• 한국재료학회지
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• 제28권11호
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• pp.663-670
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• 2018

H13 tool steels are widely used as metallic mold materials due to their high hardness and thermal stability. Recently, many studies are undertaken to satisfy the demands for manufacturing the complex shape of the mold using a 3D printing technique. It is reported that the mechanical properties of 3D printed materials are lower than those of commercial forged alloys owing to micropores. In this study, we investigate the effect of microstructures and defects on mechanical properties in the 3D printed H13 tool steels. H13 tool steel is fabricated using a selective laser melting(SLM) process with a scan speed of 200 mm/s and a layer thickness of $25{\mu}m$. Microstructures are observed and porosities are measured by optical and scanning electron microscopy in the X-, Y-, and Z-directions with various the build heights. Tiny keyhole type pores are observed with a porosity of 0.4 %, which shows the lowest porosity in the center region. The measured Vickers hardness is around 550 HV and the yield and tensile strength are 1400 and 1700 MPa, respectively. The tensile properties are predicted using two empirical equations through the measured values of the Vickers hardness. The prediction of tensile strength has high accuracy with the experimental data of the 3D printed H13 tool steel. The effects of porosities and unmelted powders on mechanical properties are also elucidated by the metallic fractography analysis to understand tensile and fracture behavior.

• Corrosion Science and Technology
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• 제11권5호
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• pp.165-172
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• 2012

The corrosion resistance of epoxy-coated carbon steel was evaluated. The carbon steel surface was subjected to different treatment methods such as steel grit blasting with different size, steel shot ball blasting and power tool treatment. To study the effect of the treatments, the topology of the treated surface was observed by optical 3D microscopy and a pull-off adhesion test was conducted. The corrosion resistance of the epoxy-coated carbon steel was further examined by electrochemical impedance spectroscopy (EIS) combined with hygrothermal cyclic testing. The results of EIS indicated that the epoxy-coated carbon steel treated with steel grit blasting showed an improved corrosion resistance compared to untreated epoxy-coated surfaces or surfaces subjected to shot ball blasting and power tool treatments.

• 한국표면공학회지
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• 제39권3호
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• pp.87-92
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• 2006

The characteristic of interface layer and the effect of bias voltage on the microstructure of c-BN films were studied in the microwave plasma hot filament C.V.D process. c-BN films were deposited on a high speed steel(SKH-51) substrate by hot filament CVD technique assisted with a microwave plasma to develop a high performance of resistance coating tool. c-BN films were obtained at a gas pressure of 20 Torr, vias voltage of 300 V and substrate temperature of $800^{\circ}C$ in $B_2H_6-NH_3-H_2$ gas system. It was found that a thin layer of hexagonal boron nitride(h-BN) phase exists at the interface between c-BN layer and substrate.

• 동력기계공학회지
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• 제11권3호
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• pp.47-52
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• 2007

Punching, blanking, trimming and slitting are widely used in shearing processes in sheet metal forming of automotive parts. In this paper the effects of clearance, cutting angle and tool sharpness on the formation of burr were investigated by experimental method in shearing processes of steel sheets, SPCEN and SPRC35E. The amount of burr and the shapes of burr were different between two kinds of steel sheets. It has been shown that the cutting angle of the shearing blade had no effects on the height of burr when the clearance was below the 10% of the steel sheet thickness, and also that the height and shape of burr were not affected by the cutting angle when the wear of shearing blade was below the 10% of the steel sheet thickness. It was known that there had been existing the critical clearance of 10 to 15% for the tested steel sheet, SPCEN and SPRC35E.

• International Journal of Precision Engineering and Manufacturing
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• 제8권2호
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• pp.38-43
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• 2007

This paper describes ultrasonically assisted grinding used to obtain a glossy surface quickly and precisely. High-quality surfaces are required for plastic injection molding dies used in the production of plastic parts such as dials for cellular phones. Traditionally, in order to finish the dies, manual polishing by a skilled worker has been required after the machining processes, such as electro discharge machining (EDM), which leaves an affected layer, and milling, which leaves tooling marks. However, manual polishing causes detrimental geometrical deviations of the die and consumes several days to finish a die surface. Therefore, a machining process for finishing dies without manual polishing to improve the surface roughness and form accuracy would be extremely valuable. In this study, a 3D positioning machine equipped with an ultrasonic spindle was used to conduct grinding experiments. An electroplated diamond tool was used for these experiments. Generally, diamond tools cannot grind steel because of excessive wear as a result of carbon atoms diffusing into bulk steel and chips. However, ultrasonically assisted grinding can achieve a fine surface (roughness Rz of $0.4{\mu}m$) on die steel without severe tool wear. The final aim of this study is to realize mirror surface grinding for injection molding dies without manual polishing. To do this, it is necessary to fabricate an electroplated diamond tool with high form accuracy and low run-out. This paper describes a tool-making method for high precision grinding and the grinding performance of a self-electroplated tool. The ground surface textures, tool performance and tool life were investigated A ground surface roughness Rz of 0.14 um was achieved Our results show that the spindle speed, feed rate and cross feed affected the surface texture. One tool could finish $5000mm^2$ of die steel surface without any deterioration of the ground surface roughness.

• 열처리공학회지
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• 제2권4호
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• pp.27-39
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• 1989

The effect of gas mixing ratios during gas nitrocarburizing treatment on the formation of compound layer and the mechanical properties has been studied for hot work tool steel by using a combined heat treating technique. The thickness of compound and diffusion layers has been shown to grow as a parabolic relation with increasing the amount of ammonia at a given flow quantity of $CO_2$ gas. The compound layer consists mainly of ${\varepsilon}-Fe_3$(C, N) with small amounts of ${\gamma}^{\prime}-Fe_4N$ and ${\alpha}$-Fe. The combined heat treated hot work tool steel has shown that the thickness of compound layer increases with increasing nitrocarburizing time, but the rate of growth slows down as gas nitrocarburizing time goes more than two hours. Tensile properties have given a remarkable improvement. In particular, the wear resistance of combined heat treated hot work tool steel has exhibited an improvement of about 165% greater than that obtained from conventional quenching and multi-tempering treatments.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2004년도 추계학술대회 논문집
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• pp.327-330
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• 2004

Laser aided direct metal deposition (LADMD) process offers the ability to make a metal component directly from 3-D CAD dimensions. A 3-D object can be formed by repeating laser cladding layer by layer. The key of the build-up mechanism is the effective control of powder delivery and laser power to be irradiated into the melt-pool. A feedback control system using optical sensors is introduced to control laser power and powder mass flow rate. Using H13 tool steel and $CO_2$ laser system, comprehensive analysis are executed to test the efficiency of the system. In addition, the dimensional characteristics of directed deposited material are investigated with the parameters of deposition thickness, laser power, traverse speed and powder mass flow rate.

• 대한금속재료학회지
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• 제48권6호
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• pp.533-542
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• 2010

The specific motivation for joining an Al alloy and Zn-coated steel arises from the need to save fuel consumption by weight reduction and to enhance the durability of vehicle structures in the automobile industry. In this study, the lap joining A6K31 Al alloy (top) and SGARC340 Zn-coated steel (bottom) sheets with a thickness of 1.0 mm and 0.8 mm, respectively, was carried out using the friction stir weld (FSW) technique. The probe of a tool did not contact the surface of the lower Zn-coated steel sheet. The friction stir welding was carried out at rotation speeds of 1500 rpm and travel speeds of 80~200 mm/min. The effects of tool geometry and welding speed on the mechanical properties and the structure of a joint were investigated. The tensile properties for the joints welded with a larger tool were better than those for the joints done with a smaller tool. A good correlation between the tensile load and area of the welded region were observed. The bond strength using a larger tool (M4 and M3) decreased with an increase in welding speed. Most fractures occurred along the interface between the Zn-coated steel and the Al alloy. However, in certain conditions with a lower welding speed, fractures occurred at the A6K31 Al alloy.

• 한국분말재료학회지
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• 제25권2호
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• pp.137-143
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• 2018

In this study, two types of SKD61 tool-steel samples are built by a selective laser melting (SLM) process using the different laser scan speeds. The characteristics of two kinds of SKD61 tool-steel powders used in the SLM process are evaluated. Commercial SKD61 tool-steel power has a flowability of 16.68 sec/50 g and its Hausner ratio is calculated to be 1.25 by apparent and tapped density. Also, the fabricated SKD61 tool steel powder fabricated by a gas atomization process has a flowability of 21.3 sec/50 g and its Hausner ratio is calculated to be 1.18. Therefore, we confirmed that the two powders used in this study have excellent flowability. Samples are fabricated to measure mechanical properties. The highest densities of the SKD61 tool-steel samples, fabricated under the same conditions, are $7.734g/cm^3$ (using commercial SKD61 powder) and $7.652g/cm^3$ (using fabricated SKD61 powder), measured with Archimedes method. Hardness is measured by Rockwell hardness testing equipment 5 times and the highest hardnesses of the samples are 54.56 HRC (commercial powder) and 52.62 HRC (fabricated powder). Also, the measured tensile strengths are approximately 1,721 MPa (commercial SKD61 powder) and 1,552 MPa (fabricated SKD61 powder), respectively.

• 한국재료학회지
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• 제32권3호
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• pp.139-145
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• 2022

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.