RESERCH ON QUENCH SENSITIVITY AND MICROSTRUCTURE ANALYSIS OF 7050 ALUMINUM ALLOY

7050 aluminum alloy is an important structural material widely used in aerospace industry with a high quench sensitivity. In this work, the time-temperature-transformation (TTT) curves of 7050 aluminum alloy were determined by interrupted quench method. The microstructure evolution and phase transformation kinetics during solid solution, isothermal quenching, ageing treatment were studied by TEM and JMA equation. The results show that the nose temperature of TTT curves is about 330 degrees C, with the quench sensitivity range of 300380 degrees C. The quenching sensitivity of high temperature range of 400 similar to 450 degrees C is lower than that of low temperature range of 210 similar to 270 degrees C. The laminar equilibrium eta phases characterized with nucleus of Al3Zr particles and several needle-shaped S phases are the main precipitations of the supersaturated solid solution decomposes during isothermal holding process. With the increase of holding time, the volume fraction of precipitated phases rises quickly, which coarsen the grain boundary continuously and broaden precipitation free zone (PFZ). While holding far away from the nose temperature, the speed of precipitation slow down and the degree of continuity and coarsening of grain boundary decrease. The values of n in JMA equation vary from 0.50 to 0.65, indicating that the precipitations are mainly laminar precipitated phases and supplementarily needle-shaped phases.