by casting or powder metallurgy is the realization of near-net-shape production of castings and sintered parts, thereby minimizing cutting processing and drastically short- ening the process chains due to fewer process stages. The process chain is dominated up to the finished part by chipless shaping.

 

1: workpiece; 2: upper tool / die; 3: lower tool; 4: bottom die; 5: plasticizing jaw.

Development in shaping by casting is focused on two directions. First, the components become increasingly closer to the finished parts. Second, many single parts are aggregated to one casting (integral casting). Both directions of development are realized in all variants of casting technology. However, in precision casting, casting by low pressure, and pressure die casting, the material and energy savings to be achieved are especially pronounced.

For evaluation, the manufacturing examples in Figures 4 and 5 were considered starting from melting up to a commensurable part state.

Figure 4 shows a technical drawing of a flat part that had previously been produced by cutting starting from a bar, that is now made as a casting (malleable cast iron) using the sand-molding principle. In cutting from the semifinished material, material is utilized at only 25.5%. As a result of shaping by casting, utilization of material was increased to 40%. The effects of shaping by casting become evident in the energy balance (primary energy). For cutting the flat part from the solid,

GJ / t parts are required. For shaping by casting, 17.462 GJ / t parts are required. Consequently, 64.6% of the energy can be saved. Compared to cutting of semifinished steel material, for part manufacturing about a third as much primary energy is required.

The doorway structure of the Airbus passenger door (PAX door: height about 2100 mm; width about 1200 mm) is illustrated in Figure 5.

In conventional manufacturing of the doorway structure as practiced until now, apart from the standard parts such as rivets, rings, and pegs, 64 milling parts were cut from semifinished aluminum materials with very low utilization of material. Afterwards, those parts were joined by about 500 rivets.

As an alternative technological variant, it is proposed that the doorway structure be made of three cast segments (die casting—low pressure). Assuming almost the same mass, in production from semifinished materials, the ratio of chips amounted to about 63 kg, whereas in casting, it can be reduced to about 0.7 kg. Thus, in casting, the chip ratio amounts to only 1% in comparison to the present manufacturing strategy. In the method starting from the semifinished material, about 175 kg of materials have to be molten, however, in shaping by casting, this value is about 78 kg—that is, 44.6%.

As a result of the energy balance (primary energy), about 34,483 MJ are required for manufac- turing the doorway structure from the semifinished material. However, in shaping by casting, 15,002

MJ are needed—that is about 46%. The result of having drastically diminished the cutting volume due to nns casting can be clearly proven in the energy balance: in the variant starting from the semifinished material, 173 MJ were consumed for cutting; in casting, less than 2 MJ.

Today, the Airbus door constructions that have, in contrast to the studies mentioned above, cast as one part only are tried out. In this variant, 64 parts are aggregated to one casting (integral casting).

Figure 6 illustrates a welding-constructed tool holder consisting of seven steel parts (on the left), for which one consisting of two precision castings has been substituted (steel casting, GS-42CrMo4, on the right). Comprehensive cutting and joining operations can be saved through substitution by investment casting. In the variant where the nns design is based on precision casting, only a little cutting rework is required.

Powder Metallurgy: Manufacturing Examples

Hot Isostatic Pressing (HIP)

Since the 1950s, hot isostatic pressing technology has been developing rapidly in the field of powder metallurgy. Assuming appropriate powders are used, precision parts of minimum allowances can be produced. Applications provide solutions especially in cases of maraging steels, wear steels with high carbon content, titanium and superalloys, and ceramic materials.

As usual in this process, after the production of the appropriate powder, powder is enclosed in an envelope under an almost complete vacuum or inert gas atmosphere. Within this envelope, the powder is compacted under high pressure and temperature (hot isostatic pressing or compaction, see Figure 7). Almost isotropic structural properties are achieved by identical pressure acting from all sides. Thereby, the pressing temperature is partially far below the usual sintering temperature, which results in a fine-grained structure apart from the almost 100% density.

HIP technology can be applied to

• Postcompaction of (tungsten) carbide

• Postcompaction of ceramics

• Powder compaction

• Postcuring of casting structures

• Reuse and heat treatment of gas turbine blades

• Diffusion welding

• Manufacturing of billets from powders of difficult-to-form metals for further processing

Workpieces made using the HIP process are shown in Figure 8. These parts are characterized by high form complexity and dimensional accuracy. With regard to the mechanical properties, parts made using HIP are often much better than conventionally produced components due to their isotropic structure.

Powder Forging

The properties of powder metallurgical workpieces are greatly influenced by their percentage of voids and entrapments (see Figure 9). For enhanced material characteristics, a density of 100% density must be striven for.

The application of powder or sintering forging techniques can provide one solution to achieving this objective. The corresponding process flow variants are given in Figure 10. Regarding the process sequence and the equipment applied, the powder metallurgical background of the process basically conforms to shaping by sintering.

According to their manufacturing principle, the powder-forging techniques have to be assigned to the group of precision-forging methods. Depending on part geometry, either forging starting from the sintering heat or forging with inductive reheating (rapid heating up) is applied. When forging the final shape, an almost 100% density is simultaneously generated in the highly stressed part sections. The part properties resulting from manufacturing by powder forging are equivalent to or frequently even more advantageous than those of usual castings. This becomes evident especially in the dynamic characteristics.

The advantages of powder-forged parts can be summarised as follows:

• Material savings (flashless process)

• Low mass fluctuations between the individual parts

• High accuracy (IT8 to IT11)

• High surface quality

• Low distortion during heat treatment due to the isotropic structure

• High static and dynamic characteristic values

Some typical powder forging parts are shown in Figure 12.

Cold-formed Near-Net-Shape Parts: Examples of Applications

Extrusion

Cylindrical solid and hollow pieces including an axial direction slot of defined width and depth at the face are examples of typical mechanical engineering and car components. Parts like these can be found in diesel engines and pumps (valve tappets), in braking devices (as pushing elements), and for magnetic cores.

The valve tappet for large diesel motors made of case-hardening steel (16MnCr5), illustrated in Figure 13, is usually produced by cutting from the solid material, whereby the material is utilized at a rate of :::40%. A near-net-shape cold-extrusion process was developed to overcome the high part- manufacturing times and low utilization of material and cope with large part quantities.

Starting from soft annealed, polished round steel, the valve tappet was cold extruded following the process chain given in Figure 14. The initial shapes were produced by cold shearing and setting, but also by sawing in the case of greater diameters. Surface treatment (phosphatizing and lubricating) of the initial forms is required for coping with the tribological conditions during forming. Near-net- shape extrusion can be carried out both in two forming steps (forward and backward extrusion) and in a one-step procedure by combined forward / backward extrusion. The number of forming steps depends on the workpiece geometry, particularly the dimensional ratios b / d and D / d.

Modified knee presses up to a nominal force of 6300 kN are employed to extrude the valve tappets. The output capacity is 25–40 parts per minute. As a result of the near-net-shape process, the material is utilized to a higher extent—about 80%. The sizes of slot width and hole diameter are delivered at an accuracy ready for installation, whereas the outer diameter is pressed at IT quality 9–10 ready for grinding. The number of process steps and part manufacturing time were reduced.

Apart from these effects, material cold solidification—caused by the nature of this technology—and continuous fiber orientation resulted in an increase in internal part strength.

Swaging

Swaging is a chipless incremental forming technique characterized by tool segments radially quickly pressing in an opposite direction on the closed-in workpiece. A distinction is made between the feeding method for producing long reduced profiles at relatively flat transition angles and the plunge method for locally reducing the cross-section at steep transition angles (see Figure 15).

A sequence as used in practice, from blank to nns finished part, consisting of six operations is described with the aid of the process chain shown in Figure 16. In the first process step, feed swaging is performed upon a mandrel to realize the required inner contour. The blank is moved in the working direction by oscillating tools. The forming work is executed in the intake taper. The produced cross-

section is calibrated in the following cylinder section. Feeding is carried out when the tools are out of contact. In the second stage of operation, the required internal serration is formed by means of a profiled mandrel. The semifinished material having been turned 180° in the third process step, the opposite workpiece flank is formed upon the mandrel by feed swaging. To further diminish the cross- section in the marked section, plunge swaging is following as the fourth process step. This is done by opening and closing the dies in a controlled way superimposed upon the actual tool stroke. The workpiece taper angles are steeper than in feed swaging. The required forming work is performed in both the taper and the cylinder. In the plunge technique, the size of the forming zone is defined by the tools’ length. A mandrel shaped as a hexagon is used to realize the target geometry at one workpiece face by plunge swaging (fifth step). In the sixth and final process step, an internal thread is shaped.

The obtainable tolerances to guarantee the nns quality at the outer diameter are basically com- mensurate with the results achieved on precise machine tools by cutting. Depending on material, workpiece dimensions, and deformation ratio, the tolerances range from ±0.01 mm to ±0.1 mm. When the inner diameter is formed upon the mandrel, tolerances of <0.03 mm are obtainable.

The surfaces of swaged workpieces are characterized by very low roughness and a high bearing percentage. As a rule, for plunging, the roughness is usually Ra < 0.1/-Lm, whereas in feed swaging, Ra is less than 1.0 /-Lm. During plastic deformation, fiber orientation is maintained rather than inter- rupted as in cutting. Thus, enhanced functionality of the final product is guaranteed.

The material that can be saved and the optimization of weight are additional key issues in this forming procedure. The initial mass necessary for the production of bars and tubes diminishes because the usual cutting procedure is replaced by a procedure that redistributes the material. For an abundance of workpieces, it is even possible to substitute tubes for the bar material, thus reducing the component mass.

In swaging, the material has an almost constant cold work hardening over the entire cross-section. Nevertheless, even at the highest deformation ratios, a residual strain remains that is sufficient for a follow-up forming process.

Swaging rotationally symmetric initial shapes offers the following advantages:

• Short processing time

• High deformation ratios

• Manifold producible shapes

• Material savings

• Favourable fiber orientation

• Smooth surfaces

• Close tolerances

• Environmental friendliness because lubricant film is unnecessary

• Easy automation

Swaging can be applied to almost all metals, even sintered materials, if the corresponding material is sufficiently strained.

Orbital Pressing

Orbital pressing is an incremental forming technique for producing demanding parts that may also have asymmetric geometries. The method is particularly suited to nns processing due to its high achievable dimensional accuracy. The principle design of orbital pressing process chains is illustrated in Figure 17.

An accuracy of about IT 12 is typical for the upper die region. In the lower die, an accuracy up to IT 8 is achievable. Parts are reproducible at an accuracy of ±0.05 mm over their total length. Roughness values of Ra 0.3 /-Lm can be realized at functional surfaces. The accuracies as to size and shape are mainly determined by the preform’s volumetric accuracy, whereas the obtainable rough- ness values depend on die surface and friction.

The advantages of orbital pressing compared to nonincremental forming techniques such as ex- trusion are a reduced forming force (roughly by factor 10) and higher achievable deformation ratios. As a result of reducing forming forces, the requirements for die dimensioning are lowered at dimin- ished tool costs. Orbital pressing can be economically employed for the production of small to medium part quantities. For lower dies, tool life quantities of 6,000 to 40,000 parts are obtainable. The lifetime is about four times higher for upper dies. Concerning the usual machine forces, orbital presses are usually available at 2,000, 4,000, and 6,300 kN. For special applications, manufacturing facilities up to a pressing force of 1,600,000 kN are also available.

Generally, workpieces of up to 5 kg maximum weight, 250 mm diameter, and 220 mm maximum blank height are formed of steel types with low carbon content and nonferrous metals, such as copper and aluminum alloys.

The example in Figure 18 indicates an extremely high-stressed component of a large diesel motor (part weight 1.1 kg; flange diameter 115 mm; flange height 16 mm; total height 55 mm). With the use of the orbital pressing technique, for this workpiece, the process chain—originally consisting of turning, preforging, sandblasting, and cold coining of the profile—could be reduced to only two steps. Furthermore, the cavity could be formed at a greater depth and at higher accuracy, thus yielding significantly higher load capacity, reliability, and component life. In this example, the part costs were reduced by about 60% due to the application of orbital forging.

Semihot-formed Near-Net-Shape Components: Examples of Applications

Semihot Extrusion

In the automotive industry, the functional parts in particular are subjected to high-quality require- ments. The successful use of semihot extrusion is demonstrated, for example, by the manufacture of triple-recess hollow shafts used in homokinematic joints in the drive system. The complex inner contours of the shaft have to be produced to high accuracy and surface quality. Producing these parts by cutting is not economically viable due to the high losses of material and the high investment volume required, so the inner contour is shaped completely (net shape) by forming. In forming technology, the required inner contour tolerance of ±0.03 mm can only by achieved by calibrating a preform of already high accuracy at room temperature. However, the material Cf 53, employed due to the necessary induction hardening of the inner contour, is characterized by poor cold formability. For that reason, shaping has to be performed at increased forming temperature but still with a high demand for accuracy. Shaping can be realized by means of a multistep semihot extrusion procedure carried out within a temperature interval between 820°C and 760°C in only one forming step. Semihot extrusion is performed on an automatic transfer press at the required accuracy, at optimum cost and also avoiding scaling. The component is shaped by semihot extrusion starting out from a cylindrical rod section without thermal and chemical pretreatment. The corresponding steps of operation are reducing the peg, upsetting the head, centering the head, and pressing and compacting the cup (see Figures 19 and 20). The advantages most relevant for calculation of profitability in comparing semihot

extrusion of shafts to hot extrusion are caused by the low material and energy costs as well as postprocessing and investment expenditures.

Semihot Forging

Forging within the semihot forming temperature interval is usually performed as precision forging in a closed die. With closed upper and lower dies, a punch penetrates into the closed die and pushes the material into the still-free cavity. Bevel gears completely formed, including the gear being ready for installation (net shape process), are a typical example of application. Such straight bevel gears (outer diameter 60 mm or 45 mm and module about 3.5 mm) are employed in the differential gears of car driving systems. For tooth thickness, manufacturing tolerances of ±0.05 mm have to be maintained. In forming technology, the required tolerance can only be obtained by calibrating the teeth preformed at high accuracy at room temperature. The preform’s tooth flank thickness is not to exceed about 0.1 mm compared to the final dimension. The shaping requirements resulting from accuracy, shape complexity, and the solid, alloyed case-hardening steel can only be met by semihot forming. Processing starts with a rod section that is preshaped without the teeth in the first stage (see Figure 21). The upset part is of a truncated cone shape. The actual closed die forging of the teeth is carried out as a semihot operation in the second stage. The bottom (thickness about 8 mm) is pierced in the last semihot stage. The cost efficiency effects of the semihot forming process compared to precision forging carried out at hot temperatures are summarized in Table 4.

Today, differential bevel gears of accuracy classes 8 to 10 are still produced by cutting, despite their large quantity. Low-cost forming technologies perspectively represent the better alternative, even though they require relatively high development costs.

Hot-formed Near-Net-Shape Components: Examples of Applications

Precision Forging

Which hot-forming technique to select for a given workpiece from a low-cost manufacturing view- point depends mainly on the part quantity and the extent to which the forming part is similar to the finished shape. In precision forging, where the forming part can be better approximated to the finished

part, the total cost, including tool and material costs as well as costs for the forming and cutting processes, can be decreased despite increasing blank costs.

The accuracy values obtained are above the die forge standards (e.g. in Germany, DIN 7526, forging quality E). As a goal, one process step in finishing by cutting is to be saved (this means either grinding or finishing is necessary). In forging, some dimensions of importance for the near- net-shape have closer tolerances.

The steering swivel shown in Figure 22 is an example of nns manufacturing using precision forging. The fixing points determining the spatial location are fitted to the dimensions of the finished part by additional hot calibrating.

Forged parts (hot forming) that make the subsequent cutting operation unnecessary or whose accuracy need only be enhanced at certain shape elements of the part (IT9 to IT6) by follow-up cold forming can also be produced by precision forging. Thus, formed components of even cold-pressed parts’ accuracy can be made this way. This is especially important if cold forming cannot be realized due to too-high forming forces and tool stresses or too-low formability of the part material. From an economic viewpoint, precision forging is also more advantageous if the surfaces to be generated could otherwise only be produced by relatively expensive cutting techniques.

Hot Extrusion

Field spiders are usually made by die forging with burr and subsequent cold calibrating. High pressing forces are required due to formation of burr. The forged shape is approximated to the final shape only to a minor extent. For that reason, high forces also have to act in cold calibration. A nns manufacturing variant for field spiders consists of combining hot extrusion and cold forming.

The process chain is characterized by the following procedural stages:

1. Shearing a rod section of hot-rolled round steel (unalloyed steel with low carbon content) with a mass tolerance of ±1.5%

2. Heating in an induction heating device (780°C)

3. Hot extrusion on a 6,300-kN knee press performing the following forming stages (see also Figure 23):

• Upsetting

• Cross-extrusion

• Setting of the pressed arms

• Clipping

4. Cold forming (see Figure 24):

• Backward extrusion of the hub

• Forming of erected arms

• Piercing of the hub

• Calibrating.

An automated part-handling and special tool-cooling and lubricating system are installed to keep the forming temperatures and the tribologic conditions exactly according to the requirements for nns processes.

The advantages of hot extrusion / cold calibrating vs. die forging with burr / cold calibrating of field spiders can be summarized as follows:

In calibrating the hot extruded part, forming is required to a lower extent due to better approxi- mation to the final shape. This excludes the danger of cracks in the field spider.

Axial Die Rolling

In axial forging–die rolling, the advantages of die forging and hot rolling are linked. Disk-like parts with or without an internal hole can be manufactured. On the component, surfaces (e.g., clamping surfaces) can be shaped in nns quality, thus enabling later part finishing by cutting in only one clamping. Accuracy values of IT9 to IT11 are obtained because the axial die rolling machines are very stiff and the forming forces relatively low. Furthermore, the desired component shape can be approximated much better because no draft angles are necessary and small radii or sharp-edged contours can be formed. Within the workpiece, a favorable fiber orientation, resulting in reduced distortion after heat treatment, is achieved by burrless forming. The advantages of the nns hot-forming technology using axial die rolling vs. conventional die forging can be shown by a comparison of manufacturing costs of bevel gears for a car gear unit (see Figure 25). Cost reductions up to 22% as well as mass savings up to 35% are achieved by the nns technology.

Special Technologies: Manufacturing Examples

Thixoforging

Thixoforming processes make use of the favorable thixotropic material behavior in a semiliquid / semi solid state between the solid and liquid temperatures. Within this range, material cohesion, which makes the metal still able to be handled as a solid body and inserted into a die mold, is kept at a liquid rate of 40–60%. This suspension is liquefied under shear stresses during pressing into a die mold. Due to the suspension’s high flowability, complex geometries can be filled at very low forces, which represents the main benefit of this technique because part shapes of intricate geometry (thin-walled components) can be produced at low forming forces.

The process stages between thixocasting and thixoforging are different only in their last stage of operation (see Figure 26). With an appropriately produced feedstock material, the strands are cut into billets, heated up quickly, and formed inside a forging or casting tool.

A component made of an aluminum wrought alloy by thixoforging is illustrated in Figure 27. The forging has a very complex geometry but it was reproducible very accurately. The thick-walled component regions as well as parts with essential differences in wall thickness are characterized by a homogeneous structure free of voids.

Industrial experience shows that components with filigree and complex part geometries, unfavor- able mass distribution, undercuts, and cross-holes can be produced in one stage of operation in a near-net-shape manner by means of thixoprocesses.

Comparison of thixoforming to die casting shows the following results:

• 20% improvement in cycle times

• About 20% increase in tool life

• Raw material costs still up to 20% higher

The thixoprocesses are regarded as an interesting option for both the forging industries and foun- dries. With them near-net-shape technology is added to conventional procedures.

Near-Net-Shape Processes for Prototyping

The ability to create prototypes of properties conforming totally with the series properties quickly at low cost has become increasingly important because the engineering lead time dictated by the market has drastically diminished. Apart from the classical methods of generating prototypes, such as cutting and casting, generic manufacturing techniques are increasingly being used. In cutting or casting, tools, dies, or fixtures are required for fabricating the prototype, whereas prototyping by means of generic techniques is based on joining incremental solid elements. Thus, complex components can immediately be made from computerized data, such as from a CAD file, without comprehensive rework being necessary.

In contrast to cutting technologies, where the material is removed, in generic manufacturing techniques (rapid prototyping), the material is supplied layer by layer or is transformed from a liquid or powder into a solid state (state transition) by means of a laser beam. The most essential rapid prototyping techniques (also called solid freeform manufacturing or desktop manufacturing) are:

• Stereolithography (STL)

• Fused deposition modeling (FDM)

• Laminated object manufacturing (LOM)

• Selective laser sintering (SLS)

These techniques are able to process both metals and non metals. As a result of intensive research, disadvantages of these techniques, such as high fabrication time, insufficient accuracy, and high cost, have been widely eliminated. Today, maximum part dimension is limited to less than 1000 mm.

Near-net-shape processes for prototyping and small-batch production by means of rapid proto- typing techniques can be characterised by the process chains listed in Figure 28.

In principle, prototypes or patterns that in turn enable the manufacture of prototypes and small batches in combination with casting processes can be produced directly by means of rapid prototyping techniques. Rapid prototyping can also be applied to manufacturing primary shaping and forming tools to be used for the fabrication of prototypes.