Microstructural changes during tension of friction-stir welded AZ31 magnesium alloy

In this work, microstructural changes occurring during transverse tension tests of friction-stir welded AZ31 magnesium alloy were systematically studied in order to clarify the microstructure-properties relationship. To this end, digital image correlation and electron backscatter diffraction (EBSD) were employed. As expected, plastic deformation in the stir zone was found to be preferentially concentrated at its advancing- and retreating sides. In both these locations, the microstructure evolution was shown to be a complex process consisting of two stages. The first stage was characterized by a profuse {101―2} twinning giving rise to a 0001<112¯0> texture component. In the second stage, enhanced slip activity led to extensive formation of low-angle boundaries and to the development of a fiber hkil<112¯0> texture. The twin-induced 0001<112¯0> orientation was shown to promote double {101¯1}–{101¯2} twinning which is known to lead to final failure. It was therefore suggested that the cause of the well-known premature fracture of magnesium welds at the stir zone extremity may be due to the profuse {101¯2} twinning.