The Research On Diffusion Bonding Of Dissimilar Materials Ti-6Al-4V And ZQSn10-10

The joining technology of new materials or dissimilar materials is foreland and hot point in the materials joining field nowadays. To research and develop the bonding technology of titanium alloys and other materials is an effective way to extend the application of titanium alloys. But certain difficulties are encountered when titanium and copper alloys are bonded because of different physical and chemical properties, especially bonding sleeve structures such as complex precise friction pairs. And it is not successful until now. This work was supported by National Natural Science Foundation of China (NSFC, No. 50375065) and State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology (No.04005). Diffusion bonding of Ti-6Al-4V and ZQSn10-10 was systemically carried out, during which the diffusion pressure was produced by different coefficients of linear expansion of dissimilar metals. This new technique will exploit wider application fields for diffusion bonding technology and promote the development of modern bonding technology.Direct diffusion bonding of Ti-6Al-4V/ZQSn10-10 in abutting joints was performed and the results indicated that the maximum tensile strength was only 84MPa, about 35% that of the matrix copper alloy under the optimum bonding parameters(T=850℃, P=10MPa,t=30min). And fractography of the joints showed the intermetallic compounds of CuSn3Ti5 and Cu3Ti forming on the ZQSn10-10 side and some Pb particles were also present; while on the Ti-6Al-4V side, the intermetallic compounds of CuSn3Ti5 and Pb particles could be detected. Therefore CuSn3Ti5 and Pb particles are the main reason to lead to fracture of the joints although the presence of Cu3Ti compounds on the interface.Both Ni interlayer and combined metal interlayers of Ni/Cu could raise the tensile strength of Ti-6Al-4V/ZQSn10-10 bonded joints. Using Ni as interlayer, the tensile strength under the optimum bonding parameters( T=830℃,P=10MPa,t=30min) was up to 155MPa, which reached to 64% that of ZQSn10-10 base metal. The fracture occurred on the Ti-6Al-4V/Ni interface, on which there were most intermetallic compounds of Ni3Ti. And using Ni/Cu as interlayer, the tensile strength was up to 155.8MPa under the optimum bonding parameters(T=850℃, P=10MPa, t=20min), which corresponded to that of Ti-6Al-4V/Ni/ZQSn10-10 joint. The fracture occurred on the Ti-6Al-4V/Ni interface, and NiTi and Ni3Ti compounds could be found out on the fractographes.Diffusion bonding of Ti-6Al-4V/ZQSn10-10 with Cu interlayer was carried out and the tensile strength of the joint was appreciably increased up to 192MPa under the optimum bonding parameters( T=850℃, P=10MPa, t=30min), about 80% that of ZQSn10-10 base metal. The fracture appeared on the Ti-6Al-4V/Cu interface where the Cu3Ti2 and CuTi compounds were present.When Ta was taken as the interlayer, the microstructures of the joint appeared compact without cracking but there was not adequate elemental diffusion around the interface. If using Ag as the interlayer, there were lots of intermetallic compounds on the interface from the bonding metallographes. The joint performance change from good to bad was Ta(Ag)< Ni(Ni/Cu)< Cu interlayers successively. Among the experimental results Cu interlayer was the best.Diffusion bonding of Ti-6Al-4V/ZQSn10-10 sleeve structure was performed and the diffusion bonding process included physics contact, Ti(ss.Cu)formation, Cu3Ti and CuSn3Ti5 formation and growth, andβ-Ti transforming toα-Ti. The fracture occurred in the zone of compound mixtures of Cu3Ti and CuSn3Ti5. When the intermetallic compound thickness was less than 6μm , the shear strength of the bonded joint increased gradually up to 55.9Mpa. On the contrary, when the intermetallic compound thickness was more than 6μm , the shear strength decreased with the increase of intermetallic compound thickness.When Ni interlayer was adopted in diffusion bonding of Ti-6Al-4V/ ZQSn10-10 sleeve structure, three layers ofα-Ti,Ni3Ti and NiTi appeared on the Ti-6Al-4V/Ni side. The joint was fractured on the interface of Ti-6Al-4V/Ni where Ni3Ti and NiTi compounds were present. And when the intermetallic compound thickness was less than 5μm , the shear strength was gradually increased up to 70.2MPa. On the contrary, when the intermetallic compound thickness was above 5μm , the shear strength decreased with the increase of intermetallic compound thickness.When Cu interlayer was adopted in diffusion bonding of Ti-6Al-4V/ ZQSn10-10 sleeve structure, the process of the diffusion bonding could be divided into five steps. They were physics contact, Ti(ss.Cu)formation, Cu3Ti2 and CuTi formation, Cu3Ti2 and CuTi growth andβ-Ti transforming toα-Ti. The fracture occurred in the zone of compound mixtures of Cu3Ti2 and CuTi. And when the intermetallic compound thickness was less than 5μm , the shear strength increased with increasing thickness up to 107.8MPa. it was appreciably increased compared with direct diffusion bonding. The main reason was Cu as a interlayer could not only prevent Sn and Pb from diffusion but also relieve the residual stresses. On the contrary, when the intermetallic compound thickness was more than 5μm , the shear strength was decreasing with the increase of intermetallic compound thickness.Residual stress has a great influence on the joint performance but it is difficult to measure it. So the finite element method was adopted in the text. The residual stress distribution of the diffusion bonding joint of Ti-6Al-4V/ ZQSn10-10 sleeve structure was simulated using ANSYS9.0 to supply the theoretical base for diffusion bonding technology. The results indicated that the residual stress harmful to the joint appeared in the narrow area adjacent to Ti-6Al-4V side where intermetallic compounds formed; and the stress uniformly distributed along the welding axis. But the stress concentrated on the both upper and down surfaces because of brinkmanship. When Cu was taken as interlayer, the residual stress value was much less than that of the direct diffusion bonding joint. When Ni was taken as interlayer, the residual stress distribution resembled that of Ti-6Al-4V/Cu/ZQSn10-10 joint, but the value was 100MPa, larger than the maximum value while Cu was used as interlayer.Concept of the residual stress coefficient (Rsc) was put forward in this dissertation. It is defined as Rsc = ?α? E ?σs?w, and qualitatively reflects the amount of residual stresses of the diffusion bonding joint. The smaller the value of Rsc, the less the residual stress of the joint and the better the properties. Using Cu as interlayer, the value of Rsc is smaller than that of Ni interlayer, so the residual stress of the joint is less, that accords with the simulation result.The principle for selecting the suitable interlayer was founded based on crystal structures, atomic radius and electronegativity for the layer choice. The interface diffusion model of the sleeve structure was established which can be utilized to analyze the elemental diffusion qualitatively or half-qualitatively.Based on diffusion theory, the forming capacity of the intermetallic phases on the diffusion bonding interface is determined by characteristics of the elements in the diffusion couple, the composition and proportion of the formed phases is in agreement with the atom diffusion flux. When J Ni :J Tiis larger than 3, the formation probability of Ni3Ti is the biggest by kinetics. Then Ni3Ti phase is born first and grows up breadthwise to a unity, and then turns to grow lengthways. And when J Ni :J Ticloses to 1, NiTi phase begins to form on the interface because of meeting the crystal nucleus growing condition by thermodynamics, and grows up with the increasing bonding time. The thermodynamics of element diffusion indicates that there is a latent period for intermetallic compound growth. And the relation between the intermetallic compound thickness and diffusion time conforms to parabola rule. When the bonding temperature holds invariable, the intermetallic compound thickness increases with bonding time. However, when the bonding time is up to a certain value, the element diffusion velocity decreases with prolonging the bonding time and the growth velocity of intermetallic compound reduces, too.