In order to estimate the residual stresses in Ti2AlNb alloy jointed by electron beam welding (EBW), a computational approach based on finite element method was developed. Meanwhile, experiments were carried out to ver...In order to estimate the residual stresses in Ti2AlNb alloy jointed by electron beam welding (EBW), a computational approach based on finite element method was developed. Meanwhile, experiments were carried out to verify the numerical results. The comparison between the simulation results and measurements suggests that the developed computational approach has sufficient accuracy to predict the welding residual stress distributions. The results show that the central area of the fusion zone suffers tensile stresses in three directions. When the other parameters remain unchanged, the focus current has great impact on the weld shape and size, and then affects the residual stress level significantly. Moreover, the thick plate full-penetrated EBW weld suffers near 1000 MPa tensile stress of Z-direction in the center of the fusion zone. The wider weld has lower tensile stress in Z-direction, resulting in lower risk for cracking.展开更多
In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments,a series of heat treatment conditions were performed to the circular joints welded by electron beam,and then ...In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments,a series of heat treatment conditions were performed to the circular joints welded by electron beam,and then the macrostructures and microstructures were investigated using optical microscopy,scanning electron microscopy,X-ray diffractometry,and transmission electron microscopy.The results show that the reheat cracking occurs primarily along the grain boundaries in the weld when the Ti2AlNb circular welded joints are heated up to about 700℃.During the heat treatment,an almost complete transformation of B2→O happens while the temperature goes up through the O single-phase region.Then,O→B2+O phase transformation occurs primarily along the grain boundaries as the weld metal continues to heat up to the B2+O dual-phase region.Under the high tension stress consisting of welding residual stress and phase transformation stress,reheat cracking occurs at the interface between the B2+O dual-phase layer and the O-phase matrix.展开更多
Numerical simulation and experimental methods were used to investigate the effects of weld penetration on tensile properties of 2219 aluminum alloy tungsten inert gas(TIG)welded joints.The results show that when other...Numerical simulation and experimental methods were used to investigate the effects of weld penetration on tensile properties of 2219 aluminum alloy tungsten inert gas(TIG)welded joints.The results show that when other geometric parameters are consistent,within a certain range,the deeper the weld penetration of the capping weld is,the lower the tensile strength of the j oint is.The deeper weld penetration of the capping weld can cause the more concentrated stress at the weld toe and the joint is more likely to crack accordingly.Based on necessary assumptions,a model for analyzing the mathematical relation between the weld penetration of the capping weld and the tensile strength of the joint was proposed to validate the experimental results. The decrease of weld penetration of capping weld can be controlled by decreasing welding current,helium content or increasing welding voltage.展开更多
Wall structures were made by cold metal transfer-based wire and arc additive manufacturing using two kinds of ER2319 welding wires with and without Cd elements. T6 heat treatment was used to improve mechanical propert...Wall structures were made by cold metal transfer-based wire and arc additive manufacturing using two kinds of ER2319 welding wires with and without Cd elements. T6 heat treatment was used to improve mechanical properties of these wall structures. Due to the higher vacancy binding energy of Cd, Cd-vacancy clusters are formed in the aging process and provide a large number of nucleation locations for θ′ phases. The higher diffusion coefficient of the Cd-vacancy cluster and the lower interfacial energy of θ′ phase lead to the formation of dense θ′ phases in the heat-treated α(Al). According to the strengthening model, after adding Cd in ER2319 welding wires, the yield strength increases by 43 MPa in the building direction of the heat-treated wall structures.展开更多
Microstructural evolution and its effect on mechanical properties in different regions of 2219-C10S aluminum alloy tungsten inert gas(TIG)welded joint were analyzed in detail.In weld zone(WZ),α+θeutectic structure f...Microstructural evolution and its effect on mechanical properties in different regions of 2219-C10S aluminum alloy tungsten inert gas(TIG)welded joint were analyzed in detail.In weld zone(WZ),α+θeutectic structure formed at grain boundaries with no precipitates inside the grains.In partially melted zone(PMZ),symbiotic eutectic or divorced eutectic formed at grain boundaries and needle-likeθ′phases appeared in the secondary heated zone.In over aged zone(OAZ),the coarsening and dissolution ofθ′phases occurred and mostθ′phases transformed intoθphases.In general heat affected zone(HAZ),θ′phases coarsened.Factors such as the strengthening phases,the grain size,the Cu content in matrix and the dislocation density can affect the mechanical properties in different regions of the joint.Moreover,a model describing the relationship between mechanical properties of the material and the volume fraction of precipitates,the average diameter of precipitates and the concentration of soluble elements was proposed.展开更多
Transient liquid phase(TLP)bonding is a potential high-temperature(HT)electron packaging technology that is used inthe interconnection of wide band-gap semiconductors.This study focused on the mechanism of intermetall...Transient liquid phase(TLP)bonding is a potential high-temperature(HT)electron packaging technology that is used inthe interconnection of wide band-gap semiconductors.This study focused on the mechanism of intermetallic compounds(IMCs)evolution in Ag/Sn TLP soldering at different temperatures.Experimental results indicated that morphologies of Ag3Sn grains mainlywere scallop-type,and some other shapes such as prism,needle,hollow column,sheet and wire of Ag3Sn grains were also observed,which was resulted from their anisotropic growths.However,the scallop-type Ag3Sn layer turned into more planar with prolongingsoldering time,due to grain coarsening and anisotropic mass flow of Ag atoms from substrate.Furthermore,a great amount ofnano-Ag3Sn particles were found on the surfaces of Ag3Sn grains,which were formed in Ag-rich areas of the molten Sn and adsorbedby the Ag3Sn grains during solidification process.Growth kinetics of the Ag3Sn IMCs in TLP soldering followed a parabolicrelationship with soldering time,and the growth rate constants of250,280and320°C were calculated as5.83×10-15m2/s,7.83×10-15m2/s and2.83×10-14m2/s,respectively.Accordingly,the activation energy of the reaction was estimated about58.89kJ/mol.展开更多
基金Project(CALT201309)supported by Joint Innovation Fund for China Academy of Launch Vehicle Technology and Colleges
文摘In order to estimate the residual stresses in Ti2AlNb alloy jointed by electron beam welding (EBW), a computational approach based on finite element method was developed. Meanwhile, experiments were carried out to verify the numerical results. The comparison between the simulation results and measurements suggests that the developed computational approach has sufficient accuracy to predict the welding residual stress distributions. The results show that the central area of the fusion zone suffers tensile stresses in three directions. When the other parameters remain unchanged, the focus current has great impact on the weld shape and size, and then affects the residual stress level significantly. Moreover, the thick plate full-penetrated EBW weld suffers near 1000 MPa tensile stress of Z-direction in the center of the fusion zone. The wider weld has lower tensile stress in Z-direction, resulting in lower risk for cracking.
文摘In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments,a series of heat treatment conditions were performed to the circular joints welded by electron beam,and then the macrostructures and microstructures were investigated using optical microscopy,scanning electron microscopy,X-ray diffractometry,and transmission electron microscopy.The results show that the reheat cracking occurs primarily along the grain boundaries in the weld when the Ti2AlNb circular welded joints are heated up to about 700℃.During the heat treatment,an almost complete transformation of B2→O happens while the temperature goes up through the O single-phase region.Then,O→B2+O phase transformation occurs primarily along the grain boundaries as the weld metal continues to heat up to the B2+O dual-phase region.Under the high tension stress consisting of welding residual stress and phase transformation stress,reheat cracking occurs at the interface between the B2+O dual-phase layer and the O-phase matrix.
基金Project(U1637601)supported by the Joint Funds of the National Natural Science Foundation of China
文摘Numerical simulation and experimental methods were used to investigate the effects of weld penetration on tensile properties of 2219 aluminum alloy tungsten inert gas(TIG)welded joints.The results show that when other geometric parameters are consistent,within a certain range,the deeper the weld penetration of the capping weld is,the lower the tensile strength of the j oint is.The deeper weld penetration of the capping weld can cause the more concentrated stress at the weld toe and the joint is more likely to crack accordingly.Based on necessary assumptions,a model for analyzing the mathematical relation between the weld penetration of the capping weld and the tensile strength of the joint was proposed to validate the experimental results. The decrease of weld penetration of capping weld can be controlled by decreasing welding current,helium content or increasing welding voltage.
基金the financial support from the National Key Technologies Research & Development Program of China (No. 2018YFB1106000)the Youth Talent Project of CAST (No. 2019QNRC001)。
文摘Wall structures were made by cold metal transfer-based wire and arc additive manufacturing using two kinds of ER2319 welding wires with and without Cd elements. T6 heat treatment was used to improve mechanical properties of these wall structures. Due to the higher vacancy binding energy of Cd, Cd-vacancy clusters are formed in the aging process and provide a large number of nucleation locations for θ′ phases. The higher diffusion coefficient of the Cd-vacancy cluster and the lower interfacial energy of θ′ phase lead to the formation of dense θ′ phases in the heat-treated α(Al). According to the strengthening model, after adding Cd in ER2319 welding wires, the yield strength increases by 43 MPa in the building direction of the heat-treated wall structures.
基金Project(U1637601)supported by the Joint Funds of the National Natural Science Foundation of China。
文摘Microstructural evolution and its effect on mechanical properties in different regions of 2219-C10S aluminum alloy tungsten inert gas(TIG)welded joint were analyzed in detail.In weld zone(WZ),α+θeutectic structure formed at grain boundaries with no precipitates inside the grains.In partially melted zone(PMZ),symbiotic eutectic or divorced eutectic formed at grain boundaries and needle-likeθ′phases appeared in the secondary heated zone.In over aged zone(OAZ),the coarsening and dissolution ofθ′phases occurred and mostθ′phases transformed intoθphases.In general heat affected zone(HAZ),θ′phases coarsened.Factors such as the strengthening phases,the grain size,the Cu content in matrix and the dislocation density can affect the mechanical properties in different regions of the joint.Moreover,a model describing the relationship between mechanical properties of the material and the volume fraction of precipitates,the average diameter of precipitates and the concentration of soluble elements was proposed.
基金Project(51375260) supported by the National Natural Science Foundation of China
文摘Transient liquid phase(TLP)bonding is a potential high-temperature(HT)electron packaging technology that is used inthe interconnection of wide band-gap semiconductors.This study focused on the mechanism of intermetallic compounds(IMCs)evolution in Ag/Sn TLP soldering at different temperatures.Experimental results indicated that morphologies of Ag3Sn grains mainlywere scallop-type,and some other shapes such as prism,needle,hollow column,sheet and wire of Ag3Sn grains were also observed,which was resulted from their anisotropic growths.However,the scallop-type Ag3Sn layer turned into more planar with prolongingsoldering time,due to grain coarsening and anisotropic mass flow of Ag atoms from substrate.Furthermore,a great amount ofnano-Ag3Sn particles were found on the surfaces of Ag3Sn grains,which were formed in Ag-rich areas of the molten Sn and adsorbedby the Ag3Sn grains during solidification process.Growth kinetics of the Ag3Sn IMCs in TLP soldering followed a parabolicrelationship with soldering time,and the growth rate constants of250,280and320°C were calculated as5.83×10-15m2/s,7.83×10-15m2/s and2.83×10-14m2/s,respectively.Accordingly,the activation energy of the reaction was estimated about58.89kJ/mol.
