Materials Science Forum Vols. 638-642

Abstract: Friction stir welding (FSW) offers many potential benefits including reduced distortion, lower cost, no harmful airborne emissions, semi-automated, etc. Although initially developed for Al alloys, considerable work now has been completed to explore the ability of FSW to weld relatively thin (6 mm) ferrous alloys including many alloys of interest to the ship building industry such as HSLA-65. The status of current progress for ferrous alloys is presented offering insight into capabilities and opportunities as well as areas of FSW activity requiring additional improvement. Practical issues of weld penetration depth, gap tolerance, post-FSW mechanical properties, and distortion are addressed. Special consideration will be made to address the ability to friction stir weld 6 mm thick HSLA-65 steel with no/low distortion. Tool materials discussed will include polycrystalline cubic boron nitride (PCBN) and a new composite tool fabricated from PCBN and W-25Re. Last, data will be presented illustrating the complete absence of harmful airborne emissions when welding an austenitic stainless steel.

Abstract: The effect of fine precipitates, excess dislocations and sub-boundary hardening on creep strain behavior in the transient region has been investigated for tempered martensitic 9%Cr steel at 600 and 650oC. The fine precipitates that form during tempering or during creep decrease the creep rate in the transient region, while excess dislocations produced by cold rolling promote the recovery of dislocations during creep, resulting in higher creep rates. The sub-boundary hardening is enhanced by fine precipitates along lath and block boundaries, which retards the onset of acceleration creep. The movement and annihilation process of dislocations in the transient region is controlled by not only the movement of dislocations in the matrix but also the absorption of dislocations at boundaries. The minimum creep rate is basically determined by the time to minimum creep rate.

Abstract: The mechanical properties of nickel superalloys are related to the spatial distribution of hardening phases, their size and composition, and on the configurations of dislocations introduced by plastic and viscoplastic straining. Heterogeneous plastic flow in relation with dynamic strain aging is examined and synthesized. Dislocations are usually faced with the alternative of shearing or bypassing the ’ phase occupying up to 60 vol.%. Depending on ’ size, several Orowan bypassing mechanisms are observed, alternatively shearing by dislocation pairs or complex configurations involving S-ISF and S-ESF. Variables such as temperature, strain rate and Schmid factor play a decisive role in determining the dislocation configurations which either percolate through the matrix or shear the ’ structure. Various dislocation strategies and microstructures are analyzed and illustrated; they are reviewed critically and summarized in a strain rate versus 1/T mechanism map.

Abstract: Aluminium foams produced according to the powder metallurgical/foaming agent process are currently being used in several industrial sectors, such as automotive, rail transport or machine tools. Nevertheless there still is a high further application potential to be exploited. Especially in hybrid structures, e.g. in automotive structures that are locally filled with aluminium foam, great improvements regarding the energy absorption capacity and the sound absorption behaviour can be obtained. In the present paper several methods that allow for filling or local filling of hollow structures are investigated and presented. The effect of the foam filling on the energy absorption behaviour of the hybrid structure is discussed. Similar effects were also observed in compression tests on foam filled hollow profiles. The results of these investigations are presented.

Abstract: The formation mechanisms of hollow metal oxide through the oxidation of several metal nanoparticles have been studied by transmission electron microscopy. For Zn, Al, Cu, Ni and Fe nanoparticles, hollow oxide nanoparticles were obtained as a result of vacancy aggregation in the oxidation processes. The formation of the hollow morphology is attributed to the faster outward diffusion of metal ions through the oxide layer in the oxidation processes. Further changes in morphology during the annealing of hollow Cu, Ni and Fe oxides at higher temperatures in air were examined.

Abstract: The present paper presents the development status of creep resistant magnesium materials. It reviews the creep deformation mechanisms of magnesium alloys. Based on these mechanisms, several effective approaches to improve the creep resistance are discussed, including alloying, microstructural control and the development of new monolithic alloys and composites.

Abstract: Many nanocomposite materials are obtained by dispersing a charge in a matrix. Due to the conditions of mixing, the arrangement of the charge usually presents some heterogeneity at different scales. In order to predict the effective properties of such composites (like the dielectric permittivity or the elastic moduli), it is necessary to know the properties of the two components (charge and matrix), and their spatial distribution. To fulfil this project, we developed a general methodology in several steps: the morphology is summarized by multi-scale random models accounting for the heterogeneous distribution of aggregates. The identification of models is made from image analysis. It is then used for the prediction of effective properties by estimation, or by numerical simulations. Our approach is illustrated by various examples of multi-scale models: Boolean random sets based on Cox point processes and various random grains (spheres, cylinders), showing a very low percolation threshold and therefore a high conductivity or elastic moduli for a low charge content; multi-scale iterations of random media.

Abstract: This paper presents the results of microstructural and mechanical characterization of Friction Stir Welding joints of two aluminum-based particles reinforced composites. The composites were FSW in the extruded and T6 heat treated condition. No post-weld heat treatment was carried out on the FSW joints. Hardness, tensile, low-cycle fatigue and impact tests were carried out. Microstructural and fractographic characterization were performed both on the base and FSW material, in order to investigate the effect of the solid-state welding process on the reinforcement particles and aluminum matrix. The FSW produced high quality joints with good microstructural characteristics: the welded zone displayed a refinement of the Al matrix grain size and reinforcement particles, and a better particle distribution. The FSW specimens showed high efficiency, both in the tensile, impact and fatigue tests.

Abstract: The key concept of nanopowder agglomerate sintering (NAS) process is based on the optimization of structure design and full density processing of nanopowder into nanostructured micro-components. The kinetics of NAS process is characteristic of being controlled by material transport through hierarchical interface structures of nanopowder agglomerates. Through optimal design of those hierarchical interfaces such as nano grain boundary and agglomerate boundary, thus, full density nanopowder materials can be fabricated by pressureless sintering. In this paper we overview recent studies on the role of hierarchical interfaces for processing of full density nanopowder materials.

Abstract: The evolution mechanisms of ultrafine grains processed by severe plastic deformation are studied in ferritic steel, copper and aluminum alloys. The structural changes are characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be subdivided in the following three stages irrespective of deformation temperature: i.e. an incubation period for new grain evolution in low strain; grain fragmentation by frequent development of MSBs in medium strain, and a full development of new grains in large strain. A mechanism of new grain formation during SPD, i.e. the MSB-based model, is proposed and discussed comparing with the subgrain-based model.