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Vacuum Brazing: A Three-In-One Process
Metal joining is necessary for components too complex to machine. There are many metal joining processes, each having advantages for certain applications.
The vacuum brazing method of material joining offers many important advantages that manufacturers of instrument and part assemblies should consider when selecting a metal joining approach.
Vacuum brazing's three step process creates a bond that is leak tight, non-corrosive, and stronger than alternative joining methods.
The first step in vacuum brazing, positioning together parts to be joined, is usually very easy. Because of tight tolerances, many components fit tightly together and are ready brazing filler material to be applied to the joining area. More complex components are assembled with special fixturing, tack welding, staking, or a combination of these methods.
The second step is applying the braze alloy to the joining area. Most braze joint areas lend themselves to a slurry of braze alloy powder and a gel binder. The slurry is often applied by a technician using needle point tips with foot controlled pneumatic pumps supplying the alloy. Other alloy forms, such as wire, preforms, or foil can be manually applied to the braze area.
The assembly can have an infinite variety of configurations. The process enables different component types such as tubing, capillary, machined, and formed parts, to be joined. And, with a pneumatic assisted application, daily production quantities can be hundreds or thousands, depending on assembly configuration.
The final step, vacuum furnace treatment, is a programmed computerized cycle based on the component material, size or quantity of assemblies, and alloy composition. The vacuum thermal process includes a heat up, preheat, holding period, braze alloy solidification, and quenching.
Vacuum brazing doesn't just join metal components, it also enhances the quality and value of the resulting assembles in other ways. An often overlooked ability of vacuum brazing is to simultaneously combine three metal treatments -- bonding, cleaning, and heat treating -- in one process.
Vacuum brazing usually uses a nickel braze alloy as a filler material in the gap between the parts being joined. The filler material melts at a lower temperature than the parts being joined and diffuses into the metal, creating the non-corrosive bond. After brazing, the transgranular diffusion of individual elements forms a new alloy that requires a higher remelt temperature than that of the original braze alloy.
Nickel alloy fillers have extensive applications, including all commonly used stainless steels, tool steels, low carbon steel, and kovar®. For extremely corrosive applications, such as harsh chemical or oil environments, parts can be joined with a gold/nickel filler, but the need must justify the cost.
In most applications, the nickel alloy, at a very reasonable cost, will provide the non-corrosive bond. This can be illustrated by an oil lamp manufactured in Pennsylvania by the Amish. Formally, the lamp bases were made of brass and soldered around the base. The soldered oil tank seam eventually corroded. The manufacturers changed the assembly to tanks made of stainless steel components, vacuum brazed with a nickel alloy. This new process resulted in a leak-free, non-corrosive seal.
Besides the materials already listed, vacuum brazed components can be cobalt, titanium, carbide, or copper. Other brazing filler options include copper, silver, gold, and various combinations of these.
Filler alloys come in paste, powder, foil, pre-forms, and even a two-sided tape. Because of the alloy variety, many non-metal materials can be joined such as ceramic to metal, diamond, and other precious minerals to metal.
A critical characteristic and benefit of the vacuum brazing process is its cleaning and brightening ability. The process is sometimes viewed as the new and improved 'Tide' of the metal joining industry. This is immediately evident when parts come out of the furnace.
During the vacuum braze process, the part assembly goes through a bake-out that removes oxides, oils, and other contaminants from the part assembly more effectively than chemical cleaning. This bake-out is able to reach machined part crevices, the internal dimensions of tubing, and short capillary tubes. Productivity improves because no flux or other contaminants need to be removed after the vacuum braze process.
This cosmetic benefit has particular importance for the instrument and medical device industries.
Finally, because the total assembly is also heat treated, up to 2175F for nickel alloy joining of stainless steel, the part assembly is metallurgically consistent. This means the assembly has consistent tensile strength throughout. Not only is the strength of the bonded area greater than alternative metal joining methods, the overall assembly strength and ductility is enhanced.
Vacuum brazing can provide several simultaneous thermal treatments with some stainless steels and/or precipitation hardened alloys. The potential exists to braze, harden, diffuse and temper 400 series stainless steel; braze and anneal 300 series stainless steel; braze, solution anneal, and age PH grade alloys without ever removing the parts from the vacuum furnace. For example, the nickel brazed oil tanks for the Amish lamps are annealed during the brazing process.
Single assemblies, or large quantities of assemblies, can be efficiently brazed in a vacuum furnace. This enables a commercial vacuum braze operation to process a small assembly with other jobs needing the same brazing cycle. In this way, vacuum braze production is very suitable for the daily draw of parts for just in time delivery.
The size of a vacuum brazed piece is only limited by the capacity of the furnace. The typical brazed part is small enough to run hundreds of brazed assemblies in a three-foot diameter, four foot deep furnace with multiple layered furnace baskets. A heat treater with various sized furnaces can schedule efficient runs of thumbnail sized or large single piece assemblies.
Sometimes, because of various machining, alloy, configuration, and component variables, it can be very difficult to immediately determine if vacuum brazing is the best method for joining. In such situations, experimentation and prototype work is needed.
Selecting the correct brazing alloy requires an understanding of the assembly application as well as metallurgical expertise because of the tolerance requirements for braze joint clearance.
An assembly should have a joint clearance of 0.0013 inch to 0.0022 inch. As the braze clearance range widens, joint strength decreases drastically. Vacuum brazing utilizes capillary action to draw braze alloy into the interface between parts. Capillary action is reduced to nil at 0.005 inch gap or greater; gravity takes over, and a loss of brazeability occurs.
Many nickel braze alloys, usually preferred to bond stainless steels, contain elements such as boron, phosphorus, silicon, etc. that lower the melting point. A wide joint gap will cause very brittle centerline bands to form consisting of borides, silicides, and phosphides. These bands can be minimized, or eliminated, with proper control over such variables as time, temperature, and braze joint clearance. (1)
The capacity to handle a specified load is jeopardized if due consideration is not given to these variables. Dual quality control procedures are necessary to prevent substandard joint bonding. This includes metallurgical qualifications for the alloy composition and the components to be brazed.
Most instrument and part assemblies need a leak-tight joint that vacuum brazing provides. A visual inspection can determine if the assembly is leak tight. However, a helium mass spectrometer should be used when certification of a leak tight assembly is critical.
When proper procedures are followed, the bonding area is not a weak link. In fact, because of the diffusion bond, the strength of the joint area is greater than with alternative joining methods.
(1) A. Sakamoto, C. Fujiwara, T. Hattori, S. Saka; Optimizing Processing Variables in High-Temperature Brazing with Nickel-Based Filler Metal; March, 1989.
Kovar is a registered trademark of Carpenter Technology Corporation
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