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• The preheating was introduced into the friction stir welded 7Mn steel to tailor austenitic stability. • The unprecedented the product of strength and elongation was obtained in the as-preheated NZ. • The deformation behavior of austenite in the NZ was systematically studied. • Multiple strain hardening mechanisms in the NZ were revealed. The martensite often appears in the nugget zone (NZ) of friction stir welding (FSW) 7 wt.% Mn steel due to low austenite stability, deteriorating ductility and toughness. In this work, a 7 wt.% Mn steel was subjected to FSW, and preheating was used to tailor the austenitic stability to greatly improve the strength–ductility combination of the NZ. The austenitic deformation behavior and strain hardening mechanism in the NZ were systematically investigated. The microstructure of the as-welded NZ was composed of ultrafine blocky ferrite, austenite, and small amounts of martensite, whereas the as-preheated NZ contained ultrafine blocky ferrite and austenite, and the concentration of Mn in austenite was increased from 8.4 wt.% to 10.7 wt.%. This enhanced the austenitic stability, resulting in a significant increase in the volume fraction of austenite in the as-preheated NZ from 37.3% to 66.4%. The product of strength and elongation (PSE) in the as-preheated NZ increased dramatically from 42.6 GPa% to 67.1 GPa%, depending on a persistent high strain hardening rate (SHR). Multiple strain-hardening mechanisms were revealed. The austenite with enhanced stability can provoke sustained transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects, and massive dislocation multiplication occurs during tension, resulting in strong strain hardening. [Display omitted] [ABSTRACT FROM AUTHOR]

Compared to nonferrous alloy, the microstructure in the nugget zone (NZ) of steel is easy to coarsen during friction stir welding (FSW) because of higher melting point of steel, leading to lower strength and plasticity of the joint. Here, high-strength bainitic steel was subjected to FSW in water + solid CO2medium, and mechanical properties and deformation behavior of the NZ were studied. The excellent yield strength (617 MPa) being 111% of the basal metal (BM) was obtained in the NZ owing to fine grain strengthening by fine effective grains of 4.3 μm and dislocation strengthening by the high density of dislocations of 6.7 × 1014 m−2. A perfect tensile strength of 664 MPa and uniform elongation of 13.1% were achieved in the NZ depending on the highly sustained work hardening rate, which were up to 112% and 113% of that of the BM. This can be partly explained by the fact that fine effective grains can obviously reduce the mean free path of dislocations. Moreover, dynamic recrystallization of ferrite with low density dislocations provided sufficient space for the multiplication of dislocations during deformation. Furthermore, refined effective grain and dual-phase microstructure (ferrite/bainite) promoted ductile fracture, and the total elongation of sample reached 108% of that of the BM.

• The thermo-mechanical process during FSW pipeline steel in the NZ was simulated by multi-pass thermal compression. • The influence of the dynamic recrystallization (DRX) process was studied during thermal deformation. • The austenitic non-DRX caused a significant grain refinement and an improvement in toughness simultaneously. High-strength pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 400–700 rpm, and the grain refinement mechanism of the nugget zone (NZ) was determined. The thermo-mechanical process during FSW in the NZ was simulated by multi-pass thermal compression, thereby achieving the austenitic non-recrystallization temperature (T nr). The austenitic non-recrystallization in the NZ at the lowest rotation rate of 400 rpm caused a significant grain refinement. Furthermore, the reduced rotation rate also resulted in the formation of a high ratio of island-like martensite-austenite (M-A) constituent. The toughness of the NZs was enhanced as the rotation rate decreased, which is attributed to the fine effective grains and homogeneously distributed fine M-A constituents dramatically inhibiting crack initiation and propagation. [Display omitted] [ABSTRACT FROM AUTHOR]

DP780 steel sheets consisting of ferrite and martensite were successfully friction stir spot welded (FSSW) at the rotation rates of 500 to 1500 r/min using a W-Re alloy tool. The effect of rotation rate on microstructure and mechanical properties of the FSSW DP780 was investigated. The peak temperatures in the welds at various rotation rates were identified to be above A 3 temperature. FSSW caused the dynamic recrystallization in the stir zone (SZ), thereby producing the fine equiaxed grain structures. At the higher rotation rates of ≥1000 r/min, a full martensitic structure was observed throughout the SZs, whereas at the lower rotation rate of 500 r/min, the SZ consisted of a fine dual phase structure of ferrite and martensite due to the action of deformation induced ferrite transformation. The maximum average failure load as high as 18.2 kN was obtained at the rotation rate of 1000 r/min and the fracture occurred at the thinned upper sheet. [ABSTRACT FROM AUTHOR]

As-extruded ZK60 and ZK60-Y magnesium alloy plates were successfully processed via friction stir processing (FSP) at a tool rotation rate of 1600 r/min and a traverse speed of 200 mm/min. FSP resulted in the formation of equiaxed recrystallized microstructures with the average grain sizes of ∼8.5 and ∼4.7 μm in the ZK60 and ZK60-Y alloys, respectively. Moreover, FSP broke and dispersed the MgZn2 and W-phase (Mg3Zn3Y2) particles and dissolved MgZn2 phase in the FSP ZK60 alloy. With the addition of rare earth element yttrium (Y) into the ZK60 alloy, the ratio of the high angle grain boundaries (HAGBs) in the FSP alloys increased from 64% to 90%, and a certain amount of twins appeared in the FSP ZK60-Y alloy. The maximum elongation of 1200% and optimum strain rate of 3 × 10−3 s−1 achieved at 450 °C in the FSP ZK60-Y alloy were substantially higher than those of the FSP ZK60 alloy. This is attributed to the fine grains with high ratio of HAGBs and the distribution of a large number of dispersed second phase particles with high thermal stability in the FSP ZK60-Y alloy. Grain boundary sliding was identified as the primary deformation mechanism in the FSP ZK60 and ZK60-Y alloys from the superplastic data analyses and surficial morphology observations. [ABSTRACT FROM AUTHOR]

Friction stir welding (FSW) of Mg-Zn-Y-Zr plates with 6 mm in thickness was successfully carried out under a wide range of rotation rates of 600-1200 r/min with a constant traverse speed of 100 mm/min. After FSW, the coarse grains in the parent material (PM) were changed into fine equiaxed recrystallized grains at the nugget zone (NZ). Furthermore, the coarse Mg-Zn-Y particles (W-phase) were broken up and dispersed homogenously into the Mg matrix. With increasing rotation rates, the size of the W-phase particles at the NZ significantly decreased, but the recrystallized grain size tended to increase. The hardness values of the NZs for all the FSW joints were higher than those of the PM, and the lowest hardness values were detected in the heat affected zone (HAZ). The fracture occurred in the thermo-mechanical affected zone (TMAZ) on the advancing side for all the FSW joints in the tensile test, due to the incompatibility of the plastic deformation between the NZ and TMAZ caused by remarkably different orientation of grains and W-phase particles. The strength of FSW joint reaches 90% of that of its PM. [ABSTRACT FROM AUTHOR]