Analysis and discussion of solution treatment experiment results of 7075 rheo squeeze casting parts at different solution temperatures

(a) 460℃ (b) 475℃ (c)490℃ (d)505℃ (e)520℃ (f)535℃

It can be seen from the metallographic structure figure 1 after solution treatment at different temperatures for 2 h that after solution treatment at 460 ℃ for 2 h, the primary phase α – Al in the structure has little change compared with that before solution treatment, and the grain size is 41.1 μ m, which has a certain ripening effect, but not obvious. The second phase is still concentrated in the grain boundary, and the solid solution effect is not obvious. When the solution temperature increases to 475 ℃, the average size of primary grains is 34.4 μ M. at the same time, the number of second phase particles at the grain boundary decreases obviously, the grain boundary becomes fine, compact and clear, and a small part of the grain boundary appears fracture and grain fusion. With the temperature further increasing to 490 ℃, the average size of primary grains is 41.5 μ m, most of the second phase particles in the structure have been dissolved into the matrix, a certain amount of hard and refractory phases remain at some grain boundaries, some grain boundaries have fusion phenomenon, and the grain boundaries become difficult to distinguish. When the solution temperature is increased to 505 ℃, the second phase particles in the structure are almost invisible, the abnormal growth and coalescence of grains increase obviously, the average size of primary α – Al is 44.4 μ m, and the grain boundary becomes obvious and has a growth trend, which indicates that the solution treatment temperature is too high. When the solution temperature reaches 520 ℃, the primary phase grains grow further, with an average size of 54.1 μ m, and the grain boundaries change from continuous to intermittent. However, at 535 ℃, the average grain size of primary phase is 57.1 μ m, the grain boundary becomes larger and separation occurs, and the over burned dendrite (literal) structure appears.

It can be seen from the hardness test results (Fig. 2) that with the increase of solution temperature to 475 ℃, the hardness of the two parts reaches the highest value. When the temperature reaches 490 ℃, the hardness decreases obviously. After that, the hardness varies from high to low at 505 ~ 535 ℃, but the fluctuation range is not large. When the temperature is about 475 ℃, the best strength can be obtained.

It can be seen from the above analysis that the suitable solution treatment temperature for 7075 rheo squeeze casting parts is around 475 ℃.