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Effect of extrusion on the microstructure and thermomechanical behavior of continuously electromagnetic casting AZ31 To cite this article: Qiwei Wang et al 2019 Mater. Res. Express 6 1065b2

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Mater. Res. Express 6 (2019) 1065b2

https://doi.org/10.1088/2053-1591/ab3cd8

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Effect of extrusion on the microstructure and thermomechanical behavior of continuously electromagnetic casting AZ31

REVISED

21 July 2019 ACCEPTED FOR PUBLICATION

20 August 2019 PUBLISHED

Qiwei Wang1, Dan Li1, Yingfei Yang1 , Xiaoming Wang2, Qing Chang2, Guofeng Han2, Sheng Zhu2 and Wei Li1 1

4 September 2019 2

Institute of Advanced Wear & Corrosion Resistant and Functional Material, Jinan University Guangzhou 510632, People’s Republic of China Academy of Army Armored Forces, National Key Laboratory for Remanufacturing, Beijing, 100072, People’s Republic of China

E-mail: [email protected] Keywords: magnesium, AZ31, thermal deformation, dynamic recrystallization

Abstract The thermal deformation behaviour of semi-continuously electromagnetic casting AZ31 magnesium alloy was investigated by compression tests with strain rate controlled for 0.001∼0.1 s−1 at 473 K∼623 K. A detailed analysis with respect to the influencing parameters was then conducted based on the true stress-strain curve, including calculating the deformation activation energy under various magnetic field conditions. Extrusion tests of AZ31 alloy under different casting conditions were performed. The testing parameters were optimized by the available hot compression data. The fracture morphology of preand post-deformation were compared to evaluate the effects of casting conditions on the microstructures and performances of the extrusion bars. The results show that dynamic recrystallization takes place under all the experimental conditions. The stress-strain curve exhibits a peak firstly, and then a drop followed to a stable stage with the subsequent strain accumulation. The value of the stable stage decreases with the increase of temperature but decrease of the strain rate. The dynamic recrystallization leads to a refinement of the grains and hence enhanced the mechanical performance of the materials. In addition, a hereditary effect was found on the properties of the extrusion AZ31 alloys for electromagnetic field frequency.

1. Introduction In recent years, the application of AZ31 wrought magnesium alloy has shown a noticeable increase in industrial usage [1]. With the assistance of various processing techniques like rolling and extrusion, it becomes possible to manufacture the as-cast alloys into products with different shapes, such as rod, profiles and pipes, etc Due to its excellent mechanical and special corrosion performance, magnesium alloys have already shown great potential in the fields of aerospace, vehicle, sports equipment, 3 C device and other industries [2–8]. It is now widely accepted that the deformation of AZ31 magnesium alloy is a thermally activated process [9, 10]. Therefore, it’s of great significance to study the thermomechanical behavior of the casting materials at elevated temperatures. When metals deform at low temperature, deformation hardening occurs. In contrast, deformation hardening as well as deformation softening are simultaneously performed when metals deform at high temperature as they are in a highly plastic state [11–14]. The deformation hardening often leads to an increase in dislocation density and formation of various dislocation configurations [15]. However, the deformation softening reduces the dislocation density and forms a low-energy state structure [11]. When deformation softening proceeds under elevated temperature, the softening process is main composed by dynamic recovery and dynamic recrystallization. In order to figure out the deformation softening behavior and the dynamic recrystallization characteristic of AZ31, the thermal compression was conducted under different temperature and different strain rates.

© 2019 IOP Publishing Ltd

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Table 1. Casting conditions of the ingots. Sample

Casting condition

Ingot radius

V=200 mm min−1, f=15 Hz, I=120 A V=200 mm min−1, f=30 Hz, I=120 A, with 0.0075% Be V=230 mm min−1, no electromagnetic field, with 0.0075% Be

1 2 3

100 mm

Table 2. Chemical compositions of AZ31 alloys (wt%).

ASTM As-cast As-cast with Ca As-cast with Be

A1

Zn

Mn

Be

Fe

Ca

Mg

2.5–3.5 3.3–3.7 3.2–3.7 3.3–3.7

0.6–1.4 1.17–1.27 1.15–1.25 1.17–1.27

0.2–1.0 0.106–0.27 0.106–0.28 0.106–0.27

<0.1 0.001 <0.1 0.0075

<0.005 0.009–0.012 <0.005 <0.005

− − 2 −

Bal. Bal. Bal. Bal.

Table 3. Hot compression processing parameters. temperature strain rate

473 K

523 K

573 K

623 K

0.001 s−1 0.01 s−1 0.1 s−1

+ + +

+ + +

+ + +

+ + +

On the other hand, continuously electromagnetic casting is a very common process for producing magnesium ingots [16–19]. The hot compression behavior of AZ31 casted by various conditions were evaluated through a systematic comparative study and the corresponding deformation activation energy was measured. Based on the obtained data, the effect of casting conditions and extrusion on the microstructure and mechanical property were discussed.

2. Experimental procedures 2.1. Experimental materials Samples for hot compression test were machined from the annealed magnesium produced under different electromagnetic casting conditions. The sample size was Φ8 mm×15 mm. The ingot are casted according to parameters listed in table 1. The ASTM standard composition and chemical analysis results of the as-cast AZ31 magnesium alloy are shown in table 2. 2.2. Hot compression test The hot compression test was carried out on a Gleeble-2000 thermal simulator. The heating rate of the sample was maintained at 10 °C/s. The alloy was holding for 1 min when the deformation temperature was reached before compressing. The total amount of compression strain was kept for 0.6 (true strain). A subsequent water quenching was immediately continued after compression in order to acquire the retained original deformation microstructure. The detailed parameters for hot compression are listed in table 3. 2.3. Extrusion test The extrusion test of AZ31 magnesium alloy was conducted on a 5-ton vertical hydraulic machine. The extruded billet was taken from the center of the low-frequency electromagnetic semi-continuous casting ingot. After homogenization and annealing at 693 K for 12 h, the billets were machined as a dimension of 40 mm×120 mm and then followed by the extrusion test. A reversal extrusion method was conducted with the extrusion ratio setting at 25 and extrusion speed maintained at 12 mm s−1. Molybdenum disulfide and graphite powder were used as lubricant during the extrusion. The mold temperature and the extrusion temperature were maintained at 613 K. 2

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Figure 1. True stress-strain curves at different strain rate: (a) 0.001 s−1, (b) 0.01 s−1, (c) 0.1 s−1.

2.4. Microstructure For metallographic characterization, the working surface of the samples was sequentially grounded from 400# to 2000# abrasive papers and then polished by a grinding machine with finely stained magnesium oxide. Finally, a cleaning with ethanol was conducted to minimize the effect of water on the magnesium alloys. The prepared samples were etched before OM and SEM observation. For the as-cast sample, etching were conducted with HNO3 solution (5 vol% HNO3 and 95 vol% ethanol) for 5 to 10 s before consequently rinsing in water and anhydrous ethanol. While for the deformed sample, etching solution was composed of 10 ml acetic acid, 5 g Picric acid, 10 ml water, 100 m1 ethyl alcohol. The grain size was measured by the cut line method according to the ASTM E112-95 standard and the average grain size was achieved by measuring grain size of three different locations in the sample surface. 2.5. Mechanical properties test The macro-hardness test was carried out on a Vickers Hardness Tester (450SVD-TM) with the load maintained at 5 kg for 30 s. For the accuracy, the average hardness was calculated from four points for each sample.

3. Result and discussion 3.1. Effect of strain rate and deformation temperature on the dynamic recrystallization behavior and flow stress of the as-cast AZ31 Figure 1 shows the true stress-strain under different temperature and different strain rate. With the true strain increasing, the flow stress firstly increases and then slowly decreases after a peak flow stress for the strain rate controlled at 0.001 s−1 and 0.01 s−1 and temperature ranged from 473 K to 623 K. All the flow stress reached its peak when the strain is in the range of 0.1–0.2 and then kept nearly constant when the train is larger than 0.6. This actually is a typical character of the true stress-strain curve for dynamic recrystallization, indicating the occurring of the completely dynamic recrystallization under the strain rate of 0.001 s−1 and 0.01 s−1. Same trends were observed in the true stress-strain curves when the deformation temperature are 523 K, 573 K and 623 K with the strain rate kept for 0.1 s−1. However, it is intriguing to notice that the flow stress for deformation at 473 K kept almost unchanged when the peak stress is reached, which is totally different from the true stress3

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Figure 2. Microstructures of the as-cast AZ31 deformed at a settled strain rate 0.01 s−1 with different temperature: (a) T=473 K, (b) T=523 K, (c) T=573 K, (d) T=623 K.

strain curve under other conditions. This may because that dynamic recovery has occurred already, hinting that the alloy did not undergo completely dynamic recrystallization at lower deformation temperatures and higher strain rates. When the alloy deformed at a settled strain rate, the flow stress increases as the deformation temperature decreases, meaning a larger obstacle for recrystallization under low temperature. However, the stress peak nearly disappeared in the stress-strain curve under the condition of small strain rate (0.001 s−1) and high deformation temperature (623 K) and a basically steady state continued during the whole strain range (figure 1(a)). Theoretically, the higher temperature would accelerate the diffusion of atoms inside the alloy and intensify the movement of the screw dislocation and blade dislocation [20]. For magnesium alloy, whose deformation is mainly based on the slip of the dislocation, the elevated temperature would motivate the sliding both in cylindrical surface and conical surface. As a result, dislocation accumulation is greatly reduced and the flow stress decreased as well. Meanwhile, when the temperature increases, the number of nucleation as well as the growth rate of nuclei during the dynamic recrystallization increase, which would significantly speed up the dynamic recrystallization process as the nucleation rate of dynamic recrystallization is controlled by thermal activation [21]. On the other hand, strengthening effect of the second phase particles (parts of the second phase were tagged out by the red arrows in figures 2 and 3) on the alloy reduced when the temperature increases, which means the slipping obstacle of dislocations decreased. Hence, deformation resistance of the alloy decreased correspondingly. When make a comparison between the three true stress-strain curves in figure 1, it can be noted that the flow stress increases with the increase of strain rate under a constant temperature. For example, the peaks of the flow stress at 637 K increases from 47 MPa to 83 MPa when the strain rate increases from 0.001 s−1 to 0.1 s−1, which indicates that the as-cast AZ31 magnesium is sensitive to positive strain. In addition, as the strain rate increases, the peaks of flow stress shifted towards higher deformation strain, hinting the difficulty in recrystallization for the as-cast AZ31 under higher strain rate. This may be because the dislocation density increases rapidly under high strain rate and the extent of work hardening of the alloy enlarged correspondingly, which lead to the increase of the flow stress directly [22–24]. On the other hand, the deformation process of the as-cast AZ31 sustained for a very short time when the strain rate increases, which lead to a decrease in the number of nuclei during recrystallization and the extent of work softening. As a result, the flow stress increased indirectly [25]. The magnesium used in the current work is electromagnetically casted. Figure 2 shows the microstructure of the as-cast AZ31 (V=200 mm min−1, f=30 Hz, and I=120 A) deform at a settled strain rate of 0.01 s−1 with various temperatures. It can be noted from figure 2 that the alloy recrystallized under all temperature and the recrystallized grain size grown with the increase of temperature. Figure 3 shows the microstructure of the as-cast 4

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Figure 3. Microstructures of the as-cast AZ31 deformed at 573 K with different stain rate: (a) 0.1 s−1, (b) 0.01 s−1, (c) 0.001 s−1.

Table 4. Peak stresses (MPa) obtained from the flow stress-strain curves under continuous electromagnetic field with V=200 mm min−1, f=30 Hz, and I=120 A (MPa). Strain rate / s−1

473 K

523 K

573 K

623 K

0.001 0.01 0.1

142.54 177.38 190.62

102.61 120.01 160.35

72.07 88.82 117.40

52.40 79.58 100.31

AZ31 after deformation under different strain rate at 573 K. In comparison, the recrystallized grain size decreased when the strain rate gradually increased from 0.001 s−1 to 0. 1 s−1. Based on the experimental results and analysis above, it can be concluded that the dynamic recrystallization state of the magnesium and grain refinement can be quickly reached by increasing deformation temperature and decreasing the strain rate, which provided a guidance to deciding the parameters for the following extrusion in order to achieve a satisfactory dynamic recrystallization during the extrusion process. 3.2. Effect of electromagnetic field on thermal deformation activation energy of AZ31 magnesium alloy A potential barrier, which is so-called activation energy, should be overcome during the recrystallization transformation when the metal is plastically deformed [26]. The activation energy is important for recrystallization transformation process. In addition, knowing the exact activation energy is of great help to formulate the recrystallization process reasonably. Therefore, the recrystallization activation energy of AZ31 magnesium casted by the continuous electromagnetic field (V=200 mm min−1, f=30 Hz, and I=120A) was calculated under different casting parameters. Table 4 shows the values of peak stress obtained from the flow stress-strain curve. When metals deform at elevated temperature, the stress σ is determined by deformation temperature T and strain rate e , which could be described by various formulations according to different strain conditions [27–30]. For deformation under low strain and high strain, the flow stress σ can be expressed by equations (1) and (2), respectively. However, for all strain rate, the flow stress σ can be depicted by a sine relation as in equation (3) without considering the value of strain rate: ⎛ Q ⎞ ⎟ e = A1 s n1 exp ⎜ ⎝ RT ⎠

5

(1)

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⎛ Q ⎞ ⎟ e = A2 exp (bs ) exp ⎜ ⎝ RT ⎠

(2)

⎛ Q ⎞ ⎟ e = A3 [sinh (as )]n1 exp ⎜ ⎝ RT ⎠

(3)

Where e is the strain rate, A1, A2, A3, n1, b and a are constants, and α=β=/n1, Q is the activation energy, R is the gas constant, and T is the deformation temperature. On the other hand, the relationships between e and s can be portrayed by a Z parameters as in equations (4) or (5) [31]: ⎛ Q ⎞ ⎟ = A [ sinh (as )]n1 Z = e exp ⎜ ⎝ RT ⎠

(4)

or: 1

sinh (as ) = (Z / A) n1

(5)

According to the definition of hyperbolic sine function: sinh (as ) = (e as - e -¥) / 2

(6)

Equation (5) can be displayed as: as = ln {sinh (as ) +

[sinh (as )]2 + 1 }

(7)

Combining equations (5) and (7), the flow stress can be expressed as a function of Z: ⎡⎛ 2 ⎞ ⎤1 / 2 1 ⎧⎛⎜ Z ⎞⎟1 / n ⎫ ⎢⎜ Z ⎞⎟ n ⎟ ⎬+ ln ⎨ s= + 1⎥ ⎥⎦ a ⎩⎝ A ⎠ ⎭ ⎢⎣⎜⎝ A ⎠ ⎟⎠

(8)

Equation (8) can be used to determine the flow stress under all conditions if the values of A, Q, n and α are obtained, which is convenient to calculate the relationship between flow stress and strain rate, temperature and so on during thermal processing of materials. For the sake of convenience, take the logarithm of the equations (1) and (2): ln (e ) = ln (A1) + n1 ln (s )

(9)

ln (e ) = ln (A2 ) + bs

(10)

Input peak value of flow stress (table 4) into equations (9) and (10) and the relationship between e and σ can be drawn as figure 4, where the solid line was the results of linear regression. n1 and β are the slop of the lines in figures 4(a) and (b), respectively. For the accuracy, the average of n1 and β are calculated for five lines from each figure, which are 10.17 and 0.0902 MPa. Then the value of α can be obtain by α=β/ n1=0.00887 MPa−1. Take the natural logarithm of both sides of equation (3), and assume the deformation activation energy is independent to temperature, then a linear relationship can be achieved between ln e and ln [sinh(ασ) as equation (11) ln e = A + nln [sinh (as )]

(11)

Input the value of peak flow stress and strain rate obtained at different temperature, the curves of ln e - ln [sinh(ασ)] can be drawn as figure 4(c). The solid line in figure 4(c) was acquired by linear regression, which is in good agreement with calculated results, indicating the hyperbolic sinusoidal relationship between stress and strain rate for as-cast AZ31 during high temperature compression deformation. For deformation under a constant strain rate, equation (12) can be derived from equation (4) if the activation energy are presumed to remains constant over a certain temperature range: ln e + ln [sinh (as )] =

Q - ln A 1000 RT = A3 + B n T

(12)

Input the peak value of flow stress into equation (12) and figure 5 can be obtained. The solid line in figure 5 is the results of linear regression, which concurs well with the calculated results and it implies the conformation of the hyperbolic sinusoidal relationship between stress and temperature. Differentiate equation (12) then equation (13) can be achieved as: ⎧ ⎫ ⎧ ¶ ln [ sinh (as )] ⎫ ¶ ln e ⎬ ⎨ ⎬ Q = R⎨ ⎩ ¶ ln [ sinh (as )] ⎭T ⎩ ⎭e ¶ (1 / T )

The term in the bracket refers to the slope of the lines in figures 4 and 5, which are 7.457 and 2.1875, respectively. Therefore, the average activation energy Q is calculated as 135.55 KJ·mol−1. 6

(13)

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Figure 4. Relationships between peak flow stress and strain rate.

Figure 5. Relationships between peak stress and temperature.

The activation energy under different temperatures and different strain rates can be calculated by the same method as well, which is displayed in figure 6(a). Similarly, activation energy of the alloy casted under other conditions were calculated also using the value of peak flow stress listed in tables 5 and 6 and the results are shown in figures 6(b) and (c). For the sake of comparing expediently, the average activation energy at different casting conditions are calculated and compared as displayed in figure 7. It can be noted that that the average activation energy for samples casted under electromagnetic field is higher than that without electromagnetic field. The state of V=200 mm min−1, f=30 Hz, I=120 A has the highest average activation energy. Furthermore, the less activation energy the material required, the easier the recrystallization will be. 7

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Figure 6. Activation energy at different conditions: (a) V=200 mm min−1, f=30 Hz, I=120 A, with 0.0075 wt% Be, (b) V=200 mm min−1, I=120 A, f=15 Hz, (c) V=230 mm min−1, no electromagnetic field, with 0.0075 wt% Be.

Table 5. Peak stresses (MPa) obtained from the flow stress-strain curves under continuous electromagnetic field with V=200 mm min−1, f=15 Hz, and I=120 A (MPa). Strain rate / s−1 0.001 0.01 0.1

473 K

523 K

573 K

98.795 115.421 154.089

66.533 84.464 107.926

48.282 62.773 91.092

Table 6. Peak stresses (MPa) obtained from the flow stress-strain curves without electromagnetic field (MPa). Strain rate / s−1

473 K

523 K

573 K

0.001 0.01 0.1

104.73 123.27 152.05

60.20 92.83 112.23

50.05 60.93 96.32

3.3. Influence of extrusion on the microstructure and mechanical properties of AZ31 AZ31 under different casting conditions (table 7) were extruded and the microstructure as well as the mechanical properties after extrusion were explored. In order to eliminate the dendrite segregation and internal stress in the magnesium alloy, the ingot samples are annealed at 693 K for 12 h to obtain a homogeneous microstructure before extrusion. However, it is inevitable that grain grown during the annealing, which will result in a much larger grain size and a change in overall performance of the alloy. The macroscopic grain of AZ31 after annealing is shown in figure 8, where the grain size is about 1000 μm. The inserted figure in figure 8 shows the macro morphology of the extruded bars, which displayed a good surface quality after extrusion. 8

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Figure 7. Activation energy at different casting conditions: (a) V=200 mm min−1, f=30 Hz, I=120 A, with 0.0075 wt% Be, (b) V=200 mm min−1, I=120 A, f=15 Hz, (c) V=230 mm min−1, no electromagnetic field, with 0.0075 wt% Be.

Table 7. Casting conditions and components of extrusion billets. Samples 1 2 3 4

Casting speed /mm/min

Current intensity /A

Electromagnetic field frequency /Hz

Alloying element

200 200 200 200

120 120 120 120

15 15 15 30

null 2 wt%Ca 0.0075 wt%Be 0.0075 wt% Be

Figure 8. Macrostructure of AZ31 magnesium alloy annealed at 693 K for 12 h with the inserted figure showing the macro surface morphology of the bars after extrusion.

3.3.1. Microstructure after extrusion Figure 9 shows the microstructure of extruded bars from ingots with different electromagnetic casting conditions. It can be seen that recrystallization occurs after extrusion, and fine equiaxed grains are obtained. The recrystallized grain structure is particularly sensitive to the original grain size. When the original grain size was larger, the new grain size is also coarser, and vice versa. By comparing the figures 9(a) and (b) with figures 9(c) and (d), it can be noted that the addition of Ca led to the refinement of grain size. The average grain size casting at the same electromagnetic field frequency with and without Ca and is 8 μm and 10 μm, respectively. However, average grain size is 18 μm after addition of Be (figures 9(g) and (h)), hinting coarsening effective of grains by adding Be. When the electromagnetic field frequency increase from 15 Hz (figures 9(e) and (f)) to 30 Hz (figures 9(g) and (h)), the average grain size decreases from 18 μm to 15 μm, which means the intense of electromagnetic field frequency played a role in grain refinement. 9

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Figure 9. Microstructures of the bars after extrusion under different electromagnetic condition: (a) f=15 Hz, cross section, (b) f=15 Hz longitudinal section, (c) f=15 Hz, with 2 wt% Ca, cross section, (d) f=15 Hz, with 2 wt% Ca, longitudinal section, (e) f=30 Hz, with 0.0075 wt% Be, cross section, (f) f=30 Hz, with 0.0075 wt% Be, longitudinal section, (g) f=15 Hz, with 0.0075 wt% Be, cross section, (h) f=15 Hz, with 0.0075 wt% Be, longitudinal section.

3.3.2. Mechanical properties after extrusion Table 8 shows the mechanical properties of the ingots prepared by different electromagnetic casting processes before and after extrusion. It can be seen that the tensile strength and yield strength after extrusion are 20% higher than the corresponding as-cast state under the all conditions. The elongation is even more than twice after extrusion and further increased after adding Ca, where the elongation is ten times larger than the original condition. Figure 10 compared the fractural surfaces of the as-cast and extruded materials under the condition of V=200 mm min−1, I=120 A, f=15 Hz, without Be and with 2 wt% Ca. It can be seen from figure 10(a) that the as-cast alloy is in the form of brittle fracture and only a small amount of dimples appeared. However, the 10

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Table 8. Mechanical properties before and after extrusion. σb/MPa

Casting condition V=200 mm min−1, I=120 A, f=15 Hz V=200 mm min−1, I=120 A, f=15 Hz, with 2 wt% Ca V=200 mm min−1, I=120 A, f=15 Hz, with 0.0075 wt% Be V=200 mm min−1, I=120 A, f=30 Hz, with 0.0075 wt% Be

δ

σ0.2/MPa

Before

After

Before

After

Before

After

213 168 172 195

255 288 255 270

11% 1.4% 6.2% 6.7%

22.4% 15% 15.8% 16.3%

85 100 81 83.5

139 199 147 151

Figure 10. SEM images of tensile fracture surface before and after extrusion (V=200 mm min−1, I=120 A, f=15 Hz): (a) As-cast, without Be, (b) Extruded, without Be (c) As-cast, with 2 wt% Ca, (d) Extruded, with 2 wt% Ca.

fracture surface after extrusion (figure 10(b) shows a typical ductile fracture morphology and the dimple is elongated. Figures 10(c) and (d) show the fracture surfaces of the as-cast and extruded materials under V=200 mm min−1, I=120 A, f=15 Hz plus 2 wt% Ca. In contrast, the amount of the fracture dimples increases and its size decreases by adding Ca. It is also intriguing to observe that an obvious transition from brittle fracture to ductile fracture appeared after extrusion. Based on the above results, the addition of Ca played a significant role in refining grain size and improving mechanical properties. Actually, Ca is a common element used in magnesium to refine grains. The solubility of Ca in magnesium is limit and its diffusion rate is low. As a result, Ca would prefer to aggregate at the solid/liquid interface during the solidification process and forming a constitutional supercooling zone, which prevent the growth of the grains [32, 33]. Besides, the Al2Ca phase, whose melting temperature is 1352 K, would precipitate prior to Mg17Al12 ( whose melting temperature is 710 K) and prevented the grains from growing in the following treatments. According to the Hall-Petch relationship in equation (14), the mechanical properties would be improved by when the grains are refined by addition of Ca. s = s0 + kd1 / 2

(14)

where σ is the yield stress, σ0 is the friction stress and k is a positive constant of yielding associated with the stress required to extend dislocation activity into adjacent unyielded grains and d is the grain size. On the other hand, the mechanical properties of AZ31 magnesium alloy have been improved to a large extent after extrusion. This mainly stems from the refined microstructure due to dynamic recrystallization. 11

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Figure 11. Effects of frequency (a) on tensile strengths, (b) on yield strength and (c) on elongation after extrusion.

Meanwhile, the extrusion eliminates the formation of the various defects in the casting process: firstly, the columnar crystals and coarse dendrites are broken during the extrusion process, which will make the segregation uniform; secondly, the density of the material is increased by the compressing of the pores of the ingots; finally, under triaxial compressive stress, the shrinkage and looseness produced by casting can be reduced largely. By controlling the extrusion process, an ultimate grain size of less than 10 μm can be obtained, which shows an excellent ductility and strength. 3.3.3. Effects of electromagnetic field frequency on mechanical properties of the extruded alloy Figure 11 shows uniform elongation, yield strength and tensile strength of the extruded materials with different electromagnetic field frequency. When the electromagnetic field frequency increase, the elongation, yield strength and tensile strength were improved, which means that the influence of electromagnetic field frequency on mechanical properties is hereditary and it even remains unchanged after extrusion.

4. Conclusions (1) The flow stress-strain curve of AZ31 magnesium alloy cast under different casting conditions exhibits the characteristic of dynamic recrystallization. The stress decreases with the increase of temperature and/or decrease of strain rate, which shows a positive strain rate sensitivity. (2) The activation energy is larger in the condition of electromagnetic field and it is increases with the increase of electromagnetic field frequency. (3) AZ31 alloys casted under all various electromagnetic field conditions experienced dynamic recrystallization during extrusion. The grains were remarkably refined which results in the greatly improved mechanical properties. (4) The mechanical properties of the as-cast AZ31 alloy was significantly enhanced by addition of Ca and/or increasing electromagnetic field frequency. In comparison, addition of Be played a role in reducing the ductility of the as-cast AZ31 alloy. (5) The increase in electromagnetic field frequency resulted in an improvement of the mechanical properties both for the as-cast ingots and the extrusion billets, where a hereditary effect on the mechanical properties was shown by the electromagnetic field. 12

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Acknowledgments This work was financially supported by the Postdoctoral Science Foundation of China (Grant No2014M562617) and the ‘Guangdong Province Science and Technology Plan’ (Grant No. 2017B090903005). This project was also sponsored by Scientific Research Funds for the Talents and Innovation Foundation of Jinan University, Guangzhou, China.

ORCID iDs Yingfei Yang

https://orcid.org/0000-0001-9965-3033

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