Solidification of aluminum in electric field
- PDF / 915,175 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 75 Downloads / 219 Views
Solidification of Aluminum in Electric Field A. PRODHAN,C.S. SIVARAMAKRISHNAN, and A.K. CHAKRABARTI Solidification of aluminum on alternate current (AC) or direct current (DC) fields improves casting properties. It increases casting yield by eliminating shrinkage porosity. Mechanical properties are improved due to reduction of pinhole porosity and columnar-to-equiaxed transition. However, removal of oxide dross is not possible by this treatment. Hydrogen content may be reduced to 70 pct of the original level without any degassing treatment, but use of hexacholoroethane cannot be avoided entirely. Some of the common problems encountered during melting and casting of aluminum and its alloys are[1,2,3] pinhole porosity due to dissolved hydrogen, entrapped oxide dross, high solidification shrinkage, and build up of Ti-B debris as inclusions in the recycled scrap. A review of the literature[4–14] indicates that structural refinement can be carried out by means of increased fluid flow induced by magnetic or electric field or a combination of both. This effect is better appreciated with increasing superheat and solute element concentration. It is known that the melt undercooling during solidification increases under the influence of either electric or magnetic field. This has been interpreted as the net outcome of two competing phenomena.[4] (a) faster growth of crystallites due to higher rate of mass transfer around each crystallite resulting in reduced superheating by accelerated release of latent heat and reduced nucleation rate; and A. PRODHAN, Scientist-E1, and C.S. SIVARAMAKRISHNAN, Senior Deputy Director, are with the Materials Processing Division, National Metallurgical Laboratory, Jamshedpur-831 007, India. A.K. CHAKRABARTI, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721 302, India. Manuscript submitted February 10, 2000. 372—VOLUME 32B, APRIL 2001
(b) faster removal of heat along the solidification front, the effect of which is to enhance the tendency of the liquid to undercool and, hence, accelerate the process of nucleation. It is reported[5–12] that electromagnetic stirring imposed at temperatures above liquidus causes grain refinement by nucleation of primary crystals, spherical growth, agglomeration, etc. When the same is imposed at temperatures below liquidus (at a two-phase region), there is fracture of dendrites, spherodization, and agglomeration of particles with increased stirring time. Low intensity vibration[5] results in large equiaxed dendrites, which become smaller as the intensity and electromagnetic pressure are increased. At higher pressure, isolated grains start appearing. Vives[8,9] reported the effect of electromagnetic pressure wave on cavitation phenomena and oscillatory flow and its effect on microstructure. The fragmentation rate[10] depends on the magnitude of particle velocity near the interface. The interdendritic fluid flow[11] does not contribute directly to crystal multiplication by causing dendrite fragmen
Data Loading...
