Study of Nano-treating Effects on High Strength Aluminum Alloys
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Study of Nano-treating Effects on High Strength Aluminum Alloys


High strength aluminum alloys have been used widely in the aerospace and automobile industries because of their high specific strength and good corrosion resistance. The rapid development of electric vehicles promotes the demand for these alloys for energy saving. High-strength aluminum alloys, including heat-treatable 7xxx and 2xxx series, have been studied for decades. Their strength mainly comes from uniformly distributed precipitates inside the grains. High alloying is often required to provide a high-volume fraction of the precipitates to improve their mechanical properties. However, this alloying strategy brings along drawbacks. First of all, a higher strength usually compromises other properties such as ductility, electrical conductivity, and stress corrosion resistance, etc. Secondly, a higher alloying often deteriorates the manufacturing capability of the alloys. For example, AA7034 aluminum alloy containing 11.0-12.0% Zn and 2.5% Mg offers the highest strength among all commercial aluminum alloys. Rapid cooling is required for manufacturing AA7034 because of the need to suppress solidification cracking, porosity, and inhomogeneity induced by the high alloying. Moreover, high alloying often results in bulky secondary phases after common solidification, such as casting, thus making solutionization more difficult. Lastly, thermodynamics imposes inherent limits for alloy element/s solubility, thus unable to bring out more precipitates. In the 2xxx series, an extra amount of Mg cannot be dissolved during solutionization because of this solubility limit. The metallurgical barriers (e.g., manufacturing capability and thermodynamic limits) pose a grand challenge to further design and improve the overall properties of the high-strength aluminum alloys for widespread applications.A new nanotechnology-enabled metallurgy method, Nano-Treating (NT), has been proposed in recent years. By adding a low volume fraction of nano-reinforcements into an alloy melt (i.e., nano-treating), it can tune the solidification/manufacturing process, microstructure, and properties of the alloy. Researchers have found that nano-reinforcements, especially nanoparticles, can induce heterogeneous nucleation and effectively restrict grain growth during solidification, thus significantly reducing grain size and inhibiting dendritic arm development. When nanoparticles are added to multi-phase alloys, the secondary phase can be altered, refining secondary phases and modifying phase morphology. Moreover, nanoparticles can influence the precipitation behaviors of heat-treatable aluminum alloys. The precipitate free zone was found to be much reduced by nanoparticles. Improved mechanical properties were achieved by nano-treating too. Nano-Treating has emerged as a powerful new metallurgical method in addition to the traditional ones like alloying, grain refining, heat treatment, and plastic deformation. However, the nano-treating effects have never been systematically studied despite their great potential. To empower this new nanotechnology to improve high-strength aluminum alloys' manufacturing capability and properties, this thesis focuses on the nano-treating effects on solidification behavior and heat-treatment of high-strength aluminum alloys, and a systematic study was conducted. There are two methods to incorporate and disperse ceramic nanoparticles into the metal matrix. The first method is ex-situ incorporation that requires the addition of existing ceramic nanoparticles into the metal melt from outside. This method is more limited for manufacturing due to the cost of ex-situ ceramic nanoparticles. More often, the interfacial bonding between nanoparticles and matrix is not ideal. The second, more economical method is the in-situ synthesis of nanoparticles inside the molten metals. However, effective control of nanoparticle sizes during in-situ synthesis is challenging. Available controlling processes like rapid solidification, ultrasonication-assist fabrication, and high-energy ball milling are expensive and difficult to scale up for mass production. To enable nano-treating for mass production of metals, a new cost-effective production method must be developed. The diffusion-controlled growth of a particle is related to the supersaturation of the reactants, diffusivity of reactants, and growth time. As the reaction is typically at high temperatures in molten metal, no surfactants can survive the temperature, thus unable to be utilized for diffusion and growth time control. Therefore, diluting the reactants would be effective in reducing supersaturation for size control of synthesized nanoparticles. A new controlling mechanism on growth time is also needed to achieve a more uniform size distribution. This study discovered an interface-controlled mechanism to successfully fabricate in-situ TiB2, TiC, WC/W, ZrB2 nanoparticles with small size and narrow size distribution. The new nanoparticle synthesis method builds a solid foundation for the industry-scale application of nano-treating. A systematic study of nano-treating effects on solidification and heat treatment was then conducted on high strength aluminum alloys. Examination of the microstructures of the nano-treated alloys and solidification curves suggest a promoted nucleation and effective restriction of grain growth. During the last stage of solidification, the grain coherent point (GCP) is effectively postponed by nanoparticles, thus allowing sufficient time for liquid feeding to prevent solidification shrinkage and hot tearing. These nanoparticles-induced effects can significantly improve the casting capability and mechanical integrity as well as properties of alloys. The volume fraction of the secondary phases is also altered by nano-treating, possibly due to a higher permeability of the coherent solid network and a higher viscosity of the liquid. Furthermore, the study suggests that a promoted diffusion is achieved by interface and dislocation mediated diffusion. It is believed that nano-treating can effectively promote the solution treatment due to a reduced volume fraction and refined size of secondary phases, which facilitate the dissolution of the secondary phases during solutionization. This promotion effect enables the successful manufacturing of high alloying alloys like AA7034 at slow cooling by traditional casting. Improved dissolution of the secondary phases results in a higher level of supersaturation of the solute atoms within a specific time limit. Dislocations can also serve as heterogeneous nucleation sites to facilitate precipitates. A promoted aging also allows natural aging to yield adequate strength at a relatively short time. In general, nano-treating has two effects: “antibody effect” to improve the manufacturing capability and “vitamin effect,” to tune the microstructure and properties of the alloy. To demonstrate the nano-treating effects, three application case studies are conducted. The first application case study is to enable the manufacturing of high-strength Al-Zn-Mg-Cu alloy by casting. While the highly alloyed AA7034 alloy offers the highest tensile strength among commercial aluminum alloys, its manufacturing typically requires a rapid solidification process, such as spray casting. Under any regular cooling rate, its high alloying content makes the secondary phase large and difficult to be dissolved. By nano-treating, AA7034 is successfully cast at a slow cooling rate with significantly modified microstructure, including the volume and size of the secondary phases. These refined phases were then more easily dissolved during solutionization to offer a higher mechanical property. The second application case study demonstrates a solution to eliminating the need for post-welding heat treatment in arc welding of AA7075 alloy, as the post-welding heat treatment is never ideal for large parts and field welding. Unfortunately, it would take years for the natural aging of AA7075 alloy to reach its peak strength. When this alloy was butt welded by a nano-treated 7075 filler wire with a reduced Cu content, the natural aging is promoted, and the weld recovered 77% of the base metal strength after 40 days, which is 50.2% stronger than the control sample welded by pure 7075 filler wire. The third application case study is to develop and cast high-strength Al-Cu-Mg alloy. AA2024 alloy, a typical high-strength wrought alloy, contains 4.5% Cu and a max 1.8% Mg. Its high alloying makes it prone to solidification defects. Thus, plastic deformation is required to close shrinkage porosity, making this alloy for wrought products only. Moreover, this alloy has a limit of maximum Mg content at 1.8% as a high Mg content would form bulky Al2CuMg phase in the matrix, which has a limited solubility during solutionizing. Nanoparticles successfully reduced the solidification defects and greatly recovered the ductility of the cast alloy. In summary, this dissertation first discussed a new interface-controlled mechanism for in-situ synthesis of ceramic nanoparticles to build a solid foundation for economical nano-treating of molten metals. Then the nano-treating effects on solidification and heat treatment were systematically studied. Finally, two critical effects from nano-treating were summarized: “antibody effect” to enable scalable manufacturing of various high-strength aluminum alloys without cracking and shrinkage porosities and “vitamin effect” to improve microstructure and properties of alloys. The effects were successfully demonstrated to develop and manufacture high-performance aluminum alloys: manufacturing high-strength Al-Zn-Mg-Cu alloy, natural aging to AA7075 welds with nano-treated welding wires, and casting of high-strength Al-Cu-Mg alloys.

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