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.
Effect of external forces on microstructural evolution and mechanical properties of high pressure die cast AA5754 alloy
Abstract The effects of ultrasonic treatment (UT) and electromagnetic stirring (EMS) forces, individually and in combination, prior to high pressure die-casting of AA5754 alloy were assessed. The liquid of alloy was subjected to UT, EMS and the combined style and then transferred to the shot chamber. The grain size, mechanical properties, gas content and hot tearing susceptibility were analyzed. The results suggest that the application of each process enhances grain refinement and decreases the porosity of the specimens. UT reduces the grain size from 140 to 82 μm and decreases the porosity from 5.5% to 1.4%. EMS reduces the grain size to 107 μm and the porosity to 3.3%. The combination of UT and EMS decreases the grain size and the porosity to 65 μm and 1.1%, respectively, which are the lowest grain size and porosity ever reported for this alloy achieved via physical processing. Intensive cavitation and stirring not only resulted in a refined microstructure but also significantly decreased the hot tearing susceptibility and improved the mechanical properties. All of the aforementioned characteristics are due to cavity formation, during each process. By combination of techniques, EMS could promote the nucleation process by providing more exogenous particles in the strong cavitation field of UT. Ultrasonic plays a major role in grain refining, decreasing the hot tearing susceptibility and the gas content by introducing a strong cavitation field.
Stress corrosion cracking of aluminum alloys
Stress corrosion cracking of aluminum alloys is reviewed. An extensive failure analysis shows how many service failures occurred in the aerospace industry over a ten year period and what kind of alloys and stresses led to initiation and propagation of stress corrosion cracks which caused these service failures. The paper contains most of the results of stress corrosion tests with aluminum alloys that have been obtained to date with fracture mechanics techniques. Stress corrosion crack growth rate measurements are compared with the results from smooth specimen testing and it is shown that the correlation between the different test results is very satisfactory. The present and limited status of theoretical understanding of stress corrosion cracking is outlined.A major part of the paper is devoted to the results of the latest alloy development. High strength aluminum alloys of dramatically increased stress corrosion resistance are available now. In the near future, stress corrosion resistant alloys of even higher strength might become available. What is still lacking is a detailed understanding of the mechanisms by which stress corrosion cracks initiate and propagate.