A: Review the fundamentals when looking at your designs. You need good fit and proper clearance. What does this mean? Most braze joints can be made with a depth of 5 mm (0.200 in.). One habit designers tend to have is to make the joints too deep, and then management says you must fill up the entire joint volume. Figure 1 is a good example of a joint that is too deep. In looking at this joint, a couple of things come to mind. Being so deep, it is more difficult to heat. You must bring the entire joint up to braze temperature and then focus the heat such that the alloy is drawn into the joint. Being copper, the heat wants to be drawn away from the joint. Heating it so the alloy is drawn into the joint may make it colder at the point you add the alloy. The way to overcome this is to keep adding braze filler metal. Proper Joint Sizing This joint also appears to be quite a tight fit, further adding to the difficulty of getting braze filler metal to flow into the joint. The alignment seems to be poor also. It makes one side of the joint wider than the other. Braze filler metal has trouble penetrating and the operator keeps adding alloy. We recommend a fit no greater than 0.10 mm (0.004 in.), so even in a well-designed joint, there will be limited room in the joint to fill. Adding a bit of a flare cupping to the end of the female tube will help direct braze filler metal into the joint. A benefit to making the joint easier to heat, i.e., tighter fit with smaller overlap, is the heating time will decrease. An associated plus is there will be fuel gas and oxygen savings. Temperature and Torch Tip There are a few things to keep in mind, however, when the subject is proper heating. Torch tip selection is important. Some joints need a broad flame and some are more targeted. Also, using a double tip may make the heating more uniform and faster. There are a wide variety of tips on the market to select from. Additionally, you want to make sure your operators are using a reducing flame. The tendency of most operators, especially new ones, is to crank up the oxygen. The thinking is the hotter the better so they can get the joint done faster. This increases the oxidation on the assembly, so when you add the braze filler metal, it acts to inhibit flow. Again, most operators try to overcome this with adding more rod — Fig. 2. The problem changes when you have dissimilar metals. In Figure 2, the joint was not brought up to proper brazing temperature. It is referred to as a cold joint. The steel wants to retain heat and the copper wants to draw it away from the joint. In this case, when the braze filler metal was added, the point where the rod was touched was underheated. Touching it with the braze rod cools it further, as the heat in the base metals is used to melt the rod. Adding more filler metal makes matters worse. Figure 3 shows what can happen when joint accessibility is an issue. The close proximity of the tubes to each other makes them difficult to heat. It also makes it hard to add the filler metal. If not heated properly and BRAZING Q&A 26 WELDING JOURNAL / JUNE 2016 BY TIM P. HIRTHE Q: We are a manufacturer of airconditioning and heating products. We have several issues with our brazing. The biggest and most obvious problem we seem to have is using too much braze alloy. You can see this in the enclosed photos of some of our joints. At this time, we're not sure if it is due to the design of our joints, the skill of our brazers, or both. Perhaps it is something else we are not considering. Using the wrong braze alloy, perhaps? In our internal discussions we have a lot of opinions, and the consensus is that it is several things. Most of our joints are coppertocopper tube assemblies, but there are some where we join copper to steel. We don't have an outofcontrol leak problem in the field, but we feel that achieving a sealed system comes at the cost of using far too much braze material and rework. Hand in hand with using too much alloy is the fact the joint aesthetics are poor. Our customers are not thrilled with the appearance of our joints. While the product doesn't leak, a lot of times it sure looks like it will. We use a 2% silver braze rod. The heating is manual torch brazing using oxyacetylene. On some joints, we use single tip torches, and on others, we use double tip. It depends on the size and accessibility of the joint. Operator preference is also a factor. We have an internal training program that is hard to implement because we have a lot of turnover in our operators. We're constantly faced with the issue of bringing new people up to speed. An additional challenge is out on the shop floor. After our operators have been trained, they develop bad habits and we don't have a lot of resources to give them oversight and remedial training. What steps can we take to reduce the excessive use of braze alloy and improve the appearance of our joints? I need a place to start. Fig. 1 — Sectioned coppertocopper tube joint. Fig. 2 — Coppertosteel connection.
Welding Journal | June 2016
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