A number of corporations use radiography as an important part of their accept/reject criteria for in-house brazing operations, believing that X-ray films can always show any imperfections, voids, etc. in any of the components they braze.

A number of corporations use radiography as an important part of their accept/reject criteria for in-house brazing operations, believing that X-ray films can always show any imperfections, voids, etc. in any of the components they braze. Unfortunately, X-ray radiography has some severe limitations to its capabilities that need to be understood if companies want to be able to get meaningful results from their use of this inspection technique.

Radiographic inspection can be justified as a brazing inspection method only if each of the following items can be clearly demonstrated:
  • The radiographs can be easily read and accurately interpreted.
  • Radiographs can be taken of 100% of critical surfaces in the brazed joints.
  • The thickness of the brazed joint being radiographed is greater than 2% of the total metal thickness through which the X-rays must travel.
  • The use of radiography does not interfere with the optimization of other braze-process variables, e.g., filler-metal selection, part cost, etc.
Large-diameter, thin-walled cylindrical combustion-chamber assemblies and honeycomb panels are two examples of brazed components that routinely can use radiography as a viable inspection technique. One company I was visiting was using real-time X-ray radiography to find voids in braze joints in large-diameter, thin-walled cylinders in which overlapping nickel-alloy cylinders were being brazed together with a nickel-based brazing filler metal. The total wall thickness of the assembly when brazed was only about 0.083 inches (2 mm), and the thickness of the braze-gap itself was about 0.003" (0.08 mm). The X-ray-source probe was placed inside the cylinder and moved along close to the ID wall of the cylinder while the receiver followed it along the OD of the cylinder wall. Thus, the X-rays were only shooting through the 0.083 inches (2 mm) wall thickness. Because the 0.003 inches (0.08 mm) braze-gap clearance represented almost 4% of the total brazed-wall thickness – well in excess of the minimum 2% needed for x-ray interpretation to be meaningful – the radiographs were able to clearly show any voids in the braze joint. Additionally, because of the configuration of the overlapping cylinders and the ease of being able to access 100% of the brazed surfaces by this method, X-ray radiography proved to be an excellent, preferred method for internal inspection of these brazed joints. Notice that in this example, the assemblies being radiographed met all four of the requirements above.

All too often I encounter companies trying to use X-ray radiography for inspection of their brazed assemblies when they meet few, if any, of the requirements above. One company routinely uses X-ray inspection on brazed components that range in thickness from about 1-2 inches (25-50 mm) in which they must shoot the X-rays through that entire thickness. There is absolutely no way that they come anywhere close to meeting the 2% rule, but they continue to "insist" on using X-ray inspection. In fact, they (and other similar companies) will actually change the type of brazing filler metal (BFM) being used for brazing from an optimal choice (such as a nickel-based BFM or a copper-based BFM) to a very expensive gold-based BFM because they erroneously believe that the higher density of the gold-based BFMs will overcome the 2% rule sufficiently to allow meaningful interpretation of the very cloudy radiographs they get from their inspection. I've seen many of these radiographs, and I still shake my head in wonder and disbelief at their claims for these now very expensive brazed assemblies.

By the way, even when performed correctly, users of X-ray inspection techniques need to understand that even if the BFM has flowed into the joint, X-rays cannot indicate if the BFM has actually alloyed with (bonded to) the base metals. That is why X-ray inspection should never be used for inspecting braze joints in which the BFM has been pre-placed (X-ray inspection results would be meaningless in such a case).

When each of the four requirements for X-ray use (as outlined above) cannot be used, seek other inspection techniques such as ultrasonic testing. Performance testing should be required according to what the brazed component will see in actual service. If it will see fluid flow, then pressure testing with air, gas or water should be done as part of a company's QC plan. Helium leak testing is widely used for similar needs. Visual inspection should always be a requirement for all brazed assemblies prior to shipment to your customer.