Influence of antimony on the mechanical properties and gas content of alloy AlSi6Cu4

Transcription

1 A R C H I V E S of F O U N D R Y E N G I N E E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 11 Issue 1/ /1 Influence of antimony on the mechanical properties and gas content of alloy AlSi6Cu4 D. Medlen *, D. Bolibruchova a Department of technological engineering, University of Žilina, Univerzitná 1, Žilina, Slovakia *Corresponding author. address: dusan.medlen@fstroj.uniza.sk Received ; accepted in revised form Abstract Aluminium alloys based on Al-Si are used in automotive and aerospace industries. AlSi6Cu4 alloy is used the complicated castings, which must comply high strength requirements. Strength characteristics can also be affected by the modifiers: Na, Sr, Sb. In the literature is mentioned, that AlSi6Cu4 modified by sodium and strontium has negative effect - increases of the gas absorption. Modification of AlSi6Cu4 alloy by antimony, is still not mentioned in the literature. The article gives the effect of antimony on selected mechanical properties and gas content of foundry alloy AlSi6Cu4. Keywords: Mechanical properties; Metallography; Modification; Antimony; Gas content 1. Introduction AlSi6Cu4 alloys are used for castings of complicated shapes with different wall thicknesses. The most commonly are used for castings, which are moderately to highly stressed. (Engine blocks, heads and pistons, clutch housings, exhaust end, die-cast chassis). AlSi6Cu4 alloy is used in the automotive industry for 2.2 l Opel engine also BMW, Fiat, Citroën use this alloy to manufacture engine blocks and cylinder heads. In Slovakia is AlSi6Cu4 alloy used for cylinder heads for 1.6 l engines for Kia Ceed and in the Czech Republic for Skoda Fabia 1.2 TSI. [1, 2] The structure and mechanical properties of AlSi6Cu4 alloys can be improved not only through modifying, grain refinement but also through applying heat treatment and other technologies. In practice, the most common elements with the modifying effect are strontium, sodium and antimony. Adding these elements leads to a change in the shape of eutectic silicon, resulting in an increase of the mechanical characteristics of alloys and toughness. [3] AlSi6Cu4 castings must meet the high requirements not only on the mechanical properties but must meet high requirements on pressure tightness as well. Strontium is now widely used in practice for modification of AlSi6Cu4 alloy. When modifying AlSi6Cu4 alloys by strontium was found, that the alloy subjected to this type of modification has higher values of gas content, leading to higher porosity and thus reducing the pressure tightness. The aim of experimental work was to investigate the influence of antimony on mechanical properties and gas content of aluminum alloy AlSi6Cu4. 2. Experimental procedure 2.1. Melt treatment and casting procedures The experiments were carried out in the foundry laboratory of the Department of Technological engineering at the University of Zilina, where as the experimental material was used alloy AlSi6Cu4. Chemical composition of the alloy is listed in Table 1. The melting process and the modification were carried out in a graphite-chamotte melting crucible in an resistance oven. The grain refinement process using refining salt AlCuAB6 was A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

2 carried out while overheating the metal bath to 730 C ± 5 C. The modification process using antimony was carried out under the same technological conditions. compared with the unmodified state of alloy AlSi6Cu4 was up to 9% reduction in Brinell hardness. Table 1. Chemical composition of the AlSi6Cu4 alloy Elements Si Fe Cu Mn Mg (wt.%) 6,52 0,43 3,88 0,45 0,29 Elements Ni Zn Ti Cr (wt.%) 0,01 0,46 0,15 0,01 The amount of antimony chosen for each cast is listed in the Table 2. This amount was determined based on the most widely used quantities shown in the literature for Al-Si-Cu based alloys. Table 2. Amount of antimony for each cast of AlSi6Cu4 alloy used in the present work Cast number Fig. 1. Relationship between ultimate tensile strength and amount of antimony (ppm) Sb amount (ppm) Cast number Sb amount (ppm) Mechanical properties There were 3 samples cast, of which they were made test bars for tensile testing according to EN The tensile test was performed on a tensile machine ZDM 30 at 21 C. Values of ultimate tensile strength (UTS) determines the average value of 3 test bars (Fig. 1). Percent elongation (Fig. 2) was for each sample calculated using mathematical formulas. When measuring the Brinell hardness on the measuring device CV-3000 LDB, the parameters used were HBS 5/250/15. Hardness values were determined as average values of 5 measurements on one sample of the each cast (Fig. 3). Fig. 2. Relationship between elongation and amount of antimony (ppm) Fig. 1 shows increasing amount of the modifying element - Sb, increases ultimate tensile strength. To increase the ultimate tensile strength of not heat-treated alloy AlSi6Cu4 about 5 to 10%, is the most appropriate amount of Sb in the range from 1000 to 2500 ppm. Higher amount of modifier has led to a reduction of ultimate tensile strength. Fig. 2 demonstrates the AlSi6Cu4 alloy subjected to modification by antimony exhibits very low values of percent elongation, where the highest value of A5 = 0.464% was measured in amount of 2000 ppm Sb. Fig. 3 shows the relationship between Brinell hardness and amount of antimony. Alloy AlSi6Cu4 reported, in unmodified state, HBS = and the highest hardness was measured at 100 ppm Sb and that was HBS = The lowest measured value was HBS = corresponding to 2500 ppm Sb, which Fig. 3. Relationship between Brinell hardness and amount of antimony (ppm) 74 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

3 2.3. Microstructure analysis For metallographic examination, samples (20 mm x 20 mm) were sectioned from each cast with graduated Sb amount. The microstructures were analyzed according to EN - STN using NEOPHOT 32 optical microscope in conjunction with image analyzer. Microstructure and morphology of silicon AlSi6Cu4 alloy from each cast are shown in Figure 4 to Figure 14. In all samples, the structure consists of dendrites α - phase, in which the plane metallographic sample is observed in the form of white bodies and eutectic silicon excreted in the form of gray bodies. Based on the alloy microstructure observation in Figure 4 to Figure 14 and comparing them with each other, it can be concluded that the microstructure of the 250 x magnification has not essentially changed. Further analysis of AlSi6Cu4 microstructure using higher magnification will be carried out in further works. Fig. 6. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by 300 ppm Sb Fig. 4. Optical micrographs showing microstructure of unmodified AlSi6Cu4 alloy Fig. 7. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by 500 ppm Sb Fig. 5. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by 100 ppm Sb Fig. 8. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by 800 ppm Sb A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

4 Fig. 9. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by ppm Sb Fig. 12. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by ppm Sb Fig. 10. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by ppm Sb Fig. 13. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by ppm Sb Fig. 11. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by ppm Sb Fig. 14. Optical micrographs showing microstructure of AlSi6Cu4 alloy modified by ppm Sb 76 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

5 2.4. Gas content of AlSi6Cu4 AlSi6Cu4 alloy was subjected to the same technological processes as in the analysis of mechanical properties and the same amount of Sb was used in the casts listed in the Table 2. To detect the gas content was used equipment Vacuum Density Tester 3VT- LC, vacuum forming and weighing-machine MK 2200LC, which were used for weight identification of the experimental samples. Weighing-machine evaluated the density of samples and also a percentage, based on mathematical and physical relationships and formulas, determined the density index - DI. In each cast were made two samples where the first one was solidifying on the air and the second one was placed in vacuum for 7 minutes. Crosssection cuts of individual samples can be seen in Figures In Table 3 shows the calculated values of the density index of AlSi6Cu4 alloy modified by graduated amounts of antimony on the basis of mathematical and physical relationships. Based on the comparison Table 3 and Figures 4-14 can be noted that mathematically determined amount of gas content was confirmed by observing cross-section cuts of experimental samples. Table 3. Density and density index of AlSi6Cu4 modified by Sb Cast number Density on air (g/cm 3 ) Density in vacuum Density index DI (%) (g/cm 3 ) air 2 vacuum Fig. 16. Cast number 2 (100 ppm Sb) 3 air 3 vacuum Fig. 17. Cast number 3 (300 ppm) 4 air 4 vacuum Fig. 18. Cast number 4 (500 ppm Sb) Unmodified AlSi6Cu4 alloy showed DI = 11.11%, while by adding a Sb modifier were obtained significant changes, about 60 percent reduction, in gas content corresponding to 500 ppm Sb. 5 air 5 vacuum Fig. 19. Cast number 5 (800 ppm Sb) 1 air 1 vacuum Fig. 15. Cast number 1 (0 ppm Sb) 6 air 6 vacuum Fig. 20. Cast number 6 (1 000 ppm Sb) A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,

6 7 air 7 vacuum Fig. 21. Cast number 7 (1 500 ppm Sb) 11 air 11 vacuum Fig. 25. Cast number 11 ( ppm Sb) 3. Conclusions 8 air 8 vacuum Fig. 22. Cast number 8 (2 000 ppm Sb) Based on the experiments can be claimed, that the most appropriate amount of antimony for non heat-treated AlSi6Cu4 alloy is 2000 ppm Sb. The results of mechanical tests show that for such amount of the modifying element, alloy has an ultimate tensile strength Rm = MPa at the highest measured elongation A5 = 0.464% and Brinell hardness HBS = Gas content of the test sample AlSi6Cu4 alloys modified by ppm Sb, was reduced by 30% compared to original unmodified alloy. 4. Acknowledgements 9 air 9 vacuum Fig. 23. Cast number 9 (2 500 ppm Sb) The authors wish to thank European Regional Development Fund for received financial support to create present paper within execution of the project "Equipment for the production of prototype parts by casting on a computer-controlled base" with ITMS code References 10 air 10 vacuum Fig. 24. Cast number 10 (3 000 ppm Sb) [1] D. Medlen, D. Bolibruchová, Teoretical studies of AlSi6Cu4 alloys, Technológ vol. 2, (2010) (in Slovak) [2] D. Bolibruchová E.Tillová.: Foundry alloys Al-Si, EDIS Žilina, (2005) 180 (in Slovak) [3] R. Romankiewicz, F Romankiewicz, Influence of odification of silumin AlSi6Cu4 on fracture morphology, journal of machine manufacturing, vol. XLIX, (2009), A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 1, I s s u e 1 / ,