Surface quality study of AZ31B Mg alloy in electric pulse-ultrasonic assisted incremental sheet forming

doi:10.21203/rs.3.rs-1644548/v1

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Surface quality study of AZ31B Mg alloy in electric pulse-ultrasonic assisted incremental sheet forming

https://doi.org/10.21203/rs.3.rs-1644548/v1

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Poor surface quality is one of the main problems for the forming and application of magnesium alloys. In this study, electric pulse and ultrasonic vibration were simultaneously applied to the incremental sheet forming (ISF) process of magnesium alloys, and the effect of electric pulse - ultrasonic composite energy field on the surface quality was investigated. Compared with electric pulse assisted forming, composite energy field could improve the surface quality at the condition of higher current frequency (≥ 350HZ), lower step depth (0.1mm) and keep the effective current density less than 30A/mm². Moreover, although the surface quality can be improved by ultrasonic energy in both directions, the effect is more significantly in direction perpendicular to the feed direction than that in the direction parallel to the feed direction. The repeated rolling caused by ultrasonic vibration is benefit to the rearrangement of the surface gully, and makes the surface profile height distribution relatively uniform. The surface quality also would be aggravated due to the “jumping” effect of the tool head and serious surface oxidative, which caused by excessive temperature increasing at the condition of very current density.

Magnesium alloy

surface quality

incremental sheet forming

electric pules

ultrasonic vibration

Magnesium alloys are one kind of lightweight structural materials, which had applied in the automotive, aerospace, defense, and biomedical industries[1, 2]. But its low ductility and strong texture makes it difficult to obtain large angle and high precision in forming process[3–5]. Incremental sheet forming (ISF) is based on incremental forming theory, manufactureing the sheet part by means of localized plastic deformation accumulation[6, 7]. For materials such as magnesium alloys which are difficult to form at room temperature, a variety of ISF processes using external physical fields have been developed. Hino et al.[8] successfully applied laser to incremental forming. Their research showed that laser-assisted single point incremental forming is suitable for a variety of materials, and can accurately control temperature, significantly reduce forming force, improve forming accuracy. Ambrogio et al. [9] carried out an experimental study on electromagnetic assisted ISF, which showed that electromagnetic induction technology can heat the sheet in a short time and easily adjust the forming temperature. Xu et al. [10] used electromagnetic pulses and heated aluminum alloy backing plates to assist the ISF of magnesium alloys, and verified the feasibility of improving the formability of magnesium alloys. Zhang et al. [11] achieved warm ISF of magnesium alloy sheets by heating in an oil bath. Zhang et al. [12] studied the microstructure evolution of AZ31B sheet deformed by electric hot temperature-controlled single point incremental forming. Yang et al. [13] reported a friction stir-assisted ISF research. By studying the effect of ultrasonic vibration on the ISF process of aluminum alloys, Li et al. [14–16] proved that ultrasonic vibration can reduce the forming force and friction.

However, the high surface roughness in the ISF process affects its application in industry, low surface roughness requires small step depths in the ISF process, which means longer forming times[17]. In recent years, the research on surface quality in the ISF process has mainly focused on the effect of process parameters on surface roughness. Chang et al. [17]established a roughness prediction model that comprehensively described the effects of tool radius, wall angle, step depth, sheet thickness, and material properties. Kumar et al. [18] investigated the effect of tool diameter, step size, and spindle speed on surface roughness during ISF process. The results showed that surface roughness increases with the decrease in tool diameter and spindle speed, decreases with the decrease in step size.

As a simpler and more economical way than laser-assisted, the electric-assisted ISF process has received extensive attention. Bao et al. [19] and Ao et al. [20] respectively demonstrated that electric assistance could improve the formability of magnesium and titanium alloys during ISF process. But their study didn't mention the issue of surface quality. The study by Fan et al. [21] demonstrated poor surface quality during electric-assisted ISF process, and they improved the surface quality by using different lubricants. Ambrogio et al. [22] pointed out that the current-induced oxidation and tool impact jointly could significantly affect the surface quality of the sheet in electric-assisted ISF of titanium alloys. Najafabady et al. [23] used alternating current instead of direct current in electric-assisted ISF process, and found that the surface quality was susceptible to the step depth and sheet thickness.

In the forming process, the influence of ultrasonic vibration on the material is mainly divided into volume effect and surface effect. Surface effects are considered to improve the lubrication and friction between the mold and the workpiece[24, 25]. Li et al. [26] carried out research on ultrasonic-assisted incremental forming based on finite element simulation. The results show that ultrasonic improve the part surface quality. Liao et al. [27] studied the surface quality change of AZ31B magnesium alloy sheet by hot single point incremental forming under ultrasonic assistance. They found that temperature has a significant effect on orange peel patterns, and the scratches and adhesive wear of the contact surface also increased with increasing of temperature. Amini et al. [28] found that ultrasonic vibration can improve formability and reduce surface roughness in the ISF process of aluminum sheet.

In addition, the current research on the coupling-assisted forming of electric pulse and ultrasonic vibration (EP/U) is mostly concentrated in the field of local plastic deformation. Sun et al. [29] used EP/U assisted surface rolling process to treat nickel-aluminum bronze materials, and found that the surface strength would first increase and then decrease with the enhancement of electric pulse parameters. The EP/U assisted nano-grain surface modification research carried out by Ye et al. showed that the EP/U coupling effect can significantly improve the surface roughness, strength and hardness of parts[30].

According to the above literature, it can be seen that the electric pulse and ultrasonic composite energy field has great potential in improving the surface quality of metal sheet during ISF process, but its influence mechanism is rarely studied. In this paper, based on the self-designed EP/U composite energy field assisted single point incremental forming experimental platform, the effect of EP/U coupling on the surface quality of AZ31B magnesium alloy during the ISF process was studied. The effect of EP/U coupling field on the surface quality of parts was studied under the conditions of different effective current density, frequency and step depth of the tool head.

2.1 Materials

Commercial AZ31B magnesium alloy sheet with 1 mm thickness was selected as research material in this article. The chemical composition (w.t.%) of the material is shown in Table 1.

Table 1

Chemical composition of AZ31B sheet(mass fraction wt.%)
Al	Zn	Mn	Ca	Si	Mg
2.99	0.64	0.34	0.16	0.08	Bal.

2.2 Experimental device and method

2.2.1 Electric pulse-ultrasonic composite field assisted incremental forming experimental platform

The EP/U composite field-assisted incremental forming experimental platform built on the XKA714 traditional vertical three-axis CNC milling machine was shown in Fig. 1. The ultrasonic vibration unit was connected with the CNC milling machine spindle through the BT40-C32-110 high-strength tool holder, and the natural frequency of the tool head is 23.4 kHz. During the incremental forming process, the electric current generated a complete circuit through the vibration unit housing, tool head, workpiece sheet and pressure plate. The temperature in the top area of the tool head was measured by K-type contact thermocouple, and the LDCZL-MFS3 three-dimensional force sensor was used to measure the vertical forming force in the three directions of the Z axis of the machine tool coordinate system during the forming process. Table 2 showed the technical parameters of the THDM-1 high-energy pulse power supply used in this study, which can output high-energy pulse current with a sinusoidal waveform, as shown in Fig. 2.

Table 2

Technical parameters of THDM-1 high-energy electropulse generator
Power (kW)	Output frequency (Hz)	Output voltage (V)	Peak current (A)	Pulse width (µs)
15	120–900	30–140	≥ 5000	60–80

Eq. 1 was the function of the output current value and time. i(t) is instantaneous current, I_p is the peak current, n is the number of pulses, T is the pulse period. In this research, the oscilloscope and transformer were used to monitor and collect the actual output peak current, effect current and frequency. The peak current of the electrical pulse was controlled by the output voltage of the power supply, and the effect current value is related to the peak current and frequency:

$$i(t)=\left\{ \begin{gathered} {I_p}\sin (\frac{{2\pi }}{{{t_i}}} \times \frac{t}{2}) \hfill \\ 0 \hfill \\ \end{gathered} \right.\begin{array}{*{20}{c}} {}&{\begin{array}{*{20}{c}} {Tn \leqslant t \leqslant Tn+{t_i}} \\ {Tn+{t_i} \leqslant t \leqslant T(n+1)} \end{array}} \end{array}$$

$${I_r}={I_p}\sqrt {f\int_{0}^{t} {{{\sin }^2}({{\pi t} \mathord{\left/ {\vphantom {{\pi t} {{t_i}}}} \right. \kern-0pt} {{t_i}}})dt} } ={I_p}\sqrt {{{f{t_i}} \mathord{\left/ {\vphantom {{f{t_i}} 2}} \right. \kern-0pt} 2}}$$

where I_r is effective current value, f is the pulse frequency, t is operation time and t_i is pulse width. The peak current and effective current value divided by the effective cross-sectional area is the corresponding current density.

Based on the truncated cone model, three groups of control experiments were carried out in this study, as shown in Table 3.

Table 3

Experimental parameters of incremental forming for surface quality
Groups	NO.	Condition	Step depth (mm)	Pulse current parameter		Ultrasonic frequency f_u (kHz)
Groups	NO.	Condition	Step depth (mm)	J_r (A/mm²)	Current frequency f_e (Hz)	Ultrasonic frequency f_u (kHz)
1	1	EP	0.1	21.1	400	-
	2	EP/U	0.1	21.1	400	23.4
	3	EP	0.1	25.7	400	-
	4	EP/U	0.1	25.6	400	23.4
	5	EP	0.1	29.9	400	-
	6	EP/U	0.1	30.0	400	23.4
	7	EP	0.1	33.3	400	-
	8	EP/U	0.1	33.4	400	23.4
2	9	EP	0.1	30.0	300	-
	10	EP/U	0.1	30.1	300	23.4
	11	EP	0.1	30.1	350	-
	12	EP/U	0.1	30.2	350	23.4
3	13	EP	0.2	29.8	400	-
	14	EP/U	0.2	29.9	400	23.4
	15	EP	0.3	29.9	400	-
	16	EP/U	0.3	30.1	400	23.4
Note: J_r represents effective current density, EP represents for electric pulse assisted forming, EP/U represents for electric pulse and ultrasonic composite energy field assisted forming.

2.2.2 ISF experiments and surface roughness measurement

The radius of the tool head is 4 mm, the forming angle is 40°, and the corresponding contact area is about 5.88 mm². During the forming process, the temperature at the top of the tool head was measured in real time using a K-type contact thermocouple and regarded as the contact zone temperature. At the same time, the vertical forming force was recorded based on the force sensor. The surface morphology and roughness of the truncated cone shaped parts were measured by laser confocal microscope (VK-X1000 3D, KEYENCE), the location distribution of sampling points and the position of the reference line for line roughness measurement was shown in Fig. 3.

In addition to directly observing the surface morphology, the surface quality can also be quantitatively characterized by surface roughness and line roughness. In this study, the arithmetic average roughness Ra and Sa were used as the reference values for evaluating the roughness, which could be obtained by Eq. 3 and Eq. 4, respectively.

Assumed that the profile curve on the baseline of the specimen is Z = z(x), defined the Z axis to be perpendicular to the X axis, while the X-axis was the mean line of the profile curve. Then the arithmetic average line roughness R_a was:

$${R_a}=\frac{1}{l}\int_{0}^{l} {\left| {z(x)} \right|} dx$$

where l was the length of the reference line corresponding to the measurement range.

Accordingly, assumed that the height of the contour surface in a certain area of the specimen was Z = z(x,y), defined the Z axis to be perpendicular to the XY plane, while the XY plane corresponded to the average height of the contour surface in the plane. Then the arithmetic average surface roughness S_a was:

$${S_a}=\frac{1}{A}\iint\limits_{A} {\left| {z(x,y)} \right|dxdy}$$

where A was reference plane area corresponding to the average.

3.1 Forming force and temperature

Figure 4 shows the sheet surface temperature and vertical forming force corresponding to Experiments 1–8 in Table 3. It could be seen from Fig. 4 that the temperature increased and the forming force decreases as the effective current density increases. When the frequency of the electrical pulse remains constant, the effective current density increased, based on Eq. 2, the peak current density of the electrical pulse increased accordingly. It means that the current heating effect became more pronounced and the temperature of the sheet rose. Compared the surface temperature of the sheet under the assistance of EP and EP/U coupling field, it could be seen that EP/U coupling caused further temperature increase, and this phenomenon was more pronounced at conditions of lower effective current density, while the average percent of temperature rise reached a maximum of about 17% when effective current density was 21.1 A/mm². Correspondingly, the forming force was further deceased in the EP/U coupling assisted forming process, while the average percent of forming force decrease reached a maximum of about 20% when effective current density was 33.4 A/mm².

The influence of EP/U coupling on the forming force showed different characteristics with the forming depth increase, as shown in Fig. 4(b). Firstly, the evolution of the forming force decreased slightly at the initial stage, and then increased gradually; secondly, the forming force with EP/U coupling field is less than that of only EP condition due the higher temperature, especially at the later stage of forming process. AZ31B magnesium alloy sheet was softened under the assistance of EP or EP/U coupling field at the beginning of ISF forming, however, the effect of work hardening was gradually enhanced with the increasing of feeding depth, and the dynamic softening also played an important role at the later stage of forming process, which lead to vertical force decreasing.

The contact area temperature and variation of forming force with different current frequency were shown in Fig. 5. The contact area temperature was higher when high frequency was applied. Therefore, with the increase of the forming depth, the high frequency was more likely to cause heat accumulation than the low frequency, so the material exhibited more pronounced softening. Due to the above phenomenon, the forming force was significantly reduced at high frequency. The influence of ultrasonic vibration is mainly reflected in the early stage of forming. From Fig. 5 (a), ultrasonic vibration made the contact area temperature significantly increase in the initial stage of forming, which further softened the material and reduced the forming force. As the forming progressed, the temperature gap between EP assisted and EP/U assisted gradually decreased, so that the forming force tends to be consistent.

Figure 6 shows the effect of step depth on temperature and forming force when the effective current density was 29.9 A/mm² and frequency was 400 Hz. Under the premise of the same forming height, larger step depth could effectively shorten the forming time. The temperature rise trend was basically the same with Fig. 5. Meanwhile, increasing the step depth led to a larger contact area between the tool head and the sheet during the feeding process, and resulted in an increase in the forming force. In addition, it could be seen from Fig. 6 (b) that when the step depth was set to 0.1 mm, the reduction of forming force caused by the EP/U coupling effect in the initial stage was more significantly.

3.2 Surface quality

Selected the area corresponding to the forming depth of 5 mm (sampling point B in Fig. 3) as the control area, surface morphology and roughness of the EP assisted and EP/U assisted forming parts under different effective current densities was shown in Fig. 7. Compared Fig. 7 (a), (c), (e), (g), it could be seen that the increase of the current density makes surface gullies more obvious and the roughness increase. After superimposing ultrasonic vibration, as shown in Fig. 7 (b), (d), (f), (h), the surface roughness all showed a downward trend, except when the effective current density was 33.3A/mm².

In order to analyze the variation law of surface quality with forming depth, take the formed parts under the condition of electric pulse frequency of 400 Hz and effective current density of 29.9 A/mm², and observed the surface morphology of sampling points A and C, as shown in Fig. 8. Combined with the surface morphology of the sampling point B shown in Fig. 7 (e), it could be seen that the surface quality deteriorated with the increase of the forming depth at the condition of EP assisted, the corresponding value of Sa increases from 0.653 (Point A) to 1.190 (Point C). However, the application of ultrasonic vibration made the surface quality no longer decrease sharply with the increase of forming depth and Sa remained around 0.7.

Figure 9 shows the effect of current frequency on surface topography of the formed parts when the current density was 30 A/mm² and step depth was 0.1 mm. Compared with Fig. 7 (e) (f) which conducted at current frequency of 400 Hz, it was clearly seen that when the effective current density remained constant, low frequency conditions resulted in an increasing of Sa which means deterioration of the surface quality. When the effective current density remained constant, increasing the electric pulse frequency caused peak current density decreased, which showed a negative correlation with the frequency. Due to the contact area between the tool head and the plate was small, the current would concentrate on the surface of the plate and generate a high-density area due to the "skin effect"[30], so that the high peak current density which corresponding to low current frequency had a more significantly negative effect on the surface quality. For this reason, increasing current frequency can improve the surface quality. In addition, the application of ultrasonic vibration resulted in a substantial improvement in surface quality at each frequency, and the EP/U coupling effect was the most significant at 350 Hz, where the Sa reduced by about 50% compared with EP assisted.

Figure 10 shows the surface topography of the parts with different step depths when current density was 30 A/mm² and frequency was 400 Hz. As can be seen from Fig. 10, increasing the step depth resulted in increase of surface roughness, however, the surface quality of parts can be improved under ultrasonic vibration condition. The surface of magnesium alloy incremental formed parts is periodically distributed along the depth direction with "gully" formed by the action of the tool head, which is one of the main reasons for the poor surface quality of parts. The reasons for these "gully" can be concluded as follow: (1) Due to the hemispherical shape of the tool head, feeding along discrete layers will leave unformed protrusions between layers. Theoretically, the height of protrusions, also known as residual wave crest height, is equal to the vertical height from the intersection of the tool head contour to the theoretical contour, as shown in Fig. 11, in which the h_s is the residual height, r is the radius of the tool head, h is the interlayer feed in depth direction and α is the wall angle of truncated cone. (2) Due to the extrusion and friction of the tool head, the surface material flowing lead to local accumulation on both sides of the feeding direction of the tool head, and a furrow effect is generated at the top of the ball head. (3) The mechanical wear and grinding particles also occurred in the process of tool head feeding, and the effect of oxidative wear would become much more serious with surface temperature increasing. The increasing of the step depth would increase the theoretical residual height, resulting in the increase of surface roughness. However, the dynamic softening and impact effects caused by ultrasonic vibration could enhance the formability of materials, which resulted in the decreasing of Sa and improvement of surface quality with assistance of EP/U coupling field.

3.3 Line roughness in different directions

The line roughness of the workpiece under different effective current density was shown in Fig. 12, and the position of the reference line for line roughness measurement was shown in Fig. 3 (b). In this study, $\Delta Ra$(the difference between the linear roughness of the EP assisted part minus the linear roughness of the EP/U assisted part) was used to characterize the influence of the coupling effect on the roughness.

From Fig. 12, the line roughness perpendicular to the feeding direction of the EP assisted and EP/U assisted forming parts showed the same trend, they both increased with the increase of the effective current density. The main factor affected the line roughness perpendicular to the feed direction was surface gully. The increase of surface temperature caused by the increasing of effective current density led to the surface material soften, and more serious mechanical wear and oxidative wear generated. For this reason, the roughness in parallel direction also increased with the increasing of effective current density in both EP assisted and EP/U assisted ISF process. As for the$\Delta Ra$in perpendicular direction, it showed a trend of initial increased and then decreased with the increasing of the effective current density. $\Delta Ra$reached a maximum at the effective current density of 25.6 A/mm², which means that the influence of EP/U coupling effect was the most significant under this condition. When the effective current density reached 33.3 A/mm², $\Delta Ra$was less than zero, which means that the EP/U coupling effect further increased the line roughness in this direction. After applying ultrasonic vibration, the superimposed ultrasonic energy can further soften the surface material. Meanwhile, with the up and down movement of the tool head in the axial direction, more local surface materials will participate as the deformation continued, thereby effectively improving the flow of surface materials. However, the impact caused by ultrasonic vibration enhances the rolling action of the tool head on the sheet surface. On the premise of improving the formability of the surface material, the repeated rolling of the sheet by the tool head is conducive to the rearrangement of the surface gully, makes the surface profile height distribution relatively uniform. Under the combined effect of the above factors, when effective current density was lower than 30A/mm², EP/U composite field assistance effectively reduced the roughness perpendicular to the direction of the feed direction. Nevertheless, when the effective current density was higher than 30 A/mm², the surface temperature was too high and would cause excessive softening of the surface material. In this case, the dynamic impact caused by ultrasonic vibration leads to more severe adhesive wear, oxidative wear and local surface quality deterioration.

The$\Delta Ra$in parallel direction showed a different trend, with the corresponding maximum value reached 0.32 when effective current density is 29.9 A/mm². Same as above, this trend could be partly attributed to the combined effect of material softening and surface wear caused by higher temperature. In addition, it was also affected by the "jumping" of the tool head and the ultrasonic dynamic impact. Figure 13 shows the morphology of the EP/U assisted forming parts under the conditions of effective current of 21.1 A/mm² and 33.4 A/mm². Due to the “jumping” action of the tool head on the surface of the sheet, a continuous arrangement of “fish scale” ripples were formed in the surface gullies at condition of effective current of 21.1 A/mm², as shown in Fig. 13(a). The reason for its formation is shown in Fig. 14.

The tool head was subjected to the reaction force of the sheet during the feeding process, so it can be regarded as a fully constrained cantilever beam. At the condition of lower current density (≤ 30.0 A/mm²) or frequency (≤ 400 Hz) of electric pulse, the deformation resistance of the sheet was larger, tool head and fixtures did not guarantee an ideal rigid connection. Therefore, elastic bending deformation in the horizontal direction occurred during the feeding process. On one hand, there was no relative movement between the tool head and sheet at the moment, and the friction mode was static friction. With the position offset of the tool head, the actual contact area between the tool head and the sheet became smaller, which reduced the amount of material involved in the deformation and reduced the corresponding forming force, then the tool head had a certain degree of elastic recovery. This phenomenon caused alternating static and dynamic friction between the tool head and the sheet, which made the tool head “jumping” in the feed direction. At the same time, the sheet was elastically deformed, resulted in residual bulges in the feed path. On the other hand, at lower current density, the material flow ability was poor, surface material accumulated in front of the tool head, and increased tool head deflection. These accumulated materials will be destroyed under the pressure of the tool head, which cause the tool head elastically recover in a short period time in the case of sudden resistance changes and cause an impact on the front surface material and left a residue “hollow” in the original position, as Fig. 14 (a) showed.

Ultrasonic vibration would have different effects on the above phenomena. As shown in Fig. 14 (b), the tool head moves up and down along the axial direction under the action of ultrasonic vibration, which would form a hollow on the surface of the sheet. This hallow would be superimposed with the "jumping" of the tool head, causing a greater impact on the front surface material, resulting in further accumulation of the material in front of the tool head. At the same time, the upward movement helps the tool head to pass over the material accumulated in front, which will effectively avoid material damage, and reduced the elastic bending degree, weaken the impact caused by “jumping”. However, the accumulated material is not peeled off, it means high residual height. The above reasons lead to the fact that the roughness parallel to the feeding direction increased after ultrasonic vibration was applied under the condition of low effective current density. The primary cause of the above phenomenon was the limited flow ability of the surface material and the high deformation resistance. When the effective current density increased, the plastic deformation ability of the surface material was improved by temperature increasing and electroplastic effect, and the “fish scale” ripples also disappeared, as shown in Fig. 13 (b).

Figure 15 (a) (b) show the line roughness for different forming depths, which correspond to the position of A, B and C, as shown in Fig. 3 (b). It can be seen that the line roughness in both directions showed an increasing trend with depth increasing. The larger the tool head feed depth, the shorter feed cycle between layers. That meant the interval between the tool head running along the contour line was shorter, heat accumulation faster, then the actual temperature of the sheet increased further. The increase of temperature would lead to further softening and wear of the material at the position, which resulted in increasing of roughness. Meanwhile, compared the$\Delta Ra$in the two directions, it was found that the surface quality improvement effect caused by ultrasonic vibration was most significant at the forming depth of 5 mm (Position B), where the$\Delta Ra$reached a maximum 0.2 in perpendicular direction and 0.07 in parallel direction. Therefore, with the increasing of forming depth, the line roughness in both directions will increase due to the material softening caused by temperature rise. Moreover, EP/U coupling field could decrease the line roughness and improve the surface quality compared with EP assisted forming. It should be noted that the parallel direction$\Delta Ra$was less than zero at a forming depth of 2 mm (Position A), which was due to significant dynamic impact effect of the tool head and the lateral “jumping” caused by the low temperature and limited deformation ability of magnesium alloy sheet.

It can be seen from Fig. 15 (c) (d) that the roughness in both directions of the EP and EP/U assisted forming parts basically showed a trend of decrease with the increasing of frequency. This is because the high peak current density corresponding to the low frequency would cause excessive softening and wear of the surface material. For the perpendicular direction, the line roughness decreased by 0.42, 0.28 and 0.18 corresponding to the frequency was 300 Hz, 350 Hz and 400Hz under the action of ultrasonic vibration. The higher peak current density caused by lower frequency led to stronger electroplastic effect and better plasticity of surface material. Under the impact effect caused by ultrasonic vibration, the gullies were partially filled, so that the roughness perpendicular to the feed direction was significantly reduced. However, the line roughness in the parallel direction increased due to the higher forming force and more material accumulating and flaking caused by ultrasonic vibration at low frequency. With the increasing of frequency, the line roughness decreased with the action of EP/U coupling effect, and $\Delta Ra$ reached a maximum of 0.09 when current frequency was 350 Hz, which means that the surface quality was significantly improved. Therefore, the line roughness in both directions of perpendicular and parallel to feed direction could be reduced by EP/U coupling effect.

Based on the self-designed electric pulse-ultrasonic composite energy field assisted single point incremental forming experimental platform, high-energy electropulse and ultrasonic vibration were simultaneously applied in the incremental forming process of AZ31B magnesium alloy sheets. The effects of effective current density, step depth and frequency on the surface quality of composite energy field-assisted ISF sheet were discussed. The main conclusions are as follows:

The temperature increased with the increasing of current density and frequency, and further increased with the action of EP/U coupling field, which resulted in the decreasing of forming force.

The surface quality of AZ31B magnesium alloy was improved by EP/U coupling field in ISF process, especially at the condition of effective current density less than 30A/mm², higher current frequency (≥ 350 Hz) and lower step depth (0.1 mm). The dynamic softening and impact effects caused by ultrasonic vibration could enhance the formability of materials, and correspondingly enhance the local plastic flow capacity of workpiece and improve the surface quality. However, the excessive temperature caused by the higher current density would lead to further softening of surface materials, which resulted in a local damage due to the great impact caused by ultrasonic vibration. The “jumping” of the tool head and serious surface oxidative wear would also further aggravate the surface quality.

The surface roughness decreased with the increasing of frequency in EP assisted forming process, and the surface quality could be further improved by the EP/U coupling effect. The peak current density decreased with increasing of frequency, and weaken the energy concentration on the surface of the plate due to the “skin effect”, which reduced the local surface destroy, especially when coupled with the ultrasonic energy. Therefore, the best surface quality was obtained at the frequency of 400 HZ with the assistance of EP/U composite energy field.

The line roughness of the workpiece could be decreased by EP/U coupling effect in both directions of perpendicular and parallel to feed direction. The impact caused by ultrasonic vibration enhances the rolling action of the tool head on the sheet surface, and the repeated rolling on the sheet is conducive to the rearrangement of the surface gully, which makes the surface profile height distribution relatively uniform.

Acknowledgements The authors would like to thank the Physical-Chemical Materials Analytical & Testing Center of Shandong University at Weihai for providing the optical microscope in order to components microstructure observation.

Author contribution Zimo Song: Investigation, Conceptualization, Methodology, Formal analysis, Writing - original draft. Haoran Zhang: Investigation, Data curation, Formal analysis, Validation. Xingrong Chu: Conceptualization, Project administration, Formal analysis, Funding acquisition, Writing - original draft. Zhiyong Zhao: Supervision，Resources, Funding acquisition, Writing - review & editing. Jun Gao: Supervision, Writing - review & editing.

Funding This work was supported by the financial support from National Natural Science Foundation of China [grant number 52175339, 52001185].

Competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Availability of data and material The processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Code availability Not applicable.

Ethics approval Not applicable.

Consent to participate Not applicable.

Consent to publication Not applicable.

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Editorial decision: Major Revisions Needed
14 Jul, 2022
Reviews received at journal
15 May, 2022
Reviewers invited by journal
15 May, 2022
Editor assigned by journal
13 May, 2022
First submitted to journal
10 May, 2022

You are reading this latest preprint version

Surface quality study of AZ31B Mg alloy in electric pulse-ultrasonic assisted incremental sheet forming

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Version 1

Abstract

Figures

1. Introduction

2. Experiments

2.1 Materials

2.2 Experimental device and method

2.2.1 Electric pulse-ultrasonic composite field assisted incremental forming experimental platform

2.2.2 ISF experiments and surface roughness measurement

3. Results And Discussion

3.1 Forming force and temperature

3.2 Surface quality

3.3 Line roughness in different directions

4. Conclusion

Declarations

References

Status:

Version 1