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Metallurgy

Union sealed

Unprecedented technique makes it possible to solder alloys of titanium and steel for use in the aeronautical industry

MIGUEL BOYAYANLaser soldering: high-energy source does the soldering with precisionMIGUEL BOYAYAN

The manufacture of aircraft and the production of equipment for the petrochemical industry have won a new technology that is going to make it possible to solder titanium and steel, two materials that are important for these areas and, at the same time, chemically incompatible. The new method is based on the use of a nickel alloy between these two materials and on the employment of laser soldering. The innovation is important, because it will make it possible to expand the use of these materials in the production of parts of the fuselage, turbines and landing gear of aircraft, besides making tubes and storage tanks in the chemical and petroleum area. The novelty is a result of a partnership between researchers from the Institute of Nuclear Energy and Research (Ipen), the Technological Center of the Navy in São Paulo (CTMSP) and the Aerospace Technical Center (CTA), which has already filed for a patent for the new process with the National Institute of Industrial Property (INPI).

“It is probably a question of a worldwide innovation. As far as we know, no group in Brazil or abroad has managed to solder these two materials successfully”, says physicist Milton Sérgio Fernandes de Lima, from the CTA’s Advanced Studies Institute (IEA). According to the researcher, titanium and its alloys are used increasingly as structural materials in many industrial applications, as a replacement for steel and aluminum. The advantages of titanium lie mainly in the fact that it is an inert material that has a surface layer that is impermeable to various acid and alkaline substances, besides fluids from the human body. This makes it more resistant to corrosion, and it is biocompatible.

“Titanium also has an excellent relation between mechanical resistance and weight, and its alloys have replaced aluminum when the operating temperature is located above 130°, at the moment at which this material can no longer be used”, he says. “However, one obstacle to the widespread use of titanium as structural material lies in its low resistance to wear and tear. To make good this shortcoming, alloys need to be employed, with better properties relating to friction, wear and fatigue, such as steel.” Nickel was chosen as intermediary material because it has properties that make it metallurgically compatible with steel and titanium.

Until now, the only industrial way of joining these two materials has been by the use of rivets, small cylindrical pieces of metal similar to screws. Mechanical junction with these pieces, though, shows serious problems that result from the perforation of the structure. Besides causing tensions in the material, the rivets deform the region around the hole. This means that there is a reduction in the lifetime of the junction. The new technique developed by the Brazilian researchers solves these problems. With the use of a thin layer of nickel, of 0.1 to 0.3 mm in thickness, between the parts of steel and titanium, and the application of a pulsed laser, developed at Ipen, it is possible to solder the two materials.

“Actually, the mechanical junction of titanium and steel was already known. Our innovation was to use a high energy source, like the laser, to do the soldering”, says mechanical engineer José Roberto Berreta, from the Technological Center of the Navy in São Paulo. “With the laser, it is possible to control the soldering parameters better, like the focusing of the light beam and the rate of thermal intake, which is very difficult to do in other processes.”

After demonstrating the technical viability of obtaining the soldering in test samples, the researchers are working on the development of two prototypes of interest to industry, to corroborate the success of their discovery. The first demonstrator will be an electronic circuit encapsulating device, made up of a dome of stainless steel on a thick (from 1 to 3 mm) titanium base. This device has a widespread use in the aeronautical industry and is usually fixed to the fuselage of the plane. The second prototype is a nozzle made with a diameter of 1.27 cm in stainless steel and a diameter of 0.6 cm in titanium, for use in sectors of the chemical and petrochemical industries. “We hope to have the two prototypes ready early this year”, says physicist Wagner de Rossi, from the Lasers and Applications Center at Ipen.

Technology transfer
The project to develop this new laser soldering technique was able to rely on support from FAPESP, and it was carried out in the ambit of the Millennium Factor Institute (IFM), an organization supported by the Ministry of Science and Technology that brings together 600 researchers from 31 research groups, with Ipen and the CTA  amongst them. The IFM’s profile of work is focused on research into manufacturing aimed at the needs of Brazilian industry. According to Milton Lima, of the CTA, to start with, the researches for welding steel and titanium were purely theoretical. “I wanted to study the metallurgical reactions that occurred between iron and titanium. It was a scientific work, but I soon realized that it had a strong commercial appeal”, he says.

“We are optimistic as to the applicability of this new technique.” In May 2005, Lima presented the results of the project at the 38th International Production Systems Seminar of the Collège International pour la Recherche en Productique (CIRP), held in Florianópolis, Santa Catarina. “It is one of the most prestigious international congresses in materials processing, and our studies were very well received”, says Lima.

The objective of the group is to license one or more companies to exploit this technology. “We intend to make a transfer to the industrial sector, but we have not yet started the contacts with companies. First, we are going to prepare the demonstration prototype. We imagine that the main companies interested in the process will be from the chemical, petrochemical and, in particular, aerospace sector, since the fuselages will probably have their raw material changed from aluminum to titanium.” This is an area in which the requirements of tests and certifications are extreme. “And the existence of a process that successfully solders this material to steel will be fundament.”

The Projects
1.
Transformations of phases in the iron-titanium system induced by high power lasers (nº 02/10009-0); Modality Regular Line of Research Grants; Coordinator Milton Sérgio Fernandes de Lima – IEA-CTA; Investment R$ 33,275.00 and US$ 835.23 (FAPESP)
2. Laser soldering process between steel and titanium; Modality Millennium Factory Institute; Coordinator Milton Sérgio Fernandes de Lima – IEA-CTA; Investment R$ 18,500.00 (IFM-MCT)

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