Semi-Solid Slurry Casting Using Gas Induced Semi-Solid Technique to Enhance the Microstructural Characteristics of Al-4.3Cu Alloy

Transcription

1 Semi-Solid Slurry Casting Using Gas Induced Semi-Solid Technique to Enhance the Microstructural Characteristics of Al-4.3Cu Alloy M. Abdi 1a, S.G. Shabestari 2, b * 1 Ph.D. student, Center of Excellence for High Strength Alloys Technology (CEHSAT), School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST) Tehran, Iran 2 Professor, Center of Excellence for High Strength Alloys Technology (CEHSAT), School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST) Tehran, Iran a mohsen_abdi@metaleng.iust.ac.ir, b shabestari@iust.ac.ir Keywords: Al-4.3%Cu, long freezing range alloy, Thermal analysis, Gas Induced Semi-Solid (GISS). Abstract. Semi-solid processing of Al-4.3%Cu (A206) alloy was performed by Gas Induced Semi-Solid (GISS) process in different condition. The flow rate of argon gas, starting temperature for gas purging (the temperature of superheated-melt) and the duration of gas purging were three key process variables which were changed during this investigation. It was found that inert gas purging near liquidus, significantly, led to the microstructural modification from fully dendritic to globular structure. Thermal analysis was successfully implemented through CA-CCTA technique to understand the cause of the microstructure change during GISS process. The results showed that gas purging into the melt leads to temperature drop of the melt to its liquidus just after a few seconds from start of gas purging. In fact, copious nucleation was induced by cooling effect of inert gas bubbles. Microstructural features were characterized in semi-solid as well as on conventionally cast samples. The optimum gas purging temperature, injection time, and inert gas flow rate was determined in semi-solid processing to obtain the best globularity in the microstructure of a long freezing range alloy. However, the microstructure of the conventionally cast sample was fully dendritic with shrinkage which affects the soundness of casting products. Introduction Aluminum alloys are vital choices for the automotive industry due to the growing demand for more fuel-efficient vehicles to reduce energy consumption and air pollution [1, 2]. Al-Cu based alloys are commonly used in automotive industry. These alloys achieve mechanical properties similar to many commonly used cast irons and a significantly greater specific strength [3]. As known, Al-Cu alloys have a wide freezing temperature range [4]. Therefore, casting of Al-Cu alloys via conventional methods leads to defects in castings [5, 6]. Flemings et al. [7-9], introduced semi-solid as a new concept in metals in Since then, a variety of semi-solid techniques have been developed. Semi-solid processes postpone the onset of dendrites impingement and lead to produce sound casting parts [10]. Semi-solid processes are cheaper compared to other metal forming processes. Semi-solid processes are less energy-intensive and lead to superior quality [11].

2 In this study, the effect of semi-solid process on the solidification parameters and microstructural features has been investigated in Al-4.3% as conventionally cast sample to study solidification characteristics. Experimental Material and Melting Cu alloy. Thermal analysiss was also implemented for semi-solid samples as well Al-4..3%Cu casting alloy was used in this investigation. The alloy was prepared by adding a pre-determinate amount of pure cathodic copper in the form of thin strip to 5kg molten aluminum in an induction melting furnace at temperature of 780 c. The melt was protected with coveralll flux. The produced alloy was cast as ingots. The chemical composition of prepared ingots is given in Table 1. The manufactured ingots were cut to small pieces of 300g. Table 1. Chemical composition of Al-4.3%Cu alloy. Elements (wt Pct.) Cu 4.33 Fe 0.12 Si 0.08 Al Bal. Semi-Solid Process Gas induced semi-solid (GISS) technique was conducted in this research. This process is carried out by purging an inert gas in ambient temperature into the molten metal via a graphite diffuser [12, 13]. A schematic of the process equipment which was used in this study is illustrated in Fig. 1. Fig. 1. Schematic of the equipment used for GISSS process.

3 Inert gas flow rate, gas purging duration and temperature are 3 key parameters in GISS process. During this study, an argon gas was used and its flow rate was changed from 1 to 4 L.min -1. Gas purging durations were chosen from 10 to 30 second and the starting temperatures for gas purging or in other words the superheats of the melt were changed between 660 to 670 C. The samples nomenclatures as well as their respective semi-solid parameters are shown in Table 2. Table 2. The nomenclatures of semi-solid samples and their respective GISS variables. Nomenclature Starting temperature ( C) Gas purge duration (s) Gas flow rate (L/min) S-1L S-1L S-1L S-2L S-2L S-2L S-4L Thermal Analysis Thermal analysis was implemented on gas GISS samples as well as conventionally cast sample via CA-CCTA 1 technique which was described elsewhere [14, 15]. It is worth mentioning that to eliminate errors during thermal analysis the thermo-couples were fixed by a refractory tube during GISS process. Microstructural Evaluation GISS samples as well as conventionally cast sample were sectioned and prepared through standard metallography processes and were etched via Keller s reagent (10ml hydrofluoric acid, 20ml hydrochloric acid, 40ml nitric acid, and 130ml distilled water). The microstructures were studied by an optical microscope. Image J image analyzer software was used to quantify the microstructural features. Result and Discussion Microstructure Fig.2 shows the microstructures of conventionally cast Al-4.3%Cu in a thin steel cup. The microstructure of Al-4.3%Cu alloy consists of primary α-al dendrites and inter-dendritic Al-Al 2 Cu eutectic phases (Fig. 2(a)). Shrinkage porosity also exists in the alloy microstructure (Fig. 2(b)). 1.Computer Aided-Cooling Curve Thermal Analysis

4 Fig. 3. Microstructures of Al-4.3%Cu GISS samples. a) S-1L. b) S-1L. c) S-1L. d) S-2L. e) S-2L. f) S-2L. Fig. 2. Microstructures of Al-4.3% %Cu alloy conventionally cast in a thin steel cup. The microstructures of GISS samples are illustrated in Fig. 3. As seen, the microstructure has been changed from dendritic to globular through GISS process.

5 Treating Al-4.3% %Cu alloy via GISS process has significantly decreasedd casting defects like inter-dendritic shrinkage. The average grain sizes and shape factors for GISS samples are shown in Fig. 4. Fig 4. Average grain sizes and shape factors of GISS samples It is clearly observed that by increasing the inert gas flow rate and the gas purgingg duration, the average grain size decreases and the globularity increases which leads to the microstructural improvement. The best globularity and grain size occurred when the alloy was treated at 670 C with inert gas injection of 2 L.min -1 for 20 seconds. This happens becausee of the increasing of the heat extraction from the melt and the agitation which result in the homogeneity in temperature and composition of the melt. Therefore, numerous nucleation [16] and multi- rate of directional grain growth occur and cause fine globular grain structure. Thermal Analysis The cooling curve of conventionally cast Al-4.3%Cu alloy in a thin steel cup at cooling 0.25 Css -1 and its first and second derivative curves are shown in Fig. 5. According to this figure, the solidification starts by the nucleation of primary α-al dendrites at 652 C. The solidification continues as the melt cools down and the remained melt enriches with copper. Finally, the enriched melt solidifies with the formation of Al-Al 2 2Cu eutecticc phases in inter-dendritic spaces at 541 C. As seen, the freezing temperature range of Al-4.3%Cu alloy is 111 C and therefore, it is a long freezing range alloy and suitable to be treated through semi-solid process. The cooling curve of one of the GISS samples (670-20S-4L) is illustrated in Fig. 6. According to this figure, a sharp drop in temperature occurs simultaneous with the commencing the inert gas purging into the molten metal via the graphite diffuser. This part of the curve is enlarged in Fig. 6(b) for better observation. The results showed that the melt temperature dropped to liquidus temperature at once the gas purging was started. Then, the rate of temperature drop decreased because of the latent heat development due to α-al nucleation. The cooling effect of the inert gas bubble induces copious nucleation which results in a globular and fine grain structure and decreases the casting defects.

6 Fig. 5. Thermal analysis parameters of conventionally cast Al-4.3%Cu at cooling rate of 0.25 Cs -1 Fig. 6. Cooling curve of GISS sample of S-4L Conclusions Al-4..3%Cu casting alloy was treated through GISS semi-solid technique and the effects of GISS parameters on the solidification characteristics and microstructure of the alloy were studied. The main results are as follows: 1. Gas inducedd semi-solid treating of molten metal leads to microstructural change from dendritic to globular.

7 2. The best globularity and grain size occurred when Al-4.3%Cu alloy was GISS treated at 670 C with inert gas injection of 2 L/min for 20 seconds. 3. The cooling curve of the GISS sample shows a significant drop in temperature simultaneous with the commencing the inert gas purging into the molten metal. The rate of temperature drop decreases as the temperature reaches to liquidus due to the latent heat development of α-al nucleation. References [1] W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P. De Smet, A. Haszler, A. Vieregge, Recent development in aluminium alloys for the automotive industry, Materials Science and Engineering A280 (2000) [2] A. Lombardi, D. Sediako, A. Machin, C. Ravindran, R. MacKay, Effect of solution heat treatment on residual stress in Al alloy engine blocks using neutron diffraction, Materials Science & Engineering A 697 (2017) [3] A. Lombardi, W. Mu, C. Ravindran, N. Dogan, M. Barati, Influence of Al2Cu morphology on the incipient melting characteristics in B206 Al alloy, Journal of Alloys and Compounds 747 (2018) [4] S.Geng, P.Jiang, X.Shao, G.Mi, H.Wu, Y.Ai, C.Wang, C.Han, R.Chen, W.Liu, Comparison of solidification cracking susceptibility between Al-Mg and Al-Cu alloys during welding: A phase-field study, Scripta Materialia 150 (2018) [5] J.Wannasin, D.Schwam, J.A.Yurko, C.Rohloff, G.Woycik, Hot tearing susceptibility and fluidity of semi-solid gravity cast Al-Cu alloy, Solid State Phenomena (2006) [6] S.Janudom, J.Wannasin, P.Kapranos, S.Wisutmethangoon, The effect of hot tearing in semi-solid casting of aluminum A201 alloy, Advanced Materials Research 739 (2013) [7] D.B.Spencer, R.Mehrabian, M.C.Flemings, Rheological behavior of Sn-15 Pct. Pb in the crystallization range, Metallurgical Transactions 3 (1972) [8] M.C.Flemings, R.G.Riek, K.P.Young, Rheocasting, Materials Science and Engineering 25 (1976) [9] M.C.Flemings, Behavior of metal alloys in the semisolid state, Metallurgical Transactions A 22A (1991) [10] M.H.Ghoncheh, S.G.Shabestari, Effect of cooling rate on the dendrite coherency point during solidification of Al2024 alloy, Metallurgical and Materials Transactions 46A (2015) [11] S.Nafisi, R.Ghomashchi, Semi-Solid Processing of Aluminum Alloys, Springer, 2016, Chapter 1. [12] J.Wannasin, R.A.Martinez, M.C.Flemings, A novel technique to produce metal slurries for semi-solid metal processing, Solid State Phenomena (2006) [13] J.Wannasin, R.A.Martinez, M.C.Flemings, Grain refinement of an aluminum alloy by introducing gas bubbles during solidification, Scripta Materialia 55 (2006) [14] M.Malekan, S.G.Shabestari, Computer-aided cooling curve thermal analysis used to predict the quality of aluminum alloys, Thermal Analysis and Calorimetry 103 (2011) [15] M.H.Ghoncheh, S.G.Shabestari, M.H.Abbasi, Effect of cooling rate on the microstructure and solidification characteristics of Al2024 alloy using computer-aided thermal analysis technique, Thermal Analysis and Calorimetry 117 (2014) [16] F.Czerwinski, Modern aspects of liquid metal engineering, Metallurgical and Materials Transactions B 48 (2017)