Technical Paper: PIM International, Vol.3 No. 3 September 2009, pages 58-63, 2590 words

Authors: B. S. Zlatkov [1], U. Bavdek [2], M.V. Nikolic [3], O.S. Aleksic [3], H. Danninger [4], C. Gierl [4] and A. Erman [2]

[1] FOTEC Forschungs- und Technologietransfer GmbH, Viktor Kaplan-Strasse 2, 2700 Wiener Neustadt, Austria
[2] Magneti Ljubljana d.d., Stegne 37, 1000 Ljubljana, Slovenia
[3] Institute for Multidisciplinary Research, Kneza Viseslava 1, 11000 Beograd, Serbia
[4] Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, 1060 Wien, Austria

Abstract

Magnets of the alnico group have a several times lower energy product compared to Re-Co and NdFeB magnets. However application of these latter magnets is limited by the maximum operating temperature of 180°C for NdFeB and about 350°C for Re-Co compared to 540°C for alnico. Additionally, mechanical properties and thermal stability of magnetic properties, which are also important in practice, are much higher for alnico. PIM (powder injection moulding technology) is used for large scale manufacturing of small and highly complex parts, making it attractive to introduce alnico in PIM since complex-shaped anisotropic components can be obtained. This was studied in the present work. First the alnico 8 starting powder was additionally milled until a mean particle size of d50=8 µm was attained. Properties of the original – as-received and as-milled powder were investigated using SEM and XRD analysis. The milled powder was then mixed with a solvent-type binder. The feedstock thus prepared was injected into a cylindrical mould. This step was then followed by debinding and sintering. Different sintering temperatures and times were used in order to optimise the sintering process. The C, S, N and O contents in the samples were measured. Thermomagnetic treatments of the sintered samples were performed in order to obtain optimal magnetic properties. These were measured on sintered samples before, between and after thermomagnetic treatment processes. The final maximum magnetic energy product value obtained for the currently applied optimal sintering conditions ((BxH)max = 22.95 kJ/m³) was not as high as the ones obtained using classical methods for alnico manufacturing, but acceptable for a first attempt. Thus it can be concluded that application of PIM in manufacturing of alnico magnets with complex shapes is possible but requires further optimisation of the sintering and thermomagnetic treatment processes and also of the debinding procedure since the content of C and O in the samples also needs to be reduced.

Introduction

Alnico alloys are well-known representatives of hard magnetic alloys. Alnico stands for an alloy consisting of aluminium, nickel and cobalt. Besides these elements, alnico alloys contain iron and small amounts of copper and titanium. By varying the composition (amount of added copper and titanium and even niobium) it is possible to tailor the magnetic properties to meet the needs of a wide variety of end use applications. First members of the alnico series discovered in 1932 by Mishima contained iron, nickel and aluminium, no cobalt [1]. Addition of cobalt to the Fe-Ni-Al alloys increased the Curie temperature and improved magnetic properties. Much experimental research has been done on this alloy group to study the correlation between composition and thermomagnetic treatment regime in relation to the resulting magnetic properties. Alnico is widely applied in industrial applications such as: electric motors, microphones, sensors, generators, hand tools, loudspeakers, actuators, cow magnets, electric guitar pickups, military devices etc. [2-5]. Different alnico grade designations are found in the literature and in practical applications, such as Ticonal, Koerzit, Oerstit, Alcomax etc. According to the standard DIN 17410/63 designations such as AlNiCo 120, AlNiCo 300 etc. are used, while American standards use the terms alnico 1, alnico 2, alnico 3 etc....

Further sections of this paper include:

- Introduction
- Experimental
Powder and feedstock preparation
Injection Moulding
Debinding and sintering
Thermomagnetic treatment
Measurements
- Results and Discussion
- Acknowledgement

Figures and Tables:

Fig. 1 SEM micrographs of the powder before (a) and after (b) additional milling

Fig. 2 DTA/TG of as-milled alnico 8 powder measured in argon

Fig. 3 XR diffraction pattern of as-milled alnico 8 powder

Fig. 4 PIM shaped green alnico 8 cylindrical components (a) cross section and (b) contour

Fig. 5 Green (top) and sintered (bottom) alnico 8 MIM parts

Fig. 6 Hysteresis graphs and demagnetisation curves after thermomagnetic treatment for alnico 8 PIM samples sintered for 2h at 1300°C a), c) and 1325°C b), d) respectively

Fig. 7 a) Shape distortion, b) exudation of a liquid phase

Table 1 Density and dimensional change values of alnico 8 samples made by PIM as obtained for different sintering temperatures

Table 2 Analysis of the C, S, N and O content

Table 3 Magnetic properties after sintering and heat treatment of alnico 8 PIM specimens