3 Developments R
Lost Foam
Several new methods of improving the lost foam process could mean more high strength castings on the horizon.
Vacuum-Assisted Filling Improves Mechanical Properties Harry Littleton and Alan Druschitz, University of Alabama at Birmingham, Birmingham, Alabama
esearch has shown that the pressure differential between the sand and molding cavity in lost foam casting plays a dominant role in the removal of pattern pyrolysis products. This phenomenon was fi rst discovered during the pouring of high silicon
aluminum alloys that historically produce large amounts of surface defects commonly named “alligator skin.” A study at the Univ. of Alabama at Birmingham, Birming-
ham, Ala., revealed alligator skin could be eliminated in cylinder head castings with a 9% silicon alloy by increasing the pouring temperature to 1,500F (815C) or with vacuum assisted fi lling. Since the elevated temperature is not practical due to increased hydrogen solubility and reduced furnace life, vacuum assisted fi lling was selected for further research. The goals in the second study were to minimize the effect
of the changing metal head pres- sure by applying a vacuum to the sand/coating to control the metal fi lling rate from the sand side of the casting cav- ity and lower the pouring tem- perature to take advantage of hydrogen solu- bility at lower temperatures. A four-cylin-
der engine block poured with A319 aluminum was selected for the study. Ther- mocouples and pressure probes measured data at
36
For more information, download the full paper, “Vacuum-Assisted Filling of Lost Foam Castings,” at
www.moderncasting.com.
pouring temperatures of 1,450F (788C), 1,350F (732C) and 1,250F (677C) and a pressure of -4.5 psig (-233 mm Hg). After compacting sand around the cluster, a plastic fi lm was placed over the top of the sand and sprue. A pouring cup was installed, sealing the upper surfaces of the fl ask and sand. A vacuum was applied and metal poured in the nor- mal manner. The melt was degassed using argon distributed through a rotating graphite tube for six minutes. The results of the study indicated mechanical properties— particularly percent elongation—of A319 castings can be im- proved with vacuum assistance as a result of percent porosity and pore size decreasing as pour temperatures decrease (Fig. 1). A follow-up study is planned using A356 or a 9% silicon alloy, which contains microstructure that is not as limited in achiev- able properties as A319’s. The applica-
No Vacuum 1,450F-Vacuum 1,350F-Vacuum 1,250F-Vacuum
Fig. 1. Photomicrographs of lost foam casting samples show that pore size and frequency of porosity was reduced as pouring temperatures were reduced.
tion of a vacuum to lost foam cast- ing is relative- ly simple and should not affect production rates. Pouring at lower temperatures of- fers energy and cost savings during melting and holding and could extend fur- nace life.
MODERN CASTING / December 2010
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