The Influence of Process Variables on Microshrinkage Formation
in Thin Wall Ductile Iron
The University of Alabama
Department of Metallurgical and Materials Engineering
December 2005
Abstract of Thesis
Recent research on thin wall ductile iron castings has demonstrated their excellent static mechanical properties which makes them a material of choice for automotive applications. However, to reach the full potential of thin wall ductile iron castings it is necessary to identify the metallurgical and processing conditions necessary to obtain sound castings with the desired microstructure. This thesis presents an analysis of the effect of process variables on microshrinkage formation in thin wall ductile iron.
Casting design has a critical influence on the formation of microshrinkage, since this determines the extent to which liquid metal can be fed to each plate. Accordingly, three different patterns with horizontal plates were used to physically simulate feeding of horizontal plates from adjacent sections, the absence of feeding in horizontal plates, and the absence of feeding in plates that filled at a 15° incline from the horizontal plane. Other variables included the pouring temperature (1330 to 1457 °C), the carbon equivalent (4.58 to 4.96), the cooling rate (in the range of 4.8 to 47.7 °C/s), and the casting modulus (M = volume/area 1 to 3.4 mm). The influence of these variables on the formation of microshrinkage has been evaluated in ductile iron plates of thicknesses of 2 to 6 mm.
It was concluded that microshrinkage can be easily avoided with appropriate feeding from adjacent sections under the processing conditions for thin wall ductile iron castings used in this study. The experimental results demonstrate that, with proper feeding, it is possible to cast sound horizontal plates with dimensions of 2x60x100 mm.
For the horizontal independent plates that were not fed it was found that the microshrinkage increases with plate thickness, or conversely is smaller at higher cooling rates. These results prompted the investigation of microshrinkage formation in independent plates which are filled at an angle of 15° from horizontal. These plates fill in a controlled manner without turbulence. The inclined independent plate design was evaluated by comparing modeling results obtained with NovaCast & NovaFlow® software with images obtained through high speed video. The experimental results demonstrate that thin ductile iron plates that solidify with no feeding from a riser or an adjacent section, microshrinkage decreases as cooling rate increases. In line with this, a higher pouring temperature promotes lower microshrinkage, although the data scatter is rather significant. When comparing the results of the horizontal independent plates to the inclined independent plates, it is reasonable to conclude that by controlling the mold filling, microshrinkage data scatter is dramatically reduced as a much higher correlation between microshrinkage and cooling rate is observed. The reduction of the thermal conductivity of the mold through the addition of a low-density alumina-silicate ceramic (ASC) has been demonstrated to improve results consistency because of a more uniform solidification pattern.
Two microshrinkage characterization methods were employed in this study. One method involved characterizing test plates by the amount of microshrinkage present on the metallographic planes along the longitudinal and transverse axes. The other approach involved the use of Archimedes’ principle to perform a differential density analysis which yields a percent volume of microshrinkage present in the plate.
Several established thermal criteria functions were applied to the microshrinkage results of an inclined independent plate casting. The thermal variables were estimated from NovaCast & NovaFlow ® solidification simulation software. A reliable correlation between the criteria function values and the microshrinkage results was not established.
Acknowledgments
This research was performed under
the supervision of my advisor, Dr. Doru M. Stefanescu, at the University of Alabama. This work was a part of
Phase II of the Thin Wall Iron Group (TWIG) research effort. This work
was a product of teamwork, including the talents of post-doctoral researchers
Dr. Frank Juretzko and Dr. Roxana Ruxanda, fellow graduate students Luke Dix
and John Torrance, undergraduate research assistant Wes Nicholson, and foundry
technician Bob Fanning.
