ALUSIL®-Cylinder Blocks for the new V6 and V8 SI Engines 2 ALUSIL®-cylinder blocks

V8 cylinder block V6 cylinder block (fuel stratified injection [FSI])

Fig. 1: ALUSIL® cylinder block of the new Audi V6 and V8 SI engine generation:

V8 cylinder block cylinder block of the (multi-point injection) existing Audi

1. Audi setting standards in lightweight design

With its vehicle model series, Audi is setting new stand- ards in lightweight design. So it is only logical for the carmaker again to count on in developing the cylinder blocks for its new spark-ignition V-engines (Fig. 1). On account of the reduced wall thickness (cylinder land width of only 5.5 mm (Fig. 2) between cylinder bores), Audi relies on the hypereu- tectic aluminium-silicon AlSi17Cu4Mg registered for KS ATAG under the brand name of ALUSIL®. This concept al- 5.5 mm lows the pistons to move directly along the honed cylinder bore surfaces of the aluminium casting – an ideal condition for high-performing engines.

Fig. 2: Cylinder head flange surface with a land width of only 5.5 mm

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2. Reasons in favor of ALUSIL® from the viewpoint of The above described assets of the ALUSIL® alloy are cer- Audi: tainly significant arguments in favor of the use of this materi- al. But also the low-pressure die casting method (Fig. 5) which There are many technical reasons which induced the car- has since then proven to be the best by far is an important maker Audi to adhere to the proven ALUSIL® concept: prerequisite for process reliability in making mass-produced cylinder block castings of ALUSIL® . • ALUSIL® makes a minimum weight at a high degree of in- tegration of engine functions like lubricant, coolant and engine venting circuits.

• ALUSIL® enables minimum overall cylinder block lengths to be implemented because the engines can be operated without inserted cylinder liners. To achieve the shortest possible engine length at a specified swept volume and stroke, the cylinder bore diameter should be kept as large as possible, at minimum land width. In such cases, the land width of the jointly cast cylinders is determined in practice by the safe and reliable performance of the cylin- der head gasket, by the cutting pressure applied for ma- chining and the cylinder distortion in engine operation.

• ALUSIL® boasts excellent tribological characteristics. As the pistons and piston rings slide along the exposed sil- icon crystals, their susceptibility to seizing is minimized Fig. 3: Material structure of the alloy AlSi17Cu4Mg / ALUSIL® with (Fig. 3). primarily precipitated silicon crystals (dark areas in the picture)

• ALUSIL® displays optimum thermal conductivity; Audi is thus in a position to achieve a high specific engine per- formance.

• ALUSIL® does not imply any recycling problems because the cylinder block does not contain extraneous materials – such as cast-in cylinder liners made of grey cast iron.

• ALUSIL® allows cylinder blocks to be cast monolithically without cylinder liners or subsequent coating of the cylin- der bores. This makes it possible to achieve:

- components of optimum structural rigidity through the benefit of a by 12% higher Young’s modulus of the ALUSIL® alloy compared to a hypoeutectic standard alloy,

- as well as process reliability in machining without hav- Fig. 4: Mechanically exposed ALUSIL® cylinder bore surface ing to interrupt the process flow for costly additional working steps specifically for the cylinder bore surfaces. A decisive milestone in this area was the mechanical ex- posure of the silicon crystals by means of a third honing step (Fig. 4) as a substitute for the chemical laying bare (etching) after the two-step honing process which was a must before. Laying bare the silicon grains by mechani- cal means allows perfect on-line production.

KS Aluminium-Technologie AG 4 ALUSIL®-cylinder blocks

3. Reasons in favor of low-pressure die casting from crystals in the cylinder area (criteria: distribution, grain the viewpoint of Audi: size range (Fig. 8) and number of crystals per cm²), low porosity and minimizing casting flaws like blowholes, • Low-pressure die casting, LPDC (Figs. 5 + 6) allows the use pores, cold fusion, etc. This process control ensures con- sand cores where necessary, e.g. for water jackets (Fig. 7). stant quality of the castings. In this way, structurally rigid closed-deck cylinder blocks can be produced, a prerequisite for high specific engine • LPDC permits unrestricted heat treatment of the casting. outputs. It is already possible to achieve a certain increase in hard- ness and strength by applying controlled cooling of the • LPDC allows controlled, low-turbulence die filling, and cylinder blocks from the casting temperature by means what is even more important, controlled cooling of the die of customary T5 heat treatment. The subsequent artificial to ensure component-specific, virtually ideal directional aging not only serves to enhance hardness and strength, solidification. The compilation of all casting-relevant data but primarily also to stabilize the volume, i.e. avoid an and the resulting control of the casting process are imple- irreversible expansion in length and volume (distortion) mented today computer-assisted. Especially a purpose- referred to as “growth” when the casting is exposed to the designed cylinder sleeve cooling system is an indispen- operating temperature of the engine. sable prerequisite for uniform precipitation of the silicon For still higher thermal stresses in engine operation, a modified T5 heat treatment method is available by which the components of casting temperature are chilled locally, for example in the cylinder deck or bearing bulkhead areas by means of water showers, and their hardness and strength are sleeve yoke with raised through precipitation hardening. The heat treatment is movable die half additionally utilized for stabilizing the alloy. hydraulic cylinder sleeve lifters

cylinder sleevers open die casting

stationary die half hydraulic lateral shifters

riser

holding furnace with melt

Fig. 6: Design of a low-pressure die for aluminium cylinder block lifting table, stroke about 2m

Fig. 5: Low-pressure die casting, illustrated principle of a casting cell with opened die (movable die half in lifted position)

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Fig. 7: View of a water-jacket core box in the core shooter: water-jacket sand cores for Audi V6 cylinder block (left) and V8 cylinder block (right)

For absolute high-performance engines, Audi will in the the engine, the artificial aging time is frequently even extend- future apply full heat treatment (T6) comprising homogeniz- ed. This extension of the aging time improves the expansion ing, chilling and artificial aging. This method contributes to a characteristics. further distinct improvement in terms of static and dynamic strength. As “mere” T6 heat treatment without specific artifi- cial aging only leads to a low degree of volume stabilization and involves the risk of distortion at operating temperature of

25.0

20.0

15.0

10.0

5.0

100.2 micron 0 10 20 30 40 50 60 70 80 Number of grains in the area measured in the area grains Number of

Grain size [µm] Area measured

Fig. 8: Representative grain distribution of the primarily precipitated silicon crystals

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4. Audi’s new V-engine generation: 5. The Audi V6 and V8 engine concept:

The new V-engine generation of Audi – both petrol and The new V6 in the large displacement version of 3.2 l and diesel engines – is setting standards with respect to compact- the new V8 with a swept volume of 4.2 l originate from the ness and overall length. Customers’ requests for more power- new Audi V-engine family with a stroke of 92.8 mm (Fig. 9), a ful engines even in smaller vehicle model series implied the V angle of the cylinder banks of 90° and a central division of need for shorter overall engine lengths and for reducing the the cylinder block (bedplate concept). The distance between vehicle front-part weight. As a result a new V-engine genera- cylinders is 90 mm, the cylinder bank offset being 18.5 mm. tion has been created. For the V6 of 3.2 l as well as the V8 of 4.2 l swept volume, the cylinder bore is 84.5 mm. For the smaller V6 engine of 2.4 l Audi is the market leader in lightweight design, specifi- swept volume, it is 81 mm. The cylinders are cast jointly at cally in the premium segment. The lightweight strategy is a land width of 5.5 mm and of 9 mm, respectively for the V6 impressively implemented in the car body through the Audi with the smaller swept volume. space frame aluminium technology. It was a logical conse- quence therefore to rely on a weight-optimizing all-aluminium The V6 of the large swept volume version operates with solution for the petrol V engines – i.e. ALUSIL® cylinder blocks. fuel stratified injection (FSI), and in the small version, with Thanks to its high competence as the European market leader multi-point injection (MPI). The different cylinder bore diam- KS ATAG was able to convince the market with ALUSIL® for cyl- eters do not require an adjustment on the water-jacket side. inder blocks of passenger-car SI engines in the respective mar- The V8 SI engines are available in three versions. To the ex- ket segment. Attractive contracts from Audi are contributing to isting two engine versions with MPI, a new version with FSI the further strengthening of KS ATAG’s location in Neckarsulm was added which also distinguishes itself by its cylinder and its market leadership. block design.

Fig. 9 : The new Audi V6 (left) and V8 (right) SI engines

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6. Description of the cylinder block top:

As mentioned above, the Audi cylinder blocks (Fig. 10) are made of the hypereutectic alloy AlSi17Cu4Mg. The low- pressure die casting method is applied, with controlled cool- ing from the casting temperature. Cooling takes place at ambi- ent air or its partly assisted by an air blower. This is followed by artificial aging for volume stabilizing (T5 heat treatment), a process which only implies a slight decrease in hardness. The associated bedplates – not included in the scope of supply of KS ATAG – are high-pressure die castings made from the hypoeutectic alloy AlSi9Cu3 with cast-in bearing bulkheads of spheroidal cast iron in the case of the V6. In the new V8, high-pressure die castings from alloy AlSi12Cu1(Fe) are em- ployed for the bearing brackets. The cylinder block of the V8 FSI engine was submitted to further structural optimization for the purpose of boosting performance output; this is recogniz- able from the outside on the basis of the cross members in the V space between the cylinder banks. Further modifications relate to the oil filter flange area.

The water jackets of the cylinder banks produced by using sand cores as well as the water supply channels are integrated on the left and right sides in the cylinder blocks. In the case of the V6, in addition the part of the water pump housing on Fig. 10: Aluminium cylinder block concept, consisting of monolithic the discharge-nozzle side is arranged at the front, above the V cylinder block top and bedplate with cast-in bearing bulkheads made space but connected with it. Water supply to the jackets of the of spheroidal casting two cylinder banks is accomplished by means of a manifold, flange-mounted in the V space. Further machining steps in the first and second clamping The oil circuit ducts are partly pre-cast, partly drilled. The operations are the drilling of the oil ducts, pre-drilling of the two deep-hole bores centrally arranged in the V, for the main cylinders, pre-milling of the main bearing area and cubing of oil and injection nozzle channels for piston cooling, which vir- the external faces. In this last step, all “important” surfaces as tually cover the whole block length, are pre-cast for the V8 in re-milled in order to remove casting fins, for instance in order the same way as the non-pressurized oil drain backs and cyl- to enable a subsequent leak test to be carried out. inder block vent ducts. They are arranged externally on the side walls, in parallel with the cylinders. These channels are For warranting absolute casting quality, every cylinder connected with the bedplate through bores in order to ex- block is submitted to an x-ray, hardness and ultrasonic test. clude the risk of casting fins being left. The latter serves to check the bearing bulkheads for porosity. The oil ducts and water jackets are submitted to a leak test It goes without saying that after removing the sand cores applying the differential-pressure method as well as a 100 % the cylinder blocks have to be submitted to first-cut machin- visual inspection before the cylinder blocks are delivered to ing (Fig. 11). Audi.

Adjustment in the crankcase area is achieved by align- ment in the first clamping step, starting from cast locating points at the interface to the bedplate. To this effect, the three locating points on the gearbox side are machined and two index bores (fitting bores) are set.

KS Aluminium-Technologie AG 8 ALUSIL®-cylinder blocks

Fig. 11: Preparation of the Audi V6 cylinder block 3 Top: 1st clamping with machining 1 of the locating points (1), the index bore (2) and the pres- surized-oil ducts (3)) 3 3 1 Bottom: 1 2nd clamping with pre-drilling 2 of the cylinders (a), pre-milling 2 of the main bearing area (b), and cubing of the remaining external surfaces b a a b a a b a a

b

At AUDI HUNGARIA MOTOR Kft. (AHM), the cylinder blocks deposited iron or a plastic layer applied to the piston skirt by are integrated into the machining line on the locating points screen-printing. The piston rings also need a specific adjust- and aligned on gearbox flange level by means of the fitting ment to the ALUSIL® bore surfaces, although nowadays there bores provided in the gearbox flange. Next, the cylinder block are not necessarily any significant differences with respect to is machined as an individual part, with most of the machining the piston ring protection usually applied in the case of grey work being done in these working steps. Subsequently, the casting cylinders. finished bedplate is “matched” with the cylinder block, i.e. pinned and bolted. From this moment, the cylinder block and ALUSIL® features sufficiently high strength so that even the bedplate constitute one unit and move jointly through the for the high-strength bolt joints like those of main bearing assembly line. An important machining step is the mechani- cap and cylinder head the nut thread can be cut directly into cal exposure of the silicon crystals in a third honing step with the cast material. When “overtightening” the bolt, it will in- “soft” hones which means hones with cutting material em- variably tear off before the nut threads are shorn. The high bedded in a non-rigid matrix. In the process, the hones virtu- compressive strength of ALUSIL® has an extremely positive ally “erase” any adhering aluminium from the silicon crystals effect on the bolted joints. It exceeds the tensile strength by (50 % is already removed in the second honing step when nearly 100%, depending on the heat treatment condition. As eliminating the silicon crystals destroyed on the surface) a result, commercial bolt heads hardly lead to an appreciable and, this is decisive, the aluminium matrix is thus recessed setting of the aluminium even if the bolt was tightened up to to a certain extent. Given the hardness of the silicon crystals, the tensile yield point. the pistons require reinforcement composed of galvanically

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7. Description of the casting die and development / adjustment of the casting process

Audi‘s intention to achieve a synergy effect between V6 achievement was preceded by a series of parallel activities in and V8 cylinder blocks has been fully implemented with the the sense of consistently practiced simultaneous engineering die design, the concept of the machining installations and between Audi, KS ATAG and the die manufacturer by exploit- testing equipment as well as, in particular, the development / ing all possibilities in terms of virtual product development in adjustment of the casting process. Never before had KS ATAG an uninterrupted CAD, CAE, CAM sequence (Fig. 12): succeeded in supplying V8 cylinder blocks to the Audi plants in such a record time, quasi in fast motion from contract award by Audi through to the production and delivery of the dies and the prompt launching of the casting process. This

Fig. 12: Acceleration of develop- Conventional product development is largely serial ment phases through simultane- ous engineering: utilization of all available resources on the way to virtual product development abt. 18 months

die correction

process development

die manufacture

simulation

sand casting prototypes

product development

1. Dataset serial 0

The challenge is: virtual development!

abt. 6 months

die correction

process development

die manufacture

simulation

product development

1. Dataset serial 0

KS Aluminium-Technologie AG 10 ALUSIL®-cylinder blocks

The following prerequisites, among others, had to be fulfilled to achieve this:

• Installation of an experienced and assertive project man- • Activation of the full casting technique competence of KS agement as well as an interdisciplinary project team en- ATAG starting with the melting operation, via melt prepara- compassing all necessary functions. tion in the holding furnace, the filling of the die adjusted to the component geometry, i.e. the cross-sectional de- • Design engineering optimization based on the results of velopment in the horizontal section, through to controlled previously completed die filling and solidification simula- die cooling. In all those cases, efforts were concentrated tions (Fig. 13). These indicated in an early stage any filling on local heat dissipation through the steel sleeves for the inadequacies or unfavourable timing of the die filling pro- cylinders and the die bottom (bearing bulkhead area) and cess (e.g. local surging of the melt, Fig. 14) as well as iso- its correct timing. lated residual solidification zones making re-feeding im- possible. • Timely preparation and checking of the clamping fixtures for pre-machining; installation of the pre-machining sys- • Knowledge of casting problems previously occurring with tem and necessary inspections of the mechanical work the V6 cylinder blocks, which are basically of the same including availability of suitable labour and its specific geometric concept, and consequently provision of suit- qualification for the new product family. able remedies as a preventive measure.

• Exploiting the full know how available at KS ATAG and at 8. Summary, conclusion and outlook the die manufacturer; application of the latest experiences concerning design and engineering of the casting dies; For its state-of-the-art high-performance SI engines of V avoidance of time losses as a result of adjustment difficul- design, Audi stakes on the combination of ALUSIL® low-pres- ties to the casting support; on-schedule preparation of the sure die casting and mechanical silicon grain exposure. In the casting cell including all peripheral installations; program- present situation, this combination offers optimum conditions ming of the handling robot, etc. for the engine functions, production and process reliability as well as quality. Hence there are quite a number of factors speaking in favor of ALUSIL® to be rated as the best material available today for high-performance spark-ignition V engines

Critical residual solidification zone

Fig. 13: Solidification simulation with the example of the V6 cylinder block Top: Residual solidification zone (risk of blowholes) in the gearbox flange area without active cooling measures Bottom: Optimized condition; avoidance of blowholes (no residual solidification zones) through selective active cooling measures

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1a 2a 3a

1b 2b 3b

Fig. 14: Die filling simulation of the gearbox flange with the example of the V8 cylinder block Top (pictures 1a-3a): turbulent filling (surging melt due to abrupt cross-sectional change Bottom (pictures 1b-3b): stabilized filling by adjusting the geometry and optimum filling parameters

when weighing the many convincing positive properties up Other casting methods are being taken into consideration, against occasionally adduced less favourable characteristics again and again. Whereas for in-line cylinder blocks it may be like low ductility and the somewhat higher machining costs. conceivable also to apply gravity die casting implying slightly What is a decisive aspect is the potential it implies for further lower production costs, this still appears to be fairly problem- strength improvements through full hardening (T6) the entire atic for V cylinder blocks in the present situation. Process-reli- component or local, so-called modified T5 heat treatment. able application of high-pressure die cast ALUSIL® has so far failed because of problems with silicon pre-precipitation and For decades, KS ATAG has focused on the continuous hot cracking. Nonetheless there is hope for further progress optimization of the ALUSIL® concept. The result of these ef- to be achieved. Active cylinder sleeve cooling, a necessity for forts also involves the associated progress achieved in the process-reliable production according to the present state of low-pressure die casting method which is firmly integrated the art, is a constraint to sand core casting methods which with this concept today. There are numerous suggestions for only allow the more elaborate passive cooling due to the prin- a further reduction of cycle times (cost curbing), but on the ciple on which they are based. other hand, the physical laws cannot be disregarded. Dissi- pating more local heat per time unit can only be accomplished KS Aluminium-Technologie AG would like to thank AUDI AG partly without sensibly disturbing the course of the solidifica- for the opportunity of a joint presentation of the ALUSIL® con- tion fronts, e.g. in the cylinder-block skirt area through shrink- cept with the example of the new V6 / V8 engine generation. fit cooling at the bearing bulkhead level. Another approach We also convey our thanks to the outstandingly committed would be thin-wall dies to achieve a distinct reduction in the staff of AUDI who have actively contributed to the component “thermal inertia” of the die. However, this would increase the development and thus made the common success possible. monitoring requirements in those areas which cannot yet be We would also express our special thanks to Dr. Franz Bäumel reliably controlled for the time being. All in all, improvements (V6) and Mr. Armin Bauder (V8) on behalf of all the others who will only be reached in small steps, however in the right direc- have made best efforts to keep our enthusiasm going. We also tion, including the reduction of rejects in fully operational owe a debt of gratitude to Mr. Armin Bauder for his valuable condition. contribution to this article.

KS Aluminium-Technologie AG www.kolbenschmidt-pierburg.com

KS Aluminium-Technologie AG Hafenstrasse 25 74172 Neckarsulm, Germany Phone: +49 (0) 71 32 33-1 Subject to alterations. Printed in Germany. A|IX|e Fax: +49 (0) 71 32 33-43 57