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

Fundamental Study of Nanoparticle Effects on Functional Properties of Metals

The objective of this study is to provide insights and guidance for the rational design of high-performance metal matrix nanocomposites (MMNCs) with tunable functional properties for widespread applications. Microstructure-property relationship is a long-term focus for MMNCs, and incorporation of different nanoparticles would change these properties differently. Since the metals/alloys are unique with their high electron concentration and strongly coupled interaction between electrons and other configurational (micro-)structures, introducing nanoparticles into them will significantly influence electrical, thermal, chemical, and electrochemical properties. Given the various demands from industrial fields, MMNCs with predictable and reliable functional properties are urgently needed. However, there still exist significant challenges in utilizing optimal functional properties with a suitable combination of metals/alloys and nanoparticles. Several reasons contribute to the deadlock situations: First, the supreme functional properties would limit the selection of nanoparticles, but specific desired nanoparticles may be hard to fabricate, incorporate, and uniformly disperse in metals/alloys. Second, due to the lack of studies into nanoparticles-affected functional properties, the relationships between microstructures, processing routes, and final performance are too complicated to be determined. Therefore, though mechanical properties have been studied for decades, the lack of functional performance study hinders the use of metal matrix nanocomposites. In this dissertation, a wide variety of MMNCs with a rational selection of nanoparticles were fabricated both in situ and ex situ to experimentally study the nanoparticle effects on their functional properties. Electrical, thermal, chemical (mainly anti-oxidation), electrochemical, and other selective functional properties (mainly tribological performance) were studied. The underlying mechanisms and (semi-)quantitative models have been investigated and developed to reveal nanoparticles’ role in tuning these properties, providing insightful guidelines for the rational design of high-performance MMNCs. The metal matrices in MMNCs are full of free electrons, and the high concentration of electrons is the most crucial factor to determine the electrical performance and other functional properties in MMNCs. In this study, the electrical performance of MMNCs fabricated by both in situ and ex situ methods have been investigated. Both Cu and Al alloys as the highly conductive matrices have been studied, and the metal-like ceramic nanoparticles such as WC, TiC, TiB2, and ZrB2 were used. First, the electron performance was measured with large temperature scanning on physical property measurement system (PPMS), and the role of the electron concentration and the electron diffusivity have been decoupled in the MMNC system. The experiments demonstrated that the reduced electrical conductivity is associated with a reduced electron concentration by the interfacial electron localization. Second, with the understanding of the reduced apparent free electron concentration in MMNCs, a quantitative model was developed to depict the electrical conductivity change after the nanoparticle incorporation, and the feasibility of this model has been confirmed in Cu- and Al-based MMNC systems. This part of the study illustrates the fundamental role of the matrix-nanoparticle interface and explain the quantitative influences of the interfaces on the electronic parameters related to electrical conductivity. The developed model to predict electrical conductivity is necessary for the MMNC applications in electronic sectors. With this understanding, since electrons are the dual carrier for both electricity and heat, the thermal performance of the MMNCs has been systematically investigated. In this study, ex situ nanocomposites of Cu alloy (i.e., Cu-Ag/WC) and in situ nanocomposites of Al (i.e., Al-TiC, Al-TiB2, and Al-ZrB2) were the primary focus. With the detailed microstructure investigation, matrix-nanoparticle interfacial characterization, and thermal parameter analyses, the influence of nanoparticles on the thermal performance of metal matrix nanocomposites has been clarified. The changes in heat capacity (by differential scanning calorimetry), thermal diffusivity (by laser flash method), and thermal conductivity have been decoupled. The contribution from electronic and phonic thermal transport has been compared. This part of the study confirms the electron behavior change by nanoparticles. It illustrates a semi-quantitative relationship and close links between the investigated electronic and thermal properties in MMNCs. The analyzed systems (i.e., Cu and Al nanocomposites) would be critical to rational design and applications of MMNCs in thermal management fields. Similarly, due to the high concentration of electrons of metal matrices and their relatively high activity, even with the conductive ceramic nanoparticles, it is of interest to investigate how metal matrix nanocomposites respond to environments. Oxidation and corrosion are the two primary degradation forms of metals in the environment, potentially compromising their service life and significantly limiting their applications in various conditions. This part of the study would be divided into two sections accordingly: Firstly, how the high-temperature oxidation process is influenced and how the temperature-dependent stability is tuned by nanoparticles have been investigated. Quantitative information about the thermal oxidation in Cu-based (i.e., Cu-40 wt.% Zn/WC) and Al-based (i.e., Al/ZrB2) nanocomposites have been obtained via ex situ and in situ (mainly in situ XRD) measurement methods. Two distinctive oxidation layer growth modes (e.g., continuous growth in Cu-40 wt.% Zn/WC and self-limiting growth in Al/ZrB2 nanocomposites) have been identified, respectively. The thermal oxidation kinetics and dynamics in MMNCs have also been clarified. The interactions among nanoparticles, microstructures, and oxidation driving force have been studied, and the potential applications and effective prevention measures have also been proposed. Second, metal corrosion is a process associated with electron transfer and ion transport. During the process, corroded by-products (mainly oxides and hydro-oxides) will appear on the metal surface as a protection layer. Therefore, to understand the corrosion performance in MMNCs, the electron behavior and oxide growth were integrated. In this part of the study, aluminum alloy 7075 and A206 nanocomposites (i.e., wrought AA7075-TiB2 and AA7075-TiC as well as cast A206-TiC) were the main focus. During the experiments, the corrosion processes on the freshly exposed surface, passivated surface (with oxide layer), and passivated and then immersed surface (after being pitted) were compared, and their corrosion dynamical characteristics have been depicted with polarization potential scanning and electrochemical impedance scanning (EIS). Different corrosion performances, including pitting, intergranular corrosion, and stress crack corrosion, have been investigated under ASTM standards. The interplay between microstructures, oxidation, and corrosion has been quantitatively studied. In short, the corrosion study of Al nanocomposites has advanced the understanding of the corrosive degradation process. It would also fundamentally shed light on possible measures of promoting the overall anti-corrosion performance in MMNCs. Moreover, given other essential applications of MMNCs, other functional properties linked with tribological and manufacturing fields have been investigated. In summary, this dissertation’s extensive experimental studies have provided a useful fundamental understanding of the promising and tunable functional properties in various MMNCs. The starting point of electron behavior has created a unique angle to look into these functional properties by linking microstructures, electrons, and incorporated nanoparticles. Then, thermal properties, anti-oxidation performance, and anti-corrosion performance have been systematically studied, and the (semi-)quantitative models have been developed. Finally, tribological properties have been studied. This study advances the knowledge for rational design and manufacturing of high-performance MMNCs with desirable predictable functional properties for numerous applications

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

Manufacturing and Characterization of Zn-WC as Potential Biodegradable Material.

This work presents the manufacturing and characterization of zinc-tungsten carbide (Zn-WC) nanocomposite as a potential biodegradable material. A highly homogeneous WC nanoparticle dispersion in a Zn matrix was achieved by molten salt assisted stir casting followed with hot rolling. The Vickers microhardness and ultimate tensile strength of zinc were enhanced more than 50% and 87%, respectively, with the incorporation of up to 4.4 vol. % WC nanoparticles. Additionally, Zn-WC nanocomposite retained high ductility (> 65%). However, the electrical and thermal conductivities were reduced by 12% and 21%, respectively. The significant enhancement in mechanical strength makes nanoparticle-reinforced zinc a promising candidate material for biodegradable metallic implants for a wide range of clinical applications, including orthopaedic and cardiovascular implants as well as bioresorbable electronics

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

Experimental study on novel biodegradable Zn-Fe-Si alloys.

Bioabsorbable metals are increasingly attracting attention for their potential use as materials for degradable implant devices. Zinc (Zn) alloys have shown great promises due to their good biocompatibility and favorable degradation rate. However, it has been difficult to maintain an appropriate balance among strength, ductility, biocompatibility, and corrosion rate for Zn alloys historically. In this study, the microstructure, chemical composition, mechanical properties, biocompatibility, and corrosion rate of a new ternary zinc-iron-silicon (Zn-Fe-Si) alloy system was studied as a novel material for potential biodegradable implant applications. The results demonstrated that the in situ formed Fe-Si intermetallic phases enhanced the mechanical strength of the material while maintaining a favorable ductility. With Fe-Si reinforcements, the microhardness of the Zn alloys was enhanced by up to 43%. The tensile strength was increased by up to 76% while elongation to failure remained above 30%. Indirect cytotoxicity testing showed the Zn-Fe-Si system had good biocompatibility. Immersion testing revealed the corrosion rate of Zn-Fe-Si system was not statistically different from pure Zn. To understand the underlying phase formation mechanism, the reaction process in this ternary system during the processing was also studied via phase evolution and Gibbs free energy analysis. The results suggest the Zn-Fe-Si ternary system is a promising new material for bioabsorbable metallic medical devices

Highly Ductile Zn-2Fe-WC Nanocomposite as Biodegradable Material.

Zinc (Zn) has been widely investigated as a biodegradable metal for orthopedic implants and vascular stents due to its ideal corrosion in vivo and biocompatibility. However, pure Zn lacks adequate mechanical properties for load-bearing applications. Alloying elements, such as iron (Fe), have been shown to improve the strength significantly, but at the cost of compromised ductility and corrosion rate. In this study, tungsten carbide (WC) nanoparticles were incorporated into the Zn-2Fe alloy system for strengthening, microstructure modification, and ductility enhancement. Thermally stable WC nanoparticles modified the intermetallic ζ-FeZn13 interface morphology from faceted to non-faceted. Consequently, WC nanoparticles simultaneously enhance mechanical strength and ductility while maintaining a reasonable corrosion rate. Overall, this novel Zn-Fe-WC nanocomposite could be used as biodegradable material for biomedical applications where pure Zn is inadequate

Nano-Treating Promoted Natural Aging Al-Zn-Mg-Cu Alloys

Natural aging reduces the cost of alloy manufacturing while saving input energy but takes too long to complete for most Al-Zn-Mg-Cu alloys. Research has proved that nano-treating can facilitate precipitation in heat-treatable alloys. In this study, nano-treated Al-6.0Zn-2.6Mg-xCu samples containing different Cu contents were fabricated to investigate the influence of nano-treating on natural aging. TiC nanoparticles were used for nano-treating. Three cooling conditions after solution treatment (water quenching, air cooling, and as-cast) were investigated to check their quench sensitivities. The study shows the alloy’s microstructure was modified by nano-treating, and the growth of dendritic arms was inhibited. Compared to the control samples, nano-treating also increased both the microhardness and tensile strength of the alloy after natural aging. Out of the three different solution treatments, the air-cooled samples presented the highest UTS and microhardness values. The precipitation process was sped up by nano-treating by approximately 50%, and a higher volume fraction of GPII zones were formed in the nano-treated samples. HRTEM results also confirm the formation of more GPI and GPII zones in a nano-treated samples. With the help of natural aging, the Al-6.0Zn-2.6Mg-0.5Cu alloy reached a UTS of 455.7 ± 40.2 MPa and elongation of 4.52 ± 1.34% which makes it a great candidate for a naturally aged Al-Zn-Mg-Cu alloy

Zn–Mg–WC Nanocomposites for Bioresorbable Cardiovascular Stents: Microstructure, Mechanical Properties, Fatigue, Shelf Life, and Corrosion

Zinc (Zn) and Zn alloys have been studied as potential materials for bioresorbable stents (BRSs) in the last decade due to their favorable biodegradability and biocompatibility. However, most Zn alloys lack the necessary combination of strength, ductility, fatigue resistance, corrosion rate (CR), and thermal stability needed for such applications. In this study, nanoparticles made of tungsten carbide (WC) were successfully incorporated into Zn alloyed with 0.5 wt % magnesium (Mg) and evaluated for their suitability for BRS applications. Specifically, the resulting Zn-0.5Mg-WC nanocomposite's microstructure, mechanical properties, in vitro CR, and thermal stability were evaluated. The Zn-0.5Mg-WC nanocomposite had excellent mechanical strength [ultimate tensile strength (UTS) > 250 MPa], elongation to failure (>30%), and a suitable in vitro CR (∼0.02 mm/y) for this clinical application. Moreover, the Zn-0.5Mg-WC nanocomposite survived 10 million cycles of tensile loading (stress ratio, R = 0.053) when the maximum stress was 80% of the yield stress. Its ductility was also retained during a 90-day thermal stability study, indicating an excellent shelf life. Stent prototypes were fabricated using this composition and were successfully deployed during bench testing without fracture. These results show that the Zn-0.5Mg-WC nanocomposite is a promising material for BRS applications. In vivo studies are underway to validate both biocompatibility, stent function, and degradation