As discussed in last month’s blog-article, a lap-joint with an overlap of “3T-to-6T” (where “T” is the thickness of the thinner of the two members being brazed) is all that is needed to provide full strength and hereticity in a properly designed brazed joint (1T-to-3T for aluminum alloys).
By this I am saying that we need to look at the amount of GOOD braze coverage, rather than being overly concerned with trying to count the number of voids in a joint! Counting voids is really the wrong way to approach the “goodness” of a brazed joint.
Please note, that if someone brazed a lap-joint that used a 6T-overlap, and it contained about 50% void content (assuming the voids to be randomly distributed throughout the joint), that would still leave a net-overlap (after deducting all the areas of voids) of about 3T. As we saw in last month’s article, 3T is all that is needed to insure full strength and hermeticity in a brazed joint for most metals (again, only 1T-to-3T is needed for aluminum alloys).
Some readers are probably saying: “Hold on, there! Suppose the 50% void-content is actually just one or two big, huge voids in the middle of the joint, or if a similarly large group of voids is positioned such that less than 10% of the joint width is brazed from one side of the joint to the other. Doesn’t that make it dangerous to assume that a 50% void-content can be okay in a brazed joint? Doesn’t the AWS Specification C3.6 for Furnace Brazing have void-content criteria based on the class of brazed joint? Doesn’t AWS Specification C3.3 “Recommended Practices for The Design, Manufacture, and Examination of Critical Brazed Components” have requirements in it re void content?”
The answer to your objections here is a qualified: “Yes, you MAY have a point there. But it also says in paragraph 7 of the same C3.3 specification, when talking about voids: “7.2.1 Lack of Fill (Voids, Porosity). Lack of fill can be the result of improper cleaning, excessive clearances, insufficient filler metal, entrapped gas, improper flux or atmosphere, competing capillaries, or the movement of the faying parts caused by improper fixturing while the filler metal is in the liquid or partially liquid state. This imperfection reduces the strength of the joint by reducing the load-carrying area, and it may provide a path for leakage.”
Are you, the reader, then suggesting that your own brazing operations have one or more of these deficiencies inherent in them, such that you need to be concerned about huge large voids showing up in the joints you are brazing? I would certainly hope not. I would hope that you are taking proper care to be sure that proper clearance, cleanliness, BFM quantity, brazing temperatures, etc., are being adequately controlled such that any trapped voids (as I have seen in many, many brazed joints) are thus only very small, randomly distributed tiny bubbles within the joint, no single one of which is large enough to cause any concern. And, if there were enough of them trapped inside the joint to bring their total up to 50% of the joint area through which the BFM has flowed, my opinion is that such a joint could very well prove to be quite acceptable in service! The question to you is, how do you feel about the “control” being exhibited by the people brazing the assemblies that you are producing?
First of all, since no one ever wants to have 50% void content or more in a joint, there should never be any need to extend the overlap from 3T all the way up to 6T, since nothing is really gained by so doing. Please remember that this is not because overlaps of 6T or more don’t work, but because it is wasteful of brazing filler metal (BFM) to have to fill a much-longer-than-needed overlap. It also wastes a lot of base metal to have to make an overlap that long! 3T-to-4T is perfectly adequate for ALL braze joints (1T-to-3T for aluminum joints; AND, a general analysis of void content of brazed joint over the years indicates an interesting phenomenon: void content of the brazed joint increases with overlap distance! Not just the quantity of voids, but the percentage of voids relative to the entire area of the brazed joint!
Note: This assumes, of course, that the brazing filler metal (BFM) is fed from one end of the joint, and is being asked to flow all the way through from its front edge all the way through the 4T, 6T, or 10T length of joint. It does NOT apply to long joints into which a preform of BFM has already been placed, and it does not apply to aluminum brazing sheet (on which the BFM has already been clad). In these latter cases, since the BFM is already pre-placed throughout the entire joint, then this void-content issue, related to distance of BFM flow, does not apply.
It has been found that when overlaps exceed about 4T, the percentage of voids in the joints begins to rise rapidly since you are asking the BFM to flow farther and farther through longer and longer joints. Analysis of joints with long overlaps (up to 10T and greater) show a much higher percentage of void content than those made with 3T-to-4T overlap. Once again, not only did the actual number of voids increase as the overlap got longer, but the percentage of voids in the entire joint also increased!
Figure 1 shows a simplified cross section of an actual part that was brazed some time ago.
The customer in this case-study had heard that some industries had specifications requiring brazing shops to keep the void content in their brazed joints below about 20% maximum. Because the customer wasn’t too concerned about the void content of the joint, he suggested to the brazing shop personnel that he’d allow up to 40% voids in the joint (double the so-called “industry standard”). He thought he was being generous. However, the customer submitted a poorly designed joint to be brazed, even though the customer was not aware that it was a poor design. As shown in Fig. 1, the design called for an overlap of about ½” (13mm), even though the wall-thickness of the components was only 1/32” (0.032 inches/0.8mm). Thus, the amount of overlap was 16T, far more than needed for a good braze!
When the brazing shop test-brazed some of the initial parts and then “peel tested” some of them to verify the void-content, they found that the brazed joints had approximately 48% void content, with the voids randomly spread throughout the joint area. Unfortunately, therefore, the parts had to be rejected, since they did not meet the so-called “generous” void-content criteria given to the brazing shop by the customer!
When some of the brazed assemblies in that rejected lot of parts were later submitted to pressure and vibration testing that duplicated the end-use service conditions the part was supposed to see, the parts passed all expected service conditions with flying colors!
How could the parts behave so well, even though they failed the void-content requirement? It was because the parts had more GOOD coverage in the brazed joint than was needed for full-strength and leak-tightness (hermeticity) of the part! For the sake of our discussion, let’s just round upwards the 48% void content to an even 50%. In such a case, half the joint would then be filled with voids. That means, of course, that half of the overlap distance was filled with brazing filler metal (BFM). Thus, the joint actually had about 8T of good braze coverage, and, from last month’s article, the reader should recall that only 3T of braze-coverage is needed for full strength, leak-tightness (hermeticity) of the brazed joint in service!
Thus, the braze shop in this current case-study had to reject good parts that would have nicely worked in end-use service, all because the customer erroneously decided to specify a void-content criteria for his parts — simply because he thought that doing so was a standard procedure in the world of brazing.
When the shop in this case study used a proper amount of overlap on subsequent parts, namely 3T-to-4T (3/32”-to-1/8”, 2.5-3.0 mm), they found that the void content of the joints dropped below 10% in all cases.
I personally do NOT recommend using a void-content criterion as an accept/reject standard by customers for a brazing-shop’s brazing-production. In my experience and opinion, void-content criteria, if used, should only be an in-house quality control item in the brazing shop itself, as a measure of determining how much the brazing processes in the shop are in control. If you see that the void content of joints is increasing, for whatever reason, then that may be an indication that your shop personnel are getting a little careless in their manufacturing, cleaning, assembly, or handling processes, and that such processes need to be brought back into control.
Thus, using “the glass is half-full” vs. “the glass is half-empty” thinking, brazing shops and their customers should be looking at the amount of actual braze coverage in a brazed joint, and not concentrating instead on the amount of voids in the joint.
In next month’s blog article I’ll look at some different methods you can use to measure void content of a brazed joint.