Influences of alloying composition and cooling rate on precipitation behaviors of primary Al3Zr-D023 intermetallics in Al-Mg-Li-Zr alloys

In the industrial process, the potential benefit of Al3Zr precipitates is usually weakened because coarse Al3Zr precipitates have no positive effect on the dynamic recovery, recrystallization and grain growth. Effects of alloying composition and cooling rate on precipitation and growth behaviors of primary Al3Zr-D023 intermetallics were investigated, especially in a large compositional space. Four types of elements exist in the Al3Zr-D023 phase. Increasing initial Mg, Li, Zr, and Ti contents is beneficial to increase the start precipitation temperature of Al3Zr-D023 particles. Increasing initial Mg, Li, Zr, Ti contents and reducing cooling rate are beneficial to increase the mean length, decrease the nucleation rate and the number density of Al3Zr-D023 precipitates. Furthermore, Zr and Ti element contents show a strong positive correlation with the start precipitation temperature and the final length of Al3Zr-D023 precipitates. Reducing slightly initial Ti content is beneficial to reduce the final length of Al3Zr-D023 precipitates. Furthermore, two empirical equations were obtained using the multiple linear regression method, which reveal the complex interactions rather than single-element effect of Mg, Li, Zr, and Ti.


Introduction
Al-Mg-Li-Zr alloy was successfully developed by adding 1.8-2.2wt.% Li into primary rustproof aluminum alloy (LF6) in 1960s, and the density is in the range of 2.48 to 2.50 g/cm 3 [1][2][3].These advantages, such as low density, high specific strength, high specific modulus, good corrosion resistance and high temperature mechanical properties, have made this alloy a competitive material in the aerospace field [4][5][6].The influences of different chemical compositions on peculiarities and susceptibilities of Al-Mg-Li-Zr ingot were clearly clarified in previous studies, promoting the development technology of stable ingot casting and forging [7].During the industrial production process, adding Zr element into the melt is to form fine Al3Zr intermetallic, which becomes the crystallization core of grain nucleation during solidification to refine grain size of casting ingot, and fine Al3Zr precipitates also have a positive effect on the dynamic recovery, recrystallization and grain growth [8].
However, the potential benefit of Al3Zr precipitates is usually weakened because it is easy to form excess and coarse precipitates with improper Zr addition and process control [9].Meanwhile, the hereditary influence of excess and coarse precipitates (with a size greater than a few micrometers) on the structure and properties of semi-finished products deepened concerns about performance of Al-Mg-Li-Zr alloy [10].Because excess and coarse Al3Zr-D023 primary intermetallics have been proved to induce the formation of laminations defect in Al-Mg-Li-Zr alloy die forgings, which is a mentioned cause of defects in semi-finished products [8].
Grushko et al. [8,11] investigated in detail experimental Al-Mg-Li-Zr alloys prepared by adding pure materials, revealed that the zirconium content lower than 0.11 wt.% can effectively avoid the gross precipitation of coarse Al3Zr-D023 particles.Moreover, solid nonmetallic inclusions or dross particles are able to become as heterogeneous nucleation cores, and induce Al3Zr-D023 to crystallize and grow on their surfaces under industrial conditions [10,12,13].Meanwhile, in order to reduce the products cost, the addition of large amounts of waste aluminum alloy will increase the accumulation of trace elements in the melts, such as iron, titanium, and manganese etc..However, the effect of accumulation of trace elements in the melts on precipitation behavior of Al3Zr-D023 intermetallics is difficult to be fully investigated using experimental Al-Mg-Li-Zr alloys which prepared by pure materials.Thus, few reports about these accumulation effects have gone into detail up to now.However, the accumulation effect of trace elements on primary Al3Zr-D023 intermetallics couldn't be selectively ignored under large amounts of waste aluminum alloy addition [7,8].Additionally, the effect of every alloying element on precipitation and growth behaviors of primary Al3Zr-D023 intermetallics require the large number of phase diagrams in a large compositional space, and the interaction between alloying elements in a multicomponent Al-Mg-Li-Zr alloy is still missing.Thus, the aim of the current study is attempt to investigate the effects of alloying composition and cooling rate on precipitation and growth behaviors of primary Al3Zr-D023 intermetallics, especially in a large compositional space.
In our research work, the Thermo-Calc software was used to investigate the solidification path of industrial Al-Mg-Li-Zr alloy and the precipitation behavior of Al3Zr-D023 particles.The thermodynamic database, TCAl6, and Mobility database, MOBAL5, were utilized to calculate the phase diagram under the equilibrium solidification and the nucleation and growth of Al3Zr-D023 within a large range of Mg, Li, Zr, Ti levels under the non-equilibrium solidification.Meanwhile, some non-equilibrium solidification calculations using the Scheil solidification module and precipitation module were performed to illustrate the influence of cooling rate on the precipitation and growth behaviors of Al3Zr-D023 intermetallics.Finally, regression equations were also derived to quantitatively study the influence of each alloying elements and cooling rate on the precipitation and growth behaviors of Al3Zr-D023intermetallics.The aim of the regression processing is to gain a further understanding for the precipitation and growth behaviors, and dominant element of primary Al3Zr-D023 intermetallics in the melts under industrial conditions.

Materials and Methods
In the current investigation, the industrial Al-Mg-Li-Zr alloy was supplied by Southwest Aluminum (group) Co., Ltd.A more detailed production process is available in the literature reported by Fan et al. [14].The chemical compositions of the research alloy in mass fraction (wt.%) are listed in table 1.
Table 1.Chemical composition of the investigated Al-Mg-Li-Zr alloy (mass fraction).Thermo-Calc software was used to predict phase formation, phase amounts, number density and mean length during the solidification of industrial Al-Mg-Li-Zr alloys.For phase diagrams under the equilibrium solidification, the calculated temperature range was selected between 100 and 900 ℃.The adjacent temperature interval was defined as 2 ℃.For non-equilibrium solidification behavior and nucleation and growth behaviors of Al3Zr-D023, Scheil module and Precipitation module were utilized to calculate within a large Mg, Li, Zr, Ti compositional space and various cooling rates.The nucleation site of Al3Zr-D023 was define as bulk, the nucleation morphology was define as plate, and the aspect ratio of Al3Zr-D023 was define as 1.5.For the quantified estimation of alloying element effects, the start precipitation temperatures of Al3Zr-D023 precipitates from phase diagrams were first extracted, the final lengths of Al3Zr-D023 precipitates were also extracted from non-equilibrium calculation results, and then used as the input for regression equations.These regression results provides an insight to understand effects of alloying composition and cooling rate on precipitation and growth behaviors of Al3Zr-D023 intermetallics.

Sequence of the solidification of industrial Al-Mg-Li-Zr alloy
Calculated results of the precipitates sequence in industrial Al-Mg-Li-Zr melts are illustrated in figure 1.The results indicate that liquidus temperature (TL) and solidus temperature (TS) are 635.8°C and 595.1 °C, respectively.Furthermore, about six types of particles would precipitate from the liquid or solid phases during the solidification, as shown in figure 1(a).The sequence of six types of particles is Al3Zr-D023→Al3Fe→Mg2Si→Al2MgLi→Al2CuMg→Al3Mg2, as shown in figure 1(b)-(c).The content of particles is Al2MgLi＞Al3Mg2＞Al3Zr-D023＞Al3Fe＞Mg2Si＞Al2CuMg.These calculation results show that only Al3Zr-D023 particle precipitates from the pure liquid phase region, whose precipitation temperature is about 800 °C.The non-equilibrium solidification behavior of industrial Al-Mg-Li-Zr alloy is shown in figure 1(d).
During the non-equilibrium solidification, Al3Zr-D023 phase would be precipitated from the liquid phase before the solidification occurrence.Al3Fe phase precipitates from the liquid phase when the mass fraction of solid phase is 0.65.Mg2Si phase precipitates from the liquid phase when the mass fraction of solid phase is 0.84.Moreover, Al2MgLi precipitates from the residual liquid phase when the mass fraction of solid phase is 0.93.The precipitation temperature of Al3Zr-D023 particle is about 800 °C in the industrial Al-Mg-Li-Zr alloy whether under equilibrium or non-equilibrium conditions.It means that the growth time of the Al3Zr-D023 particle is sufficient, which leads to form excess and coarse Al3Zr-D023 precipitates before the solidification of the liquid phase takes place.Furthermore, solidus temperature is reduce to about 474 °C during non-equilibrium solidification, which broadens the width of the mushy zone and makes Al3Zr-D023 precipitates take more time to grow.In order to further investigate the precipitation temperature of Al3Zr-D023 during the solidification, these coarse Al3Zr-D023 particles, which are random and uniform distribute in the matrix, were observed by using SEM and EDS.Microscopic morphology for coarse Al3Zr-D023 precipitates in the ingot is shown in figure 2. In the present study, the area percentage of coarse Al3Zr-D023 particles were measured using image analyzer software.Next, the result of the area percentage was converted to the percentage of volume.Finally, the volume percentage was converted to the mass fraction by combining densities of the Al3Zr-D023 particle and Al-Mg-Li-Zr matrix.The calculation equation is shown as follows: wt.%(Al3Zr-D023) = vol.%(Al3Zr-D023)×(ρ(Al3Zr-D023)/ρ(matrix)) (1) where wt.%(Al3Zr-D023) is the mass fraction of coarse Al3Zr-D023 particles, %; vol.%(Al3Zr-D023) is the volume percentage of coarse Al3Zr-D023 particles, %; ρ(matrix) is the density of the matrix, g/cm 3 ; ρ(Al3Zr-D023) is the density of the Al3Zr-D023 particle, g/cm 3 ; the usual value for ρ(Al3Zr-D023) in the literature is 4.1 g/cm 3 [3,15].

Effect of initial Li content on Al3Zr-D023 precipitation and growth
For the sake of simplicity, the effect of composition variation typically make used of calculated phase diagrams in quaternary systems sometimes.The initial components were defined as 5.0 wt.% Mg, 0.1 wt.% Zr, and 0.01 wt.% Ti.The cooling rate was defined as 40 °C/min, and the results are shown in figure 5.As shown in figure 5

Effect of initial Mg content on Al3Zr-D023 precipitation and growth
The initial components were defined as 2.0 wt.% Li, 0.1 wt.% Zr, and 0.01 wt.% Ti.The cooling rate was defined as 40 °C/min, and the results are shown in figure 6.As shown in figure 6

Effect of initial Zr content on Al3Zr-D023 precipitation and growth
The initial components were defined as 5.0 wt.% Mg, 2.0 wt.% Li, and 0.01 wt.% Ti.The cooling rate was defined as 40 °C/min, and the results are shown in figure 7.As shown in figure 7(a), increasing initial Zr content will result in an obvious increase of Al3Zr-D023 precipitation temperature.Moreover, the total mass fractions of Al3Zr-D023 precipitates also increase dramatically.The change of Zr content has an obvious effect on the nucleation and growth behaviors of Al3Zr-D023 precipitates during the nonequilibrium solidification.Increasing initial Zr content will result in the decrease of the nucleation rate of Al3Zr-D023, and the decrease of the number density of Al3Zr-D023 precipitates.Increasing initial Zr content will result in the increase of the mean length of Al3Zr-D023 precipitates.Comparing that to Li and Mg content variations, the results show that the content of Zr in the Al-Mg-Li-Zr alloy need to excessive control, while it is unfavorable to over addition for eliminating follow-up laminations defect of Al-Mg-Li-Zr alloy.IOP Publishing doi:10.1088/1742-6596/2635/1/0120218 effect of excess and coarse Al3Zr-D023 precipitates is also apparent even when titanium content is about 0.005%.So controlling Ti content is an effective method to reduce the Al3Zr-D023 precipitates amount and formation temperature.

Effect of cooling rate on Al3Zr-D023 precipitation and growth
To figure out the effect of cooling rate on Al3Zr-D023 precipitation and growth behaviors, four cooling rates were defined as 20, 40, 60 and 80 °C/min, respectively.The defined initial components were defined as 5.0 wt.% Mg, 2.0 wt.% Li, 0.1 wt.% Zr, and 0.01 wt.% Ti. Figure 9 illustrates the calculated results of Al3Zr-D023 precipitates during the non-equilibrium solidification.As shown in figure 9(a), Al3Zr-D023 precipitates growth tendency is obviously suppressed with the cooling rate increase.Increasing cooling rate will result in the noteworthy decrease of the mean length of Al3Zr-D023 precipitates.As shown in figure 9(b)-(d), increasing cooling rate of the melt will increase the degree of undercooling and increase the nucleation rate and number density of Al3Zr-D023 precipitates, so as to achieve the effect of size refinement of Al3Zr-D023 precipitates.The increasing rate volume fraction of Al3Zr-D023 precipitates significantly varies depending on the cooling rate during crystallization.

Discussion
In the industrial production of Al-Mg-Li-Zr alloy, the effect of every alloying element about the start precipitation temperature and the final length of Al3Zr-D023 precipitates all need to be balanced and to reduce the genetic effect of primary Al3Zr-D023 particles [17].In this work, the influence of alloying element estimated and discussed by using the slopes of linear fitting process.The fitted results are shown in figure 10.As shown in figure 10

Conclusions
(1) During the equilibrium solidification, the precipitates sequence in industrial Al-Mg-Li-Zr melts is Al3Zr-D023→Al3Fe→Mg2Si→Al2MgLi→Al2CuMg→Al3Mg2, therein Al3Zr-D023 particle precipitates from the liquid phase region.However Al3Fe, Mg2Si, and Al2MgLi particles will also precipitate from the liquid phase during the mushy zone of non-equilibrium solidification.The start precipitation temperature of Al3Zr-D023 particle is about 800 °C under equilibrium or non-equilibrium conditions.
(2) Four types of elements exist in the Al3Zr-D023 phase, which are Al, Li, Ti and Zr, respectively.Increasing initial Mg, Li, Zr, and Ti contents are all beneficial to increase the start precipitation temperature of Al3Zr-D023 particles.The contents of Zr and Ti show a strong positive correlation with the start precipitation temperature of Al3Zr-D023 particle, Li and Mg elements show weak positive correlation with the start precipitation temperature of Al3Zr-D023 particle.
(3) During the non-equilibrium solidification, increasing initial Mg, Li, Zr, Ti contents and reducing cooling rate are beneficial to increase the mean length of Al3Zr-D023 precipitates, decrease the nucleation rate and the number density of Al3Zr-D023 precipitates.Therein, Zr and Ti elements show a strong positive correlation with the final length of Al3Zr-D023 precipitates, Li and Mg elements show weak positive correlation with the final length of Al3Zr-D023 precipitates.
(4) Lowering of the initial Ti content in the Al-Mg-Li-Zr alloy is beneficial to reduce substantially the final length of Al3Zr-D023 precipitates.Based on equilibrium and non-equilibrium calculated results, the empirical equation about the start precipitation temperature of Al3Zr-D023 precipitates is defined as Tpre.=614.52+9.67[Mg]+29.53[Li] +722.17

Figure 1 .
Figure 1.Equilibrium and non-equilibrium results of the investigated Al-Mg-Li-Zr alloy calculated by Thermo-calc software: (a)-(c) equilibrium result, (d) non-equilibrium result.

Figure 3
Figure3illustrates the equilibrium mass fraction of Al3Zr-D023 precipitates and the calculated result of mass fraction of coarse Al3Zr-D023 particles based on microscopic images.The calculated result shows that the mass fraction of coarse Al3Zr-D023 particles in the matrix is in the range of 0.1 to 0.2 wt.%, the corresponding end temperature of the precipitation is in the range of 747.0 to 635.8 °C.It supports that excess and coarse Al3Zr -D023 particles, during the actual industrial solidification process, would precipitate from the pure liquid phase even without counting the effect of segregation.

Figure 3 .
Figure 3. Equilibrium result and actual measurement result of mass fraction of Al3Zr-D023 particles during the solidification.Element types and element contents existing in the Al3Zr-D023 phase are shown in figure 4. It's clearly that four types of elements exist in the Al3Zr-D023 particle, which are Al, Li, Ti and Zr, respectively.However, Li content is extremely low in the Al3Zr-D023 particle.Based on research results, it is extrapolated that chemical formula of the Al3Zr-D023 particle may be (Al,Ti)3(Zr,Li).The precipitation behavior of the Al3Zr-D023 particle is controlled by main alloying agents and controlled

Figure 4 .
Figure 4. Element types and element contents in the Al3Zr-D023 phase.
(a), increasing initial Li content will result in the increase of Al3Zr-D023 precipitation temperature.As shown in figure 5(b), under the non-equilibrium solidification, the change of Li content has slight effect on the size distribution of Al3Zr-D023 precipitates.As shown in figure 5(c), increasing initial Li content will lead the start precipitation time of Al3Zr-D023 to advance.But the mean particle length of Al3Zr-D023 precipitates are almost equal in size.As shown in figure 5(d), increasing initial Li content will lead the nucleation rate of Al3Zr-D023 to decrease, which leads the number density of Al3Zr-D023 precipitates to slight reduce when the non-equilibrium solidification finishes.The results show that the change of Li content has slight effect on the growth behavior of Al3Zr-D023 precipitates under non-equilibrium conditions.

Figure 5 .
Figure 5.Effect of initial Li content on Al3Zr-D023 precipitation and growth behaviors during solidification: (a) mass fraction, (b) size distribution, (c) mean particle length, (d) number density.
(a), increasing initial Mg content will result in the increase of Al3Zr-D023 precipitation temperature.As shown in figure 6(b), the change of Mg content has obvious effect on the size distribution of Al3Zr-D023 precipitates.Increasing initial Mg content will result in the decrease of the size distribution of Al3Zr-D023 precipitates.As shown in figure 6(c), increasing initial Mg content will lead the start precipitation time of Al3Zr-D023 to advance.However, the mean particle length of Al3Zr-D023 precipitates are not obvious different in size when the non-equilibrium solidification finishes.As shown in figure 6(d), increasing initial Li content will lead the nucleation rate of Al3Zr-D023 to decrease, which leads the number density of Al3Zr-D023 precipitates to reduce when the non-equilibrium solidification finishes.Comparing that to Li content variation, the results shown that the change of Mg content has more obvious effect on growth behavior of Al3Zr-D023 precipitates during the non-equilibrium solidification.

Figure 6 .
Figure 6.Effect of initial Mg content on Al3Zr-D023 precipitation and growth behaviors during solidification: (a) mass fraction, (b) size distribution, (c) mean particle length, (d) number density.

Figure 7 .
Figure 7. Effect of initial Zr content on Al3Zr-D023 precipitation and growth behaviors during solidification: (a) mass fraction,(b) size distribution, (c) mean particle length, (d) number density.

Figure 8 .
Figure 8.Effect of initial Ti content on Al3Zr-D023 precipitation and growth behaviors during solidification: (a) mass fraction, (b) size distribution, (c) mean particle length, (d) number density.The initial components were defined as 4.0 wt.% Mg, 1.0 wt.% Li, and 0.1 wt.% Zr.The cooling rate was defined as 40 °C/min, and the results are shown in figure 8.As shown in figure 8(a), in similar, increasing initial Ti content will result in the increase of Al3Zr-D023 precipitation temperature.The change of Ti content has an obvious effect on the nucleation and growth behaviors of Al3Zr-D023 precipitates.Increasing initial Ti content will result in the decrease of the nucleation rate of Al3Zr-D023, and the decrease of the number density of Al3Zr-D023 precipitates.Increasing initial Ti content will result in the increase of the mean length of Al3Zr-D023 precipitates.It is noteworthy that the negative

Figure 9 .
Figure 9. Effects of different cooling rates on Al3Zr-D023 precipitation and growth during solidification: (a) mean particle length, (b) size distribution, (c) volume fraction, (d) number density.
(a), Zr and Ti show a strong positive correlation with the precipitation temperature of Al3Zr-D023 precipitates, Li and Mg show weak positive correlation.It is of interest to point out that Mg shows very week positive correlation, and negligible correlation with the precipitation temperature of Al3Zr-D023 precipitates.The effect of Mg, Li, Zr and Ti on the final length of Al3Zr-D023 precipitates, as shown in figure 10(b), is quite similar as the one in figure 10(a) in positive correlation, with the difference lying in the corresponding number values.In practical production of Al-Mg-Li-Zr alloy or the development of new Al-Mg-Li-Zr alloys, the effect of Zr on the start precipitation temperature and the final length of Al3Zr-D023 precipitates will always be able to attention, and the effect of Ti always be a sake of simplicity to overlook.

Figure 10 .
Figure 10.Effects of different alloying elements on Al3Zr-D023 precipitation temperature and final length during solidification.For the convenience of applications, two empirical equations are attempted to obtain in the current work, which consider contributions of different alloying elements and cooling rate based on linear addition rules, and expressions are shown as follows.These two empirical equations could be as a complement for the study of alloying composition and cooling rate on precipitation and growth behaviors of primary Al3Zr r-D023 intermetallics in a larger compositional space if interested.Tpre.=614.52+9.67[Mg]+29.53[Li]+722.17[Zr]+109.77[Ti](R 2 =0.95) (2) Lmean=12.47+0.13[Mg]+0.14[Li]+76.97[Zr]+84.83[Ti]-0.15Vc(R 2 =0.92) (3) where Tpre. is the start precipitation temperature of Al3Zr-D023 precipitates in the pure liquid phase zone, °C; is the final mean length of Al3Zr-D023 precipitates after end of solidification, μm; [Mg], [Li], [Zr], and [Ti] are initial components of Mg, Li, Zr, and Ti elements, respectively, wt.%; Vc is the cooling rate, °C/min.