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A forming limit diagram (FLD) is used to define the strain limit of a material fracture during sheet formation. Owing to its usefulness in the FE analysis of hot stamping, numerous studies have been conducted regarding the FLD based on temperature. There have been few studies on the forming limit of martensite, which must be investigated to measure the impact characteristics and impact toughness to evaluate the safety of car components. Moreover, the need for FLDs of hot press products to evaluate the collision characteristics of automobile parts is increasing. In this study, a stretch test is performed to measure the forming limit of 22MnB5 steel before and after hot press forming. To minimize the error between the experimental data and FE simulation, fracture energy theory was applied to the damage evolution technique to enhance the accuracy of the fracture results. Accordingly, an FE analysis of the FLD was performed using the ABAQUS software, and the method for minimizing the error compared to the actual data of FLD is examined. The results were validated between an experiment and FE analysis by applying forming limit analysis data to the FE analysis of the tensile test.

Hot press forming (HPF) steels with a full martensite structure and 2.0 GPa strength (HPF2.0) were friction stir welded as bead-on-plate using a WC–Co12% tool. The stir zone was composed of recrystallized grains of various sizes and had comparable to or higher hardness than the base material. However, the intercritical heat-affected zone (ICHAZ) and subcritical HAZ experience softening. During the tensile test, fractures consistently occurred at the softened HAZ. The minimum hardness of the ICHAZ in the friction stir welds was 300 HV, which was similar to that in the arc welds with polygonal ferrite (328 HV) and lower than that in the laser welds with needle-like ferrite (400 HV). Remarkably, the softened width of the friction stir welds was narrower than half that of the arc welds. These results confirm the possibility of expanding the application of friction stir welding to next-generation HPF steels.

A high strength Al-12Si has been prepared through mechanical alloying and press forming without any additional alloy components. The alloy exhibited a high tensile strength of 458 MPa at room temperature and retained excellent tensile properties at elevated temperatures with a UTS of 118 MPa at 350 °C after 1000 h of exposure. Furthermore, after 1000 h of heat exposure testing, the mechanical properties of the alloy showed no significant decrease. X-ray diffraction characterizations indicated that the alloy consists solely of an Al matrix and Si phase. Microstructural characterization through HRTEM revealed that the grain size of the Al matrix was approximately 300 nm, with a high-density of stacking faults present. The grain refinement strengthening and stacking fault strengthening contributed to the alloy’s excellent mechanical properties at both room temperature and elevated temperatures.

The low ductility of Ti–6Al–4V alloy at forming temperatures below 780°C is still a problem. Ti–6Al–IV alloy is usually hot press formed at 780 to 900°C. Hence, it is necessary to develop a new press forming for Ti–6Al–4V alloy sheets at forming temperatures below 780 °C to improve productivity. A new method to achieve of warm and cold press forming of Ti–6Al–IV alloy sheets by combining press motion control and clearance between a die and a blank was developed. In this study, the various forming conditions for developed press forming method were confirmed. In this method, the following two processes were separated: punch motion process without Blank Holding Force(BHF) and BHF motion process without punch motion. These two processes were carried out alternately during deep drawing or drawing and ironing. Three forming parameters, namely, the punch speed S pi , punch stroke P si , and blank holding force BHF i , were controlled separately and the effect of forming conditions on deformation behavior was investigated by using experiment and finite element analysis(FEA). As a result, the circular and square cups drawing and ironing of Ti alloy sheets were achieved by applying developed method.

Binderless poplar/bismuth oxide wood alloy is prepared using the warm-press forming technology. The effect of the forming temperatures on color changes and mechanical properties of the poplar/bismuth oxide wood alloy is studied. The results show that the surface color of the specimen gradually darkened as the forming temperature increased. There is the most obvious change from 140 to 160 °C. The CIE lightness color coordinate L* and chroma coordinate b* decrease with the increase of the forming temperature, while chroma coordinate a* decreases initially, but later increases with treatment severity. The static bending strength (MOR), the elastic modulus (MOE) and the surface hardness (HV) increase first and then decrease with the increase of the forming temperature. The X-ray diffraction (XRD) analysis shows that the wood underwent carbonization at 180 °C, resulting in a decrease in the density and mechanical properties of poplar/bismuth oxide wood alloy, and a deepening of the surface color. The Fourier transform infrared spectroscopy (FTIR) analysis reveals that the pyrolysis of cellulose and hemicellulose, as well as the pyrolysis and condensation of lignin led to the color of poplar/bismuth oxide wood alloy deepening. The hydroxyl groups between the cellulose molecular chains are reduced and hydrogen bonds are formed at 140 °C, which improve the mechanical properties of poplar/bismuth oxide wood alloy. However, the massive degradation of hemicellulose weakens binding strength with cellulose and lignin at 160 °C above. It greatly reduces the mechanical properties of specimen.

Using a tailor-welded blank (TWB) and hot-press forming (HPF), a 22MnB5 blank was surface-treated under four conditions. The penetration rates of the FexAly compounds under the four surface-treatment conditions were investigated, and the hardness values were measured. A finite element (FE) simulation was performed for the characteristics of the heat-affected zone (HAZ), using the hardness value and results of previous researchers. In particular, the mechanical property settings of the mesh were designed to realize the conditions for the FexAly compounds in the HAZ. Fine meshing was performed by partitioning the HAZ sections. For the mechanical properties of the HAZ with the FexAly compounds, the strength was predicted from the hardness value, and the elongation values investigated by other researchers were used. The forming limit diagram, which was proportional to the elongation, was predicted. Specific elements were defined as the areas with FexAly compounds, which played the same role as impurities. Tensile TWB–HPF specimens with different HAZ characteristics under four surface-treatment conditions were fabricated. Experiments and FE simulations were performed and compared. Details are as follows: For loads, a minimum error rate of 3% and a maximum error rate of 6% were obtained. For displacement, a minimum error rate of 9% and a maximum error of 25% were obtained. The feasibility of the simulation was verified by comparing the simulation and experimental results. A match of more than 75% was obtained.

In the automotive industry, the demand for ultra-high-strength steel is increasing due to the CO 2 emission and safety regulations. Hot-press forming (HPF) steels are a type of boron-alloyed high-strength steel fabricated via hot-press forming, which enables both high strength and elongation. Because HPF is conducted at high temperatures (900–950 °C) for a few minutes, its surface is coated with Al–Si or Zn to prevent surface oxidation and decarburizing. However, the coating layer often influences the properties of the welds. In this study, friction stir spot welding (FSSW) is used to weld dissimilar metals, i.e., an Al alloy and Al–Si-coated HPF steel. The effects of Al–Si coating on the mechanical and metallurgical properties of the hook formation on the weld are investigated. The shape of the hook, which is formed during FSSW, changes from bent to straight shape due to the presence of Al–Si and the HPF process. The joint strength of the straight hook-shaped specimens is demonstrated to be lower than that of the bent hook-shaped specimens. This difference in strength is because the hard Fe–Al–Si intermetallic (IMC) layer on the outer surface of the hook disturbs the bending of the hook during the welding. On the outer surface of the hook, a Fe–Al–Si IMC layer of chemical composition similar to that of the coating layer formed during HPF is observed. This formation is different from the inner surface of the hook, wherein a thin Fe–Al IMC layer is reconstructed between aluminum and steel. Thus, the hard Fe–Al–Si intermetallic layer transformed during the HPF process is the primary cause of the straight hook shape.

The partial strengthening method for pre-stamped panels both avoids excessive strength enhancement over the entire product and substantially reduces the processing cost compared to conventional hot press forming. In this paper, we propose two types of partial strengthening processes to selectively increase the local strength of a cold stamped B-pillar by utilizing an induction coil system and water spray quenching to induce the martensitic transformation in the target region. Thermo-mechanical FE analyses were conducted to evaluate the effect of the process parameters such as the quenching rate, quenching time, and heating width on the shape changes during the partial strengthening processes while simultaneously considering the thermal dilatation and transformation plasticity.

The advantages of multi-point warm press forming Corian sheet are fewly studied. In this study, the principle of multi-point warm press forming process and the geometrical relationship between multi-point punch elements and objective surface were firstly illustrated. The multi-point CAD/CAM software was used for punch height calculation and punch location adjustment. The effects of elastic cushion, forming temperature and forming force on the forming accuracy, were studied through multi-point warm press forming experiments. Then, the Corian sheets for spherical and saddle-shaped parts based on suitable forming parameters were formed through multi-point warm press forming process, and the shape errors between experimental parts and objective ones were compared. The high surface quality and shape accuracy of both spherical and saddle-shaped parts were obtained at 165 °C with the forming force of 100 kN. It confirms that the multi-point warm press forming process is feasible for manufacturing Corian sheets.

In this paper, hot and cold deep drawing processes are determined with direct deep drawing process and indirect deep drawing process. To predict the friction coefficient, the finite-element method, which can predict deformation behavior until the fracture of a blank sheet, was proposed using the forming limit diagram (FLD) curve. The effect of fracturing of the coating layer on the friction coefficient during the hot and cold deep drawing processes was investigated. The deformation behavior of the coating layer of the boron steel sheet that affects the friction coefficient in the hot and cold deep drawing processes was also proposed. A forming method that can control the surface condition of the formed product is further proposed by explaining the fracture of the coating due to the forming process.