Papers by Keyword: Microstructure Modification

Abstract: The most popular aluminium alloy used for the automotive applications is the wrought-Al alloy, where its popularity arises from its intrinsic characteristics such as, excellent formability, crash resistance, corrosion resistance and excellent specific strength. In the coming decades the use of aluminium alloys is expected to increase within automotive and aerospace industries, where this will source for an upsurge in Al recycling. Problems arise during Al recycling, where there is a steady build-up of Fe content, as this is recognised as being an impurity element. Fe has very little solubility in Al in its solid state and precipitation of these Fe intermetallics (IMC), in the Al matrix decrease mechanical properties, due to the Fe IMC brittle nature. These Fe-rich IMC also have very little cohesion to the Al matrix and can separate from the Al matrix resulting in the development of voids, where the initiation of microcracks becomes ostensible when subjected to thermomechanical processing. In order to curtail the damaging effect of the Fe IMC it so of importance to alter the nucleation and growth characteristic of the Fe rich IMC during solidification. Addition of trace elements and the manipulation of cooling rates have shown to be an effective technique to alter the Fe IMC morphology. In order observe the morphological evolution of the Fe IMC, various experiments were conducted using Al-1Si-1Mg-1Fe alloy with the addition of Mn and TiB₂-based commercial grain refiner. Microstructural analysis of the primary α-Al and Fe IMC are observed and the morphological evolution of the Fe IMC is analysed with respects to the addition of Mn and TiB₂. How the addition of these trace elements influence the growth characteristics and chemistry of the Al melt is also presented in this work.

Abstract: Friction stir processing is a solid-state welding technology capable of joining metal parts without the melting. The microstructure of the material evolved during the process vary from columnar grain along the thermal gradient in the melt pool to fine equiaxed grains. Evaluation on its mechanical properties in terms of micro-hardness was performed. A significant decrease in microhardness was observed in the processed region. The decrease in the microhardness is mainly attributed to the dissolution of hardening precipitates in the aluminium matrix.

Abstract: The propagation of cracking in concrete is a mechanism governing many physical and mechanical properties of the material. The aim of this study was to experimentally investigate the crack propagation of new concrete compositions using image analysis. Several concrete mixes containing microsilica and nanosilica were made. For each composition, Compact Tension (CT) specimens were prepared with dimensions 150x150x12mm. Specimens were subjected to a tensile load. The formation and propagation of the tensile cracks was traced on the surface of the specimens using a high resolution digital camera with 60 mm focal length. Images were captured during testing with a time interval of one second. The compression strength and modulus of elasticity were also determined for reference. The results obtained with this method have shown that it is possible to monitor relatively small displacements on the specimen surface regardless of the scale of the representative area of interest and to evaluate the influence of filler on the cracking properties of concrete.

Abstract: A series of solidified TiC-TiB₂ were prepared by combustion synthesis in enhanced high-gravity field, and the ceramics were comprised of TiB₂ primary phases, irregular TiC secondary phases, a few of Al₂O₃ inclusions and Cr-based metallic phases. Because of the accelerated phase separation of Al₂O₃ droplets from the molten reaction products, the enhanced high-gravity field not only caused both Al₂O₃ inclusions and shrinkage cavities to be sharply reduced, but also brought about the refined microstructure and the improved homogeneity in the solidified ceramic. As high gravity acceleration reached 2500 g, the ultrafine-grained microstructure with the thickness of TiB₂ platelets smaller than 1 μm was achieved, and FESEM fractographs showed a whole of ultrafine TiB₂ platelets almost accomplished the pullout process, and the maximum fracture toughness of 16.5 ± 1.0 MP · m^0.5 was achieved, whereas the maximum flexural strength of 982 ± 20 MPa was also simultaneously obtained as a coupled result of sharply-reduced defects, the refined microstructure, the improved homogeneity and the high defect tolerance achieved in the near-full-density composite.

Abstract: Based on increasing the acceleration of high-gravity field from 500 g to 2500 g, a series of TiC-TiB₂ composites are prepared by combustion synthesis in high-gravity field, and microstructure modification, microstructure homogeneity and mechanical properties of TiC-TiB₂ composites are discussed in terms of liquid-liquid separation of TiC-TiB₂ and Al₂O₃ in high gravity field. XRD, FESEM and EDS results showed that by increasing the acceleration of high gravity field, Al₂O₃ inclusions in the ceramic decreased both in volume fraction and size due to the enhanced separation of TiC-TiB₂ liquid and Al₂O₃ liquid, while the enhanced Stokes flow induced by the enhanced liquid-liquid separation also promoted the constitutional homogeneity of TiC-TiB₂ melt, thereby bringing about the refinement of the solidified microstructures, finally, making Vickers hardness and fracture toughness of the composites increase with the acceleration of high gravity field.

Abstract: Compared to the many fusion welding processes that are routinely used for joining stainless steel 316L, the friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and is being recast. The welding parameters play a major role in deciding the weld quality. In this investigation an attempt has been made to understand the influences of rotational speed and traverse speed of the tool on the microstructure of the friction stir processing zone in stainless steel 316L. Five different tool rotational speeds have been used to fabricate the joints at four different traverse speeds from this investigation which is the optimum for the tool speed and higher or lower amounts of these parameters are not useful for the process.

Abstract: Friction stir processing (FSP) is a solid state process to modify microstructure and mechanical properties of sheet metals and as-cast materials. In this process stirring action of the tool causes the material to intense plastic deformation that yields a dynamical recrystalyzation. In this study the effect of FSP and process parameters on hardness, and microstructure of stainless steel 316L has been investigated. Also by using of FSP, a composite layer of 316L/SiC has been produced. Results show that FSP leads to a finer and homogenized grain structure, as well as increased hardness, strength, toughness, and elongation at failure of the material. The composites produced by FSP have a uniform distribution of SiC particles between the grains of the base metal.

Abstract: In this paper, the influence of deep cryogenic treatment (DCT) on mechanical properties of two commercial steels is analysed. Hardened AISI 302 stainless steel and 18NiCrMo5 carburized steel specimens were subjected to DCT after standard treatments. For both materials, the fatigue behaviour is a key property considering their usual applications requirements. Surface hardness, tensile properties and axial fatigue resistance of both materials were measured and compared with and without DCT. From the analysis of the experimental results and from their interpretation in the light of the previous literature, some useful indications are obtained about the DCT potential fallout on design and construction of structural components.

Abstract: In this paper the influence of DCT (Deep Cryogenic Treatment) and a CrN arc-deposited PVD (Physical Vapor Deposition) coating on the fatigue behaviour of AISI 302 stainless steel was studied. Rotating bending tests were carried out on standard specimens to evaluate the fatigue limit at 300000 load cycles. The single and the combined effects of the two treatments were investigated by addressing untreated, PVD-coated and both PVD-coated and DC-treated specimens to the tests. All the series of specimens were also tested statically and laboratory analyses including fracture surface SEM observations and hardness measurements were performed.