Study On Microstructure And Properties Of Superplastic Diffusion Joints Between Ti₂AlNb And TC4 Alloys

Titanium alloy is widely used in aerospace industry due to its light weight,high strength,low thermal expansion coefficient,excellent creep resistance and corrosion resistance.However,its application is affected by the poor stability and oxidation resistance.It can only be used for a long time under600?.Ti₂AlNb alloy has some characteristics such as low density,high specific strength,good plasticity and toughness at room temperature and excellent hightemperature behavior,therefore Ti₂AlNb alloy is the most suitable materialin the range of 600⁷50°C.The question of how to weld these two alloys has become a hot research topic.In this paper,TC4?Ti-6Al-4V?and Ti₂AlNb?Ti-22Al-25Nb?alloys are taken as the research objects.Both TC4/TC4joints and TC4/Ti₂AlNb joints are obtained by means of isothermal superplastic diffusion bonding and phase transformation superplasticdiffusion bonding.Through the analysis and study of interfacial structure,mechanical properties and fracture morphology,the influence of temperature,time and phase transformation on joints'properties is revealed.From the perspective of element diffusion to expound interfacial effect and the mechanism of hole closure in welding,and to realize the designing and controlling of joints'microstructure and performance.Isothermal superplastic diffusion bonding of TC4 homologous alloy and TC4/Ti₂AlNb heterologous alloys was carried out under non-vacuum conditions.Best quality of joints was obtained underthe condition of 950°C/100 min/15MPa.An obvious diffusion layer formed around the interface between the TC4 and Ti₂AlNbbase alloys.Its width is 40?m.On Ti₂AlNb side,the diffusion layer is only composed of B2 phase.On TC4 side,adjacent to the interface is a discontinuous?/?₂ phase layer and between the interface and TC4 base alloy is a necklace-shaped?+?'martensitic structure layer.Downhill diffusion for all elements occurred across the interface.In the diffusion layer,the concentration of any element at certain point mainly depends on the distance of this point from the interface and the phase type where the point locates.The diffusion occurred not only in transverse direction but also in longitudinal direction.The Vickers microhardness of?+?'diffusion layer on TC4 side is HV385,which is higher than HV347 for?_p grain and HV308 for?+?_s colony structure.The hardness of B2 layer on Ti₂AlNb side was decreased to HV309 from HV324 for Ti₂AlNb base alloy.The strength of the joint has approached 926 MPa,almost equal to the UTS of the Ti₂AlNb base alloy.Some regions of the fracture surface reveal a relatively flat surface and occupy most of the surface area,which fractured along the bonding interface,belonging to brittle fracture.The other regions on TC4 side are composed of a moderate number of irregularly-shaped cavities,while on Ti₂AlNb side,are composed of as many irregularly-shaped bulges as the cavities,both resulting from the fracture along boundaries between?+?'layers and primary?_p grains or from the transcrystalline fracture of?p grains.Join's tensile strength under 700°C condition is 305MPa.It reaches the tensile strength of TC4 base material at 700°C.The mechanical properties of the joints obtained from the phase transformation superplastic diffusion bonding process under T_max=950°C,T_min=850°C,P=15MPa,N=4,t_s=100min are almost the same to those obtained from 950°C constant temperature diffusion bonding,but the average bonding temperature decreases by 50°C.The results of this project are expected to be applied in the preparation of some key components in hypersonic vehicles.