Definitions - Soldering and Brazing

Nothing has the potential to cause more confusion than the use of the terms soldering and brazing when talking with a silversmith. The aim of this post is to define what the difference is between a brazing alloy and a solder according to International Standards, explain how a silversmith uses the term solder in a different way to the standard definition and what the basic principles are of how to get a good brazed or soldered joint.

Both soldering and brazing use a filler metal which melts below the melting point of the parent metals being joined. This filler metal ‘wets’ onto both parent metals and is drawn into the joint gap by capillary attraction where it solidifies to give a strong, ductile bond. Different filler metal alloys melt at different temperatures, meaning it is possible to ‘step’ joints that are close together starting with a higher melting point filler metals and then progressively using lower melting point filler metals. In addition some filler metals flow better than others. The more free-flowing filler metals are better for very tight narrow joint gaps, the more stodgy filler metals are better at filling wider joint gaps.

Brazing operations are defined as taking place at 450C (840F) or above and soldering takes place below 450C (otherwise the processes are the same).

Silversmiths however frequently refer to the higher temperature brazing process as soldering and this can be a cause of some confusion. Therefore in this post when I am referring to the higher temperature joining process I will call it soldering (brazing) to differentiate it from the low temperature soldering process.

Although the staring point for soldering (brazing) alloys is 450C the silver containing solder (brazing) alloy family generally have a much higher melting ranges, starting at about 640C (1184F). The correct solder (brazing) alloy for silversmithing use depends greatly on the hallmarking requirements of each different territory. For example the United Kingdom Hallmarking Act specifies that any soldering (brazing) alloy used on a silver article must have a minimum silver content of 65%. However the different hallmarking criteria in other European countries allow the use of lower silver content soldering (brazing) alloys, typically containing 55% silver which melt at lower temperatures and are generally much easier to use than those with higher silver contents.

To get a good joint it is important that the soldering (brazing) alloy wets and flows well on the parent metals being joined. This is achieved by having very clean surfaces on the parent metals being joined, correct joint gaps and developing a heat pattern to promote the flow of the soldering (brazing) alloy.

To obtain a clean surface a flux is generally used. The flux reacts on the surface of the parent metals to give a chemically clean surface, free from oxides, to enable a good sound joint to form. When choosing a flux it is important to look at the temperature range over which it works. You want it to start working at least 50C (122F) before your soldering (brazing) alloy starts to flow and to remain active at least 50C above the highest temperature that you will reach when heating the parent metals to carry out the joining process.

The correct joint gap is also important to ensure that you get good fill along the length of the capillary joint length with no voids present. Remember these are joint gaps at the joining temperature, as heated metals expand your joint gaps will alter from those at room temperature. The joint gap at temperature for the more free-flowing soldering (brazing) alloys should be between 0.05 - 0.15mm (0.002 - 0.006'') and for the stodgier soldering (brazing) alloys this should be between 0.05 - 0.2mm (0.002 - 0.008'').

The part of the soldering (brazing) process which requires the most skill is the control of the heat pattern in the pieces being joined to encourage the flow of the soldering (brazing) alloy into the joint gap. The simplest way to visualise this is that the molten soldering (brazing) alloy is drawn towards the heat source. You should therefore aim to heat the entire joint area to close to the joining temperature for your soldering (brazing) alloy and then when you apply your filler metal allow the conduction of the heat to draw the alloy into the joint and encourage the filler metal to flow towards the heat source.

The soldering (brazing) process is one where the best way to get good joints is to practice as often as you can using small scrap pieces of metal so that you can control the heat pattern that you are creating with your torch.

Definitions - Solidus and Liquidus

I was asked yesterday what solidus and liquidus meant. This made me think of all the terms we use in silversmithing or metallurgy which we assume people understand but may mean different things in different contexts. So I thought that it might be of interest if I defined and tried to explain some technical terms.
If we start with the basics; pure metals have a single melting point. This is the point at which they change from a solid to a liquid. For silver this is 960.8C (1761F), copper 1083.4C (1982F) and germanium 937C (1719F); so all the main constituents in Argentium silver alloys have very similar melting points.Similarly the boiling point of a pure metal is the temperature at which it changes from a liquid to a gas. For silver this is 2163C (3925F), copper 2560C (4640F) and germanium 2830C (5126F).

One metal which is commonly used in silver alloys which has a very low boiling point is zinc. Its melting point is 419.5C (787F) but its boiling point is only 911C (1672F). If you are making an alloy of silver and copper which also contains zinc you can see that at a temperature when you have the silver and copper molten you are already above the boiling point of zinc. This is why zinc is such a difficult constituent to control in silver alloys and why we refer to zinc loss on melting, the zinc does literally boil away!

If we now think about what happens when we add copper to silver we will be able to understand what we mean by solidus and liquidus. As we have said, pure silver melts at 960.8C (1761F) and you would think that by adding copper to it which has a higher melting point of 1083.4C (1982F) the overall melting point would increase, but that is not the case it actually falls.

To explain this we have to think of what is happening to the silver atoms when we add copper to them. The atoms of silver and copper have different sizes (silver is larger than copper) and in a crystal of pure silver the bonds between each atom are of equal length. As you add copper to silver the smaller copper atom replaces a silver atom in the crystal structure and as a consequence increases the length of the bonds between itself and the silver atoms because of its smaller size. This longer copper-silver bond is not as strong as a silver-silver bond and so it is more easily broken when we apply heat to the solid silver-copper alloy when we want to melt it.

This is why we have a melting range with alloys, the weaker bonds are broken first at the point the alloy starts to melt and all the bonds are broken when it is completely molten. So the point at which melting starts (or if you think of the molten metal cooling, the point at which the molten metal becomes completely solid) is called the solidus. The point at which the metal is fully molten (or again to think of the metal cooling, the point at which the completely molten metal stops being completely liquid) is the liquidus. So these two terms, the solidus and liquidus define the melting range of the alloy.

The best definition though is that of the temperature range between the solidus and liquidus when the alloy is not completely molten; that is called either the ‘mushy’ or ‘pasty’ range. You have to smile when you can legitimately call the state of a metal ‘mushy’!