Effects of electromagnetic field on structure and heat treatment behavior of Mg-Li-Al alloys

MA Lei-juan(马蕾娟), HAO Hai(郝 海), DONG Han-wei(董汉伟),

ZHANG Xing-guo(张兴国), JIN Jun-ze(金俊泽)

School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The effects of external fields such as electromagnetic field on the structure and heat treatment behavior of Mg-Li-Al alloys were studied. Mg-8Li-3Al alloys cast with and without electromagnetic stirring were used for solution treatment and aging treatment. Experimental results show that the dendritic arms are broken and a large quantity of equiaxed grains appear in the microstructure of specimens with electromagnetic stirring (EMS). With the increase of the quenching temperature(150-350 ℃), the solution of Mg and Al in solid β phase increases, and the Brinell hardness of the alloy increases as well. Aging peak and over aging happen because θ(MgLi₂Al) which precipitates in the β matrix and strengthens the alloy is unstable and transforms to stable AlLi phase. Aging curves of EMS specimens change in a smaller amplitude.

Key words: Mg-Li-Al alloys; electromagnetic stirring (EMS); solution treatment; artificial aging; hardness

1 Introduction

With energy crisis and attention of environment protection, light-weight and recyclable materials are in great need. As the lightest alloys among the known metals and alloys, Mg-Li alloys (the density between 1.3 and 1.6 g/cm³) have been attractively investigated and have the application potential in the fields of aerospace, aircraft structures, automobile and electric industry as well as for structural components in ultra-light communication systems[1-4].

At room temperature, Mg-Li alloys are of either α hexagonal close packed (hcp) structure (0-5% Li, mass fraction) or β body-centered cubic (bcc) structure (above 12% Li) or α+β phases (5%-12% Li)[5-6]. But binary Mg-Li alloys have some problems such as over ageing tendency, low creep strength and poor corrosion resistance[6-8]. Many authors attempted to add third alloying elements such as Al and Zn to Mg-Li alloys to strengthen the matrix[9-10]. The commercial magnesium alloy, LA141, containing 14% Li and 1% Al, has been considered for space applications, and it presents good mechanical properties[1]. However, there has been little work reported on the casting process of Mg-Li based alloys.

Electromagnetic stirring is known to be effective to improve the structure and properties in the solidification of alloys[11-13]. The purpose of this work is to study the effect of electromagnetic field on the structure and heat treatment behavior of Mg-Li-Al alloys.

2 Experimental

2.1 Alloy preparation

The raw materials were Mg (99.9%), Li (99.95%) and Al (99.99%). Mg-8Li-3Al alloys were prepared by melting solid metals in a tall stainless steel tube (the ratio of height to diameter is 3:1) in an electric furnace. In order to keep the melt from oxidation, the melt was covered with LiCl+LiF flux (3:1 in mass ratio) and the entire operation was carried out in a high pure argon atmosphere. A plunger was used to force the lithium pieces into the metal liquid of magnesium and aluminum and taken out until the lithium was melted totally. The melt was poured into a stainless steel mould to solidify. The mould was placed in an electromagnetic stirring equipment (Fig.1) with the frequency of 50 Hz and the voltage of 80 V, and the time of the electromagnetic stirring was 3 min.

Fig.1 Schematic illustration of electromagnetic stirring system: 1—Pouring cup; 2—Crucible; 3—Heating apparatus; 4—Coil; 5—Heat insulator; 6—Firebricks

2.2 Heat treatment and hardness testing

In order to keep the Mg-8Li-3Al alloys from the oxidation, every specimen prepared for heat treatment was buried in the graphite powder with aluminum foil packaged. Some specimens were heated for 1 h at 150 ℃, 250 ℃ and 350 ℃ respectively and then quenched into ice water. The other specimens were heated at 350 ℃ for 1 h and then quenched into ice water for aging tests. Aging tests were carried out at room temperature, at   55 ℃ and at 100 ℃ respectively. The Brinell hardness tester HB-3000-I was used to study the hardness change of Mg-8Li-3Al alloys. The load employed was 9 807 N and maintained for 30 s.

2.3 Microstructure observation

All the specimens used for microstructure observation were prepared by cutting, grinding, polishing and etching. An acetic-picral solution (i.e., 10 mL acetic acid, 4.2 g picric acid, 10 mL distil water and 70 mL ethanol) and oxalic acid solution were used to etch. The specimen microstructure was examined with the optical microscope MEF-3.

3 Results and discussion

3.1 Microstructures at as-cast state

The billets were cast with and without electromagnetic field, and the specimens were cut from the bottom and the middle part of the billets, respectively. Fig.2 shows the (α+β) two phase structures of as-cast Mg-8Li-3Al alloys. In the condition of permanent casting without EMS, the dendritic arms become greater from the bottom to the middle part due to different cooling rate at different zones. Compared with the coarse structure of samples without EMS, the grains of samples with EMS are small and equiaxed, no matter at the bottom or at the middle part. This is due to a strong electromagnetic stirring in the liquid pool during the solidification. The melt flow in the liquid pool is obviously strongly turbulent[14-15]. This intensely forced convection promotes the evacuation superheat and breaks the dendrite arm, which leads to grain multiplication. The suspended nuclei localized in the near vicinity of the solid/liquid interface are carried away and dispersed in a slightly undercooled melt. Crystalliza-

Fig.2 Microstructures at as-cast state: (a) Bottom, samples without EMS; (b) Bottom, samples with EMS; (c) Middle part, samples without EMS; (d) Middle part, samples with EMS

tion takes place simultaneously in most of the sump, and around a number of floating nuclei. This increased nucleation results in the appearance of a fine-equiaxed structure. Thus, the grain size of the EMS specimens becomes more uniform and the final structure is more homogeneous.

3.2 Solution treatment

Figs.3 and 4 show the hardness variation curves and microstructures respectively of Mg-8Li-3Al alloys quenched at various temperatures. With the increase of the quenching temperature, the hardness values of the specimens with and without EMS become higher (Fig.3), and β phase of the specimens both grows bigger and thicker due to the solution of Mg and Al in solid β phase(Fig.4). It can be seen in Fig.4, the specimens with EMS show round and uniform grain structure, compared with the dendritic structure of the specimens without EMS, even though the hardness of the latter is a little higher than that of the former.

Fig.3 Brinell hardness as function of quenching temperature

3.3 Aging behavior

Figs.5-7 show hardness variations aged at room temperature, 55 ℃ and 100 ℃ respectively for Mg-8Li- 3Al alloys.

Fig.4 Metallographs of Mg-8Li-3Al alloys quenched at various temperatures: (a) 150 ℃, without EMS; (b) 150 ℃, with EMS;    (c) 250 ℃, without EMS; (d) 250 ℃, with EMS; (e) 350 ℃, without EMS; (f) 350 ℃, with EMS

Fig.5 Aging curves of Mg-Li-Al alloy aged at room temperature

Fig.6 Aging curves of Mg-Li-Al alloy aged at 55 ℃

Fig.7 Aging curves of Mg-Li-Al alloy aged at 100 ℃

For specimens without EMS, aging peak is obvious when the aging time is 2 h and then the hardness curve decreases quickly at room temperature as shown in Fig.5. Aging peak of Mg-Li-Al alloy appears because θ(MgLi₂Al) phase precipitates in the β matrix and strengthens the alloy, and the overage of Mg-Li-Al alloy contributes to the precipitation of stable AlLi phase. However, the hardness of specimens with EMS keeps stable since the peak value appears when the specimens are aged for 2 h. It is supposed that the effects of electromagnetic field on the solidification of Mg-Li-Al alloys cause this phenomenon. The uniform distribution of α and β phase caused by electromagnetic stirring results in the dispersion precipitation of AlLi phase, which is beneficial to keeping a stable hardness of the alloys.

Figs.6 and 7 show that the aging behavior is not clear at 55 ℃ and 100 ℃. Compared with the specimens without EMS, the hardness curve of specimens with EMS keeps more stable with the aging time, and the hardness change of specimens with EMS delays and occurs in a smaller amplitude.

Fig.8 shows the microstructures of specimens without EMS aged for different times at 55 ℃. With the increase of aging time, θ(MgLi₂Al) which precipitates in the β matrix and strengthens the alloy transforms to

Fig.8 Microstructures of specimens without EMS aged for different times at 55 ℃: (a) 0.5 h; (b) 1 h; (c) 2 h

stable AlLi phase，as shown in Figs.8(a) and (b), which coincides with the decrease of hardness in Fig.6. It can be speculated that with the precipitation and increase of AlLi phase a new coherent relation between AlLi phase and β phase creates, which coincides with the hardness increase in Fig.6. Further study is needed on this point.

4 Conclusions

1) Electromagnetic stirring breaks the dendritic arm and leads to the EMS specimen having homogeneous structure.

2) With the increase of the quenching temperature (150-350 ℃), the solution of Mg and Al in solid β phase increases, and the Brinell hardness of the alloy increases as well.

3) Aging peak and over aging happen because θ(MgLi₂Al) which precipitates in the β matrix and strengthens the alloy is unstable and transforms to stable AlLi phase.

4) Aging curves of specimens with EMS change in a smaller amplitude.

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(Edited by ZHAO Jun)

Foundation item: Project (107031) supported by the Key Grant of Science & Technology of the Ministry of Education of China

Corresponding author: HAO Hai; Tel: +86-411-84709458; E-mail: haohai@dlut.edu.cn