BIW design with aluminium

Steel BIW have been traditionally fabricated from stamped sheet parts joined by resistance spot welding. Newer developments included the introduction of the hydroforming technology and the laser beam welding technique. Together with the market introduction of new high and ultra high strength steel grades, it was thus possible to improve the stiffness and crashworthiness and/or reduce the weight of the steel car bodies at no or little additional cost. Laser welded, continuous joints significantly increase the rigidity of the monocoque body structure and structural components and subframes manufactured from thin, hydroformed steel tubes enable further improvements of the body strength and stiffness. Similar design and manufacturing principles as used for steel body structures can be applied to realize an all-aluminium car body. However, simple material substitution leads not always to cost efficient solutions. It is essential to take a holistic approach and to consider the total system consisting of the construction material, appropriate design concepts and applicable fabrication methods. Technically and economically promising aluminium car body concepts are the result of aluminium-oriented design concepts and properly adapted fabrication technologies. With its different product forms (sheets, extrusions, castings, etc.), aluminium offers a wide variety of design options. Therefore an appropriate substitution of steel by aluminium in the body structure enables not only a significant weight reduction, but influences the cost efficiency too. The selection of the most appropriate product form ? depending on the type of car and the planned production volume – also allows the optimisation of the technical performance under the given economical and ecological boundary conditions.

The main elements of a self-supporting car body structure (“unibody”) are: - load-carrying profiles - stiffening sheets and the required joining elements (nodes). The profile structure provides the basis for the required high bending and torsion stiffness of the car body within certain package restrictions. The basic body frame given by profiles and nodes is further stiffened by the addition of sheets which are also used to form the overall body enclosure. An additional design requirement is an excellent crash worthiness of the car body (high energy absorption capability by deformation without crack initiation and fracture). A most important advantage of aluminium compared to steel is the additional availability of extruded, single- or multi-hole profiles with complicated cross sections and thin-walled, intricately shaped castings with excellent mechanical properties. These components cannot be only beneficially used for load-carrying and/or stiffening functions, but may also serve as joining elements. The proper use of extruded (and formed) or die cast products enables the development of new, innovative structural design solutions and, consequently, significant weight and cost savings by parts integration and the incorporation of additional functions. Aluminium sheets show similar denting and bending stiffness as steel sheets when their thickness is increased by 40 %, i.e. the weight reduction resulting from a material substitution reaches up to 50 %. In case of the profiles, the substitution of steel by aluminium offers in particular potential for weight reduction when the profile geometry (cross section) can be varied, e.g. by changing from an open to a closed profile or by the introduction of multi chamber profiles. Furthermore, there is a clear potential for the beneficial application of extruded aluminium profiles when the profile diameter can be increased. The decisive factor in the selection of the most effective aluminium body design concept is the envisaged production volume. High volume production looks for minimum material (part) cost and low assembly cost, but can afford relatively high capital investments (both in tools and manufacturing equipment). In contrast, low volume production asks for minimum capital expenditures whereas component and assembly costs play a less important role.the cost efficiency too. The selection of the most appropriate product form ? depending on the type of car and the planned production volume – also allows the optimisation of the technical performance under the given economical and ecological boundary conditions. The main elements of a self-supporting car body structure (“unibody”) are: - load-carrying profiles - stiffening sheets and the required joining elements (nodes). The profile structure provides the basis for the required high bending and torsion stiffness of the car body within certain package restrictions. The basic body frame given by profiles and nodes is further stiffened by the addition of sheets which are also used to form the overall body enclosure. An additional design requirement is an excellent crash worthiness of the car body (high energy absorption capability by deformation without crack initiation and fracture). A most important advantage of aluminium compared to steel is the additional availability of extruded, single- or multi-hole profiles with complicated cross sections and thin-walled, intricately shaped castings with excellent mechanical properties. These components cannot be only beneficially used for load-carrying and/or stiffening functions, but may also serve as joining elements. The proper use of extruded (and formed) or die cast products enables the development of new, innovative structural design solutions and, consequently, significant weight and cost savings by parts integration and the incorporation of additional functions. Aluminium sheets show similar denting and bending stiffness as steel sheets when their thickness is increased by 40 %, i.e. the weight reduction resulting from a material substitution reaches up to 50 %. In case of the profiles, the substitution of steel by aluminium offers in particular potential for weight reduction when the profile geometry (cross section) can be varied, e.g. by changing from an open to a closed profile or by the introduction of multi chamber profiles.

Furthermore, there is a clear potential for the beneficial application of extruded aluminium profiles when the profile diameter can be increased. The decisive factor in the selection of the most effective aluminium body design concept is the envisaged production volume. High volume production looks for minimum material (part) cost and low assembly cost, but can afford relatively high capital investments (both in tools and manufacturing equipment). In contrast, low volume production asks for minimum capital expenditures whereas component and assembly costs play a less important role. 

Joining technology – the key to success

The realisation of a sheet-intensive (“monocoque” or “unibody”) structure requires a very large amount of joining. As a consequence, the properties of the joints have a significant effect on the overall properties of the whole structure including global stiffness, NVH (noise, vibration, harshness) and crashworthiness. The most important difference between aluminium and steel designs is the prevailing joining technique. Compared to steel, aluminium alloys show a high electrical and thermal conductivity and ? correspondingly ? low electrical resistance. When the resistance spot welding technique, traditionally used with steel, is applied to aluminium, the welding current must be significantly higher. Consequently, conventional resistance spot welding of aluminium proves to be energy-intensive, unreliable and costly (need for special welding equipment, sheet surface preparation prior to welding, frequent electrode cleaning, etc.). Proper solutions for these problems have been developed, but resistance spot welding of aluminium nevertheless require special effort. A specific problem is the low electrode lifetime. A possible solution is the frequent cleaning of the electrode surface, e.g. by regular surface machining or brushing (“electrode buffing”). Successful resistance spot welding of aluminium can be also achieved with the Fronius DeltaSpot technology. In this case, the robot welding gun is equipped with a process tape which runs between the electrodes and the sheets being joined. The continuous forwards movement of the process tape results in an uninterrupted process producing constant quality, reproducible welding points and ensuring high electrode service life. 

Most important for the break-through of aluminium in car body construction was, however, the further development of mechanical joining techniques and in particular the application of clinching and self pierce riveting processes in the assembly plant. Whereas the use of clinching processes is in practice limited to non-load bearing joints, self-piercing rivets (SPRs) are also suitable for joining of structural components.

The mechanical joining methods are less energy-intensive than resistance spot welding and can be highly automated. Furthermore, the resulting SPR joints have better fatigue strength properties than spot-welded aluminium joints. Self-pierce riveting is also suitable for mixed material joints (as long as both materials are significantly ductile) and is often combined with adhesive bonding. The other important joining technology for aluminium body designs is adhesive bonding. The properties of joints can be significantly improved by use of heat-cured epoxy adhesives. Normally adhesive bonds are applied in a linear form. Such joints exhibit excellent stiffness and fatigue characteristics, but should normally be used in conjunction with spot-welding, riveting or other mechanical fastening methods in order to improve resistance to peel in large deformation (i.e. during crash). Also, surface pretreatment is necessary for long-term durability of adhesively-bonded structural joints. MIG welding is usually applied for joining of structural aluminium components (extrusions, castings and thicker sheets (> 2 mm). This is also the case for laser welding although laser welding can be also used for thinner sheets. These joining techniques are suitable for situations where there is no access to both sides of the joint or where a continuous joint is required. In special applications, also friction stir welding can be beneficially applied. Today, adhesively bonded joints are increasingly used in the body assembly plant. Adhesive bonding offers a range of potential advantages:

1. High rigidity of the joint
2. Good dimensional accuracy of the design
3. Excellent performance under fatigue loading
4. Noise and vibration dampening capability
5. Additional corrosion protection
6. Blemish-free surface appearance
7. Possibility to join different materials 

Sources of tolerances

The selected joining method determines the size, location and configuration of joining flanges or overlaps and the precision of edge trim required. This in turn impacts on the choice of the part manufacturing process as well as the acceptable tolerances. The precision by which the separate parts of an assembly come together has a critical impact on joining. Thus appropriate part shape tolerances must be maintained to avoid making poor or unsound joints. The problem is of particular concern in large assemblies as shape and tolerance errors buildup and significant force may be needed to bring final parts together for joining. This is another reason why every opportunity should be taken to reduce the part count by part integration. Parts which are cold formed (stamped sheet components, bent extrusion, etc.) generally present the biggest problem in regards to tolerances. This is due to springback and is larger for aluminium than steel due to its lower elastic modulus. Distortion problems may result also when cast aluminium components are subjected to a heat treatment involving a quench.

Material selection criteria 

While a wide range of aluminium alloys are produced by the aluminium industry, not all are suitable for automotive applications. The industry has therefore developed certain aluminium materials especially for automotive use, using its experience in working with the auto industry to determine the final product requirements and then to optimize the required material properties and characteristics. Thus, the normal material of choice should be one of those specifically developed for automotive applications. This ensures availability and production to the quality and tolerances required for automotive production. The specific alloy compositions and heat treatments differ somewhat for sheet and extrusion applications and more so for castings.