Materials Science Forum Vols. 475-479

Abstract: Models and theories of the microstructure evolution in a directional solidification (DS) process will be firstly addressed. Discrepancies between theories and experiments will be presented. For the thin film sample growth in a temperature gradient stage, the geometrical constraint makes it difficult to compare the experimental primary spacing with theoretical predictions. While for a bulk sample growth, fluid flow always exists in the solidification process despite that the growth interface rejects a lighter or heavier solute. Based on these analyses, the appropriate techniques to conduct DS experiments are proposed that can be used to test models. One is the solidification in a capillary sample where a single cell/dendrite grows. While for array growth in 3 dimensions, long-duration microgravity experiments will be the only viable way to obtain meaningful results.

Abstract: In this work an applicable mathematical method is described in the analysis and description of the profile of the drop obtained in the sessile droplet test. The proposed method bases on the study of the differential geometry of surfaces and in the analysis of phenomena related with the creation, expansion and contraction of interfaces to calculate the surface tension of metals. Liquid vapor surface energy of tin and indium were calculated by using sessile droplet method. Tests were carried out at 280 and 350oC temperature over a titanium nitride substrate. Results showed a very good agreement between both methodologies. For example, Sn at 280oC presented a surface tension of 545 mN/m, which is closer to the value reported in the literature (550 mN/m).

Abstract: Welding is a non-equilibrium process. However, some weldability issues, as the extension of the heat-affected zone (HAZ) can be addressed using equilibrium phase diagrams. The 70 wt% Fe-Cr-Ni pseudo-binary phase diagram is commonly used to establish the phase transformations during welding of duplex stainless steels. The predicted results are assumed to be reasonably good for most of the duplex stainless steels. Thermodynamic calculations were used to determine multicomponent phase diagrams and volumetric fraction of phases present as a function of temperature several commercial duplex stainless steels. Results showed that simplified pseudobinary phase diagram approach is valid to estimate welded joint microstructures only for the low alloy duplex stainless steels as UNS S32304, but phase transformations and mainly solidification paths of high alloy duplex stainless steels should predicted only using a multi-component phase diagram.

Abstract: In order to clarify the effect of antimony on graphite morphology and mechanical properties in heavy section ductile iron, experiments were carried out using a newly developed thermal simulation system. Results show that with 0.011% addition of antimony, tensile strength of heavy section ductile iron are obviously improved by elimination of graphite degeneration in it, and the effect of antimony is increased when the addition is increased to 0.02%. In the range of 0.01~0.04%, antimony is beneficial to graphite morphology and tensile strength although the advantageous effect on them become not so obvious when addition is exceed 0.02%.

Abstract: A unique pilot low-pressure spray forming plant was established and its spray atomisation and deposition process developed to study the new processing methods for high performance materials and to develop spray forming technology suitable for making sound superalloy preforms. The results indicated that high density (>99%) preforms (billets and rings) with little gas pick-ups and with the microstructural features of rapidly solidified superalloys, i.e. refined equiaxed grains and uniform microstructure, could be achieved after the optimisation of the spray atomisation and deposition process. The effects of subsequent thermal processing on the density, microstructure and mechanical properties of the spray formed superalloy were investigated. Compared to the turbine disks and rings made by wrought superalloys, the spray formed superalloys with identical chemistry showed significantly improved metallurgical quality, higher mechanical properties, and better hot workability.

Abstract: The recent investigations on spray formed ultrahigh-carbon steels (UHCSs) are reviewed. A satisfactory combination of strength and ductility in spray formed UHCSs can be obtained by hot rolling and annealing. The composition and hot rolling have a marked effect on the formation of graphite in UHCSs. The possibility of achieving high strain rate superplasticity in the spray formed UHCS was first revealed by very recent investigations in Shanghai Baosteel Research Institute. The UHCS processed by a combination of spray forming and hot rolling exhibited high strain rate superplasticity.

Abstract: An ultra-high strength Al-11Zn-2.9Mg-1.7Cu alloy has been prepared by spray forming process. The microstructures of as-cast and as-deposited alloys have been studied. XRD analysis result shows that the phases of spray formed Al-11Zn-2.9Mg-1.7Cu alloy consist of a-Al, MgZn2, Al2Cu and Al2CuMg. T6 heat treatment process is used to strengthen the alloy. The ultimate tensile strength reach up to 810MPa, and the elongation is about 9~11%. This kind of aluminum alloy is an ideal structural material for the aerospace and automobile industries.

Abstract: ray forming was employed to refine the microstructures of AZ91 and AZ91-3.34wt%Si Mg alloys during solidification by means of rapid solidification generated at atomization and droplet flight stages. The process parameters of spray forming were varied to reduce the porosity level and improve the morphology of the billet preforms. As-spray-formed microstructures were characterized using OM, SEM/EDS, and XRD, which were compared with as-cast microstructures. The significant differences of grains, Mg2Si and Mg17Al12 in morphology and in size between as-spray-fromed and as-cast materials are mainly due to fast cooling and solidification rates of spray forming process. Supersaturated matrix is found in as-spray-formed materials due to rapid solidification generated by spray forming. Overall, spray-formed materials are shown to have four characteristics, which are much finer microstructures, uniformly distributed phases, equiaxed-shaped phases, and supersaturated matrix.

Abstract: To minimize the distortion potential of bearing steels, 100Cr6 steel has been spray formed for the manufacture of ring shape components in this study. Structure characteristics of the spray formed bearing steel have been investigated. Evaluation of distortion of bearing components after machining and heat treatment have been made in comparison with continuous cast material. Spray forming has been approved to be an alternative approach to conventional continuous casting for the production of homogeneous 100Cr6 bearing steel of lower distortion potential.

Abstract: A novel spray deposition technology with characteristic of the movable crucible, has been developed to prepare aluminum alloy and aluminum alloy matrix composite rings with large dimensions. The influences of process parameters on the microstructure and properties of the as-deposited rings were systematically investigated and the process parameters were optimized. Up to now, 7075Al/SiCp and A356/SiCp composite rings with the outer diameter of 1200mm, the inner diameter of 600mm and the thickness of 250mm, and LF6 aluminum alloy ring with the outer diameter of 3500mm, the inner diameter of 3100mm and the thickness of 250mm have been produced respectively. All the as-deposited performs were featured with the grain size about 3~5µm and a homogeneous distribution of ceramic particles the cooling rate up to 104K/s. Moreover, no cracking occurred in the performs because of the lower inner stress. The experimental results indicated that this novel technology was a promising and viable mean for the preparation of metal rings.