Fabrication And Microstructural Evolution Of The Semi-Solid Al-4Cu-Mg Alloy

Semi-solid forming (SSF) is a novel and promising technique in the metal forming of 21st century, which could meet the requirement of advanced manufacturing technology and offer significant advantages over conventional casting and forging technologies. The key to SSF is fabricating the semi-solid materials with fine, uniform and spheroidal microstructure.Although several fabrication techniques, including Electromagnetic Stirring (ES), have been applied for a wide variety of alloys, they are difficult and/or expensive to produce the semi-solid materials of some wrought alloys. Strain Induced Melt Activation (SIMA) is an ideal technique for solving this problem with the significant commercial advantages of simplicity and low equipment cost in the fabrication process. In this paper, the semi-solid Al-4Cu-Mg alloy has been fabricated by plastic deformation below solidus temperature, which include cold deformation and hot deformation, and isothermal heat treatment in semi-solid temperature range. The as-received material used in the experiments was hot extruded and treated by a solid solution and natural aging. The experimental results demonstrated that the fine, uniform and spheroidal microstructures can be obtained by controlling the process parameters such as height reduction, deformation temperature, deformation velocity, isothermal temperature and holding time.The SIMA process parameters and microstructural evolution of the semi-solid Al-4Cu-Mg alloy have been investigated systematically. In order to provide scientific foundation for determining the SIMA process parameters and improving the service quality of the semi-solid Al-4Cu-Mg alloy, effect of process parameters, including height reduction, deformation temperature, deformation velocity, isothermal temperature and holding time, on microstructure variables, including grain size, fractal dimension and the distribution of grain size, has been investigated. Upsetting tests have been carried out to investigate the mechanical properties of semi-solid Al-4Cu-Mg alloy fabricated by the SIMA process. Although the upsetting tests show that the ultimate strength and yield strength of semi-solid Al-4Cu-Mg alloy are less than those of the as-received, its mechanical properties are enough to satisfy the service life.The distributions of major elements have been investigated using SEM and linescanning in the semi-solid Al-4Cu-Mg alloy after isothermal heat treatment. The dislocation morphologies of the as-received, hot-deformed and cold-deformed Al-4Cu-Mg alloy during the heating stage of isothermal heat treatment have been observed using TEM. The dislocation morphologies of the cold-deformed Al-4Cu-Mg alloy during the holding stage of isothermal heat treatment have been observed using TEM, too. Finally, the mechanisms of microstructural evolution have been discussed for the Al-4Cu-Mg alloy during the fabrication process and the microstructural evolution of the cold-deformed Al-4Cu-Mg alloy during isothermal heat treatment can be concluded as follows: deformation and elongated grains -? recovery -* recrystallization-*- fragmentation-* spheroidization-* coarsening.Based on the microstructural evolution of the Al-4Cu-Mg alloy during the SIMA process, several assumptions have been proposed for the recrystallization, grain growth, fragmentation, spheroidization and coarsening processes during isothermal heat treatment. Then, a series of models to describe different processes have been put forward according to the theories of plastic deformation and solidification of metals. The comparison of the experimental results with the calculated results indicated that the models for microstructural evolution of the Al-4Cu-Mg alloy during isothermal heat treatment in the SIMA process are reliable.