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 107 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.
Highlights
Friction stir processing (FSP), which is based on the principle of friction stir welding, is an effective surface-modification technique using a rotational tool to refine and homogenize microstructure of various metallic materials [1]
Fatigue failure occurred at the base metals (BMs) and the stir zone (SZ), respectively, in the welded joints modified by FSP for HT490 and HT780, in comparison with the heat-affected zone (HAZ) 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
Summary
Friction stir processing (FSP), which is based on the principle of friction stir welding, is an effective surface-modification technique using a rotational tool to refine and homogenize microstructure of various metallic materials [1]. Material flow during FSP contributes to the improvement of fatigue property of cast alloys [8] [9]. Costa et al [10] [11] [12] [13] reported that grain refinement and stress concentration reduction due to FSP at the weld toe increased fatigue strength of several aluminum alloy joints fabricated by metal inert gas welding. FSP can be used as a new technique to increase fatigue strength of the fusion-welded joints, and provide advantages different from grinding [14] [15] [16] and remelting [17] [18] [19] for the weld geometry modification or peening [20] [21] [22] and post-weld heat treatment [23] for decreasing tensile residual stress. FSP was conducted on the HAZ surface of butt-welded joints for two high-strength steels with 490 MPa and 780 MPa in tensile strength grades (hereafter HT490 and HT780, respectively), and the effects on microstructure and fatigue strength were investigated
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