35. Braze Alloys (Silver Solders)

The difference in braze alloy performance can be tremendous

 even though they appear very similar based on chemistry.   

Saw tips are brazed onto a steel saw using braze alloy.   A common term in the industry is silver solder.  Technically these are braze alloys because they melt above 840 degrees F.   

When a tungsten carbide saw tip breaks it is usually bad carbide, the wrong grade of carbide, the wrong braze alloy or a combination of these. 

The brazing process forms a three part composite.  The success of the composite depends on the tungsten carbide, the steel, the braze alloy and the way it is all put together.  

The braze alloy has to do three things.  1.  It has to keep the tip on the saw.  2.  It has to cushion the tip because the tip suffers a lot of impact stress when the saw cuts.  3.  It has to compensate for the difference in expansion between steel and tungsten carbide as they are heated and cooled during brazing.    

History

Ten years ago the standard braze alloy was a 50% silver with Cadmium.  Over the years the government tightened the regulations on Cadmium and levied some big fines on people using Cadmium.  Because of these actions there was a switch to a 50% silver solder without Cadmium.  This was done on a guess basis.  It seemed to make sense that the next best alloy was 50% silver without Cadmium.  There was the assumption that more silver meant a better alloy or they would not have put the silver in since it was expensive.

The use of silver solders without Cadmium resulted in increased tip loss and tip breakage because the 50% Cadmium free solder did not provide the impact protection.  Cadmium is extremely soft so it contributed a cushioning effect as well as lowering the melting point. 

The 50% Cadmium free never did work as well as the alloy with Cadmium.  We did tests with Weyerhaeuser about ten years ago on the two alloys.  The Cadmium free alloy was not as good as the Cadmium alloy but it worked pretty well and it was safer so it became the standard.  Worker safety and avoiding government fines were considered important enough to put up with increased breakage and tip loss. 

At the same time Sandvik was also testing braze alloys.  The showed different test results than the Weyerhaeuser tests.  Sandvik used a side pressure push off test.  Pressure was slowly applied from the side until the tips were pushed off.

Sandvik, side-pressure, pushoff tests

We suggested these test to Don Anderson at Weyerhaeuser.  He contacted Keith Dietrich at Systi Matic.  Systi Matic laser cut and brazed some saw sections.  Weyerhaeuser then ran impact tests.  In the impact tests, the force was delivered by a sixteen-ounce arm traveling at eleven feet per second.

Both tests drew the conclusion that there was a cadmium-free braze alloy suitable for use in sawmills.  Weyerhaeuser recommended the cadmium free because of concern for health and safety of the employees.

What we seemed to have missed was the fact that different solders behaved differently in a lot of ways.  At this time we thought that solder just held the tips on.  The concept of solders (braze alloys) providing impact protection came later.

In 1996 we developed a braze alloy we call High Impact.   

How Solder Works

The silver solder we use is an alloy which means it is a combination of metals each of which adds something unique to the mixture so that the combination works much better than any of the individual metals.

In these silver solders it is the combination of silver, copper and nickel that provides the strength.  Nickel also improves the flow of the alloy.  Zinc and Cadmium are added to lower the melting point of the materials.   Cadmium is a very soft metal so it adds a cushioning effect to the braze alloy.  High Impact has unique properties to absorb shock so it also cushions the braze joint.   

Kinds of alloys

In brazing tungsten carbide there are typically four kinds of alloys used.  The Cadmium alloy is not used much now.  The 50% Cadmium free is used but is being replaced by High Impact braze alloy.  The 56% with tin is used in special applications.

This is a list of the alloys and their melting ranges.  The solidus it the highest point at which the alloy is solid.  The liquidus is the point at which the alloys are fully liquid. 

                                                      Solidus   Liquidus

BAg-3        50% with Cadmium    1170    -    1270                                      

BAg-22      High Impact                1260    -    1290                 

BAg-24      50% Cadmium free     1220    -    1305                                  

Bag-8         56% with tin                1145    -    1205

Even if the temperatures are close the alloys still melt differently.   50% with Cadmium has been the standard alloy.  By comparison the 50% alloy without Cadmium takes a little more heat.  When it does get to temperature it wants to run faster and farther.    High Impact brazes pretty much like the other alloys but it does have a tendency to form little nodules or lumps. 

Heating the Alloys

Eutectic alloys melt at a single point.  The brazing alloys we are discussing melt over a range. The individual alloys take on the properties of the metals in them. The Zinc and Cadmium melt first and that starts the other metals melting sooner because a liquid transfers thermal energy much more rapidly than a solid.   However it is not that simple.  The different mixtures also have unique properties of their own.   

This range-melting is both good and bad.  The good is that there is a certain amount of plasticity of stretch in the material while it is cooling which helps to relieve stress after the brazing operation.  The disadvantage is that it can make it difficult to know when the solder is hot enough.  It is possible to braze a saw tip to a saw so that the solder achieves full flow on one side and one side only so that the tip is really only partly fastened to the saw.  We have seen this type of situation when brazing wide kerf tips to the point where the solder will flow on the side near the torch but the heat won’t penetrate all the way through the joint and the result is a tip that is only brazed on one side.

Theoretically the shorter the range the less likely there is to be trouble with the bond strength from not enough heating.  The cooling period, after brazing the tungsten carbide to the steel, is extremely important.  This is controlled by pulling the torch away from the tip at a somewhat slow steady rate.

There is a process called Liquation that applies to some metals.  Liquation is the tendency of some materials to fuse together when heated.  High Impact will fuse together until it melts and then it will dissolve.  When you heat this alloy the High Impact wants to lump together and these lumps will be the last part to melt.   

This is a chart showing the various properties of the braze alloy components. 

Metal              Atomic    Melt      Boil    Density  Coefficient      specific heat     Thermal

                        wt           pt          pt                  of expansion                               conductivity

Cadmium       112.41    619        1412      8.69          30.8                0.232                0.968                

Copper             63.5    1983       4643      8.96         16.5                0.385                4.01

High Impact    54.93    2271       3563      7.20         21.7                0.479                0.0782

Nickel            58.69    2651       5275       8.90          13.4                0.444                0.907 

Silver            107.87   1762       3924      10.5           18.9                0.235                4.29 

Zinc             65.39       786       1665       7.14         30.2                0.388                1.16 

Atomic weight - the weight of one atom expressed in atomic mass units

Melting point in Fahrenheit

Boiling point in Fahrenheit

Density - the ratio of a mass of an object to its volume

Coefficient of linear expansion - a measurement of how much a material grows as a percent of its original length

Specific heat - the ability of a metal to absorb heat

Thermal conductivity - the ability of the metal to transfer heat

Practical considerations:

When brazing it is extremely important to avoid overheating the parts.  This can put heat stress in the tungsten carbide.  It can put a chill line in the steel.  It can boil out components in the braze alloy.

It is also important not to underheat the braze alloy.  The High Impact braze alloy can be used very successfully if it is underheated.   It will bond well and it will prevent impact breakage because it has diffused into the alloy.

If the High Impact alloy is underheated then there will be little lumps left in the alloy when a tip is removed for replacement. 

Important:  There are some really excellent torch brazers making saws.  They can catch the temperature within a few degrees and within a tenth of second.   A really good brazer is so sensitive that they can catch the braze alloy before it has full flow.  If there is good feathering or fillets on both sides of the braze joint then it is a good braze job and is good all the way through.

With the High Impact alloy there is the possibility that the brazer can be causing the Liquation of the High Impact when they heat the alloy to remove the old tip.  Once again, liquation is term to describe the fact that some metals lump together before they melt.     This can be done when applying the first tip or when removing a tip for replacement. 

The critical point is to use enough heat to make a good braze joint when inserting the tip.  The best indicator here is the flow back onto the plate.  With the High Impact alloy you do not want to get the tip red and you want to stay in the notch just a little bit longer. 

When removing the old tip, a good brazer will be able to pull the old tip out at a temperature below the full flow point of the alloy.  If the brazer is really good they may see some little lumps or nodules from the liquation.  These will melt when the new tip is brazed in.  If these little bumps cause problems inserting a replacement tip then a little more heat during the tip removal should solve the problem. 

Note:  Once again some brazers are just incredibly good at what they do.  Five years ago “Everybody knew” that brazers were sloppy and would not notice a difference in alloys.  In the last five years we went out and watched brazers.  A good brazer can braze more accurately than I can run a stopwatch.  They can catch the braze alloy within a few degrees and they can definitely pick up the difference of a few percent in the braze alloy. 

Testing the High Impact Alloy

High Impact does not make sense on paper when compared to other alloys.  Fortunately we did try it in the real world. 

This is a new alloy and its heating range makes it attractive when compared to other alloys.  It melts between 1260 - 1290 F.  This compares to S50N with Cadmium at 1170 - 1270.  A50N melts at 1220 - 1305 and A 56T melts at 1145-1205.

Report on tip breakage with High Impact Alloy

The test results on this new alloy were spectacular.  In equivalent destructive tests the traditional Cadmium alloy had zero failures. The new alloy also had zero failures.  The Cadmium free alloys had failure rates from 25% to 100%.

The tips were identical tips brazed on the same plate by the same brazer.  This is an extremely good brazer.  In tests over the years it has been established that this brazer and other really good brazers can recognize very small differences.  A really good brazer can feel differences as small as seven degrees of temperature and a two per cent change in alloy concentration.  A good brazer will also reliably braze tips within one tenth of a second repeatability.

We ended up running tests of 20, 19, 8 and 8 parts.  The traditional Cadmium alloy did not have any lost tips out of twenty tested.  The most common Cadmium free alloy had a tip loss of six out of nineteen.  The next most common Cadmium free alloy had a tip loss of eight out of eight.  The High Impact had a zero tip loss out of eight parts.  When tips were brazed with other Cadmium free alloys they seemed to almost spring off the saw under relatively mild impact.  When the same tips were brazed onto the same saw under identical conditions the tips could not be beaten off the saw.

Tests  

Test Series # 1

A50N -             50%     Silver - Cadmium free                64%

S50N - 50%     Silver with Cadmium                              100%

A50N               with copper spheres added                     67%

Test Series # 2

A50N -             50% Silver - Cadmium free                    75%

S50N - 50% Silver with Cadmium                                  100%

A56T -             56% Silver with Tin                                           0%

High Impact                                                                  100%

Tests                                        Number broken / # of samples

A50N                                                               4 / 11     

S50N                                                                0 / 12

A50N with copper spheres                                5 / 15

A50N                                                               2 / 8

S50N                                                                0 / 8

A56 T                                                               8 / 8   

High Impact                                                      0 / 8

The brazer was extremely confident in this new alloy.   We mentioned to the brazer that we respected his judgment but that eight tips was not enough to really tell for sure.  The brazer said that he liked the way it worked and he knew it was good.  Then he bounced the saw up and down on the concrete floor to show that the tungsten carbide would not break.  We had to admit that he had a point.

I am definitely not making any promises about bouncing saws on concrete but it was a very impressive demonstration and it sure showed the kind of difference this alloy can make.

S50N is the standard. 50%  braze alloy with Cadmium.  A 50N is the same 50% silver alloy without Cadmium.  A56T is a 56% silver alloy without Cadmium but with tin added. 

The initial analysis is that the High Impact alloy is the best Cadmium free braze alloy.   These numbers are more than supported by comments from the participants in the tests and the people observing the test.

These tests were run in March and April of 1996.  As I write this in June of 1997, the new alloy is rapidly gaining in popularity and is being used in production all around the county.  It is working extremely well wherever it is being used.   

Once the parts are properly pretinned they are extremely easy to use.  The brazer in the tests made the following comments:

1.  It seemed to be more liquid than the standard solders.

2.  It sort of felt like there was a cushion in the middle of the joint.

3.  It seemed to slide in a bit differently.

Generally there was just a difference in feel but no problem converting to the new alloy. 

Brazing High Impact Alloy

This alloy melts between 1260 - 1290 F.  S50N with Cadmium melts at 1170 – 1270,  A50N melts at 1220 - 1305 and A 56T melts at 1145-1205.

A good brazer will notice the difference and adjust to it.  It does take a bit of adjustment.  The alloy needs some heat to get the High Impact bumps fully melted.  When you drop an ice cube into boiling water it takes it a bit to melt.  Brazers who helped us develop this alloy recommend a little slower heating cycle.  Watch the heat.  Do not let the tip get red.  Put the heat into the alloy.  Try to bring it up to temperature slowly and then hold it at temperature for a couple seconds.  Use just enough heat to keep the temperature in the 1320 – 1340 range without heating it any hotter. 

Difference in appearance

Tips pretinned with a High Impact alloy have a different chemistry and different physical properties than other alloys which is why it works differently.

This alloy has a short heating range of 30 degrees F.  We use equipment that is sensitive to +/- 2 degrees F to catch this alloy at exactly the right melt point.   We are not fully melting the alloy. 

This alloy is composed of various metals that melt at different temperatures.  When we pretin we do not fully melt the alloy.  We melt the alloy enough to cover and protect the surface.  First we clean and activate the surface then we flow the alloy over the surface to form the bond.    The alloys are sensitive to heating as is the tungsten carbide.  Generally the less we heat the alloy and the tungsten carbide the better it is.  Also, by underheating the braze alloy or solder; we can leave a hump or slug of alloy in the middle of the tip which is where it is needed for maximum brazing effectiveness.  

With other alloys we can create a smooth crest.  With this alloy the appearance is more of a hump in the center and the hump is just a bit rough.

Bond Strength

There are two types of bond strength we need to consider the tensile strength (think of the kind of strength in a butt joint) and the shear strength.  The tensile strength relates to the tip being pulled directly away from the saw.  Shear strength relates to any other force acting to remove the tip that is not directly applied.  Shear strength includes twisting as well as forces acting from the side. 

Tensile strength is important because it is easily measured.  Whenever someone hits a saw tip with an oak bat or a plastic hammer they are running bond strength tests.  This test should be done with a steadily increasing pull.  On a practical basis, if a good man with a hammer can’t knock the tip out then it probably will not come loose while the saw is running. 

There is a relationship between shear strength and tensile strength.  Theoretically if the bond strength is good then the shear strength will be good in this type of a situation.

50% silver solder with Cadmium when brazed to steels has a strength of 50,000 to 100,000 psi. while the 50% silver solder without Cadmium has a strength of 69,500 to 88,000 psi. on 18-8 annealed stainless steel and 66,000 to 73,300 psi. on cold rolled 1020 steel. 

Where the tensile strength is 50,000 to 100,000 psi. the shear strength is 25,000 to 50,000 on steel and 25,000 psi. on tungsten carbide.  It is safest to assume that the shear strength of the tungsten carbide braze is 25,000 psi. in a standard brazing operation. 

An Experiment to Demonstrate the Importance of Braze Alloy in Preventing Tip Breakage.

This photo shows a trimetal braze alloy inserted in an indexable tool.  The alloy was not brazed. 

Typically these tools see more movement than brazed tools.  Some times they actually chatter. 

The experiment was to see if a soft cushion would reduce breakage.

Reducing Tungsten carbide Breakage in Turning Operations Using Ductile Precious Metals

By: Robert L. Martin BSME Ph.D.

Vice President Engineering / Carbide Processors, Inc.

EXPERIMENT:  To measure the benefits of “Super Cushion” shock absorbers in reducing

cutting tip chipping and breakage within a controlled production environment.  Three jobs were

observed for the purpose of this analysis.  Each job was identical in terms of machine

configuration, speed rates and materials.  “Super Cushions” were inserted under the cutting

tips of two of the four lathes used in this experiment. 

The results indicate a significant reduction in cutting tip breakage on inserts using “Super Cushion” shock absorbers.  While replacement ratios varied depending on the materials being cut, overall performance of “Super Cushion” greatly reduced tooling operation cost.

INTRODUCTION:  The “Super Cushion” is an engineered, multi-layered cushion that acts as a shock absorber for inserted cutting tools tips.  The product was designed and developed by Tungsten carbide Processors, Inc. of Tacoma, Washington. The “Super Cushion” consists of two soft outer layers of precious metal based alloys and a hard inner layer of a base material.  These materials form a ductile cushion that absorbs shocks applied to the cutting tool tip during cutting operations.  A cutting tool tip is constantly and routinely subjected to impact stresses.  In a stand tool/holder configuration the stresses directly impact the insert which is backed by relatively unyielding steel.  This is equivalent to placing the insert between a hammer and an anvil.  The “Super Cushion” yields and absorbs impact shock to gradually dampen and eliminate it so the insert survives.  The “Super Cushion” shock absorber is described as “A Means of Reducing or Eliminating Breakage in Cutting Tool Tips” in patent application 07/448752.

DEMONSTRATION:  Computer simulation was initially used to evaluate the effectiveness of the “Super Cushion.”  The results showed conclusively that the presence of “Super Cushions” substantially reduced the incidence of cutting tool tip breakage.

We now needed a real world environment to fully test the “Super Cushion” under controlled production conditions.  We were very fortunate in securing the cooperation of a production machining facility in the Pacific Northwest.  The company has been in business over twenty-five years and has just completed a major overhaul involving the building of a new plant and the installation of state of the art machinery.  The company also has very sophisticated production controls for costing.  The company is well over the $10,000,000 gross sales level and is certified to do military and aircraft work.

METHODOLOGY:  Four Mazak CNC lathes were used for testing purposes.  Each lathe had its own operator.  Machine ages varied from less than a year old to approximately two years old.  Sandvik CNMG straight tungsten carbide negative/negative inserts were used on all lathes.  “Super Cushions” were used on the cutting tips of two of the four lathes.  The testing period covered three jobs and ran for a total of 104 man-hours.  Materials machined were identical for all three jobs and included Titanium, Inconel x 635, Aluminum 6061 and Aluminum 7075.  Cutting conditions were equally similar in tooling and specified feed rates.  Approximately 20% of the test involved interrupted cuts, 75% inside diameter turning, and 25% outside diameter turning.  Production rates were roughly equivalent.

RESULTS:  The following chart illustrates the effectiveness of the “Super Cushion” during actual production operations.  The same four materials were used for each of the three jobs.  Comparisons are made between inserts using a “Super Cushion” and inserts not using “Super Cushions”

ANALYSIS

Reduction in inserts used on “Super Cushion” equipped machines by material type: