Effect of thermomechanical processing on the surface roughening of AA6061
Master Thesis
2001
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University of Cape Town
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Abstract
Surface roughening is an undesirable feature in materials. This work investigates the cause of surface roughening in four plates of AA6061 with different thicknesses (Le. 6 mm, 9 mm and two 12 mm). Production of these plates requires thermomechanical processing of the cast slab via a two stage process, namely the initial rolling stage on a hot roughing mill (HRM) and final rolling on the hot finishing mill (HFM). After rolling. the plates are levelled by a tensile stretching process and it is during this process that surface roughening of the plates can arise. Three of these plates exhibited severe roughening and the other 12 mm thick plate exhibited mild surface roughening during the stretch levelling process. Investigations looked closely at how the cause of surface roughening is related to the thermomechanical processing. The four products studied exhibited different degrees of surface roughening during uniaxial tensile deformation. Detailed investigation of the microstructure of these four products was carried out. It was found that the plates that exhibited severe roughening had large flat and elongated grains at the surface. It is the rotation of these surface grains during tensile deformation that was thought to be inducing the roughness in the material. The microstructure evolution was explained relative to the thermomechanical history of the plates. Plane strain compression (PSC) tests were used to simulate the thermomechanical processing cycles on the HFM. Large strain rates used in the industry were compensated with an adjustment in temperature so that an equivalent Zener-Holloman parameter (Z) could be achieved. The microstructure of the PSC specimen was characterised using the electron back scattered technique. It was found that the grain size increases with a decrease in the value of Z.
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Aphane, A. 2001. Effect of thermomechanical processing on the surface roughening of AA6061. University of Cape Town.