PURPOSES : Pavement surface friction depends significantly on pavement surface texture characteristics. The mean texture depth (MTD), which is an index representing pavement surface texture characteristics, is typically used to predict pavement surface friction. However, the MTD may not be sufficient to represent the texture characteristics to predict friction. To enhance the prediction of pavement surface friction, one must select additional variables that can explain complex pavement surface textures. METHODS : In this study, pavement surface texture characteristics that affect pavement surface friction were analyzed based on the friction mechanism. The wavelength, pavement surface texture shape, and pavement texture depth were hypothesized to significantly affect the surface friction of pavement. To verify this, the effects of the three abovementioned pavement surface texture characteristics on pavement surface friction must be investigated. However, because the surface texture of actual pavements is irregular, examining the individual effects of these characteristics is difficult. To achieve this goal, the selected pavement surface texture characteristics were formed quantitatively, and the irregularities of the actual pavement surface texture were improved by artificially forming the pavement surface texture using threedimensionally printed specimens. To reflect the pavement surface texture characteristics in the specimen, the MTD was set as the pavement surface texture depth, and the exposed aggregate number (EAN) was set as a variable. Additionally, the aggregate shape was controlled to reflect the characteristics of the pavement surface texture of the specimen. Subsequently, a shape index was proposed and implemented in a statistical analysis to investigate its effect on pavement friction. The pavement surface friction was measured via the British pendulum test, which enables measurement to be performed in narrow areas, considering the limited size of the three-dimensionally printed specimens. On wet pavement surfaces, the pavement surface friction reduced significantly because of the water film, which intensified the effect of the pavement surface texture. Therefore, the pavement surface friction was measured under wet conditions. Accordingly, a BPN (wet) prediction model was proposed by statistically analyzing the relationship among the MTD, EAN, aggregate shape, and BPN (wet). RESULTS : Pavement surface friction is affected by adhesion and hysteresis, with hysteresis being the predominant factor under wet conditions. Because hysteresis is caused by the deformation of rubber, pavement surface friction can be secured through the formation of a pavement surface texture that causes rubber deformation. Hysteresis occurs through the function of macro-textures among pavement surface textures, and the effects of macro-texture factors such as the EAN, MTD, and aggregate shape on the BPN (wet) are as follows: 1) The MTD ranges set in this study are 0.8, 1.0, and 1.2, and under the experimental conditions, the BPN (wet) increases linearly with the MTD. 2) An optimum EAN is indicated when the BPN (wet) is the maximum, and the BPN decreases after its maximum value is attained. This may be because when the EAN increases excessively, the space for the rubber to penetrate decreases, thereby reducing the hysteresis. 3) The shape of the aggregate is closely related to the EAN; meanwhile, the maximum value of the pavement surface friction and the optimum EAN change depending on the aggregate shape. This is believed to be due to changes in the rubber penetration volume based on the aggregate shape. Based on the results above, a statistical prediction model for the BPN (wet) is proposed using the MTD, EAN, and shape index as variables. CONCLUSIONS : The EAN, MTD, and aggregate shape are crucial factors in predicting skid resistance. Notably, the EAN and aggregate shape, which are not incorporated into existing pavement surface friction prediction models, affect the pavement surface friction. However, the texture of the specimen created via three-dimensional printing differs significantly from the actual pavement surface texture. Therefore, the pavement surface friction prediction model proposed in this study should be supplemented with comparisons with actual pavement surface data in the future.

High-Manganese (Mn) austenitic steel, with over 24 wt% Mn content, offers outstanding mechanical properties in cryogenic settings, making it a potential replacement for existing cryogenic materials. This high manganese steel exhibits high strength, ductility, and wear resistance, making it promising for applications like LNG tanks, flanges, and valves. To operate in cryogenic environments, hot forging and heat treatment processes are vital, especially in flange production. The cooling rate during high-temperature cooling after hot forging plays a critical role in influencing the microstructure and mechanical properties of high manganese steel. The rate at which cooling occurs during this process influences the size of the grains and the distribution of manganese and consequently has an impact on mechanical properties. This study assessed the microstructure and mechanical properties based on different cooling rates during the hot forging of High-Mn steel flanges. Comparing air and water cooling after hot forging, followed by heat treatment, revealed notable differences in grain size. These differences directly impacted mechanical properties such as tensile strength, hardness, and Charpy impact property. Understanding these effects is crucial for optimizing the performance and reliability of High-Mn steel in cryogenic applications.

The effects of annealing on the microstructure and mechanical properties of Al–Zn–Mg–Cu–Si alloys fabricated by high-energy ball milling (HEBM) and spark plasma sintering (SPS) were investigated. The HEBM-free sintered alloy primarily contained Mg2Si, Q-AlCuMgSi, and Si phases. Meanwhile, the HEBM-sintered alloy contains Mg-free Si and θ-Al2Cu phases due to the formation of MgO, which causes Mg depletion in the Al matrix. Annealing without and with HEBM at 500oC causes partial dissolution and coarsening of the Q-AlCuMgSi and Mg2Si phases in the alloy and dissolution of the θ-Al2Cu phase in the alloy, respectively. In both alloys, a thermally stable α-AlFeSi phase was formed after long-term heat treatment. The grain size of the sintered alloys with and without HEBM increased from 0.5 to 1.0 μm and from 2.9 to 6.3 μm, respectively. The hardness of the sintered alloy increases after annealing for 1 h but decreases significantly after 24 h of annealing. Extending the annealing time to 168 h improved the hardness of the alloy without HEBM but had little effect on the alloy with HEBM. The relationship between the microstructural factors and the hardness of the sintered and annealed alloys is discussed.

The purpose of this study was to analyze microstructural changes and evaluate the mechanical properties of TWIP steel subjected to variations in heat treatment, in order to identify optimal process conditions for enhancing the performance of TWIP steel. For this purpose, a homogenization heat treatment was conducted at 1,200 °C for 2 h, followed by hot rolling at temperature exceeding 1,100 °C and cold rolling. Annealing heat treatment is achieved using a muffle furnace in the range of 600 °C to 1,000 °C. The microstructure characterization was performed with an optical microscope and X-ray diffraction. Mechanical properties are evaluated using micro Vickers hardness, tensile test, and ECO index (UTS × Elongation). The specimens annealed at 900 °C and 1,000 °C experienced a significant decrease in hardness and strength due to decarburization. Consequently, the decarburization phenomenon is closely related to the heat treatment process and mechanical properties of TWIP steel, and the effect of the microstructure change during annealing heat treatment.

In this study, changes in the microstructure and mechanical properties of cast and extruded Al-2Li-1Ce alloy materials were investigated as the Mg content was varied. The density decreased to 2.485, 2.46 and 2.435 g/cm3 when the Mg content in the Al-2Li-1Ce alloy was increased to 2, 4 and 6 wt%, respectively. Intermetallic compounds of Al11Ce3 were observed in all alloys, while the β-phase of Al3Mg2 was observed in alloys containing 6 wt% of Mg. In the extruded material, with increasing Mg content the average grain size decreased to 84.8, 71.6 and 36.2 μm, and the fraction of high-angle grain boundaries (greater than 15°) increased to 82.8 %, 88.6 %, and 91.8 %, respectively. This occurred because the increased Mg content promotes dynamic recrystallization during hot extrusion. Tensile test results showed that as the Mg content increased, both the yield strength and tensile strength increased. The yield strength reached 86.1, 107.3, and 186.4 MPa, and the tensile strength reached 215.2, 285, and 360.5 MPa, respectively. However, it is worth noting that the ductility decreased to 27.78 %, 25.65 %, and 20.72 % as the Mg content increased. This reduction in ductility is attributed to the strengthening effect resulting from the increased amount of dissolved Mg, and grain refinement due to dynamic recrystallization.

Boron carbide (B4C) is highly significant in the production of lightweight protective materials when added to aluminum owing to its exceptional mechanical properties. In this study, a method for fabricating Al-B4C composites using high-energy ball milling and directed energy deposition (DED) is presented. Al-4 wt.% B4C composites were fabricated under 21 different laser conditions to analyze the microstructure and mechanical properties at different values of laser power and scan speeds. The composites fabricated at a laser power of 600 W and the same scan speed exhibited the highest hardness and generated the fewest pores. In contrast, the composites fabricated at a laser power of 1000 W exhibited the lowest hardness and generated a significant number of large pores. This can be explained by the influence of the microstructure on the energy density at different values of laser power.

This study aims to examine the correlation between microstructures and the mechanical properties of two highstrength API X70 linepipe steels with different specimen directions and Moaddition. The microstructure of the Mo-added steel has an irregularly shaped AF, GB matrix with pearlite because of the relatively large deformation in the non-recrystallization temperature region, while that of the Mo-free steel shows a PF matrix with bainitic microstructure. In the Mo-added steel, the M/A (martensite-austenite) in granular bainite (GB) and pearlite act as crack initiation sites with decreased upper shelf energy and an increased ductile to brittle transition temperature (DBTT). Regardless of Mo addition, all of the steels demonstrate higher strength and lower elongation in the T direction than in the L direction because of the short dislocation glide path and ease of pile-up at grain boundaries. In addition, the impact test specimens with T-L direction had a lower impact absorbed energy and higher DBTT than those with the L-T direction because the former exhibit shorter unit crack path compared to the latter.

The Ag/WC electrical contacts were prepared via powder metallurgy using 60 wt% Ag, 40 wt% WC, and small amounts of Co3O4 with varying WC particle sizes. After the fabrication of the contact materials, microstructure observations confirmed that WC-1 had an average grain size (AGS) of 0.27 μm, and WC-2 had an AGS of 0.35 μm. The Ag matrix in WC-1 formed fine grains, whereas a significantly larger and continuous growth of the Ag matrix was observed in WC-2. This indicates the different flow behaviors of liquid Ag during the sintering process owing to the different WC sizes. The electrical conductivities of WC-1 and WC-2 were 47.8% and 60.4%, respectively, and had a significant influence on the Ag matrix. In particular, WC-2 exhibited extremely high electrical conductivity owing to its large and continuous Ag-grain matrix. The yield strengths of WC-1 and WC-2 after compression tests were 349.9 MPa and 280.7 MPa, respectively. The high yield strength of WC-1 can be attributed to the Hall–Petch effect, whereas the low yield strength of WC-2 can be explained by the high fraction of high-angle boundaries (HAB) between the WC grains. Furthermore, the relationships between the microstructure, electrical/mechanical properties, and deformation mechanisms were evaluated.

Titanium, which has excellent strength and toughness characteristics, is increasingly used in the aerospace field. Among the titanium alloys used for body parts, more than 80 % are Ti-6Al-4V alloys with a tensile strength of 931 MPa. The spark plasma sintering (SPS) method is used for solidification molding of powder manufactured by the mechanical milling (MM) method, by sintering at low temperature for a short time. This sintering method avoids coarsening of the fine crystal grains or dispersed particles of the MM powder. To improve the mechanical properties of pure titanium without adding alloying elements, stearic acid was added to pure titanium powder as a process control agent (PCA), and MM treatment was performed. The properties of the MM powder and SPS material produced by solidifying the powder were investigated by hardness measurement, X-ray diffraction, density measurement and structure observation. The processing deformation of the pure titanium powder depends on the amount of stearic acid added and the MM treatment time. TiN was also generated in powder treated by MM 8 h with 0.50 g of added stearic acid, and the hardness of the powder was higher than that of Ti-6Al-4V alloy when treated with MM for 8 h. When the MM-treated powder was solidified in the SPS equipment, TiC was formed by the solid phase reaction. The SPS material prepared as a powder treated with MM 8 h by adding 0.50 g of stearic acid also formed TiN and exhibited the highest hardness of Hv1253.

Because magnets fabricated using Nd-Fe-B exhibit excellent magnetic properties, this novel material is used in various high-tech industries. However, because of the brittleness and low formability of Nd-Fe-B magnets, the design freedom of shapes for improving the performance is limited based on conventional tooling and postprocessing. Laserpowder bed fusion (L-PBF), the most famous additive manufacturing (AM) technique, has recently emerged as a novel process for producing geometrically complex shapes of Nd-Fe-B parts owing to its high precision and good spatial resolution. However, because of the repeated thermal shock applied to the materials during L-PBF, it is difficult to fabricate a dense Nd-Fe-B magnet. In this study, a high-density (>96%) Nd-Fe-B magnet is successfully fabricated by minimizing the thermal residual stress caused by substrate heating during L-PBF.

This research investigated how adding Sb (0.75, 1.0, 2.0 and 5.0 wt%) to as-extruded aluminum alloys affected their microstructure, mechanical properties, electric and thermal conductivity. The addition of Sb resulted in the formation of AlSb intermetallic compounds. It was observed that intermetallic compounds in the alloys were distributed homogenously in the Al matrix. As the content of Sb increased, the area fraction of intermetallic compounds increased. It can be clearly seen that the intermetallic compounds were crushed into fine particles and homogenously arrayed during the extrusion process. As the Sb content increased, the average grain size decreased remarkably from 282.6 μm (0.75 wt%) to 109.2 μm (5.0 wt%) due to dynamic recrystallization by the dispersed intermetallic compounds in the aluminum matrix during the hot extrusion. As the Sb content increased from 0.75 to 2.0 wt%, the electrical conductivity decreased from 61.0 to 59.8 % of the International Annealed Copper Standard. Also, as the Sb content increased from 0.75 to 2.0 wt%, the ultimate tensile strength did not significantly change, from 67.3 to 67.8 MPa.

Changes in the mechanical properties and microstructure of an IN 939 W alloy according to the sintering heating rate were evaluated. IN 939 W alloy samples were fabricated by spark plasma sintering. The phase fraction, number density, and mean radius of the IN 939W alloy were calculated using a thermodynamic calculation. A universal testing machine and micro-Vickers hardness tester were employed to confirm the mechanical properties of the IN 939W alloy. X-ray diffraction, optical microscopy, field-emission scanning electron microscopy, Cs-corrected-field emission transmission electron microscopy, and energy dispersive X-ray spectrometry were used to evaluate the microstructure of the alloy. The rapid sintering heating rate resulted in a slightly dispersed γ' phase and chromium oxide. It also suppressed the precipitation of the η phase. These helped to reinforce the mechanical properties.

A typical trade-off relationship exists between strength and elongation in face-centered cubic metals. Studies have recently been conducted to enhance strength without ductility reduction through surface-treatment-based ultrasonic nanocrystalline surface modification (UNSM), which creates a gradient microstructure in which grains become smaller from the inside to the surface. The transformation-induced plasticity effect in Fe-Mn alloys results in excellent strength and ductility due to their high work-hardening rate. This rate is achieved through strain-induced martensitic transformation when an alloy is plastically deformed. In this study, Fe-6%Mn powders with different sizes were prepared by high-energy ball milling and sintered through spark plasma sintering to produce Fe-6%Mn samples. A gradient microstructure was obtained by stacking the different-sized powders to achieve similar effects as those derived from UNSM. A compressive test was performed to investigate the mechanical properties, including the yielding behavior. The deformed microstructure was observed through electron backscatter diffraction to determine the effects of gradient plastic deformation.

Beta-titanium alloys are used in many industries due to their increased elongation resulting from their BCC structure and low modulus of elasticity. However, there are many limitations to their use due to the high cost of betastabilizer elements. In this study, biocompatible Ti-Mo-Fe beta titanium alloys are designed by replacing costly betastabilizer elements (e.g., Nb, Zr, or Ta) with inexpensive Mo and Fe elements. Additionally, Ti-Mo-Fe alloys designed with different Fe contents are fabricated using powder metallurgy. Fe is a strong, biocompatible beta-stabilizer element and a low-cost alloying element. The mechanical properties of the Ti-Mo-Fe metastable beta titanium alloys are analyzed in relation to the microstructural changes. When the Fe content increases, the tensile strength and elongation decrease due to brittle fracture despite a decreasing pore fraction. It is confirmed that the hardness and tensile strength of Ti-5Mo-2Fe P/M improve to more than 360 Hv and 900 MPa, respectively.

In this study, additive manufacturing of a functionally graded material (FGM) as an alternative to joining dissimilar metals is investigated using directed energy deposition (DED). FGM consists of five different layers, which are mixtures of austenitic stainless steel (type 316 L) and low-alloy steel (LAS, ferritic steel) at ratios of 100:0 (A layer), 75:25 (B layer), 50:50 (C layer), 25:75 (D layer), and 0:100 (E layer), respectively, in each deposition layer. The FGM samples are successfully fabricated without cracks or delamination using the DED method, and specimens are characterized using optical and scanning electron microscopy to monitor their microstructures. In layers C and D of the sample, the tensile strength is determined to be very high owing to the formation of ferrite and martensite structures. However, the elongation is high in layers A and B, which contain a large fraction of austenite.

A Cu-15Ag-5P filler metal (BCuP-5) is fabricated on a Ag substrate using a high-velocity oxygen fuel (HVOF) thermal spray process, followed by post-heat treatment (300oC for 1 h and 400oC for 1 h) of the HVOF coating layers to control its microstructure and mechanical properties. Additionally, the microstructure and mechanical properties are evaluated according to the post-heat treatment conditions. The porosity of the heat-treated coating layers are significantly reduced to less than half those of the as-sprayed coating layer, and the pore shape changes to a spherical shape. The constituent phases of the coating layers are Cu, Ag, and Cu-Ag-Cu3P eutectic, which is identical to the initial powder feedstock. A more uniform microstructure is obtained as the heat-treatment temperature increases. The hardness of the coating layer is 154.6 Hv (as-sprayed), 161.2 Hv (300oC for 1 h), and 167.0 Hv (400oC for 1 h), which increases with increasing heat-treatment temperature, and is 2.35 times higher than that of the conventional cast alloy. As a result of the pull-out test, loss or separation of the coating layer rarely occurs in the heat-treated coating layer.

In this study, a new manufacturing process for a multilayer-clad electrical contact material is suggested. A thin and dense BCuP-5 (Cu-15Ag-5P filler metal) coating layer is fabricated on a Ag plate using a high-velocity oxygen-fuel (HVOF) process. Subsequently, the microstructure and bonding properties of the HVOF BCuP-5 coating layer are evaluated. The thickness of the HVOF BCuP-5 coating layer is determined as 34.8 μm, and the surface fluctuation is measured as approximately 3.2 μm. The microstructure of the coating layer is composed of Cu, Ag, and Cu-Ag-Cu3P ternary eutectic phases, similar to the initial BCuP-5 powder feedstock. The average hardness of the coating layer is 154.6 HV, which is confirmed to be higher than that of the conventional BCuP-5 alloy. The pull-off strength of the Ag/BCup-5 layer is determined as 21.6 MPa. Thus, the possibility of manufacturing a multilayer-clad electrical contact material using the HVOF process is also discussed.

본 연구는 보험설계사의 코칭역량과 일터환경특성이 조직성과에 미치는 영향 을 규명하고, 대인관계의 매개효과를 실증적으로 규명하는 데 있다. 본 연구는 보험설계사를 대상으로 총 521부의 구글 설문을 수집하였고, SPSS와 AMOS를 사용하여 분석한 결과는 다음과 같다. 첫째, 보험설계사의 코칭역량과 조직성과 간의 상관관계를 살펴보면, 통계적 으로 유의미한 정적 상관관계가 있으며, 이는 보험설계사의 코칭역량이 높으면 조직성과도 높아짐을 의미한다. 둘째, 보험설계사의 대인관계와 조직성과, 코칭 역량과 대인관계, 일터환경특성과 대인관계 간의 상관관계를 살펴보면, 대부분 의 측정 변인들 간에 통계적으로 유의미한 정적 상관이 있는 것으로 나타났다. 셋째, 보험설계사의 코칭역량과 일터환경특성은 조직성과에 직접적으로 긍정적 인 영향을 미치고, 대인관계의 매개를 통해서도 조직성과에 영향을 미치는 것으 로 예측되어, 대인관계는 코칭역량과 조직성과, 일터환경특성과 조직성과의 관 계를 부분 매개하고 있음을 알 수 있었다. 이 연구를 통해서 보험설계사가 상담 시 사용하는 상담스킬이 코칭에서 사용 되는 코칭역량과 대단히 유사함을 규명하는 연구들이 더 진행되기를 제안한다.

Reheat treatment process of mold is necessary when partial machining of the mold is required, such as shape correction for an existing mold. If defects such as cracks or significant deterioration of mechanical properties of the mold occur during reheat treatment, it is impossible to reuse the mold. In this study, reheat treatment was performed up to two times for STD11 tool steel, and microstructure and mechanical properties according to the reheat treatment were evaluated. Carbide fraction and grain size of prior austenite were almost unchanged after the reheat treatment. Hardness and impact toughness increased significantly after QT treatment, and these properties were maintained without significant change even after the reheat treatment. It is concluded that up to two iterations of reheat treatment does not cause deterioration of properties of STD11 tool steel. Based on these results, a mold for a face-lifted front bumper was manufactured through machining and reheat-treating of an existing mold.

In this study, we report the microstructure and characteristics of Ag-SnO2-Bi2O3 contact materials using a controlled milling process with a subsequent compaction process. Using magnetic pulsed compaction (MPC), the milled Ag-SnO2-Bi2O3 powders have been consolidated into bulk samples. The effects of the compaction conditions on the microstructure and characteristics have been investigated in detail. The nanoscale SnO2 phase and microscale Bi2O3 phase are well-distributed homogeneously in the Ag matrix after the consolidation process. The successful consolidation of Ag-SnO2-Bi2O3 contact materials was achieved by an MPC process with subsequent atmospheric sintering, after which the hardness and electrical conductivity of the Ag-SnO2-Bi2O3 contact materials were found to be 62–75 HV and 52–63% IACS, respectively, which is related to the interfacial stability between the Ag matrix, the SnO2 phase, and the Bi2O3 phase.