Hot Press Forming Research Articles

Characterization of relaxation behaviour of CF/PEKK aerospace composites using the time-temperature-crystallinity superposition principle

In this study, the Time-Temperature-Crystallinity Superposition Principle (TTCSP) was applied to determine the viscoelastic behavior of Thermo-rheological Complex Materials (TCM), specifically Carbon fibre/Poly-Ether-Ketone-Ketone (CF/PEKK) composites. The study investigated the effects of various parameters on the viscoelastic behavior of the composites, such as the degree of crystallinity after different melting temperatures, relaxation, and crystallization times. The TTCSP was utilized on the relaxation data to generate great-grand master curves for the degree of crystallinity for different laminate lay-ups. Hot press forming was employed to manufacture samples under different processing conditions, including various melting and cold crystallization temperatures. Differential Scanning Calorimetry (DSC) was employed to calculate the degree of crystallinity of CF/PEKK composites, while the Dynamic Mechanical Analyzer (DMA) was used to obtain the relaxation data. The generated great-grand master curves proved effective in predicting the relaxation behavior of the composites consolidated using single and double hold cycles at different melting temperatures and crystallization times, respectively. The great-grand master curves presented in this study can serve as valuable tool to calibrate key viscoelastic and/or thermo-viscoelastic material models for aerospace-grade CF/PEKK composites. These models are crucial for simulations aimed at predicting residual stresses and process-induced deformations during the thermoforming process.

Effect of Al-based intermetallic coating layer on H uptake during austenitization in 31MnB5 steel

Currently, the Al-Si-coated steel is widely utilized in hot press forming (HPF) processes. The Al-Si coating serves as a protective barrier against oxidation and decarburization, as the HPF steel undergoes austenitization at temperatures exceeding approximately 900 °C. However, the formation of liquid Al during austenitization triggers an oxidation reaction with atmospheric moisture, generating H atoms. This leads to H uptake and subsequent embrittlement. Therefore, in the present study, to reduce the generation and absorption of H during the austenitization of Al-Si-coated HPF 31MnB5 steel, we modified the Al-Si coating layer to a fully intermetallic coating layer by performing annealing before HPF process. Consequently, there was a notable reduction in both H uptake and subsequent embrittlement. This can be attributed to three main reasons. (1) Restricted interaction between the Al-based intermetallic phase on the surface of the coating layer and the water vapor in the furnace atmosphere. (2) Low H diffusivity within the intermetallic coating layer. (3) Efficient entrapment of H within surface voids found in the intermetallic coating layer.

Comparative Analysis of SPR and SPR-Bonded Joints of Hot Press Forming Steel and AA5052 under Corrosive Conditions

The increasing demand for effective joining techniques in dissimilar materials has led to the widespread use of self-piercing rivet (SPR) mechanical joints, particularly in aluminum combinations. Studies in this field aim to enhance joint strength by optimizing process conditions, including the use of SPR-bonded processes with structural adhesives. Structural adhesives in SPR-bonded processes introduce different corrosion behavior owing to galvanic corrosion caused by potential differences between sheets over time. In this study, we utilize SORPAS® simulation to optimize SPR conditions and analyze stress and deformation. A tensile shear test was performed on a hot pressforming steel/AA5052-H32 combination for SPR and SPR-bonded processes. The experimental results indicated die sticking during SPR optimization, increased tensile strength through AA5052-H32 surface cleaning in the SPR-bonded process, varying corrosion conditions, and consistent SPR-bonded strength after salt spray exposure. The difference in failure modes between the processes concerning corrosion time can be attributed to the corrosion- mitigating effect of structural adhesive within the galvanic coupling region formed by potential differences. Additionally, adhesive and rivet failure modes were observed. Moreover, adhesive failure occurred in the SPR-bonded process after shear tensile tests, characterized by a fracture between the lower AA5052-H32 interface and adhesive. Furthermore, we found a linear relationship between the remaining adhesive area and shear tensile strength.

Effects of Ni-precoating on hydrogen intrusion/emission behavior in Al-Si-coated hot-press-forming steel sheets

In Al-Si-coated hot-press-forming (HPF) steels, high reactivity between Al and moisture often leads to the generation of hydrogen (H) atoms at elevated temperatures, resulting in hydrogen embrittlement. In this study, a thin Ni layer of approximately 0.5 μm in thickness was electroplated on the steel substrate surface before the conventional AlSi coating process. The formation of a Ni-enriched zone was found to affect hydrogen intrusion and emission behavior positively. Prior to the HPF simulation, the Ni-enriched zone was formed inside Fe2SiAl7 intermetallic compounds, and migrated toward the outer FeAl(Si) layer during the subsequent HPF simulation. This migration served as an impediment to elemental diffusion, thereby resulting in a prolonged simulation time for the complete formation of coating microstructures. Consequently, the presence of the Ni-enriched zone functioned as a high-energy barrier, impeding the diffusion of H and rendering H intrusion more challenging, while simultaneously enhancing corrosion resistance. Complete H emission occurred within 3 days in the Ni-precoated sheet, whereas a significant amount of H content remained even after 17 days in the non-Ni-precoated sheet. This difference could be attributed to the reduced amount of H intrusion in the Ni-precoated sheet, despite similar H emission rates observed in both sheets.

Low Temperature Hot Press Forming of a Zinc Coated Third Generation Advanced High Strength Steel

A 7 wt.% Mn steel was designed and was cast and processed to 1.5 mm sheets. The sheets were continuous annealed and coated with a Zn alloy by hot dip galvanizing before subjecting them to hot press forming cycles. The final microstructure was characterized by ultrafine ferrite grains and a high fraction of retained austenite. Excellent combinations of in-service strength-ductility-bendability were achieved for hot forming in the temperature range of 530-675 °C. The use of the low temperature hot forming minimized the liquid metal embrittlement induced cracking with Zn coating during hot forming and spot welding. Various application properties such as oxidation resistance, corrosion resistance, and springback were found at optimum levels for hot forming at 675 °C.

Materialization of the Heat-Affected Zone with Laser Tailor-Welded HPF 22MnB5 Steel Using FLD and the Fracture Displacement Method in FE Simulation

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.

Improvement of hydrogen embrittlement by void formation at coating layers of Sb-added Al-Si-coated hot-press-forming steels

Al-Si coating prevents the oxidation or decarburization of hot-press forming (HPF) steel. However, the high reactivity of Al with moisture promotes H uptake, and the coating hinders the out-diffusion of absorbed H atoms inside the steel, leading to hydrogen embrittlement (HE). This study added Sb to adjust Fe and Al/Si diffusivity, and forming H-trapping voids within the coating. Simultanesouly, these voids provided routes for the emission of H atoms, thereby improving the resistance to crack propagation during bending. The optimal design of the Al-Si coating by adjusting the Sb content and void fraction would enable extensive applications of HPF steel.

Optimization of Hot Forming Temperature to Minimize Liquid Metal Embrittlement Induced Cracking in Resistance Spot Welded Zinc‐Coated Medium Manganese Steel

Herein, an investigation on the influence of the hot press forming (HPF) temperature cycles of a medium manganese, high ductility, zinc‐coated steel with ≈1000 MPa final tensile strength (HPF1000‐GI), with a specific emphasis on the risks of liquid metal embrittlement (LME) induced cracking during resistance spot welding (RSW) is presented. The HPF1000‐GI steel is hot press formed using several temperature cycles, and subsequently the LME sensitivity of the variously hot press formed materials are investigated using both dedicated RSW experiments as well as high‐temperature tensile testing. It is shown that the LME sensitivity of the HPF1000‐GI steel is significantly reduced after hot forming at higher intercritical temperatures, and that this favorable behavior is caused by microstructural changes at the coating‐substrate interface.

Microstructural evolution of liquid metal embrittlement in 1.5-GPa-grade Zn-coated hot-press-forming steels

Zn-assisted liquid metal embrittlement (LME) in hot press forming (HPF) steels is attributed mainly to the cracking inside both Zn coating and HPF substrate, but very few studies on how the Zn coating composed of Fe–Zn intermetallic compound of Γ-Fe3Zn10 and Zn-containing ferrite (α-Fe(Zn)) and alloying elements influence the LME cracking have been conducted. In this study, mechanisms of LME were investigated by using laboratory-scale HPF simulation tests in relation with microstructural evolutions of Zn coating and LME cracking. The coating and steel substrate were composed mostly of α-Fe(Zn) and full martensite, respectively, in both reference (Ref) and 1-wt.%-Si-containing (1Si) HPF steels, while the Γ phase was additionally formed at the 1Si steel. The high-temperature tensile test results indicated that the loss of elongation increased with decreasing exposure time near or at 900 °C and with increasing test temperature or Si content. Based on the equilibrium Fe–Zn binary phase diagram, the amount of liquid remained after the austenitization was larger in the 1Si steel than in the Ref steel because the addition of 1 wt% Si retarded the Fe–Zn alloying reaction between the Zn coating and steel substrate to raise the susceptibility to LME. Thus, the Si content should be carefully controlled by appropriately using the HPF process parameters such as the time of austenitization at 900 °C to sufficiently achieve the Fe–Zn alloying reaction.

Microstructure and mechanical properties of laser welded hot-press-formed steel with varying thicknesses of Al–Si coatings cleaned by nanosecond pulsed laser

The Al–Si coatings are commonly used for hot-press-forming (HPF) steel to prevent oxidation in the automobile industry. However, due to the presence of Al–Si coating, undesirable intermetallic compounds are easily formed in the welds of HPF steel. In this work, an environment-friendly and effective nanosecond pulsed laser cleaning process was used to remove the Al–Si coating, and HPF steel with varying thicknesses of coatings was prepared for laser welding. The microstructure and mechanical properties of laser welded HPF steel with different thicknesses of coatings were further investigated. The results showed that there was a threshold of power to remove the Al–Si coatings in the laser cleaning process. The reduced thickness of coatings not only could improve the stability of the welding process due to the diminishing of metal vapor generated by coating ablation, but also decrease the content of the Fe–Al phase in the welded joint, which is detrimental to weld performance. The tensile strength and cupping test pass rate of weld joints gradually increased with the decreasing thickness of coatings to 5 μm. After that, the Al–Si coating almost had no effect on the mechanical properties of the laser weld joints. The laser weld joint with 5 μm-thick of Al–Si coating showed, a tensile strength of 1435.05 MPa, a microhardness of 488.38 HV and a pass rate of 100% for the cupping test.

Investigation of Collision Toughness and Energy Distribution for Hot Press Forming Center Pillar Applied with Combination Techniques of Patchwork and Partial Softening Using Side Crash Simulation

Various techniques can be applied to center pillars to enhance collision characteristics during side crashes. For instance, patchwork (PW) can be welded to the center pillar to increase its stiffness, and partial softening (PS) can be applied to provide ductility. Side crash tests are conducted by the Insurance Institute for Highway Safety (IIHS) to evaluate collision resistance. However, it is difficult to evaluate collision toughness and energy distribution flow for each automobile component. In this study, a side crash simulation was performed with IIHS instruction. We investigated the effect of hot press forming (HPF) a center pillar with a combination of PW and PS techniques on collision toughness and energy distribution flow. As a result, the role of PW and PS techniques were verified during side crashes. PW improved the strain energy and intrusion displacement by 10% and 7.5%, respectively, and PS improved the plastic deformation energy and intrusion displacement by 10%. When PW and PS were applied to the HPF center pillar simultaneously, a synergistic effect was achieved.

Biomimetic laminated basalt fiber-reinforced composite with sinusoidally architected helicoidal structure integrating superior mechanical properties and microwave-transmissibility

High-efficient microwave-transmissibility, mechanical durability, and lightweight are in significant demand for wireless communication components. Unfortunately, conventional materials can hardly meet the requirements for emerging radio-frequency (RF) devices so far. Herein, inspired by the sinusoidally architected helicoidal structure of mantis shrimp's dactyl club, a biomimetic laminated basalt fiber-reinforced composite (BL-BFRC) is designed and manufactured through a method combining linear helicoidal layup and hot press forming. Remarkably, the flexural strength and flexural modulus of BL-BFRC are 259.71 MPa and 22.61 GPa, which increase by 28.4% and 36.9% over those of conventional BFRC (202.27 MPa and 16.52 GPa). In addition, BL-BFRC exhibits a dielectric constant of 2.2858 and a loss tangent of 1.55 × 10−3, which are much lower than those of conventional BFRC (3.6459 and 9.73 × 10−3). Accordingly, the microwave-transmissibility increases from 93.53% (conventional BFRC) to 98.34% (BL-BFRC). In fact, the superior mechanical durability of BL-BFRC is mainly attributed to the crack deflection of helicoidal structure and stress dissipation of sinusoidal morphology. And the efficient microwave-transmissibility benefits from dielectric transition effect on non-smooth sinusoidal surface. This work uniquely combines the features of high-efficient microwave-transmissibility and mechanical durability into RF devices and would enable a broad range of wireless communication applications.

Rice husk ash: Economical and high-quality natural-based reinforcing filler for linear low-density and high-density polyethylene

Rice husk ash (RiHA) was employed as the bio-originated and inexpensive filler prepared from agricultural wastes for reinforcing high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). X-ray fluorescence (XRF) spectroscopy showed ∼80.82% for the silica content of RiHA as well as the values of other components present in this bio-based filler. The composites were obtained via melt mixing followed by the compression molding by the hot press forming. Characterization of the composites by FT-IR spectroscopy revealed that the filler has the sheer effects on the vibrational bands of the polymers. The usage of X-ray diffraction (XRD) analysis to investigate the d-spacing values and the crystallinity of the samples, exhibited the increase of d-spacing upon reinforcing the polymers with RiHA. The scanning electron microscopy (SEM) images showed an average size of 32 µm for the irregular RiHA particles which uniformly dispersed in the polymeric matrices. The energy dispersive X-ray (EDX) analysis displayed C, O, and Si as the main constituting elements of the composites and alternatively confirmed the well dispersion of the filler particles into the polymer matrices. The mechanical measurements showed the significant improvements in Young’s modulus, yield stress, and hardness results of the polymers after reinforcing with the rice husk ash. For example, Young’s modulus of HDPE was increased ∼15% after incorporating 7 wt.% of RiHA into this polymer. These mechanical properties of the polymers were increased upon increasing the RiHA content, while the parameter of elongation at break was decreased.

Prediction of the peak and steady-state ply–ply friction response for UD C/PAEK tapes

Excessive ply–ply friction can lead to process-induced defects during the hot press forming of thermoplastic composite laminates. Process simulations can be used to enable first-time-right defect-free manufacturing. However, a good understanding of the ply–ply friction is required to improve the constitutive models to allow simulation of more complex parts. We performed friction experiments on UD C/PAEK tapes, showing a typical response with a peak followed by a steady-state friction. Micrographs of tested specimens were analyzed to evaluate the matrix interlayer thickness distribution in the ply–ply interface, which we combined with the matrix viscosity to predict the peak friction. Generated fiber distributions, mimicking the ply–ply interface, were successfully used to obtain the matrix interlayer as well. The steady-state friction was accurately described by including a critical shear stress to represent wall slip, substantiating the concept of wall slip as the dominant underlying mechanism for the start-up friction response.

Impact resistant basalt fiber-reinforced aluminum laminate with Janus helical structures inspired by lobster and mantis shrimp

Fiber metal laminate (FML) is an extremely strong and light material that is widely applied in the structural components of aircraft for weight reduction and improving mechanical properties. However, the effective integration of excellent structural failure tolerance property and superior stress dissipation capacity into one FML material remains a huge challenge. Herein, inspired by the non-linear helical structure of lobster and the linear helical structure of mantis shrimp, a novel bioinspired FML with Janus helical structure was designed and manufactured through the method of hot press forming. Afterwards, low velocity impact tests were carried out to evaluate the impact resistant property. Remarkably, the peak impact force of the bioinspired Janus FML (BJFML) at the impact energy of 40 J was 8216.36 N, which increased by 10.22% compared with the 7454.52 N of traditional cross-ply FML (CPFML). Moreover, the dent depth of the BJFML (5.95 mm) decreased by 28.23% compared with that of CPFML (8.29 mm). Finally, the finite element simulation was conducted systematically. In fact, the non-linear helical arrangement greatly hindered crack propagation and the periodic linear structure improved its load homogenizing ability for impact forces, thus improving structural failure tolerance, strengthening stress dissipation, and synergistically enhancing the impact resistance.

On the origin of micro-cracking in zinc-coated press hardened steels

Zn-coated press hardened steels (PHS) are in high demand for automotive mass reduction and enhanced passenger safety applications while the Zn coating supplies robust cathodic corrosion protection. However, the mechanism of micro-crack formation during direct hot-press forming (DHPF) has not been adequately described. Thus, the objective of this work was to determine the mechanism for micro-crack formation in Zn-coated DHPF PHS that addressed the relationship between micro-cracking and the coating microstructure created during substrate austenitization. Zn-coated 22MnB5 steel sheets were annealed at 900 °C for annealing times ranging from 30 to 780 s and DHPF at 75 °C s−1 to obtain a fully martensitic substrate microstructure. The inward diffusion between the Zn coating and the substrate during annealing resulted in a dual phase coating microstructure initially comprising Γ-Fe3Zn10 + α-Fe(Zn), transitioning to a single phase α-Fe(Zn) coating after annealing for 240–420 s. Coincident coating α-Fe(Zn) and substrate Zn-enriched austenite (γ-Fe(Zn)) grain boundaries became Zn-enriched, forming a thin layer of α-Fe(Zn) along the γ-Fe(Zn) grain boundaries. It is proposed that coincident coating α-Fe(Zn) and substrate prior austenite grain boundaries (PAGBs) were weakened by this grain boundary α-Fe(Zn) layer. Upon the application of tensile stress, intergranular fracture occurred along the coincident coating α-Fe(Zn) and Zn-enriched PAGBs in the Zn-enriched martensite (M(Zn)) layer. It was further determined that crack propagation ceased and the crack tip was blunted when Zn-enrichment along the PAGBs in the M(Zn) layer was exhausted.

Numerical and experimental formability analysis of aluminum 3105 sandwich panels produced by continuous hot-press forming

AbstractAluminum–plastic composites as building materials are widely used for different construction purposes, for instance, for exterior wall cladding, ventilated facades, and interior decoration of buildings, in billboards, trains, and automotive industry. The main goal of this study is to investigate the formability of AA3105/LDPE/AA3105 sandwich composite in detail, which would exhibit a higher formability compared to AA3105 due to its higher strain rate sensitivity and strain hardening exponent. This is of decisive importance for the manufacturing process. The Nakajima tests are performed to experimentally determine the forming limit curves (FLCs) usingin situoptical technique digital image correlation. Furthermore, numerical simulations of Nakajima tests are conducted using a modified Gurson–Tvergaard–Needleman damage model to compare the numerically and experimentally determined FLCs. The results show that the sandwich composite underwent inhomogeneous deformation during the Nakajima test, and that the FLC has no typical patterns exhibited by metal sheets. The FLCs predicted by the numerical model used in this study exhibit a very good correlation with the FLCs determined experimentally. The results of the present study provide new insights into the analysis and understanding of the deformation behavior of the sandwich composite sheet that may undergo complex stress and strain states.

Effects of Al-Si coating structures on bendability and resistance to hydrogen embrittlement in 1.5-GPa-grade hot-press-forming steel

Hot-press-forming (HPF) steels have attracted great attention as automotive reinforcement parts, but are exposed to the potential risk of hydrogen embrittlement (HE) because H introduced easily during HPF processes is hardly de-trapped through the solidified coating. In particular, H seriously deteriorates bendability, which is one of the main properties to be considered. In this study, the Al-Si coating structures were modified to improve H emission by increasing H diffusivity. The effects of coating structures on bendability and H desorption were investigated by interrupted bending tests, H-permeation tests, and thermal desorption analyses according to elapsed time after H-charging. Immersion in an Al-10%Si bath and the subsequent HPF process (930 °C for 6 min) produced a 33 μm-thick multiple coating structure composed of Fe2Al5, FeAl, and ferrite layers. On the other hand, the reduced Al-Si adhesion amount from the dip bath and the increased time and temperature (950 °C for 30 min) produced a 30 µm-thick body-centered-cubic (BCC)-based coating structure composed of FeAl and ferrite layers. The BCC-based crystal structure, reduced Al content in the FeAl layer, and coarsened ferrite grains effectively enhanced H diffusivity and suppressed H-induced degradation. Moreover, the softened FeAl and thick ferrite layers improved bendability by allowing the large strain accommodation of bending deformation. Thus, this work proposes an optimal Al-Si coating design that enhances both bendability and resistance to H-induced degradation for secure HPF steel applications.

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.

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.