Nanostructured β-type titanium alloy fabricated by ultrasonic nanocrystal surface modification

Kheradmandfard, Mehdi,Kashani-Bozorg, Seyed Farshid,Kim, Chang-Lae,Hanzaki, Abbas Zarei,Pyoun, Young-Shik,Kim, Jung-Hyong,Amanov, Auezhan,Kim, Dae-Eun Elsevier 2017 Ultrasonics sonochemistry Vol.39 No.-

<P><B>Abstract</B></P> <P>The surface of β-type Ti-Nb-Ta-Zr (TNTZ) alloy, which is a promising material for biomedical applications, was treated with the ultrasonic nanocrystal surface modification (UNSM) technique to enhance its hardness. As a result, a gradient nanostructured (GNS) layer was generated in the surface; the microstructure of the top surface layer consisted of nanoscale lamellae with a width of about 60–200nm. In addition, there were lamellar grains consisting of nanostructured subgrains having unclear and wavy boundaries. The treated surface exhibited a hardness value of ∼385HV compared to 190HV for the untreated alloy. It was further determined that highly dense deformation twins were generated at a depth of ∼40–150µm below the UNSM-treated surface. These deformation twins led to a significant work hardening effect which aided in enhancing the mechanical properties. It was also found that UNSM treatment resulted in the formation of micropatterns on the surface, which would be beneficial for high bioactivity and bone regeneration performance of TNTZ implants.</P> <P><B>Highlights</B></P> <P> <UL> <LI> UNSM is a method that utilizes ultrasonic vibration to generate nanostructured surface. </LI> <LI> The surface of β-type Ti-Nb-Ta-Zr (TNTZ) alloy was treated with the UNSM technique. </LI> <LI> A gradient nanostructured layer was generated in the surface. </LI> <LI> Top surface layer consisted of nanoscale lamellae with a width of about 60–200nm. </LI> <LI> Hardness of treated surface was ∼385HV compared to 190HV for the untreated alloy. </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.

UNSM 처리한 베어링 강재의 VHCF 피로 및 파단특성

남승훈(Seung-Hoon Nahm),서민수(Min-Soo Suh),서창민(Chang-Min Suh),편영식(Young-Sik Pyun) 대한기계학회 2018 大韓機械學會論文集A Vol.42 No.6

사용된 베어링강의 UNSM처리효과와 기가사이클 설계기준에 필요한 VHCF 회전굽힘피로특성을 구하기 위한 피로시험을 거쳐 미시적인 파면해석과 EDS 분석을 통하여 표면과 내부에서 파괴되는 기구를 파괴역학적으로 연구하였다. 베어링강은 UNSM처리재와 미처리재중 경면상태의 경우는 duplex S-N 곡선이 얻어졌고, 가공상태의 경우는 표면조도가 크므로 이중 S-N곡선이 형성되지 않았다. 또 미처리재에 비해서 UNSM처리재는 표면의 나노(nano)조직화, 표면경도 상승, 표면조도 감소 및 압축잔류응력형성에 의하여 10⁶ 사이클 피로한도에서 약 28 % 정도 상승하는 경향을 나타내었다. 또 HCF(장수명피로) 범위에서는 표면을 기점으로 하는 표면결함에서 발생한 표면균열에 의해 파괴되는 형태를 취하였고, VHCF(초장수명피로, Nf >10⁶ 사이클) 범위에서는 내부에서 피로균열이 형성되는 어안균열이 주요 파괴기구로 되었다. 이 어안균열은 개재물의 흔적이 명확한 TiN개재물에서 주로 발생하였다. 개재물의 △K는 5 ㎫√m에 접근하며, 또 어안균열의 △K는 10 ㎫√m에 접근하였다. This study was to investigate the influence of ultrasonic nanocrystal surface modification (UNSM) effects on very high cycle fatigue(VHCF) strength and fracture mechanism of a bearing steel. In case of mirror finished specimens with and without UNSM treatment after machined surface, the duplex S-N curves were obtained, but in case of unpolished machined surface condition, the duplex S-N curves were not formed. The UNSM treated specimen showed a tendency to increase about 28% in the fatigue limit of 10⁶ cycles due to the refined structure to nanocrystal, the increased surface hardness, the decreased surface roughness and the increased compression residual stress. In the range of HCF, the fracture was caused by the surface crack generated by the surface defects originating from the surface. In the VHCF range, the dominant fracture mechanism was the fish-eye crack in which the internal originating fracture type was formed. The central part of the fish-eye where cracks initiated was inclusion consist of TiN. The stress intensity factor range (ΔK) of inclusions and fish-eye cracks approached 5 ㎫√m and 10 ㎫√m respectively.

Numerical and Experimental Studies on Subscale Behaviors of Ultrasonic Surface Peening

Hyeonuk Park,Junhyung Kim,Youngsik Pyun,Amanov Auezhan,최윤석 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.3

Displacement-controlled and force-controlled surface-striking conditions were applied for finite element method simulationsof the single path ultrasonic nanocrystal surface modification (UNSM) surface peening process. The single path UNSMtreatment was done on a 4 mm-thick 6061 T6 Al sheet, and the surface topology and through-thickness residual stresseswere measured and compared to the simulation results. Both displacement- and force-controlled simulations predicted maximumcompressive residual stresses near the surface and subsurface for the stresses along the strike progression directionand transverse direction, respectively. The experimentally measured through-thickness residual stress profile also showedthe maximum compressive residual stress near the subsurface. Both simulation conditions showed a topological pile-up onthe surface in front of the leading strike. However, only the force-controlled simulation showed the pile-up breakdown at acritical number of strikes followed by the redistribution of subsurface strain fields, and lead to topological undulations alongthe edge and valley of the UNSM path, which were confirmed to be consistent with the surface topology measured fromthe experiment. It was concluded that the actual UNSM strike is close to the force-controlled surface-striking condition.

Control of Local Hardness Gradient of Metal Surface by Inclined Surface Treatment Using Ultrasonic Nanocrystal Surface Modification

Yeong-Kwan Jo,Yeong-Wook Gil,Do Sik Shim,Young-Sik Pyun,박상후 한국정밀공학회 2021 International Journal of Precision Engineering and Vol.8 No.2

We propose an effective method to control the local hardness and morphology of a metal surface by tilting the incident angle of a horn during ultrasonic nanocrystal surface modification (UNSM). In this study, surface treatment using UNSM was performed on an S45C specimen and a parameter study was conducted for optimization. The process parameters were the feeding rate, static load, striking force, and processing angle ( Ф ). In particular, the Ф was analyzed by tilting the horn by 0°, 10°, 20°, 30°, 40°, and 45° to understand its effect on surface hardness and changes in the morphology. From fundamental experiments, some important phenomena were observed, such as grain-microstructure changes along the processing and thickness directions. Furthermore, to verify the practical usefulness of this study, a flat and a hemispherical specimen of S45C material were treated using UNSM with various values of Ф . A significant change in hardness (an increase from 2–45%) and a gradual hardness gradient on the tested specimens could be easily realized by the proposed method. Therefore, we believe that the method is effective for controlling the mechanical hardness of a metal surface.

초음파 미세 표면 냉간단조법(UNSM)을 적용한 AZ31 마그네슘 판재의 단면 미세조직 및 강도 향상 효과 분석

강명성,강주희,박현욱,김준형,편영식,백민재,이동준,이승진,박상후,김홍물,박성혁,최윤석 대한금속·재료학회 2022 대한금속·재료학회지 Vol.60 No.3

An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a 1-mm thick AZ31 magnesium sheet. UNSM is a relatively new surface modification technique in which a hard, hemispherical tip (2.38 mm in diameter) strikes the surface at an ultrasonic frequency to induce plastically deformed gradient microstructures and deep compressive residual stresses through the thickness. After the UNSM treatment, the through-thickness microstructures were thoroughly investigated using electron microscopy and electron backscatter diffraction analysis. The through-thickness microstructures revealed zones that were severely deformed (down to 200 µm from the surface) and twin-dominated (200~300 µm deep from the surface). The severely deformed zone consisted of shear banding, grain subdivision and reorientation, due to the strong plastic deformation, accompanied by the formation of { } tensile twins (despite compressive strikes by the hemispherical tip), { }-{ } double twins and { } compression twins. The cause for tensile twinning was examined through a literature survey. In the twin-dominated zone, the twining activity prevailed as the slip activity gradually decayed through the thickness. The UNSM-induced hardness and microstructure enhancement was found to be effective down to about 300~400 μm deep from the surface. Finally, the source of the increase in yield strength after the UNSM treatment of the AZ31 sheet was analyzed, and focused on individual cases of microstructural enhancement in the severely deformed zone and the twin zone, and the compressive residual stress.

Simultaneous grain refinement and nanoscale spinodal decomposition of β phase in Ti-Nb-Ta-Zr alloy induced by ultrasonic mechanical impacts

Kheradmandfard, Mehdi,Kashani-Bozorg, Seyed Farshid,Kang, Kyeong-Hee,Penkov, Oleksiy V.,Zarei Hanzaki, Abbas,Pyoun, Young-Shik,Amanov, Auezhan,Kim, Dae-Eun Elsevier 2018 JOURNAL OF ALLOYS AND COMPOUNDS Vol.738 No.-

<P><B>Abstract</B></P> <P>It was found that ultrasonic nanocrystal surface modification (UNSM) treatment performed on Ti-Nb-Ta-Zr (TNTZ) alloy surface to produce a gradient nanostructured surface layer also resulted in the nano-scale spinodal decomposition of β phase. For the first time, nano-scale spinodal decomposition of β phase induced by ultrasonic mechanical impacts was observed in a short time without any age treatment. The peak shift of XRD to lower angles, (110) β XRD peak splitting, overlapped and asymmetric XRD peaks, satellite reflections in the SAED pattern, and lattice straining in HR-TEM images confirmed nano-scale spinodal decomposition of β phase in the TNTZ alloy induced by UNSM treatment. Nano-scale EDS line-scan probe analysis revealed the β phase separation into nano-scale domains of Ti-rich (β<SUB>1</SUB>) and Ti-depleted (β<SUB>2</SUB>) phases. HR-TEM images showed the semi-coherent arrangement of Ti-rich (β<SUB>1</SUB>) and Ti-depleted (β<SUB>2</SUB>) regions. Nanosized grain formation and spinodal decomposition of β phase were induced simultaneously by UNSM treatment. The nano-scale grains, high dislocation density, and semi-coherent interface between the spinodally separated phases contributed to the high hardness of the UNSM-treated TNTZ alloy specimen.</P> <P><B>Highlights</B></P> <P> <UL> <LI> UNSM is a method that applies ultrasonic impacts to generate nanostructured surface. </LI> <LI> UNSM was applied to the surface of TNTZ alloy. </LI> <LI> Gradient nanostructured layer was produced in the top surface. </LI> <LI> Nano-scale spinodal decomposition of β phase induced by ultrasonic impacts. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

전조 가공을 통해 제작된 볼트에 대한 초음파나노표면개질 효과에 관한 연구

하형진,박한별,정태형,심도식 한국정밀공학회 2023 한국정밀공학회지 Vol.40 No.8

This study aimed to determine effects of ultrasonic nanocrystal surface modification (UNSM) as a surface pre-process on performance and surface characteristics of bolts manufactured through a screw rolling process. Surface roughness,hardness, and microstructural changes after UNSM treatment were examined. Results showed no significant defects such as cracks in all fabricated samples after screw rolling of bolt pre-processed by UNSM treatment. In addition, material flow was continuously maintained without disconnection. After UNSM treatment, surface roughness was improved for both body and screw parts. The surface roughness of the UNSM treated screw part was improved the most at 43%. Hardness test showed the greatest increase in hardness on the surface hit by the UNSM ball tip, with hardness improved to about 500 μm deep from the surface. The hardness at the screw part was the highest at 471 HV, which was attributed to the fact that grains near the surface were deformed and refined by UNSM treatment followed by screw rolling. Near the surface of the screw, refined grains and high dislocation density were clearly observed by EBSD mapping. These results confirm that UNSM treatment before screw rolling is effective in improving mechanical properties of screw rolled bolts.

적층제조된 스테인레스 316L에 대한 초음파나노표면개질에 따른 표면특성 및 부식성에 관한 연구

김준호(Jun-Ho Kim),오영택(Yeong-Taek Oh),박한별(Han-Byeol Park),김화정(Hwa-Jeong Kim),김의준(Ui-Jun Kim),심도식(Do-Sik Shim) 한국기계가공학회 2020 한국기계가공학회지 Vol.19 No.8

This study investigated the effects of ultrasonic nanocrystal surface modification (UNSM) on the deteriorated surface of AISI SUS316L additively manufactured (AM) using the powder bed fusion (PBF) technique. Specifically, the effects of UNSM conditions on surface topology, hardness, and anti-corrosion were examined. Before UNSM treatment, the stainless steel 316L powder was processed via the PBF machine to prepare a substrate. We observed surface changes due to UNSM treatments in PBF SUS316L substrates and examined the correlation between topology changes, roughness, hardness, and anti-corrosion. After UNSM treatment, the coarse as-built surface was refined, and a regular micro-profile was implemented. Compared to the non-treated PBF sample, the waviness and roughness of the surfaces after UNSM treatment decreased by up to 56.0% and 94.5%, respectively, and decreased further as the interval decreased. The hardness improved by up to 63.0% at a maximum depth of 500 μm from top surface by the UNSM treatment. The results of the corrosion test showed that the corrosion resistance of the UNSM specimens was moderately improved compared to that of the untreated surface. This study confirmed that UNSM is an effective post-processing technique for additively manufactured parts.

레일의 내마모성에 대한 타당성 연구

장세기(Seky Chang),편영식(Young Sik Pyu) 한국생산제조학회 2014 한국생산제조학회지 Vol.23 No.6

Rails are continuously exposed to severe environments due to increases in train service frequency, passing tonnage, and speed. Wear is one of the most common types of damage observed on rails. The surface of railsis heat treated to secure safety at critical sites, such as curved tracks or turnouts. Ultrasonic nanocrystal surface modification (UNSM), a new approach for rails, was examined to strengthen the rail surface. The effect of UNSM on the wear resistance of rail specimens was compared with that of heat treatment. Wear resistance was improved after UNSM treatment due to increased hardness and surface compressive residual stress.