FATIGUE & FRACTURE STUDIES ON ULTRAFINE GRAINED 2014 AL ALLOY

Abstract

Al alloys are extensively used in high strength applications due to its high strength to weight ratio. Among Al alloys, Al 2014 alloy is generally used for aircraft and automobile applications and is key engineering material for aircraft fitting vessels, roof structures and bridge decks. Therefore, it is imperative to enhance the mechanical and physical properties of these alloys through different thermo mechanical techniques without changing the alloy composition in order to increase service life of the structural components. In recent years, producing ultrafine grained material (UFG) through various severe plastic deformation techniques is growing enormously for achieving significant improvement in mechanical and physical properties hitherto unachieved in their bulk materials. In addition to the primary properties of these ultrafine grain (UFG) alloys such as tensile strength and hardness, fatigue and fracture behavior are very essential for the structural components experiencing dynamic loads during service conditions. Therefore, the fatigue and fracture studies can provide comprehensive deformation behavior of material under cyclic loading as compared to monotonic loading. Cryorolling and cryoforging are the novel deformation processing techniques used widely to produce ultrafine and nanostructures in the pure metals and alloys. In this technique, dynamic recovery is suppressed to accumulate high density of dislocation in the materials during processing at liquid nitrogen temperature. The dislocation density acts as a source of nucleation sites for the formation of ultrafine grains and nanostructures in the bulk materials through dynamic recrystallisation. The literature on the effect of cryorolling & cryoforging on tensile properties, fatigue and fracture behavior of Al 2014 alloys is scarce. Thus, the present work is focused on fabricating ultrafine grained Al 2014 alloy through cryorolling and cryoforging and to investigate i) Effect of cryorolling and followed by annealing on the mechanical properties such as tensile strength, yield strength, fracture toughness, fatigue crack growth rate and high cycle fatigue of Al 2014 alloy ii) To correlate the above mechanical properties with the microstructural features obtained through optical microscopy, SEM, TEM and EBSD characterization techniques iii) To identify the optimum processing conditions for the bulk UFG Al 2014 alloy usable in high strength structural applications iv) Development of ultrafine grain microstructure in coarser grained Al 2014 alloy through multidirectional room temperature and cryoforging v) A comparative study of ii mechanical properties (Ultimate tensile strength, yield strength and Fracture toughness) and microstructure of bulk UFG Al 2014 developed by multidirectional room temperature forging and cryoforging vi) To study the influence of annealing on the tensile properties, strain hardening behavior, fracture toughness, and fracture mechanism of bulk UFG Al 2014 alloy produced by multidirectional cryoforging. The key results obtained in each piece of work are discussed below. To understand the effect of cryorolling and followed by annealing on microstructural evolution, precipitation sequence, tensile properties and fracture toughness of Al 2014 alloy, the sample was solutionised (ST) and subjected to cryorolling (CR) up to effective true strain of 2.3. The CR Al 2014 alloy samples were annealed (AN) at temperatures ranging from 100°C to 350°C for the duration of 45 minutes. Study reveals the improvement in ultimate tensile strength (245 MPa - 447 MPa) and fracture toughness Kee (23.06 MPa√𝑚 - 37.8 MPa√𝑚) of cryorolled Al 2014 alloy as compared to solution treated alloy with reduction in ductility from 18.5% to 4.8%. When cryorolled samples were annealed in the temperature range of 100°C to 350°C, the strength and fracture toughness were retained up to 200°C, while continuous drop in these properties were observed when samples were annealed beyond 200°C. However, the ductility was improved with increasing annealing temperature when cryorolled samples were annealed from 100°C to 350°C due to softening facilitated by dynamic recovery and recrystallisation, which led to formation of dislocation free grains. The improvement in mechanical properties of cryorolled Al 2014 alloy at low temperature annealing from 100°C to 200°C is attributed to formation of GP zones and metastable phase θ' in this temperature range, while the reduction in mechanical properties beyond temperature 200°C is observed due to combined recovery, recrystallisation and formation of stable coarser phase θ and λ. Studies on the effect of cryorolling and followed by annealing on high cycle fatigue behaviour of bulk UFG Al 2014 alloy revealed the improved high cycle fatigue (HCF) strength of cryo rolled (CR) alloy as compared to solution treated (ST) alloy due to grain refinement. The improvement in high cycle fatigue properties of cryorolled followed by annealed alloy up to 200°C as compared to ST alloy observed is due to improved crack tip plasticity facilitated by crack tip/precipitate interaction at grain boundaries (GBs).The high cycle fatigue (HCF) strength is observed to be maximum on annealing at 100°C, while beyond iii this temperature, a gradual decrease on high cycle fatigue (HCF) strength is observed as compared to sample annealed at 100°C due to gradual coarsening of metastable precipitate (θ' phase), which transformed in to stable coarser precipitate ‘θ’ phase at 250°C. On investigating the fatigue crack growth (FCG) behaviour of cryorolled and followed by annealed alloy at low stress intensity factor (ΔK) range, fatigue crack growth rate (FCGR) of cryorolled (CR) alloy was observed to be more as compared to coarser grain solution treated (ST) alloy due to formation of UFG microstructure resulting reduced crack path tortousity, while on annealing in the temperature range 100°C to 250°C, the fatigue crack growth rate (FCGR) decreases significantly and it was observed to be minimum for sample annealed at 100°C. The decrease in fatigue crack growth rate (FCGR) at 100°C is attributed to evolution of fine metastable spherical phase θ', which obstructs the crack growth during FCGR testing. To investigate the effect of multidirectional room temperature forging and cryoforging on the microstructure evolution, tensile properties and fracture toughness, Al 2014 alloy is multidirectional forged (MDFed) at room temperature and cryogenic temperature up to cumulative true strains of 1.2 (2 cycles), 1.8 (3 Cycles) and 2.4 (4 cycles). This study revealed that, multidirectional cryoforged sample up to cumulative strain of 2.4 showed an improvement of ultimate tensile strength, hardness and apparent fracture toughness (KQ) from 245 MPa to 470 MPa, 115 HV to 171 HV, and 23.93 MPa√𝑚 to 37.7 MPa√𝑚 , respectively, with decrease in ductility from 18.5% to 6% as compared to solution treated alloy. The substantial improvement in the ultimate tensile strength (7%), yield strength (3%) and hardness (3%) of multidirectional cryoforged Al 2014 alloy is observed as compared to room temperature forged alloy due to suppression of dynamic recovery at liquid nitrogen temperature. To understand the effect of multidirectional cryoforging (MDCF) and followed by annealing on the tensile properties, strain hardening behavior, fracture toughness, and fracture mechanism in MDCFed Al 2014 alloy, the sample is post annealed in the temperature range from 150°C to 350°C with the interval of 50°C for the duration of 1 hour. The mechanical properties are correlated with the microstructural evolution during deformation and post deformation annealing through optical microscopy and TEM studies, while fracture mechanism in processed and annealed alloy is established using macrograph analysis and SEM studies. The study reveals that strain hardening ability, fracture mechanisms and fracture iv toughness of deformed and post annealed samples are influenced by the shear banding, combined recovery /recrystallisation process, and evolution of second phase precipitates during annealing treatment at various temperatures. Significant improvement in the ultimate tensile strength (UTS) and fracture toughness is observed on annealing the processed alloy at 100°C, while beyond this temperature, a gradual drop in these properties is seen as compared to sample annealed at 100°C. Macroscale fracture behaviour in tensile testing of multidirectional cryoforged (MDCFed) alloy consists of shear fracture while upon annealing, it is transformed gradually in to mixed mode of fracture consisting of shear plus necking. Finally, it was concluded that fine spherical semi coherent phase θ' is responsible for improving the mechanical properties of bulk UFG Al 2014 alloy. Finally, strengthening contributions to the yield strength from different strengthening mechanism were evaluated for the multidirectional cryoforged (MDCFed) and MDCFed followed by annealed Al 2014 alloy. The study revealed that grain boundary strengthening (𝜎𝐺𝑏𝑠), is dominating strength contributor for raising the yield strength of MDCFed Al 2014 alloy, while for annealed samples up to the temperature range 350°C, the grain boundary (𝜎𝐺𝑏𝑠) and precipitation strengthening (𝜎𝑝𝑠) are found to be dominating strengthening mechanism in raising the yield strength of bulk UFG Al 2014 alloy.