The relationship between the Zener­-Hollomon parameter and transfer strip Creep in hot rolled AA3104

Borrageiro, Roberto

The relationship between the Zener-Hollomon parameter and transfer strip Creep in hot rolled AA3104

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Excessive sagging has been experienced in AA3104 transfer bar after hot rough rolling, during transport between the hot roughing and finishing mills. This sagging has resulted in bottom scuffing and surface defects as a result of localised contact between the slab and the roller table. This creep of the material is caused by excessive recrystallisation in the microstructure of the material, as a result of the deformation parameters used during hot rough rolling. The presented research aims to assess the mechanical response of the material at varying deformation parameters and to verify their link to the resulting creep experienced by the material. The effects of hot rough rolling and in-situ creep will be simulated with plane strain compression tests performed on a Gleeble 3800 and the resulting test data will be linked with the deformation parameters through the use of the Zener-Hollomon Equation. The level of recrystallisation experienced by a material is heavily impacted by the parame-ters at which the material was deformed. Recrystallisation is driven by the energy stored in the material as a result of deformation processes. It has been found that materials deformed at higher strain rates (ϵ˙) possess increased levels of strain accumulation, and therefore experience elevated levels of recrystallisation when exposed to temperatures (T ) greater than 50% of the materials absolute melting temperature. However, it has also been determined that recrystallisation can be minimised, if the material is deformed at low strain rates and low temperatures [36]. The objectives of this research are to use the simulated hot rolling and in-situ creep data acquired from the Gleeble, to determine threshold values for Z and the deformation parameters, that will result in material slabs which possess a resistance to creep. Plane strain compression tests were performed on a Gleeble 3800 at deformation strain rates and temperatures in the range of ϵ˙ = 1 − 21s−1, and T = 420 − 500◦C, to assess the effects varied parameters had on the creep experienced by the slabs as well as the level of recrystallisation produced in the resulting microstructure of the material. The effects of strain rate and temperature during hot deformation were represented by Zener-Hollomon as well as Arrhenius-type values. These values were compared with the creep strain experienced by samples to identify a threshold value for Z above which creep was minimised. A series of micrographs were acquired to assess the level of recrystallisation experienced by samples deformed at certain parameters. The results from these micrographs were compared with the creep results to highlight the relationship between the deformation parameters and the resulting recrystallisation and creep experienced by the material. Tests performed at deformation parameters which resulted in Z ≥ 1e14 or ln(Z) ≥ 32 were found to experience less recrystallisation and hence were more resistant to creep. It was also found that samples deformed at T > 450◦C experienced significantly more creep than samples that were deformed at T ≤ 450◦C. In both temperature ranges, higher ϵ˙ values commonly resulted in samples experiencing more recrystallisation, and this was heavily accentuated at T > 450◦C. The threshold values required for creating a creep resistant slab of AA3104 during hot rough rolling are Z ≊ 1e14, ln(Z) = 32, and T = 450◦C. Materials deformed at parameters resulting in Z ≥ 1e14 or ln(Z) ≥ 32 will result in a creep resistant slab of material if the deformation temperature T ≤ 450◦C. However increased values for strain rate ϵ˙, will still result in more recrystallisation being experienced by the material, even at T ≤ 450◦C, therefore higher ϵ˙ should be avoided where possible.

Keywords

AA3104 transfer bar

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