Hot Deformation Behavior and Processing Map of GH901 Superalloy

During the forging process GH901 superalloys easily produce cracks and defects, such as coarse crystals in tissues, which affect the performance of the alloy. Using GH901 nickel-based alloy, high-temperature compression tests at deformation temperatures of 990, 1040, 1090 and 1140 °C were carried out in a Thermecmastor-Z thermal simulator, with strain rates 0.001, 0.01, 0.1 and 1 s−1. Next, the isothermal forging process of a GH901 disc was simulated using DEFORM finite element simulation software. The results showed that with the increase in deformation temperatures and the decrease in strain rates, the flow stress clearly decreased. The flow stress constitutive model of GH901 superalloy under ε0.3 and the flow stress constitutive model for strain compensation were obtained. The processing map was built, and a reasonable range of thermal processing was obtained. Meanwhile, the isothermal forging simulation verified the reliability of the thermal processing range of the alloy.

Practical Use of Hot-Forged-Type Ti-42Al-5Mn and Various Recent Improvements

The use of a hot-forged TiAl alloy enables the fabrication of large parts that are difficult to manufacture by casting or isothermal forging. Ti-42Al-5Mn (at%) is the world’s first TiAl alloy in this category and has been used to manufacture practical large-scale structural defense components since around 2010. This paper discusses the developmental status and practical applications of this alloy. In addition, recent developments in process stabilization and improvements in material properties, which have been issues for the practical use of this TiAl alloy in the past, are also discussed.

Temperature Increase during Isothermal Forging of Ti-5Al-2Sn-2Zr-4Cr-4Mo Alloy Using a 1500-Ton Forging Press

In this study, the temperature increase of the Ti-17 alloy (Ti–5Al–2Sn–2Zr–4Cr–4Mo, wt.%) during isothermal forging in the (α + β) dual-phase region was investigated using large-size workpieces forged between hot dies in a 1500-ton forging press. The temperature increase was predicted using finite element analysis (FEA). The tip of a thermocouple was inserted into the center of the workpiece (diameter: 100 mm; height: 50 mm). The forging temperatures were 1023 K (750 °C) and 1073 K (800 °C) in the (α + β) dual-phase region. The strain rate was 0.05 s−1 and 0.5 s−1 at each temperature. Meanwhile, the compression percentage was 75%. The true stress–true strain curves were inversely obtained by fitting the load–stroke curves using the FEA. The predicted temperature was in a good agreement with that in the experimental results when the value of 1.0 was used as the conversion factor for plastic deformation energy to heat energy in the FEA under the current forging conditions. At the strain rate of 0.5 s−1, the temperature instantly increased to a β-transus temperature in 3 s at 1073 K (800 °C). In contrast, the temperature logarithmically increased at both 1023 K (750 °C) and 1073 K (800 °C) at 0.05 s−1 in 28 s (e.g., 42 K at 1023 K (750 °C)). The obtained true stress–true strain curves indicate that flow softening occurred during the forgings, which is attributed to dynamic recrystallization and/or dynamic recovery. The temperature increase in the Ti-17 alloy was smaller than that in the Ti–6Al–4V alloy under the same forging condition.

Microstructure Control of Welded Joints of Dissimilar Titanium Alloys by Isothermal Forging

In this study, the welded joints of dissimilar titanium alloys Ti600/Ti-22Al-25Nb were strengthened by isothermal forging. Different deformation parameters, including temperature, deformation speed, and reduction, were chosen. By isothermal forging, the original coarse dendritic grains of the welded joints were broken up effectively to form a large number of equiaxed grains. Meanwhile, many second phases were precipitated in the grain. Additionally, the dynamic globularization kinetics of second phases within the welded joints were quantitatively characterized and investigated. The results showed that the dynamic globularization kinetics and globularization rate were sensitive to the deformation conditions, and were promoted by a reduced strain rate and an elevated deformation temperature.