Brazing is a way of joining metal parts by melting a filler metal between them. Brazing takes place at higher temperatures – typically above 450°C – than soldering. But, like soldering, a flux may help dissolve or dissipate surface oxide films which can affect its wettability. And like soldering (but unlike welding), the process is not so hot that it melts the base materials. When done correctly, the process creates joints that are at least as strong as the substrate.
Because of the way the brazing filler metal behaves when molten, it tends to flow easily. Ian Hawkes, technical advisor at service provider Inductoheat, describes the process: “You might want to arrange it so it’s slightly hotter at the end away from the braze material, so that when the braze material melts, it tends to pull through the joint, and the capillary action, the surface tension, [will make] the liquid will tend to run towards the hotter point.”
Part gaps for many silver-based fillers are very small: 0.05-.15mm, because the molten material runs very easily. With alloys it typically uses, the highest percentage of silver is 55%; other alloying materials in the filler material might be copper and tin to adjust the melting point to 650-670°C. One other feature is that brazing makes its mark; while from one perspective that might make it unsightly, on the other hand it makes it easier to inspect. Adds Hawkes: “If braze material is at one end of joint and is pulled through to other end of joint, the fact that you see witness marks at both ends when is finished is a pretty good indicator that the braze has melted and run through.”
This process, used for joining pipework (among other parts), is induction brazing, which uses an electromagnetic coil close (10-20mm) but not touching to heat both parts. Unlike a hob, the parts need not be ferrous to be heated by induction, so it is also suitable for copper, brass and bronze. Aluminium is less commonly processed in this way.
Compared to torch brazing (when the parts are heated by blowtorch), the advantage of induction brazing is the efficiency and repeatability of the process, which can scale up to produce thousands of parts. It is less suitable for small numbers, because of the tooling requirements; ideally, the shape of the coil has to match the shape of the part. When considering developing a production process, Hawkes adds: “Possibly the most important thing for us is that customers are willing to reconsider some of the details of the design. They may need a slight tweak in joint gap, for instance.”
FLUX FOCUS
Across the channel, Belgian chemical company Solvay Fluor has produced brazing fluxes since the early 1980s, originally for Canadian aluminium producer Alcan. Solvay sells them under the branding ‘NOCOLOCK’, which stands for non-corrosive locking. The main application is furnace brazing of vehicle and stationary aluminium heat exchangers in a controlled atmosphere. Particularly important is (reduced) oxygen content, which limits the formation of the tough oxide layer on the surface of the metal, which impedes wetting.
When asked why brazing joining is so well-suited to working with aluminium, global key account manager Werner Schmitt replies that it is all about temperature. The melting point of pure aluminium is 660°C. The aluminium alloy-based filler metal melts at 577°C, and the flux 10-15° below that, so it dissolves the oxides, and then the filler flows. “If you wanted to braze steel, it wouldn’t work, because its melting point is about 1,000°C.”
The company now produces a number of different fluxes in addition to the standard product, potassium fluoroaluminate. They include fluxes with caesium – which scavenges magnesium in the alloy, and so is suitable for lightweight, thinner-wall parts, lithium – which is less water-soluble, so less prone to corrode in wet environments, such as rain-soaked HVAC compressors, zinc – also for corrosion protection of aluminium extrusions, and silicon – for unclad aluminium sheet. The technology also provides solutions for heat exchangers on new energy vehicles. Solvay also offers an annual English-language training course in Hanover, Germany (this year’s took place in March).
R&D
Despite being an old dog in industry, brazing continues to reveal new tricks. Russell Goodall, professor of metallurgy at Sheffield University, has experimented with cheaper alternatives to silver-based alloys. As a precious metal, Ag 155 is expensive, so there is a need to reduce the quantity of silver while maintaining performance.
Another recent area of interest requires lowering the brazing temperature. High-tech materials called thermoelectrics can capture waste heat in vehicle applications and turn it into electrical energy, but to do so they need to be attached to particular components, and if the process is too hot, it irreparably damages them.
He says: “Traditionally, there’s this boundary of 450°C that separates soldering and brazing. As we develop more novel materials, interesting materials to do different things, sometimes we want to join something around that temperature.”
This has coincided with the development of a new area of materials science over the past 20 years that sees alloys blended from four or five elements in similar quantities, rather than the standard mix of a single dominant material with fractional additional quantities. As these multimaterial alloys, also known as high-entropy alloys, are so new, their properties are not well-understood. Hence the academic interest, or, as he puts it, predictive or modelling approaches, to help design new alloys.
This has been the subject of a three-year publicly funded R&D project in collaboration with TWI, and also the University of Leicester. “We’ve undertaken sort of a design series of design challenges. We’ve manufactured [alloys] in our laboratories. We then pass that on TWI, which has great experience in actually doing joining processes. They’ve made joints and then we’ve helped them test the joints, they’ve done some testing themselves. And it’s all been supported by our colleagues at [the University of] Leicester who’ve done various different modelling activities.”
Two particular focus points have been brazing nickel superalloys for aerospace (pictured above and left) and carrying out dissimilar material joining in nuclear fusion reactors, connecting refractory tungsten parts to a conductor.
One intriguing finding of the work was a higher-than-expected degree of interaction between multimaterial fillers and the substrate. While that could be undesirable when joining a thin-walled structure, it can also create strong bonds in difficult materials. The work also suggests a future where metallurgists specify the composition of a brazing filler to accomplish a specific job: “if we could achieve such a deep degree of understanding of the alloy behaviour that we could say, ‘You want this? Okay, that’s the composition.’ You don’t even need to test it, you can just go and use it.” He stresses, however, that this is a long way away. In the nearer term, such techniques might help manufacturers avoid elements like cobalt, which is in heavy demand for batteries, but has a limited supply chain.
BOX: SUSTAINABILITY
The environmental impact of the brazing process can be modified, which Solvay is investigating in several areas. First is applying flux to base metal. Brazing flux starts as a powder, and is formulated with organic or water-based carriers to reach the part as a slurry. That can result in customers disposing of excess product. It is examining these mixtures for selective fluxing that would be dispensed by robot. Another area of research seeks to collect used (but unwanted) brazing flux and process it into new product.
