Brazing and Soldering

Richard Lofting

• Publisher: Crowood
• Year: 2014
• ISBN:
  9781847978370
• Topic: Tecnología e ingeniería
• Subtopic: Oficios técnicos y manufactureros

Brazing and soldering are essential metal joining techniques for a range of jobs in the workshop. This new practical guide will introduce you to the methods and show you how to enhance these skills safely and effectively, with step-by-step photography throughout. Included in this new book is advice on equipment and building a brazing hearth; an overview of alloys and fluxes; lead loading, body and electrical soldering and pickling and cleaning. The main brazing and soldering types are also covered including soft soldering, silver soldering and brazing. There is also a range of case studies to show practical technique applications. Fully illustrated with 298 colour photographs and 2 tables.

Alloy rods and fluxes required for soldering and brazing operations.

1 Alloys and Fluxes

The three categories of soldering and brazing are:

◆ soft soldering, which is mainly carried out with a soldering iron and low temperature alloys

◆ silver soldering, also known as hard soldering, which is performed in a brazing hearth using alloys containing silver

◆ brazing, which is also performed in a brazing hearth, but using higher temperatures and higher temperature melting alloys, producing the strongest joints.

SOLDER AND BRAZING RODS

To help understand the process of soldering it will be advantageous to look at the composition of the alloys used in the rods for each process. First, the basic question needs to be answered: what is solder? Traditionally, soft solder was mainly based on alloys predominantly made of lead, but with the increasing awareness of the poisonous effects of lead on persons and the environment, lead solders are no longer used in plumbing. In addition, lead-based electrical solders are now virtually banned worldwide due to the possibility of the lead content leaching into water supplies from landfill rubbish sites, as a result of the ever increasing amount of discarded electronic equipment. European Union directives are now in place to prohibit the use of lead in various industrial processes and the construction industry, with it being replaced by other non- or less poisonous metals such as tin, copper and silver as the main alloy. Apart from the attributes that lead gave to solder alloys, such as good wettability, it was a cheap and readily available commodity, whereas the alternatives are relatively more expensive to source and extract from the ore containing the metal.

Another question is why use alloys for the filler material instead of a pure metal? There are several reasons. A pure metal melts at a higher temperature than an alloy containing the same metal and by combining two or more metals the attributes from each can be taken advantage of in the alloy. Also, varying the mixture will vary the melting temperature and other factors such as wettability or the tensile strength of the resultant solder alloy.

Soft solder is available as sticks or wire on a reel; most soft solders are predominantly non-lead based today, due to environmental concerns.

As already seen in the introduction, soldering has been neatly divided up into three sections, namely soft soldering, silver (or hard) soldering and brazing. These are arbitrary boundaries that have been historically developed over the years. As far as the alloys used for all three categories are concerned, there are no delineating boundaries, as the only difference between any of the alloys used for soldering or brazing is the temperature at which they melt and consequently can be used at, and their service life. It will be explained later on in this chapter that most alloys have two temperatures given in their specification, unless they are a eutectic alloy and therefore have a single melting point.

Silver solder comes as wire or rods depending on the diameter being used and for what purpose; thin foils are also available for specific purposes.

Brazing rods are very much like silver soldering rods, the difference being the melting temperature of the alloy they contain and that they are usually brassy coloured.

NAME THAT ALLOY

There are various ways that manufacturers of soldering and brazing rods describe their particular soldering and brazing alloys, sometimes using the main alloying metal’s percentage with a range description, for example Silver-flo 40. This alloy, which is from the Johnson Matthey range, has a silver content of 40 per cent, copper 30 per cent, zinc 28 per cent with 2 per cent tin, a melting range of 650–710°C and a tensile/shear strength of 450/155 N/mm2. This particular alloy mix meets the widely recognized international standards and is variously known as AG20, L-Ag 40 Sn and AG105, depending upon whether you are looking at the BS1845, DIN 8513 or the EN 1044 standards. Or the alloy’s name may contain the temperature as part of its description, for example Indalloy 281, this being a eutectic alloy having a single melting temperature of 281°F or 138°C. The use of the Fahrenheit temperature scale in the description usually means that the manufacturer is from America, where the Fahrenheit scale is standard. Where a manufacturer produces a range of alloys in a series it makes it relatively easy to pick various alloys from the range so that soldering in a sequence or step soldering can be performed in confidence, without disturbing the previously made joints. This very useful technique will be covered in detail in a later chapter.

THE DEFINITION OF AN ALLOY

The term ‘alloy’ describes a mixture of two or more pure metals. The most common feature of an alloy is that its melting temperature is always lower than any of its constituent parts. Solders use the differing alloying contents mainly to determine the melting characteristics of the solder. Alloys are not confined to soldering and brazing, but have many uses, such as brass made from copper and zinc, bronze made from copper and tin, and of course the world would be a different place without the steel alloys that are commonly used. Alloys have many uses in producing properties in materials, such as hardness and wear-resistance, which the parent materials do not possess.

ALLOY MELTING TEMPERATURE

A feature of an alloy is that it does not usually go directly from a solid to a liquid, as does a pure element such as copper, although there are exceptions to this. Two terms are used in defining the melting and solidifying of an alloy – these are the liquidus and solidus points.

Liquidus

The liquidus point in an alloy describes where the whole alloy is deemed to be a homogenous liquid without any remaining solid crystals from any of the constituencies of the alloy. This is the upper temperature quoted for a particular alloy. For example, a 5 per cent silver (Ag), 95 per cent tin (Sn) soft solder is quoted in data charts as having a melting temperature range of 221–235°C; in this case, the liquidus point is 235°C. As the liquid alloy cools down, the liquidus point marks where crystals once again start to form within the liquid alloy, with the amount of crystals increasing as the temperature falls. Once cooled to the solidus point, the alloy is once again a solid.

Solidus

The solidus point is the start of a range at which an alloy will melt, as different crystals within the alloy will melt at differing temperatures. As in the example above, the lower figure of 221°C given in the range for that particular soldering alloy is the solidus point. The alloy will not be completely melted until the liquidus point is reached; this is when the whole alloy is a homogenous liquid.

The difference between the liquidus and solidus points can be small or relatively large, depending on the alloying materials. Within the range of the two points, the alloy will become a mush, not unlike melting snow, and is neither completely solid nor a liquid. Normally during soldering and brazing operations it is nothing more than an inconvenience, but movement of items being soldered during this phase will possibly result in a dry or porous joint. However, as will be seen later in Chapter 8, this mushiness can be used to our advantage. For example, when lead loading an automotive body panel with body solder, if the alloy is kept between the solidus and liquidus points, it can be shaped and contoured, not unlike plastic. But of course should a little bit more heat be applied, so that the liquidus point is reached or superseded, the body solder will be all over the floor!

Liquation

As stated above, generally the solidus and liquidus range is nothing more than an inconvenience while soldering. However, with some alloys that have a large temperature range and particular alloying elements a problem called liquation can occur. This is where on reaching the solidus temperature, as the alloy begins to melt, the prolonged heating cycle can cause the alloying elements to separate. The ones with the lower melting temperature melt first, leaving crystals of the remaining alloys behind. Once this has occurred, the only effective remedy is to allow everything to cool and then clean off the separated constituents of the alloy, and the inevitable surface oxides, and start all over again. In order to prevent liquation, the best solution is to complete the joint as quickly as possible by rapid heating of the items being soldered or brazed through the solidus and liquidus points. Apart from the unsightly appearance of a joint where liquation has occurred, the resultant joint will possibly be brittle and porous and thus unreliable in service.

Eutectic Alloys

The exception to an alloy with both a solidus and liquidus point is a eutectic alloy. This type of alloy has a single melting point, with the solidus and liquidus points being effectively one and the same, as in a pure element such as iron or copper.

Although not connected to soldering, it is of interest to note that low-temperature eutectic alloys are used in the fusible plug within automatic fire-prevention sprinklers, whereby an alloy with a m...