Zhenzhen Yu

Transient Heat and Material Flow Modeling of Friction Stir Processing of Magnesium Alloy using Threaded Tool

A three-dimensional transient computational fluid dynamics (CFD) model was developed to investigate the material flow and heat transfer during friction stir processing (FSP) in an AZ31B magnesium alloy. The material was assumed to be a non-Newtonian viscoplastic fluid, and the Zener-Hollomon parameter was used to describe the dependence of material viscosity on temperature and strain rate. The material constants used in the constitutive equation were determined experimentally from compression tests of the AZ31B Mg alloy under a wide range of strain rates and temperatures. A dynamic mesh method, combining both Lagrangian and Eulerian formulations, was used to capture the material flow induced by the movement of the threaded tool pin. Massless inert particles were embedded in the simulation domain to track the detailed history of material flow. The actual FSP was also carried out on a wrought Mg plate where temperature profiles were recorded by embedding thermocouples. The predicted transient temperature history was found to be consistent with that measured during FSP. Finally, the influence of the thread on the simulated results of thermal history and material flow was studied by comparing two models: one with threaded pin and the other with smooth pin surface.

Interphase Strain Gradients in Multilayered Steel Composite from Microdiffraction

Multilayered steel composites consisting of alternating martensite and austenite layers and exhibiting a combination of high strength and ductility were successfully fabricated. To understand the microplasticity mechanisms responsible for such exceptional mechanical behavior, 3D X-ray microscopy with a submicron beam size was employed to probe the stress/strain distribution within the top two layers during incremental tensile loading. The 3D depth-dependent strain gradients were monitored in situ near the martensite/austenite interfaces as a function of the load level. It was observed that the strain gradients redistributed during loading. Specifically, large compressive strains developed in the top martensite layer transverse to the loading direction, while small tensile strains were found across the layer interface into the underneath austenite layer.

RHEGAL: Resistive heating gas enclosure loadframe for in situ neutron scattering

In situ neutron scattering is a powerful tool to reveal materials atomic structural response such as phase transformation, lattice straining, and texture under external stimuli. The advent of a high flux neutron source such as the Spallation Neutron Source (SNS) allows fast measurement in even non-equilibrium conditions, i.e., phase transformation in steels. However, the commercial fast heating apparatus such as commercial physical simulation equipment is not designed for in situ neutron scattering, which limits its application to in situ materials research by using neutrons. Here we present a resistive heating gas enclosure loadframe (RHEGAL) for non-equilibrium phase transformation studies by using in situ neutron scattering, which takes the advantage of high flux neutron sources like SNS. RHEGAL enables fast resistive heating of metal samples to 1200 °C at a rate up to 60 °C/s in an inert atmosphere. It provides both horizontal and vertical positions for scattering optimization. The mechanical loading capability also allows in situ high temperature tension above the oxidation temperature limit. The optimized translucent neutron scattering window by silicon allows both reflection and transmission measurements, making this equipment applicable for neutron diffraction, small angle scattering, and imaging. To demonstrate the fast heating capability, the phase transformations of an example of advanced high strength steel heated at 3 °C/s and 30 °C/s were measured with the VULCAN engineering diffractometer, and the different phase transformation kinetics by neutron diffraction were presented.

Texture Modification and Ductility Enhancement in Mg Alloy Through Friction Stir Processing

The effects of friction stir processing (FSP) parameters, i.e., rotation and travel rates of the processing tool, on the texture modification and ductility enhancement of an Mg alloy AZ31B were investigated. With the systematic change in processing parameters as a function of the Zener-Hollomon parameter, a transition of different crystallographic texture was observed through neutron diffraction measurement, which correlated well with the changes in deformation and recrystallization mechanism activated during the processing. The variation in the texture leads to dramatic changes in the strength and ductility in the stir zone of the processed Mg plate. A maximum of three-fold increase in the ductility was achieved in the Mg alloy through FSP when the Zener-Hollomon parameter exceeds 1012 s−1 which is associated with low rotation speed and high travel speed processing conditions.Copyright