Based on the indirect hot-stamping test system, the effect of pre-forming on the microstructure evolution (grain size, dislocation density, martensite phase transformation) and mechanical properties of the blank in indirect hot stamping is systematically studied using ultra-high-strength steel 22MnB5. It is found that the average austenite grain size slightly decreases with the increase in pre-forming. After quenching, the martensite also becomes finer and more uniformly distributed. Although the dislocation density after quenching slightly decreases with the increase in pre-forming, the overall mechanical properties of the quenched blank are not greatly affected by pre-forming under the combined effect of the grain size and dislocation density. Then, this paper discusses the effect of the pre-forming volume on part formability in indirect hot stamping by manufacturing a typical beam part. According to the numerical simulations and experimental results, when the pre-forming volume increases from 30% to 90%, the maximum thickness thinning rate of the beam part decreases from 30.1% to 19.1%, and the final beam part has better formability and more uniform thickness distribution results when the pre-forming volume is 90%.

Through the study of the thermal rheological behavior of Ti6Al4V alloy at different temperatures (500 °C, 600 °C, 700 °C, and 800 °C) and different strain rates (0.1 s⁻¹, 0.05 s⁻¹, 0.01 s⁻¹, and 0.005 s⁻¹), a constitutive model was developed for Ti6Al4V alloy across a wide temperature range in the hot stamping process. The model’s correlation coefficient reached 0.9847, indicating its high predictive accuracy. Hot processing maps suitable for the hot stamping process of Ti6Al4V alloy were developed, demonstrating the significant impact of the strain rate on the hot formability of Ti6Al4V alloy. At higher strain rates (>0.05 s⁻¹), the hot processing of Ti6Al4V alloy is less prone to instability. Combining hot processing maps with hot stamping experiments, it was found that the forming quality and thickness uniformity of parts improved significantly with the increase in stamping speed. The phase composition and microstructures of the forming parts under different heating temperature conditions have been investigated using SEM, EBSD, XRD, and TEM, and the maximum heating temperature of hot stamping forming was determined to be 875 °C. The recrystallization mechanism in hot stamping of Ti6Al4V alloys was proposed based on EBSD tests on different sections of a hot stamping formed box-shaped component. With increasing deformation, the effect of dynamic recrystallization (DRX) was enhanced. When the thinning rate reached 15%, DRX surpassed dynamic recovery (DRV) as the dominant softening mechanism. DRX grains at different thinning rates were formed through both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), with CDRX always being the dominant mechanism.

Titanium alloy hot stamping technology has a wide range of application prospects in the field of titanium alloy part processing due to its high production efficiency and low manufacturing cost. However, the challenges of forming titanium alloy parts with large depths and deformations have restricted its development. In this study, the hot stamping process of a Ti6Al4V alloy box-shaped part was investigated using ABAQUS 2020 software. The thermodynamic properties of a Ti6Al4V alloy sheet were explored at different temperatures (400 °C, 500 °C, 600 °C, 700 °C, 800 °C) and different strain rates (0.1 s⁻¹, 0.05 s⁻¹, 0.01 s⁻¹). In addition, the influence law of hot stamping process parameters on the minimum thickness of the formed part was revealed through the analysis of response surface methodology (RSM), ultimately obtaining the optimal combination of process parameters for Ti6Al4V alloy hot stamping. The experimental results of the hot stamping process exhibited a favorable correlation with the simulated outcomes, confirming the accuracy of the numerical simulation. The study on the microstructure evolution of the formed parts showed that grain refinement strengthening occurred in the part with large deformation, and the formed box-shaped parts exhibited a uniform and fine microstructure overall, demonstrating high forming quality. The achievements of the work provide important guidance for the fabrication of titanium alloy parts with large depths and deformations used in heavy industrial production.

This article is dedicated to quantitatively and systematically revealing the changes of mechanical properties and bake hardening properties of AA6014 alloy during six-month natural aging in detail. Three directions (0, 45, and 90° relative to the rolling direction) of the aluminum alloy sheets and 16 time points within six months were selected to conduct experiments. The change trend of six mechanical properties (0.2% offset yield strength, ultimate tensile strength, plastic extension at maximum force, elongation after fracture, and strain hardening exponent) were obtained by a large number of micro-hardness measurements and tensile tests. The results show that elongations along the three directions are basically the same and do not drop significantly with the progress of natural aging but fluctuate within a certain range. The trends of the n value during natural aging before and after bake hardening are opposite and bake hardening leads to ~0.07 decrease of the n value. The PLC phenomenon disappears after 90 days of natural aging, and the yield strengths along the three directions also stabilize; thus, it can be inferred that the cluster changes tend to stabilize after 90 days natural aging. The large and systematic dataset are clearly and intuitively presented, which can not only be used to provide data reference for industrial production of automobile manufacturers but also be used to reveal the microscopic mechanism of the natural aging process.

To meet the development and application needs of advanced high-strength steel, the laser welding of 22MnB5 hot-forming steel plates with thicknesses of 2 mm, 3 mm, and 4 mm was studied in this paper. Mechanical testing revealed that as plate thickness increased, the tensile strength of welded joints decreased from 1489 MPa to 1357 MPa and 1275 MPa, equating to 96%, 91%, and 88% of the corresponding base metal strength, respectively. The heat-affected zone exhibited the lowest mechanical properties. Microstructural characterization showed that with increasing plate thickness, martensite grains in the welded joints grew larger, transitioning from fine acicular to larger island structures. Concurrently, dislocation density in the welded joints decreased gradually. Furthermore, microstructural changes in the heat-affected zone were more pronounced than those in the fusion zone. The larger grain size and reduced dislocation density softened the joint structure, which consequently decreased the strength and hardness of the welded joint. Laser-welded joints of three thicknesses can exceed 85% of the corresponding base metal strength, demonstrating strong industrial application potential.

Aiming at the need for lightweight requirements of the components in the bus, combined with the advantages of the hot stamping ultra-high-strength steel, a new television (TV) bracket was proposed. The finite element (FE) simulation of the beam part in the TV bracket during the indirect hot stamping process was discussed. After two-stages of pre-forming, the blank was in good formability and without visible cracks. According to the FE simulation results, the punch speed, quenching force, and quenching time significantly affected the temperature, microstructure, hardness, and mechanical properties of the beam part during hot stamping. With the increase of the quenching force and quenching time, the martensite fraction of the beam part was increased. For the beam part, the punch speed should be at least 80 mm/s during the forming stage. For complete quenching, the quenching force should be above 1000 kN and quenching time should be up to 10 s. Based on the parameters from the FE simulation, the forming experiment of the beam part was discussed. Microstructure analyses and microhardness tests as well as tensile tests of the hot stamping beam part were performed. The results confirmed that the FE simulation of the beam part was reliable.