PRE-AGING OF RETROGRESSION AND RE-AGING OF SPRAY FORMED 7075 ALLOY

Retrogression and re-aging (RRA) treatments are divided into pre-aging, retrogression and re-aging. Although peak aging was used as the pre-aging of RRA treatment in the past, some different opinions were reported in recent years. The effects of pre-aging of RBA treatment on microstructure, mechanical properties and conductivity of spray formed 7075 aluminum alloy were investigated by TEM, tensile and conductivity test. The results show that the mechanical properties and conductivity of spray formed 7075 alloy could be improved by the under aging at 120 degrees C for 16 h as the pre-aging of RRA treatment. The properties of the 7xxx series aluminum alloys depend on matrix precipitates (MPt), grain boundary precipitates (GBPs) and precipitate free zones (PFZs). The tiny MPt can increase the tensile strength. The connected GBPs and narrow PFZs will lower the conductivity and the elongation of the alloy. The under aging is more beneficial for the re-dissolution of the MPts at retrogression treatment at 200 degrees C for 10 min, and is more conducive to interrupt distributions of the GBPs than the early aging after RRA treatment. With the under aging as the pre-aging treatment, the growth of the MPts was actively suppressed and the GBPs at grain boundaries are continuous. During retrogression treatment, the MPts were re-dissolved absolutely and the GBPs were transformed from a chain to dissociation. After re-aging treatment, lots of tiny dispersive MPts precipitated out again in matrix, the GBPs were totally separated and the PFZ were widened. After pre-aging at 120 degrees C for 16 h and RRA treatment, the tensile strength and yield strength of the alloy are 782 and 726 MPa, respectively, which are higher than that after peak aging treatment or conventional RRA treatment, the conductivity of the 7075 alloy is excellent with 22.7 MS/m.