Effectiveness of Zn-Ti Based Refiner of Al and Zn Foundry Alloys

Transcription

1 Effectiveness of Zn-Ti Based Refiner of Al and Zn Foundry Alloys W. K. Krajewski* 1, A. L. Greer**, J. Zych* and J. Buraś* * AGH University of Science and Technology, Faculty of Foundry Engineering. Reymonta Krakow. Poland. ** University of Cambridge, Department of Materials Science and Metallurgy. Pembroke Street, Cambridge CB2 3QZ, UK. Abstract The presented work is devoted to structural characteristics and performance of the new family of grain-refiners, based on the Zn-Ti system. The system studied was Zn-25wt%Al alloy (ZnAl25), Al-20wt%Zn alloy (AlZn20), Zn-4.6wt%Ti master alloy (ZnTi4 MA), ZnAl-4 wt%ti (ZnAl-Ti4 MA) as well as the traditional refiners Al-5wt%Ti-1wt%B (AlTi5B1 MA) and Al-3wt%Ti-0.15wt%C (AlTi3C0.15 MA). SEM (scanning electron microscopy), LM (light microscopy) and TA (thermal analysis) investigations showed high effectiveness of the ZnTi-based master alloys as refiners of the inoculated with them AlZn20 and ZnAl25 alloys. The obtained structure refinement is comparable to those obtained after addition of the traditional AlTi5B1 MA or AlTi3C0.15 MA. The initial examinations of the damping properties showed, that attenuation coefficient of the refined AlZn20 alloy decreases together with increased structural fineness of the inoculated alloy. Key words Aluminium-Zinc cast alloys, Grain refinement, Heterogeneous nucleation, Zn-Ti master alloy. 50/1

2 Introduction Grain refinement of the Al based cast alloys is a common practice, which allows obtaining fine structure with increased strength properties. On the other hand, many structural materials are required to have improved damping properties. However, damping capacity and strength properties are believed to be contradictory factors [1]. On the other hand, recent literature brings information that grain refinement of A356 alloy increases its damping capacity together with increased mechanical properties [2]. It should be noted that AlZn-based cast alloys are numbered among the structural materials of improved damping capacity. It is well known, that high-aluminium zinc alloys, for example ZA-27 alloy, fall in the category of HiDAlloys (High Damping Alloys) [3]. It was also stated that high-zinc aluminium alloys show increased damping properties [4]. The both groups, i.e. high-aluminium zinc and high-zinc aluminium alloys solidify naturally with a coarse structure and using the refinement process allows obtaining a highly refined structure [5 7]. Surprisingly, the literature lacks the information of how the refining process influences damping properties of the AlZn-based cast alloys. The extensive investigations of the strength properties of sand-cast and chill-cast Al-Zn alloys showed that the latter have higher elongation, most probably due to the finer structure [8], Fig. 1. In practice, there are two main groups of refiners based on the Al-Ti-B and Al-Ti-C systems, commonly used in melting technology of Al-alloys. However, the range of melt temperatures of o C required for the dissolution of AlTiB and Al- TiC refiners is K higher than the o C range recommended for the Zn-Al alloys. Also the high-zinc aluminium alloys, for example Al- (20-30) wt%zn are very prone to surplus overheating, which causes melt oxidation and gases pick-up, detrimentally influencing their properties. A newly introduced alternative - a master alloy based on the Zn-Ti system - requires a melt temperature of only about 500 o C, which avoids detrimental overheating, reducing the costs of energy and material, and improving the mechanical properties of castings [5-7]. This work is aimed at presenting to the foundrymen's community the characteristics of structure and performance of the ZnTi-based refiners. It presents examples of structure and properties changes of the selected Znbased and Al-based cast alloys inoculated with these refiners. The paper describes also changes of attenuation coefficient of the Al - 20 wt%zn alloy (AlZn20) after its grain refinement, which was unavailable in literature until recently. Experimental The examined alloys ZnAl25, AlZn20 and the master alloys Zn-4.6wt%Ti (ZnTi4) and Zn-20wt%Al-4wt%Ti (ZnAl-Ti4) were prepared from electrolytic aluminium (minimum purity 99.96%), electrolytic zinc (99.995%) and titanium sponge ( %, from Johnson Matthey Alfa). The AlZn20 and ZnAl25 alloys were melted in an electric resistance furnace, in an alumina crucible of 0.2 litre capacity. The AlZn20 melt was superheated to 50/2

3 ~720 o C, while the ZnAl25 melt to ~600 o C. After introducing a master alloy the melt was held for 2 minutes, then the melt was stirred for 2 minutes with an alumina rod, and the alloy was cast into a dried sand mould (Fig. 2(a)) or into a preheated graphite-chamote crucible (Fig. 2(b)). To monitor the cooling process - two thermocouples NiCr-NiAl mm were mounted in the sand mould cavity or were introduced from the top into the melt in a preheated graphite-chamote crucible. Temperatures (accuracy ± 1 o C) were recorded using a multi-channel recorder Agilent 34970A (Agilent Technologies Inc., USA). Microsections for LM examinations were ground on abrasive paper (grit ) and then were polished using sub-microscopic aluminium oxide in water-alcohol suspension. The AlZn20 samples, used in macrostructure examinations, were etched with Keller's reagent. LM observations of microstructures were performed using Leica- DM IRM microscope. SEM investigations were performed using ESEM Philips XL30 microscope equipped with an EDS system EDAX Gemini Another microscope used was SEM Jeol JSM 5800 WV equipped with Noran Voyager 3 EDS system. The examinations of damping properties were performed on the AlZn20 alloy using ultrasonic technique. The used samples were discs 32x7mm cut from the sand-castings (Fig. 2(a)), whose parallel surfaces were ground on abrasive paper of 600 grit. A DI-4P ultrasonic defectoscope (UNIPAN Poland), operating at constant frequency of 2.5 MHz, was used as a source of the longitudinal ultrasonic wave. The attenuation coefficient α was obtained using an echo-method. Results Figs 3(a) and 4(a) show microstructure of the sand-cast, unrefined ZnAl25 and AlZn20 alloys. It can be seen that the both alloys solidify with coarse, branched dendrites of the solid solution of Zn in Al. However, after inoculation the same alloys show highly refined microstructure, as it was shown in Figs 3(b) and 4(b). It should be noted that the refined structures were obtained after using the new ZnTi-based master alloys. Fig 5 and Fig. 6 show microstructures of the master alloys used in inoculation of the examined alloys, i.e. a binary Zn-4.6 wt%ti (ZnTi4) master alloy Fig. 5, introduced into the alloy ZnAl25 Fig. 3(b); and a ternary ZnAl-4 wt%ti (ZnAl- Ti4) Fig 6, introduced into the AlZn20 alloy Fig. 4(b). The chemical compositions of the Ti-based intermetallic particles, existing in microstructures of the ZnTi4 and ZnAl-Ti4 master alloys, are collected in Table 1. In Figs 7 and 8 there are shown results of temperature measurements of the examined alloys slowly solidifying in the graphite-chamote crucible the ZnAl25 alloy, or in the sand mould the AlZn20 alloy. The Figures 7-8 contain also first derivatives of the temperature after time. It can be seen that both the examined alloys solidify with reduced undercooling after inoculation with the ZnTi-based master alloys, which is typical after using a refiner of heterogeneous nucleation. Results of the ultrasonic examination of damping properties of the AlZn20 alloy are shown in Fig. 9. It is evident after comparing results from Fig. 9 with the macrostructures 50/3

4 shown in Fig. 10, that attenuation coefficient of the examined AlZn20 alloy decreases together with the decrease of the grain size. Discussion As noted in the introduction, the main refiners of Al alloys are those built on the systems Al-Ti-B and Al-Ti-C. For the Al-Ti-B MA the crucial role as a direct substrate of α-al nucleation is played by Al 3 Ti thin layer adsorbed on TiB 2 while TiC is the direct nucleant particle when Al-Ti-C master alloy is used [9]. This is because of the similar crystal structure and lattice parameters of α-al and Al 3 Ti or TiC. The presented here new family of grain refiners based on the Zn-Ti system has TiZn 3 phase in its structure Fig. 5, which has the same crystal symmetry as the α-al and closely matches its lattice parameter. In a Al-Zn melt the TiZn 3 phase transforms into a ternary, more stable, Ti(Al,Zn) 3, which has also the same features as TiZn 3, and whose particles appear to be active centres of heterogeneous nucleation in the inoculated Zn-Al alloys [10] (the heterogeneous nature of nucleation supports observed reduced undercooling of the inoculated alloys, as it was shown in Figs 7 and 8). This is the most probable reason why the ZnTi-based master alloys are effective as refiners of the examined Al-Zn alloys. It should be pointed out once more, that the ZnTi-based master alloys require relatively low melt temperatures, which is an advantage as compared with the traditional Al-Ti-B or Al-Ti-C master alloys. Macrostructures shown in Fig. 10 indicate that ternary master alloys AlTi5B1, AlTi3C0.15 and ZnAl-Ti4 cause strong refinement of the inoculated AlZn20 alloy, while the binary ZnTi4 master alloy causes rather moderate refinement. However, at the same time the AlZn20 alloy inoculated with the mentioned above ternary master alloys shows also strong decrease of its attenuation coefficient, as it is seen in Fig. 9, which is a disadvantage, when preserved high damping properties are important. As noted in [9], the inoculation is rather aimed at obtaining uniform, equiaxed grain structure, and not very fine grains per se. Taking into consideration observed decrease of damping together with the decrease of the grain size, one can conclude that the refinement process should be controlled to obtain a compromise between changes of strength and damping properties. Conclusions The elaborated new family of the ZnTi-based grain refiners shows high effectiveness of inoculating of high-aluminium zinc alloys (e.g. ZnAl25 inoculated with binary ZnTi4 MA) and high-zinc aluminium alloys (e.g. AlZn20 inoculated with ternary ZnAl-Ti4 MA). The observed refinement of the examined AlZn20 alloy, inoculated with the ZnAl-Ti4 MA, is of the same order as the observed one after using the traditional AlTi5B1 or AlTi3C0.15 master alloys. However, the ZnTi-based master alloys require lower melt temperatures, which allows avoiding detrimental overheating. The initial investigations of damping properties showed, that attenuation coefficient of the AlZn20 alloy decreases together with the decrease of the grain size. Thus, in case of the damping Al-Zn alloys the refinement should be performed to such extent that would allow improving strength 50/4

5 properties with the damping ones preserved. The detail investigations are in progress and will be presented elsewhere in a close future. References 1. R. Schaller, Metal matrix composites, a smart choice for high damping materials. Journal of Alloys and Compounds, 355, 2003, pp Y. Zhang, N. Ma, Y. Le and H. Wang, Mechanical properties and damping capacity after grain refinement in A356 alloy. Materials Letters, 59, 2005, pp I.G. Ritchie, Z-L Pan and F.E. Goodwin, Characterization of the damping properties of die-cast zinc-aluminium alloys. Metallurgical Transactions A, 22A, March 1991, pp S. Rzadkosz, Effect of chemical composition and phase transformation on the damping and mechanical characteristics of alloys from aluminium-zinc system. Habilitation thesis (in Polish). University of Mining and Metallurgy, Krakow W. Krajewski, Investigation of the high-aluminium zinc alloys grain refinement process due to Ti addition, Archives of Metallurgy, 44, 1999, pp W. K. Krajewski, A. L. Greer, T. E. Quested and W. Wolczynski, Solidification and Crystallization, chapter 16, Identification of the substrate of heterogeneous nucleation in Zn-Al alloy inoculated with ZnTi-based master alloy, pp , Euromat 2003 papers. Viley-VCH, Ed. D.M. Herlach, Köln 2004, ISBN W.K. Krajewski, A.L. Greer and J. Buraś, Grain refinement of Al- Zn alloys by melt inoculation with Zn-Ti based refiner, European Congress on Advanced Materials and Processes Euromat 2005, Prague, Czech Republic, 5-8 Sept, 2005 ( 8. A. E. Wol, Strojenije i swojstwa metalliczeskich sistem. Vol. 1. Fizmatgiz, Moskwa A.L. Greer, Grain refinement of aluminum alloys. In: Solidification of Aluminum Alloys. Proceedings of the TMS Annual Meeting, Charlotte, North Carolina, USA, March, 2004, pp W.K. Krajewski and A.L. Greer, EBSD Study of ZnAl25 alloy inoculated with ZnTi4 master alloy, Materials Science Forum. Vol. 508 Solidification and Gravity IV, 2006, pp Acknowledgements The authors acknowledge the KBN - Polish State Committee for Scientific Research for financial support under research grant 4 T08A The provision of laboratory facilities in the Department of Materials Science and Metallurgy, University of Cambridge, is also kindly acknowledged. 50/5

6 Tables Table 1. Chemical composition of the randomly chosen Ti-based particles existing in microstructures of the ZnTi4 and ZnAl-Ti4 master alloys. ZnTi4 MA Fig. 5 ZnAl-Ti4 MA Fig. 6 No. 1 No. 2 No. 3 No. 4 No. 1 No. 2 No. 3 No. 4 Ti, at. % Zn, at. % Al, at. % NORAN - Voyager 3 EDS EDAX - Gemini 4000 EDS Figures UTS, [MPa] SAND CHILL Elongation, [%] SAND CHILL Zn, [wt%] Zn, [wt%] Fig. 1. UTS and elongation of cast Al-Zn alloys as a function of Zn content. (Diagrams based on data published in [8]) (a) Riser Ø33/ Ø44x30 Casting Ø31/ Ø33x80 Sample Ø31x25 for LM, SEM, XRD, TEM T c centre thermocouple T w wall thermocouple Dry sand mould 50/6

7 (b) Graphite crucible 35/ 45 x 50 mm Melt T c centre thermocoule T w wall thermocouple Fig. 2. Schematic sketch of the system: (a) sand mould casting, with mounted thermocouples. (b) chamote graphite crucible with thermocouples introduced from the top into a melt. Fig. 3(a). Coarse microstructure of the initial sand-cast ZnAl25 alloy. Leica DM IRM LM. Fig. 3(b). Refined microstructure of the sand-cast ZnAl25 alloy inoculated with ZnTi4 MA (0.05 wt% Ti). Leica DM IRM LM. 50/7

8 500 µm 500 µm Fig. 4. (a) - Coarse microstructure of the initial sand-cast AlZn20 alloy. (b) - Refined microstructure of the sand-cast AlZn20 alloy inoculated with ZnTi4 MA (0.04 wt% Ti). Philips XL30 ESEM. Fig. 5. Microstructure of the binary ZnTi4 MA. JEOL JSM 5800 WV SEM. Chemical composition of the particles 1 to 4 are collected in Table µm Fig. 6. Microstructure of the ternary ZnAl-Ti4 MA. Philips XL30 ESEM. Chemical composition measured in areas 1 to 4 are collected in Table 1. 50/8

9 (a) 515 ZnAl (b) 515 ZnAl25Ti Temperature Tw, [ o C] Tw dtw/dt dtw/dt, [K/s] Temperature Tw, [ o C] Tw dtw/dt dtw/dt, [K/s] Time t, [s] Time t, [s] Fig. 7. Cooling curves of the ZnAl25 alloy solidifying in the chamotegraphite crucible shown in Fig. 2(b). (a) ZnAl25 alloy without Ti addition; and (b) ZnAl25 alloy inoculated with the ZnTi4 MA wt%Ti addition. Temperatures registered by T w wall thermocouple, dt w /dt first derivative of the T w temperature after time. (a) 630 AlZn (b) 630 AlZn20Ti Temperature T, [ o C] Tc Tw dtw/dt dtw/dt, [K/s] Temperature T, [ o C] Tw Tc dtw/dt dtw/dt, [K/s] Time t, [s] Time t, [s] Fig.8. Cooling curves of the AlZn20 alloy solidifying in the sand mould shown in Fig. 2(a). (a) Initial AlZn20 alloy and (b) AlZn20 alloy inoculated with ZnAl-Ti4 MA, 0.04wt%Ti addition, T w and T c accordingly, wall and centre temperatures, dt w /dt first derivative of the T w temperature after time [7]. 50/9

10 1 AlZn20 Attenuation coefficient, [db/mm] : Unrefined 2: AlTi5B1 3: AlTi3C0.15 4: ZnAl-Ti4 5: ZnTi4 Type of grain refiner Fig. 9. Attenuation coefficient of the: 1 initial, unrefined AlZn20 alloy, and 2 to 5 AlZn20 alloy with addition of 0.04 wt% Ti introduced with the examined refiners mm 4 5 Fig. 10. Macrostructures of the AlZn20 samples which attenuation coefficient is shown in Fig unrefined AlZn20 initial alloy; and the same alloy inoculated with addition of 0.04 wt% Ti introduced with the master alloy 2 AlTi5B1, 3 AlTi3C0.15; 4 ZnAl-Ti4, 5 ZnTi4. 1 To whom all correspondence should be adressed: krajwit@agh.edu.pl (W.K. Krajewski) 50/10