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The key in-service requirements for automotive closures (bonnets, boots, wings, rear quarter panels, doors) are panel bending stiffness and dent resistance, corrosion resistance and surface appearance. The main manufacturing requirements are good formability and joinability.
Here we investigate the mechanical performance (bending stiffness and dent resistance) of two steels and an aluminum alloy, with respect to panel mass.
Bending stiffness depends on Young's modulus, so changing alloy within steels or within Al alloys has little effect. Dent resistance requires a sufficiently high yield strength to withstand permanent deformation. Therefore Al alloys for closures are generally selected from the age-hardened 6xxx series. Bending stiffness, dent resistance and panel mass all increase with panel thickness.
The graphs below show the effects of alloy and thickness on these attributes.
With the thickness for both steels set to 1 mm, use the slider to adjust the thickness of Al alloy 6016 to give the same bending stiffness as the steels.
Using aluminium sheet 1.44 times thicker than steel will result in 50% weight reduction for the same panel bending stiffness, without loss of dent resistance. Aluminium also competes favourably with steel on corrosion performance. However, the wider uptake of aluminium requires development of manufacturing technologies at a competitive cost, notably for forming and joining. One example of an efficient processing operation is the integration of forming, heat treatment and paint-bake cycle for 6xxx sheet. The sheet is solution treated and naturally aged condition (T4), formed in this relatively soft condition, and finally age hardened (typically 160-180 °C for 30 minutes) to simultaneously increase the yield strength (for dent resistance) while curing the paint coating.
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