The Effect of Solution Treatment Time on the ...

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018.

The Effect of Solution Treatment Time on the Microstructure and Ductility of Naturally-Aged A383 Alloy Die Castings G. Eisaabadi B.a, GY Yeomb, c, Murat Tiryakioğlud,1 , Nelson Nettod, R. Beygia, M.Z. Mehrizia, SK Kimb a

Department of Materials Science and Engineering, Faculty of Engineering, Arak University, Arak 38138-5-3945, Iran. b Korea Institute of Industrial Technology, Incheon 406-840, South Korea. c Department of Materials Science and Engineering, Inha University, Incheon 402-751, Korea. d School of Engineering, University of North Florida, Jacksonville, FL 32256, USA

Abstract A383 aluminum alloy high pressure die castings were solution treated at 490°C for six duration ranging between 15 and 180 minutes, subsequently quenched in water and naturally aged for 4 days. The effect of solution treatment time on the evolution of microstructure and tensile properties were determined. As expected, Si particles became larger and rounder with increasing solution treatment time. In all cases, the size and aspect ratio of the Si particles followed the lognormal distribution. Moreover, the coarsening of Si particles during solution treatment was found to follow the Lifshitz –Slyozov-Wagner model. A new equation was developed for the evolution of the aspect ratio during solution treatment of Al-Si-Mg alloys. Analysis of tensile properties showed that elongation and quality index increased steadily with increasing Si particle size, a result that is in contrast with the widely accepted notion that large Si particles impairs the ductility of cast AlSi-Mg alloys. The positive correlation between Si particle size and quality index was interpreted to be due to partial healing of oxide bifilms entrained in the castings. Keywords: Heat treatment; Homogenization; ADC12 alloy; aspect ratio; coarsening.

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Corresponding Author: 1

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. Introduction A383 (ADC12) is one of the most widely used aluminum alloys in high-pressure die casting (HPDC) of thin-wall components for automotive applications, due to the combination of its high specific strength, high fluidity, high corrosion resistance and low volumetric shrinkage during solidification [1-10]. Typically, the heat treatment schedule for A383 castings involves a solution treatment [11-16] to homogenize the solutes, dissolve Al2Cu and Mg2Si to maximize the amount of hardening solutes in the aluminum matrix, and finally spheroidize eutectic silicon. Solution treatment needs to be long enough to dissolve Mg2Si (β) and Al2Cu particles completely. Simultaneously, solution treatment time, tST, should be as short as possible to save energy costs. Moreover, HPDC components usually contain internal pores in which gases such as air, hydrogen and/or vapors formed by the decomposition of organic die wall lubricants have been entrapped. The gases entrapped inside the pores expand during conventional solution treatment i.e., temperature above 500°C for more than 8 hours, and result in surface blistering and dimensional instability. Recently, it has been demonstrated that solution treatment temperatures lower than 500°C can be used in HPDC components without causing blistering [17, 18]. Although the results of solution treatment at lower temperatures are promising, the effect of the solution treatment time on microstructure and tensile properties of HPDC components has not been undertaken, to the authors’ knowledge. This study is intended to fill this gap. Background Tensile deformation of cast Al-Si-Mg(-Cu) alloys has been investigated extensively [19-28]. In one of the earlier studies, Gangulee and Gurland [29] observed in situ that Si eutectic particles fractured early in plastic deformation and intense slip bands appeared between fractured Si particles, which led to cracks and eventually to final fracture. It has been shown [27, 29-31] that the fraction of cracked Si particles increases linearly with plastic deformation. Moreover, it was shown [32] that the probability of a Si particle to crack at a given plastic strain is related to the product of its equivalent diameter (deq) and its aspect ratio (RA). Hence, the statement by Zhang et al. [33] that ductility in cast Al-7%Si-Mg alloys is determined by morphology and size of Si particles, is widely accepted in the literature.

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. Recently Alexopoulos et al. [34] investigated the effect of Cu in A357 casting alloy produced by the Sophia process, coupled with Sr, Sm and Ag additions on microstructure and tensile properties. They found a correlation between mean particle size and mean elongation (eF): 3.12 eF  9.48 deq

(1)

where elongation is in percent and Si particle size is in microns. Alexopoulos also reported an empirical equation for the relationship between mean elongation and the product of Si particle size and aspect ratio: eF  25.7 ( deq R A ) 1.47

(2)

Equations 1 and 2 provided respectable fits to data, which is in agreement with previous results in the literature. However, Alexopoulos also observed that the Weibull distribution for elongation in A357-Cu-Sr castings was bimodal although the lognormal distribution for Si particle size was unimodal. Consequently, they concluded that the strong correlations between elongation and Si particle size and shape were probably incidental and did not indicate causation. The as-cast microstructure in Al-Si-Cu-Mg alloys has a profound effect on the response of the alloy to solution treatment [35]: while only several minutes are necessary to complete dissolution and homogenization in fine microstructures [36-39], several hours are necessary for coarser microstructures. In a study on A356 aluminum alloy, Shivkumar et al. [40] investigated the effect of solution treatment time at 540oC on the tensile properties of sand and permanent mold castings. The solution treatment time varied between 25 and 800 minutes for permanent mold castings and between 50 and 1600 minutes for sand castings. They found that with solution treatment time, Si eutectic particles got rounder and coarsened and tensile ductility increased even though yield strength (σY) either remained essentially the same or even increased. These results were later confirmed in another study by Shivkumar et al. [41]. Similar observations about the beneficial effects of prolonged solution treatment time on ductility were made by Drouzy et al. [22] and Tiryakioğlu [42]. Pan et al. [43] observed that both yield strength and tensile strength (ST) of A357 alloy castings initially increased, then decreased with increasing solution treatment time while elongation increased steadily in almost every dataset. Highest strength values corresponded to the Si morphology obtained at the end of spheroidization and in the early stages of particle 3

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. coarsening. These results are in contrast with those of Meyer [44] who found that all tensile properties of A357 alloy castings increased with solution time, despite significant coarsening of Si particles during solution treatment. The beneficial effects on tensile properties, especially ductility, despite coarser Si particles after solution treatment are in contrast with the findings that larger Si particles fracture at lower strains and therefore reduce elongation. The present study is motivated by this contrast in the literature. Experimental Details The chemical composition of the recycled A383 alloy used in the study, as determined by optical emission spectrometry, is provided in Table 1. The alloy was received as five kg ingots, which were then melted at 750°C in an electric furnace. Even though no Sr was added to the melt, residual Sr was in the melt (0.015wt.%) because the alloy was recycled. The melt was subsequently held at 680°C in a holder. Plates with the dimensions of 65 mm×120 mm×3 mm, as shown in Figure 1, were produced by using TOSHIBA cold chamber die casting machine without applying any vacuum to the die cavities. The chill-vent (labyrinth extension) on top of each plate was designed to increase the soundness of the castings. The casting and HPDC machine parameters are listed in Table 2. Table 1. Chemical composition of the alloy used in this study. Element wt. %

Si 11.83

Fe 0.827

Cu 2.365

Mn 0.169

4

Mg 0.255

Ni 0.054

Zn 0.516

Ti 0.028

Al Bal

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018.

(c)

Figure 1. (a) 3D representation of die cavity illustrating the orientation of the excised specimens, and (b) casting produced by HPDC machine. (c) Dimension of tensile specimens. Table 2. Casting and HPDC machine parameters. Parameter Pouring temperature Mold temperature Clamping force Diameter Plunger Initial speed Injection speed

Level 690±10 °C 120 °C 350 ton 50 mm 0.23 m/s 3.74 m/s

Castings were solution treated at 490°C [15] for 15, 30, 60, 90, 120 and 180 minutes in an air circulating furnace and were subsequently quenched in water at 25°C. Specimens were naturally aged for four days at room temperature. Tensile test specimens were machined according to ASTM B557M-10. The dimensions of the tensile specimens are provided in Figure 1.c. Four tensile test 5

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. specimens were prepared for each solution treatment time. Microstructural changes were examined on polished surfaces of metallographic specimens obtained from the as-cast and solution-treated test bars. The specimens were chemically etched using Keller's reagent. A Nikon Eclipse MA 200 optical microscope, a FEI Quanta 200F FE-SEM equipped with EDS and Image-Pro Plus software were used to characterize the microstructure of the samples.

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. Results and Discussion The as-cast microstructure is presented in Figure 2 which shows that the Si phase has a coral structure. Image analysis of the as-cast microstructure revealed that the average diameter of α-Al dendrites was 12.4µm. It is noteworthy that there was no primary Si particles, most probably due to the residual Sr in the melt [45], which inhibits heterogeneous nucleation of Si on inclusions, such as oxide bifilms.

Figure 2. The as-cast microstructure showing coral-like Si eutectic phase. The evolution of the size and shape of Si particles during solution treatment is presented in Figure 3. The coral structure observed in the as-cast condition transformed rapidly to fine and fibrous particles in solution treated samples. Si particles became larger and more spherical with increasing solution treatment time, which is consistent with previous results [16, 46-49]. It is also noteworthy that no blisters were observed in the solution treated specimens. Micrographs were analyzed digitally to determine the size and aspect ratio of each Si particle. The size and aspect ratio of Si particles were shown [16] to follow the lognormal distribution, the density function (f) for which is written as; f(x) =

1 (x − τ)σ√2π

exp [

−(ln(x − τ) − μ)2 ] 2σ2

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(3)

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. where τ is the threshold, σ is the shape parameter and μ is the scale parameter. Therefore, lognormal distributions were fitted to the data by using the maximum likelihood method. The estimated parameters for the lognormal distributions for Si particle size and aspect ratio are given in Table 3. Note that the threshold, τ, for equivalent diameter fits are zero, effectively reducing Equation 3 to a two-parameter lognormal distribution. Results of the Anderson-Darling [50] goodness-of-fit tests showed that lognormal distributions in Table 3 could not be rejected.

Table 3. Estimated parameters of the lognormal distributions for the size and shape of the Si particles. deq

RA

τ (μm) μ σ 𝑥̅ (μm) τ μ σ 𝑥̅

As cast 0 -0.8572 0.6189 0.5139 0.8887 0.0993 0.7405 2.3414

15 min 0 -0.3032 0.4278 0.8092 0.9089 -0.0520 0.8238 2.2418

30min 0 0.2027 0.6714 1.5343 0.8470 -0.0201 0.7619 2.1572

60 min 0 0.3501 0.5556 1.6561 0.9187 -0.3149 0.8350 1.9530

90 min 0 0.5347 0.3485 1.8138 0.9398 -0.3649 0.7115 1.8340

120 min 0 0.6155 0.3971 2.0024 0.9678 -0.5269 0.7427 1.7457

180 min 0 0.6926 0.4167 2.1802 0.9946 -0.7493 0.9007 1.7038

The lognormal distributions for the size of Si particles plotted by using Equation 3 and the estimated parameters in Table 3 are presented in Figure 4.a. Note that the distributions shift right to larger Si particle sizes with increasing solution treatment time. The lognormal distributions for aspect ratio of Si particles are presented in Figure 4.b. The peaks of the distributions shift to lower aspect ratio values with increasing solution treatment time. The average of a three-parameter lognormal distribution, 𝑥̅ , is found by; σ2 𝑥̅ = τ + exp [μ + ] 2

(4)

The estimated averages of the lognormal distributions are also provided in Table 3.

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. a

b

c

d

e

f

Figure 3. The evolution of microstructure with solution treatment time: (a) 15, (b) 30, (c) 60, (d) 90, (e) 120, and (f) 180 minutes.

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. The coarsening of Si particles during solution treatment of Al-Si alloys was investigated in several studies [16, 51-54] and was found to follow the coarsening model developed by Lifshitz and Slyozov [55] and Wagner [56] (LSW): d̅3 − d̅30 = k ∙ t ST

(5)

whered is the average diameter,d0 is the initial diameter, k is a temperature dependent constant and tST is solution treatment time. By using the equivalent diameter averages in Table 3 and taking as-cast size as the initial diameter, LSW coarsening model was fitted to experimental data. Results are presented in Figure 5, which shows that the change in the volume of particles with solution treatment time becomes linear after 30 minutes, consistent with the LSW model. The change in average aspect ratio of Si particles, given in Table 3, with solution treatment time is presented in Figure 6. The best fit curve, indicated in Figure 6, has the following form: t n ̅A = R ̅ A0 − (R ̅ A0 − R ̅ AL ) ∙ exp [− ( ST ) ] R t0

(6)

̅ A0 is the aspect ratio of the as-cast condition, R ̅ AL is the aspect ratio limit, t0 is the time where R ̅ AL=1.69, t0 = 65 minutes, n = constant and n is the exponent. The fit as shown in Figure 6 with R 1.35, has a coefficient of determination, R2, of 0.999, which implies an almost perfect fit to the data. To the authors’ knowledge, Equation 6 is the first equation developed for the evolution of aspect ratio with solution treatment time.

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018.

(a)

(b)

Figure 4. (a) Si size and (b) aspect ratio distributions for as cast and all solution treatment times investigated in this study.

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018.

Figure 5. The change in the size of Si particles with solution treatment time.

Figure 6. The change in the average aspect ratio of the Si particles with solution treatment time.

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. The change in tensile properties with solution treatment time is presented in Figure 7. Note that there is a drop in tensile and yield strengths after a solution treatment of 90 minutes. The exact reason for this drop is unknown. It can be speculated that the natural aging time of four days may not have been sufficient to develop a steady strength level after 90 minutes of solution treatment. Also note that elongation increases with solution treatment time even for times less than 90 minutes.

Figure 7. The change in tensile properties with solution treatment time. To evaluate whether solution treatment time has any effect on the structural quality, the quality index proposed by Tiryakioǧlu et al. [57-59] for Al-7%Si-Mg alloys was used: QT =

eF β0 − β1 σY

(7)

where β0 and β1 are 36.0 and 0.064 MPa-1 for Al-7%Si-Mg alloys, respectively. Tensile properties presented in Figure 7 were converted to the quality index values by using Equation 7. The results 13

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. are shown in Figure 8. Note that the trend in the structural quality is the same as in elongation; solution treatment, even for low durations, increases the structural quality of A383 alloy die castings. The magnitude of this beneficial effect increases with solution treatment time. This result is consistent with the finding in previous studies [22, 40-44].

Figure 8. The effect of solution treatment time on the quality index, QT. Possible correlations between quality index and Si particle size and aspect ratio are also investigated. The results are presented in Figure 9. The correlation between average equivalent diameter of Si particles and quality index is shown in Figure 9.a. The structural quality of the castings increases with larger Si particles, which can be anticipated from the discussion above and previous results in the literature [22, 40-44]. However, when Equation 1 is plotted in Figure 9.a after converting elongation to QT values for the A357 alloy used by Alexopoulos et al. (indicated with dashed lines), the two correlations are opposite of each other. Hence as an improvement in structural quality can be expected with larger Si particles due to coarsening during solution treatment, the same increase in average Si particle size will produce a significant reduction in structural quality, as in insufficient or no modification [32, 34]. The contrast between the two correlations is remarkable. It is well known that correlation between two variables does not necessarily mean causation [60]. Therefore, the ductility (and therefore the structural quality) of 14

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. cast Al-Si-Mg(-Cu) alloys is determined by factors other than Si particles size, as suggested by Alexopoulos et al. [34] and Tiryakioğlu et al. [57]. Hence, it can be stated that the intrinsic effect of Si particle size is not known at this point. A similar conclusion can be made about the product of Si particle size and aspect ratio, as presented in Figure 9.b. The trend in change in QT is similar to the one in Figure 9.a; QT increases with increasing the product of Si particle and aspect ratio. This trend is the reverse of what would be expected from the literature; the probability of fracture of Si particles is related to deq.RA [32], and if the elongation of cast Al-Si alloys is determined by the fractured Si particles as suggested by Zhang et al. [33], then an increase in deq.RA should reduce ductility, as shown by the dashed lines for the data of Alexopoulos et al. The reverse trend reported in the present study is also consistent with the results of Shivkumar et al. for the A356 alloy [40]. Recent research [61, 62] showed that the mechanical properties and performance of cast Al-Si-Mg alloys are determined mainly by extrinsic factors, namely oxide bifilms that get entrained into the melt during mold filling and/or melt processing. Therefore, it is extremely challenging to determine the intrinsic effects of processing variables such as solution treatment time and/or microstructural factors such as particle size and aspect ratio on mechanical properties in the presence of the strong effect of oxide bifilms, masking the true effect of the microstructure. This challenge has been demonstrated by the trends in the quality index with Si particle size found in this study, that are the opposite of those found in an investigation on the effect of modifying additions. The increase in the quality index of the A383 castings found in this study can be attributed to the healing of bifilm defects with solution treatment. Although coarse “old” bifilms coming from the melt are resistant to healing [63, 64], “young” bifilms that are created during mold filling can be healed. Therefore, the positive correlation between Si particle size and quality index is not because of the coarsening of Si particles but as a result of oxide bifilms healing with longer solution treatment times. It can also be stated that the true relationship between Si particle size and ductility is not completely known. Research is needed to determine the exact relationship in the absence of oxide bifilms, which have been shown [65] to act as heterogeneous nucleation sites for Si particles. Although correlations between QT and the two Si particle parameters in Figures 9.a and b reported in this study is the opposite of those found by Alexopoulos et al., the results of the two studies are remarkably consistent when QT is plotted as a function of aspect ratio, as presented in 15

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. Figure 9.c. The best fit curve to the data is also shown in the figure. The consistency between the two studies may be due to the true underlying effect but more research is needed to validate it.

(a)

(b)

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G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018.

(c) Figure 9. The change in the quality index with (a) average size of Si particles, (b) the product of mean size and aspect ratio of Si particles, and (c) aspect ratio of Si particles. Conclusions The present study aimed to investigate the effect of solution treatment time on the microstructure, tensile properties and structural quality of the AlDC12 alloy high pressure die castings. Microstructural evolution was assessed by digital image analysis of the micrographs obtained from the samples solution treated from 15 to 180 minutes. Analysis of microstructural data showed that (i) Si particles got larger in size and rounder with increasing solution treatment time, (ii) size and aspect ratio of the Si particles followed the lognormal distribution for each solution treatment condition, (iii) Si particles coarsened during solution treatment according to the LSW model. Moreover, a new equation was proposed for the evolution of the aspect ratio of the Si particles with solution treatment time: t n ̅A = R ̅ A0 − (R ̅ A0 − R ̅ AL ) ∙ exp [− ( ST ) ] R t0 that yielded an R2 value of 0.999, which implies an almost perfect fit to the data. Analysis of tensile properties revealed that with increasing of the solution treatment time, the yield strength and tensile strength of the alloy remained essentially unchanged but elongation increased. 17

G. Eisaabadi B., G.Y. Yeom, M. Tiryakioğlu , N. Netto, R. Beygi, M.Z. Mehrizi, S.K. Kim: Materials Science and Engineering A, v. 722, pp. 1-7, 2018. To evaluate the effect of solution treatment time on the structural quality of the alloy, the tensile data were converted to the quality index. Result of these analysis showed a positive correlation between quality index with the size of Si particles. This result is in contrast to several studies in which a negative correlation between the two was reported. Therefore, the ductility (and therefore the structural quality) of cast Al-Si-Mg(-Cu) alloys is determined by factors different from Si particles size. Similar conclusions can also be made about the product of Si particle size and aspect ratio, a parameter used in the literature to measure the probability of cracking of a Si particle. It is the authors’ opinion that it is impossible to evaluate the intrinsic effect of the size and aspect ratio of the Si particles on tensile properties of cast Al-Si-Mg alloys in the presence of oxide bifilms. The positive influence of solution treatment on the structural quality of high pressure die cast A383 alloy can only be explained by the partial healing of bifilm defects.

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