Materials Transactions, Vol. 50, No. 12 (2009) pp. 2820 to 2825 #2009 The Japan Institute of Metals
Effects of Process Parameters on the Macrostructure of a Squeeze-Cast Mg-2.5 mass%Nd Alloy Yanling Yang1 , Liming Peng1;2; * , Penghuai Fu1 , Bin Hu1 , Wenjiang Ding1 and Baozheng Yu3 1
National Engineering Research Center of Light Alloys Net Forming, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Key State Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China 3 Beijing Institute of Aeronautical Materials, China Aviation Industry Group Corporation, 100095, P. R. China 2
The effects of applied pressure, pouring and die temperatures on the macrostructure of a squeeze-cast Mg-2.5 mass%Nd alloy were investigated. The grain size of the Mg-2.5 mass%Nd alloy increased with increasing applied pressure from 60 to 180 MPa when the pouring and die temperatures were 725 C and 330 C. This was attributed to the melting temperature at the time when pressure was applied under the pouring temperature of 725 C, higher than the liquid temperature. Whilst, the pouring and die temperatures are 700 C and 330 C, the grain size of the alloy decreased with increasing applied pressure from 0 to 180 MPa. Lower die and pouring temperatures were favorable to refine grains, but the effects of the die temperature on the distribution and structure of grains were much smaller than those of the pouring temperature. [doi:10.2320/matertrans.MRA2008452] (Received December 8, 2008; Accepted September 9, 2009; Published November 4, 2009) Keywords: squeeze cast, magnesium-neodymium alloy, macrostructure, grain size
1.
Introduction
Magnesium alloys are considered as the most promising structural material in the 21st century. As they can satisfy the need of weight saving and reduction of CO2 emission, magnesium alloys are used more and more in automotive industry.1–3) So far, most of magnesium alloys are used as casting products. However, the main disadvantage of conventional casting processes such as high-pressure die casting is the formation of defects such as gas and shrinkage porosities, which would significantly decrease mechanical properties, integrity and reliability of the products. Squeeze casting is a process that has been employed for making products with better properties and near net shapes. In the squeeze casting process, a molten metal is solidified under an applied pressure during solidification, which leads to a high cooling rate and temperature gradient. Compared with the conventional HPDC process, the squeeze casting has a number of advantages, such as low density of porosities, heat treatability, consistency and soundness of mechanical properties. The key of the squeeze casting process is to control the processing parameters. Maleki et al.4) had studied the effects of squeeze casting parameters on the macrostructure of the LM13 (Al-1013 mass%Si) alloy. They reported that the increased applied pressure resulted in the smaller grain size and the decreased pouring or die temperature rendered the similar effects. In Mg alloys, AZ91 (Mg8.98 mass% Al-1.02 mass% Zn) alloy, AM50 (Mg-5 mass% Al-0.4 mass% Mn) alloy and RZ5DF (Mg-4.2 mass% Zn1 mass% RE) alloy have been investigated in the squeeze casting.5–7) Yong et al.7) studied the effect of the applied pressure on the grain size of the RZ5DF alloy and found that the increasing applied pressure in the squeeze cast promoted rapid solidification and fine grains. But they did not show the microstructure of the samples solidified under different pouring and die temperatures. In fact, the macrostructure as *Corresponding
author, E-mail:
[email protected]
well as the soundness and interior quality of squeeze castings are influenced by more parameters besides the applied pressure, such as the melt volume and quality, the duration of the applied pressure, die temperature, pouring temperature and time delay before pressure application. Therefore, all these parameters are required to be optimized for each individual alloy system and casting.8) Moreover, the studies on the squeeze-cast magnesium alloys have been more focused on Mg-Al and Mg-Zn series alloys, and rarely on Mg-Rare earth elements (RE) series alloys. It has been demonstrated that rare earth metals (RE) are the most effective elements to improve the mechanical properties of magnesium alloys especially at elevated temperature. The Mg-Nd alloy is one of the typical agehardening Mg-RE alloys.9) In the present work, the effects of process parameters on the macrostructure of the squeezecast Mg-2.5 mass% Nd alloy were investigated and the key factors affecting the grain size of the squeeze castings were discussed. 2.
Experiment
An alloy of nominal composition Mg-2.5 mass% Nd was melted in an electric resistance furnace using a steel crucible with the protection of a mixed gas atmosphere of SF6 (1 vol%) and CO2 (99 vol%). The melt temperature was controlled with a thermocouple before pouring into a cylindrical die. The die is with a double-layer as schematically shown in Fig. 1. The outer layer is preheated with an electric heater to maintain a temperature at 200 C in order to retard the temperature dropping of the inner layer. The inner layer is preheated to the designed high temperature in the furnaces, which can accurately control the die temperature. The die is coated with a graphite suspension before each experiment. A 80-tonne hydrostatic press is used for the direct squeeze casting. In all experiments, the delay time after the pouring of melt into the die and before the pressure practically applied to the melt in the die is 7 s. After
Effects of Process Parameters on the Macrostructure of a Squeeze-Cast Mg-2.5 mass%Nd Alloy
(a)
(b)
(c)
(d)
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Fig. 1
Table 1
Schematic drawing of the squeeze-cast process.
The process parameters used in the present squeeze cast process. Fixed parameters
Variable parameter
Pressure,
Tpouring ¼ 700 C Tdie ¼ 330 C
P/MPa
Tpouring ¼ 725 C Tdie ¼ 330 C
Pouring
Tdie ¼ 250 C P ¼ 120 MPa
temperature,
Tdie ¼ 280 C P ¼ 120 MPa
Tpouring / C
Tdie ¼ 330 C P ¼ 120 MPa
P ¼ 0; 60; 120; 180
Tpouring ¼ 675; 700; 725
Die temperature, Tpouring ¼ 675 C P ¼ 120 MPa Tdie ¼ 250; 280; 330 Tdie / C
Tpouring ¼ 700 C P ¼ 120 MPa Tpouring ¼ 725 C P ¼ 120 MPa
solidification, the casting was removed from the die and quenched in cold water at about 25 C. When the applied pressure is zero, the process is in fact gravity casting. The squeeze-cast sample is about 80 mm height. Macrostructure investigations were carried out on a transverse section which is vertical to the extrusion direction and 15 mm away from the upper surface of the sample and observed by a scanner (EPSON PERFECTION 4990 PHOTO). The positions for the grain size measurement are decided by the grain structure. When the sample is composed of fine grains, the measured positions of the grain size are about 1/4 diameter of the samples away from the edge; when the sample is composed of columnar grains and equiaxed grains, only the sizes of equiaxed grains are measured. The process parameters of the present squeeze-casting process are shown in Table 1. 3.
Fig. 2 Macrostructure of the squeeze-cast Mg-2.5%Nd alloy solidified under different applied pressures. (a) 0 MPa; (b) 60 MPa; (c) 120 MPa; (d) 180 MPa (Tpouring ¼ 700 C and Tdie ¼ 330 C).
Results and Discussion
3.1 Effect of the applied pressure on macrostructure Figure 2 shows the macrostructures of the squeeze-cast Mg-2.5%Nd alloy solidified under the different applied pressure with the pouring and die temperatures of 700 C and
330 C, respectively. The macrostructures of the samples solidified under 0 MPa and 60 MPa (Figs. 2(a) and (b)) consist of bands of thick columnar grains surrounding some equiaxed grains in the center, and the applied pressure leads to finer columnar grains as well as equiaxed grains and limits the extent of the columnar growth from the die surface. As the applied pressure increased to 120 MPa, much finer grains are obtained. The grain structure of the sample solidified under 120 MPa (Fig. 2(c)) is significantly different from the samples solidified under 0 MPa and 60 MPa. Further increase of applied pressure has no significant effect on the grain structure (Fig. 2(d)). Figure 3 shows the macrostructures of the squeeze-cast Mg-2.5%Nd alloy solidified under different applied pressures with the pouring and die temperatures of 725 C and 330 C, respectively. All of the grain structures are not obviously different and consist of a band of thick columnar grains surrounding some large equiaxed grains in the center. The average grain size of the sample solidified under 60 MPa is the smallest, and there is a tendency of grain coarsening with the increasing pressure. Figure 4 illustrates the average grain size distribution of the squeeze-cast Mg-2.5%Nd alloy solidified under different pouring temperature and applied pressure. When the pouring temperature is 725 C, the average grain size of the sample solidified under the applied pressure of 60 MPa is smaller than that of the sample solidified under the atmospheric pressure, which is probably because that the applied pressure eliminates air gaps at the mold-liquid metal interface and increases the cooling rate. Further increase of the applied pressure can not refine grains again. On the contrary, it leads to grain coarsening. When the pouring temperature is 700 C, the average grain size decreases with increasing applied pressure. When the applied pressure increases from 60 to
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effect of applied pressure on the melting point of an alloy is given by the Claucius-Clapeyron equation as follows.
(b)
dT Tm V ¼ dP Hf
(c)
(d)
11mm
Fig. 3 Macrostructure of the squeeze-cast Mg-2.5%Nd alloy solidified under different applied pressures. (a) 0 MPa; (b) 60 MPa; (c) 120 MPa; (d) 180 MPa (Tpouring ¼ 725 C and Tdie ¼ 330 C).
Fig. 4 The average grain size distribution of the squeeze-cast Mg-2.5%Nd alloy solidified under different pouring temperatures and applied pressures.
120 MPa, the average grain size decreases from 947 to 71 mm. It is believed from Fig. 4 that there is a critical pouring temperature between 700 C and 725 C, which plays an important role to change the grain size evolution tendency with the applied pressure. When the pouring temperature is lower than the critical value, the applied pressure can obviously refine grains. On the contrary, when the pouring temperature is higher than the critical value, the applied pressure leads to the grain size coarsening slightly. The applied pressure as a primary parameter in the squeeze-cast process has the most significant effect on a component via a variety of approaches which basically include the change in the melting point of an alloy and the increase of the heat-transfer rates across the casting/die interface by eliminating air gaps at the interface.10,11) The
ð1Þ
Where P is the applied pressure, Tm is the melting point in the standard atmosphere, V is the volume change during solidification, and Hf is the latent heat of fusion. During the solidification process, both V and Hf are normally negative due to the shrinkage of metals and heat release, respectively. Thus, dT=dP is positive, which indicates that the applied pressure will increase the melting point of a metal having a volume decrease tendency during solidification. Yue12) has investigated the cooling curve of a squeeze-cast AA7010 alloy and observed a sudden temperature rise of 18 K as the pressure applied. It suggests that the liquidus temperature increases with the increase of applied pressure. There are three cases for the practical melting temperature at the time of pressure application Tmelt : 1) Tmelt is lower than the liquidus temperature under atmospheric pressure Tl ; 2) Tmelt is higher than the liquidus temperature under applied pressure TlP ; 3) Tmelt is between Tl and TlP . Figure 5 shows the relationship between the undercooling temperature and the practical melting temperature at the time of pressure application. When Tmelt is lower than Tl , the total undercooling temperature T consists of temperature undercooling Tt (which is caused by the temperature decrease) and pressure undercooling TP (which is caused by the pressure application). When Tmelt is between Tl and TlP , the T only includes TP . When Tmelt is higher than TlP , there is no undercooling. From Figs. 5(a) and (b), it can be seen that a sudden pressure undercooling will occur and increase with the increasing applied pressure as the pressure applied to the melt at or below TlP . But when the melting temperature at the time of pressure application is above TlP , as shown in Fig. 5(c), the melt will cool first and then solidify under the applied pressure. In general, the grain nucleation begins at some degree of undercooling and the volume free energy change is given by:13) GV ¼
Vh T Tl
ð2Þ
Where V is the volume of nucleus, h is the enthalpy difference between the liquid and solid, Tl is the liquidus temperature under atmospheric pressure. When the pressure is applied to the melt, the liquidus temperature Tl will increase to TlP . Therefore, under the applied pressure, Formula (2) will change into GPV ¼
Vh Vh T ¼ T TlP Tl þ tP
ð3Þ
Where GPV is the volume free energy change under applied pressure and tP is the increment of the melting point under applied pressure. When the pressure is applied to the melt at or below TlP , T ¼ Tt þ TP . Comparing Formula (2) with (3), GPV will be decreased in this case, which leads to the increased nucleation rate and refined grains. When the pressure is applied to the melt above TlP , the melt will cool to a certain undercooling temperature, then solidify under the
Effects of Process Parameters on the Macrostructure of a Squeeze-Cast Mg-2.5 mass%Nd Alloy
(a) ∆TP
∆Tt
C*
C*
C*
Tmelt
TP l TsP
(c)
(b)
Tmelt
∆ TP
T melt
TP l
Tl
TsP
Ts
Tl
TsP
TP l Tl
Ts
Ts
Composition
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Composition
Composition
Fig. 5 The relationship between the undercooling and the melt temperature at the time of pressure application. (Ts is the solidus temperature under atmospheric pressure, Tl is the liquidus temperature under atmospheric pressure, TsP is the solidus temperature under the applied pressure, TlP is the liquidus temperature under the applied pressure, Tmelt is the melt temperature at the time of pressure application, Tt is the temperature undercooling, TP is the pressure undercooling)
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
11mm Fig. 6 Macrostructures of the squeeze-cast Mg-2.5%Nd alloy solidified under different pouring and die temperatures (P ¼ 120 MPa). (a) Tpouring ¼ 675 C, Tdie ¼ 250 C; (b) Tpouring ¼ 700 C, Tdie ¼ 250 C; (c) Tpouring ¼ 725 C, Tdie ¼ 250 C; (d) Tpouring ¼ 675 C, Tdie ¼ 280 C; (e) Tpouring ¼ 700 C, Tdie ¼ 280 C; (f) Tpouring ¼ 725 C, Tdie ¼ 280 C; (g) Tpouring ¼ 675 C, Tdie ¼ 330 C; (h) Tpouring ¼ 700 C, Tdie ¼ 330 C; (i) Tpouring ¼ 725 C, Tdie ¼ 330 C.
applied pressure, and the certain undercooling is constant for a material. So in this case, due to the increase of Tl to TlP , GPV will be increased, indicating that the nucleation rate will decrease and the grains are coarsened with increasing applied pressure. It can be inferred from Fig. 4 and Fig. 5 that the practical melting temperature at the time of pressure application is higher than TlP when the pouring temperature is 725 C, while, the melting temperature is lower than the TlP when the
pouring temperature is 700 C. It may be the basic reason for the quite difference of the macrostructures of the samples solidified under the same applied pressure. 3.2
Effects of die and pouring temperature on macrostructure Figure 6 shows macrostructures of samples solidified under different die and pouring temperatures with the applied pressure of 120 MPa. Under the pouring temperature
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enhanced by eliminating air gaps between the die and casting, but the transfer enhancement can be counterbalanced by a decrease of temperature difference between the die and casting.14) Therefore under the same pouring temperature and applied pressure, higher die temperature leads to slower solidification rate, which leads to coarser grains. Also, a heat balance at the liquid/solid interface in the crystal growth from the melt is given by13) Ks Gs Kl Gl ¼ s HR
Fig. 7 The grain size of the squeeze-cast Mg-2.5%Nd alloy solidified under different pouring and die temperatures.
of 675 C and 700 C, the alloys consist of fine grains; whist under the pouring temperature of 725 C, the macrostructure consists of a band of thick columnar grains surrounding some large equiaxed grains in the center. The variation of the die temperature has no significant effect on the grain structure, but a tendency of slight increase in the grain size. Figure 7 illustrates the effect of die and pouring temperatures on the grain size of samples solidified under the applied pressure of 120 MPa. As shown in Fig. 7, the decrease of the die temperature is favorable to refine grains. The grain size of the samples solidified under the pouring temperature of 725 C is quite larger than those of the samples solidified under the pouring temperature of 700 C and 675 C. Therefore, it can be reasonably deduced that there is also a critical pouring temperature between 700 C and 725 C, which remarkably affects the grain size at a certain die temperature and applied pressure. It is believed that only when the pouring temperature is lower than the critical value, high applied pressure can get fine grains. The average grain size of the sample solidified under the die and pouring temperatures of 250 C and 675 C is the smallest, which is 54 mm. From Fig. 6 and Fig. 7, it is obvious that the pouring temperature has a greater influence on the grain size of the squeeze-cast components than the die temperature. The die temperature is an important parameter which affects the heat-transfer rates and consequently the cooling behavior of materials in the squeeze casting. The heat transfer at the casting/die interface can be determined by the following formula. q ¼ hðTc Td Þ
ð4Þ
Where q is the heat flux across the casting/die interface, h is the heat transfer coefficient, Tc is the casting temperature at the casting/die interface, Td is the die temperature at the casting/die interface. In order to enhance the heat transfer at the interface, the independent variable h and/or (Tc Td ) should be increased. In the squeeze casting process, the heat transfer coefficient h across the casting/die interface is
ð5Þ
Where Ks is a thermal conductivity of a solid metal, Kl is thermal conductivity of a liquid metal, Gs is temperature gradient in the solid at the liquid/solid interface, Gl is temperature gradient in liquid at the liquid/solid interface, R is solidification rate, s is density of a solid metal, H is heat of fusion. The solidification rate R is dependent, not on absolute thermal gradient, but on the difference between Ks Gs and Kl Gl . Under the same die temperature, Gl is increased with the increasing pouring temperature, which leads to a slower solidification rate R and coarser grains. Generally the fluidity of melt is a less crucial issue for the squeeze casting processes, therefore the pouring temperature of the squeeze casting can be lower than other casting processes.15) However, it must offer sufficient heat to avert premature solidification of the metals before pressured. A suitable pouring temperature is dependent on several factors, such as the liquidus temperature, freezing range of the metal and die complexity.8,12) 4.
Conclusions
The following results were obtained in this work on the squeeze-cast Mg-2.5%Nd alloy: (1) The effects of the applied pressure on the macrostructure of the Mg-2.5 mass% Nd alloy can be divided into two cases: one is that the applied pressure can refine grains obviously; the other is that the grain size is coarsened with increasing applied pressure. The essential factor is the difference of the practical melting temperature at the time of pressure application to the melt for the two cases. The applied pressure is not always beneficial to refine grain size in squeeze cast. It is determined by the practical melting temperature at the time of pressure application to the melt Tmelt . When Tmelt is higher than the liquidus temperature under the applied pressure TlP , the applied pressure leads to grain coarsening; when Tmelt is at or below TlP , the applied pressure can refine grains. (2) Under the same applied pressure, lower die and pouring temperatures are all favorable to refine grains. The pouring temperature has larger influence on the macrostructure of the squeeze-cast Mg-2.5%Nd alloy than the die temperature. Acknowledgement The authors would like to appreciate the project support by Shanghai Qi-Ming-Xing Plan for Young Scientists (Project No. 07QA14031) and The National Grand Fundamental Research Program (973 plan) of China (No. 5133001A).
Effects of Process Parameters on the Macrostructure of a Squeeze-Cast Mg-2.5 mass%Nd Alloy
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