Section 4 Others MOLD DESIGN AND TEMPERATURE FIELD ANALYSIS ON ELECTRIC CURRENT SINTERING OF ELECTRICALLY CONDUCTIVE POWDERS YANG Junyi, LI Yuanyuan, LI Xiaoqiang, Guo Liang *School of Mechani cal Engi neer ing, South China University o f Technology, Guangzhou 510640, China Fax: (86) 20-87112948, E-mail: [email protected]Keywords: Field activated sintering, temper ature field, fini te element modelling, powder metallurgy. Abstract In order to probe and improve temperature field during elect ric current sintering, high st ren gth ceramic and graphite were used respectively as female die during sintering. Experiments and finite element method (FEM) were conducted to characterize the electric current density, resistance heati g and temperature distribution in the s pe ci me n/ di e/ pu nc h. O bv io us ly higher heating r at e, higher final sintering temperature and higher gradient temperature w er e f oun d in sp ec im en s w he n h ig h st re ng th c er am ic s w as used as female die, compared with graphite female die under t he c on di ti on s of the same die dimension and sintering eliminate the differences as well as to improve heating rate of the specimen. The experimental res ults are in good agreement with calculated values by FEM, which indicates that FEM may be an effective analysis method to simulate temperature field o f electric current sintering. 1 Introduction Electric current sintering (ECS) techniques, especially spark plasma sintering (SPS), have shown some great mer its such as faster heating and cooling, shorter holding time, fewer p ro ce ssi ng steps, l owe r si nt er in g t em pe ra tu re , c on tr ol le d a tm os ph er e a nd e xt er na l p re ss , e li mi na ti on of the need for s in te ri ng a id s, a nd n ea r n et s ha pe c ap ab il it y [1]. T he y h av e been widely used in fabrication and investigation of magnetic m ate ria ls, C er am ic s, b ul k a mo rp ho us al loy , i nt er me ta ll ic c om po un ds , m eta l m at ri x c om pos it es , fu nc ti ona l g ra di en t materials (FGM), ultra-fine grained and nano st ruc tu re materials, and so on, which have shown excellent performance [2- 8]. Currently, the main die material for ECS is graphite because of its benefits such as high melting point, medium electric resistance, low cost and so on. However, graphite has d is ad va nt ag es f or s in te ri ng . It s l ow s tr en gt h m ak es g ra ph it e punch not bear a relatively high uniaxial-pressure, otherwise the horizontal pressure generated by uniaxial-press can destroy the female die or shorten its working- life. In addition, s in te ri ng v el oc it y is a ls o l ow er ed by l ow s in te ri ng p re ss ur e. A no th er a dv er se fa ct or o f graphite is its high thermal conductivity ( abo ut three times t ha n t ha t o f st ee l) w he n graphite is used in female die. Due to the high thermal c on du ct iv it y, its h ea t t ra ns fe r is fast, wh ic h a cc el er at es t he h ea t lo ss o f d ie s ur fa ce . So , w it ho ut a pp ro pr ia te i ns ul at ed measures, the temperature of g ra ph it e d ie is d if fi cu lt t o r is e, which the maximal pulsed electric current is relatively low. It can also be imagined that the heat transfer in powders, which c on ta ct i nt er io r di e su rf ac e, is a lso fa st b ec au se o f t he h igh thermal conductivity of graphite, this is one o f the causes that result in large te perature gradient in powders [9], in particular for SPS which has fast heating rate. It is known that the heat energy in powder specimen during ECS is transferred from graphite punches and comes from its Joule heat [10,11]. Hence, the heating rate may be greater when using high strength ceramic die, becaus of its lower thermal conductivity and higher electrical resistance compared with graphite material . This makes lower electric current sintering possible. In this paper, to further clarify the advantages of cer amics die , both experime ts and FEM were adopted to ana yse the temperature field during electric current sintering of electrically conductive powders. 2 E xp er im en ta l s tu dy Experiments were carried out using an e le ct ri c c ur re nt sintering equipment, which had a maximal uniaxial-pressure o f 1 OOOKN. T he p ow er sup pl y c ou ld p ro vi de a p ul se d d ire ct e le ct ri c c ur re nt u p t o 3 00 0 A t hr ou gh t he r am s i nt o t he d ie se t that contained specimen. Fig.l shows the Schematic diagram o f heating system. A 5 mm thick celluloid plane was used to separate graphite punch electrode from the system. Graphite and alumina ceramic were adopted as female die materials respectively. Since the thermal stress generated by the temperature difference between int erior and outer die surface can destroy the die when it is beyond the ceramics thermal shock resistance [11]. In this paper alumina ceramic die surface was enveloped by iron sheath by thermal interference fit. A thermocouple was placed 3mm away from the top of the lower punch. E xpe ri me nt al p ar am et er s: t he ba se , d ut y r at io, f re que ncy , holding time and peak of pulsed direct current were 360 A, 50 %, 50Hz, 6minutes and 2400 A, respectively. Uniaxial
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Mold Design and Temperature Field Analysis on Electrical Current Sintering of Electrically Conductive Powders
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8/2/2019 Mold Design and Temperature Field Analysis on Electrical Current Sintering of Electrically Conductive Powders