This study focuses on press-formed parts made from medium-to-thick EN AW 6082-T651 aluminum alloy plates. Addressing the issue of cracking during the press-forming process, the study analyzes the root causes from multiple perspectives-including chemical composition, directional differences in bending properties, and process parameters-and identifies the fundamental mechanism of crack formation through experimental verification. Based on these findings, a systematic process optimization scheme is proposed, covering mold surface roughness control, optimization of forming speed, material preheating, mold lubrication, and post-forming flaw detection. Practical application demonstrates that the optimized process significantly reduces the cracking rate and ensures forming quality meets technical requirements, providing a reliable technical reference for the press-forming of similar medium-to-thick aluminum alloy plates.
Introduction
Due to advantages such as low density and corrosion resistance, aluminum alloys have become the preferred material for lightweighting high-speed trains, metro trains, and dedicated coal transport wagons in my country, seeing widespread use in the country's railway transport sector [1]. EN AW 6082-T651 is a heat-treatable Al-Mg-Si series aluminum alloy; following solution treatment and artificial aging, it exhibits a combination of high strength, good weldability, and corrosion resistance, making it a core material in equipment manufacturing. This paper investigates the manufacturing process regarding a sudden cracking issue occurring on the outer side of the R35 mm arc at the end of a U-shaped panel during press forming.
Product Structure and Forming Issues
The U-shaped panel is manufactured from a 10 mm thick EN AW 6082-T651 plate using a press-forming die. Its main structure is a symmetrical U-shape with a forming radius 3.5 times the plate thickness and a forming angle of 108°. The finished dimensions are 2552 mm in length, 480 mm in width, and 240 mm in height, as shown in Figure 1. The component demands high standards for surface quality and internal microstructural integrity in the formed corner regions; visible cracks and hidden defects are prohibited during the forming process, and the part must meet requirements for subsequent machining and load-bearing performance. Image
Figure 1: Schematic of the U-shaped panel's profile and dimensions
During manufacturing, a fracture occurred on the outer side of the R35mm arc at the end section following the forming process. The fracture spanned two-thirds of the panel's width, rendering the panel scrap (see Figure 2).
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Figure 2: Fracture after forming
Root Cause Analysis and Verification
The U-shaped panel is produced using a process involving laser contour cutting followed by die forming. To leverage the material's directional properties and ensure the formed area remained free of defects such as cracks, the forming operation was performed perpendicular to the rolling direction. To pinpoint the cause of the fracture, a systematic verification was conducted, examining both material characteristics and the forming process.
3.1 Verification using scrap material from the central section
Product specifications required the bend to be perpendicular to the rolling direction, with a bend radius of 35 mm and an angle of 108°. Re-testing specifications required a bend parallel to the rolling direction, with a radius of 50 mm and an angle of 180°; bending performance perpendicular to the rolling direction is superior to that in other directions. To verify whether the bending direction of the failed component was correct, die-forming tests were conducted on scrap material from near the failure zone (specifically, the area with the end-section cutout) in both directions-perpendicular and parallel to the rolling direction. In the test perpendicular to the rolling direction, the outer surface fractured almost completely (see Figure 3); in the test parallel to the rolling direction, the material fractured completely, with the damage being more severe than in the perpendicular test (see Figure 4). Thus, it was confirmed that the cracking during forming was not caused by an incorrect choice of bending direction relative to the rolling direction.
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Figure 3: Verification perpendicular to rolling direction
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Figure 4: Verification parallel to rolling direction
3.2 Material composition confirmation
The batch of sheet metal was inspected; re-testing of chemical composition (see Table 1) and mechanical properties (see Table 2) confirmed compliance with EN485 standards. This ruled out the possibility that the failure was caused by non-compliant chemical composition or mechanical properties. Table 1 Chemical composition of the material (mass fraction) (%)
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Table 2 Mechanical properties of the material
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The bending performance can be quantitatively evaluated based on the sheet's elongation. The relationship between elongation, sheet thickness, and bending radius can be expressed by the following formula [2]:
δ = (t/2) / (t/2 + r) (1)
Where δ is the elongation (%); t is the sheet thickness (mm); and r is the bending radius (mm).
Based on the elongation and thickness values in Table 2, the calculated bending radius for the sheet is approximately 36.6 mm. This is close to the product requirement of 35 mm, indicating that the material's bending performance is satisfactory.
3.3 Inspection of the forming die
Inspection of the forming die revealed a relatively rough surface. However, as this die had previously produced other batches of sheets without issues, and a test pressing using a 12 mm sheet of the same material also showed no abnormalities, it was concluded that while the die's surface roughness increased surface damage and resistance during the forming process, it was not the primary factor.
3.4 Verification of bending performance
(1) Verification of specimen bending performance (details below):
1) Samples were taken from the 130 mm wide central section of the U-shaped panel (Area A, see Figure 5). Bending tests were conducted on two specimens oriented perpendicular to the rolling direction and two parallel to the rolling direction. The tests followed technical requirements for sheet metal bending re-inspection, using a forming radius of 50 mm and a bending angle of 180°.
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Figure 5 Schematic of test areas on the panel
2) Since the material's performance is weaker when bent parallel to the rolling direction compared to perpendicular to it, bending parallel to the rolling direction was performed first. Based on the sheet thickness and material specifications, a 50 mm radius (R50) punch was used at a speed of 20 mm/min to achieve a 180° bend. No cracks were observed (see Figure 6), demonstrating that the bending performance of the material in the central section of the U-shaped panel meets the requirements. Image
Figure 6: Bending behavior of Area A parallel to the rolling direction
3) To further verify the bending performance of the sheet and align with the product's bending radius requirements, a bending test was conducted using an R30 mm punch perpendicular to the rolling direction. Two sets of test speeds were selected for comparison: 20 mm/min (standard bending test speed) and 300 mm/min (production forming speed). The results showed no cracking in either set after a 180° bend, as illustrated in Figure 7.
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Figure 7: Bending behavior of Area A perpendicular to the rolling direction
Thus, it is evident that the U-shaped shroud-which exhibited cracking at the ends-demonstrates good bending performance in its central section (Area A), meeting both the material's bending performance specifications and the product's forming radius requirements.
(2) Forming verification of the defective component (end section). Details are as follows:
1) Based on the bending performance verification results for the central section of the U-shaped shroud, further testing was conducted to determine if there were significant differences in material properties in the end regions. Forming verification was performed on areas B, C, and D of the defective (cracked) component; the locations are shown in Figure 5. A comparison of the bending tests is shown in Figure 8, and the results are listed in Table 3.
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Figure 8: Comparison of bending results for areas B, C, and D
Table 3: Bending results
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2) Based on the results in Table 3, samples were prepared from the sections on either side between Area C and Area D for additional bending tests. Two samples were taken from the "cracked side" and two from the "non-cracked side."
Verification of bending performance according to raw material re-inspection standards (perpendicular to the rolling direction; R50 mm punch; speed: 20 mm/min): No cracking was observed at 180° for samples from either the cracked or non-cracked sides, as shown in Figure 9. This confirms that the material's bending performance meets the required specifications.
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a) Cracked side
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b) Non-cracked side
Figure 9: Bending behavior (perpendicular to rolling direction) of the section between areas C and D
Verification of simulated product forming (R30 mm punch; speed: 300 mm/min): The sample from the non-cracked side showed no cracking at 180°; the sample from the cracked side exhibited cracking at approximately 60°, as shown in Figure 10. This demonstrates the existence of non-uniform mechanical properties at the ends of the material.
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a) Non-cracking side
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b) Cracking side
Figure 10: Verification of the product's forming and bending behavior
Thus, it is evident that the primary cause of cracking during forming is the non-uniformity of mechanical properties across the sheet; performing room-temperature compression molding under conditions exceeding the re-test bending standards poses a risk of cracking.
Process Optimization and Verification
Based on the analysis above, optimization measures were formulated and verified across several dimensions, including die condition, forming speed, material pre-treatment, lubrication, and quality inspection.
4.1 Optimization of Die Surface Roughness
In the original process, the die surface roughness (Ra) was 1.6 μm; high surface friction made the material prone to surface stress concentration. Precision polishing was applied to the working surfaces of the molding die to reduce the roughness (Ra) to below 0.4 μm. This reduced friction between the material and the die and lowered surface stress concentration, resulting in more uniform material deformation and preventing cracks caused by localized stress overload.
4.2 Optimization of Forming Speed
The original forming speed was 300 mm/min, a rate that may have been incompatible with the material's plastic response limits. The speed was optimized to a range of 120–180 mm/min using a slow, constant-speed forming method. This allowed the material sufficient time to undergo plastic deformation and fully dissipate deformation stresses, thereby avoiding stress concentrations and cracking caused by excessive deformation rates.
4.3 Material Preheating Process
The EN AW-6082-T651 aluminum alloy exhibits relatively low plasticity at room temperature; preheating can enhance plasticity and reduce deformation resistance. Currently, the aging process parameters for producing 6082-T6 aluminum plates cover a broad range (aging temperatures of 155–190°C and holding times of 2.5–20 hours), with mechanical properties consistently meeting relevant standards (such as EN 485) [3-5]. Preheating the forming zone of the workpiece to approximately 100°C increases plasticity reserves, reduces deformation resistance and stress concentration during the molding process, and inhibits crack formation. 4.4 Optimization of Die Lubrication
The original process lacked lubrication measures or specialized films to reduce frictional resistance between the workpiece and the die. The optimized process employs a specialized lubricant for aluminum alloy die forming, applied evenly via spraying. This creates a uniform lubricating film at the contact interface, lowering the friction coefficient and reducing surface scratches and stress concentrations, while also preventing localized material degradation caused by frictional overheating.
4.5 Post-Forming Flaw Detection
To detect surface defects promptly, a post-forming flaw detection step was introduced. Liquid penetrant testing is used to conduct 100% inspection of the outer arc and transition zones of the formed part, screening for surface and near-surface cracks to ensure the absence of surface defects.
4.6 Verification of Optimization Results
Two pieces of scrap material exhibiting similar forming cracks were selected for verification. The process involved: grinding the edges of the formed area to remove burrs and flash; preheating the bending zone to approximately 100°C; applying lubricant to the female die to reduce friction; and finally, reducing the forming speed followed by penetrant testing. Verification results showed only minor surface traces (approximately 3 mm in length) in localized areas, which were eliminated by light grinding, demonstrating significant optimization effectiveness (see Figure 11).
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a) Specimen 1
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b) Specimen 2
Figure 11 Flaw detection of optimized specimens
Conclusion
This paper addresses the issue of cracking in U-shaped formed parts made from EN AW 6082-T651 aluminum alloy medium-thick plate. Through systematic analysis and verification, the root cause of the cracking was identified as a combination of factors: relatively low mechanical properties at the material ends (despite meeting standard requirements), a tight bending angle, and an incompatible forming process. Based on these findings, a comprehensive process optimization scheme was proposed, covering die surface roughness control, forming speed optimization, material preheating, die lubrication, and post-forming flaw detection. Practical verification confirmed that this approach effectively resolved the cracking issue, achieving forming quality that meets technical requirements.
