Shashank Tiwari

Recovery quantification and onset of recrystallization in aluminium alloys

Novel forming processes for light metal alloys utilize recovery and recrystallization to extend their total elongation and enhance formability. To attain optimum efficiency in such processes, it is necessary to understand and quantify the kinetics of recovery and recrystallization in work-hardened metal alloys. An electron backscatter diffraction based method, using local average orientation spread, is shown to identify the end of recovery as well as the onset of recrystallization. Local average orientation spread results from dislocation flux and storage during plastic deformation and hence, captures the evolution of static recovery process. The method has been demonstrated using pre-strained Al–Mg alloys. The recovery kinetics is shown to be consistent with results from dislocation density based recovery models. In addition, a direct observation of the coexistence of static recrystallization and recovery illustrates competing processes for energy minimization.

Influence of Microstructure on Uniaxial Strain Localization in AA5754 Aluminum Sheets Produced by Various Processing Routes

The application of lightweight aluminum sheets to fabricate automotive components for vehicle weight reduction continues to be limited due to their low formability and high cost. This report summarizes a metallurgical investigation of the influence of various microstructural attributes on the forming and failure characteristics of aluminum sheets produced by lower cost continuous casting processes. The study has identified the combination of microstructural attributes, such as grain size, texture, and second phase particle distribution, in the sheets which make some sheets more formable than others and has traced the origin of these features to the processing history. The results show that the microstructural features present in the sheets have their origin in the casting, rolling, and recrystallization processes involved in their fabrication.

Annealing response of AA5182 deformed in plane strain and equibiaxial strain paths

The annealing response of AA5182 Al–Mg alloy deformed to an effective prestrain of  = 0.15 via plane strain and equibiaxial strain paths is compared. The comparison is done at two temperatures namely, 623 and 673 K. The recovery, recrystallization and grain growth behaviour of this alloy is studied by electron backscatter diffraction and dislocation density estimation using X-ray line broadening analysis. It is found that recrystallization is slower in equibiaxial deformation condition compared to that in plane strain deformation condition during annealing. Significant recrystallization is observed after annealing for 60 s at 673 K and for 480 s at 623 K following plane strain deformation. Furthermore, significant recrystallization is associated with lower grain growth at 673 K (∼55 μm) as opposed to that at 623 K (∼75 μm). The results are explained on the basis of differences in both the strain paths.

Cluster identification in AA5754 aluminium sheets using mathematical morphology analysis

Quantitative image analysis of particle distribution in the microstructure of continuous cast (CC) and direct chill cast (DC) AA5754 aluminium alloy sheets have been conducted. This information can be used as an input for modelling mechanical deformation and instability in these materials. The quantitative analysis reveals that there are significant differences in the microstructure of the two materials even though the total content of second‐phase particles is statistically similar. Qualitative observation shows the second‐phase particles to be arranged in the form of streaks parallel to the rolling direction in the CC sheets and in a uniform random manner in the DC sheets. The main difference in the geometric microstructure of the CC and DC material is the spatial arrangement of the second‐phase particles. A new mathematical technique called proximity analysis is developed to identify clusters and group of particles belonging to a cluster. Quantification through proximity analysis reveals that the particle clusters in CC sheet are in the form of long clusters (streaks) parallel to the rolling direction and are significantly longer than those in DC sheets (with the largest cluster in CC being four times larger than DC), and also have anisotropic angular orientation parallel to the rolling direction. The lower value of fracture strain observed in the CC sheets compared to DC sheets is attributed to a combination of large sizes of clusters and their preferential alignment along the rolling direction in the CC microstructure.