Effect of post-homogenisation cooling rate and Mn addition on Mg2Si precipitation and hot workability of AA6060 alloys

ABSTRACT

The microstructure evolution for different post-homogenisation cooling rates and the flow stress behaviour in direct chill cast AA6060 alloys were studied. Results revealed that decreasing cooling rates reduced the flow stress owing to the precipitation of Mg₂Si and reduction of the solid solution level. Micro-alloying of Mn generated a distribution of α-Al(FeMn)Si dispersoids during the homogenisation, with the size and number density decreasing at higher homogenisation temperatures. TEM studies confirmed that the dispersoids acted as favourable nucleation sites for Mg₂Si and significantly promoted the precipitation of Mg₂Si during subsequent cooling. The high-temperature flow stress was controlled by the solid solution levels of Mg, Si, and Mn resulting from the interaction between dispersoids and Mg₂Si. The combination of the Mn addition, a low cooling rate, and a low homogenisation temperature provided the lowest flow stress, which improved the hot workability of the alloy and promoted ready dissolution of Mg₂Si during extrusion.