Effects of Annular Electromagnetic Stirring Melt Treatment on Microstructure and Mechanical Properties of 7050 Rheo-casting

Tianyang Guan1,2a, Zhifeng Zhang1b*, Min He1c, Yuelong Bai1d and Ping Wang2e

1 General Research Institute for Non-Ferrous Metals, Beijing, China2 Northeastern University, Shen yang, China

anmg_guan@163.com, bzhangzf@grinm.com, chemin5209@126.com, dbai_yuelong@163.com, e wping@epm.neu.edu.cn

Keywords: Annular Electromagnetic Stirring; Melt Treatment; 7050 alloy; Rheo-casting; Microstructure; Mechanical Properties;

Abstract. The microstructure and mechanical properties of 7050 alloy rheo-castings after treated by Annular Electromagnetic Stirring (A-EMS) melt treatment were investigated. The results revealed that, under A-EMS, the refinement and homogeneity of the solidification structure could be improved greatly and the slurry was suitable for the following rheo-casting; and also the hot-cracking defects in the casting process were significantly alleviated, meanwhile, the strength and ductility of the alloy casting were found to be comparable to those of conventionally forged 7000 series alloys.

1.Introduction

The 7050 alloy with high strength and heat treatability, as one of important lightweighting materials, has been successfully applied in industries such as aerospace, rail transit and defense[1]. But because of its high alloy content and large density difference among elements, the inhomogeneous microstructure, serious segregation and hot cracking susceptibility in the casting process are not avoided especially for large-sized ingots and castings [2].

Past research works [3-5] showed that homogeneous composition distribution, uniformly fine microstructure during solidification, and the defects-related properties of these alloys could be effectively achieved by melt treatment including both chemical (chemical additions to the melt) and physical (intensive shearing) methods, leading to a superior combination of higher strength levels and acceptable ductility even in the casting condition [6, 7].

Electromagnetic stirring technology has been promoted as an effective melt treatment method for aluminium castings, however, inhomogeneous microstructure still occurs in the casting products especially large-sized ones due to the skin effect resulting from alternating electromagnetic field [8]. In order to achieve uniformly fine microstructure in aluminum alloy castings, an optimized electromagnetic stirring—annular electromagnetic stirring (A-EMS) was developed, and it can provide intensive stirring in the melt so as to improve uniformity of temperature field and composition field. Moreover, melt can be treated continuously for large-sized ingots and castings.

In this work, based on the past research work [5], 7050 alloy with a high alloying content and typical castings was chosen, and the effects of melt treatment processes by A-EMS on microstructure and mechanical properties of rheo-casting were investigated.

2.Experimental

The melt treatment equipment based on the annular electromagnetic stirring technology is schematically illustrated in Fig.1. The starting materials were commercial aluminium (99.98%), copper (99.99%), zinc (99.92%), magnesium (99.95%), and the Al-4%Zr (wt%) master alloy. Ten kilograms of test alloy were melted in a graphite crucible using a pit type resistance furnace and the smelting temperature was controlled at 750 °C. The measured composition of the test alloy is given

in Table 1. The liquidus and solidus temperature measured by DSC of the test alloy are 629° C and 477° C. After stirring and skimming off the dross, the melt was held for 10 minutes, then fed into the melt treatment equipment for further annular electromagnetic stirring. The stirring current was 30A and stirring frequency was 30 Hz. To compare the effect with and without melt treatment, some amount of melt treated with A-EMS from 750 °C to 635 °C and other amount of melt was cooling down from 750 °C to 640 °C in the air. To evaluate the grain refining effect, both melt poured into the standard TP-1 mould. And these samples were examined in an optical microscope with polarized light after anodizing (4% HBF4 in distilled water). The grain sizes were measured using the standard ASTM E112-96, linear intercept method [9].

Figure 1. Schematic view of melt treatment apparatus by A-EMS(1—smelting crucible 2—annular cooler 3—electromagnetic stirring 4—melt collector)

Table 1. The composition of the test alloy (wt. %).Zn Mg Cu Zr Fe Si Other, total Al

6.45 2.13 1.97 0.12 0.009 0.007 <0.05 Bal.

On the other hand, to compare the mechanical properties, the two kinds of melt were transferred to a 400-ton squeeze machine to produce test samples at the same condition. The die temperature was maintained at about 300 °C, the specific pressure for squeeze casting was controlled at 100 MPa and the dwell time was kept for 30 seconds. The squeeze casting samples with an external size of 100 mm × 100 mm × 120 mm, and wall thicknesses of its four sides were 5 mm, 10 mm, 15 mm and 20 mm, respectively. In order to obtain maximum strength, T6 heat treatment was used for all specimens. The heat treatment technology consisted of homogenization treatment for 12 h at 475 °C, followed by quenching to room temperature with water and finally aging at 120 °C for 24 h. Tensile specimens were cut from the middle section of the test squeeze casting samples. The microstructures were observed by the optical microscopy (OM), scanning electron microscope (SEM).

3.Results and Discussion

3.1 Effect of A-EMS on Microstructures

Figure 2. Optical micrographs of 7050 alloy in the TP-1 casting under different melt treatment conditions: (a) without A-EMS, (b) with A-EMS.

Figure 3. The average grain size of 7050 alloy in the TP-1 casting under different melt treatment condition

Fig.2 shows the optical micrographs of the TP-1 samples treated without and with A-EMS. The microstructure of the samples treated without A-EMS mainly consisted of coarse and non- uniform grains (Fig.2 a). In the case with A-EMS, the grain morphology becomes mainly rosettes with a mount of globular grains and the grain size was sharply decreased (Fig.2 b). It demonstrated that A-EMS melt treatment has the efficiency to restrain grain growth up and change the grain morphology. Fig.3 shows that the average grain size reduced from 342μm without A-EMS to 97μm with A-EMS melt treatment. Besides, the range of error bars indicates a smaller variation in the grain size in the samples treated with A-EMS, which points out a fine and uniform microstructure across the samples.

In general, addition of grain refiners (such as Al-Zr) in 7000 series alloy was found to be beneficial towards grain refinement. The presence of both potent nucleant particles and sufficient solutes is essential for effective grain refinement [10, 11]. In the case without A-EMS, since the melt solidifies from outer part to inner one of castings gradually, the grain size of the casting is not uniform along the solidification direction. What’s worse, the effective nuclei may gather together so that weaken the role of heterogeneous particles. By A-EMS treatment, the intensive shearing flow through the narrow annular gap can accelerate to reach more uniform melt temperature and composition field, the stronger stirring thins the diffusing boundary layer of the solute, decreases the undercooling of the solute, thus, fine equiaxed and uniform microstructure can be obtained.

3.2 Effect of A-EMS on Mechanical Properties

Figure 4. The squeeze casting samples at different melt treatment conditions: (a) without A-EMS, (b) with A-EMS

Fig. 4 shows hot-cracking of the squeeze casting samples under different conditions. It is noted that hot-cracking occurred at almost all the transition positions of different wall thickness in the cross section of the cast samples. While being treated with A-EMS, the hot-cracking disappeared completely.

There exist a large solidification interval and serious hot-cracking susceptibility for the 7050 alloy, the distribution of solute elements and the local temperature in the primary melt is not

uniform. So, in the case without A-EMS, the transition positions of the casting, which solidifies finally, suffers from a large temperature gradient and a large tensile stress, so shrinkage porosity and hot-cracking defects easily take place.

After treated with A-EMS, much latent heat has been released ahead of time during the process of stirring. At the same time the effective nuclei in the melt are more homogeneously distributed compared with that without A-EMS. According to the calculation method applied by Spencer and Flemings[12], the average shear rate can be calculated by Eq.(1):

γ̇=2RrR2- r2 ⋅

2πn60

(1)

where γ̇ is the average shear rate; R is the inner radius of the slurry-preparation house; r is the outer radius of the cooling pipe; n is the rotation speed of the magnetic field and it is determined by Eq.(2):

n =60fp

(2)

where p is the pole number; and f is the frequency. There is a gap between the slurry-preparation house and the cooling pipe. From Eq.(1) and Eq.(2), it can be seen that when the stirring frequency is constant, the narrower the gap is, the larger the average shear rate is. Larger shear rate strengthens the stirring of molten metal and heat transmission, makes the temperature and the distribution of solute elements uniform. [13]. As a result, small spherical microstructures are obtained.

. Figure 5. Mechanical properties of the 7050 alloy in squeeze casting process under different melt treatment

conditions.

Fig.5 shows the mechanical properties of the test alloy in the squeeze casting process. It is noted that, in the case without A-EMS, the average ultimate tensile strength, yield strength and elongation are 523 MPa, 498 MPa and 6.2%； however， in the case with A-EMS, the mechanical properties of the samples increase greatly. The average ultimate tensile strength, yield strength and elongation are 586 MPa, 532 MPa and 10.8%, respectively. Furthermore, this result can be well explained by the Hall Petch relationship [14]. The grain refinement benefits for properties improvement of not only strength but also elongation. Because the microstructure is very homogeneous, the uniformity of the mechanical properties are also improved greatly.

4. Conclusions

1) Processed by A-EMS, uniformly fine microstructures of the test alloy in TP-1 casting process can be obtained, and the average grain size decreases from 342μm to 97μm; also the hot-cracking of the 7050 alloy castings can be suppressed.

2) Processed by A-EMS, the mechanical properties are improved greatly, and the ultimate tensile strength, yield strength and elongation of the test alloy samples in squeeze casting process are 586 MPa, 523 MPa and 10.8%, respectively.

Acknowledgements

The authors gratefully acknowledge the financial support by the International Science and Technology Cooperation Program of China (No. 2015DFA51230)

References

[1] Jürgen H. Recent development in aluminium for automotive applications. Transaction of Nonferrous Metal Society of China, 2014, 24(7), 1995-2002.

[2] Nadella R, Eskin DG, Du Q, Katgerman L. Macrosegregation in direct-chill casting of aluminium alloys. Progress in Materials Science. 2008;53:421-480.

[3] Lu Guimin, Dong Jie, Cui Jianzhong. New technology in Semi-solid making—liquidus casting. Special Casting & Nonferrous Alloys, 2001(S1), 221-223.

[4] Dong Jie, Lu Guimin, Cui Jianzhong. Discussion on the formation mechanism of nondendritic semi-solid microstructures during liquidus casting. Acta Matallurgica Sinica, 2012, 60(4), 1528-1537.

[5] Luo Yajun, Zhang Zhifeng, Gao Mingwei. Study on Grain Refinement and Homogenization of Large-sized DC Casting Ingot by A-EMS Melt Treatment. Materials Science Forum, 2016, 850, 653-658.

[6] H.-T. Li, Y. Wang, Z. Fan. Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al–Mg alloys. Acta Materialia, 2012, 60(4), 1528-1537.

[7] Wang Ping, Liu Jing. Microstructure and Mechanical Property of 6061 Alloy Near-Liquidus Electromagnetic Semi-Continuous Casting and Thixoforging. Rare Metal Materials and Engineering. 2014(08), 1969-1973.

[8] F.C. Robles Hern´andez, J.H. Sokolowski. Comparison among chemical and electromagnetic stirring and vibration melt treatments for Al–Si hypereutectic alloys. Journal of Alloy and Componds, 2006, 426(1–2), 205-212.

[9] Standard Test for Determining Average Grain Size, vol. 14, ASTM International, PA, USA, 2002, pp. 256–281.

[10] Wang Feng, Qiu Dong, Liu Zhilin, John A. Taylor. The grain refinement mechanism of cast aluminium by zirconium. Acta Materialia, 2013, 61(15), 5636-5645.

[11] P Schumacher, A. L. Greer, J. Worth, P. V. Evans. New studies of nucleation mechanisms in aluminium alloys: implications for grain refinement practice. Materials Science and Technology. 1998, 14(5), 394-404.

[12] Spencer D, Flemings M C. Rheological behavior of Sn-15%Pb in the crystallization range. Metallurgical and Materials Transactions, 1972, 3(7): 1925í1932

[13] Z. Fan, X. Fang, S. Ji. Microstructure and mechanical properties of rheo-diecast (RDC). Materials Science Engineering A. 2005,412,298-306

[14] N. Hansen. Hall–Petch relation and boundary strengthening. Scripta Materialia, 2004, 51(8), 801-806.