Materials Science Forum Vols. 604-605

Abstract: A model for describing the plastic flow has been developed. The model is based on a strain dependent Garofalo equation and predicts the variation with strain of grain size refinement by dynamic recrystallization using non-linear optimization methods. The predictions have been applied to two wrought magnesium alloys, AZ31 and AZ61 and are in good agreement with experimental data.

Abstract: Processing through the application of severe plastic deformation (SPD) provides an opportunity for achieving very significant grain refinement in bulk metals. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 µm, it is reasonable to anticipate that materials processed by SPD will exhibit superplastic ductilities when pulled in tension at elevated temperatures. This paper summarizes the fundamental principles of SPD processing and describes recent results demonstrating the occurrence of exceptional superplastic flow in these ultrafine-grained materials.

Abstract: Small additions of Sc and Zr may considerably improve both strength and ductility of 7000 series aluminum alloys, which is associated with precipitation of fine coherent Al3(Sc,Zr) particles. These particles also increase resistance to recrystallization and promote formation of a stable refined microstructure. Because of very low equilibrium solubility of Sc and Zr in Al alloys, a supersaturated solid solution of these elements in the Al matrix presented after casting is responsible for precipitation of the Al3(Sc,Zr) particles during subsequent heat treatment, while the strengthening effect depends on the particle size and number density. Therefore, it is necessary to optimize precipitation of the Al3(Sc,Zr) particles, in order to achieve superior balance of mechanical properties. In the present work, the effect of heat treatment conditions on microstructure and tensile properties of a developmental 7000 series Al alloy SSA018 modified with Sc and Zr was studied, with special emphasis on the analysis of size and number density of the Al3(Sc,Zr) particles, and their effect on the strength increase.

Abstract: In this paper a review of several papers published on seam weld formation also by the authors is presented and discussed: the process mechanics is deeply investigated and several experimental results are related to process parameters by means of FEM simulations. The relation between die design and local welding parameters, such as contact pressure, temperature, time of contact, strain and strain rate paths is analyzed. In particular, by testing profiles extruded in different conditions, it was found that it is possible to find optimum processing conditions for a welded profile to behave like a not welded one. The possibility to adopt criteria for assessing the welding quality is finally discussed, together with its implementation directly inside process FEM codes.

Abstract: The process involving cold rolling and annealing of Al-steel-Al sandwiches allows to produce an alloy containing more than 7 wt.% of Al. Unfortunately many α-Al2O3 scales tend to spall during thermal cycling because of thermal expansion mismatch between the alloy and the scale. Small additions to the ferritic stainless steel of oxygen reactive elements are known to improve the adhesion of α-Al2O3 scales. In this work hafnium and zirconium were added to the aluminium foil before roll bonding and their behaviour during the diffusion bonding was evaluated. This novel doping method was proven to be effective in improving scale adhesion and alloy lifetime. Moreover it allows to overcome problems related to steel alloying with reactive elements and reduces the quantity of expensive reactive elements that needs to be added to the steel foil.

Abstract: A fully austenitic steel containing 0.5 wt.% C and 22 wt.% Mn was recently proposed for the fabrication of automotive body structures by room-temperature sheet forming, the goal being weight reduction and better crash performance owing to its much higher yield stress and elongation (as compared to presently employed ferritic and multiphase steels). Full-thickness tensile specimens, cut from as-produced sheets, were polished and tested at different strain rates, and the macroscopic surface relief eventually induced by the plastic deformation was recorded with a video camera. Between 0.3 and 0.4 true strain, successive macroscopic deformation bands (forming about 45° angle with the tensile axis and involving the full specimen width) travel along the specimen, a new one being nucleated as the previous reaches one of the specimen heads, whereas the gage displacement vs. stress curve shows a series of steps, each corresponding to the transit of a band through the gage length, and the cross-head displacement vs. stress curve shows isolated stress peaks, each immediately preceding the nucleation of a new band. Afterwards, and up to rupture, a series of stationary deformation bands appear, most being immediately adjacent to the preceding ones, with the stress vs. strain curve showing a series of serrations with large stress drops. As the strain rate is increased from 0.0004 to 0.4 s-1, the overall flow stress slightly decreases and the mentioned plastic localization phenomena become less evident.

Abstract: The formability of AZ31 magnesium alloy sheets, with two different thicknesses, has been investigated at room temperature and 250°C by means of Nakazima tests. The different straining conditions have been studied by using sheet blanks with several length to width ratios, and Forming Limit Diagrams were then obtained with and without using lubricant. As expected, an increase in temperature was observed to enhance the formality of the alloy. The formability increases also by increasing the thickness as well as by using Teflon foil as lubricant. The microstructure of the deformed samples was analysed by means of light optical microscope.

Abstract: Semi-amorphous aluminum alloy powders produced by gas atomization were compacted using equal channel angular extrusion (ECAE) in the temperature range of 200-300°C. Microstructures of the compacts were studied after different levels of ECAE deformation, which allowed analysis of the mechanism of particle interactions, deformation and bonding during different stages of consolidation. Very non-homogeneous deformation of the compacted material was detected. At the temperatures of compaction, amorphous powder particles were much softer than fully crystalline powder particles, and they were severely deformed while the crystalline particles remained nearly spherical. Almost full consolidation occurred in regions predominantly consisting of amorphous and semi-amorphous powder particles at true strains of ~2. However, further excessive deformation led to strain localization and shear crack formation in the consolidated regions. Regions dominated by fully crystalline spherical particles with brittle intermetallic phases showed poor consolidation due to particle rotations and particle fracture; leading to weak particle bonding. The results show that consolidation is impeded by the presence of fully crystalline particles in the powder.

Abstract: ECAP (Equal Channel Angular Pressing) is a very interesting method for modifying microstructure in producing UFG (Ultra Fine Grained) materials. It consists of pressing test samples through a die containing two channels, equal in cross section and intersecting at an angle Φ. As a result of pressing, the sample theoretically deforms by simple shear and retains the same cross sectional area to repeat the pressing for several cycles. 2-D and 3-D FEM simulations of both one and four ECAP passes of two modified aluminium alloys were performed in order to investigate the deformation state of processed workpiece and, moreover, the effect of the different alloy related Strain Hardening Rate (SHR), die geometry (in terms of variation of channel outer angle) and friction on deformation distribution and magnitude. FEM results showed a lower equivalent plastic strain on the outer side of both cross and longitudinal sections of the billets after one and four passes. Microhardness tests performed on the same sections of ECAP processed billets supported these findings. Moreover, FEM analysis indicated that an higher SHR means a greater strain inhomogeneity on cross section of the processed billet. The same effect was observed by increasing the channel outer angle by computing friction.

Abstract: In the present work, effects of loading scheme and strain reversal on structure evolution are studied by using high pressure torsion (HPT) and twist extrusion (TE) techniques. High purity aluminum (99.99%) was processed at room temperature up to a total average equivalent strain of ~4.8 by TE and HPT with two deformation modes: monotonic and reversal deformation with a step of 12˚ rotation. It was revealed that microstructural change with straining observed in pure Al was a common consequence of the SPD processing and was not affected significantly by the loading scheme. At the same time, it was found that strain reversal retarded grain refinement in comparison with monotonic deformation.