The final mechanical properties of hot-stamped steel are determined by the microstructures which are greatly influenced by the cooling process after hot stamping. This research studied the effect of the cooling path on the microstructures and hardness of 22MnB5 hot-stamped steel. The cooling path was divided into continuous and discontinuous (primary and secondary) processes. After cooling, the Vickers hardness along the thickness of the specimens was measured. The results indicate that, for a continuous cooling process, there was a critical cooling rate of 25 °C/s to obtain fully martensitic microstructure. For the discontinuous cooling process, the slower was the cooling rate, the higher was the degree of auto-tempering that occurred, and the greater was the amount of carbides that formed, regardless of the primary or secondary cooling rate. When the cooling rate was lower than the critical value, a higher primary cooling rate suppressed the auto-tempering of lath martensite and increased the quenched hardness. By contrast, the hardness was not sensitive to the cooling rate when it exceeded the critical value.

Effect of Cooling Path on Microstructures and Hardness of Hot-Stamped Steel

H13 die steel often fails as a result of physical and chemical effects such as wear, erosion and cyclic stress. Accordingly, the study evaluates Co-based gradient coating on an H13 steel featuring a stress-relieving effect. Scanning electron microscope and X-ray diffraction were used to analyze the microstructure and phase of the coatings. A microhardness tester and friction and wear tester were used to compare the hardness and wear resistance of the coatings and the substrate, and the wear morphology was observed. A pendulum impact test was used to compare the impact resistance of the coatings and the substrate, and the fracture morphology was observed. Finally, a corrosion test was used to compare the corrosion resistance of coatings and substrate. The results show that the Co-based gradient coatings have good combinations with the substrate, the hard phase content gradually increases from the bottom to the top of the coating, and the crystal microstructure generally maintains a distribution trend from coarse to fine. The hardness of the gradient coatings is significantly higher than the substrate, and from the coating surface to the substrate, the hardness decreases slowly. The wear loss of the coatings is much lower than that of the substrate, the main wear mechanism of the substrate is abrasive wear, and the main wear mechanism of the coatings is brittle spalling. While the gradient coatings increase the surface hardness, the brittleness also increases, the impact resistance of the coatings is lower than that of the substrate, the fracture form of the substrate is a ductile fracture, and the fracture form of the coating is a brittle fracture. The gradient coatings effectively improve the corrosion resistance of the substrate surface, and the higher the content of the reinforcing phase, the better the corrosion resistance of the coatings.

Preparation of WC Reinforced Co-Based Alloy Gradient Coatings on a H13 Mold Steel Substrate by Laser Cladding

Investigation of different lubricant classes for aluminium warm and hot forming based on a strip drawing test

Friction during contact between metals can be very complex in pulse current-assisted forming. Based on stamping process characteristics, a reciprocating friction tester was designed to study the friction characteristics between AA7075 aluminum alloy and P20 steel under different current densities. Origin software was used to process the experimental data, and a current friction coefficient model was established for the pulse current densities. The results show that the friction coefficient of the aluminum alloy sheet decreased with the increase in the pulse current density (2–10 A/mm2). After that, the friction mechanism was determined by observing microscopic morphology and SEM: some oxide cracked on the friction surface when the current was large. Finally, finite element simulations with Abaqus software and a cylindrical case validated the constant and current friction coefficient models. The thickness distribution patterns of the fixed friction coefficient and the current coefficient model were compared with an actual cylindrical drawing part. The results indicate that the new current friction model had a better fit than the fixed one. The simulation results are consistent with the actual verification results. The maximum thinning was at the corner of the stamping die, which improved the simulation accuracy by 7.31%. This indicates the effectiveness of the pulse current friction model.

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Study on Friction Characteristics of AA7075 Aluminum Alloy under Pulse Current-Assisted Hot Stamping

Hot stamping is an important manufacturing process for sheet metal parts. However, it is easy to produce defects such as thinning and cracking in the drawing area during the stamping process. In this paper, the finite element solver ABAQUS/Explicit was used to establish the numerical model of the magnesium alloy hot-stamping process. The stamping speed (2~10 mm/s), the blank-holder force (3~7 kN), and the friction coefficient (0.12~0.18) were selected as the influencing factors. Taking the maximum thinning rate obtained through simulation as the optimization objective, the response surface methodology (RSM) was applied to optimize the influencing factors in sheet hot stamping at a forming temperature of 200 °C. The results showed that the maximum thinning rate of sheet metal was most influenced by the blank-holder force, and the interaction between the stamping speed and the blank-holder force/friction coefficient had a great influence on the maximum thinning rate. The optimal value of the maximum thinning rate of the hot-stamped sheet was 7.37%. Through the experimental verification for the hot-stamping process scheme, the maximum relative error between the simulation and the experimental results was 8.72%. This proves the accuracy of the established finite element model and the response surface model. This research provides a feasible optimization scheme for the analysis of the hot-stamping process of magnesium alloys.

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Process Optimization of the Hot Stamping of AZ31 Magnesium Alloy Sheets Based on Response Surface Methodology

Aluminum alloy hot stamping technology can improve the formability of materials and obtain parts with high dimensional accuracy. Friction behavior in the hot stamping process is very important for forming quality. Accurate friction coefficient is helpful to improve the prediction accuracy of forming defects. It is hard to obtain the friction coefficient by simple experiments due to the complicated thermal–mechanical coupling and contact conditions during the hot stamping of aluminum alloys. In order to explore the effect of friction behavior on forming quality, hot stamping experiments of 7075 aluminum alloy U-shaped parts with different lubricants were carried out. The influence of different lubricants on the force–displacement curve, material inflow, surface appearance, and thickness distribution of the formed part was analyzed. The results showed that a good lubrication effect could be obtained with the molybdenum disulfide lubricant. The friction coefficient under different lubrication conditions was obtained by using the inverse problem optimization method. Compared with the experimental results, the determined friction coefficients could accurately predict the force–displacement curves and the thickness distributions of formed parts under different lubrication conditions.

Determination of Hot Stamping Friction Coefficient of 7075 Aluminum

Different from traditional hot stamping components with full martensite, the new tailored hot stamping (THS) components have different quenched microstructures, which results in their lower shape accuracy. To investigate the influence of different quenched phases on the springback of a component, a THS experiment of a U-shaped component was performed with segmented heating and a cooling tool. The area fractions of phases at different tool temperatures were obtained by a two-stage color tint etching procedure. Results showed that the quenched phase of the cold zone was almost full martensite. The area fraction of martensite in the hot zone was reduced to the lowest 13% at the tool temperature of 600 °C, while the bainite content reached the highest at 70%. The springback angles at different tool temperatures for quenching were measured by 3D scanning technology and the reverse modeling method. It was revealed that the springback angle increased with the increase of martensite and yet decreased with the increase of bainite. The relationship between the springback angle and the area fractions of the quenched phases was established by means of multiple linear regression analyses. The error analysis results of the predictions and measurements showed that the springback analysis model, based on the area fractions of quenched phases, could be used to predict the springback of hot stamping components with tailored properties.

https://www.mdpi.com/2075-4701/9/6/694/pdf

Xu Yan

Papers

Determination of Hot Stamping Friction Coefficient of 7075 Aluminum

Springback Prediction of a Hot Stamping Component Based on the Area Fractions of Phases