چكيده به لاتين
In this work, the workability of the as-cast AZ91 alloy was investigated by the equal channel angular pressing (ECAP) method. The hot compressive behavior of the alloy was first studied and the constitutive equation was extracted. Based on this constitutive equation, the ECAP processing was simulated by ABAQUS FEM software package to design the ECAP die and to predict the pressing speed. Since due to the initial coarse structure of the alloy, the ECAP processing was not successful at below 350˚C, a two-step ECAP processing was employed to avoid cracking the sample. The main benefit of such procedure was that no preceding thermomechanical process was required prior to ECAP. The first pass was done at 350˚C, while the next passes were carried out at a lower temperature, to minimize the grain growth.
To study the effect of temperature, ECAP processing was done at three temperatures of 320˚C, 290˚C, and 260˚C via route BC. The process was repeated at 290˚C via two other routes of A and C and the microstructure and the mechanical properties of the alloy were investigated. A hot rolling process was employed to study the effect of a thermomechanical processing prior to ECAP. The mechanical properties and microstructure evolution of ECAPed samples were investigated by tensile tests and optical and scanning electron microscopy, respectively. The X-ray diffraction analysis was used to identify the phases and to determine the crystallite size of the samples.
According to the microstructural observations, simultaneously occurrence of dynamic recrystallization (DRX) and dynamic precipitation (DP) are the main reasons for the grain refinement during the ECAP processing. Increasing the strain level led to the increasing the fraction of DRXed grains as well as Mg17Al12 precipitates. The results reveal that the Mg17Al12 precipitates have two effects on the development of the refined structure. The coarse precipitates (bigger than 1μm) act as a nucleation site and promote dynamic recrystallization. The smaller precipitates with pinning the new grain boundaries can also retard the grain growth and provide more grain refinement.
The tensile test results showed that an increase in strength and ductility was obtained after ECAP processing because of grain refinement in the microstructure. The strength of the AZ91 alloy was affected by grain refinement, dislocation density, precipitates and solid solution strengthening mechanism. The effect of each mechanism was calculated 47%, 26%, 5% and 16% respectively.
Using route C resulted in a finer grain size. However, the ECAP processing via route BC led to the formation of a homogenous microstructure in lower strain levels. Nonhomogeneous microstructure, the presence of shear bands in the structure, and non-uniform distribution of precipitates are the main reasons of weak mechanical properties of the sample after ECAP processing via route A.
Doing an initial rolling process prior to ECAP resulted in decreasing the temperature at the first ECAP pass. Nevertheless, the two-step ECAP processing led to the higher grain refinement compared to rolling+ECAP processing.
A model based on microstructural observations and quantitive results was proposed for development of the microstructure of AZ91 alloy during ECAP. In this model, the effect of ECAP temperature, DRX, DP and twinning on the grain refining and development of structure has been considered.
Keywords: Equal channel angular pressing; magnesium-aluminum alloy; grain refinement; dynamic recrystallization; dynamic precipitation.
