Cryomilling and Spark Plasma Sintering of 2024 Aluminium Alloy

Aluminium alloys are characterized by a low specific weight, which make them highly interesting for structural applications. Mechanical properties are lower than those of steels, so the possibility to obtain an increase by means of the structural refining (either nano- or ultra-fine grained structure) would extend their applications in several fields. Bulk nanocrystalline metals and alloys can be produced by high energy milling of powders and their consolidation by sintering techniques characterized by a low thermal load in order to minimize grain growth. This is an alternative approach to other methods based on severe plastic deformation, with the advantage of obtaining near-net shape parts, within the limits of the Powder Metallurgy (PM) route. Even in the case of the part cannot be obtained directly a preform can be produced by Powder Metallurgy and finished by hot working. In this case, Powder Metallurgy is used to produce preforms with geometry closer to the final one than that attainable by other technologies, reducing production costs and raw material consumption. It is well known that nanostructure (D < 100 nm) of Al alloys can be obtained by high energy milling technique. During milling, the grain size is determined by equilibrium between recovery and formation of defects due to heavy plastic deformation. Face centered cubic (FCC) materials, as Al and alloys, are difficult to reduce by mechanical milling. The opposite occurs with body centred cubic (BCC) and hexagonal close packet (HCP) metals due to relatively defects accumulation and difficult of fast recovery kinetics. A valid alternative is the cryogenic milling, where the powders are milled in slurry formed with liquid nitrogen. Cryomilling takes advantage due to low temperature of the liquid nitrogen that either suppresses or limits recovery and recrystallization and leads to finer grain structure faster. In addition cryogenic milling does not require use of process control agent (PCA) that can contaminate the powder with carbon and oxygen. A very important factor to preserve the nanostructure of a material is its thermal stability that depends on the balance between driving and resisting forces. It is well known that the smaller the grain size, the bigger the tendency to grain growth. In most cases, the thermal stability of a nanostructure depends on the lattice defects stored between and within grains, and on the particles such as nitrides and oxides precipitated at the grain boundaries. It is really important achieve an equilibrium between grain size and thermal stability of the material to avoid grain growth on sintering. Moreover, if the powder particles are very fine, sintering becomes hard because of the oxide layer that surrounds the particles. Bulk nanomaterials can be produced through several PM techniques. Hot isostatic press (HIP), dynamic consolidation, hot extrusion and spark plasma sintering (SPS) are effective to achieve a full dense material. In the frame of the near-net shape technologies, SPS is a novel technology that has large potentiality, because of the lower temperature and shorter time required. In this process a pulse electric current flows directly on the powders and a high heating efficiency is offered. It is known that Al powders are hardly sinterable due to oxide layer on their surface. This layer has to be broken in order to form a solid neck between the particles. SPS has been used to produce nanostructured Al and iron alloys starting from nanostructured powders. A bimodal microstructure can be formed during SPS sintering due to the localized overheating generated by the sparks and low thermal stability of the material. It is well known that a bimodal microstructure reveals an improvement of ductility which is the most critical characteristics of nanostructured metals. In a simplistic view, ultra-fine/nano crystallites are responsible for high strength and micrometric grains provide increased ductility. Additional strategies of ductility improvement provides deformation at low temperatures/high strain rates, which furnishes accumulation of dislocations within nanocrystalline/UFG, resulting in increased strain hardening and enhancement of strain rate sensitivity of the flow stress. Hot workability of metals depends on several parameters. Temperature and strain rate affect the flow stress and the strain rate sensitivity. The former increases on decreasing grain size, until the deformation process is determined by dislocation motion. In FCC materials, particularly in Al and its alloys, refining grains to UFG level promotes an increase in strain rate sensitivity. The hot workability is usually defined as the quantity of deformation that a material can undergo without cracking and reaching desirable deformed microstructures at a given temperature and strain rate. Improving workability means increasing the processing ability and the properties of the materials. Hot workability can be studied by the approach of the power dissipation maps. In this PhD work, the production of nanometric Al 2024 alloy powder by cryomilling, ultra-fine grained/micrometric material consolidated by SPS, and its further deformability at high temperature was studied. The results are presented in three chapters. Chapter 1 reports the methodology to obtain the nanostructured 2024 alloy powder. Many aspects such as the evolution of the microstructure, the role of liquid nitrogen during milling and the thermal stability are studied in order to have an insight on the kinetics (1). The study of the thermal stability of the nanostructured powder is presented, as well. Chapter 2 describes the SPS experiments of the as-atomized and as-milled powders and the characterization of the consolidated material. Chapter 3 reports the hot compression experiments on the atomized and milled samples, and discusses the differences in the deformation behaviour on the basis of the starting microstructure and of its evolution during deformation.