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So to offer a data to optimize aging treatments on spray formed 7075 alloy and
references for the next step research, this paper studies the retrogression on microstruc-
ture mechanical properties and SCC behavior of spray formed 7075 alloy via under aging
by transmission electron microscope (TEM), tensile test and slow strain rate test (SSRT).
Experimental. The experimental material was 7075 alloy, with composition
(wt.%): 5.48 Zn; 2.21 Mg; 1.48 Cu; 0.189 Cr; 0.371 Fe and 0.121 Si.
The technological parameters of spray forming were as follows: atomization gas
was nitrogen (N 2), spray distance – 370…380 mm, substrate eccentricity – 60…65 mm,
conduit bore – 3.6 mm, angle of incidence – 37°…39°, spray temperature – 770…780°C,
crucible temperature – 735…745°C, horizontal velocity – 0.15 mm/s, and vertical
velocity – 0.18 mm/s.
The bars after hot extrusion (temperature – 400°C; ratio – 30:1; rate – 1.5 mm/s) were
made into test samples of the diameter 12.8 mm for two-stage solid solution (450°C for
1 h and 475°C for 2 h, water quenched to room temperature). Specimens were pre-aged
at 120°C for 16 h, retrogressed at 160; 200 and 240°C for ~4 h, and re-aged at 120°C
for 24 h.
SCC behaviors were tested by SCC-1 stress corrosion experimental system
corresponding to international standard ISO 7539-7: 2005 (Corrosion of metals and
alloys – Stress corrosion testing, Part 7: Method for slow strain rate testing), strain rate
–6 –1
was 10 s in dry air or 3.5 wt.% NaCl solution at 35±1°C until cracking.
The 3 mm diameter disks for TEM observation were punched out directly from
samples which were mechanically ground down to 60 mm thickness after aging. These
disks were electropolished using a DJ-2000 twin-jet electropolisher with a 30% nitric
acid solution in methanol at −30°C. TEM examinations were performed using a JEM-
2100 transmission electron microscope.
Results. Figure 1 shows the ultimate tensile strength (UTS) and conductivity of
the alloy during retrogression at 160; 200 and 240°C and corresponding re-aging. From
Fig. 1, it can be seen that all tensile strength curves of retrogression decline abruptly at
first, subsequently rise since falling to a certain degree, and finally declined again.
Three tensile strength curves during retrogression have the similar characteristics, but the
times of reaching the minimum and peak of strength are different, which depend on
retrogression temperatures. Because the diffusion rate of solute and vacancy is positive
correlation with retrogression temperature, it implies that the time is shortened by high
retrogression temperature.
The tensile strength curves during RRA are also similar. With the retrogression
time extension, the tensile strength increased firstly and then reduced after a peak value.
The timing of peak strength is between the minimum and the peak of tensile strength of
the retrogression curve. With the retrogression temperature increase the time of reaching
peak strength is obviously shortened after RRA. The strengths are also influenced by
retrogression temperature. When the samples are retrogressed at 160°C, the peak
tensile strength of the alloy after RRA is 772 MPa (see Table). With the retrogression
temperature increase, tensile strength of the alloy increases gradually. When the samples
are retrogressed at 200°C, the peak tensile strength of the alloy after RRA is 791 MPa.
Then, when the retrogression temperature is increased to 240°C, the peak tensile
strength of the alloy after RRA is only 773 MPa, which is less than that via retrogres-
sion at 200°C.
It also can be seen that the conductivities rise sharply incipiently and then gently
with the retrogression time extension. The regularity of conductivities is also influenced
by retrogression temperature. When the samples are retrogressed at 160°C, the rangea-
bility of conductivity is small. In case of the retrogression above 200°C, the amplifi-

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