20 research outputs found
Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075.
Lightweight materials are of paramount importance to reduce energy consumption and emissions in today's society. For materials to qualify for widespread use in lightweight structural assembly, they must be weldable or joinable, which has been a long-standing issue for high strength aluminum alloys, such as 7075 (AA7075) due to their hot crack susceptibility during fusion welding. Here, we show that AA7075 can be safely arc welded without hot cracks by introducing nanoparticle-enabled phase control during welding. Joints welded with an AA7075 filler rod containing TiC nanoparticles not only exhibit fine globular grains and a modified secondary phase, both which intrinsically eliminate the materials hot crack susceptibility, but moreover show exceptional tensile strength in both as-welded and post-weld heat-treated conditions. This rather simple twist to the filler material of a fusion weld could be generally applied to a wide range of hot crack susceptible materials
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Microstructure Control and Performance Evolution of Aluminum Alloy 7075 by Nano-Treating.
Nano-treating is a novel concept wherein a low percentage of nanoparticles is used for microstructural control and property tuning in metals and alloys. The nano-treating of AA7075 was investigated to control its microstructure and improve its structural stability for high performance. After treatment with TiC nanoparticles, the grains were significantly refined from coarse dendrites of hundreds of micrometers to fine equiaxial ones smaller than 20 μm. After T6 heat treatment, the grains, with an average size of 18.5 μm, remained almost unchanged, demonstrating an excellent thermal stability. It was found that besides of growth restriction factor by pinning behavior on grain boundries, TiC nanoparticles served as both an effective nucleation agent for primary grains and an effective secondary phase modifier in AA7075. Furthermore, the mechanical properties of nano-treated AA7075 were improved over those of the pure alloy. Thus, nano-treating provides a new method to enhance the performance of aluminum alloys for numerous applications
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Stretching Micro Metal Particles into Uniformly Dispersed and Sized Nanoparticles in Polymer.
There is a longstanding challenge to disperse metal nanoparticles uniformly in bulk polymers for widespread applications. Conventional scale-down techniques often are only able to shrink larger elements (such as microparticles and microfibers) into micro/nano-elements (i.e. nanoparticles and nanofibers) without much altering their relative spatial and size distributions. Here we show an unusual phenomenon that tin (Sn) microparticles with both poor size distribution and spatial dispersion were stretched into uniformly dispersed and sized Sn nanoparticles in polyethersulfone (PES) through a stack and draw technique in thermal drawing. It is believed that the capillary instability plays a crucial role during thermal drawing. This novel, inexpensive, and scalable method overcomes the longstanding challenge to produce bulk polymer-metal nanocomposites (PMNCs) with a uniform dispersion of metallic nano-elements
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Aluminum with dispersed nanoparticles by laser additive manufacturing.
While laser-printed metals do not tend to match the mechanical properties and thermal stability of conventionally-processed metals, incorporating and dispersing nanoparticles in them should enhance their performance. However, this remains difficult to do during laser additive manufacturing. Here, we show that aluminum reinforced by nanoparticles can be deposited layer-by-layer via laser melting of nanocomposite powders, which enhance the laser absorption by almost one order of magnitude compared to pure aluminum powders. The laser printed nanocomposite delivers a yield strength of up to 1000 MPa, plasticity over 10%, and Young's modulus of approximately 200 GPa, offering one of the highest specific Young's modulus and specific yield strengths among structural metals, as well as an improved specific strength and thermal stability up to 400 °C compared to other aluminum-based materials. The improved performance is attributed to a high density of well-dispersed nanoparticles, strong interfacial bonding between nanoparticles and Al matrix, and ultrafine grain sizes
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Bulk ultrafine grained/nanocrystalline metals via slow cooling.
Cooling, nucleation, and phase growth are ubiquitous processes in nature. Effective control of nucleation and phase growth is of significance to yield refined microstructures with enhanced performance for materials. Recent studies reveal that ultrafine grained (UFG)/nanocrystalline metals exhibit extraordinary properties. However, conventional microstructure refinement methods, such as fast cooling and inoculation, have reached certain fundamental limits. It has been considered impossible to fabricate bulk UFG/nanocrystalline metals via slow cooling. Here, we report a new discovery that nanoparticles can refine metal grains to ultrafine/nanoscale by instilling a continuous nucleation and growth control mechanism during slow cooling. The bulk UFG/nanocrystalline metal with nanoparticles also reveals an unprecedented thermal stability. This method overcomes the grain refinement limits and may be extended to any other processes that involve cooling, nucleation, and phase growth for widespread applications
Nanoparticle Enabled Phase Control in Solidification Processing
Solidification processing such as casting is of paramount significance for the mass production of complex materials and components throughout human history. Phase control during solidification processing is vital to achieve desired structures and properties for numerous applications. However, it is a long-standing challenge to achieve effective phase control during solidification processing of materials. Conventional phase control approaches have gradually encountered certain technical and/or fundamental limits. The emerging nanotechnology could provide a new pathway to control the phase evolution during solidification to break these limits. The objective of this study is to significantly advance the fundamental understanding of nanoparticles enabled phase control during solidification processing of materials and to overcome the limits of the current methods by the incorporation of nanoparticles. More specifically, fundamental studies were conducted to understand how nanoparticles interact with liquid-liquid phase in immiscible alloys, solid-liquid phase during solidification of pure metals for unprecedented grain refinement and solid-solid phase for the grain structure stability at high temperatures.To investigate the nanoparticle enabled phase control in solidification processing, key issues of nanoparticles incorporation and dispersion should be tackled first. In this study, a novel salt-assisted incorporation method was developed to fabricate metals containing uniformly distributed and dispersed nanoparticles. Molten salt such as KAlF4 can effectively remove the oxide layer at the top of the metal melt and readily assist the incorporation of nanoparticles into molten metal. This new salt-assisted incorporation paves a new pathway for mass production of metal matrix nanocomposites. In this thesis, metal matrix nanocomposites of Al-TiC, Al-TiB2 and Cu-WC were successfully fabricated by this method through solidification process. In addition, to enhance the Orowan strengthening and study the phase control effect from nanoparticles of smaller diameter, a novel and facile molten salt reaction method was developed to synthesis small TiC nanoparticles with a diameter about 10 nm, which is not commercially available. The size of the TiC nanoparticles can be well controlled by the reaction template made of diamond nanoparticles in this study. To further improve and simplify the processing of Al nanocomposites reinforced by TiC (10 nm) nanoparticles, a novel in-situ reaction and incorporation method was explored, which enable one-step synthesis and incorporation. Al-TiC nanocomposites with 10 nm TiC nanoparticles show significantly improved mechanical properties. Cu-WC nanocomposites with various volume fractions of nanoparticles were also successfully fabricated by this molten salt method after different salts were tested for optimized processing. KAlF4, borax and NaCl were all effective for the incorporation of WC nanoparticles into the molten Cu. WC nanoparticles can be uniformly distributed and dispersed in Cu matrix by this method and provide much enhanced mechanical properties (strength, hardness and Young’s modulus) without significant deterioration of the electrical conductivity, which suggests that the new Cu-WC nanocomposites could serve as high strength and high electrical conductivity materials for a widespread range of applications. . The nanoparticles enabled phase control for liquid-liquid phase were studied in immiscible alloys both experimentally and theoretically to advance the fundamental understanding and to solve the long-standing challenges in solidification processing of immiscible alloys. It is shown that nanoparticles can move to the interface between the primary phase and secondary phase forming a coating and slowing down the diffusional growth of the secondary phase, which result in a significantly refined microstructure. TiC and TiC0.7N0.3 nanoparticles were successfully utilized in Al-Bi immiscible alloy for scalable manufacturing of high performance self-lubrication bearing materials. It was also found that the addition of Cu element can significantly enhance the mechanical properties without microstructure deterioration. Tungsten (W) nanoparticle is also demonstrated to be effective for the processing Zn-Bi immiscible alloys. In addition, our study show that nanoparticles not only worked for processing of metallic immiscible alloys, but also function well in organic immiscible systems such as B4C nanoparticles in the organic SCN-CTB immiscible system. The solid-liquid phase control by nanoparticles were investigated by studying the grain refinement effect during solidification. The grain refinement by nanoparticles for pure metals is studied in different model material systems such as Cu-WC, Al-TiB2 and Zn-WC. In this study, a new discovery that nanoparticles can refine metal grains down to ultrafine or even nanoscale by instilling a continuous nucleation and grain growth restriction mechanism during slow cooling (< 100 K/s) is reported for the first time. When casting pure Cu with WC nanoparticles, the grain sizes of Cu are refined substantially ultrafine and even nanoscale, which is approximate to the inter-particle spacing. Differential scanning calorimetry (DSC) studies showed that Cu-WC samples need significantly longer time (by 83%) to complete the solidification than pure Cu, indicating nanoparticles can substantially slow down the solidification process. It is proposed that nanoparticles can restrict the grain growth by the Gibbs-Thompson effect, thus allowing a continuous nucleation throughout the solidification process. Furthermore, this newly revealed grain control mechanism is successfully applied in other materials systems such as Al-TiB2 and Zn-WC for ultrafine grains via slow cooling. This revolutionary method paves a pathway for the mass production of bulk stable ultrafine-grained (UFG)/nanocrystalline materials. This method may be readily extended to any other processes that involve cooling, nucleation and phase growth for widespread applications. The solid-solid phase control by nanoparticles was investigated by studying the thermal stability of UFG/nanocrystalline Cu-WC nanocomposites. It is shown that the as-solidified bulk ultrafine/nanocrystalline Cu reveals an unprecedented thermal stability up to 1023 K (0.75 melting point of Cu) by the Zener pinning effect from WC nanoparticles. In summary, this dissertation presents experimental methods and a theoretical framework to understand and utilize the newly discovered nanoparticles enabled phase control mechanisms during solidification processing. Cases studies of liquid-liquid phase (e.g. immiscible alloys), solid-liquid phase (e.g. grain refinement) and solid-solid phase (grain structure thermal stability) of pure metals show that nanoparticle is extremely effective to control phase evolution. This approach breaks the fundamental and/or technical limits for solidification processing of numerous metals with unusual properties for broad applications
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Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075.
Lightweight materials are of paramount importance to reduce energy consumption and emissions in today's society. For materials to qualify for widespread use in lightweight structural assembly, they must be weldable or joinable, which has been a long-standing issue for high strength aluminum alloys, such as 7075 (AA7075) due to their hot crack susceptibility during fusion welding. Here, we show that AA7075 can be safely arc welded without hot cracks by introducing nanoparticle-enabled phase control during welding. Joints welded with an AA7075 filler rod containing TiC nanoparticles not only exhibit fine globular grains and a modified secondary phase, both which intrinsically eliminate the materials hot crack susceptibility, but moreover show exceptional tensile strength in both as-welded and post-weld heat-treated conditions. This rather simple twist to the filler material of a fusion weld could be generally applied to a wide range of hot crack susceptible materials
High Strength and High Electrical Conductivity Al Nanocomposites for DC Transmission Cable Applications
Aluminum is one of the most abundant lightweight metals on Earth with broad practical applications, such as in electrical wires. Although traditional aluminum manufacturing by alloying, deformation and thermomechanical means addresses the balance between high strength and high conductivity, adding metallic ceramic nanoparticles into the aluminum matrix can be an exciting alternative approach to mass produce aluminum electrical wires. Here, we show a new class of aluminum nanocomposite electrical conductors (ANECs), with significantly higher hardness (130 HV) and good electrical conductivity (41% IACS). This ANEC is composed of Al and dispersed TiB2 nanoparticles, as confirmed by XRD scanning and SEM imaging. We further observed an unusual ultra-fine grain (UFG) size when slow cooling ANEC samples, as a grain as small as 300 nm was clearly captured in FIB images. We believe that the significant hardness enhancement can be partially attributed to the UFG. Our investigation and theoretical analysis further validated that UFG can be achieved when nanoparticles are uniformly dispersed and distributed in the aluminum matrix, and this understanding is important for the development of Al nanocomposite wires with high strength and high electrical conductivity