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Właściwości kompozytów AK7-SiC po ponownych przetopach

Tomczyński S., Mitko M.

Kompozyty

2002

R. 2, nr 4

217-219

Przedstawiono wyniki badań właściwości mechanicznych, fizycznych i trybologicznych kompozytu metalowego poddanego wielokrotnemu przetopieniu. Kompozyt wyjściowy AK7-10%SiC, otrzymany metodą mieszania mechanicznego i odlwania do kokil, przetapiano ponownie, mieszano i odlewano do kokili. Dokonano pomiaru właściwości Rm, A i E podczas próby rozciągania statycznego, a także twardości HB, własności trybologicznych (Zc,f) i fizycznych (p, c, l). Badania przeprowadzono na kompozycie wyjściowym oraz otrzymanym po pierwszym, drugim, trzecim i czwartym przetopach. Odlewy kompozytowe uzyskane w pierwszych dwóch przetopach swoimi właściwościami zasadniczo nie odbiegają od odlewów wykonanych z kompozytów wyjściowych. Kompozyty uzyskane w następnych przetopach wykazują już znaczne obniżenie właściwości wytrzymałościowych i trybologicznych.

The basic AK7-SJC composite (10 vol. % of SiC particles of dimensions 63:1OO um) has been prepared by mechanical mixing and gravity casting [1]. Then it has been rcmelted, mixed and poured into metal molds. Static tensile test (Rm, A, E) have been performed, and measurements concerning hardness (HB), tribological properties (Zc,f), and physical properties have been taken both for basic composite and for the material obtained after the first, the second, the third and the fourth remelting (Fig. 1). Remelting of composites deteriorate their mechanical and tribological properties. Though after the first remelting changes are small (about 5%), after the fourth remelting they should be already considered as large: tensile strength decreases by 50 MPa, and hardness by 10 HB as compared with the basic composite. Elongation (A) and the elastic modulus (E) changes insignificantly after remelting. The repeated melting causes an increase in abrasive wear and in the coefficient of friction of the examined composites. After two cycles of remelting the increase in abrasive wear Zc and in the f coefficient is still insignificant. However, after the third and the fourth remelting abrasive wear increases by 30%, and the coefficient of friction by 50% as compared with the basic composite. Density (p) of composites after subsequent remelting cycles slightly decrease, but there have not been found changes in specific heat (c) or in the thermal conductivity (l). The performed examinations have shown that, in the case of the multiple remelting of AK7-SiC composite, the properties of castings obtained after the first and the second remelting cycle are do not differ much from the ones exhibited by castings obtained directly from basic composite. However, after the third and the fourth remelting composites reveal already significant decrease in strength and tribological properties. This is caused by introducing some impurities during each remelting cycle, and also by degradation of the particle-matrix bonding zone. Hard SiC particles which are not well-bonded with the matrix crumble out and act as abrasive particles at the frictional surface of a kinematic pair. Physical properties, as density, specific heat, and thermal conductivity remain almost unchanged after repeated melting of the composites under investigation. The performed examinations show that the composite waste can be successfully reused as charging material for producing composite castings.