Lightweight Stamping Driven by New Energy Vehicles: Deep Analysis of Ultra-high Strength Steel and Aluminum Alloy Forming Technology
Introduction: Game Balance between Lightweight and Safety
The endurance anxiety and collision safety regulations of new energy vehicles have jointly pushed the lightweight of the body to an unprecedented height. For every 100kg weight loss, the cruising range of pure electric vehicles can be increased by about 8-10km. At the same time, the global NCAP and China C-NCAP continue to increase the requirements for occupant protection. This requires materials with both ultra-high strength and excellent formability. Advanced high-strength steel (AHSS) and aluminum alloy have become the two main materials, while hot-stamped boron steel has broken through the most difficult area of the contradiction between strength and formability.
However, these materials expose their own unique technical problems during the stamping process: high springback of AHSS and die wear, low elongation of aluminum alloy and surface scratch sensitivity, narrow hot stamping process window and complex die cooling design. This paper conducts a comprehensive technical analysis of the stamping lightweight technology of new energy vehicles from four dimensions: material characteristics - process parameters - die design - defect control.
Cold stamping technology for advanced high-strength steel (AHSS)
1.1 From DP steel to CP steel and Q & P steel
Duplex steel (DP, composed of ferrite + martensite) is currently the most used AHSS, with typical grades DP590, DP780, DP980. It is characterized by continuous yield, high processing hardening rate, but limited flanging performance. Duplex steel (CP) adds bainite and dispersed precipitates on the basis of martensite, and has a higher porosity, which is suitable for chassis structural parts. The latest generation of quenched-partition steel (Q & P) obtains stable residual austenite through carbon partitioning process. The strength and elongation are improved simultaneously. The elongation of Q & P steel of DP1180 grade can reach more than 12%.
1.2 Core pain points and countermeasures of cold stamping process
(1) Accurate compensation for rebound
The yield strength of AHSS is high, and the modulus of elasticity is basically unchanged, resulting in a large proportion of elastic recovery after unloading. The rebound of complex three-dimensional curved parts (such as A-pillar reinforcement plates) can reach 3-5 of the design angle. Traditional molds are compensated by repeated mold trials and manual grinding. The current mainstream method is based on CAE reverse iterative compensation - the reverse offset of the simulated rebound mesh to obtain a new mold profile. Usually 2-3 rounds of iterations can control the rebound error within ±0.2.
For the more extreme springback dispersion problem (the difference in springback between different coils of the same batch of materials exceeds ±1), a closed-loop adjustment system of the mold needs to be introduced: a controllable gasket or an electrostrictive ejector bar is arranged at the key position of the mold, and dynamic compensation is achieved by scanning the springback angle online and adjusting the local load in milliseconds.
(2) Intelligent control of high drawability and blank holder force
DP980 is prone to longitudinal cracking when the drawing depth is large. Optimized strategies include: using the pulsation curve of servo stamping, "pause-reload" multiple times during the stamping stroke to improve material flow; or using segmented variable blank holder force, applying increased blank holder force to inhibit wrinkling in the early stage of forming, reducing the blank holder force in the middle stage to promote material inflow, and increasing the blank holder force in the later stage to shape.
(3) Mold wear and nano-coating
The high hardness of AHSS results in severe wear on the flanges and corners of the die. The AlCrN/TiSiN coating mentioned earlier has become the standard choice. In addition, cemented carbide inserts or dispersion-strengthened copper alloys are used as heat-conducting and wear-resistant composites in the stretched rounded corners.
Second, the precision stamping process of aluminum alloy plates
2.1 6-series aluminum alloy (Al-Mg-Si) and 5-series aluminum alloy (Al-Mg)
6000 series aluminum alloys (such as AA6016 and AA6022) can be strengthened by heat treatment, and the strength can be further improved after coating and baking. They are the first choice for outer covers (engine covers, doors). However, their room temperature formability is poor, the elongation is generally only 20% to 25%, and they are prone to age hardening. 5000 series (such as AA5182) have better formability, but the surface is prone to Lüdes bands, which are mainly used for inner panels.
2.2 Core challenges and solutions for aluminum sheet stamping
(1) The risk of cracking due to low elongation
The safe forming range of aluminum plate is much narrower than that of steel plate. Solution: ① Use hydraulic forming or pneumatic assisted forming to make the plate stick to the mold under liquid pressure to avoid local excessive thinning caused by rigid punch; ② Use the forming limit diagram (FLD) in the mold design stage to strictly restrict the primary and secondary strains and not allow to exceed the thinning limit; ③ Develop local heating assistance - heat the aluminum plate to 200-250 ° C through induction coils in the complex flanging area to temporarily increase the elongation.
(2) Surface scratches and aluminum powder accumulation
The oxide film on the surface of the aluminum plate is easily scratched by the mold, and the aluminum powder generated by wear will adhere to the surface of the mold, further worsening the scratches. A mirror polishing mold (roughness Ra≤0.05μm) must be used, with special low viscosity stamping oil (including extreme pressure additives), and regular automatic cleaning of the mold surface. In addition, the hard DLC coating has been shown to be effective in anti-stick aluminum.
(3) Rebound characteristics
Although the springback of aluminum plate is smaller than that of AHSS, its anisotropy is obvious, and it is easy to produce twisted springback. It is necessary to use a more refined material model (such as the Barlat YLD2000 yield criterion) for simulation, and at the same time use the bottom dead center pressure holding function of servo stamping to extend the pressure holding time to 2-3 seconds to release the elastic internal stress.
Third, hot stamping technology: an all-in-one solution for ultra-high strength
3.1 boron steel (22MnB5) hot stamping principle
The core logic of hot stamping is to heat a boron steel plate with a tensile strength of about 600MPa to 930 ° C for austenitization, and then transfer it to a mold with a cooling pipe in a few seconds. Fast stamping and pressure-holding quenching, martensitic phase transformation occurs, and finally parts with a tensile strength of more than 1500MPa and a hardness of 450~ 520HV are obtained. This process eliminates springback (hardening fixed shapes after high temperature forming) and can shape complex geometries.
3.2 Process window and mold cooling design
The key to the success or failure of hot stamping lies in the cooling rate: it must be greater than the critical cooling rate of martensite (about 27 ° C/s). Therefore, a high-density cooling water channel 5-10mm away from the die surface must be designed inside the die, and the temperature of the die surface must be uniform through heat flow coupling simulation. In addition, the edge of the part may have cooled to below Ar3 before the die is closed, forming ferrite and reducing strength - the transfer time from the heating furnace to the press needs to be optimized (usually ≤10 seconds).
3.3 Integrated door ring and welding plate hot stamping
The latest technological development is to connect multiple parts such as A-pillars, B-pillars, thresholds, etc. through laser tailor-welded plates, and then hot-stamped as a whole into an integrated door ring. This can reduce weight by about 15% and reduce the welding joint and assembly process. The difficulty lies in the consistent control of the temperature field in different plate thickness or coating (aluminum-silicon coating) areas, as well as the risk of cracking of the weld during the hot stamping process.
3.4 Hot stamping + cold stamping mixing process
Some automakers have started to adopt the concept of local heating cold stamping: induction heating is only used to heat areas that require high strength and are difficult to cold form, and the rest of the areas are kept at room temperature. Hot stamping and cold forming are completed on the same servo press. The technology is still in the laboratory verification stage, but it is regarded as the next generation of lightweight processes.
IV. Hydraulic forming and internal high pressure forming technology
For hollow structural parts such as chassis sub-frame and torque beam, internal high pressure forming of pipes is a lightweight and efficient means. The pipe is placed into a closed mold, axial force is applied at both ends, and high pressure liquid (up to 400MPa) is filled inside to make the pipe stick to the mold cavity. Compared with stamping and welding parts, it can reduce weight by 20% to 30% and improve stiffness. With the complexity of new energy vehicle battery pack frames, the application of internal high pressure forming of aluminum alloy extruded profiles is rapidly expanding.
V. 2026 young quantitative stamping materials application prospects
Multi-material hybrid body: steel (AHSS thermoformed parts) + aluminum (cladding parts) + magnesium (instrument panel beams) + carbon fiber (local reinforcement).
Short-process hot stamping production line: from heating-stamping-quenching-laser cutting integration, the tempo is increased to 4 to 5 pieces per minute.
Uncoated hot stamped steel: Develop new oxidation-resistant surface treatments to replace expensive aluminum-silicon coatings that pose a risk of hydrogen embrittlement.
Aluminum-steel dissimilar material connection: stamping simultaneously completes FDS (hot melt self-tapping) or self-piercing riveting to reduce post-processing.
Conclusion
Lightweight stamping for new energy vehicles is a comprehensive competition of materials, processes and equipment. Cold stamping by AHSS must solve the "precise control" of springback and wear; aluminum alloy must overcome the "fine care" of forming limit and surface quality; hot stamping requires "lean control" of heat-force-phase transition coupling. In the next five years, with the competitive integration of integrated die casting and hot stamping, the stamping process will still maintain an irreplaceable position in the field of safety parts with extremely high strength requirements, and the new stamping workshop with data and closed-loop control as the core will become the core competitiveness of the whole vehicle factory.
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