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Title: Microstructure formability relationships in new generation high strength aluminium automotive alloys
Author: Nolan, Ross Andrew
ISNI:       0000 0004 5993 8351
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2015
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The desire to reduce weight in automotive products is driven by a need to improve efficiency. As such, to allow further weight reduction, higher performance aluminium alloys are in demand for sheet metal body structures. Due to their high strength to weight ratio 7xxx alloys are seen as an ideal candidate for this, however their use to date has been limited by poor formability. Previous work indicated that by moving to high temperatures (>350°C) or by using a soft temper (W), good formability could be achieved but the samples required further heat treatment post-forming. This work explored the warm forming temperature range to improve formability whilst developing the required properties during processing. The performance of a 7xxx candidate alloy, 7021, has been assessed in stretching and drawing operations, both at room temperature and over the elevated temperature range of 150-250°C. The microstructure and other properties of the alloy were investigated in W, T4 and T6 tempers, before and after testing, through a range of techniques, including DSC, DMTA, SEM, EBSD and TEM.In the T4 temper, UTS and proof stress increased with temperature up to 190°C, due to dynamic precipitation. Increasing temperature only provided a modest increase in strain to failure for both the T4 and T6 temper. Cup height was not significantly improved in the warm forming temperature range during Erichsen cup testing. By deep drawing at 250°C it was possible to fully draw a cup (with an LDR of 2.2) in both the T4 and T6 temper of 7021, with both tempers having comparable post-forming hardness. This indicates that at 250°C the starting condition has no impact on drawability. Although full drawability is achieved at 250°C the final product would require further heat treatment if it were to replace 6016. However, by deep drawing 7021-T4 at 190°C, a fully formed cup was produced with a hardness between that of the T4 and T6 temper. The microstructure of the formed cup showed no grain boundary precipitation and a fine distribution of the strengthening phase η', suggesting there is a dynamic effect on the precipitation during deep drawing at this temperature. In conclusion, the work has shown that warm forming does not significantly improve stretching behaviour of 7021, but by using warm forming temperatures deep drawing is improved.
Supervisor: Quinta Da Fonseca, Joao Sponsor: EPSRC ; Constellium
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Formability ; Aluminium Alloys ; 7xxx series alloys ; Deep drawing ; Forming limit diagram