Response to T6 heat treatment of extruded and thixoformed EN AW 2014 alloys

Response to T6 heat treatment of extruded and thixoformed 2014 aluminium alloy was investigated in the present work. Peak hardness is attained after 16 h at an ageing temperature of 160 °C following a solution heat treatment at 500 °C in both alloys. While the time-to-peak hardness is the same in extruded and thixoformed 2014 alloys, the peak hardness obtained with the latter is higher, suggesting a superior age hardening capacity for the latter. The favourable impact of the thixoforming process is attributed to the change in grain boundary chemistry upon thixoforming. The segregation of Si, as well as of Cu, to the grain boundaries during soaking prior to thixoforming facilitates the formation of the ternary Al–Cu–Si eutectic with a relatively lower melting point at the grain boundaries. It is easier to dissolve the eutectic network at the grain boundaries and at triple junctions than coarse Al2Cu particles entrapped inside the grains. The thixoformed alloy thus enjoys a higher amount of solute Cu after the solution heat treatment and a higher age hardening capacity.

► Age hardening of the thixoformed 2014 is superior than its extruded counterpart.
► Change in grain boundary chemistry during semi-solid soaking is responsible.
► Si and Cu segregate to the grain boundaries during soaking prior to thixoforming.
► This promotes ternary Al–Cu–Si eutectic with a relatively lower melting point.