Effect of Tool Angle on Friction Stir Weldability of AISI 430 Friction stir welding (FSW), invented at The Welding Institute in 1991 (Ref. 1), is a solid-state joining process in which welding defects formed in conventional fusion welds are not observed (Ref. 2). The shoulder provides the fundamental source of heat by friction, prevents the material expulsion, assists the material movement around the tool, and mixes the material under the tool shoulder. During the process, heat is generated by plastic deformation as well as by the friction between the tool and workpieces. Friction stir welding is carried out in several steps. First, the pin is plunged into the joint formed by the two sheets to be welded, until the shoulder gets in contact. The sheets are heated until melting temperature by rotation at a high velocity without any translational motion. Then, the tool is moved along the weld interface, heating the material further by the stirring action and moving the softened (but always solid) material from the front of the tool, and depositing it behind its trailing edge to produce the weld. Advantages of Friction Stir Welds The advantages of FSW include low residual stress, low energy input, and fine grain size compared to the conventional welding methods. Ferritic steels are generally more difficult to weld than austenitic steels. This is the main reason they are not used to the same extent as austenitic steels. AISI 42 JANUARY 2013 430 has greatly reduced ductility in the weld. This is mainly due to strong grain growth in the heat-affected zone (HAZ), but also to precipitation of martensite in the HAZ. Many parameters affect the welding success and quality of FSW, especially stainless steels, which are difficult to weld due to their high melting points compared with aluminum, brass, and copper. Researchers and scientists are still working on the effects of these parameters on the welding and welded joint strength. Results of Earlier Studies Alptekin (Ref. 3) obtained the best weld joint in 304 austenitic stainless steel using 1000 rev/min rotational speed, 63 mm/min traverse speed, and tool angle of 1 deg 45 s using a 20-mm-diameter tungsten carbide tool. Meran and Canyurt (Ref. 4) studied the effect of tool angle on friction stir weldability of AISI 304 austenitic stainless steels, and they found the highest tensile and impact test results at 2-deg tool angle at a constant 1180 rev/min, 60 mm/min welding speed, and 9 kN axial force. Hattingh et al. (Ref. 5) investigated the characterization of the effect of FSW MEHMET B. BILGIN (mb_bilgin@hotmail.com) is an Assistant Prof. Dr.; and CEMAL MERAN and OLCAY E. CANYURT are Associate Prof. Drs., Dept. of Mechanical Engineering, Technology Faculty, Amasya University, Amasya, Turkey. tool geometry on welding forces and weld tensile strength using an instrumented tool. They searched the effect of important parameters including flute design (e.g., number, depth, and taper angle), the tool pin diameter and taper, and the pitch of any thread form on the pin. They analyzed the force footprint and flute angle. They obtained the highest strength welds with low angular rotation values of the maximum force and high ratios of maximum to minimum force on the tool. Buffa et al. (Ref. 6) studied the design of the FSW tool using the continuumbased FEM model. They suggested that tool geometry plays a fundamental role in obtaining desirable microstructures in the weld and HAZs, and consequently improves the strength and fatigue resistance of the joint. They concluded that increasing the pin angle enlarges both the HAZ and thermomechanical zone resulting in a bigger weld nugget. Also, they determined that the overall temperature in the weld zone increases with pin angle. They suggested that an increase in the pin angle leads to uniform temperature distribution along sheet thickness, which is favorable for the reduction of distortion. They also suggested that the plastic deformation in the nugget increases with the pin angle. Tests were conducted to determine the best tool angle for making friction stir welds BY MEHMET B. BILGIN, CEMAL MERAN, AND OLCAY E. CANYURT Table 1 — Chemical Composition of AISI 430 (%) C Cr Si Mn P S Fe 0.01 16.48 0.31 0.52 0.05 0.01 Bal.
Welding Journal | January 2013
To see the actual publication please follow the link above