Fig 1

Fig 2

Most metals can have stringers, including low vapor pressure metals. Some of these include:

• ●  Steel
• ●  Cu-Ni
• ●  Monel
• ●  SST
• ●  Iron (Fe)
• ●  Kovar

Intrinsic leak paths can also be found in cast metals and are generally the result of gas and oxide inclusions.  Fortunately some materials are stringer free, such as OFE Copper.

When designing for high vacuum applications, you should avoid thin wall sections that are perpendicular to the grain direction in the material.   It is a good practice to specify grain direction on your component drawing and also specify the raw material form.  For example, bar stock and tube have grains running axially or length-wise and sheet stock has grains running horizontally.

When thin weld flanges or other thin components are needed and have features in both axes or multiple directions (see Fig. 3), you may want to consider forming or punch pressing the component from think sheet stock.  The drawing process will allow the grains to stretch and follow the “bends” or features in parallel eliminating the possibility of a perpendicular stringer.

Fig 3: Stamped sheet with grains directed parallel to the material surface

Also, when very thin sections of metal are being used, say less than 0.015”, there’s always potential for a leakage path through an inherent stringer and the risk can be greatly increased if this material is “over-brazed” resulting in “erosion” (see Fig. 4), where the parent material is “thinned” and/or a significant portion is brought into solution with the molten braze filler/braze alloy.

Fig 4: Erosion and enlarged grain size due to over-brazing of thin wall material

Once the business charter was laid out they needed to come up with a name.  Greg Cody wanted to name the company “Andromeda Technologies” after the street where he lived in one of our own Bay Area cities.  The other founders thought it would be too hard to spell and after the stardust settled, the founders finally agreed to call the new company “Altair Technologies, Inc”.  One would think they chose the name “Altair” since it is the brightest star in the constellation Aquila. Actually, it was much simpler than that. These fine gentlemen chose the name “Altair” because it was the next street over.

Thus, 1991 saw the humble beginnings of “Altair Technologies, Inc”.

Originally located on Bransten Ave in San Carlos, California, Altair bought its first braze furnace out of a barn in the state of Illinois.  Although Altair wasn’t the first “Brazer” in Silicon Valley, it was the first Brazer in the SF Bay Area to focus on the Electron Beam market. 

The initial business model was to sell RF Windows to Varian Medical, which proved more difficult than originally planned.  It was definitely a long 3 years but Altair finally became self-sufficient selling brazing services.  The mid 90’s brought some additional success stories and achievements and around 1995 Altair introduced its first product, the S-Band RF Window.  This same RF Window design is still in use today in many Medical Accelerators for the treatment of Cancer.

In the late 90’s Altair moved down the street sharing a building with Eimac and in 2004 the team moved to a 24,000 square foot facility in Menlo Park, California. With its headquarters in Menlo Park, CA, Altair Technologies Inc. serves customers throughout the United States and internationally by providing innovative brazing services and industry leading support.

Nineteen years have passed and Altair has become one of the most successful braze service providers in the industry.  Our dedicated team of professionals are some of the most respected people in the industry.  Altair is quickly becoming known for Design Engineering services as well as collaborating with customers to improve their existing products.  Over the years, the business model expanded to include Medical / Security, Defense, Semiconductor and Research market segments. 

Altair’s proud history allows us to boast a number of products, such as Electron Guns, Ion Pumps and RF Windows. Altair shipped its first product overseas in 2001 and today is a global contract manufacturer with a significant amount of revenue outside of the US.  
We welcome the opportunity to show you the brazing expertise behind the Altair logo and encourage you to call upon our services for your Electron Guns, RF Windows, HV Feed Throughs, Target Assemblies, Custom Waveguides and Ion Pump requirements. 
Join the many customers that have partnered with us as we continue to build on our reputation by providing extraordinary value and quality for their customers and their investors.

CTE is used to calculate the dimensions needed at the time of fabrication, in order to achieve a desirable braze gap (.000” to .005”) at braze temperature, between two dissimilar materials.

 For example, if you needed to design a SST weld sleeve to be brazed onto a 6” diameter Titanium tube with a .07” wall thickness, you would determine the fabrication dimensions for the SST weld sleeve as follows:

First you would determine the ID of the Titanium at the braze temperature (950º C).

The CTE for Titanium at 950º C = 10.21 μin/in

If the ID of the Titanium Tube at room temperature = Ø5.860”

Titanium ID at 950ºC   =   IDØ + (CTE (IDØ (ΔT) + IDØ))

                                 =   5.860 + (0.00001021 (5.860 (925) + 5.860))

Therefore the Titanium ID at braze temperature would be = Ø5.915”

A desirable braze gap would be ~.005”.  Therefore, we would want the OD of the SST to  be Ø5.910” at braze temperature; where by, the inverse functions are applied. 

The CTE for SST @ 950ºC = 18.59 μin/in

If the OD of the SST sleeve at braze temperature will be = Ø5.910”

SST OD at room temperature  =  5.910 / (CTE x ΔT) + 1

                                            =  5.910 / (.00001859 x 925) + 1

Therefore the SST OD at room temperature should be =  Ø5.810”

The aforementioned example clearly illustrates the SST sleeve will fit very loose at room temperature to achieve an ideal braze gap distance at braze temperature.  It should be noted that this calculation is a numerical example only and is intended to demonstrate the significant thermal expansion changes that can occur among dissimilar materials when brazing.  A designer must also evaluate the stresses that will be apparent during cooling, since a solidified alloy will now be present in the braze gap and the parent materials will no longer be able to “return” to their original dimensions or geometry.  This concept was briefly discussed in the previous post and is an essential component of braze design that must be addressed.  Altair is vastly experienced with creating innovative solutions for such scenarios.

To accomplish this, braze joint fit-up at room temperature may need to be made with “excessive” clearance or even interference such that at braze temperature, proper joint clearance exists to allow capillary action to work.

When dealing with materials with vastly different thermal expansion coefficients (CTE), brazing copper or stainless steel to ceramic for instance, fixturing becomes essential in creating the correct joint clearance.  Fixtures work by using thermal expansion relative to one part in order to “re-size” (plastically deform) it to create the proper clearance between its mating part at temperature.  What usually determines the type of fixturing needed (restraint vs. pusher) is the component that is the easiest to deform at braze temperature and its position relative to the stronger component.  A combination of yield strength and geometry will determine ease of deformation.  Fixture material must have higher yield strength at temperature than the material that is being deformed.  Therefore, molybdenum and other refractory metals are commonly used for fixturing due to their high strength at high temperature properties.

After brazing is complete and cooling is occurring, stress will be induced into assemblies from differential shrinkage.  The greater the CTE difference, the greater the stresses will be in the final assembly.  Be mindful of joint location relative to geometry as joints can see multiple shear, tension, and/or compressive forces acting towards stress concentrations.  When dealing with ceramics, it is especially important to design joints as to not create tension and shear stresses since cracking the ceramic or de-lamination of metallization can occur with minimal force.  Additionally, furnace process controls become increasingly important since the temperature ramp rate is ultimately what controls the instantaneous differential expansion between components that heat differently.

Sample Cracking from CTE Mismatch

On the surface, brazing may seem like a simple process.  Although some aspects are fundamental, the reality is that as materials and configurations become more complex, so does the underlying process.  This is where Altair’s expertise in joint design, experience with fixturing, and precise process controls make the difference between a successful braze and scrap metal.

We have years of experience bonding dissimilar materials with varying coefficients of thermal expansion.  Our next blog entry will detail a sample CTE calculation depicting the approach of determining the nominal room-temperature dimensions to achieve an ideal braze gap distance at braze temperature. Stay tuned.

Why do we experience excessive spreading with some filler (braze alloys)/substrate interactions and poor spreading in others? Let’s examine phase diagrams to explain the following observations:

• Cusil on Cu blushes (excessive flow)
• Cu/Au on Cu flows well
• Cu on Ni flows poorly
• Cu on Fe flows very well (blush potential)

It is widely known by many in the brazing industry, that “Cusil” an Ag/Cu eutectic, flows like crazy on copper. In this interaction, molten Cusil dissolves copper (Cu) thereby increasing the melt volume, but the solidus temp doesn’t rise at all!

The viscosity of the melt is very low so flow is rapid and solidification is controlled by solid state diffusion of Ag into the Cu substrate, ~ 100X slower than liquid-solid interactions. In this system, a huge amount of Ag needs to diffuse and subsequently a “Freeze Out” condition is virtually impossible. See diagram below:

Looking at a Au/Cu we see a braze alloy like 35Au/65Cu flows well on Cu but not excessively, why is that? In this system there is no eutectic and no solubility limit, and consequently no fixed solidus temperature. Non-eutectic melts have higher viscosity, so flow is slower than with Cusil. As the melt flows away from filler source, it continues to pick up Cu and the composition enters the “mush zone” resulting in a sharp increase in viscosity and reduced flow or what is called a “freeze out.” See diagram below:

Next, let’s look at why Cu flows poorly on Ni? Like Au/Cu, Ni & Cu are a non-eutectic with higher melt viscosity. The raised temperature is only 8 ºC below braze temperature, so the viscosity of the liquid phase is relatively high. Since the solid and liquid phases have similar composition, small increase in Ni pushes filler into the mush zone. High brazing temperature speeds up the solid-state diffusion and here we experience a rapid “Freeze Out”. See diagram below:

Why does Copper flow well on Iron? As copper melts, it dissolves iron, but the solubility of copper in iron is limited. That solubility limit results in a fixed solidus temperature of 1096C for Fe/Cu alloys that are < 91.2% Fe. Once the initial 3% of Fe is in solution, this system acts much like the classical wetting model because of little additional interaction. A considerable amount of copper would need to diffuse into iron otherwise “freeze out” is slow like the Cusil example above. See diagram below:

Let’s look closer at Cusil on copper in a system where the braze temperature is 20 ºC above the liquidus. Here the molten filler dissolves copper changing melt composition to a liquidus/braze temperature intersection. The volume of melt increases ~ 10% by eroding ~ 10% of the filler volume of copper substrate. This happens very fast, within seconds: 9% = 37% – 28% (increase in Cu in an Ag/Cu filler). If we “over-brazed” by 20 ºC: 20% = 48% – 28% (increase in Cu in an Ag/Cu filler), then twice the volume of substrate is eroded. See diagram below:

Here’s a method for predicting erosion potential for a filler/substrate combination:

Using the angle of liquidus line (ºC per % dissolved substrate) and the width of “mush zone” in % dissolved substrate (where narrower implies less erosion), we get a qualitative erosion-potential figure of merit when we take the width of the “mush zone” divided by liquidus angle squared:

Substrate Filler Liquidus Line Angle, C per % Width of Mush Zone Mush over Angle Squared
Cu CuSil 2.2 55.0 11.1 High erosion potential
Ni Cu 3.8 4.0 0.3 Low erosion potential

The erosion behavior of Cusil and 35Au/65Cu are similar but the freeze out behaviors is very different. As braze temperature is held, composition starts to move along the line towards solidus. The solid phase forms by diffusion of either Ag or Au into the Cu substrate and the liquid volume drops accordingly (AKA “mush zone”). See diagram below:
• Ag/Cu solid phase is 8% Ag, whereas filler is 72% Ag.
• Ag must diffuse into a volume of Cu that is 9 times that of the original filler (9 = 72% / 8%).
• Au/Cu solid phase is 16% Au, whereas filler is 35% Au.
• Au must diffuse into a volume of Cu 2 times that of the original filler (~2 = 35% / 16%).
• Cusil must do more diffusion “work” at a lower temperature (810 ºC vs. 1030 ºC for 35/65), which slows solid state diffusion; therefore, Cusil is much slower to “freeze out”.

We are pleased to announce that our resident Engineer, Adam Mitchell, and QA Engineer, Agyei Sowande were both recently inducted into the “Decade Club”, having reached a pinnacle of 10 years continuous employment with Altair Technologies.   It has been an absolute pleasure to work with and see Adam and Agyei grow and excel here at Altair.  We all look forward to their next ten years of smiles and contributions.

L-to-R: Chris Ferrari, Adam Mitchell, Curtis Allen

L-to-R: Agyei Sowande, Chris Wallace

Applications for brazing ceramic to metal are most commonly found in microwave tube, semiconductor feedthru and laser devices, where high vacuum integrity and dielectrical properties are required.

More recently, medical and military devices have brought forward new demands for ceramic to metal seals, which require biocompatibility and high joint strength.

The difficulty in brazing ceramics to metals is largely due to the inability of most braze alloys to “wet” directly to ceramic materials and the accumulation of residual stress once the materials have been successfully “wetted”.  The three contributing factors for successfully brazing ceramics to metals are as follows:

Material selection: 

 Metals are generally selected based on their CTE (Coefficient of Thermal Expansion) value.  In general, the ceramic and metal being bonded should have CTE values which are as close as possible.  However, the yield strength of almost any metal can be lowered by reducing its cross sectional area; therefore, no metal should be discounted (for a ceramic to metal seal) based strictly on its CTE value.
 
 Metals with a low elastic modulus and/or a low yield strength will generally result in less residual stress in the braze joint.

Metals, with low CTE values, typically used when brazing to ceramic

•     Molybdenum (low CTE)
•     Niobium (low CTE)
•     Kovar (low CTE)
•     Invar (low CTE)
•     Titanium (low CTE)

Joint Design:

Joints are generally designed so that plastic deformation of the metal member will absorb a significant amount of the CTE difference; there by, reducing the residual stress. 

 Joints are generally designed to put the metal to ceramic joint in compression upon cooling.

“Wettabilty”:

For a ceramic to metal braze to be successful, the two adjoining surfaces must be “wettable” by a braze alloy.  Typically, when two metals are brazed together, an alloying reaction takes place between the two parent materials.  In order for this reaction to occur, the surfaces of the parent materials must be free of non-metallic materials; therefore, common brazing alloys do not “wet” directly to oxide ceramics. 

There are two processes commonly used to “wet” the surfaces of oxide ceramics, making an alloying reaction with metals possible.

• Moly-Manganese Metallization:

Although there are several types of metallization, the most common method for producing a wettable surface on an oxide ceramic is moly-manganese.

For this process, a mixture of molybdenum and manganese  powders are suspended in paint and applied to the surface of the ceramic.  The mixture is then sinter fired onto the surface of the ceramic.  During the sintering process, the manganese is oxidized, forming MnO, which penetrates into the ceramic grain boundary and allows the glass phase of the ceramic to permeate the molybdenum layer.  This creates a metallized surface on the ceramic whose interface with the ceramic is hermetic.

To make the moly-manganese surface more readily wettable by most common braze alloys, a layer of Ni plating is usually applied. The Ni provides a barrier to keep the metallizing from oxidizing, but more importantly, it’s used as a barrier layer to keep the braze alloy from gong into solution with the metallizing.   In the case of say 35Au/65Cu alloy, the gold (Au) is soluble with the moly and will destroy the metallization when it becomes molten.

• Active-Metal Brazing:

Another common method makes use of Active-Metal braze alloys.  These alloys contain an “active” element (typically titanium) which chemically reacts with the surface of the parent ceramic, allowing the braze alloy to wet directly onto the surface.

The titanium’s ability to react chemically with oxides, forming a reaction layer of metal-oxides, in effect creates a metallized layer which is wettable by the other constituents of the active-metal alloy.

Things to consider when deciding between Moly-Manganese metallization and Active-Metal brazing:

Active-Metal braze alloys eliminate process steps, allowing you to braze directly to raw ceramics (with prior heal firing), without having to metallize/plate the surface prior to brazing.  The result is time and cost savings.  Depending on joint geometry, Active-Metal bonds should not necessarily be considered as strong as moly-manganese bonds and may require empirical testing.  Active-Metal alloys are typically brazed in vacuum atmospheres and do offer wetting compatibilities with some oxides that are otherwise very difficult to braze using moly-manganese, such as zirconia.

Moly-Manganese metallization allows for more precise alloy flow control over the surface of the ceramic.  This metallization can also be brazed in oxidizing atmospheres with a wider alloy selection than Active-Metal alloys. For example, if nickel plated, moly-manganese bonds can be brazed in wet or dry hydrogen, high vacuum or just about any inert atmosphere. If not plated, the particular hydrogen atmosphere dew point used depends on the braze temperature (see wettability and phase diagrams).

Selected References

G. Humpston and D.M. Jacobson, Principles of Soldering And Brazing, AMS International, 1993

H. Mizuhara and T. Oyama, Ceramic/Metal Seals, AMS International, 1993

So how do you read a phase diagram? This phase diagram Phase Diagram shows a typical binary system that happens to contain a eutectic. This is a fairly common characteristic of bimetallic alloys, the copper-silver system for instance has a phase diagram very similar to the one in the link. First lets consider the information that is displayed.

The horizontal axis displays the range of possible compositions of the alloy. The far left hand side indicates pure element A and as you move to the right element A decreases and element B increases until you reach the far right which is a composition of pure element B. So the horizontal position indicates the A-B composition in percent. Phase diagrams can be expressed in either atomic or weight percentage. The two ways of expressing the chart are equivalent and you can convert between the two by using the atomic mass of each element to convert.

The vertical axis represents temperature. It’s very straight forward. The higher on the chart the hotter. So if you pick a point on the chart. You read along to the x-axis and read off the composition. You read along the y-axis to read off the temperature. Therefore any single point on the phase diagram represents a specific alloy composition at a specific temperature. So I like to think of the phase diagram as depicting a composition-temperature space. Great, but how does that relate to the phases?

Metallurgical phase diagrams typically only present liquid and solid phases. Note that it is possible for there to be more than one liquid phase present, think about water and oil for instance. If you mix them you get two distinct liquid phases forming that are immiscible, meaning they don’t mix. One is mostly oil and the other is mostly water. Since they don’t mix and are separately identifiable they are considered separate phases. immiscible liquid phases are not common in metal alloys, but immiscible solid phases are very common and that is what has caused the eutectic in the linked phase diagram. Most typically these separate solid phases are the result of two different crystal structures and are favored by the particular element or compound.
The various phases are depicted using the curved lines on phase diagrams, actually to be more precise the lines represent phase boundaries whereas the space between the lines depicts the areas where a particular phase or phases are present in the composition-temperature space. Note that these areas can contain either a single phase or two phases, but never more than two. In the case of the example eutectic diagram, the several phase regions are labeled: alpha, beta, alpha+beta, L (= liquid), liquid + alpha, and liquid + beta. Once you understand the logic of phase diagrams it’s possible to deduce what phases must be present in what regions.

The alpha and beta phases are phases that represent a solid solution of elements A and B. The alpha phase will have a crystal structure like pure A and will contain some B substituting in the crystal lattice. Likewise, beta phase will have an element B crystal structure with A substituting for some B atoms. In both cases if the substituting element exceeds some percentage limit, the limit is a function of the thermodynamics of the particular elements in question, then further substitution becomes energetically unfavorable and the “extra” substituting element will start to form a phase made up of its own crystal structure (also with substitutions of the other element). That is what is occurring in the alpha + beta region. In that region there will be two solid phases present, one will be the alpha phase and the other will be the beta phase. Both phases will be “saturated” with the substituting element and the relative volumes of each phase will be determined by the relative richness of the A and B elements at that composition.

The relative stability of the substituting elements in the foreign crystal lattice is temperature dependent, recall that stability is a matter of specific thermodynamics. That is why the phase boundaries curve. Also the melting point of the phase is dependent upon the percentage of substitution causing the curve in the liquidus lines.
In the case where a eutectic forms it just so happens that substitution lowers the melting points of both the alpha and beta phases. That means in those cases a pure crystal lattice is more stable than a crystal lattice with substitutions. That is true for both the A and B elements in this case and also it just so happens that there is a composition where both the alpha and beta phases have the same melting point. That point is known as the eutectic. Its the point at the bottom of the “V” formed by the liquidus lines.
Solidus and Liquidus are terms that name the phase boundaries between a solid phase and a phase that contains both a solid and a liquid (solidus) or between that solid+liquid phase and a liquid phase (liquidus). Eutectic is a term that names the point of the “V” described above.

There is a lot more information that you can glean about the characteristics of the alloy formed between the two elements from this phase diagram and also information about how this alloy might perform in relationship to a braze application but that will be the subject of a future post or two.