Stdudy On Investment Casting Stress And Defects Of Ti-48Al-2Cr-2Nb

Ti-Al based alloy features for its performance at high temperature with low density,high melting point and large strength attracts increasing attention. Recently, themanufacturing of Ti-Al based alloy using precision investment casting has beendeveloped. This paper investigates the fluidity and capability of fulfilling the mesh ofTi-48Al-2Cr-2Nb alloy to evaluate the filling property. The residue stress of the castingsis measured using neutron diffraction. The mechanism of the crack emergence isanalyzed. In order to modify the crack defect, botanical organic material is added intothe casting mold. The shrinkage rate, microhardness, grain size and properties ofvarying percent of botanical organic material have been determined. Moreover, thecrack strength and thermal expansion coefficient of the shell after the addition ofpowder have been measured in order to establish necessary foundation for the furtherinvestigation.The result indicates, when the shell is heated at300℃, Ti-48Al-2Cr-2Nb alloy canperfectly fill the1.3mm thick casting, the residue stress is entirely tensile stress, andincreases with the increasing blocked area. The crack varies from black to grey as thetemperature decreases. The fracture pattern varies from brittle intergranular fracture toductile fracture and finally brittle cleavage fracture, and is in good agreement with filmtheory. After the addition of botanical organic powder, the deformability of the mold issignificantly improved and the tendency of crack is smoothed. After the comparison ofthe microstructure, surface quality of the as-cast sample, together with the strength,thermal expansion coefficient of the casting, this paper finds that addition of5%botanical powder is most appropriate.