Carbide Precipitate Deactivation and Module Characteristics in Thin Wall Ductile Iron (TWDI) Castings
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Ductile iron (DI) thin section profiles (≤ 3mm) known as Thin Wall Ductile Iron (TWDI) offer significant potentials in automotive applications because of their high strength with good ductility, wear resistance, castability, machinability and good fatigue properties. “Globally” as-cast TWDIs in many foundries suffer macrostructural and microsructural defects such as solidification shrinkages, coldshuts, misrun, incomplete filling, carbide precipitation, poor graphite morphological characteristics, which jeopardize the enhancement of mechanical properties required for automotive applications. Production of “defect-free” TWDI castings is hindered by difficulty in channeling molten metal appropriately into the mould cavity. This study considers the effect of casting design and sprue/metal head height on mould filling, microstructure and mechanical properties of TWDI casting. The suppression of carbide formation and non-nodular graphite is dependent on the nodularisation and inoculation treatments (liquid treatments), hence the effect of nodularisation treatment temperatures of 14500C-15500C is investigated. This study also investigates effect of incorporation of 2-12 wt. % aluminum dross (AlDr) and 1-6 wt.% rice husk ash (RHA) on thermal properties of moulding sand, the microstructure and mechanical properties of TWDI castings, as literature on these is found to be scanty. The results show that side feeding casting design enabled completely filled mould cavities. At metal/sprue height of 50 mm the melt could not push through the gating channel, but as it is increased from 100-350 mm, complete mould-fill is achieved. However, between 200 mm and 350 mm, carbide precipitates and flow related defects namely; misrun, coldshut, incomplete filling and air entrapment are evident. These defects significantly lower the strength and ductility of TWDI. Desirable nodularity ratings above 90%, good nodule count of 450-610 nodules/mm2, reduction of deleterious carbides with improved elongations are achieved at 14500C-14700C melt treatment temperatures. Below 14500C and above 14900C, poor nodularity and nodule count with reduction in ductility are obtained. RHA significantly reduced the thermal conductivity of the moulding sand from 1.631 to 1.141 W/m. K, for 0 to 6 wt. % RHA in sand mix respectively, (a 30% reduction). TWDI castings with nodularity ratings above 90%, high nodule count above 1000 nodules/mm2, high strengths of 564, 575 and 596 MPa for 2, 3 and 4 mm respectively, occurred at 4 wt. % RHA addition. High ductility of 4.7, 5.8 and 6.1 for 2, 3 and 4 mm thick samples respectively occurred at 6 wt. % RHA additions. This study has shown that sound TWDI castings are achievable with proper gating design and dimensions. Sprue/metal height is an important gating system parameter to be considered during casting of TWDIs. Lower treatment temperatures of 1450 0C and 1470 0C are adequate for good nodularity and nodule count. Incorporation of RHA to silica moulding sand significantly reduced thermal conductivity leading to reduction in cooling rate with controlled carbide precipitation and enhancement of nodular graphite of desired morphological characteristics.