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Investigations on ultrasonic welding of thermoplastic composites always focus on carbon fibre-reinforced thermoplastics so far. This study marks the first attempt at joining glass fibre-reinforced polyphenylene sulphide (GF/PPS) using ultrasonic welding. Attributed to the high observability of the glass fibre-reinforced thermoplastic, new discoveries on bonding formation and mechanism of void defect evolution were acquired. A special pattern of alternating high and low heat input regions was revealed, which was independent of welding energy variations and presented an inherent characteristic of ultrasonic welded GF/PPS joints. Squeeze-out and thermal degradation of matrix induced two primary defect types: larger voids with a size of approximately 100 μm were formed in the matrix between fibre bundles at the joint edge at lower welding energies and were diminished at excessive welding energy; smaller microvoids within 10 μm were formed within the fibre bundles at moderate welding energies and were increased in prevalence with higher energy levels. The research identified four distinct fracture modes that are strongly influenced by the welding energy and closely linked to the lap shear strength (LSS) of the joint. The GF/PPS joint achieved a maximum LSS of 22.6 MPa, which is the state-of-the-art welding joints, typically resulting from fracturing in the welded adherend. These findings provide valuable insights for refining the ultrasonic welding process of thermoplastic composites. [Display omitted] [ABSTRACT FROM AUTHOR]

Resistance welding, a favored method for joining thermoplastic composites, often faces the challenge of nonuniform bonding which leads to weak bonding at the overlap edge and compromises joint integrity. This study proposes a novel modification to traditional resistance welding by incorporating ultrasonic during the final stage of the process. We employed various durations of resistance welding and ultrasonic to join the GF/PPS composites, which yielded significant improvements in bonding strength and welding efficiency. Applying ultrasonic introduced a self-adaptive temperature compensation mechanism, enhancing the temperature at the overlap edge while diminishing the temperature difference between the center and edge of the overlap. This method also improved fusion bonding by encouraging resin flow along the unfused interface at the overlap edge, thereby expanding the bonding area. Notably, the introduction of ultrasonic revealed a new bonding mechanism involving the interaction between molten PPS and unmelted PPS, which was thoroughly examined. The results revealed a 19% increase in lap shear strength and a 40% reduction in welding time for joints with ultrasonic compared to those without ultrasonic. [ABSTRACT FROM AUTHOR]

The metallization of the SiC ceramic was realized with Sn0.5Zn at a relatively low temperature of 250 °C with the assistance of ultrasonication. This process avoided the use of high temperature, vacuum, or long holding time in traditional metallization methods. Results indicate that fine metallization without defects can be completed under ultrasonication excitation for 20 s. The bonding mechanism was forming a nano-scale layer of non-uniform but continuous amorphous zinc suboxide (ZnO 1-x) (even when the Zn was only 0.5 (wt%)), which acted as a transition region from the SiC ceramic substrate to the Sn0.5Zn metal layer. The tensile strength between the Sn0.5Zn layer and the SiC was 12.3 MPa. The acoustic pressure in the liquid Sn0.5Zn pool was calculated via COMSOL Multiphysics software. The simulation results show that alternating positive and negative acoustic pressure existed in the liquid Sn0.5Zn metal, with the highest amplitude of more than 107 Pa, thereby generating the cavitation effect. The collapse of cavitation bubbles resulted in micro-jets, hot spots, and a high cooling rate, promoting the movement of Zn atoms toward the SiC surface and the formation of ZnO 1-x. [ABSTRACT FROM AUTHOR]

Joint seam is always the weak region of the welded joint. The soldered/brazed ceramic joint always fails through the joint seam due to lower strength of the solder/braze. In this work, to fabricate ceramic joint stronger than its substrate, ultrasonic vibration was used when brazing porous Si 3 N 4 ceramics. Results show that the brazing can be directly realized at 450 °C in air because ultrasonic cavitation can break the oxide layers of braze. Under ultrasonication, high acoustic pressure is obtained in braze, thus it rapidly infiltrates into the porous ceramics. Then, tight bond is realized between the ceramic and braze within several seconds because of the cavitation phenomenon. The transmission electron microscope results show that the main bonding medium at the ceramic/braze interface is an amorphous Al 2 O 3 layer. Shear test reveals that long ultrasonic time is beneficial to the shear strength of the joint. Wetting times longer than 10 s produce joint stronger than the ceramic substrate. This work provides a new insight when joining other porous materials. • Porous Si 3 N 4 ceramics joints stronger than the substrate was fabricated by ultrasonic brazing. • Short ultrasonic time of 10 s and low temperature of 450 °C produce joint stronger than the ceramic substrate. • The wetting interface is characterized by amorphous Al 2 O 3. [ABSTRACT FROM AUTHOR]

AlN ceramic was successfully wetted and then joined with nonactive Sn9Zn eutectic solder assisted by ultrasonication in air. The effect of ultrasonic time on the formation of joint was studied. Results indicated that the defect-free joint can be obtained at an ultrasonic time of 5 s. Two regions, namely, AlN/Sn (s,_s) and AlN/Zn (s,_s), were found in the bonding interface. Zn and O accumulated in the AlN/Sn (s,_s) interface. An amorphous and nanocrystalline layer of ZnO formed in the hard-wet AlN surface. And Zn (s,_s) directly bonded with AlN. The low temperature and fast bonding of the AlN was attributed to the high pressure and temperature caused by cavitation effect. The shear strength of the joint increased from 10.6 MPa to 30.7 MPa when the ultrasonic treatment time increased from 5 s to 150 s. With the prolongation of ultrasonic time, more AlN ceramic particles entered the solder and acted as the reinforcing phase. • AlN ceramic was rapidly wetted with a nonactive solder (Sn9Zn) in 5 s assisted by ultrasonication at 230 °C. • Wetting was realized by an amorphous and nanocrystalline ZnO at AlN/Sn interface, and Zn directly bonded with AlN. • The shear strength of the joint was 30.7 MPa with a short wetting time of 150 s. • The triggering mechanism of acoustic cavitation on wetting was analyzed through finite element simulation and calculation. [ABSTRACT FROM AUTHOR]

• Cavitation characteristics of GaIn melt during the ultrasonic-induced spreading process are studied in situ by high-speed photography. • The cavitation cloud, single cavitation bubble, stable cavitation bubble, their sizes and life periods are identified. • The bubble density decreases with the spreading of the droplet, which is explained by acoustic pressure. • A non-cavitation region showing increasing width as the droplet spreads consistently appears at the spreading front due to the acoustic pressure loss. • A cavitation threshold measuring method is proposed, and the cavitation threshold of eutectic GaIn alloy is identified to be approximately 0.65 MPa. Cavitation is widely used in many fields, but the nature of cavitation, especially in metallic liquids, remains unclear. In this work, the cavitation bubble characteristics during the spreading of eutectic GaIn liquid droplet subjected to ultrasonication were studied in-situ by high-speed photography. The types, diameters, variations, and life cycles of cavitation bubbles were identified. Results show that the bubble density gradually decreases during the droplet spreading process because of the decreased acoustic pressure inside the thinned liquid. The cavitation presents strong/weak change characteristics with a period of dozens of acoustic periods (T, 50 µs). Cavitation clouds with large sizes and long lifetimes are observed at the early-spreading stage. Tiny cavitation bubbles emerging and collapsing within 1 T dominate the cavitation field in the late-spreading stage. Stable cavitation bubble showing continuous nucleation and collapse but minimal changes in its position is also observed. A non-cavitation region consistently appears at the spreading front because the acoustic pressure is below the cavitation threshold. The width of the non-cavitation region gradually increased as the size of the spreading area increased. A cavitation threshold measuring method is proposed based on the cavitation observation and simulation and the cavitation threshold of eutectic GaIn alloy is identified to be approximately 0.65 MPa. The cavitation bubble dynamics result shows that the cavitation bubbles with different nucleation diameters have different life periods the same growth velocity. [Display omitted] [ABSTRACT FROM AUTHOR]