Thermo Mechanical Processing And Fatigue Of Aluminum Alloys A Review

A review is made of the available information on thermomechanical processing of aluminum alloys, especially with respect to their fatigue behavior. An attempt is made to evaluate from the scarce and sometimes contradictory experimental evidence the possibility of improving the fatigue resistance of high-strength aluminum alloys by these treatments. To do so, it is also necessary to review the important microstructural features and deformation characteristics of these alloys, which seem to control the various stages of fatigue, and which may be influenced by the thermomechanical treatments. The discussion is limited to thermomechanical treatments subsequent to solution annealing. It is concluded that suitable processing offers the advantage of improving both strength and ductility in Al-Cu and Al-Zn-Mg-Cu alloys. If the effects of inclusions can be minimized, significant benefits are possible in fatigue, at least in Stage I crack resistance, by thermomechanical processing.

The application of advanced metallurgical processing techniques, e.g., intermediate thermomechanical treatment (ITMT) and powder metallurgy (P/M), has produced a recrystallized X7091 P/M alloy having fine grain and particle structures and weak texture with the best overall mechanical properties compared to those of ingot metallurgical and conventional P/M alloys. The dependence of mechanical properties on grain size and particle distribution has been demonstrated among these alloys. The study of the effect of the last microstructural feature, crystallographic texture, is necessary for further property improvement through texture control.

The effects produced by thermomechanical treatments on the mechanical, microstructural, and stress corrosion resistance properties of 2024, 7049, and 7075 aluminum alloys were investigated. These properties are compared with those obtained in the standard commercial tempers. The optimum benefits of thermomechanical processing are obtained when the material has previously been heattreated to its maximum age-hardenable condition. A thermomechanical response is developed with as little as 5 percent mechanical deformation. The uniformity of this plastic strain throughout the entire mass of the material is a critical factor. This mechanical deformation is carried out at a temperature high enough to develop a homogeneous random distribution of dislocations and to stabilize this configuration by precipitation along a portion of their lengths. The optimum temperature for a postdeformation heat treatment to produce the necessary resistance to stress corrosion cracking is just above the Guinier-Preston zone solvus for the hardening phase. (Author).