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Frictional heat induced morphological responses of austenitic alloys SUS304, A286, and Inconel 718 at the interface in rotary friction welding was focused in this study, addressing initiation, evolution of corona-bond and the formation of heat-pattern. Summative models that describe the location and width of corona-bond at initiation, the corona-bond evolution mode and the formation of heat-patterns were given. The results show that when the corona-bond initiates at 0.33 R ∼ R, it fills the interface to form a lens-shaped heat-pattern. Inside this morphology, recrystallized ultrafine grains are formed to provide a superior performance. When the corona-bond initiates at 0–0.33 R with a width >0.4 R, it spreads to periphery to form a straight-line-shaped heat-pattern. Inside this heat-pattern, deformed grains and sub-boundaries are formed. The tensile strength of straight-line heat-pattern is lower than that of lens-shaped heat-pattern. When the corona-bond initiates at 0–0.33 R with a width ≤0.4 R, it does not spread but concentrates itself at center to form a spindle-shaped heat-pattern consisted of a ‘spindle body’ at center and a ‘friction line’ at periphery. Spindle body corresponds to a region made up of equiaxed recrystallized ultrafine grains, whereas the friction line corresponds to recrystallized grains and substructured grains. The formation of the friction line makes neglectable effect on local the strength but it does lower the elongation, where the local elongation of the friction line decreases to 6%–9% compared to 18% of a spindle body.

Alclad layer is a challenge to ensure the mechanical properties of refill friction stir spot welding (RFSSW) alclad 7050/2524 joint. In this study, RFSSW with suspension rotating process (SRP) was designed to improve the mechanical properties by strengthening the alclad layer. The thermal cycles, microstructures and mechanical properties of the joints welded without and with SRP were compared. The results indicated that SRP increased the high temperature dwell time of the stir zone bottom, which promoted more alloy being stirred into and mixing with the alclad, beneficial for strengthening the alclad. When with SRP, the tensile shear strength of joint reached 11 kN, improved by about 30% compared with joint without SRP.

TC4 (Ti–6Al–4V) titanium alloy and Al5(HfNbTiZr)95 refractory high-entropy alloy were successfully diffusion bonded at a temperature range of 800–1000 °C for 60 min under 5 MPa. The microstructure and mechanical properties of the diffusion bonded joints were investigated. The diffusion bonding seam of the joint contained two different zones, namely: a reaction zone with acicular titanium structures (Zone I); and a continuous solid solution reaction zone (Zone II) which inherited the orientation of the matrix Al5(HfNbTiZr)95. The highest microhardness of reaction zone reached 6.82 GPa, located in the boundary of TC4 and the acicular titanium structure zone. The effects of bonding temperature on the microstructure and mechanical properties of the joints were revealed. As bonding temperature increased, all the reaction zones grew in a parabolic manner with an increase in the bonding temperature. The shear strength of the joints ranged from 55.56 MPa to 777.06 MPa as the bonding temperature increased. The maximum shear strength of 770.06 MPa was obtained at a bonding temperature of 950 °C for 60 min under 5 MPa, and the fracture located in the TC4 substrate illustrated a typical ductile fracture mode.

A new 70Mn–25Ni–5Cr (wt. %) filler with no traditional melting-point depressants (MPDs) of Si/B was used for transient liquid phase (TLP) bonding a directionally solidified (DS) superalloy based on Ni3Al intermetallic compound (IMC) at 1150 °C, a pressure of 2 MPa for 10–120 min. Experimental results show that the joint can be divided into 3 zones, isothermally solidified zone (ISZ), athermally solidified zone (ASZ) and diffusion-affected zone (DAZ), which was similar to microstructure of typical TLP joint bonded by Ni based filler. The joint microstructures were mainly composed of γ-Ni, γ′-Ni3(Al, Ta), MC and M23C6 carbides. Besides, in the ISZ, due to the enrichment of Al, which diffused from the base metal (BM), strip-shaped Ni(Mn, Al) phases formed in such region. The ASZ included γ-Ni and compounds of “equiaxed Ni(Mn, Al) + acicular Ni3Ta” owing to the composition segregation during the later stage of solidification. Moreover, as the bonding time increased, the volume fraction of Ni(Mn, Al) and MC carbides decreased owing to homogenization of alloying elements. After bonding for 120 min, sufficient isothermal solidification (IS) of filler can also decrease segregation of Mn/Al in front of the S/L interface and the Ni(Mn, Al)+Ni3Ta compounds were eliminated almost. Thus, the highest tensile strength was 816.9 MPa of the TLP joint of 120 min, which has been equivalent to that of BM (812 MPa). However, due to the existence of a few residual large MC carbides in the joint, which can induce the stress concentration and decreased the ductility of the joints, the highest joint elongation of 4.4% was only about 40% of the BM.

Nickel-based single crystal superalloy DD6 was diffusion bonded in vacuum at 1170 °C under 55 MPa for 2–6 h. Two processes, direct bonding and bonding with interlayer, were designed for comparison. A pure 4 μm nickel (Ni) foil was selected for the interlayer. The joint strength was measured via shear test, whereas the joint microstructure and element distribution were analyzed via scanning electron microscope, electron probe analyzer and transmission electron microscope. The results indicated that the joint direct bonded for 6 h achieved the best joint quality at approximately 353 MPa of shear strength. In this case, the joint morphology and microstructure were characterized by microvoids and carbide precipitation. In contrast, in the joints bonded with interlayer, both carbide precipitation and microvoids disappeared. When 4 μm nickel foil was used as an interlayer, the foil/DD6 interface became less distinct as the bonding time prolonged. When the bonding time reached 6 h, the γ′ phase crystals connected each other with the misorientation distribution of the joint less than 5°, and maximum shear strength of 727 MPa was obtained. The fracture morphology has a mixed ductile-brittle feature.