Tecnología de fabricación, caracterización y propiedades de piezas de fundición nodular de pequeño espesor

Abstract

During the last years, ductile iron applied to produce small thickness parts has arose the interest of researchers, foundrymen and users, because it is a viable alternative to compete with lighter materials, especially aluminum. In the same way, many ductile irons parts of conventional thickness (above 10 mm) could be made like hollow pieces, diminishing the weight and improving the performance. The development of Thin Wall Ductile Iron (TWDI) provides the designers a new option to select the most appropriate material, not only based on the weight/strength relation, but also on the cost/properties ratio. This development creates a new technological challenge, since in the industrial practice it is considered that a ductile iron cast part must have a thickness not smaller than 6 mm so as to be able to be cast free from defects, using conventional sand molding techniques. In general, there are three aspects to be solved in the manufacture of TWDI: a) to adequate pattern and mold designs to achieve a right feeding, so as to obtain castings macro and micro structurally sound, in spite of the high cooling rate and the fluidity reduction. b) to adjust the liquid during melting to get homogeneous and carbide–free solidification structures. c) to characterize mechanical properties in order to supply design values to be used in the fabrication of TWDI components. In the research carried out for this thesis, pattern evaluation and optimization, melting techniques, melt treatment and molding practice have been investigated. Likewise, the effect that graphite morphology, nodule count, chemical composition, heat treatment and solidification defects, exert on mechanical properties of TWDI have been studied. It has been achieved plates with thickness between 1.5 and 4 mm using horizontal and vertical molds, been the latter process the most efficient one. It was established that using chemical compositions with carbon equivalent under 4.6%, melt treated in two stages with post-inoculation in the second ladle and gravity pouring procedure, it is possible to get thin plates with good metallurgical quality, free from carbides and anomalies of the graphite phase. Mechanical properties evaluated using tensile test reach, and in many cases surpass, the minimum values established by the ASTM standard for conventional thickness ductile iron with ferritic and ausferritic (ADI) matrices. It has been also possible to identify three different types of microshrinkage cavities in TWDI plates and to determine their effect on the mechanical properties. Microshrinkage cavities, which are located in the plate feeders already considered in the design stage, do not influence the mechanical properties. However, the disperse macroshrinkage, originated by deficiencies of the feeding process, greatly affects the properties and increases the scatter of elongation values. Besides, disperse microshrinkage which is located in microsegregated zones and originated by a nucleation and growth mechanism and closely linked to the presence of non-metallic inclusions, has little influence on the mechanical properties. This research gives comprehensive and useful technical knowledge for the development of TWDI parts.