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Annealing behaviour of cold deformed AA3003 aluminum alloys

University of British Columbia

AA3xxx series aluminum alloys which are used in automotive heat exchangers usually experience a complicated thermomechanical history with a number of processing steps including homogenization, extrusion, cold deformation and then annealing. The cold deformation can involve a wide range of strains, for example, a few percent strain for micro-multiport tubing and large strains for tube drawing. Often the parts are annealed either as a separate processing step or in conjunction with the brazing operation. Results from industry suggest that a wide range of microstructures can be observed after brazing, ranging from coarse multi-crystals to fine grained polycrystalline microstructures. As such, annealing behaviour of cold deformed AA3003 based alloys after extrusion was investigated in this work. Experimental work was conducted on two alloys homogenized prior to extrusion: 3003 (0.54 wt% Fe) and low Fe 3003 (0.09 wt% Fe). A variety of homogenization heat treatments were examined in order to produce the starting materials. Microstructure, yield stress, work hardening and recrystallization behavior of these alloys with different initial microstructures were investigated. A wide range of pre-strain (1-80%) was applied at room temperature using tensile test and rolling. Although most of the annealing treatments were done at 600 °C, samples with pre-strains larger than 0.1 were also annealed at 350-600 °C to study the effect of temperature on microstructure evolution. The minimum strain required to initiate recrystallization was experimentally measured for each condition using tapered samples. As expected, it was found that dispersoids significantly inhibit recrystallization process and can change the critical strain for recrystallization as high as 18%. In addition, contribution from different strengthening mechanisms on the yield stress and work hardening behaviour was calculated and then a model was developed to describe the stress-strain response. The model represents experimental data within ±10% for both yield stress and UTS over a wide range of conditions. Consequently, a physically based model was developed for the critical strain required to initiate recrystallization. This is the first attempt, to the author’s best knowledge, to model critical strain in a system with distribution of fine and relatively large precipitates.

Text

2015-08-26

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/54656

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/