J.K. CHEN | National Taipei University of Technology (original) (raw)

Papers by J.K. CHEN

Ceramics International, Nov 30, 2023

International Journal of Heat and Mass Transfer, Feb 1, 2017

In this study, the effects of casting processes and heat treatments on the thermal conductivity o... more In this study, the effects of casting processes and heat treatments on the thermal conductivity of an Al-10 wt% Si-0.6 wt% Cu-0.9 wt% Fe-0.7 wt% Zn aluminum alloy were studied. Both gravity-and die-casting processes were used. The porosity in die castings was controlled by varying the injection pressure and plunger velocity. The porosity, microstructures, and thermal and electrical conductivities of the die castings were characterized. The thermal conductivity of gravity-cast material achieved 143.4 W m À1 K À1 , whereas those of die-cast materials ranged from 110.8 to 126.8 W m À1 K À1. The die-cast aluminum alloys demonstrated considerably finer interconnecting precipitates than did the gravity castings. The closely spaced precipitates acted as barriers for thermal conduction, leading to reduced thermal conductivity. Die-casting pressures and velocities were controlled to form die castings with up to 3.73% porosity, which led to a 12.6% decrease in thermal conductivity. The die-cast Al alloys were then subject to solutionization (T4) and aging treatments (T6). The silicon precipitates were spheroidized, thus widening the paths for heat conduction. The thermal conductivity was greatly enhanced, increasing from 126.8 W m À1 K À1 in the die-cast condition to 151.6 W m À1 K À1 after 550°C solutionizing and 4 h of aging at 200°C. The thermal conductivity of the gravity-cast materials remained unchanged after the same heat treatments. Improving the thermal conductivity of die castings through proper heat treatments is crucial for aluminum alloys in rigorous heat-dissipating applications.

Proceedings of SPIE, Jun 1, 2012

Fluid driven devices have been widely used in many applications, such as pumping, circulating, an... more Fluid driven devices have been widely used in many applications, such as pumping, circulating, and cooling systems in handling liquid. Their driving conditions are highly dependent on the operation purposes. Some of them work with high pressure and high flow rate without the need of flow stability. On the other hand, the steady flow with low pressure and flow rate is required for bio-applications. In a perfusion system for culturing cells, a suitable shear stress from a cultivated fluid is one of key factors to reproduce the fluid conditions of cells in a living organism. A special pump is needed to provide a steady flow rate and stress in such system. In this study, a novel design of the pump constituted by a housing and a screw-type rotor with micro-channels was proposed. To understand the flow phenomena in this design, both computational modeling and real experiment are utilized. In the experiment, a minimum rotational speed is needed to drive the fluid flow. In the modeling, the steady state with low pulsation was achieved within a short period of time. A perfusion system with 7.8% variation in flow rate could be obtained in comparison with traditional peristaltic pump with up to 29% variation in flow rate. Steady fluid flow for a perfusion system then could be obtained in this screw-type pump.

Social Science Research Network, 2022

The minerals, metals & materials series, 2020

In this study, compression testing on Ti6Al4V cuboctahedron cellular sample is performed. In the ... more In this study, compression testing on Ti6Al4V cuboctahedron cellular sample is performed. In the experiment, by varying sample porosity number for selective laser melted structure we ended up with brittle and nonlinear response. Simulation is performed using bilinear material model with ANSYS workbench. Different simulations are modelled to include nonlinear and plastic effects in the results. Results of compression simulation at strain rate of 0.00208 s−1 and deformed up to 50% strain are extracted. The usual way of computing stress from force reaction and strain from total deformation does not fit with experiment exactly. To study the equivalent stress and strain directly, the numbers of solid element distributed at each stress/strain levels is listed and the solid density of the structure is used to transform the equivalent stress and strain for cellular structure. Experimental stress and strain for cellular structure are compared with the stress–strain curves derived from various models described in static structural module. The structure with yield stress of 361.23 MPa starts to get highly distorted after a strain of 35.5%. The compression and failure of cellular structures can be considered by introduction of density ratio of the structures.

Nucleation and Atmospheric Aerosols, 2019

This study investigates the structural compression properties of the additive manufactured (AM) T... more This study investigates the structural compression properties of the additive manufactured (AM) Ti-6Al-4V lattice structure using LS-Dyna simulation analysis software and calculates its mechanical properties under different porosities and different pattern designs. Ti-6Al-4V alloys formed by using additive manufacturing produced elliptic holes with long and short axes due to the difference in the direction of construction. These holes could be eliminated and the materials could be compacted by hot isostatic pressing (HIP) treatment. Compactness of the frame element achieved 99.9% and drastically reduced the anisotropic behavior caused by the fabrication process. For the experimental testing, the samples with different porosities and the A-type and AZ-type designs were employed in compression experiment. In the meantime, the Young’s modulus, the yield strength, plateau stress and other mechanical behavior were investigated by CAE simulation. The plateau stress, which is an important property for some biomedical applications, was calculated with satisfactory accuracy compared with experimental measurements. The simulated results of the designed lattice structures with different porosity contents were able to adopt the Gibson-Ashby model to determine their failure mechanisms. Finally, the CAE simulation for the design of an auxetic material was demonstrated.This study investigates the structural compression properties of the additive manufactured (AM) Ti-6Al-4V lattice structure using LS-Dyna simulation analysis software and calculates its mechanical properties under different porosities and different pattern designs. Ti-6Al-4V alloys formed by using additive manufacturing produced elliptic holes with long and short axes due to the difference in the direction of construction. These holes could be eliminated and the materials could be compacted by hot isostatic pressing (HIP) treatment. Compactness of the frame element achieved 99.9% and drastically reduced the anisotropic behavior caused by the fabrication process. For the experimental testing, the samples with different porosities and the A-type and AZ-type designs were employed in compression experiment. In the meantime, the Young’s modulus, the yield strength, plateau stress and other mechanical behavior were investigated by CAE simulation. The plateau stress, which is an important property for some biome...

Metallurgical and Materials Transactions, Dec 1, 1994

Causes are investigated for the changes in precipitate crystal structure (fcc to 9R) and in morph... more Causes are investigated for the changes in precipitate crystal structure (fcc to 9R) and in morphology (degenerate plate to plate), which are observed to take place in Cr-Ni alloys as the reaction temperature decreases. Transmission electron microscopy (TEM) study is performed to determine the matrix/precipitate orientation relationship, habit plane, and growth ledge spacing. O-lattice modeling is used to show that it is likely that the metastable 9R phase forms as a transition phase at lower reaction temperatures because lattice matching at the bcc/9R habit plane is better than the matching at the bcc/fcc habit plane. The ability of the phenomenological theory of martensite crystallography (PTMC) to predict the habit plane of 9R plates precipitated by a diffusional mechanism is explained by the small lattice invariant deformation required to produce an invariant plane in Cr-Ni. Under this circumstance, the PTMC habit plane nearly coincides with the best-matching interface that presumably appears and is predicted by O-lattice theory.

Materials transactions, 2013

This study investigated Ni50 mass% Cr alloys produced by the vacuum sintering and hot isostatic p... more This study investigated Ni50 mass% Cr alloys produced by the vacuum sintering and hot isostatic pressing (HIP) of powder metallurgy technology. The experimental results showed that the relative density of Ni50 mass% Cr alloys reached 98.67%, the apparent porosity decreased to 1.33%, transverse rupture strength (TRS) increased to 454.29 MPa and electrical resistivity decreased to 4.284 © 10 ¹4 ³•cm after 1345°C sintering for 1 h. Meanwhile, laminar eutectic precipitations appeared in the sintered Ni50 mass% Cr alloys. In addition, the relative density increased to 99.73%, the apparent porosity decreases to 0.27% and TRS was obviously enhanced to 1181.4 MPa after 1260°C 175 MPa 4 h HIP treatment. Moreover, the electrical resistivity decreased to 3.346 © 10 ¹4 ³•cm after the optimal HIP treatment. This study showed that the HIP process is effective in eliminating internal pores and improving the mechanical and electrical properties of the sintered Ni50 mass% Cr alloys, thus obtaining the high density and optimum properties of the sintered materials.

Materials transactions, 2015

In this study, different amounts of tantalum carbide (TaC) powders (10, 20 and 30 mass%) were mix... more In this study, different amounts of tantalum carbide (TaC) powders (10, 20 and 30 mass%) were mixed and added to 440C steel powders. The composite powders were sintered at 1270, 1280 and 1290°C, respectively, for 1 h. The experimental results showed that a suitable amount of TaC particle addition was effective in improving the strength and hardness of the composite materials. Conversely, excess TaC hindered the liquid diffusion of the Fe elements. As a result, the 440C specimens with 30% TaC addition still had 1.3% porosity. The 440C specimens with 10% TaC addition sintered at 1270°C possessed the highest TRS values (2260.3 MPa), while those with 20% TaC addition sintered at 1290°C had the highest hardness values (HRA 85.2). The microstructural evaluation of the 440C specimens with 10% TaC addition revealed that all the rod-shaped M 7 C 3 carbides transformed to M 23 C 6 carbides and precipitated within the grains as a strengthening phase after heat treatment, which was advantageous to the transverse rupture strength (TRS). Consequently, the TRS value of the 440C specimens with 10% TaC addition was significantly increased (2260.3 ¼ 2458.4 MPa) after heat treatment, thus possessing a high hardness (HRA 83.8).

Isij International, 2012

This study conducted gas nitriding on AISI H13 steels, followed by oxidization treatments at vari... more This study conducted gas nitriding on AISI H13 steels, followed by oxidization treatments at various temperatures (560°C, 580°C, 600°C) and soaking times (1 h, 2 h, 3 h), to investigate the melting loss and corrosion resistance of A380 aluminum alloy. The experimental results indicated that the samples of oxynitriding (post-oxidizing treatment after nitrided) at 580°C for 3 h, 600°C for 2 h and 600°C for 3 h have better ability to resist melting loss due to their thicker oxidized layers. The weight loss rate also showed that they are near zero after a 4 h erosion test. The increasing weight loss rate is relative to the appearance of the FexAlySiz compound. In addition, the difference in corrosion current is closely related to the corrosion rate. A smaller corrosion current will lead to a smaller corrosion rate. As a result, increasing the temperature and soaking time of oxidization is advantageous to the corrosion resistance, and a small variation in corrosion current will result in a significant change to the polarization resistance of corrosion. Consequently, the oxide layer can enhance the polarization resistance, and quickly generate a passivation layer that increases the corrosion resistance ability.

Philosophical Magazine A, Aug 1, 1998

The interfacial structure of bcc laths precipitated from a nickel-rich fcc matrix were studied us... more The interfacial structure of bcc laths precipitated from a nickel-rich fcc matrix were studied using transmission electron microscopy and high-resolution electron microscopy (HREM). 50 precipitates and more than 200 defects in their vicinity were analysed. HREM images indicate the parallel conjugate planes of the Kurdjumov± Sachs orientation relationship are continuous across the broad face of the laths. In agreement with earlier studies, the (121) f habit plane contains structural ledges, irregularly spaced dislocations but no regular array of mis® t dislocations. 87% of dislocations in the habit plane have Burgers vectors in the conjugate plane. These dislocations are found in the risers of growth ledges and must climb as the ledges migrate. The dislocations accommodate mis® t in the conjugate planes of the orientation relationship while mis® t perpendicular to these planes is compensated by elastic strain. Stacking faults found extending into the matrix from the precipitate habit plane accommodate elastic strain and appear to aid the formation of growth ledges. On the side facet of the precipitate lath, two types of linear defect are found: mis® t dislocations spaced 12 nm apart with 1 3 [ 111 ]f Burgers vectors, and structural ledges parallel to the mis® t dislocations spaced 1.5 nm apart. The mis® t dislocations are in a sessile orientation with respect to growth of the side facet.

Materials Science and Technology, 2002

ABSTRACT

ACS applied electronic materials, Feb 4, 2022

Steel Research International, Sep 1, 2008

LOders strain and aging index were employed to characterize the aging properties of hot rolled lo... more LOders strain and aging index were employed to characterize the aging properties of hot rolled low carbon steels during production. The LOders strain was found to increase with aging time while the aging index decreased. It is interpreted that the aging index represents the susceptibility of steels to aging, while the LOders strain measures the effects of carbon and nitrogen atmosphere pinning on dislocations. By comparing accelerated aging with room temperature aging behaviours of low carbon steels for up to two years, an algorithm deriving the activation energy of solute carbon and nitrogen migration from aging property measurements was proposed. The equation derived for accelerated aging of hot-rolled low carbon steels investigated was in reasonable agreement with that reported in the literature.

Advanced Materials Research, 2013

Nanocomposite Cr-W-B-N coatings with various tungsten contents were synthesized on silicon wafer ... more Nanocomposite Cr-W-B-N coatings with various tungsten contents were synthesized on silicon wafer substrates. The used technique is a DC reactive magnetron co-sputtering deposition equipped with a Cr-B alloy target with 20 at.% B and a W target in a mixed argon/nitrogen plasma atmosphere. Composition and microstructure of the obtained coatings were investigated using X-ray diffraction, X-ray Photoelectron Spectroscope and transmission electron microscope while the micro-hardness was measured using a depth-sensing nano-indenter. The results have shown that the microstructure and the mechanical properties of Cr-W-B-N coatings were strongly dependent on either the tungsten content or the volume fraction of W-N crystalline phases. It was observed that the micro-hardness of Cr-W-B-N coatings is lower than that of Cr-B-N coating as the tungsten content is less than 24 at.% and the volume fraction of W-N crystalline phases is lower than 37 vol.%. As the tungsten content further increased to 30 at.% and the volume fraction of W-N crystalline phases increased to 55 vol.%, the micro-hardness of Cr-W-B-N coating was found enhanced to 19 GPa and higher than Cr-B-N film. It was also obtained that the volume fraction of Cr-N crystalline phases is inversely proportional to the volume fraction of W-N crystalline phases.

Steel Research International, Mar 23, 2015

In this study, commercial FCD700 ductile irons are austenitized in the intercritical region to ob... more In this study, commercial FCD700 ductile irons are austenitized in the intercritical region to observe their effects upon the microstructures and mechanical properties of austempered ductile irons (ADIs). The austenitizing temperature is chosen such that ferrite, austensite, and graphite coexist in the microstructures. Strength of 974 MPa, 16.4% elongation, and impact energy of 166 J are achieved by the ADI austenitized at 1103 K and subsequently austemperted at 673 K. These properties are in contrast to 790 MPa strength, 8.2% elongation, and 42 J impact energy in as-cast FCD700. The properties are also superior to the traditionally austempered ductile iron with 967 MPa strength, 12.7% elongation, and 124 J impact energy. Combinations of the optimum ductility and strength are achieved by 8-19% of proeutectoid ferrite and over 20% of retained austenite in the intercritically treated microstructures. Nanoscale twinned and lenticular martensite forms within the microsize blocky retained austentite islands during plastic deformation leading to the transformation induced plasticity. The intercritically austenitizing and austempering treatments provide unique combination of strength and elongation for commercial ductile irons.

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE OF COMPUTATIONAL METHODS IN SCIENCES AND ENGINEERING 2019 (ICCMSE-2019), 2019

Materials Science and Engineering: C, 2019

Cellular structures often show fluctuating stresses in compression stress-strain curves. Such flu... more Cellular structures often show fluctuating stresses in compression stress-strain curves. Such fluctuating stresses correspond to strut fractures. In this study, the cellular Ti-6Al-4V alloy with cuboctahedron structure was prepared by selective laser melting. The cuboctahedron cellular structures showed reduced fluctuations in their compressive stress-strain curves after the initial yielding peak. Their moduli were modulated via the porosity of the structure by changing the strut diameter. A compressive modulus of between 1.3 and 4.868 GPa can be achieved by varying the porosity in the cellular structures between 33% and 84%. Both heat treatment and hot isostatic press (HIP) treatment reduced the fracture strength of the struts during compression due to the conversion of the α′ martensite phase into the more ductile α + β phase. The cellular structure with HIP treatment produced a continuous stress-strain curve during compression, indicating uniform strain distribution behavior. The continuous compressive stress-strain curve can lead to reduced debris formation during compression processes. The deformation showed either bending or stretching mechanisms depending on whether the supports were included along the building direction. The design concepts of cellular structures demonstrated in this study will be valuable in future biomedical applications.