Ultrasonic Characterization of Diffusion Bonds

The characterization of weak (low ultimate tensile stress) diffusion bonds is a generic problem which continues to challenge the QNDE community.1 Here we present preliminary results for an approach based on ultrasound reflection from, and transmission through, a planar diffusion bond. The samples consisted of two bonded stainless steel cylinders, each 12.7 mm in diameter and 63.5 mm in length. The bonds were made by means of thin Ag layers deposited on each of the matching ends of the cylinders. Here the ultrasonic transducers are placed at the ends of the cylinders. For this geometry, some of the ultrasonic energy impinges on the walls of the cylinders and undergoes mode conversion, leading to a complex train of pulses. The appropriate formulae were obtained to identify the various signals in the pulse train and to associate their amplitudes and arrival times with their ray path histories. The technique used is based on a pulse-echo method in which the reflection and transmission coefficients of the bond are combined in such a way that the transducer transfer functions, including coupling variations, are eliminated. Two types of measurements were made on each sample: ultrasonic measurements at 15 MHz and tensile tests to ultimate failure. The samples were bonded under controlled conditions in which bonding time, temperature, and pressure were varied. The ultrasonic measurements suggest that the effective reflection coefficient of the bond may be a candidate for a potential correlation with bond strength