Thesis (PhD ) - Open University. BLDSC no.DXN075767.

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Thesis (Ph. D.)--Open University. BLDSC no. DX237328.

In recent years, considerable concern has been shown about the effects SPF cavitation has on the mechanical properties of superplastic alloys. This investigation was undertaken to ascertain whether Hot Isostatic Pressure (HIP) eliminated this cavitation in Supral 150 and correspondingly brought about an improvement in the mechanical properties. It was found that the density increased with various isothermal anneals; the activation energy for this process was close to that for grain boundary diffusion in aluminium (61.93 KJ mol-1). The rate of cavity sintering was seen experimentally to be enhanced by the application of pressures greater than 7 HPa, and had an activation energy of 62.42KJ mol-1. Complete cavity closure occurred when the external pressure was greater than the flow stress of the material at thaý temperature and strain-rate; the ratio of external pressure (Pe) to flow stress (of) increased with falling HIP temperature. The activation energy at constant strain-rate (Q-) associated with plastic flow under conditions of hole closure was found to be 53.54KJ mol-1; giving a corresponding activation energy at constant stress (Qa), which is close to that for lattice diffusion in aluminium. The alloy used contained a high level of hydrogen which caused blistering on heat treatment, and was also responsible for the reappearance of porosity in subsequently heat-treated material previously returned to theoretical density; the extent of which was decreased by the use of higher temperatures and pressures or by vacuum degassing the material prior to HIP. Post SPF room temperature ductility was enhanced by HIP. The scatter in the 0.2% PS and UTS values found in as-received SPF specimens was not altered by the use of low pressure HIP (up to 35 MPa), although higher pressures (100 NPa) did slightly enhance these values and drastically reduced the scatter. Room temperature fracture of as-received Supral was by a 450 ductile shear mechanism. In the SPF cavitated material, the external characteristics Of fracture had a more jagged appearance, as the cavitation alters the route of the propagating crack. SPF material which has been HIPped to remove cavitation, fails in a manner similar to the as-received material.

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The aims of this project are to present an understanding in what happens when aluminium-(Al) alloys are welded, and to investigate how the Mg-, Si- and Cr-contents in AA6005A influence the weld strength and cracking susceptibility. It is known that heat from welding affects the mechanical properties (strength) of the material. Different heat cycles during welding are one of the main reasons that the strength varies. Welding can cause various phenomena such as decreased strength, porosity, deformation, cracks and corrosion. To minimize these phenomena one has to have a balance between the welding parameters, alloy composition and welding fixture setup. Al alloys are sensitive to heat from welding because they have high heat conductivity and high thermal expansion coefficient. They also deform easily when the material is heated locally. If the material is deformed too much then cracking easily occurs. This project has examined how the Mg-, Si- and Cr-contents in AA6005A, affect the welded material. A V-joint with MIG welding is used for producing weld samples. For evaluation Vickers micro-hardness, tensile testing, radiography (X-ray), LOM and SEM with EBSD and EDS was used. The evaluation focuses on mechanical properties and microstructure. The results show that small variations of Mg-, Si- and Cr-content do not have any clear effects on the welded material. The results from tensile testing show that all samples have failed in the heat affected zone (HAZ). The tensile strength of all samples are higher than standard but the yield strength are lower than standard (EN ISO 10042:2005). The lowering in hardness and tensile strength in the HAZ are believed to be a result from beta-phase (AlFeSi), lead to transformation and coarsening of the strengthening and metastable precipitate. The HAZ is wide, ranging about 20 mm from the fusion line in 5 mm thick plate. The microstructure evaluation has shown that the grain size in the HAZ has been influenced while welding.  The EDS analysis shows that a small amount of AlFeSi particles occur in the base material and HAZ but not in the weld seam. Future research is suggested to focus on understanding more about ageing, coarsening of beta-phase and precipitation of intermetallic phases.

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