ERI: Understanding Porosity Formation in Ductile Iron Castings Induced by Graphite Precipitation and Casting Wall Movement

This Engineering Research Initiation (ERI) grant enables fundamental understanding of porosity formation in ductile iron castings induced by graphite precipitation and casting wall movement. Compared to steel and non-ferrous alloys, ductile iron castings are inexpensive to manufacture, and are easily recycled. Ductile iron castings are commonly found in automotive, agriculture, mining, municipal water/sewerage, defense, and other industries.

However, defects such as porosity can form in ductile iron castings when the processing conditions are not optimal. This research is aimed at understanding the complex nature of porosity formation in ductile iron castings by studying the effects of graphite formation and casting wall movement via simulation and experiment. The knowledge gained from this work leads to empirical models that can be applied to make sound, defect-free ductile iron castings.

The research outcomes are advanced process control, improved casting quality, and reduced scrap resulting in energy and material savings and economic benefits for the U.S. foundry industry. This research involves several disciplines including metallurgy, materials science, and manufacturing. The multi-disciplinary approach helps broaden participation of women and underrepresented students in research and positively impacts engineering education.

Liquid metal shrinks as it solidifies in a casting mold causing shrinkage porosities in the last-to-freeze regions of the casting. Metal casting foundries try to minimize porosity formation in ductile iron castings through better mold cavity and riser design and alloy chemistry modification. However, excessive casting wall movement due to graphite precipitation and inadequate mold strength during solidification as the root cause of porosity is often overlooked.

This research aims to corelate porosity formation in ductile iron castings to graphitization and casting wall movement via experiment and simulation. For the experiments, graphitization is controlled by different inoculation treatments, and casting wall movement is varied with molds of different strengths. This research plans to develop a novel experimental apparatus for in-situ multi-axis measurement of casting wall movement during ductile iron solidification and cooling using high-accuracy displacement sensors.

Coupling displacement data and cooling curves acquired with thermocouples, the technique provides insights into the thermo-mechanical behavior of the solidifying casting. Size distribution of graphite and porosity in the final castings are statistically characterized with X-ray radiography and automated scanning electron microscopy. Empirical models that correlate porosity formation and casting distortion with graphitization and casting wall movement are developed via numerical simulation using commercially available metal casting simulation software.

This fundamental research is generalizable and could be applied to understand porosity formation in other cast engineering alloys such as steel, aluminum, copper, and titanium.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.