Metallic materials for coins of higher security standards [Materiali metallici per la produzione di monete PIÙ sicure]

This paper describes a patented method to produce coins of higher security standards [1]. Today, in addition to interpersonal exchange, a lot of coin transactions are performed through vending machines. Vending machines use technical means (mechanical and electronic coin validators) for rejecting slugs (counterfeited coins, lower value foreign coins, tokens etc.). Many tests are carried out, among them the alloy test is the most relevant. In mechanical validators coins move along a rail through a magnetic field which induces Eddy currents giving rise to a force opposite to the motion direction: higher the electrical conductivity of the coin, larger the braking. In the case of ferromagnetic materials also magnetic permeability plays a role enhancing the braking effect. In electronic validators there are more coils symmetrically allocated with respect to a runway on which the coin moves (see Fig. 1). They are part of a resonant circuit; when the coin passes through the coils the frequency increases and the amplitude decreases. The paramagnetic metals influence the circuit response through their electrical conductivity, the ferromagnetic ones also through the magnetic permeability. The current density inside the coin exponentially diminishes from the surface inwards, The skin depth d, i.e. the depth where the current density becomes 37% (1/e) of the surface value, is expressed by eq. (1) being f the frequency, μ the magnetic permeability and a the electrical conductivity. On the basis of the tests performed by coin validators different routes can be envisaged to produce safer coins; the present paper focuses the attention on materials with uncommon magnetic-resistive characteristics. Electrical conductivity and magnetic permeability of some metals are reported in Tab. 1. The values of electrical conductivity are quite scattered, in addition this property is affected by crystal defects and impurities, also in very small amounts, as shown in Fig. 2 for copper [2]. Since electrical conductivity can be modified in an extended range of values 13], the measurement of this single property is not a sufficient guarantee to identify the slugs. Table 1 clearly shows that metals exhibit great differences of magnetic permeability (low values for paramagnetic materials, high values for ferromagnetic materials). No metal has intermediate values. This consideration induced us to investigate metallurgical treatments able to suitably modify some alloys available on the market to get values of magnetic permeability in the middle range. The task can be accomplished by joining on macro-scale paramagnetic and ferromagnetic metals (for example by cladding), however the material is not homogeneous along its thickness thus the combination of electrical conductivity and magnetic permeability "experienced" by sensors depends on the frequency f employed in the test (skin depth d is a function of f). Two possible routes have been considered to obtain materials homogeneous on macro-scale: 1. composition tailoring of copper alloys, 2. martensitic transformation induced by thermo-mechanical treatments in some austenitic steels. The magnetic properties of copper can be modified by addition of alloying elements such as iron, chromium and manganese also in very small quantities (tens of ppm). Magnetic-resistive characteristics of such alloys are uncommon and not easily reproducible by counterfeiters. When the content of impurities is of the order of 100 ppm or lower, their homogeneous distribution in the matrix could be a critical aspect in the industrial process of blank production. On the other hand, the solubility of these elements in copper is very low. An amount of impurities above 100 ppm gives rise to the formation of second phases with consequent hardening, a serious drawback for rolling and coining operations. For instance, at temperatures below 800° C about 100 ppm of Cr can be dissolved in the Cu matrix (see Cu-Cr phase diagram in Fig. 3 [4]). If a higher Cr quantity is present in the alloy, a second phase forms and is made of nearly pure Cr. Cr precipitates increase the recrystallzation temperature, refine the grain and harden the metal. Therefore, a careful tailoring of composition is of the utmost importance to avoid an excessive hardening and an inhomogeneous distribution of solute in the matrix. Among copper alloys of particular interest are Heusler alloys, intermetallic ternary compounds (A2BC). The alloy Cu2MnAl, whose structure is displayed in Fig. 4, is ferromagnetic with Curie temperature of 330°C [5]. Details of its electrical and magnetic properties can be found in ref.[6-8]. The other route investigated by us to prepare metals with uncommon magnetic properties regards those austenitic stainless steels, which undergo a martensitic transformation induced by plastic deformation [9]. Austenite (paramagnetic γ phase) transforms to martensite (ferromagnetic α phase); higher the deformation degree, larger the amount of martensite. The transformation can be reverted by suitable heat treatments [10-13]. In the following AISI 304 steel, one of the most used metals exhibiting such phenomenon, has been chosen as example. Fig. 5 shows how the martensite fraction increases with the deformation degree by cold rolling (left) and decreases after heat treatment at 400°C (right). The physical process of austenite to martensite transformation induced by plastic deformation consists of successive steps, which can be described as follows [9]: 1- formation of parallel slip bands, twins and stacking faults (Fig. 6 a); 2- martensite embryos originate from crystal shears at the intersection of slip bands, twins and stacking faults (Fig. 6 b); 3- above a critical size (10nm) embryos become stable blocks of martensite (Fig. 6 c); 4- when the degree o plastic deformation is further increased blocks coalesce forming blocks of larger size. As displayed in Fig. 6, the two phases (austenite and martensite) have microscopic size and are mixed on microscopic scale. In conclusion the relative amounts of martensite and austenite can be suitably changed through plastic deformation and heat treatments, the γ→α′ transformation involves a change of magnetic permeability and the two phases are mixed on microscopic scale. On the contrary of other methods (cladding, bimetallic coins etc.) where metals with different characteristics are joined on macroscopic scale, coin validators detect always the same value of magnetic permeability independently on position and size of coils or frequency used in the test. Both the routes considered here to get materials with uncommon magnetic-resistive properties require industrial equipments and scientific know-how not easily accessible to counterfeiters. In addition, they do not require the introduction of new coin validation devices but only a different calibration of those currently in use.