Multiple pass axi-symmetrical forward spiral extrusion of interstitial-free (IF) steel
Introduction
The ever increasing demand for safer and lighter vehicles has been behind many developments in material science and engineering. This includes generation of new “crash energy absorption materials” [1], [2]. The high strain rate sensitivity of flow stress at room temperature in some steels makes them suitable constructional material for energy absorbing parts for crash structure.
Ti-IF steels have a single-phase ferritic microstructure with a decreased amount of interstitial carbon and nitrogen, which is achieved by micro-alloying with Ti and/or Nb and formation of very small volume fractions of carbides and nitrides [3]. Although Ti-IF steel has the potential for energy absorption applications, its microstructure has low strength because of the decrease in the solid solution hardening effect of the interstitial atoms [4]. Considering the single-phase microstructure of Ti-IF steels, grain boundary and/or dislocation strengthening seem feasible methods to improve their strength.
Numerous studies have shown that severe plastic deformation (SPD) is an effective method to produce materials with refined microstructure by imposing intensive plastic strain. Since deformation under combined shear and high pressure is the most effective mode for grain refinement [5], the SPD techniques that feature shear as the dominant deformation are of more interest. Examples of such processes include equal channel angular extrusion (ECAE) [6], high pressure torsion [7], twist extrusion (TE) [8] and axi-symmetric forward spiral extrusion (AFSE) [9].
It has been established that there is a close relationship between the microstructure of the deformed materials and the corresponding mechanical properties. Mean boundary spacing and mean misorientation angle are the key factors in understanding the relationship [10], [11].
The microstructure evolution and the corresponding mechanical properties of IF steel processed by ECAE has been previously investigated [12]. It was found that the evolution of high angle grain boundaries (HAGBs) varies with processing route, but the overall trend suggests a significant increase in the HAGB area fraction with higher number of ECAE passes. Continuous ECAE [13] has been implemented in the processing of IF steel sheets. It was found that the microstructure includes dislocation cell bands with mainly low angle grain boundaries (LAGBs). An increase in the strength of the processed sheets with an increase in the number of passes was observed but there was a considerable decrease in the elongation to failure with the number of passes. Another study of ECAE [14] showed that for a higher rate of microstructure evolution and grain refinement, Route A is more efficient than Route C.
The AFSE is a SPD technique that applies both rotation and forward extrusion to the sample. The engraved grooves inside the AFSE die enables a combined rotational and longitudinal extrusion of material along the extrusion direction [9]. The tangential flow in the plane normal to extrusion direction imposes a shear deformation leading to structural evolution in the material [15].
Heterogeneous microstructure developed by most of the SPD techniques is a serious disadvantage in many cases. Recently, multiple pass SPD processing has been used to improve the microstructural homogenization, to improve grain refinement, or to achieve both [16], [17], [18]. However, analytical and experimental aspects of the multiple pass processing are complex. This makes understanding of the mechanisms responsible for these improvements difficult. In the case of AFSE, the existing simple analytical derivations are only applicable for the single pass processing.
To compare the homogenization merits and potential of multiple pass AFSE (MPAFSE) with other multiple pass SPDs, it is necessary to study the mechanical properties and microstructural evolution by this process. Ti-IF steel samples will be processed by the MPAFSE in the current study. Owing to the lack of analytical models to understand grain refinement and homogenization mechanisms during MPAFSE, a number of complementary experimental assessments will be carried out. These include the torsion test, micro-hardness tests and EBSD analysis on Ti-IF steel samples processed by the MPAFSE. The results of these assessments will be used to explain the homogenization mechanisms during the MPAFSE.
Section snippets
Sample preparation
Ultra-low-carbon Ti-IF steel with chemical composition shown in Table 1 was selected for this research. The cylindrical samples of 9 mm diameter and 20 mm length were processed for 1, 2 and 4 passes of AFSE, respectively, at room temperature. The AFSE die parameters which are helix angle γ, γ, chamfer length L1 and twist length L2 were 23°, 3 mm and 17 mm, respectively (details of AFSE can be found in [9]).
According to an existing kinematics solution of AFSE, the equivalent plastic strain of a
Mechanical properties
The stress–strain curves obtained from the hollow torsion results are shown in Fig. 3. Decreased ductility is a general feature of strain-hardened metallic materials [24] and has been reported for some SPD processes. This behaviour is attributed to the dislocation dynamics in ultra-fine grained materials and is a pronounced specification of most SPD products [25]. However, the results in Fig. 3 cannot be considered as proof of reduced ductility in the post AFSE samples. Due to experimental
Conclusion
The effect of deformation by AFSE on mechanical and microstructure evolution of Ti-IF steel were studied. Mechanical properties were assessed by torsion and hardness tests while microstructure investigations were performed by EBSD. The improvement of strength after AFSE can be related to the deformed microstructure which includes sub grains with relatively high density of dislocations. However, the limited ductility of the processed Ti-IF steel by AFSE can be attributed to the plastic
Acknowledgement
The authors acknowledge the use of facilities within the Monash Centre for Electron Microscopy (MCEM).
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