Active Metal-matrix Composites with Embedded Smart Materials by Ultrasonic Additive Manufacturing

Transcription

1 Active Metal-matrix Composites with Embedded Smart Materials by Ultrasonic Additive Manufacturing Ryan Hahnlen, Marcelo J. Dapino Smart Vehicle Concepts Center, The Ohio State University, Columbus, OH, USA, ABSTRACT This paper presents the development of active aluminum-matrix composites manufactured by Ultrasonic Additive Manufacturing (UAM), an emerging rapid prototyping process based on ultrasonic metal welding. Composites created through this process experience temperatures as low as 25 C during fabrication, in contrast to current metal-matrix fabrication processes which require temperatures of 500 C and above. UAM thus provides unprecedented opportunities to develop adaptive structures with seamlessly embedded smart materials and electronic components without degrading the properties that make these materials and components attractive. This research focuses on developing UAM composites with aluminum matrices and embedded shape memory NiTi, magnetostrictive Galfenol, and electroactive PVDF phases. The research on these composites will focus on: (i) electrical insulation between NiTi and Al phases for strain sensors, investigation and modeling of NiTi-Al composites as tunable stiffness materials and thermally invariant structures based on the shape memory effect; (ii) process development and composite testing for Galfenol-Al composites; and (iii) development of PVDF-Al composites for embedded sensing applications. We demonstrate a method to electrically insulate embedded materials from the UAM matrix, the ability create composites containing up to 22.3% NiTi, and their resulting dimensional stability and thermal actuation characteristics. Also demonstrated is Galfenol-Al composite magnetic actuation of up to 54 μɛ, and creation of a PVDF-Al composite sensor. Keywords: NiTi, Galfenol, magnetostriction, PVDF, metal-matrix composite, active composites, ultrasonic additive manufacturing, ultrasonic consolidation 1. INTRODUCTION Metal-matrix composites have been investigated as a way to create multifunctionality or achieve improved mechanical properties relative to the parent matrix material. 1 Methods for embedding long fibers, preforms, or particles generally require temperatures near or above the melting point of the matrix material. Even in powder metallurgy processes involving isostatic pressing and diffusion bonding, temperatures can reach up to 565 C. 2 This makes it challenging to embed smart materials into metallic matrices as the unique properties of smart materials often degrade with exposure to high temperatures. In contrast, composites created through Ultrasonic Additive Manufacturing (UAM) are constructed at room temperature. Ultrasonic Additive Manufacturing, also known as Ultrasonic Consolidation (UC), is an emerging rapid prototyping technology that is used in this study to create composites consisting of an aluminum matrix and seamlessly embedded smart materials. This research focuses on creating three types of UAM composites, each with different embedded smart materials. The first type of samples are NiTi-Al composites which were studied for their stiffness tuning properties 3 and thermomechanical characteristics. Multiple electrical insulation coatings were investigated to enable NiTi based strain sensing. The second type of composite was created by embedding magnetostrictive Galfenol (FeGa) to investigate magnetic actuation and sensing properties. A PVDF-Al composite was created as the third type of composite for this research to investigate its sensing properties. Ultrasonic Additive Manufacturing incorporates the principles of ultrasonic metal welding and subtractive processes to create metal parts with arbitrary shapes and seamlessly embedded materials. 4 Being a lowtemperature process, UAM offers unprecedented opportunities to create parts with embedded smart materials (e.g., shape memory alloys, fiber optics, and polymers) and electronic components. Further, the subtractive Further author information: (Send correspondence to M.J.D) R.H.: hahnlen.1@osu.edu, Telephone: M.J.D.: dapino.1@osu.edu, Telephone: Industrial and Commercial Applications of Smart Structures Technologies 2010, edited by M. Brett McMickell, Kevin M. Farinholt, Proc. of SPIE Vol. 7645, 76450O 2010 SPIE CCC code: X/10/$18 doi: / Proc. of SPIE Vol O-1

2 stage integrated within commercial UAM systems allows for the simultaneous incorporation of arbitrarily shaped internal features such as cooling channels or designed anisotropies. The key features of ultrasonic metal welding are shown in Fig. 1 (a). A piezoelectric ultrasonic transducer drives the transversely vibrating tip of the sonotrode which is pressed against the top work piece. The motion imparted to the top piece creates a relative, friction-like action at the interface of the work pieces. This relative interface motion causes shear deformations of contacting surface asperities, dispersing interface oxides and ultimately bringing clean metal-to-metal contact and metallic bonding between the work pieces. 5 Ultrasonic metal welding has been adapted into a rotating transducer, booster, and horn system for UAM, creating a new and distinct manufacturing process with capabilities not possible to achieve with conventional ultrasonic metal welding processes. As shown in Fig. 1 (b), instead of a spot contact, vibrations generated by a piezoelectric ultrasonic transducer are transmitted into the parts through a rolling sonotrode. The vibrations propagate longitudinally from the transducer to the sonotrode through tuned waveguides while normal force is applied to the vibrating sonotrode as it rolls along the work piece. The vibrations transmitted to the weld interface cause a solid state bond between the parts. The current UAM systems achieve the most effective bonding on thin metal layers of approximately 152 μm (0.006 in) thickness. The UAM system therefore creates larger bulk builds by welding successive layers of metal tapes or sheets. UAM makes it possible to not only embed hard reinforcement materials, such as sigma fibers (TiB fibers with a tungsten core and SiC casing) but also brittle materials such as fiber optics (Fig. 2 (a) and (c)). Being a solid state process, UAM has been utilized to embed and join dissimilar materials, such as copper and aluminum (Fig. 2 (b)), and to create materials with arbitrary internal spaces (Fig. 2 (d)). Possibilities of advanced UAM builds include augmented structural panels with embedded reinforcement that could be monitored for damage using embedded sensors, or thermal control using embedded thermocouples for sensing with integrated internal channels for on-demand cooling at specific locations. With UAM it is possible to have a multifunctional build capable of meeting structural, sensing, motion control, and stiffness control requirements. In addition to sensors and fibers, UAM offers the opportunity to embed entire electronic components or circuit boards, allowing for sophisticated data acquisition, control, or monitoring systems to be fully integrated into a structural package. Other concepts such as reversible adhesion, in which ultrasonic bonds between metal parts are created and then separated on demand, have also been investigated using UAM. Embedding materials using UAM can be accomplished through one of two general procedures, depending on the shape and size of the embedded objects. Common to both methods, the UAM process is paused at the desired height of the embedded material. In the most simple method the embedded material is oriented as desired and the next tape layer is welded as normal. This method relies entirely upon the normal force and ultrasonic vibrations to plastically deform and flow the matrix material around the embedded object. As shown in Fig. 3 (a) and (b), this method has proven to work well for relatively small wires and ribbons. As the fiber Figure 1. (a) Schematic representation of ultrasonic metal welding, a solid-state joining process which forms the basis for ultrasonic additive manufacturing (UAM). (b) In the UAM process, successive layers of metal tape are bonding together for creating metallic composites with seamlessly embedded materials and features. Proc. of SPIE Vol O-2

3 dimensions increase, more normal force and ultrasonic power are required in order to produce sufficient material flow to fully envelop the embedded material. The second method of embedding involves machining a pocket in the previously consolidated layers. Once the embedded objects are placed in the machined pocket, successive tapes are welded on top to fully enclose the embedded material. This method is used for embedded materials or objects of large size or irregular shape and has been proven as a viable way of embedding sensors and electronic components.7 Figure 2. (a) Micrograph of 100 μm (0.004 in) diameter sigma fibers embedded in aluminum.6 (b) Aluminum UAM build with embedded copper block. (c) Fiber optics embedded between aluminum tapes. (d) An X-ray image of a UAM build with arbitrary multi-level internal channels made using subtractive processes. (Photographs (b) and (c) courtesy of Solidica, Inc.) Figure 3. (a) Micrograph of 75 μm (0.003 in) diameter NiTi wire embedded in Al matrix. (b) Micrograph of 254 μm (0.010 in) thick NiTi ribbon embedded in Al matrix. Both cases utilize plastic flow of the matrix material alone to encapsulate the NiTi feature. 2. NICKEL TITANIUM-ALUMINUM UAM COMPOSITES 2.1 NiTi composite methods NiTi composite construction In order to investigate electrical insulation methods, the stiffness tuning capabilities, and thermomechanical behavior of NiTi-Al UAM composites, multiple NiTi-Al composites were created. Single fiber builds were created using plastic flow and subtractive processes for insulation trials. In previous research it was shown that increasing the relative amount of NiTi increases the stiffness tuning response of the composite and that there is a critical NiTi area ratio, approximately 4.5%, below which stiffness will decrease with increasing temperature.3 For this reason, the two additional composites were constructed with NiTi area ratios greater than 10%. NiTi-Al composite 1 has eight 203 μm (0.008 in) diameter wires embedded between two Al 3003-H18 tapes and NiTi-Al composite 2 has six 203 μm diameter wires embedded between two Al 3003-H18 tapes. After consolidation, both NiTi composites were machined to final dimensions. After machining, NiTi-Al composite 1 had a NiTi area ratio of 22.3% while NiTi-Al composite 2 had a NiTi area ratio of 13.4%. The area ratio was calculated by dividing the cross-section area of the embedded NiTi by the total cross-sectional area of the composite. Proc. of SPIE Vol O-3

4 2.1.2 Electrical insulation testing Several insulation trials were conducted to test different coatings and their compatibility with the embedded materials as well as the UAM process. Insulation methods for NiTi considered here include an oxide coating induced through anodization, polytetrafluoroethylene (PTFE), polyimide film (Kapton) tape, and a commercial spray-on enamel coating. An additional build was created by embedding copper wires that utilize an enamel varnish insulation. Before embedding, all insulation samples were tested to determine if the insulation adhered to the embedded material and if the coating provided sufficient electrical insulation. If a coating did not adhere to or insulate the material, it was not embedded. After embedding, a multimeter was used to measure the resistance between the embedded materials and the Al matrix. A successfully embedded insulation coating maintains high electrical resistance between the embedded material and matrix. Electrical continuity between the embedded material and the matrix indicates that the coating was worn through by the relative motion of the faying surfaces during the welding process NiTi composite stiffness testing Stiffness tests were conducted on NiTi-Al composite 1. The composite was placed in a thermal chamber and an axial load was applied while the temperature, force, and strain were measured. Load was applied via an electrically controlled linear positioner excited by a ramp input starting from a 13.3 N (3 lbf) preload to a maximum force value. The preload was applied to ensure that any slack in the load train was removed prior to taking measurements. Multiple experiments were run with maximum force values of 17.8 N, 22.2 N, and 26.7 N (4 lbf, 5 lbf, and 6 lbf, respectively). Measurements were taken for increasing and decreasing load at 25 C and 150 C. For all trials, the slope of the force-displacement test plot was used to determine composite stiffness NiTi composite thermomechanical testing NiTi-Al composite 2 was used to observe the unloaded thermally induced strain. The composite was machined as shown in Fig. 4. A strain gage was bonded onto the narrow portion of the sample, which is the region with embedded NiTi wires. A reference strain gage was bonded on one of the Al 3003 side wings. Once the strain gauges were applied, the composite was placed in a thermal chamber and thermally cycled from 25 C to approximately 130 C multiple times while the strain gauge output was monitored. The reference and composite gauge were oriented in a half bridge configuration as to electrically subtract the reference strain signal from the composite strain signal as described by Lanza di Scalea. 8 This negates thermal response of the strain gauges, allowing the composite Coefficient of Thermal Expansion (CTE) to be calculated as α comp = α ref + ɛ sig ΔT, (1) and the composite strain to be accurately calculated as ɛ comp = ɛ sig + α ref ΔT. (2) Here, α ref is the CTE of the reference material, Al 3003 (23.2 ppm/ C 9 ), ɛ sig is the reference strain signal subtracted from the composite strain signal combined into a single output, and ΔT is the change in temperature. Figure 4. NiTi-Al UAM composite 2 used for thermomechanical characterization. Proc. of SPIE Vol O-4

5 2.2 NiTi composite experimental results Electrical insulation results The NiTi trial insulation coatings utilizing an oxide layer and PTFE did not adhere well to NiTi and were not embedded in an Al matrix. Although the commercial spray-on enamel did adhere well to NiTi and was embedded between two Al 3003 H-18 tapes, the electrical resistance between the NiTi phase and Al matrix was less than 1 Ω, indicating failure of the coating during the embedding process. Similarly, the copper wire build with enamel varnish insulation exhibited electrical continuity with electrical resistance of less than 1 Ω. The NiTi build utilizing polyimide film insulation maintained high electrical resistance between the embedded material and the Al matrix NiTi composite stiffness results NiTi-Al composite stiffness results are shown in Tables 1 and 2 for the room temperature and elevated temperature trials, respectively. The results indicate that the composite stiffness decreases with increasing temperature. Previous trials on a composite with a 13.4% NiTi area ratio have shown up to a 5% increase in stiffness with 3, 10 increasing temperature. A solid Al 3003-H18 sample of the same dimensions is expected to have an overall stiffness value and change in stiffness similar to what NiTi-Al composite 1 exhibits over the given temperature range. This indicates that the NiTi wires and Al matrix do not have satisfactory mechanical coupling allowing the wires to contract within the Al matrix without carrying the applied load. A material cross section was taken from one end of NiTi-Al composite 1 and prepared for optical microscopy. The resulting micrograph, Fig. 5, shows that during the UAM process the wires moved in the direction of ultrasonic vibrations, rolling on top of each other and disrupting the matrix flow. The disrupted matrix flow was not able to provide intimate contact between the wires and matrix thereby resulting in insufficient coupling. These results suggest that wire spacing must be considered during sample construction as interaction between wires during embedding will negatively impact coupling between the wires and matrix. Table 1. NiTi-Al UAM composite 25 C stiffness trial results. Max Load [N] Rising Load Stiffness [N/m] Falling Load Stiffness [N/m] E E E E E E+06 Average 6.04E E+06 Standard Deviation 4.56E E+05 C v 8% 5% Table 2. NiTi-Al UAM composite 150 C stiffness trial results. Max Load [N] Rising Load Stiffness [N/m] Falling Load Stiffness [N/m] E E E E E E+06 Average 5.78E E+06 Standard Deviation 9.88E E+05 C v 17% 3% Proc. of SPIE Vol O-5

6 Figure 5. Optical micrograph of NiTi-Al UAM composite 1 showing interaction of embedded wires resulting in poor mechanical coupling with the Al matrix NiTi composite thermomechanical results NiTi-Al composite 2 was thermally cycled in two separate trials. The first trial included two thermal cycles while the second trial included three thermal cycles. The results from the thermomechanical characterization tests are shown in Fig. 6 and Fig. 7. In each trial, the initial cycle showed a sharp strain recovery between 100 C and 130 C. Prior to this recovery, the composite exhibited a nearly linear expansion in response to increasing temperature. Upon cooling, the composite exhibited a linear contraction with decreasing temperature resulting in a net decrease in strain upon returning to 25 C. In trial 1, the composite exhibited an initial strain recovery of 1486 μɛ while in trial 2, the composite exhibited a 721 μɛ initial strain recovery. Subsequent thermal cycles showed significantly less strain recovery in both trials. This suggests that the behavior is due to initially detwinned embedded NiTi wires which recovered residual strain during the first thermal cycle and returned to a state in which the NiTi wires and Al matrix are in equilibrium. The behavior of composite 2 is similar to that of non-embedded shape memory NiTi: heating from an initial deformed state will result in strain recovery but subsequent thermal cycles will not exhibit strain recovery without additional detwinning of the NiTi from further deformation. In all cases, NiTi-Al composite 2 exhibited significantly less thermally induced strain than that expected from solid Al 3003-H18, as indicated by the red dashed line representing solid Al 3003 in Fig. 6 and Fig. 7. This result indicates that NiTi-Al composites are useful as a method to improve the dimensional stability of Al based structures exposed to temperature changes. Figure 6. Strain response to two successive temperature cycles for NiTi-Al composite 2, compared to the calculated strain response of solid Al 3003 H-18, trial 1. Proc. of SPIE Vol O-6

7 Figure 7. Strain response to three successive temperature cycles for NiTi-Al composite 2, compared to the calculated strain response of solid Al 3003 H-18, trial NiTi composite modeling NiTi composite stiffness modeling For development of future NiTi-Al composites, a model was created to determine the stiffness of NiTi-Al composite 1 as a function of temperature. This model assumes perfect coupling between the NiTi wires and Al matrix. In developing this model, values for the elastic moduli of Al 3003 and NiTi were obtained from the literature as 11, 12 shown in Table 3. For long fiber reinforced matrices, stiffness is calculated as a rule of mixtures, k comp (ξ,t) = E NiTi (ξ) A NiTi + E Al (T ) A Al. (3) L The elastic modulus of Al 3003 is linearly interpolated with temperature using modulus values at 24 C, 100 C, 149 C, and 177 C. The elastic modulus of NiTi is calculated as a function of volume fraction using the Brinson model, which describes the relation between volume fraction and modulus, 13 E NiTi (ξ) =(1 ξ) E A + ξe M. (4) Proc. of SPIE Vol O-7

8 Table 3. Elastic moduli values used for composite stiffness model. Property Description Value [GPa] 14 E A Austenite elastic modulus E M Martensite elastic modulus 26 E Al (24 C) 15 Al 3003 elastic modulus at 24 C 68 E Al (100 C) 15 Al 3003 elastic modulus at 100 C 65 E Al (149 C) 15 Al 3003 elastic modulus at 149 C 62 E Al (177 C) 15 Al 3003 elastic modulus at 177 C 59 A maximum percent change in stiffness is calculated by difference in composite stiffness when the embedded NiTi is fully austenitic and when it is fully martensitic, normalized by the martensitic composite stiffness, Δk comp (ξ,t)%= k comp (ξ,t) k comp (1, 24 C) k comp (1, 24. (5) C) The model results for expected percent stiffness change as a function of temperature are shown in Figure 8. The expected stiffness response from NiTi-Al composite 1 includes a peak at 88 C of approximately 12% increase in stiffness relative to room temperature. Due to the poor mechanical coupling of the NiTi wires and Al matrix, the measured behavior is closer to that of the black dotted line representing a homogenous Al 3003-H18 sample becoming softer with increasing temperature. Figure 8. Composite stiffness model for NiTi-Al UAM composite NiTi composite thermomechanical modeling The initial linear regions of the thermomechanical behavior of NiTi-Al composite 2, shown in Fig. 6 and Fig. 7, 11, 12 can be modeled as the thermal expansion of a long fiber reinforced composite using a rule of mixtures, α 1 = E Alα Al (1 f)+e NiTi α NiTi f, (6) E Al (1 f)+e NiTi f where E Al, E NiTi, α Al, α NiTi, are the elastic moduli and CTEs for Al 3003-H18 and NiTi, respectively. The value for martensitic NiTi is used for E NiTi, as once transformation to austenite begins, the composite is no longer in the linear region. The values used for elastic moduli are shown in Table 3. The values used for α Al and α NiTi are 23.2 ppm/ C 9 and 10 ppm/ C, 13 respectively. In (6), f represents the NiTi area ratio, which is 13.4% for composite 2. The resulting strain predicted by the model is shown in Fig. 9. Similar to the experimental results, the linear region of the thermally induced strain in the composite is less than that of solid Al 3003-H18 due to the lower CTE of the embedded NiTi. However, the strain recovery observed in the composite is due to the transformation Proc. of SPIE Vol O-8

9 of NiTi from martensite to austenite (M-A) which is currently not described by the model. Ongoing work includes modeling the M-A transition of the NiTi which takes into account the stress generated by differential thermal expansion of the Al matrix and NiTi wires and its impact on the transition temperatures of the NiTi phase. Figure 9. Linear thermal expansion model for NiTi-Al UAM composite GALFENOL-ALUMINUM UAM COMPOSITES 3.1 FeGa composite methods FeGa composite construction Three FeGa-Al composites were constructed via UAM. In all cases, the Galfenol piece was 381 μm thick and placed in a 381 μm deep groove machined in Al 3003-H18 plate as to remain flush with the welding surface. Each Galfenol piece was secured to the bottom of the plate using strain gauge adhesive and the remaining gap was filled with glazing putty, as shown in Fig. 10 (a), so as to keep the the material in place during the embedding process. FeGa-Al composites 1 and 2 were constructed to test the ability to embed Galfenol via UAM and to observe the Galfenol-Al interface via optical microscopy upon successful construction. The material embedded in FeGa-Al composites 1 and 2 is Fe81.6 Ga18.4. FeGa-Al composite 3 was created for actuation testing. The material in FeGa-Al composite 3 is Galfenol steel consisting of 18.4 at.% Ga and 1002 low carbon steel additions with magnetic domains oriented in the 100 direction, along the length of the strip. After embedding, FeGa-Al composite 3 was machined to a width of 25.4 mm (1 in) and a thickness of 1.27 mm (0.050 in). After machining, composite 3 had a small section of exposed Galfenol, as shown in Fig. 11, to be used as reference material in acuation testing. Figure 10. Galfenol-Al composites 1, 2, and 3 from top to bottom: (a) before embedding and (b) after embedding FeGa composite actuation testing Strain gauges were applied to FeGa-Al composite 3 on the exposed Galfenol and on the surface of the composite directly above the embedded Galfenol and were used to measure the strain generated along the direction of the Galfenol strip. The composite was suspended in an electromagnetic drive coil while flux was measured at the tip of the exposed Galfenol using a Hall probe. The drive coil was used to create an alternating field of ±30 ka/m while signals from both strain gauges were recorded. Proc. of SPIE Vol O-9

10 Figure 11. Machined FeGa-Al UAM composite FeGa composite experimental results FeGa embedding results FeGa-Al composites 1, 2, and 3 are shown in Fig. 10 (b) after the UAM process. The Galfenol piece in composite 1 dislodged during welding resulting in an empty cavity in the build. FeGa-Al composite 2 was well embedded in the Al matrix as indicated by the intimate contact observed in section micrographs (Fig. 12). This intimate contact indicates that there is mechanical coupling between the FeGa and Al and any strain in the FeGa sample induced by a magnetic field will be transferred to the magnetically inactive Al matrix, resulting in a magnetically active composite. Figure 12. Micrograph of material section of Galfenol-Al composite 2 showing intimate contact between the matrix and embedded material FeGa composite actuation results FeGa-Al composite 3 exhibited a strain response to the applied magnetic field as shown in Fig. 13. The strain responses of both the exposed Galfenol and composite show flat regions indicative of magnetic saturation of the FeGa. The exposed Galfenol exhibits a maximum strain of μ, which is typical for this alloy.16 The composite exhibits a 52.4 μ response resulting in a 27.1% transmission of magnetostriction at the outer fiber of the composite. Figure 13. Strain response of FeGa-Al composite and exposed FeGa versus applied magnetic field. Proc. of SPIE Vol O-10

11 4. PVDF-AL UAM COMPOSITES 4.1 PVDF composite construction method The third type of UAM composite was created by embedding a thin electroactive PVDF film. Prior to embedding, a 140 mm (5.5 in) long, mm (0.125 in) wide piece of PVDF with deposited electrodes was encased in polyimide tape (Fig. 14 (a)) resulting in a total thickness of 152 μm (0.006 in). The resulting sensor was attached to an amplifying circuit to observe its response to external pressure excitation prior to embedding. The sensor exhibited a capacitance of approximately 2 nf and no electrical continuity between the electrodes. The sensor was embedded by placing it in a 152 μm deep channel in an Al 3003-H18 baseplate and welding over top of the channel. Figure 14. PVDF film with deposited electrodes encapsulated in insulating tape: (a) prior to embedding via UAM and (b) after embedding. 4.2 PVDF composite construction results After embedding, a multimeter was used to measure capacitance of the sensor, resistance between the embedded sensor and the Al matrix, and resistance between the sensor s electrodes. The PVDF composite exhibited the same capacitance as it did before embedding, maintained high electrical resistance between the electrodes and the Al matrix, and did not have electrically shorted electrodes, indicating that the UAM process did not affect the integrity of the PVDF strip or insulation. The PVDF-Al composite was attached to the same amplifying circuit used to test the original sensor. Upon subjecting the composite to vibrations in initial testing, the embedded sensor produced a signal demonstrating that it remains electroactive after UAM processing, and furthermore, indicating that the average temperature of the welding interface did not exceed the Curie temperature of PVDF, 195 C, 17 during construction. Future tests will characterize the dynamic response of the composite which will act as a starting point for modeling and the design of more advanced PVDF-Al active composites. 5. SUMMARY In this research UAM has been used to embed three different smart materials, NiTi, Galfenol, and electroactive PVDF, in Al 3003-H18 matrices. Electrical insulation, stiffness tuning, and thermomechanical response were investigated for NiTi-Al composites. This research found that polyimide tape provides a durable coating capable of maintaining electrical insulation between an embedded material and the surrounding UAM matrix. With electrical insulation, it is now possible to create embedded NiTi sensors via UAM allowing for monitoring of stresses and strains inside of a metallic structure. While the stiffness tuning model predicts a 12% increase in stiffness for a 22.3% NiTi composite, poor coupling between the embedded wires and Al matrix resulted in a net softening of NiTi-Al composite 1. Future work will focus on composites with new geometries of NiTi to avoid movement during the embedding process. Thermomechanical experiments on NiTi-Al composite 2 indicate these MMCs are useful as thermal actuators, exhibiting over 1000 μɛ recovery, and as dimensionally stable Proc. of SPIE Vol O-11

12 structures in thermally varying environments. Modeling is continuing in order to more accurately describe the thermomechanical behavior of NiTi-Al UAM composites. UAM has also been successfully utilized to create FeGa-Al composites that exhibit magnetostriction when exposed to magnetic fields. The observed deformations are lower than the unloaded deformation of the Galfenol piece, which is due to loading of the aluminum matrix or imperfect coupling between the Galfenol piece and aluminum matrix. Ongoing work with FeGa-Al composites include non-contact sensing of composite stress and strain utilizing the embedded magnetostrictive material. A PVDF-Al composite was also successfully created via UAM. This composite demonstrates that UAM is capable of embedding polymers in metal matrices and also that bulk composite temperature remain low during the UAM process. The resulting composite has exhibited vibration sensing properties. Further testing will determine the frequency response and sensitivity of the composite to aid in designing new PVDF-Al builds. ACKNOWLEDGMENTS The authors would like to acknowledge Matt Short and Karl Graff from the Edison Welding Institute for their assistance and use of UAM equipment. Financial support for this research was provided by the Smart Vehicle Concepts Center ( a National Science Foundation Industry/University Collaborative Research Center, and by the Ohio State University through a Smart Vehicle Concepts Graduate Fellowship. REFERENCES [1] Krainer, K., [Metal Matrix Composites], Wiley-Vch Wevlag GmbH & Co., Weinheim, 2 52 (2006). [2] Chawla, N., Metal matrix composites in automotive applications, Advanced Materials and Processes, 164, (2006). [3] Hahnlen, R., Dapino, M., Short, M., and Graff, K., Aluminum-matrix composites with embedded Ni-Ti wires by ultrasonic consolidation, Proc. SPIE 7290 (2009). [4] Graff, K., [New Developments in Advanced Welding], Woodhead Publishing Limited, Cambridge, 241 (2005). [5] Vries, E. D., Mechanics and Mechanisms of Ultrasonic Metal Welding, PhD Thesis, The Ohio State University, Columbus, OH (2004). [6] Kong, C. and Soar, R., Fabrication of metal-matrix composites and adaptive composites using ultrasonic consolidation process, Materials Science and Engineering A, 412, (2005). [7] Siggard, E., Investigative Research into the Structural Embedding of Electrical and Mechanical Systems using Ultrasonic Consolidation, M.S. Thesis, Utah State University, Logan, UT (2007). [8] Scalea, F. L., Measurement of thermal expansion coefficients of composites using strain gauges, Experimental Mechanics, 328, (1998). [9] Kaufman, J., Aluminum alloy database, knovel.com (2004). [10] Hahnlen, R., Development and Characterization of NiTi Joining Methods and Metal Matrix Composite Transducers with Embedded NiTi by Ultrasonic Consolidation, M.S. Thesis, The Ohio State University, Columbus, OH (2009). [11] Clyne, T. and Withers, P., [An Introduction to Metal Matrix Composites], Cambridge University Press, Cambridge, (1993). [12] Staab, G., [Laminar Composites], Butterworth-Heinemann, Boston, 92 (1999). [13] Hartl, D. and Lagoudas, D., [Shape Memory Alloys], Science and Business Media, LLC, New York (2008). [14] Dynalloy, Inc., (2008). [15] Kaufman, J., ed., [Properties of Aluminum Alloys: Tensile, Creep, and Fatigue Data], The Aluminum Association, Inc. and ASM International, Materials Park (1999). [16] Mahadevan, A., Force and Torque Sensing with Galfenol Alloys, M.S. Thesis, The Ohio State University, Columbus, OH (2009). [17] Dargaville, T., Celina, M., Elliot, J., Chaplya, P., Jones, G., Mowery, D., Assink, R., Clough, R., and Martin, J., Characterization, performance, and optimization of PVDF as a piezoelectric film for advanced space mirror concepts, Sandia National Laboratory Report (2005). Proc. of SPIE Vol O-12