Ryan Hahnlen

Active metal-matrix composites with embedded smart materials by ultrasonic additive manufacturing

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 μ(see manuscript), and creation of a PVDF-Al composite sensor.

Fusion welding of nickel–titanium and 304 stainless steel tubes: Part II: tungsten inert gas welding

Shape memory nickel–titanium is attractive for lightweight actuators as it can generate large blocking stresses and high recovery strains through solid-state operation. A key challenge is the integration of the nickel–titanium components into systems; this alloy is difficult and expensive to machine and challenging to weld to itself and other materials. In this research, we join nickel–titanium and 304 stainless steel tubes of 9.53 mm (0.375 in) in diameter through tungsten inert gas welding. By joining nickel–titanium to a common structural material that is easily machined and readily welded to other materials, the system integration challenges are greatly reduced. The joints prepared in this study were subjected to optical microscopic inspection, hardness mapping, energy dispersive X-ray spectroscopy, mechanical testing, and failure surface analysis via scanning electron microscopy. The affected zone from welding is approximately 125 µm (0.005 in) wide including partially mixed zones with a maximum hardness of 817 HV and a possible heat-affected zone of 1–2 µm (39–79 µin) wide. The maximum average ultimate torsional strength is 415 MPa (60.2 x 103 lbf/in2). Implementation of this joining method is demonstrated in the construction of a solid-state torsional actuator that can lift a weight of 2.3 kg (5 lb) to a distance of 610 mm (24 in). The laser and TIG welding processes are compared.

Fusion welding of nickel–titanium and 304 stainless steel tubes: Part I: laser welding

Due to their large blocking stresses, high recovery strains, and solid-state operation, nickel–titanium actuators can offer substantial weight and space savings relative to traditional electric or hydraulic systems. A challenge surrounding NiTi-based actuators is integration of the NiTi components into a given system; this alloy is difficult and expensive to machine and challenging to weld to itself and structural materials. In this research, we join NiTi and 304 stainless steel tubes of 9.53 mm (0.375 in) in diameter through laser welding to create joints with weld depths up to 1.65 mm (0.065 in). By joining NiTi to a common structural material that is easily machined and readily welded to other materials, system integration is greatly improved. The joints prepared in this study were characterized through optical microscopy, hardness mapping, energy dispersive X-ray spectroscopy, mechanical testing, and analysis of the resulting fracture surfaces. The average ultimate shear strength of these joints is 429 MPa (62.2 103 lbf/in2) and the resulting fusion zone has a maximum width of 21.9 μm with a maximum hardness of 929 HV, while a possible heat-affected zone in NiTi is limited between 1 and 2 μm over most of the weld.

Aluminum-matrix composites with embedded Ni-Ti wires by ultrasonic consolidation

[Smart Vehicle Workshop] 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 UAM experience process temperatures as low as 20°C, in contrast to current metal-matrix fabrication processes which require fusion of materials and hence reach temperatures of 500°C and above. UAM thus creates unprecedented opportunities to develop adaptive structures with seamlessly embedded smart materials and electronic components without degrading the properties that make embedding these materials and components attractive. This research focuses on three aspects of developing UAM Ni-Ti/Al composites which have not been accomplished before: (i) Characterization of the mechanical properties of the composite matrix; (ii) Investigation of Ni-Ti/Al composites as tunable stiffness materials and as strain sensors based on the shape memory effect; and (iii) Development of constitutive models for UAM Ni-Ti/Al composites. The mechanical characterization shows an increase in tensile strength of aluminum UAM builds over the parent material (Al 3003-H18), likely due to grain refinement caused by the UAM process. We demonstrate the ability to embed Ni-Ti wires up to 203 μm in diameter in an aluminum matrix, compared with only 100 μm in previous studies. The resulting Ni-Ti/Al UAM composites have cross sectional area ratios of up to 13.4% Ni-Ti. These composites exhibit a change in stiffness of 6% and a resistivity change of -3% when the Ni- Ti wires undergo martensite to austenite transformation. The Ni-Ti area ratios and associated strength of the shape memory effect are expected to increase as the UAM process becomes better understood and is perfected. The Brinson constitutive model for shape memory transformations is used to describe the stiffness and the strain sensing of Ni-Ti/Al composites in response to temperature changes.

Performance and modeling of active metal-matrix composites manufactured by ultrasonic additive manufacturing

This paper presents the development and characterization of active aluminum-matrix composites manufactured by Ultrasonic Additive Manufacturing (UAM), an emerging rapid prototyping process based on ultrasonic metal welding. The primary benefit of UAM over other metal-matrix fabrication processes is the low process temperatures, as low as 25 °C. 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. The objective of this research is to develop UAM composites with aluminum matrices and embedded shape memory NiTi, magnetostrictive Galfenol (FeGa), and polyvinylidene fluoride (PVDF) phases. The paper is focused on the thermally induced strain response and stiffness behavior of NiTi-Al composites, the actuation properties of FeGa-Al composites, and the embedded sensing capabilities of PVDF-Al composites. We observe up to a 10% increase over room temperature stiffness for NiTi-Al composites and a magnetomechanical response in the FeGa-Al composite up to 52.4 με. The response of the PVDF-Al composite to harmonic loads is observed over a frequency range of 10 to 1000 Hz.

Dimensionally Stable Optical Metering Structures With NiTi Composites Fabricated Through Ultrasonic Additive Manufacturing

High performance optical metering structures in airborne and space applications need to exhibit dimensional stability in demanding thermal and mechanical environments. Materials for this application should have a low coefficient of thermal expansion, high thermal diffusivity, high specific stiffness and exhibit good ductility. Current materials are limited in one or more of these properties. Common choices are invar, carbonfiber composite, and silicon-carbide. The former has low specific stiffness and thermal diffusivity and the latter choices are brittle materials that require special care and have slow manufacturing processes. In this work, the development of a thermally invariant metal matrix composite will be described along with its incorporation into a high performance optical metering structure. The material is a composite of super-elastic NiTi ribbons and aluminum, where the ribbons are embedded using ultrasonic additive manufacturing. Measurements and modeling of the thermo-elastic response will be presented followed by the design and manufacture of a metering structure. The metering structure design eases integration with an optical bench and lens bezels while leveraging the advantageous properties of this new metal matrix composite.© 2013 ASME

Stress-induced tuning of ultrasonic additive manufacturing Al-NiTi composites

This paper addresses the development of active metal-matrix composites manufactured by Ultrasonic Additive Manufacturing (UAM), an emerging manufacturing process that allows the embedding of materials into seemingly solid metal components. In the UAM process, successive layers of metal tapes are ultrasonically bonded together at low temperatures to form a metal-matrix. Being a low-temperature process, UAM offers unprecedented opportunities to create metal components with embedded thermally-sensitive materials, such as shape memory alloys. In this study UAM is used to create composites with aluminum matrices and embedded NiTi ribbons. These composites exhibit tunability of both the coefficient of thermal expansion and natural frequencies. These effects are due to the phase-dependent modulus and transformation stresses developed by the prestrained NiTi phase. Since the embedded NiTi ribbons are constrained by the matrix, thermally-induced transformation from detwinned martensite to austenite will be accompanied by the generation of transformation stresses. The effect of transformation stress and changing phase of NiTi on thermally-induced strain is observed and modeled by combining strain matching algorithms with thermodynamic-based constitutive models. The composite model accurately describes effects due to changing NiTi modulus and strain recovery due to initial stress-induced martensitic volume fractions including a 200 με contraction with increasing temperature. The observed dynamic behaviors include up to a 16.6% increase in natural frequency at 100°C as compared to room temperature tests. No substantial increase in damping ratio was observed relative to solid aluminum.

Thermomechanical Behavior of Low CTE Metal-Matrix Composites Fabricated Through Ultrasonic Additive Manufacturing

Shape memory and superelastic NiTi are often utilized for their large strain recovery and actuation properties. The objective of this research is to utilize the stresses generated by pre-strained NiTi as it is heated in order to tailor the CTE of metal-matrix composites. The composites studied consist of an Al 3003-H18 matrix with embedded NiTi ribbons fabricated through an emerging rapid prototyping process called Ultrasonic Additive Manufacturing (UAM). The thermally-induced strain of the composites is characterized and results show that the two key parameters in adjusting the effective CTE are the NiTi volume fraction and prestrain of the embedded NiTi. From the observed behavior, a constitutive composite model is developed based constitutive SMA models and strain matching composite models. Additional composites were fabricated to characterize the NiTi-Al interface through EDS and DSC. These methods were used to investigate the possibility of metallurgical bonding between the ribbon and matrix and determine interface shear strength. Interface investigation indicates that mechanical coupling is accomplished primarily through friction and the shear strength of the interface is 7.28 MPa. Finally, using the developed model, a composite was designed and fabricated to achieve a near zero CTE. The model suggests that the finished composite will have a zero CTE at a temperature of 135°C.Copyright

TIG Welding of Nickel-Titanium to 304 Stainless Steel

Shape memory nickel-titanium (NiTi) is attractive for use in solid-state actuators as it exhibits large recoverable stresses, limited by its ultimate shear strength of over 120 ksi (960 MPa), and large recoverable strains, up to 8%. Broad application of NiTi is hindered by the expense, complexity, and lack of reliability in machining and joining it to structural materials. This paper investigates the use of orbital Tungsten Inert Gas (TIG) welding to join NiTi to 304 stainless steel (304 SS), a common structural material that can be readily machined and welded. Tubes of NiTi and 304 SS were joined using a nickel filler to mitigate the formation of brittle intermetallics. Both tubes had a 0.375 in (9.53 mm) outer diameter with wall thicknesses of 0.065 in and 0.075 in (1.7 mm and 1.9 mm) for the 304 SS and NiTi tubes, respectively. Viable joints were created and characterized through X-ray analysis, optical microscopy, hardness mapping, and strength testing. The joints had an average failure torque of 450 in-lb (52 N-m), corresponding to an ultimate shear strength of approximately 50 ksi (350 MPa). This was sufficient to detwin the NiTi in the tubes, which occurs at a shear stress of 16 ksi (110 MPa), and plastically deform the annealed 304 SS tubes. Optical microscopy and hardness mapping revealed a heat-affected zone 0.005 in (125 μm) wide with a maximum hardness of 817 HV. Outside of this heat-affected zone the hardness was not affected, indicating that no large-scale loss of superelastic or shape memory properties arises from TIG welding.Copyright

Laser Welding of Nickel Titanium and 304 Stainless Steel Tubes

Shape memory nickel-titanium (NiTi) can generate large blocking stresses and high recovery strains, up to 8%, which make NiTi a good candidate for solid state actuators, resulting in substantial weight and space savings when they replace traditional electric or hydraulic systems. A challenge surrounding NiTi based actuators is integration of the NiTi components into a given system; this alloy is difficult and expensive to machine and weld to itself and structural materials. In this research, we join NiTi and 304 stainless steel tubes 9.52 mm (0.375 in) in diameter through laser welding to create joints with weld depths up to 1650 μm (0.065 in). By joining NiTi to a common structural material that is easily machined and readily welded to other materials, the challenges surrounding system integration are reduced. The joints prepared in this study were characterized through optical microscopy, hardness mapping, and mechanical testing. The average ultimate shear strength of these joints is 423 MPa (61.3 ksi) and the resulting HAZ has a maximum width of 21.9 μm with a maximum hardness of 929 HV.Copyright