Investigation on ultrasonic assisted embossing process of micro-features with variable curvature as functional surface using ultra-thin sheet of 5052 aluminum alloy

doi:10.21203/rs.3.rs-1603443/v1

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Investigation on ultrasonic assisted embossing process of micro-features with variable curvature as functional surface using ultra-thin sheet of 5052 aluminum alloy

https://doi.org/10.21203/rs.3.rs-1603443/v1

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Functional surface with pattern micro-features of sine waves fabricated by ultra-thin metallic sheet is very useful as a shield charactered by variable curvature to diffusely reflect strong light on the severe condition and reduce weight. A novel ultrasonic assisted embossing is developed to precisely fabricate the complex curved surface using its advantages of “Blaha effect”. Effects of embossing parameters were studied by considering the change of strain states. Improvement of shape accuracy by ultrasonic energy and duration time was investigated in the reducing of the springback and increasing of the amplitude of formed sine waves micro-features. Analysis and discussion of the deformation behaviors and springback were carried out in ultrasonic assisted embossing from the viewpoint of residual acoustic softening effect and eliminating residual stress. Experimental results indicate that the ultrasonic assisted embossing process is helpful for precisely manufacturing the complex surface with variable curvature using ultra-thin metallic sheet.

Functional surface

Ultrasonic assisted embossing

Micro-features with variable curvature

Springback

Residual softening effects

In the course of evolution, nature has arrived at startling surface micropattern solutions to ensure survival. Investigations into functional biological surface, ranging from plants, insects and aquatic animals, have inspired the intricate surface patterns to create useful functionalities [1]. Benefit from these investigations, large numbers of products have emerged where the surface micropattern has been specifically designed to provide a particular function, such as superhydrophobic, wettability, anti-adhesion, and lower-friction, etc.[2]. However, manufacturing of surface micropattern directly on the parts is difficult to be realized for some reasons, such as structure, materials of products, etc. A kind of functional surface, complex curved micro-features, is designed as a shield to diffusely reflect strong light to avoid damaging precision measuring instruments, and ultra-thin metallic sheet is chosen for the severe working condition and reducing weight of parts, which brings great difficulties to manufacturing processes. Generally, this kind of parts are often fabricated using embossing process, charactered by bulging and bending deformation behavior. However, the workpiece size is in microscale and has only a few grains involved in a deformation zone, leading to the deformation behavior different from those in macroscale, and the situation becomes much more difficult for the existing of scaling effects induced by the small dimension of feature and foil thickness. In micro embossing, the deformation load and the deviation tends to increase with the increase of grain size [3]. Ductile fracture of metallic materials in micro/meso scale plastic deformation is influenced by geometry, surface roughening and grain sizes and the so-called size effect thus exists [4]. The forming limit curve shifts down with the decreasing ratio of the thickness to grain size [5].

Cao J. indicates that springback is another difficult problem to solve in ultra-thin sheet metal forming due to lesser plastic deformation in the pure bending and their variation in the microstructure [6]. Wang, et al. [7] reports that thickness, grain size and their interactive effect has an obvious effect on the springback of brass foils. Experimental results obtained via micro-tensile revealed that the yield strength, Young’ modulus and elongation had a close correlation with the thickness-to-average grain size ratio. Both springback and negative springback are observed in the micro W-bending. The springback increases with the decrease in the foil thickness, and increase of t/d ratio.

To improve the accuracy and forming limitation of the products, external physical field assisted microforming processes were carried out [8]. In recent years, ultrasonic vibration was widely used in many forming processes for the existing of Blaha effect, such as micro-bulging, micro-deep drawing, micro-bending, micro-forging [9] and micro-extrusion [10], etc. Both yield stress and flow stress decrease immediately, and max. reduction of yield stress of Al 1100-0 foils is up to 82%, when the ultrasonic vibration or low-frequency vibration with micro-amplitudes is applied in uniaxial tension tests and upsetting tests [11], which means that acoustic softening effect occurs. Experimental results showed that the stress decrease due to the acoustic softening is proportional to the vibration amplitude [12]. When vibration was removed, residual softening was observed, which was attributed to the decline of the dislocation density [13]. The ductility of the superalloy sheet is notably enhanced because that the grain misorientations are significantly changed [14]. Also, the acoustic softening and size effects which can reduce the elastic limit of copper foil are enhanced [15]. To realize the mechanism of acoustic softening, several models were proposed in terms of stress superposition, thermal activation theory [16], crystal plastic theory [17] and other mechanisms associated with dislocation evolution.

Several kings of ultrasonic assisted forming processes were carried out, such as deep drawing, bulging, and bending, etc. Pasierb and Wojnar [18] found that deformation force was decreased in the defined part of vibration period in the deep drawing process of thin-walled products. Huang et al. [19] performed micro-deep drawing process combined with ultrasonic vibration, experimental results shown that the limit drawing ratio (LDR) was increased from 1.67 to 1.83, from 1.75 to 1.92, and from 1.83 to 2 for thickness of 50, 75, and 100µm, respectively. Also, improvement of formability e.g. forming limit was observed during ultrasonic assisted incremental sheet forming for the occurrence of dynamic recovery [20], uniform residual stress and improving the elongation of the material [21]. Plastic powder melted by ultrasonic vibration as flexible punch was proposed by Luo et al. [22], and results showed that the replication degree could reach up to 98% for cross-section micro channels [23]. Spherical caps array as surface texturing was fabricated using ultrasonic flexible bulging process, and the surface quality and height of bulged cap were improved clearly for the acoustic softening effect [24]. And, springback rate of aluminum alloy component was reduced for the application of ultrasonic waves [25]. However, ultrasonic assisted embossing process of micro-features with complex surface were seldom investigated with ultra-thin metallic sheet.

In the investigation, ultrasonic assisted embossing process has been proposed with rigid die to fabricate micro-features with sine wave curves of variable curvature using ultra-thin sheet of 5052 aluminum alloy. Effects of embossing parameters such as punch displacement, temperature of treatment, and rolling direction were studied by analyzing the amplitude of sine waves and springback. Then, the influence of ultrasonic energy and duration time were investigated in the improvement of shape accuracy. And, the mechanism of ultrasonic assisted embossing is discussed by considering strain states, the residual acoustic softening effect and eliminating residual stress.

2.1 Die and principle

As a kind of surface texture, micro-features with sine wave curves are designed to diffuse lines and protect the precision optical equipment, and manufactured with ultra-thin metallic sheet to reduce weight. Figure 1 (a) gives dimensions of micro-features, and their outline is agreement with the sine wave Eq. (1),

y = 0.3cos(πx) (1)

The length of single wave is 2.0mm, and the amplitude is 0.6mm. In the investigation, the waves were repeated for 26 times. Then, the length of parts is 52mm, and the width is selected as 30mm. Considering the blank holder, the dimensions of workpiece are set as 66mm in length and 30mm in width.

Since sine wave micro-features owns complex surface with variable curvature, it is difficult to manufacture this kind of parts for the existing of the springback using traditional sheet forming process. Then, ultrasonic assisted embossing was developed which could improve the shape accuracy for the existing “Blaha” effect. As shown in Fig. 2 (b), the forming equipment mainly consists of two parts, embossing mold and ultrasonic vibrator. Figure 2 (c) indicates the schematic of embossing mold, which consists of male die, female die and guide holder, etc. Sine wave micro-features were machined on bottom surface of male die, top surface of female die using fine electrical discharge machine process (EDM), respectively. The clearance between male die and female die is equal to the thickness of ultra-thin sheet. To get high accuracy of micro-features, material JIS-SKD61 with high stiffness and hardness 48–52 is used in the mold, which can minimize the effect induced by the elastic deformation of male and female die.

As shown in Fig. 1 (b), the ultrasonic vibrator (Shenzhen Rifa Ultrasonic Equipment Co. LTD) was selected, and its parameters were max. power of 2kW, and output vibration frequency of 20kHz. By using an electrical universal testing machine (MTS-SANS UTM6140, max. load 10kN), embossing experiments were carried out at room temperature without any lubricants. During the tests, the ultrasonic vibrator moves down with the drive of testing machine, and apply the load and vibration to the male die directly. When the punch displacement reaches 0.3mm, the vibrator stops moving down, and the power of ultrasonic vibrator is turned on, works for the set time. During the process, the effect of vibration is studied by changing ultrasonic energy (65%, 75% and 85% of max. output energy) and duration time (30s, 60s and 90s).

(a) Dimension of sine wave micro-features, (b) Quipment of ultrasonic assisted embossing, (c)Schematic of ultrasonic assisted embossing mold, (d) Photograph of ultrasonic assisted embossing mold, (e) female die, (f) male die

2.2 Experimental materials and parameters

For its excellent properties, such as corrosion resistance, high strength and fatigue strength etc., rolled ultra-thin sheet of aluminum alloy GB 5052 with 50µm in thickness was selected in the embossing experiments. Its chemical composition is listed in Table 1.

Table 1

Chemical composition limits of aluminum alloy GB 5052 (Wang, et al., 2020a)
Weight%	Al	Cu	Si	Fe	Mn	Mg	Zn	Cr	Others
5052	Bla	≤ 0.10	≤ 0.25	≤ 0.40	≤ 0.10	2.2/2.8	≤ 0.10	0.15/0.35	≤ 0.05 (each) ≤ 0.15 (total)

To study the effect of the aluminum alloy microstructure, heat treatment process was performed at temperature of 300 ̊ C, 400 ̊ C, 500 ̊ C, and 600 ̊ C in the vacuum condition for 1 hour, and the obtained grain sizes were 13.68µm, 18.7µm, 25.55µm, and 47.65µm, respectively. The properties were investigated by the universal extension tests. Experimental results indicate that acoustic softening effect occurs obviously. Ultrasonic assisted uniaxial tensile experiments were carried out with punch speed of 0.3mm/min using duration time of 30s under different vibration amplitude of 5µm, 5.6µm and 6.2µm, respectively. It is found that the flow stress decreases immediately, and the reduction of flow stress are 16.43MPa, 21.67MPa and 24.82MPa, respectively. The details of acoustic soften effects can be found in the reference [24, 26].

The formed micro-features of sine wave were machined by EDM, and polished to get cross section. Then, the curves of formed micro-feature are distinguished and measured by a video measuring system (VMS 2515F).

3.1 Effect of punch displacement

Since the dimensions of micro-features and thickness of sheet are very small, the displacement of punch has an obvious effect on the deformation behaviors in embossing process. Punch displacement, such as 0.3mm, 0.35mm, 0.40mm, 0.45mm, and 0.50mm were selected, and ultra-thin sheet treated at temperature of 500℃ was used. The rolling direction is parallel with the curves of sine waves. Then, embossing experiments without ultrasonic vibration and duration time were carried out at room temperature. The formed parts with micro-features are in shown in Fig. 2. In the investigation, the length and amplitude of waves as main parameters were measured by using a video measuring system as shown in Fig. 3 and Fig. 4.

Photographs in Fig. 3 show that there are no cracks on the formed parts. Sideview of sine wave micro-features indicates that the plastic deformation of parts increases with the increasing of punch displacement. Then, dimensions of formed sine waves were measure, and shown in Fig. 4. It was found that the amplitude of sine waves becomes larger for the bigger punch displacements. Also, we notice that the difference of wave length is almost not changed for different punch displacements, which is less than 0.02mm, and meeting the requirement of design. So, the analysis is focused on the amplitude of sine wave in the following investigations.

Since the sine wave is a kind of complex surface with variable curvature, the springback has an obvious effect on the amplitude of sine wave of formed parts when the embossing is performed using ultra-thin sheet. In the investigation, the springback is evluated using the difference between the punch displacement and amplitude of formed waves. The results indicate that springback increases with the increasing of punch displacement, and then decreases when punch displacement is bigger than 0.45mm as shown in Fig. 5. At the beginning of embossing process, bending dominates the plastic deformation for the small puch displacements, and strain gradient increases with the increasing of punch displacements, which leads to the increase of springback. With the further increase of punch displacement, tensile strain dominates the deformation behavior, which decreases the difference of strain in the direction of the thickness. And aslo, the female die corrects the shap of sine wave. As a result, springback of sine wave decreases with the increasing of punch displacement.

3.2 Effect of heat treatment temperature

Heat treatment has an obvious effect on the properties of rolled ultra-thin sheet, i.e. microstructure, flow stress, etc.. In the investigation, ultra-thin sheets treated at the temperatures of 200℃, 300℃, 400℃, 500℃, and 600℃ were chosen, and the punch displacement of 0.55mm is applied. The rolling direction is parallel with the curves of sine waves. Then, embossing experiments without ultrasonic vibration and duration time were carried out at room temperature. However, the thin sheet was broken during embossing experiment when the sheet treated at 200℃ was performed, and the data was not considered in the analysis. To reduce the effect of rough specimen side and improve the measurement accuracy, the formed parts were inlayed a container and polished as shown in Fig. 6. The outlines of formed sine waves were atuomatically distinguished and measured using the vedio measurement system. The measured outlines are shown in Fig. 7, and the amplitude and the calculated sprinback of formed sine waves are shown in Fig. 8.

Experimental results (Fig. 8 (a)) show that the amplitude of sine wave increaes with the increasing of treatment temperature. It can be anlyzed from the viewpoint of the influence of treatement temperature on the properties of metallic sheet. Base on the experimental results of tensile tests [26], harding index becomes bigger for high temprature, which improves the ability of homogeneous deformation. Then, the strain gradient can be reduced, and the springback decreases accordingly (Fig. 8 (b)). As a result, the aplitude of formed sine waves increases when the sheet treated by higher temperature is used.

3.3 Effect of rolling direction

During rolling process of metallic sheet, microstructure of materials is deformed along the rolling direction, and distribution of some small particles is also parallel to the rolling direction. The properties of sheet are not same for different directions. And, these kinds of effects become larger for ultra-thin sheet. In the studies, two directions, such as parallel and vertical ones, were considered in the embossing process using sheets treated at the temperature of 500℃. Results in Fig. 9 indicates that the amplitude (0.4522mm) for vertical one is bigger than that (0.4132mm) for parallel one. The reason maybe that the hardening index and elastic modulus for vertical direction are larger, which leads to small springback of sine waves. Then, higher amplitude is obtained when the sheet is put on the male die using the vertical direction.

3.4 Effect of duration time

Sine wave micro-features is very small, and it needs long time to complete the deformation, especially for the extremely small thickness of metallic sheet. Different duration time, such as 0, 1min, 2min and 3min, were selected to emboss the parts with micro-features. Other parameters are the punch displacement of 0.55mm, heat treatment temperature of 500℃, and vertical direction. Experimental results are shown in Fig. 10 and Fig. 11. It was found that the amplitude of sine wave increases obviously with the increasing of the duration time (Fig. 11 (a)). This means that the increase of duration time is helpful for improving the accuracy of sine wave micro-feature. Accordingly, the springback is decreased obviously (Fig. 11 (b)). Also, we notice that the amplitude for 2min is almost the same with that for 3min, and then, duration time of 2min is accepted in the following experiments considering the production efficiency.

3.5 Effect of vibrator energy

Based on the analysis shown above, the amplitude of formed sine wave micro-features can not meet the requirement of design dimension. Acoustic assisted embossing process was carried out using punch displacement of 0.6mm with vertical direction metallic sheet treated at 500℃. When the punch displacement is reached at 0.3mm, the punch is stopped, and then the ultrasonic vibration is turned on and works for 60s. After that, the ultrasonic vibration is turned off, the punch is going down until that it reaches at punch displacement of 0.6mm. Experimental results without duration time of vibration are shown in Fig. 12 and Fig. 13. It is found that the amplitude of sine wave becomes bigger obviously (Fig. 13 (a)), and increases with the increasing of ultrasonic energy. Accordingly, the springbcak is decreased by ultrasonic vibration (Fig. 13 (b)). The minimum value of springback is 0.0457 when the vibrator energy of 85% is applied, which is much lower than that without ultrasonic as shown in Fig. 5.

These results can be analyzed from the viewpoint of acoustic softening effects and residual effects of ultrasonic vibration. For the existing softening effects, the flow stress is decreased because that ultrasonic vibrator provides the energy needed for dislocation sliding. Even that, the ultrasonic vibration is turned off, residual softening effect exists since that the microstructure of metallic sheet is changed. Then, the lower flow stress leads to the increase of amplitude of formed sine waves. On another side, hardening index of ultra-thin sheet is increased by the ultrasonic vibration. This means that uniform deformation ability of metallic sheet is improved, which can lower the strain gradient along the thickness direction both for the bending and bulging in the embossing process. Then, the springback of ultra-thin sheet is also reduced during the embossing process. Bigger the acoustic energy is, larger the effects as shown above are. As a result, the amplitude of formed sine wave increases with the increasing of ultrasonic energy.

3.6 Effect of duration time of ultrasonic vibration

Using the same experimental parameters shown in section 3.5, the effect of duration time of ultrasonic vibration, such as 30s, 60s, and 90s, is investigated at the punch displacement of 0.3mm using the ultrasonic energy of 75%. Figure 14 and Fig. 15 show experimental results. It is found that the amplitude of formed sine wave increases (Fig. 15 (a)), and the springback decreases (Fig. 15 (b)) with the duration time of ultrasonic vibration. For the long duration time of ultrasonic vibration, residual softening effect becomes much clearer, which is a kind of permanent effect. The lower flow stress and springback leads to the increase of amplitude of formed sine waves.

Sine wave micro-features own the complex surface with variable curvature, and the springback is critical issue in the micro-manufacture process, espcially for the ultra-thin sheet. In generally speaking, the springback is induced by the strain gradient in the thickness direction. In order to change the situation, embossing process charactered by bulging deformation is performed instead of bending process to reduce the strain gradient as shown in Fig. 16. The major stress is a kind of tensile stress, and the compressive stress is very small at the contact interface between die and sheet. Accordingly, the major stain is a tensile strain, which is helpful for reducing the springback of sine wave micro-features, and also is demonstrated by the experimental results.

The formed sine wave micro-feature was polished from the sideview, and its thickness was measured as shown in Fig. 17. Thicknesses are 52.5µm at peak point and 52.0µm at valley point of sine wave. Between the peak point and valley point, the thicknesses of 3 points were measured, and results are 48.3µm, 47.1µm and 48.2µm, respectively. The interesting thing is that the thickness of part at peak and valley point is increased. As shown in Fig. 18, the reason is that the clearance 37µm of female and male die is a little smaller than the sheet thickness of 50µm at the area between peak and valley of wave when their outlines were machined according to the standard sine wave respectively. In the investigation, the female and male die with equal distance of 50µm were machined to get the outline of sine wave with high accuracy.

Since the thickness of the sheet is extremely small, and the sine wave micro-feature is a kind of complex surface, the springback is also obvious in the embossing process. Ultrasonic vibration, as a kind of physical energy, is performed in the micro-manufacturing process. Considering the fact that the punch load near to the end of embossing is much bigger than the allowable value of a vibrator, and the ultra-thin sheet is easily broken by the ultrasonic vibration of high energy as shown in Fig. 19, the ultrasonic vibration is applied when the punch displacement is equal to the half of amplitude of sine wave. When the ultrasonic vibration is applied, acoustic soften effect has an important influence on the deformation behaviors of ultra-thin sheet. With ultrasonic vibration, the deformation becomes easy and uniform, which is helpful for reducing springback of ultra-thin sheet in the embossing. Even that the ultrasonic vibration is turned off, the microstructure of sheet is changed [16], and residual softening effect also exists which is demonstrated in reference [26], which affects the deformation of thin sheet in the following embossing process. As a result, the springback of formed sine wave micro-features is evidently reduced, and the accuracy of parts is improved obviously.

Another effect of ultrasonic vibration is that it can eliminate residual stress induced by nonhomogeneous plastic deformation [20]. When the punch displacement reaches 0.3mm, nonuniform plastic deformation occurs at the peak and valley of sine wave, which leads to the appearance of residual stress as the influence parameter for the springback. In the investigation, the application of ultrasonic vibration reduces the residual stress, and then, the sprinback of formed micro-features becomes smaller, and parts with high accuracy can be formed by using ultrasonic vibration.

Based on the analysis shown above, the parameters including ultrasonic vibration and embossing are optimized, and sine wave micro-features with high quality is formed as shown in Fig. 20. The experimental results indicates that the length of 1.9983mm and amplitude of 0.5801mm are obtained for the sine wave micro-feature, which can meet the requirement of design.

To fabricate functional micro-features of sine waves with variable curvature, a novel ultrasonic assisted embossing process is developed with ultra-thin metallic sheet. The following results can be concluded:

(1) With the increasing of punch displacement, the amplitude of formed sine waves increase obviously, and wave length is almost not changed. The sprinback increases at the beginning, and then decreases when punch displacement is bigger than 0.45mm.

(2) For higher temperature of heat treatment of thin sheet, the amplitude of sine waves becomes bigger, and springback decreases for the increasing of forming ability and homogenous deformation induced changing of hardening index.

(3) When the sheet is put on the male die using the vertical direction, higher amplitude is obtained for the small elastic modulus and hardening index. The amplitude of sine wave increases obviously with the increasing of the duration time, and the springback is decreased obviously.

(4) It is found that the amplitude of sine wave becomes bigger obviously (Fig. 13 (a)), and increases with the increasing of ultrasonic energy. Accordingly, the springbcak is decreased. These results can be analyzed from the viewpoint of acoustic softening effects and residual effects of ultrasonic vibration.

(5) For the long duration time of ultrasonic vibration, residual softening effect becomes much clearer, which is a kind of permanent effect, and the lower flow stress and springback leads to the increase of amplitude of formed sine waves.

(6) Changing of strain states, and acoustic softening effect and eliminating residual stress of vibrator dominate the deformation behaviors and springback during the ultrasonic assisted embossing process, which clearly improve the shape accuracy of micro-features with sine wave outlines.

Funding This work was supported by financial support from the Natural Science Foundation of Jiangsu Province (BK20192007), the National Natural Science Foundation of China (No. 51875128, No. 51905362) , Six Talent Peaks in Jiangsu Province (GDZB-069), the Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 19KJB460022), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2020043).

Conflicts of interest/Competing interests The authors declare no competing interests.

Code availability Not applicable

Ethics approval This research did not involve any human participants or animals.

Consent to participate All authors agree participate the manuscript and their order.

Consent for publication All authors agree to publish the manuscript in the journal named The International Journal of Advanced Manufacturing Technology.

Authors' contributions All authors contributed to the study conception and design. Material preparation was performed by Lidong Cheng, Chunju Wang and Shuhan Zhang. Data collection was performed by Shuhan Zhang, Qi Zhong and Zhenwu Ma. Analysis was performed by Lidong Cheng, Chunju Wang, Zhenwu Ma and Haidong He. All authors read and approved the final manuscript.

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You are reading this latest preprint version

Investigation on ultrasonic assisted embossing process of micro-features with variable curvature as functional surface using ultra-thin sheet of 5052 aluminum alloy

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Version 1

Abstract

Figures

1 Introduction

2 Experimental Setup

2.1 Die and principle

2.2 Experimental materials and parameters

3 Experimental Results And Analysis

3.1 Effect of punch displacement

3.2 Effect of heat treatment temperature

3.3 Effect of rolling direction

3.4 Effect of duration time

3.5 Effect of vibrator energy

3.6 Effect of duration time of ultrasonic vibration

4 Discussion

5 Summary

Declarations

References

Status:

Version 1