Thermal Stir Welds in Titanium
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INTRODUCTION
FRICTION stir welding (FSW) is a solid-state joining process that was originally developed to join aluminum alloys, although it has since been adapted for many other alloys including steels and titanium.[1] In the FSW process, a rotating, non-consumable tool is plunged into the material to be joined, where it generates heat in the surrounding material through frictional and plastic deformation heating. The heat reduces the flow stress in the surrounding material, allowing the rotating tool to deform or ‘stir’ that material. The resultant material flow transfers material around the tool to be deposited in its wake as a consolidated weld.[1,2] There have been many proposed shapes of FSW tools, but conventional tools often consist of a cylindrical or truncated conical pin and a shoulder. The pin produces heat within the workpiece and generates the rotational (usually with a vertical component) flow around the tool.[3] The shoulder introduces additional heat into the top surface and subsurface regions of the workpiece while constraining the deformed material. The tool shoulder produces a majority of the heating in thin sheet welds, while the pin produces a majority of the heating in thick workpieces.[4] While there have been many studies on FSW in titanium alloys, few of those[5–15] have focused on alpha or near-alpha titanium alloys. Most of those studies have focused on thin section (from 2- to 6.35-mm-thick sheet) welding, where the heat generated by the shoulder can more readily be transferred to the base of the weld during welding. While FSW of thicker (12.7 mm) plate has been performed on alpha[7] and near-alpha[6,10,15] titanium alloys, no stable welding conditions (tool design, tool rotation, weld speed, and welding forces) RICHARD W. FONDA, Section Head, and KEITH E. KNIPLING, Metallurgist, are with the Materials Science & Technology Division, Naval Research Laboratory, Washington, DC. Contact email: [email protected] ADAM L. PILCHAK, Materials Research Engineer, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB, OH. Manuscript submitted March 26, 2015. Article published online November 3, 2015 360—VOLUME 47A, JANUARY 2016
have been found to consistently achieve defect-free welds in such thicker sections of these alloys. The difficulty in FSW alpha and near-alpha titanium alloys arises from a combination of tool limitations, low thermal conductivity, and the temperature dependence of flow stress. The high strength and elevated temperatures required to friction stir weld titanium, in combination with the chemical reactivity of the titanium workpiece, limit possible tool materials that can be used to friction stir weld these alloys. The tool materials used, typically a strengthened tungsten alloy, cannot retain many of the complex geometries used to friction stir weld more conventional materials such as aluminum. Thus, most tool designs for welding titanium are relatively simple and do not generate the vertical material flow typically obs
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