Trans. Nonferrous Met. Soc. China 31(2021) 865-877

Eliminating shrinkage defects and improving mechanical performance of large thin-walled ZL205A alloy castings by coupling travelling magnetic fields with sequential solidification

Lei LUO, Hong-ying XIA, Liang-shun LUO, Yan-qing SU, Chao-jun CAI, Liang WANG, Jing-jie GUO, Heng-zhi FU

National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

Received 16 May 2020; accepted 22 December 2020

Abstract: ZL205A alloys with large thin-walled shape were continuously processed by coupling travelling magnetic fields (TMF) with sequential solidification, to eliminate the shrinkage defects and optimize the mechanical performance. Through experiments and simulations, the parameter optimization of TMF and the influence on feeding behavior, microstructure and properties were systematically studied. The results indicate that the magnetic force maximizes at the excitation current of 20 A and frequency of 200 Hz under the experimental conditions of this study, and increases from center to side-walls, which is more convenient to process thin-walled castings. TMF can break secondary dendritic arm and dendrites overlaps, widen feeding channels, prolong the feeding time, optimize the feeding paths, eliminate shrinkage defects and improve properties. Specifically, for as-cast state, TMF with excitation current of 20 A increases ultimate tensile strength, elongation and micro-hardness from 186 MPa, 7.3% and 82.1 kg/mm² to 221 MPa, 11.7% and 100.5 kg/mm², decreases porosity from 1.71% to 0.22%, and alters brittle fracture to ductile fracture.

Key words: ZL205A alloys; large thin-walled alloy castings; travelling magnetic fields; sequential solidification; shrinkage defects; mechanical performance

1 Introduction

ZL205A alloys are the Al-Cu based alloys with the outstanding comprehensive mechanical properties such as the low density, high specific strength, ductility and toughness [1,2]. Accordingly, they are extensively employed in the automobiles, aerospace, aviation and military fields, and especially used as large thin-walled castings [3,4]. While, continuous casting is one of the main forming methods for large castings being  explored [5]. However, during the continuous casting process, the large solidification intervals of ZL205A alloys, i.e., the difference between the liquid and solid temperatures, are prone to inducing the unstable solid-liquid interface to form cellular and dendritic crystals [6-9] and easily forming the compositional undercooling in mushy zones, so that the deflected and disordered growth of the primary dendrites ensues, accompanied by the prolific formation of the secondary dendrites [10]. In this case, the feeding channels between the dendrites in the mushy zones become blocked, resulting in the increase of the shrinkage defects [11-13]. Additionally, all these results are more harmful to the large thin-walled alloy castings, because the melt flows and feeding capacity are seriously restricted and reduced due to their size and shape. Ultimately, the mechanical properties, production efficiency and material utilization will be slashed. Consequently, finding ways to effectively solve the feeding problems and improve the mechanical performance of the large thin-walled ZL205A alloy castings during the solidification process is of great significance.

Currently, many methods have been proposed to enhance the feeding capacity of the alloy melt, such as changing the gating system [14], altering the casting modes [15,16], regulating the melt condition [17,18] and applying external physical fields [19-21]. However, changing the gating system, altering the casting modes and regulating the melt condition will increase the complexity of the solidification process and even cause secondary pollution to the alloy melt. In addition, these methods will increase the waste of materials and the manufacturing costs. Contrastively, applying an external physical field, especially the travelling magnetic fields (TMF), is more suitable for rapid, efficient and automatic casting, because of their non-contacting, pollution-free and easily-controlled advantages [22,23]. Meanwhile, the sequential solidification (SS) process is often used as one of the most important methods to regulate the stability of the mass and heat distribution in mushy zones, and it can effectively replace the continuous casting process to obtain more stable solidification control conditions [24].

Correlational studies [25,26] have suggested that TMF processing can promote the crushing and breaking of the secondary dendrite arms, and transfer the shredded fragments to the high- temperature melt zone or the undercooled liquid region for re-melting or regenerating nucleation. Noteworthily, appropriate parameters of TMF can promote the directional growth of matrix phase  α(Al) and restrict the growth of secondary dendrite together in Al-Cu based alloys [27]. Moreover, buoyancy and Marangoni forces as well as the Lorentz forces induced by TMF will generate various sequential forced melt flows instead of natural convection in the alloy melt, resulting in effective heat and mass transmission [28,29], as well as the microstructural evolution [30]. Besides, other literatures [31,32] have also proposed that magnetic fields stirring can effectively penetrate the mushy zone so as to improve the solidification feeding and eliminate the porosity. However, most of the studies focused on the rod-like samples of Al-Cu binary model alloy, regardless of the size and shape of alloys, nor of the multi-element and multi-phase of alloys. In summary, none of the current research involves applying the TMF to the ZL205A alloys castings with large solidification intervals and multi-phase, especially, with the large thin-walled shape. Therefore, the study on whether TMF can effectively improve the feeding capacity, reduce the shrinkage porosity and increase the mechanical properties of large thin-walled ZL205A alloys castings is scarce. Similarly, the influence mechanism of TMF on the feeding behavior is also relatively lacking.

In this regard, in the current work, we couple TMF with SS process to improve the feeding capability of ZL205A alloys with large solidification intervals, multi-phase and large thin-walled shapes by continuously regulating the mushy zone and solidification behavior. Furthermore, by combining the simulations with experiments, we conduct systematic studies on the optimization of the TMF parameters, and on the variations of solidification process, microstructures, and mechanical properties induced by TMF, as well as the related mechanisms. Our objective is to propose an innovative method and idea for decreasing the shrinkage defects and increasing the mechanical properties of large thin-walled castings made of ZL205A alloys with a large solidification interval, and to elaborate the mechanisms of them.

2 Experimental

2.1 Materials preparation

Fig. 1 Preparation schematic diagram and simulation model

The ZL205A alloy samples (Al-5.0wt.%Cu- 0.3wt.%Mn-0.15wt.%Ti) were prepared by a self-designed TMF processing equipment with SS function as shown in Fig. 1(a). The TMF was regulated and characterized by the alternating excitation current, of which the intensity was controlled by the Autotransformer (XTSGC1, China), while the frequency was controlled by the Variable-frequency Drive (MTB030G, China). In addition, the temperature gradient (G_T) and cooling rate (v_c) measured by several times were 2 K/mm and 0.3 K/s during the SS process. It is worth mentioning that, the temperature measurements were performed by using the combination of K-thermocouple with multichannel temperature recorder (MIK-4000D, China). Wherein, the temperature gradients (G_T) were measured by using three sets of thermocouples distributed equidistantly along the direction of temperature gradient. While, the cooling rates (v_c) were calculated by the time- temperature curves recorded by multichannel temperature recorder. Furthermore, a constant downward drag velocity (v_d =150 μm/s) was used to make sure that the solid-liquid interface can move up at the same speed as the samples move down, so as to effectively fix the mushy zones into the area of TMF. And the constant downward drag velocity (v_d) was given by

v_d=v_c/G_T                                                  (1)

Each sample was melted at 1000 K in a resistance furnace and degassed with high-purity argon. After holding for 15 min, the alloy melt was poured into a cylindrical graphite mold with an inner diameter of 170 mm, an outer diameter of 180 mm and a length of 240 mm. The cylindrical graphite mold was preheated in the tube furnace at 1000 K (Fig. 1(a)), where the SS process with or without TMF was carried out. Finally, the large thin-walled castings with a wall thickness of 5 mm

and a height of 240 mm were obtained by SS process with the constant downward drag velocity v_d =150 μm/s, under the different TMF processes. It is worth noting that the excitation current was used to represent the strength of the magnetic fields, because of the strong spatial and temporal dependence of TMF [33]. And the different excitation currents (I_e=0, 5, 10, 15, 20 A) were selected to perform a semi-quantitative study of the effects of TMF on the microstructure and the feeding capacity of alloys. In this study, due to the limitation of equipment, the maximum current was set to 20 A, and related laws of TMF were studied under these conditions.

2.2 Measurement and analysis methods

We selected the same positions of samples along the direction of temperature gradient to analyze the microstructure in longitudinal and cross sections, as well as the performance (Fig. 1(b)).  The microstructural morphology and chemical composition were respectively analyzed by scanning electron microscopy (SEM, Quanta 200FEG, FEI, USA) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA). The measurements of tensile strength were performed at room temperature with a strain rate of 10^-3 s^-1 by universal testing machine (Instron5569, Instron, USA), and the tensile axis was chosen parallel to the direction of temperature gradient. The tensile tests were repeated 3 times and the average value was taken, of which the standard deviation was approximately 5%. Microhardness was measured by a hardness tester (HVS-1000A, Laizhou Huayin, China) with a 300 g load and  dwell time of 10 s. Similarly, the average value of 10 times measurements was taken, of which the standard deviation was about 5%. The density tests of the samples were conducted by the high precision digital display density meter (QL-202GR, Shenzhen Qunlong, China); and the average of 30 times measurements was selected, of which the standard deviation was about 5%. Besides, the strength of the magnetic fields used in the experiments was measured by tesla-meter (HT201A, Shanghai Hengtong, China).

2.3 Simulations and calculations

The magnetic fields and magnetic force density were simulated by the Ansoft-Maxwell software (ANSYS Inc., USA). ESI-Procast software (ESI, FRA) was utilized to emulate the SS process, including the distributions of temperature and shrinkage defects in the alloy castings. Relevant parameters applied in the simulations are displayed in Table 1, and the simulated models are shown in Figs. 1(c) and (d). In the simulations of magnetic fields and magnetic force density, the following assumptions were made: (1) the permittivity, permeability and material conductivity involved in the calculations were assumed to be constant, regardless of the influence of temperature; (2) each set of coils was considered as a unit, with the magnetic flux leakage between each two winds neglected; (3) the refractories attached to the TMF generator and the insulation materials in the experiment were modeled as regions of vacuum, because of their low permeability and non-conductivity; (4) the heating of the melt caused by TMF was not considered in the simulations. Analogously, in the calculations of the solidification behavior produced by TMF during the SS process, the following assumptions were made: (1) MILE algorithm was used to simulate the SS process; (2) the magnetic force added into the alloy melt were regarded as the average magnetic force; (3) the average magnetic force applied to each position in the melt was considered as equal; (4) the Niyama criterion was used to represent the quantity of shrinkage defects.

Table 1 Related parameters used in simulations of magnetic fields and solidification behavior [27]

3 Results

3.1 Relevant laws of magnetic fields and magnetic force

Firstly, by using a small TMF generator, we simulated the magnetic fields in the alloy melt during the SS process; meanwhile, we measured and verified the results by a Tesla-meter. The intensity of magnetic fields along longitudinal direction shown in Fig. 2(a) indicates an edge effect [34] appearing in the alloy melt. In other words, the magnetic field intensity is relatively stable in the middle position of magnetic field action space and decreases sharply at both ends. Therefore, during SS process, the mushy zone of alloy melt should be kept in the middle position of magnetic fields generator along the longitudinal direction by the constant downward drag velocity (v_d). In addition, the changes in excitation current parameters, including the intensity and the frequency, will affect the values of induced current density, magnetic intensity and magnetic force in the alloy melt, as well as their distributions in the alloy melt.

Fig. 2 Correlation law of TMF

From Figs. 2(b) and (d), it can be concluded that magnifying the excitation current can largely improve the intensity of induced current density and magnetic fields in the alloy melt. Additionally, by enhancing the frequency of excitation current, the induced current will be increased largely (Fig. 2(c)). Oppositely, the magnetic intensity will decrease as the frequency of excitation current increases (Fig. 2(e)). Besides, along the radial direction, the intensity of induced current density and magnetic fields are increased as the distance to the side walls decreases, and will maximize at the side walls of TMF generator. Similarly, the magnetic force is also influenced by the different excitation currents and frequencies. As shown in Fig. 2(f), the maximum of average magnetic force can be greatly improved by about 16 times with the increase of current from 5 to 20 A. Nonetheless, with increasing the frequency from 0 to 2000 Hz, the average magnetic force first increases and then decreases, and turning out to be the maximum at 200 Hz. Accordingly, under our study conditions, we confirmed the excitation current parameters of 20 A and 200 Hz to be the optimal parameters for the generation of TMF with the purpose of providing a more effective magnetic force in the alloy melt. In addition, the optimal area for TMF to process the alloy melt is adjacent to the side walls of the TMF generator; in this regard, the alloy castings with large thin-walled shape can be processed more efficiently by TMF than the castings with other shapes.

3.2 Influence of TMF on elimination of shrinkage defect

Fig. 3 Simulation model of SS with or without TMF

The average magnetic force calculated suing the Ansoft-Maxwell was imported into ESI-Procast software for the simulations of SS process, and  the results are shown in Fig. 3. Among them, the models in Figs. 3(a) and (b) are the temperature distributions in the whole casting and in the sections of side walls during the SS process, respectively.

It is visible from Fig. 3(c) that the temperature distributions are unidirectional and the values increase from bottom to the top position along the longitudinal direction, which are consistent with the characteristic of sequential solidification process. Besides, in the SS process without TMF, the temperature distributions in the sections of side walls indicate that the isolated liquid phase regions will appear in the alloy castings, which are surrounded by the solid phase regions (Fig. 3(c)). Under the circumstances, the alloy melt cannot flow into the isolated liquid phase regions or feed the regions, so as to form the shrinkage cavity or the porosity at the alloy castings. Therefore, in Fig. 3(d), quantities of shrinkage defects occur at the side walls of the casting, and the porosity reaches about 10%. Here, we used the Niyama criterion [35,36] to represent the quantity of shrinkage defects. It is noteworthy that when TMF is applied into the SS process, the general trend of temperature distributions will not be changed, but the isolated liquid phase region no longer occurs (Fig. 3(e)). This result suggests that the movement and the feeding capacity of the alloy melt in the mushy zones have been significantly increased by TMF process. Additionally, from Fig. 3(f), it can be illustrated that the shrinkage defects in the castings have completely disappeared, which also strongly confirms that TMF process can effectively reduce the shrinkage defects for the large thin-walled castings prepared by ZL205A alloys with large solidification intervals.

In order to further verify the above simulated results, we carried out the related experiments based on the parameters of simulation. Firstly, we selected the same position at the side walls of the castings to observe the macrostructure and microstructure, as exhibited in Fig. 4. Comparing Figs. 4(a) and (b), it is clear that a series of shrinkage defects are formed in the alloy castings by the SS process without  TMF. While, when TMF is applied, the shrinkage defects are largely eliminated. Furthermore, from Figs. 4(c) and (d), it can also be demonstrated that in the microstructures, the number of shrinkage defects is decreased greatly by the TMF process.  In addition, the microstructure in Fig. 4(d) also presents an obvious sequential growth of dendrites with the TMF process; nevertheless, the growth of dendrites in the process without TMF will form a certain deflection due to natural convection in the alloy melt (Fig. 4(c)). Therefore, the addition of TMF can efficaciously reduce the shrinkage defects in both the macrostructure and microstructure, and promote the sequential growth of dendrites along the direction of temperature gradient.

Fig. 4 Shrinkage defects in alloys

3.3 Influence of TMF on mechanical performance

We selected the same position of castings to perform the measurements of the tensile, micro- hardness and porosity, and to analyze the fracture morphologies, of which the relevant results are shown in Fig. 5. Wherein, the porosity was calculated by [37]

P_p=1-ρ_s/ρ₀                                                (2)

where P_p is the porosity of alloy casting caused by shrinkage defects, ρ_s is the density of the alloy casting measured by the related instrument, and ρ₀ is the theoretical density of alloys. It can be obtained that, the porosity sharply decreases from 1.71% (without TMF) to 0.22% (with TMF and the excitation current of 20 A). Additionally, the ultimate tensile strength and micro-hardness can be significantly improved from 186 MPa and 82.1 kg/mm² (without TMF) to 221 MPa and 100.5 kg/mm² (with TMF and the excitation current of 20 A). Similarly, the elongation increases from 7.3% to 11.7%, which confirms that TMF can promote the ductility of the alloy. Furthermore, these improvements are more significant with the increase of excitation current from 0 to 20 A.

Fig. 5 Performance test (a) and fracture morphologies of alloys with excitation current of 0 A (b), 5 A (c), 10 A (d) and 20 A (e)

Moreover, the SEM images of fracture morphologies in Fig. 5(b) reveal that there are so many shrinkage cavities and aggregative precipitated phases in the alloys without TMF process; wherein, the dimples are rare and the fracture mode is brittle. With the increase of excitation current, the magnetic field intensity can be increased; subsequently, the porosities are gradually eliminated and the precipitated phases can be uniformly distributed. Contrastively, the dimples are substantially increased and the fracture modes change from brittle mode to ductile mode by the TMF process (Figs. 5(c), (d) and (e)). In summary, TMF can effectively optimize the macrostructure and microstructure, promote the growth of dendrites along the direction of temperature gradient, remove the shrinkage defects, and improve the mechanical properties of the large thin-walled castings prepared by ZL205A alloys with large solidification intervals; additionally, the improvement effects of TMF will be enlarged with the increase of magnetic fields intensity.

4 Discussion

4.1 Distribution of magnetic force in alloy melt

Many literatures have shown that, the magnetic force is mainly related to the induced current density and magnetic intensity, which will be influenced by the excitation current. The area of TMF can be assumed as a complete alloy casting, so the relationship between the magnetic force and the excitation current can be expressed as [38-41]

J₀=I_e/S                              (3)

J=J₀exp(-d/d_s)                       (4)

                            (5)

F=JB                                  (6)

where J₀ is the induced current density on the alloy casting surface, J is the induced current density inside the alloy casting, S is the cross-sectional area of the electromagnetic coil, d is the distance to the coil, d_s is the depth of skin effect dependent on the alloy casting, B is the magnetic intensity, μ₀ is the permeability of vacuum, r is the radius of coil and equal to d here, and F is the magnetic force. From Eqs. (3) and (4), we can obtain that the induced current density inside the alloy casting will be enlarged with the increase of excitation current intensity, but will be decreased with the increase of the distance to the coil. Similarly, Eq. (5) also indicates that the magnetic intensity is positively correlated with the excitation current, but is negatively correlated with the radius of coil, i.e., equal to d. Consequently, both the induced current density and magnetic intensity will increase as the distance to the magnetic fields generator decreases, and reach the maximum at the side wall of magnetic fields generator. In addition, we can conclude from Eq. (6) that the magnetic force can increase dramatically as the excitation current increases, likewise, it will increase with the decrease of distance to the magnetic fields generator, finally reach the maximum at the side walls of TMF generator. Therefore, the results shown in Fig. 2 are verified. Furthermore, it can be demonstrated that the alloy castings with large thin-walled shape can be processed more efficiently by TMF than the castings with other shapes. Besides, when the alloy casting has a large thin-walled shape, the value of J is the same as J₀, so that Eq. (6) can be converted to

                         (7)

Therefore, it can be easily obtained that the magnetic force is positively correlated with the square of current, as demonstrated in Fig. 2. In addition, the excitation current has a close association with frequency (f ), as

                        (8)

where I₀ is the maximum current, f is the current frequency, φ is the phase angle and t is the time. It can be calculated that the current and frequency have a sinusoidal relationship, so that there can be a specific frequency value that maximizes the current, in turn to maximize the magnetic field strength and magnetic force, as shown in Fig. 2.

4.2 Effect of TMF on elimination of shrinkage defect

Fig. 6 Solidification behavior of alloy melt in SS process [42]

To analyze the shrinkage behavior in the solidification process, we established the sequential solidification models, as shown in Fig. 6. In this case, the transverse sections of the feeding channels are considered to be circular sections, of which the radius will decrease with the increase of solid phase fraction. The alloy melt in the feeding channels can form the continuous laminar flows, and the feeding capacity can be affected by the solidified rate and dynamic conditions. When the feeding channels are small and the feeding rate is less than the solidified rate, the feeding behavior will be difficult to continue, resulting in isolated liquid phase regions surrounded by the solid phase regions. As a result, the isolated liquid phase regions cannot be fed during the solidification, and the shrinkage defects are formed. On the other hand, with respect to the dynamic conditions, the change of pressure in the alloy melt will also influence the feeding behavior during the SS process, as follows [42-44]:

△P_s=σR^-1                                     (9)

where ΔP_s is the increased pressure generated by the surface tension, σ is the surface tension of alloy melt in the feeding channels, and R is the radius of cross section of the feeding channels. Additionally, the increased pressure induced by the volume force (ΔP_v) can be expressed as [42-44]

△P_v=(ρg+F_m)L                        (10)

where ρ is the density of alloy melt, g is the gravity coefficient, F_m is the magnetic force, and L is the length that can be reached by the feeding capacity in the feeding channels. However, in the solidification process, the growth of dendrites will produce a reduction in feeding pressure (ΔP_f), as follows [42-44]:

                  (11)

where u is the feeding rate of alloy melt, and η is the viscosity of alloy melt. Since the melt flows formed in the feeding channels are continuous, Eq. (11) can be evolved as

△P_f=8uηR^-2L                          (12)

Therefore, according to the law of conservation of energy, following formulas can be obtained [42]:

P_l-P₀+△P_s+△P_v-△P_f=0                  (13)

L=(P_l-P₀)/(8uηR^-2-ρg-F_m)                (14)

where P₀ is the static pressure at the beginning of the feeding channels, and P_l is the pressure at a position of L in the feeding channels. Moreover,  the values of the constant parameters including ρ,  g and η are listed in Table 1. By combining Eqs. (10)-(14), it can be confirmed that with the increase of F_m, the length L representing the feeding capacity will be increased accordingly. That is, TMF during the SS process is very effective to increase the feeding pressure in the feeding channels, further to improve the feeding capacity of alloy melt in the mushy zone, so as to reduce the shrinkage defects for alloys with large solidification intervals.

Additionally, ZL205A alloys as the Al-Cu based alloys with large solidification intervals, will have a low critical velocity for the steady growth of a flat solid-liquid interface during solidification, which can easily produce the instability of interface to form cellular and dendritic crystals [6-8]. In this case, the distribution of temperature and solute will be serious chaotic, and the growth of dendrites will be deflected and disordered. Meanwhile, secondary dendrites will be relatively developed [45]. As a result, the feeding channels between the dendrites in the mushy zones become easily blocked and the feeding capacity is decreased, so as to badly form the isolated liquid phase regions (Fig. 3(c)) and  the shrinkage defects (Fig. 3(d) and Figs. 4(a, b)) during the sequential solidification process. Comparatively, according to the previous work, it can be indicated that the TMF can generate the strong long-range sequential melt flows by the combination of gravity, buoyancy, Marangoni forces and the Lorentz forces [27]. Moreover, these melt flows can effectively regulate the heat and mass transfer, in turn to distribute the temperature and solute in the alloy melt more uniform and stable. In this case, the isolate liquid phase can be removed (Fig. 3(e)), and the feeding capacity can be enhanced, so that the shrinkage defects can be eliminated availably (Fig. 3(f) and Figs. 4(c, d)). Besides, the strong melt flows induced by TMF  can crush and break up the secondary dendrite  arms [46,47], so as to reduce the formation of secondary dendrites and overlaps, further to largely widen the feeding channels. Hence, the solidification rate is decreased, and the feeding time is delayed. Consequently, TMF can widen the feeding channels and prolong the feeding time, leading to an increase of feeding capacity of alloy melt in the mushy zone during the SS process.

4.3 Effect of TMF on mechanical performance

The effect of TMF on optimization of shrinkage defects and microstructure is mainly determined by the value of magnetic force, in other words, the intensity of excitation current (as discussed in Eq. (7)). When the intensity of excitation current increases, the intensity of TMF and the magnetic force can be enhanced accordingly, so that the efficiency of the sequential transfer of mass and heat will be improved, as well as the feeding capacity. Meanwhile, the growth of matrix phase α(Al) along the direction of temperature gradient can be promoted effectively, and the crushing capacity to the secondary dendrites and overlaps will be increased accordingly, as the intensity of excitation current increases (Fig. 4). Consequently, the elimination of shrinkage defects and the optimization of microstructure can be more efficient when the intensity of excitation current is large (Fig. 5). Therefore, with the increase of the intensity of excitation current, the formation of shrinkage defects can be correspondingly reduced, and the grain size will be refined effectively, further to greatly increase the tensile strength and the micro-hardness. Additionally, the growth of matrix phase α(Al) along the direction of temperature gradient can be improved and uniformed, so as to increase the elongation of alloy castings. In summary, TMF can increase the feeding capacity of alloy melt in the mushy zone during the SS  process, promote and uniform the growth of of matrix phase α(Al) along the direction of temperature gradient and reduce the secondary dendrites and overlaps. Ultimately, the TMF process can eliminate the shrinkage defects and optimize the microstructure, in turn to effectively improve the mechanical performance.

5 Conclusions

(1) Magnetic force can be increased with the increase of the exciting current, while as the exciting current frequency increases, it will first increase and then decrease, and reach the maximum value at 200 Hz. Additionally, the magnetic force increases as the distance to the side walls of TMF generator decreases, so as to more easily regulate the alloy castings with large thin-walled shape.

(2) TMF can significantly improve the feeding capacity of alloy melt by breaking the secondary dendrites and overlaps between dendrites, prolonging the feeding time, optimizing the feeding channels and promoting the growth of dendrites along the direction of temperature gradient.

(3) TMF coupling with SS process can effectively eliminate the shrinkage defects and improve the mechanical performance of alloys. Specifically, this process can increase the ultimate tensile strength, elongation and microhardness of alloys from 186 MPa, 7.3% and 82.1 kg/mm² without TMF to 221 MPa, 11.7% and 100.5 kg/mm² under TMF process with an excitation current of 20 A, while decrease the porosity from 1.71% to 0.22%; meanwhile, it can alter the brittle fracture mode to ductile fracture mode.

Acknowledgments

The authors are grateful for the financial supports from the National Key Research and Development Program of China (2017YFA0403804) and the National Natural Science Foundation of China (51425402, 51671073).

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行波磁场耦合顺序凝固改善ZL205A大型薄壁件缺陷及性能

罗 磊，夏宏营，骆良顺，苏彦庆，蔡超军，王 亮，郭景杰，傅恒志

哈尔滨工业大学 材料科学与工程学院 金属精密热加工国家重点实验室，哈尔滨 150001

摘  要：利用行波磁场耦合顺序凝固连续地处理大型薄壁ZL205A合金铸件，消除收缩缺陷，提高力学性能。实验结合模拟，针对行波磁场参数优化对补缩行为、显微组织和性能的影响进行系统的研究。结果表明，本研究条件下，当励磁电流为20 A、频率为200 Hz时，磁场力达到最大值；磁场力随着到磁场发生器距离越近而越大，更有利于对薄壁铸件进行处理。行波磁场可以有效破碎二次枝晶臂和枝晶间的搭接，拓宽补缩通道，延长补缩时间，优化补缩路径，最终消除收缩缺陷并提高力学性能。当励磁电流为20 A时，铸态合金极限抗拉强度、伸长率和显微硬度分别由186 MPa、7.3%和82.1 kg/mm²提高至221 MPa、11.7%和100.5 kg/mm²，孔隙率由1.71%降至0.22%，断裂模式由脆性转变为韧性断裂。

关键词：ZL205A合金；大型薄壁合金铸件；行波磁场；顺序凝固；收缩缺陷；力学性能

(Edited by Xiang-qun LI)

Corresponding author: Yan-qing SU; Tel: +86-451-86418740; E-mail: suyq@hit.edu.cn

DOI: 10.1016/S1003-6326(21)65545-0

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press