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 RRA 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 ℃ 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 ℃ 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 ℃ 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.
Fund: Supported by Fok Ying-Tong Education Foundation (No.121054) and Program for Innovative Research Team in University of Liaoning Province (No.LT2012004)
Fig.1 Tensile properties (a) and conductivity (b) of 7075 alloy pre-aged at 120 ℃ for different times
Fig.2 Tensile properties (a) and conductivity (b) of 7075 alloy after retrogression at 200 ℃ for 10 min with pre-aging at 120 ℃ for different times
Fig.3 Tensile properties (a) and conductivity (b) of 7075 alloy after retrogression and re-aging at 200 ℃ for 10 min and at 120 ℃ for 24 h with pre-aging at 120 ℃ for different times
Fig.4 TEM images of 7075 alloy pre-aged at 120 ℃ for 8 h (a), 16 h (b), 24 h (c) and 32 h (d)
Fig.5 TEM images of 7075 alloy after retrogression treatment at 200 ℃ for 10 min with pre-aging at 120 ℃ for 8 h (a), 16 h (b), 24 h (c) and 32 h (d)
Fig.6 TEM images of 7075 alloy after retrogression and re-aging (RRA) treatment at 200 ℃ for 10 min and at 120 ℃ for 24 h with pre-aging at 120 ℃ for 8 h (a), 16 h (b), 24 h (c) and 32 h (d)
Fig.7 Schematic of precipitate transformations of 7075 alloy during RRA with different pre-aging treatments
[1]
Wang Y Q, Wang Y, Chen P M, Shao Y W, Wang F H. Acta Metall Sin, 2011; 47: 455
(王艳秋, 王 岳, 陈派明, 邵亚薇, 王福会. 金属学报, 2011; 47: 455)
[2]
Marlaud T, Deschamps A, Bley F, Lefebvrec W, Baroux B. Acta Mater, 2010; 58: 248
[3]
Marlaud T, Deschamps A, Bley F, Lefebvrec W, Baroux B. Acta Mater, 2010; 58: 4814
[4]
George S L, Knutsen R D. J Mater Sci, 2012; 47: 4716
[5]
Su R M, Qu Y D, Li R X, Li R D. Appl Mech Mater, 2012; 217: 1835
[6]
Jeyakumar M, Kumar S, Gupta G S. Mater Manuf Proc, 2010; 25: 777
[7]
Silva G, Rivolta B, Gerosa R, Derudi U. J Mater Eng Perform, 2013; 22: 210
[8]
Ricker R E, Lee E U, Taylor R, Lei C, Pregger B, Lipnickas E. Metall Mater Trans, 2013; 44A: 1353
[9]
Ning A L, Liu Z Y, Feng C, Zeng S M. Acta Metall Sin, 2006; 42: 1253
(宁爱林, 刘志义, 冯 春, 曾苏民. 金属学报, 2006; 42: 1253)
[10]
Fooladfar H, Hasnemi B, Younesi M. J Mater Eng Perform, 2010; 19: 852
[11]
Arnold E M, Schubbe J J, Moran P J, Bayles R A. J Mater Eng Perform, 2012; 21: 2480
[12]
Cina B M. US Pat, 3856584, 1974
[13]
Li R D, Su R M, Qu Y D. J Mech Eng, 2013; 49(20): 22
(李荣德, 苏睿明, 曲迎东. 机械工程学报, 2013; 49(20): 22)
[14]
Su R M, Qu Y D, Li R D, Xie Q M, Wu Y S. Adv Mater Res, 2013; 774: 872
[15]
Peng G, Chen K, Chen S, Fang H. Mater Sci Eng, 2011; A528: 4014
[16]
Reda Y, Abdel-Karim R, Elmahallawi I. Mater Sci Eng, 2008; A485: 468
[17]
Ohnishi T, Ibaraki Y, Ito T. Mater Trans JIM, 1989; 30: 601
[18]
Higashi K, Ohnishi T, Tsukuda I. US Pat, 4713216, 1987
[19]
Lin J, Kersker M M. US Pat, 5108520, 1992
[20]
Han X L, Xiong B Q, Zhang Y A, Zhu B H, Li Z H, Li X W, Wang F, Liu H W. Chin J Nonferrous Met, 2012; 22: 3006
