FORMING LIMIT IMPROVEMENT OF AZ31 MAGNESIUM SHEET AT ROOM TEMPERATURE USING PRESTRAIN ANNEALING

Abstract

The formability of automotive magnesium sheet at room temperature is limited due to its HCP crystalline structure with limited slip systems. To improve the formability at room temperature, a new method of pre-strain annealing and subsequent forming (PASF) has been assessed. Uniaxial tensile tests are conducted on fully annealed AZ31 sheets as well as on a range of pre-strain annealed samples at an annealing temperature of 250 . The test data is used to characterize improvements to tensile elongations as well as changes in the work hardening behaviour arising from the PASF process as well as for prediction of forming limit diagrams. The formability improvements from PASF process are further assessed by conducting interrupted hemispherical punch stretching tests at room temperature, annealing, and then continuing the dome stretching process to fracture. Several different specimen geometries are tested to analyze the punch load versus punch displacement characteristics. Further, the tests are used to construct the room temperature forming limit diagrams (FLD) of fully annealed sheets as well as PASF process dome samples. The improvement in limit strain by PASF process is strongly dependent on the pre-strain, annealing temperature and time. An improvement of 20% in limit strain values could be achieved with one annealing step under optimized conditions of annealing and pre-straining. Experimental FLDs are also compared with theoretical FLDs based on well-known Marciniak-Kuczynski (M-K) analysis and an existing anisotropic yield criterion with appropriate modifications for anisotropic characteristics and changes in the work hardening behavior of AZ31 sheet from the PASF process. The M-K theory is also used to calculate FLD of AZ31 sheet at 300 and compared with an existing FLD determined in the McMaster Metal Forming Laboratory. Domes test samples are also examined by optical metallography to rationalize the differences in fracture behavior of fully annealed and PASF samples.