Braze Alloy Forms
Braze Alloy Forms
Braze alloy comes in several forms; wires of various diameters, flux coated wires, ribbon or strip, tri-metal or sandwich ribbon or strip and paste.
The selection of the form of the braze alloy is typically governed by the following requirements; 1. Producing a quality joint, 2. Throughput required and 3. Total manufacturing cost.
Alloy Form Manufacturing
Braze alloy is typically made as a wire. It can be rolled flat to form a ribbon. A copper ribbon is placed between two ribbons of braze alloy and this is pressure rolled to bond the three ribbons and form a sandwich. The braze alloy wire can be heated to the point where it vaporizes. This braze alloy vapor then condenses into droplets which are sorted and used to make brazing paste. Brazing paste is braze alloy droplets, flux and the binder to keep the braze alloy in the flux from separating during dispensing.
Alloy Form Relative Cost
In terms of cost per Troy Ounce of braze alloy; wire is the least expensive followed by ribbon than tri-metal with paste being the most expensive.
Alloy Form Advantages and Disadvantages
Braze alloy in wire form (including rod) is the least expensive per Troy ounce and is a very versatile form. Because the wire melts out flat due to capillary attraction it can expose flaws, such as non-wedding areas, in the surface preparation. Wire can also be an excellent indicator of temperature at the interior of the braze joint. When the braze alloy wire hits the proper temperature, the wire melts and the part settles in somewhat so it is very obvious when the proper internal temperature has been achieved.
Ribbon is somewhat more expensive than wire but not horribly so. Ribbon can be very handy when you are preparing large areas. Ribbon also has an advantage over wire in furnace brazing or other applications where the parts cannot be adjusted during brazing. When wire hits the proper temperature it melts and flattens thus moving the parts a wee bit. When ribbon hits the proper temperature it typically melts out without moving the parts significantly.
Ribbon material seems to be much easier to use than wire in automated brazing systems such as automatic machines to braze carbide tips onto saw blades.
In welding wire is the preferred form. This is not necessarily true in brazing. In welding you work from the outside. In brazing you work from the inside. An example would be brazing carbide into a slot. It is common for welders to want to put the carbide in the slot and then run a bead of braze alloy around the outer edges and have the braze alloy wick down into the joint. This can be a very valid technique. However it is typically easier and much surer to place the braze alloy in the bottom of the notch, flux it properly and then place the carbide on top of the braze alloy. Once heated to the proper temperature the carbide part will settle in and the braze alloy will wick up and out.
3. Sandwich material
Sandwich material typically uses a standard braze alloy for the bread and a layer of copper for the sandwich filling. This is typically in a ratio of 1:2:1. The primary advantage of this material is to relieve the stress caused by brazing. As steel and carbide heat up the steel grows at a rate about 3 to 4 times that of the carbide. As soon as the heat is removed in the brazing operation the parts start cooling and lock into place. Just as the steel grew much more than the carbide, so does it want to shrink much more than the carbide. This creates stress in the carbide and the steel. This can create enough stress in the relatively brittle carbide so that the carbide can be highly resistant to breaking during use. In extreme cases the carbide will crack during the cooling process.
Caution: Sandwich braze alloy is often seen as a universal solution to preventing carbide breakage and carbide loss. This is not necessarily true. The surfaces of the steel and the carbide must be in a condition where both wet completely. If you have large sections of non-wetting surfaces the stresses will be applied in a highly uneven manner and all the sandwich braze alloy will do is disguise the non-wetting areas.
If you use too much flux you can have a very weak joint because of entrapped gas. Standard black flux can be as much as 10 to 20% inert materials which can cause weakness in joints. There is now available a “purified” black flux which contains almost no inert materials and thus creates a much stronger braze joint.
Paste is far and away the most expensive form of braze alloy to buy. It can also be far and away the least expensive form of braze alloy to use. Paste lends itself extremely well to manual or automatic dispensing. Paste is typically loaded into a syringe and then an air powered machine will dispense a precise amount when the switch is triggered.
The switch can be triggered by hand, foot or some sort of an automatic device such as an optical or mechanical sensor.
Following is a website of a well-known maker of paste dispensers.
Paste does have the drawback in that it must contain a binder of some sort. If you try and make a brazing paste that is nothing but flux and tiny bits of braze alloy you will squeeze all the flux out when you try to dispense it and you will be left with a syringe or tube full of the braze alloy bits. The paste is typically described as an inert material. However it can create smoke and flame. Thus the term inert applies only to the fact that it does not interfere with the brazing process.
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