Influence of repetitive pulsed laser irradiation on the surface characteristics of an aluminum alloy in the melting regime

최성호,장경영 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.1

We have investigated the influence of repetitive near-infrared (NIR) pulsed laser shots in the melting regime on the surface characteristicsof an aluminum 6061-T6 alloy. Characteristics of interest include surface morphology, surface roughness, and surface hardness in themelted zone as well as the size of the melted zone. For this study, the proper pulse energy for inducing surface melting at one shot isselected using numerical simulations that calculate the variation in temperature at the laser beam spot for various input pulse energies inorder to find the proper pulse energy for raising the temperature to the melting point. In this study, 130 mJ was selected as the input energyfor a Nd:YAG laser pulse with a duration of 5 ns. The size of the melted zone measured using optical microscopy (OM) increasedlogarithmically with an increasing shot number. The surface morphology observed by scanning electron microscopy (SEM) clearlyshowed a re-solidified microstructure evolution after surface melting. The surface roughness and hardness were measured by atomicforce microscopy (AFM) and nano-indentation, respectively. The surface roughness showed almost no variation due to the surface texturingafter laser shots over 10. The hardness inside the melted zone was lower than that outside the zone because the β'' phase was transformedto a β phase or dissolved into a matrix.

Study on surface shape control of pure Ti fabricated by electron beam melting using electrolytic polishing

Jung, Jae-Hyun,Park, Hyung-Ki,Lee, Byoung Soo,Choi, Jaeho,Seo, Bosung,Kim, Hyo Kyu,Kim, Gun Hee,Kim, Hyung Giun Elsevier 2017 Surface & Coatings Technology Vol.324 No.-

<P><B>Abstract</B></P> <P>Titanium parts in special industries such as vacuum, catalyst and medical application require a variety of surface properties with a unique structure for high efficiency. In this study, the unique surface of commercially pure titanium (CP Ti) parts fabricated using Electron Beam Melting (EBM) in the process of additive manufacturing is post-treated by the electrolytic polishing process under conditions ranging from 10 to 30V and 10–600s in order to study the surface characteristics. The possibility of controlling microstructural surface shape according to electrolytic polishing process conditions is presented from the increase of surface area through the formation of micro-dimples on protruding residual powder on the surface to the flattening of entire surface.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Microstructural study on the surface of additive manufactured pure Ti part after electrolytic polishing. </LI> <LI> It is possible to maximize surface area by forming micro dimpling at the initial stage of electrolytic polishing. </LI> <LI> Identification of surface flattening mechanism of additive manufactured Ti part using electron beam melting. </LI> </UL> </P>

Application of ultrasonic nanocrystal surface modification for improving surface profile of DEDed AISI 316L

김민섭,조영관,박상후,심도식 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.12

This study investigated the effect of ultrasonic nanocrystal surface modification (UNSM) on the deteriorated surface of AISI 316L deposited using the direct energy deposition (DED) technology. After UNSM treatment, the coarse DEDed surface was fined, and a regular micro-surface profile was implemented. Compared to the case before UNSM treatment, the waviness and roughness of the surface after UNSM treatment decreased by up to 73.8 % and 86.2 %, respectively, and reduced further as the UNSM interval was decreased. Surface severe plastic deformation (S 2 PD) was induced in the dendrite structure of the UNSM-treated DEDed sample surface. The microstructure was deformed till a depth of up to 92.13 mm from the surface and was significantly affected by the interval. After UNSM treatment, the hardness improved by up to 71.5 % and gradually decreased from the surface to the inside; the hardness was improved by UNSM up to a maximum depth of 400 μm. Although the UNSM interval condition had a significant effect on the DEDed surface, it barely affected the relationship between the directions of DED deposition and UNSM treatment. This study confirmed that the UNSM technology can effectively improve a DEDed surface.

Novel functional treatment of polymer surface by surface dissolution and directional melt crystallization

안수영,김현진,이종휘 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.0

In the domain of porous materials preparation technology, directional melt crystallization(DMC) is novel promising method because it is simple step and applicable to various polymer. In our study, for these advantages, we developed this method as a surface treatment technology. We treated various polymer surfaces such as polystyrene, polyvinyl alcohol and polyurethane by DMC. We dissolved polymer surfaces by proper solvents and then crystallized the solvents by a directional temperature gradient on the liquid nitrogen. In crystallization step, freezing rate of polymer surface was an important parameter, since polymer dissolution time and dissolved structure were changed by freezing rate. So we controlled freezing rate by pre-cooling polymer surface method. The key achievement of our study is that we successfully prepared treated polymer surfaces of porous and rough surfaces. In addition, we pretty confirmed changing modified surface morphology as for changing freezing rate and polymers. We also confirmed that the hydrophobicity of treated surfaces increased. With this versatile method, various polymer surfaces can be treated conveniently without complex and delicate processing steps.

Molecular Dynamics Study on Atomistic Details of the Melting of Solid Argon

Han, Joo-Hwan The Korean Ceramic Society 2007 한국세라믹학회지 Vol.44 No.8

The atomic scale details of the melting of solid argon were monitored with the aid of molecular dynamics simulations. The potential energy distribution is substantially disturbed by an increase in the interatomic distance and the random of set distance from the lattice points, with increasing temperature. The potential energy barriers between the lattice points decrease in magnitude with the temperature. Eventually, at the melting point, these barriers can be overcome by atoms that are excited with the entropy gain acquired when the atoms obtain rotational freedom in their atomic motion, and the rotational freedom leads to the collapse of the crystal structure. Furthermore, it was found that the surface of crystals plays an important role in the melting process: the surface eliminates the barrier for the nucleation of the liquid phase and facilitates the melting process. Moreover, the atomic structure of the surface varies with increasing temperature, first via surface roughening and then, before the bulk melts, via surface melting.

Surface characteristics of aluminum 6061-T6 subjected to Nd:YAG pulsed-laser irradiation

최성호,김정석,장경영,Wan-Soon Shin 대한기계학회 2012 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.26 No.7

The objective of this study was to investigate the surface characteristics of an aluminum 6061-T6 alloy subjected to Nd:YAG pulsedlaser irradiation. The test specimens were prepared by a mechanical polishing process using diamond paste and emery polishing paper to obtain different levels of initial surface roughness. After ten pulsed-laser shots, the surface morphology was observed via optical microscopy (OM) and scanning electron microscopy (SEM). Nano-indentation hardness testing was also conducted on the irradiated surface. The diameter of the melted zone increased with surface roughness because of the multiple reflection and absorption of the laser beam. The relative absorbance was measured as a function of the diameter of the melted zone with varied surface roughness.

Microstructural, hardening, and wear characteristics of surface re-melted AISI 410S stainless steel via fiber laser process

Behdad Nayebi,Hamidreza Najafi,Amirreza Farnia 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.10

AISI 410S ferritic stainless steel was surface melted via a continuous fiber laser at different scan rates. The microstructural characteristics, thickness, hardness, and tribological behavior of the laser melted (LSMed) surface were studied after different scan rates. The hardened layer thickness was inversely related to the scan rate. The thickness reduced from 146 μm at the scan rate of 15 mm.s -1 to 110 μm at the scan rate of 24 mm.s -1 . The microstructural studies revealed that the microstructure of the hardened layers mostly consisted of martensite. At the lowest scan rate, the martensite was in two forms of fine and coarse packets. The surface hardness enhanced with decreasing the scan rate. The maximum average hardness (330 HV) was obtained from the scan speed of 15 mm.s -1 . However, the most surface hardness scattering was observed in the sample treated by the lowest scan speed. The sample treated using the moderate scan rate (20 mm.s -1 ) demonstrated superior wear resistance due to its lowest coefficient of friction (COF) and weight loss. The reason for this behavior could be related to the appropriate melted thickness. Microstructural studies revealed that abrasive wear and adhesive wear were the dominant mechanisms in the worn surfaces.

SKD61 열간합금강의 레이저 표면 합금화 경화처리 공정에서 SKH51 분말 송급속도에 따른 균열 형성에 대한 고찰

최성원 ( Seong-won Choi ),이광현 ( Kwang-hyeon Lee ),서정 ( Jeong Suh ),오명환 ( Myeong-hwan Oh ),강정윤 ( Chung-yun Kang ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.3

A laser surface-melting alloying process using a laser beam is a new surface-hardening process of obtaining an alloying layer that melts the surfaces of substrates and alloying powder at the same time. This study used SKD61 hot die steel as a substrate and SKH51 powder as an alloying powder. The laser beam speed and the laser power were fixed at 70 mm/sec and 2 kW. The power feeding rate was changed from 0 rpm to 6 rpm (step: 1 rpm). The alloying layer showed high hardness (710~830 Hv), but cracks occur at a high powder feeding rate. Cracks occur at more than 5 rpm, and the lengths of cracks become longer as the powder feeding rate increases. Moreover, cracks were observed at the dendrite boundary, and dendrite protrusions were observed on the fracture surfaces. As the powder feeding rate increases, the concentration of the Mo, V, and W in alloying layer increase. The liquidus and solidus temperatures decreased by as much as 6 ℃ and 26 ℃. As a result of calculating the aspect ratio (penetration depth/width) of the molten zone, it was found that there is no difference as the powder feeding rate increases. Therefore, strain by solidification contraction was constant with an increase in the powder feeding rate, but cracks occur, and the number of cracks increases because the solidus temperature decreases and the ductility of alloying layer reduces.

Polymer surface treatment by directional melt crystallization method

안수영,김현진,이종휘 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.1

Directional Melt Crystallization (DMC) is a novel promising method in the domains of porous materials preparation. Since this method is simple and applicable to various polymers, we developed this method as a surface treatment technology. We treated various polymer surfaces such as polystyrene and polyurethane by DMC. We dissolved polymer surfaces by proper solvents and then crystallized the solvents by a directional temperature gradient. The key achievement of our study is that we successfully prepared treated polymer surfaces of porous and rough surfaces. We also confirmed that the hydrophobicity of treated surfaces increased. With this versatile method, various polymer surfaces can be treated conveniently without complex and delicate processing steps.

YAG LASER에 의한 공구강의 표면개질에 관한 연구

옥철호,강형식,박흥식,전태옥 韓國工作機械學會 2000 한국생산제조학회지 Vol.9 No.2

Laser induced surface hardening of Tool steel(STC5) can be achieved either with or without surface melting. In trans-formation hardening, as the surface is heated to a temperature below its melting point and is rapidly cooled, solidified microstructures are usually much finer and stronger than those of the base matals. For this reason, surface modification of tool steel by YAG laser irradiation has been studied as a fuction of processing parameters such as, power density, pulse width, defocusing distance, and molten depth. The high energy density changes and refines the microstructure of the near surface layer. In the case of beam passes, martensite formed in the melt zone exhibited very high vickers hardness values. Molten depth and width depend on defocusing distance, and energy of black color painting is more absorptive than other color painting.