Impurities Friend or Foe?

The process of welding is an age old process thousands of years old discovered in the ’Iron Age’ and been refined ever since. To put this statement into perspective the dictionary definition for welding states;

’Fastening two pieces of metal together by softening with heat and applying pressure.’

This simple statement however does not fully describe the different attributes imparted to the weld by the various techniques for applying heat and pressure, from the simple forge to the sophisticated arc welding robots used in the industrial manufacturing lines. The definition also does not eliminate the possibility of soldering or brazing, which are two separate and distinct processes.

Soldering

’Soldering is the process in which a fusible metal is melted to join metallic surfaces, the filler metal must have a lower melting point than the adjoining metals. Metals that melt between 180 and 190’C are the most commonly used.’

Brazing

’A metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal.’

So now we have the basic differences between the three definitions, all of which apply heat but both soldering and brazing use filler metals with a lower melting point than the adjoining metal, unlike welding which uses the same metal. For the purposes of this article, welding includes soldering and brazing.

BONDING

To make a bond between two pieces of metal the molten metal must coalesce, to obtain coalescence there must be a combination of proximity and activity between the pieces being joined, sufficient to cause the formation of common metallic crystals. Proximity and activity can be increased by plastic deformation (by the application of pressure) or by melting the two surfaces so that fusion occurs.

The essential feature of a soldered joint is that a metallurgical bond is produced at the filler-metal/base-metal interface. The solder reacts with a small amount of the base metal and wets the metal by forming intermetallic compounds. Upon solidification, the joint is held together by the same attraction between adjacent atoms that holds a piece of solid metal together.

In both instances the bond/joint is produced by the formation of metallic compounds; their inherent properties dependent upon the base metals used and their related impurities.

ALLOYS

So far only pure base metals have been considered, to understand what an alloy is, its’ industrial applications and its’ chemical complexity we need to understand what a metal is:

Any of a category of electropositive elements that usually have a shiny surface, are generally good conductors of heat and electricity, and can be melted or fused, hammered into thin sheets, or drawn into wires. Typical metals form salts with non-metals, basic oxides with oxygen, and alloys with one another.

Therefore an alloy is

‘a metal made by melting and mixing two or more metals or a metal and another material together.’

An alloy is distinct from an impure metal, in that, with an alloy, the added impurities are usually desirable and will typically have some useful benefit, such as; hardness, shininess, corrosion resistant, melting point, etc. But by definition the act of welding, soldering or brazing and the bonding of two metals will form an alloy, the strength of which will be dependent upon the elements present at the bonding interface including atmospheric gases and surface contamination.

IMPURITIES

In order to comprehend and appreciate the welding/bonding mechanism within metals and alloys, it is necessary to understand the difference between an impurity (something contained within the fabric of the material) and a contaminant (something that is added during the process).

An impurity is an undesirable element or substance commonly or naturally contained in something that lowers the thing’s quality or value, but (depending on its amount) may or may not make it unfit for its intended use.

In comparison, a contaminant is an external agent that is (or gets) added to something and usually renders it unfit for its intended use.

Therefore, an impurity is dependent upon the original quality of the base metal(s) or alloy(s), it isn’t added to the weld but forms an integral part of the joint during the welding process, whereas, a contaminant is added to the weld because of poor working practices.

DISCUSSION

Metals form oxides, salts and alloys when welded, the strength of the weld is dependent upon the bonding process and the bonding process is dependent upon the resulting alloy produced from the base metals and their impurities. The fact that two or more chemical elements can form a metal alloy, simply by welding, is too simplistic; initially, upon melting, the elements must be soluble in the liquid state, upon cooling this state may change.

• If the metals remain soluble when solid, the alloy forms a solid solution, becoming a homogeneous structure consisting of identical crystals.
• If the mixture cools and the constituents become insoluble, they may separate to form two or more different types of crystals, creating a heterogeneous microstructure of different phases.
• However, in other alloys, the insoluble elements may not separate until after crystallization occurs. These alloys are called intermetallic alloys, intermetallic phases reinforce the crystals internally.

So now we can update our information; metals form oxides, salts, and a host of alloys with three different types of microcrystalline structures (homogenous, heterogeneous and intermetallic phases). The quality, repeatability and reproducibility of the welded product is dependent upon the quality, repeatability and reproducibility of the raw materials, their storage, pre and post weld treatment and the welding process itself. Because of the nature of metals, their impurities and contaminants a lack of control in any one area can result in the fabrication of an unreliable product.

CONCLUSION

Welding is a fundamental industrial fabrication process, the type of alloy used dependent upon the physico-chemical demands of the final structure, from buildings to bridges to aircraft. As discussed earlier, the welding process is a complex chemical process, sensitive to impurities and contaminants in the raw materials and the fabrication process itself to such an extent that it can affect the microcrystalline structure of the resulting alloy or produce a different alloy altogether.

The only way to guarantee the success of a welding process is to control every part of the fabrication technique, from sourcing the appropriate raw materials to pre and post treatment of any metals. Bulk solutions for the pre and post treatment of metals can be manufactured and supplied by Waterfall & O’Brien Ltd.

Fortunately the micro-crystalline structure of any metal or alloy that has been welded as part of the fabrication process can be investigated using a series of chemical etchants designed for this purpose. These etchants are specific for the type of alloy being investigated and the type of information required to guarantee the quality of any finished metal product. A wide selection micro-etchants indicated for use for a wide variety of metals and alloys, are given on the EtchantsUK website.