Effects of Microstructural Modification Using Friction Stir Processing on Fatigue Strength of Butt-Welded Joints for High-Strength Steels

Abstract

Friction stir processing (FSP) is an effective surface-microstructure modification technique using a rotational tool to refine and homogenize microstructure of metallic materials. In this study, FSP was conducted on the surface of the heat-affected zone (HAZ), which is a region exhibiting degraded mechanical properties and shown to have microstructural changes, of butt-welded joints for two high-strength steels with tensile strength grades of 490 MPa and 780 MPa (hereafter HT490 and HT780, respectively). Inhomogeneous mixing of materials derived from weld metals and base metals (BMs) in a stir zone (SZ) produced inhomogeneous distribution of elements and microstructure depending on the set of the advancing side and retreating side in the SZs. The welded joints with FSP for HT490 exhibited higher hardness than that of the BM through whole of the SZ surface (fine polygonal ferrite grains and bainite structure with laths at the Mn-rich and Mn-poor regions, respectively). On the other hand, those for HT780 exhibited the minimum hardness value similar to that of the BM at the SZ surface (a few polygonal ferrite grains in the matrix of martensite laths). Fatigue strength increased by about 35 MPa and 15 MPa in stress amplitude at 10 7 cycles as fatigue limit due to FSP. Fatigue failure occurred at the BM and the SZ, respectively, in the welded joints modified by FSP for HT490 and HT780, in comparison with the HAZs in the as-welded joints for both grade steels. The difference in fatigue strength increase due to FSP and failure location between the welded joints for HT490 and HT780 can be attributed to the topmost SZ microstructures and their distribution.

Material positioning influence towards fatigue strength of the friction stir processed AA8011/AA6082 joints

“…Additionally, the fatigue strength was found to increase with a decrease in the microstructural grain size. Similar results were in consistent with those reported in the literature [21]. FSP was employed on the TIG welded AA6061 joint with the aim of increasing the fatigue life of the joint [22].…”

Friction Stir Processing Influence on Microstructure, Mechanical, and Corrosion Behavior of Steels: A Review

“…It was also recommended that slow FSW speed is best suited for improved fatigue performance due to the development of a highly refined, defect-free microstructure in the welded zone [ 47 ]. Fatigue strength of butt welded joints after FSP was investigated by Yamamoto and Ito [ 48 ] on two high-strength steels. Double-sided FSP was performed using a WC-6% Co tool with a rotational speed of 400 rpm and travel speed of 140 mm/min.…”

Thermo mechanical and microstructural constituents of dissimilar S700MC-S960QC high-strength steel welded joints using overmatched filler wire

“…By increasing softening the heat-affected zone (HAZ) of dissimilar welded joints of high-strength and ultra-high-strength steel when using overmatched filler wire can lead to the beneficial outcomes of lighter weight and increase of strength in the HAZ [5], [7], [8]. The gas metal arc welding process (GMAW) is an arc welding process that is suitable for welding high and ultra-high-strength steels [9][10][11][12][13]. Welding of dissimilar high-strength and ultra-high-strength steel is a difficult task that depends mainly on the heat source parameter for it to improve mechanical properties in the HAZ [14][15][16].…”