چكيده به لاتين
Severe plastic deformation is a method to achieve ultra fine grain (UFG) structures in metals and alloys. Among the various methods of plastic deformation, equal channels angular pressing (ECAP) has greater efficiency and therefore, the most commonly used method. Achieving further grain refinement during ECAP requires increased workability which has been done with using partial annealing and application of back pressure.
In this investigation, the effect of choked and relieved configurations of die exit channel on the workability of AA6063 alloy during equal channel angular pressing (ECAP) is investigated. Finite element simulation is used to predict variation in maximum principal stress (1) to be consequently correlated to the occurrence of surface cracking. It was found that the critical regions for fracture are at the top surface of the sample at which tensile 1 exists. Tensile stresses are involved when dies with parallel or relieved exit channels are used while the stresses are compressive in case of die with choked exit channel. Choke angle of 0.2 is found as the optimum value guaranteeing compressive 1 in addition to avoiding significant increase in strain inhomogeneity and pressing pressure. In order to verify the FEM predictions, two different configurations are considered for the exit channel of the ECAP die, i.e., relieved and choked, with angles of 0.2 . In the die with relieved exit channel, the sample was extruded for 6 passes with no surface cracks. By using the die with choked exit channel, it was possible to deform the sample for up to 14 passes with no sign of surface cracking. Increasing the number of passes has helped the alloy to re-strengthen after it acquires a saturated level of hardness and tensile strength in 4 passes. By significant increase in the number of passes due to utilization of the die with choked exit channel, a second mechanism of grain refinement, i.e., progressive lattice rotation, has been activated leading to extremely fine grains within the size range of less than 100 nm.
