Create extremely strong joints without melting the base metals
By Greg Holster
Preparing flux paste
Brazing can be described as the joining of two or more pieces of base metals together using a copper or silver-based filler rod. This filler rod when melted flows into or across the joint and solidifies when cool to form a solid bond.
Unlike soldering, brazing creates an extremely strong joint without melting or deforming the parent materials and this joint is usually stronger than the base metal pieces themselves. Dissimilar metals or base metals can be brazed together. Copper and brass are perfect for brazing as are steels and stainless steels.
Because the parent materials are not fused, the silver brazing alloy becomes the common joint.
The low surface tension of this molten brazing alloy enables the adhesion between the parts of the brazed joint. Such brazing alloys are renowned for their capillary action which makes this process excellent for watertight or air-tight joints. Copper plumbing is the best example.
Brazing temperatures can be defined as over 450º C, meaning the brazing filler material will become a molten liquid above this temperature. Soldering is defined as having a molten liquid temperature below 450º C. This process is often referred to as low-temperature brazing, silver brazing, or silver soldering.
Such low-temperature brazing has been around for a few thousand years with gold and silver documented as being used to braze goblets and jewellery around 2500BC.
While this filler material has remained in use over the ages, modern heating processes have certainly changed. There are not a lot of ancient furnaces, forges, or charcoal fires complete with blowpipes floating around these days.
Common brazing alloys
Trade names like Easyflo 50, Silverflo 55, and Comweld 356T encompass brazing filler metal with 50-56 percent silver content and a melting temperature between 620-630º C. These alloys are normally used on a tight or narrow joint such as a lap joint where a joint gap of 0.05-0.15mm is ideal. This will produce a neat joint with optimum strength and offers the highest level of ductility and corrosion resistance.
Because of their excellent flowing characteristics, Easyflo 50 and its namesakes have historically been regarded as among the best silver brazing alloys available. The 50-56 percent silver alloys have fantastic capillary action and their molten state flowing characteristics are good for both tight and capillary joints.
The 50-56 percent silver alloys can be used to join steels that include mild, carbon, tool steel, stainless steels, low alloy steel, and tungsten carbide. They are also ideal for copper alloys including, brasses, bronzes, gunmetal, nickel silvers, aluminum bronze, and copper-nickel.
They are not so suitable for fillet-type brazed joints. The fillet size will always be too small to add any strength so consumables with a lower silver base are far better for these fillets and build-up.
Joints requiring fillet welds or where fitments maybe not as tight as joints requiring capillary action use brazing alloys with a 34-45 percent silver content.
These alloys still have great flow characteristics and a melting temperature of 610-700º C but can create a small fillet which adds to their strength. A similar flowing cadmium-free alloy would be Silverflo 452 or Comweld 345T.
Joint design and preparation
A successful application of low-temperature brazing depends on
* Joint design. What are the gap tolerances, and is it a good fitting or a loose one?
* Use of a good-quality brazing alloy and flux. Do you know what flux and alloy you have available? Number 8 wire and baking soda won’t work here.
* Good preparation of the surfaces to be brazed. How clean are they?
* Correct heat input and technique. Brazing with a cigarette lighter, believe it or not, doesn’t work.
Fluxes
Fluxes are crucial to a good clean bond when it comes to brazing. Standard fluxes are readily available. But a common problem when learning to braze is over-heating the joint and burning the flux. So ask at your local welding house for a higher temperature flux.
Most fluxes contain boron compounds and simple and complex fluorides. It sounds unhealthy and probably is. Don’t go sniffing the fumes.
Mixing
If you put the flux on as a powder and apply the flame, the powder usually disappears. It is best to mix the flux into a paste which can be applied exactly where it is needed. Note how the paste turns white and then clear which is the temperature indicator to start adding your silver alloy.
The other option is a flux-coated rod. The quality on these coatings is usually excellent and I have used quite a few different brands. Flux-coated brazing alloys have always been popular, for two reasons:
* Firstly, the flux coating is usually the perfect match for the alloy it is covering. I don’t remember ever having problems with pre-fluxed gas rods of any type or brand.
* Secondly, the rods and flux are colour-coded. Silver brazing alloys that are uncoated have a coloured tip on one end of the rod to indicate the alloy mix. This is not always clear and if on a small job you use the wrong end where the colour code is, there is no telling what the rod is next time you go to use it.
Many companies like using flux-coated rods because they are coloured according to the amount of silver alloy in each rod. The wrong alloy can’t be used for the wrong application.
Torch with silver ring brazing
Cadmium
Alloys using cadmium alloyed with silver and copper make excellent silver solders with a good tensile strength in the brazing temperature range. Cadmium enhances the flow of the molten filler rod while lowering the melting temperature of the filler alloy.
But cadmium is also a “welding nasty” releasing potentially harmful cadmium oxide fumes during the brazing process. Cadmium is an extremely toxic metal commonly found in industrial workplaces particularly where any ore is being processed or smelted.
Due to its low permissible exposure limit (PEL), overexposures may occur even in situations where trace quantities of cadmium are found in the welding consumable or welding fume.
Cadmium-free
Cadmium-free alloys are now sold more widely because of the environmental benefits. With so much emphasis being put on health and safety, more cadmium-free alloys are available. These products make an excellent contribution to protecting the health of those doing a lot of brazing work. Higher silver content plus other alloys are needed to achieve similar molten flow and melting temperatures.
Silverflo 55 and is the non-cadmium version of 50 percent cadmium-bearing silver alloy. To get a similar ductility, flow, and strength, extra silver, copper, and zinc have been.
Bronze brazing, showing capillary action with brazing alloy
Consumables
Consumables most commonly are rods, flux-coated and uncoated, but they are also rings, shim / foil, pastes (complete with fluxes), wire and powder. The joint on one of our gas torches is brazed using a 45 percent silver ring. This keeps it tidy and makes for a safe and gas tight joint. Believe it or not, some welding torches are glued together.
Pastes are fun to use. One version which Kendall distributors in Auckland sell comes in a syringe, great for small fiddly bits.
Low-silver alloys
Low-silver alloys are mostly used in the plumbing industry. These are generally 0-15 percent silver.
The most common silver percentages are 2, 5, and 15 percent. Phosphorus makes up 6-7 percent and the rest is copper. These alloys have trade names such as Silfos, Silbralloy, and Copperflo. They are not suitable for use with sulphur-containing media. In Rotorua, for example, houses cannot use standard-brazed hot water cylinders. The sulphur in the water erodes the low-silver alloys, causing leaky hot water cupboard syndrome.
These lower silver alloys are normally used only for copper or brass alloys. On mild and stainless steels, for example, the phosphorous causes embrittlement and inhibits adhesion.
Fluxes are available, but flux is not necessary for brazing copper as long as the material is clean. When we brazed the copper pipes for this article, they were brazed straight off the shelf. Some brasses are OK but beware of the oxides.
Which alloy?
A number of criteria must be taken into account when selecting a brazing alloy:
You need to ask: WHAT IS
1. The base metal?
2. The intended use and treatment of the base materials?
3. The brazing equipment you have access to?
4. The load on the joints,(weight, stresses, etc)?
5. The operating temperature and pressure?
6. The amount of corrosion that may attack the brazed connections?
You also need to consider the dimensions and manufacturing tolerances of the workpieces.
Pink, blue and uncoated brazing alloys. Colours indicate different silver percentages
Clean
As with most welding or brazing, clean work is essential. Oxide layers and foreign matter such as rust and scale should be removed from the joint area, mechanically or chemically. Silver solders flow, spread, and bind only on clean metallic surfaces.
Grease or oil can be wiped off or removed with solvents or detergents in the case of delicate workpieces. Polished machined pieces do not require cleaning. If any oxide remains on the workpiece after pre-cleaning, it should be dissolved by the flux once heating begins. But beware of over-heating or burning the flux.
Techniques
The oxy-acetylene flame is usually recommended because of the rapid input of heat. This is particularly noticeable with copper which has a high thermal conductivity. An LPG turbo-torch or similar are also great on tube and pipe.
Oxy / LPG is being used more and more and for the learner, oxy / LPG has the advantage of taking slightly longer to heat up. This also reduces the likelihood of overheating, which is common when you are coming to grips with brazing the first few times.
Ideally, a neutral flame to a slightly carburizing flame is used for 20-55 percentage silver brazing alloys, and neutral to slightly oxidizing flame for 20 percent silver and below on copper. This is all according to the brazing bibles. But I use a neutral flame for everything.
Start heating the joint approx 20-40mm away from the fitting or braze area. Heat evenly to get a uniform expansion of all parts being brazed. For example, if you are brazing a light tube into a brass block, obviously more heat should be concentrated on the larger piece.
The silver brazing alloy should be applied only when the work is hot enough to melt the silver alloy. A common problem is sticking the rod under the flame when the parent material is too cold. You’ll recognize the bird poo blob; we’ve all done it. Wait for the clear-looking flux and the right temperature will be very close.
The outer envelope of the flame should then be used to heat the parts evenly, the heat input being general over the area to be joined. The parts should be heated as stated previously until the flux is molten and clear along the line of the joint. The filler material can then be added and heated.
Continue just long enough to flow the alloy between the parts to be joined then pull the flame further back. This will avoid overheating, while still maintaining an even heat. For larger or longer joints, use a light brushing motion The silver alloy will flow freely into and along the joint.
Silver brazing like many welding techniques can be a matter of trial and error. Hopefully, you can pick up some useful info from this instruction. I have harped on about the flux paste turning clear as a heat indicator; this is the secret to good brazing. You cannot go wrong.
Jewellery soldering
By Peter Minturn
Having a range of melting point solders enables a jeweller to work on complex jobs with many solder joins with relative safety.
The first joints on the piece will be soldered with hard solder. As the job progresses the goldsmith will use medium and then easy solders. If a job has just three solder joins, you can choose to use hard all the way through the job (hard usually has the best colour match for the host metal).
If the job requires a lot of forging, I have found that medium solder has more ductility than hard which will often break at the join under hammer work. For jewellers, the metal to be soldered can be any of copper, bronze, sterling silver, gold or platinum.
The alloy solder used in the case of the “royal” metals gold, silver or platinum, is of the same carat value as the host metal. These solder alloys have been designed to melt at a lower temperature than the host metal. In the case of copper and bronze, jeweller’s use silver solder alloys. Sterling silver 96 percent pure melts at 960º C, hard silver solder melts at between 770 – 780º C, medium silver solder melts at between 740º – 760º C, easy silver solder melts at between 680º – 690º C.
If the joint is a perfect fit and clean, the soldered join should be nearly invisible, after finishing.
