Stir zone anisotropic work hardening behavior in friction stir processed EN8 medium carbon steel

Abstract

The SZ anisotropic work hardening behavior in duplex ferrite-martensite phase (DFM) microstructures has not been investigated yet. It is an important consideration for the selection of the stretch formability direction of the friction stir processed blank used for sheet metal forming applications. The anisotropic work hardening behavior is characterized by using miniature tensile specimens prepared along longitudinal (LD) and transverse (TD) directions. The tribological parameters such as coefficient of friction (COF) and wear rate at LE value of 6 are 0.57 and 0.5 × 10 − 10 m 3 / Nm . The effect of surface texture on the work hardening response is observed to be negligible. The plastic anisotropy in terms of three and multi-stage work hardening behaviors is investigated at a linear energy ( LE) value of 6. The global true stress-strain responses and fracture surface morphologies suggested that the deformation occurs in different stages. In the initial stage, the deformation is mainly localized in the soft ferrite phase followed by the distinct work hardening stages in the plastic flow curves. In the elastic and elastoplastic region of the flow curve, the soft ferrite phase plastically deformed, followed by stress transfer to the martensite/retained austenite phases in the fully plastic region. The (110) and (111) pole figure suggested that the active in-habit-plane slip system is (110) and [ 1 1 ‾ 1 ] . The strain localizes more in the V19 + V22 block having the critical resolved shear stress (CRSS) value of 445.5 MPa. The retained austenite (RA) is having ( − 11 − 1 ) A / / ( 0 − 11 ) F and [ 011 ] A / / [ 111 ] F type Kurdjumov-Sachs (K–S) orientation relationship with the ferrite phase. A BCC {112} <111> -type twinned laths is found to be present within the martensite laths. The recommended stretch formability direction (SFD) should coincide with the longitudinal or processing direction of the friction stir processed blank.