MECHANICAL CHARACTERIZATION OF Al- 6061 STRENGTHENED BY ECAP

Abstract

A study was carried out to investigate the effects of equal channel angular pressing (ECAP) on mechanical properties and microstructures of Aluminum alloy Al-6061. Teflon and Nylon was also investigated to visualization deformation because of their high elastic plastic behaviour to validate the mathematical model. The severe plastic deformation (SPD) techniques were used to increase strength through grain refinement. The ECAP process has improved upon the mechanical properties of light metal alloys like Al-6061. In this process the specimen size remain unchanged and the required mechanical properties are increased by SPD technique. The design and development of ECAP experimentation was done on specimens and equipment used to do with suitable measurements and with safety precautions. The experiments were carried out on Teflon, Nylon, and Aluminum 6061. The die and ECAP fixture were designed and fabricated in the Mechanical Engineering Department at University of Engineering & Technology Taxila. A purpose built hydraulic press having capacity of 100 tons with special fixture was used to squeeze the material through ECAP die. In situ heating of die with specimen was employed for smooth flow of material through the die. The temperature was maintained at upper critical temperature (i.e. 450 0C) for two hours for obtaining homogeneous temperature. A total number of eight experiments were performed using the ECAP process on this experimental setup. II The shear strain mathematical model for ECAP was developed considering the elastic recovery of materials after angular extrusion. In addition to shear strain mathematical model, the load required to push the material through ECAP die was calculated to achieve results. The numerical simulation through ABAQUSTM 6.10.1 was performed to validate the mathematical model. The mathematical model of shear strain and orientation in axis was validated successfully with the 3% error along major axis and 2% along minor axis of ellipse. A series of tests including tensile, Vickers hardness, micro hardness, and three points bend fatigue (crack mouth opening displacement) test were performed for mechanical characterisation. In addition, metallographic and fractographic analyses were performed for microstructure verification. The 3-point bend fatigue tests were performed on as-received and ECAP specimens. The specimens were made according to ASTM-E647 standard from Al-6061 alloy. The fatigue crack growth (FCG) behaviour of as-received and ECAP was investigated and compared against different stress ratios. Different plots were drawn between different geometric parameters and found that fatigue crack growth was slower in ECAP specimens as compared to asreceived specimens. This slow rate of FCG was mainly due to the increase in strength by grain refinement introducing severe plastic deformation. The mechanical and microstructure analyses validated grain refinement through ECAP process. A 25% increase in Vickers hardness, 15% improvement in yield strength and 35% enhancement in ultimate tensile strength was recorded