Analysis of the effects of different metal sheets (e.g., aluminum, stainless steel, galvanized sheet) on edge folding and rolling performance

The folding hemming processes is core process of automobile manufacture, household appliance production and precision machinery processing, which directly influences the structural strength, sealing performance and surface quality of the product. Because of differences in crystal structure, mechanical properties and surface characteristics of different metallic materials, the process characteristics are obvious in the process of folding and fringing. Taking aluminum alloy, stainless steel and galvanized steel sheet as examples, the influence of their material properties on the folding and winding process is systematically analyzed, and the optimization strategies is put forward according to the engineering example.
1.Mechanism of Material Characteristics Influencing Fringe Process
1.1 Edge Folding Characteristics of Aluminum Alloy
Aluminum alloys (such as the 6016 series) have unique edge in hem due to low yield strength (approximately 140–180 MPa) and high elongation (≥25%). By means of finite element analysis, the material flow in the deformation zone is uniform the tangential tensile stress distribution distribution is more uniform than that carbon steel steel in the process of opening and turning the edges of 6016 aluminum, effectively reducing the risk of edge cracking. For example, in the folding process of a motor flue gas engine, 6016 aluminum alloy can have a limit turning factor of 0.68, 9.7% higher than the DC04 steel sheet limit turning factor (0.62), allowing for greater turning height and more complex geometry.
However, the high strain hardening index (n value) of aluminum alloy (0.2-0.3) results in greater rebound after edge folding than steel. Measurement data from the front cover of the electric car showed the aluminium edge fold had a the springback angle of 3.2 degrees, 77.8% higher than the same thickness of steel sheets (1.8°). In order to control the rebound, the following measures must be taken:
Increase the flanging fillet radius (recommended r ≥ 0.5t, t is sheet thickness).
Optimized die compensation coefficient (K = 1.05–1.10).
Implement secondary calibration.
1.2 Stainless Steel Side Fold Challenge
Austenitic stainless steel (e.g 304) faces two major challenges in folding due to yield strength ≥ 205 MPa and relatively low elongation ≥≥40%:
Edge Cracking: the high strength leads to a concentration of concentrated tangential tensile stress in the deformation zone, and the edge of the hole is prone to microcracks when the turning coefficient is less than 0.58. A case study from a kitchen equipment company shows that 304 stainless steel had a crack crack rate 12 12% when it had an 8mm the flanging height, while 6016 aluminum had a crack rate of only 2% under the same conditions.
Work Hardening: When the n-value is 0.3 -0.5, the hardness of the material increases by 30%–50% behind edge folding, greatly increasing mold wear in die forging.
To solve the problem of stainless steel hem, engineering practices usually include:
the pre-punched hole diameter increased by 5%–8% to compensate for the rebound.
Liquid nitrogen was used to reduce the flow stress of the material.
The friction coefficient was reduced by nano lubricant (μ≤ 0.08).
1.3 Process characteristics of Galvanized Steel Sheet.
The edge folding properties of galvanized steel sheet (e.g. DC04+ZE) are strongly influenced by the coating:
Galvanized sheet: Galvanized sheet is 5 – 10 μm thick, with strong adhesion to substrates. In the process of edge folding, the zinc coating deforms in sync with substrates and is not easy to fall off. However, the hardness of the zinc coating (HV 180-220) is higher than that of the substrate (HV 140-160), resulting in a concentration of stress at sharp corners when the edges fold.
Hot-Dip Galvanized Sheet: With a coating thickness of 20–40 μm and relatively poor plasticity, the zinc coating is prone to network cracking when the flanging height exceeds 6 mm. Tests by one home appliance company shows that when the rims were turned up to 8mm high, the thermal galvanized layer was only 65 only 65% rate, while the electrogalvanized sheet was 92 percent complete.
Optimization solutions include:
Control the hem velocity (≤50 mm/s) to reduce coating flaking.
The step folding process is adopted (forming in two steps).
Increase the stripping angle (1°–2°) to reduce friction.
2. Reaction of the material during ripping
2.1 Hemming Pressure and Material Deformation
Hemming pressure is an important index of material formability. Based on Dynaform simulation data:
6016 aluminum alloy pre-roll pressure averaged 502 N and the final hemming pressure is 1,327 N.
The preroll pressure of DC04 steel sheet averaged 860N and the final hemming pressure is 1,852 N.
aluminum alloy requires 40%–42% lower bending pressure than steel, mainly due to its low elastic modulus of elasticity (70GPa vs 70GPa). 210 GPa) and a high plastic-strain ratio (r value 1.2: 0.8).
2.2 Wave Effect Control
The yield strength of the material directly affects the surface quality after the curling. 6016 aluminum alloy has a yield strength of 140 MPa and a wave height of 0.15 mm after curling, which is 53% lower than the wave-height (0.32 mm) of DC04 steel sheet under the same curling force. This makes it ideal for automotive outer panel hemming paneling. The surface roughness of aluminum alloy hemming parts can reach 0.8 μm, which meets the requirements of A-Class surface of high-end models.
2.3 Indentation Management
In the curling (indentation) process, the amount of material flowing into the flange must be strictly controlled. 6016 aluminum alloy indentation is 15%–20% larger than steel plate indentation. If process parameters are not controlled properly, they may lead to:
Incomplete hemming (clearance > 0.1 mm).
Edge stress concentration (leading to fatigue cracks).
A car company controls indentations to within 0.3 mm by:
segmented pressure control (initial pressure reduced by 30%) is used for precurling.
Increases the length of stay (from 2 to 4 ss) during final hemming.
Optimize die clearance (1.1t vs.
3. Engineering Practice of Material Selection and Process Optimization
3.1 Case study: Automotive Body Panels
The new car front cover exterior plate uses 6016 aluminum material to replace the traditional steel material, realizes quality improvements through the following process innovations:
Material Pretreatment: T4 heat treatment (solution treatment + natural aging) resulted in a yield control of 160 MPa and an increase in elongation to 28%.
Die Design: Reduction of friction and extension of die life from 50,000 to 200,000 weeks with DLC coating (hardness HV2500).
Process Monitoring: Install pressure sensors (accuracy ±1 N), adjust the curling force in real time and control wave height in ±0.05 mm range.
3.2 Case study: stainless Stainless Steel Inner Liner of Home Appliances
The high-end refrigerator lining, made of 304 stainless steel, can solve the problem of refrigerator edge cracking by:
Lubrication Upgrade: The friction coefficient is reduced from 0.2 to 0.06 using graphene-containing nano-lube.
Process improvement: using ``pre-stamping → cryogenic folding → annealing treatment"three-step process to increase the edge height from 6 mm to 10 mm.
Die optimization: increase the flanging radius punch fillet from 0.3t to 0.5t and decrease the crack rate from 8% to 0.5%.
3.3 Case study: Galvanized Steel Sheet for Building Structures
In the steel structure engineering of hot-dip galvanized sheet to make roof tiles, the zinc coating peeling problem in the process of wall folding is solved by the following measures:
Coating control: Reduce coating thickness from 30 microns to 20 microns to balance corrosion resistance and shaping.
Process Parameters: Reduced hem speed from 80mm/s to 40mm/s and increased dwell time from 1s to 3 s.
Post-treatment: Increased injection pellets (Almen intensity 0.15A) to remove residual stress from edge folding.
4. Future Development Trends and challenges
The increasing demand for lightweight aluminum alloys (such as the 7075 series) and advanced high-strength steel (such as DP980) has led to increasing applications, posing new challenges to hem and curling processes:
High-strength aluminium alloys: yield strengths in excess 500 MPa requires the development of thermal molding processes (150–250°C) to reduce deformation resistance.
Third-Generation High-Strength Steels: only 10%–15%, requires to be hydraulic forming in combination with local heating techniques.
Composites: The interfacial bonding issues between dissimilar materials needs to be solved in the side fold of steel-aluminum composite plate.
Conclusion:
Different metal sheets vary greatly in the process of folding and edging: aluminum alloy is the preferred material for external plates due to low yield strength and high elongation, but requires strict control of bounce and indentation; stainless steel requires lubrication upgrades and process innovations to address cracking; galvanized steel sheet requires balance of coating thickness and formability. In the future, with the development of materials science and forming technology, the folding and curling process of multi-material hybrid automobile body will become a hot topic, which requires collaborative innovation in material design, mold optimization and process control.