Hydrogen Absorption and Desorption Behavior on Aluminum-Coated Hot-Stamped Boron Steel during Hot Press Forming and Automotive Manufacturing Processes

Our study mainly focused on diffusible hydrogen in aluminum–silicon-coated hot-stamped boron steel during a hot press forming process and in pre-treatment sequential lines of the automotive manufacturing process using a thermal desorption spectroscopy (TDS) technique. First, in the hot stamping procedure, as the soaking time increased in the heating furnace at a specific dew point when austenitizing, a high concentration of diffusible hydrogen was absorbed into the hot-stamped boron steel. Based on the TDS analysis of hydrogen absorbed from hot stamping, the activation energy value of hydrogen trapping in 1.8 GPa grade steel is lower than that of 1.5 GPa grade steel. This means that diffusible hydrogen can be more easily diffused into defective sites of the microstructure at a higher level of the tensile strength grade. Second, in sequential pre-treatment lines of the automotive manufacturing process, additional hydrogen did not flow into the surface, and an electro-deposition process, including a baking procedure, was effective in removing diffusible hydrogen, which was similar to the residual hydrogen of the as-received state (i.e., initial cold rolled blank). Based on these results, the hydrogen absorption was facilitated during hot press forming, but the hydrogen was sequentially desorbed during automotive sequential lines on aluminum-coated hot-stamped steel parts.

X-ray Radiography Inspection of Pores of Thin Aluminum Foam during Press Forming Immediately after Foaming

Forming aluminum foam to the desired shape while retaining its pore structures is essential for manufacturing aluminum foam products. Recently, a press forming process for aluminum foam that is performed after precursor foaming but before solidification has been proposed. In this study, to track individual pores throughout press forming immediately after foaming, X-ray radiography inspection was applied. A thin precursor was used, and foaming was constrained to the X-ray transmission direction. It was shown that, although some pores coalesced with other pores, the pores did not collapse during press forming. In addition, the porosity of aluminum foam evaluated from X-ray transmission images was constant during press forming. Some pores retained their shape during press forming but their position was changed by the material flow generated by press forming. These results show that by press forming before the solidification of aluminum foam, aluminum foam can be shaped without the collapse of pores.

Investigation of Press Forming Process for CF/PEEK and GF/PP Thermoplastic Composite Materials

Aerospace industry requires engineering materials with significant mechanical strength, low cost, easy to recycle, easy to process, and high chemical resistance. Thermoplastic composites are considered as new generation aerospace structural materials due to their superior characteristics. In this study, one of the most common methods used in manufacturing of thermoplastic composite parts from unidirectional (UD) pre-consolidated prepreg (blank), press forming process is investigated and crack behavior of the samples is examined. The forming process is utilized, and consolidated blanks of UD CF/PEEK and GF/PP composite materials are shaped by pressing, and mechanical tests are performed. Micro defects on the samples are monitored by microscopic examination and scanning electron microscopy (SEM) analysis. Results show that when the temperatures of CF/PEEK and GF/PP blank are increased by preheating, the formability of both composites are enhanced. Final outcomes reveal that both CF/PEEK and GF/PP composite materials can easily be used for possible aircraft structures. Additionally, CF/PEEK is found to be appropriate not only secondary structures but also for primary structures.

Interpretation of surficial shear crack propagation mechanisms in bending for Zn or AlSi coated hot press forming steels

The bending angle at the peak load is regarded as the most important parameter for evaluating bending properties of hot-press-forming (HPF) steels. However, it is not a mechanics-based parameter for the bending criterion, and the data interpretation is difficult because bending criteria in relation with microstructures and associated bending mechanisms have not been verified yet. In this study, effects of coating and baking treatments on bending angles at the peak load of three kinds of 1470 MPa-grade HPF steels were investigated by interrupted three-point bending tests coupled with direct microstructural observation. According to direct observations of sequential cracking processes of V-shaped crack (V-crack), bending procedures were classified into four stages: (1) formation of small V-crack, (2) increase in number and size of V-cracks, (3) initiation of shear-crack propagation from the V-crack tip, and (4) further propagation and opening of the shear crack. The minimum bending angle required for initiating the shear-crack propagation from the V-crack tip was defined as a critical angle, which meant the boundary between the 2nd and 3rd stages. The present bending behavior related with critical bending angle and V-cracking could be interpreted similarly by the fracture-mechanics concept, i.e., the initiation of shear-crack propagation.

Effects of press-forming parameters on the dimensional stability of paperboard trays

The dimensional stability of press-formed paperboard trays was investigated during heating and cooling of trays packed with oatmeal. Female mold tool temperature, dwell time, pressing force, and blank holding force were altered in the press-forming of the trays to observe their impacts on the dimensional stability. Dimensional measurements of the trays showed reduced tray width, and the trays exhibited distortions on the tray flange and outer wall. The results showed smaller effects on the tray length, parallel to the machine direction of the material. Improved dimensional stability of the trays was found with a 180 °C female mold tool temperature, a 600-ms dwell time, a 150-kN pressing force, and a 1.44-kN blank holding force. The optimal press-forming parameters were concluded to enhance bonding of the paperboard fibers during the press-forming. The optimization of the press-forming parameters was found necessary to reduce the observed negative response of the material to the challenging environmental conditions in the production of prepared food.

On the origin of start-up effects in ply-ply friction for UD fiber-reinforced thermoplastics in melt

Hot press forming is an attractive production technology to fulfil the increasing demand for complex fiber-reinforced thermoplastic parts. Over the years, process simulation tools on press forming have shown to be very helpful in facilitating the design stage for defect free parts production. One of the important deformation mechanisms considered in process simulations is the relative slip of successive plies or ply-ply friction, of which the underlying principles need to be better understood in order to improve the overall predictive simulation quality. In particular the use of steady-state friction values, neglecting the transient response, is questionable as experiments showed that shear stress overshoots can be as high as three times the long-time value. The phenomenon of the overshoot at start-up shear is analyzed. Possible explanations include nonlinear viscoelasticity and a slip relaxation effect giving rise to wall slip, which are discussed using relevant ply-ply friction measurements carried out on a dedicated friction test set-up. Experimental results on UD C/PEEK show that the shear stress build up and subsequent relaxation comply with nonlinear viscoelasticity. However, the long-time shear stress fails to match the matrix material’s viscosity, possibly due to a yield stress. The flow curve corrected for a yield stress resembles the effects of wall slip. A transient model according to these findings will enhance the accuracy of press forming simulation software.