3. Characteristics, Design & Manufacturing Considerations
Material characteristics, design compatible properties and availability of efficient and affordable manufacturing technology are very important attributes that need to be considered before any material is put to use in automotive applications. Steel is known to posses all such characteristics and attributes and as such went on to become the most favored material for automotive applications. However, steel having a higher density or weight has practical limitations for application in modern light weight vehicles, and hence this has openedakhtarauk.ibm.com up the possibility for the introduction of other favorable materials. Aluminum has a wide range of characteristics and properties that can be engineered precisely to the demands of specific automotive applications through the choice of alloy, temper and fabrication processes (The Aluminum Association Inc., AT7-2001). A comparative account of important characteristics of aluminum and steel from the standpoint of automobile engineering and design requirements is given below:
Yield Strength: In general steel has higher yield strength compared to aluminum when both are of same material thickness. However, aluminum has the advantage of being lighter (2/3 less heavy) when compared to steel and this gives a scope for increasing aluminum’s yield strength to levels comparable to that of steel by doubling the thickness of aluminum. Selection of better aluminum alloys also prove effective in this case. Moreover, the combination of the strength of aluminum with its low density results in a higher strength to weight ratio than steel (Aluminum Manual, 2001) and thus enables significant weight reduction where strength is the design limiting criterion.
Impact: Aluminum vehicle structures absorb energy exactly the same as steel by the deformation, folding and concertinaing of the front longitudinal−box−structural beam members. The amount of energy absorbed is related to the yield strength of the material, its thickness and the rate at which the material work hardens as it is deformed. The aluminum can be in the form of sheet structural assemblies, extruded beams or even as ductile castings. Comparative tests with steel show that a spot welded and bonded aluminum box beam will absorb as much energy as a similar steel beam at 55% of steel’s weight. This same relationship applies for bending collapse. Also, just as with steel, the geometric design and dimensioning of the energy absorption members are critical in ensuring that folding collapse develops and premature buckling does not occur at the base of these units (Aluminum Association; www://aluminum.org).
Corrosion Resistance: Aluminum does not rust away on exposure to the environment like steel; its natural oxide coating blocks further oxidation. The risk of galvanic corrosion can be minimized by the appropriate choice of alloy, component design, and protective measures.
Forming, Fabricating & Joining: Aluminum can be formed and fabricated & joined by all common metalworking methods including casting, stamping, forging, bending, extruding, cutting, drilling, punching, machining, finishing, and slide-on, snap together or interlocking joints.
The main focus of an automotive design engineer is very much directed towards three material properties – strength (ultimate tensile and yield, as well as elongation), stiffness (modulus of elasticity) and fatigue life. A good designer overall strives for integration of functions and components, maintaining stiffness and strength along with improved performance and reducing total scraps or wastes. While designing components and structures in aluminum, it is important to take advantage of the available product forms to reduce the number of individual parts that have to be made and assembled. This will reduce costs, both by reducing tooling and assembly operations, and will also improve dimensional accuracy, fatigue durability and structural stiffness. The combination of the strength of aluminum with its low density results in a higher strength to weight ratio than steel, and thus enables significant weight reduction where strength is the design limiting criterion. In closure panels for example, aluminum automotive closure sheet can replace steel with a 40-50% weight saving. However, when aluminum is being substituted for steel in an existing component or structural design, there is generally less scope for accommodating larger beams. Aluminum also has a particular advantage over steel for body structure protection in crash situations. Its lower modulus allows for greater elastic deflection and higher energy absorption at weight savings of up to 64%. Even when the beam is designed to match the steel stiffness, energy absorption is higher and 40-50% weight is saved. The utility of aluminum under certain specific design considerations have been illustrated below to see where it stands compared to conventional steel.