Materials Technology - Aluminium Alloys Examination of the Influence of Heat Treatment on the Properties of AI-Si Alloys Darko VUksanovic 1 ), Srdjan Markovic 2 ), Stanka Tcmovle Petrovic 3 ), Sneiana Rakocevic 1 ), Kata Kovacevic 4 ), Sneiana Tripkovic 5 ) 1) Faculty of Metallurgy, Cetinjski put bb, 81000 Podgorica, Serbia and Montenegro; 2) Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia and Montenegro; 3) Institute "Kirilo SaviC", Vojvode Stepe 51,11000 Belgrade, Serbia and Montenegro; 4) Institute for Ferrous Metallurgy, 83000 Serbia and Montenegro; 5) "H. K. Petar DrapSin", 11 400 Mladenovac, Serbia and Montenegro; Corresponding author: Stanka Tomovic Petrovic, Helgerudvegen 105 B, 2816 Gj0vik, Norway, stomovicpetrovic@yahoo.es In this paper the influence of heat treatment on the structural and mechanical properties of AI-Si alloys was investigated. Silicon content in the examined alloys was in the range 11 to 14%, the contents of the other alloying elements were in the standard range [1] but all alloys were modified with strontium. The regime of the applied heat treatment was quenching (520°C/6h - cooling in water) + aging (205°Cl7h - air cooling). The examinations were carried out at room temperature as well as at 250°C and 300°C. The obtained results showed a posi- tive influence of the applied heat treatment on the mechanical properties of the examined alloys. The improvement of the mechanical prop- erties can be considered as a consequence of a redistribution and change of morphology of the phases present in the structure of the al- loys. Keywords: alloys of AI-Si system, heat treatment, mechanical properties, inter-metallic phase, microstructure. Introduction Following the contemporary development of aluminium alloys, in order to obtain the best possible combination and value of required properties, complex multi-phase alloys are produced. In the process of crystallization of Al-Si alloys, as a con- sequence of non-uniform conditions, beside the eutectic, primary silicon crystals are present in the microstructure as well. Their size and shape may vary, depending on cooling velocity. The bigger they are and the more they differ from regular polyhedrons, the more tension is entailed, which has a negative influence on the mechanical properties of the al- loys [2-3]. These alloys are usually exposed to heat treat- ment in order to improve their exploitation properties. In the structure of heat-resistant Al-Si alloys, soluble and insoluble particles of inter-metallic phases are present. The soluble inter-metallic phases improve the mechanical prop- erties of the respective alloys. The extent of the improve- ment depends on their share and distribution in the mi- crostructure of the alloys. Percentage and morphology of insoluble inter-metallic phases in the alloy structure also has an important influence on the mechanical properties. Besides the influence of alloying elements and inter- metallic phases, the quantity of silicon crystals, size of sili- con particles and average distance between such particles are also of influence on the general properties of multi- phase Al-Si alloys. Since these alloys tend to liquate pri- mary silicon crystals, certain deviations from these rules de- pending on the liquation degree can be expected [ 4-7]. Experimental Work The examined alloys were produced in an electric furnace with a graphite pot with a power of 5.5 kW, and the operat- ing temperature was 1100°C. The bar-shaped examined probes were cast in a grey iron permanent mould. The applied heat treatment was the following: (1) Quenching: Heating of the samples at 520±5°C during 6 hours and quenching from that temperature in water previ- 648 ously heated up to 70°C; (2) Aging: Heating of the samples at 205±5°C during 7 hours and air cooling. The heat treat- ment regime was chosen on the basis of literature data [8- 9]. The heat treated samples were investigated in regard to examination of mechanical properties at room temperature as well as at 250°C and 300°C, structural examinations and fractography. Mechanical properties. Examinations of tensile strength and relative extension at room temperature were carried out on a universal electronic tensile strength device "1195 IN- STRON" whose loading range was 20 N to 20 kN, in ac- cordance with the standard JUS CA4-002/54. Examinations of tensile strength and relative extension at higher temperatures were carried out on the Amsler Univer- sal Testing Machine MG-EMC, with the tensile test samples <1>4. These examinations were performed after heating the tensile test samples during one hour. Hardness (HB) was measured on the device Karl Frank GmbH, type 38532. The measurements were carried out ac- cording to Brinell, and the applied standard was JUS.C.A4.l03. Microstructure. Structural examinations were per- formed on the optical microscope Neophot 21 connected with a device consisting of a camera, a PC and a laser print- er. The device contains 3 wave-dispersing (TDS) spectrom- eters and an energy-dispersing spectrometer for rapid analy- sis (EDS), thus providing a modem combination of a scan- ning microscope and an electronic microprobe. Fracto- graphic analysis of the samples was obtained from the sec- ondary electrons (SSE). Results and Discussion The chemical composition of the examined alloys, deter- mined by the non-destructive method with X-ray quan- tometer, is given in Table 1. All alloys were modified with strontium. steel research int. 78 (2007) No. 8