Abstract:

The material used in this investigation was LC4 aluminum alloy （e-quivalent to 7075 in ASTM） in sheet form with thickness 2.4mm. It contains 6.03% Zn, 2.70% Mg. 1.54% Cu, 0.30%Mn, 0.48% Fe, 0.23% Si and 0.13% Cr. Specimens were solution treated for 30 min in salt bath at 470±2℃, quenched in water at room temperature, pre-aged at 100℃ for 4 h, rolled at room temperature with a reduction of 0, 5, 10 and 20%, then aged at 120, 140 or 160℃ for different times. The standard T6 and T73 tempers were also performed for comparison. The hardness, tensile strength, fatigue properties and resistance to SCC of the material were measured. The microstructures in different states were investigated with optical metallography, SEM and TEM.The results show that the tensile and yield strengths of the material are increased by TMT, and the higher the reduction, the larger the strengthening effect. After TMT with 10-20% reduction, the T. S. and Y. S. are 10% higher than that of the T6 temper. TMT gives the alloy higher resistance to SCC. For example, in the aqueous medium of 3.5% NaCl+ 0.5% H2O2, the critical fracture stress for 200 h of TMT specimens is 85% higher than that of the T6 temper and comes near to the level for the T73 temper. However, the tensile strength of TMT specimens is 20% higher than that of the T73 temper. This indicates that the advantages of T6 and T73 tempers are combined in TMT.There is no apparent difference in fatigue properties after different processes. The observation of microstructure and fracture finds that the deformation during TMT makes inclusion particles crack and the fatigue origins are often located at the cracked particles near surface or on the clad layer which is not affected by TMT. The transmission electron micrographs show that there are chain precipitates distributed along grain boundaries and a PFZ in the T6 temper, whereas there is only separated intergranular precipitates and no PFZ in the TMT specimens. Furthermore, the dislocation density in the substructure of TMT specimens is much higher than that of the T6 temper. Such beneficial changes in microstructure alter the slip characteristics, eliminate the intergranular fracture tendency and lead to the increase in tensile strength and resistance to SCC of the alloy.