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Advanced Aluminum Alloy Powders
Chapter 13
Advanced Aluminum Alloy Powders Oleg D. Neikov, Frantsevich Institute for Problems of Materials Science (IPMS), Kiev, Ukraine
Emerging processes, such as rapid solidification, mechanical alloying and spray forming, create powders that upon subsequent consolidation provide significant improvements in room and elevated temperature strength, fracture toughness, fatigue life and corrosion resistance. The real advantage of powder metallurgy processing is in the production of new alloys and composites with metallurgical structures and compositions that cannot be produced by ingot metallurgy. Rapid solidification extends the solubility of alloying elements, particularly transition and rare earth elements, and refines the structure of intermetallic phases responsible for improved mechanical properties. Mechanical alloying (MA) is a dry, high-energy milling process producing dispersions of insoluble oxides and carbides that stabilize the microstructure leading to high strength at elevated temperatures in the consolidated materials. By blending the alloy powder with a strengthening phase, discontinuously reinforced aluminum–matrix composites containing insoluble dispersoids (oxides and carbides), particulates, whiskers or fibers are produced for highperformance structural applications [1–5]. Table 13.1 contains the chemical composition commercially available of aluminum PM alloys and dispersionstrengthened composites. The discontinuously reinforced aluminum–matrix composites are generally isotropic and less costly in comparison with continuous-fiber-reinforced aluminum–matrix composites. Silicon carbide or alumina-particle-reinforced aluminum composites have higher stiffness and, principally, high wear resistance in comparison with the unreinforced aluminum alloys. The PM route for producing the discontinuously reinforced composites involves blending elemental or pre-alloyed powder with the reinforcement, followed by canning, vacuum degassing and some form of consolidation, such as hot pressing or hot isostatic pressing (HIP), into a billet that is subsequently rolled, forged or extruded.
Mechanical Alloying This technique is used for fabricating oxidedispersion-strengthened alloys and discontinuously reinforced composites. High-energy ball milling results in mechanically alloying of pure metal powder and alloying components. During this process, intimate alloying by repeated welding, fracturing and rewelding take place. More detailed discussion of mechanical alloying can be found in Chapter 3. In the case of aluminum alloys, carbon derived from process contribution agents and incorporated into the processed powder reacts with aluminum to form very fine carbides. These carbides and the fine oxide particles derived from the breakup of surface films on the initial powder particles create a dispersion that affects the fine-grained microstructure. This processing technique promotes the solution of the problem of low solubility according to the phase diagram or possibility of forming low-melting equilibrium or non-equilibrium phases. Thus, a metastable supersaturation can be achieved in systems such as aluminum–iron, aluminum–nickel, aluminum–copper, aluminum–titanium and aluminum–chromium. Another potential of MA is the generation of amorphous alloys that, following controlled crystallization, can create nanometer-scale microstructures [10]. IncoMAP alloy Al-9052 has a specific density 5% less than that of conventional age-hardenable aluminum alloy of comparable strength such as 2024 [11] and its combination of light weight, high strength and corrosion resistance makes it advantageous in aerospace applications. In Ref. [12], MA is used to prepare composite powders of 7010 Al from elemental powder with or without SiC particulates. The addition of SiC improves the mechanical properties. The mechanical alloyed composites had higher modulus but lower strength values than the matrix at room temperature. However,
Advanced Aluminum Alloy Powders Oleg D. Neikov, Frantsevich Institute for Problems of Materials Science (IPMS), Kiev, Ukraine
Emerging processes, such as rapid solidification, mechanical alloying and spray forming, create powders that upon subsequent consolidation provide significant improvements in room and elevated temperature strength, fracture toughness, fatigue life and corrosion resistance. The real advantage of powder metallurgy processing is in the production of new alloys and composites with metallurgical structures and compositions that cannot be produced by ingot metallurgy. Rapid solidification extends the solubility of alloying elements, particularly transition and rare earth elements, and refines the structure of intermetallic phases responsible for improved mechanical properties. Mechanical alloying (MA) is a dry, high-energy milling process producing dispersions of insoluble oxides and carbides that stabilize the microstructure leading to high strength at elevated temperatures in the consolidated materials. By blending the alloy powder with a strengthening phase, discontinuously reinforced aluminum–matrix composites containing insoluble dispersoids (oxides and carbides), particulates, whiskers or fibers are produced for highperformance structural applications [1–5]. Table 13.1 contains the chemical composition commercially available of aluminum PM alloys and dispersionstrengthened composites. The discontinuously reinforced aluminum–matrix composites are generally isotropic and less costly in comparison with continuous-fiber-reinforced aluminum–matrix composites. Silicon carbide or alumina-particle-reinforced aluminum composites have higher stiffness and, principally, high wear resistance in comparison with the unreinforced aluminum alloys. The PM route for producing the discontinuously reinforced composites involves blending elemental or pre-alloyed powder with the reinforcement, followed by canning, vacuum degassing and some form of consolidation, such as hot pressing or hot isostatic pressing (HIP), into a billet that is subsequently rolled, forged or extruded.
Mechanical Alloying This technique is used for fabricating oxidedispersion-strengthened alloys and discontinuously reinforced composites. High-energy ball milling results in mechanically alloying of pure metal powder and alloying components. During this process, intimate alloying by repeated welding, fracturing and rewelding take place. More detailed discussion of mechanical alloying can be found in Chapter 3. In the case of aluminum alloys, carbon derived from process contribution agents and incorporated into the processed powder reacts with aluminum to form very fine carbides. These carbides and the fine oxide particles derived from the breakup of surface films on the initial powder particles create a dispersion that affects the fine-grained microstructure. This processing technique promotes the solution of the problem of low solubility according to the phase diagram or possibility of forming low-melting equilibrium or non-equilibrium phases. Thus, a metastable supersaturation can be achieved in systems such as aluminum–iron, aluminum–nickel, aluminum–copper, aluminum–titanium and aluminum–chromium. Another potential of MA is the generation of amorphous alloys that, following controlled crystallization, can create nanometer-scale microstructures [10]. IncoMAP alloy Al-9052 has a specific density 5% less than that of conventional age-hardenable aluminum alloy of comparable strength such as 2024 [11] and its combination of light weight, high strength and corrosion resistance makes it advantageous in aerospace applications. In Ref. [12], MA is used to prepare composite powders of 7010 Al from elemental powder with or without SiC particulates. The addition of SiC improves the mechanical properties. The mechanical alloyed composites had higher modulus but lower strength values than the matrix at room temperature. However,