발간논문

Home > KJMM 논문 > 발간논문

A coating process for a multilayer coating of aluminide and yttrium was developed for high temperature applications. Aluminide coating was carried out by pack cementation method, while yttrium was incorporated by an ion plating method. Various combinations of pack alumininzing and yttrium (Y) ion plating were examined with respect to coating sequence, aluminum activity, and corrosion environment. The best corrosion resistance for high temperature hot corrosion was obtained from a multilayer coating of H/A＋Y(high activity aluminizing＋Y-ion plating) type. Uniformity of the Y deposition was greatly dependent upon the surface condition of the aluminide coating layer. The high activity aluminide coating gave better uniformity of Y deposition than did the low activity aluminide coating. Improvement of corrosion resistance by the Y-modified aluminide multilayer coatings of H/A＋Y type was attributed to the fact that presence of Y along the Al₂O₃columnar grain boundaries prohibits substantially depletion of Al contained in the aluminide coating layer. The consistent and prolonged supply of Al thus improves the Al₂O₃scale adherence and stability.

keyword :

Synthetic chromite consisting of FeO and Cr₂O₃, was reduced by graphite powder to investigate the effects of variation of FeO content in it. The summaries of result are as follows; 1) The reduction rate of FeO was faster than that of Cr₂O₃in synthetic chromite. The reduction rate of two kinds of synthetic chromite(CF1 and CF2) having different compositions of FeO and Cr₂O₃increased with increasing temperature. 2) The reduction rate increased with increasing the amount of addition of FeO on synthetic chromite. That is, the reduction rate of CF2 was faster than that of CF1. It seemed that because reduction rate of FeO was faster than that of Cr₂O₃a portion of FeO in spinel structure of chromite was destroyed as a result of reduction of FeO. Therefore the increase of vacant site of structure was caused and this vacancy supplied the reduction site in the spinel structure of chromite. This phenomena was remarkable at low temperature than high temperature. 3) The reduction of FeO and Cr₂O₃in synthetic chromite was simultaneously occurred above 1200℃, but Cr₂O₃was reduced after reduction of FeO below 1200℃.

keyword :

The main objective of this study is to establish powder metallurgy (P/M) fabrication processes for a high performace 2009 Al composite reinforced with SiC whiskers. Rapidly solidified 2009 Al powders produced by commerical atomization technique were mixed with SiC whishers. The results of mixing processes indicated that fluidized zone mixing technique was considerably proper for the large scale production of the mixture of Al powders and whishers, although a combined ultrasonic/mechanical mixing technique gave the better mixtures. In order to consolidate these Al-SiCw mixtures into Al-SiCw composite billets, a vacuum hot press was set up, and hot processing variables were investigated. Using the hot pressing temperature of 620℃ under the pressure of 50 MPa, good quality Al-SiCw composite billets having relatively homogeneous microstructure and sound Al/SiC interfacial bonding were obtained.

keyword :

The purpose of this study is to establish hot extrusion variables for the powder metallurgy (P/M) processed 2009 Al-SiCw composites and to evaluated tensile properties of extruded composite bars. In order to extrude Al-SiCw billets, 550 ton hot extruder was used, and extrusion variables such as billet temperature, die temperature, ram speed and reduction ratio were investigated. Experimental results indicated that formation of circumferential cracks was associated with fast ram speeds and/or high die temperatures, suggesting that low ram speeds and low die temperatures are required to prevent, the circumferential cracks. Using the extrusion temperature of 450∼500℃ under the extrusion pressure of 700∼1000㎫, composite billets were extruded to bars having relatively homogeneous microstructures. Mechanical properties of the extruded bars were found to be comparable with those of composite processed by Advanced Composite Materials Corp.

keyword :

Al₂O₃has been joined to STS304 stainless steel by a brazing method using Al-10 wt% Si insert-metal. The joining has been performed in a vacuum hot press under 4 mPa vacuum varying the conditions such as temperature(850-953 K), holding time(0-3.6 ksec) and applied contact pressure(0-9. 8 ㎫). With use of SEM, EDX and XRD observation of the morphology and microstructure for a bonded interface and the analyses for existing elements and phases have been carried out to investigate possible reaction(s) at the bonded interfaces. No reaction at the interface between Al₂O₃and insert-metal has occured but a formation of reaction layer between STS304 and insert-metal layer has been observed which is composed of Al, Si, Fe and Cr. The thickness of reaction layer increases with a joining temperature and a holding time. In the insert-metal layer the acicular eutectic Si-phase and the new ternary compound of (Al, Fe, Si) exist in the Al-matrix after the joining. The strength of joined body measured by a tensile test has been varied with the joining conditions. The maximum average strength of 32 ㎫ has been obtained from the joints which were joined at 873 K for 1.8 ksec under a contact pressure of 4. 9 ㎫.

keyword :

The densification behavior has been analyzed for a liquid phase sintering system with closepacked spherical particles(face-centered cubic structure) and limited volume of liquid. The specimen shrinkage has been calculated on the basis of the two-particle model up to the moment of the liquid filling of tetrahedral sites. As the volume of liquid(V¹) decreases, the shrinkage and densification rates increase because of high capillary pressure. The closure of interconnected pore channels and the liquid filling of tetrahedral sites, however, occur in shorter sintering time for the particles array with higher liquid volume. As V¹decreases, the dominant mechanism for densification changes from the pore filling to the contact flattening. When V¹is less than 0.1 vol% the condition for the liquid filling of tetrahedral sites can not be reached until the densification by contact up to 97% relative density. The applicability and limiation of the present calculation are discussed.

keyword :