Effect of low-frequency electromagnetic field on the as-cast microstructure of a new super high strength aluminum alloy by horizontal continuous casting
The super high strength aluminum alloy ingots with 100 mm in diameter were cast by the process of low-frequency electromagnetic horizontal continuous casting (LFEHC) and the effect of electromagnet ic field on the as-cast microstructure was studied. Results show that microstructure of the sample prepared by the LFEHC process was greatly refined. Microstructures at the border and the center of the ingots were fine, uniform and rosette-shaped. Electromagnetic frequency plays a key role in microstructure refining. Fine and uniform microstructures can be obtained with optimal electromagnetic frequency. In this experiment, under a frequency of 30 Hz the microstructure was the finest and the most uniform.
Temperature field in the hot-top during casting a new super-high strength Al-Zn-Mg-Cu alloy by low frequency electromagnetic process *
The billets of a new super-high strength Al-Zn-Mg-Cu alloy in 200mm diameter were produced by the processes of low frequency electromagnetic casting (LFEC) and conventional direct chill (DC) casting, respectively. The effects of low frequency electromagnetic field on temperature field of the melt in the hot-top were investigated by temperature measurement method. Temperature curves were measured from the surface to the center of the billets by locating type K thermocouples into the casting during the processes. The results show that during LFEC process the temperature field in the melt applying the hot-top is very uniform, which is helpful to reduce the difference of thermal gradients between the surface and the center, and then to reduce the thermal stress and to eliminate casting crack.
Retrogression and reaging and the role of dislocations in the stress corrosion of 7000-type aluminum alloys
The 7000-type aluminum alloys in the T6 temper are known to be highly susceptible to stress-corrosion cracking (SCC). Some years ago, a heat treatment known as retrogression and reaging (RR) was developed by one of the authors (B.C.), providing for enhanced stress-corrosion resistance without any sacrifice of yield or tensile strength in 7075 aluminum alloy. The idea behind the process was based on the suggestion that dislocations developed during quenching from the solution treatment were responsible for susceptibility to stress corrosion. In spite of considerable practical development of the RR process, the above basic hypothesis as to the role of dislocations has never been investigated. In the present work, the effect of the RR treatments on the dislocation structure of 7000-type aluminum alloys was studied using transmission electron microscopy (TEM). A clear relationship has been found between the presence of dislocations adjacent to grain boundaries and the susceptibility to stress corrosion of 7000-type aluminum alloys. The beneficial effect of the RR treatment on the SCC of 7000-type aluminum alloys in the T6 temper is believed to be due to the disappearance of the above dislocations as a result of RR treatment.