Ultrasonic NDT
Braze Joint Testing
Published on 26th March 2020
Background
Brazing is a common process for joining sections of small-diameter metal tubing and certain other small metal parts, such as electrical contacts. It bonds two pieces of metal by means of a metallic filler that is melted and then flowed into the space between them by capillary attraction. The process is similar to soldering. The filler metal, which is commonly an alloy of copper, silver, tin, and/or zinc with a relatively low melting point, must fill the space between the two parts being joined, wet the surfaces, and fuse them when it cools and solidifies. If the braze metal does not flow properly or does not fuse, the integrity of the joint is affected. This problem can usually be identified with an ultrasonic flaw detector.
Equipment
Most braze joints can be tested with an Olympus EPOCH® 6LT,
EPOCH 650, or EPOCH 1000 ultrasonic flaw detector. Given the typical geometry
of braze joints, small-diameter contact or delay line transducers are usually
used. The transducer should be selected with respect to the geometry of the
specific joint being tested, but for typical small-diameter tubing, the V208-RM
(20 MHz) and V260 Sonopen® (15 MHz) transducers are most commonly recommended.
Spring-loaded holders are available with these transducers as an alignment aid.
Procedure
Braze joints are typically tested using a straight beam test
that monitors the amplitude of the echo from the braze location at the inner
surface of the outer metal part. In the case of a good braze joint at the test
point, the metal-to-metal bond will return a relatively small reflection, while
an air gap at the test point will return a much larger reflection. These levels
should be established in each specific case by setting up on reference samples
representing known brazed and un-brazed conditions.
The example below represents typical echoes from a braze
joint between 9.5 mm (0.4 in.) and 12.5 mm (0.5 in.) copper tubing that was
tested with a V208-RM delay line
transducer and a flaw detector. The first waveform shows a reference
echo from an un-brazed area of the outer tube, which has been marked with a
gate and set to nearly full-screen height. The second waveform shows the
response from a properly brazed joint, in which the echo in the gated region
drops to a very low level. It is followed by a larger peak representing the
echo from the inside surface of the inner tube. By observing the amplitude of
the echo within the gate, the operator can determine the joint condition at
each test point by comparing the echo amplitude to these reference levels.
A partial braze within the area of the sound beam, due to
conditions such as porosity, will typically be represented by a mid-level echo.
Note: it is important to remember that very small discontinuities, such as
pinholes that are a small fraction of the beam diameter, will not be detected
through this procedure.
Typical echo response from a brazed joint, with a minimal
echo in the gate.
Typical echo response from an unbrazed joint, with a large
echo in the gate.
Products used for this application
EPOCH 6LT
NEW
The EPOCH 6LT portable ultrasonic flaw detector is optimized
for one-handed operation and delivers excellent performance in rope access and
high-portability applications. Lightweight with an ergonomic design, the
instrument fits securely in a user’s hand or can be strapped to the leg for
rope access applications.
EPOCH 650
The EPOCH 650 is a conventional ultrasonic flaw detector
with excellent inspection performance and usability for a wide variety of applications.
This intuitive, rugged instrument is a continuation of the popular EPOCH 600
flaw detector with additional capabilities.
EPOCH 1000 Series
The EPOCH 1000 is an advanced conventional ultrasonic flaw
detector that can be upgraded with phased array imaging at an authorized
Olympus service center. Key features include: EN12668-1 compliant, 37 digital
receiver filter selections, and 6 kHz pulse repetition rate for high speed
scanning.