Mechanical properties of thixoformed hypoeutectic gray cast iron

• Fe-2.6 wt%C-1.5 wt%Si cast iron exhibited excellent potential as a thixoformable raw material.
• Increased liquid fraction decreases the force values require for the thixoforming process.
• Ultimate tensile strength, elongation and hardness of the thixoformed alloy are relatively high compared to conventional gray cast iron.

The effects of thixoforming process variables on the final mechanical properties of a specially designed hypoeutectic cast iron were studied. An Fe-2.6 wt%C-1.5 wt%Si alloy was prepared using conventional sand casting molds and thixoformed in an eccentric press. Different temperatures and heat-treatment holding times were tested. After being heated to the semisolid state at 1160 °C and 1180 °C and held at these temperatures for 0, 30, 60 and 90 s, the samples were subjected to the thixoforming process. The holding time in the semisolid range simulates the industrial heating process, which is time controlled rather than temperature controlled. The results indicate that the forces applied during thixoforming are closely related to the liquid fraction of the hypoeutectic gray cast iron. A comparative analysis of the behavior of the tensile properties, Vickers microhardness and porosity before and after thixoforming was carried out. The stress-strain curves were similar for the different liquid fractions studied: at 1160 °C the average values were YE = 282 MPa, UTS = 289 MPa, E = 0.21% and Vickers microhardness = 274HV, while at 1180 °C the corresponding figures were YE = 272 MPa, UTS = 277 MPa, E = 0.2% and Vickers microhardness = 257HV. The small variations in tensile properties can be attributed to the final graphite morphology of the thixoformed alloy. The results also show that after thixoforming, the porosity of the Fe-2.6 wt%C-1.5 wt%Si alloy decreased for all experimental conditions and had a maximum value of 5%. There was no significant difference in hardness between the thixoformed material and the as-cast alloy.

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