Papers by Author: Sri Lathabai

Abstract: Friction stir processing (FSP) is a novel thermo-mechanical technique for modifying the microstructure of metals and alloys at targeted locations. In the present study, the microstructures and mechanical properties of friction stir processed Mg-9Al-1Zn (AZ91) alloy were evaluated. 4 mm thick sand cast AZ91 plates with a coarse dendritic microstructure and visible intermetallic phase were processed using single-pass FSP with different combinations of tool rotational and traverse speeds. Significant grain refinement (<10 μm), elimination of casting defects and the dissolution of intermetallic phase were observed at the stir zone (SZ) of all tested specimens. Microhardness tests showed increased microhardness along the SZ with a more uniform microhardness profile as compared to the regions outside the SZ. Mechanical properties evaluation using shear punch testing and subsequent microstructure analysis performed using scanning electron microscopy and microchemical analysis using Energy Dispersive Spectroscopy are discussed in this paper.

Abstract: Friction stir processing (FSP) was performed on AA 7075-T6, a heat treatable high strength Al-Zn-Mg-Cu alloy. The two main FSP parameters, the tool rotational and travel speed, were varied systematically in order to understand their influence on the microstructure and mechanical properties of the processed zone. At a given rotational speed, increasing the travel speed increased the microhardness of the nugget (stir) zone; for a given travel speed there appeared to be an optimum rotational speed which resulted in the highest microhardness. The range of FSP parameters used did not significantly influence the nugget zone grain size. It is suggested that the observed mechanical properties are a result of the complex interactions between the FSP thermo-mechanical effects and the processes of dissolution, coarsening and re-precipitation of the strengthening precipitates in this alloy.

Abstract: Commercially pure cast aluminium was subjected to equal channel angular pressing (ECAP) at room temperature using routes A, Bc and C. Microhardness distribution maps were produced on sections of extruded billets after one, two, three and four passes for each of the processing routes. It was found that the mean hardness increased significantly already after the first pass. With subsequent passes, the hardness increase was smaller but the hardness distribution became narrower, indicating increasing homogeneity. For route Bc, a slight decrease in average hardness was observed after the fourth pass. The mean hardness after four passes was highest for the route C sample, followed by the route A and route Bc samples.

Abstract: Major research and development efforts both in CSIRO and elsewhere aim to develop processes for commercial production of low cost titanium powder. These processes could reduce the current cost of titanium, but the major savings are expected to come by enabling powder metallurgical processes which transform these powders into products. Powder metallurgical (PM) processes (e.g. CIP/HIP) are well established for discrete components, but technologies for continuous production of “mill” product are immature. New processes will be needed for the economic manufacture of mill product (e.g. sheet and tube) from the large quantities of low-cost powder which will become available when the emerging powder production processes enter commercial production. The paper will present a process for the production of commercial purity (CP) sheet by direct rolling of powder. It is novel in that it avoids both the use of binders and densification via batch sintering. A roll compacted green sheet is rapidly heated under a controlled atmosphere before being consolidated to nominally 100% density by hot rolling. Following conventional batch annealing, strip samples exhibit properties approaching those of commercial wrought sheet of an equivalent grade. In order to achieve this, a number of key variables including powder chemistry, morphology and particle packing, the roll compaction and hot rolling parameters needed to be understood and closely controlled.

Abstract: Friction stir processing (FSP) combines frictional heating and severe plastic deformation to produce microstructural modification, either locally targeted at the near-surface regions or through the bulk, of metallic components fabricated by conventional processing routes. In this paper, we highlight the capabilities of this process by applying it to a high-pressure die cast Al-Si-Mg-(Cu) alloy and examining the resulting microstructure and mechanical properties.