Potential of cast EN AW7075 billet as thixoforging feedstock

Abstract

The potential of cast 7075 billets for thixoforging without prior thermomechanical processing was investigated. Slugs machined from EN AW7075 cast billet were thixoformed at 615°C with a liquid fraction of 20%. The dendritic as cast matrix has transformed into a more compact structure during heating into the semisolid temperature range. This structural change was facilitated by the reversion of the fine precipitates along the dendrite boundries inherited from dendritic segregation of the as cast billet and the coalescence of the neighbouring dendrite arms. The equiaxed polygonal grains thus obtained have become increasingly globular upon holding at 615°C. The liquid phase penetrated between the grains, forming a more or less continous intergranular network. Microstructural features essential for forming in the semisolid state were obtained after ∼10 min at this temperature. The forming process has taken place in the semisolid state with no evidence of grain deformation. The thixoformed 7075 part was solutionised at 470°C for 6 h and was subsequently quenched in water. Artificial aging at 121°C has produced hardness values as high as 175 HV after 16 h. It is thus fair to conclude that the cast EN AW7075 billet can be readily thixoformed after a 3 min heating ramp and a 10 min isothermal hold at 615°C.