| The bonding of Ti/Cu dissimilar metals can take into account the excellent properties of high-strength and high-toughness titanium alloys and high-conductivity copper alloys,which has good application potentiality in the field of aviation,aerospace,shipbuilding and chemical industry.However,significant differences in physical and chemical properties of Ti/Cu dissimilar metals,including thermal conductivity,thermal expansion coefficient,melting point and metallurgical incompatibility,make it difficult to weld titanium/copper joints.And Ti/Cu-IMCs layer is the main problem.Aiming at this problem,a welding method of ex-situ remelting welding for Ti/Cu-IMCs layer of dissimilar bonding interface was proposed in this paper.Based on the characteristics of high melting point and low thermal conductivity of titanium alloy,the technological scheme of ex-situ remelting welding on Ti side after Ti/Cu welding-brazing was designed.By constructing the controlled temperature field of Ti side,the remelting of Ti/Cu-IMCs layer was realized.The microscopic effects such as morphological reconstruction,solidification control and stress relaxation of Ti/Cu-IMCs layer were achieved,finally the bonding strength of Ti/Cu joint was improved.In this paper,FEM simulation was carried out.OM,SEM,EDS,XRD,TEM were conducted to analyze the microstructure and composition.Mechanical properties such as tensile strength and microhardness were investigated.The welding characteristics of Ti/Cu dissimilar metals and control mechanism of ex-situ remelting welding were studied.The results have a general reference for the welding of other dissimilar metals.The ex-situ remelting method includes two welding processes:one is electron beam welding-brazing on Cu side to obtain a metallurgical joint;the other is ex-situ remelting welding on Ti side to realize the microcosmic control of Ti/Cu-IMCs layer.The thermodynamic model of the ex-situ remelting welding on Ti side was established by FEM,and the results show that the interface of dissimilar bonding interface can reach the temperature range of 980℃ to 1400℃ which is over the melting point temperature of Ti/Cu-IMCs and the IMCs layer will remelt.At the same time,the stress state was improved and the peak stress was shifted to the weld of Ti side from the bonding interface.Microstructure and composition studies show that the interatomic reaction between Ti and Cu will produce a variety of IMCs in the joints.AIMCs layer growing on the dissimilar bonding interface of Ti side determines the bonding strength of Ti/Cu joints.The eutectic products of Ti2Cu and TiCu phases in IMCs layer are the main brittle factors causing joints failure.Reducing the width of IMCs layer and the dispersion distribution of IMCs have significant significance for improving the bonding strength of the joints.IMCs layer is located in the coarse grain zone of the heat-affected zone of the Ti side molten pool.So the IMCs layer will be melted,which results in its lamellar characteristics disappearing and a fragmentary morphology forming.At the same time,the width of IMCs layer decreases,and the diffusion trend of Cu atoms increases.The mechanical property showed that the tensile strength of the joint increased to 210 MPa after ex-situ remelting of Ti/Cu-IMCs layer,which was equivalent to 89%of the tensile strength of the copper substrate.Based on the accurate phase identification,thermodynamic free energy calculation and diffusion dynamics analysis of Ti/Cu-IMCs layer,the formation mechanism of IMCs layer and the enhancement mechanism of ex-situ remelting welding were revealed.When the molten pool solidifies,IMCs grow in the solidification sequence of Ti/Cu eutectic reaction with high Ti content to peritectic reaction with high Cu content,and each IMCs grows in a layered form and finally a continuous IMCs layer is formed.After ex-situ remelting welding process,the solidification sequence will evolve into peritectic Ti/Cu reaction with high Cu content to eutectic reaction with high Ti content.The change of solidification sequence promotes the fragmentation of continuous lamellar morphology,inhibits the precipitation of Ti/Cu eutectic products,decreases the width of IMCs layer,and improves the joint performance.The joints of Ti/Al and Ti/Steel dissimilar metals have similar welding problems to Ti/Cu joints.Based on the control mechanism of ex-situ remelting welding of Ti/Cu dissimilar metals,the applications of the control mechanism in the Ti/Al and Ti/Steel dissimilar metals with copper as interlayer were studied,and the metallurgical bonding properties of Ti/Al and Ti/Steel dissimilar metals were further improved.The proposed welding method further complements the control system of welding quality of dissimilar metals. |
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