Forming limitations of pre-tempered 7xxx-series aluminium alloy automotive sheet

  • Elliot Cooksey-Nash

Student thesis: Phd


Decreasing automobile weight would improve fuel efficiency and significantly reduce the impact of transportation on greenhouse gas emissions and roadside pollution. High-strength 7xxx-series aluminium alloys are the ideal candidate for decreasing the weight of automotive sheet components due to their high strength-to-weight ratio. However, one fundamental limitation of peak-aged 7xxx-series aluminium alloys is their poor formability in common sheet metal forming operations. One potential solution is to use a pre-tempered sample, offering a stabilised state with potentially high formability. To achieve adequate strength for in-service properties, the pre-temper could be followed by the paint baking step, which would act as a second ageing treatment. This study assessed the stability, paint-bake response, mechanical properties, and formability of several pre-tempering conditions and compared them to the T6 (peak-aged) and T73 (over-aged) tempered conditions. The precipitate size and volume fraction have been used to characterise the different tempering conditions. Formability has been examined in different strain states using uniaxial and notched plane strain tensile experiments. Lower precipitate volume fractions resulted in higher strain hardening and a greater extent of uniform strain. Despite the significant difference in uniform strain, the strain to fracture in uniaxial tension and plane strain was similar for all the tempering conditions. Fracture surface analysis revealed that substantial void growth and coalescence had occurred for all the tempering conditions, suggesting that large intermetallic particles and dispersoids dominated fracture. An exception to these trends was observed for 80 °C (8 h) pre-tempering condition, which showed a lower extent of uniform strain and strain to fracture. Strain rate jump tests revealed that the pre-tempered samples showed a negative strain rate sensitivity during deformation, likely decreasing formability. The premature failure of the 80 °C (8 h) pretempering condition was linked to its negative strain sensitivity, which assisted the formation of localised shear instability bands resulting in rapid fracture. The notched tensile plane strain experiments were also used to test the accuracy of a finite element model (FEM) to predict strain distribution. Strain hardening laws were used to incorporate and compare the effect of temper in the FEMs. It was observed that high strain hardening resisted strain localisation during deformation. The texture was also explored using the notched plane strain FEMs by incorporating plastic anisotropy using a yield function. The yield function was fit using a novel method developed by Plowman et al. [1]. Crystal plasticity models that incorporate strain hardening and crystallographic texture are used to run virtual experiments to acquire a yield surface in full 6D stress space, which is used to fit the yield functions. The FEM with plastic anisotropy was better able to predict strain distribution. It also showed more strain locations, indicating that the texture of the AA7075 aluminium sheet had a negative effect on the formability.
Date of Award31 Dec 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPhilip Prangnell (Supervisor) & Joao Quinta Da Fonseca (Supervisor)


  • Crystallographic texture
  • 7xxx-series aluminium
  • Formability
  • Pre-temper

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