Jiuchun Yan

Interaction behaviors at the interface between liquid Al–Si and solid Ti–6Al–4V in ultrasonic-assisted brazing in air

Power ultrasonic vibration (20 kHz, 6 μm) was applied to assist the interaction between a liquid Al-Si alloy and solid Ti-6Al-4V substrate in air. The interaction behaviors, including breakage of the oxide film on the Ti-6Al-4V surface, chemical dissolution of solid Ti-6Al-4V, and interfacial chemical reactions, were investigated. Experimental results showed that numerous 2-20 μm diameter-sized pits formed on the Ti-6Al-4V surface. Propagation of ultrasonic waves in the liquid Al-Si alloy resulted in ultrasonic cavitation. When this cavitation occurred at or near the liquid/solid interface, many complex effects were generated at the small zones during the bubble implosion, including micro-jets, hot spots, and acoustic streaming. The breakage behavior of oxide films on the solid Ti-6Al-4V substrate, excessive chemical dissolution of solid Ti-6Al-4V into liquid Al-Si, abnormal interfacial chemical reactions at the interface, and phase transformation between the intermetallic compounds could be wholly ascribed to these ultrasonic effects. An effective bond between Al-Si and Ti-6Al-4V can be produced by ultrasonic-assisted brazing in air.

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi3, TiNi, Ti2Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s–1.

Effects of Fe−Al intermetallic compounds on interfacial bonding of clad materials

Abstract The growth of intermetallic compounds at the interface between solid Al and Fe and the effects of intermetallic compound layers on the interfacial bonding of clad materials were investigated. The results showed that the interface between the solid Fe and Al formed by heat-treatment consisted of Fe 2 Al 5 and FeAl 3 intermetallic compound layers, which deteriorated the interfacial bonding strength. Fractures occurred in the intermetallic compound layer during the shear testing. The location of the fracture depended on the defects of microcracks or voids in the intermetallic compound layers. The microcracks in the intermetallic compound layer were caused by the mismatch of thermal expansion coefficients of materials during cooling, and the voids were consistent with the Kirkendall effect. The work will lay an important foundation for welding and joining of aluminum and steel, especially for fabrication of Al-Fe clad materials.

Vibration assisted brazing of SiCp/A356 composites: microstructure and mechanical behaviour

Abstract Vibration assisted brazing of SiCp/A356 composites in air was investigated. A vibration was applied on one of the samples to be bonded at 375°C during brazing. The filler metal was extruded reciprocally and impacted intensively and reciprocally on the two surfaces of the base materials during vibrating. It can be found that most of the oxide film on the surface of the composites was disrupted and removed through the observation by SEM. The metallurgical bonds formed between the filler metal and the base materials. However, a lot of porosities were entrapped in the bond metal. A vibration was once more applied after the samples were continuously heated up to 520°C, and any porosity could not be found. The bonds mainly composed of a new alloy, which have a higher content of aluminium and free of porosities, show a higher shear strength of 178 MPa comparing to that of the base materials.

Building a nano-crystalline α-alumina layer at a liquid metal/sapphire interface by ultrasound

Transitional layers at the metal/ceramic interface play an very important role in ceramic joining. In this study, sapphire blocks were ultrasonically dipped in liquid Sn-Zn-Al alloy. It is found that the ultrasound promoted rapid oxidation reaction of aluminum at the Sn-Zn-Al/sapphire interface at 230°C in the ambient atmosphere, resulting in the formation of a nano-crystalline α-Al2O3 layer (NCAL). In a ∼2nm boundary layer of the NCAL, the lattice matches the sapphire substrate well. Thus, a smooth transition of the lattice from sapphire to metal was formed through the NCAL. Ultrasonically soldered sapphire joints were made with Sn-Zn-Al as the filler alloy. Compressive shear strength of the joints reached 43-48MPa, which is relatively high comparing to other Al2O3 joints made of Sn alloys doped with Ti or Rear Earth elements. Thus, a new mechanism of ultrasonic soldering, i.e. building an oxide transitional layer on the surface of the solid, was revealed. We expect this sonochemical process to be applicable to other metal/oxide systems.

Rapid ultrasound-induced transient-liquid-phase bonding of Al-50Si alloys with Zn interlayer in air for electrical packaging application

Al-50Si alloys were joined by rapid ultrasound-induced transient-liquid-phase bonding method using Zn foil as interlayer at 390°C in air, below the melt point of interlayer. The fracture of oxide films along the edge of Si particles led to contact and inter-diffusion between aluminum substrate and Zn interlayer, and liquefied Zn-Al alloys were developed. The width of Zn-Al alloys gradually decreased with increasing the ultrasonic vibration time due to liquid squeezing out and accelerated diffusion. A stage of isothermal solidification existed, and the completion time was significantly shortened. In the liquid metal, the acoustic streaming and ultrasonic cavitations were induced. As the process developed, much more Si particles, which were particulate-reinforced phases of Al-50Si, gradually migrated to the center of soldering seam. The highest average shear strength of joints reached to 94.2MPa, and the fracture mainly occurred at the base metal.

Interfacial structure and mechanical properties of hot-roll bonded joints between titanium alloy and stainless steel using niobium interlayer

Abstract The hot-roll bonding was carried out in vacuum between titanium alloy and stainless steel using niobium interlayer. The interfacial structure and mechanical properties were analyzed. The results show that the plasticity of bonded joint is improved significantly. When the bonding temperature is 800 °C or 900 °C, there is not intermetallic layer at the interface between stainless steel and niobium. When the bonding temperature is 1000 °C or 1050 °C, Fe-Nb intermetallic layer forms at the interface. When the bonding temperature is 1050 °C, cracking occurs between stainless steel and intermetallic layer. The maximum strength of ∼417.5 MPa is obtained at the bonding temperature of 900 °C, the reduction of 25% and the rolling speed of 38 mm/s, and the tensile specimen fractures in the niobium interlayer with plastic fracture characteristics. When the hot-roll bonded transition joints were TIG welded with titanium alloy and stainless steel respectively, the tensile strength of the transition joints after TIG welding is ∼410.3 MPa, and the specimen fractures in the niobium interlayer.

Capillary filling process during ultrasonically brazing of aluminium matrix composites

Abstract Alumina borate whisker reinforced aluminium metal matrix composites were ultrasonically brazed in air. The liquid filler metal (FM) of a Zn–Al eutectic alloy could get into the clearance between composite couples by the action of ultrasonic capillarity. The velocity of the FM getting into the clearance decreases with increasing clearance width. The time exhausted to remove completely the oxide film on the surface increases with increasing clearance width. The oxide film could be removed completely and the metallurgical bonding formed under the action of ultrasonic vibration for sufficient time. The shear strength of the bond could reach up to 145 MPa (the nominal strength of the Zn–Al filler alloy).

Effect of PVP on the low temperature bonding process using polyol prepared Ag nanoparticle paste for electronic packaging application

There has been an increasing interest in developing low temperature interconnection process using metal nanoparticles. In this study the Ag nanoparticles (NPs) for this low temperature bonding process applications were prepared based on the polyol method using polyvinylpyrrolidone (PVP) as the protecting agent. The effect of PVP on the Ag nanoparticle size, nanoparticle solution viscosity and the bondability of the Ag nanoparticle paste were studied. The silver nanoparticle with diameter of 90 nm, 40 nm and 20 nm were synthesized by adjusting PVP concentration. The bonding processes using different kind of Ag nanoparticle paste were conducted. The joint with shear strength of 50 MPa were formed using Ag nanoparticle (NP) paste prepared with appropriate PVP concentration.

Ultrasonic-promoted rapid TLP bonding of fine-grained 7034 high strength aluminum alloys

High strength aluminum alloys are extremely sensitive to the thermal cycle of welding. An ultrasonic-promoted rapid TLP bonding with an interlayer of pure Zn was developed to join fine-grained 7034 aluminum alloys at the temperature of lower 400°C. The oxide film could be successfully removed with the ultrasonic vibration, and the Al-Zn eutectic liquid phase generated once Al and Zn contacted with each other. Longer ultrasonic time can promote the diffusion of Zn into the base metal, which would shorten the holding time to complete isothermal solidification. The joints with the full solid solution of α-Al can be realized with the ultrasonic action time of 60s and holding time of only 3min at 400°C, and the shear strength of joints could reach 223MPa. The joint formation mechanism and effects of ultrasounds were discussed in details.