Continuous high strength aluminum bolt manufacturing by the spring-loaded ECAP system

Jin, Young Gwan,Baek, Hyun Moo,Hwang, Sun Kwang,Im, Yong-Taek,Jeon, Byung Cheol Elsevier 2012 Journal of materials processing technology Vol.212 No.4

<P><B>Highlights</B></P><P>► A ultrafine-grained aluminum bolt was made by a continuous spring-loaded ECAP system. ► Flange cracking and underfilling were observed in the bolt based on first pass ECAP. ► They were removed by introducing surface coating and a modified route C of 2nd pass. ► The UTS and elongation of the UFG bolt increased compared to the conventional one.</P> <P><B>Abstract</B></P><P>Aluminum alloy 6061-T6 was used to produce an ultrafine-grained specimen by spring-loaded equal channel angular pressing (ECAP), which was continuously applied to a commercial multi-stage former at a production rate of 30rpm at room temperature to manufacture high strength aluminum bolts. In order to increase the mechanical strength of the manufactured bolt, the bolt-forming process was introduced based on extrusion process in the present work. With the specimens processed by a single pass ECAP, underfilling at the bolt head and surface cracking at the flange of the produced bolts were observed due to the geometry of the ECAPed specimen and friction at the die interface, respectively. To remove these defects, modified route C for the second pass ECAP and aluminum fluoride coating with metal soap for the initial specimen were applied. Finally, the ultrafine-grained aluminum bolts were successfully manufactured in a continuous sequence. Both the ultimate tensile strength (UTS) and elongation to fracture of the manufactured bolts increased compared to the conventionally made bolts. According to the present investigation, an innovative approach utilizing the ECAP to develop high strength bolts without modifying conventional material is demonstrated.</P>

HPT 가공기술을 이용하여 초미세립 금속재료를 개발하는 연구기관별 경쟁력 분석에 관한 연구

김환태(H. T. Kim),남수우(S.W. Nam),길상철(S.C. Kil) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.10

The increasing interest in the severe plastic deformation to conventional coarse-grained metals is placing stringent demands on the manufacturing techniques and performance requirements, and the engineer employs the high quality and efficiency processing technology for producing ultrafine grained materials. This paper covers recent research trends of the high pressure tortion technology of the metals and alloys including the SCI-E analysis of published papers, research subjects and research institute analysis.

ECAP 가공 초미세립 조직 금속재료개발의 국가 경쟁력 분석

길상철(S.C. Kil),남수우(S.W. Nam),김환태(H. T. Kim) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.10

The increasing interest in the severe plastic deformation to conventional coarse-grained metals is placing stringent demands on the manufacturing techniques and performance requirements, and the engineer employs the high quality and efficiency processing technology for producing ultrafine grained materials. This paper covers recent research trends for of the equal channel angular pressing technology of the metals and alloys including the SCI-E analysis of published papers, research subjects and research institute analysis.

Structural characterization of ultrafine-grained interstitial-free steel prepared by severe plastic deformation

Č,í,ž,ek, J.,Janeč,ek, M.,Krajň,á,k, T.,Strá,ská,, J.,Hruš,ka, P.,Gubicza, J.,Kim, H.S. Elsevier 2016 Acta materialia Vol.105 No.-

<P>Interstitial free steel with ultrafine-grained (UFG) structure was prepared by high-pressure torsion (HPT). The development of the microstructure as a function of the number of HPT turns was studied at the centre, half-radius and periphery of the HPT-processed disks by X-ray line profile analysis (XLPA), positron annihilation spectroscopy (PAS) and electron microscopy. The dislocation densities and the dislocation cell sizes determined by XLPA were found to be in good agreement with those obtained by PAS. The evolution of the dislocation density, the dislocation cell and grain sizes, the vacancy cluster size, as well as the high-angle grain boundary (HAGB) fraction was determined as a function of the equivalent strain. It was found that first the dislocation density saturated, then the dislocation cell size reached its minimum value and finally the grain size got saturated. For very high strains after the saturation of grain size the HAGB fraction further increased. The PAS investigations revealed that vacancies introduced by severe plastic deformation agglomerated into small clusters consisting of 9-14 vacancies. The evolution of the yield strength calculated from the microhardness as a function of strain was explained by the development of the defect structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>

The mechanism of hydrogen embrittlement in intercritically annealed medium Mn TRIP steel

Han, J.,Nam, J.H.,Lee, Y.K. Elsevier Science 2016 Acta materialia Vol.113 No.-

<P>The objective of this study was to investigate the mechanisms of hydrogen embrittlement (HE) in intercritically annealed medium Mn steel. For this purpose, both hot-rolled and cold-rolled Fe-7Mn-0.1C-0.5Si (wt.%) steels were annealed at 640 degrees C for 30 min. The annealed specimens had a dual-phase microstructure of retained austenite (gamma R) and ferrite (alpha) with different morphologies; a lath shape for the hot-rolled and annealed (HRA) specimen and a globular shape for the cold-rolled and annealed (CRA) specimen. Although the difference in microstructural morphology did not influence the H permeation, it significantly affected the HE behavior. The H-charged HRA (HRA(H)) specimen was fractured by inter granular cracking occurring along the boundaries of prior gamma grains by the H-enhanced decohesion (HEDE) mechanism. The intergranular cracking leaved both flat and rugged facets, which appeared at the prior gamma grain boundaries without and with gamma R, respectively. The H-charged CRA (CRA(H)) specimen was fractured to leave both dimples filled with grains and empty dimples at the fractured surface. The dimples filled with grains were generated by intergranular cracking occurring along the boundaries of gamma R grains by the HEDE mechanism. The empty dimples were made by intragranular cracking occurring inside the alpha grains by the H-enhanced local plasticity (HELP) mechanism. The CRA(H) specimen exhibited a smaller elongation loss than the HRA(H) specimen because cracks were propagated by frequently changing their direction along the boundaries of nano-sized gamma R grains or into alpha grains. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>

The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel

Han, Jeongho,da Silva, Alisson Kwiatkowski,Ponge, Dirk,Raabe, Dierk,Lee, Sang-Min,Lee, Young-Kook,Lee, Sang-In,Hwang, Byoungchul Elsevier 2017 Acta materialia Vol.122 No.-

<P><B>Abstract</B></P> <P>The effects of prior austenite (γ) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (γ<SUB>R</SUB>) and ferrite (α) after intercritical annealing at 640 °C for 30 min. The phase fractions and the chemical composition of γ<SUB>R</SUB> were almost identical in both types of specimens. However, their microstructural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to −196 °C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior γ grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine α and α′ martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior γ grains through the recrystallization of αʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Fatigue Crack Growth Behavior in Ultrafine Grained Low Carbon Steel

Kim, Ho-Kyung,Park, Myung-Il,Chung, Chin-Sung,Shin, Dong-Hyuk The Korean Society of Mechanical Engineers 2002 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.16 No.10

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angula. pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of asreceived coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower △Kth with an increase of R ratio. The R ratio effect on crack growth rates and △Kth was basically indistinguishable at lower load ratio (R >0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.

Wear properties of high pressure torsion processed ultrafine grained Al-7%Si alloy

Aal, M.I.A.E.,Kim, H.S. Scientific and Technical Press ; Elsevier Science 2014 Materials & design Vol.53 No.-

In this paper, Al-7wt% Si alloy was processed via high pressure torsion (HPT) at an applied pressure 8GPa for 10 revolutions at room temperature. The microstructure and hardness of the HPT samples were investigated and compared with those of the as-cast samples. The wear properties of as-cast and the HPT samples under dry sliding conditions using different sliding distances and loads were investigated by reciprocated sliding wear tests. The HPT process successfully resulted in nanostructure Al-7wt% Si samples with a higher microhardness due to the finer Al matrix grains and Si particles sizes with more homogeneous distribution of the Si particles than those in the as-cast samples. The wear mass loss and coefficient of friction values were decreased after the HPT process. The wear mechanism was observed to be adhesive, delamination, plastic deformation bands and oxidization in the case of the as-cast alloy. Then, the wear mechanism was transformed into a combination of abrasive and adhesive wear after the HPT process. The oxidization cannot be considered as a mechanism that contributes to wear in the case of HPT samples, because O<SUB>2</SUB> was not detected in all conditions.

Nanocrystalline and Ultrafine Grained Materials by Mechanical Alloying

Wang Erde,Hu Lianxi 한국분말야금학회 2006 한국분말야금학회 학술대회논문집 Vol.2006 No.1

Recent research at Harbin Institute of Technology on the synthesis of nanocrystalline and untrafine grained materials by mechanical alloying/milling is reviewed. Examples of the materials include aluminum alloy, copper alloy, magnesium-based hydrogen storage material, and Nd2Fe14B/α-Fe magnetic nanocomposite. Details of the processes of mechanical alloying and consolidation of the mechanically alloyed nanocrystalline powder materials are presented. The microstructure characteristics and properties of the synthesized materials are addressed.

The effects of the initial martensite microstructure on the microstructure and tensile properties of intercritically annealed Fe-9Mn-0.05C steel

Han, J.,Lee, S.J.,Jung, J.G.,Lee, Y.K. Elsevier Science 2014 Acta materialia Vol.78 No.-

The effects of the initial microstructure of α' martensite on the microstructural evolution during intercritical annealing and the tensile properties of annealed specimens were investigated for Fe-9Mn-0.05C (wt.%) steel. The hot-rolled specimen with fully α' martensitic microstructure showed a mixed microstructure of lath-shaped ferrite (α<SUB>L</SUB>) and austenite (γ<SUB>L</SUB>) after intercritical annealing. The α<SUB>L</SUB> grains had a high density of dislocations due to inactive recovery, and also had a low Mn concentration. The γ<SUB>L</SUB> grains had a low density of dislocations and high Mn and C concentrations. The α<SUB>L</SUB> and γ<SUB>L</SUB> grains were deformed simultaneously during the tensile test because the α<SUB>L</SUB> grains were as hard as the γ<SUB>L</SUB> grains due to their high dislocation density, resulting in continuous yielding. The cold-rolled specimen with a deformed α' martensite microstructure exhibited a mixed microstructure of globular-shaped ferrite (α<SUB>G</SUB>) and austenite (γ<SUB>G</SUB>) after intercritical annealing. The α<SUB>G</SUB> grains had a low dislocation density due to active recovery, and also had a low Mn concentration. The γ<SUB>G</SUB> grains had a low dislocation density and high Mn and C concentrations. The soft α<SUB>G</SUB> grains with a low dislocation density were easily deformed at the early stage of the tensile test, resulting in discontinuous yielding and a large yield point elongation.