The development of a new Iron-cobalt soft magnetic alloy was proposed with a smart combination of both high mechanical strength and moderated iron losses. The former allows higher speed of electric generator, increasing the mass power density. The later improves the thermal transfer of dissipated energies, the sustainability of the generator and help then to a further power density increase. In the complete continuation of Aperam RTD efforts these last years, this new material very recently patented has been demonstrated on laboratory scale and - in some few cases of working features – on industrial tools for small quantities.
But this new material is roughly defined as a new alloys family – Iron-cobalt fully processed (i.e. annealed) and workable material – and it needs to be optimised and fully characterized with respect to working solicitations. In the framework of this project, such a high strength new FeCo alloy was defined in terms of processing parameters corresponding to 800MPa yield stress (R0,2) whereas several metallurgical ways will be studied to get lower iron losses than previous know ones (280W/kg at 2T/400Hz for 0,4mm thickness) without a marked decrease of R0,2. Complementary processes such as thin foils assembly, recovery or texturization will be used to reach this later target, with the help of a specialized partner (LPCES).
After optimisation of the lamination structure and of its microstructure, the new material was deeply characterised mechanically and magnetomechanically (including influence of uni- or bi-axial stresses on the magnetic properties) by another specialized partner (LMT). At the same moment some small quantities (300 to 500kg) of the optimised alloy were processed on industrial tools and punched in pieces to provide 2 rotor stacks to the Topic Manager Company.
The equiatomic Fe-Co alloy (with 2%V) – as the AFK502R grade of Aperam Alloys Imphy – was produced to fulfil every requirement in power electrical engineering at low-medium frequency asking firstly and strongly for lightening or volume reduction. Such advantage was allowed by the the high magnetic saturation together with its rather high permeability. BUT a very fast ordering transformation acting below 700°C brings brittleness in the strip: then at the end of hot rolling the Fe-Co tapes had to be intensively water-quenched to prevent brittleness and in order to bear cold transformation. Through cold rolling the laminations were then sold to customers in the as-rolled and ductile state, with a good ability to be stamped into pieces BEFORE static annealing which involved recrystallization and then restores the soft magnetic properties. Such a process is called “semi-process” (strips sold in the as-rolled state). After static annealing the pieces (or stacks of them) become very brittle and can’t bear any stamping, punching etc. The box static annealing allows only low or medium yield mechanical strength (<500MPa), low or medium magnetic losses, and induces out of shape deformation during the final annealing
Facing this lack of high yield stress (>500MPa) and/or this absence of fully process grade (delivered as annealed and electrically insulated) and/or this out of shape during the final annealing, the FeCoProGen project aims to develop an original metallurgical path, starting from industrially annealed hot bands (usual AFK502 grade) through a very dedicated and well-controlled continuous annealing (called HSP for High Strength Processing) in place of the final static annealing, and able to prevent from ordering base embrittlement together with the availability to get precise high strength AND improved compromises between mechanical and magnetic properties.
Peculiarly the project target consisted in decreasing the magnetic losses down to less than 200W/kg at 2T/400Hz for a required yield strength (YS) of at least 800MPa in every measurement direction of the rolling plane, called Min(YS).
The 2 years of project have shown that:
- a reduced lamination thickness has to be favoured (for example 0,2mm), that a double stage process (one intermediate annealing between the cold rolling) can be at least as interesting - in terms of compromise between magnetic losses and yield stress – as the single stage process delivering High Strength (HSP). 250W/kg at 2T/400Hz could be obtained with a minimum isotropic yield stress of 800MPa, in case of 0,2mm thick laminations.
- downstream industrial processes (after final annealing) may lead to drastic and damaging effects on magnetic properties and have to be well controlled.
- multiply product, made of glued thin HSP laminations, have been done successfully, without high magnetic damage (15% on losses), with a medium ability to stamping, allowing a potential reduction of stacking factor and production cost with nearly the technical characteristics of thin HSP laminations.