Saurabh Basu

Effect of Severe Plastic Deformation in Machining Elucidated via Rate-Strain-Microstructure Mappings

Machining induces severe plastic deformation (SPD) in the chip and on the surface to stimulate dramatic microstructural transformations which can often result in a manufactured component with a fine-grained surface. The aim of this paper is to study the one-to-one mappings between the thermomechanics of deformation during chip formation and an array of resulting microstructural characteristics in terms of central deformation parameters–strain, strain-rate, temperature, and the corresponding Zener–Hollomon (ZH) parameter. Here, we propose a generalizable rate-strain-microstructure (RSM) framework for relating the deformation parameters to the resulting deformed grain size and interface characteristics. We utilize Oxley’s model to calculate the strain and strain-rate for a given orthogonal machining condition which was also validated using digital imaging correlation-based deformation field characterization. Complementary infrared thermography in combination with a modified-Oxley’s analysis was utilized to characterize the temperature in the deformation zone where the SPD at high strain-rates is imposed. These characterizations were utilized to delineate a suitable RSM phase-space composed of the strain as one axis and the ZH parameter as the other. Distinctive one-to-one mappings of various microstructures corresponding to an array of grain sizes and grain boundary distributions onto unique subspaces of this RSM space are shown. Building on the realization that the microstructure on machined surfaces is closely related to the chip microstructure derived from the primary deformation zone, this elucidation is expected to offer a reliable approach for controlling surface microstructures from orthogonal machining.

Crystallographic Textures Resulting from Severe Shear Deformation in Machining

Evolution of textures on surfaces created using plane strain machining (PSM) under machining-relevant thermomechanical conditions is studied and compared against that in the chips. By analyzing orientation distribution functions, it is shown that the texture on the surface is comprised of prominent and distinct fibers. By analyzing the pole figures of chips and the surface, it is shown that the two textures have features distinct from one other, even though the scale of the microstructure on the surface and the chips are traditionally considered to be comparable. In situ characterization using high speed imaging of PSM is coupled with a visco-plastic self-consistent (VPSC) model and is used to predict the pole figures in the chip and the surface. A finite element model of PSM is generated and coupled with the VPSC model to create a fully computational route for predicting textures from machining.

Mechanics of Intermittent Plasticity Punctuated by Fracture During Shear Deformation of Mg Alloys at Near-Ambient Temperatures

Microstructure evolution of basal-textured Mg alloy AZ31B (Mg: Al: Zn; 96: 3: 1 wt pct) during simple shear deformation at near-ambient temperatures was studied by plane-strain machining. Using Schmid factor calculations in conjunction with quantitative electron microscopy, it was found that plastic deformation in AZ31B in the primary deformation zone of machining commences by extension twinning followed by basal slip. Characteristics of twinning in individual grains were described by correlating the direction of twinning with the principal stress state. The implications of these deformation mechanics for the microstructure inherited by the freshly generated surfaces in shear-based material removal processes are examined. These include the identification of extensive surface texture reorientation at machined surfaces via extension twins, limits on surface integrities wrought by fracture events that punctuate plastic deformation, and their relationship to the cutting tool geometry.

Design and development of strain wave generating cam for a new concept ‘harmonic drive’

A split cam design is proposed to solve the problem of assembly of the single piece cam in the flexible raced bearing of an earlier proposed novel harmonic drive system, which shows better torque characteristics and capacities in comparison to the conventional one of same size with oval-shaped strain wave generating cam. The cam profile has circular arcs at two working zones at 180° phases. The proposed profile shape is identified as the cause of trouble in assembly if the cam is made single piece. The split cam is made of two identical pieces having circular arc edges. These pieces constitute the cam in assembly after putting it inside the inner race of the flex bearing and adjusted by an adjuster. The design, kinematics, and the assembly method of the proposed split cam are presented in this article. The split cam arrangement not only solves the assembly problem but also gives a scope of fine adjustment of center distance (eccentricity). Such an adjustment is not possible in conventional oval wave generating cam. Stresses in flex gear cups assembled with both type cams at load and no-load conditions are estimated using finite element method. Some results are verified experimentally. Although the flex gear cup with the proposed split cam experiences lower stresses at load transmitting active gear contact zones, it shows higher stresses at some non-active zones (where teeth are free of load). It is apparent from results that stresses at those non-active zones do not increase substantially with the increase in torque, as they are away from active zones.

Deformation heterogeneity and texture in surface severe plastic deformation of copper

Comprehensive understanding of thermomechanical response and microstructure evolution during surface severe plastic deformation (S2PD) is important towards establishing controllable processing frameworks. In this study, the evolution of crystallographic textures during directional surface mechanical attrition treatment on copper was studied and modelled using the visco-plastic self-consistent framework. In situ high-speed imaging and digital image correlation of surface deformation in circular indentation were employed to elucidate mechanics occurring in a unit process deformation and to calibrate texture model parameters. Material response during directional surface mechanical attrition was simulated using a finite-element model coupled with the calibrated texture model. The crystallographic textures developed during S2PD were observed to be similar to those resultant from uniaxial compression. The implications of these results towards facilitating a processing-based framework to predict deformation mechanics and resulting crystallographic texture in S2PD configurations are briefly discussed.

Interactive Effects of Strain, Strain-Rate and Temperatures on Microstructure Evolution in High Rate Severe Plastic Deformation

During high rate severe plastic deformation (HRSPD), strain and strain-rate are not the only external factors that determine microstructural transformations in materials, temperature-rise due to heat generation from deformation processes, also plays an important role. Temperature may influence the microstructure directly by controlling grain growth kinetics and it may also have an indirect effect through the interactive effect on material behavior, which in turn, influences strain and strain-rate parameters. This complex thermomechanics of HRSPD can lead to myriad of microstructure and consequently, material properties and phenomenon. These deformation parameters can be utilized as a ‘fingerprint’ for the resulting microstructure, and the properties and phenomenon related to it. Here, we capture some of these microstructural transformations by relating grain and sub-grain sizes, to the deformation parameters. In doing so, we find evidence of continuous dynamic recrystallization operative under these HRSPD conditions, where the interplay of strain, strain rate and temperatures offer varying degrees of multimodality in the grain-size distributions.

Creation of ultrafine-grained surfaces by large strain extrusion machining (LSEM)

ABSTRACT Large strain extrusion machining (LSEM) is examined as a route for achieving controlled microstructure refinement at freshly generated surfaces in a single pass of the machining tool. It is shown that the extrusion ratio λ of LSEM, which is the ratio of the thickness of the chip to that of the preset depth of cut, controls the extent of the ultrafine-grained (UFG) zone. Microstructure analysis was performed using orientation imaging microscopy (OIM) and mechanical testing using nanoindentation was used to characterize the UFG microstructure beneath the freshly generated surfaces. The mechanics of deformation in LSEM were examined using infrared thermography and modeled. The present research demonstrates LSEM as a novel platform for tailoring surficial microstructures and controlling their spatial extents in fabricated components.

Low-temperature machining in a fully submerged cryogenic environment

ABSTRACT The role of an immersive cryogenic environment in affecting material response in machining was explored using dynamometry, calorimetry, electron microscopy, and microindentation. Effects of tool rake angle on energy dissipation, stored energy of cold work, deformed microstructure, and hardening were evaluated for machining under a fully submerged cryogenic cutting environment and a dry cutting environment. Sustained immersion of the cutting zone in liquid nitrogen resulted in greater energy dissipation and hardening in the work and machined subsurface. This increased hardening at low temperature was directly linked to greater microstructure refinement and a lower fraction of dissipated energy stored in the form of added defects and grain boundaries. Various microstructure types with domain sizes from microscale to nanoscale were developed in the machined chips, depending on the rake angle and temperature used.

Characterization of Deformation Mechanics and Microstructure Evolution During Indirect Extrusion in Small Length Scales

In situ characterization of mechanics of deformation including dynamic strain, strain-rate and rotation of material elements was performed in prototypical small length-scale forming operation- indirect extrusion (IE) of commercially pure Lead (Pb) and Aluminum (Al 1100) using Digital Image Correlation (DIC) technique. Effects of scaling and deformation rate over a range of dimensions and velocities on mechanical response of CP Pb and the resultant anomalies were studied. Additionally, previous and post-deformation characterization of microstructure in Al was accomplished by performing Orientation Imaging Microscopy (OIM) on workpiece materials with different grain sizes. Finally, In situ characterization aided by high speed imaging of indirect extrusion of Al coupled with a Visco-Plastic Self-Consistent (VPSC) model was used to predict the evolved textures in various regions of deformation zone and the results were compared to OIM observations.Copyright

Crystallographic Textures Produced During Sand Blasting

Evolution of crystallographic textures in sand blasted surfaces is studied. Sand blasting is approximated as a series of ideal cylindrical, flat punch and wedge indents. Features of the deformation field produced during indentation with these geometries is studied using in-situ techniques. Subsequently, crystallographic textures produced during indentation with these geometries is delineated using orientation imaging microscopy and simulated using the visco plastic self consistent framework. Finally, using a novel heuristic, the crystallographic textures created during indentation are related to those produced during sand blasting. It was seen that the heuristic is able perform a first order replication of empirically observed crystallographic textures.Copyright