Machining of iron–gallium alloy for microactuator

Sensors and Actuators A 137 (2007) 134–140

Machining of iron–gallium alloy for microactuator T. Ueno a,∗ , E. Summers b , T. Higuchi a a

The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan Etrema Products, Inc., 2500 North Loop Drive Ames, IA, USA

b

Received 10 July 2006; received in revised form 17 January 2007; accepted 21 February 2007 Available online 3 March 2007

Abstract We study the micromachining of iron–gallium alloy for use in a microactuator. Iron–gallium (Galfenol) is an iron-based magnetostrictive material with magnetostriction exceeding 200 ppm, Young’s modulus of 70 GPa, and distinctive ductile and machinable properties. An actuator made by small Galfenol component therefore should be simple, robust against external forces and drivable at low voltage. A rod of Galfenol (Fe81.6 Ga18.4 ) prepared by the free stand zone melting technique was machined to create pillars of l mm2 by an ultra precision cutting technique to study the suitability of Galfenol in micro components. Comparison of strain gage measurements for the pillars and non-machined specimens verifies the magnetostriction, with variation arising from the grain distribution that is not significantly dispelled by the milling process. Measurement of displacements by a Laser Doppler vibrometer supports our results and discussion. The successful fabrication of pillars of 0.7 and 0.5 mm2 and length 5 mm shows the forgiveness of the material under high-speed cutting, and its potential in miniaturization. © 2007 Elsevier B.V. All rights reserved. Keywords: Galfenol; Microactuator; Magnetostriction; Ultra precision machining

1. Introduction The trend toward miniaturization of consumer electronics demands innovation in materials and fabrication technology. For example, high-speed positioning of an optical lens in a digital camera has been realized by a small component of piezoelectric material [1,2]. The combination of the smooth impact driving mechanism and small PZT actuator enables positioning with high speed and high resolution with long traveling distance. Active materials, such as piezoelectric (PZT) or giant magnetostrictive (Terfenol-d) materials, that are capable of generating vibrations of high frequency, are strong candidates in novel microactuators. Even with small displacements of micro or sub-micro order (much smaller than electromagnetic or SMA type), a large force and high resonance frequency (rigidity) can provide mechanism of high-speed motion of a frictionally supported heavy mass. The demand is for a miniature microactuator while maintaining the simplicity and material performance. Piezoelectric material or PZT is currently considered superior to Terfenol-d, because it is available in the configuration of a

∗

Corresponding author. Tel.: +81 3 5841 6466; fax: +81 3 56847532. E-mail address: [email protected] (T. Ueno).

0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.02.026

single plate with electrodes for actuation, even with the low efficiency of the transverse effect and its brittle nature. Terfenol-d is limited in size and, must still be equipped with a magnetic circuit and prestress mechanism to utilize the magnetostriction effectively. The situation is changing with the advent of a new class of magnetostrictive materials, iron–gallium alloy (Galfenol) [3–5]. Galfenol is an iron-based magnetostrictive material with properties sufficient for actuators, such as magnetostriction ranging from 200 to 300 ppm, Young’s modulus of 70 GPa and high relative permeability (60–200). It has ductile and machinable properties, such that a complex structure made of the material can be fabricated through conventional machining techniques. The use of Galfenol in microactuators is particularly promising. Problems arising from the miniaturization of active materials, such as PZT or Terfenol-d do not arise with Galfenol. Micro components fabricated by micromachining technique can maintain effective magnetostriction, and are robust against external forces, bending or tensile. The actuator can therefore be miniaturized and shaped without prestressing. As the size diminishes, the eddy current loss in high frequency driving falls, and the driving voltage is less with small winding of the coil. The machining techniques available for Galfenol include cutting (such as milling and lathing), laser cutting and wire

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Fig. 2. Cutting procedure.

rod of Galfenol, and their magnetostrictions were measured by strain gages. The results of the pillars and non-machined material are compared, to seek an explanation for the variation in the slope and saturation. Magnetostriction measurements made by a Laser Doppler vibrometer on the square rods support the analysis and dependence of micro component properties on the grain distribution. 2. Fabrication of distributed pillars A rod of iron–gallium, of nominal composition 18.4 at% Ga, and diameter 6.35 mm, was machined to the shape shown in

Fig. 1. Machined iron–gallium (top) and its surface (bottom).

EDM. We prefer the ultra precision cutting technique, which can give complex 3D structure of micrometer order in fast time. High-speed cutting using a carbide end mill with small diameter enables the fabrication of a square rod, hole and ditch of micrometer order. A further issue is whether Galfenol is suitable for cutting and whether its magnetostrictive property is maintained thereafter. Below we evaluate the technique for the fabrication of micro components. For this purpose, pillars of 1 mm2 cross section with high aspect ratio were shaped from a Table 1 Technical specifications of the ultraprecision machine tool used for the fabrication Controllable axis

Four axes: X, Y, Z and C

Linear axis (special roller guide)

Strokes X: 220 mm, Y: 100 mm, Z: 150 mm Scale resolution: l nm Maximum speed: 1000 mm/min Straightness: 0.1 ␮m

C spindle (air bearing)

Rotary encoder: 1/100,000 Maximum speed: 3000 rpm Rotation precision: 0.05 ␮m

Grinding spindle (air bearing)

Maximum speed: 40,000 rpm Rotation precision: 0.05 ␮m

Fig. 3. Machined sample with strain gages attached (top) and experimental setup (bottom).

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Fig. 6. Magnetostrictions of un-machined portion under zero-compressive preload (measured by large strain gage, 6 mm × 1.7 mm).

Fig. 4. Magnetostrictions of pillars of center (top) and left (bottom) under zerocompression.

Fig. 1. The rod was prepared by the free stand zone melting (FSZM) technique under research grade conditions passing the molten zone up the rod at rates less than 75 mm per hour. FSZM is a crystal growth technique used to produce high-performance smart materials. In this technique, a feedstock rod is held in a

fixed position while an induction coil moves along the length of the rod melting small sections. Surface tension from the alloy system being zoned prevents the molten section from dropping out. During solidification, the rod develops a strong orientation (i.e. typically a fiber texture) due to thermal gradients established in the process. The grain size of this sample is typically around 1350 ± 150 ␮m according to orientation image microscopy measurements [6]. The rod was performed milling by an ultraprecision machine tool. The machine tool has three linear axes (XYZ axis) and one rotational axis (C axis), and a high-speed spindle with an air bearing. Table 1 shows the performance of the machine tool. First, the rod was fixed along the C axis and shaped to a 4 mm2 with a carbide end mill of 0.8 mm diameter, as shown in Fig. 2. It was then divided into nine rods of 1 mm2 by cutting two grooves on the four faces with a carbide end mill of diameter 0.5 mm. In the first trial, a depth of cut 2 ␮m and feeding speed 40 mm/min were used to reduce the effect of the cutting force, but these values degrade the surface condition and the magnetostrictive properties. The cutting condition used for fabrication of the grooves was 8 ␮m depth of cut, a feeding speed 100 mm/min and rotation at 20,000 rpm with an oil mist coolant, generating a fine surface without transgranular chipping. 3. Measurement of magnetostriction At first, the magnetostriction was measured by the experimental setup shown in Fig. 3. Strain gages (grid length l mm

Fig. 5. Magnetostrictions of un-machined portion under zero-compressive preload (measured by small strain gage, 1 mm × 0.6 mm).

Fig. 7. Displacement measurement by Laser Doppler vibrometer.

T. Ueno et al. / Sensors and Actuators A 137 (2007) 134–140

Fig. 8. Displacements of pillars of 1.0 mm2 under zero-compressive pre-load measured by Laser Doppler vibrometer at 100 Hz.

and width 0.6 mm) were attached in the longitudinal direction of the pillars at the center (Pc) and left (P1) of four faces. For comparison, four strain gages were also attached in the longitudinal direction on the non-machined material (Pn). The sample was placed inside an excitation coil of inner diameter 11 mm, and a magnetic field up to 78 kA/m was applied with the frequency of 0.1 Hz under a 0 MPa compressive load. The sample was placed between iron rods, so that the flux passes inside the pillars with small leakage. Figs. 4 and 5, respectively, show the results for the pillars and non-machined material. The pillars were saturated at lower magnetic field than the non-machined material due to the small area. All points lay on clear butterfly curves, but their shapes were different. The saturation values also ranged from 70 to 200 ppm. We thought that the difference was due to the error of measurements caused by the insufficient bonding or arrangement of the strain gage. However, the results of un-machined also exhibit the difference. The rod and pillar consist of large grains, some larger than l mm in diameter. Thus, the strain gage with area 1 mm × 0.6 mm is regarded monitoring the behaviors of one or several grains including their boundary, which result in individual difference in the measured magnetostriction. In fact, the results with large area strain gages of grid length 6 mm and width 1.7 mm in the same position exhibited a more averaged behavior with the saturation value close to the reference data of the bulk material as shown in Fig. 6. The effect of the milling is not therefore clear; however, the saturation close to 200 ppm obtained with some pillars indicates that the milling process does not significantly impair the magnetostrictive property when compared to the saturation magnetostriction values of the non-machined part.

Fig. 9. Pillars of 0.7 and 0.5 mm2 .

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Fig. 10. Displacements of pillars of 0.7 mm2 (top) and 0.5 mm2 (bottom) under zero-compressive pre-load measured by Laser Doppler vibrometer at 100 Hz.

The displacement of nine pillars was measured by a Laser Doppler vibrometer as shown in Fig. 7 in order to confirm the validity of the strain gage measurements. The spot diameter of laser is 50 ␮m small enough to measure the displacement of the surface. The sample was set in an air coil (l000 turn/at 18 mm length) and excited by a magnetic field at l00 Hz frequency to generate clear signals from the displacement. Fig. 8 shows the results, in which the number on the graphs corresponds to the arrangement of the pillars. In this case, the displacement is the sum of the magnetostriction of the pillar and the non-machined portion, and is affected by the demagnetizing effect. Even the magnetostrictions are not saturated, their slope are different. In addition, the results of the distribution seem reasonably consistent with the sum of the magnetostrictions measured by strain gage. This supports the validity of the strain gage measurement and that the difference is inherited from the local behavior of the grains, which consists the pillars. The high-speed cutting process can realize a high aspect ratio of machining without bending or chipping. We have successfully fabricated eight pillars of 0.7 and 0.5 mm2 , as shown in Fig. 9. The rod (Fe81.6 Ga18.4 ) was finished to shape by cutting grooves on four faces of a 4.3 mm2 rod,

Fig. 11. Fabricated square rods on brass fixture.

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Fig. 12. Magnetostrictions of nine points of 1 mm2 rod (no. 1) with zero-compressive pre-load.

with the end mill of diameter 0.5 mm under the same cutting condition. The displacement of the pillars is plotted in Fig. 10 using the Laser vibrometer. A difference in the slopes was observed but no trend was confirmed depending on the geometry.

4. Magnetostriction of square rods The magnetostriction was measured using square rods to determine whether the milling affects the magnetostriction. Two samples of the rod, l mm2 and 5 mm long, were fabricated from

Fig. 13. Magnetostrictions of nine points of 1 mm2 rod (no. 2) with zero-compressive pre-load.

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5. Conclusion

Fig. 14. B–H of 1 mm2 rods with zero-compressive pre-load.

the plate of Galfenol (Fe81.6 Ga18.4 ) having thickness of l mm. The milling process, cutting the top face and grooves for the division, was performed on the plate, which was glued to a brass fixture as shown in Fig. 11. To the rod was then applied a magnetic field with an exciting frequency of 100 Hz. The magnetostriction of nine points, with interval of 0.35 mm on the end, was measured by the Laser vibrometer. The induction was measured with a pick-up coil (five turns of 0.1 mm diameter copper wire) wound on the rod integrating the induced voltage with the flux changes. Figs. 12 and 13 show the results of magnetostriction. One rod gave a relatively uniform distribution with saturation more than 200 ppm, close to that of the bulk material with no pre-load. Another sample showed some non-uniformity in the distribution, with lower magnetostriction. This means that the rod includes several grains with the orientations, which may not be ideal of 1 0 0 direction. On the other hand, the B–H curves of the two samples are almost the same with relative permeability around 20, Fig. 14. The small value of the permeability compared with bulk of 60–200 is due to demagnetizing effect. The difference in the results for these two samples shows an issue in fabricating micro components made of Galfenol, and is attributed to the microstructure (grain size and shape) along with variations in grain texture or orientation.

We have studied the milling of iron–gallium alloy for use as a microactuator. Cutting conditions were adjusted to realize a high aspect ratio of machining without significant deterioration of the magnetostrictive property. Variations in the magnetostriction appear to remain as results of the variation in microstructure and texture. We shall try to identify other potential milling effects possibly induced by tool compression with alternative fabrication techniques, such as wire EDM, or conducting thermal annealing, relieving machining induced stresses. From the results, a rod of length 5 mm should provide a displacement of more than 1 ␮m, which is comparable to the performance of conventional stack PZT actuators. The present results clearly show the potential of ultra precision cutting for the micromachining of iron–gallium alloy. Cutting with a high rotation speed using a micro end mill is capable of providing small structures of sub-micro order for novel applications. Acknowledgements The authors thank Etrema Product Inc. for the polycrystalline Galfenol samples. This research was funded under ONR contract N00014-06-M-0136, with the support of Jan Lindberg. References [1] Y. Yamagata, T. Higuch, O. Omiti, A micro-mobile mechanism driven by  impulsive inertial force, in: Proc. 5th Int. Conf. on New Actuator 96, 1996, pp. 68–71. [2] R. Yoshida, Y. Okamoto, H. Okada, Development of smooth impact drive mechanism (2nd report), J. Jpn. Soc. Precision. Eng. 68 (2002) 536–542. [3] A.E. Clark, M. Wun-Fogle, J.B. Restorff, Magnetostrictive properties of body-centered cubic Galfenol and Galfenol–Al Alloy, IEEE Trans. Mag. 37 (2000) 3238–3240. [4] A.E. Clark, M. Wun-Fogle, J.B. Restorff, Magnetostrictive property of Galfenol alloys under compressive stress, Mater. Trans. 43 (2002) 881–886. [5] M. Wun-Fogle, J.B. Restorff, A.E. Clark, Magnetostriction of stress annealed Galfenol–Al and Galfenol alloys under compressive and tensile stress, Proc. SPIE Smt. Str. Mater. 5387 (2004) 468–475. [6] E. Summers, T.A. Lograsso, J.D. Snodgrass, J. Slaughter, Magnetic and mechanical properties of polycrystalline Galfenol, Proc. SPIE Smt. Str. Mater. 5387 (2004) 448–459.