PROPERTIES OF EN AW-2024 WROUGHT ALUMINUM ALLOY AFTER CASTING WITH CRYSTALLIZATION UNDER PRESSURE Branislav Vanko 1, Ladislav Stanček 1, Michal Čeretka 1, Eduard Sedláček 1, Roman Moravčík 2 1 Institute of Technologies and Materials, Faculty of Mechanical Engineering, Slovak University of Technology in Bratislava, Námestie slobody 17, 812 31 Bratislava, Slovak Republic, branislav.vanko@stuba.sk, ladislav.stancek@stuba.sk, michal.ceretka@stuba.sk, eduard.sedlacek@stuba.sk 2 Institute of Materials Science, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Paulínska 16, 917 24 Trnava, Slovak Republic, roman.moravcik@stuba.sk Keywords: casting with crystallization under pressure, wrought aluminum alloy, 2024, heat treatment, mechanical properties Abstract Establishing of wrought aluminum alloys casting to manufacture is now a global trend, for example due to lower production costs compare to forging or due to the ability to produce parts with thinner sections and more complex shapes. The aim of using these alloys in the foundry industry is in particular the creation of castings with higher mechanical properties than achieve castings made of standard casting aluminum alloys. Most often are cast wrought aluminum alloys of the 2xxx, 6xxx and 7xxx series. In the experiment, an alloy EN AW-2024 has been cast by modified technology of casting with crystallization under pressure. They were measured basic mechanical properties of the castings in the as-cast state and after heat treatment. 1 INTRODUCTION Potential of casting aluminum alloys is probably already exhausted from a most part and perhaps even no new casting technology does not allow achieve higher mechanical properties of castings such as those we already know. In order to take advantages of foundry technologies compared with forging such as higher dimensional complexity and thinner sections of the products or lower production costs, began the trend of investigating utilizability of high strength wrought aluminum alloys in the manufacture of castings by different casting methods, for example CDS process [1], direct and indirect squeeze casting [2, 3], LSPSF rheoforming process [4, 5], or SIM process [6]. The most commonly examined are alloys of 2xxx, 6xxx and 7xxx series [7] because they offer high mechanical properties. The problem with the casting of these alloys is but (mostly) hot cracking (tearing) [8-10]. The aim of this research is to investigate the effect of basic parameters of casting with crystallization under pressure with forced flow [11] on mechanical properties of castings made of the EN AW-2024 wrought aluminum alloy (further only 2024 alloy). Part of the experiment is to verify the effectiveness of the proposed heat treatment T6.
2 MATERIAL AND METHODS The experimental material was prepared using four different ways of casting with crystallization under pressure with forced flow (Tab. 1). Castings shape is presented in Fig. 1. Table 1: Casting parameters. Casting number No. 1 No. 2 No. 3 No. 4 Melt temperature [ C] 650 650 700 700 Die temperature [ C] 200 200 250 250 Pressure [MPa] 100 200 100 200 Pressing time [s] 30 30 30 30 Figure 1: The shape of the castings. Chemical composition of the 2024 wrought aluminum alloy was Al 92.783 %, Cu 4.264 %, Mg 1.446 %, Mn 0.8 %, Fe 0.356 %, Si 0.22 %, Zn 0.06 %, Ti 0.025 %, Cr 0.013 %, and the rest were other elements contained only in trace amounts. Alloy was treated with covering flux during melting. When the melt temperature was at required value, the preheated die was opened and the melt was dosed into the die mounted on hydraulic press. Within few seconds (max. 5 sec.) of the beginning of dosing, the die was closed and the value of pressing force reached the preset maximum, which corresponds to the desired pressure in the die cavity. After 30 seconds of pressing the die was opened and the casting was ejected. Measured and evaluated were following quantities: melt temperature T, die temperature T D, punch displacement h, and pressure p h measured in a hydraulic system. The tensile specimens (Fig. 2) for tensile testing were made by milling axial sections of castings. Some of them were heat treated in a thermocouple controlled electric resistance muffle
furnace. Temperature was 495 C, and the time of the solution annealing was set at 3 hours. After removal from the furnace the specimens were quenched into water with a temperature of 20 C. Artificial aging was made in the same furnace at a temperature of 190 C, and the time was set at 12 hours. Cooling down of the tensile specimens after aging was carried out in the open air. Figure 2: The shape of tensile specimens for tensile testing. Mechanical properties were determined by the tensile testing which was carried out in accordance with the standard STN EN ISO 6892-1 at ambient room temperature on a universal electromechanical testing machine with the crosshead separation rate of 0.5 mm.min -1. The tensile strength R m, yield strength R p0.2, as well as the percentage elongation to fracture (further only elongation) A was determined. 3 RESULTS AND DISCUSSION In the experiment, influence of the melt and die temperature, and the pressure during the solidification on the mechanical properties of the castings was investigated. From the records of casting were evaluated these parameters: melt temperature at the pressuring start, solidification time in the middle of the casting wall (interval between the melt temperature at the pressuring start and the equilibrium temperature of solidus 503 C), initial and average cooling rate during solidification in the middle of the casting wall, punch movement and average punch speed during solidification (Tab. 2). These parameters determine the conditions of solidification and exactly classify the production process of each casting. On the basis of these parameters it is possible during repeated cycles detect deviations in the casting (solidifying) and cannot come to an error in reproducibility of castings. The melt has at each casting a slightly different temperature at the pressuring start (Tab. 2) and thus different initial ratio of the liquid and solid phase (Tab. 3). Fraction of solid (liquid) phase was determined approximately on curve of DSC analysis [12]. A different amount of solid phase during the die closure and initial pressure application provide differences in the efficiency of utilization of punch movement to fragmentation of dendrites and their spheroidization through shear stresses according to the Newton's law of viscosity. Such modified non-dendritic microstructure (Fig. 3) ensure higher permeability of the skeleton of solid phase for better feeding of liquid phase to the areas of final solidification. This procedure prevent the formation of hot cracks (tears) and allow their treatment until the final stage of solidification. According to the
results of processing aluminum alloys in semi-solid state the non-dendritic structure is capable to provide castings also higher mechanical properties [4]. The resulting mechanical properties of the castings in the as-cast and heat treated state are shown in Tab. 4 and Fig. 4. Table 2: Evaluated casting parameters. Casting number No. 1 No. 2 No. 3 No. 4 Melt temperature at the pressuring start [ C] 619 617 631 632 Solidification time [s] 4 4.3 7.8 7.7 Initial cooling rate [ C.s -1 ] 565 550 390 383 Average cooling rate during solidification [ C.s -1 ] 29 26.5 16.4 16.8 Punch movement during solidification [mm] 4.3 5.5 5.3 6.5 Average punch speed during solidification [mm.s -1 ] 1.1 1.3 0.7 0.8 Table 3: Initial ratio of the solid and liquid phase at the pressuring start. Casting number No. 1 No. 2 No. 3 No. 4 Initial fraction of solid phase [%] 47 49 32 31 Initial fraction of liquid phase [%] 53 51 68 69 Figure 3: Typical microstructure in the middle of the casting walls (casting No. 2). Table 4: Mechanical properties of the castings in the as-cast and heat treated state. Casting state Casting number No. 1 No. 2 No. 3 No. 4 Rp0.2 [MPa] 120 108 115 106 As-cast Rm [MPa] 293 317 309 340 A [%] 2 3.5 2.5 4.5 Rp0.2 [MPa] 211 368 378 344 Heat treated Rm [MPa] 356 420 418 400 A [%] 2 2.5 2.5 3
Figure 4: Tensile strength (R m ), yield strength (R p0.2 ), and elongation (A) in the as-cast and heat treated state. The effect of the melt and die temperature (i.e. the effect of the initial fraction of solid phase during pressure application, cooling rate during solidification, solidification time, punch movement and its speed) on the mechanical properties of the castings in the as-cast state is clear. Castings made of lower melt and die temperature (no. 1 and 2) in compare with castings made of higher melt and die temperature (no. 3 and 4) in the as-cast state (according to the level of pressure, castings no. 1 and 3 casted under pressure of 100 MPa and castings no. 2 and 4 casted under pressure of 200 MPa) showed lower tensile strength and elongation, but higher yield strength. In the as-cast state, the highest tensile strength and elongation was obtained in the casting no. 4 (340 MPa, 4.5 %). Yield strength in the casting no. 4 in the as-cast state (106 MPa) was the lowest of all castings, but only 14 MPa below the highest yield strength in the as-cast state in the casting no. 1 (120 MPa). After heat treatment, the positive effect of higher melt and die temperature on all observed mechanical properties was showed only in the pair of castings no. 1 and 3. Casting no. 4
cast under pressure of 200 MPa showed a very low sensitivity to heat treatment and final tensile strength (400 MPa) and a yield strength (344 MPa) were lower than in the casting no. 2 (tensile strength of 420 MPa and yield strength of 368 MPa), even were lower than in the casting no. 3 (tensile strength of 418 MPa and yield strength of 378 MPa), which was cast with the same melt and die temperature, but at a lower pressure of 100 MPa. The lowest values of all observed mechanical properties after heat treatment were achieved in the casting no. 1 (tensile strength of 356 MPa, yield strength of 211 MPa and elongation of 2 %). After the heat treatment, the highest tensile strength was achieved in the casting no. 2 (420 MPa), yield strength in casting no. 3 (378 MPa) and elongation in the casting no. 4 (3 %). Casting with the highest rating in terms of mechanical properties obtained after heat treatment is casting no. 3, the tensile strength of 418 MPa is only about 2 MPa less than the highest achieved tensile strength (420 MPa) in the casting no. 2, yield strength 378 MPa is the highest achieved value and elongation of 2.5 % is only 0.5 % less than the highest achieved elongation (3 %) in the casting no. 4. The effect of the higher pressure during solidification on the mechanical properties of castings in the as-cast state is also clear. The values of tensile strengths and elongations in the ascast state were higher in both pairs of castings (no. 1 and 2, and no. 3 and 4) if it was applied the higher pressure to the solidifying melt, what is in accordance with the theoretical and practical knowledge of the solidification of wrought aluminum alloys under pressure [13]. This does not apply to the yield strength, which was lower in the castings produced under higher pressure (no. 2 and 4). The benefits of the higher applied pressure during solidification of castings is the better elongation in the as-cast and heat treated state too. The positive effect of the higher pressure during solidification was observed on all mechanical properties of the heat treated castings no. 1 and 2. Expected benefit of the higher pressure in the castings no. 3 and 4 after the heat treatment was showed only on elongation. The values of tensile and yield strength in the casting no. 4 cast under pressure of 200 MPa were below the casting no. 3 cast under pressure of 100 MPa. Castings no. 2, 3 and 4 achieve the tensile strength values comparable to the results of other laboratories [4, 13]. For example, the tensile strength of the casting no. 3 in the as-cast state (309 MPa) is about 3 MPa higher than the highest tensile strength achieved in the experiment [4], and the tensile strength of the heat treated casting no. 3 (418 MPa) is only 10 MPa lower than the highest tensile strength achieved in the same experiments. Yield strength of the casting no. 3 in the as-cast state (115 MPa) is up to 83 MPa below the maximum value of the experiment [4], but the yield strength of heat treated casting no. 3 (378 MPa) is 57 MPa higher than the highest yield strength achieved in these experiments. Compared with the results of other laboratories the elongation in the as-cast state (2 to 4.5 %) and even after heat treatment (2 to 3 %) is on very low level. Typical elongation of castings made of 2024 alloy in the as-cast state is within the range of 6.7 to 10.4 % [4] and after heat treatment of 8 to 13 % [4, 13]. 4 CONCLUSION In the experiments, 2024 wrought aluminum alloy was successfully cast via casting with crystallization under pressure with forced flow. The highest tensile strength and elongation in ascast state was achieved in the casting no. 4 (340 MPa, 4.5 %). Yield strength of this casting in the
as-cast state was only 106 MPa. After the heat treatment T6 give the highest mechanical properties casting no. 3, tensile strength of 418 MPa, yield strength of 378 MPa, and elongation of 2.5 %. The effect of higher melt and die temperature on the tensile strength and elongation of castings in the as-cast state can be evaluated on both pairs of castings (no. 1 and 3, and no. 2 and 4) as a positive, a negligible negative effect this had only on yield strength. The positive effect of higher melt and die temperature on all observed mechanical properties after heat treatment can be evaluated only by comparing the castings no. 1 and 3. The effect of higher pressure applied during solidification on the tensile strength and elongation of castings in the as-cast state can be also evaluated on both pairs of castings (no. 1 and 2, and no. 3 and 4) as a positive, but again this had a negative effect on yield strength. The positive effect of the higher pressure applied during solidification on all observed mechanical properties after heat treatment can be evaluated only by comparing the castings no. 1 and 2. Values of the tensile and yield strength of observed castings (no. 2, 3 and 4) are comparable with other laboratories, but elongations are at very low level. It is necessary, in additional experiments aim to increase it. It is also appropriate to verify the measurement of the casting no. 4 to determine if the measured values of mechanical properties after heat treatment are relevant. REFERENCES [1] Shankar, S., Saha, D., Makhlouf, M. M., Apelian, D.: Casting of wrought aluminumbased alloys. Die Casting Engineer, March 2004, p. 52-56. [2] Chinnathambi, K.: Study on mass effect of indirect squeeze cast 2014 wrought aluminum alloy. Die Casting Engineer, September 2006, p. 40-47. [3] Hajjari, E., Divandari, M.: An investigation on the microstructure and tensile properties of direct squeeze cast and gravity die cast 2024 wrought Al alloy. Materials and Design, 29 (2008), p. 1685-1689. [4] Guo, H., Yang, X., Zhang, M.: Microstructure characteristics and mechanical properties of rheoformed wrought aluminum alloy 2024. Transactions of Nonferrous Metals Society of China, 18 (2008), p. 555-561. [5] Guo, H., Yang, X.: Preparation of semi-solid slurry containing fine and globular particles for wrought aluminum alloy 2024. Transactions of Nonferrous Metals Society of China, 17 (2007), p. 799-804. [6] Yanlei, L., Yuandong, L., Chun, L., Huihui, W.: Microstructure characteristics and solidification behavior of wrought aluminum alloy 2024 rheo-diecast with selfinoculation method. China Foundry, Vol. 9, No. 4 (2012), p. 328-336. [7] Curle, U. A.: Semi-solid near-net shape rheocasting of heat treatable wrought aluminum alloys. Transactions of Nonferrous Metals Society of China, 20 (2010), p. 1719-1724. [8] Eskin, D. G., Suyitno, Katgerman, L.: Mechanical properties in the semi-solid state and hot tearing of aluminium alloys. Progres in Materials Science Forum, 49 (2004), p. 629-711.
[9] Viano, D., StJohn, D., Grandfield, J., Cáceres, C.: Hot tearing in aluminium copper alloys. Light metals 2005. TMS (The Minerals, Metals & Materials Society), 2005, p. 1069-1073. [10] Kamguo Kamga, H., Larouche, D., Bournane, M., Rahem, A.: Hot tearing of aluminumcopper B206 alloys with iron and silicon additions. Materials Science and Engineering A, 527 (2010), p. 7413-7423. [11] Stanček, L., Vanko, B.: Instrument for casting with crystallization under pressure with increasing of flow intensity. 10 th International Conference Technology 2007, September 19 20, Bratislava, Slovakia, 2007. (In Slovak) [12] Kim, W. Y., Kang, C. G., Kim, B. M.: The effect of solid fraction on rheological behavior of wrought aluminum alloys in incremental compression experiments with a closed die. Materials Science and Engineering A, 447 (2007), p. 1-10. [13] Zhong, Y., Su, G., Yang, K.: Microsegregation and improved methods of squeeze casting 2024 aluminium alloy. Journal of Materials Science & Technology, Vol. 19, No. 5, 2003, p. 413-416. ACKNOWLEDGEMENT We would like to thank the Slovak grant agency VEGA for the financial support of the project VEGA 1/0876/14 Study of obtaining spheroidal morphology of primary solid solution of wrought aluminum alloy and its effect on mechanical properties.