주철의 피로전파거동에 미치는 미세조직의 영향

김석원 ( Sug Won Kim ),박진성 ( Jin Sung Park ),이형철 ( Hyung Chul Lee ),김동건 ( Dong Keun Kim ),우기도 ( Kee Do Woo ) 한국주조공학회 2005 한국주조공학회지 Vol.25 No.1

N/A This study aims to investigate the effects of the microstructures and nodule type on the fatigue characteristics of cast iron. Fatigue tests were carried out in tension-tension mode using a servo-hydraulic testing machine with load control mode operating at a frequency of 15 Hz. The tests were conducted at stress ratio R=Kmin/Kmax, of 0.1. Initial crack ΔK values were highly per-formed with increase in tensile strength of DCI fatigue specimens. ΔK_(th) region, fatigue crack propagation was primarily advanced through cell boundary and in periphery of near nodule. Fatigue crack propagation rate of D2 consisted with 2Phase(Ferrite + Pearlite) was slow due to crack closure; enhanced by crack deflection and occurred crack branching. The generation of crack branch was occurred due to interaction of crack-nodule. At Threshold and Paris zone, the fractographs of the fatigue fracture surface for DCI show typical striations of a ductile fracture and isolated cleavage planes near graphite. The effect of microstructure on fatigue crack propagation of GC strongly depends on the type of take. The generation of crack branch occurred due to interaction of crack-nodule. The fractographs of the fatigue fracture surface for GC show cleavage plane along the flake graphite.

SiCp 입자강화 Al 복합재료의 내열 및 마모특성

김석원(Sug Won Kim),김완기(Wan Ki Kim),우기도(Kee Do Woo),안행근(Haeng Keun Ahn) 한국주조공학회 2000 한국주조공학회지 Vol.20 No.6

N/A Al matrix composites as the most promising MMCs can be expected to be excellent engineering materials in the nearest future. So as to improve material properties of composite, many manufacturing processes have been developed. Among them, squeeze casting process which offers fine microstructure and near-net-shape is one of the most successful MMCs manufacturing processes. But, in case of with subsieve size particles (under 44㎛), it is very difficult to homogeneously distribute particles in matrix of Al matrix composite by various casting processes, including squeeze casting used so far. Duplex process which was developed in previous study was used to distribute the particle of subsieve size more homogeneously in matrix of Al matrix composite. Microstructures, wear and heat resistance characterization of Al-Si-Cu-Mg-(Ni)/SiCp manufactured by duplex process were examined to clarify the effect of manufacturing conditions, particle size of reinforcement and alloying elements. Al matrix composites reinforced with SiCp(10㎛) have the lowest wear amount among composites reinforced with 3㎛, 5㎛ and 10㎛ SiCp. The wear amount of Al matrix composites with 10wt% SiCp(3, 5, 10㎛) was decreased according to the increase of the sliding speed because abrasive wear takes place at high sliding speed of 4㎧ and worn debris with block type occurs at low sliding speed of 1㎧. As for heat resistance, it is made clear that remarkable heat resistance property can be obtained by addition of Ni element in Al matrix composites.

SiCp 입자강화 Al 복합재료에 대한 합금원소의 영향과 시효특성에 관한 연구

김석원(Sug Won Kim),이의종(Ui Jong Lee),우기도(Kee Do Woo),김동건(Dong Keun Kim) 한국주조공학회 2001 한국주조공학회지 Vol.21 No.1

The research on new DRA(discontinuous reinforced alloy) and CRA(continuos reinforced alloy) composites has been carried out to improve the properties of ceramic fiber and particle reinforced metal matrix composites(MMCs). Effects of alloying elements and aging conditions on the microstructures and aging behavior of Al-Si-Cu-Mg-(Ni)-SiCp composite have been examined. The specimens used in this study were manufactured by duplex process. The first squeeze casting is the process to make precomposite and the second squeeze casting is the process to make final composite. The hardening behavior was accelerated with decreasing the size of SiCp particle in the composites. It is considered that the dislocation density increased with increasing SiCp size, due to the different thermal deformation between Al matrix and SiCp during quenching after the solution treatment. Peak aging time to obtain the maximum hardness in 3㎛ SiCp reinforced Al composite was reduced than that in large size(5, 10㎛) of SiCp because of difference in dislocation density. Aging hardening responce(ΔH = H_(max)-H_(S.T)) of composites was greater than that of unreinforced Al alloy because of higher density of second phases in matrix.

중력주조 및 직접가압주조 7XXX 계 Al 합금의 미세조직에 관한 연구

김석원(Sug Won Kim),김대영(Dae Young Kim),우기도(Ki Do Woo),김동건(Dong Kun Kim) 한국주조공학회 1999 한국주조공학회지 Vol.19 No.1

N/A Squeeze casting process has been used in the field of a commercial manufacturing method, in which metal is enforcedly solidified under pressure enough to prevent the cast defects such as either gas porosity or shrinkage defect. In this paper, to clarify the relationship between applied pressures and macro·microstructural behaviors in gravity and direct squeeze casts, specimens were cast by various squeezing pressures during solidification of 7000 series Al wrought alloy in the metal die designed specially. The applied pressures used in this study were 0, 25, 50, and 75 MPa. The microstructural morphologies of squeeze cast were more fine and dense with increasing the applied pressures, because of the greater solidification rate of billet resulting from the applied pressure. A normal segregation phenomenon of an increasing in amount of eutectics towards the center of the billet was observed for squeeze casts, whereas gravity cast showed an inverse segregation phenomenon of an increasing in amount of eutectics towards the edge in the billet. This change in segregation pattern which is normal or inverse is due to a higher radial temperature gradient and reduced time in the semi solid state for squeeze casting. (Received September 2, 1999)

Sc을 첨가한 Al-7.7wt%Zn-2.0wt%Mg-1.9wt%Cu합금의 고온 변형거동

우기도,유용석,김석원,Woo, Kee-Do,Ryu, Yong-Seok,Kim, Sug-Won,Deliang Zhang 한국재료학회 2003 한국재료학회지 Vol.13 No.12

The Al-7.7Zn-2.0Mg-1.9Cu-0.1Zr-0.1Sc alloy exhibited excellent elongation by the new thermomechanical treatment (TMT) process; solution treatment and furnace cooling\longrightarrowhot and cold rolling and then annealing for short time. Tensile test at high temperature from 430 to $500^{\circ}C$ has been performed with various strain rates using for the Al-7.7Zn-2.0Mg-1.9Cu-0.1Zr-0.1Sc alloy obtained by the TMT process. The elongation of the Al-7.7Zn-2.0Mg-1.9Cu-0.1Zr-0.1Sc was 550% tensile tested at $470^{\circ}C$ temperature and 2.2 $\times$ $10^{-3}$ $s^{-1}$ strain rate. The m value of Al-7.7Zn-2.0Mg-1.9Cu-0.1Zr-0.1Sc alloy deformed 85% increased from 0.33 to 0.46 with increasing total elongation. This new TMT process was very simple and easy to make the sheets in the company.

고탄소강의 흑연화에 미치는 Mn , Cr의 영향

우기도,김석원,김대영,박영구,류재화 ( Kee Do Woo,Sug Won Kim,Dae Young Kim,Young Koo Park,Jae Hwa Ryu ) 한국열처리공학회 1997 熱處理工學會誌 Vol.10 No.2

In high carbon steels, the cementite phase is more unstable thermally than graphite, and it can lead to a marked deterioration in the formability. Many studies have been carried out to evaluate the effect of the fundamental elements on the graphitization of cementite in high carbon steels. In this present work, the effect of Mn, Cr addition on graphitization in Fe-0.65%C-1.0%Si steel has been investigated by means of hardness testing, optical microscopy and EPMA. The nucleation of graphite may mostly depend on the dissolution rate of cementite into ferrite and the number of the nucleation sites of graphite. The graphitization was promoted by the addition of 0.1%Mn in high carbon steel, but retarded by more addition than 0.5% of Mn. By the more addition of Cr than 0.1%, the graphitization of high carbon steel was strongly retarded. Because Mn was moderate anti-graphitizer, but Cr was stroug antigraphitizer.

직접가압주조한 Al-5%Ni-5%Mg-(Mm) 합금의 조직 및 기계적 성질에 미치는 가압력의 영향

우기도(Kee Do Woo),정동석(Dong Sug Chung),황인오(In O Hwang),김석원(Sug Won Kim) 한국주조공학회 2001 한국주조공학회지 Vol.21 No.2

Misch metal (rare earth element, Ce, La, Nd, Pr) which has large influence on high-temperature stability and toughness was added to the Al-5%Ni-5%Mg alloy, and squeeze casting was used for Al-5%Ni-5%Mg-(Mm) alloys. The effect of applied pressure and misch metal additions on mechanical properties in Al-5%Ni-5%Mg alloy by direct squeeze casting has been investigated. The applied pressure were 0㎫(gravity casting), 25, 50 and 75㎫. Squeeze-cast Al-5%Ni-5%Mg-(Mm) alloys had better mechanical properties than those of non-pressurized cast alloys because of the increased cooling rate by the application of pressure during solidification. By the addition of misch metal in Al-5%Ni-5%Mg alloy, better combination of strength and elongation was obtained. The addition of 0.3%Mm in Al-5%Ni-5%Mg alloy improved the heat resistant property due to the formation of fine eutectic phases.

자동차 차체재료용 Al-Mg-Si-(Cr,Be) 합금의 기계적 성질에 관한 연구

우기도(Kee Do Woo),이종수(Jong Soo Lee),이민상(Min Sang Lee),김석원(Sug Won Kim) 한국주조공학회 1999 한국주조공학회지 Vol.19 No.2

N/A Al-Mg-Si alloys have good strength, formability, corrosion resistance, and bake hardenability. But there is a weak point to use for the auto body materials. This is because the strength of this alloy was decreased by the precipitation of G.P.I zone formed at room temperature for a long holding time. In practical usage, it is inevitable to hold at room temperature prior to paint baking. The present work is aimed to improve bake hardenability using special heat treatments such as two step aging, and retrogressing and reaging(RRA) in Al-Mg-Si-(Cr, Be) alloys. A differencial scanning calorimeter and tensile test were employed to investigate the precipitation behaviors and the mechanical properties in Al-Mg-Si alloys. The tensile strength of Al-Mg-Si-Be and Al-Mg-Si-Cr-Be alloys were improved by special heat treatments such as two step aging and RRA treatment after pre-aged at 120 and 150˚C for 4 hrs. The combination between strength and ductility of the Cr, Be containing Al-Mg-Si alloy is better than those of no addition of Cr,Be in Al-Mg-Si alloy. (Received February 2,1999)

ADI 의 기계적 성질에 미치는 합금원소 및 열처리의 영향

김원석,우기도 ( Sug Won Kim,Sang Won Han,Kee Do Woo ) 한국열처리공학회 1998 熱處理工學會誌 Vol.11 No.2

N/A The effects of alloying elements(Mo, Cu, Ni) and austempering temperature conditions on the microstructural morphologies and mechanical properties in austempered ductile cast iron has been investigated. The austempering at 350℃ for 2hrs after austenitizing at 900℃ for 2hrs in all specimens with various alloying elements was optimum because the good combination of tensile and yield strength, hardness and impact value was obtained. The microstructures of these ADIs treated by a forementioned austempering condition are nearly a mixture type of needle and feathery bainite. Among those alloys, Mo-Cu alloyed DCI had the best optimum mechanical properties of hardness and toughness for automobile parts by austempering treatment for 2hrs at 900℃ followed by 350℃ for 2hrs.

Squeeze Casting 에 의한 SiC 입자강화 Al 합금기 복합재료의 미세조직 특성

김석원,우기도,한상원 ( Sug Won Kim,Kee Do Woo,Sang Won Han ) 한국주조공학회 1995 한국주조공학회지 Vol.15 No.6

N/A In this study, the microstructural characteristics such as primary silicon, eutectic silicon, SiC_p dispersion behavior, compound amount and Si solubility in Al/SiC_p composite fabricated by the squeeze casting under various conditions were investigated systematically. As applied pressure(㎫) increases, cooling rate and compound amount are increased. In gravity casting, the cooling rate of hypereutectic composite is slower than of hypoeutectic composite by exothermic reaction of primary Si crystallization. But the cooling rate of hypereutectic composite is faster than that of hypoeutectic composite fabricated by same applied pressure, because amount of primary Si crystallization in hypereutectic composite was decreased, on the contrary, primary α-Al in hypoeutetic composite was increased due to increase of Si solubility in matrix by applied pressure. The crystalized primary silicon in hypereutectic composite fabricated by squeeze casting become more fine than that in non-pressure casting. This is because mush zone became narrow due to increase of Si content of eutectic composition by pressure and time for growth of primary silicon got shorter according to applied pressure. It is turned out that eutectic temperature and liquidus are decreased by the increasing of squeeze pressure in all the composite due to thermal unstability of matrix owing to increasing of Si solubility in matrix by the increasing of applied pressure, as indicated in thermal anaiysis(DSC) results.