Brazing Q & A
The following Brazing Q & A articles were published in the Welding Journal and are available online.
- October 2015 by Dan Kay
Q: We're furnace brazing some carbon-steel tubular assemblies together, using different diameter tubes, some of which are supposed to be brazed side by side along their longitudinal axes. After brazing we see that a number of the assemblies did not braze properly, were somewhat distorted, and the small brackets we had tack welded onto the tubes to hold them together in alignment had actually broken away entirely. The steel is a 1010 steel, and the filler metal we're using is pure copper. Have you seen this before? Are we doing something wrong?
A: Yes, I've encountered this a number of times in the past. What you are seeing is a problem uniquely related to the expansion characteristics of steel. All metals expand when they are heated, and contract when they are cooled. This fact has been thoroughly explored over the years, and data tables have been published showing the coefficients of thermal expansion (CTE) for each of the many metals available for use in product design and construction...
- August 2015 by Alexander E. Shapiro
Q: We manufacture fuel collectors by brazing in vacuum with BNi-2 wire rings. Thin-wall tubes are brazed with cast and machined guides and nozzle bodies. The base material is stainless steel 316. We test brazed tubes periodically for fatigue resistance at 80 Hz, and we found that sometimes the fatigue endurance decreased significantly. Cracks always appeared and propagated in the steel, but never in the joint metal. How can we improve and stabilize the fatigue strength of the brazed joints? Maybe it is reasonable to switch to induction brazing in order to avoid annealing of the steel tubes, which definitely affects the strength of stainless steel?
A: First of all, I recommend checking the design of your brazed joints. Simple tube-in-tube design is characterized by a stress concentration coefficient of ˜4.5 at the edge of the outside part, and this can be greater and more dangerous if a thin-wall tube is brazed into a thick-wall part...
- June 2015 by Tim Hirthe
Q: We are brazing carbon steel tubes to fittings using several induction heating units. I have included two photos to show typical examples of what we are doing. There are many different sizes and configurations of fittings and tube styles. We use a number of induction coils depending on the fitting size and configuration. We have large, complex jigs to hold the components during brazing. The braze alloy is BAg-7 and is in the form of rings. These rings are placed inside the joint on a counterbore between the fitting and the end of the tube. We use a black flux that we thin with water. The assemblies range in size from quite small and simple to very large with many complex tube configurations. Some of our heating times can be quite long. Due to the large variety of what we do and the complex fixtures we employ, we find some joints are better brazed in the horizontal position and some in the vertical. We have issues with both conditions.
In the horizontal position, we find that the braze alloy flows out and tear drops at the bottom of the joint and is starved at the top. This is more common on the larger fitting sizes. When we braze the assemblies in the vertical position, we find the joint on the bottom has difficulty achieving a full fillet even though we have gravity helping us. We find this more common on the larger assemblies also. The parts normally leak-check okay, but are unacceptable in appearance to our customer. What can we do to correct these conditions?
A: You don't explicitly state it, but the assumption I will make is that you are brazing both joints at the same time. You are expecting the braze alloy to flow in a variety of ways in this application. Understanding and utilizing capillary attraction is an important factor in what you are trying to accomplish...
- April 2015 by Dan Kay
Q: We pipe gaseous atmosphere into our brazing furnace from a liquid argon tank located in the company's back lot. The problem is the dew point we measure for the gas at the furnace is too high. Our gas supplier claims the liquid storage tank and equipment are not causing this problem. Why is the dew point so poor and what can we do to get drier gas at the furnace?
A: In simple terms, the dew point is the temperature at which the gas becomes saturated with moisture. For most brazing operations, the dew point must be –60°F/–50°C or colder to achieve good brazes. You are quite correct to be concerned about a high dew point since excess moisture in the gas can ruin your work...
- February 2015 by Alexander E. Shapiro
Q: We tried to braze aluminum Alloy 6061-T6 in a vacuum using BAlSi-4 as the filler metal because it has the lowest brazing temperature (1080°–1100°F) of all the standard brazing alloys listed in AWS A5.8M/A5.8:2011, Specification for Filler Metals for Brazing and Braze Welding, Table B.2. However, during the first few trials, we faced such problems as remelted edges of brazed parts that locally changed shape and a significant loss in mechanical properties of the base material 6061-T6. According to the specification, the tensile yield strength of this alloy is in the range 36–40 ksi, while after our brazing cycle, the alloy exhibited only 27.3–29.6 ksi. We suggest that this drop in tensile strength resulted from using a brazing temperature much higher than the solution temperature (985°F) of this alloy. In other words, we destroyed the hardening effect of thermal treatment of the 6061-T6 alloy, including solution and aging. Can you recommend an aluminum brazing filler metal with a brazing temperature below 985°F?
A: The problem with the supplying and application of low-temperature aluminum filler metals for vacuum brazing is quite old. Since the first filler metals based on aluminumsilicon eutectic were used during the Second World War, many attempts were made to develop the filler metal having a brazing temperature below 550°C (1022°F)...
- December 2014 by Tim Hirthe
Q: We are making a joint between a braided copper (Cu) cable and a Cu post/rivet using a filler metal of 95% cadmium (Cd) and 5% silver (Ag). The flux we are using leaves a residue and the design of the assembly makes it difficult, if not impossible, to clean. The problem is that the residue is very corrosive. It is a riveted, electrical connection so it does not require significant strength, just good electrical properties. Since we are joining Cu, it has been suggested we use a solder and a noncorrosive rosin-based flux. These assemblies operate at an elevated temperature and we need to perform our joining operation at 600°F (315°C) minimum so we have ruled out soldering. We have investigated flux materials on the market that are used for joining Cu to aluminum (Al) that have noncorrosive residues. If they are good on Cu to Al, we wonder whether they can be used on Cu to Cu. It seems Cu to Cu should be easier to join. They appear to be in an acceptable temperature range also. What do you recommend?
A: First, I must address your use of cadmium (Cd). Historically, Cd was commonly found in metal-joining workplaces, but as its toxicity became better understood, its use has declined. Cadmium is one of six substances specifically restricted in the European Restriction of Hazardous Substances directive (RoHS). The most dangerous forms of exposure are through dust and fumes...
- October 2014 by Dan Kay
Q: What should we do when a customer has a critical "hot job" that he wants brazed immediately and cannot wait for us to properly clean the furnace and run the necessary leak checks? How can we make a good braze when the furnace isn't ready?
A: Most brazing shops will occasionally be given rush jobs by their good customers and they do the best they can to comply; however, no brazer wants to risk losing a good customer by delivering a poor-quality product...
- August 2014 by Alexander E. Shapiro
Q: I must braze a Grade 2 titanium tube coil to an oxygen-free high-conductivity (OFHC) copper bushing, which is inside of a titanium tube. The 1/2 in. diameter copper bushing has a 1/8 in. wall thickness. The titanium tube has 1/16 in. wall thickness. We tried to braze in a vacuum with a BAg-8 wire 1/16in. diameter but had an air leakage from the joints during pneumatic testing. We have several parts in a furnace run, but still don't have a stable process. Please look at the drawing and tell me what you think.
A: Despite different base materials, the brazed parts in both cases require the same approach to make quality joints. Both combinations of base metals to be brazed...
- June 2014 by Tim P. Hirthe
Q: Our metal fabrication job shop does mostly welding and some occasional brazing. We have a job that calls for soldering and we have no expertise in that process. The base materials to be joined are copper tubing and a bronze fitting. The customer has given us great leeway in the solders we are allowed use, only stipulating that it be lead free. Our dilemma is the choice of flux. We believe the general-purpose brazing flux we have on hand is inappropriate for a soldering application. What do you recommend?
A: You are correct in saying that the brazing flux you have in-house is inappropriate for the job. It becomes active at too high a temperature. Regardless of whether you are soldering or brazing, if you are not performing the process in a protective atmosphere that reduces oxides on the base metals and prevents oxide formation during the process, you need to use flux. There are exceptions...
- April 2014 by Dan Kay
Q: When we brazed Inconel® 718 components in our vacuum furnace, the initial results were marginal at best. The parts are not bright and the filler metal did not flow well. Someone suggested we may need to more effectively clean the furnace prior to brazing and use some titanium material as a "getter" in the furnace beforehand. Please explain what a getter is, and how its use may help us.
A: By definition, a getter is a deposit of reactive material that is placed inside a vacuum system for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption. The use of clean titanium as a getter prior to vacuum brazing sensitive base metals is recommended, since all vacuum chambers have leaks from the many fittings, connections, and seals.
- February 2014 by Tony Anderson
Q: What etchants do you use to etch brazed joints to visualize the joint metal for metallography using an optical microscope? For instance, stainless steel brazed with BAg-24, or titanium Grade 5 brazed with TiBraze200? I cannot find etchant formulations in the literature, and I am having a hard time seeing the brazed joint properly. Many times we don't have time to send samples for electron scanning microscope study. I may be overetching my samples, but I want to make sure that I am seeing the correct things.
A: You are right. We rarely see optical metallography of brazed joints, etching procedure, and etchant formulations in the literature of the last two decades. Electron scanning microscopy is mostly used today, which does not require etching. However, for practical purposes, many companies still use optical metallography of polished and etched brazed joints. The following offers some practical information about etchants and other recommendations related to the polishing and etching procedure of cross sections of brazed joints.
- December 2013 by Tim P. Hirthe
Q: We make connections between copper and steel tubing on our assembly line using manual torch brazing. We are hand feeding a silver bearing braze alloy (38% silver) and using a white flux. After having a high leak rate on these joints, we identified significant differences between brazers in the heating process. In order to apply more consistent heating, we have been trying induction heating using a hand-held induction unit. The only other variable we changed is that we use a preform ring rather than hand feeding brazing rod. The leak rate has not dropped as we anticipated. We obviously are missing something. Can you offer suggestions as to how we can evaluate this?
A: Induction heating is a good choice for brazing as long as the joints you are making are of a consistent nature. It is a very uniform and consistent heating method, but it requires the parts being brazed to be similarly uniform and consistent.
- October 2013 by Dan Kay
Q: What is the correct amount of overlap to use when brazing tubular aluminum pieces together? Is the amount of overlap for aluminum different than for other metals such as stainless to stainless?
A: As a general guideline, I suggest that the amount of overlap for aluminum brazed joints be about 1T to 3T, where T is the thickness of the thinner of the two aluminum pieces being joined.
- August 2013 by Alexander E. Shapiro
Q: We need assistance to design and manufacture a structure for cryogenic work that will be immersed in liquid nitrogen with the upper tubing staying at ambient temperature. The application of low-weight metals is very desirable but the bottom plate or cup should be made from copper as the metal with the best thermal conductivity. The cylinder can be made of aluminum, titanium, or magnesium alloys. As any combination of these metals with copper is not weldable, we have to join the cylinder, the bottom, and tubing by brazing or soldering. But we cannot find any data about the mechanical behavior of brazed joints of dissimilar metals at cryogenic thermal cycling. How should we select a filler metal or solder for this job? What would be the best material combinations to provide reliable work for at least 2000 hours?
A: You are right, the properties of brazed joints subjected to low temperatures are not known for many base metal/filler metal combinations. Some data were published for brazed joints designed for use in nuclear industry applications, but the joining technique is definitely not suitable in your case. Read More
- June 2013 by Tim P. Hirthe
Q: We manufacture an aluminum heat exchanger for an automotive application. It is of a round tube-to-fin design. The open ends of the tubes in the circuit are joined with a U-bend, and are brazed with a flux bearing aluminum-zinc braze alloy. We use a relatively simple fixture with four torches. We have set up to use natural gas and oxygen. The design of the heat exchanger includes a mounting bracket that makes it difficult to heat the entire circumference of one of the braze joints. We are having significant leak issues in the area where accessibility is hindered. Is there equipment available or some technique that will allow us to get a quality braze in this joint?
A: To state the obvious, the best situation would be to not have the bracket in the way in the first place. You mentioned that it is an automotive application, so the odds of removing it from the design are probably quite low. Read More
- April 2013 by Dan Kay
Q: Our brazing shop has been doing nickel brazing in our vacuum furnaces for a number of years. I heard recently that I can use isothermal solidification in some high-temperature brazing processes in order to significantly raise the remelt temperature of a nickel brazement in subsequent service. How is this accomplished?
A: Yes, isothermal solidification can be a very useful brazing process for some brazing filler metals (BFMs), and can result in a significant increase in the re-melt temperature of the BFM in that brazed joint. Read More
- February 2013 by Alexander E. Shapiro
Q: We are very interested in brazing titanium products. My question concerns brazing titanium with steel. Basically, we would like to join titanium Grade 5 plate with stainless steel 304 round bars (1 or 7⁄8 in. in diameter) and require a strength of 40 ksi at the joint. Please suggest a suitable filler metal and a brazing process for us to try.
A: Technically, vacuum brazing of titanium Grade 5 (Ti-6Al-4V alloy) to stainless steel is not a problem. Read More
- December 2012 by Tim P. Hirthe
Q: We are manufacturing refrigeration lines consisting of a variety of copper tubing joints. The joints are primarily copper to copper but we do have some brass valves and steel connections. We employ dozens of hand brazers over several shifts and have a tremendous variation in quality. We are using oxyacetylene torches. It seems that everyone makes the joints their own way. One of the most aggravating issues is that we have a great deal of braze alloy teardrops and spatter. I amsure we are using more braze alloy than is necessary. I have included a photo for your reference – Fig. 1. My preference is to find some other method of making these joints. We are continually told that we need to improve our training but it seems to me to be a training nightmare. There must be equipment we can use to minimize our brazer variation. What can we do to take the manual aspects out of the operation to try to get some consistency?
A: In my experience, manual torch brazing is the most difficult brazing process to get under control. Generally, the process and resultant quality are in the hands of the operator. Read More
- October 2012 by Dan Kay
Q: I am wondering whether there is an industry standard dealing with the shelf life of brazing pastes. I notice that manufacturers of brazing pastes seem to use different statements on their products, some giving a shelf life of up to two years, while others merely specify a date of manufacture. Since I'm working in a critical environment, how can I know when a brazing paste is actually no longer good or out of date if the manufacturer doesn't print an actual shelf life date on their containers?
A: I am not aware of any industry standard that has guidelines dealing with the shelf life or expiration dates of brazing pastes. Read More
- August 2012 by Alexander E. Shapiro
Q: Many times in our repair business, we encounter the problem of brazing steel tubes that have deviations in diameters, so there is no consistency in the size of the joint clearance of the joints to be brazed. Sometimes the tubes match each other and the joint clearance is small, but often they do not match and the joint clearances are extensive, and often the centering of tubes is far from symmetrical. In these cases, we get voids on the side of the wider joint clearance. What can be done to braze wide nonuniform joint clearances properly, without these voids that cause leakages and require rebrazing after testing under pressure? We use torch and induction brazing with BAg-1a for carbon steel tubes and BAg-22 for stainless steel tubes.
A: Successful brazing or soldering requires a capillarity force in the joint clearance between the parts to be joined. Read More
- June 2012 by Tim Hirthe
Q: We are brazing an assembly fabricated from 439 stainless steel and are encountering a severe problem with failure due to corrosion. The parts are showing rust at the fillet edges between the braze alloy and the stainless parts. The braze alloy is BAg-24 and the joints are made using torch heating and a black brazing flux. We had an understanding that by selecting a braze alloy that contains nickel we would not have a problem with corrosion. I have included photos showing the problem and our attempt at a solution using BAg-21 as an alternative. These parts are subjected to moisture in service and the corrosion appears after only a short time in tap water. We also find that the steel rusts on the surface well away from the joint after thoroughly removing the flux. The BAg-21 joints are better but it is a difficult alloy to work with. What is causing this condition and are there any other remedies we can try?
A: Every once in a while a classic brazing problem rears its head and that's what we see here. What you are experiencing is called interfacial corrosion. Read More
- April 2012 by Dan Kay
Q: We are brazing high-temperature alloys together with nickel-based brazing filler metals (AMS 4777), and have had problems with parts failing in service due to cracks right through the brazed joints. The vacuum furnaces we use for brazing appear to be fine, and cross-section evaluation of the failed parts showed that joint clearances were about 0.006 in. (0.15 mm) in the joints before they cracked. Why are these joints failing in service if we're doing our best to keep the parts really clean before and during brazing, and our vacuum brazing cycle appears to be correct according to our customer specification requirements?
A: You are experiencing a problem that many brazing shops face, namely that of correctly dealing with joint-clearance issues during nickel-brazing processes. Read More
- February 2012 by Alexander E. Shapiro
Q: We want to braze a 1/16-in.-diameter tungsten rod into a quartz glass tube about 11/2 in. in diameter. The rod will work as an electrode inside the glass tube. A similar device that we have used for many years looks like it was made using a glass solder. How do we select the correct glass solder? What is the soldering technique that is applicable for use in a university lab? In other words, how can we make a sealed joint between the quartz glass tube and the tungsten rod?
A: Brazing or soldering glass to metals has a long history of applications. Since ancient times, they were used for decoration of glassware, in jewelry, and later, in the manufacture of optical and chemical devices. Read More