The 7000 series of aluminum alloy is commonly used with many varieties. Due to its low specific gravity, high strength, and good processing performance, these alloys are widely used in the fields of aviation and national defense.1-3 However, the stress corrosion cracking (SCC) susceptibility of aluminum alloy is high, which limits their broader application.4-5
The stress corrosion behavior of high-strength aluminum alloys has been studied by many scholars over the past several years.6-8 Researchers have seldom considered the effect of two-stage double peaks aging on the SCC resistance of these alloys. Therefore, the effect of two-stage aging on the SCC susceptibility of 7075 aluminum alloy (AA7075) was studied and described in this article.
Experimental Procedures
Materials and Heat Treatment
The material used in the present study was AA7075 (wt%: 6.02 Zn, 2.20 Mg, 1.56 Cu, 0.20 Ti, 0.30 Mn, 0.25 Cr, 0.50 Fe, 0.40 Si, and balance Al). All samples were machined to tensile sample size as described in Reference 9.9 The sample was put into a box-type electric furnace for a solution treatment of 120 min at a solution temperature of 470 °C.
After quenching in cold water, it was put into the stainless steel electric air-drying box for artificial aging. After the first aging at 120 °C for 6 h, it was stored at room temperature for 10 h, and then the second aging began at 157 °C. The second aging time was ~50 h. After heat treatment, the samples were rinsed with deionized water, degreased in acetone, and dried.
Rockwell Hardness Test
Before the test, sandpaper of grits 200, 400, 600, 800, 1,000, and 1,200 were successively used to polish the surface of the samples to remove the oxide film formed during the heat treatment process. The samples were placed on the Rockwell B hardness (HRB) tester to test HRB. After testing five points for each sample, the values were averaged.
Slow Strain Rate Tensile Test
The slow strain rate tensile test (SSRT) was performed on an electromechanical machine at a strain rate of 1 × 10–6 s–1. The samples were divided into two groups for the experiment. One group was tested in air. The other group was tested in a neutral solution of 3.5% sodium chloride (NaCl), and only the gauge section was exposed to the corrosive medium. The remaining surface was wrapped with Teflon†
The SCC susceptibility is defined in Equation (1):
ISCC = 1 – δNaCl/δair
where ISCC is the SSC susceptibility of the alloy, δNaCl is the elongation to the fracture in the corrosion environment, and δair is the elongation in the air. ISCC reflects the extent of SCC susceptibility—the higher the ISCC value, the higher susceptibility to SCC.
Fracture Surfaces Observation
The fracture surfaces were observed by scanning electron microscopy using a Model: Hitachi S-4700†.
Results and Discussion
Two-Stage Aging Hardening Characteristics
Figure 1 shows the aging hardening curve of AA7075 under two-stage aging conditions. The result showed that AA7075 has the characteristics of double peaks aging during the two-stage aging process (i.e., the first peak aging appears at 15 h while the second peak aging appears at 30 h). Also, there is little difference in hardness between the two peaks. The two-stage aging can accelerate the forward shift of the double peaks aging, which greatly accelerates the production speed and improves the production efficiency.10
SCC Susceptibility of Double Peaks
Table 1 shows the mechanical properties and SCC susceptibilities of AA7075 under different aging conditions. These mechanical properties were measured in air and in 3.5% NaCl solution, respectively. The strength showed the same trend as the hardness during aging (i.e., there were also double peaks for the strength).
The ultimate tensile stress (σb) and elongation (δ) for the alloy tested in 3.5% NaCl solution were lower than for the same alloy tested in air. These results implied that AA7075 underwent severe SCC during the SSRT in the 3.5% NaCl solution. In addition, the SCC susceptibility decreased dramatically with increasing aging time. It is worth noticing that the SCC susceptibility of the second peak aging (0.17) was much lower than that of the first peak aging (0.36), although the ultimate tensile stresses of both peaks were almost the same.
The stress corrosion resistance of Al-Zn-Mg-Cu high-strength aluminum alloy is improved with increasing the aging time, which is due to the sequence of the precipitation phase in the aging process: α(super-saturated solid solution) → Guinier-Preston (GP) zone → ηʹ phase (MgZn2) → η phase (MgZn2).11 In the first aging peak state, the matrix structure of the alloy is a small GP zone with a small amount of ηʹ semi-coherent phase. The strength and hardness reach the maximum value, but the toughness and stress corrosion properties are not as good as those in the second aging peak state.
In the second aging peak state, the uniformly dispersed ηʹ phase and a small bit of η phase (MgZn2) have been formed in the matrix. At this time, the matrix structure is dominated by the semi-coherent or noncoherent precipitation phase, which can reduce the aggregation of hydrogen atoms near the grain boundary in the matrix, thus improving the anti-stress corrosion performance of the alloy.12
Fracture Surface Morphologies
Figure 2 shows the fracture surface morphologies of the specimens under double peaks aging conditions tested in air and 3.5% NaCl solution. It can be seen from Figure 2 (top) that the fracture of the specimens tested in 3.5% NaCl solution showed obvious embrittlement tendency and the deformation capacity obviously decreased compared with that of the specimens tested in air. In terms of morphology, the SCC fracture surfaces were both transgranular and intergranular. With prolonging the aging time, the proportion of intergranular fracture became less and less, and the dimples changed from small and shallow into large and deep.12 The macroscopic expression is the increase of deformation, which is the embodiment of good plasticity.
Conclusions
1. There are two aging hardening peaks during the two-stage aging process of AA7075.
2. The SCC susceptibility of the second peaking was much lower than that of the first peak aging, although the strengths of both peaks were almost the same.
Acknowledgments
The financial support of the National Natural Science Foundation of China under grant No. 51871031 and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) are gratefully acknowledged.
† Trade name.
References and About the Authors