Welcome to Heat Treating: 101 Series
Introduction to Vacuum Aluminum Brazing
What is Brazing?
There are a lot of ways to permanently attach one metal part to another metal part without using a fastener. The most common methods are soldering, welding, and brazing.
Soldering
Most people will have seen or done some sort of soldering in their life. A common middle-school science fair project might include drizzling a bead of molten metal solder onto copper wires to connect a battery to a light bulb. To do this, a soldering gun heats up a wire containing an alloy that frequently is made up mostly of tin, alloyed with other metals like copper, antimony, or silver. The alloy is suspended in a flux that aids the flow of the solder, designed to keep the surface of the component being soldered clean and oxide-free, then evaporate leaving behind only the desired metal alloy. Because of the flux and the nature of the alloy, solder melts at a lower temperature than the copper wires, so it can be used like a bead of glue that, once cool, holds together the two wires. And because of the metallic nature of the alloy, the solder can aid in conducting electricity across the connection.
Welding
Welding takes connecting metals a little further. Instead of only heating up the filler metal, a weld also heats up both metallic pieces to be joined in such a way that the two metals fuse together. You may have seen a welder in action at an auto body shop. An auto accident may deform a piece of metal that needs to be removed and replaced, so a body shop technician might use an acetylene torch to cut off the damaged part and replace it with a new part, cut to match the original and welded into place.
Welding two metals together creates a strong bond, as the metals from both parts at the junction have fused together. The crystalline structure of the two distinct pieces have intermingled and can act as though they were always part of the same structure. Optimal welding conditions rely on the two parts being welded having a similar original crystalline structure, which usually means being made from the same, or a fairly similar, metal alloy.
WARNING: Never look directly at a welding spark. The temperature at the point of the weld can be well over 6,000 degrees Fahrenheit, and the spark can be brighter than the sun!
Brazing
Brazing lives in the world between soldering and welding. Like soldering, brazing uses a filler material that melts and cools, either connecting two parts together along a seam or using capillary action to fill in a gap. According to the American Welding Society, soldering uses a filler material that melts at or below 840 degrees Fahrenheit (450 Celsius), while brazing uses a filler metal that requires higher temperatures to flow. Brazing temperatures, however, are not so high as to melt and merge the crystalline structures of the source material together.
Brazing material may have similar qualities as the source material, but it is alloyed in ways that give the brazing filler a slightly lower melting point. To ensure the filler material melts without changing the structure of the source material, the brazing process requires a higher degree of thermal uniformity and process control. By exerting these controls, the filler material can act predictably, using capillary action to fill seams or voids evenly enough to create strong, air-tight bonds.
Brazing tends to leave clean, attractive joints that typically don’t require significant additional processing. Once parts have been brazed, no further heat-treatment is needed to restore the part’s original metallic qualities because only the brazing material was heated to the point of liquid transformation — the integrity of the metal in the original parts has not been affected.
Brazed parts have a curved meniscus at the joint that often has a look reminiscent of a fresh bead of caulk around a bathtub. This meniscus adds stability to the part, reducing stress concentrations and improving fatigue properties at the joint.
Why Vacuum Brazing?
Vacuum brazing can allow the bonding of materials using a filler material that doesn’t require a separate flux to keep oxides at bay. Maintaining a vacuum throughout the braze does most of the work of keeping oxygen out of the process. This matters for customers that have very specific parts where any of the chemicals or metals that may be included in a flux could create unwanted impurities within the final bond. It results in delivering exceptionally clean parts.
Because vacuum furnaces are designed with temperature control in mind, metallurgists know that a vacuum furnace can heat up the brazing material and the parts to be joined with precision, uniformity, and repeatability. Parts with architectural complexity that need to be joined in hard-to-reach places, or assemblies with a large number of joints are more easily brazed in a vacuum furnace environment.
Vacuum brazing is also outstanding when two parts with either very thin or very thick cross sections need to be connected with some structural integrity. Thin stock parts rely on the precision temperature control not to warp, while thicker cross-section parts can count on the thermocouple monitoring to ensure a proper soak time, heating the part evenly.
The atmosphere and temperature control found in a vacuum furnace allows for a consistent temperature for parts with complex surfaces. By heating and quenching parts at a predictable rate, a vacuum furnace provides exceptional control over transforming the brazing alloy to liquid at a temperature that will not transform or distort the parts to be bonded.
Another significant advantage of using a vacuum brazing solution is the ability to control process byproducts. Vacuum furnace systems are built to control and process exhaust better than a comparable salt bath or atmospheric solution, and don’t require harsh chemicals or molten salts that would need to be carefully contained and disposed of. Clean vacuum brazing recipes don’t require additional post-processing parts cleaning, which also reduces chemical waste.
Why Aluminum Brazing?
While many different metals can be used to braze parts, with copper, silver, brass, and nickel alloy all popular brazing materials, aluminum offers a lightweight but strong material that can bond with a wide variety of other metal alloys.
You might find aluminum brazed parts wherever you have the need to transfer heat from a liquid to the surrounding air in a closed liquid system. Items like car radiators, intercoolers, air conditioning units, or evaporators. The complex geometries of pipes winding back and forth along a strict path with a large surface area is an ideal situation to use vacuum aluminum brazing to ensure a complete seal throughout the bends and valleys of the part.
Vacuum aluminum brazing is also appealing for parts that demand cleanliness — aerospace components, medical devices, or other high-precision parts. Since part cleaning is a necessary step prior to the brazing process, an atmosphere-free process will likely result in an oxide-free, clean finished product.
Why Buy a Dedicated Vacuum Aluminum Brazing Furnace?
Vacuum aluminum brazing can technically be done in practically any vacuum furnace with a metallic hot zone, but it is not recommended. A dedicated vacuum aluminum brazing furnace, like Ipsen’s VAB furnace, has features that a standard vacuum furnace doesn’t, and the process is hard on a furnace in a way that many other heat-treating processes aren’t.
While most Ipsen vacuum furnaces are optimized for operating at temperatures between 1,000 °F and 2,400 °F (540 °C – 1320 °C), the VAB furnace is focused on operations between 600 °F and 1,200 °F (320 °C – 650 °C), where the liquidus state of most aluminum filler material occurs.
Additional thermocouple controls ensure that the temperatures throughout the chamber maintain a tight 50 °F (10 °C) variable range from front to back, and top to bottom. This intense precision at lower temperatures is necessary to meet the exacting standards of industries like aerospace and aviation.
Did You Know?
NASA recently completed a study aboard the ISS on aluminum brazing in microgravity, dubbed “BRazing of Aluminum alloys IN Space” – ensuring the acronym “BRAINS.”
The study examined the effects of microgravity on aluminum brazing, whether in a low-pressure nitrogen atmosphere or a high vacuum.
“In addition to the fundamental science associated with aluminum brazing under microgravity conditions, pragmatic objectives are twofold.
“First, a brazing process may be used to mitigate the consequences of high velocity micro meteoroid and/or space debris impacts, and possible catastrophic penetration of aluminum manufactured objects in space.
“Second, construction in space would necessitate joining complex assemblies (e.g., habitat systems), with brazing providing one meaningful manufacturing option of permanent bonding (both in the vacuum of open space, and under controlled atmosphere in confined space habitats).”
Source: nasa.gov
“Most 4000-series aluminum material goes into a liquidus state around 565 °C to 575 °C. Atmosphere set points are usually around 625 °C. It is important not to let the base material (typically 3000-series aluminum) exceed 603 °C to prevent erosion.”
– Jim Haggerty, Global Aluminum Brazing
When brazing aluminum in a vacuum furnace, one of the biggest issues operators have to deal with is magnesium oxide build-up. Magnesium is used as an additive to the aluminum filler material, acting very much like a flux. Alloys of aluminum that include magnesium have a lower melting point than pure aluminum or other common alloys. Then, as the brazing material heats up, the released magnesium will vaporize and seek out any remaining oxygen atoms in the vacuum furnace, keeping it away from the joint. Magnesium will also attack any aluminum oxide that exists on the surface of the aluminum to ensure the remaining brazing material adheres to the parts uniformly.
These chemical reactions occur as the furnace reaches and passes 1,060 °F (570 °C), creating a “magburst” – a spike in outgassing. The optimal furnace design for handling this jump in outgassing needs to have a robust enough pumping system to maintain a vacuum of 10-4 to 10-5 torr. As the vacuum furnace continues to pump out the new gasses, the process may leave outgassing residue on the walls of the hot zone.
Handling magnesium oxide residue is tricky, as the residue has a similar chemical makeup to a road flare. The majority of maintenance time spent on a vacuum furnace doing aluminum brazing will be devoted to cleaning out these magnesium oxide deposits. Scraping the chamber walls and hot zone shields must be done with a non-ferrous scraping material to avoid creating a spark that would ignite the fast-burning oxide. Should the magnesium oxide buildup be too heavy and hard to scrape off, an air burnout cycle may have to be used to break apart the larger magnesium oxide clusters. Once the majority of the residue has been removed, a normal vacuum burnout is needed to condition the furnace for future brazing production. Diffusion pumps will also need to be cleaned regularly, following the specifications of the manufacturer.
Currently, Ipsen is producing VAB furnaces that include magnesium collector plates. This design innovation helps customers reduce maintenance downtime, and prevents magnesium buildup in areas like the furnace power feed-through and thermocouple feed-throughs — areas that can be difficult to keep clean.
Some of the chemical reactions that are happening during the aluminum brazing process include:
Mg + H2O → MgO + H2
Mg + O2 → 2 MgO
Mg + Al2O3 → MgO + Al
Mg + N2 → Mg3N2
Conclusion
Vacuum aluminum brazing is a tested and proven solution for connecting metal pieces within deep valleys or complex geometries. Businesses looking to do vacuum aluminum brazing need a furnace that can provide deep vacuum levels, precise temperature control and excellent temperature uniformity.
The aluminum brazing process requires a higher degree of process control and temperature uniformity at a lower temperature than a conventional vacuum furnace. Using a standard vacuum furnace to handle regular aluminum brazing processes is a little like using a sledgehammer to hang a picture in your living room — aluminum brazing wants lower temperatures with higher thermal accuracy.
A dedicated vacuum aluminum brazing furnace, like Ipsen’s VAB furnace, provides the optimal conditions by creating a consistent vacuum that can withstand a magburst, maintain precise temperature and vacuum control, and minimize costly downtime and maintenance typically needed after brazing in other furnaces.
Contributors: Jim Grann – Ipsen Technical Director, Mark Heninger – Ipsen Director of Equipment Sales, John Pease – Ipsen Regional Sales Manager, Jim Haggerty – Owner of Global Aluminum Brazing, LLC