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Effect of post weld artificial aging and water cooling on microstructure and mechanical properties of friction stir welded 2198-T8 Al-Li joints

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Journal of Materials Science & Technology 2022年第28期123卷 92-112页

作者： Y.Tao Z.Zhang P.Xue D.R.Ni B.L.Xiao Z.Y.Ma Institute of Metal Research Chinese Academy of SciencesShenyang 110016China

Friction stir welding(FSW)under both air cooling and water cooling conditions with welding parameters of 800-1200 rpm rotation rates and 50-200 mm/min welding speeds was carried out on 2198-T8 Al-Li alloys,and post weld artificial aging was performed on the air cooled joints.No welding defects other than lazy S were observed in the nugget zone(NZ)for all joints.Under air cooling condition,the lowest hardness zone(LHZ)occurred in the heat affected zone(HAZ).FSW resulted in gradual dissolution of original T_(1),θ′andδ′/β′from the base material(BM)to the thermo-mechanically affected zone(TMAZ),and complete dissolution of all precipitates in the NZ withδ′/β′and Guinier-Preston zones precipitating during cooling.The air cooled joints exhibited no noticeable changes in intrinsic tensile strength with a joint strength reaching 81.3%of the BM,but varied elongation with welding parameters,which was closely related to failure in the NZ and fracture along lazy S.Post weld artificial aging led to the largest hardness recovery in the TMAZ but smaller hardness recovery in the initial LHZ and the NZ.Different aging kinetics across the joint was determined by volume fraction of both original precipitate dissolu-tion during welding and coarse particles formed during aging,and by dislocation density inherited from welding.Post weld artificial aging greatly enhanced the joint strength with the ultimate tensile strength reaching 87.3%of the BM.As compared to air cooling condition,water cooling hardly affected the NZ hardness and did not improve the joint strength,and the reason was discussed in light of precipitates,hardness changes and fracture behavior.

关键词： Friction stir welding Aluminum-lithium alloys Microstructure Mechanical properties

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Characterization of the prior particle boundaries in a powder metallurgy Ti2AlNb alloy

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Journal of Materials Science & Technology 2019年第11期35卷 2513-2525页

作者： Jinhu Zhang Jinmin Liu Dongsheng Xu Jie Wu Lei Xu Rui Yang Institute of Metal Research Chinese Academy of Sciences

Ti2 AlNb-based alloy powder metallurgy(PM)compacts were prepared via hot isostatic pressing(HIP)under relatively low temperature(920 and 980℃)and at certain pressure(130 MPa).The microstructure,composition and orientation of B2,α2 and O phases in the compacts were characterized and analyzed with an aim to investigate the effect of unsuitable HIPping parameters on the appearance of prior particle boundary(PPB),which seriously affects the mechanical properties of the alloy.The results show that moreα2 phase is the characteristics of the PPB in Ti2AlNb-based alloy when HIPped at relatively low temperature.Increasing HIPping temperature to the upper part of the two-phase region can effectively inhibit the formation of PPB.Electron backscatter diffraction measurements show the specific orientation relationship between phases,which helps us understand the origin of a2 and O phase and the corresponding transformation path.The HIPping at a higher temperature can weaken the micro-texture intensity of theα2 and O phase due to the increase of misorientation in B2 phase.Theα2 phase at cell wall keeps the Burgers orientation relationship(BOR)with the grain on one side,and does not satisfy the BOR with the other.It is found that some O phase variants inside the cell HIPped at 980℃can only maintainα2-O OR withα2 owing to theα2→O phase transformation forming the O phase,while these O variants deviate from B2-O OR with B2 phase.

关键词： Powder metallurgy Titanium alloys Hot isostatic pressing Prior particle boundary(PPB) Microtexture Ti-22Al-24Nb-0.5Mo alloy

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Stress rupture properties and deformation mechanisms of K4750 alloy at the range of 650℃ to 800℃

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Journal of Materials Science & Technology 2019年第7期35卷 1270-1277页

作者： Meiqiong Ou Yingche Ma Weiwei Xing Xianchao Hao Bo Chen Leilei Ding Kui Liu Institute of Metal Research Chinese Academy of Sciences

The stress rupture properties and deform ation mechanism s of K4750 alloy at 650 ℃, 700 ℃, 750 ℃ and 800 ℃ were investigated. As the decrease of tem perature and stress, the stress rupture life gradually increased. A Larson-Miller Parameter (LMP) method was used for analyzing the stress rupture life under different conditions. The linear fitting formula between stress (σ) and LMP was derived as σ= 3166.455-119.969 ×LMP and the fitting coefficient w as 0.98. After testing, the dislocation configurations of all stress rupture samples w ere investigated by transm ission electron microscopy (TEM). The tem perature and stress had a significant impact on the deform ation m echanism, thereby affected the stress rupture life of K4750 alloy. As the increasing stress at a given tem perature, the deform ation m echanism gradually transform ed from Orowan looping to stacking fault shearing. Based on experim ental results, the threshold stress at 650 ℃, 700 ℃, 750 ℃ and 800 ℃ for the transition of deformation mechanism was estimated to be about 650 MPa, 530 MPa, 430 MPa and 350 MPa, respectively. Below the threshold stress,γ phase effectively hindered dislocation motion by Orowan looping mechanism, K4750 alloy had along stress rupture life. Slightly above the threshold stress, Orowan looping combining stacking fault shearing was the dom inant mechanism ,the stress rupture life decreased. As the further increase of stress, stacking fault shearing acted as the dominant deformation mechanism , the resistance to dislocation motion decreased rapidly, so the stress rupture life reduced significantly.

关键词： Nickel based superalloy Stress rupture properties Dislocation Deformation mechanisms Transmission electron microscopy

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Strain-induced Precipitation in Ti Micro-alloyed Interstitial-free Steel

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Journal of Iron and Steel research(International) 2016年第4期23卷 385-392页

作者： Ya-jun HUI Yang YU Lin WANG Chang WANG Wen-yuan LI Bin CHEN Sheet metal research institute Shougang Research Institute of Technology

Stress relaxation method was carried out on a Ti micro-alloyed interstitial-free （IF） steel at the temperature ranging from 800 to 1000℃. The results show that the softening kinetics curves of deformed austenite can be divided into three stages. At the first stage, the stress has a sharp drop due to the onset of recrystallization. At the second stage, a plateau appears on the relaxation curves indicating the start and finish of strain-induced precipitation. At the third stage, the stress curves begin to descend again because of coarsening of precipitates. Precipitation-time temperature （PTT） diagram exhibited a ＂C＂ shape, and the nose point of the PTT diagram is located at 900 ℃ and the start precipitation time of 10 s. The theoretical calculation shows that the strain-induced precipitates were confirmed as almost pure TiC particles. The TiC precipitates were heterogeneously distributed in either a chain-like or cell-like manner observed by transmission electron microscopy （TEM）, which indicates the precipitates nucleated on dislocations or dislocation substructures. In addition, a thermodynamic analytical model was presented to describe the precipitation in Ti micro-alloyed IF steel, which shows a good agreement between the experimental observation and the predictions of the model.

关键词： Ti micro-alloyed IF steel stress relaxation strain-induced precipitation analytical model

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Flow behavior and processing map for hot deformation of ATI425 titanium alloy

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Journal of Materials Science & Technology 2018年第4期34卷 679-688页

作者： Qinggang Meng Chunguang Bai Dongsheng Xu Institute of Metal Research Chinese Academy of Sciences

Flow behavior and processing map play important roles in the hot deformation process of titanium alloys. In this research, compression Gleeble tests have been carried out to investigate the stress-strain relationship at temperatures ranging from 700 to 1000 ℃ and strain rates ranging from 0.001 to 1 s-1 for ATI 425 titanium alloy. Arrhenius type constitutive equation was obtained to describe the compressive flow behavior with modification of additional deformation dead zone, friction model, temperature model and strain rate. The introduction of novel calculation method for α value in Arrhenius equation gives more accurate fitting than traditional one. Processing maps were drawn based on the distribution of dissipator co-content, and optimized deformation temperature and strain rate range obtained. It is proven to be accurate and effective through the experimental results. The microstructure analysis shows that more dynamic recrystallization can be achieved in the area with larger ηvalue on the processing map.

关键词： Titanium alloy Flow stress Constitutive equation Processing map

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Solidification of Immiscible Alloys Under the Effect of Electric and Magnetic Fields

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Acta metallurgica Sinica(English Letters) 2018年第12期31卷 1240-1248页

作者： Hong-Xiang Jiang Jiu-Zhou Zhao Institute of Metal Research Chinese Academy of Sciences

Immiscible alloys have aroused considerable interests in the last decades on account of their special physical andmechanical properties and potential applications. A considerable number of researches have been implemented toinvestigate the solidification behaviors of immiscible alloys in electric and magnetic fields. It has been indicated that themagnetic field and electric current can remarkably affect the solidification process and microstructures of immisciblealloys. The solidification techniques under the effects of electric and magnetic fields have great potentials for the fabri-cation of immiscible alloys. This paper reviews the research work in this field in recent years.

关键词： Immiscible alloys Solidification Microstructure evolution Electric current Magnetic field Electric-magnetic fields

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Investigation of Solidification and Segregation Characteristics of Cast Ni-Base Superalloy K417G

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Journal of Materials Science & Technology 2018年第3期34卷 541-550页

作者： Li Gong Bo Chen Zhanhui Du Mengshu Zhang Ruichang Liu Kui Liu Institute of Metal Research Chinese Academy of Sciences

Differential scanning calorimetry （DSC） analysis, isothermal solidification experiment and Thermo-Calc simulation were employed to investigate solidification characteristics of K417G Ni-base superalloy. Elec- tron probe microanalysis （EPMA） was employed to analyze the segregation characteristics. Liquidus, solidus and the formation temperatures of main phases were measured. In the process of solidification, the volume fraction of liquid dropped dramatically in the initial stage, while the dropping rate became very low in the final stage due to severe segregation of positive segregation elements into the residual liquid. The solidification began with the formation of primary γ. Then with solidification proceeding, Ti and Mo were enriched in the liquid interdendrite, which resulted in the precipitation of MC carbides in the interdendrite. A1 accumulated into liquid at the initial stage, but gathered to solid later due to the precipitation of γ/γ＇ eutectic at the intermediate stage of solidification. However, Co tended to segregate toward the solid phase. In the case of K417G alloy, combining DSC analysis and isothermal solidification experiment is a good way to investigate the solidification characteristics. Thermo-Calc simulation can serve as reference to investigate K417G alloy.

关键词： K417G superalloy Solidification characteristics Segregation Isothermal solidification Thermo-Calc simulation

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Crevice Corrosion of Several Supper Stainless Steels in the Simulated LT-MED Environment

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Acta metallurgica Sinica(English Letters) 2018年第11期31卷 1137-1147页

作者： Chang-Gang Wang Xiao-Fang Li Jie Wei Xin Wei Fang Xue Rong-Yao Ma Jun-Hua Dong Wei Ke Institute of Metal Research Chinese Academy of Sciences

Susceptibility and morphological characteristics of crevice corrosion for SS316, SS904L, SS254sMo and SS2507 in thesimulated low-temperature multi-effect distillation environment were investigated by cyclic polarization test, scanningelectron microscope and laser microscope. The results show that the crevice corrosion resistance of four kinds of stainlesssteel is ranked as SS254sMo ≈ SS2507 〉 SS316 〉 SS904L. There are ＂cover＂ structures over the edge of active crevicecorrosion regions of SS904L, SS254sMo and SS2507, but SS316 is an exception. Galvanic corrosion characteristicsappeared in the crevice of duplex supper stainless steel SS2507.

关键词： Supper austenite stainless steel Super duplex stainless steel Low-temperature multi-effect distillation （LT-MED） Crevice corrosion ＂Cover＂ structure

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On the γ' precipitates of the normal and inverse Portevin-Le Chatelier effect in a wrought Ni-base superalloy

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Journal of Materials Science & Technology 2019年第1期35卷 84-87页

作者： Xinguang Wang Guoming Han Chuanyong Cui Shuai Guan Jinguo Li Guichen Hou Yizhou Zhou Xiaofeng Sun Institute of Metal Research Chinese Academy of Sciences

The Portevin-Le Chatelier(PLC) effects in a wrought Ni-base superalloy with different γ' precipitates contents have been investigated. Detailed analysis on the serration type of the tensile curves indicates that the γ' precipitates have a decisive influence on the transformation from normal to inverse PLC behavior, which is rarely proposed in other works. It is considered that the γ' precipitates play the same role in PLC effect as temperature and strain rate for the investigated wrought Ni-base superalloy.

关键词： Normal and inverse PLC effec γ’ precipitates Dynamic strain aging Critical strain Wrought Ni-base superalloy

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Effect of cooling rate on microstructure,microsegregation and mechanical properties of cast Ni-based superalloy K417G

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Journal of Materials Science & Technology 2018年第5期34卷 811-820页

作者： Li Gong Bo Chen Long Zhang Yingche Ma Kui Liu Institute of Metal Research Chinese Academy of Sciences

The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900℃ and stress rupture test was performed at 950℃ with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ＇phase and the size of γ/γ＇eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ＇ eutectic decreased.In the cooling rate range of 1.42℃s^-1–0.849 s^-1,the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ＇ eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84℃ s^-1 showed the best comprehensive mechanical properties.

关键词： Ni-based cast superalloy Cooling rate Microstructure Microsegregation Mechanical property

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