Ultrasonic-Assisted Zn-Al Brazing of SiC Joints
Ultrasonic-Assisted Zn-Al Brazing of SiC Joints – Sonic4lab
Silicon carbide ceramics, with their superior mechanical properties, excellent thermal conductivity, and low coefficient of thermal expansion, are widely used in aerospace, automotive, energy, and electronics fields, and are particularly attractive for high-performance microelectronic packaging. Reliable bonding between silicon carbide ceramics and themselves or metals is required, and brazing is a common connection method.
Traditional brazing of silicon carbide ceramics often uses Ag-Cu-Ti and Co-based brazing filler metals, with brazing temperatures reaching 800~1200℃, which can easily damage electronic components and is difficult to use in electronic engineering. Tin-based lead-free brazing filler metals have melting points below 250℃, which cannot meet the requirements of high-temperature power electronic devices. Zn-Al alloys have moderate melting points (above 350℃) and excellent corrosion resistance, thermal conductivity, electrical conductivity, and mechanical properties, making them ideal materials for high-temperature connections. Eutectic Zn-5Al brazing filler metal, with a melting point of 381℃, is suitable for silicon carbide ceramic connections, but a large amount of brittle Zn-Al eutectic will reduce the mechanical properties of the joint.
Therefore, this study investigated the ultrasonic-assisted air brazing of silicon carbide ceramics using Zn-5Al and Zn-5Al-3Cu alloys as fillers. Two alloys were prepared in a glove box using high-purity Al, Zn, and Cu, with preheated Al-5Ti-1B grain-refining intermediate alloy rods also added. During brazing, silicon carbide samples were stacked and heated to 420℃ at a rate of 50℃/min, followed by ultrasonic vibration. The ultrasonic amplitude was 3.5 μm, and the frequency, pressure, and power were fixed at 20 kHz, 0.4 MPa, and 300 W, respectively. The ultrasonic treatment time was 5–20 seconds, followed by a 3-minute holding period and then air cooling.
The results showed that the joints of both brazing alloys subjected to ultrasonic treatment for 5 seconds exhibited no obvious defects or reaction products. The Zn-5Al brazing filler joint layer contains the η-Zn phase and a large-area Zn-Al eutectic dendrite structure. The Zn-5Al-3Cu brazing filler joint layer, in addition to the η-Zn phase and layered Zn-Al eutectic phase, also contains a non-layered eutectic structure containing the α-CuZn₄ phase. Furthermore, the volume fraction of the primary η-Zn phase increases, while the eutectic structure decreases. This is because Cu promotes the heterogeneous nucleation of the η-Zn phase and inhibits eutectic formation, which is beneficial for improving mechanical properties.
When the ultrasonic treatment time of the Zn-5Al brazing filler is extended to 20 seconds, there is no welding gap in the joint, the layered Zn-Al eutectic structure disappears, and more fine non-layered eutectic phases appear. The α-Al phase is also smaller and more uniformly distributed. In addition, the shear strength of the Zn-5Al brazing filler joint reaches approximately 138 MPa, approaching the level of the Zn-5Al-3Cu brazing filler. This research provides an effective solution for reliable low-temperature bonding of silicon carbide ceramics.
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