Purpose of Flux in Welding Copper Joints

Purpose

• The primary purpose of flux when used in welding is to protect the surfaces of the joint from the oxides that form on copper during heating and to absorb the oxides that copper produces. This absorption aids in the flow of the solder and ensures a stronger joint. Flux also protects the joint surface from air to prevent further oxide formation and it aids in the heat transfer from your heat source to the copper. Copper pipes and fittings that are cleaned, fluxed and assembled properly will draw more liquid solder into the joint during heating which creates a better bond.

Types

• Flux is made from inorganic or organic materials and is available in liquid, powder or paste form, depending on the type of flux and its intended application. In welding, liquid or paste flux is typically used. Liquid flux contains a powerful cleaning agent which makes it ideal for fast soldering joints. Paste flux cleans and absorbs oxides at the same time and is lead-free. Water-soluble paste flux is lead-free and ideal for water supply lines. Most paste fluxes can be used on most metals except aluminum or stainless steel. Flux is also manufactured as a hot weather formula paste for use in arid climates or when working with copper outdoors on hot days.

Applications

• Flux is used when bonding copper and other metals through brazing, welding or soldering. These are sometimes confused as the same process, but each is slightly different. Brazing creates a bond by heating the copper and a filler metal, such as solder, to brazing temperature, which is above 840 degrees Fahrenheit. The solder, or alloy used in brazing is typically a base of silver that is combined with nickel, copper and zinc. Soldering is very similar to brazing, but the filler metal used is typically a base of tin combined with lead, silver, bismuth, indium and antimony. These filler metals reach liquid state below 840 degrees Fahrenheit. Welding uses direct heat to melt both the copper, or base metal, and a filler metal to create a solid finished joint. Most metal fillers used in welding are iron-based, using alloys such as steel. Flux is used in all of these processes for the same purpose -- to clean the metal and protect it from further oxidation.

How to Use Flux

• Flux, in liquid or paste form, is typically brushed onto the surface of each fitting to be welded after the metal is cleaned. The joints are then assembled and heat is added along with the solder or filler metal. Flux may also be blended into the filler metal in the form or a coated wire. When applying flux to copper water pipes, it’s best to avoid getting the flux inside the pipes. Flux that is manufactured specifically for use with water supply lines is available at most hardware stores, and its intended use is clearly stated on the label.