LASER SHOCK PEENING INDUCED BACK STRESS MITIGATION IN ROLLED STAINLESS STEEL

Laser shock peening (LSP) is investigated as a potential tool for reducing tensile back stress, shown here applied to rolled and annealed 304L austenitic steel. The back stress of treated and untreated dog-bone samples is extracted from hysteresis tensile testing. Electron back scatter diffraction (EBSD) and orientation imaging microscopy (OIM) analysis quantify the geometrically necessary dislocation (GND) density distribution of unstrained and strained as well as un-peened and peened conditions. Finite element analysis (FEA) simulation models back stress and residual stress development through tensile testing and LSP treatment using known LSP pressure models and Ziegler's non-linear kinematic hardening law. Non-linear regression fitting of tensile testing stress-strain in as-received specimens extracts the kinematic hardening parameters that are used in numerical study. This research shows LSP may be used to overcome manufacturing design challenges presented by yield asymmetry due to back stress in rolled steel.

Wire Drawing Effect on Microstructural and Textural Evolution in Medium Carbon Steel Wires

This study was mainly oriented on the evolution of the crystallographic texture as a function of the deformation resulting from the industrial wire drawing process. This, in fact, will make it possible to establish a relationship between the microstructure and the crystallographic texture in the medium carbon steel wires obtained by industrial wire drawing process and used in the manufacture of spring mattresses in order to minimize the loss of material and to satisfy the users of this product.During this study, a medium-carbon steel wires was characterized by two analytical techniques. The scanning electron microscopy (SEM) to monitor the microstructure evolution and the electron backscatter diffraction (EBSD) for the crystallographic texture analysis. The EBSD results are processed with OIM (Orientation Imaging Microscopy) analysis software.

Study of diffusionless and diffusional transformations using in situ cooling and heating techniques in a scanning electron microscope

In situ electron microscopy can be an effective tool to investigate the underlying science of many transformation mechanisms in materials science. Useful utilization of these experimentations will provide greater insight into many of the existing theories, as microstructural changes can be visualized in real time under some applied constraints. In this study, we have investigated two basic phase transformation phenomena: diffusionless and diffusional mechanisms with the help of in situ cooling and heating techniques in scanning electron microscope (SEM). In situ cooling experiments have been carried out on secondary hardening ultra-high-strength steels to understand the diffusionless transformation of austenite to martensite. Nucleation and growth of the martensites have been observed with cooling in different steps to −194°C. Details of the formation of different variants of martensites in steel were studied with the help of orientation imaging microscopy. Diffusional transformations were studied in terms of oxidation of pure copper in SEM using in situ heating technique. Different heating cycles were adopted for different samples by in situ heating to a maximum temperature of 950°C for the oxidation study. Nucleation of copper oxides and subsequent growth of the copper oxides at different temperatures were studied systematically. Raman spectroscopy and orientation imaging were done to confirm the formation of oxides and their orientations. The thermal cycling phenomenon was replicated inside SEM with heating and cooling and it has been demonstrated how the nature of copper and its oxides changes with the thermal cycle. This article is part of a discussion meeting issue ‘Dynamic in situ microscopy relating structure and function’.

Deformation Behavior of Grains Near Defects in Direct Metal Laser Sintered Inconel 718 During Indentation

The present work utilizes Orientation Imaging Microscopy and Finite Element Modelling to analyse microstructure evolution in grains near defects during plane strain indentation of direct metal laser sintered Inconel 718. Defects are inevitably produced during printing of metals and they degrade the mechanical behaviour of parent components. Understanding microstructure evolution of grains present near defects can help create better predictive models of mechanical behaviour of components resulting from additive manufacturing. In this work, an ex-situ study of microstructure evolution during plane strain indentation of DMLS Inconel 718 specimens is performed. Regions that lie near volumetric porosity defects were studied. Grain Orientation Spread was utilized as a metric to quantify intra-granular deformation. It was seen that microstructure evolution of grains near defects is enhanced due to strain concentrations whereby they exhibit larger orientation spread after plastic deformation. Finite Element Analysis was used to simulate the plane strain indentation test on the specimen in which, porosity defects and roughness textures similar to those seen in the as-received specimen were programmed using the python scripting interface of Abaqus. Results from finite element analysis were compared with insights from microstructure analysis to describe evolution of microstructure during deformation near defects.

Analysis of Twinning Behavior of Ti-2V Alloy Compressed at High Strain Rate

The spilt Hopkinson pressure bar was employed to study dynamic compression mechanical response of Ti-2V alloy. The dynamic compression experiment was carried at a strain rate of 3000s-1. The microstructure of deformed specimen with ε=0.05, 0.18, 0.26 was observed by optical microscope. Electron Back-Scattered Diffraction (EBSD) technique was applied to confirm the types of twinning. Through analyzing mechanical response and microstructure evolution rule, the effect of element vanadium and deformation degree on dynamic mechanical properties and twinning deformation behavior was revealed. The results indicate that twinning is the prime dynamic deformation mechanism in Ti-2V alloy and the twinning fraction is increasingly raised during the deformation process. The twinning types, confirmed by Orientation Imaging Microscopy software, are namely {102}, {112} and {111} twinning. And the number of {111} twinning is far less than the other two types of twinning.

Modeling Evolution of Microstructures Beneath Topographically Textured Surfaces Produced Using Shear Based Material Removal

Tool chatter is envisaged as a technique to create undulations on fabricated biomedical components. Herein, a-priori designed topographies were fabricated using modulate assisted machining of oxygen free high conductivity copper. Subsequently, underpinnings of microstructure evolution in this machining process were characterized using electron back scattered diffraction based orientation imaging microscopy. These underpinnings were related to the unsteady mechanical states present during modulated assisted machining, this numerically modeled using data obtained from simpler machining configurations. In this manner, relationships between final microstructural states and the underlying mechanics were found. Finally, these results were discussed in the context of unsteady mechanics present during tool chatter, it was shown that statistically predictable microstructural outcomes result during tool chatter.