EFFECT OF CRYOROLLING ON MECHANICAL AND FATIGUE CRACK GROWTH BEHAVIOR OF AL ALLOY 6351

Abstract

Mechanical and fatigue behavior of Al alloy has studied after cryorolling (CR) in this experimental project. The Al alloy is heated to temperature 540°C and hold approximately for two hours and quenched into water to form supersaturated solid solution in alpha phase. Heat treated and quenched alloy is called solution treated or bulk alloy (ST). Cryorolling is done at extremely low temperature using liquid nitrogen and four different cases (with 20 % reduction, 40 % CR, 60 % CR and 80 % CR) are taken to calculate the Yield strength (YS), ultimate tensile strength (UTS), hardness and fatigue crack growth rate in AL alloy 6351. To study the microstructure of 6351 alloy Field Emission Scanning Electron Microscopy (FESEM), X-Ray Diffraction (XRD) and optical microscope are used. The cryorolled alloy produced ultrafine grains (ufg) which improve mechanical properties (YS, UTS and Hardness) and fatigue crack growth resistance but simultaneously reduce ductility. To optimize mechanical and fatigue properties aging treatment of Al 6351 is done for temperature range 100ºC to 200°C and optimum mechanical and fatigue properties are found at aging temperature and time of 100ºC and 12 hours. The yield strength (Y.S), ultimate tensile strength (U.T.S) and hardness (365 Mpa, 415 Mpa and 170 Hv) for 80 % CR alloy increased as compared to the bulk 6351 Al alloy (YS-140 MPa, UTS-202 MPa and hardness 67 Hv). The cryorolled Al alloy shows higher mechanical properties as compare to bulk or solution treated alloy. It is due to increase in dislocation density and suppression of dynamic recovery and production of ultrafine grains. Fatigue crack growth resistance of cryorolled samples also improve as compare to solution treated alloy it is due to increase the number of grain boundaries produce through cryorolling and formation of finer precipitates of magnesium silicide (Mg2Si) and interaction between propagating crack and grain boundaries