Informacije MIDEM 22(1992)4, Ljubljana UDK 621.3:(53+54+621+66). ISSN0352-9045 WATER ATOMIZED Fe .. Co-BASED POWDERS FOR Alnico MAGNETS Prepared from poster presentation on the 8th International Conference on Powder Metallurgy in CSFR, Piestany, october 1992 B. Šuštaršič, S. Beseničar, J. Holc, Z. Lengar, S. Tašner KEY WORDS: permanent magnets, alnico magnets, magnet manufacturing, metal powders, Fe~Co powder, powder preparation, water atomization, powder compaction, powder sintering, magnetie properties ABSTRACT: Sintered Alnico magnets remain commercially of interest and therefore a subject of R&D works in spite of increased atractivity of new rare~earth super magnets. Sintered AINiCo anisotropic permanent magnets are produced from a mixture of powders. The master alloy powder is produced conventionally by grinding and milling of a east alloy. Our aim was to replace this procedure with technologically, economically and eco logi cally a more effective powder preparation method. We therefore investigated the applicability ofwater atomization for the preparation of Fe~Co~based metal powders. The appropriate chemical compositions and process parameters were determined on the basis of the significant features of water atomization. The morphologlcal properties of the prepared powders we re determined as functions of the main influent parameters of water atomization. Water atomized powders were then mixed with an appropriate amount of different additions and compacted into green sam ples. Samples were vacuum sintered and thermomagnetically treated under different conditions on the laboratory and industrial scale. The resulting magnetie properties of the prepared materials are equal in comparison with commercially available samples and better than those of conventionally prepa red materials. Vodno atomizirani Fe-Co prahovi primerni za izdelavo Alnico magnetov Pripravljeno po posterski predstavitvi na 8. mednarodni konferenci o metalurgiji prahov na Ceškoslovaškem, Piestany, oktobra 1992. K LJUČNE BESEDE: magneti trajni, magneti alnico, izdelava magnetov, prahovi kovinski, prah Fe~Co, priprava prahov, atomizacija vodna, stiskanje prahov, sintranje prahov, lastnosti magnetne POVZETEK: Kljub temu, da danes večino raziskovalcev s področja magnetnih materialov privlačijo predvsem še nepojasnjeni pojavi in nadaljne izboljšanje lastnosti redkozemeljskih supermagnetov, ostajajo tržno in zato tudi raziskovalno še vedno zanimivi, že dolgo uveljavljeni trdomagnetni materiali, kot so feriti in tudi Alnico magneti. Drobljenje, mletje in sejanje ulite predzlitine je ustaljen in precej zastarel postopek priprave kovinskih prahov za izdelavo AINiCo anizotropnih trajnih magnetov. Ta postopek smo zato želeli zamenjati s tehnološko, ekonomsko in ekološko učinkovitejšim postopkom. Zato smo raziskovali možnosti uporabe postopka vodne atomizaciJe za izdelavo prahov na osnovi kobalta in železa primernih za izdelavo AINiCo anizotropnih trajnih magnetov. Praktično delo nam Je omogočila predvsem nova laboratorijsko pilotna naprava za vodno atomizacijo z induktivnim talil nim sistemom. Glede na tehnološke značilnosti postopka vodne atomizacije smo izbrali primerno kemično sestavo vložka in določili osnovne tehnološke parametre atomizacije. Izdelanim prahovom smo določili osnovne morfološke lastnosti v odvisnosti od parametrov atomizacije. Lastnosti izdelanega prahu so bile v okviru pričakovanj in tehnoloških možnosti postopka vodne atomizaciJe. Izdelanim vodno atomiziranim prahovom smo primešali potrebne dodatke za korekcijo sestave in izboljšanje stisljivosti in Jih nato stiskali v preizkusne valjčke. Stisnjene surovce smo nato sistematično sintrali in toplotno obdelali v magnetnem polju pri različnih pogojih tako v laboratoriju, kot tudi na industrijskih napravah. Dosežene magnetne lastnosti obeh preiskovanih materialov (tip 400K in 1500) so bile podobne komercialnim vzorcem te vrste. ~ INTRODUCTION Standard procedure of Alnico (AI-Ni-Co-Fe based al- loys) permanent magnets production in ISKRA Magnets factory, Ljubljana, Slovenia is based on melting and casting of Fe-Co master alloy with additions of other alloying elements, such as Ni, Al, Cu, Si, Nb and Ti. The next production step is powder fabrication via grinding, milling, sieving (see Fig. 1) and mixing with a proper amount of binder and lubricant for automatic die compac 240 tion. Compacted green parts are then sintered in a protective atmosphere or vacuum, as well as thermo- magnetically and mechanically treated. Nowadays it is possible to produce metal powders with more effect ive powder preparation methods. The most important are the physical methods of powder prepara- tion and among them fluid atomization (inert-gas and water atomization) is the most wide spread. The atomi- zation or powder production directly from melt has many B. Šuštaršič, S. Beseničar, J. Holc, Z. Lengar, S. Tašner: Water atomized Fe-Co-based powders for ... Fig. 1.· SEM micrograph of the powder prepa red by conventional powder fabrication method, Fraction: <; 63 pm advantages. In particular, casting, grinding and milling of master alloy is not necessary. For a powder particie size in the 10 to 100 pm range, the cooling rate of disintegrated particles during water atomization is esti- mated on approximately 102 to 106 K/s. This me ans that we can expect better chemical homogeneity and micro- structure refinement because of rapid solidification. An- other advantage of water atomization is simple control of mean powder particie size by water pressure and relatively narrow particie size distribution of produced powder. On the other hand we can expect some difficul- ties in preparing metal powder with the selected final chemical composition because of oxidation of presented re active elements such as Al, Ti and Nb during water atomization, available for our experiments. Also, we did not find out references 1.2) yet, that it is possible to prepare powder by water atomization, appropriate for the production of sintered Alnico permanent magnets. EXPERIMENTS AND RES UL TS For our R&D work, two conventional hard magnetic materials were selected. The first one has trade-mark SIMAG 1500 (equivalent to DIN Alnico 39/12) with the nominal remanence Br = 0.85 T and coercivity HcB = 118 kAfm and the other one is SIMAG 400K (DIN Alnico 28/6) with Br = 0.9 T and HcB = 55 kAfm. Water atomized powders with various chemical compositions were pre- pared by new pilot water atomizer (05/2 Davy McKee, England, recently installed at the Institute of Metals and Technologies in Ljubljana, Slovenia) at different process parameters of atomization to optimize process and pow- der properties. All prepared metal powders were exa- mined by light microscope and SEM (Scanning Electron Microscopy). Basic physical and morphological proper- ties (particie shape, mean particie size, particie size 241 Informacije MIDEM 22(1992)4, str. 240-244 distribution, apparent density and flowability)3) of pow- ders were also determined. In tables 1 and 2 are col- lected the most important controlled parameters of water atomization. With the assistance of classical chemical analysis and X-ray fluorescence melts and final chemical composi- tions of atomized powders were controlled and cor- rected, respectively. Oxygen content of prepared pow- ders was between 0.1 and 0.15 wt.%. Another very important parameter in chemical composition of AINiCo magnets is carbon content which does not exceed 0.05 wt.%. All prepared powders have irregular particie Fig. 2. Fig. 3: SEM fTJ/crograph of Alnico water atomized AINiCo powder. Fraction: <; 6~m SEM micrograph of gas atomized AINiCo 1500 Fraetion: <; 63 11m Informacije MIDEM 22(1992)4, str. 240-244 shapes (see Fig. 2) which is characteristic of vJater atomization2A). This means that we can expect better compressibility but lower flowability of water atomized Fig. 4: SEM micrograph of particle of gas atomized AINiCo 1500 powder. powders compared Witll gas atomized powders (see figures 3 and 4). Mean particie size of prepared powders varies between 45 and 100 pm, depending on selected chemical com- position, superheating and water pressure. For consoli- dation only powder fraction :::: 120 pm was used. The optimal chemical composition was selected on ex- perimental basis, so that the combination with other additions (Ti-hydrid powder was replaced with cheaper CoTiAI or NiTiAI commercially available powders) pre- vents oxidation of the most reactive elements du ring water atomization. The compressibility of powders was improved with the reduction of iron content in water atomized powders and adding of soft pure Hoeganaes iron to powder mixtures. The systematical experimental cold pressing (600 to 750 MPa with 0.5 wt.% wax) and sintering (in vacuum 10.2 mbar, 1350 to 1370°C and 1 to 3 hours) of prepared water atomized powders in mixtures with other additions were carried out on laboratory and industrial scale. Green density of samples was between 5.7 and 6 g/cm3 and sintering density was from 7.05 to 7.30 g/cm3 de- pending on pressing and sintering conditions, respec- tively. The main magnetic properties were determined after thermomagnetical and mechanical treatment of samples. Obtained average magnet ic properties of sintered Alni- co magnets prepared from water atomized powders are presented in table 3. 242 B. Šuštaršič, S. Beseničar, J. Holc, Z. Lengar, S. Tašner: Water atomized Fe-Co-based powders for ... Table 1 : Experimental process parametsrs for prepara- tion of Alnico powders types 400K and 1500 (Davy Mc KeeD5/2 water atomizer - IMT Ljubljana). I " Remarks I Process parameters I Superheating of 1550 to 16500C measured by opt~ ~ m,kl"C) cal pyrometer ----- Temperature of 1250 (± 20°C) thermocouple Pt- tundish (0c) PtRh13 Tundish nou ki 4.5 fused quartz .1 diarneter (mm) f----- 1-- Water jets main 1.20x1.05 type 1503 diameter (mm) side 1.10 x 0.85 type 1502 -f------- Jet apex main 25° original ang le side 20° manifold ------- '-- i--- _.- Water prossure 180to 230 (bars) _._- --_._--------- --- flow rate oi 0.8 m3/h N2 in flow-meter protective g2.5 atom iz. chamber 0.3m3/h Ar over melt during melting Table 2: Parameters of water atomization for Alnico powders (Davy McKee D5/2 water atomizer-IMT Ljubljana). Material Water Water Metal Flow d50 pressur8 flow rate flow rate ratio (bars) (I/min) (kg/min) (water/ (11m) SIMAG 130 56 3,54 15.82 50 400K ------------. SIMAG 200 55 6.65 8.30 60 1500 =~c_=,~~~~~o=---============= Table 3: Magnetic properties of sintered Alnico magnets prepared from water atomized powders. Material SIMAG 400K SIMAG 1500 Magnetic properties Contents Br (T) Hcr (kA/m) (BH)m~x wt.% 0:2 wt.% C (kJ/~=._====== 110 60.0 28.5 0.10 0.050 0,90 124.0 42.0 0.15 0.035 It is necessary to point out that besides the above mentioned advantages of water atomization, water atomized powders have different (see figure 5) and better sinterability, respectively. This results in finer microstructures with narrower grain size and pore dis- tribution (see figures 6 and 7). The final result are more uniform magnetic properties and improved dimensional stability of sintered Alnico products. B. Šuštaršič, S. Beseničar, J. Holc, Z. Lengar, Informacije MIDEM 22(1992)4, str. 240-244 S. Tašner: Water atomized Fe-Co-based powders for .. , 2 -=--= .--- '';::-'" ~ ••••• ., • OJ e o. • IJ ••• ~ oil o .... . ...... .. ~ . --- " '. ..... ... ~,'\ ., ...... ~ " J ., ..... " .~--- \ . \ . CD 400 K I k • . . . \ . cz) 400 Ka/WA . • \. . , , r-'----'-" · ---i' CD 400 Kb/WA "o. · I · ~. · · (0 1500 WA · , ~ -r .. , _. '" ; \. , ; \' I : ., -.-~~ r=-," -t--2- ; ~ I ® ...... ~;;:::'--1 .' ,----.. -- . ......-. ~- 1--- ~.-.-~.- .~6~ ~~.~ CD- ............ - -- O o "" o - .c -2 +J 01 C cl> --' -4 4- o cl> -6 o) c o ..c u cl> -8 .> ......, o ...,..J -10 cl> o:::: -12 O 200 400 600 800 1000 Temperature ( °C ) 1200 1400 k .... standard procedure WA ... water atomization a ..... Co-Ti-Al addition b .... Ni -Ti -Al addition Fig_ 5: Comparison of dilatometric curves for conventional!y prepared Alnico powder type 400K and powders prepa red by water atomization. Fig. 6: Optical micrograph of the typical microstructure of sintered Alnico type 400K magnet prepa red by standard procedure. 243 Fig. 7: Optical micrograph of the typical microstructure of sintered Alnico type 400 K magnet prepared from the water atomized powder. Informacije MI DEM 22(1992)4, str. 240-244 CONCLUSIONS We investigated the applicability of water atomization for the preparation of Fe-Co-based metal powders suitable for the production of Alnico permanent magnets. On the basis of the significant features of water atomization the appropriate chemical compositions and process par- ameters were selected. The basic physical and chemlcal properties of prepa red water atomized powders were determined. Prepared powders were then mixed with other commercially available powders and additions, pressed and sintered at different conditions, thermo- magnetically treated and ground. The measured mag- netic properties of the prepared sintered AINiCo mag- nets are equal in comparison with commercially avail- able samples and better than those of conventionally prepared materials. REFERENCES 1) F. V Lenel Magnetic Applications, Metal Handbook, 91h edition, Volume 7, Powder Metallurgy, p.: 641, 244 B. Šuštaršič, S. Beseničar, J. Holc, Z. Lengar, S. Tašner: Water atomized Fe-Co-based powders for ... 2) J. J. Dunkley The Production of Metal Powders by Water Atomi· zation, Powder Metallurgy International, Vol.:1 0, 1/78, 3) MPIF Standard Test Methods for Metal Powders and Powder Metallurgy Products, Metal Powder Industries Federation, Edition 1985/1986, Princeton, New Yersey, 4) J. J. DunkleY,J Palmer Factors affecting particie size of atomized metal powders, Powder Metallurgy 29/1986, No.:4. Borivoj ŠUŠTARŠiČ dipl.ing., Institute of metals and technologies, Lepi pot 11, 61000 Ljubljana, Slovenia dr. Spomenka BESENiČAR dipl.ing., dr. Janez HOLC dipl.ing., Jožef Stefan Institute, Jamova 39, 61000 Ljubljana, Slovenia mag. Zvonko LENGAR dipl.ing, Stojan TAŠNER dipUng, ISKRA Magnets, Stegne 37, 61000 Ljubljana, Slovenia Prispelo: 20.10.92 Sprejeto: 30.11.92