The research on semisolid metal forming of Ni-based superalloys is limited because of the difficulties in preparing semisolid billets. Traditional methods for fabricating semisolid billets are SIMA and RAP. In order to decrease the processing procedures, a new method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was used to fabricate semisolid billets in this paper. In order to get the optimal parameters to fabricate Ni-based GH4037 semisolid billets, the effects of isothermal temperature and soaking time on microstructure evolution were investigated. The isothermal temperature ranges from solidus to liquidus, and the soaking time ranges from 10min to 60min. The results show that near-globular grains can be obtained only in the semi-solid state with appropriate solid fractions. When the solid fraction was high, the shape of grains was irregular and the average size of grains was large and inhomogeneous. When the isothermal temperature reached 1350°C, spherical grains appeared, and many small intragranular liquid droplets existed. During the appropriate isothermal temperature range, from 1350°C to 1380°C, the shape of solid grains was globular and the size of solid grains was homogeneous. At the same time, the size of the grains and the roundness of the grains increased with the increasing of isothermal temperature.
Semi-solid processing combines the advantages of traditional forging and casting methods, so it has received much attention recently. However, the research on semi-solid behaviors of Nickel-based superalloys has been rarely reported. In order to investigate the behaviors of Nickel-based superalloy at solid and semi-solid states, oxidation experiments, isothermal treatment experiments and deformation experiments of GH4037 alloy were studied. Short-term oxidation experiments of GH4037 alloy were carried out at a solid temperature (1200 °C) and a semi-solid temperature (1360 °C). The results indicated that the oxides formed at 1200 °C were mainly composed of TiO2, Cr2O3 and a small amount of spinels NiCr2O4, while the oxides formed at 1360 °C consisted of the spinels of NiCr2O4, NiWO4 and NiMoO4 besides TiO2 and Cr2O3. Microstructure evolution of GH4037 alloy after semi-solid isothermal treatment at 1370 °C and 1380 °C was studied. The results indicated that semi-solid microstructures consisted of equiaxed solid grains and liquid phases. The average grains size and shape factor of solid grains were affected by melting mechanism and grain growth mechanism. Compression behaviors of GH4037 alloy after compressed at 1200 °C and 1360 °C were investigated. The results indicated that the flow stress of 1360 °C decreased significantly compared to that of 1200 °C. The deformation zones in the specimens were divided into three parts: the difficult deformation zone, the large deformation zone, and the free deformation zone. At 1200 °C, the deformation mechanism was plastic deformation mechanism. At 1360 °C, sliding between solid particles (SS), liquid flow (LF), flow of liquid incorporating solid particles (FLS), plastic deformation of solid particles (PDS) coexisted in the compression specimen.
Semisolid slurries of four wrought alloys were fabricated via partial melting of commerical wrought aluminum alloy. Thixoforming experiments of four typical parts were performed. The results showed that a large amount of equiaxed grains before soaking in semisolid state were created due to recrystallization occurred in the continuous heating from room temperature to a given temperature above recrystallization temperature. It provides a desirable microstructure to form spheroidal grains during the next soaking process in semisolid state. The microstructure of the 2A12,7A04 and 7075 semisolid slurry consisted of fine and spheroidal grains. The elongation of the thixoformed parts were higher those of the hot-rolled plate. The UTS of the thixoformed parts were close or ever higher than those of the hot-rolled plate. Although the grain size and roundness of the 5A06 semisolid slurry are not very desirable, the mechanical properties of the thixoformed part are close or ever than those of the hot-rolled plate. The high mechanical properties of the thixoformed parts further confirmed the feasibility of short-process thixoforming route
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