Showing papers by "Yung C. Shin published in 2009"

Multi-step 3-D finite element modeling of subsurface damage in machining particulate reinforced metal matrix composites

Abstract: A multi-step 3-D finite element model using the commercial finite element packages Third Wave Systems AdvantEdge © and ABAQUS/Explicit © is developed for predicting the sub-surface damage after machining of particle reinforced metal matrix composites. The composite material considered for this study is an A359 aluminum matrix composite reinforced with 20 vol% fraction silicon carbide particles (A359/SiC/20p). The effect of machining conditions on the measured cutting force and damage is modeled by means of a multi-step fully-coupled thermo-mechanical model. Material properties are defined by applying the Equivalent Homogenous Material (EHM) model for the machining simulation while the damage prediction is attained by applying the resulting stress and temperature distribution to a multi-phase sub-model. In the multi-phase approach the particles and matrix are modeled as continuum elements with isotropic properties separated by a layer of cohesive zone elements representing the interfacial layer to simulate the extent of particle–matrix debonding and subsequent sub-surface damage. A random particle dispersion algorithm is applied for the random distribution of the particles in the composite. Experimental measurements of the cutting forces and the sub-surface damage are compared with simulation results, showing promising results.

Predictive modeling and experimental results for residual stresses in laser hardening of AISI 4140 steel by a high power diode laser

Abstract: A predictive model for residual stresses induced in a laser hardened workpiece of AISI 4140 steel with no melting has been developed and experimentally verified. A transient three-dimensional thermal and kinetic model is first solved to obtain the temperature and solid phase history of the workpiece, which is then sequentially coupled to a three-dimensional stress model to predict residual stresses. The phase transformation strains are added to the thermal strains at each time step during the heating and cooling cycles to obtain the resultant residual stresses in the workpiece. The importance of considering phase transformation has been explained through the comparison of the magnitudes of residual stresses with and without the inclusion of phase transformation kinetics. The model predicted strong compressive residual stresses of about 200 MPa in the heat affected zone due to austenite-to-martensite transformation. The predictions matched well with the X-ray diffraction measurements.

Laser-Assisted Machining of a Fiber Reinforced Metal Matrix Composite

Abstract: Metal matrix composites due to their excellent properties of high specific strength, fracture resistance and corrosion resistance are highly sought after over their non-ferrous alloys, but these materials also present difficulty in machining. Excessive tool wear and high tooling costs of diamond tools makes the cost associated with machining of these composites very high. This paper is concerned with machining of high volume fraction long-fiber MMC’s, which has seldom been studied. The composite material considered for this study is an Al-2%Cu aluminum matrix composite reinforced with 62% by volume fraction alumina fibers (Al-2%Cu/Al2 O3 ). Laser-machining is utilized to improve the tool life and the material removal rate while minimizing the sub-surface damage. The effectiveness of the laser-assisted machining process is studied by measuring the cutting forces, specific cutting energy, surface roughness, sub-surface damage and tool wear under various material removal temperatures. A multi-phase finite element model is developed in ABAQUS/Standard to identify and assist in selection of cutting parameters such as; tool rake angle, cutting speed and material removal temperature. The multi-phase model is also successful in predicting the damage depth on machining. The optimum material removal temperature is established as 300°C at a cutting speed of 30 m/min. LAM provides a 65% reduction in the surface roughness, specific cutting energy, the tool wear rate and minimum sub-surface damage over conventional machining using the same cutting conditions.Copyright © 2009 by ASME

Microstructural analysis and machinability improvement of udimet 720 via cryogenic milling

Abstract: The objective of this research is to improve the milling process performance for Udimet 720. In order to do so, both material microstructure and tool failure conditions are studied with different milling processes, such as cryogenic-assisted machining and conventional milling. To gain an understanding of the mechanism behind the smearing and plucking phenomena, which are used as a measure of quality and also a limiting factor to machining speed, the post machined microstructure has been investigated. The dynamic forces of machining, chip morphology, microstructure, deformation layer, and maximum cutting velocity are considered for the optimization of the milling process of Udimet 720. The tool life during milling of Udimet 720 is also determined under two different machining arrangements with an oil-base coolant and cryogenic cooling. In cryogenic milling, it has been discovered through these experiments, that there exists a concentration of gamma phase in the material. The experimental cryogenic milling ...

High Speed Machining of Titanium Alloys

Abstract: Removal rates for machining titanium alloys are an order of magnitude slower than those for aluminum. The high strength and hardness coupled with the relatively low elastic modulus and poor thermal conductivity of titanium contribute to the slow speeds and feeds that are required to machine titanium with acceptable tool life. Titanium has extremely attractive properties for air vehicles ranging from excellent corrosion resistance to good compatibility with graphite reinforced composites and very good damage tolerance characteristics. At current Buy to Fly ratios, the F-35 Program will consume as much as seven million pounds of titanium a year at rate production. This figure is nearly double that of the F-22 Program, which has a much higher titanium content. As much as 50% of the final cost of titanium parts can be attributed to machining. Specifically, in this task, we are working to improve the material removal rate of titanium to reduce cost. Lockheed Martin is evaluating the potential to use lasers to heat the material ahead of the tool to reduce its strength. Coupled with other technologies that can improve the tool life and prevent the titanium material from welding to the tool, there is hope for a practical solution using similar milling machines to those which exist today, if not a simple retro-fit option. This presentation will present the current progress of this project and its potential impact to the Joint Strike Fighter.

Integration of thermo-dynamic spindle and machining simulation models for a digital machining system

Abstract: A digital machining system is a core subsystem of a virtual machining system at the lowest level, and it provides physical attributes of both the machining process and machine tool to the upper application level. A digital machining system based on mechanistic models is developed. An expandable general base model is built in the system and the interfaces for extending to upper level models are provided for easy integration. The system consists of many integrative dynamic machining process simulation models including milling, turning, boring and grinding. The development of the digital machining system is completed by integrating a spindle analysis model through a modular interface using modal superposition methods. The digital machining system is evaluated in aspects of determining spindle related machining process constraints, predicting spindle condition-dependent chatter boundaries and selecting cutting tools.

Growing radial basis function networks using genetic algorithm and orthogonalization

Abstract: With the recent popularity of the neural networks for modeling nonlinear systems, there is a growing need for a systematic and autonomous way to simultaneously determine the optimal network size and parameters. This paper tries to meet such demand by presenting a new growing algorithm for the radial basis function networks (RBFN). In the proposed algorithm, a new hidden node is added to the RBFN in sequence while previously found hidden nodes remain intact. Genetic algorithm searches for each new hidden node after calculating the fitness of candidate hidden nodes via a computationally efficient orthogonalization procedure. The proposed algorithm is compared with conventional GA-based methods and non GA-based growing algorithms in the literature through benchmark examples to demonstrate its superior performance in model-building without any priori knowledge. Lastly, the proposed algorithm is successfully applied to a task of modeling the disk grinding process based on the actual process data collected from electronics industry.

Optimization of Laser Hardening Processes for Industrial Parts With Complex Geometry via Predictive Modeling

Abstract: A predictive laser hardening model for industrial parts with complex geometric features has been developed and used for optimization of hardening processes. A transient three-dimensional thermal model is combined with a three-dimensional kinetic model for steel phase transformation and solved in order to predict the temperature history and solid phase history of the workpiece while considering latent heat of phase transformation. Further, back-tempering is also added to the model to determine the phase transformation during multitrack laser hardening. The integrated model is designed to accurately predict temperature, phase distributions and hardness inside complex geometric domains. The laser hardening parameters for two industrial workpieces are optimized for two different industrial laser systems using this model. Experimental results confirm the validity of predicted results.Copyright © 2009 by ASME

Modeling of the Off-Axis High Power Diode Laser (HPDL) Cladding Process

Abstract: Off-axis high power diode laser (HPDL) cladding is commonly used for surface quality enhancement such as coating, part repairing, etc. Although some laser cladding models are available in the literature, little has been reported on modeling of powder flow and molten pool for a rectangular beam with side powder injection. In this article, a custom-designed flat nozzle delivers the powder material into a distinct molten pool formed by a high power diode laser (HPDL) with a rectangular beam. A powder model is first presented to reveal the powder flow behavior below the flat nozzle. Key parameters such as the nozzle inclination angle, the rectangular beam profile, shielding gas flow rates and powder feed rate are incorporated so that spatial powder density, powder velocity and temperature distribution are distinctly investigated. Then in order to describe thermal and fluidic behavior around the molten pool formed by the rectangular beam, a three dimensional self-consistent cladding model is developed with the incorporation of the distributed powder properties as input. The level set method is adopted to track the complex free surface evolution. Temperature fields and fluid motion in the molten pool area resulting from the profile of rectangular beam are distinctly revealed. The effect of continuous mass addition is also embedded into the governing equations, making the model more accurate. A HPDL cladding with little dilution is formed and the simulated result agrees well with the experiment.Copyright © 2009 by ASME

Shock Wave Propagation and Spallation Study in Laser Shock Peening

Abstract: This paper deals with the spallation induced by shock wave propagation in targets during the laser shock peening process. Physical aspects concerning laser-matter interaction, shock wave propagation, and spallation are considered. A continuous kinetic model for the spallation process is included in a one-dimensional finite difference hydrodynamic code using Lagrangian coordinates in order to calculate the laser-induced spallation phenomena. Shock wave propagation in solids is calculated and validated by experimental data. The spallation zone location is then calculated for various materials with different thickness of foils and various laser shock peening parameters. The numerical simulations are compared with previously reported experimental results, and good agreement is obtained for the spallation threshold and damage zone location.Copyright © 2009 by ASME

A Unified Simple Predictive Model for High Fluence Ultra-Short Pulsed Laser Ablation of Metal, Semiconductor and Dielectric

Abstract: A unified simple predictive model has been presented for high fluence ultrashort laser ablation of metal, semiconductor and dielectric, which has very low computational cost and is very easy to apply. Unlike many other simplified models, this model does not involve any free adjustable variables. The model predictions agree well with experimental measurements for femtosecond laser ablation, while the model is not very applicable for pulse durations more than ∼10 picosecond.Copyright © 2009 by ASME

A stable hierarchical fuzzy control design for certain non-linear systems based on input-output passivity theory

Abstract: In this paper, a hierarchical fuzzy control algorithm is designed with the auto-tuning ability for certain non-linear systems. The resultant closed-loop system remains stable not only for an open-loop stable system but also for a certain open-loop unstable system, whose augmentation is passive stable. The design of the fuzzy control scheme is straightforward, which is based on human knowledge in terms of linguistic rules instead of mathematical models. The stability of the resultant closed-loop system is mathematically proven in both continuous and discrete domains. The proposed hierarchical fuzzy control scheme can be used to control both single variable systems and multivariable systems. In the simulation example, the control performance of the hierarchical fuzzy controller is compared with an input-state linearization non-linear controller for an unstable non-linear magnetic bearing system and a non-linear adaptive controller for a multivariable chemical pressure tank system. Much better system performance are achieved with the hierarchical fuzzy controller to stabilize the system in the presence of unknown external disturbances and model parameter variations.