Investigations on the suitability of some ferrous alloys for semi-solid processing

Purpose: the work analyses the thixoability of SAE 1070, SAE 1548 and SAE 4340 steels; the possibility of producing thixotropic semi-solid by partial melting of these alloys and the phase transformations taking place during the process. Design/methodology/approach: thixoability was characterized by differential scanning calorimetry (DSC) to determine transformation temperatures involving liquid formation and dfl/dT within the solidification range. Thixotropic slurries were produced by heat treatments at different temperatures above Ts, and distinct holding times. Microstructures were analysed by RX diffractometry, optical and electronic microscopy and EDS microanalysis. Findings: results show that the three alloys investigated present high thixoability, given by wide solidification ranges associated with affordable sensitivity of liquid fraction with temperature within these ranges. Higher thixoability is presented by SAE 1070, followed by SAE 1548 and SAE 4340. Results show also that is perfectly feasible the production of thixotropic slurries of all investigated alloys, by simply heating to temperatures where a liquid phase can be present. Spheroidisation of solid primary phase is fast and increasing holding time at the semi-solid temperature leads to excessive growth of the globules in the thixocast material. Research limitations/implications: thixoability prediction models rely on sensitive experiments as thermoanalysis, with results strongly dependent on experimental conditions; and on thermodynamic data, sometimes not available or reliable for a specific alloy composition. Practical implications: the prediction of the thixoability of a certain alloy can make it more effective its thixoprocessing, allows better control of processing parameters and of the quality of final product. The viability of producing thixotropic semi-solid of ferrous alloys by simple partial melting can definitively insert these families of metallic alloys in the semi-solid processing field. Originality/value: ferrous alloys have become part of the thixoforming scenario more recently, when compared to aluminium and magnesium alloys. Therefore, the study of the thixoability of three commercial steels can bring important information as far as their utilization in thixoforming processing is concerned.