Audi A8: aluminum and steel
In 1994 year began to produce the car Audi A8 (D2) - a four-door executive class sedan. Since then, it has come a long way: from an almost all-aluminum body to an increase in the proportion of steel in the body., as well as the application of new materials – CFRP and magnesium – in Audi A8 (D5).
Audi A8 D2 – aluminum body
The novelty of the car Audi A8 was in, that during its development, the concept of a spatial self-supporting body structure - ASF (Audi Space Frame) was used for the first time. This concept is continued in subsequent models Audi, including, aluminum body Audi A2.
Figure 2 -Aluminum body Audi A8 (D2)
The essence of this concept is that, to reduce body weight as much as possible by taking advantage of the various forms of aluminum products in the vehicle:
- aluminum sheets,
- aluminum profiles and
- aluminum castings.
In this concept, the roles of the various components of a self-supporting vehicle body are clearly separated:
- profiles - carry loads;
- sheets - provide rigidity;
- casting - are connecting elements.
Self-supporting body consists of Audi skeletal structure, made of aluminum hollow profiles. These aluminum profiles can be straight or bent (2D or 3D). If necessary, use aluminum profiles with multiple cavities with a specially designed cross-section. In the most loaded corners and other compounds of elements of the body are usually connected via complex, thin-walled aluminum hub parts. These high-pressure aluminum castings cast by vacuum casting.
Figure 3 - Exploded View
aluminum body Audi A8 (D2)
Types of aluminum in bodywork
The Audi A8 (D2) car was produced with 1994 by 2002 year. Its aluminum body weight was 249 kg (without doors) - on 200 kg lighter steel body. This aluminum body includes 334 details:
- 47 aluminum profiles (14 % weight);
- 50 castings (15 %) and
- 237 sheet metal stampings (71 %).
Compared with the previous – steel – bodywork number of parts decreased by 25 %.
Audi's first self-supporting aluminum bodies contained:
- Curved aluminum profiles, flat (2D) and spatial (3D), alloy 6060
- The outer sheet body panels made of aluminum alloy 6016
- Internal sheet panels of aluminum alloy 6009
- Bearing sheet panels of aluminum alloy 5182
- Cast anchor parts of aluminum alloy A356
The production of the aluminum body of the first model A8 (D2) was characterized by a low degree of automation.. body assembly was carried out on approximately 75 % manually. Compounds to nodular cast parts, who made MIG welding, serves to compensate tolerances.
Heat treatment of the body
Another feature of the manufacturing D2 body was heat treatment the whole body assembly when 210 ° C within 30 minutes immediately prior to painting. The purpose of this operation was, to ensure the necessary strength of the body due to the ideal - artificial - aging of profiles and sheets of 6xxx series alloys (AlMgSi).
Subsequently, however, experience has shown, in a separate step of heat treatment of the body is not necessary, and it was abolished as early as next Audi model with an aluminum body - the Audi A2. Instead, the desired level of strength of the body was achieved during the heating time in a furnace at body painting operation. The thing is, that the coloring step comprises a body cataphoretic dipping bath and then treating it in an oven for about 20 minutes at 180 oC. This heating was sufficient for effective hardening aluminum alloys 6xxx series due to artificial aging.
The chemical composition of aluminum alloys
Highlighted in red are the main alloying elements, blue – Auxiliary alloying elements and impurities stricter requirements.
Audi A8 D5: return to steel
40 % steel
However, it became known , that the body of the new Audi A8 D5 model release 2018 contains more than a year 40 % steel. This represents a reversal in 180 degrees from the almost all-aluminum body, developed for the Audi A8 D2 in 1994 year.
The design center of the company Audi Leichtbauzentrum said , that there will be cars with a fully aluminum body in the future. The development of the car will play a special role steel, are thermally hardened in hardening dies. These steels are processed at high temperatures by heating in an oven and hot stamping using a cooled tool (Figure 4). High cooling rate in the die formation of the structure provides, which is 100 % It consists of martensite. To improve the quenchability of these steels generally contain boron.
Debut magnesium Audi A8
Audi bit late with magnesium, that other manufacturers have used in one way or another. Magnesium is lighter than aluminum yet: its density is only 1,74 g / sm3. The new Audi A8 already includes a magnesium component in the form of a spacer between the front suspension cups (picture 4). Its main goal - to increase the torsional stiffness.
Figure 4 – Body Audi A8 D5 aluminum, steel, CFRP and magnesium
The turn from aluminum to steel is confirmed by the data on the change in the ratio of the weight of materials in the body structure of the Audi A8 (table 1). As a result, for example, Body weight increased to about 280 kg instead of 230 kg from the previous model .
Table – Changes in the composition of materials for the body of the Audi A8 
Why more steel?
Three reasons are named in the Audi Design Center :
- The most important characteristic of a car is its crash strength.. New enhanced security requirements, and the need for additional protection of lithium-ion battery, It required the use of high-strength hot-deformed steel for strengthening the internal “cell” Smart car.
- Moreover, the use of steel enhances the stiffness of the body torsion - to 24 % compared with the old design. This improves handling car.
- Finally, the steel has a more favorable acoustic properties, reducing noise inside the vehicle.
Aluminum is a major player
Aluminum has a population of about 58 % body weight Audi A8 (D5) and is used in three main types: castings, extruded profiles and sheets. Cast components form the main part of the structure, and are "nodes", to which are attached other details. Despite the increase in the weight of this model compared to previous models, update on 40 % easier, than, if it were made entirely of steel.
1. The Aluminium Automotive Manual, The European Aluminum Association, 2013