Madhu Vadali

Melt Pool Flow and Surface Evolution During Pulsed Laser Micro Polishing of Ti6Al4V

Extensive experimental work has shown that pulsed laser micro polishing (PLμP) is effective for polishing micro metallic parts. However, the process physics have not been fully understood yet, especially with respect to the melt pool flow. A reliable physical model can be of significant assistance in understanding the fluid flow in the melt pool and its effect on PLμP. In this paper, a two-dimensional axisymmetric transient model that couples heat transfer and fluid flow is described that was constructed using the finite element method. The model not only provided the solutions to the temperature and velocity fields but also predicted the surface profile evolution on a free deformable surface. The simulated melt depth and resolidified surface profiles matched those obtained from optical images of PLμPed Ti6Al4V sample cross-sections. The model was also used to study the effect of laser pulse duration on the melt pool flow. The study suggests that longer pulses produce more significant fluid flows. The cut-off pulse duration between capillary and thermocapillary regimes, below which minimal Maragoni flow should be expected, was estimated to be 0.66 μs for Ti6Al4V, which also matched well with the experimental results. It is evident that the coupled model offers reliable predictions and thus can be extended for a more complex parametric study to provide further insights for PLμP.

Effects of Pulse Duration on Laser Micro Polishing

Pulsed laser micro polishing (PLμP) has been shown to be an effective method of polishing micro metallic parts whose surface roughness can approach the feature size. Laser pulse duration in the PLμP process is an important parameter that significantly affects the achievable surface finish. This paper describes the influence of laser pulse duration on surface roughness reduction during PLμP. For this purpose, near-infrared laser pulses have been used to polish Ti6Al4V at three different pulse durations: 0.65 μs, 1.91 μs, and 3.60 μs. PLμP at longer pulse durations resulted in dominating Marangoni convective flows, yet significantly higher reductions in the average surface roughness were observed compared to the short pulse duration regime without convection.

Coordination of capacity adjustment modes in work systems with autonomous WIP regulation

A method is presented in this paper for coordinating multiple modes of capacity adjustment in work systems with autonomous WIP regulation with the goal of maintaining desired fundamental dynamic behavior. To prevent overcorrection of capacity, adjustments involving floaters, temporary workers, overtime, etc. need to be coordinated, and it is shown that control-theoretic analysis can be used to develop algorithms for determining combinations of adjustments that result in WIP regulation that is as fast-acting as possible yet non-oscillatory. Results of discrete event simulations in Arena, driven by industrial data, are used to illustrate the dynamic behavior of WIP regulation in an autonomous work system that incorporates such an algorithm and multiple modes of capacity adjustment.

Control of Rotary Steerable Toolface in Directional Drilling

This paper presents a detailed mathematical model of a rotary steerable drilling system (RSS) that adopts hydro-electromechanical devices to generate bending torque in adjusting the toolface (TF). Key requirements of RSS are to adjust the TF promptly to track the TF command, to maintain the TF in presence of the external disturbances, and to do so during the drilling process. Accordingly, a controller with a fast response time and effective disturbance rejection capability is desired for the RSS. The complexity and non-linearities of the RSS creates additional challenges to the controller design. This paper describes a simple and effective controller scheme that is designed based on the analysis of the system’s dynamics model. By decoupling the disturbances, physical state feedback, and non-linearities, the RSS can be controlled by using a simple and effective proportional-integral-derivative (PID) controller with the desired performance. The simulation results show that the proposed controller is effective against the disturbance and the variations of the parameters.Copyright

Pulsed Laser Micro Polishing: An Analytical Method for Predicting Surface Finish

This project is focused on developing physics based models to predict the outcome of pulsed laser micro polishing (PLμP). Perry et al. [1–3] have modeled PLμP as oscillations of capillary waves with damping resulting from the forces of surface tension and viscosity. They have proposed a critical spatial frequency, fcr , above which a significant reduction in the amplitude of the spatial Fourier components is expected. The current work extends the concept of critical spatial frequency to the prediction of the spatial frequency content and average surface roughness after polishing, given the features of the original surface, the material properties, and laser parameters used for PLμP. The proposed prediction methodology was tested using PLμP results for Nickel, Ti6Al4V, and stainless steel 316L with initial average surface roughnesses from 70 nm to 190 nm. The predicted average surface roughnesses were within 10% to 15% of the values measured on the polished surfaces. The results show that the critical frequency continues to be a useful predictor of polishing results in the spatial frequency domain. The laser processing parameters, as represented by the critical frequency and the initial surface texture therefore can be used to predict the final surface roughness before actually implementing PLμP.Copyright

Dynamic Modeling of Bottomhole Assembly

This paper presents a comprehensive 4D dynamic model of a bottomhole assembly (BHA) used for directional drilling of oil and gas wells. Although directional drilling has been in practice for some time, it still poses several challenges, particularly related to building an autonomous drilling system. The difficulty with drilling automation derives from the complexity of the process that includes interaction with the borehole and fluid (mud) flow and complex downhole vibrations, such as bit-bounce (axial), whirl (lateral), and stick/slip (torsional). Moreover, the measurements from a limited number of downhole sensors are usually contaminated with high noise levels, and can only be transmitted at low rates with long transmission delays using mud pulsing, or at a high cost using wired pipe. Therefore, it is preferable that the directional drilling system work autonomously with limited communication to the surface. To facilitate this, a compressive physics-based model of the BHA behavior was created to be used in control system development.In this work, the 4D dynamic model of the BHA accounts for the dynamics in rotation, axial motion, and bending along two lateral directions. The model uses a lumped mass-spring system and the system parameters (mass and stiffness) are derived from the shear beam theory of a flexible beam under certain boundary conditions.Simulation results of the model were successful in qualitatively replicating the three types of downhole vibrations, namely bit-bounce, whirl, and stick/slip, and are discussed in this paper. The model is shown to qualitatively replicate downhole conditions and can be implemented in real-time, thereby making it suitable for autonomous directional drilling control.Copyright