Use of the ultrasonic technique in measuring inclusions in aluminum-silicon alloy melts.

摘要

The presence of inclusions in aluminum alloys is one of the most serious problems encountered in the production of aluminum castings. Inclusions may reduce mechanical properties, are detrimental to surface finish, increase porosity, and display a tendency to increase corrosion. Some brittle non-metallic inclusions act as stress raisers, and can cause premature failure of a component. With the increasing demand for higher quality aluminum cast products, especially in the automotive and aerospace industries, much attention has been paid to the cleanliness of molten aluminum.;The control of melt cleanliness may be carried out through inclusion-removal and inclusion-monitoring techniques. From the monitoring point of view, a number of techniques, such as PoDFA (Porous Disc Filtration Analysis), LAIS (Liquid Aluminum Inclusion Sampler), Prefil (Pressure Filtration), and Qualiflash have been developed for the measurement of inclusions in aluminum melts. These techniques however are time-consuming and can only provide off-line results; consequently, the information is often obtained too late to make timely adjustments in the casting process. Currently, although there is also an on-line inclusion-measuring technique available, namely LiMCA II, it is too expensive and hence not generally accessible.;The Ultrasonic detection technique appears to be a promising method to resolve this problem because of its capacity for probing the interior of materials. The present study was undertaken to investigate the capacity of the ultrasonic technique for measuring various inclusions in liquid aluminum and commercial 356 alloy at two different temperatures each. Inclusions of TiB2, AlSr, Al3Ti, and Al2O3 were studied using a Metalvision MV20/20 ultrasonic machine. The data provided plots of (i) the cleanliness value, (ii) average particle size, and (iii) particle count for each particle size range, as a function of testing time. An examination of the microstructure of solidified samples obtained from samplings of the melt was also carried out using Jeol JXA-8900L electron probe microanalyzer.;Analysis of the ultrasonic data and their corresponding microstructures showed that the ultrasonic machine may be used as an on-line device for determining the melt cleanliness for a long period of time, 5 h in the present study. The melt cleanliness curves obtained by the ultrasonic machine are reliable and may be used as a guide for casting provided that the melt temperature and stirring conditions are properly adjusted.;Increasing the concentration of inclusions reduces the cleanliness level of the melt, and is reflected by a corresponding decrease in the melt cleanliness curve of the ultrasonic machine, indicating its response to the change in inclusion level. Increasing the melt superheat, however, accelerates the decomposition of the master alloy, thereby increasing the number of inclusion particles in the melt and reducing the melt cleanliness value.;Normally, when TiB2 inclusions are added to aluminum melt, the fluidity of the melt will decrease significantly. Other techniques, including LiMCA and all the filtration techniques, can only detect TiB2 inclusions when the concentration is very low (less than 10 ppm B). By applying the ultrasonic technique, measurements may be conducted properly for concentrations as high as 90 ppm B; thus the ultrasonic technique provides an improved alternative for the measurement of TiB2 inclusions in liquid aluminum. This fact is of significance, since inclusions are invariably present because the addition of TiB2 is essential to the grain refining processes used for aluminum.