COMPARATIVE HOT WORKABILITY OF 7012-ALLOY AND 7075-ALLOY AFTER DIFFERENT PRETREATMENTS

Hot torsion tests, in the range 250-450 degrees C and 0.05-5.0 s(-1), were performed on Al-Zn-Mg-Cu alloys (7012 and 7075), which had been d irect chill cast, homogenized and precipitation treated to give fine, well-dispersed precipitates. Additional tests were conducted on materi al that had been extruded, solution treated or precipitation treated a t deformation temperature. The peak flow stress was related to the str ain rate by the hyperbolic sine equation; the activation energy for pr ecipitated alloys was close to that of the bulk self-diffusion of pure aluminium. For solution-treated metal, the peak stress was very high at low temperatures due to dynamic precipitation; as a consequence, th e activation energy was about 50% higher than that of precipitated all oys. The ductility was almost independent of temperature in the invest igated range, but decreased with rising strain rate. The ductility of the extruded alloys was almost double that of the as-cast material, wi th the exception of the solution-treated material where, at low temper ature, the ductility of the extruded alloy was lower. The original gra ins were elongated with precipitates on the boundaries. The dynamicall y recovered subgrains exhibited sub-boundaries with a high density of fine precipitates and an interior network of dislocations also tied to precipitates.