126 CHAPTER 3 Materials for consideration and use
3.4 ALUMINUM
In general terms, the attraction of aluminum is based on its low density (2.69 g/cm3), the relevance of which in automotive terms is discussed in Chapter 2. The historical rule-of-thumb when considering structures or subassemblies made of alternatives to steel is that the weight can be approximately halved but the cost is doubled.
Although aluminum’s density is one third that of steel, the full down-weighting potential cannot be realized as the modulus (69 GPa) is considerably lower than that of steel (210 GPa), and as stiffness is a primary influence on the design of most body parts some compensation must be made and thickness increased. Any comment on cost must be qualified by the fact that this can fluctuate with the rise and fall of the commodity markets. Thus, for planning purposes some means of stabilizing future costs, such as buying ahead (or an alternative strategy), must be considered. The doubling of cost in net terms compared with steel also includes a factor for increased manufacturing costs, such as those incurred by modification of welding equipment, faster electrode tip wear, cold joining and the need for additional changes to the paint process. Total ownership costs must also be considered. One disadvantage of the more recent models featuring aluminum is the expensive repair of specialist parts such as cast nodes, which form part of a complex integrated substructure, which could also result in higher insurance premiums and difficulty locating a specialist repair shop. In summary then, the major advantages and disadvantages of aluminum as an autobody material are as follows.
Advantages:
• low density;
• corrosion resistance;
• strong supply base;
• recyclability.
Disadvantages:
• high and fluctuating cost;
• poorer formability than steel;
• less readily welded than steel.
3.4.1 Production process
Aluminum is the most common metal in the earth’s crust (8% as opposed to 5% for iron). However, it has only been smelted for industrial use in the last 100 years. The HalleHeroult process is used to extract the metal from alumina dissolved in molten cryolite (a fluoride of sodium and aluminum) by electrolysis using carbon anodes.
A flow chart showing the production process is shown inFigure 3.15.
Following casting the slabs are milled to remove the tenacious oxide film and annealed for up to 8 hours at a rolling temperature of 440e550C. They are then hot-rolled to 10 mm before undergoing a continuous heat treatment prior to a final 128 CHAPTER 3
Materials for consideration and use
cold rolling on a four-high roll stand. The strip may then be straightened and cut to length.
3.4.2 Alloys for use in body structures
The common alloys used for the manufacture of body panels are shown inTable 3.11 and are designated according to the internationally recognized four-digit system.
The 5xxx series refers to aluminum-magnesium alloys while the 6xxx alloys refer to those with additions of magnesium plus silicon. The second digit indicates alloy modifications, and if zero indicates the original alloy; the last two digits have no special significance beyond identifying different alloys.
The 5xxx ‘wrought’ series alloys have traditionally been used for panel production in the UK due to their relatively low cost (three times that of zinc-coated steel (ZCS), compared with five times ZCS for 6xxx) and formability. The main concern has been that they are prone to stretcher-strain markings or Lu¨ders bands, which can appear as flamboyant ‘type A’ coarse markings on the sheet surface, and are coincident with yielding. Alternatively they may appear as finer, more regular ‘type B’ markings, which appear during the plastic stage of deformation. Despite the rolling and heat treatment solutions claimed by some to be effective, these marks tend to re-appear on forming and can show through the paint finish unless reworked by linishing.
The 6xxx series alloys are characterized by higher yield strength than Al-Mg alloys and are heat-treatable, imparting a significant degree of bake hardening at temperatures approaching 200C (seeFigure 3.16). Despite the increased cost, the 6xxx series alloys (6016 in particular) are proving most versatile, and are in use by the majority of car producers using aluminum in Europe, providing a combination of good stretching and drawing characteristics, dent resistance and consistent surface.
With regard to 6016, as well as being stretcher-strain free the use of a 1.0mm EDT textured finish allows a similar quality of finish to be obtained to the steel outer panels, and this tends to be the universal specification. This is in spite of the advantages being claimed for the EBT finish (see below). The other commonly used mill finish applied to internal panels and utility vehicles is less popular due to directionality effects on painting of vertical surfaces. For maximum economy, current designs often feature internal panels in 5xxx alloy with outers in 6xxx where critical quality is required.
The developments in alloys are summarized in Table 3.12 and include the emergence of an internal 6xxx quality, 6181A, and a 6022 alloy with higher proof stress value than 6016, which may give further opportunities for down-gauging providing forming and hemming performance can be sustained at realistic levels. It has been noted that the industry in the USA has adopted the copper-bearing 2036 alloy (not favored in Europe for recycling reasons) for selected panels such as hoods at gauges down to 0.8 mm, compared with the more normal 1.2 mm in the UK. The potential for reducing thickness is now being explored with higher PS (Proof Stress) materials. Very high Al-Mg alloys (5.5% Mg content) are also being evaluated as elongation figures sometimes in excess of 30% are achievable, but high rolling load requirements make it very difficult to produce material with consistent properties.
Table 3.11 Automotive Aluminum Alloys in Current Use Alloy AA DIN
AA6016 AIMg0.4Si1.2
AA6111
AIMgO.7SiO.9CuO.7
AA6009
AIMg0.5Si0.8CuMn
AA5251 AIMg2Mn0.3
AA5754 AIMg3
AA5182 AIMg5Mn
Temper T4 T4 T4 H22 (Grade 3) O/H111 O/H111
UTS (MPa) 210 290 250 190 215 270
0.2 proof stress (MPa)
105 160 130 120 110 140
Elongation A80 (%)
26 25 24 18 23 24
r (mean value) 0.61 0.55 0.64 0.70 0.80
n 5% (mean value)
0.30 0.28 0.29 0.35 0.33
Advantages Formability, no stretcher-strain marks, balanced properties
No stretcher-strain marks, improved bake- hardening response
No stretcher-strain marks, mechanical strength
Corrosion resistance, cost
Good formability
Very good formability
Disadvantages Limited bake- hardening response at Rover paint temperature
Corrosion concerns, limited formability
Limited hemming and forming properties
Possible stretcher-strain marks (Lu¨ders lines) after deep drawing
Alloy type Bake hardening Non-bake hardening
Typical use Skin panels Inner panels
130CHAPTER3Materialsforconsiderationanduse
Alusuisse DIN (AIMg0.6Si1.3) (AIMg0.8Si0.9) (AIMg4.5Cu) (AIMg5.5Cu) Pe-600
Temper T4 T4 O/H111 O/H111 O/H111
UTS (MPa) 270 230 275 285 270
0.2 proof stress (MPa)
150 125 135 130 140
Elongation A80 (%) 26 24 28 29 29
r (mean value) 0.60 0.65 0.70 0.70 0.72
n 5% (mean value) 0.26 0.28 0.34 0.36 0.34
Advantages Improved bake- hardening response
Improved bake- hardening response
Improved formability Improved formability Improved formability Disadvantages Directional hemming
properties
Limited hemming properties
Corrosion, susceptible to stretcher-strain
Corrosion, susceptible to stretcher-strain
Alloy type Bake hardening Slightly bakehardening Non-bake
hardening
Typical use Skin panels Inner panels 3.4Aluminum131
As stated by Dieffenbach1 the aluminum spaceframe represents the second leading body architecture and the history and current use of aluminum in design is presented in Chapter 2.