Tag: Brazing

Dissimilar metals, like Aluminum, Copper, Brass, and Steel are some of the main metals one may experience in their trade and few are comfortable working with. They don’t trust their brazed joint, or they are afraid of melting away material. To put some of the nerves of those in the field at ease let’s understand our metals with a specific focus on thermal effects on structures of metals.

 

To review the extreme, we theoretically come upon a copper and aluminum joint. The plan is to braze the two together with my copper being my male and the swaged aluminum as my female. I find an applicable alloy and bam!!! Would you look at that nice joint, I quickly cool the joint off by applying a wet rag to it, then tug on both ends which separates the two Alloys completely.

 

What the Flux!!! Only thing I can think of that I did not do was clean my joining metals.

 

Our issue here goes much deeper than a proper cleaning.

 

Thermal Expansion Coefficient of:Thermal Expansion (Microinch per °F)
Aluminum13.1
Brass10.4
Copper9.8
Steel7.3

Iron

6.8

Each material has a different response to the heat, which is characterized by its thermal expansion coefficient. The thermal expansion coefficient represents the amount that the material expands per each degree increase.  It is important to know that whatever the solid expands when heated it also contracts when cooled, but not necessarily at the same rate of expansion.

 

Each time you are cycling through expansion and contraction you are altering the structure of the metal as well as reducing its ductility or ability to flex without becoming brittle.

 

What should we consider when working with two different (Dissimilar) metals?

  • Melting temp of brazing alloy and brazing metal
  • Metal thickness
  • Brazing Metal Ductility
  • Heating and cooling rate of joint
  • The heating rate of the torch being used
  • Galvanic Corrosion between the two dissimilar metals
  • Oxidation of the metals heated

Referencing Copper to Aluminum, the vastly different thermal expansion rates of two metals make this joint very difficult to make.  When joining the two, we would use a brazing alloy that has a fluid melting temperature below our brazing metal as we want our brazing alloy to be pulled and filled into our joint through capillary action. The alloy must also be ductile enough to maintain the bond throughout the expected temperature range of the joint while in service.

Without the proper clearance between the aluminum and the copper, there is no guarantee that the brazing alloy will properly fill the joint and create a trustworthy bond and when working with dissimilar metals you will generally want to allow the work to cool naturally rather than cooling instantaneously with water.

— Sal Hamidi w/ Productsbypros.com

In Brazing and soldering tubing, we have a few things we need to accomplish to make a proper connection

  1. We can’t overheat the joint to the point that it overheats the base metal or the flux where applicable
  2. We must bring the entire joint above the melting temperature of the brazing alloy
  3. Draw the alloy deep into the joint

New techs often underheat, overheat or take too long to complete a joint because they don’t use visual cues to apply the alloy at the proper time.

Every metal and alloy responds a little differently but we always use indicators of some sort to know when to start and stop.

Some things you need to know before making a connection are

  • What is the melt temperature of the base metal?
  • What is the working temperature of the alloy?
  • Is flux required and is it external or integral to the alloy you are using?
  • What is the thermal conductivity of the metals?

Most metals we work with will respond to heat in the same way with a color change shown in the chart below. Notably, aluminum will show no change in color before it hits its melt point.

Copper to Copper

Copper to copper connections require no flux when rods that contain phosphorus are used. This is why rods using a small amount of silver with the remainder of the rod being made of copper and phosphorus are common.

Most of these phos/copper alloys have a working temperature around 1200°F and copper has a melting temperature of around 1950°F. A quick look at the chart above will show you that a good brazing indicator would be copper in the “cherry” range of 1175 – 1275°F when applying the rod. We use the Solderweld 15% round alloy at Kalos.

Copper to Brass 

Brass is a metal that is made up of a mixture of copper and zinc. Brass has a lower melting point than copper but is great for casting so many valves and other refrigeration components will be made of brass. It is preferable to use a high silver content alloy with either an external flux or a flux coated rod like the Solderweld 56% rod.

You will then heat up both sides of the joint until you see the proper color on the copper and to a lesser extent in the brass as well. The flux will also act as an indicator because it will go completely clear and flat giving both base metals a “wetted” look at about 1100°F (for most appropriate fluxes). Both the color change and the clear flux can act as indicators that the temperature is correct on copper to brass.

Copper or Brass to Steel 

Working with copper or brass to steel will definitely require a brazing alloy with no phosphorus and flux. Steel changes color in much the same way as copper but it has less thermal conductivity which means that the heat you apply tends to concentrate in one spot rather that travel the way it does with copper. Steel doesn’t melt until 2500°F but the working range for the flux is generally 1100°F – 1600°F (depending on brand/type) so you can easily overheat the flux when working with steel as well as bump into the copper melting temperature of 1950°F if you aren’t careful.

When working with copper or brass to steel use the metal color in that “cherry” zone as well as the quiet, clear flux as an indicator of proper brazing temperature.

Aluminum

Aluminum gives you no indication of when it’s going to melt which makes it more tricky to work with. It also melts at 1220°F which means that if you are working with aluminum to other metals you are in the danger zone as soon as there is any redness in the other metals. In brazing aluminum to aluminum, patching aluminum or working with aluminum to other metals you need to rely heavily on the aluminum flux to tell you when it is time to apply the alloy.

For aluminum to aluminum work we use Alloy-sol from Solderweld because you can apply as much flux as you need and it gives you a great indication of when to start applying the rod when the flux goes clear at around 600°F.

For aluminum to copper we use Al-Cop, it has a flux built into a channel on the rod that can be helf to the joint and will melt and run out when the proper temperature is reached.

In all cases we are looking for visual cues rather that overheating and damaging the base material or burning flux or underheating and globbing up rod onto the joint.

— Bryan

My technician (and brother in law) Bert made a good point the day (It’s hard for me to admit it, but it’s true). When he needs to open the refrigerant circuit to make a repair regardless of whether he is recovering or pumping down, he pulls out his nitrogen tank and his regulator (We like the VN500 shown above).

Once the refrigerant has been fully pumped down or recovered, instead of opening the system to the atmosphere and exposing it to air and moisture, he simply puts it on “BRZ” mode and introduces a very low flow of nitrogen. Now when he cuts into the system to replace a line drier, or a coil, or a compressor, or an accumulator (you get the idea) the system will stay dry and it will be less likely that anything undesirable enters the system. You simply connect the regulator to your center hose and direct the flow to the high side, low side or both depending on what part of the system you have open.

Once the system is all dry fit into place you are then ready to flow nitrogen while brazing, pressure test and even triple evacuate if nitrogen is needed for that.

The biggest hurdle to getting techs to flow nitrogen while brazing is getting the nitrogen tank off the truck. If you get in the habit of connecting nitrogen before you ever cut or open the lines it even further reduces the chance that you “forget” and increases the chances that your system is clean and dry.

Just a thought (from Bert)

— Bryan

Flow_Nitrogen

In this episode of HVAC School Bryan talks with Tim Bagnall about flowing nitrogen. We discuss:

  • The Proper tools and flow settings for brazing
  • How the pressures should be set to SCFM and not PSI
  • The possibility that geography may contribute to scale
  • How to flow nitrogen in a practical way
  • And More…

As always if you have an iPhone subscribe HEREand if you have an Android phone subscribe HERE

 

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