Papers by Keyword: Grain Refinement

Abstract: AE 42 is rare earth containing magnesium alloy which was developed for creep resistance and automobile applications. This alloy was subjected to friction stir processing and the effect of processing parameters on the properties was studied in detail. Mechanical and metallurgical properties of stir zone were evaluated and compared with parent metal. Thermal cycling of stir zone was carried out to study the stability of it at high temperature. Due to processing, stir zone microstructure was refined to 5 micron. The second phase inter-metallic particles Mg₁₇Al₁₂, Al₁₁Ce₃ and A_l2Ce were refined to tiny pieces of 1-2 micron and evenly distributed in the matrix. The stir zone was stable up to 250°C during thermal cycling. Mechanical properties of friction stir zone were superior to parent metal.

Abstract: The influence of material processing conditions for preparing aluminium based metal matrix nanocomposites through stir casting route is reviewed. The role of particle size with respect to Brownian motion, Stokes settling velocity and strengthening mechanism is assessed from theoretical understandings. Variation of microstructural features and mechanical properties of the nanocomposites are predicted from theoretical concepts and related mathematical models. Experiments conducted to validate the theoretical predictions show that both Orowan and grain refinement strengthening mechanisms remain operative which is the key to the improved strength property of the nanocomposites.

Abstract: Friction stir process (FSP) is a severe plastic deformation based secondary processing technique that can be utilized to engineer novel microstructures in metallic alloys. It is well known that such techniques are cumbersome and require significant experimental work and material to determine optimum processing conditions. Therefore in this work, we propose a new two step numerical approach, where: (i) CFD simulations coupled with Zener-Holloman relation are used to predict microstructure evolution in stirred, transition and heat affected zones of friction stir processed AZ31 Mg alloy sheets, (ii) Finite element simulations are carried out to evaluate superplastic forming characteristics of different microstructures developed after FSP. Simulation trends including forming pressure profiles, dome height evolution, and thickness distribution of friction stir processed sheets are compared with those of the base material. The proposed combination of numerical approaches to model both processing and forming aspects yields a powerful tool to study and optimize processing and forming technologies with limited experimentation.

Abstract: Friction stir processing is applied for property improvement of cast alloys for last two decades and many developmental studies were carried out in this topic on various alloys. In the current work, friction stir processing was carried out on rare earth containing magnesium alloy AE42. This alloy was specially developed for automobile application as it has better creep resistance than commercial magnesium alloys. Multi-pass Friction Stir Processing was carried out with varying the distance between passes from 0.5 mm to 2.5 mm using 12 mm shoulder diameter tool. Pin was with conical (tapered) and flat configurations with 3 mm height. After processing, the resultant mechanical and metallurgical properties were evaluated. Microstructure was refined to 5 micron and the secondary phases were made in to tiny pieces of 0.5-1 micron and evenly distributed in the matrix. Continuous network of grain boundary which is reason for poor mechanical properties was eliminated. Mechanical properties were improved by 30%. The variation of mechanical properties of processed material with respect to variation of distance between passes was negligible from 1 mm to 1.5 mm for flat pin tool.

Abstract: Seamless stainless microtube (SUS 304L) was selected for a detailed investigation of the strain-hardening influencing grain refinement and microstructural evolution during processing by varying press bending angles. The problem was approached in three stages. First, the microtubes were annealed to examine the initial grain size and analyzed the effect of grain size on mechanical properties by performing a tensile test. Second, a microtube press bending system was developed to observe springback behavior. Third, the microstructure of the microtube with different press bending angle was observed to investigate the effect of strain-hardening induced grain refinement on springback behavior. As a result, the occurrence of grain refinement reduced the springback amount. The mechanism of microstructure development in the shear zone during the press bending process is needle like, twin band and equiaxed and small rounded grains sequentially.

Abstract: Cast light metal alloys have retained their importance and unique characteristics as first candidates when cost-function relationship is considered. Hypoeutectic aluminum silicon alloys as (A356) exhibit several specific and interesting properties that qualify them to be used in many automotive and aeronautical applications. Evidence of significant enhancement in strength in the properties of Al-Si cast alloys by incorporating nano-particles have been recently presented. The present study aims at developing nano-dispersed Al-Si alloys with suitable casting methods that assure the dispersion of the nano-particles. In this work a number of cast samples of A356 were prepared by rheo-casting in a specially designed and built furnace unit allowing for the addition of the nano-particles into the molten Al-Si alloy in the semi-solid state with mechanical stirring. The microstructural features and the mechanical properties of the cast and T6 heat treated samples were investigated. The results obtained in this work showed enhancement in the mechanical strength of the nano-dispersed alloys, accompanied by significant increase in the elongation percentage, supported by evidence of refined dendrite arms length, and inter-lamellar spacing.

Abstract: In this paper, Zn-6Ti master alloy was produced and characterized by XRD and SEM. It was found that α-Al dendritic grains in Zn-25Al alloy were remarkably refined after the addition of Zn-6Ti master alloy, with the size remarkably reduced and the morphology changed from complex dendrite to rose-like or granular (equiaxed particle) structure. Lower solidification temperature of the Zn-Al melt was found to further promote the formation of granular (equiaxed particle) α-Al grain in Zn-25Al alloy with the addition of Zn-6Ti master alloy. The mechanism for the grain refinement is discussed based on the SEM observation of TiAl _3-x Zn_x particles at the center of α-Al grain in Zn-25Al alloy.

Abstract: In this paper, two types of Zn-Al-Ti-B-C master alloys were produced by a two-step method and were found to have good refinement effect for Zn-50Al alloy. SEM results show that TiC and TiB2 particles act as the nucleating center of α-Al grains in Zn-50Al alloy. The presence of TiAl3-xZnx phase in the matrix of Zn-Al-Ti-B-C master alloy was found to further enhance the refinement effect. The melt thermal-rate treatment process present good grain refinement effect for Zn-50Al alloy and it was further promoted by the addition of Zn-Al-Ti-B-C master alloy into Zn-Al matrix.

Abstract: In this paper, the possibility of refining grain of Ti-48Al-2Cr-2Nb alloy in the processes of multi-stage heat treatment consisted of initial heat treatment, cyclic heat treatment and under-annealing was evaluated. Microstructural changes that take place during the particular heat treatment procedures were also described. It was demonstrated that due to the application of combined cyclic heat treatment and under-annealing almost 24-fold grain refinement in relation to the state after homogenising could be obtained. Probable mechanisms of grain refinement in the proposed heat treatment processes were also presented and influence of individual procedures of the proposed treatment on selected properties of the investigated alloy was described