The main goal of the work was to produce ecological brass that would exhibit better plastic properties and machinability, as well as a lower content of harmful chemical elements that negatively affect the quality of drinking water. Improved plastic properties are particularly important in hot forging processes, for example in the production of plumbing fixtures for drinking water. Nowadays, in order to be able to fully fill the commercial brass, such as ‘Ecobrass’, void, it is imperative to raise the forging temperature to over 800 °C, which, in conditions of traditional forging efficiency (30–50 pieces/min), triggers rapid processes of thermoplastic wear of the forging tools. As a result, the forges are compelled to reduce their efficiency in order not to damage the tools. This is one of the major factors influencing the market prices of the fixtures. For flashless forging, it is required to use forging tools with superior strength in comparison to their traditional forging analogs [
1,
2,
3]. During flashless forging, there may be difficulties filling the forging impression, for example the narrow ribs or sharp corners, which may then lead to excessive loading and cracking of the material [
4,
5]. The results of the research on lead-free alloys have been presented in experiments [
6,
7,
8] on the influence of selected elements on corrosion, microstructure and machinability.
The issue of production efficiency is also very important during the machining of forged fixtures. The better the machinability, the greater the adjustability of the machining parameters. When modifying the chemical composition, special attention should be paid to the toxicity of the alloy components. The objective of this paper was to determine the influence of selected alloying agents, as well as the impact that the morphology of the microstructure of developed alloys has on machinability and corrosion resistance, which in turn determines the washout of harmful elements into drinking water. The solution to the problem that is being applied at this point in time is the production of a variety of silicon-containing brasses with more complex chemical compositions [
3]. An example of such a silicon brass developed and marketed in recent years is EcoBrass
® (Tokyo, Japan), which is manufactured both as a casting alloy in the form of pigs and as a product subjected to plastic deformation. It is characterized by very good castability (considerably better than the castability of CuSn5Zn5Pb5), as well as high strength, favorable machinability (thanks to the Cu8Zn2Si and Cu4ZnSi stages in the matrix of the α phase of the microstructure), very good resistance to dezincification and stress corrosion and, above all, by very good suitability for hot plastic deformation. Despite their many advantages and very encouraging descriptions and specifications provided by their manufacturers, currently available commercial lead-free alloys have many flaws: poorer castability leading to significantly worse casting tightness and their corresponding high price (according to various sources, between 25% and 50% higher than that of lead-containing alloys). In the case of alloys with added selenium, the evident toxicity of the latter may pose problems in the future. In the case of alloys containing silicon applied in the manufacture of products galvanically coated with chromium, significant problems associated with hard inclusions may arise, too.
The demand for copper alloys with low lead content, or completely devoid of it, in the manufacture of products intended for use as drinking water vessels arises from, among other things, legal and administrative issues in the United States and the European Union. The three fundamental documents pertaining to this are the “Reduction of the Lead in Drinking Water Act” No. 3874 of the US Congress, which was passed in 2011 and restricts the content of lead in drinking water; European Union Directive No. 98/83/EC on the “Quality of Water Intended for Human Consumption”, amended in 2015, and the Common Approach of 2011 adopted by Germany, France, the Netherlands and the United Kingdom (4 MSI) on the “Acceptance of Metals Used for Products that Come into Contact with Drinking Water”, amended in 2016. The last directive defines the array of metals to be used with drinking water. Such materials are copper alloys containing added bismuth and mischmetal as a modifier. They were developed and patented in the USA by Singh [
9,
10] and marketed under the brand name Federalloy
® (Bedford, OH, USA) by Federal Metal Company, Bedford, OH, USA. This group covers over 15 alloys (different types of bronzes and brasses) which are currently quite common in the US and other countries. The appropriate mechanical properties and machinability are achieved thanks to the simultaneous addition of bismuth and mischmetal as a modifier or an equivalent admixture of rare-earth metals. According to reports published by the manufacturer of these alloys [
11,
12], they contain less than 0.10% lead and have technical and technological characteristics similar to that of the lead-containing alloys that have been used until date. They are also distinguished by a better microstructural homogeneity than alloys containing Pb and they exhibit very good mechanical grinding, polishing and galvanic coating characteristics. Copper alloys in which bismuth and selenium are simultaneously applied as additives constitute a separate group of materials. These alloys are the result of an extensive research program implemented by a consortium headed by the American Foundry Society, in response to curbs on the amount of lead permissible in drinking water, which was introduced in the USA [
13]. The alloys have found their commercial applications as a group of alloys initially named SeBiLoy
® (Salt Lake City, UT, USA) and currently sold under the EnviroBrass
® (Salt Lake City, UT, USA) brand name. The simultaneous application of bismuth and selenium additives comes from the fact that their combined beneficial impact on machinability is stronger than the aggregate total of their separate effects [
14,
15]. Bismuth inhibits washouts of selenium into water, which is particularly important considering the harmfulness of the latter and the need to limit its content in drinking water. While the beneficial effects of these alloying agents are being employed, other grades of casting alloys have also been developed [
16,
17]. However, selenium as an alloying agent (in the production of these amalgams) that generates toxic smoke, which is also why it has proven to be more favorable as a bismuth selenide bath [
18,
19]—a compound specially produced by Asarco Company as an additive in the manufacture of EnviroBrass
® alloys. The EnviroBrass
® group of alloys comprises three casting alloys, of which the first two are zinc- and tin-containing bronzes, besides bismuth and selenium, while the third is a brass additionally containing tin and aluminum components. As a result of the quest for alloys that could replace lead-containing bronzes and brasses, work on the application of alloys containing silicon is now at an advanced stage. Studies have shown that silicon brass CuZn14Si4 can be a likely alternative to CuSn5Zn5Pb5 [
20], while silicon bronzes like CuZn5.5Si4.5 and CuZn14Si4 can serve as substitutes for bronzes like CuZn5Sn5Pb5 and CuZn9Sn3Pb7 [
21,
22]. Alloys containing silicon have comparable elevated castability values, as well as greater levels of hardness and strength. They, however, have slightly poorer, yet acceptable, machinability, comparable corrosion resistance, and poorer solderability. They are distinguished by their relatively good suitability for surface finishing treatment (grinding, polishing, slightly poorer adhesion to galvanically applied coatings). The main disadvantage of alternative CuZnSi alloys is their significant higher price, which is mainly due to the lack of sources of cheap scrap that can be used for their production. In view of that, this paper presents the results of research work aimed at finding new alloys that will serve as alternatives to those already present on the market. Most products intended to come into contact with drinking water are manufactured using hot forging technology by means of flash or flashless forging.