The Effect of Microstructure on Mechanical Properties of Forged 6061 Aluminum Alloy

Transcription

1 Proceedings of the 9 th International Conference on Aluminium Alloys (2004) Edited by J.F. Nie, A.J. Morton and B.C. Muddle Institute of Materials Engineering Australasia Ltd 1382 The Effect of Microstructure on Mechanical Properties of Forged 6061 Aluminum Alloy N. Hosoda, M. Nakai, T. Eto Kobe Steel Ltd., Takatsukadai, Nishi-ku, Kobe , Japan Keywords : 6061 aluminum alloys, Zener-Hollomon parameter, subgrains, yield stress, fracture toughness, constituents Abstract A novel process to improve mechanical properties of forged 6061 aluminum alloy has been developed. The retention of subgrains developed during hot forging at low Z, Zener-Hollomon, parameter conditions leads to enhancement of mechanical properties compared to those of conventionally recrystallized ones. The yield strength of the subgrained 6061 aluminum alloy is 350MPa, which is higher than 6X51 aluminum alloys with excess Si. The strengthening stems from dislocation strengthening and fine grained strengthening through Hall-Petch relation. Fracture toughness of the subgrained 6061 aluminum alloy, which breaks by transgranular fracture, is 10% higher than that of recrystallized ones, which fail by intergranular fracture. It is proposed that development of Al-Mg-Si alloys with excellent mechanical properties requires a subgrain structure that can be achieved by hot-forging at low Z parameter conditions. 1. Introduction In 6XXX aluminum alloys, high strength is obtained by increasing the amount of Mg 2 Si. It is reported that, however, there are problems of reduced fracture toughness and corrosion resistance in 6X51 type aluminum alloy with excess Si [1]. The requirement is for materials which have not only high strength but high fracture toughness and corrosion resistance for applications. It is known that those aluminum alloys which have high stacking fault energy easily form subgrains due to polygonization during hot working. It is assumed that mechanical properties are affected by these subgrains formed during hot working. The object of the present paper was to clarify the interrelationship among alloy/process-subgrain microstructure and mechanical properties combined with micromechanics, and to improve strength and fracture toughness of forged 6061 aluminum alloy having good corrosion resistance.

2 Processing and Experimental Procedure 2. Material and Methods The present work has been carried out on an AA6061 alloy. The chemical composition is shown in Table 1. Table 1: Chemical composition of 6061 aluminum alloy. (wt%) Si Fe Cu Mn Mg Cr Zn Ti Zr Al < < <0.01 bal. The material which was prepared by melting and DC casting as φ80mm billets was homogenized at 500 for 4hours, and was then hot forged. The hot forging conditions were as follows: All the specimens were hot forged at 350, 400, 500, at strain rates of s -1, s -1, with 75% reduction. In this work, the hot forging conditions are expressed by Zener-Hollomon parameter, as shown by the following equation: Z (dε / dt) exp(q / RT), (1) where Z is Zener-Hollomon parameter, dε / dt is the strain rate, T is the temperature in hot forging, Q is the activation energy, R is the gas constant. The hot forged specimens were solution heat treated at 540 for 3h and subsequently quenched in water at 25. The cooling rate from 400 to 290 was 250 /s. All the specimens were then aged at 180 for 8h in T6 temper. Three specimens were used in this work, High Z, Middle Z and Low Z. Tensile tests were carried out at s -1 in the long transverse (LT) direction using a JIS Z 2241 test piece. The fracture toughness Kc was estimated by the following equation [2]: Kc=36.6 NTR-20.82, (2) where NTR is Notch tensile strength / Tensile yield strength Ratio. The mechanical properties, yield strength and fracture toughness, of Middle Z and Low Z were higher than that of High Z. Microstructure characterizations of the specimens were made by optical microscopy (OM) and scanning electron microscopy (SEM). OM observations were prepared using electrochemical etching and caustic soda etching. SEM observations were carried out using the specimens after fracture toughness tests. 3.1 Subgrain Microstructure 3. Results and Discussion Figure 1 (A-C) shows OM micrographs of the conventional T6 tempered specimens, these show the coarse grains at approximately 300µm in High Z (A), the fine subgrains at 7µm in Middle Z (B) and the grown up subgrains at 15µm in Low Z (C) respectively. The formation of subgrains depends on Z parameter which is represented by equation (2), as shown in Figure 2.

3 1384 A B C 200µm 20µm Figure 1: Optical micrographs showing the effect of hot forging on the grain or subgrain structure of T6 tempered 6061 aluminum alloy. (A) High Z; (B) Middle Z; (C) Low Z. The subgrain structure was developed during hot forging at low Z parameters, development of subgrains was especially significant in the range less than s -1. In alloys with high stacking fault energies, such as aluminum alloys, the recovery kinetics of dislocation structures occurred during hot working, and there are possibilities of cross-slip or dislocation climb. The dislocations are typically arranged, after deformation, in the form of a cell structure, the cell walls being complex dislocation tangles, this process is sometimes referred to as polygonization. During recovery, a significant number of the dislocations migrate away from the pile-ups 0 1.0E E E E E E E+11 and rearrange themselves to form the subgrain boundaries [3]. In the hot forging conditions at low Z parameters the recovery kinetic is more active and, as a result, nucleation of recrystallization does not occur and subgrain structure is obtained. In addition, lower Z parameters lead to more complete polygonization, and the subgrain size gradually increases, as shown in Figure 2(c). 3.2 Mechanical Properties Sub-grains rate (%) Low Z Sub-grained Middle Z Recrystallized High Z Zener-Hollomon parameter (s -1 ) 6061-T6 sub-grained recrystaliized Figure 2: The effect of Z parameter on development of subgrains. High yield strength and estimated fracture toughness Kc were obtained by retention of subgrains as shown in Figure 3. The yield strength of recrystallized High Z was 293MPa, and it was almost the same as that of conventional T6 tempered 6061 aluminum alloy. In subgrained ones as Middle Z and Low Z, however, these were 348MPa and 324MPa respectively, and these values are higher than Al-Mg-Si alloys with excess Si, such as 6X51 type Al-Mg-Si alloys. Estimated fracture toughness Kc of High Z, Middle Z, Low Z was 38.3MPam 1/2, 40.0 MPam 1/2, 42.0 MPam 1/2 respectively. Subgrain leads to the enhancement of mechanical properties. 3.3 Subgrain Strengthening Figure 4 shows the influence of the subgrain or grain sizes on yield stress. Generally, the Hall-Petch relationship was apparent as shown by the following equation (3): σ=σ 0 +K y d -1/2, (3)

4 T6 sub-grained recrystaliized Sub-grained Low Z T6 sub-grained recrystaliized Middle Z Kc (MPa m 1/2 ) 40 σ 0.2 (MPa) 320 Low Z 39 High Z Middle Z Recrystallized High Z Figure 3: The effect σ 0.2 of (MPa) sub-grain structure on mechanical properties Figure 4: Strengthening d -1/2 (mm by -1/2 ) refining grain or subgrain sizes as shown by Hall-Petch relation. where σ is yield strength, σ 0 is yield strength of the materials with single crystal, K y is the constant and d is the subgrain or grain sizes. It is assumed that equation (3) shows an increase with resistance to the moving of the dislocations on the subgrain or grain boundaries in connection with a decrease in the subgrain or grain sizes. It was reported that there was the relationship in a wide range of coarse grains at 200µm to the fine grains at approximately 20µm in recrystallized 6061 aluminum alloys [4]. The retention of subgrains leads to a marked effect on the strengthening, compared with the fine grain strengthening. The subgrain strengthening is up to 50MPa by comparison with recrystallized 6061 aluminum alloys. According to a conventional study [3, 6-7], the subgrain strengthening was found to be more useful than the fine grain strengthening in the range of less than 10µm subgrains. In this study, however, there was the potential that the 15µm subgrains were a sufficient strengthening mechanism. It is assumed that the subgrain strengthening depends on the enhancement of the dislocations in conjunction with the fixing force represented by the Hall-petch relationship in equation (3), the subgrain boundaries are more resistant to the moving of the dislocations than the grain boundaries. The strengthening with fining subgrains depends on an increase in resistance to the dislocations due to the enhancement of dislocation density. In the iron alloys, the fine grain strengthening was found to be more useful than the subgrain strengthening [8-9], but it was found that the subgrain strengthening is an extremely useful mechanism in this alloy. A B C 20µm Figure 5: SEM fractographs of specimens after fracture toughness test for 6061 aluminum alloys with High Z (A), Middle Z (B) and Low Z (C).

5 The Effect of Microstructure on Fracture Toughness Kc As shown in Figure 3, estimated fracture toughness Kc of subgrained Middle Z and Low Z were higher than for recrystallized High Z. In order to clarify the reason why estimated fracture toughness Kc was increasing, the fracture surfaces were investigated. Figure 5 shows SEM fractographs of three specimens after fracture toughness testing. The fractograph (A) of recrystallized High Z showed an intergranular mode of fracture by cleavage, accordingly the estimated fracture toughness Kc was lower. On the other hand, subgrained Middle Z (B) and Low Z (C) with high fracture toughness Kc showed equiaxed dimples initiating from cracked constituents spacing, and a transgranular mode of fracture was dominant. It was reported that the subgrains lead to highest fracture toughness due to a decrease with stress concentration to the boundaries [10-11]. On the other hand, recrystallized High Z is subjected to an intergranular mode of fracture due to the enhancement of concentrating stress to the grain boundaries. Figure 6 shows schematic illustrations of fracture modes of Middle Z (a) and Low Z(b), respectively. The dimples in Low Z (b) are larger in size than those in Middle Z (a), as shown in Figure 5 and 6. Fracture occurred by incorporated dimples, the dimple linkage is delayed with an increase in the dimple sizes [2]. In Middle Z (a) with high yield strength, the dimple sizes are lower by fracture nucleation on the fine constituents (b). In the Low Z (b), the fine constituents did not show fracture nucleation, so that the dimple sizes of Low Z (b) were increased, and the spacing of fracture nucleation S2 was wider than S1 in Middle Z (a). As a result, fracture toughness for Low Z was higher than that for Middle Z. Dimples Spacing of constituents, S1 Constituents Fracture surface Spacing of constituents, S2 Figure 6: Schematic figures of fracture modes for 6061 aluminum alloys with Middle Z and Low Z. Subgrains lead to a high level of mechanical properties in 6061 aluminum alloy. It was reported that subgrains lead to a high level of corrosion resistance and mechanical properties after corrosion [12]. We concluded that development of subgrains is useful for improving mechanical properties and corrosion resistance on forged 6061 aluminum alloy. 4. Conclusions The effect of microstructure on mechanical properties of 6061 aluminum alloy was investigated. 1. A subgrained structure was obtained by hot forging at low Z conditions, at high temperature or low strain rate. The retention of subgrains was an extremely novel process to improve strength and fracture toughness. 2. Yield strength of subgrained 6061 alloy, Middle Z and Low Z were 348MPa and 324MPa respectively, due to subgrain strengthening. The development of subgrains leads to higher strength than Al-Mg-Si alloys with excess Si, such as 6X51 type Al-Mg-Si alloys. It is assumed that the subgrain boundaries are more effective in impeding moving dislocations than grain boundaries. 3. Fracture toughness Kc of subgrained 6061 was higher than that of recrystallized ones. It is assumed that subgrains lead to higher fracture toughness due to a decrease in stress concentration at the boundaries, in comparison with high stress concentrations in coarse grains.

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