The present invention relates to an alloy for use in friction materials, such as a precipitation hardened aluminum alloy and, in particular, to a precipitation hardened aluminum alloy in the form of chips, fibers or powder to be used as a raw material in formulations of friction materials for braking of vehicles or industrial machines.

Background of the invention Metallic fibers have emerged as the dominant fiber in the semi-metallic class of materials. These fibers are produced by drawing serrated knives along the surface of a metallic rod to form thin filament of metal. Metallic fibers can also be in the form of finely chopped wire. Regardless of the method of production, metallic fibers are added to friction compositions to improve many performance aspects including enhancing structural reinforcement, improving wear resistance, and increasing the thermal transport characteristic of the lining or pad. Metallic fibers at the surface of a friction material composite aid in the formation of load carrying plateaus and provided beneficial wear enhancement, as well as increases in the real contact area. The creation mechanism was described as an accumulation of wear debris particles lodged behind the fiber. The particles begin to accrete together to form a plateau that grow in size and remain until the fiber is worn away.

The pressure distribution across the surface of the friction material is altered, which impacts frictional performance as well. This process can also be expected to occur with any fiber that is thermally stable at the operating temperatures developed during use. The more thermally stable (not melting or not dropping in their mechanical properties) will be the more adequate.

Steel fibers are perhaps the most common form and are extensively employed in semi-metallic and low steel materials, but copper, brass, and other non-ferrous metallic fibers are also used in the other material types, like NAO, to a great extent. They provide wear resistance while maintaining the friction level during operation at elevated temperatures, but steel fibers have high wear rates, especially of the mating member.

The use of copper in modern friction material composites is due to a multiple of reasons. These include beneficial improvements in the thermal conductivity of the Friction Material ; reduction in the wear of the friction material and the rotor; an overall improvement in frictional performance and coefficient of friction stability, which is most notable in the area of fade resistance; beneficial improvements in

NVH; improvements in integrity when fibrous morphology used, with also the excellent thermal conductivity of copper helping to achieve a more efficient/uniform cure of the final product. The benefits of brass fiber are similar to copper and, in many cases, the metals are described interchangeably. The role of brass is similar to that of copper, and brass provides improved thermal dissipation because of the favorable conductivity of the metal. Any metallic material or alloy can potentially be used as a metallic fiber in friction material. The importance of metallic fiber additions to friction materials continues to be a means of imparting structure while simultaneously improving thermal transport. The only limitation is based on the ability of the metal to be drawn into wire or shaved to form wool.

Regarding Copper, there is an emerging trend for some Friction Materials in some geographic areas, in particular in the USA, to move to lower Copper inclusions, due to cost and also environmental reasons. Aluminum fiber and powder has been seen as one of the potential replacement materials for copper and copper based alloys in friction materials. But there are some concerns with aluminum being used in friction material, especially in disc pads applications. The commercial aluminum fibers are commonly made with pure aluminum or with Al-Mg alloy (wrought wire), and are processed by drawing and then shaving. From studies of the surface temperatures reached by sliding solids, it was determined that certain metals such as aluminum will undergo an exothermic reaction. The exothermic reaction elevates the interface temperature and can lead to hot spots with temperatures in excess of 2000°C. The formation of aluminum oxide is an exothermic reaction. By increasing the temperature at the interface makes the fiber less effective in evacuating heat from this area and contributes to a decrease on the friction level at high temperatures and/or high pressures.

This effect can be seen in AK Master results, when replacing in volume basis copper powder by aluminum and Al-Mg alloy fibers in a commercial NAO, especially in the effectiveness sections. This effect produces a drop in the coefficient of friction. A K Master is the test program used by the brake industry to measure and develop brake pads for use and assesses their performance at the more extreme values of braking. It evaluates "Green" performance, Bedding, Performance v Speed, Cold stop, Motorway stops, Fade, Recovery, Performance v Temperature and Hot

Performance.

An additional concern is in this case, the aluminum oxide served as a hard abrasive and led to issues relative to abrasion and wear. These are concerns that prevent the general acceptance of the aluminum alloys by the friction industry as alternative to copper and copper-based alloys.

Therefore, the objective of the present invention is to provide a new alloy for friction materials that can be processed in form of fibers, powder or chips, with a unique combination of properties that minimizes the weaknesses of traditional aluminum alloys.

Description of the invention With the alloy of the invention said drawbacks can be solved, presenting other advantages that will be disclosed hereinafter.

The alloy for friction material according to the present invention comprises - aluminum in a percentage in weight between 65% and 95%,

- tin in a percentage in weight between 3% and 20%, and

- manganese, iron, copper and/or nickel, i.e. at least one of manganese, iron, copper or nickel, or a mixture of some or all of manganese, iron, copper or nickel.

Preferably, the alloy according to the present invention comprises aluminum in a percentage in weight between 77% and 87%, and tin in a percentage in weight between 5% and 10%.

According to preferred embodiments, the alloy according to the present invention can comprise also:

- manganese in a percentage in weight between 0% and 10%, preferably between 1 % and 5%;

- iron in a percentage in weight between 0% and 5%, preferably between 1 % and 3%;

- bismuth in a percentage in weight between 0% and 5%, preferably between 0.5% and 2%;

- zinc in a percentage in weight between 0% and 10%, preferably between 0% and 3%;

- copper in a percentage in weight between 0% and 10%, preferably between 0.35% and 7%;

- nickel in a percentage in weight between 0% and 2%, preferably between 0.05% and 1 %;

- zirconium in a percentage in weight between 0% and 2%, preferably between 0.1 % and 1 %;

- silicon in a percentage in weight between 0% and 5%, preferably between 0% and 2%; and/or

- titanium in a percentage in weight between 0% and 2%, preferably between 0% and 0.5%.

Furthermore, the alloy for use in friction materials according to the present invention is preferably in the form of chips, fibers or powder.

The alloy for use in friction materials according to present invention at least provides the following advantages:

- The alloy can be processed to have the correct shape and particle size distribution to be well mixed and process in a friction material; - The presence of well distributed of high melting point intermetallics, in this case aluminides of Cu, Mn, Fe and/or Ni, and especially important are the binary AI ₂ Cu, FeAI ₃ , MnAI ₆ and the ternary AI ₁₃ (Fe,Mn) ₄ and AI ₆ (Fe,Mn). The hard intermetallics have a role of strengthening the alloy and improving the wear resistance. - The presence of well distributed low melting temperature segregated phases, especially Sn and/or Bi, that contributes to the behavior by lubricating the interface and absorbing part of the heat generated at the interface when melt, being the melting a endothermic reaction. Description of the drawings

For a better understanding of what has been disclosed, some drawings are attached in which, diagrammatically and only as a non-limitative example, one embodiment is shown.

Fig. 1 is a diagrammatical view of one fiber made from the alloy according to the present invention;

Fig. 2 is a diagrammatical view of a friction material comprising a plurality of fibers made from the alloy according to the present invention; and

Fig. 3 shows some graphics and charts of the maximum temperature of the working surface of the disc during the effectiveness section 4.3, 4.4 and 4.5 of AKM test done in a commercial NAO with:

a) 5% vol Cu powder;

b) 5%vol pure Aluminum fiber;

c) 5%vol commercial Aluminum Alloy (AI-4Mg) fiber;

c) 5%vol alloy A according to the invention (example A according to the following table);

d) 5%vol alloy B according to the invention (example B according to the following table); and

e) 5%vol alloy C according to the invention (example C according to the following table)

All the other components are kept constant in the formulation. Description of preferred embodiments

Firstly, the exact composition of three non-limitative examples of the alloy for friction material according to the invention is disclosed in the following table:

Element (wt%) Example A Example B Example C

Aluminum (Al) 78.6 85.3 86.4

Tin (Sn) 8.0 8.0 6.0

Copper (Cu) 6.4 0.5 0.0

Bismuth (Bi) 0.8 0.5 0.0

Iron (Fe) 0.9 1 .0 0.0

Nickel (Ni) 0.3 0.1 5.3

Titanium (Ti) 0.2 0.0 0.0

Zirconium (Zr) 0.1 0.6 0.6

Manganese (Mn) 0.6 4.0 1 .8

Silicon (Si) 1 .8 0.0 0.0

Zinc (Zn) 2.3 0.0 0.0 It must be pointed out that Examples A, B and C in the graphics of Fig. 3 are identified as RIMSA example A, RIMSA example B and RIMSA example C, respectively.

The alloys have the same type of microstructure, and show a similar behavior when tested in the same friction material as copper replacement but different from the pure aluminum or traditional aluminum alloys (in this case AI-4Mg).

This microstructure can be seen in Fig. 1 , which is a diagrammatical view of a particle (in this case a fiber) made from the alloy according to the present invention, identified generally by numeral 1 , showing intermetallic phase particles, identified by numeral 2, and low melting temperature segregated phases, identified by numeral 3 in a matrix of aluminum, identified by numeral 4.

This alloy is preferably processed to be transformed in particles with the right size and shape (specifically fiber, chip or powder) to be used as one of the raw materials in a friction material. In the case of fiber or chip morphology, this is achieved by the microstructure that allows the alloy to break on the right size under mechanical cutting forces.

As an example of the complete process follow by the applicant, the alloy is melted in a cupola and then poured at a temperature between 720°C and 850°C into a metallic mould to form a billet. Once the billet is cooled down, it can be machined (for example in a CNC lathe) with the right parameters to obtain fibers. Shape and size is a function of chemical composition and machining parameters.

The following table shows the typical particle size distribution (in percentage) attained with the material according to Example 1 with different machining conditions.

The machining conditions are the following:

CONDITION 1 : Speed 470m/s - Feed 0.05mm

CONDITION 2: Speed 425m/s - Feed 0.03mm Sieve size in mm (according to ISO 3310- Condition 1 Condition 2 1 :2000)

0.850 0 0

0.425 97 15

0.150 3 84

< 0.150 0 1

The fibers can be subsequently milled to adjust the particle size distribution. Typical particle size distributions (in percentage) of the obtained particles with this method (condition 1 ) are show in the following table.

Milling conditions are the following:

MILLING 1 : Mill sieve 2 mm

MILLING 2: Mill sieve 1 .5 mm

The fibers obtained can be mixed with all the other components of the formula to obtain a friction material. The fibers 1 (or chips or powder) made from the alloy according to the present invention are well distributed into a matrix of friction material 10, as schematically is shown on Figure 2.

The wear rate of aluminum will affect the extent of the exothermic reactions on the interface. By minimizing the wear rate this effect will decrease. A material with high mechanical properties will behave better than a soft material in terms of wear. The principal goal of the introduction of Mn and such transition metals as Fe, Cu, Ti, Zr,

Ni, and Zn is in achieving an additional strengthening due to the formation of anomalously supersaturated solid solutions under the conditions of non-equilibrium solidification.

The dispersoids of aluminides themselves enhance strengthening somewhat, especially at elevated temperatures. Nickel, iron and manganese are eutectic- forming transition metals. The presence of well distributed hard and thermally stable aluminides (intermetalic) particles limit the wear of the alloy particle, controlling the release of Al to the interface and then controlling the level of the exothermic reaction. When a low melting point phase (like Sn) is included in the material, a part of the energy will be dissipated in melting this phase, because melting is an endothermic process. Tin is distributed in aluminum matrix as a separate phase in form of a reticular (network) structure along the edges of aluminum grains. During melting process the temperature of this phase will be keep nearly constant, helping to control the overheating of the interface and having a extra heat dissipation effect.

As a result of these unique properties, the new alloy developed attains a behavior compared with traditional aluminum based fibers in the critical sections of AKM test. Note the close thermal profiles between copper-based formulations and the alloy fiber formulations according to the invention, and the differences with the ones using traditional aluminum-based products (Fig 3). The maximum temperature attained at the disc working surface during each snub is much lower when the new alloys are used in the friction material formulation than when other commercial Al based products are used the attained temperature is much higher. This can be attributed to limiting the exothermic reaction that occurs during the oxidation of the Al debris.

Pure aluminum fibers or commercial Al-Mg fibers have a high wear even at low pressures, causing the exothermic reactions and then the increase on the disc surface temperature. Because of their microstructure, the wear of the fibers of the new alloys is limited and there's also the lubricating and cooling effect of low melting point segregated phases, limiting the exothermic reactions and then keeping the disc surface at lower temperature during the effectiveness tests. The new alloys behave in this sense closer to the copper-based formulations, where the disc surface temperature is keep at even lower temperature. Even though reference has been made to specific embodiments of the invention, it is apparent for a person skilled in the art that the disclosed alloy is susceptible of variations and modifications, and that all the details cited can be substituted by other technically equivalent ones, without departing from the scope of protection defined by the attached claims.