A study of surface quality and mechanical strength in 5083 aluminum alloy plates using pulsed laser cleaning

Ji Lin,Chunming Wang 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.10

In this work, pulsed laser cleaning was conducted on 5083 aluminum alloy plates for removing the oxide film and improving the surface quality. The chroma was innovatively introduced to evaluate the surface quality of cleaned samples. Samples with good cleaning quality were successfully achieved by means of process optimization involving single factor and orthogonal laser cleaning tests. The optimum cleaning parameters for desirable effect consisted mainly of average power of 40 W, scanning speed of 2500 mm/s and repetitive frequency of 85 kHz. On this basis, a well-formed welded joint can be obtained because of the removal of the oxide film on the surface of aluminum alloy. It has been found that the maximum stress and strain of joints subjected to laser cleaning were up to 102.5 % and 114.81 % of joints laser welded in the absence of laser cleaning, respectively. Significant improvement in mechanical strength was mainly attributable to effective suppression of porosities in welds on account of the removal of oxides with the assist of pulsed laser cleaning.

A two-level finite element method for the steady-state Navier--Stokes/Darcy model

Jilin Fang,Pengzhan Huang,Yi Qin 대한수학회 2020 대한수학회지 Vol.57 No.4

A two-level finite element method based on the Newton iterative method is proposed for solving the Navier--Stokes/Darcy model. The algorithm solves a nonlinear system on a coarse mesh $H$ and two linearized problems of different loads on a fine mesh $h=O(H^{4-\epsilon})$. Compared with the common two-grid finite element methods for the considered problem, the presented two-level method allows for larger scaling between the coarse and fine meshes. Moreover, we prove the stability and convergence of the considered two-level method. Finally, we provide numerical experiment to exhibit the effectiveness of the presented method.

Effect of AlN content on the properties and microstructure of pressureless-sintered Al2O3-AlN composites

Jilin Hu,Qizhong Huang,Hongxia Peng,Xiuying Tian,Zhanjun Chen,Yangxi Peng 한양대학교 세라믹연구소 2018 Journal of Ceramic Processing Research Vol.19 No.3

Aluminium oxide (Al2O3)-aluminium nitride (AlN) composites were prepared at 1500-1600 oC by pressureless sintering. Themain raw materials used were AlN and Al2O3, and appropriate amounts of TiO2-Y2O3-CaO-La2O3 quaternary sintering aidswere added. The effects of different AlN contents and sintering temperatures on the bulk density, bending strength, Rockwellhardness, thermal conductivity, and other properties of Al2O3-based ceramics were studied. The phase composition andmicrostructure of the samples were analysed with an X-ray diffractometer and scanning electron microscope. Results showedthat the Al2O3-AlN composites had excellent overall performance in terms of a flexural strength of 295.7MPa, a Rockwellhardness of 61.5 HRA, and a thermal conductivity of 38.8W/(m·K) (at 25 oC) after pressureless sintering at 1600 oC for 3 hwith 10 wt% AlN content. These properties met the requirements of high-power ceramic LED lamp holders. The Al2O3-10 wt%AlN composites sintered at 1600 oC had a dense microstructure, tight bonding among grains, and few pores, whichexplained the excellent performance of the samples.

Effects of different aluminum sources and calcination temperatures on the synthesis of ultrafine AlN powder via carbothermal reduction nitridation

Jilin Hu 한양대학교 세라믹연구소 2017 Journal of Ceramic Processing Research Vol.18 No.3

AlN ultrafine powder was synthesized in N2 via carbothermal reduction nitridation with different aluminum sources (i.e.,Al(OH)3, Al(NO3)3·9H2O, and α-Al2O3) and high-activity nanocarbon black as raw materials. The effects of differentaluminum sources, aluminum dosage, and calcination temperature on the phase composition and microstructure of thesynthesized AlN powder were studied. Thermodynamic analysis of the reaction process was performed and the reactionmechanism of AlN powder was discussed. Results showed that fully nitrided phase-pure AlN powder was synthesized afterincubating the precursor powder samples at 1500 oC for 2 hrs with Al(OH)3 or Al(NO3)3·9H2O as the aluminum source. Ata 1 : 5 molar ratio of Al2O3/C, a weak Al2O3 diffraction peak was present in the AlN powder samples that were synthesizedat 1550 oC with α-Al2O3 as the aluminum source. The quality of AlN powder samples that were synthesized withAl(NO3)3·9H2O as the aluminum source was optimal relative to those of AlN powder samples that were synthesized with thetwo other aluminum sources. Flake-like particles (~ 100-300 nm) in the powder samples exhibited reduced agglomeration. Moreover, approximately spherical structures that were 0.5-1.0 μm in size formed between the flake-like particles. Duringcarbothermal reduction nitriding, Al2O3 in the raw material first underwent carbothermal reduction to produce metallicaluminum vapor and aluminum low valence gas oxides (Al2O), and then underwent further nitriding to form AlN.

Consolidation of Thawing Permafrost Considering Phase Change

Jilin Qi,Xiaoliang Yao,Fan Yu 대한토목학회 2013 KSCE JOURNAL OF CIVIL ENGINEERING Vol.17 No.6

When permafrost thaws, consolidation occurs under surcharge loads, leading to settlement which is one of the largest threats to infrastructures constructed in permafrost regions. This paper proposes a three dimensional thaw consolidation theory combining Biot’s consolidation theory with heat conductive equations considering ice-water phase change effects through specific heat. Numerical simulation was implemented. The theory was verified by comparing the degree of consolidation and pore pressure ratio calculated by this theory with that from closed form solutions for a simplified one dimensional problem. Thaw consolidation tests were carried out on frozen soil samples with four water contents and under two surcharge loads. Strains occurring in the tests were compared with those from the numerical calculations. It is found that with low water contents in the frozen soil, strain from thaw consolidation was relatively small, and was well simulated by the proposed theory. With increase in water content of frozen soils, the discrepancy between calculated and test results increases.

Preparation of Al2O3-ZrO2 composite powder by co-precipitation method in an alcohol-water solution and its sintering behavior

Jilin Hu,Xiuying Tian,Chuanyue Hu,Yige Luo,Hongxia Peng,Jinqiu Luo 한양대학교 세라믹연구소 2016 Journal of Ceramic Processing Research Vol.17 No.11

Different types of aluminum sources (such as Al2O3 and Al(OH)3) were used to prepare an Al2O3-ZrO2 composite powder inan alcohol-water system, in which ZrOCl2·8H2O and NH4HCO3 were used as starting material and precipitant, respectively. This study explores the effects of different types of aluminum sources and heat treatment on the preparation of Al2O3-ZrO2composite powder. The sintering property of the Al2O3-ZrO2 composite powder prepared from different types of aluminumsources was also investigated. Results indicate that Al2O3-ZrO2 composite powder prepared from Al(OH)3-containingprecursors after calcination at 600 oC possesses a relatively uniform spherical structure, with particle size ranging from 50 nmto 100 nm. As the calcination temperature increases, the particle size of powder prepared using precursors that containAl(OH)3 increases and the micro appearance of the particles gradually shifts from a mainly spherical structure to a diversifiedstructure, which can be spherical, sheet-like or rod-like. The Al2O3-ZrO2 composite powder prepared from Al(OH)3-containingprecursors exhibits satisfactory sintering property. The relative density of the sintered sample reaches 98.5%, and the Rockwellhardness reaches 85.5 HRA after sintering of the composite powder at 1550 oC for 2 hrs

Effects of different carbon sources and reaction temperatures on the synthesis of SiC-TiC composite powders by carbothermal reduction

Jilin Hu 한양대학교 세라믹연구소 2017 Journal of Ceramic Processing Research Vol.18 No.1

SiC-TiC composite powders were synthesised in argon atmosphere by a carbothermal reduction method using silica sol, TiO2,and different carbon sources (i.e., graphite and starch) as the raw materials. The effects of carbon source types and reactiontemperatures on the phase composition, particle size distribution, and microstructure of SiC-TiC composite powders wereinvestigated. The reaction mechanism of SiC-TiC composite powders was also discussed. Results showed that SiC-TiCcomposite powders can be synthesized under optimum conditions (i.e., 1600 oC for 2 hrs using graphite as carbon source and1550 oC for 2 hrs using starch as carbon source). The SiC-TiC composite powders prepared at 1600 oC comprised a smallamount of large-sized tabular grains, a large number of fine irregular rod-like and approximately spherical particles 200-400 nm in size using graphite as carbon source. The SiC-TiC composite powders prepared at 1550 oC were composed of flakyparticles, spherical particles (~ 100-200 nm), and whiskers (50-200 nm in diameter) using starch as carbon source. The growthmechanism of SiC whiskers synthesised in powder samples followed the vapor-solid mechanism.

Synthesis of SiC-TiC composite powders by carbothermal reduction and their sintering behavior

Jilin Hu 한양대학교 세라믹연구소 2013 Journal of Ceramic Processing Research Vol.14 No.1

SiC-TiC composite powders were synthesized in Ar at 1300 ~ 1700 ο C by carbothermal reduction using TiO2, carbon black and silica sol as the starting materials. The synthesized products were characterized by XRD and SEM. The ignition loss was calculated to determine the degree of the carbothermal reaction. The results show that the SiC-TiC composite powders synthesized at 1600 ο C for 1 h are composed of round-like and plate-like particles and a small amount of whiskers with a size of 50 ~ 100 nm. The influence of a AlN-Y2O3 additive on the sintering behavior of the SiC-TiC composite powders was investigated by microstructural observation and Rockwell hardness testing. The SiC-TiC composites with 10 vol% AlN-Y2O3 hot pressed at 1900 ο C under 25 MPa possess a relative density of 98.9% and Rockwell hardness of 93.2HRA.

A TWO-LEVEL FINITE ELEMENT METHOD FOR THE STEADY-STATE NAVIER-STOKES/DARCY MODEL

Fang, Jilin,Huang, Pengzhan,Qin, Yi Korean Mathematical Society 2020 대한수학회지 Vol.57 No.4

A two-level finite element method based on the Newton iterative method is proposed for solving the Navier-Stokes/Darcy model. The algorithm solves a nonlinear system on a coarse mesh H and two linearized problems of different loads on a fine mesh h = O(H^4-𝜖). Compared with the common two-grid finite element methods for the considered problem, the presented two-level method allows for larger scaling between the coarse and fine meshes. Moreover, we prove the stability and convergence of the considered two-level method. Finally, we provide numerical experiment to exhibit the effectiveness of the presented method.

May Fourth: A Patriotic Movement of Cosmopolitanism

Xu Jilin 성균관대학교 동아시아학술원 2009 Sungkyun Journal of East Asian Studies Vol.9 No.1

The May Fourth Movement, which occurred ninety years ago in 1919, has long been characterized as a patriotic event in textbooks and mainstream media, a collective memory to be refreshed and glorified periodically. The author seeks to characterize what kind of patriotism it was and its relationships with the cosmopolitanism, social regeneration, and individualism of the New Culture Movement of the period. The May Fourth Movement, after turning into a social movement, would inevitably become more ideology-driven and more political. Post-1922 fervor for politics, having emerged from the New Cultural Movement, had a distinctive doctrinal commitment and was fueled by specific ideology. But after 1922, in the wake of the upheaval of the May Fourth Movement and the continuous social movements, all social classes became dissatisfied with the warlord rule and the Beiyang government. The catalyzing effects of both ideology and social movement paved the way for the Nationalist Revolution. Like the May Fourth Movement, the Nationalist Revolution had its catalyst, which was nothing other than the consciousness of nationalism, being suppressed by the cosmopolitanism of the May Fourth period. What is stressed here is that social movement was not cultural movement. New Culture Movement could simply import theories and tolerate diversity, but a social movement needed mass mobilization and the propagation of ideas.