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Surface properties, microstructures and formability development in Al-strip casting for foils and other novel applications
Development and characterisation of new soft magnetic and getter materials

Projects

CO-AMM-RP 1
Surface properties, microstructures and formability development in Al-strip casting for foils and other novel applications

Strategic decision of Impol d.o.o. – rolling restructuring with passage on continuous cast strip as base for production of foils, means conquesting the technology for producing continuously casted Al-strips and determing parameters of termo-mechanical transforming for foil production, that will have comparable physical-chemical properties with the foils conventionaly produced from hot rolled strips.

Parallelly with the introduction of TRC procedure of Al strips of thickness 6mm between water-cooled rolls and setting up of new highly competitive rolling mill for foils, the developmental work was performing on: a) development on surface properties of strip casting, b) development and introduction of new modified alloys for continuous casting, c) complete microstructure controlling, phase compositions, distributions of intermetallic phases and reformability of strip castings by cold rolling to the final foils products of thickness 6µm, d) development of new products and applications from strip casting.

The crystalization speed and simultaneously hot rolling between water-cooled rolls have the influence of alloying elements (Fe,Mn,Si) on αAl supersaturation and on size, distribution and chemical composition of binary and multicomponent intermetallic phases. With the aim to achieve equally fine-grained microstructure containing equally distributed phases, which is the basic for achieving required physical-chemical properties of foils, our research has been focused on the influence parameters, on the quality of casted strip surface and microstructural characterization of as-casted strips and on different thermo-mechanical treatments of alloys. We followed the microstructural development after heat treatment and cold rolling, where the knowledge about the influence of annealing temperature and degree of deformation is important on: recrystalization kinetics, microstructure (size, distribution, type and amount of phases), workability, anisotrophy, mechanical properties, heat conductibility and surface quality of foils.

The results of entire Research&Developmental team are handed in 7 final reports R&D work, 20 know-how technologies on the continuous casting TRC field and in 17 know-how technologies on the cold work field. We achieved mechanical characteristics by EN 546 1-4, required surface quality and thermostability. I would like particularly expose know-how production for blister foils made from thin casted strips, which is patent protected.

Team constituted of researchers from IMT, NTF- University of Ljubljana, private researcher V. Kevorkijan and Impol, worked very successfully, since research findings were immediately checked by technologically possibilities in Impol and of long standing experience technologists on the filed of aluminium alloys forming. Bachelor degree students and two students which have finished Ph.D. study participated in this project.

Important effects of realized program of project CoE-AMM RRP1 are:


CO-AMM-RP 2
Development and characterisation of new soft magnetic and getter materials

deals with 4 themes:

Theme 1:
Development of soft magnetic powders

In first phase of the project a whole spectrum of soft and hard magnetic materials were developed by using ultra high solidification procedures and therefore amorphous or nanocrystalline structure was achieved, which assure unique magnetic performances. By atomization procedure soft-magnetic nanocrystalline powders on the base of Fe, Fe-Si, Fe-Si-B-Ni in Fe-P-Si-B were produced. Melt spinning technique was applied to manufacture Fe-Nd-B ribbons. Second phase of the project involved the production of bulk soft-magnetic materials by rapid solidification procedure by casting into the copper mold. By this technique some complex alloys as Fe-Si-B-P-Ga-Al, Fe-Si-B-Ni, Fe-Si-B-Ni-Cu were produced. Appropriate combinations of alloying elements enable amorphous solidification at moderate cooling rate. In third phase of the project the SMC technology was developed in total. SMC technology means execution of appropriate isolation layer around each particle, further compaction of powder together with organic or inorganic dielectric resin, which binds together the particles and isolates them from each other not to be in contact. Beside SMC technology development for the production of soft-magnetic stator-cores, similar technology for the production of multi-pole magnetic hard-magnetic rotor-cores was developed. Tiny particles of crushed Nd-Fe-B ribbons were equally distributed in polymer resin. Effects of particle size, share of polymer resin and compaction density were analyzed as well.

Strategy
Previous material development for micromotor developments was oriented above all towards low and medium frequencies. Due to the needs of partners and industrial requirements the research of motor development is oriented towards high frequency range (MHz). Therefore, nanocrystalline soft-magnetic powders of only a few µm size and perfect round shape with superior isolation layer must be developed. Commercially available powders on the base of pure iron do not achieve desired permeability and Q-factor. The development will be oriented in the production of powder alloys, which will have particle size as low as possible.

Theme 2:
Development of high permeability non-oriented electrical steel sheets.

Advanced electrical steels must have high permeability and low core loss with low cost production. The quality of electrical steels depends on chemical composition, influenced by alloying elements such as Si and Al and proper technology from the production of steel melt (secondary metallurgy) using the VOD process, hot and cold forming to final heat treatment; annealing for decarburisation, recrystallisation and grain growth. The production of high permeability non-oriented electrical steels for economic electro motors needs a lot of knowledge and skills of researchers and experts from the industrial environment; besides interdisciplinary applied knowledge and skills, fundamental knowledge is also required. The project joined researchers from the fields of steelmaking, hot and cold forming, characterisation of materials, composition, mechanical properties, microstructure, texture, magnetic properties and information technologies, databases and the preparation of suitable software.
For small and medium electro motors the high permeability of electrical steel sheets is very important because the loss in the copper (coil) relative to total losses is high. High permeability is important due to the decrease of current needed for magnetisation, and this current is a source of losses. ACRONI is producer of non-oriented electrical sheets of F330 up to F800, which are largely used for small and medium electro motors while, on the contrary, in EU countries and the USA the trend is to use high permeability electrical steel sheets M45 to M65. Project research led to development and production of high permeability electrical steels along with the simultaneous optimisation of chemical composition, evolution of a favourable texture and findings on the influence of inclusions and impurity elements on magnetic properties. The results are transferred to Slovenian industry to serve as a base for the development and introduction of high permeability electrical steel to industrial production.

The development of high permeability electrical steels in an industrial environment is very expensive and demanding and, for that reason, it is more suitable to manufacture the model alloys in the laboratory at the IMT because it has all the equipment required for FeSi alloys synthesis from very pure base materials. After vacuum induction, melting follows casting into ingots and immediately hot and cold forming, then rolling to a final thickness of 0.35 mm. For the required magnetic properties the samples are annealed in a gas mixture of hydrogen and water vapour for decarburisation, recrystallisation and grain growth. Upon recrystallisation, the faceting and reconstruction of the surface occurs.

High permeability electrical steels are made from no alloyed charge of controlled chemical composition and alloying with Si and Al. The alloys FeSi always contain residual impurities in very small, trace amounts. During the final heat treatment the impurity elements segregate to the surface and interface and affect the texture’s evolution and magnetic properties. Our investigations have shown that some surface-active elements in small contents (up to 0.05%) have a positive effect on the evolution of the sheet texture. For this reason it is believed that knowledge of the mechanism and kinetics of the segregation of residual impurities, in which several elements compete, lead to the data needed for obtaining non-oriented electrical sheets with tailored properties. An investigation at the nano level is necessary to understand the mechanism and the phases of the surface reconstruction, which are essential for the formation of a determined type of texture.

Theme 3:
Development of analytical methods for determining the sorption characteristics of new getter materials.

To maintain ultrahigh vacuum (UHV) to extreme high vacuum (XHV), non-evaporable getters (NEGs) working on chemical pumping principle are incorporated both in large dynamic vacuum systems (particle accelerators, synchrotron radiation light sources, nuclear fusion and physics experiments) and small static vacuum systems (Field-Emission Displays – FEDs, Micro-Electro-Mechanical Systems – MEMs, Image-Intensifier Tubes – I2Ts, Chanel Photo-Multipliers – CPMs). Due to special requirements at both large dynamic and small static vacuum systems, the incorporation of additional H2 pumps is based on the use of NEGs: in first case due to several expositions to the ambient atmosphere, and in second case due to lack of room. Materials exhibiting large H affinity at room temperature (RT) as well as high solubility for O at elevated temperatures are elements from the IVB column (Ti, Zr, Hf) of the Periodic System.

According to the ASTM F 798-97 (2002) Standard Practice Recommendation, the characterization of NEGs starts with getter activation followed by determination of getter pumping performance. NEG activation takes place at elevated temperatures in UHV. According to this standard, NEG sorption characteristics are determined at RT using the dynamic method. Its main characteristics are as follows: use of hot-cathode ionisation gauges as total pressure gauges in the gas manifold and in the test chamber, variable gas flow through the known conductance, constant pressure above the getter surface and pumping of non-getterable gases out of the test chamber. The standard gases are CO and H2 as representative of irreversibly and reversibly gettered gases.

The main characteristics of the static sorption method, developed at IMT during the course of CoE RR project, are constant gas flow and variable pressure above the getter surface. It is essential that an inert vacuum gauge such as spinning rotor gauge (SRG) is used for both determining gas flow by the rate of pressure rise method and measuring pressure above the getter surface. For the static sorption method, it is characteristic that during the sorption test non-getterable gases (noble gases and CH4) are able to accumulate in the test chamber. Therefore, the test gas flow is interrupted few times in order to measure the increase of background pressure. To determine NEG sorption properties by the static sorption method, a new measurement UHV system has been built up, equipped with SRG, variable leak valve, UHV valves, residual gas analyser (RGA) and dry turbomolecular pumping unit.

There is a big advantage of the developed static sorption method over the widely used standard dynamic sorption method, since total pressure measurements are performed using the inert SRG in static mode without affecting the working atmosphere. In addition, the static method enables discrete monitoring the background pressure due to accumulation of inert gases throughout the test and, at the end of test, analysis of non-getterable gas composition using RGA in dynamic mode.

Theme 4:
Improvement of analytical methods for determination of hydrogen content in metals and alloys

We upgraded and improved methods to precisely determine the hydrogen concentration in stainless steel and its kinetics of evolution at elevated temperatures. Special emphasis was devoted to the determination of the energy levels that could be occupied by hydrogen in the solid metal at moderate temperatures. These could be determined from the experimentally measured hydrogen release kinetics at a constant or linearly increasing temperature. Our decision to investigate stainless steel relies on its use in several modern devices: ultra high vacuum components, hydrogen storage vessels and planned fusion reactors, like ITER. In fusion reactors, one of the unsolved issues is tritium retention in all surfaces exposed to plasma during the shot and subsequent exposure to gaseous tritium that was not spent in the fusion. Our approach was to instead apply deuterium whose handling is safe comparing to tritium. Precise long-term pressure sensing and subsequent mass spectrometry enables to follow the isotope exchange reactions. Regular hydrogen dissolved in the bulk and chemically bound water at the surface both enable exchange with deuterium or tritium. This reaction thus represents an important channel to enrich the surface concentration of tritium which can then influence the diffusion rate into the bulk.