@article{heng_tu_im_kim_chanchamnan_kim_mun_2023, title={A novel auto-gaping magnetic pole system for inner surface finishing of non-circular pipes using magnetic abrasive finishing process}, volume={580}, ISSN={["1873-4766"]}, DOI={10.1016/j.jmmm.2023.170909}, abstractNote={This study explored the design of a novel auto-gaping magnetic pole system for finishing the inner surface of non-circular pipes using a magnetic abrasive finishing (MAF) process. Non-circular pipes are widely used for the transportation of ultra-high purity substances, including gases and fluids in many industries for semiconductor, medical, and aerospace applications. MAF process typically utilizes non-movable magnetic poles. However, due to the noncircular cross-section of the pipe, the magnetic flux density, produced by non-movable magnetic poles, across the polishing surface will exhibit huge variations, resulting in uneven and poor finish at some areas. The proposed auto-gaping magnetic pole system allows the magnetic poles to move so that they can track the workpiece surface to keep the gap between the magnetic pole and workpiece's external surface constant. As a result, the magnetic densities can be maintained at desired values to achieve even and effective finishing by MAF process. Elliptical pipes were used as the non-circular pipes for experimental validation under different MAF process parameters. The results showed that this novel technique for inner surface finishing of a non-circular pipe is capable of improving the surface roughness, reducing the Ra values from over 0.20 μm to 0.05 μm or lower, across the entire inner surface of the elliptical pipe. In addition, superior material removal efficiency was achieved. The results demonstrate that the MAF process combined with a novel auto-gaping magnetic pole system is feasible to produce the ultra-smooth inner surface of non-circular pipe.}, journal={JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS}, author={Heng, Lida and Tu, Jay F. and Im, Hongcheol and Kim, Hwi-Joong and Chanchamnan, Sieb and Kim, Jeong Su and Mun, Sang Don}, year={2023}, month={Aug} }
 @article{susi_tu_2022, title={Digital Synthesis of Realistically Clustered Carbon Nanotubes}, volume={8}, ISSN={["2311-5629"]}, DOI={10.3390/c8030034}, abstractNote={A computational approach for creating realistically structured carbon nanotubes is presented to enable more accurate and impactful multi-scale modeling and simulation techniques for nanotube research. Much of the published literature to date involving computational modeling of carbon nanotubes simplifies their structure as being long and straight, and often existing as isolated individual nanotubes. However, imagery of nanotubes has shown over several decades that nanotubes agglomerate together and exhibit looping and curvature due both to inter- and intra-nanotube attraction. The research presented in this paper leverages multi-scale simulations consisting of a simple bead-spring model for initial nanotube relaxation followed by a differential geometry approach to create an atomic representation of carbon nanotubes, and then finalized with molecular dynamics simulations using the Tersoff potential model for carbon that allows dynamic bonding and cleavage. The result is atomically accurate representations of carbon nanotubes that exist as single nanotubes, or as clusters of multiple nanotubes. The presented approach is demonstrated using (5,5) single-walled carbon nanotubes. The synthesized nanotubes are shown to relax into the curving and looping structures observed in transmission or scanning electron microscopy, but also exhibit nano-scale defects due to buckling, crimping, and twisting that are resolved during the molecular dynamics simulations. These features locally compromise the desired strength characteristics of nanotubes and therefore the presented procedure will enable more accurate modeling and simulation of nanotubes in subsequent research by representing them less as the theoretically straight and independent entities, but as realistically imperfect.}, number={3}, journal={C-JOURNAL OF CARBON RESEARCH}, author={Susi, Bryan T. and Tu, Jay F.}, year={2022}, month={Sep} }
 @article{tu_rajule_mun_2021, title={Laser Spot Welding and Electric Contact Points Using Copper/Single-Walled Carbon Nanotube Nanocomposite Synthesized by Laser Surface Implanting}, volume={5}, ISSN={["2504-477X"]}, DOI={10.3390/jcs5030087}, abstractNote={In our previous studies, we have developed a wet process, denoted as laser surface implanting (LSI), to synthesize a copper/single-walled carbon nanotube (Cu–SWCNT) metal nanocomposite. The nanostructure of this Cu–SWCNT composite was shown to contain discernable SWCNT clusters in nanosizes inside the copper matrix. Its hardness could achieve up to three times that of pure copper, verified by micro-hardness and nano-hardness tests. A focus ion beam bombardment test and a plane strain compression test show 2.5 times toughness improvement for the Cu-SWCNT composite. Based on these strength improvements, two potential applications for the Cu-SWCNT nanocomposite are proposed and their feasibilities are verified using specially design test rigs. The first application is related to creating long lasting electric contacts. The result shows that the Cu-SWCNT nanocomposite is highly wear-resistant. The contact area of the simulated electric contacts increases after repeated impact loading, which potentially could lower the contact resistance. The second application is to use the Cu-SWCNT implants as high strength spot weld for joining copper foils. A smaller weld with a higher strength reduces the power requirement of the laser and, consequently, the thermal distortion for higher-dimensional precision. The specially designed test rig for the weld strength characterization is a new contribution, providing a new testing capability for small and non-homogeneous samples not suitable for a standard tensile test machine.}, number={3}, journal={JOURNAL OF COMPOSITES SCIENCE}, author={Tu, Jay F. and Rajule, Nilesh and Mun, Sang Don}, year={2021}, month={Mar} }
 @misc{tu_2020, title={Difficult Engineering Concepts Better Explained}, ISBN={9789811213786 9789811213793}, url={http://dx.doi.org/10.1142/11652}, DOI={10.1142/11652}, publisher={WORLD SCIENTIFIC}, author={Tu, Jay F}, year={2020}, month={Aug} }
 @article{heng_kim_tu_mun_2020, title={Fabrication of precision meso-scale diameter ZrO2 ceramic bars using new magnetic pole designs in ultra-precision magnetic abrasive finishing}, volume={46}, ISSN={["1873-3956"]}, DOI={10.1016/j.ceramint.2020.04.022}, abstractNote={Ultra-Precision Magnetic Abrasive Finishing (UPMAF) is a technique for enhancing the surface quality of products, such as medical devices. In the UPMAF process, magnetic poles connect magnets and magnetic abrasive particles to form magnetic abrasive brushes for controlling the surface finishing process. Different edge shapes of the magnetic pole achieve different surface finish results. In this study, three magnetic pole designs, with a sharp edge, a square edge, and a round edge, were studied regarding their resulting surface quality and roundness precision for meso-scale diameter ZrO2 bars. Experiments were conducted to quantify critical input parameters, including magnetic pole shape, diamond particle grain size, and magnetic pole vibration frequency at an ultra-high rotational speed of 35000 rpm. An FEM model for magnetic field is used to evaluate the magnetic flux density results of the different magnetic pole shapes. The results show that stock ZrO2 bars with an Ra surface roughness of 0.18 μm and a roundness error of 3.5 μm can be improved to 0.02 μm and 0.2 μm, respectively, with the following process parameters: magnetic poles with 2-mm square edges, 1-μm diamond abrasive particles, magnetic pole vibration frequency of 8 Hz, and a workpiece rotational speed of 35000 rpm for 40 s of finishing time.}, number={11}, journal={CERAMICS INTERNATIONAL}, author={Heng, Lida and Kim, Jeong Su and Tu, Juei-Feng and Mun, Sang Don}, year={2020}, month={Aug}, pages={17335–17346} }
 @article{tu_rajule_mun_2020, title={Novel Characterizations of Mechanical Properties for a Copper/Single-Walled Carbon Nanotube Nanocomposite Synthesized by Laser Surface Implanting}, volume={6}, ISSN={["2311-5629"]}, DOI={10.3390/c6010010}, abstractNote={In our previous studies, we have developed a wet process, denoted laser surface implanting (LSI), to synthesize a copper/single-walled carbon nanotube (Cu–SWCNT) metal nanocomposite as an implant onto the surface of a pure copper substrate. The nanostructure of this Cu–SWCNT composite was confirmed independently by several methods, including transmission electron microscope (TEM) images, which show discernable SWCNT clusters in nano sizes inside the copper matrix. The hardness was measured by micro-hardness tests to indicate over three times hardness over that of pure copper could be achieved. In this paper, we present several unique ways to further characterize the mechanical properties of the Cu-SWCNT nanocomposite. Nano-hardness tests are first performed to confirm that hardness improvement, about three times that of pure copper, is achieved, consistent with the micro-hardness test results. A new toughness measurement based on focus ion beam (FIB) bombardment was performed to demonstrate 2.5 times toughness improvement. Finally, a new compression test rig was designed to conduct plane strain compression test for an array of Cu-SWCNT implants. The results confirmed that the Cu-SWCNT nanocomposite exhibits a stress-strain behavior consistent with the results of the hardness and FIB tests.}, number={1}, journal={C-JOURNAL OF CARBON RESEARCH}, author={Tu, Jay F. and Rajule, Nilesh and Mun, Sang Don}, year={2020}, month={Mar} }
 @article{yin_tu_lee_yang_mun_2019, title={Effect of the Magnetic Pole Arrangement on the Surface Roughness of STS 304 by Magnetic Abrasive Machining (vol 15, pg 1275, 2014)}, volume={20}, ISSN={["2005-4602"]}, DOI={10.1007/s12541-019-00158-1}, abstractNote={One of the authors of this article wishes to change his name given based on Korean pronunciation to a Chinese version. Accordingly, the author name “Sung Yoon” should instead appear as “Cheng Yin”.}, number={6}, journal={INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING}, author={Yin, Cheng and Tu, Juei-Feng and Lee, Jun Ho and Yang, Gyun Eui and Mun, Sang Don}, year={2019}, month={Jun}, pages={1069–1069} }
 @article{tu_reeves_2019, title={Feasibility Study of Microneedle Fabrication from a thin Nitinol Wire Using a CW Single-Mode Fiber Laser}, volume={9}, ISSN={["2391-5439"]}, DOI={10.1515/eng-2019-0023}, abstractNote={Abstract}, number={1}, journal={OPEN ENGINEERING}, author={Tu, Jay and Reeves, Nicolas}, year={2019}, month={Jun}, pages={167–177} }
 @article{susi_tu_2018, title={Molecular dynamics simulations of the wetting behavior of carbon nanotubes in liquid copper}, volume={172}, ISSN={["1879-0747"]}, DOI={10.1016/j.compfluid.2018.06.004}, abstractNote={Although it is generally agreed that carbon is not wetted by liquid copper, the degree of rejection for a Single Wall Carbon Nanotube (SWCNT) has not been quantified. This paper presents Molecular Dynamics (MD) simulations to quantify the magnitude of resistance liquid copper imposes on a (5,5) SWCNT under static and dynamic intrusion scenarios. Two new sets of coefficients for the Morse potential model are proposed that better predict interfacial behavior between liquid copper and carbon. The proposed models, after being validated using empirically observed contact angle data for liquid copper and carbon, are used to investigate the wettability of a (5,5) single-walled carbon nanotube in liquid copper. It was found that the force required to submerge an initially un-wetted SWCNT into liquid copper under static conditions is higher than the expected force calculated from macro-scale fluid dynamic theory. The results indicate that a perturbation in the liquid copper surface reduces the force required for the SWCNTs to become incorporated in the liquid copper.}, journal={COMPUTERS & FLUIDS}, author={Susi, Bryan T. and Tu, Jay F.}, year={2018}, month={Aug}, pages={19–28} }
 @article{tu_rajule_liu_martin_2017, title={Nanostructure diffraction analysis of a copper/single walled carbon nanotube nanocomposite synthesized by Laser Surface Implanting}, volume={113}, ISSN={["1873-3891"]}, DOI={10.1016/j.carbon.2016.11.004}, abstractNote={A new wet process, denoted as Laser Surface Implanting (LSI), has been developed to synthesize a Copper-Single Wall Carbon NanoTube (Cu-SWCNT) metal nanocomposite by dispersing SWCNTs into molten copper, followed by rapid and non-equilibrium solidification to form the Cu-SWCNT nanocomposite such that dispersed SWCNTs could locked in positions without agglomerating into large clusters. However, the nanometer sizes of the SWCNT clusters inside this nanocomposite make it extremely difficult to obtain TEM images with discernable SWCNT clusters in the copper matrix. In this paper, TEM images and their diffraction patterns for annealed pure copper, quenched pure copper (by the same synthesis process without introducing SWCNTs), and Cu-SWCNT nanocomposite are compared. It is concluded that TEM images with discernable SWCNT clusters are rare. Therefore, diffraction patterns are better tools to identify SWCNTs within the copper matrix. The indexed diffraction patterns confirm that the copper fcc lattice is preserved. However, the Cu-SWCNT nanocomposite samples also exhibit ordered diffuse scattering, consisting of at least two polyhedra of diffuse-scattering bounded by the {110}* and {200}* family of reciprocal lattice planes, respectively. In addition several samples exhibit super-lattice Bragg diffraction indicative expanded unit cells. It thus appears that the SWCNTs are incorporated into the Cu matrix with precise arrangements commensurate with specific Cu lattice planes.}, journal={CARBON}, author={Tu, Jay F. and Rajule, Nilesh and Liu, Yi and Martin, James}, year={2017}, month={Mar}, pages={1–9} }
 @article{tu_rajule_molian_liu_2016, title={Laser synthesis of a copper-single-walled carbon nanotube nanocomposite via molecular-level mixing and non-equilibrium solidification}, volume={49}, ISSN={["1361-6463"]}, DOI={10.1088/0022-3727/49/49/495301}, abstractNote={A copper–single-walled carbon nanotube (Cu–SWCNT) metal nanocomposite could be an ideal material if it can substantially improve the strength of copper while preserving the metal’s excellent thermal and electrical properties. However, synthesis of such a nanocomposite is highly challenging, because copper and SWCNTs do not form intermetallic compounds and are insoluble; as a result, there are serious issues regarding wettability and fine dispersion of SWCNTs within the copper matrix. In this paper we present a novel wet process, called the laser surface implantation process (LSI), to synthesize Cu–SWCNT nanocomposites by mixing SWCNTs into molten copper. The LSI process includes drilling several microholes on a copper substrate, filling the microholes with SWCNTs suspended in solution, and melting the copper substrate to create a micro-well of molten copper. The molten copper advances radially outward to engulf the microholes with pre-deposited SWCNTs to form the Cu–SWCNT implant upon solidification. Rapid and non-equilibrium solidification is achieved due to copper’s excellent heat conductivity, so that SWCNTs are locked in position within the copper matrix without agglomerating into large clusters. This wet process is very different from the typical dry processes used in powder metallurgy. Very high hardness improvement, up to 527% over pure copper, was achieved, confirmed by micro-indentation tests, with only a 0.23% SWCNT volume fraction. The nanostructure of the nanocomposite was characterized by TEM imaging, energy-dispersive x-ray spectroscopy mapping and spectroscopy measurements. The SWCNTs were found to be finely dispersed within the copper matrix with cluster sizes in the range of nanometers, achieving the goal of molecular-level mixing.}, number={49}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Tu, Jay F. and Rajule, Nilesh and Molian, Pal and Liu, Yi}, year={2016}, month={Dec} }
 @article{li_wei_li_kim_wen_duan_guo_wang_liu_yin_2016, title={Regulation in free amino acid profile and protein synthesis pathway of growing pig skeletal muscles by low-protein diets for different time periods1,2}, volume={94}, ISSN={0021-8812 1525-3163}, url={http://dx.doi.org/10.2527/jas.2016-0917}, DOI={10.2527/jas.2016-0917}, abstractNote={The objective of the study was to explore the extent to which the dietary CP level can be reduced for maintaining muscle protein deposition in growing pigs as well as the related mechanism and whether the response to dietary protein restriction is diversely modified throughout the 2 trial periods. A total of 36 pigs (9.57 ± 0.64 kg initial BW) were individually penned and fed 1 of 3 diets for 10 or 25 d. During each period, the diets contained 20, 17, and 14% CP, respectively. Both the 17% CP diet and the 14% CP diet were supplemented with Lys, Met, Thr, and Trp to provide the same total concentrations as those in the 20% CP diet. Results showed that feeding the 14% CP diet for 10 or 25 d seriously impaired ( < 0.05) growth performance of the pigs compared with those fed the 20 or 17% CP diets. Pigs fed the 20% CP diet for 25 d had a higher ( < 0.05) serum content of urea nitrogen than those fed the 17 and 14% CP diets. In addition, the free AA (FAA) profile in skeletal muscle of the pigs was evidently changed ( < 0.05) by the low-protein diets for 25 d; of note, the 14% CP diet increased ( < 0.05) the size of muscle FAA pool compared with the 20% CP diet. Meanwhile, on d 25, reducing dietary CP levels also influenced ( < 0.05) mRNA levels of specific AA transceptors expressed in skeletal muscle, especially revealing the striking differences between the 14 and 20% CP diet-fed pigs. Most importantly, we observed a globally decreased ( < 0.05) activation of the mammalian target of rapamycin complex 1 (mTORC1) pathway in skeletal muscle of pigs fed the 14% CP diet, whereas only partial inhibition was observed for those fed the 17% CP diet compared with those fed the 20% CP diet. However, feeding the low-protein diets for 10 d had minimal effects on serum parameters, muscle FAA profile, and muscle mTORC1 pathway of the pigs. Taken together, our results indicate that supplementing with limiting AA to the 14% CP diet is not highly effective for the pigs in restoring protein synthesis and muscle growth, whereas the 17% CP diet likely maintains the pigs' muscle mass, which were regulated, at least in part, by mediating AA transceptors expression, FAA profile, and activation of the mTORC1 pathway.}, number={12}, journal={Journal of Animal Science}, publisher={Oxford University Press (OUP)}, author={Li, Y. H. and Wei, H. K. and Li, F. N. and Kim, S. W. and Wen, C. Y. and Duan, Y. H. and Guo, Q. P. and Wang, W. L. and Liu, H. N. and Yin, Y. L.}, year={2016}, month={Dec}, pages={5192–5205} }
 @article{yoon_tu_lee_yang_mun_2014, title={Effect of the Magnetic Pole Arrangement on the Surface Roughness of STS 304 by Magnetic Abrasive Machining}, volume={15}, ISSN={["2005-4602"]}, DOI={10.1007/s12541-014-0467-x}, number={7}, journal={INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING}, author={Yoon, Sung and Tu, Juei-Feng and Lee, Jun Ho and Yang, Gyun Eui and Mun, Sang Don}, year={2014}, month={Jul}, pages={1275–1281} }
 @article{tu_paleocrassas_reeves_rajule_2014, title={Experimental characterization of a micro-hole drilling process with short micro-second pulses by a CW single-mode fiber laser}, volume={55}, ISSN={0143-8166}, url={http://dx.doi.org/10.1016/J.OPTLASENG.2013.11.002}, DOI={10.1016/J.OPTLASENG.2013.11.002}, abstractNote={Laser ablation with pulse durations in a few microseconds is a viable solution for micro-hole drilling applications which require large material removal rate (MRR) with moderate hole quality. However, the body of work regarding short microsecond laser drilling/ablation is small. The objective of this paper is to experimentally characterize this short micro-second laser micro-hole drilling technique using a 300 W, CW, single-mode fiber laser. This CW fiber laser is controlled to produce modulated pulses from 1 μs to 8 μs and these modulated laser pulses have a unique profile which contains an initial spike with a peak power of 1500 W for 1 μs, followed by the steady state power of 300 W. Because of its excellent beam quality, the laser beam produced by this fiber laser can be focused to a small spot size of 10 μm to achieve very high power density up to 1.9 GW/cm2. With one single laser pulse at approximately 1 μs, a blind hole of 167 μm in depth and 23 μm in opening diameter can be created in a stainless substrate. The experimental characterization of this micro-hole drilling process includes laser control, laser beam characterization, hole formation, photodiode measurements of the vapor intensity, high-speed photography of vapor/plasma formation, and spectroscopic measurements of plasma. The results show that, due to very high irradiance of the fiber laser beam, the absorbed energy not only is sufficient to melt and vaporize the material, but also is able to dissociate vapor into intense plasma at temperatures over 16,000 K. The hole drilling mechanism by this short microsecond laser ablation is due to a combination of adiabatic evaporation and ejection of fine droplets.}, journal={Optics and Lasers in Engineering}, publisher={Elsevier BV}, author={Tu, Jay and Paleocrassas, Alexander G. and Reeves, Nicholas and Rajule, Nilesh}, year={2014}, month={Apr}, pages={275–283} }
 @article{tu_riley_2013, title={Low-temperature evaporative glass scoring using a single-mode ytterbium fiber laser}, volume={51}, ISSN={["1873-0302"]}, DOI={10.1016/j.optlaseng.2013.01.005}, abstractNote={Glass cutting is increasingly important in industry to cut glass into various sizes for high definition televisions, cell phones, laptops, and tablet computers. A conventional mechanical cutter is usually used to score the glass before a bending force is applied to separate the glass along the scoring mark. This paper presents a laser glass scoring technique aimed at replacing the mechanical cutter to reduce cracks. This scoring technique, denoted as the Low-temperature Evaporative Glass Scoring process (LEGS), is different because laser energy is not directly absorbed by the glass. To achieve the proposed laser scoring, a laser beam is focused through the glass onto a metal substrate. The metal substrate absorbs the laser energy to generate a metal vapor to etch the glass, forming a scoring mark. The feasibility of this glass scoring technique is demonstrated using a continuous-wave fiber laser, at a low power of 60 W, and a 7075-T6 Aluminum alloy plate as the metal substrate. When the laser beam scans across the substrate, the laser energy creates a quasi-static aluminum molten pool, covered by an aluminum vapor at a temperature about 3000 K. At an optimal setting of 51 μm gap distance, 60 W laser power, and 6 mm/s scoring speed, a uniform scoring mark of 37 μm width and 120 μm depth was successfully generated on a piece of soda-lime glass without visible micro-cracks. The paper also discussed the uncertainties and their remedies involved in the LEGS process. To facilitate the process design, a model for predicting the aluminum vapor temperature was developed. This model accounted for the laser focus, reflection, absorption and transmission, laser energy distribution, and the aluminum melting and vaporization processes. Finally, this model was validated by comparing the actual melt depth of the aluminum substrate with the one predicted by the model.}, number={6}, journal={OPTICS AND LASERS IN ENGINEERING}, author={Tu, J. F. and Riley, P. E. B.}, year={2013}, month={Jun}, pages={696–706} }
 @article{tu_paleocrassas_2011, title={Fatigue crack fusion in thin-sheet aluminum alloys AA7075-T6 using low-speed fiber laser welding}, volume={211}, DOI={10.1016/j.jmatprotec.2010.09.001}, abstractNote={Reinforcing cracked aluminum structures with composite patches have been recognized as an efficient and economical method to extend the service life of cracked aluminum components. To further enhance the effectiveness of composite patches, it is envisioned that the crack can be first fused by laser welding to remove the high stress concentration at the crack front before applying the composite patch. In this paper, the feasibility of the envisioned fusion repair is investigated. A systematic approach for the fusion process design is proposed to overcome challenges related to alloy strength recovery, crack tracing, focusing position, welding speed, plate flatness, shielding gas pressure, thin-sheet factors, and skewed cracks. A thick-sheet, partial penetration model is first used to determine the starting point of laser welding conditions. A systematic approach to transfer the thick-sheet condition to successful thin-sheet welding is then presented. Based on successfully fused crack samples of AA 7075-T6, the ultimate tensile strength tests show that in average 74% of the alloy's original strength was recovered for a single-pass repair and 68% for a double-pass repair and the results are highly repeatable. It should be clear to see the benefit of the crack fusion because without crack fusion, the composite patch is bonded to a part with zero UTS at the crack region and with a high stress intensity factor at the crack front.}, number={1}, journal={Journal of Materials Processing Technology}, author={Tu, J. F. and Paleocrassas, A. G.}, year={2011}, pages={95–102} }
 @article{paleocrassas_tu_2010, title={Inherent instability investigation for low speed laser welding of aluminum using a single-mode fiber laser}, volume={210}, ISSN={["0924-0136"]}, DOI={10.1016/j.jmatprotec.2010.04.002}, abstractNote={The causes of instability are investigated for low speed welding of aluminum from 10 mm/s down to 1 mm/s using a 300 W single-mode fiber laser. Results show that the welding is stable until the speed drops below a certain threshold (∼1 mm/s) at which there was a significant change in the process mechanism, causing shallow, inefficient welds with many defects. A power distribution model and several tests are used to examine different types of power losses at low speeds. It is then hypothesized that, at low speeds, the CW laser beam mainly irradiates at the molten pool, which absorbs a large portion of the beam energy near the surface. The majority of this absorbed energy subsequently is either lost via evaporation or transferred into the bulk material via convection and conduction without being used for melting the solid at the welding front. A laser pulsing scheme was used to test the above hypothesis. It was found that, through proper control of the duty cycle and frequency to prevent overheating of the molten pool, a high aspect ratio weld shape can be restored at low speeds, thus, confirming the hypothesis. In addition, the 1 mm/s low speed threshold is found to be related to the initial molten pool propagation speed, which is found to be approximately 1.4 mm/s. Although this paper does not propose a solution to restore process stability, the understanding of the instability origin will be helpful in the search of such a solution to overcome the process instability for slow speed welding of aluminum.}, number={10}, journal={JOURNAL OF MATERIALS PROCESSING TECHNOLOGY}, author={Paleocrassas, A. G. and Tu, J. F.}, year={2010}, month={Jul}, pages={1411–1418} }
 @article{harp_paleocrassas_tu_2008, title={A Practical method for determining the beam profile near the focal spot}, volume={37}, ISSN={["1433-3015"]}, DOI={10.1007/s00170-007-1067-z}, number={11-12}, journal={INTERNATIONAL JOURNAL OF ADVANCED MANUFACTURING TECHNOLOGY}, author={Harp, W. R. and Paleocrassas, A. G. and Tu, J. F.}, year={2008}, month={Jul}, pages={1113–1119} }
 @article{dennis_tu_2008, title={Development of a localized heat treatment system for shape memory alloy wires using an ytterbium fiber laser}, volume={199}, ISSN={["0924-0136"]}, DOI={10.1016/j.jmatprotec.2007.07.050}, abstractNote={In order to create localized areas of shape memory effect or superelastic effect in a shape memory alloy (SMA) wire, it is necessary to precisely control the temporal and spatial temperature profiles of the treated wire in order to change its composition at specific locations. In this paper, a localized heat treatment system (LHTS) has been developed based on a one-dimensional heat transfer model coupled with a high power fiber laser. This LHTS is capable of achieving various temporal temperature profiles in a point or a section of an SMA wire prescribed by a user. Furthermore, a variable flow inert gas heat treatment chamber has been designed to prevent oxide formation in shape memory alloy heat treatments without causing significant forced convection cooling. Although the method of how to prescribe a suitable temperature profile for a specific SMA property is beyond the scope of this paper, several experiments were carried out to demonstrate the feasibility of localized heat treatment by the proposed LHTS. It has been shown that a region of a superelastic wire can be treated to demonstrate shape memory effects and even show signs of precipitate hardening when over-aged.}, number={1-3}, journal={JOURNAL OF MATERIALS PROCESSING TECHNOLOGY}, author={Dennis, R. S. and Tu, J. F.}, year={2008}, month={Apr}, pages={245–255} }
 @article{harp_dilwith_tu_2008, title={Laser ablation using a long-pulsed, high-fluence, CW single-mode fiber laser}, volume={198}, ISSN={["0924-0136"]}, DOI={10.1016/j.jmatprotec.2007.06.062}, abstractNote={Laser ablation is commonly used to produce microsized holes that are difficult to produce by conventional methods. To achieve non-thermal ablation, ultra-short-pulse lasers in the range of femtoseconds 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. In this paper, the feasibility of using a continuous-wave fiber laser to perform thermal ablation via long-period pulses is investigated. The fiber laser has excellent beam quality and can be modulated to operate at different pulse widths in the range of tens of microseconds with high repetition rates. Results show that a pulse of 18 μs 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 μm thick using only seven pulses.}, number={1-3}, journal={JOURNAL OF MATERIALS PROCESSING TECHNOLOGY}, author={Harp, W. R. and Dilwith, J. R. and Tu, J. F.}, year={2008}, month={Mar}, pages={22–30} }
 @article{paleocrassas_tu_2007, title={Low-speed laser welding of aluminum alloy 7075-T6 using a 300-W,single-mode, ytterbium fiber laser}, volume={86}, number={6}, journal={Welding Journal}, author={Paleocrassas, A. G. and Tu, J. F.}, year={2007}, pages={179S–186} }
 @article{lin_tu_2007, title={Model-Based Design of Motorized Spindle Systems to Improve Dynamic Performance at High Speeds}, volume={9}, ISSN={1526-6125}, url={http://dx.doi.org/10.1016/S1526-6125(07)70111-1}, DOI={10.1016/S1526-6125(07)70111-1}, abstractNote={High-speed motorized spindle systems are subjected to several effects during high-speed rotations that can cause substantial changes in their dynamic and thermal behaviors, leading to chatter, bearing thermal seizure, or premature spindle bearing failures. Therefore, it is important to consider these high-speed effects in the design stage of high-speed motorized spindles. This paper first develops a design flow chart to represent the overall spindle design problems. Based on this flow chart, eight design parameters are identified. A design sensitivity analysis of these eight design parameters is then conducted based on an integrated finite element method model to investigate their influence on the natural frequencies of the spindle system. Based on the rule of Maximum Improvement First, a set of systematic design procedures is proposed to suggest design changes to a custom-designed motorized spindle rated at 32 kW and 25,000 rpm. Based on the simulation results, it is shown that the first-mode frequency of this spindle system can be improved from 790.7 Hz to 934 Hz at 25,000 rpm by simply adjusting the front and rear bearing locations. At the optimal design, the first-mode frequency can reach 1454.3 Hz at 25,000 rpm, which represents more than 80 percent improvement over the original design.}, number={2}, journal={Journal of Manufacturing Processes}, publisher={Elsevier BV}, author={Lin, Chi-Wei and Tu, Jay F.}, year={2007}, month={Jan}, pages={94–108} }
 @article{sweet_tuz_2007, title={Tolerance design for the fit between bore and shaft for precision assemblies with significant error- scaling problems}, volume={45}, ISSN={["0020-7543"]}, DOI={10.1080/00207540600871244}, abstractNote={An inspection procedure may be necessary when a process does not have the capability to produce parts that meet specifications. After inspection, the accepted parts then have a probability density function which is truncated. In a previous paper, the authors presented a method for the computation of tolerances for the fit between a bore and a shaft, based on the construction of a probabilistic model of the fit, using truncated probability density functions for the bore and shaft. In this paper, a method of design is presented for the computation of tolerances for the bore and shaft that are necessary to satisfy the tolerance of the fit and at the same time minimize the probability of rejection for the processes that produce the bore and shaft. Application is made to the situation where the variations in the dimensions of the parts may be large compared with their nominal dimension or the fit requirement of the assembly. Such a situation will be denoted as the error-scaling problem.}, number={22}, journal={INTERNATIONAL JOURNAL OF PRODUCTION RESEARCH}, author={Sweet, Arnold L. and Tuz, Jay F.}, year={2007}, pages={5223–5241} }
 @article{nakkiew_lin_tu_2006, title={A new method to quantify radial error of a motorized end-milling cutter/spindle system at very high speed rotations}, volume={46}, ISSN={["1879-2170"]}, DOI={10.1016/j.ijmachtools.2005.04.021}, abstractNote={The radial error motion of a machine tool cutter/spindle system is critical to the dimensional accuracy of the parts to be machined. The spindle's radial error motions can be measured by mounting a sphere target onto the spindle as a reference. A set of sensors is used to measure displacements of the reference sphere in various directions to determine spindle error motions. This measurement technique can be reliably carried out when the spindle is at rest or at low rotational speeds. However, at very high speeds, the reference sphere must be carefully centered and balanced to avoid introducing additional error motions. In addition, the sensors must be held with very rigid mounts in order to avoid measurement errors caused by vibrations. For high-speed end milling spindles, the spindle is operated with a cutter. The cutter must be removed when mounting a reference sphere. Because the cutter itself can introduce errors due to centering and unbalancing effects, the error motions measured by the reference sphere method do not include the error caused by the cutter. This paper introduces a new and practical method to provide an indicator of the radial error of a motorized end-milling cutter/spindle system at very high speed rotations without the need of a reference sphere. This indicator of the radial error is based on the size of the cutting marks produced by the end mill, which is attached to the spindle. The cutting marks are circular, and their diameters are related to the radial error of the cutter/spindle system. Quantitative precision analysis was carried out to confirm the accuracy and repeatability of this new measurement technique. This technique has been implemented in order to determine the effects of the spindle speed, the level of unbalanced mass, and the spindle stiffness on the cutter/spindle's radial error. The results reveal that the centrifugal force generated by the unbalanced mass is the main factor causing the increase in radial error. One way to compensate for the effect of unbalanced mass is to increase the spindle stiffness. Experimental results confirm that a higher front bearing preload can render the spindle stiffer, thus reducing the radial error of the cutter/spindle system. Finally, it should be pointed out that the proposed cutting mark measurement cannot replace the sphere method because it cannot provide time-resolved or angle-resolved information as those obtained from polar charts. However, the proposed cutting mark measurement can provide the characterization of the spindle with the cutter attached. As a result, both methods can complement each other to provide a more complete picture of the behavior of the cutter/spindle system at high speeds.}, number={7-8}, journal={INTERNATIONAL JOURNAL OF MACHINE TOOLS & MANUFACTURE}, author={Nakkiew, Wasawat and Lin, Chi-Wei and Tu, Jay F.}, year={2006}, month={Jun}, pages={877–889} }
 @article{sweet_tu_2006, title={Evaluating tolerances and process capability when using truncated probability density functions}, volume={44}, ISSN={["0020-7543"]}, DOI={10.1080/00207540500521576}, abstractNote={An approach is presented on the evaluation of tolerances for the fit between a bore and a shaft, based on the construction of a statistical model of the fit, using truncated probability density functions. The model yields relationships between the probabilities that the bore, shaft and fit meet their specifications, and thus relates the parameters that define the probability density functions. Equations are presented in sufficient detail so that numerical values of capability indices, operating characteristics and Taguchi loss functions can be computed using currently available software. As an example, truncated normal distributions are used to illustrate the capabilities of the model.}, number={17}, journal={INTERNATIONAL JOURNAL OF PRODUCTION RESEARCH}, author={Sweet, AL and Tu, JF}, year={2006}, month={Sep}, pages={3493–3508} }
 @article{tu_corless_jeppsson_2004, title={Robust control of high speed end milling with unknown process parameter and CNC delay}, volume={17}, DOI={10.1061/(asce)0893-1321(2004)17:1(1)}, abstractNote={Lack of complete knowledge of process characteristics, particularly uncertainties associated with milling processes and spindle behavior, severely limits the reliability of high speed spindles to support high speed machining. Attempts to regulate high speed milling processes based on sensing spindle power and controlling feed rate to prevent spindle stalls have not been successful, mainly due to the fact that power is often not the limiting factor. Instead, problems such as chatter, cutter and/or part deflection, milling forces, and its effects on the rapid wear of the taper of the tool holder and the spindle bearings can easily lead to scrapped parts and/or spindle breakdowns unless extreme care is taken. This paper addresses the stability analysis and design of a novel, robust, nonlinear milling controller implemented at Boeing Co. This controller is aimed at driving an end mill at the highest possible feedrate without damaging the cutter and the spindle. The successful implementation of this controller at Boeing has significantly reduced spindle failures in their extremely demanding high speed machining production lines.}, number={1}, journal={Journal of Aerospace Engineering}, author={Tu, J. F. and Corless, M. and Jeppsson, J.}, year={2004}, pages={09-} }