A model for predicting the effect of deformation after solution treatment on the subsequent artificial aging behavior of
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I. INTRODUCTION
THERE is currently a large interest in increasing the utilization of aluminum alloys in automotive applications, due to increasing concerns regarding vehicle weight. One area where aluminum has made a significant impact is in its usage as a bumper material. Over 1 million aluminum bumpers are currently being produced annually, primarily for European automanufacturers. For this product, the forming and the heat-treatment operations are intimately linked to the manufacturing process. The bumpers are fabricated from 7000-series aluminum alloys using a stretch-forming operation immediately after the solution treatment. The magnitude and strain path of deformation are spatially heterogeneous within the bumper (i.e., the strain values range from 0 to 0.3) and this has an important impact on the final mechanical properties after the subsequent aging treatment. Traditionally, 7000-series alloys have found applications in the aerospace industry due to their good combination of specific strength and fracture toughness. It has been well established that these alloys are very sensitive to their processing history.[1–7] Typically, these alloys will undergo multistep aging treatments, including natural aging and, often, two-step artificial aging treatments. The alloys also often receive plastic deformation after the solution heat treatment to remove residual stresses and quench distortion,[6] W.J. POOLE, Associate Professor, is with the Department of Metals and Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. J.A. SÆTER, Research Scientist, is with the R&D Materials Technology Group, Hydro Aluminium a.s., N-6600 Sunndalsøra, Norway. S. SKJERVOLD, Manager, and G. WATERLOO, Research Scientist, are with the Process Development Group, Hydro Automotive Structures, N-2831 Raufoss, Norway. Manuscript submitted September 16, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
although usually at much smaller levels (i.e., strains of less 0.05) than in bumper manufacturing. The precipitation sequence in these alloys is generally accepted to be Supersaturated solid solution → Guinier–Preston zones → h8 → h where h8 is the metastable precipitate and h is the equilibrium MgZn2 precipitate.[5,8] The peak-strength condition is associated with a fine distribution of h8 precipitates.[5] The equilibrium precipitates can form numerous crystallographic orientations with respect to the matrix (the most commonly observed are designated as h1, h2, and h4[5]), and it has been found that the presence of dislocations can significantly alter the precipitation process.[1,9,10] Nucleation of precipitates can occur more readily on dislocations, as the free-energy change required for nucleation can be substantially reduced[11] and, also, the growth and coarsening of precipitates can be enhanced due to short-circuit diffusion along the dislocation core.[12] Furthermore, it has been shown that prestraining has the effect of (1) increasing the kinetics of the aging process,[13,14,15] (2) changing the sequen
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