X80 pipeline steel plates were friction stir welded(FSW)under air,water,liquid CO2+water,and liquid CO2 cooling conditions,producing defect-free welds.The microstructural evolution and mechanical properties of these F...X80 pipeline steel plates were friction stir welded(FSW)under air,water,liquid CO2+water,and liquid CO2 cooling conditions,producing defect-free welds.The microstructural evolution and mechanical properties of these FSW joints were studied.Coarse granular bainite was observed in the nugget zone(NZ)under air cooling,and lath bainite and lath martensite increased signifi cantly as the cooling medium temperature reduced.In particular,under the liquid CO2 cooling condition,a dual phase structure of lath martensite and fi ne ferrite appeared in the NZ.Compared to the case under air cooling,a strong shear texture was identifi ed in the NZs under other rapid cooling conditions,because the partial deformation at elevated temperature was retained through higher cooling rates.Under liquid CO2 cooling,the highest transverse tensile strength and elongation of the joint reached 92%and 82%of those of the basal metal(BM),respectively,due to the weak tempering softening.A maximum impact energy of up to 93%of that of the BM was obtained in the NZ under liquid CO2 cooling,which was attributed to the operation of the dual phase of lath martensite and fi ne ferrite.展开更多
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/rain and a traverse speed of 200 mm/min. FSP resulted in the format...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/rain 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 X 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.展开更多
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 sub-jected to FSW,a...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 sub-jected 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 con-tained 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.展开更多
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 thermomechanical process during FSW ...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 thermomechanical 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.展开更多
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 micro- st...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 micro- structure and mechanical properties of the FSSW DP780 was investigated. The peak temperatures in the welds at various rotation rates were identified to be above A3 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 mar- tensite 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.展开更多
基金financially supported by the National Nature Science Foundation of China(Nos.51774085 and 51671190)the Fundamental Research for the Chinese Central Universities(No.N170704013)the National Key Research and Development Program of China(No.2017YFB0305004).
文摘X80 pipeline steel plates were friction stir welded(FSW)under air,water,liquid CO2+water,and liquid CO2 cooling conditions,producing defect-free welds.The microstructural evolution and mechanical properties of these FSW joints were studied.Coarse granular bainite was observed in the nugget zone(NZ)under air cooling,and lath bainite and lath martensite increased signifi cantly as the cooling medium temperature reduced.In particular,under the liquid CO2 cooling condition,a dual phase structure of lath martensite and fi ne ferrite appeared in the NZ.Compared to the case under air cooling,a strong shear texture was identifi ed in the NZs under other rapid cooling conditions,because the partial deformation at elevated temperature was retained through higher cooling rates.Under liquid CO2 cooling,the highest transverse tensile strength and elongation of the joint reached 92%and 82%of those of the basal metal(BM),respectively,due to the weak tempering softening.A maximum impact energy of up to 93%of that of the BM was obtained in the NZ under liquid CO2 cooling,which was attributed to the operation of the dual phase of lath martensite and fi ne ferrite.
基金supported by the National Natural Science Foundation of China(No.51001023)the Fundamental Research Funds for the Chinese Central Universities(No.N120407004)
文摘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/rain 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 X 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.
基金supported by the National Nature Science Foundation of China(Nos.52274378,51774085)the Liaoning Province Excellent Youth Foundation(No.2020-YQ-03)the Open Research Fund from the National Key Research and Development Program(No.2018YFA0707304).
文摘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 sub-jected 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 con-tained 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.
基金supported by the National Nature Science Foundation of China(No.51774085)Liaoning Province Excellent Youth Foundation(2020-YQ-03)+1 种基金the Open Research Fund from the State Key Laboratory of Rolling and AutomationNortheastern University(NEU)(2020RALKFKT009)。
文摘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 thermomechanical 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.
基金supported by the National Natural Science Foundation of China (No. 51001023)the Fundamental Research for the Chinese Central Universities (No. N120407004)the National High Technology Research and Development Program of China (No. 2015AA03A501)
文摘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 micro- structure and mechanical properties of the FSSW DP780 was investigated. The peak temperatures in the welds at various rotation rates were identified to be above A3 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 mar- tensite 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.
