Search Results (112)

Predicting bending fractures in advanced high-strength steel (AHSS) is challenging due to complex microstructural behaviors and strain rate dependencies, particularly in industrial forming processes. Current models and standards primarily focus on quasi-static tension or slow bending speeds, leaving a gap in understanding the accelerated failure of AHSS without necking. In this study, direct bending experiments were conducted on dual-phase, complex-phase, and martensitic AHSS grades under varying bending speeds and radii. Since the bending crack is irrelevant to the load drop, surface crack evolution was measured using three-dimensional surface profile analysis. The results showed that accelerated bending significantly delayed crack initiation across all tested materials, with small-radius bending showing reduced strain localization due to strain rate hardening. Larger-radius bending benefited primarily from increased fracture strain. Full article

Additive manufacturing techniques, such as laser-based powder bed fusion of metals (PBF-LB/M), have now gained high industrial and academic interest. Despite its design flexibility and the ability to fabricate intricate components, LPBF has not yet reached its full potential, partly due to the challenges associated with microstructure control. The precise manipulation of the microstructure in LPBF is a formidable yet highly rewarding endeavor, offering the capability to engineer components at a local level. This work introduces an innovative parallelized Cellular Automaton (CA) framework for modeling the evolution of the microstructure during the LPBF process. LPBF involves remelting and subsequent nucleation followed by crystal growth during solidification, which complicates and burdens microstructure simulations. In this research, a novel approach to nucleation seeding and crystal growth is implemented, focusing exclusively on the final stages of melting and solidification, enhancing the computational efficiency by 30%. This approach streamlines the simulation process, making it more efficient and effective. The developed model was employed to simulate the microstructure of an austenitic advanced high-strength steel (AHSS). The model was validated by comparing the simulation results qualitatively and quantitatively with the experimental data obtained under the same process parameters. The predicted microstructure closely aligned with the experimental findings. Simulations were also conducted at varying resolutions of CA cells, enabling a comprehensive study of their impact on microstructure evolution. Furthermore, the computational efficiency was critically evaluated. Full article

In recent decades, the automotive industry has faced challenges around improving energy efficiency, reducing pollutant emissions, increasing occupant safety, and reducing production costs. To solve these challenges, it is necessary to reduce the weight of vehicle bodies. In this way, the steel industry has developed more efficient metal alloys. To combine vehicle mass reduction with improved performance in deformations in cases of impact, a new family of advanced steels is present, AHSS (Advanced High-Strength Steels). However, this family of steels has lower formability and greater springback compared to conventional steels; if it is not properly controlled, it will directly affect the accuracy of the product and its quality. Different regions of a stamped component, such as the flange, the body wall, and the punch pole, are subjected to different states of stress and deformation, determined by numerous process variables, such as friction/lubrication and tool geometry, in addition to blank holder force and drawbead geometry, which induce the material to different deformation modes. Thus, it is understood that the degree of work hardening in each of these regions can be evaluated by grain morphology and material hardening, defining critical regions of embrittlement that, consequently, will affect the material’s stampability. This work aims to study the formability of the cold-formed DP600 steel sheets in the die radius region using a Modified Nakazima test, varying drawbead geometry, followed by a nanohardness evaluation and material characterization through the electron backscatter diffraction (EBSD). The main objective is to analyze the work hardening in the critical blank regions by applying these techniques. The nanoindentation evaluations were consistent in die radius and demonstrated the hardening influence, proving that the circular drawbead presented the most uniform hardness variation along the profile of the stamped blank and presented lower hardness values in relation to the other geometries, concluding that the drawbead attenuates this variation, contributing to better sheet formability, which corroborates the Forming Limit Curve results. Full article

In the stamping process of automotive parts, springback is a major problem when using Advanced High-Strength Steel (AHSS). This phenomenon significantly impacts the shape accuracy of products and is difficult to control. This study aims to optimize process parameters such as blank holder force (BHF), die clearance, and blank width to minimize springback in the workpiece. Using optimal process parameters will enhance the efficiency of die compensation processes. The study uses the Finite Element Method (FEM) simulation to predict forming behavior. The case study, Reinforcement-CTR PLR, is made from AHSS grade JSC980Y with a thickness of 1 mm. Four material model combinations were evaluated against actual experiment results to select the most accurate springback prediction model. A full factorial design was used for experiments with varied process parameters. The optimization process used regression and various Artificial Neural Networks (ANNs). From the result, a Deep Neural Network (DNN) with two hidden layers performed with the highest accuracy compared to the other models. The optimal process parameters were identified as 27.62 tons BHF, 1 mm die clearance, and a 290 mm blank width. These optimal results achieved 98.05% of the part area within a displacement tolerance of −1 to 1 mm, closely matching FEM-based validation. Full article

Advanced high-strength steels (AHSSs) are used as lightweight solutions for vehicles, mainly focusing on the Body-in-White. However, the implementation of such steels for chassis parts requires a profound knowledge of the key design parameters for these components, particularly those concerning fatigue performance. Manufacturing of chassis parts include mechanical cutting operations. Therefore, the deformation and damage induced at the cut edge may affect the fatigue resistance of the parts in service. To characterize and study this critical area, damage and micromechanical properties have been evaluated at the cut edge for three different AHSS grades, CP800, CP980, and DP600, analyzing the impact of cutting parameters and post-processing treatments, such as sandblasting. Large high-speed nanoindentation maps of 400 × 200 µm² have been carried out along the cut edge in the three different target zones: burnish, fracture, and burr. In the hardness maps, the deformation lines and the gradient of hardness with increasing distance from the cut edge are perfectly observed. Residual stresses at the target zones of the cut edges were measured using the FIB-DIC method for CP980 to complement the micromechanical study in these critical areas. The results found show that reduced cutting clearance leads to larger hardened zones and favorable compressive stress distributions, correlating with improved fatigue resistance. Hardened zones extending up to 100 µm from the cut edge and compressive residual stresses exceeding −300 MPa were observed at low clearance. These findings are consistent with numerical simulations and previous fatigue tests, highlighting the potential of combining high-speed nanoindentation and local stress analysis for optimizing shear cutting processes in AHSS components. Full article

As a third-generation advanced high-strength steel (AHSS), δ-TRIP steel exhibits the characteristics of high strength, high plasticity, and low density. However, the addition of Al to steel will affect solidification and segregation, which may impact the final microstructure and mechanical properties of the product. In this study, thermodynamic calculations and microsegregation model analysis were employed to investigate the effects of Al addition on the solidification path, peritectic reaction range, equilibrium partition coefficients, and microsegregation behavior of δ-TRIP steel. The results show that with an increase in the Al content, the carbon content range in which δ ferrite is retained without complete transformation during the solid-state phase transition becomes broader. Simultaneously, the carbon concentration range of the peritectic zone expands. The segregation of the C, Si, Mn, P, and S elements increases with increasing Al content, whereas the segregation of Al decreases as the Al content increases. Under non-equilibrium solidification conditions, unlike equilibrium solidification, the temperature difference between the solid and liquid phases initially increases, then decreases, and subsequently levels off with further Al addition. This study provides information for the composition design and production process optimization of δ-TRIP steel, and the research results can provide a reference for similar high-aluminum, low-density steels. Full article

This study analyzes a three-stage roll flattening process to improve the flatness of 1.5 GPa grade AHSS sheets. Unlike conventional leveler rolls, which mainly relieve residual stress through longitudinal tension-compression, the second roll has a sloped pattern to induce transverse deformation and redistribute local residual stresses. A twisted sheet was processed under different roll gap settings (1.3 mm, 1.1 mm, 0.9 mm, and 0.7 mm), and experimental measurements were compared with Abaqus Explicit simulations. At a 1.1 mm gap, the RMSE between experiment and simulation is 0.22 mm, showing the highest agreement. Both twist and crossbow defects are reduced by over 80%, achieving optimal flattening. At 1.3 mm, the simulation overestimates the second roll’s effect, causing excessive localized deformation. Reducing the gap to 0.9 mm or 0.7 mm increases discrepancies due to roll fixation differences. Experiments allow more central bending, amplifying crossbow, while simulations assume rigid rolls, underestimating curvature. Adjusting the second roll’s geometry to enhance transverse tension-compression and setting the gap to 1.1 mm effectively reduces defects. This method improves flatness while minimizing the number of rolls needed in high-strength steel sheet production. Full article

Liquid metal embrittlement (LME) in Zn-coated advanced high-strength steels (AHSSs) is an increasing concern, particularly in automotive assembly, where it can cause early failure and reduce ductility during resistance spot welding (RSW). This study explores the impact of adding 0.2 wt% Mo on the LME susceptibility of 0.2C-2Mn-1.5Si AHSS through hot tensile testing, RSW, and advanced microstructural analyses, including atom probe tomography (APT) and transmission electron microscopy (TEM). The results suggest that Mo enhances resistance to LME, as evidenced by the increased tensile stroke from 2 mm in the case of the 0 Mo alloy and to 2.75 mm in the case of the 0.2 Mo sample. Also, the average crack length in the shoulder of the welded samples decreased from 109 ± 7 μm to 28 ± 3 μm by adding 0.2 wt% Mo to the base alloy. APT analysis revealed that, in the presence of Mo, there is increased boron (B) segregation at austenite grain boundaries, improving cohesion, while TEM suggested more diffusion of Zn into the substrate, facilitating the formation of Zn-ferrite. These findings highlight Mo’s potential to reduce LME susceptibility of AHSS for automotive applications. Full article

The automotive industry is currently in a paradigm shift transferring the fleet over from internal combustion vehicles to battery electric vehicles (BEV). This introduces new challenges when designing the body-in-white (BIW) due to the sensitive and energy-dense battery that needs to be protected in a crash scenario. Press-hardening steels (PHS) have emerged as an excellent choice when designing crash safety parts due to their ability to be manufactured to complex parts with ultra-high strength. It is, however, crucial to evaluate the crash performance of the selected materials before producing parts. Component testing is cumbersome and expensive, often geometry dependent, and it is difficult to separate the bulk material behaviour from other influences such as spot welds. Fracture toughness measured using the essential work of fracture method is a material property which has shown to be able to rationalise crash resistance of advanced high-strength steel (AHSS) grades and is thereby an interesting parameter in classifying steel grades for automotive applications. However, most of the published studies have been performed at quasi-static loading rates, which are vastly different from the strain rates involved in a crash. These higher strain rates may also lead to adiabatic self-heating which might influence the fracture toughness of the material. In this work, two PHS grades, high strength and very high strength, intended for automotive applications were investigated at lower and higher strain rates to determine the rate-dependence on the conventional tensile properties as well as the fracture toughness. Both PHS grades showed a small increase in conventional mechanical properties with increasing strain rate, while only the high-strength PHS grade showed a significant increase in fracture toughness with increasing loading rate. The adiabatic heating in the fracture process zone was estimated with a high-speed thermal camera showing a significant temperature increase up to 300 °C. Full article

TBF 1180 steel was plastically deformed under different strain paths in order to study both the ductility and RA transformation rates. Specimens were prepared from a 1 mm thick sheet and then tested incrementally under uniaxial tension, plane-strain tension, and biaxial tension. The retained austenite (RA) levels were measured, as a function of the plastic strain, using electron backscatter diffraction (EBSD). The plane-strain tension specimens had the fastest rate of RA transformation as a function of strain, followed by uniaxial tension, and then biaxial tension. The forming limits were measured for each strain path, yielding major limit strains of 0.12 under uniaxial tension, 0.09 under plane-strain tension, and 0.16 under biaxial tension. These results were compared to prior work on a 1.2 mm Q&P 1180 steel sheet, which had a similar yield and ultimate tensile strength, but exhibited slightly greater forming limits than the TBF material. The visual inspection of the micrographs appeared to show an equiaxed RA morphology in the Q&P 1180 steel and a mixture of equiaxed and lamellar RA grains in the TBF 1180 steel. However, the statistics generated by EBSD revealed that both alloys had RA grains with essentially the same aspect ratios. The average RA grain size in the Q&P alloy was found to be about three times larger than that of the TBF alloy. As such, the small but consistent formability advantage exhibited by the Q&P 1180 alloy along all three strain paths can be attributed to its larger average RA grain size, where larger RA grain sizes correlated with a more gradual transformation rate. Full article

Auxetic honeycomb structures, known for their exceptional mechanical properties, are widely used as sacrificial layers to protect critical targets from extreme explosive loads. However, conventional double arrowhead auxetic honeycomb-core structures (DA-AHSs) encounter significant interfacial connectivity challenges, and scaling auxetic honeycombs with alternative cellular microstructures introduces further complexity. To overcome these issues, riveted and assembled double-trapezoidal auxetic honeycomb-core structures (DT-AHSs) were developed as a replacement for DA-AHSs. The deformation modes and energy absorption mechanisms of DT-AHSs were analyzed through theoretical methods and quasi-static testing. The results show that DT-AHSs energy absorption primarily relies on the yield deformation of the longer inclined walls and rotational deformation of the shorter inclined walls. Additionally, the shorter walls support auxetic behavior by stabilizing the deformation of the longer walls. These findings provide a basis for further exploration of the protective potential of DT-AHSs. Full article

Sheet metal forming is one of the key processes in the manufacturing of parts for several industries, such as automotive, aerospace and packaging. However, it is often constrained by the onset of plastic instability, which limits uniform deformation. To address this challenge, considerable attention has been given to methods that enhance strength, formability, and energy absorption during forming processes. One such method is the continuous-bending-under-tension (CBT) deformation mechanism, which has shown potential in mitigating localized instability during plastic deformation. This study presents a numerical investigation of the CBT process using Abaqus 2017 to evaluate the forces generated in both the CBT equipment and material when processing high-strength materials. The reference material used is the USS CR980XG3™ AHSS, a third-generation, high-strength, high-elongation steel grade with retained austenite (980T/600Y). The study systematically analyzes the effects of key parameters, such as roll diameter, distance between rolls, depth setting, specimen thickness, and the number of deformation cycles, on the evolution of forces during the CBT process. The results demonstrate that both the forces applied by the rolls and those experienced by the specimen are significantly influenced by the distance between rolls, depth setting, and specimen thickness. In contrast, the roll diameters have minimal influence. These findings contribute to the optimization of the CBT process and provide valuable insights for future studies aimed at enhancing the performance and formability of various materials. Full article

The automotive industry employs structural steels with E-coats to reduce weight and increase the corrosion resistance of chassis, reducing CO₂ emissions. Due to their mechanical properties, part of the chassis is a composite of advanced high-strength steels (AHSS). AHSSs are coated by conversion methods such as phosphate to increase epoxy coating adherence and corrosion resistance. The main point of this research is to characterize an AHSS complex-phase (CP) 780 in blank, with a phosphate coating and an E-coat organic coating using electrochemical noise, employing time-domain, frequency-domain, time–frequency-domain, and chaotic system methods to determine the type and corrosion kinetics of each system. The electrochemical noise technique was made with a conventional three-electrode cell, using a saturated calomel as a reference electrode. Data were recorded at 1024 s, at 1 data per second in a 3.5 wt. % NaCl electrolyte, according to ASTM G199-09. The results show how AHSS CP 780 presented uniform corrosion, similarly to the phosphate sample; however, the E-coat presented a trend of a localized process when analyzed by Wavelets transform. On the other hand, corrosion resistance increased for the E-coat sample, with values of 2.58 × 10⁶ Ω·cm². According to the results of the research, all the samples are susceptible to present localized corrosion. Full article

In this study, the carbon footprint of a steel-based windshield reinforcement component assembled in a sport utility vehicle was calculated in three different stages: steelmaking, stamping, and middle-of-use. Possible solutions to decrease carbon emissions were evidenced, such as the purchasing of steel made through low-impact technologies and the exploitation of the green energy grid to power up stamping machines. The life cycle assessment methodology was used to calculate the carbon footprint. Four different steels provided by different suppliers were compared in order to highlight the greenest material for both the steelmaking and stamping processes and the best supplier from an environmental point of view. In addition, the carbon footprint related to the component weight in vehicles with different traction set-ups, i.e., internal combustion engine, mild hybrid electric, and battery electric, was reported. To reduce the carbon footprint, electric arc furnace-based steelmaking and cold stamping were the best options. In addition, component weight reduction (for instance, changing materials) allowed a decrease in fuel and/or energy consumption, with carbon footprint benefits. Full article

Automotive structural components from Advanced High-Strength Steels (AHSS) can be manufactured with Flexible Roll Forming (FRF). The application of FRF in the automotive industry is limited due to flange wrinkling defects that increase with material strength. The new Incremental Shape Rolling process (ISR) has been shown to reduce wrinkling severity compared to FRF and therefore presents a promising alternative for the manufacture of high-strength automotive components. The current work analyzes for the first time the mechanisms that lead to wrinkling reduction in ISR based on the critical stress conditions that develop in the flange. For this, finite element process models are validated with experimental forming trials and used to investigate the material deformation and the forming stresses that occur in FRF and ISR when forming a variable-width automotive component. The results show that in ISR, the undeformed flange height decreases with increasing forming; this increases the critical buckling and wrinkling stresses with each forming pass and prevents the development of wrinkles towards the end of the forming process. In contrast, in FRF, the critical buckling or wrinkling stress is constant, while the longitudinal compressive stress in the flange increases with the number of forming passes and exceeds the critical stress. This leads to the development of severe wrinkles in the flange. Full article