Microstructure Evolution During Friction Stir Processing and Hot Torsion Simulation of Ti-6Al-4V
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I.
INTRODUCTION
FRICTION stir processing (FSP) is a variant of friction stir welding (FSW) that can be used to modify the microstructure of the substrate material. In most cases, this modification leads to a dramatic reduction in grain size via a recrystallization mechanism.[1–3] FSP of aluminum alloys has been demonstrated[1–3] and more recent studies involving FSP have focused on highmelting temperature, non-ferrous materials (Ti and Ni). The joining of Ti-structural components by fusion welding often degrades mechanical properties relative to the base material. It is well known that arc welding processes such as gas tungsten arc welding (GTAW) produce large columnar grains in the weld metal. The large grain size reduces the mechanical properties, particularly toughness and ductility.[4,5] FSP has the potential to modify both wrought and cast base metal and deposited weld metal in a manner that may improve mechanical properties. FSP has been shown to produce severe deformation and develop a fully consolidated (defect free) process JOHN C. LIPPOLD, Professor, and JASON J. LIVINGSTON, former Graduate Student, are with the Welding Engineering Program, The Ohio State University, Columbus, OH. Contact e-mail: lippold.1@ osu.edu Manuscript submitted October 22, 2012. Article published online May 7, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
zone. During FSP, friction between the workpiece and tool causes thermal and mechanical modifications generating three distinct regions, namely the stir zone (SZ), the thermomechanically affected zone (TMAZ), and the heat-affected zone (HAZ), as shown schematically in Figure 1. For Ti alloys which are characterized by highly temperature-dependent flow stress, the TMAZ may not always be evident and will be visible only in a narrow region known as the transition zone (TZ).[6,7]
A. Friction Stir Welding of Titanium Alloys Numerous studies have focused on friction stir welding of titanium and its alloys with emphasis on the development of grain structure and texture evolution. The following is an overview of recent work performed in friction stir welding of titanium and titanium alloys. Cheng et al.[8] investigated the localized mechanical behavior and texture evolution of friction stir welds in a near-a alloy, Ti-5111 (Ti-5Al-1Sn-1Zr-1V-0.8Mo). Evidence of continuous grain boundary a in the center SZ indicated that the region experienced temperatures exceeding the b-transus. Transverse tensile tests revealed a reduction in ultimate tensile strength, yield strength, and elongation relative to the base metal. They suggested that this loss of mechanical properties was the result of defects in the welds. However, micro-sample VOLUME 44A, AUGUST 2013—3815
tensile tests localized in the SZ showed an increase in ultimate tensile strength and yield strength. The effect of spindle rotational speed on the microstructure of Ti-6Al-4V friction stir welds was studied by Zhou et al.[9] Rotation speed was varied from 400 to 600 RPM at a constant travel speed of 75 mm/min produced either a fully
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