Production and properties of thin-wall ductile iron castings

Fuel efficiency requirement for cars and trucks has conducted to an increased use of low-density materials such as aluminium in the automotive industry. However, such materials, which require tremendous amounts of energy for their production, have also significant drawbacks with respect to strength and cost. Therefore, they do not represent the panacea for weight reduction. Ductile iron remains a material of choice for automotive applications due to its castability, strength-to-weight ratio and cost. Indeed, the mechanical properties of Ductile Iron allow the reduction of the casting size, assuming that such parts could be produced carbide free in the as-cast condition. A comprehensive study was carried out to design a production route for thin wall Ductile Iron castings free of eutectic carbides. In this study, a test casting assembly with section thicknesses from 3 to 10 min representative of actual castings was designed using computer modeling technique. Castings of various chemical compositions produced using different processing parameters were manufactured. The study showed that carbide free 3 mm thick castings can be manufactured when powerful inoculants, such as Bi-bearing ferro-silicon, are used. In such cases, carbide free castings with nodule count in excess of 1200 can be produced, while maintaining silicon content at a reasonable level (< 2.8%). Under these conditions, cooling rate and inoculation rather than silicon content control nodule count and matrix structure. The mechanical properties of the test castings were found slightly inferior to those of conventional section castings, but sufficiently high to compete with light metals in many applications. Routes to further improve the mechanical properties were investigated, including filtering which was found to improve the consistency of the mechanical properties.