Education

Research Description

Dr. Tu is interested in laser material processing, high speed machining/spindle technology, precision engineering, mechatronics, modeling, monitoring, and control of dynamic systems. Dr. Tu's long term goal is to contribute to the advancement of lasers in engineering. Currently, Dr. Tu is 1) studying the fundamental mechanisms in laser welding of aluminum to improve the repair of fatigue cracks in aerospace materials, 2) using lasers as a manufacturing tool and design approach to improve the energy efficiency of solar panels, and 3) studying laser micro-hole drilling for fuel injectors, nozzles, and medical devices. Laser micro-hole drilling offers significant advantages resulting from its ability to produce high depth-to-cross-section ratios and fast material removal rates.

Publications

Molecular dynamics simulations of the wetting behavior of carbon nanotubes in liquid copper

Susi, B. T., & Tu, J. F. (2018), COMPUTERS & FLUIDS, 172, 19–28. https://doi.org/10.1016/j.compfluid.2018.06.004

Nanostructure diffraction analysis of a copper/single walled carbon nanotube nanocomposite synthesized by laser surface implanting

Tu, J. F., Rajule, N., Liu, Y., & Martin, J. (2017), Carbon, 113, 1–9. https://doi.org/10.1016/j.carbon.2016.11.004

Laser synthesis of a copper-single-walled carbon nanotube nanocomposite via molecular-level mixing and non-equilibrium solidification

Tu, J. F., Rajule, N., Molian, P., & Liu, Y. (2016), Journal of Physics. D, Applied Physics, 49(49).

Regulation in free amino acid profile and protein synthesis pathway of growing pig skeletal muscles by low-protein diets for different time periods

Li, Y. H., Wei, H. K., Li, F. N., Kim, S. W., Wen, C. Y., Duan, Y. H., … Yin, Y. L. (2016), Journal of Animal Science, 94(12), 5192–5205. https://doi.org/10.2527/jas.2016-0917

Effect of the magnetic pole arrangement on the surface roughness of STS 304 by magnetic abrasive machining

Yoon, S., Tu, J. F., Lee, J. H., Yang, G. E., & Mun, S. D. (2014), International Journal of Precision Engineering and Manufacturing, 15(7), 1275–1281.

Low-temperature evaporative glass scoring using a single-mode ytterbium fiber laser

Tu, J. F., & Riley, P. E. B. (2013), Optics and Lasers in Engineering, 51(6), 696–706.

Fatigue crack fusion in thin-sheet aluminum alloys AA7075-T6 using low-speed fiber laser welding

Tu, J. F., & Paleocrassas, A. G. (2011), Journal of Materials Processing Technology, 211(1), 95–102.

Inherent instability investigation for low speed laser welding of aluminum using a single-mode fiber laser

Paleocrassas, A. G., & Tu, J. F. (2010), Journal of Materials Processing Technology, 210(10), 1411–1418.

Development of a localized heat treatment system for shape memory alloy wires using an ytterbium fiber laser

Dennis, R. S., & Tu, J. F. (2008), Journal of Materials Processing Technology, 199(1-3), 245–255. https://doi.org/10.1016/j.jmatprotec.2007.07.050

Laser ablation using a long-pulsed, high-fluence, CW single-mode fiber laser

Harp, W. R., Dilwith, J. R., & Tu, J. F. (2008), Journal of Materials Processing Technology, 198(1-3), 22–30. https://doi.org/10.1016/j.jmatprotec.2007.06.062

View all publications via NC State Libraries

Grants

EAGER Collaborative Research: Nanocomposite Copper Tooling for Faster Cycle and Improved Precision in Plastic Molding Proof of Concept

National Science Foundation (NSF)(7/15/09 - 12/31/10)

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EAGER Collaborative Research: Nanocomposite Copper Tooling for Faster Cycle and Improved Precision in Plastic Molding Proof of Concept

Sponsor: National Science Foundation (NSF)

Start Date: 7/15/09

End Date: 12/31/10

Abstract

EAGER Collaborative Research: Nanocomposite Copper Tooling for Faster Cycle and Improved Precision in Plastic Molding ? Proof of Concept Project Summary Research Objective To test the hypothesis that the laws of continuum mechanics hold inside a micro-well of molten copper so that when Single Wall Carbon Nanotubes (SWNTs) are introduced into the micro-well, they will follow the fluid streamline due to SWNTs? high aspect ratio (approximately 1.2 nm in diameter and 1.5 µm in length) assuming that the SWNT and the molten copper are not interacting (i.e., no chemical reactions). The motivation for this work is that copper alloy molds, due to its high thermal conductivity, offer reduced cycle time and improved product precision in plastic injection molding over the traditional, industry standard H13 steel and aluminum molds. However, copper molds have relatively lower fatigue strength, hardness, erosion resistance, and higher coefficient of friction, which drastically lower mold life. It is envisioned that the copper mold can be reinforced with a dense array of micro SWNT/Cu composite implants without compromising its thermal conductivity and netshape. This idea is similar to the enforcement of ground foundation by driving concrete piles into the soil mass. Instead of driving SWNT/Cu implants into the copper substrate by mechanical force, we propose to use a laser to create a micro-well of molten copper, and then to introduce SWNTs into the micro-well to form the SWNT/Cu composite implant upon solidification. This technology is denoted as Laser Surface Implanting (LSI) in this proposal. As the mechanical properties of the nanocomposite implants are critically influenced by the alignment of the SWNTs in the Cu matrix, we would like to determine if vertical alignment can be achieved because it has been observed that there exists a vertical laminar flow pattern in a laser-induced micro-well. Research Plan To test the above hypothesis, we plan to conduct several experiments which will first create micro-wells of proper sizes by a single-mode, CW, 300W fiber laser on a copper substrate. SWNTs will be produced and introduced into the micro-wells before the molten copper solidifies. The micro-well will then be sectioned and polished to examine its micro-structure and the alignment of the SWNTs and their distribution. Justification as an EAGER Proposal The justifications for an EAGER proposal include high risks and high returns nature of the proposed research, the demand of substantial preliminary results as a regular proposal, societal/economic impacts, and time factor. High Risks: There are several high risks associated with the proposed idea which make this proposal ?vulnerable? as a regular proposal as reviewers often question any of these risks and demand substantial preliminary results. Substantial resources are needed to conduct preliminary studies of the proposed research due to many unknown behaviors of the proposed LSI process. Some of these unknowns are: Oxidation, Short laser-material interaction time, Embedment Vertical Alignment, and Straightening of SWNT. An EAGER funding will allow the PIs to overcome these formidable challenges and demonstrate the proof of concept. Societal/Economic Impacts: A potentially transformative aspect of the research is the changes expected in the $65 billion die and mold making industry that would improve the U.S.?s leading edge in this fiercely competitive market. Building dies and molds from the proposed process and Cu-SWNT composite will simplify the manufacturing process, reduce the lead time and prolong die life. Consequently a new workforce of new die and mold makers will be created as the die and mold making is in the danger of becoming a dying trade, as the age of the average die-maker in the U.S. has risen to 56 years.

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SGER: Synergistic and Inherently Stable Laser/Plasma-Jet Welding Processes: Proof of Concept

National Science Foundation (NSF)(9/15/07 - 8/31/09)

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SGER: Synergistic and Inherently Stable Laser/Plasma-Jet Welding Processes: Proof of Concept

Sponsor: National Science Foundation (NSF)

Start Date: 9/15/07

End Date: 8/31/09

Abstract

Laser based manufacturing is a technology capable of drastically increasing productivity while reducing production cost. Laser welding of various materials of limited thickness (under 10 mm) has become a standard production in industry. Applications to heavy industry (shipyards, trucks, heavy machinery, etc) require deep laser welding from 10 mm to 30 mm or beyond. This has proved to be challenging because of the prohibitive cost of very high power laser systems and instability of the deep penetration welding process. Intellectual merits: To date, in spite of many attempts made to increase the stability of high power laser welding, only modest improvements under specific test conditions have been observed. This lack of success results from the use of essentially open-loop approaches which in turn comes from the lack of reliable real time sensor data about key process information. The inherent instability of the high power welding process compounds those difficulties. This SGER proposal aims at developing a pilot system to achieve synergistic and stable deep penetration welding by combining a fiber laser and a plasma jet. The success of this pilot system heavily relies on a real-time plasma sensor and the integration of the fiber laser and the plasma jet. To develop this real-time plasma sensor, we propose to acquire a high speed photography system together with a high performance computing system for image processing at a frame rate up to 200,000 fps. This high speed photography system allows the PIs to observe the oscillation of keyhole and plasma ejection in order to validate and refine the design of the proposed plasma sensor array. The observation also allows the validation of rigorous mathematical modeling of the laser/plasma-jet process currently under development. The second acquisition is a 3kW plasma-jet system to be integrated with a 300W single-mode fiber laser to form the proposed pilot system. The key technology resides in synergistic energy deposition of a laser beam with moderate power and a plasma torch with a special nozzle design. The laser, with its much higher power density, will create the keyhole while a finely focused plasma jet from the plasma torch will provide additional energy deposition. The keyhole will be stabilized through active regulation of the plasma jet and the laser-induced plasma. The successful development of closed-loop deep penetration laser welding processes will be a breakthrough in this area. The request SGER grant will allow the PIs to prove the synergistic energy deposition concept as well as the inherent stability of the proposed process, thus, establishing the foundation for a closed-loop welding system. Such a closed-loop system has not been attempted because the lack of real-time feedback sensors. Broader Impact: The proposed technology will render laser welding economically viable for a much larger range of industries. The US industry is in need of this kind of advanced technology to boost competitiveness in order to turn back the tide of off-shoring jobs by many major corporations. Further, US laser job shops (over 10,000) will see their service capability significantly increased by the availability of such a technology. Computationally, the project will use novel combination of numerical strategies. The proposed work will be performed in an interdisciplinary group environment that integrates research with undergraduate and graduate students through both courses and projects. The PIs will continue to actively recruit female and minority students through participation in prospective students? visits and recruiting talks.

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Quantitative Roundness Design and Quality Assurance For Ultra-Precision Assemblies With Significant Error-Scaling Problems

National Science Foundation (NSF)(6/01/04 - 5/31/08)

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Quantitative Roundness Design and Quality Assurance For Ultra-Precision Assemblies With Significant Error-Scaling Problems

Sponsor: National Science Foundation (NSF)

Start Date: 6/01/04

End Date: 5/31/08

Abstract

The PI has established a Precision Laser Processing Laboratory (PL2) at NCSU based on a 300 W CW Ytterbium single-mode fiber laser. Single-mode means that it can provide a near Gaussian beam with a M2 value of 1.04 for 1.0 representing a perfect Gaussian beam. With proper focusing, this laser can achieve a power density several thousand times of a multi-kW laser. Laser ablation is commonly used to produce micro sized holes that are difficult to produce with conventional methods. To achieve non-thermal ablation, ultra-short pulse lasers in the range of femto-seconds are often used. However, they are very expensive and their material removal rates are very limited due to the small amount of energy they can deposit and their low repetition rates. The NCSU team has shown the feasibility of the fiber laser to perform thermal ablation via a special control scheme to produce high fluence pulses at different pulse duration in the range of microseconds with high repetition rates. Results show that a pulse of 18 ?Ýs produces the optimum hole characteristics. Also, the removal rate is superior to other lasers used for laser ablation. A though-hole was generated in a piece of stainless steel 100 ?Ým thick using only seven pulses. Further studies indicate that the fiber laser can be further controlled in a specific modulated mode to generate a spiked pulse with an initial spike of 2 microsecond at a power level of 3 kW, followed by a flat pulse at the power level of the CW mode. By choosing different total pulse durations, it was found that micro spot welding could be achieved when the total pulse width was longer than a specific value, while micro drilling was achieved when the total pulse duration was shorter. Initial explanation for this observation is that micro drilling is a result of thermal ablation by the initial spike, while the spot welding is mainly due to the melting caused by the following flat pulse. Because the duration of the initial spike is fixed, a longer total pulse duration allows for more melting to occur and to refill the cavity created by the ablation. Upon solidification, a change in density causes the material to occupy more volume, and thus, creating a micro welding bump. However, it is desirable to obtain detailed information of the transition mechanism between ablation and spot-welding so that the laser can be better controlled to achieve holes or bumps with desirable roundness and depth. Proposed Research Collaboration In this proposal, we propose to develop a model along with Prof. E. Ohmura at Osaka University, Japan to investigate the transition between these two results. Professor Ohmura is a pioneer in Molecular Dynamic (MD) simulation to study the mechanism of laser ablation using ultra-short-pulse lasers. In his MD simulation, he typically considers about 100,000 atoms and each atom is accounted for its absorption of laser power and its interaction with neighboring atoms. The computation is intensive but it offers unique insight of different laser ablation mechanisms. The MD simulation, in fact, is not suitable for micro-sized laser ablation because it involves with too many atoms (in the range of tens of millions) but the insight can be used to improve a Finite Volume Method model which is under development by Mr. William Ray Harp, Ph.D. student of the PI, who has been identified as the guest researcher to Osaka University.

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