Don’t Blame the Braze because Joint was Poorly Designed, and NO, larger fillets won’t compensate for that!
Have you ever heard someone tell you something like this: “Well, brazing may be okay, but if you really want a strong joint, you should weld it!” Such comments are often made when someone sees what appears to be a cracked brazed-joint, such as that shown in Figure 1, and they then assume that (1) the crack they are looking at probably extends all the way through the brazed-joint, and that (2) if the joint had been welded it would not have cracked.
In the actual, real-life case shown in the close-up drawing in Figure 1, the comment about brazing vs. welding was actually made by an engineer who saw the part. But he was very wrong! The actual cause of the joint-failure was a poor joint design that placed very high stress concentration right at the edge of the brazed joint. In actual fact, the brazed joint itself held up very well, and the crack proceeded from the joint-edge down through the base metal. Engineers were quite surprised!
So let me start with this:
WARNING: Be very careful about judging the overall quality of a brazed joint by only looking at its fillet!
In service, there was high stress placed on the outer edge of the brazed joint shown in Fig. 1, when vibration and flexing/bending (some of it hard and sudden) occurred on a regular basis. The crack started where the stresses were most heavily concentrated — at the outer edge of the brazed joint, where the two metals being joined met at a sharp right angle. Under continued stress the crack continued into and through the base metal until an open leak-path was formed in the part, and it had to be replaced.
NOTE: Because of proprietary nature of component, only drawings are used of key portions of brazed assembly, in which the lower part of the assembly shown is approximately 0.125” (3 mm) thick.
During component-evaluation, people began to assume that the braze was bad as soon as they saw the crack at the edge of the joint, thinking that the crack extended all the way through the joint, thus causing the leak path to occur. When the component was cut open to find the actual leak path, they were surprised to find that the actual crack went through the base metal, and not through the joint, as shown in the cross-section diagram in Figure 2. It’s interesting to note that the entire brazed joint was sound!
ROOT CAUSE OF PROBLEM? POOR JOINT-EDGE DESIGN FOR SERVICE CONDITIONS TO BE ENCOUNTERED!
Design engineers need to learn how to design parts for brazing, and they must NOT assume that the same principles apply for both “weld-design” and “braze-design”. The two joining processes are very different, and require different design guidelines! If the assembly shown in Fig. 2 were welded instead of brazed, what might a cross section of that joint have looked like after welding? See the illustration in Fig. 3.
Note that the weld illustrated in Fig. 3 is a partial-penetration weld, and would only cover a small portion of the joint area at the left side of the joint overlap, leaving the rest of the joint unbonded. Also note that the shape of the external portion of the weld-bead is such that it can help to spread stresses that might otherwise be concentrated at the sharp corner if the weld-bead were not there. However, the value of that weld is limited to just the area of the joint-overlap where the physical weld is located. The rest of the overlap has no support, unless a second weld were added at the other end of the joint-overlap. Additionally, due to the high weld-heat used to melt the base-metal in the weld area, distortion of the metal in the rest of the joint overlap is quite common.
By comparison, the braze joint shown in Fig. 2 covers the entire surface in the overlap area between the two components, and, because of the even heating of the part in the vacuum brazing furnace used to join the assembly, no distortion of the lower sheet portion of the joint occurred.
BUT — due to the small size of the brazing fillet (often also referred to as the braze-meniscus because of its small size), there is no large fillet to help spread the stresses that will concentrate at that sharp corner. This is where the designers need to become educated re braze-design! Because there is no large fillet to help spread the stress, the base metal itself needs to be shaped so as to spread the stress, rather than depending on a large fillet to do so. Thus, a proper design for the joint would look like the one illustrated in Fig. 4, where the sharp corner is removed.
Many years of experience with similar designs as this have clearly shown that such joint-edge contouring (by machining, or edge-rolling at 45-degree angle, etc.) virtually eliminates joint-edge cracking of brazed-joints, even under severe conditions of vibration, bending, twisting, etc.
WARNING#2: Do NOT think that you could merely build up large braze-fillets in order to help spread the stresses at the sharp corner. Braze fillets are castings, and the larger the casting, the greater the risk of joint-edge cracks! Period!
CONCLUSION: Design engineers — please learn the distinct difference between designing joints for welding and for brazing. For welding, you may believe in the ability of the weld to help spread stresses at a sharp corner, but the entire joint depends on the overall goodness of the limited coverage of that weld bead. For brazing, you have the complete integrity of a well-brazed joint whose joint-length is about 4-to-5 times the thickness of the thinner of the two components being joined. But, because a braze-fillet (meniscus) should be very small, you MUST spread the joint-edge stress by designing that “stress-spreading” capability into the base metal itself, and NOT depend on large brazing filler metal (BFM) “castings” to spread that stress.
Dan Kay – Tel: 860-651-5595: – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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