Influence of Heat Input on Friction Stir Welding for the ODS Steel MA956

Abstract

The oxide dispersion strengthened steel MA956 was friction stir welded using eight different rotational speed/translational speed combinations using a polycrystalline cubic boron nitride tool. Weld parameter conditions with high thermal input produced defect-free, full penetration welds. Electron backscatter diffraction showed a significant increase in grain size in the stir zone, a body centered cubic torsional texture in the stir zone, and a sharp transition in grain size across the thermo-mechanically affected zone. Micro-indentation results showed an asymmetric reduction in hardness across the transverse section of the weld that was sensitive to the heat input. This change in hardness is explained by the increase in grain size and may be described using a Hall-Petch type relationship.

Grain size and particle dispersion effects on the tensile behavior of friction stir welded MA956 oxide dispersion strengthened steel from low to elevated temperatures

“…Additional work has conclusively shown that FSW causes significant oxide particle coarsening which reduces the strengthening contribution of the dispersed oxides considerably [22]. The research presented herein combined with related reports on the evolved microstructure [21] and oxide particle morphology [22] aims to establish the processing, microstructure, and material property relationships of the ODS alloy MA956 following FSW with a variety of welding conditions. When combined the results show that MA956 can be manufactured and joined in a manner that maintains sufficient high temperature strength for use in nuclear applications.…”

“…Plunge force was maintained constant at 17.8 kN (4000 lbf). For the current paper, only fully consolidated welds were analyzed although previous research on processing and microstructure relationships included defective welds [21]. The three defect free FSW parameter conditions analyzed in this paper are (1) 400 RPM/100 MMPM (the lowest heat input condition), (2) 300 RPM/50 MMPM (slightly higher heat condition), and (3) 500 RPM/25 MMPM (the highest heat input condition).…”

“…Previous research on FSW of MA956 has shown that FSW is a viable joining mechanism that can produce consolidated welds in MA956 while maintaining a majority of the base material hardness [21,29,37]. Research on FSW with an initial fine grain size was shown to cause significant grain coarsening and softening in the weld nugget that was a function of the welding parameters [21].…”

“…The solid state joining technique friction stir welding (FSW), has been shown to be effective for MA956 and other ODS alloys by several authors [15][16][17][18][19][20], but a systematic review of FSW conditions, evolved microstructure, and final material properties has not been completed. Baker et al have recently shown that FSW can produce consolidated welds in ODS steels with varying degrees of grain coarsening depending on the starting grain size [21]. Additional work has conclusively shown that FSW causes significant oxide particle coarsening which reduces the strengthening contribution of the dispersed oxides considerably [22].…”

“…Research on FSW with an initial fine grain size was shown to cause significant grain coarsening and softening in the weld nugget that was a function of the welding parameters [21]. Weld consolidation, grain growth, texture, and hardness were correlated to welding parameters using a ratio of tool rotational speed (ω) in revolutions per minute (RPM) to tool traverse speed (ν) in millimeters per minute (MMPM) denoted as heat index (HI) (Eq.…”