Effect Analysis of Process Parameters on Geometric Dimensions during Belt-Heated Incremental Sheet Forming of AA2024 Aluminum Alloy

Abstract

The effect of process parameters on geometric dimensions is complex during belt-heated incremental sheet forming, leading to control difficulties in the geometric dimension of parts. In this study, the geometric errors were analyzed under the action of different process parameters, and the corresponding influence law was obtained through macro- and microexperiments. On this basis, the micromorphology of the deformation region section was analyzed in detail, and the effect of process parameters on micromorphology characteristics was obtained during belt-heated incremental sheet forming. Meanwhile, the dislocation density of the deformation region section was analyzed through XRD, and the change law of the dislocation density was revealed under the action of different process parameters. Therefore, the macro and micro effect mechanism of process parameters on geometric dimensions was verified, and then the control method of the geometric dimension was obtained in belt-heated incremental sheet forming.

1. Introduction

The incremental sheet forming process is a type of rapid prototyping technology that can replace stamping to produce small batches of sheet metal parts, enhancing production efficiency and reducing costs. However, incremental sheet forming involves point-by-point extrusion, which tends to increase the material springback. Meanwhile, the springback reduces the geometrical accuracy of parts, and so the application of this technology can be limited. To overcome the aforementioned disadvantage, various studies, such as structural design [1,2,3], geometrical error compensation [4,5], process parameter optimization [6,7,8], and forming scheme optimization [9,10], have been conducted to enhance the forming accuracy of parts. Allwood et al. [2] concluded that the geometrical error is about ±3 mm in dieless incremental sheet forming and found that the part errors primarily consist of the clamping error, the non-clamping error, and the final error. To enhance the forming accuracy, two methods, such as auxiliary compensation and process parameter optimization, are commonly employed [4,5]. The optimization of process parameters becomes the primary method for improving the forming accuracy after that the forming condition is determined.

As the industry advances, the use of lightweight alloys is becoming more prevalent. However, lightweight alloys typically exhibit poor plasticity at room temperature and good plasticity at high temperatures. Therefore, incremental sheet forming technology is unsuitable for forming lightweight alloy parts at room temperature. To improve the above problem, thermal-assisted incremental sheet forming processes have been proposed, including laser-heated incremental sheet forming [11], heated medium incremental sheet forming [12], and electrically heated incremental sheet forming (EHIF) [13,14]. Among these, the electrically heated incremental sheet forming process has the advantages of efficient heating and simple equipment structure. Therefore, the forming process is widely applied and researched. Fan et al. [15] used a method combining reverse stretching and self-resistance electrically heated incremental sheet forming to weaken the reverse bulge at the bottom of titanium alloy parts. Ambrogio et al. [16] further investigated the relationship between the forming limit angle and the current density in self-resistance electrically heated incremental sheet forming for three types of lightweight alloys (AA2024-T3, AZ31B-O, and Ti-6Al-4V), and the corresponding forming limit curve of each alloy was also obtained. Meanwhile, Ullah [17] proposed an optimal forming path to improve the forming accuracy of the part. In addition to this, increasing the surface quality can also improve the forming accuracy of the part, according to the study of Ao [18], Saidi [19], and Xu [20]. However, the arc burn defect of the part surface can eliminate the forming accuracy in the self-resistance electrically heated incremental sheet forming process. Therefore, a belt-heated incremental sheet forming process (as shown in Figure 1), which not only suppresses arc burn defects on the part surface but also enables rapid heating of the material deformation area, is proposed by our team [21]. For electrically heated incremental sheet forming (EHIF) of lightweight alloys, the effect of process parameters on the geometrical accuracy is more complex in hot incremental sheet forming, particularly the interaction between process parameters and thermal expansion [22,23,24]. Currently, the optimization of process parameters is typically employed to enhance the forming accuracy of parts during hot incremental sheet forming [25].

Based on the aforementioned research, some methods, such as process parameter optimization, forming process optimization, and path optimization, are proposed to control the geometrical accuracy during hot incremental sheet forming of lightweight alloys. However, the interaction mechanisms between process parameters and microfeatures are often overlooked in this forming process. In this work, a coordinated control method between process parameters and microfeatures is proposed based on the belt-heated incremental sheet forming process of AA2024 aluminum alloy through the macro- and microanalysis. Meanwhile, a relationship between forming accuracy, process parameters, and microfeatures is established to achieve effective control of the forming accuracy.

2. Materials and Methods

An AA2024-T4 alloy sheet, with a size of 200 mm × 200 mm and a thickness of 1.0 mm, was adopted to fabricate the part. The chemical composition of AA2024-T4 alloy is shown in

Table 1. Chemical composition of AA2024-T4.

Al	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti
Balanced	0.5	0.5	3.8	0.3	1.2	0.1	0.25	0.15

. A high-temperature chain oil of 600 °C was used to ensure the surface quality of materials in this work.

Recently, Li [19] proposed a belt-heated method to fabricate an aluminum alloy sheet, and a temperature model of the heating method was established. Therefore, the above heating method has been adopted in this work, since it provides the features of fast heating and simple structure. As shown in Figure 2, the hot-working die steel of H13, which has good strength at high temperatures, was used to fabricate the clamp and support plates. Mica was adopted to fabricate the insulating plates. Meanwhile, the use of insulating plates can limit the heat loss at the edge. In addition to this, a thermal imager, Shanghai Thermal Imaging Technology Co., Ltd., Shanghai, China (Range: −20 °C to 1300 °C; Error: ±1 °C) was adopted to collect the temperature for the forming region.

In order to test the microstructure, a square cone, which had a 145 mm opening length, 25 mm height, and 85 mm bottom length, as shown in Figure 3a, was adopted to analyze the microstructure based on the different experimental parameters. On this basis, the effect of process parameters, such as tool diameter, feed rate, and step down, on the geometric accuracy was further analyzed at 180 °C, and the corresponding experimental scheme is shown in

Table 2. The experimental scheme.

Number	Tool Diameter (mm)	Feed Rate (mm/min)	Step Down (mm)
1	8	1000	0.15
2	10	1000	0.15
3	12	1000	0.15
4	10	1500	0.15
5	10	2000	0.15
6	10	1500	0.2
7	10	1500	0.25

. Meanwhile, the three parts of each experimental group were fabricated, and the average error of the three parts was viewed as the geometric error of each experimental group.

The inclined wall is the major deformed region, as shown in Figure 3b. Therefore, the deformed region can clearly describe material microscopic characteristics. In order to establish a relation between process parameters and microstructures, a cross-section of 14 mm length and 7 mm width was cut to analyze the change law of microstructures. Meanwhile, an X-ray diffraction of Japan Rigaku Ultima IV (Rigaku Corporation, Akishima-shi, Japan) and a scanning electron microscope of Germany ZEISS GeminiSEM 300 (Carl Zeiss AG, Oberkochen, Germany) were separately used to analyze the dislocation density; the morphology of the deformation cross-section in belt heating incremental sheet forming of AA2024 aluminum alloy, in which Cu target was used, and the scanning range, with 2°/min scanning speed, of 5–90° were set to analyze the dislocation density of the cross-section; and magnifications from 20× to 100,000× and an accelerating voltage of 15 kV were separately adopted to analyze the morphology of the test region.

3. Results and Discussion

3.1. Effect of Process Parameters

Based on the experimental design shown in Table 2, the absolute values of the axial geometrical errors of the parts were collected for each experimental group to determine the effect of different process parameters on geometrical errors, as shown in Figure 4. Due to the symmetry of the part, the geometrical error collection points started from the center of the part, as shown in Figure 4a. In addition to this, larger geometrical errors were observed at center distances of 50 mm and 60 mm, and the tool diameter and the step down were major influence factors for the bottom and opening geometricals, as shown in Figure 4b,d. In contrast, the effect of the feed rate on the geometrical errors was weaker than the first two factors, as shown in Figure 4c.

According to the above analysis, the three targets (center distance range: 40–60 mm), such as the average error, the maximum error, and the percentage of the maximum error, were further calculated under different process parameters, as shown in

Table 3. The result of each experiment.

Number	Average Error (mm)	Maximum Error (mm)	Maximum Error Percentage (%)
1	0.567	0.77	3.85
2	0.737	1.16	5.8
3	1.323	1.7	8.5
4	0.603	1.03	5.15
5	0.707	0.9	4.5
6	0.683	0.93	4.65
7	0.733	0.93	9.3

. The maximum axial geometrical error of each experiment could be controlled within 10%, in which the largest average error was 1.323 mm, which was obtained in experiment number 3. The maximum errors of experiment numbers 1–6 were both obtained at a center distance of 50 mm, and the maximum error of experiment number 7 was obtained at a center distance of 60 mm. In addition to this, experiment number 7 had the highest error percentage of 9.3%. The next highest was experiment number 3, and smallest maximum error percentage was obtained in experiment number 1. In summary, the larger step down and the larger tool diameter both significantly increased geometrical errors. Therefore, the oversized large tool diameter and the oversized step down should be avoided in belt-heated incremental sheet forming of AA2024 aluminum alloy.

3.2. Analysis of Dislocation Density

Since the dislocation movement in materials can enhance the macroscopic material flow, improvement of the geometric accuracy can be achieved. Therefore, the dislocation density of the material can intuitively reflect the internal dislocation movement, namely, the dislocation movement on a specific plane will result in an increase in the dislocation density of the corresponding crystal plane. To analyze the distribution of dislocation density within the material. The dislocation density of each experiment was further analyzed to obtain the effective law of process parameters on the dislocation density, as shown in Figure 5.

Figure 5a shows the effect of different tool diameters on the dislocation density, and the dislocation density increases with the increase in tool diameter. Meanwhile, some new planes with dislocation movement were also obtained, and the main dislocation movement plane of the material was the (111) plane, which aligns with the dislocation slip characteristic of FCC crystals. Meanwhile, the maximum error percentage of each tool increased with the increase in tool diameter. According to the above analysis, the increase in dislocation density caused by the increase in tool diameter will reduce the forming accuracy of the parts. Therefore, a tool of 10 mm diameter should be adopted due to the fact that the tool of 8 mm diameter may have a lower stiffness. On this basis, the effect of different feed rates on the dislocation density was analyzed; a feed rate of 1000 mm/min can activate more dislocation movement planes, and the number of planes decreases with the increase in the feed rate. In addition to this, the (111) plane exhibited the highest dislocation density when the feed rate was 1500 mm/min, and a more pronounced dislocation movement was obtained at this rate, as shown in Figure 5b. Meanwhile, the effect of the feed rate on the forming accuracy was weaker, according to Table 3; then, a feed rate of 2000 mm/min was adopted considering the forming efficiency. Moreover, Figure 5c shows the effect of step down on dislocation density. The primary and secondary dislocation movement planes were consistent when the step down is 0.15 mm and 0.2 mm, and the dislocation density of each plane was roughly the same under the action of the two rates. Meanwhile, the number of dislocation movement planes significantly decreased when the step down was 0.25 mm, and the increase in dislocation density of each plane was not significant. Therefore, the material flow was limited, which can easily lead to the fracturing of the part, as shown in Figure 5d. In addition to this, the forming accuracy from 0.2 mm was less than that of 0.15 mm, so the step down of 0.2 mm should be adopted in the belt-heated incremental sheet forming process.

3.3. Analysis of Microscopic Morphology

The plastic deformability of materials can be estimated through the microscopic morphology of materials. Good plasticity can promote material flow, which can ensure successful deformation of the material, and then the forming accuracy of the part can be effectively enhanced. Therefore, the microscopic morphology of the part needs to be analyzed to evaluate the effect of different process parameters on plastic deformability. Figure 6 shows the microscopic morphology of the part under different process parameters. As the tool diameter increases, a tearing ridge appears on the test surface and the dimples gradually become shallower. The tearing ridge of the material was significant when the tool diameter was 12 mm, as shown in Figure 6a. Figure 6b shows the effect of feed rates on the microscopic morphology; the ripple surface characteristic was significant under the action of the random feed rate and the dimples were both small and deep. Therefore, the feed rate does not significantly affect plastic deformability, and forming accuracy is not influenced by the change in feed rate. In addition to this, the microscopic morphology of the part changed from the ripple surface to the tearing ridge when the step down increased from 0.15 mm to 0.25 mm, and the dimples gradually became shallower, as shown in Figure 6c. The tearing ridge was especially significant at the step down of 0.25 mm. Meanwhile, a mixed fracture of brittleness and ductility was obtained, which lead to the premature fracture of the material; thus, the part was not successfully formed. Therefore, the step down of 0.25 mm should not be used in the belt-heated incremental sheet forming of AA2024 aluminum alloy.

4. Conclusions

The primary effective parameters of the geometrical accuracy are the tool diameter and the step down in the belt-heated incremental sheet forming of AA2024 aluminum alloy when the forming temperature is constant. Meanwhile, the effect of the feed rate on the geometrical accuracy is slight and can be ignored. In addition to this, the tool diameter and the step down primarily influence the geometrical dimensions at center distances of 50 mm and 60 mm. The tool diameter affects the geometrical dimensions at the opening and bottom regions. Moreover, the step down mainly impacts the opening geometrical dimensions. From a microscopic mechanism perspective, some new dislocation movement planes are activated with the increase in the tool diameter, in which the dislocation movement is promoted. However, the tearing ridge, which influences the forming ability and the forming accuracy, is significant when the tool diameter is 12 mm. Furthermore, the number of dislocation movement planes decreases at a step down of 0.25 mm, and the tearing ridge morphological feature is extremely significant, which causes premature fracturing of the material. Based on the above analysis, considering the overall processing efficiency, the optimal process parameters, namely 10 mm tool diameter, 2000 mm/min feed rate, and 0.2 mm step down, necessary to achieve high-accuracy belt-heated incremental sheet forming of AA2024 aluminum alloy were obtained. In future research, the quantitative relationship between geometric accuracy and microscopic features needs to be further investigated in detail. Meanwhile, these finding can be further extended and applied to the forming of other grades of aluminum or magnesium alloys; the forming accuracy of light alloys can also be improved in rapid manufacturing fields, such as aerospace, automotive industry, rail transit, etc.