Figure 1. Cap dimensions do not match tube top.

Brazers commonly encounter voids in brazed joints and often wonder where they come from and how to avoid them in future brazements.

Some common sources of voids in braze joints are:

  1. Surface contamination
  2. Base metal and brazing filler-metal (BFM) constituents
  3. Brazing methods/temperatures used
  4. Avoiding/suppressing voids in brazed joints
  5. Poor joint fitup

The first three items listed above can often result in gas bubbles being formed in brazed joints. Such gas-bubble voids will usually try to form in spherical shape as they move through a joint. The “rounded” edges of such bubble-voids can often be clearly seen in cross-section photomicrographs of brazed joints, especially under high magnification. The inside surfaces of a bubble-void will often appear “clean” or “shiny” as well.

Let’s briefly look a little more closely at these sources of voids in brazed joints with a look at surface contamination.

1. Surface Contamination

It is not uncommon to hear some brazers say, “Don’t worry about surface contamination – the furnace will take care of it,” or “Don’t worry about surface contamination – just put more flux on the part to take care of it.” Both statements are dangerous and can lead to weakened joints and, in many cases, to failed joints.

Oils, greases, lubricants, dirt, etc. left on the surface of assembled parts in a brazement often contain ingredients that will tend to volatilize and outgas at the elevated temperatures involved in brazing. These “gases” will attempt to expand and move out of the confining braze joint, and, as long as there is an “escape path” open to the outside atmosphere for these expanding gases, they will not tend to be a problem during brazing. Since contaminants on the outside surface of parts being brazed are readily open to the atmosphere, such contaminants may completely volatilize, causing no problems to the brazement.

When the inside surfaces (faying surfaces) of a brazement are contaminated and not cleaned off prior to assembly, however, it frequently becomes impossible for those contaminants to volatilize (turn to a gas). Even if much of it could volatilize, those gases usually find it difficult, if not impossible, to make way to the outside of the joint where they can finally be released to the atmosphere. They will become entrapped (as bubbles) inside the brazed joint when the joint finally solidifies.

Additionally, be aware that surface contaminants and oxides inside a joint that do not volatilize can ruin a braze! Since molten brazing filler metal (BFM) does not want to bond to (or flow over) oils, dirt, greases or oxides on the faying surfaces inside a joint, the BFM may not be able to penetrate through the braze joint at all when surfaces have not been adequately cleaned prior to joint assembly.

Thus, it is very important that all surfaces to be brazed must be cleaned PRIOR to assembly for braze. Surfaces must then be handled with gloved hands (so that they are not recontaminated by the brazer’s fingers and hands). These precautions will minimize any voids in the brazed joint.

Let’s briefly look a little more closely at the second source of these voids in brazed joints, base metal and brazing filler-metal constituents.

2. Base Metal and BFM Constituents

Many base metals and BFMs have constituents in them that can easily volatilize when heated to brazing temperatures. Zinc, cadmium and lead are three such metals that will, in fact, outgas readily during any kind of brazing process, and proper precautions should be observed when such metals are used in brazing.

Lead may be found in some steels or brasses to enhance the machinability of those base metals. If such metals are then used as part of a brazement, however, the lead will quickly outgas, forming bubbles in the brazed joint and perhaps leaving holes in the base metal (such holes might result in leak paths through the metal, hurting hermeticity of the brazed assembly). This can obviously become a problem where leak-tight hermetic-seals are required for brazements in service.

Zinc and cadmium are used in certain BFMs as temperature depressants to help lower brazing temperatures and also to enhance the flowability of these BFMs on certain base metals. Zinc and cadmium may also be found as platings on some metal parts that are to be brazed. Zinc and cadmium, like lead, will readily outgas upon heating to brazing temperatures, and this will be seen as bubbles in the braze joint or as fumes in the workzone of the brazers. Neither situation is desirable.

Please bear in mind that because such outgassing of these three metals will, in fact, occur during heating to braze temps, they should NEVER be used in any vacuum brazing environment as they will contaminate the vacuum furnace and its pumping system, perhaps to the point of rendering the furnace non-usable! I strongly recommend that metals and BFMs containing these three elements be limited to flame brazing and induction brazing and that they not be used in any kind of furnace brazements because of the potential for contamination of furnace surfaces. Even with flame (torch) brazing or induction brazing, proper venting of the brazer’s breathing zone must be done so that the brazer does not have to breathe the fumes being generated.

Let’s briefly look a little more closely at the third source of these voids in brazed joints, brazing methods/temperatures used.

3. Brazing Methods/Temperatures Used

It is very important to control the brazing temperature and time as much as possible to minimize outgassing of metal constituents that can form bubbles in a brazed joint. It is not difficult to control the temperatures of furnace brazing since it can be programmed to within a few degrees of desired temperature. It is usually more difficult to control actual temperatures involved in flame brazing than for furnace brazing.

When brazing temperatures are allowed to go too high, there is a strong thermodynamic driving force to move the metal from solid to liquid to gas. I’ve frequently witnessed flame-brazing operations where the flame setting used by the brazer is too intense, resulting in overheating of the joint and outgassing of BFM constituents as he/she tries to “speed up the process” to get more production done. Although that person may be brazing more parts per hour, etc., the quality of such joints is open to question! Gases formed from such overheating can result in excess fumes in the breathing zone as well as lots of gas bubbles (voids) in a brazed joint.

Proper training and practice is essential to be able to bring the brazing temperature to a point where the BFM will melt and flow throughout the brazed joint but not be so high as to cause the liquid BFM to turn to a gas, resulting in imperfect joints.

Summary

To minimize the outgassing of these metallic elements, do not aggressively heat the base metals or BFMs involved but try to use temperatures that just melt and flow the BFM but no more. Do not overly extend the time of brazing either. “Get in, braze and get out.” When using base metals and/or BFMs containing these elements, you will always outgas them to some extent. Your job is to minimize the outgassing as much as possible and to be sure you only use approved brazing processes for such materials (never vacuum).

Let’s now look a bit more closely at the fourth source of these voids in brazed joints – poor joint fit ups – and then add a brief review and summary to this topic.

4. Avoiding/suppressing Voids in Brazed Joints

1. Be sure all surfaces of parts to be brazed are very clean and free from any lubricants, oils, greases or oxides that might outgas during heating.

2. When flame brazing or induction brazing, do not overheat the brazed joint or the BFM. Practice will enable the brazer to uniformly melt and flow the BFM throughout the brazed joint with a minimum of gas bubbles.

3. When furnace brazing:

a) Atmosphere continuous belt furnace – Carefully control furnace temperatures and belt speeds so that the assemblies do not overheat during their passage through the furnace. This can be determined by placing thermocouples on a set of parts going through the furnace and then examining cross-section photomicrographs of brazed joints. Modify belt speed and furnace set temperatures until cross sections show complete BFM flow with a minimum of gas bubbles in the joint.

b) Vacuum furnaces – Monitor as shown above using thermocouples, but it is also very important that the level of vacuum is such that it will not cause metallic elements to vaporize. As pressure levels get less and less in a vacuum furnace, the temperatures at which metals vaporize gets lower and lower. Standard “vapor-pressure charts” show this. It may be necessary to braze in a partial pressure of inert gas in the vacuum furnace, achieved by backfilling the vacuum furnace during the brazing process. This process, performed by many brazing companies, involves backfilling a vacuum furnace to a pressure of approximately 100 microns or more, using argon or nitrogen in order to “suppress” the outgassing of any metallic elements in the base metals or BFMs involved.

5. Poor Joint Fitup

Figure 1 shows a classic example of poor joint fit-up leading to voids in a brazed joint. This was encountered a few years ago at one of my client companies when someone copper-brazed a steel cap onto a specialized steel tube. The cap was supposed to fit nicely onto the formed top surface of the tube, but (as is readily apparent in the drawing) the cap’s dimensions were such that it did not fit the curvature of the formed top of the tube.

It is an unfortunate fact of brazing that parts seldom “self-center” during a brazing process. Extreme caution can be exercised to ensure self-centering. This can be done under lab conditions, if the brazer is carefully monitoring the brazing process visually or if furnace brazing is used where the parts that are mating to each other have been very carefully machined for tight fit-up. However, in the real world of normal brazing, in which a lot of commercial tolerances are used (or worse) and fit-ups are often less than desirable, problems with voids in brazed joints often occur because gaps are too wide.

Look again at Fig. 1. Notice that the cap is too big for the curved surface at the top of the tube. It can easily drift to one side or the other during the furnace brazing process. Since there is a layer of brazing filler metal (BFM) between the cap and the tube, the cap can “float” when the BFM has melted and becomes liquid. Unless the tube remains perfectly vertical during the furnace process, the cap can “skate” to one side or the other until solidification takes place. When parts are heated in a furnace brazing cycle, temperature differentials occur between each part of the load and within each component, since hotter sections will grow more than cooler sections. Thus, the part is likely to move, tilt, etc., as these thermal differences continue throughout the cycle.

Inlet tube braze joint

Fig. 2. Left side of assembly showing very nice braze.

If the cap is poorly matched dimensionally, as shown in Fig. 1, then there is a high probability (as occurred in fact) that the cap will shift and the braze will solidify in a position as shown in Fig. 1. Let’s look further at what happened.

As metals are heated, they expand. Of course, as they cool, they contract. This is true for liquids as well as for solids. Another fact to hold onto is that very thin brazed joints will solidify quicker than thick joints, simply due to the huge difference in liquid mass present in the joint. BFMs in a joint usually solidify from the outside to the center of the joint. And, since diffusion is also happening in any braze-joint, a very thin joint will usually solidify much faster than a thick joint with a lot more BFM in it.

Thus, the BFM in the thin section of the joint on the left side of Fig. 1 will solidify first, locking the cap in its position on top of the tube. The solidified joint of the left side is shown in Fig. 2. Note the smooth consistency of the tight braze joint, and the lack of voids.

Now see photo of the right side of same joint (Fig. 3). Note all the voids in the joint, which actually gave rise to a leak-path through that section of the joint (much like a spiraling, uneven worm hole).

Wide gap on the right side of the joint with lots of shrinkage voids

Fig. 3. Wide gap on the right side of the joint with lots of shrinkage voids.

Remember that all metals shrink when they cool down, even liquids. On the right side of the joint the liquid BFM filled the gap clearance, but upon cooling it wanted to shrink. However, the gap was “locked” because the joint had already solidified on the left side with the tight gap. Thus, the liquid BFM on the right side of the joint could not force the cap to move closer together as the BFM liquid on that thicker, right side shrank. As the liquid shrank to occupy a smaller volume and the walls of the joint could not move closer together, voids were opened up in the solidifying liquid.

This is a very real issue in a lot of brazing shops I visit. That is why I continue to “preach” about the need for good fit-up at brazing temps for brazements. For this to happen it may be necessary to tighten up on the tolerances of parts made by a shop or purchased from outside vendors. Yes, this may increase costs a bit. This cost is more than recovered in significantly reduced scrap and rework, greater productivity, and in a better name for your company in the eyes of your customers.

Concluding Thoughts

When examining voids in a braze joint via cross-sectional micrographs:

1. Gas-bubble voids usually tend to have rounded edges, and the inside of the voids typically appear clear and shiny.

2. Voids resulting from surface contamination can have very differing shapes and edges. The inside surfaces of the voids can be discolored and may show presence of residues or surface irregularities as compared to properly brazed surfaces. Microprobe analysis of the inside surfaces of a void can sometime pinpoint the elements present in the void to assist in determining its cause.

3. Voids resulting from poor gap clearances usually show measurable differences in the distance between the faying surfaces as compared to that of properly brazed joints in the same assembly.

4. To prevent gas-bubble voids, be sure parts are kept very clean prior to and during brazing, and be sure temperatures used for brazing are not excessive. In vacuum-furnace brazing, this may also necessitate the use of a backfilling gas to build up a partial pressure in the vacuum furnace.

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