喷射态7075合金回归再时效中预时效的研究

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

采用拉伸测试、TEM和电导率测试等方法,研究回归再时效(RRA)处理中预时效处理对喷射成形7075合金组织及性能的影响.结果表明,采用120℃,16 h的欠时效预处理比120℃,24 h峰值时效预处理和120℃,32 h过时效预处理更有利于在回归处理过程中合金晶内析出相的回溶,比120℃,8 h的早期预时效处理更有助于晶界相在再时效处理后断续分布.120℃,16 h预时效处理的合金经RRA处理后,其抗拉强度和屈服强度分别为782和726 MPa,均高于T6峰值时效和常规回归再时效水平;电导率为22.7MS/m,晶界析出相断续分布,合金抗腐蚀性能优良.

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.