Showing papers by "Yung C. Shin published in 2018"

Analysis of weld geometry and liquid flow in laser transmission welding between polyethylene terephthalate (PET) and Ti6Al4V based on numerical simulation

Abstract: The laser transmission welding of polyethylene terephthalate (PET) and titanium alloy Ti6Al4V involving the evaluating of the resultant geometry and quality of welds is investigated using a fiber laser in this paper. A 3D transient numerical model considering the melting and fluid flow is developed to predict the weld geometry and porosity formation. The temperature field, molten pool and liquid flow are simulated with varying laser power and welding speed based on the model. It is observed that the weld geometry predictions from the numerical simulation are in good agreement with the experimental data. The results show that the porosity consistently appears in the high temperature region due to the decomposition of PET. In addition, it has also been found that the molten pool with a vortex flow pattern is formed only in the PET layer and the welding processing parameters have significant effects on the fluid flow, which eventually affects the heat transfer, molten pool geometry and weld formation. Consequently, it is shown adopting appropriate welding processing parameters based on the proposed model is essential for the sound weld without defects.

Predictive modeling capabilities from incident powder and laser to mechanical properties for laser directed energy deposition

Abstract: This paper presents an overview of vertically integrated comprehensive predictive modeling capabilities for directed energy deposition processes, which have been developed at Purdue University. The overall predictive models consist of vertically integrated several modules, including powder flow model, molten pool model, microstructure prediction model and residual stress model, which can be used for predicting mechanical properties of additively manufactured parts by directed energy deposition processes with blown powder as well as other additive manufacturing processes. Critical governing equations of each model and how various modules are connected are illustrated. Various illustrative results along with corresponding experimental validation results are presented to illustrate the capabilities and fidelity of the models. The good correlations with experimental results prove the integrated models can be used to design the metal additive manufacturing processes and predict the resultant microstructure and mechanical properties.

Robust Tool Wear Monitoring Using Systematic Feature Selection in Turning Processes With Consideration of Uncertainties

Abstract: This paper describes a robust tool wear monitoring scheme for turning processes using low-cost sensors. A feature normalization scheme is proposed to eliminate the dependence of signal features on cutting conditions, cutting tools, and workpiece materials. In addition, a systematic feature selection procedure in conjunction with automated signal preprocessing parameter selection is presented to select the feature set that maximizes the performance of the predictive tool wear model. The tool wear model is built using a type-2 fuzzy basis function network (FBFN), which is capable of estimating the uncertainty bounds associated with tool wear measurement. Experimental results show that the tool wear model built with the selected features exhibits high accuracy, generalized applicability, and exemplary robustness: The model trained using 4140 steel turning test data could predict the tool wear for Inconel 718 turning with a root-mean-square error (RMSE) of 7.80 μm and requests tool changes with a 6% margin on average. Furthermore, the developed method was successfully applied to tool wear monitoring of Ti–6Al–4V alloy despite different mechanisms of tool wear, i.e., crater wear instead of flank wear.

Microhole Drilling by Double Laser Pulses With Different Pulse Energies

Abstract: Previous investigations on “double-pulse” nanosecond (ns) laser drilling reported in the literature typically utilize double pulses of equal or similar pulse energies. In this paper, “double-pulse” ns laser drilling using double pulses with energies differing by more than ten times has been studied, where both postprocess workpiece characterizations and in situ time-resolved shadowgraph imaging observations have been performed. A very interesting physical phenomenon has been discovered under the studied conditions: the “double-pulse” ns laser ablation process, where the low-energy pulse precedes the high-energy pulse (called “low-high double-pulse” laser ablation) by a suitable amount of time, can produce significantly higher ablation rates than “high-low double-pulse” or “single-pulse” laser ablation under a similar laser energy input. In particular, “low-high double-pulse” laser ablation at a suitable interpulse separation time can drill through a ∼0.93 mm thick aluminum 7075 workpiece in less than 200 pulse pairs, while “high-low double-pulse” or “single-pulse” laser ablation cannot drill through the workpiece even using 1000 pulse pairs or pulses, respectively. This indicates that “low-high double-pulse” laser ablation has led to a significantly enhanced average ablation rate that is more than five times those for “single-pulse” or “high-low double-pulse” laser ablation. The fundamental physical mechanism for the ablation rate enhancement has been discussed, and a hypothesized explanation has been given.

Adaptive robust control of machining force and contour error with tool deflection using global task coordinate frame

Abstract: Peripheral milling process productivity or quality can be improved by controlling either cutting force or contour error. While each means for improvement is often addressed individually, efforts to...

Manufacturing of hourglass-shaped through holes with varying diameters at different depths by dual-pulse laser drilling and laser-induced plasma-hole interaction

Abstract: Microholes in a metal with diameters varying unusually at different hole depths (e.g., microholes with a decreasing and then increasing diameter with the depth) have important applications. This paper reports studies on a novel method of drilling such microholes in a metal, which is through “double-pulse” percussion laser drilling followed by laser-induced plasma (from a backing plate placed behind the workpiece) – hole interaction. Using this method, a through microhole has been produced in a metal workpiece, which has similar diameters at the hole entrance and exit, but a much smaller diameter at a certain waist section inside the hole.

Methods of making hydrophobic contoured surfaces and hydrophobic contoured surfaces and devices made therefrom

Abstract: A method of creating a polymer surface with surface structures is disclosed. The method includes creating a mold, forming a metal sheet into the molds, creating a surface structure on a surface of the metal sheet by exposing the surface to laser pulses, and bringing a curable polymer to be in contact with the surface of the metal sheet containing the surface structure, curing the curable polymer, and separating the cured polymer from the metal sheet, resulting in a polymer surface containing the surface structure. The polymer surfaces with the surface structures can be hydrophobic or superhydrophobic depending on the micro and nano features contained by the surface structures.