Magnesium for motoring

Materials World magazine
,
1 Nov 2008

Technical Specialists from Jaguar Land Rover in Coventry, UK, Adrian Coomber and Charlie Loh, compare the performance of magnesium to other metals in car components.

The XF is Jaguar’s latest offering in the automotive world. It has won awards such as What Car? Car of the Year and Interior of the Year from CAR magazine.

An important method in achieving improved vehicle performance and handling, while also reducing carbon dioxide emissions, is weight reduction using light metals such as aluminium and magnesium. The XF instrument panel and its associated components are carried by a magnesium cross car beam. Without this beam the instrument panel would not be possible in its current form.

Cross beam construction

Cross car beams (CCB) form the backbone of cockpit modules which are subassemblies put together to improve build quality and reduce assembly time. Within the module, the CCB supports the instrument panel, steering column, heating and air conditioning unit, and crash energy absorbing devices.

The CCB is the key structural element of the cockpit module and Jaguar Land Rover (JLR) uses three versions:

• Steel: This is a welded assembly of tubes and pressings. The company uses this on the Range Rover and Jaguar X Type. The part and tool costs are relatively low, but the mass is high at approximately 12kg.

• Hybrid: A bolted and welded assembly of magnesium castings and steel tubes and pressings. This is used on the Freelander 2 4x4. The part cost is similar to steel, the tool cost is higher, but the mass is lower at approximately 8.5kg.

• Magnesium: A one-piece casting that JLR uses in the Jaguar XJ, Discovery 3/Range Rover Sport, Jaguar XK, and Jaguar XF. The part cost is 25% more than steel, and the tool cost is higher than steel but less than the hybrid. Key benefits are low mass at 5.5kg, with good dimensional stability and quality which lead to reduced squeaks and rattles, and improved craftsmanship. Magnesium is generally the favoured route for future programmes.

The part cost for a magnesium beam is significantly higher than other materials. During development there was a focus on maximum utilisation of the material to keep costs down while achieving weight benefits. Typically for a CCB, this involves working with the manufacturer on latest production techniques, maximising integration of adjacent components such as brackets, using self-cutting screws to cast bosses to avoid costly machining, and employing the latest finite element analysis techniques to develop the design to meet structural requirements.

As the CCB takes its form it changes from a basic shape into one that fulfils the structural need for durability and safety together with the physical requirement of allowing other parts to be mounted onto it.

Design and safety

Computer-aided engineering (CAE) and finite element analysis are critical. Not only does CAE identify areas where material must be added or removed, but it also defines design rules for future CCBs to minimise development time.

Usually, the first step in applying CAE to the CCB is developing its structure to meet noise, vibration and harshness (NVH) targets for parts such as the steering column and wheel. Next the analysis enables the component to meet crash requirements by ensuring the steering column, and driver’s and passenger’s airbags are properly supported, and that energy absorbing devices such as knee bolsters perform correctly. During a crash, these bolsters control the motion of the lower part of the driver’s and passenger’s bodies in markets where seat belt use is not compulsory.

Finally, the analysis examines the CCB’s stiffness to meet vehicle assembly requirements when the cockpit module is fitted and to maintain correct gaps between visible parts in the finished vehicle.

The design shown meets NVH targets and is undergoing simulation for crash loading. There are large areas where the von Mises stress is over 200MPa, indicating that there is a high risk of large-scale cracking of the casting. Subsequent development has led to the introduction of ribs and features to reduce these stresses to an acceptable level.

The guidance derived from CAE can either be product specific or more generic. As the design rules are applied to each new evolution of the CCB, the time spent is reduced and focus can be placed on optimising unique design aspects.

Carrier casting

Another component that could exploit magnesium to minimise mass is the front end carrier (FEC). An example from JLR is on the Discovery 3 and Range Rover Sport .

The front end carrier is bolted onto the body and carries the following main parts – headlamps, horn, bonnet latch, radiator grille, radiator upper mountings, and bumper cover mountings. The material options usually considered are – one piece cast magnesium, a hybrid of steel pressings and plastic injection moulding, and steel or aluminium from a welded assembly of pressings.

• Magnesium has the lowest mass, approximately 5.5kg, and has good dimensional quality and stability. However, the part cost is relatively high.
• Hybrid steel and plastic has a mass of approximately seven kilogrammes and a relatively low part cost. The tool cost is high and there are concerns about distortion and fatigue performance.
• Steel has a relatively high mass of 11kg. The part and tool costs are relatively low.
• Aluminium has a mass of about seven kilogrammes, similar to the hybrid option. The part cost is relatively high.

Magnesium has significant benefits which led to its selection for Discovery 3 and Range Rover Sport. It is an extremely lightweight material giving a high structural performance for low mass. There is a significant improvement in the durability of the body structure due to low front end mass. This is particularly important on Land Rovers which canhave high road load inputs to the body. Magnesium dimensional quality leads to accuracy, which results in good control of the vehicle front end build quality.

However, corrosion is an important consideration when using magnesium for FECs. Unlike the CCB, the FEC is in an exterior environment. Epoxy-coated aluminium spacers are added between all contact surfaces of the carrier to the body, preventing direct contact with steel parts. The FEC is also coated with epoxy powder after etching and priming. There is good drainage around fixings, and raised areas are used where possible to minimise local coating damage during vehicle build. Finally, the steel fixings have coatings that give high corrosion resistance to salt spray.

Further future applications of magnesium

Jaguar Land Rover are considering the use of magnesium to minimise the mass of the following parts –
• Seats - Cushion and back frame structures.
• Body - Cast shock towers and cast rear longitudinal.
• Doors - Inners and frames.
• Lids - Lower tailgate.
• Driveline - Transmission case and transfer case.
• Engine - Block and sump.
• Wheels.

Further information: Jaguar Land Rover