Literature Review of:
P. Li, J. Li, J. Xiong, F. Zhang and S. H. Raza, "Diffusion bonding titanium to stainless steel using Nb/Cu/Ni multi-interlayer," Materials Characterization, vol. 68, pp. 82-87, 2012
Diffusion bonding is a method of joining metallic or non-metallic materials. This method produces solid-state coalescence between two materials through the application of pressure at a temperature range between 0.5 of the absolute melting temperature of the material and the room temperature under a low pressure without causing a macroscopic plastic deformation in the material [1, 2]. Since diffusion bonding is formed from atomic migration across an interface in a solid-state, there is no metallurgical discontinuity at the interface and hence mechanical properties and microstructure at the bonded region are not different from those of the base metal [2]. The quality of joint is determined by the precise control of the type and thickness of the intermetallic phases formed at the interface region, and the adjustment of the process parameters [1].
Diffusion bonding has already proved to be a considerable potential for joining of a variety of similar and dissimilar materials. Due to the increasing use of titanium alloys in transportation, power generation, and chemical industries, a considerable amount of interests have been developed in joining titanium to steel for reducing the cost of materials. However, direct bonding of titanium to steel is very difficult due to the low solubility of iron in alpha titanium at room temperature [1]. Moreover, direct diffusion bonding of titanium and stainless steel leads to formation of a variety of intermetallic compounds such as TiFe, TiFe2, σ (with composition of Fe-Cr-Ti-Ni), Fe2Ti4O and TiC, which embrittles the joint [3]. Orhan et al. (2001) have suggested that Fe-Cr-Ti intermetallic is more important than σ-phase formation during directly bonding titanium alloys to micro-duplex stainless steel since formation of σ-phase could be controlled by adjusting the diffusion bonding parameters. On the other hand, Aleman et al. (1993) reported that a large internal stress is formed because of the difference of linear expansion and heat transmission coefficient between titanium and steel, which leads to crack formation [1]. Meanwhile, residual stress may arise from the thermal expansion mismatch because of dissimilar metals bonded [3]. Thus, it became necessary to insert metal foils as the interlayers to accommodate residual stress concentration and chemical compatibilities.
The interlayer is used for several reasons such as (1) to minimize thermal expansion mismatch, (2) to reduce bonding temperature and/ or pressure, (3) to inhibit diffusion of undesired element and also (4) to reduce or to avoid the formation of brittle intermetallic phases [4]. Single metal foil was firstly used as the interlayer for diffusion bonding of titanium and stainless steel, such as Al, Cu or Ni foil, which serves as a ductile interlayer to release the residual stress concentrated in the joint [3].
Moreover, the multi-interlayer also used in diffusion bonding of titanium and stainless steel was reported as Cu+Ni foils and Nb+Ni foils. The Ni foil was set adjacent to the steel side for better Ni-steel bonding, whereas the Cu foil was set to the titanium side. A lot of Ti-Cu intermetallic compounds were formed at the interface, of which the joint strength was found to be lower than others [3]. Therefore, Nb foil was used instead of Cu for the improvement of the bonding quality and strength. But, the Cu foil relaxes stress concentration, and the absence of Cu may cause the joint to become brittle. So, inserting Cu foil into Nb-Ni was proposed and, further investigation was done on the diffusion bonding of Ti to Stainless steel using Nb/Cu/Ni as the multi-interlayer [5].
Commercially available pure titanium (Cp-Ti, Grade 3) and AISI 321 austenitic stainless steel (SS) were used as a parent metal for bonding, and were machined into a cylindrical size. The samples were polished and ultrasonically cleaned in ethanol and dried in air. The samples were assembled in the structure of Cp-Ti/Nb/Cu/Ni/SS; loaded into a vacuum diffusion bonding furnace, and pressed under an axial pressure of 10 MPa. After obtaining the vacuum pressure, the temperature was raised to 800º C and kept for 60 min for good contact of the bonding surface. As the temperature was raised up to 800º C, the axial pressure was then reduced to 3 MPa to control the deformation. To clarify the effect of bonding temperature and time, the bonding temperatures were set from 825º C to 950º C in a step of 25º C , and bonding times of 15, 30, 45 and 60 min were used at 850º C. After cooling down the samples, those were cut along the longitudinal direction and then prepared by conventional grinding and polishing techniques for metallographic examination. The microstructure and element distribution of the joints were analyzed by a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS), and the joint strength was evaluated by a tensile testing machine.
The evolvement of interfacial microstructures and element concentration profiles of the joints bonded at 850º C, 900º C and 950º C for 45 min were presented. There are five interfacial layers that gradually emerged with the gradual increase of temperature. One layer, formed at the original SS-NI interface, represented the existence of Fe-Cr-Ni solid solution, and the others represented the residual Ni, Cu and Nb interlayers. Ni atoms aggregated at the Cu_Nb interface by uphill diffusion across the Cu interlayer. The presence of Ni atoms promoted the solution of Cu in Nb base. Though Cu-Nb is an immiscible system in equilibrium state, the supersaturated solid solution of Cu-Nb formed during the bonding process has extremely high strength. A figure was represented showing the tensile strength of the joints as a function of bonding temperature. The highest tensile strength was 314 MPa at 850º C, and gradually decreased to 150 MPa at 900º C. The optimum bonding temperature was set at 850º C to study the effect of bonding time. The microstructure evolvement of the joints bonded for 15 min, 30 min and 60 min with EDS analysis were presented. Voids were found initially at the interfaces of Cu-Nb and SS-Ni which was an indication of incomplete plastic collapse of the mating surface owing to insufficient bonding time. The voids were then vanished as the bonding time increased to 30 min or longer. The tensile strength of the joint at 850º C as a function of bonding time was measured. At 15 min, the tensile strength was around 260-270 MPa, and it increased gradually along with the increase of bonding time till 45 min. As the bonding time increased, the improved mutual contact with the vanishing of interfacial voids and the growth of the diffusion depth by Ni atom aggregation were both responsible for the promotion of the bonding strength. However, The bonding strength decreased sharply when the bonding time reached at 60 min. At 60 min, the Ni atom aggregation disappeared which destroyed the Cu-Nb solution strengthening effect, and, as a result, the bonding strength became less.
Overall, the article was easily understandable. The effects of multi-interlayers, bonding time and bonding temperature on the interfacial microstructure and tensile strength of the joints were well discussed. The strengthening effect due to the Cu-Nb solution was discussed clearly. REFERENCES

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