Mary A. Wells

Mathematical model of deformation and microstructural evolution during hot rolling of aluminium alloy 5083

Abstract A mathematical model to predict the through thickness temperature, strain and strain rate distributions during hot rolling and the subsequent microstructure evolution was developed using the commercial finite element package ABAQUS. Microstructure evolution predictions included the amount of recrystallisation through the thickness of the sheet based on its thermomechanical history during rolling and thermal history after rolling. The equations used to predict the microstructure evolution were based on semiempirical relationships found in the literature for a 5083 aluminium alloy. Validation of the model predictions was done using comprehensive experimental measurements which were conducted using the Corus research multimill, a pilot scale experimental rolling facility, in Ijmuiden, The Netherlands. The results indicate that the through thickness temperature and strain distribution predictions for the rolling operation are reasonable. Hence, the boundary conditions used in the finite element model adequately represent the interface heat transfer and friction conditions. Microstructure predictions using the literature based equations significantly underestimate the amount of recrystallisation occurring in the sheet. A sensitivity analysis indicates that the recrystallisation kinetics are extremely sensitive to the fitting parameters used in the microstructure equation, and that the gradient in the recrystallisation kinetics is the result of the temperature gradient experienced by the specimen during deformation.

Application of a mathematical model to multipass hot deformation of aluminium alloy AA5083

Abstract Sheet metal forming operations are used extensively in industry to alter the shape of the metal through plastic deformation. A critical step in the sheet manufacturing process is hot rolling which reduces the thickness of the ingot and can significantly impact the final sheet properties based on the microstructure evolution during this operation. A two-dimensional mathematical model was developed and experimentally validated to simulate deformation and microstructure evolution during multipass hot rolling for an AA5083 aluminium alloy. The details of model development and experimental validation can be found in earlier work. In this article, the application of the validated model to further understand and optimise the material stored energy and ensuing microstructure during multipass hot rolling is described. Specifically, the model was employed to examine the effect of changing the number of rolling passes as well as strain partitioning during multipass rolling on the material stored energy and the resulting microstructure. Results indicate that the number of passes has a significant effect on the stored energy which increases as the number of passes increases. In addition within a multipass rolling schedule the way in which the strain is partitioned is also shown to have an effect on the stored energy with a decreasing strain/pass schedule providing the highest material stored energy after rolling is complete. In contrast an increasing strain/pass schedule provides the lowest stored energy in the material after rolling. This overall effect is attributed to the differences in strip temperature as the lowest exit temperature strip has the highest stored energy. The model was further utilised to generate operational curves to predict the material stored energy and subsequent recrystallisation under different rolling conditions, namely at different interpass times and total strains for various start deformation temperatures.

Transient Cooling of a Hot Steel Plate by an Inclined Bottom Jet

Controlled cooling on the runout table is a crucial component in the production of highly tailored steels since it has a strong influence on the final mechanical properties. High efficiency heat transfer in impinging jet cooling makes this an important method for heat transfer enhancement. The purpose of this study is to develop an experimental database for modelling of boiling heat transfer for bottom jet impingement that occurs during runout table cooling in a steel mill. Experiments have been carried out on a pilot scale runout table using stationary plates, with focus on the effect of water flow rate and nozzle inclination to the overall heat transfer rates. Volumetric flow rates and inclination angles are in the range of 35-55 l/min and 0-30º, respectively. Temperatures on the test plates are measured internally very close to the surface during cooling for the purpose of reducing thermal lag and receiving better data responsiveness. These measurements are taken at the impingement point and several streamwise distances from the impingement point. From the above measurements transient cooling data on the hot steel plate by bottom jet impingement has been analysed.

A Comparative Study on the Softening Behaviour of Cold Rolled Ingot and Continuous Cast AA5754

An investigation was conducted on the softening behaviour of cold rolled continuous cast (CC) AA5754 Al alloy and compared to the results for the ingot cast (IC) material. The present study suggests that the CC material exhibits greater resistance to softening as compared to the IC AA5754 for the same amount of cold deformation. The differences in the softening kinetics become more noticeable with increasing level of cold deformation and from a processing point of view can be attributed to the absence of the homogenization stage during the processing of the CC material. Resistivity measurements were carried out during the annealing treatment of the CC materials to examine the possibility of concurrent precipitation, which could potentially retard the softening kinetics for these materials. In addition, the current research reveals that the CC material produces a finer recrystallized grain size as compared to the IC material.

Effect of Advanced Cooling Front (ACF) Phenomena on Film Boiling and Transition Boiling Regimes in the Secondary Cooling Zone during the Direct-Chill Casting of Aluminium Alloys

Accurate knowledge of the boundary conditions is essential when modeling the Direct-Chill (DC) casting process. Determining the surface heat flux in the secondary cooling zone, where the greater part of the heat removal takes place, is therefore of critical importance. Boiling water heat transfer phenomena are quantified with boiling curves which express the heat flux density as a function of the surface temperature. Compilations of boiling curves for the DC casting of aluminum alloys present a good agreement at low surface temperatures but a very poor agreement at higher surface temperatures, in the transition boiling and film boiling modes. Secondary cooling was simulated by spraying instrumented samples with jets of cooling water. Quenching tests were conducted first with a stationary sample, and then with a sample moving at a constant “casting speed” in order to better simulate the DC casting process. The ejection of the water film in quenching tests with a stationary sample and the relative motion between the sample and the water jets both lead to an Advanced Cooling Front (ACF) effect, in which cooling occurs through axial conduction within the sample rather than through boiling water heat transfer at the surface. The heat flux density and surface temperature were evaluated using the measured thermal history data in conjunction with a two-dimensional inverse heat conduction (IHC) model. The IHC model developed at the University of British Columbia was able to take into account the advanced cooling front effect. The effect of various parameters (initial sample temperature, casting speed, water flow rate) on the rate of heat removal in the film boiling and transition boiling regimes was investigated.

Application of a Mathematical Model to Simulate Multi-Pass Hot Rolling of Aluminium Alloy AA5083

A mathematical model has been developed and validated to predict deformation, temperature and microstructure evolution during multi-pass hot rolling of an AA5083 aluminum alloy. The validated model was employed to examine the effect of changing the number of rolling passes and the strain partitioning during multi-pass rolling on the material stored energy and the resulting microstructure. Results indicate that the number of rolling passes has a significant effect on the material stored energy. In addition, the way the strain is partitioned in two-pass rolling cases affects the material stored energy with decreasing strain/pass providing the highest stored energy in the material after rolling and vice versa. The reason behind these results was further investigated indicating that the thermal evolution during rolling may significantly influence the material stored energy and subsequent recrystallization kinetics.