alusil audi

ALUSIL®-Cylinder Blocksfor the new AUDI V6 and V8 SI Engines

KS Aluminium-Technologie AG

ALUSIL®-cylinder blocks

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 aluminium 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 alloy AlSi17Cu4Mg registered for KS ATAG under the brand name of ALUSIL®. This concept al-lows the pistons to move directly along the honed cylinder bore surfaces of the aluminium casting – an ideal condition for high-performing engines.

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

V6 cylinder blockV8 cylinder block

(fuel stratified injection [FSI])

V8 cylinder block (multi-point injection)

cylinder block of the existing Audi V8 engine

5.5 mm

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

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Fig. 3: Material structure of the alloy AlSi17Cu4Mg / ALUSIL® with primarily precipitated silicon crystals (dark areas in the picture)

2. Reasons in favor of ALUSIL® from the viewpoint of Audi:

There are many technical reasons which induced the car-maker Audi to adhere to the proven ALUSIL® concept:

• 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 cylinder 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).

• 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-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.

The above described assets of the ALUSIL® alloy are cer-tainly significant arguments in favor of the use of this materi-al. But also the low-pressure die casting method (Fig. 5) which has since then proven to be the best by far is an important prerequisite for process reliability in making mass-produced cylinder block castings of ALUSIL® .

Fig. 4: Mechanically exposed ALUSIL® cylinder bore surface

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

sleeve yoke with movable die half

open die

stationary die half

hydraulic cylinder sleeve lifters

cylinder sleevers

casting

hydraulic lateral shifters

riser

lifting table, stroke about 2m

holding furnace with melt

3. Reasons in favor of low-pressure die casting from the viewpoint of Audi:

• Low-pressure die casting, LPDC (Figs. 5 + 6) allows the use sand cores where necessary, e.g. for water jackets (Fig. 7). In this way, structurally rigid closed-deck cylinder blocks can be produced, a prerequisite for high specific engine outputs.

• LPDC allows controlled, low-turbulence die filling, and what is even more important, controlled cooling of the die to ensure component-specific, virtually ideal directional solidification. The compilation of all casting-relevant data and the resulting control of the casting process are implemented today computer-assisted. Especially a purpose-designed cylinder sleeve cooling system is an indispen-sable prerequisite for uniform precipitation of the silicon

Fig. 6: Design of a low-pressure die for aluminium cylinder block

crystals in the cylinder area (criteria: distribution, grain size range (Fig. 8) and number of crystals per cm²), low porosity and minimizing casting flaws like blowholes, pores, cold fusion, etc. This process control ensures con-stant quality of the castings.

• LPDC permits unrestricted heat treatment of the casting. It is already possible to achieve a certain increase in hard-ness and strength by applying controlled cooling of the cylinder blocks from the casting temperature by means of customary T5 heat treatment. The subsequent artificial aging not only serves to enhance hardness and strength, but primarily also to stabilize the volume, i.e. avoid an irreversible expansion in length and volume (distortion) referred to as “growth” when the casting is exposed to the operating temperature of the engine.

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 raised through precipitation hardening. The heat treatment is additionally utilized for stabilizing the alloy.

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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 future apply full heat treatment (T6) comprising homogeniz-ing, chilling and artificial aging. This method contributes to a 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

the engine, the artificial aging time is frequently even extend-ed. This extension of the aging time improves the expansion characteristics.

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

25.0

20.0

15.0

10.0

5.0

0

Num

ber o

f gra

ins

in th

e ar

ea m

easu

red

10 20 30 40 50 60 70 80

Grain size [µm]

100.2 micron

Area measured

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4. Audi’s new V-engine generation:

The new V-engine generation of Audi – both petrol and diesel engines – is setting standards with respect to compact-ness and overall length. Customers’ requests for more power-ful engines even in smaller vehicle model series implied the need for shorter overall engine lengths and for reducing the vehicle front-part weight. As a result a new V-engine genera-tion has been created.

Audi is the market leader in lightweight design, specifi-cally in the premium segment. The lightweight strategy is impressively implemented in the car body through the Audi space frame aluminium technology. It was a logical conse-quence therefore to rely on a weight-optimizing all-aluminium solution for the petrol V engines – i.e. ALUSIL® cylinder blocks. Thanks to its high competence as the European market leader KS ATAG was able to convince the market with ALUSIL® for cyl-inder blocks of passenger-car SI engines in the respective mar-ket segment. Attractive contracts from Audi are contributing to the further strengthening of KS ATAG’s location in Neckarsulm and its market leadership.


5. The Audi V6 and V8 engine concept:

The new V6 in the large displacement version of 3.2 l and the new V8 with a swept volume of 4.2 l originate from the new Audi V-engine family with a stroke of 92.8 mm (Fig. 9), a V angle of the cylinder banks of 90° and a central division of the cylinder block (bedplate concept). The distance between cylinders is 90 mm, the cylinder bank offset being 18.5 mm. 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 swept volume, it is 81 mm. The cylinders are cast jointly at a land width of 5.5 mm and of 9 mm, respectively for the V6 with the smaller swept volume.

The V6 of the large swept volume version operates with fuel stratified injection (FSI), and in the small version, with multi-point injection (MPI). The different cylinder bore diam-eters do not require an adjustment on the water-jacket side. The V8 SI engines are available in three versions. To the existing two engine versions with MPI, a new version with FSI was added which also distinguishes itself by its cylinder block design.

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

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Fig. 10: Aluminium cylinder block concept, consisting of monolithic cylinder block top and bedplate with cast-in bearing bulkheads made of spheroidal casting

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 the discharge-nozzle side is arranged at the front, above the V space but connected with it. Water supply to the jackets of the two cylinder banks is accomplished by means of a manifold, flange-mounted in the V space.

The oil circuit ducts are partly pre-cast, partly drilled. The two deep-hole bores centrally arranged in the V, for the main oil and injection nozzle channels for piston cooling, which virtually cover the whole block length, are pre-cast for the V8 in the same way as the non-pressurized oil drain backs and cylinder block vent ducts. They are arranged externally on the side walls, in parallel with the cylinders. These channels are connected with the bedplate through bores in order to ex-clude the risk of casting fins being left.

It goes without saying that after removing the sand cores the cylinder blocks have to be submitted to first-cut machin-ing (Fig. 11).

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.

Further machining steps in the first and second clamping operations are the drilling of the oil ducts, pre-drilling of the cylinders, pre-milling of the main bearing area and cubing of the external faces. In this last step, all “important” surfaces as re-milled in order to remove casting fins, for instance in order to enable a subsequent leak test to be carried out.

For warranting absolute casting quality, every cylinder block is submitted to an x-ray, hardness and ultrasonic test. The latter serves to check the bearing bulkheads for porosity. The oil ducts and water jackets are submitted to a leak test applying the differential-pressure method as well as a 100 % visual inspection before the cylinder blocks are delivered to Audi.

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Fig. 11: Preparation of the Audi V6 cylinder block

Top: 1st clamping with machining of the locating points (1), the index bore (2) and the pres-surized-oil ducts (3))

Bottom: 2nd clamping with pre-drilling of the cylinders (a), pre-milling of the main bearing area (b), and cubing of the remaining external surfaces

1

3

1

2

3

2

1

a

a

a

a

a

a

b

b

b

b

3

At AUDI HUNGARIA MOTOR Kft. (AHM), the cylinder blocks are integrated into the machining line on the locating points and aligned on gearbox flange level by means of the fitting bores provided in the gearbox flange. Next, the cylinder block is machined as an individual part, with most of the machining work being done in these working steps. Subsequently, the finished bedplate is “matched” with the cylinder block, i.e. pinned and bolted. From this moment, the cylinder block and the bedplate constitute one unit and move jointly through the assembly line. An important machining step is the mechani-cal exposure of the silicon crystals in a third honing step with “soft” hones which means hones with cutting material em-bedded in a non-rigid matrix. In the process, the hones virtu-ally “erase” any adhering aluminium from the silicon crystals (50 % is already removed in the second honing step when eliminating the silicon crystals destroyed on the surface) and, this is decisive, the aluminium matrix is thus recessed to a certain extent. Given the hardness of the silicon crystals, the pistons require reinforcement composed of galvanically

deposited iron or a plastic layer applied to the piston skirt by screen-printing. The piston rings also need a specific adjust-ment to the ALUSIL® bore surfaces, although nowadays there are not necessarily any significant differences with respect to the piston ring protection usually applied in the case of grey casting cylinders.

ALUSIL® features sufficiently high strength so that even for the high-strength bolt joints like those of main bearing cap and cylinder head the nut thread can be cut directly into the cast material. When “overtightening” the bolt, it will in-variably tear off before the nut threads are shorn. The high compressive strength of ALUSIL® has an extremely positive effect on the bolted joints. It exceeds the tensile strength by nearly 100%, depending on the heat treatment condition. As a result, commercial bolt heads hardly lead to an appreciable setting of the aluminium even if the bolt was tightened up to the tensile yield point.

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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 and V8 cylinder blocks has been fully implemented with the die design, the concept of the machining installations and testing equipment as well as, in particular, the development / adjustment of the casting process. Never before had KS ATAG 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

achievement was preceded by a series of parallel activities in the sense of consistently practiced simultaneous engineering between Audi, KS ATAG and the die manufacturer by exploit-ing all possibilities in terms of virtual product development in an uninterrupted CAD, CAE, CAM sequence (Fig. 12):

Fig. 12: Acceleration of develop-ment phases through simultane-ous engineering: utilization of all

available resources on the way to virtual product development

Conventional product development is largely serial

1. Dataset serial 0

abt. 18 months

product development

sand casting prototypes

simulation

die manufacture

process development

die correction

The challenge is: virtual development!

1. Dataset serial 0

abt. 6 months

product development

simulation

die manufacture

process development

die correction

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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

Critical residual solidification zone

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

• Installation of an experienced and assertive project man- agement as well as an interdisciplinary project team en-compassing all necessary functions.

• Design engineering optimization based on the results of previously completed die filling and solidification simula-tions (Fig. 13). These indicated in an early stage any filling inadequacies or unfavourable timing of the die filling process (e.g. local surging of the melt, Fig. 14) as well as iso- lated residual solidification zones making re-feeding im-possible.

• Knowledge of casting problems previously occurring with the V6 cylinder blocks, which are basically of the same geometric concept, and consequently provision of suit-able remedies as a preventive measure.

• Exploiting the full know how available at KS ATAG and at the die manufacturer; application of the latest experiences concerning design and engineering of the casting dies; avoidance of time losses as a result of adjustment difficul-ties to the casting support; on-schedule preparation of the casting cell including all peripheral installations; program-ming of the handling robot, etc.

• Activation of the full casting technique competence of KS ATAG starting with the melting operation, via melt prepara-tion in the holding furnace, the filling of the die adjusted to the component geometry, i.e. the cross-sectional de-velopment in the horizontal section, through to controlled die cooling. In all those cases, efforts were concentrated on local heat dissipation through the steel sleeves for the cylinders and the die bottom (bearing bulkhead area) and its correct timing.

• Timely preparation and checking of the clamping fixtures for pre-machining; installation of the pre-machining sys-tem and necessary inspections of the mechanical work including availability of suitable labour and its specific qualification for the new product family.

8. Summary, conclusion and outlook

For its state-of-the-art high-performance SI engines of V design, Audi stakes on the combination of ALUSIL® low-pres-sure die casting and mechanical silicon grain exposure. In the present situation, this combination offers optimum conditions 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

ALUSIL®-cylinder blocks10


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

1a

1b

2a

2b

3a

3b

when weighing the many convincing positive properties up against occasionally adduced less favourable characteristics like low ductility and the somewhat higher machining costs.What is a decisive aspect is the potential it implies for further strength improvements through full hardening (T6) the entire component or local, so-called modified T5 heat treatment.

For decades, KS ATAG has focused on the continuous optimization of the ALUSIL® concept. The result of these ef-forts also involves the associated progress achieved in the low-pressure die casting method which is firmly integrated with this concept today. There are numerous suggestions for a further reduction of cycle times (cost curbing), but on the other hand, the physical laws cannot be disregarded. Dissi-pating more local heat per time unit can only be accomplished partly without sensibly disturbing the course of the solidifica-tion fronts, e.g. in the cylinder-block skirt area through shrink-fit cooling at the bearing bulkhead level. Another approach would be thin-wall dies to achieve a distinct reduction in the “thermal inertia” of the die. However, this would increase the monitoring requirements in those areas which cannot yet be reliably controlled for the time being. All in all, improvements will only be reached in small steps, however in the right direc-tion, including the reduction of rejects in fully operational condition.

Other casting methods are being taken into consideration, again and again. Whereas for in-line cylinder blocks it may be conceivable also to apply gravity die casting implying slightly lower production costs, this still appears to be fairly problem-atic for V cylinder blocks in the present situation. Process-reli-able application of high-pressure die cast ALUSIL® has so far failed because of problems with silicon pre-precipitation and hot cracking. Nonetheless there is hope for further progress to be achieved. Active cylinder sleeve cooling, a necessity for process-reliable production according to the present state of the art, is a constraint to sand core casting methods which only allow the more elaborate passive cooling due to the prin-ciple on which they are based.

KS Aluminium-Technologie AG would like to thank AUDI AG for the opportunity of a joint presentation of the ALUSIL® con-cept with the example of the new V6 / V8 engine generation. We also convey our thanks to the outstandingly committed staff of AUDI who have actively contributed to the component development and thus made the common success possible. We would also express our special thanks to Dr. Franz Bäumel (V6) and Mr. Armin Bauder (V8) on behalf of all the others who have made best efforts to keep our enthusiasm going. We also owe a debt of gratitude to Mr. Armin Bauder for his valuable contribution to this article.

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www.kolbenschmidt-pierburg.com

Hafenstrasse 25 74172 Neckarsulm, Germany

Phone: +49 (0) 71 32 33-1 Fax: +49 (0) 71 32 33-43 57


Subject to alterations. Printed in Germany. A|IX|e

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