Effects of Friction Stir Processing on the Microstructure and Mechanical Properties of Fusion Welded 304L Stainless Steel

A variation of FSW, friction stir processing (FSP), has been used to modify selected regions of materials to enhance specific properties while eliminating fusion welding defects such as porosity, cracking, and the cast microstructure. The combination of fusion welding defects and high tensile residual stresses caused by the solidification of the molten weld pool adversely affect the post weld service integrity. FSP has been demonstrated to eliminate many of these problems while at the same time improving the resulting properties. FSP has been utilized to locally process regions of arc weldments in 304L stainless steel to improve the service integrity. The cast microstructure and coarse delta-ferrite is replaced with a fine-grained wrought microstructure. This paper presents a preliminary processing window for FSP of 304L stainless steel, the resulting microstructure associated with this process window, along with considerations given to mechanical properties and corrosion. Introduction and Background Friction stir welding (FSW), since it’s conception by TWI in 1991[1,2], has been used extensively and successfully in joining aluminum and other low-temperature materials. FSW is a solid-state joining process in which the material is softened and plasticised by a rotating tool consisting of a shoulder and a smaller concentric pin. The pin tool acts to transport material from the front to the back of the tool where the shoulder provides the forging pressure necessary to reconsolidate the material forming a weld. The rotating tool is translated along the weld joint leaving a homogeneous, refined grain size, wrought microstructure. The resulting joint is superior to that of a traditional arc weld in that it has lower residual stresses, and is free of the cast microstructure and defects such as cracking and porosity. A variation of FSW called friction stir processing (FSP), which uses the same tooling, has been proven effective in selectively modifying the microstructure of specific areas to improve local properties [3]. Both processes have been shown to be superior to Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 2004 2. REPORT TYPE 3. DATES COVERED 00-00-2004 to 00-00-2004 4. TITLE AND SUBTITLE Effects of Friction Stir Processing on the Microstructure and Mechanical Properties of Fusion Welded 304L Stainless Steel 5a. CONTRACT NUMBER