Saumyadeep Jana

Fatigue behaviour of magnesium to steel dissimilar friction stir lap joints

Abstract A short study has been conducted to assess the performance of friction stir welded Mg/steel lap joints under dynamic loads. The major mode of failure was found to be top Mg sheet fracture. Crack initiation is noted to have taken place at the Mg/steel interface. The fatigue life of the joints is found to be significantly different than the fatigue data of the Mg alloy obtained from the literature. The reasons behind such a difference have been examined in this work.

Friction Consolidation Processing of n-Type Bismuth-Telluride Thermoelectric Material

Refined grain sizes and texture alignment have been shown to improve transport properties in bismuth-telluride (Bi2Te3) based thermoelectric materials. In this work we demonstrate a new approach, called friction consolidation processing (FCP), for consolidating Bi2Te3 thermoelectric powders into bulk form with a high degree of grain refinement and texture alignment. FCP is a solid-state process wherein a rotating tool is used to generate severe plastic deformation within the Bi2Te3 powder, resulting in a recrystallizing flow of material. Upon cooling, the far-from-equilibrium microstructure within the flow can be retained in the material. FCP was demonstrated on n-type Bi2Te3 feedstock powder having a −325 mesh size to form pucks with a diameter of 25.4xa0mm and thickness of 4.2xa0mm. Microstructural analysis confirmed that FCP can achieve highly textured bulk materials, with sub-micrometer grain size, directly from coarse feedstock powders in a single process. An average grain size of 0.8xa0μm was determined for regions of one sample and a multiple of uniform distribution (MUD) value of 15.49 was calculated for the (0001) pole figure of another sample. These results indicate that FCP can yield ultra-fine grains and textural alignment of the (0001) basal planes in Bi2Te3. ZTxa0=xa00.37 at 336xa0K was achieved for undoped stoichiometric Bi2Te3, which approximates literature values of ZTxa0=xa00.4–0.5. These results point toward the ability to fabricate bulk thermoelectric materials with refined microstructure and desirable texture using far-from-equilibrium FCP solid-state processing.

High Shear Deformation to Produce High Strength and Energy Absorption in Mg Alloys

Magnesium alloys have the potential to reduce the mass of transportation systems however to fully realize the benefits it must be usable in more applications including those that require higher strength and ductility. It has been known that fine grain size in Mg alloys leads to high strength and ductility. However, the challenge is how to achieve this optimal microstructure in a cost effective way. This work has shown that by using optimized high shear deformation and second phase particles of Mg2Si and MgxZnZry the energy absorption of the extrusions can exceed that of AA6061. The extrusion process under development described in this presentation appears to be scalable and cost effective. In addition to process development a novel modeling approach to understand the roles of strain and state-of-strain on particle fracture and grain size control has been developed.

Detecting the Extent of Eutectoid Transformation in U-10Mo

During eutectoid transformation of U-10Mo alloy, uniform metastable γ UMo phase is expected to transform to a mixture of α-U and γ’-U2Mo phase. The presence of transformation products in final U-10Mo fuel, especially the α phase is considered detrimental for fuel irradiation performance, so it is critical to accurately evaluate the extent of transformation in the final U-10Mo alloy. This phase transformation can cause a volume change that induces a density change in final alloy. To understand this density and volume change, we developed a theoretical model to calculate the volume expansion and resultant density change of U-10Mo alloy as a function of the extent of eutectoid transformation. Based on the theoretically calculated density change for 0 to 100% transformation, we conclude that an experimental density measurement system will be challenging to employ to reliably detect and quantify the extent of transformation. Subsequently, to assess the ability of various methods to detect the transformation in U-10Mo, we annealed U-10Mo alloy samples at 500°C for various times to achieve in low, medium, and high extent of transformation. After the heat treatment at 500°C, the samples were metallographically polished and subjected to optical microscopy and x-ray diffraction (XRD) methods. Based on ourmorexa0» assessment, optical microscopy and image processing can be used to determine the transformed area fraction, which can then be correlated with the α phase volume fraction measured by XRD analysis. XRD analysis of U-10Mo aged at 500°C detected only α phase and no γ’ was detected. To further validate the XRD results, atom probe tomography (APT) was used to understand the composition of transformed regions in U-10Mo alloys aged at 500°C for 10 hours. Based on the APT results, the lamellar transformation product was found to comprise α phase with close to 0 at% Mo and γ phase with 28–32 at% Mo, and the Mo concentration was highest at the α/γ interface.«xa0less

Control of Reaction Kinetics During Friction Stir Processing

Friction stir processing (FSP) was used to successfully embed galfenol particles into aluminum (AA 1100 Al) matrix uniformly. However, intermetallic layer of Al3Fe was formed around the galfenol particles. Activation energy for Al3Fe formation during FSP was estimated, and attempts were made to minimize the Al3Fe layer thickness. By changing the processing conditions, FSP successfully eliminated the intermetallic layer. Hence, FSP, in addition to microstructural control, can successfully fabricate intermetallic-free embedded regions by controlling the reaction kinetics.

Mechanical Properties Enhancement

In Chapter 2, the effect of friction stir processing (FSP) on microstructural refinement in cast metallic systems was discussed. Microstructural modification and refinement leads to significant enhancement in both quasistatic (tensile) and dynamic (fatigue) properties. The greatest impact of FSP is on tensile ductility. Elimination of porosity, refinement of second-phase particles, removal of dendritic structure, etc. lead to dramatic improvement in tensile ductility in cast alloys. In general, there is also improvement in yield strength and tensile strength after FSP. In the following sections, the effects of FSP on mechanical properties in different cast alloy systems are discussed in more detail.

Friction Stir Processing: An Introduction

Friction stir processing is an adaptation of the friction stir welding (FSW) technique, originally invented at The Welding Institute (TWI), United Kingdom in 1991 [1]. During FSW, a rotating nonconsumable tool with a specially designed pin and shoulder is first inserted into the joint line. This is the plunge step. Once the desired plunge depth is reached, the tool travels along the joint line to complete the weld. A schematic of the process is shown in Figure 2.1.

Friction Stir Processing: A Potent Property Enhancement Tool Viable for Industry

Friction stir processing (FSP) has found commercial applications in several niche products (microelectronics, cutting blades, vacuum system hardware), but high-volume applications have yet to surface. Several industries have recognized the potential and are actively researching opportunities to use FSP to improve product performance and efficiency in automotive, aerospace, heavy vehicles, consumer electronics, power transmission, and applications in the defense sector. Only a small number of these have been reported in the open literature. Internal research groups within manufacturing companies explore new technologies, often in collaboration with Universities, National Labs or Contract Research entities, under nondisclosure environments to protect any early advantage that the new technology might provide in a competitive marketplace. As a result, it is often difficult to assess the technical readiness of a new technology until a product is revealed; at which point the technical readiness is quite high! Except for the niche commercial products, it is probably fair to put FSP at a Technology Readiness Level (TRL) of 4–5. Laboratory demonstrations of performance enhancement through FSP have been shown at full scale in relevant environments, but few have been demonstrated at the prototype part level integrated into subsystems (TRL6). To illustrate the readiness level, a few examples of some applications and current FSP research projects are described.