Effect of Direct Chill Casting Process Parameters on the Microstructure and Macrosegregation of Mg-Al-Zn Ingot

Abstract

It has been demonstrated that the microstructure and macrosegregation of ingot produced by direct chill casting would take remarkable effect on its deformation behavior. In the present study, the influence of casting speed and temperature on the microstructure and macrosegregation of AZ81 ingot prepared by a self-designed crystallizer (220 × 220 mm) was investigated. The results show that the volume fraction of β-Mg₁₇Al₁₂ predicted by the Gulliver-Scheil model agrees well with image analysis results of the ingots prepared at different casting parameters. Moreover, the casting speed affects the macrosegregation greatly while it has slight influence on the distribution of β-Mg₁₇Al₁₂, and the casting temperature has slight influence on macrosegregation of ingot while it has strong influence on the distribution of β-Mg₁₇Al₁₂. The optimal casting speed and casting temperature for the AZ81 ingot are 80 mm/min and 953 K, respectively. The corresponding maximum values of ultimate tensile strength, yield strength and elongation are 265 MPa, 210 MPa and 3.5%, respectively.

1. Introduction

Due to low density, high specific strength/stiffness, and excellent machinability, as well as satisfactory magnetic shielding capacities, magnesium alloys are promising materials in fields of automotive and aerospace industries where weight saving becomes the primary concern.¹⁾ This leads to increasing demand for cast and wrought magnesium alloy products over the last two decades.

Direct-chill (DC) casting holds a prominent position in production of large sized commercial aluminum alloy ingots or billets for its high productivity, low capital investment, simple operating feature and great product flexibility, which has been extensively investigated since it was invented in the 1930s. Nowadays, excellent reviews are available on the development together with the insights into parameters.^2,3) Studies on the DC casting of aluminum alloys show that macrosegretgation is an irreparable defect (which cannot be minimized/eliminated by heat treatment) and is bound to occur in large sized castings. Furthermore, the presence of macrosegregation sets limitation on the size and composition of the billet or ingot to be cast in a productive and economical way. Thus, emphasis on the importance of reducing macrosegregation in the DC casting ingot or billet cannot be overemphasized.²⁾ In recent years, with the increasing demand for light-weight wrought materials, more and more attentions have been paid to the DC casting of magnesium alloys. Progresses have been made on this topic in both experimental and simulating aspects during last decades. In their studies, the topics mainly focus on microstructure, hot tearing, and mechanical properties of round billets, which are the preferred raw materials used to produce various products such as rods, bars, tubes and wires by forge or extrusion.^4–10) However, very few works have been carried out about the elucidation of macrosegregation of DC casting of magnesium alloy, especially for large sized flat ingot of magnesium alloy intended to be eventually rolled into plate. Obviously, to roll magnesium sheet from flat ingot instead of round billet can effectively improve the utilization rate of raw material. This satisfies the dramatically increasing demand of magnesium sheet considering the good potential of its applications in aerospace and transportation industry where weight lighting is urgently desired.¹¹⁾ Therefore, it is meaningful to investigate the effect of process parameters on microstructure and macrosegregation of magnesium alloy ingot produced by direct chill casting.

In this paper, the nominal composition of the magnesium alloy is Mg-8%Al-0.9%Zn (AZ81, in mass percent). Ingots of AZ81 alloy were produced by DC casting using a self-designed crystallizer (220 × 220 mm). Then, the effect of process parameters (casting speed and temperature) on microstructure and macrosegregation of the AZ81 ingot was investigated in order to promote the industrial application of Mg-Al-Zn series magnesium alloys.

2. Experimental Procedure

AZ81 alloy was prepared with pure Zn, Mg and Al by melting them in an electrical resistance furnace under the protection of mixture gases of SF₆ and CO₂ in this work. The melt was degassed, refined and slag removed at 1013 K, then six ingots were prepared with different DC casting parameters. In the first casting, the ingots were cast at 50, 80 and 110 mm/min, respectively. The water flow and casting temperature were maintained constant at 80 L/min and 953 K, respectively. In the second casting, the ingots were cast at 953 K, 993 K and 1033 K, respectively. Meanwhile water flow and casting speed were maintained constant at 80 L/min and 80 mm/min, respectively. For every condition, the length of ingot was produced no shorter than 300 mm in case sufficient samples were provided for metallographic characterization, defect inspection, and mechanical testing.

In order to investigate the effects of casting speed and casting temperature on the macrosegregation, the sump depth was measured according to Ref. 12). The volume fractions of the eutectic along the thickness of the ingots prepared at different casting parameters were evaluated by image analysis software Image-Pro plus 5.0 and which was also calculated with the Gulliver-Scheil model by pandat.^13,14) The Gulliver-Scheil model is a typical non-equilibrium model, which is based on the assumption of complete mixing in the liquid but no diffusion in the solid. It can be used to predict the phase formation sequence for substitutional alloys. All the samples were analyzed in the unetched condition to estimate the volume fraction of the compounds in the alloy.

The chemical compositions along the thickness of ingot were measured by inductively coupled plasma-optical emission spectrometry (ICP-OES). The alloy powders for the ICP-OES measurement were drilled from the cross-section of ingot at each 20 mm from center to the edge.

For mechanical testing, three tensile samples were cut along the casting direction of the ingot by an electric discharging wire cutting machine. The thickness and gauge length of the sample are 2 mm and 20 mm, respectively. Tensile tests were conducted on a Zwick/Roell-20KN testing machine at a cross-head moving at a speed of 1 mm/min at room temperature.

3. Results

3.1 Crystallizer for producing ingot

The uniformity of temperature field of the DC casting plays an important role in improving quality of the ingot or billet. In other words, one of the main challenges in design of crystallizer for ingot or billet is how to obtain a uniform temperature field during DC casting. As schematically illustrated in Fig. 1, the crystallizer (220 × 220 mm) is characterized by the distance between vertical cooling channels, which increases from the middle to the corner of crystallizer frame. Another feature is about the diameter of the outlet for secondary cooling water. In our design, it is gradually reduced from the middle to the corner of crystallizer frame. For both reasons, the cooling effect between the periphery and the corner of the ingot is adjustable in considering the fact that the secondary cooling water flowing past the corner is not as sufficient as that over the periphery. As a result, the variation of cooling rate is balanced to a large degree in the present study. More details of the function principle of the crystallizer were introduced elsewhere.¹⁵⁾ The typical macrostructure of the ingot prepared by the self-designed crystallizer is shown in Fig. 2. It can be seen that the surface of the ingot suffers from little slight cold shut, but no crack and oxide inclusion.

Fig. 1

The schematic illustration of the crystallizer. 1 main body of the crystallizer, 2 inlet of the cooling water, 3 plug, 4 horizontal cooling water channel, 5 vertical cooling water channel, and 6 outlet of the secondary cooling water.

Fig. 2

The surface of the ingot.

3.2 Effect of process parameters on microstructure

It is known that microstructure of the ingot has strong influence on its deformation behavior. The influence of casting speed and temperature on the microstructure, as well as the distribution and volume fraction of eutectic in AZ81 ingot produced by DC casting is introduced as follows.

3.2.1 Effect of casting speed on microstructure

Earlier investigation shows that the AZ80 magnesium alloy is composed of α-Mg and β-Mg₁₇Al₁₂.¹⁶⁾ The microstructures of the ingots prepared at different casting speeds are shown in Fig. 3. It can be seen that casting speed has slight influence on the morphology of β-Mg₁₇Al₁₂. Discontinuous net-shaped β-Mg₁₇Al₁₂ is observed at the edge of the ingot prepared at the casting speed of 50 and 80 mm/min, while it converts into granular β-Mg₁₇Al₁₂ at the edge of the ingots prepared at the casting speeds of 110 mm/min. By contrast, the morphology of β-Mg₁₇Al₁₂ in the center of ingot is indispensable to casting speed. It is worth mentioning that the β-Mg₁₇Al₁₂ distributes evenly from the center to the edge of the ingot at the casting speed of 80 mm/min.

Fig. 3

The effect of casting speed on the microstructure of AZ81 ingot (a) 50 mm/min edge, (b) 50 mm/min center, (c) 80 mm/min edge, (d) 80 mm/min center, (e) 110 mm/min edge, and (f) 110 mm/min center.

Studies on the DC casting of Al alloy show that the drastic variation of the volume fraction of eutectic along the thickness of ingot/billet is likely to cause pores or hot tears in the ingot or billet.²⁾ Therefore, it is meaningful to understand the variation of the volume fraction of β-Mg₁₇Al₁₂ along the thickness of the ingot prepared by DC casting in this work. The cooling rate of DC casting is very high because the secondary cooling water sprays directly on the surface of ingot. Consequently, solidification path of DC casting deviates from the equilibrium solidification. It has been confirmed that the Gulliver-Scheil model is an effective way to predict the volume fraction of the compounds in the alloy prepared under non-equilibrium solidification.¹⁷⁾ Figure 4 shows the solidification path of the AZ81 alloy predicted with the Gulliver-Scheil model. The sequence of the phase formation under the Gulliver-Scheil model is: Liquid → HCP (α-Mg) + Liquid → HCP (α-Mg) + β-Mg₁₇Al₁₂. It can be seen that although the concentration of Al in AZ81 alloy is less than the maximum solid solubility of Al in magnesium alloy, β-Mg₁₇Al₁₂ can still be precipitated at the end of solidification (not a solid transform) as the temperature is lower than 765 K under the Gulliver-Scheil model. It is well known that for non-equilibrium solidification to occur a certain amount of undercooling is indispensable. The degree of undercooling increases with the cooling rate and the purity of the metal. The content of alloying elements is totally about 9% in this work, but the cooling rate is very high owing to the secondary cooling water directly sprayed on the surface of the slab during DC casting. Therefore, small proportion of the β-Mg₁₇Al₁₂ would be formed under the solidus temperature. Similar result is reported in Mg-4.9Al-2Ca-2Sn alloy prepared with directional solidification.¹⁸⁾ The volume fraction of β-Mg₁₇Al₁₂ is estimated as 11.2% at room temperature, which was also measured using image analysis for ingots of different casting speeds. Both the measurement and simulation results are shown in Table 1. The results show that the volume fraction of β-Mg₁₇Al₁₂ predicted by the Gulliver-Scheil model agrees well with that determined by image analysis. This is particularly the case at the edge of ingots prepared at different casting speeds, illustrating that the Gulliver-Scheil model is very useful in microstructure simulation of DC casting of magnesium alloy. A similar result is reported in the directional solidification of AX44 magnesium alloy.¹⁷⁾

Fig. 4

The solidification path of AZ81alloy as predicted with the Gulliver-Scheil model (f_s represents the volume fraction of solid phase).

Table 1 Measured and calculated volume fraction (vol%) of the eutectic in AZ81 ingot prepared at different casting speeds.

50 mm/min		80 mm/min		110 mm/min		The Gulliver-Scheil model
edge	middle	edge	middle	edge	middle
13.2	14.5	12.5	14.7	12.0	14.2	11.2

3.2.2 Effect of casting temperature on microstructure

Figure 5 shows the microstructures of the ingots prepared at different casting temperatures. It can be seen that the casting temperature has strong influence on the distribution of β-Mg₁₇Al₁₂, which is relatively homogeneous on grain boundary and within grain interior as the ingot is prepared at 953 K. However, β-Mg₁₇Al₁₂ segregates along the grain boundary with increasing casting temperature. Since it is definite that the segregation of second phase along the grain boundary would impair the plasticity.¹⁹⁾ Therefore, the increasing of casting temperature implies a detrimental effect on the deformability of AZ81 ingot prepared by the DC casting.

Fig. 5

The effect of casting temperature on the microstructure of AZ81 ingot (a) 953 K edge, (b) 953 K center, (c) 993 K edge, (d) 993 K center, (e) 1033 K edge, and (f) 1033 K center.

The volume fractions of β-Mg₁₇Al₁₂ were also measured using an image analysis for ingots prepared at different casting temperatures. The measurement and calculation results are shown in Table 2. The results show that the volume fraction of β-Mg₁₇Al₁₂ predicted by the Gulliver-Scheil model also agrees well with that determined by the image analysis at different casting temperatures. It becomes very prominent at the edge of the ingots.

Table 2 Measured and calculated volume fraction (vol%) of the eutectic in AZ81 ingot prepared at different casting temperatures.

953 K		993 K		1033 K		Gulliver-Scheil model
edge	middle	edge	middle	edge	middle
12.2	13.7	13.6	14.5	12.0	15.7	11.2

3.3 Effect of process parameters on macrosegregation

3.3.1 Effect of casting speed on macrosegregation

The fundamental reason of macrosegregation lies in partitioning of the solute elements between liquid and solid phases during solidification. However, the parameters of DC casting could also affect the macrosegregation according to the study on the DC casting of aluminum alloys.²⁾ Quantitatively, the macrosegregation can be evaluated by a segregation ratio calculated as follows:²⁾   

\[\text{Segregation ratio} = (c_{\rm max} - c_{\rm min})/c_0\](1)

Where $c_0$ is the average composition of ingot;

$c_{\rm max}$ is the maximum composition of ingot;

$c_{\rm min}$ is the minimum composition of ingot;

The effect of casting speed on the distribution of Zn and Al along the thickness of ingot is shown in Fig. 6. According to the eq. (1) and Fig. 6, the segregation ratios of Zn and Al are 20.9% and 4.1% at 50 mm/min, 24.0% and 6.9% at 80 mm/min, 26.7% and 10.3% at 110 mm/min, respectively. The calculation results illustrate that the segregation ratios of alloying elements, both Zn and Al, increase as the casting speed increases. On the other hand, the results also illustrate that the extent of segregation of a particular alloying element is relevant to its physical/chemical attributes rather than an absolute content of it in the alloy, e.g., the content of Al is more than that of Zn in AZ81 alloy, but the macrosegregation of Zn is more serious than that of Al.

Fig. 6

The effect of casting speed on the macrosegregation of DC casting ingot (a) Zn, and (b) Al.

3.3.2 Effect of casting temperature on macrosegregation

Casting temperature is a prominent factor which affects the distribution of β-Mg₁₇Al₁₂ in the AZ81 ingot (as shown in Fig. 5). The effect of casting temperature on the distribution of Zn and Al along the thickness of the ingot is shown in Fig. 7. According to the eq. (1) and Fig. 7, the segregation ratios of Zn and Al are 24.0% and 6.9% at 953 K, 27.7% and 8.1% at 993 K, 30.1% and 9.2% at 1033 K, respectively. The calculating results indicate that the increasing of casting temperature deteriorates the macrosegregation of Zn and Al.

Fig. 7

The effect of casting temperature on the macrosegregation of DC casting ingot (a) Zn, and (b) Al.

Overall, the segregation ratios of both Zn and Al increase with increasing casting speed or temperature. However, it seems that casting speed has stronger influence on the macrosegregation than that of casting temperature, whereas casting temperature has stronger influence on the distribution of β-Mg₁₇Al₁₂ than that of casting speed.

3.4 Tensile properties

Based on the results of the influence of casting parameters on the microstructure and macrosegregation of AZ81 ingot, the optimal casting speed and casting temperature for AZ81 ingot produced by DC casting are 80 mm/min and 953 K, respectively. The tensile properties including ultimate tensile strength (UTS), yield strength (YS) and elongation (δ) of the AZ81 ingot prepared with the optimal parameters are shown in Table 3. It can be seen that the tensile properties along the thickness of the ingot almost keep constant, which confirms that the microstructure and composition of the ingot are relatively uniform as the ingot is prepared with the optimal casting parameters. The maximum values of UTS, YS and elongation are 265 MPa, 210 MPa and 3.5%, respectively.

Table 3 Mechanical properties along the thickness of the ingot prepared at optimal parameters.

location	UTS/MPa	YS/MPa	δ/%
center	249	198	3.2
1/2 thickness	265	210	3.5
edge	257	206	2.8

4. Discussion

The influence of casting temperature and speed on microstructure and macrosegregation of AZ81 ingots was investigated in this paper. The results indicate that casting speed has slight influence on the volume fraction of β-Mg₁₇Al₁₂ in the center of the ingot, while it decreases at the subsurface of the ingot with increasing casting speed. The reason can be attributed to the fact that the melt at the edge of ingot solidifies under the primary cooling, however the secondary cooling will take effect earlier with increasing casting speed. As a result, the formation of β-Mg₁₇Al₁₂ at the subsurface of ingot is inhibited as it is cooled to room temperature. Therefore, the volume fraction of Mg₁₇Al₁₂ at the edge of the ingot slightly decreases with increasing casting speed (as shown in Fig. 2). In contrast, the melt in the center of ingot, especially for large sized casting, solidifies under the secondary cooling. It is well known that cooling rate of the secondary cooling depends on the flow rate of the secondary cooling water rather than the casting speed.²⁾ Therefore, the casting speed almost has no influence on the volume fraction of the β-Mg₁₇Al₁₂ in the center of ingot. On the other hand, the degree of superheat both in the center and at the edge of ingot increases simultaneously with increasing casting temperature as the other parameters remain constant. This testifies a prominent effect of casting temperature on the volume fractions of β-Mg₁₇Al₁₂ both in the center and at the edge of ingot.

Both zinc and aluminum exhibit negative segregation (as shown in Fig. 6 and Fig. 7). However, the fluctuating tendencies for the concentration of aluminum and zinc are somehow different. The concentration of aluminum along the thickness of AZ81 alloy ingot decreases first and then slightly increases in the center of ingot, while the concentration of zinc decreases almost monotonically. Similar findings about the variation of the alloying elements were reported in NZ30K ally billet produced by DC casting.²⁰⁾ There are two essential conditions to form macrosegregation during DC casting according to the study on the DC casting of Al alloy.²⁾ They are the relative movement between solid and liquid phases, and the solute rejection by the solid phase. These conditions are tightly related to the shape of the sump, as introduced in more detail elsewhere.^2,20) The existence of the sump during the DC casting leads to macrosegregation of the alloying elements over the cross-section of the ingot.²⁾ The depth of the sump is a characteristic feature of the solidification profile that exists upon DC casting. The sump depth (h) mainly depends on casting speed, alloy type and size of the casting ingots, and it can be calculated by the following formula as pointed out in Ref. 2).   

\[h = \frac{AL^2 \nu _{\rm cast}}{4\lambda_{\rm s}(T_{\rm m} - T_{\rm surf})}\](2)

Where A: a coefficient which depends on the alloy (latent heat of fusion, density of the solid, specific heat of the solid);

V_cast: the casting speed;

λ_s: the thermal conductivity of the solid;

T_m: the melting temperature of the alloy;

T_surf: the surface temperature of the slab;

L: the equivalent radius of the slab;

The sump depths of the AZ81 ingots prepared under different casting temperatures and speeds are shown in Fig. 8. It can be seen that the sump depth increases with increasing casting speed or temperature. However, the increasing amplitude of sump depth is about 26 mm as the casting speed increases from 50 to 110 mm/min, whereas that is only 12 mm as the casting temperature increases from 953 to 1033 K. This result shows that to the sump depth, casting speed can exert a stronger influence than casting temperature. Similar trend is observed in DC casting of NZ30K magnesium alloy.²⁰⁾

Fig. 8

The effect of casting speed and casting temperature on the sump depth of DC casting (a) casting speed, and (b) casting temperature.

Casting speed is one of the most important parameters during the DC casting, which plays a dominant role in the macrosegregation of Al alloy ingot.²⁾ The underlying mechanisms to form negative macrosegregation during DC casting are believed to be related to the floating grains and the shrinkage-induced flow.²⁾ According to eq. (2) and Fig. 8(a), the sump depth (h) increases linearly with increasing casting speed, and the steepness of the solidification front increases with increasing sump depth. It has been proved that the enhancement of steepness promotes the shrinkage-induced flow, which in turn could enhance the extent of negative macrosegregation.²⁾ On the other hand, the amount of floating grain tends to increase with increasing casting speed,²¹⁾ which is in line with the enhanced negative macrosegregation. Therefore, the casting speed has strong effect on the macrosegregation of the AZ81 magnesium alloy ingot (as shown in Fig. 6).

The sump depth increases with increasing casting temperature due to the increase in the heat needed to be removed in total via the heat transfer through the surface of the ingot, but the depth does not increase linearly with increasing casting temperature (as shown in Fig. 8(b)). The main reason is because the total amount of heat increment by increasing the casting temperature, compared with the latent heat of solidification, is relatively less, e.g., the amount of the latent heat of fusion (339 kJ/kg) released from AZ31 alloy is about ten times that of the heat generated by increasing casting temperature from 953 to 1033 K.²²⁾ Therefore, the casting temperature has limited influence on the macrosegregation of the DC casting (as shown in Fig. 7).

5. Conclusions

In this work, the effect of casting parameters on the microstructure and macrosegregation of AZ81 ingot was investigated. The main conclusions can be drawn as follows:

• (1)   The volume fraction of β-Mg₁₇Al₁₂ predicted by the Gulliver-Scheil model is 11.2% at room temperature, and the accuracy of Gulliver-Scheil model to predict the volume fraction of β-Mg₁₇Al₁₂ at the edge of ingot is better than that in the center.
• (2)   The casting speed has strong influence on the macrosegregation of the alloying element, while the casting temperature has limited influence on the macrosegregation.
• (3)   The optimal casting speed and casting temperature for the AZ81 ingot (220 × 220 mm) are 80 mm/min and 953 K, respectively. The corresponding maximum values of UTS, YS and elongation are 265 MPa, 210 MPa and 3.5%, respectively.

Acknowledgements

This work is supported by the Shanghai Rising-Star Program under the grant number 14QB1401400, and the Doctoral Scientific Research Foundation of Shanghai Ocean University (A2-0203-17-100325).

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