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Below you’ll find the most frequently asked questions and answers. If you have additional questions, Lucas-Milhaupt has chemists, engineers and metallurgists on staff to provide technical support for our customers. Please send us your questions at info(replace this text and the parentheses with the 'at' symbol)

What is the difference between soldering, brazing and welding?

Brazing - The AWS defines brazing as a group of joining processes that produce coalescence of materials by heating them to the brazing temperature and by using a filler metal (solder) having a liquidus above 840°F (450°C), and below the solidus of the base metals. For a more in-depth explanation, see How Brazing Works

Soldering - Soldering has the same definition as brazing except for the fact that the filler metal pastes used has a liquidus below 840°F (450°C) and below the solidus of the base metals.

Welding - In welding, fusion takes place with melting of both the base metal and usually a filler metal. See our Brazing vs. Welding article for more in-depth answers.

What does a brazed joint provide?

Brazing provides:

  • Strong joints
  • Lower temp/lower cost
  • Maintains integrity of base metals
  • Easily joins dissimilar metals
  • Good joint appearance
  • Skill easily acquired/automated

How does brazing work?

Brazing creates a metallurgical bond between the filler metal and the surfaces being joined. Heat is applied to the base metals and the filler metal is brought into contact with the heated parts. When the filler metal melts, it is drawn through the joint by capillary action.

What types of filler metal forms are there?

Filler metals come in wire, strip, powder, or paste form. Availability in these forms depends on the alloy that is chosen. Preforms can be made from strip and wire.

What is oxidization?

When metals are exposed to oxygen, oxides form from oxygen atoms that attach to the metal. The oxides that form prevent the molten alloy from metallurgically joining to the metal.

What is Flux?

Flux is a chemical compound that is applied and shields the joint surface from air and prevents oxide formation. Although flux will dissolve and absorb oxides, the metals that are being joined should be properly cleaned prior to brazing.

What are the available flux forms?

Flux is available from Lucas-Milhaupt in paste, slurry, liquid, and powder form depending on the type of flux. Paste, slurry, and liquid fluxes are all water based while dispensable fluxes are petroleum based. Flux can also be delivered to the joint through the torch that is being used.

How can flux be cleaned off of joint after brazing?

The easiest way to clean flux off the brazed joint is to quench and soak the assembly in hot water. HCl (up to 25%) can be added to the water for stubborn flux residue. Special cleaners may also be purchased if needed.

What is the shelf life of flux?

If stored in the original unopened container, Lucas-Milhaupt flux is under warranty for twelve months from the date of manufacture. This does not mean that after twelve months the flux is no longer useful.

How can flux be reconstituted or thinned?

Water can be used to thin water-based flux. Usually distilled or de-ionized water is used for this purpose.

What is brazing paste?

Brazing paste is a material consisting of metal alloy in a powder form mixed with a binder. When it is required, brazing flux is added to the brazing paste to provide protection from oxidization.

In what applications is using paste feasible?

Paste can be used for torch, induction, and furnace applications. In these applications flux is usually added to paste formulation. When using paste in a vacuum or atmosphere furnace flux does not need to be added to the paste.

What is the shelf life of brazing paste?

Brazing paste that is in its original unopened container is warranted for 90 days. Similarly to the flux, this warranty does not mean that after the 90 days, the paste will not function. Functionality can only be determined by melting the paste to see if flows or not. If the alloy flows then it is still functional, conversely if the alloy balls up then it is not functional.

What is the proper clearance for brazing?

In general, a joint clearance ranging from 0.002"-0.005," will produce sound high strength joints when flux brazing. When atmosphere and vacuum brazing, joint clearance should be 0.000"-0.002." Care does need to be taken when brazing metals with different coefficients of thermal expansion. The amount the materials expand needs to be factored in when determining joint clearance.

What types of joint configurations can be used when brazing?

There are several different types of joints that can be used. The most common of these are butt joints, lap joints, and butt lap joints. Pictures and calculations for determining the length of a lap joint can be seen by mouse clicking here to go to the "Principles of Joint Design" article."

At what temperature should the furnace be at when furnace brazing?

Typically the furnace will be heated to a temperature 50°F-100°F above the liquidus of the filler metal being used.

What is the strength of a brazed joint?

The strength of a brazed joint depends on several different factors. These being:

  • the base metals being joined
  • joint clearance
  • filler metal used

Joint strength varies with use of different base metals and filler metals.

Joint strength also depends on the gap between the two metals being joined. When the gap is increased the joint strength decreases. Often times, under the correct conditions, the braze joint strength will be equal to or greater than the strength of the base metals. For more in-depth information on the types of brazed joints, see our article on brazing joint design.

What is HANDY® One?

HANDY® One is a flux cored product that simplifies the brazing process. HANDY® One is a brazing alloy in strip form rolled around a measured amount of powdered flux. As the part is heated the flux is released providing protection from oxidization. It is available in multiple filler metals and flux combinations to join virtually all common metals.

What alloy is recommended for brazing copper to copper?

In most cases, when brazing copper to copper in air conditioning and refrigeration service, one of our SIL-FOS®® alloys would be recommended. The phosphorous in the alloys allow the SIL-FOS® group to be self fluxing when brazing copper to copper which eliminates the use of a separate flux.

Can stainless steel be brazed in vacuum below a temperature of 1700° F?

Stainless cannot be vacuumed brazed in at a temperature below 1700° F because of the possibility of chrome-oxide formation which will prohibit the flow of the filler metal alloy on the stainless steel. A brazing temperature of at least 1750° F is usually recommended in vacuum for stainless steel. If the stainless steel is nickel plated it can be brazed at temperatures lower than 1700° F.

What braze alloy would be appropriate for brazing 300 series steel that will be subjected to contact with water?

When brazing stainless steels that will be exposed to water, an alloy should be used that contains nickel. Examples of these would be EASY-FLO® 3, Braze 505, Braze 630 and Braze 403. The nickel in these alloys helps prohibit interface corrosion within the steel. To learn more about interface corrosion please read Technical Bulletin No. T-9 or contact Lucas-Milhaupt Technical Service.