PSI - Issue 14

Pankaj Kumar et al. / Procedia Structural Integrity 14 (2019) 96–103 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

97

2

1. Introduction

Al-Mg alloys are commonly treated as non-heat treatable aluminium alloys having inherent characteristics such as high stiffness to weight ratio, good formability and recycling potential etc. Yao et al. (2000) have reported that the mechanical and fatigue properties of aluminium alloys can be easily altered through metal forming processes. Ozturk et al. (2011) investigated that c hanging the material’s forming temperature, results in substantial improvement of formability characteristics along with enhanced mechanical properties of the Al-Mg alloy. These processes results in ultra-fined grain (UFG) structure having remarkable physical, chemical and electrical properties. Among them, rolling at cryogenic environment (cryorolling) is a potential technique to achieve bulk UFG structures. Krishna et al. (2015) have postulated that in cryorolling, UFG ’ s are achieved through hindrance of dynamic recovery during large inelastic deformation at cryogenic temperature. Singh et al. (2013) studied the influence of cryorolling on microstructure and mechanical properties of Al 5083 alloy. In this work UTS of the material is reported to increase by 22.7% and 40.8% for 30% and 50% reduction. Cryogenically processed Al-Mg alloys potentially increases its demand in automotive, aerospace, marines and cryogenic industries. In these applications the components are subjected to varied dynamic loadings. Therefore it is important to study the mechanical performance of these alloys under cyclic loads. Several studies have been reported on fatigue performance of UFG aluminium alloys. Fatigue performance of fine grained Al 5056 obtained through equal channel angular pressing (ECAP) process has been reported by Vinogradov and Hashimato (2001). They reported that the crack growth takes less time to propagate in fine grained alloy as compared to coarse grained alloy. Malekjani et al. (2011) had reported no increment in fatigue life of UFG cryorolled 2024 Al alloy during strain controlled LCF test conducted at three different strain amplitudes. The cryorolled material shows no significant cyclic hardening or softening behaviour during entire fatigue life at each of tested strain amplitudes. Researchers have also applied finite element method (FEM) is to predict the low cycle fatigue behaviour of various metals and alloys without presence of discontinuity. The cyclic hysteresis loop and cyclic hardening/softening behaviour were predicted through hardening models by using FEM. Among various kinematic hardening models ― Chaboche kinematic hardening model ‖ is preferred to simulate the experimental hysteresis loop. Bari and Hassan (2002) and Paul et al. (2010) used Chaboche kinematic hardening model for CS 1026 steel and SA333 CMn steel to simulate experimental hysteresis loop. Recently, Kumar and Singh (2017) have investigated the cyclic plastic LCF behavior of AA 5754 alloy.

Nomenclature Δ ɛ

̅

strain range

d ̅ σ o

back stress tensor

increment in plastic strain tensor

initial cyclic yielding

C (i) , γ (i), Q, b kinematic hardening coefficients From above discussion it has been concluded that in literature experimental and numerical investigations on LCF behavior of CR and ACR Al-Mg alloys are not discussed in detail. In view of the aforementioned, the present investigation deals about the investigation of post-annealing behavior of cryorolled AA 5754 on LCF performance at three different strain amplitudes. The kinematic hardening model is employed to simulate the experimental hysteresis loops. XFEM is utilized to simulate the tensile behavior of the material.

2. Experimental procedure

2.1. Cryorolling

Cryorolling has been planned for 40% thickness reduction of AA 5754. The schematic of cryorolling operation is shown in Fig. 1(a). The initial block of dimension 90 mm×75 mm×23 mm is considered for rolling. These rectangular blocks are initially heated at 530 o C for 2 hours and rapidly quenching with water results in homogenization of grain structure. Panigrahi and Jayaganthan (2011) reported that large immersion time in liquid