INFLUENCE OF REMELTING REPEATED ON THE MECHANICAL PROPERTIES AND STRUCTURE OF ALLOYS RR.350. Marek BŘUSKA, Petr LICHÝ, Michal CAGALA, Jaroslav BEŇO

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1 INFLUENCE OF REMELTING REPEATED ON THE MECHANICAL PROPERTIES AND STRUCTURE OF ALLOYS RR.350 Marek BŘUSKA, Petr LICHÝ, Michal CAGALA, Jaroslav BEŇO Department of Metallurgy and Foundry Engineering, FMME, VŠB-Technical University of Ostrava, 17.listopadu 15/2172, , Ostrava Poruba, Czech Republic, Abstrakt Influence of repeated using of alloys on base Al Cu is the goal of this study. Alloy RR350 was used for this study. Specimens for tension examination were casted to the metal forms, which were protected by preservative coating. Pouring temperature and temperature of iron mold were controlled in order to keep constant conditions of experiments for all testing samples. From poured samples the testing rods were prepared to determination of tensile strength under laboratory (20 C) and under higher temperature (with maximum about 350 C). Specimens were also taken for determination of hardness (HBS) and metallographic analysis from poured samples. Metaloographic analysis was evaluated by using microscope GX 51, which equips polarization light with magnification from 12.5 to 0. Thermo-mechanical properties were carried out at the author s working place Department of Foundry Engineering, Faculty of Metallurgy and Material Engineriing, Technical University of Ostrava. Description of this measurement methodology and metallographic analysis of microstructures are also part of this contribution. At the experimental part of this study hardness of experimental samples, which were prepared at different melts with further determination of chemical composition by spectral method analysis, are evaluated. The results of this study definite confirmed negative influence of repeated using studied alloy on thermo-mechanical and structure properties. At the conclusion of this study there are also described possibilities of further treatment and machining of studied material (inoculation and further precipitation hardening). Keywords: aluminium alloys; metallographic analysis; microstructures; thermo-mechanical properties 1. INTRODUCTION Due to the ever increasing use of aluminum alloys and the economic crisis is to use more and more foundries recycled material (multiple reflow). This way you can save material and also reduce the cost of casting. As for aluminum alloys based on aluminum-copper, their use is very important in engineering: for example, heat exchangers, equipment for oil refining, distilling equipment, pumps and pipelines. The largest use is still the automotive industry, which apply mainly for heat treated parts such as cylinder heads, gearbox, air scoops in the highway tunnels (for exhaust fumes). The cylinder head is necessary to know the cast is subjected to temperatures of C, higher pressures and burning fuel stress caused by uneven heating. As for the blades in the air tunnels, highways, selects the alloys Al-Cu (RR.350) because of the low weight, small volume expansion and can withstand higher temperatures [1]. 2. ALUMINIUM ALLOY - RR.350 The aim of my study was to evaluate thermo-mechanical properties of aluminum alloys at elevated temperatures in the die casting, and consequently their influence on the structure of the alloy. These conditions affect the mechanical properties of the alloy. To explore the selected alloy RR.350. To verify the properties after multiple reconstruction was, we decided to transform the alloy four times. The investigated material was provided tensile strength, elongation, hardness morphology and structure using metallographic cuts.

2 Table 1 Chemical composition of alloys RR.350 (wt.%) Fe % Cu % Mn % Mg % Ni % Zn % Ti % Pb % Sn % Co % Cr % Al % PREPARATION OF THE SAMPLES The casting of test bars for tensile machine was used in the form of the metal ingot mold (two pieces) equipped with a special silicone coating. The cast consisted of the inlet system, the test itself and riser bars. Since it was a bad foundry alloy is dispersed cavities forming properties had to be oversized nálitek. Smelted in a crucible resistance furnace using a graphite-clay crucible. Input at the first cast from the casting roll the exact chemical composition (Table 1). The following cast has been made up of recycled material heats earlier, we could examine changes in the structure of the alloy přetavbách. After casting the specimens were cast freely cooled in air. There has been no heat treatment, because I wanted to determine the thermomechanical properties of alloy itself. Samples were ality at temperatures from 660 C to 860 C. The actual cutting was done on a horizontal water-cooled grinder, a grinding tool was used sandpaper with different gradations: th. 4. MICROSTRUCTURE Fig. 2 Melt no. I. - magnified * Fig. 1 Melt no. II. - magnified * Fig. 3 Melt no. III. - magnified * Fig. 4 Melt no. IV. - magnified *

3 Structure of the I st melt (Figure 1) shows dendritic marks with greater dendrites and without greater segregation marks of alloying elements. It can be stated that the structure is getting near to homogeneity. From the sample there is evident the alloy tendency to gasification. The melt No II. (Figure 2) shows different granularity (non-uniform dendritic cells, cavities) with more extensive separation of intermetallic phases along the grain boundaries. The melt No III. (Figure 3) shows a dendritic type of structure with different granularity, a greater share of cavities and shrinkage cavities. In the melt No IV. (Figure 4) non-uniform structure with marks of dendritic segregation with a great share of cavities of a greater volume can be seen [4, 2]. 4. ELECTRON MICROSCOPE Fig. 5 Melt no. I. - magnified 2000 * Fig. 6 Melt no. IV. - magnified 2000 * Table 2 Content of elements in Name O Al Mn Fe Co Ni Cu Zr Sb matrice 97,2 2,8 Spectrum1 15,3 25 0,7 1,7 57,4 Spectrum2 49,8 0, ,6 Spectrum3 62,8 1,7 2,9 2,7 4,9 24,3 1 Spectrum4 76,5 0,7 2,7 3,6 12,8 3,7 Table 3 Content of elements in IV. melt Name O Al Ti Mn Fe Co Ni Cu Sb matrice 96,9 0,6 2,5 Spectrum1 16,9 22,8 0,2 1 59,1 Spectrum2 43,7 0,4 0,7 22,5 32,7 Spectrum3 58,5 2,3 3,9 2,9 4,6 27,7 Spectrum4 71,1 0,7 4,3 5,3 14,8 3,8

4 [HB] [HB] Strength [MPa] , Brno, Czech Republic, EU 5. TENSILE TEST Temperature dependence of the strength of the alloy RR Temperature [ C] Graph 1 Temperature dependence of the strength of the alloy RR.350 Tensile test showed that, at first, but also a third melting temperature to C, slightly increasing strength (I cast on the strength of MPa and III. the cast of this strength 190MPa). The difference in the strength of I and III. Heat is given due to melting, which makes us as grain size, but also deprives the structure of a component and thus the mechanical properties. When I melt strength increases, decreases at C, the temperature at 350 C makes the difference in the decrease of 26% and III. Heat seeing significant drop from 150 C, when the difference of strength drops at 350 C at 20%. The difference between the strength properties of I and ing temperature 20 C - C is 11% (Graph 1). Tensile tests were carried out only up to 350 C. With the gradual heating up we started to grow strain to fracture. 6. HARDNESS MEASUREMENT Hardness [HB] Micro hardness [HB] 50 0 I. - IV. melt I IV. melt I. - IV. melt I IV. melt Graph 2 Hardness Graph 3 Micro hardness It has been proved by measurements of microhardness and hardness that with the remelting degree the alloy hardness is changed as show the Graphs 2 and 3. Hardness was decreased up to the III rd melt but in the IV th melt it was already increased what is given by grain coarsening and separation of intermetallic phases.

5 7. CONCLUSION The properties of the reconstruction was verified with multiple (four in total). For each reconstruction was, we got about 15 samples. In this particular case we set the material tensile strength, ductility and hardness, morphology, structure and elements of loss due to multiple remelting. To determine the values, the tensile test, Brinell hardness measurement and metallographic cuts [3]. Tensile test showed that, at first, but also a third melting temperature to C, slightly increasing strength (I cast on the strength of MPa and III. the cast of this strength 190MPa). The difference in the strength of I and III. Heat is given due to melting, which makes us as grain size, but also deprives the structure of a component and thus the mechanical properties. When I melt strength increases, decreases at C, the temperature at 350 C makes the difference in the decrease of 26% and III. Heat seeing significant drop from 150 C, when the difference of strength drops at 350 C at 20%. The difference between the strength properties of I and ing temperature 20 C - C is 11% (Figure 1). Tensile tests were carried out only up to 350 C. With the gradual heating up we started to grow strain to fracture. As for the hardness measurements confirmed that the degree of superheat decreases as the strength and and hardness, which is due to the elimination of elements due to the degree of melting of the alloy. This also is closely related to the impact on the structure. We must not forget the important fact that hardness decreased mainly due to the degree of melting alloy, as shown in Figure hardness. The average hardness was measured in these terms: I melt - HB 73.3, I - 68 HB decline 7%, and - 64 HB decrease of 6% (Graph 2,3). The metallographic sample showed that the morphology of the structures (Figure. 1-4) is influenced by multiple and remelting at high temperature casting. Changes in the structure can be classified as grain coarsening, non-uniformity of dendritic cells and dendrites, higher content of voids, increased segregation, and other elements [4]. Testing alloys RR.350 offers another way to improve mechanical properties. As seems appropriate adjustment substituting elements depleted due to the multiple melting and subsequent vaccinations AlTi5B1 master alloys. Furthermore, the alloy can be cured very well due to heat treatment (strength values as high as 300 MPa). As mentioned above, the alloy is prone to various kinds of cavities, so it would be advisable to degas before pouring. Production of castings can also recommend a filter to remove inclusions, which reduce the mechanical properties [2]. ACKNOWLEDGMENT The research was realized with financial support of the specific research project VŠB-TU Ostrava SP 2012/23 Studium přípravy a vlastností materiálů na bázi litých kovových pěn. LITERATURE [1] ROUČKA, J.: Metalurgie neželezných slitin. VUT Brno [2] KOŘENÝ, R.: Možnosti zvýšení kvality vysokopevnostních a žáropevných slévárenských slitin hliníku. VŠB, Ostrava [3] PÍŠEK, F.; JENÍČEK, L.; RYŠ, P.: Nauka o materiálu I., Nauka o kovech 3. svazek: Neželezné kovy. Academia, Praha [4] JAREŠ, V.: Metalografie neželezných kovů. Česká matice technická, Praha 1950.