Texture and microtexture variations in a near-α titanium forged disk of bimodal microstructure
Introduction
Manufacturing processes are heterogeneous by nature and manufacturers have difficulties in generating large parts with homogeneous properties. During the different forging steps of a disk, the change in deformation conditions from one disk location to another significantly alters the local microstructure/texture. As a consequence, the mechanical properties cannot be optimum everywhere [1].
This is particularly true in titanium forgings, as titanium alloys are known to be significantly sensitive to process parameters [2], [3]. The desired microstructure for high-pressure-compressor α titanium disks of aero-engines is often a fine-grained bimodal microstructure formed by equiaxed primary αp grains (30%) in a matrix of secondary αs laths [4]. This microstructure is expected to ensure both optimal creep and optimal fatigue properties. It is obtained by a succession of forging steps (open and closed die forging) and heat treatments in the α/β domain, starting from a coarse α lamellar microstructure. First, the ingot breakdown in the α/β domain converts the coarse colonies of α lamellae to globular αp grains, surrounded by the deformed/partially recrystallized β matrix. During cooling, the secondary αs laths are produced from the β → αs transformation with respect to the Burgers orientation relation (BOR).
However, the αp/αs microstructure may vary at different localizations of such forged products [2], [3]. Moreover, even if, after treatment, large regions with the desired microstructure have been reported, the forged product can still present sharp local textures, which differ from one region to the next. These regions, called macrozones, are potential sites for preferential nucleation of fatal cracks and can strongly reduce the fatigue lifetime [5], [6].
Over the years, much progress has been made to help manufacturers to simulate the process [7], [8]. However, microstructure prediction is still a challenge, especially if the local texture must also be predicted. This is even more difficult for dual phase processing followed by phase transformations as it is the case in the present work.
Experimentally, the link between thermomechanical treatments and the resulting microstructures has been studied extensively in the case of near-β titanium alloys [9], [10]. However, fewer data are available in the case of near-α titanium alloys because of the difficulties in analyzing the high-temperature α/β microstructure evolution [11], [12]. Thus, few publications analyze the microstructure and the local texture variations observed in a disk as a consequence of the heterogeneous forging process [13], [14].
The aim of this paper was therefore to analyze in detail the changes in local texture/microstructure observed in a bimodal (αp/αs) forged titanium disk and show how significant they can be. Specific attention was paid to the occurrence of macrozones according to the local thermomechanical history of the disk. In particular, the texture and microtexture variations from one region to the next were analyzed by discriminating the contribution of αp grains and αs laths to the resulting texture. Different regions of a forged disk were investigated, and the corresponding αp/β and αp/αs microtextures were documented and related to the processing conditions (local strain history and cooling rate during quenching). These data then allow us to discuss how the αp/β deformation and β → αs phase transformation mechanisms influence the microtexture variations produced. The results of this research may help control the texture and microtexture homogeneities in titanium forged disks.
Section snippets
Material
It is well known that a forged disk presents various regions that have undergone different deformations during the successive thermomechanical treatments. This can be an industrial issue because the local mechanical properties may differ from one region to the next. According to the axial symmetry of the disk, a single slice provides the whole range of metallurgical states due to local deformation conditions. The present contribution focuses on such a slice to record how the industrial
Local texture and microtexture variations
The α/β forged IMI834 disk showed pronounced variations in local texture and spatial distribution of orientations (Fig. 2, Fig. 3). Region R1 displayed a pronounced local texture and heterogeneous orientation distribution (Figs. 2a and 3a). Most of the grains had their c-axes aligned in specific radial directions (RDs/TD; Fig. 2a). The intensity of these texture components reached 4.5 times the random level. A minor texture component with c-axes in the AD was also observed.
Moreover, grains with
The nature of macrozones in a forged IMI 834 disk with a bimodal microstructure
Our study shows that macrozones are present at various locations in the forged IMI834 disk, and their nature can vary from one region to the next (Fig. 3). These macrozones can be defined as regions where a majority of αp grains have their c-axes close to a unique macroscopic direction. The average αp c-axis direction varies from one macrozone to the next, and the spread of these c-axes can be more or less important (Fig. 5). This heterogeneous α orientation distribution induced by the αp
Conclusion
This research focuses on the microtexture variations observed in a bimodal αp/αs forged IMI834 disk. More precisely, we analyze how macrozones present in the initial billet may be suppressed after forging. The αp/β deformation and β → αs phase transformation mechanisms are discussed according to the local processing conditions to explain the microtextures observed in the disk.
It appears that α/β forged IMI834 disks show pronounced variations in local α texture and spatial orientation
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