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Principles for Understanding Heat Treating Materials: Brazing (Part 2)

Mike Powers, F.A.S.M

The previous article in this series, Brazing (Part 1), explored the thermodynamic basis for the brazing process including the concept of wetting, the influence of capillary action, the effect of surface roughness and the import of interfacial reactions on the resulting braze joint.  In this article we will consider some of the important braze alloy systems and their areas of application, along with a review of brazing techniques, fluxes and braze joint design.

Aluminum Based Braze Alloys

There are a plethora of different brazing alloy compositions available on the market, but they fall into a relatively small number of alloy families, based on the primary metal constituent.  Because of the vast number of braze filler compositions, the most succinct way to survey the systems is to summarize significant attributes for each of the alloy families and discuss some representative or important compositions in that family. 

We start off with the family of Al based alloys with melting temperatures in the 560 to 620 degrees C range.  These braze alloys are used primarily for fluxless vacuum brazing of Al and Al alloys with a maximum assembly service temperature of 200° C.  The eutectic Al-13Si alloy is used to braze Al radiators and heat exchangers and is routinely used in the automotive industry.  The melting point of the Al-Si alloy can be depressed by small additions of Cu, Mg or Zn, but sacrifice some corrosion resistance in service.  Mg improves the flow characteristics of Al-Si alloys in fluxless vacuum brazing operations.  Aluminum based braze alloys can be tricky to work with because Al has high solubility in these brazes and the melting points are fairly close to the melting point of the parent Al, which is 660° C.

Silver Based Braze Alloys

Although pure Ag, with a melting point of 962° C, is a satisfactory braze material for bonding Be and Ti, it is seldom used due to cost concerns.  However, a number of Ag based braze alloys are used in various applications involving steels, Cu alloys, Ni alloys and even ceramics.  Ag based braze alloys are the most numerous and most commercialized family of braze materials and have been widely used in engineering applications for decades.  They offer excellent performance and ease of use.  The alloys are characterized by their relatively low melting temperatures, their ability to wet and flow readily to form strong, ductile joints with well-formed fillets with generally attractive coloration.  This family of alloys has melting ranges between 620 and 980° C with a conservative maximum service temperature of 370° C.  In addition to both alloy and carbon steels, Ag brazes are used with cast iron and stainless steel.  The Ag-28Cu eutectic alloy with a 779° C melting temperature is quite malleable and can be produced in a wide variety of preform geometries.  It is used primarily for fluxless joining of Cu based alloys in vacuum.  Derivatives of the Ag-Cu alloy that include Zn or Pd also work well with iron, Co and Ni alloys, because the Cu interdifusses with and wets these component materials much better than Ag. 

The Ag-Cu-Zn sub-family of braze alloys have a number of compositions, with some quaternary alloys that include Ni or Mn, that are used for brazing silver jewelry, for joining brass components and for brazing carbide tool tips.  2-4% Ni is added to promote wetting of carbides and improve the resistance of joints in steel components to crevice corrosion.  The addition of Mn makes the alloy capable of wetting many grades of cast iron.  Several Ag-Cu-Ti alloys are used for direct brazing of metal-to-ceramics via the active metal brazing process.

Copper Based Braze Alloys

Cu brazes are the oldest established family of braze alloys, since brazes based on brass have been used for centuries.  Pure copper, with a melting point of 1085° C finds use as a brazing material for joining mild steel, Ni and Ni-Cu alloys in vacuum and reducing atmospheres.  The solubility of Fe in liquid Cu is low at brazing temperatures, so that the flow of pure Cu over Fe components is so good that very narrow gaps can be used.   By contrast, the mutual solubility of Cu in Ni at superheat is quite large, so wide gaps must be used for Cu brazing of Ni components.  In some select cases, thin Cu metallizations on the order of 5 microns thick are plated on silver alloys and brazed well below the melting temperature of Cu, at around 800° C.  This fluxless joining process is possible due to solid state diffusion of Ag into the Cu which establishes a thin layer of Ag-Cu eutectic at the interface, which is molten at the process temperature. 

Cu brazes based on eutectic Cu-8.3P, which melts at 714° C, are used as hypoeutectic variants containing 4 to 8% P.  The function of the P additions is to depress the melting point and provide a degree of self-fluxing in mildly oxidizing environments by covering the braze surface with a film of liquid phosphate.  Phosphorus is also added to Cu-Ag alloys where the ternary braze finds wide application for joining of Cu pipes. 

Brazes based on the Cu-Zn system, with small additions of Si, Sn or Ni,  are used for brazing a wide range of component materials including low and high alloy steels, Cu alloys and Ni alloys.  The Cu-Zn brazes have a couple of limitations.  They have relatively poor corrosion resistance, which limits their service conditions.  The volatility of Zn means that over-heating the braze can cause voids to form in the joints and the affinity of Zn for oxygen means that flux must be used in even mildly oxidizing environments.

Gold Based Braze Alloys

Historically, the primary use for gold based brazing alloys was in the jewelry industry.  More recently, gold brazing alloys have been developed for use in the aerospace, nuclear and electronics industries, primarily because of their corrosion resistance in high temperature applications.  Au-Cu brazes are used in the electronics industry because of their ability to wet and form ductile joints without excessive interdiffusion with component materials such as Cu, Fe , Ni, Co and refractory metals.  The eutectic Au-20Cu alloy, with a melting temperature of 910° C, and a number or variants with additions of Ag, Zn, Ni and Ga, are popular for use with carat jewelry alloys.  Because of the narrow melting range of off-eutectic Au-Cu alloys, they display excellent fluidity and readily form nice fillets. 

Alloys within the range of 40-90% Au undergo ordering transformations at low temperature that produce a hardening effect, but are still sufficiently ductile to be mechanically worked into foil and wire preforms.  Au-Ni brazes can also wet a variety of component metals but have the additional advantage of high strength at high temperatures, such as encountered in aircraft engines.  The eutectic Au-18Ni alloy, with a 955° C melting point, is used to join stainless steel and its elevated temperature properties of high strength and corrosion resistance are further enhanced by introduction of alloying elements such as Cr, Mn, and Pd.  These additions also promote wetting of refractory materials, such as graphite and various carbides.  A good example is the Au-34Cu-16Mn-10Ni-10Pd quinary alloy, which was developed for brazing components in the Space Shuttle main engine.  These high reliability joints demanded superior oxidation resistance and the ability to withstand a service temperature around 1000° C.  The addition of Pd to Au-Cu and Au-Ni alloys increases their service temperature and resistance to high temperature oxidation.  These alloys are used to join refractory metal components that serve in relatively aggressive environments.  Specifically, the Au-25Pd alloy has a liquidus at 1400° C, which is the highest melting temperature of any of the braze alloy families.

Nickel Based Braze Alloys

The melting temperature of pure Ni at 1453° C is too high for it to be useful as a braze, but effective Ni braze alloys with acceptable melting points have been developed in response to the need for brazing jet engine components and for exceptionally high service temperatures.  Ni brazes are attractive for high temperature applications because they are much less expensive compared to the high temp Au braze alloys.  The Ni braze family is primarily based on variants of the eutectic compositions for Ni-B, Ni-P and Ni-Si.  In addition to P, B or Si, many of the alloys contain Cr to enhance their corrosion resistance and diminish interdiffusion with Ni, Fe or Co alloys.  The fluidity and wetting behavior of these alloys, particularly for the Ni-B and Ni-P alloys is very good, but compositions that are sluggish at the brazing temperature can be effective for wide gap joints.  An example is the Ni-18Cr-10.1Si ternary alloy that melts in the range of 1080-1135° C and has wide gap capability.  This braze is used for nuclear applications where neutron irradiation can produce a marked mechanical degradation of the joints. 

A common characteristic of the Ni braze alloys is that they are rather brittle and contain many intermetallic compounds, which can cause difficulties not only for the user, but the material supplier as well.  It is a challenge to produce thin foils of these materials, but in recent years the development of the rapid solidification technique for foil fabrication has mitigated this problem for some standard compositions and enabled the development of modified Ni braze compositions.  Since Ni braze alloys have extensive intersolubility with a number of engineering base alloys, most notably Fe and Ni alloys, erosion of the parent material by the braze during the joining process can be severe.  So, close attention to process control of the brazing cycle is imperative for the ultimate success of the resulting joints.

Brazing Techniques

Although the brazing process used for joining of advanced engineering materials is generally accomplished using some type of furnace, a brief survey of common brazing techniques is useful.  As with torch soldering, torch brazing is done in air using a flux that is applied to the faying surfaces of the parts and a torch is used to focus flame on work pieces at the junction of the braze joint.  Usually the filler metal is applied as a wire or rod to the joint area.  The torch uses a reducing flame, most commonly a combination of oxygen and acetylene, which produces a high temperature and is the same mix commonly used in torch gas fusion welding.  Torch brazing is used mostly for repair or low volume applications. 

Induction brazing makes use of the electrical resistance of the work piece and the application of a high frequency alternating current.  The braze joint is pre-loaded with the filler metal and flux, then the current is applied using a custom designed, water cooled coil placed on or near the intended joint, which induces heating in the parts.  The frequency of the power source, which can range from 5 to 5000 kHz, dictates the heating characteristics.  Low frequency current results in deeper heating while higher frequency causes surface heating.  Induction brazing can be used for mass production operations. 

Furnace brazing can be done is a continuous manner using a conveyer belt furnace, or as a batch process using a retort or bell jar style furnace.  Regardless of whether the parts are introduced into the furnace

 in a batch or continuous manner, the parts must be fixtured with the braze material pre-placed in the joint location before they enter the furnace.  Furnace brazing can use an inert atmosphere, with or without flux, a reducing gas, usually without flux or vacuum without flux.  Vacuum furnaces are of the retort or bell jar type. As previously mentioned, furnace brazing is the most widely used for high tech joining applications. 

On the other end of the brazing technology spectrum lies dip brazing.  In this case the heating of the joint is accomplished by immersing it into a heated molten bath of flux covered brazing filler metal or in some cases a flux covered molten salt bath.  If a salt bath is used, the filler metal is pre-loaded in the joint and the filler metal melts when the assembly is submerged in the bath.  If a metal filler bath is used, flux is applied to the joint and the when the assembly is lowered into the bath, the filler metal fills the joint by capillary action.  In both cases, the braze joint solidifies after the parts are removed from the molten bath.  This technique is capable of rapid heating rates and if the braze pot is big enough, multiple parts can be brazed at once.  The dip brazing technique is limited to lower melting temperature brazing alloys, but has the advantage of homogenous and reproducible heating of the parts.

Brazing Fluxes

Flux is an important element of many brazing processes.  As with solder fluxes, the purpose of a braze flux is to promote joint formation by removing oxide films from the faying surfaces, to protect the molten braze filler, and to promote wetting of the substrate.  As is the case with soldering, the surfaces of the parts must be cleaned prior to the brazing operation to remove organics and oxides by solvent cleaning, chemical cleaning or mechanical abrasion.  To be effective, the braze flux must have a lower melting temperature than the braze alloy, so it can perform the function of oxide removal rapidly at the braze temperature.  To achieve this, the flux must wet the component surfaces, yet must be readily displaced by the molten braze alloy, so that the braze can form metal-to-metal bonds. 

Brazing fluxes are normally powder mixtures of compounds such as borates and fluorides that are formulated as pastes which can be applied by rollers, brushes or as sprayable liquids.   The specific formulations for commercial fluxes are often considered proprietary and are treated as trade secrets.  However, some classification is possible in terms of the primary constituents, use temperatures and materials that are joined. 

Fluxes for Mg alloys are mixtures of fluorides and chlorides that work at 480-620° C.  Fluxes for Al and Al alloys are usually mixtures of alkaline earth metals that operate in the 540-615° C range.  Fluxes for use with Ag and some Cu braze alloys for joining Cu, Cu alloys, steel and cast iron are mixtures of alkali borates and fluroborates, often with secret wetting agents.  These fluxes are designed to operate in the 565 to 870° C range.  Fluxes for higher temperature brazing of steels are mixtures of alkaline borates or boric acid and operate at 750 to 1200° C for joining with Au, Cu and Ni based brazes.  In any case, it is wise to consult the braze alloy supplier for their brazing flux recommendations and follow the suggested procedures with care.

Braze Joint Design

In spite of the myriad of joint designs used for soldering and brazing processes, they are all essentially variants of four basic joint designs;  the butt, lap, scarf or hybrid butt-lap joint.  As previously discussed, a number of factors will affect the performance of the final joint including thermal expansion mismatch between the components, the strength and elastic properties of the filler metal and the relative electrochemical potentials between the filler and parent materials.  However, the fundamental design of the joint and consideration of the material properties can go a low way to insuring a sound assembly. 

In general, soldered and brazed joints tend to be weakest in tension.  As such, designing with lap joints reduces the tensile stresses to a minimum, because the load is transferred from one component to the other predominantly in shear.  The strength of the joint then depends on the length of the overlap over which the shear stresses operate and the shear strength of the braze or solder material.  The strength will also be affected by the thickness of the solidified filler metal and the elastic modulus mismatch between the components, as well as the residual stresses with differential shrinkage across the joint. 

A simple calculation can be used to determine the optimum length of flat lap joint overlap (X), given by

X = 

where Y is the desired safety factor, usually taken at 2, T is the tensile strength of the weakest parent material in the couple, W is the thickness of the weakest member, and S is the shear strength of the filler metal alloy.  Even though the equation is simple, one must take care to pay attention to the units, so that they are consistent. 

The butt joint is the simplest joint design with a shape and cross sectional area that is defined by the components.  Although the butt joint is used in many laboratory studies, it is not always suitable for applications because it can subject the joint to fairly intense tensile loads, usually as a result of temperature cycling.  Especially in the brazing arena, the T-joint is fairly common.  This is a variant of the butt joint where the components are oriented orthogonal to each other.  The advantage of the T-joint over a straight butt joint is that the fillet formation can result in a significant increase in the bonded area, but caution is still advised when using this joint design because it is still subject to the limitations of the straight butt joint.  Both the butt-lap and scarf joints are really hybrids of the butt and lap joint.  The objective in both cases is to increase the joint contact area and avoid the tensile stresses associated with straight butt joints.  The butt-lap joint has a section that is basically a lap joint, with butt joints on either side.  The scarf joint is an angled joint that mitigates some, but not all of the tensile stress that can arise from straight butt joints.  These joint designs can also be used for cylindrical type geometries.  If there is a choice as to the joint design, it is usually best to pick the lap joint, with the butt-lap joint as a reasonable second choice.

Brazing Case Study

This case study looks at two brazed assemblies that were developed for use in a mass spectrometer system.  Once resolved ion beams have passed through a mass analyzer, they sequentially strike a detector where the ion current is monitored.  The photograph on the left shows two designs of an ion focus lens assembly, which directs the ion beam at the aperture of a Faraday cup collector assembly, shown on the right.  The Faraday collector measures the ion current in the focal plane of the mass spectrometer. 

The three lens elements in the ion focus lens assemblies are made from 304 stainless steel and are separated by alumina standoffs.  The standoffs were metallized using the Mo-Mn process, which was subsequently electroplated with Ni and a thin layer of Au.  The assembly was then vacuum brazed using a 63Ag-27Cu-10In braze alloy, referred to by the trade name Incusil, which has a 685° C solidus and 730C liquidus.  The Faraday cup collector assembly was produced using a two step brazing process.  In the first step, 303 stainless pins were brazed into the alumina header using an active metal vacuum brazing process with a quaternary active braze alloy of 63Ag-34.25Cu-1Sn-1.75Ti, which has a solidus of 775° C and a liquidus of 805° C.  This active metal braze alloy has the trade name Cusin-1-ABA, where the ABA is an acronym for “active braze alloy.”  Subsequently, the header was metallized with a proprietary two part, low temperature thick film metallization developed by DuPont, where an adhesion layer is applied using a typical print/dry/fire sequence followed by a barrier layer, both of which are fired below the solidus temperature of the active metal braze alloy.  Finally, the Kovar Faraday cup was brazed to the thick film metallization using a filler alloy of 81Au-19In with a liquidus of 480° C.  However, there was a problem with this braze alloy.  If you look closely at the photo of the Faraday cup collector assembly, you can see that the cup has separated from the metallization at its base.  This was the result of a failure in the brittle Au-In braze alloy that was manifested during temperature cycling tests.  This problem was rectified by substitution of a 70Au-20In-10Sn alloy with a liquidus temperature of 446° C.  This ternary braze alloy boasts much more ductility compared to the straight Au-In filler alloy.

Summary

All solder and braze processes are by definition, performed in a three-phase system consisting of the solid components, the liquid components, which are the molten filler metal and possibly flux, and the atmosphere surrounding the work pieces.  As such, the Young equation governs the thermodynamic considerations of wetting and spreading of the parent materials by the molten filler alloy.   Solder and braze processes are fundamentally quite similar, with the main distinction being the melting point of the filler metal.  Solder processes are inherently lower temperature with solder alloy fillers defined as having a melting point below 450° C, whereas braze processes are higher temperature with filler alloy melting points above 450° C.  Solders usually react to form intermetallic phases, where most brazes are less reactive and tend to form solid solutions.  There are many solder alloys with eutectic compositions, which is not the case for braze alloys were there are relatively few.  In both cases, wetting and spreading of the filler metal can be enhanced by use of fluxes, which serve to dissolve native oxides on the surfaces of the parent materials and help keep oxides from forming on the molten filler.  In spite of processing issues associated with thermal and elastic mismatch between the filler metal and the components, replacing tensile loading of the joint by using a design involving shear, as in the lap joint or butt-lap joint, is an effective method for assuring mechanical reliability through good design.

 

Copyright © 2018 by Michael T. Powers – All rights reserved.