7B50铝合金淬透性及其临界平均冷却速率的研究

Investigation on the hardenability and critical cooling rates of 7B50 aluminum alloy

摘要:

本研究通过7B50合金改进型Jominy样品表面喷水淬火实验，在获得实测冷却曲线、不同时效状态合金的电导率和硬度的基础上，结合自然时效状态合金的微观组织，对7B50合金的淬透性及其临界平均冷却速率展开研究。结果表明：7B50合金自然时效50天的淬透深度为70 mm，对应淬火敏感温度区间（420℃~230℃）内的平均冷却速率为1.55℃·s-1；先自然时效50天再人工峰时效合金的淬透深度减至60 mm，对应的平均冷却速率为1.95℃·s-1；与自然时效状态相比，先自然时效再人工峰时效处理后合金的淬透性变差，淬火敏感性增加。表面喷水淬火时，非均匀析出相首先在晶界/亚晶界上析出，然后在基体内的Al3Zr粒子上析出；晶界/亚晶界上观察到析出相，出现在距淬火表面仅3 mm处，对应淬火敏感温度区间内的平均冷却速率高达981℃·s-1；基体内零星析出尺寸较小的非均匀析出相，出现在距淬火表面10 mm处，对应的平均冷却速率为37.75℃·s-1。喷水淬火后，距淬火表面25 mm处的性能与淬火表面处相比变化不大，该位置对应的平均冷却速率为9.34℃·s-1，远小于淬火表面处，控制7B50合金厚板的喷水淬火过程，使厚板内部的平均冷却速率接近但不低于9.34℃/s-1，厚板淬火-时效后将获得较好的性能。

Abstract:

In present work, the hardenability and critical cooling rates of 7B50 alloy were investigated based on measured cooling curves obtained from the spray quenching test on modified Jominy specimen, electrical conductivity and hardness in different aging tempers, and microstructures evolution in natural aging temper. Results show that the hardened depth of 7B50 alloy is about 70 mm in natural aging temper while it decreases to 60 mm after natural aging for 50 days followed by artificial peak aging treatment with the corresponding average cooling rates in quench sensitive temperature range (420℃~230℃) increasing from 1.55℃·s-1 to 1.95℃·s-1, respectively, indicating worse hardenability and more quench sensitivity after artificial peak aging treatment. During water-spray quenching, the precipitates form firstly on grain/subgrain boundaries and then at some special positions in matrix, such as Al3Zr particles. The inhomogeneous precipitates on grain/subgrain boundaries are observed at a location of 3 mm from the quenching surface with the corresponding average cooling rate of 981℃·s-1. While inhomogeneous precipitates in matrix are observed at a location of 10 mm from the quenching surface with the corresponding average cooling rate of 37.75℃·s-1. Properties of the alloy at 25 mm from the quenching surface have little changed compared with those on the quenching surface and the corresponding average cooling rate at the location of 25 mm is 9.34℃·s-1 which is much smaller than that on the quenching surface. Therefore, during the spray quenching process, the average cooling rate of 7B50 alloy thick plates should be controlled to close to but not less than 9.34℃·s-1, in order to get excellent and uniform properties after quenching and aging.