February 2022 Volume 4

FORGING RESEARCH

additional uniaxial compression trial with pyramidal 〈 〉 slip was also performed, and the results are presented in Figure 6. Changing from anisotropic 〈 〉 CRSS values to isotropic 〈 〉 has a relatively minor effect overall. However, the introduction of pyramidal 〈 〉 produces a substantial change. The contrast between orientations with high and low deviation from the initial BOR increased tremendously, suggesting a signicant difference between orientations which will globularize readily and those that do not. Further evaluation of pyramidal 〈 〉 slip appears warranted.

Next, different constitutive behavior was examined by changing the relative critical resolved shear stresses (CRSSs) for slip. In the original work, the CRSS values were taken directly from Reference [3] and perfectly plastic deformation was assumed. Also, each direction was assumed to behave dierently due to the initial BOR and the anisotropy of slip transmission. However, the BOR breaks down during deformation destroying the direction anisotropy. As such, the uniaxial compression trial was run assuming isotropic CRSS values. Finally, some authors have suggested that pyramidal 〈 〉 slip in the must be included for accurate texture evolution. An

Figure 6: Deviation from the initial BOR as a function of colony loading direction for different CRSS assumptions. Figure 6: Deviati n from the initial R as functio of c lony loading direction for ie n CRSS assumptions.

Finally, the boundary conditions employed during the simulation of uniaxial compression were evaluated. In the original simulations strain control was assumed because forgings are geometry- determined. However, in real materials the strain is not uniform in every grain. For example, colonies where the compression direction is aligned with the 〈 c 〉 axis are "hard orientations." In a polycrystal, these orientations would undergo relatively little strain relative to their softer neighbors. To capture this effect, stress-controlled trials were

run in uniaxial compression. It is assumed that stress control will help to represent load-shedding and possibly the kinking behavior that the original simulations missed. The stress-controlled simulation results are presented in Figure 7. As illustrated, now loading directions aligned with the 〈 c 〉 axis are correctly predicted to be difficult to globularize. Incorporating more elements of stress- controlled deformation also appears to be a promising strategy for

improving simulation predictive power. Figure 6: Deviation from the initial BOR as a function of colony loading direction for different CRSS assumptions.

Figure 7: Deviation from the initial BOR as a function of colony loading direction for strain-controlled and stress-controlled uniaxial compression.

FIA MAGAZINE | FEBRUARY 2022 81 direction anisotropy. As such, the uniaxial compression trial was run assuming isotropic a CRSS values. Finally, some authors have suggested that pyramidal a slip in the α must be included for accurate texture evolution. An additional uniaxial compression trial with pyramidal a slip was also performed, and the results are presented in Figure 6. Changing from anisotropic a CRSS values to isotropic a has a relatively minor effect overall. However, the introduction of pyramidal a produces a substantial change. The contrast between orientations with high and low deviation from the initial BOR increased tremendously, suggesting a significant difference between orientations which will globularize readily and those that do not. Further evaluation of pyramidal a slip appears warranted. Finally, the boundary conditions employed during the simulation of uniaxial compression were evaluated. In the original simulations strain control was assumed because forgings are Figure 7: Deviation from the initial BOR as a function of colony loading direction for strain-controlled and stress-controlled uniaxial compression. direction anisotr py. As such, he uniaxial comp ession trial was run assuming isot opic a CRSS l es. Fi ally, some uthors have suggested hat pyr midal a slip in the α must Figure 7: Deviation from the ini ial BOR as a f n ion of c lony loading irection for strain-controlled and ress-controlled u iaxial compression.