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정밀주조된 A356 Al 합금의 미세조직과 기계적 성질과 미치는 HIPping의 영향
김낙준,이민하,김재중,김경훈,이성학,이의휘 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.2
The effects of HIPping on high-cycle fatigue properties of investment cast A356 Al alloys were investigated in this study. Tensile and high-cycle fatigue tests were conducted for cast alloys, two of which were HIPped, and then the test data were analyzed in relation with microstructures, tensile and fracture properties, and fatigue fracture mode. Eutectic Si particles were homogeneously dispersed in the matrix of the cast A356 Al alloys, but there were many large pores formed as cast defects. The high-cycle fatigue results indicated that fatigue strength of the HIPped alloys was higher than that of the non-HIPped alloys because of the significant reduction in volume fraction of pores by HiPping. In the non-HIPped specimens, fatigue cracks initiated at large pores adjacent to the specimen surface and then propagated down to several hundreds micrometers depth while coalescing with other large pores. On the other hand, the HIPped specimens, where pores did not affect the fatigue substantially, fatigue cracks propagated along eutectic Si particles formed at solidification cell boundaries, thereby showing improved fatigue strength by 40∼50% over the non-HIPped specimens.
김재중,김낙준 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.6
The multiple-necking phenomenon and mechanism of reinforcement wires in AM60 Mg composites reinforced with AISI 304 wires has been investigated in the present study. AM60 Mg composite reinforced with AISI 304 wires exhibits ductility greater than the ductility of wire itself due to the occurrence of multiple-necking along the length of wire. As the volume fraction of AISI 304 wires decreases, the number of wire necking and the ductility of composites increase. It has been shown that the multiple-necking phenomenon is closely related to wires necking, interface reaction, and volume fraction of reinforcement. This phenomenon is also related to diameter of wires, strain rate, structure of metal, and interface strength. The multiple-necking results from local strain hardening of the matrix surrounding the neck, enabling the matrix to decrease neck growth by attraction force. Because multiple-necking plays an important role in the ductility and deformation behavior of composite, further works is needed to characterize the causes of this phenomenon in other metal/metal composites system.
Strip Casting 법으로 제조된 304 Stainless Steel 의 응고 미세조직
김재중,표성규,김낙준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.10
A study has been made of the effects of strip casting process variables on the microstructure of strip cast SUS 304 stainless steel. The main impetus for the present investigation is to suggest the optimum process variables for the strip casting of SUS 304 stainless steel based on the systematic studies of microstructural evolution. Results slow that there is a variation of microstructure through the thickness of strip. Along the surface of strip, the microstructure consists of fine dendrite and cellular structure, and there is also a formation of α' martensite and mechanical twin due to the deformation by roll contact. On the other hand, microstructure in the middle of strip consists of equiaxed grains due to slow solidification rate and recalescence. One of the most important finding is that there is a depletion of Mn in the interdendrite region and the presence of amorphous Si-rich particle.
용탕단조법으로 제조한 Ti 합금 강화 AM60 Mg 복합재의 미세조직 및 파괴거동
김재중,김낙준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.8
Microstructure and fracture behavior of Ti alloy reinforced AM60 Mg composite have been investigated in the present study. To clarify the fracture behavior, in-situ fracture test was conducted. The manufactured composite consists of 45.1 vol.% reinforcement and does not show the pore presence. No reaction layers or reaction products are observed at the Ti/Mg interface. However, there is a presence of fine Ti particles in the matrix. It has been suggested that the Ti particles form by diffusion during squeeze casting. It also has been shown that there is a diffusion of Mg into Ti. Microcracks are nucleated mainly along the reinforcement/matrix interface, but they are less sharp than those found in the ceramic reinforced composites. Considering the high volume fraction and large size of the reinforcement used in the present study, the manufactured composite shows the strong potential for further improvement in mechanical properties.
Ca, P 의 첨가가 용탕단조 Mg-Al-Zn-Si 합금의 미세조직에 미치는 영향
박성진,김낙준,김도향,김재중,신철수 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.12
Effects of alloying elements such as Ca and P on the microstructure of squeeze cast Mg-Al-Zn-Si alloys have been investigated in the present study. Addition of Si in Mg-Al-Zn alloys results in the formation of Mg₂Si in two types of morphologies; polygonal type and chinese script type. With increasing Al content the formation of polygonal type Mg₂Si is pronounced. Addition of Ca results in the homogeneous distribution of fine polygonal Mg₂Si particles. Ca is segregated mainly around the secondary phase such as Mg_(17)Al_(12), P provides the nucleation sites for Mg₂Si, results in the homogeneous distribution of the fine polygonal Mg₂Si particles. Accordingly, Ca and P containing Mg-Al-Zn-Si alloys have the much finer microstructure than those with no Ca and P since the fine polygonal Mg₂Si particles restrict the grain growth of Mg alloys during squeeze casting and subsequent heat treatment.
고액공존 과공정 Al-Si-(X) 합금의 전자기적 교반시 응고현상
田昌日,金載中,金洛俊 대한금속재료학회 2001 대한금속·재료학회지 Vol.39 No.12
Microstructural changes of Al-Si-(X) alloys during semi-solid state processing have been investigated in the present study. The main objective of this research is to investigate the effect of the electromagnetic stirring on the microstructure of Al-Si-(X) alloys. Firstly by continuous electromagnetic strirring during cooling, primary Si particles in hypereutectic Al-Si alloy tend to distribute themselves to both mold wall sides. The high concentration of primary Si at the surface increases the surface hardness and abrasive resistance of the stirred Al-Si alloy. As the rotation velocity of the magnet increases, the trends become remarkable. It has been suggested that each element having different electric conductivity has different electromagnetic forces. Secondly by adding Fe element to the Al-Si alloy, particle having a larger conductivity than Si particle is formed. During continuous electromagnetic stirring of Al-Si-Fe alloys, primary Si particles and Al_9Fe_2Si_2 particles are distributed uniformly, and the particles are fractured and then become round. It occurs due to the lots of collision between the two particles - primary Si and Al_9Fe_2Si_2 particles, since each has different velocity.
고강도 7075 Al 합금의 가공 열처리와 미세조직의 상관관계연구
표성규,김재중,김낙준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.8
The main objective of the present investigation is to study the effect of the thermomechanical treatment on the structure-property relationships of 7075 plate. Particular emphasis has been placed on clarifying the origin of the anisotropy of mechanical properties. It shows that the application of thermomechanical treatment improves the microstructure and overall tensile properties of 7075 plate. Grain size becomes finer and the morphology of coarse intermetallic particles become favorable as the 7075 plate is thermomechanically processed. This improvement of microstructure directly influences the tensile properties of 7075 plate. While the thermomechanically treated (TMT) 7075 plate don't show any significant change in tensile properties with specimen orientation, there exists a large variation of tensile properties, in especially elongation, of heat treated (HT) 7075 plate with specimen orientation. HT 7075 plate in LT orientation has a much lower elongation than that in I. orientation. Detailed fractographic study reveals that poor ductility in LT oriented specimen is due to the large effective grain size and also due to the occurrence of cleavage fracture of coarse intermetallic particles along grain boundaries. In short, it has been found that the application of thermomechanical treatment improves the tensile properties of 7075 plate and it can be directly applied for the alloy processing for forged harts.