Showing papers by "Hamid Garmestani published in 2016"

Prediction of machining-induced phase transformation and grain growth of Ti-6Al-4 V alloy

Abstract: The high-speed machining process can significantly influence the microstructure of the machined surface in titanium alloy. A physics-based finite element method is proposed for the modeling of machining-induced phase transformation and grain size growth of Ti-6Al-4 V material. Prediction of the grain growth and phase transformation are obtained with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) dynamic recrystallization model and Avrami model, respectively. A modified Johnson Cook flow stress model is embedded into the method to account for the phase transformation. The accuracy of the proposed method is validated with experimental data. Parametric studies are conducted to investigate the effects of the cutting speed and the feed rate on the microstructure change in machining.

Efficient three-phase reconstruction of heterogeneous material from 2D cross-sections via phase-recovery algorithm.

Abstract: Digital reconstruction of a complex heterogeneous media from the limited statistical information, mostly provided by different imaging techniques, is the key to the successful computational analysis of this important class of materials. In this study, a novel approach is presented for three-dimensional (3D) reconstruction of a three-phase microstructure from its statistical information provided by two-dimensional (2D) cross-sections. In this three-step method, first two-point correlation functions (TPCFs) are extracted from the cross-section(s) using a spectral method suitable for the three-phase media. In the next step, 3D TPCFs are approximated for all vectors in a representative volume element (RVE). Finally, the 3D microstructure is realized from the full-set TPCFs obtained in the previous step, using a modified phase-recovery algorithm. The method is generally applicable to any complex three-phase media, here illustrated for an SOFC anode microstructure. The capabilities and shortcomings of the method are then investigated by performing a qualitative comparison between example cross-sections obtained computationally and their experimental equivalents. Finally, it is shown that the method almost conserves key microstructural properties of the media including tortuosity, percolation and three-phase boundary length (TPBL).

Micro-Texture Evolution in Aggressive Machining of Al Alloy 7075

Abstract: The effects of process parameters on micro-texture evolution of Al alloy 7075 in machining under extreme conditions were investigated. For the first time, the process parameters in machining are related to the micro-texture evolution below the machined surface, through a computational modeling framework verified by robust experimentation. Finite element analyses (FEA) were performed to obtain the mechanical loads that the material experiences due to chip removal. The viscoplastic self-consistent (VPSC) approach was used to follow the evolution in texture due to the applied loads. The most outstanding texture component, which was seen in the case of increased feed rate, is closely followed by the texture evolution model. The observed discrepancy in experimental and computational results of texture evolution in a few studied cases is believed to be due to the activation of other microstructural evolution phenomena rather than dislocation slip, as a result of the ultra-high levels of exerted strain and strai...

The Effect of Processing Method on Microstructure and Mechanical Behavior of Ti-6Al-4V Plate Produced by Powder Metallurgy Technique

Abstract: Three Ti-6Al-4V plate materials produced by powder metallurgy technique, included pre-alloyed hydride-dehydride (HDH) plate rolled to 75% reduction in thickness, and two blended elemental (BE) powder plates rolled to 75% and 87% reduction were evaluated. The objective of this study was to determine differences in microstructure and toughness between the pre-alloyed HDH and BE Ti-6Al-4V materials processed to the same product form. Heat treatments were performed below the beta transus temperature at 982, 871, 760, and 732°C (1800, 1600, 1400, and 1350°F) for 1, 2, and 4 hours in order to determine differences in heat treating response, and above the beta transus at 1076°C (1970°F) to determine the transformation temperature. The samples were evaluated by optical microscopy and scanning electron microscopy. Charpy impact testing was performed in order to determine differences in the energy absorbed during fracture. Pole figures (0002) of selected conditions were also performed in order to determine any differences in texture between the various conditions.

Evaluation of the Microstructure and Charpy Impact Toughness of Hot Worked Powder Ti-6Al-4V

Abstract: Ti-6Al-4V powder, produced by the hydride-dehydride (HDH) process, was hot isostatically pressed (HIP) into three bars. The 10 cm (4 in) diameter bars were hot worked (HW) to three different diameters: 5.1 cm (2 in) (75% reduction in area), 3.8 cm (1.5 in) (86% reduction in area), and 2.5 cm (1 in) (94% reduction in area). Three samples were machined out of each bar along the end, middle and transverse orientations. These samples were ground, polished, and etched. The microstructure of the samples was evaluated at 100X and 200X magnifications. The objective of this experiment was to examine the effect of deformation on the microstructure and properties of hot rolled titanium alloy bar product. Charpy impact samples were also machined out of each of the various diameter bars. Impact testing was used to quantify toughness by correlating the microstructure to the energy absorbed. The tensile properties of the hot bars were determined as well as the crystallographic texture. Scanning electron microscopy (SEM) was performed on the fractured surface of the Charpy impact samples.