Brass fittings: the choice of the alloy

Brass is a metal alloy whose main components are Copper (Cu) and Zinc (Zn). Inside the family it is possible to distinguish binary brasses, composed only of copper and zinc, and ternary brasses, which have another alloy element besides the two main ones. The percentage of zinc influences the structure of the brass that can be obtained:
- Zn <36% -> phase α: the structure is cubic with centered faces, like that of copper. Excellent cold workability and good hot workability.
- 36% phase α-β: the phase α maintains the cubic crystal structure with faces centered while the phase β has a cubic structure with a centered body. This type of brass is mainly hot workable.
- Zn> 45%: this alloy is rarely used because it has little practical interest.
The brass fittings are a widespread product worldwide and are central in the production of Gnali Bocia both for gas fittings and for incompressible fluids such as water. The use of brass for the realization of these components is motivated by its mechanical characteristics (strength, ductility, malleability), resistance to corrosion, abrasion resistance and an important characteristic such as antibacteriality. Compared to pure copper it has better characteristics of hardness, resilience and fusibility.
 

For the realization of brass fittings, some machining techniques must be used for chip removal, such as turning and milling. The workability of copper-zinc alloys is good, but the toughness leads to the formation of a very long chip and this can be problematic during processing, as its evacuation is not easy. To improve this aspect, a quantity of Lead (Pb) is added between 2% and 4%. This element, insoluble and extraneous to the crystalline structure of the alloy, tends to be placed on the grain boundary. In this way the chip becomes very short and almost dusty, obtaining at the same time a double advantage:
1. Increases the ease of chip removal during processing;
2. Less wear on the tool heads, as the alloy is on the whole more tender and easily workable.
We must pay attention to the addition of lead since the excess in the quantity of this element determines the obtaining of a too soft alloy.
 

Besides the addition of lead, the brass fittings need other alloy elements to obtain fundamental properties for their operation:
* Manganese (Mn) and Stagno (Sn): they serve to increase the resistance to corrosion, an important feature for the fittings;
* Iron (Fe): it increases the breaking load and, therefore, the resistance, as it works as a refiner of the crystalline grain. It does not present an improvement in corrosion resistance;
* Aluminum (Al): increases resistance to corrosion and abrasion;
* Antimony (Sb) and Arsenic (As): they serve to inhibit dezincification, a common problem in brass;
* Nickel (Ni): improves mechanical properties and resistance to corrosion;
* Silicon (Si): it acts as a deoxidizer and favors the formation of β phase. The limitations on this element derive from the effect it has on the structure, since it tends to modify it;
The addition of a third element to the Cu-Zn alloys can be considered equivalent, to the effects of a possible structural modification, to the addition (or subtraction of zinc) in quantities proportional to that of the element itself. The proportionality constant is called the coefficient of equivalence and varies from one element to another. These coefficients are also called "of Guillet", from the name of the one who made them., from the name of the one who made them. The coefficients of equivalence for the elements mentioned above are:
- Manganese (Mn) => 0,5;
- Iron (Fe) => 0,9;
- Stagno (Sn) => 2;
- Aluminum (Al) => 6;
- Silicon (Si) => 10;
- Lead (Pb) => 0;
- Nickel (Ni) => da -1,1 a -1,7.

Silicon is added in small quantities (maximum 1-2%) because it has a high coefficient of equivalence and therefore determines an excessive change in structure. In fact, an addition of 2%, being the coefficient equal to 10, is equivalent to an increase of 20% of zinc in the alloy. Nickel has a negative coefficient: its addition is equivalent to the shift towards higher copper levels. Lead has a zero coefficient because you will find it on the grain boundary and does not enter the lattice.
 

As already mentioned in the initial part, the brass is an excellent alloy for the realization of the fitting elements mainly for its mechanical and tribological resistance properties, as well as having an excellent workability with machine tools. Among the most used alloys there are the CW614N and CW617N, both with an average copper content of 58%.
In case there is a turning process for making brass fittings, the alloy CW614N is used. This choice derives from the form of supply of this alloy which usually consists of solid or perforated bars. Conversely, starting from a rough piece, we tend to use the CW617N alloy which is better workable as the lead percentage is slightly higher than the previous alloy.
 

In addition to the correct choice of the brass alloy with the desired elements, the choice of a surface treatment to cover the fitting is also fundamental. Nickel plating, an excellent substitute for chrome plating (a technique prohibited in many countries, given the food applications of certain fittings), is an excellent methodology. With this technique, a surface layer is deposited, which allows to protect the component from wear and also from corrosion, which is possible to meet in a humid environment.
 

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