01 March 2011

Aluminium Versus Steel in Automobile Manufacture

Below is a table that compares some of the properties of steel against those of a typical aluminium alloy.

Steel Aluminium Alloy (Al,Mg,Si)
Modulus of elasticity 190,000 – 220,000 N/mm2 60,000 – 80,000 N/mm2
Strength 290 –470 N/mm2 280 – 350 N/mm2
Density 7.85 kg/dm3 2.7 kg/dm3
Coefficient of thermal expansion linear 200C 12.6 mm/mm-0C 23.2 mm/mm-0C

Ref. Carle D. and Blount G. (1999) - modified to include CTE

Aluminium is available in sheet form, as complex castings and in extruded sections that can also be quite complex in form without the need to seam weld (a typical requirement for steel). It is the ductility of aluminium over steel that permits this more flexible extruded option, ductility being the characteristic of a material that allows it to be formed or rather deformed to the required shape.

Aluminium alloys are equal in strength to steel but have a lower rigidity due in part to a lower modulus of elasticity. The consequence of this is that to obtain higher rigidity in aluminium structures various techniques or approaches are required through the manufacturing and assembly processes that will provide the higher rigidity levels required.

Increasing the wall thickness of the aluminium will compensate to an extent for the low modulus of elasticity whilst still providing the benefit of reduced mass overall, this is because the density of aluminium is approximately 1/3 that of steel (however see below ref. bending & torsion members). The resultant mass reduction for equivalent rigidity is approximately 50% where the aluminium is 1.44 times as thick as the steel.

The use of castings for single piece complex components provides the potential for increasing torsional stiffness by up to 20% and the use of extruded box sections eliminates the rigidity losses of conventional spot welded seams.

Ref. Edmans L (1993)

It is important to consider the significant effects of the differences in moduli and usable thickness for aluminium. Similar tensile test loads of 180MPa applied to both mild steel and aluminium alloy, where the steel has a thickness of 1mm and the aluminium 1.6mm, show the aluminium to be close to its yield strength but when density is taken into account, the aluminium displays a greater 'mass specific strength'.

What this means is that the performance of the aluminium for both yielding and buckling is typically 100% better than for mild steel equivalents.

The mechanical properties of aluminium alloys are less strain rate sensitive than those of mild steel and generally they do not suffer from low impact toughness under high speed impact conditions. For comparable impact energy absorption to that of mild steel an aluminium form of similar outside dimensions will be about half the steel's weight but because aluminium has a 1/3rd the stiffness and density of steel the specific elastic moduli of both materials is about the same.

The consequences of this is that simply increasing the aluminium thickness does not provide any weight saving compared to steel in bending and torsion members unless there is an increase in the section perimeter, conversely for panels subject to out of plane bending, large savings are possible.

So ideally to optimise the stiffness and rigidity of aluminium components, it is necessary to increase the perimeter for improvements to 'in plane' stiffness and increase wall thickness for improvements in 'out of plane' stiffness.

The second modern era aluminium intensive vehicle was the Porsche 928S which used Al-Mg-Si sheet alloy (AA6016). Its steel unibody structure was replaced by aluminium stampings and was joined by spot welding. The only significant design change was to modify the rocker rails creating deeper sections to compensate for the lower stiffness modulus of aluminium. This factor tends to support the previous discussion.

Ref. Wheeler M. J. (1997)

A further factor that is important with respect to aluminium is the need to heat treat it to achieve the optimum strength characteristics of the material and this should ideally be done after forming the material in it's lower strength condition (typically T4) in order to ease that particular process.

Steel on the other hand does not require further heat treatment and can be relatively easily formed in its final condition. This is particularly true with respect to stamping sheet metal where due to the characteristics of aluminium the sheet has a tendency to tear during the stamping process, especially in its final heat treated condition (typically T6), and has different springback characteristics to steel which would need to be accommodated in any tool design

One final point is with respect to the weldability of aluminium, a feature of aluminium is that it is resistant to corrosion and the reason for that is that the surface of the material forms into a tight knit aluminium oxide film which prevents oxygen reacting with the base material beneath it.

Where on the one hand this provides the benefit of corrosion resistance, it does at the same time make the material more difficult to weld due to the oxide film interfering with any welding processes, mainly because the oxide has a higher melting point than the parent material.

Ref. Narraway (1988)

Risk Factors

It is important to examine and compare both the attraction and the physical properties of alternative materials in order to understand the potential risks involved in changing from one material to another. There are 2 primary reasons for this: -

  • The risk of technical failure.
  • The risk of market failure
Both types of risk can result in a cost to the company with the most immediate expense being related to development costs, especially if the product fails or results in an inferior product for technical reasons. There are numerous reasons why a product can fail technically, these range from material properties simply failing to meet the requirements of the product design specification to adopting inappropriate manufacturing techniques or not meeting the design constraints with respect to cost of manufacture.

Equally important however, is the risk of market failure, this assumes that the specification has been satisfied in technical terms but that the product fails to find a market. This concerns the more qualitative aspects of the product design or material choice rather than the quantative and relates to the appeal of the product to the consumer. Get this wrong and the product will still fail even if the technical aspects of the design specification are met.

Ref. Smith, Preston G & Reinertsen, Donald G.(1991)

There are clear differences in the properties of aluminium versus steel which can, for the automobile industry, on first impressions and from a technical perspective, appear to be resolvable through a simple increase in wall thickness. However on further investigation it becomes apparent that there are additional aspects which need to be considered in order to optimise the use of aluminium for weight reduction e.g. increasing the section perimeters, utilising complex shapes, how to overcome potential spot welding problems and so on. The immediate suggestion here is that a straightforward replacement of steel components with aluminium is unlikely to be a satisfactory solution.

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