Present and Future Roles of Aluminum Products in Transportation
Denesh Seksaria P.E. - Technical Consultant - ALCOA Automotive StructuresIn this talk we will look at the use of aluminum in transportation products from a mechanical engineer's perspective. There is a tendency for product engineers to be afraid to get into the unfamiliar. However, designing vehicles of aluminum has been demonstrated to give 40% to 50% weight savings, improve performance, and be more environmentally friendly than most current designs.
The first thing that must be realized is that DIRECT SUBSTITUTION DOES NOT WORK. Aluminum designs will be different from traditional designs because of differences in material properties, manufacturing and joining capabilities. Direct substitution would only be possible when the following questions could all be answered to the affirmative:
Stiff enough?
Strong and durable enough?
Corrosion controlled?
Same paintability?
Same crash loads?
Same vibration and acoustics?
Same manufacturing processes?
Same design principles?
Unfortunately this is not the case.
Elastic modulus: about 1/3 that of steel
Densisy: about 1/3 that of steel
Impulsive strength: Al is not strain rate sensitive while steel is.
Fatigue: about 1/2 that of steel
Ductility: about 2/3 that of steel (less forming range)
Hardness: lower than steel
Thermal Conductivity: about 4 times that of steel
CTE: 13 ppm/F for Al vs 8.3 ppm/F for Steel
Damping: Similar to Steel
Magnetic: None for Al, high for Steel
Electric Resistity: 1/4 as much for Al as Steel
Galvanic Potential: High for Al, Low for Steel
To be successful, aluminum must be incorporated as an integrated MULTIPRODUCT approach. This approach has been successfully demonstrated by ALCOA many times.
Major Factors to be considered are: Cost, Weight, Performance, and Manufacturability.
The multiproduct forms most commonly employed are: Steet, Extrusions, and Castings (sand, die, and investment).
Rolled sheet is best utilized for enclosure surfaces. It can be used either in a heat treated or non-heat treated state. It can be formed by traditional forming (stamping) methods, but specific tool designs must be altered, deep drawing limits are reduced. It may be necessary to split what was one steel component into two aluminum ones. Most traditional joining methods can be employed for Al to Al joints. These methods are usually very low cost.
Extruded Product is an area where aluminum can demonstrate many design advantages. Straight, and curved extrusion with 2D and 3D twist are all possible. When loading condition are predictable, this can lead to many design efficiencies. Al extrusions provide excellent structural performance, and are good in crash tests. Tooling is relatively inexpensive, but processing speeds are slower than formed steel channels. Class A surfaces are also difficult to achieve on aluminum extrusions, and assembly can be more complex due to added stiffness.
Aluminum castings can be made by several processes including sand casting, die casting and investment casting. The quality is dependent on the process chosen. These are used for very complex geometries and usually for thicker sections. Casting are usually used in the heat treated form. Aluminum castings tend to be the most expensive product form.
Joining of alumimum can be accomplished by most traditional methods, (spot welding, MIG welding, adhesive joining, etc.) but the processing parameters MUST BE DRASTICALLY ALTERED. This has led to problems with Al in vehicles where the infrastructure for repair is not educated in these differences.
Six impressive examples of aluminum designs achieving the 40% - 50% weight reduction:
1) The bumper structure of a Buick Riviera using two aluminum extrusion and riveted assembly.
2) Underbody X members of a 98 LH rear suspension using 2 Al extrusions and two Al castings joined by arc welding.
3) Instrument Panel Support (DEW) involving an Al extrusion, Al sheet metal, and a magnesium casting
4) Windshield Surround for '97 Corvette employing Al extrusions, two Al sand castings at top corners, and 2 Al die castings at supports.
5) Prowler Frame combining mostly Al complex extrusions and a few Al castings.
6) Body in White for '92 Mercury Sable using Al sheet metal, with some Al extrusions and some Al castings, joined by spot welding, MIG welding and some quench bonding.
In all of the above cases, the Aluminum designs equalled or out-performed the traditional designs for the same function in all respects, while producing the 40% to 50% weight savings.
Economics, and service infrastructure remain the final stumbling blocks for widespread Al usage in vehicle manufacture.
In conclusion: Aluminum must be applied thoughtfully. Only a multi-product approach is cost effective. Alcoa has the breadth of experience necessary to develop such applications.