Microstructural Changes Due to Friction Stir Processing of Investment-Cast Ti-6Al-4V

Abstract

Friction stir processing (FSP) was used to modify the coarse fully lamellar microstructure on the surface of investment-cast and hot isostatically pressed (HIP) Ti-6Al-4V plate. The a colony and platelet structure in the base material (BM) was refined such that the effective slip length was reduced from the a colony size of the BM, several hundred microns, to that of fine equiaxed primary a grains that are on the order of 1 lm. This change in the microstructure resulting from FSP is expected to increase fatigue crack initiation resistance making it beneficial for titanium components for aerospace applications. The as-cast coarse lamellar microstructure has superior fatigue crack growth resistance compared to other microstructures that can be obtained by thermomechanical processing. Thus, it is likely that an increase in fatigue life is obtainable by FSP. Given the growing interest in friction stir welding (FSW) and FSP of titanium alloys, we believe some consistent microstructural descriptors will help avoid confusion. Accordingly, we propose terminology to standardize the descriptions of the microstructures created during FSP and FSW of titanium alloys. We also describe the microstructure changes that occur in the stir zone (SZ), transition zone (TZ), and heat-affected zone (HAZ) during FSP of Ti-6Al-4V.

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

“…Thus, it is considerably more difficult to identify the transition zone and heat-affected zones when the starting material is in the mill-annealed condition. [7,10] While the previous discussion has focused primarily on the material within the processed zone that exceeded the b transus, there have also been reports of subtransus welding and processing. In this case, both a and b phases are present in the stir zone during welding or processing.…”

“…Additional details of the processing have been described elsewhere. [7,8,12] C. Electron Microscopy Specimens for metallographic examination were sectioned from the as friction stir processed plate using an electrical discharge machine (EDM). The samples were then ground and polished using successively finer SiC papers.…”

“…[4][5][6] Recent work by the present authors showed that FSP can also be used to refine the coarse, fully lamellar microstructure of investment-cast and hot isostatic pressed Ti-6Al-4V (Ti-6-4). [1,7,8] Different microstructures can be created depending on the peak temperature in the stir zone relative to the b transus temperature. During subtransus processing, the stir zone microstructure consisted of approximately 1-lm equiaxed a grains, while it consisted of 25-to 30-lm prior b grains containing a + b colonies during supertransus processing.…”

“…The transition zone is the region between the unaffected base material and the stir zone, which contains deformed, but unrecrystallized, constituents that were present in the base material microstructure. [7] The microstructure gradient in the transition zone provides insight into how the initially strain-free, fully lamellar microstructure was converted to fine, equiaxed a grains. The present investigation addresses the fundamental principles underlying grain refinement and the formation of high-angle grain boundaries in the a phase during FSP.…”

Formation Mechanisms of Electron Beam Processed Surface Textures on Titanium Alloys

“…As shown by the dashed lines in the images, there is layer present just beneath the fusion zone, although does not appear as bright as fusion zone it exhibits a different appearance compared with the base metal. Melting or resolidification did not occur in this layer, but solid phase transformation did happen owing to the heat effect from high energy electron beam [11][12][13]. Therefore, this layer is termed heat affected zone.…”