Microstructural Evolution of 6061 Alloy during Isothermal Heat Treatment

Transcription

1 J. Mater. Sci. Technol., 2011, 27(1), Microstructural Evolution of 6061 Alloy during Isothermal Heat Treatment Na Wang 1), Zhimin Zhou 1) and Guimin Lu 2) 1) School of Sciences, Northeastern University, Shenyang , China 2) School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai , China [Manuscript received October 13, 2009, in revised form January 1, 2010] The semi-solid billet of 6061 aluminum alloy was prepared by the near-liquidus semi-continuous casting (LSC) with rosette or near-spheroide grains. The pre-deformation processing was applied before partial remelting to further improve the microstructure and properties of the semi-solid alloy. The effects of different processing parameters, such as holding temperature and holding time, on the semisolid microstructures during partial remelting have been investigated. It was found that the optimal partial remelting parameters should be 630 C and min for 6061 alloy cold rolled with 60% reduction in height of pre-deformation. The coarsening rates were anasysed by Lifshitz-Slyozov-Wagner (LSW) theory. The pre-deformed 6061 alloy exhibits lower coarsening rate constants than that of the as-cast one, and also lower than other alloys processed by different method found in previous literature. It is because the coarsening rate is associated with the initial microstructure and composition of the alloy. The secondary phases in the alloy inhibit the migration of the liquid film grain boundaries. The microstructure obtained by using the combination of near-liquidus semicontinuous casting and pre-deformation treatment is better than that without pre-deformation processing, which demonstrates that the used method is promising for fabricating high quality semi-solid alloys. KEY WORDS: Semi-solid forming; Near-liquidus semi-continuous casting; 6061 alloy; Pre-deformation; Partial remelting; Coarsening 1. Introduction Al-Mg-Si based aluminum alloys, e.g alloy, etc., are widely used in automotive and aerospace applications because of their high properties such as good strength, formability, weldability and corrosion resistance [1 6]. A weight saving is expected in application of automotive components by replacing steels with aluminum alloys, which will result in great improvements in energy saving. To improve the strength and the formability of lightweight aluminum alloys for further industrial applications [7], semi-solid forming (SSF) [8 9] technique is used as an alternative to Corresponding author. Prof.; Tel.: ; address: zmzhou@imp.neu.edu.cn (Z.M. Zhou). traditional casting and forging processes. SSF is a method that can produce complex shape products. The process has advantages of productions of high quality and performance, and low cost. SSF is now a commercially manufacturing route producing millions of near net-shape parts per annum for the automotive industry. As a key branch of semi-solid technology, thixoforming attracted much attention due to its technical and economic advantages in processing alloys, which comprises of preparation, partial remelting and thixoforming of semi-solid billets. The preparation of semi-solid billet is fundamental to thixoforming process. The near-liquidus semi-continuous casting (LSC) [10,11] based upon pouring temperature control-

2 N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), ling for the control of solid particle morphology is a semi-solid preparation technology with high efficiency, low investment and extensive alloy application scope. The partial remelting of semi-solid billet is a critical procedure in the thixoforming process. Its purpose is not only to obtain a desirable nominal liquid fraction, but also to ensure the transformation of the solid phase to a spheroidal morphology with fine grain size. During partial remelting, the alloy was heated up to a temperature at which the solid and liquid phases coexist in equilibrium [12]. Its features lie in obtaining the desirable nominal liquid fraction through the control of temperature, and realizing long-time holding to ensure complete transition from dendritic or rosette to spherical. However, a long-time holding often results in the coarsening of grains [13], which is detrimental to the thixotropic properties of semi-solid billet and the mechanical properties of thixoformed parts [14 16]. Therefore, the rate of microstructural coarsening in the semi-solid state during reheating then determines whether a material is suitable or not. In general, coarsening is regarded as diffusion controlled process and can be described by the Lifshitz, Slyozov and Wagner (LSW) theory [17 19] which gives a simple form as follows. d 3 d 3 0 = Kt (1) where d is the average particle diameter at time t, d 0 at time t=0 and K is the coarsening rate. Many researchers have studied the coarsening behavior of different semisolid alloys by using the LSW theory. Ji et al. [20] examined the coarsening of solid particles of Mg-9Al-1Zn prepared by using a twin-screw slurry maker at 593 C, and found that the coarsening exponents in the LSW equation are 8.2 and 12.7 for 300 and 800 r/min, respectively. The previous studies on isothermal coarsening support the conclusion that the value of K is lower at lower temperature. Manson- Whitton et al. [23] discussed the inhibition of coarsening by grains with grain boundary liquid films to account for slower rates of coarsening in alloy AA2618 than that in Al-4 wt. pct Cu alloy. Freitas et al. [24] compared coarsening in alloy 2024 with Al-4 wt. pct Cu in the semi-solid state and discussed the retarding effect that intermetallics exert on particle growth. In this paper, the semi-solid billet is cast by LSC to obtain the near-spheroide and rosette microstructure. Subsequently, some samples of the billet are reheated into semisolid state for partial remelting, some others are cold rolled before heat treatment. The effect of pre-deformation and holding temperature and time on the semisolid microstructure was experimentally studied, and the coarsening rate was calculated by using Eq. (1). It is found that the alloy processed by LSC method and pre-deformation has lower coarsening rate during the process of partial remelting compared with the results obtained by previous researchers. 2. Experimental 2.1 Material The experimental 6061 alloy was prepared by using commercial purity aluminum (99.7%), Al-22%Si alloy and magnesium etc. The raw material alloys were melted at 760 C in a 2RZ30 induction furnace. After degassing, holding and deslagging, the melt was purified in a temperature precisely controlled furnace, and then cooled to the destined temperature and kept for a desired time interval. With cooling intensity of 0.05 m 3 min 1, the casting velocity of 150 mm min 1 and the pouring temperatures of 657 C, 6061 alloy is semi-continuously cast into ingot with 120 mm in diameter and 1600 mm in length. 2.2 Heat treatment The chemical composition of 6061 alloy in this paper is 0.63 wt% Si, 1.09 wt% Mg, 0.10 wt% Fe, 0.07 wt% Ti, 0.25 wt% Cu, 0.21 wt% Cr, and Al balance. The differential scanning calorimetric analysis (DSC) for the semi-solid 6061 alloy was conducted on a NETZSCH STA449C integrated thermal analyzer. The measured solidus and liquidus temperatures are C and 652 C, respectively. To study the effects of temperature, the samples were heated at the temperatures of 610, 620, 630 and 640 Cfor 15 min. And to investigate the effects of time, the samples were held at 630 C for 5, 10, 15 and 20 min, respectively. The billet was machined into 20 mm 20 mm 20 mm for partial remelting experiments. In order to investigate the influence of pre-deformation on the partial remelting microstructures, some samples were subjected to one pass cold rolling with 60% reduction. Both as-cast and pre-deformed samples were placed in the electric resistance furnace for the further semi-solid heat treatment. The heating rate and fluctuation of the temperature were controlled within 10 C min 1 and ±1 C, respectively. The specimens were wrapped by aluminum foil in advance to keep it a correct shape during the reheating process. After the semi-solid heat treatment, the samples were taken out immediately for water quenching, polished, etched with the mixed acid solution of 2 ml HF, 3 ml HCl, 5 ml HNO 3, and 190 ml H 2 O. The microstructure was observed on an optical microscope (Leica DMR), and the grain size (d=(4a/π) 1/2, where A is the area of the grain) and the grain roundness (P 2 /(4πA), where P is the perimeter of grain/rosette) were automatically calculated by the Image-Pro Plus software. 3. Results and Discussion The microstructural evolution of 6061 alloy in the process of partial remelting was experimentally studied. Figure 1 shows the microstructures of the as-cast

3 10 N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8 14 Fig. 1 Microstructure of 6061 alloy of semi-solid billet: (a) as-cast, (b) as pre-deformed Fig. 2 Microstructures of the remelted 6061 alloys at different temperatures (as cast) with holding time of 15 min: (a) 610 C, (b) 620 C, (c) 630 C, (d) 640 C and pre-deformed 6061 alloy used for remelting process. It can be seen from Fig. 1(a) that the grains of as LSC cast 6061 alloy clearly exhibits the nearspheroide and rosette character. After cold rolling with the reduction of 60%, some of the rosette grains were broken up to fine blocks, and the grains of the alloy were elongated and tended to be oriented along the rolling direction, as shown in Fig. 1(b). The series of optical micrographs of 6061 alloys experienced isothermal heat treatment at different temperatures for 5, 10, 15 and 20 min, respectively, are shown in Figs. 2 to 5. Figures 2 and 3 show the microstructures of 6061 alloy isothermally treated at different temperatures for 15 min. It could be seen that the liquid phase increases with the increase of temperature. The shape of the solid grains is more globular when the temperature is lower than 630 C, but anomalous at 640 C during the partial remelting. The shape of the solid grains has a tendency to become more globular with the temperature varying from 610 C to 640 C, as shown in Fig. 3, during the partial remelting of the pre-deformed samples. A comparison between Figs. 2 and 4 reveals that, with increasing temperature, the grain boundary liquid film becomes thicker and the grains become more spheroidal in appearance in the course of partial remelting of the pre-deformed samples. The spheroidizing degree of grains is gradually improved with the increase of temperature under the driving of the specific surface energy. This can be explained by the growth mechanism named Ostwald ripening [25 27]. Larger grains grow continuously and smaller ones dissolve quickly because of the difference in curvatures of the grains. Meanwhile, the grains spheroidize. The possible reasons were elucidated as follows. On one hand, with increasing the tempera-

4 N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), Fig. 3 Microstructures of the remelted 6061 alloys at different temperatures (pre-deformed) with holding time of 15 min: (a) 610 C, (b) 620 C, (c) 630 C, (d) 640 C Fig. 4 Microstructures of the remelted 6061 alloys at 630 C for different time (as-cast): (a) 5 min, (b) 10 min, (c) 15 min, (d) 20 min ture, the diffusion of atoms would be enhanced. According to the Ostwald ripening mechanism, the dissolution and reprecipitation can be regarded as the diffusion-controlled processes that would be enhanced and result in the coarsening of solid grains. On the other hand, the dynamic equilibrium of dissolution and reprecipitation coexist in the semisolid slurry, and the energy of the system has a trend to reduce by altering the surface of solid phase, which ultimately results in the spheroidizing of solid grains. Figures 4 and 5 show the microstructures of 6061 alloy at 630 C for 5, 10, 15 and 20 min, respectively, of the two routes. After isothermal treatment for 5 min, the eutectic phase at the grain boundary remelted and

5 12 N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8 14 Fig. 5 Microstructures of the remelted 6061 alloys at 630 C for different time (pre-deformed): (b) 10 min, (c) 15 min, (d) 20 min (a) 5 min, then the periphery of solid grains remelted partially through the solute diffusion at the solid/liquid interface (Figs. 4(a) and 5(a)). After isothermal treatment for 10 min, grains start to amalgamate and grow. The grains obviously tend to grow and spheroidize. But in this case, grain boundary is ambiguous (Figs. 4(b) and 5(b)). The grains become more spheroidic and the boundaries clearer, as shown in Fig. 5(b). After isothermal treatment for 15 min, the grains keep growing, spheroidizing and being surrounded by continuous liquid phase (see Figs. 4(c) and 5(c)). After the holding time lasts for 20 min the fraction of liquid phase increases obviously. Eutectic liquid phases in the grain interior also gradually amalgamate and interpenetrate with the surrounding liquid phase. However, some coarsen grains still exist and the size of which even exceeds 200 µm (Figs. 4(d) and 5(d)). During the partial remelting at a given holding temperature and for an appropriate time, the relative content of liquid and solid phases in semisolid slurry will reach a dynamic equilibrium state, namely the rate of the melting of solid phases corresponds to that of the melting of liquid phases. It usually needs a relatively long time to reach the said dynamic equilibrium, which means that the holding time is of significance for obtaining globular grains. As-cast microstructures of 6061 alloy has many complex compounds which distribute in the grain boundary. They were broken and distributed on the α-al matrix as large granular after cold rolled. Small needle-like Mg 2 Si precipitates during isothermal treatment and the present Mg 2 Si particles at grain boundary inhibit the migration of liquid film grain boundaries, either through a pinning mechanism or through impeding diffusion through the liquid film at the boundary. In contrast, the insoluble Fe, Mn-containing particles will be present in the semisolid state and are of a size which can obstruct the migration of the liquid boundary, either through a pinning type of mechanism or through inhibiting diffusion through the liquid from one boundary position to another [30]. The results show the inhibiting effect of second phase particles. Figure 6 demonstrates the effect of holding temperature and time on grain size and shape factor of semi-solid 6061 alloy. It can be seen that the grain size increases with the increase of holding temperature and time. The shape factor is the smallest at 630 C and for 15 min during partial remelting. The main reason might be that the liquid fraction increases rapidly when the semi-solid billet firstly remelted below 630 C and isothermally held for less than 15 min. The rapid increasing of liquid phase leads to the separation of grains and the formation of individual polygonal grains which have high interfacial energy due to high specific surface area. The protruding positions of polygonal grains remelt firstly under surface tension due to low balance melting point, in which the grains spheroidize rapidly to reduce the interfacial energy. Therefore, the grains are much closer to be round. With the increase of holding temperature and time, the small grains dissolute and large grains grow into larger ones accompanied with the reduction in amount of globular grains. When the holding tempera-

6 N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), Average grain size / m Average grain size / m (a) Grain size (as-cast) Grain size (pre-deformed) Roundness (as-cast) Roundness (pre-deformed) (b) Holding temperature / o C Grain size (as-cast) Grain sizer (pre-deformed) Roundness (as-cast) Roundess (pre-deformed) Holding time / min Average roundness Average grain roundness Fig. 6 Grain size and the shape factor of remelted 6061 alloy ture is 640 C for holding time 15 min or 630 C for 20 min, the grains might not adequately ripen though the grains spheroidize rapidly but the holding time is not long enough. However, the samples are too soft to clamp. The shape factor decreases with holding temperature and time during the partial remelting of the pre-deformed samples. The difference in the kinetics of grain growth should be attributed to the different eutectic melting activation energy. During the predeformed process, a fraction of external work remains in alloys and forms strain energy. The energy stored in the alloy will provide the driving force to promote recovery and recrystallization during subsequent partial remelting. During remelting, recovery is induced by the release of the stored energy through the virtue of dislocation motion, which accelerates the atomic diffusion at the elevated temperature. The grains are much finer than that shown in Figs. 2 and 3. The predeformed billet appears to be relatively more resistant to grain growth than the as-cast billet, in which the grain is refined in advance via recrystallization. The coarsening rates are calculated by using Eq. (1). The pre-deformed 6061 alloy after heat treatment exhibits lower coarsening rate constants than that of the as-cast one because of a number of recrystallization nuclei. During the isothermal treatment, the solid particle size of the pre-deformed 6061 alloy is smaller, which result in an increase in both grain nucleation and growth; however, the increment Coarsening coefficient K ( m 3 /s) assuming d 3 -d 3 0 =kt AA2014 spray cast [23] 2014 RAP [29] 2014 CS [29] 201 CS [28] Al-4.5Cu-1.5Mg DC-cast with grain refiner [30] A356 electromagnetic [31] 6061 RAP without MnCr [23] 6061 RAP [23] 7075 RAP [29] 6061 as-cast[this work] 6061 pre-deformed[this work] 2017 Shear Cooling Roll [31] Al-4Cu SIMA with grain refinement [32] Temp. / o C Fig. 7 Dependence of coarsening coefficient (K) on holding temperature for wrought Al alloys of nucleation rate is faster than that of the growth rate. The lower coarsening rate may be attributed to the presence of Mg 2 Si particles at grain boundaries in pre-deformed 6061 alloy, which hinder the mobility of solute atoms. Inspection of the composition shows that it includes 0.07 wt% Ti which may inhibit boundary migration and hence reduce the coarsening rate constant. It can be derived from the present work that coarsening rate is relevant to the microstructure of the alloy. As shown in Fig. 7, the value of coarsening rate constant is lower for RAP2014 than that for CS2014, and similarly lower for pre-deformed 6061 than that for as-cast 6061 [28]. In addition, Fig. 7 further supports the argument that the alloys giving relatively low coarsening rates contain stuffs pining the liquid film grain boundaries (insoluble intermetallics based around elements such as Fe, Mn, Cr, porosity, grain refining species such as those based on Ti). It is shown in Fig. 7 that the pre-deformed alloy in this work has lower coarsening rates in the process of partial remelting, which indicates that combination of nearliquidus semi-continuous casting and pre-deformation treatment is a promising method to fabricate the billet of semisolid alloy. 4. Conclusions (1) Preparation of semi-solid billet by using the combination of near-liquidus semi-continuous casting and pre-deformation treatment helps to further improve the microstructure of the semi-solid alloy during partial remelting process, which leads to a lower coarsening rate of grains. (2) The optimal remelting parameters for 6061 alloy cold rolled with 60% reduction in height are heat treated at 630 C for min. The obtained microstructure is better than that without predeformation processing. (3) The coarsening rate for partial remelting of

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