Sunday, July 28, 2019

Diffusion in aluminium alloys, Hot deformation (rolling) of aluminium Dissertation

Diffusion in aluminium alloys, Hot deformation (rolling) of aluminium alloys and its dynamic recovery, dynamic recrystallisation - Dissertation Example The activation energy and diffusion constants are calculated using a variety of factors like the solid solubility, alloying content and the state of the matrix microstructure in terms of grain boundary size, dislocation density and vacancy concentration [PAPM]. The mean distance travelled by the atom during diffusion in turn is dependent on the rate of diffusion D as follows: L = (Dt)? Where L = the mean distance travelled by the atom. t = time (s) All atoms above absolute zero (-273oC) vibrate and the frequency of vibrations acts as the driving force for the movement of the atoms or for their diffusion. The frequency of vibration and diffusion increases with the rise in temperature. However, for atoms to be able to move from one lattice point to the other, the atoms need to overcome the activation energy. This activation energy is low around the metal surface and at the grain boundaries, and this is the reason for the high concentration of the precipitating solute at the grain bound aries. ... Also, temperature has a greater effect on the diffusion distance compared to time as increasing the temperature increases both the number of vacancies in a metal as well as the energy of the diffusing atoms (Wolverton, 2007). Thus, in an increased temperature, atoms are able to diffuse faster and farther. The rate of diffusion differs for atoms of different alloys and hence the reduction of micro-segregation with homogenization differs for different alloys due to the difference in their compositions. As seen from the diffusion equations presented above, the distance that atoms need to travel (which depends on the dendrite arm spacing, the relative abundance of atoms) impact on the time and temperature needed to attain the desired level of diffusion for reduction of micro-segregation. Figure1: Relative homogenization times for given dendrite cell sizes and temperatures in common aluminum alloys. (Source: Chakrabarti, 2001) According to Verlinden et al [1990] found that the dissolution of theta and S particles in an as-cast 2024 billet during a homogenization at 460oC was not possible even after 24 hours. The volume fraction was found to decrease with time but with an associated coarsening of the remaining S and theta particles resulting in coarser particles than when in the as-cast condition. A homogenization temperature of 500oC was found to completely eliminate both the S and theta particles. Due to the distances that diffusing solute atoms travel during practical homogenization treatments, these treatments are effective at removing microsegregation effects but may have little impact on macrosegregation. 2.2 Theory related to Hot Deformation Hot rolling is the

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