Mechanical properties of thixo-formed austempered ductile iron
(School of Metallurgy and Materials Engineering, University of Tehran, 14174 Tehran, Iran)
Abstract: Semi-solid processing of metallic materials has been investigated for some time; however, this innovative production process has not fully established for ferrous alloys like ductile cast iron in the industry yet. Ferrous alloys-specially ductile cast iron-because of increasing complexity due to high temperature processing and some other limitations have been allocated a small share of this useful forming process to themselves. However, this work describes that thixo-forming of ductile iron despite all of the limitations is possible even for thin sections (less than 1 mm). Die temperature and injection velocity are the most effective parameters to thixo-forming particularly for high melting point materials such as ductile cast iron. Filling behavior, shape factor of graphites and fluidity of thixo-formed specimens were characterized. In addition, defects as well as shrinkage and cold laps were examined to achieve sound specimen. Subsequence to successful thixo-forming, two step austempering was performed to accomplish a uniform structure with minimum untransformed austenite volume (UAV). Following austempering, tensile test was carried out and the results show good combination of tensile strength and elongation for treated specimens (1 137 MPa strength and 12% elongation in best situation).
Key words: semi-solid; ductile iron; high pressure mold filling; segregation; mechanical properties