Volume 28, Issue 2 (March 2000)
Ultrasonic Monitoring of a Low-Temperature Diffusion Bonding Process
In the present study, a method for ultrasonic monitoring of the quality of the diffusion bond between two metallic materials has been developed. By using the method, one can monitor the evolving bonding process and terminate the procedure when the bond has been established.
This method determines the bonding parameters by analyzing the amplitude ratio and attenuation of acoustic waves. The method is based on ultrasonic sound waves transmitted through a pair of cylindrical samples attached longitudinally one on top of the other. The samples are held in a furnace and pressed one against the other. Data analysis is made by a computer while the bonding procedure takes place.
Diffusion bonding of some materials can be enhanced by using an interlayer applied as a thin film coated on each of the bonded sample's faces. In the present study, aluminum and copper samples were bonded using silver as a bonding interlayer. The silver interlayer was either electroplated on the copper samples or sputtered on the aluminum samples.
Several experiments to bond pairs of cylindrical samples using a silver interlayer were carried out based on the conditions (time, temperature, and pressure) obtained during the ultrasonic monitoring experiments. The bonded specimens were also characterized using destructive testing methods (metallography and tension tests).
The main conclusions of the present study are as follows:
• The suggested ultrasonic monitoring method was found to be a powerful tool in monitoring diffusion bonding. It has been proven that both the beginning and the end stages of the bonding process could be detected.
• Ultrasonic M-mode mapping was shown to be an effective means of bond-quality testing. Misalignment could be detected using this method. After fixing the misalignment, better bonds were achieved.
• Bonding could be improved by using a silver interlayer. The silver interlayer can either be electroplated or sputtered on the samples.
• Aluminum samples, coated with a sputtered silver interlayer, were bonded at temperatures as low as 150°C. Copper samples, electroplated with a silver interlayer, were bonded at temperatures of 260 to 300°C.
• The coating process of the bonded specimens is of great importance. Tension test results of specimens that were not properly cleaned before bonding were significantly inferior.
• The best tension test results carried out on bonded specimens yielded a strength of about 120 MPa, which is the tensile strength of pure silver metal. Further research is needed to optimize bonding parameters for better mechanical properties.