In the quest for reduced weight, driven by the call for reduced CO2 emissions, aluminium is gaining increased use in automotive applications. The variety of semi-finished products - plate, sheet, extrusions, castings etc. - provide numerous possibilities for designing optimal, cost-effective solutions. Few other materials offer such a span of opportunities, but along with this there lies a challenge. Forming and joining aluminium into products calls for specific technical knowledge that is not always available in the engineering departments, where steel and other materials are considered the workhorses.

Besides the trend in reducing weight in a car, there is a clear drive to reduce the number of parts, both to reduce cost and improve quality. Reducing the number of parts is often synonymous with increasing the level of functional integration in a given part. This is obvious for example in large, thin-walled aluminium or magnesium castings, where one cast part replaces a number of stamped parts.

Another complication is the effort to pack more equipment and components into a limited volume. Powerful engineering tools i.e. computer-aided design and finite element analysis packages, enable engineers to utilise available space to the maximum. Tighter packing also puts increased demands on geometrical tolerances.

These factors drive the increase in demand for aluminium components, consisting of semi-fabricated products that are formed, then joined in some way or another. All of this under the pressure of providing the lowest possible cost.

Integrated function in extruded components

Figure 1: Fantasy profile – functions such as bolt tracks, sockets and flanges are easily incorporated in the cross section of the extrusion

The extrusion process provides exceedingly large opportunities

for designing optimal components, as the cross-section of an extrusion can be tailor-made for a relatively small cost. Integrating function i.e. bolt channels, sockets, cooling flanges etc. are standard procedure (Figure 1). For parts made from straight members, this is without complication.

Automotive components are seldom straight, thus calling for some sort of forming. Bending and hydroforming are two forming processes used in manufacturing components having complex shapes.

Forming in two dimensions

Figure 2: Rotary draw bending in principle – the extrusion is clamped to and formed around a rotating tool; the tool in the illustration is the yellow part and rotation counter clockwise

Bending is done by several methods: roller bending, press bending, rotary draw bending and stretch bending. Bending radius, section complexity and tolerance demands are decisive for which method is best. Rotary draw bending is used for relatively small radii and complex sections (Figure 2).

Bending complex sections is an intricate business, especially if there are tolerance demands on cross-section dimensions or the section has thin walls.

One example is illustrated in Figure 3. Finite element analysis is useful in the engineering stage for evaluating bend ability and variation in spring-back due to factors such as material strength and friction.

Figure 3: Bent section and finite element analysis – buckling may occur in tight bends and compressed flanges

Utilising extrusions in the third dimension

Bending provides curvature along the length of a member but not local variation in cross-section. This type of forming is enabled by hydroforming.

Figure 3: FEM simulation identifies problem areas in the engineering stage

The principle is that a hollow tube or extrusion is put into a tool having a cavity with the shape of the finished part. The tool is closed, and hydraulic pressure applied inside the tube where it is then formed against the tool.

Hydroforming enables the production of complex forms with very tight tolerances. The side rails in Figure 4 have a total length of 4,500mm and are within a form tolerance of 0.7mm. This part, developed for research purposes, in theory replaces eight stamped parts that must be joined together by some means.

Figure 4: Illustration of three process steps for a car side rail – first, an extruded tube (top), second, the tube after bending (middle), and third, the tube after hydroforming (bottom)

Integrating functions in the hydroforming process

In the hydroforming process, other processes may be incorporated i.e. punching and trimming. In the same part reference holes were punched and the slug was left attached to the rail.

End trimming is shown in Figure 5. This is also done in the hydroforming tool to ensure tight tolerances and reduced cost. The trimming is done at the end of the forming step when the pressure is at its peak. With extruded members it is possible to create complex sections i.e. multi-cavity hollow sections, or hollow sections with flanges. Flanges may also be trimmed in the forming cycle.

Figure 6: Prototype engine cradle – hydroformed side rails and a rear cross member made of a cast top side and a plate bottom joined by friction stir welding

The ideal forming strain for hydroformed aluminium extrusions is around 3-10 per cent increase in circumference depending on alloy and temper. This secures form stability without exceeding the formability limits. Also, as bending is often done prior to hydroforming, strain that is developed in the bending stage must be considered in the hydroforming stage. This is strong reason for using FEM analysis when designing hydroformed components.

Hydroforming is often conducted after a bending operation. The quality of the bending has paramount importance for the quality of the finished part. Wrinkles that appear in bend will not disappear in hydroforming.

Joining

Figure 5. End trimming executed in the hydroforming process

Hydroformed parts are joined by one way or another to components. The tight tolerances enable the use of laser welding where gaps typically must be less than 0.5mm. Hybrid laser welding is more flexible, tolerating larger gaps with good quality. In Figure 6, an engine cradle is presented. The cradle is the result of a lightweight study with the objective of substantially reducing weight in the front end of the vehicle. The weight of this substructure is 16kg, as compared to 23kg for the steel version.

This creation uses various semi-fabricated products and joining methods. The side members are hydroformed extrusions. The front cross member is a straight extruded member welded to the side rails by MIG welding. The rear cross member is built up of a cast part and a plate, where the plate is joined to the casting by friction stir welding (FSW). A cast part was necessary to accommodate the geometry of the member, which has very large variations in cross-section geometry. The design was locked in that aspect with no possibility for change.

The FSW operation was executed in three dimensions, not two-dimensional, which called for special preparations. FSW joints have higher fatigue strength than MIG, which could have been an alternative, but with a somewhat different design. The rear cross member was joined to the side beams by MIG welding.

The casting was sand cast in Silafont 36 alloy. Different plate alloys were used and AA6082-T6 turned out to have the best-combined properties. AA5083 was invest-

igated and tested in a few prototypes but FSW results proved to be inferior to those in 6082.

Cost analysis showed the concept to be competitive to other competing concepts and within the framework put forward by the customer.

Future developments

Cars and other vehicles incorporate an increased amount of formed aluminium extrusions and will continue to do so in the quest for reduced vehicle weight. Integrating more function helps reduce the number of parts and helps lower cost. Making use of the technologies available is a challenge for automotive engineers and suppliers.