Srinivasan Swaminathan

Low-cost manufacturing process for nanostructured metals and alloys

In spite of their interesting properties, nanostructured materials have found limited uses because of the cost of preparation and the limited range of materials that can be synthesized. It has been shown that most of these limitations can be overcome by subjecting a material to large-scale deformation, as occurs during common machining operations. The chips produced during lathe machining of a variety of pure metals, steels, and other alloys are shown to be nanostructured with grain (crystal) sizes between 100 and 800 nm. The hardness of the chips is found to be significantly greater than that of the bulk material.

Unusual Applications of Machining: Controlled Nanostructuring of Materials and Surfaces

A class of deformation processing applications based on the severe plastic deformation (SPD) inherent to chip formation in machining is described. The SPD can be controlled, in situ, to access a range of strains, strain rates, and temperatures. These parameters are tuned to engineer nanoscale microstructures (e.g., nanocrystalline, nanotwinned, and bimodal) by in situ control of the deformation rate. By constraining the chip formation, bulk forms (e.g., foil, sheet, and rod) with nanocrystalline and ultrafine grained microstructures are produced. Scaling down of the chip formation in the presence of a superimposed modulation enables production of nanostructured particulate with controlled particle shapes, including fiber, equiaxed, and platelet types. The SPD conditions also determine the deformation history of the machined surface, enabling microstructural engineering of surfaces. Application of the machining-based SPD to obtain deformation-microstructure maps is illustrated for a model material system-99.999% pure copper. Seemingly diverse, these unusual applications of machining are united by their common origins in the SPD phenomena prevailing in the deformation zone. Implications for large-scale manufacturing of nanostructured materials and optimization of SPD microstructures are briefly discussed.

Nanostructured Materials by Machining

Large strain deformation, a key parameter in microstructure refinement by Severe Plastic Deformation (SPD) processes, is a common feature of chip formation in machining. It is shown that the imposition of large plastic strains by chip formation can create metals and alloys with nanocrystalline or ultra-fine grained microstructures. The formation of such nanostructured materials is demonstrated in a wide variety of material systems including pure metals, light-weight aluminum alloys, and high strength steels and alloys. Nanocrystalline microstructures with different morphologies are demonstrated. The hardness and strength of the nanostructured chips are significantly greater than that of the bulk material. The production of nanostructured chips by machining, when combined with comminution and powder processing methods, may be expected to lead to the creation of a number of advanced materials with new and interesting combinations of properties. These materials are expected to find wide-ranging applications in the discrete products sector encompassing ground transportation, aerospace and bio-medical industries.Copyright

Characteristics of the GBS – SDC Transition during Bi-Axial Forming of AA5083

Thermomechanical processing to enable superplasticity in AA5083 materials includes cold working followed by heating prior to hot blow forming. Upon heating for forming at 450°C, a B-type ({110}) rolling texture is replaced by a near-random texture with a weak superimposed cube orientation parallel to the sheet normal. The presence of refined grains 7 – 8μm in size reflects the predominance of particle-stimulated nucleation of recrystallization prior to forming. The subsequent evolution of microstructure, texture and cavitation behaviour during biaxial deformation in the solute drag creep (SDC) and grain boundary sliding (GBS) regimes will be presented.

Microstructural Refinement in Single-Phase Copper Solid Solutions by Machining

A study has been made of the effect of solute (Mn, Al, Ni) additions on microstructure refinement due to large strain deformation in single phase, copper solid solutions. The solutes were specifically selected for their influence on stacking fault energy (SFE) of copper, and the large strain deformation was imposed by chip formation in machining. The microstructure of Cu- 0.7at%Ni chip consists of elongated, sub-micrometer sized grains while Cu-7at%Al chip is made up of long, thin microbands with twins. The microstructure of the chip changes as the SFE of the material varies. With all of the solid solutions studied, the hardness of the chips is found to be significantly greater than that of the bulk material. Recrystallization temperature of solid solution chips is found to be higher than those of pure copper chips.

Microstructure Refinement During Severe Plastic Deformation (SPD) of FCC Materials: A Texture Based Study

FCC materials were subjected to large strain deformation by three techniques: equal channel angular pressing (ECAP), plane-strain machining and friction stir processing (FSP). Based on the orientation imaging mapping (OIM) analysis of the deformed regions, the most likely microstructure refinement mechanisms have been identified for each of the techniques and compared among one another.

Friction Stir Processing (FSP) of Cast Metals: Microstructure – Property Relationships in NiAl Bronze and AA5083

Parameters for multi-pass FSP include the pattern of tool traverse and step-over distance between successive passes. Multi-pass FSP was conducted on as-cast NiAl bronze and as-cast AA5083 in order to modify stir zone (SZ) microstructures and mechanical properties. Highly refined and homogeneous SZ microstructures may be produced by FSP. Refined and equiaxed grain structures reflect recrystallization during FSP; mechanisms leading to homogenization by redistribution of microstructure constituents remain to be determined. Refined microstructures exhibit enhanced ambient-temperature properties and superplasticity at elevated temperatures.