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Vol. 9. Issue 1.
Pages 253-262 (January - February 2020)
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Vol. 9. Issue 1.
Pages 253-262 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.053
Open Access
Effect of electrochemical polishing on surface quality of nickel-titanium shape memory alloy after milling
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Guijie Wanga,b, Zhanqiang Liua,b,
Corresponding author
melius@sdu.edu.cn

Corresponding author.
, Jintao Niua,b, Weimin Huangc, Baolin Wanga,b
a School of Mechanical Engineering, Shandong University, China
b Key Laboratory of High Efficiency and Clean Mechanical Manufacture of MOE/Key National Demonstration Center for Experimental Mechanical Engineering Education, Shandong University, Jinan 250061, China
c College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao 266590, China
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Tables (4)
Table 1. Orthogonal array design and statistics of microhardness.
Table 2. Orthogonal array design and statistics of hardening degree.
Table 3. Orthogonal array design and statistics of surface roughness.
Table 4. Orthogonal array design and statistics of XRD.
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Abstract

Nickel-titanium shape memory alloys have been widely used in medical fields due to their unique shape memory effect, superelasticity, corrosion resistance and biocompatibility. However, there are still many processing difficulties to achieve required machined surface quality due to their high toughness and low elasticity. This paper proposes a sequential processing operations including milling, burnishing and electrochemical polishing. The effects of electrochemical parameters on surface quality, such as surface roughness, work hardening and surface grain size, are investigated in detail using orthogonal experiment. It is found that the electrochemical polishing parameters have the most significant effect on improving the surface roughness. With adjusting electrochemical polishing parameters, the surface roughness can be reduced to one tenth of the original value obtained by milling operations. Moreover, the current density in electrochemical polishing has more important influence on the surface quality than the distance and polishing time does, respectively. The conditions of 1.5A/cm2 current density, 7cm electrode distance and 20s polishing time can obtain the lower surface roughness, hardening degree and the larger grain size. The effects of electrochemical parameters on surface roughness, work hardening and grain size are related to each other. The degree of work hardening can be qualitatively estimated by surface roughness and grain size to avoid damage to the machined surface.

Keywords:
Nickel-titanium
Shape memory alloy
Processing optimization
Electrochemical polishing
Milling
Surface roughness
Work hardening
Grain size
Full Text
1Introduction

Nickel-titanium (NiTi) alloys have been widely used in aerospace, transportation, actuator and medical industries due to their unique memory effect and superelasticity, good physical and mechanical properties, corrosion resistance and biocompatibility [1–3]. The machining, including turning and milling, has become the main processing method due to its good shape adaptability and high processing efficiency. However, the machinability of NiTi alloys is rather poor because of their high toughness, high strength, severe work hardening [4], sensitive nature to temperature, phase transformation and high strain rate [5]. The serious tool wear and complex thermo-mechanical coupling lead to poor surface roughness and severe work hardening [6]. The surface roughness and work hardening have great influence on shape memory effect and superelasticity of the product [7,8].

Many researchers have done a lot of research on the processing of nickel-titanium alloys to address these problems. Mehrpouya et al. [9] found that cutting force and tool wear have the minimum value in a certain turning speed range, which improves the machinability. Kaynak et al. [10,11] also found that cutting speed has a significant effect on surface integrity by using liquid nitrogen for cryogenic turning. The surface micro-hardness and the depth of the machining-induced layer decrease with the increase of cutting speed. Cryogenic turning can greatly reduce tool wear, improve surface morphology and the final quality compared with dry cutting and minimal quantity lubrication (MQL).

Huang et al. [12] made a detailed study of milling NiTi alloy. The results show that the cutting force decreases gradually with the increase of cutting speed when the cutting speed is below 50m/min. When the cutting speed reaches 200m/min, the cutting force will be saturated. Both surface roughness and surface hardness have minimum values at a certain cutting speed. Guo et al. [13] found that the minimum surface roughness was generated at a certain milling speed. It was found that the higher dynamic strength resulted in serious flank wear on the flank face. And the higher specific heat capacity led to the rapid tribo-chemical dissolving of the tool coating on the rake face. Moreover, the white layer is austenite caused by large deformation, and its hardness is higher compared with substrate material. Wang et al. [14] reported that the minimum depth of work hardening layer and hardening degree in milling process were within a specific cutting speed range. Zailani et al. [15] carried out micro-milling experiments by using cold air, MQL and their combination. The results show that the dendrite structure of the workpiece is uniform, the cutting force is reduced and the burr height is reduced by using cold air. The combination of cold air and Micro-lubrication shows great potential for reducing tool wear and achieving better surface quality.

Manjaiah et al. [16] studied non-traditional processing methods, such as laser processing, water jet machining (WJM) and electrochemical machining (ECM), but these processes were limited by the complexity and mechanical properties of components. Electrical discharge machining (EDM) and wire-cut electrical discharge machining (WEDM) have the ability to process complex shapes and sizes accurately. They can obtain lower residual stress and better surface finish, but their processing efficiency is lower than that of turning and milling. Vijaykumar et al. [17] studied the EDM of NiTi alloy. It was found that deep cryogenic treatment could improve the material removal rate, but the effect on reducing tool wear rate was limited. Because the lower temperature weaken the thermal vibrations of atoms, and these thermal vibrations make it easier for electrons to move through them. At the same time, deep cryogenic treatment could also make grain growth, which leads to the reduction of boundary and decrease of resistance coefficient. Fu et al. [18] applied the fiber laser process to fabricate vascular stents. The geometry, roughness, morphology, micro-structure and hardness of the incision were analyzed to understand the properties of heat-affected zone and recast layer. The relationship between surface integrity and process parameters was investigated. The results showed that influence of laser cutting speed on surface integrity is much greater than that of laser power. The higher cutting speed may lead to unqualified melt resulting in high surface roughness and thickness of recast layer.

Existing research has found that single processing method can’t solve processing problem of nickel-titanium alloy very well. The better efficiency and surface quality of traditional turning and milling can be obtained through certain auxiliary methods, but the subsurface quality problem could not be solved very well. Non-traditional processing methods, which are limited to the shape, surface quality and processing efficiency, can’t solve the processing problems of nickel-titanium alloy very well. Considering this reason, traditional milling is combined with non-traditional electrochemical processing to solve the problems of efficiency, shape, surface quality and subsurface quality in processing NiTi alloy.

The machined parts of milling have uneven surface, and the electrochemical polishing can get a smoother surface, which can be used as the finishing process under the condition that the metal surface dissolves least and the surface dissolves most evenly. However, there are still many challenges and difficulties in electrochemical polishing [19]. The shape of the machined surface has little effect on productivity for electrochemical polishing. If there are deep scratches on the surface of the sample, it is usually difficult to remove them by electrochemical polishing. It is found that although the boundary of scratches becomes passivated, the scratches remain unchanged and larger pits are more likely to occur along the scratches. The effect of electrochemical polishing is related to the original surface roughness and surface geometric uniformity of the pattern. The better the homogeneity of the original surface is, the better the effect of electrochemical polishing will be. After burnishing the workpiece of milling, the surface roughness and the geometric uniformity of the machined surface can be improved. In summary, milling-burnishing-electrochemical polishing process is adopted in this paper as shown in Fig. 1.

Fig. 1.

Schematic diagram of milling-burnishing-electrochemical polishing experiments.

(0.14MB).
2Experimental details2.1Experimental principles

  • (1)

    On the one hand, in electrochemical polishing, the removal of rough roughness belongs to the category of macro-level polishing, which is related to the viscous layer. At this time, the distance between the electrodes in the protrusion is smaller than that in the depression. In the protrusion, the resistance is relatively small and the current density is high which make the metal dissolve quickly and makes the metal surface tend to be smooth. On the other hand, the removal of smaller roughness belongs to the category of micro-gloss polishing, which is related to the formation of oxide film on metal surface [20]. The effect of electrochemical polishing is related to the original surface roughness and surface geometric uniformity of milling. The better the homogeneity of the milling surface is, the better the effect of electrochemical polishing will be. The burnishing workpiece after milling is able to improve surface geometric uniformity, reduce scratches and surface roughness, but does not change the shape of the product as shown in Fig. 2. Therefore, the burnishing can improve the quality of electrochemical polishing.

    Fig. 2.

    Schematic diagram of milling-burnishing machined surface.

    (0.15MB).
  • (2)

    Current density is the important factor that affecting electrochemical polishing and the power of electrochemical polishing. The speed of electrochemical polishing gradually increases with the increase of current density in most cases. On the one hand, when the current density is too high, the temperature of the polishing solution rises, the oxidation of perchloric acid is enhanced, and the phenomenon of anodic oxygen evolution is serious. Although the brightness of the surface can improve under the stirring of oxygen, it is easy to cause corrugation and over-corrosion. On the other hand, if the current density is too low, the surface is in the active dissolution state, and it is difficult to form film on the surface of the anode, which results in longer polishing time, lower efficiency and worse brightness and quality [21]. Stirring is for better electrochemical reaction in the actual process. Therefore, electrochemical polishing fluid can’t be absolutely static. Diffusion and convection mostly coexist, called convective diffusion (Fig. 3).

    Fig. 3.

    Schematic diagram of influence of current density.

    (0.14MB).
  • (3)

    Distance between cathode and anode is also the important factor on electrochemical polishing of NiTi alloy. On the one hand, a smaller distance between cathode and anode is used to reduce energy consumption and save energy. However, when the distance between cathode and anode is too small, it is not conducive to the flow of polishing solution and the diffusion of ions. It is also easy to cause the temperature of polishing solution to rise, even the local current density is too high which leads to excessive corrosion. On the other hand, when the distance between cathode and anode is too large, the current density decreases. Therefore, it is necessary to increase energy consumption in order to achieve the required current density [22] (Fig. 4).

    Fig. 4.

    Schematic diagram of influence of anode-cathode distance.

    (0.05MB).
  • (4)

    The time of electrochemical polishing is closely related to the properties of materials, polishing fluid, current density, anode-cathode distance and surface quality of milling [23,24]. If the electrochemical polishing fluid is more active and the original surface quality is better, the polishing time is shorter. On the contrary, the polishing time is longer. On the one hand, if the polishing time is too short, the polishing is not thorough and the effect is not good. On the other hand, if the polishing time is too long, excessive corrosion will occur and the electrochemical polishing quality will be destroyed.

2.2Experimental methods2.2.1Processing method

The fixed milling and burnishing parameters was selected according to experimental principle (1). Electrolyte system and concentration, and magnetic stirring used to enhance agitation and speed up flow were selected according to experimental principle (2). Then, the effects of current density, anode-cathode distance and electrochemical polishing time on surface roughness, work hardening and grain size of machined surface were investigated as shown in Fig. 5.

  • (1)

    Milling: speed (vc)=35m/min, feed rate (fz)=0.145mm/r, radial infeed depth (ae) =12.8mm, axial infeed depth (ap)=0.8mm.

  • (2)

    Burnishing: pressure (P)=30MPa, speed (vb)=3000mm/min, and feed (f)=0.5mm. The direction is parallel to the direction of milling speed.

  • (3)

    Electrochemical polishing: methanol-perchloric acid system, concentration of 10vol%, magnetic stirring, current density (i)=0.75, 1.5, 2.25A/cm2, anode-cathode distance (d)=3.5, 7, 10.5cm, polishing time (t)=10, 20, 30s.

Fig. 5.

Schematic diagram of influence factors-target.

(0.15MB).
2.2.2Measuring methods

  • (1)

    The surface roughness is observed and measured with laser scanning confocal microscope. The resolution is 0.1nm (Sa).

  • (2)

    The micro-hardness of machined surface is measured on FM-800. The load is 1000g and the load residence time is 8s.

  • (3)

    The analysis of surface gain sizes are carried out on D8 ADVANCE at room temperature. The range of degree is from 10 to 85, and the 0.02° step is used.

3Results and discussion

The surface roughness, work hardening and surface grain size have important effects on performance, such as shape memory effect and superelasticity. Therefore, the effects of electrochemical polishing parameters (current density, electrode distance and polishing time) on surface roughness, hardening degree, plastic deformation and surface grain were investigated in order to control the processing process and achieve better performance. The statistical data and analysis results are as follows:

3.1Work hardening of machined surface

Work hardening of machined surface has an important influence on the superelasticity and shape memory effect of products. The effects of current density, distance and polishing time on microhardness are investigated by orthogonal experiment in Table 1 and the value of No.0 is the microhardness of milling.

Table 1.

Orthogonal array design and statistics of microhardness.

Current density A/cm2Distance cmPolishing time sMicrohardness (HV)
Average 
No.0        458  472  463  464.3 
No.1  0.75  3.5  10  428  441  422  430.3 
No.2  0.75  20  382  377  393  384 
No.3  0.75  10.5  30  367  361  348  358.7 
No.4  1.5  3.5  20  326  295  313  311.3 
No.5  1.5  30  343  337  341  340.3 
No.6  1.5  10.5  10  362  367  366  365 
No.7  2.25  3.5  30  371  377  389  379 
No.8  2.25  10  339  334  335  336 
No.9  2.25  10.5  20  357  351  347  351.7 

The hardening degree (N) is the percentage of the increased microhardness of machined surface divided by the microhardness of matrix material, i.e.

where H is the microhardness of the machined surface; H0 (H0=200HV) is the microhardness of the matrix material (Fig. 6).

Fig. 6.

Diagram of microhardness measurement.

(0.42MB).

As shown in Table 2, the hardening degree of each electrochemical polishing parameter was calculated by using the data in Table 1 and the above formula. Then the influence of current density, distance and polishing time on the hardening degree was obtained in Fig. 7 and the value of No.0 is the hardening degree of milling.

Table 2.

Orthogonal array design and statistics of hardening degree.

  Current density A/cm2  Distance cm  Polishing time s  Hardening degree 
No.0        132.15 
No.1  0.75  3.5  10  115.15 
No.2  0.75  20  92 
No.3  0.75  10.5  30  79.35 
No.4  1.5  3.5  20  55.65 
No.5  1.5  30  70.15 
No.6  1.5  10.5  10  82.5 
No.7  2.25  3.5  30  89.5 
No.8  2.25  10  68 
No.9  2.25  10.5  20  75.85 
Fig. 7.

Effect of electrochemical polishing parameters on hardening degree.

(0.2MB).

Current density has the most important effect on work hardening. The weakening effect of current density on hardening degree is not obvious when current density is 0.75A/cm2 as shown in Fig. 7. The hardening degree is lower when the current density increases to 1.5A/cm2. However, the degree of hardening increases as the current density continues to increase to 2.25A/cm2. This is because the surface is in passive dissolution state when the current density is small, the removal effect of the current density on the surface metal material is not obvious, and then the weakening effect on the hardening degree is not obvious. Surface leveling degree gets better and metal dissolution speed are faster when the current density continues to increase to 1.5A/cm2. The weakening effect on work hardening is more obvious, and the depth of plastic deformation layer at 1.5A/cm2 is smaller than 0.75A/cm2 as shown in Fig. 8. However, the surface reaction will be severe when the current density continues to increase to 2.25A/cm2, resulting in inhomogeneous dissolution and deterioration of surface quality. Then the degree of work hardening will increase.

Fig. 8.

Depth of plastic deformation layer with different current densities.

(0.3MB).

The hardening degree first decreases, and then increases with the continuous increase of the electrode distance. This is because it is not conducive to the diffusion of products when the distance is too small. Then, the temperature of the polishing fluid increases resulting in corrosion and deterioration of surface quality. However, polishing is not optimal when the distance is too large. There are two reasons. On the one hand, in order to reduce the energy loss and achieve the required current density, small electrode distance should be adopted under normal circumstances. On the other hand, large distance between electrodes should be adopted to improve the uniform distribution of surface current density, to prevent the high currents of end and corner of parts and the heating of cathode reduction process. In summary, the appropriate intermediate electrode distance should be selected comprehensively in order to achieve the best polishing effect.

The leveling speed of uneven parts and the thickness of dissolved metal layer are related to the electrochemical polishing time to a large extent. The minimum value of work hardening is shown in Fig. 7 when the polishing time is 20s. There are two reasons. On the one hand, the polishing process can’t be completely carried out when the polishing time is too short. The work hardening can’t be weakened if the effect is not good enough. On the other hand, corrosion occurs easily when polishing time is too long. And the effect on surface leveling is small when the polishing time reaches a certain level. The dissolution conditions of the metal in the concave part are different from those in the convex part, and the speed of concave part decreases with the leveling of the convex part.

3.2Surface roughness of machined surface

The influence of electrochemical polishing parameters on surface roughness is very important because electrochemical polishing is a finishing process. Therefore, the effects of current density, distance and polishing time on surface roughness are investigated by orthogonal experiment as shown in Table 3 and the value of No.0 is the surface roughness of milling.

Table 3.

Orthogonal array design and statistics of surface roughness.

Current density A/cm2Distance cmPolishing time sSurface roughness (μm)
Average 
No.0        2.544  2.643  2.617  2.601 
No.1  0.75  3.5  10  0.756  0.751  0.741  0.749 
No.2  0.75  20  0.529  0.535  0.529  0.531 
No.3  0.75  10.5  30  0.722  0.713  0.686  0.707 
No.4  1.5  3.5  20  0.281  0.273  0.284  0.279 
No.5  1.5  30  0.264  0.261  0.27  0.265 
No.6  1.5  10.5  10  0.325  0.324  0.319  0.323 
No.7  2.25  3.5  30  1.723  1.711  1.701  1.712 
No.8  2.25  10  1.299  1.309  1.304  1.304 
No.9  2.25  10.5  20  1.501  1.504  1.487  1.497 

Current density has the most important effect on surface roughness. The value of surface roughness decreases first and then increases with the increase of current density in Fig. 9. The value of surface roughness is the minimum when current density is 1.5A/cm2. On the one hand, the improvement of surface roughness by electrochemical polishing is not obvious when the current density is too small. On the other hand, the polishing voltage of external circuit is higher and the temperature of electrolyte increases when the current density is too high. As a result, the decrease of solution viscosity and the increase of acid radical ion activity will lead to the rapid dissolution of metals and the violent oxygen evolution reaction. This is helpful for the diffusion of the adsorbed film and its adjacent ions on the surface of the anode under the stirring action of a large amount of oxygen. Although the brightness is improved, it is prone to corrosion and poor surface quality. In addition, the excessive current density causes the temperature of the anode-cathode reaction zone to rise too fast. Then it will lead to the decomposition of some components in the solution and the aging of the solution will be aggravated.

Fig. 9.

Effect of electrochemical polishing parameters on surface roughness.

(0.21MB).

The effect of electrode distance and polishing time on surface roughness is smaller than that of current density does. The surface roughness first decreases and then increases with the electrode distance increasing. The surface roughness is the lowest when the distance between electrodes is 7cm. This is because it is not conducive to the diffusion of ions in the solution when the distance between electrodes is small, which is prone to over-corrosion and other defects. However the energy consumption to achieve the required current density increases when the distance between electrodes is too large. The surface roughness first decreases and then increases with the increase of polishing time. The surface roughness is the smallest when the polishing time is 20s. This is because there are unsolved scratches on the surface, the passive film could not be completely dissolved and removed, and the polishing process is not complete when the polishing time is short. The quality of electrochemical polishing is improved with the increase of polishing time. But when the polishing time is too long, the polished surface will be damaged and corroded.

3.3Grain size of machined surface

A very small grain in positive space can be regarded as a sphere in reciprocal space, and its diffraction peak width is very wide. However, a large enough grain in positive space is a point in reciprocal space, and the corresponding peak width is very narrow. Therefore, the change of grain size can be reflected in the peak width of the diffraction peak as shown in Fig. 10, and the grain size can be calculated and measured.

Fig. 10.

XRD patterns of machined surfaces.

(0.22MB).

Shelley formula was used to calculate the grain size [25] of each machined surface in the above electrochemical polishing experiments, and the effect of electrochemical parameters on the grain size (D) was investigated as shown in Table 4 and the value of No.0 is the grain size of milling.

where k is 0.89 when β is full width at half maxima, λ is the wavelength of X-rays (λ=1.5406Å), β is the full width at half maxima of the diffraction (the unit of β is radian), θ is the Bragg angle of diffraction.

Table 4.

Orthogonal array design and statistics of XRD.

  Current density (A/cm2Distance (cm)  Polishing time (s)  Diffraction angle 2θ (°)  full width at half maxima (°)  D nm 
No.0        42.006  1.688  49.853 
No.1  0.75  3.5  10  42.340  1.533  54.955 
No.2  0.75  20  42.436  1.221  69.020 
No.3  0.75  10.5  30  42.346  1.058  79.629 
No.4  1.5  3.5  20  42.233  0.935  90.070 
No.5  1.5  30  42.241  0.964  87.363 
No.6  1.5  10.5  10  42.335  1.125  74.884 
No.7  2.25  3.5  30  42.193  1.184  71.118 
No.8  2.25  10  42.143  0.963  87.424 
No.9  2.25  10.5  20  42.151  1.013  83.112 

The grain size of all electrochemical polishing surfaces is smaller than that of milling surfaces. The surface after milling has serious work hardening compared with the surface after electrochemical polishing which results in the larger plastic deformation and smaller grain size.

The current density has the most important effect on grain size in Fig. 11. The grain size first increases and then decreases with the increase of current density. The weakening effect on work hardening is not obvious because of current density is smaller in the early stage, so the grain size is smaller. Then the hardening degree in Fig. 7 and depth of plastic deformation layer in Fig. 8 decrease that make the grain size increase with the increase of current density. As the current density continues to increase, oxide film is formed on the surface and the brightness increases. However, the surface is prone to corrosion and deterioration of quality resulting in larger grain size.

Fig. 11.

Effect of electrochemical polishing parameters on grain size.

(0.21MB).

The effect of electrode distance and polishing time on grain size is smaller than that of current density does as shown in Fig. 11. Small distance is not conducive to the diffusion of the product, and easy to cause excessive solution temperature. Work hardening removal is not obvious due to the poor surface quality caused by the larger local current density. Therefore, the grain size is smaller at this time. Then, the grain size increases gradually with the increase of distance. However, the decrease of local current density as the distance continues to increase, which leads to the decrease of electrochemical polishing quality, removal degree of work hardening and grain size.

The electrochemical polishing is not enough when the polishing time is short. Moreover, the metal is dissolved and no oxide film is formed. The polishing effect is poor and the reduction of hardening degree is not obvious. Therefore, the grain size is small. Then, the oxide film is formed when the time is 20s, and the polishing effect is the best. The plastic deformation layer is almost removed and the grain size is the biggest. However, the more metal dissolution and deterioration of surface quality occur when the polishing time continues to increase to 30s. The grain size decreases at this time.

4Conclusions

Nickel-titanium shape memory alloys have been widely used in medical fields due to their unique shape memory effect, superelasticity, corrosion resistance and biocompatibility. However, there are still many processing difficulties to achieve required machined surface quality due to their high toughness and low elasticity. This paper proposes a sequential processing operations including milling, burnishing and electrochemical polishing. The aim of milling is to quickly remove materials and process forming. The aim of burnishing is to smooth the bulge formed by milling to a certain extent. The electrochemical polishing is the finishing process. The effects of electrochemical parameters on surface quality, such as surface roughness, work hardening and surface grains, were studied. The conclusions are as follows:

  • (1)

    Orthogonal experiment results show that 1.5A/cm2 current density, 7cm distance and 20s polishing time can obtain the lower surface roughness, hardening degree and the larger grain size. Current density has more important influence on surface quality than distance and polishing time does, respectively. Current density plays a key role in improving surface quality.

  • (2)

    The experimental results show that the electrochemical polishing parameters have the more important effect on improving the surface roughness compared with improving work hardening and grain size. With adjusting electrochemical polishing parameters, the surface roughness can be reduced to one tenth of the original value obtained by milling operations.

  • (3)

    The effects of electrochemical parameters on surface roughness, work hardening and grain size are related to each other. The degree of work hardening can be qualitatively estimated by surface roughness and grain size to avoid damage to the machined surface.

  • (4)

    The sequential processing operations including milling, burnishing and electrochemical polishing can effectively improve the surface quality of milling, and has certain application value.

Conflict of interest

The authors declare no conflicts of interest.

Acknowledgments

This research was funded by National Natural Science Foundation of China (Grant numbers 51425503 and 91860207) and Taishan Scholar Foundation of Shandong Province (Grant number TS20130922). The authors would also like to acknowledge the support from Collaborative Innovation Center for Shandong’s Main Crop Production Equipment and Mechanization.

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Journal of Materials Research and Technology

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