Globularization Behavior of ELI Grade Ti-6Al-4V Alloy during Non-Isothermal Multi-Step Forging

Abstract

The globularization behavior of ELI grade Ti-6Al-4V alloy having different initial lamellar structures during multi-step forging under nonisothermal condition was investigated. The samples with either a thin or thick lamellar structure, which were produced from annealing followed by different cooling conditions, were upset and stretched repeatedly at forging start temperatures of 940 C and 900 C, and subsequently air cooled. The microstructural changes after non-isothermal multi-step forging were analyzed with respect to globularization of lamellae. After multi-step forging at 940 C, the initially thin lamellar structure changed to homogeneous equiaxed globules, but elongated globules with high aspect ratio were obtained from the initially thick lamellar structure. By forging at 900 C, the initially thin lamellar structure was changed to a mixed structure of both equiaxed and elongated globules, while severely deformed lamellae were preserved inside distorted colonies with little globularization for the initially thick lamellar structure. Globularization of the samples by forging at 900 C with both initially thin and thick lamellar structures was less effective compared to that by forging at 940 C. A quantitative microstructural analysis revealed that, in addition to the initial microstructure and forging start temperature, globularization of lamellae was significantly affected by the instantaneous microstructural development by thermal fluctuation during non-isothermal multi-step forging.

Miniaturised Experimental Simulation and Combined Modelling of Open-Die Forging of Ti-6al-4v Titanium Alloy

“…In the studies reviewed, short reheating times were used due to the small size of the specimens. For instance, Ari-Gur and Semiatin [63] and Kim et al [64] used annealing times of 3 min and 10 min respectively between forging steps. Although these short annealing times had minimal impact on the microstructure, a slight grain coarsening was observed after multiple deformation and reheating cycles [64].…”

“…For instance, Ari-Gur and Semiatin [63] and Kim et al [64] used annealing times of 3 min and 10 min respectively between forging steps. Although these short annealing times had minimal impact on the microstructure, a slight grain coarsening was observed after multiple deformation and reheating cycles [64]. However, in industrial cogging operations, the reheating time for a billet is significantly longer, often lasting several hours.…”

Transformation par forgeage à chaud des microstructures lamellaires d’alliages hexagonaux

“…Lorsqu'on veutéviter de faire grossir le grain, par exemple pour mettre en forme par superplasticité, il est judicieux de recourir seulement au forgeage [6]. La forge en usine n'étant pas isotherme comme les essais de laboratoire, divers auteurs ont mis enévidence les instabilités que provoque le refroidissement en cours du processus [11,12].…”

Effect of thermomechanical processing on microstructure characteristics of γ-TiAl-based alloy

“…Compared with other plastic deformation methods – such as equal-channel angular pressing and hot isostatic pressing – the advantages of MDIF include an unrestricted sample size, simple equipment, and high efficiency. MDIF has been successfully applied to refine microstructures in Cu alloys [14], Ni-based alloys [15], Mg alloys [16,17], pure Ti [18], Ti-6Al-4V alloys [19,20], Ti-35.8Al (wt-%) alloys [21], Ti-6Al-3.2Mo alloys [22], and γ-TiAl-based alloys such as Ti-48Al-2Nb-2Cr (at.-%) [23] and Ti-44Al-4Nb-4Hf-0.2Si-1B (at.-%) [24]. As such, MDIF can be used to refine the grain size of γ-TiAl-based alloys, decreasing their brittleness at room temperature.…”