Preform Annealing Age-hardenable Aluminum Sheet

Preform annealing is one promising technology developed by General Motors toaddress the formability challenge of aluminum sheets, which includes a two-stepstamping process with an intermediate annealing heat treatment. Compared toconventional one-step stamping, the total formability can be enhanced. However, thepost-annealing mechanical behavior is quite different from the as-received materialbecause of the precipitation of the second phases for Al-Mg-Si alloy sheets; thus it isimportant to understand the changes in material behavior during preform annealingfor formability analysis, process simulation and optimization. This dissertationcharacterized the structure-property relationship and formability of an age-hardenablealloy AAx610-T4PD that was subjected to various straining and heat treatments andpresented the effects of preform and annealing on tensile behavior, plastic anisotropy(r-value) and forming limit curves (FLCs). Meanwhile, microstructure and textureevolution were also investigated to help understanding the mechanical propertychanges.Three research topics were carried out in this dissertation:(1) Tensile behavior of AAx610-T4PD for outer panel applications duringpreform annealing: The effects of paint bake cycle (PBC), pre-straining and annealingon mechanical tensile properties of the as-received AAx610-T4PD were investigated.It is found that preform annealing does increase the ductility of the Al-Mg-Si alloysheets, but with a penalty of reduced paint bake strength. Hence, this technology isonly useful if applied locally to complex areas of the panel where increasedformability is more important than high strength. Additionally, an optimal preformand annealing condition was obtained for maximizing ductility.(2) Anisotropic behavior of AAx610-T4PD with different pre-strain andannealing conditions: Plastic anisotropy (-value) with different pre-strain paths,levels and annealing conditions have been characterized. It is found that-value isnot constant during the complex forming processes because of the variation of sheettextures, which is dependent on pre-strain path, pre-strain level and annealingcondition. Hence, it is necessary to consider the variations of-value for furtherformability analysis. (3) Forming limit criterion for preform annealing: Three different FLCs weredeveloped in this research to investigate the effect of preform annealing on theformability of the AAx610-T4PD sheets: strain-based FLCs, stress-based FLCs andpolar-effective-plastic-strain (PEPS) FLCs. In order to precisely generate thestrain-based FLCs, Marciniak test was applied and a new time dependentmeasurement method based on Digital Image Correlation (DIC) was proposed todefine the onset of localized necking. From the measured strain-based FLCs, it isfound that preform annealing does increase the forming limits. Although thehardening behavior has changed significantly after preform annealing, thestress-based and the PEPS FLCs of the post-annealed materials, converted to a singlecurve from the measured strain-based FLCs using a constantresidual-effective-plastic-strain (REPS) method, have a similar shape at sameannealing condition, which are independent on pre-strain level and path. Therefore,both tools are applicable for evaluating the forming limits of6xxx alloy sheets for thiscomplex stamping process.