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Press hardened steel (PHS) has been applied to many automotive body parts to reduce the weight of the white body, and high corrosive properties are required in PHS. Al-Si coated steel sheets are widely used for protection against scaling and decarburization. Although Al-Si coated steel sheets have better corrosion resistance than bare steels due to the barrier effect, the corrosive protection performance of Al-Si coated steel sheets is not sufficient for use in wet under-body areas compared with galvanized or galvannealed steel sheets. Zn coated PHS is appropriate for hot stamped parts in which corrosive protection is required because of its sacrificial protection property. However, micro-cracks occur in severely deformed areas of Zn-coated parts under some hot stamping conditions, and deep micro-cracks can affect the fatigue property. In this study, several hat-shaped samples were prepared, and the depth of micro-cracks was changed by altering hot stamping conditions. The effects of the micro-crack depth on the fatigue property of Zn-Ni coated PHS were investigated by a plane bending fatigue test. At the average depth of 20 μm, the fatigue life of the Zn-Ni coated samples was almost equal to that of the Al-Si coated sample which was taken from the wall side of the hat-shaped sample after crash forming. Considering the standard deviation of the micro-crack depth in practical use, the Zn-Ni coated steel sheets can be used equally with the Al-Si coated steel sheets in fatigue property when the micro-crack depth was restricted to under 10 μm. Moreover, the impacts of hot deformation conditions on the micro-crack depth were investigated. It was found that the reduction of equivalent plastic strain is effective in lowering the micro-crack depth. [ABSTRACT FROM AUTHOR]

The hot stamping of Al-Si-coated high-strength steel (HSS) sheets is an important manufacturing process that has gained significant attention in recent years. However, the friction and adhesive wear that occur during the process have become major bottlenecks, limiting the efficiency and effectiveness of the process. To address this problem, a comprehensive investigation was conducted to evaluate the tribological behavior of the contact interface between Al-Si-coated HSS sheets and tool steel during hot stamping. Specifically, micro-dimpled laser surface textures (LSTs) with varying densities were applied to the H13 tool steel surface, followed by an examination of their effectiveness in reducing friction and wear. The 3D morphology of the wear scratches was assessed using a laser confocal surface roughness system, while SEM and EDS techniques were utilized to analyze the mechanisms behind friction reduction and wear resistance. The results demonstrated that laser surface texturing (LST) effectively reduces the friction coefficient between Al-Si-coated HSS sheets and H13 tool steel under high-temperature and high-pressure conditions. Moreover, it was found that LSTs exhibiting densities of 30% retained their pitted morphology following friction experiments, indicating a remarkable capability in reducing adhesive wear and facilitating debris collection. The average coefficient of friction (COF) was observed to be significantly lower in textured specimens than in untextured specimens by approximately 64.62%, 52.3%, 51.9%, and 67.11% at pressures of 5 MPa, 10 MPa, 15 MPa, and 20 MPa, respectively. This study offers a novel concept and method that leverages the LST technology to mitigate the friction and wear of Al-Si-coated HSS sheets during the hot stamping process. The findings also shed light on the anti-wear effect and the mechanism of LST in such conditions. Overall, this study highlights the potential of LST as a promising method for enhancing the efficiency and effectiveness of the hot stamping process involving Al-Si-coated HSS sheets. [ABSTRACT FROM AUTHOR]

Inconel 718 is a critical component in many sectors such as aerospace and nuclear due to its outstanding properties. As in these sectors parts are exposed to intense environments, any failure in the system can be calamitous. Thus, there are strict requirements for manufacturing precision and quality enforcement. However, poor machinability of Inconel 718 parts has caused ongoing challenges for both researchers and industry. The difficulties arise from the poor surface integrity of the machined parts due to its high strength at elevated temperatures, susceptibility to work-hardening, presence of hard carbides, low thermal conductivity, and built-up edge formation during machining. These characteristics make conventional machining methods less effective, necessitating innovative approaches. This study introduces an innovative solution to tackle this issue —a new in-situ coating method for depositing novel lubricating materials on the tool. Comprehensive machining tests and surface integrity analyses have been performed. The results show substantial improvements, including a significant prolongation in tool life, improved surface quality, and less machining-induced microstructural alteration. The success of using novel lubricious coatings in conjunction with the new in-situ method promises improved Inconel 718 machining as it resulted in exceptional tool performance and surface finishes that meet the tight standards of industry. [ABSTRACT FROM AUTHOR]

In conventional hot stamping, an Al-Si-coated blank is first heated above the austenitization temperature and then soaked for a period of time within a furnace, prior to the stamping operation. In this work, the impacts of furnace heating rate, soaking temperature, and soaking time on the Al-Si coating evolution were investigated for two commercial coating weights, 80 and 150 g/ m 2 . These heat treatment parameters during heating and soaking affect the coating microstructure and the thickness of the interdiffusion layer, which affect the properties of the as-formed coatings. The transformation and growth of binary Fe-Al and ternary Fe-Al-Si intermetallic layers were characterized and quantified for soak times up to 240 s. The results show that the effect of the heating rate on the Al-Si intermetallic distribution and ternary phase morphology was more severe than the soaking time and soaking temperature. The F e 2 A l 5 (η) phase was the dominant layer at the beginning of the soaking stage with a F e 3 A l 2 S i 3 ( τ 1 ) layer formed within it, and then the F e 3 A l 2 S i 3 layer transformed into F e A l ( β 2) as the soaking time increased due to the interdiffusion of Fe and Al. The transformation of F e 3 A l 2 S i 3 to F e A l occurred at a higher rate for elevated soaking temperatures due to the greater diffusivity of Al and Fe. The interdiffusion layer (IDL) consisted of F e A l , F e 3 A l ( β 1) and α − F e . Higher soaking temperatures of 1000 °C resulted in a thicker IDL for the same soak time when compared with 900 °C and 950 °C, but when the heating rate was lower, the IDL was thicker than that at the higher heating rate since a longer heating time was required to reach the soaking temperature of 900 °C, which prolonged the diffusion time during the heating stage. The findings were similar for AS80. [ABSTRACT FROM AUTHOR]

Hot stamping (or press hardening) is a new technology that is widely used in the production of advanced high-strength steel parts for automotive applications. Electrochemical measurements, including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), and accelerated corrosion tests (the neutral salt spray test and periodic immersion test) were conducted on press-hardened samples produced from uncoated (cold-rolled and cold strip production (CSP) hot-rolled) and Al–Si-coated press-hardened steels to elucidate their distinct anti-corrosion mechanisms. The cross-sectional micromorphology and element distribution of three types of press-hardened steels after a neutral salt spray test were observed using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDAX). The corrosion resistance of Al–Si-coated press-hardened steel was found to be significantly diminished following the hot stamping process due to the presence of microcracks and elevated iron content in the coating subsequent to austenitizing heat treatment. On the other hand, the corrosion resistance of uncoated press-hardened sheets produced from cold-rolled and CSP hot-rolled press-hardened steel was found to be proximal due to their nearly identical composition and microstructure (fully martensite) after the hot stamping process. Considering the high efficiency and energy-saving properties of hot-rolled press-hardened steel, it holds the potential to replace cold-rolled and even aluminum–silicon-coated press-hardened steel in automobile manufacturing. [ABSTRACT FROM AUTHOR]

Al-Si coatings have been widely used for their low density and high strength-to-weight ratio in automobile industries; however, poor tribological such as low wear limits their applications. The development of surface modified Al-Si coatings to enhance the tribological properties is essential and so herein is presented an investigation of the effect of laser parameters on the wear performance of Al-Si-carbon nanotube (CNT) coatings deposited on AA7075 alloy substrates. Al-Si-CNT coatings were deposited on the AA7075 substrate using atmospheric plasma spraying (APS) and then subject the laser surface modification (LSM) with a nanosecond pulsed fibre laser. Wear tests were carried out under dry conditions to evaluate wear rate and coefficient of friction (COF). The laser operating parameters like laser power, scanning speed and line spacing were optimized using multi-objective optimization to achieve minimum wear rate and COF. [ABSTRACT FROM AUTHOR]

The flexible wear-resistant coating on Ti6Al4V substrate was successfully fabricated by plasma powder cladding with Al–Si powder. The microstructure, element distribution and phase component of the flexible wear-resistant coating were characterized by the field emission scanning electron microscope (FESEM), energy dispersive spectroscopy (EDS) and the X-ray diffractometer (XRD). The micro-hardness of coating was measured by micro-hardness tester. The wear-resistance of Al–Si coating was evaluated by sliding friction experiment. The prepared coatings have no cracks and a good metallurgical bonding (MB) with the substrate. The wear-resistance of the coating surface is about twice as much as that of Ti6Al4V substrate. Furthermore, the in-between MB zone has the more excellent wear-resistance (about 7 times). The oxygen free zone can be formed in the MB zone, providing a good toughness for the flexible wear-resistant coating. The wear mode is adhesive wear for the flexible wear-resistant Al–Si coatings. Based on the self-consistent bond length difference (SCBLD) method, the competition among atoms in MB zone as well as the formation mechanism of flexible oxygen free zone is investigated.

Al–Si coated 22MnB5 steels are one of the most important lightweight materials for automotive weight reduction and safety improvement due to its advantages of light weight, high strength and good formability at high temperature. Considering the formation of α-ferrite caused by the Al–Si coating deteriorating the weld mechanical performance, the common way in the automotive industry is to remove the Al–Si coating prior to laser welding. In this work, a patented laser-filler wire welding process was developed with a high strength steel wire to inhibit the formation of α-ferrite and achieve high quality laser weld in Al–Si coated 22MnB5 without the requirement of removing the Al–Si coating. A formula for calculating the average content of Al element in the fusion zone was provided and verified by experiment. Experimental results show that Al segregation in the fusion zone with the designated filler wire was reduced, and the microstructure was full martensite with high dislocation density. The strain of the welded joint was concentrated in the hot-stamped base metal (HSBM) and fractured in an obvious necking during the tensile shear test. The tensile strength and elongation of the welded joint with filler wire increased by 7.5% and 183.9% compared with that without filler wire.

The clarification of the type of the ferrite formed owing to the melting of the Al–Si coating into the weld metal plays a significant role in the improvement of the mechanical properties of the laser welded joint. The results obtained by optical microscope (OM) and scanning electron microscope (SEM) in the regions near the ferrite before and after thermal simulation clearly demonstrate the negligible change in microstructure morphology, even for the ferrite with the size around 2 μm. In addition, simulated by JMatPro software, it can be found that the content of the α-ferrite decreases with the increase of the cooling rate. Based on the ultra-high cooling rate during laser welding and negligible change in the morphology of the ferrite by decreasing the cooling rate, the type of the ferrite formed in the WM is clarified as the δ-ferrite, without the formation of α-ferrite. The differences in the formation condition between the δ-ferrite and α-ferrite can affect the control of their size, morphology and distribution.

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.

Al–Si coated 22MnB5 steel is a lightweight material that helps significantly reduce vehicle weight and fuel consumption. In this article, the effect of different oscillation frequencies on the fraction of α-ferrite, the cross-sectional shape, microstructure and mechanical properties of laser welded Al–Si coated 22MnB5 steel was investigated. The results showed that after stamping, the fraction of α-ferrite in the fusion zone can be significantly reduced from 47.9% to 2.2% by oscillating laser beam welding compared to static laser beam welding with oscillation frequency and amplitude equal to zero. When the oscillating frequency was increased from 0 to 320 Hz, the cross-sectional shape of the weld changed from “X'' to “Y'', and the tensile strength and elongation of the welded joint was firstly increased and then decreased from the minimum value of (426.45HV, 1159.92 MPa, 1.14%) to the maximum value of (476.98HV, 1521.39 MPa and 2.76%). The formation of α-ferrite along the strip distribution near the upper surface of the fusion zone of static laser beam welding and oscillating laser beam welding after hot stamping, which led to stress concentration and eventual failure. Under the tensile-shear load, the cracks in the weld of static laser beam welding and oscillating laser beam welding were initiated on the massive α-ferrite location in the top surface of the fusion zone and expanded to the bottom of the weld.

Abstract Laser oscillating welding was employed to fabricate Al-Si coated press-hardened steel (PHS) to improve the element homogeneity in the fusion zone. Laser oscillating welding was employed with various oscillation amplitudes (0 mm, 0.5 mm and 1.3 mm) in this present. Ni foil of 0.06 mm thickness was used as an interlayer between two tailored PHS welded. The weld morphology, elemental profile, microstructure and tensile strength of welded joints were studied. The results showed that full penetration weld without any weld defects were achieved for any oscillation amplitudes, and weld width increased with increasing oscillation amplitudes. With the oscillation amplitudes increased, Ni and Al had an uneven elemental profile due to strong stirring force, but the Ni and Al content in the weld was decreased and Ni had a sharp descent compared to Al element. Only few δ-ferrite was presented in fusion line with the oscillation amplitudes increased to 1.3 mm. The oscillation amplitudes did not have an effect on the tensile properties, which was similar to that of base metal. But if keeping increasing the oscillation amplitudes or reducing the thickness of Ni interlayer, it has a potential risk to form more and more δ ferrite such that deteriorate the mechnical properties of welded joints.

22MnB5 hot forming steel is widely used in the automotive industry due to the increasing demand for lightweight vehicles. As surface oxidation and decarburization occur during hot stamping processes, an Al-Si coating is often precoated on surfaces. The coating tends to melt into the melt pool during the laser welding of the matrix and reduce the strength of the welded joint; therefore, it should be removed. The decoating process by sub-nanosecond and picosecond lasers and process parameter optimization were conducted in this paper. The corresponding analysis of the different decoating processes, the mechanical properties and the elemental distribution was carried out after laser welding and heat treatment. It was found that the Al element has an influence on the strength and elongation of the welded joint. The high-power picosecond laser has a better removal effect than the lower power sub-nanosecond laser. The best mechanical properties of the welded joint were obtained under the process conditions of 1064 nm center wavelength, 15 kW power, 100 kHz frequency, and 0.1 m/s speed. In addition, the content of the coating metal elements (mainly Al) melted into the welded joint is reduced with increasing coating removal width, which significantly improves the mechanical properties of the welded joints. Al in the coating rarely melts into the welding pool when the coating removal width is not less than 0.4 mm, and its mechanical properties can meet the automotive stamping requirements for the welded plate. [ABSTRACT FROM AUTHOR]

22MnB5 steel can be used in light-weighting applications by down gaging. In order to prevent oxidation of steel sheets at high temperatures, Al-Si coatings are usually prefabricated on their surfaces. The Al-Si coating severely deteriorates the mechanical properties of the joint. In this paper, 22MnB5 steel with different Al-Si coating removal approaches was laser welded. The effects of the coating distribution on microstructure, chemical composition, Al distribution and mechanical properties of welded joint were investigated. The results showed that the microstructure was δ-ferrite when the Al content in the fusion zone (FZ) ranged between 1.99 ~ 3.22 wt.%. When Al content in the FZ was 0.08 ~ 1.21 wt.%, the microstructure was martensite. After hot stamping, Al element segregation was always found near the fusion line of the welds, which were obtained in coated, lower surface-decoated and upper surface-decoated conditions. The decreasing level of hardness value of the FZ was present from large to small: coating (420 HV), lower surface-decoated coating (440 HV), upper surface-decoated coating (485 HV), all surface-decoated coating (497 HV). The tensile strength and the elongation of the coated (1459.7 MPa, 2.0%) and lower surface-decoated (1461.3 MPa, 1.9%) welded joint were severely reduced. The tensile strength and elongation of the upper surface-decoated welded joint were 1597.7 MPa and 3.6%, respectively, reaching 99.7 and 52.1% of the HSBM (1603.2 MPa, 6.9%). The long strip of α-ferrite was the main cause of stress concentration and failure of the welded joint during tensile process. The tensile strength and the elongation of all surface-decoated welded joint were 1598.4 MPa, and 4.1%, respectively, reaching 99.7 and 59.4% of the HSBM. [ABSTRACT FROM AUTHOR]

The non-uniform mixture microstructure is always revealed in laser welds of Al–Si coated hot stamping steel. It is very necessary to understand aluminum-rich segregation on phase morphology evolution. At same time the uniform and high aluminum welds were tried to obtain by replacing keyhole mode with conduction mode. The uniform fusion zones contained 3–6 wt.% Al, higher than normal keyhole welds. The weld microstructure was mixed of delta ferrite, lath martensite and few discussed needle phases with different morphology. Lower Al region revealed high fraction of island martensite, when aluminum content w(Al) > 5 wt.%, the needle phase was precipitated from the matrix of delta ferrite. High resolution transmission electron microscopy confirmed it long periodical stacking ordered structure in body-cubic crystal matrix.

• More compact coating surface by Mg and Sr segregation with enhanced Fe-Al reactions at inner coating matrix. • Segregated Mg and Sr formed oxides at outer-most surface and replaced Al oxides. • Enhanced hydrogen-uptake resistance of Al-Si-Mg-(Sr) coating compared to reference Al-Si coating, during hot-press-forming. This study explores the hydrogen permeation resistance of Al-Si-Mg and Al-Si-Mg-Sr coated 22MnB5 boron steel in hot-press-forming. The Al-Si-Mg coating exhibits structural changes and Mg segregation, replacing the oxide layer. Sr addition is found segregated at the surface, further transforming the oxide layer. These modifications improve hydrogen permeation resistance, reducing both reversible and irreversible hydrogen permeation during hot-press-forming. Mg or Sr addition presents promising avenues for enhancing hydrogen permeation resistance in steel, potentially extending the service life of ultra-high-strength steel. [ABSTRACT FROM AUTHOR]

Under the condition of long-time high-temperature oxidation, oxidation in Al-Si coating will lead to degradation of the coating. To solve this problem, the Al-Si coating was treated before the oxidation experiment at different pre-oxidation temperatures and times. The structure, morphology and element distribution of the oxide film were characterized by XRD, SEM and EPMA, and the oxidation kinetics curves were drawn. The results show that the oxidation resistance of the pre-oxidized samples is improved in the process of constant temperature oxidation at 1000 °C. When the pre-oxidation temperature is set at 950 °C, the mass gain of the samples is the lowest, and the accelerated decomposition of Cr2O3 caused by high pre-oxidation temperature is avoided. After oxidation for 300 h, the oxide film on the surface is still continuous and dense without peeling. The samples pre-oxidized at 950 °C for 7 h show the best oxidation resistance in a 1000 °C constant temperature oxidation process, more α-Al2O3 oxides are generated on the coating surface and the element diffusion between the matrix and the coating caused by overly long oxidation time is avoided. [ABSTRACT FROM AUTHOR]

The effect of diffusion heat treatment (DHT) on the cracking of the coating layer in Al-Si coated hot press forming (HPF) steel was investigated. Cracking-induced micro-delamination in the coating layer has limited applications of the Al-Si coated HPF steels. Introduction of DHT process can modify the microstructure and constituent phase in the coating layer which can reduce the cracking. DHT was performed at 650 °C for 360 min to drive Fe diffusion into the coating layer. Formation of band-shaped τ1-Fe3Al2Si3 in the coating layer during the DHT was observed by EBSD and SEM. After HPF process of the diffusion heat treated coating specimen, the phase fraction of FeAl increased and that of η-Fe2Al5 decreased compared to the conventional Al-Si coating specimen, due to the increase of τ1-Fe3Al2Si3. Owing to the higher fracture toughness of FeAl compared to η-Fe2Al5, application of DHT improved the toughness of the coating layer and reduced the crack density in it. DHT did not significantly affect the mechanical properties of the base steel. [ABSTRACT FROM AUTHOR]

Laser oscillating welding was employed to fabricate Al-Si coated press-hardened steel (PHS) to improve the element homogeneity in the fusion zone. Laser oscillating welding was employed with various oscillation amplitudes (0 mm, 0.5 mm and 1.3 mm) in this present. Ni foil of 0.06 mm thickness was used as an interlayer between two tailored PHS welded. The weld morphology, elemental profile, microstructure and tensile strength of welded joints were studied. The results showed that full penetration weld without any weld defects were achieved for any oscillation amplitudes, and weld width increased with increasing oscillation amplitudes. With the oscillation amplitudes increased, Ni and Al had an uneven elemental profile due to strong stirring force, but the Ni and Al content in the weld was decreased and Ni had a sharp descent compared to Al element. Only few δ-ferrite was presented in fusion line with the oscillation amplitudes increased to 1.3 mm. The oscillation amplitudes did not have an effect on the tensile properties, which was similar to that of base metal. But if keeping increasing the oscillation amplitudes or reducing the thickness of Ni interlayer, it has a potential risk to form more and more δ ferrite such that deteriorate the mechnical properties of welded joints. [ABSTRACT FROM AUTHOR]

Press-hardened steel (PHS) is widely applied to fabricate vehicle body structures for attaining mass reduction and fuel economy without sacrificing occupant safety. The VDA bendability test is often used to characterize the fracture resistance of PHS under plane-strain bending conditions. As lightweighting continues to be a design imperative in the automotive industry, it is desirable to develop and adopt more press-hardened components with higher fracture resistance. In this work, four Al–Si-coated 22MnB5 steels with various initial thicknesses and coating weights were studied. A newly developed methodology was used to calculate the fracture limit strain under plane-strain bending. The results indicate that although the four investigated 22MnB5 steels exhibit similar tensile properties under uniaxial tension, their bending performance per the VDA 238-100 standard differs significantly. The PHS with a low coating weight possesses a higher bending angle and, hence, a larger fracture limit strain. Meanwhile, the peak bending force can be 10% higher than the PHS with a standard coating weight at the same sheet thickness. Therefore, it is expected that PHS with higher fracture strain will have the potential for lightweighting due to its enhanced resistance to fracture and higher energy absorption capability. [ABSTRACT FROM AUTHOR]

The paper presents the results of laboratory and industrial tests on the role of Al-10%Si coatings in the manufacture and use of welded aluminized steel tubes. The tubes were fabricated from DX53D + AS120 steel tubes coated with Al-10%Si coating. Investigations were carried out on individual processes in the manufacture of welded tubes aimed at determining the effect of the coating properties on the conditions of the forming process and vice versa. In the next step, a quality assessment has been conducted on the finished tubes. Then, selected tests simulating the operating conditions of tubes used for the elements of exhaust systems are presented. Analyses of the susceptibility of strips to plastic deformation and evaluation of the adhesion of the Al-10%Si coating to the steel base metal were carried out using bending tests. From the results, it was proved that the Al-10%Si coating determines the technical features of the workpiece material, affects the manufacturing process, and determines the quality of aluminized steel tubes. The quality of the coating surface depended on the annealing temperature and annealing time. The higher the temperature, the shorter the time needed to produce a change in the coating properties. A p-value reported from a tests is less than 0.05. [ABSTRACT FROM AUTHOR]

For the automotive industry, it is important to predict the fracture mode of resistance spot-welded joints. Traditional 4t0.5 (where t is the sheet thickness) criterion for the transition from interfacial fracture (IF) to button pullout fracture (BPF) mode is not applicable for press hardened steel, press-hardened steel (PHS), welds. This study aims to investigate the effect of Al-Si coating and critical heat affected zone (CHAZ) location on the lap-shear fracture mode transition mechanism of Al-Si-coated PHS welds. It was found that in-situ tempering pulse after the welding stage could change the relative location of the CHAZ and weld nugget, which in turn, had a significant effect upon the fracture mode transition. Thus, a new analytical model was built to predict the critical nugget size for Al-Si-coated PHS, wherein the Al-Si coating and the CHAZ location are considered as critical factors for predicting the fracture mode transition of PHS spot welds and are incorporated into this model. A reasonable correlation of the model to experimental data was achieved. [ABSTRACT FROM AUTHOR]

The hot-stamped steel with ultrahigh strength is a promising material for the fabrication of automotive components. However, the coating on the sheet surface leads to a softening problem in the welded joint. Instead of the costly coating removal process, heat treatment is an economical and effective method for the diffusion process, which can decrease the Al concentration in the coating. In this study, a preheating treatment was carried out on Al-Si-coated 22MnB5 hot-stamped steels for the homogeneity of Al, followed by laser welding and hot stamping. The effects of the preheating on the microstructure and mechanical properties of the laser-welded joints were investigated. With the preheating treatment, the Al-Si coating transformed into an Fe-Al intermetallic compound and the difference in Al content between the coating and substrate was reduced. The Al content in the weld of the specimen with the preheating treatment was reduced, compared with that without the preheating treatment. The amount of δ-ferrite in the weld after laser welding was reduced largely. The distribution of long-bland-like segregation was changed to a fine and uniform distribution. With the preheating treatment, the tensile strength of the welded joint was significantly improved and comparable to that of the decoated joint. In conclusion, the preheating treatment before the welding is an effective method to suppress the formation of δ-ferrite and improve the mechanical properties of the welded joint. [ABSTRACT FROM AUTHOR]

Laser welding of Al-Si coated press-hardened steel (PHS) was still a challenging due to the formation of δ-ferrite in the fusion zone. However, it was ambiguous on the δ-ferrite evolution during post-weld heat treatment (PWHT). In this study, Al-Si coated PHS was welded by laser fiber and then heat-treated at various temperatures (650 °C, 950 °C and 1050 °C), and the microstructure evolution was investigated by in situ-observation. Mechanical properties and formability of welded joints was analyzed. The results showed that δ-ferrite did not go through phase transformation during PWHT regardless of any temperature, but with a grain boundary flatted and movement. The presence of coarse δ-ferrite after PWHT was the responsible for the decreasing of tensile strength and formability of welded joints. The initiation crack was easy to generate in the phase interface between δ-ferrite and martensite to form a local brittle fracture zone finally. This study results provides a new theoretical criterion for the properties improvement on the laser welding of PHS without removing Al-Si coating.

The coating on the 22MnB5 steel is intended to protect it against oxidation during the forming process. This steel is hot-pressed. A preheating before the pressing and subsequent hardening in the tool affects the properties of the AlSi coating. This study summarizes the results of investigating the effect of heat treatment parameters on the formation of intermetallics in the AlSi coating. The chemical analysis of the coating was performed by the EDX and EBSD method and the mechanical properties were determined by the Hysitron TI 950 TriboIndenter™ system. The result of this study is that, due to a diffusion during the heat treatment, the brittle coating was transformed into a tougher phase.

A ferrite layer is formed between the outer intermetallics layers and the martensite substrate during the hot stamping process of Al-Si coated press-hardened steel (PHS). Unexpectedly, it is found here that the ferrite layer does not effectively prevent brittle cracks propagating continuously from the intermetallics to the martensite substrate. This leads to a high stress intensity factor (SIF) at the crack tip, therefore initiating highly localized shear deformation in the martensite substrate, degrading the bendability of the steel. A thinner Al-Si coating produces thinner brittle intermetallics and ferrite layers, and therefore shorter coating cracks and smaller SIF at the crack tip, thus improving the bendability. In addition, a thinner Al-Si coating also has a lower material cost while keeping similar paintability and corrosion resistance. The thin Al-Si coating could potentially change the current practice of Al-Si coating, impacting the global automotive industry. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Laser spot welding method was employed to fabricate AlSi coated press-hardened steel (PHS) joints in overlapped configuration. Al and Si segregate at the notch root of a laser spot weld of AlSi coated PHS resulting in pre-mature cracking and weakening of the joint strength. The thickness-oriented δ-ferrite strips induced by the Al and Si segregation was mainly responsible for the mechanical deterioration though the dispersive δ-ferrite in the fusion zone also contributes. A supplemental annular stirring with beam oscillation is proposed to break up the segregation. With the annular stirring, the lap-shear peak load and energy absorption increased 57% and 80% respectively, which is attributed to the removal of δ-ferrite strips and the reduction in the fraction of dispersive δ-ferrite. Thermo-fluid numerical simulations suggest that the annular stirring promoted the melt flow and extended the duration of the melt pool, which accelerated and prolonged the diffusion of Al and Si atoms into the fusion zone, thus preventing the segregation of Al and Si and formation of δ-ferrite strips. Meanwhile, the prolonged melt flow pulled additional dissolved Al onto the molten pool surface forming the Al oxide film, resulting in further reduction of Al content and fraction of dispersive δ-ferrite.

An aluminide–silicide coating was prepared on a pure tungsten substrate by halide-activated pack cementation. Thickness, morphology, and phase structure of the Al–Si coating were systematically investigated. Thermal cyclic oxidation tests of the Al–Si coating were carried out at 1,000°C. The results revealed that the aluminide and silicide coating possessed relatively high cyclic oxidation resistance against high temperature. After oxidation, dense SiO2 and Al2O3 were formed in the coating. Oxidation rate of the Al–Si coating was lower than that of the pure tungsten.

In the automotive industry, the demand for ultra-high-strength steel is increasing due to the CO2 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. [ABSTRACT FROM AUTHOR]

Resistance spot welding (RSW) of Al-Si-coated PHS was undertaken, and the effect of Al-Si coating on nugget formation and mechanical properties was investigated. Press-hardened steel (PHS) has long been applied to automotive body structure construction to support mass and corresponding greenhouse gas emission reductions. PHS materials are often combined with an Al-Si coating applied as an oxidation barrier though unfortunately, the Al-Si coating poses a challenge to the resistance spot welding (RSW) of PHS containing stack-ups. As a newly developed coating for the hot stamping process, the property of Al-Si coating is different from base metal and traditional coatings, and the influence mechanism of Al-Si coating on the welding process is not clear. It would remain at nugget edge and might cause severe stress concentration. To investigate this problem, RSW of 1.5-mm Al-Si-coated PHS was undertaken, and the results indicate that a large portion of the Al-Si coating is extruded and forms a sharp notch close to the nugget edge during the welding process. During the post-weld cooling stage, a thin layer of residual coating is formed between the nugget and notch root. The mechanical performance of the welded joints is limited by the thin residual Al-Si layer which acts as a preexisting crack and supports the interfacial fracture. The presence of the Al-Si coating at the faying interface also significantly delays nugget formation, though it contributes to a larger nugget size by inhibiting expulsion events at the faying interface. [ABSTRACT FROM AUTHOR]

This paper investigated the hot corrosion behavior of two types of coatings, namely NiCoCrAlYHfSi and Al Si coatings, used in gas turbines operating in a moderate-temperature sea salt environment. Prior to the corrosion experiment, pre-oxidation treatment was employed to generate a continuous α-Al 2 O 3 thin layer on the surface of the coatings, leading to a significant reduction in coating roughness. The results demonstrated distinct failure behaviors of the two coatings in the presence of sea salt, providing insights into the mechanism of hot corrosion. The NiCoCrAlYHfSi coating effectively resisted corrosion through a mixed oxide layer consisting of Al 2 O 3 and Cr 2 O 3 , while the Al Si coating utilized the continuous formation of Al 2 O 3 to combat corrosion. In comparison to the NiCoCrAlYHfSi coating, the Al Si coating exhibited superior resistance to sea salt erosion. In addition, pre-oxidation is able to effectively reduce the erosion of the coating by sea salt, thereby extending the lifespan of the NiCoCrAlYHfSi coating and the Al Si coating by 40 % and 30 % respectively. • The hot corrosion mechanism of NiCoCrAlYHfSi coating and Al Si coating was explored. • Pre-oxidation could reduce the coating roughness and prolong the coatings lifetime. • NiCoCrAlYHfSi coating and Al Si coating exhibit different failure modes in sea salt. • Al Si coating exhibit more resistance to sea salt erosion than NiCoCrAlYHfSi coating. [ABSTRACT FROM AUTHOR]

Al-Si is the most popular coating used to prevent oxidation on the surfaces of hot-stamped steel sheets during the heating process. However, like other coatings, it affects the strength of the spot welds in joining the hot-stamped steel parts. In this study, the effects of Al-Si coating on the tensile strength of the resistance spot-welded joints in hot-stamped steel are discussed. Two types of 1.8 mm hot-stamped steel, in uncoated and Al-Si coated forms, were resistance spot-welded, and the tensile shear behavior of the welded joints was studied in both static and dynamic tests. To do this, a special fixture for impact tensile shear tests was designed and fabricated. In the case of the Al-Si coated steel, the presence of the molten Al-Si over the fusion zone, especially its aggregation in the edge of the weld nugget, caused a decrease in the maximum tensile load capacity and a failure of energy absorption in static and dynamic tests, respectively. Additionally, it increased the probability of changing its failure mode from pull out to interfacial fracture in the dynamic test. This study shows that the tensile strength behavior of the welded joints for the Al-Si coated hot-stamped steel is lower than the uncoated steel during static, and especially dynamic, force.

Boron steels are used in hot stamping process due to their good mechanical properties. During the stamping process, the dies are exposed to aggressive conditions including adhesive wear, abrasion, thermal stresses and fatigue. In the present work, QRO 90 and UNIMAX slid against 22MnB5 steel in four conditions: with and without hardening treatment and, with and without Al–10%Si coating, in order to evaluate the influence of both coating and austenitization treatment on friction and wear of tool steels. The results showed that Al–10%Si reduces the friction coefficient, while the hardening treatment results in an increase of COF due to Fe2Al5 brittle compounds. Wear mechanism of both tool steels is adhesive and oxidative when tested against coated and uncoated 22MnB5, respectively.

Inconel 718 suffers from very poor machinability which limits its application and causes very high machining costs. Therefore, maximizing the tool performance is of prime importance for improving its machinability and reducing the high costs. Physical vapor deposition (PVD), of coatings on the tool material, is the common method, suggested by many studies, to improve the tool performance during cutting of difficult-to-cut materials. Previously, authors have introduced an easy and cost-effective process for depositing the target material on the tool as an alternative for PVD coatings. In the novel proposed process, referred to by the authors as the "pre-machining" method, the material is deposited on the tool surface during a few seconds of machining of the target workpiece. In the current study, the performance of the new proposed technique is compared with the conventional PVD method. The target material used for deposition is a very soft material (Al-Si) which itself has not been used before for coatings used for difficult-to-cut materials. Thus, for this study, a single layer of soft Al-Si material is deposited on the tool through two different techniques; a typical PVD technique under a high vacuum environment in a coater chamfer and the "pre-machining" technique. The comprehensive machinability analysis performed on the tools showed that application of Al-Si as the target materials through both techniques is very beneficial for the cutting tools used for cutting Inconel 718. Moreover, results showed that the performance of the new, easy to perform, and cost-effective deposition technique is very similar to the performance of conventional PVD coatings and even outperforms a PVD coating in some respects. The results showed that the tool life of the Al-Si-coated tool with 1- and 0.7-μm thicknesses and Al-Si pre-machined tool are significantly higher than that of the uncoated tool by 321%, 231%, and 205% respectively. They also outperform the conventional TiAlN hard coating used for machining of this class of material. The study also demonstrated that the cutting force decreased significantly by using Al-Si-coated and pre-machined tools. Furthermore, chipping, abrasion wear, adhesion, BUE formation, and machining-induced work hardening of the workpiece material reduced significantly by using the Al-Si pre-machined/coated tools. Investigation of the chips produced during the machining process and coefficient of friction measurements also confirmed the friction reduction and better flow of the material at the tool-chip interface as a result of Al-Si deposition with both techniques. [ABSTRACT FROM AUTHOR]

Press-hardened steel (PHS), used for automotive safety-related structure parts, is sensitive to hydrogen embrittlement due to its martensitic microstructure. Hydrogen is introduced in PHS during the hot press forming (HPF) process, by an atmospheric corrosion process. In this study, the hydrogen embrittlement behavior of uncoated, aluminized, and galvanized PHSs was investigated. The Al-10%Si coating promoted the absorption of diffusible hydrogen at elevated temperature during the HPF while the reacted coating layer prevented the absorbed hydrogen from out-diffusing through the reacted coating surface layer at room temperature. Therefore, the aluminized PHS showed a greater sensitivity to both the hydrogen uptake and the resultant embrittlement, as compared to the uncoated and galvanized PHSs. Use of galvanized PHS for HPF application reduces the risk of hydrogen embrittlement, since the Zn coating effectively prevents the hydrogen uptake. The greater embrittlement resistance of the galvanized PHS is possibly due to the inhibition of the hydrogen generation reaction by the surface ZnO oxide layer and the low rate of hydrogen transport through the liquid Zn phase. • Aluminized press hardened steel is most susceptible to the H uptake and embrittlement. • The H-induced loss of the plasticity is more pronounced at low strain rates. • H atoms absorbed in the coated steel are trapped mostly in the martensitic matrix. • Zn coating effectively prevents the H uptake during hot press forming. • Coating-type dependent H absorption and desorption mechanisms are proposed. [ABSTRACT FROM AUTHOR]

• Al-Si coating affects the microstructure of 22MnB5 steel fiber laser welded joints. • In fatigue test, strain localization occurs at the martensite/martensite interfaces. • Coated welded joints performs lower anti-deformation capacity during Erichsen tests. Fiber laser welding of as-received 22MnB5 steel with Al-10wt.%Si coating in butt configuration was performed. The influence of Al-Si coating on microstructure transformation and mechanical properties including tensile tests, fatigue tests and Erichsen cupping tests was investigated. Ferrite with Al enrichment (1.37 wt%) was found in the fusion zone (FZ) of coated welded joints with 15% area proportion, while nearly entire martensite structure composed the FZ in the case of de-coated joint. The strain localization was more likely to accumulate at the martensite/martensite interface rather than the ferrite/martensite interface due to the higher kernel average misorientation (KAM) values. Microhardness of the FZ in coated welded joints dropped by 50HV compared with that of de-coated welded joints. However, tensile properties were not affected since failure occurred at base metal (BM). Fatigue samples of coated welded joints showed better performance than the de-coated ones, suggesting that the ferrite in the FZ had positive effect on fatigue lives under cycling load conditions. The Erichsen cupping tests showed similar Erichsen values (5.60 mm and 5.67 mm) for de-coated and coated welded joints. An opposite effect of ferrite on Erichsen cupping tests was obtained. The peak force required for deformation dropped by 35% with Al-10wt.%Si coating. [ABSTRACT FROM AUTHOR]

Currently, continuous implementation of safety and CO2 emission standards in automotive vehicles has driven the search for weight reduction and safety enhancement solutions. In this context, the development of high mechanical strength in steels has been highlight as the most applicable. Those pieces have complex geometric and require greater mechanical strength, thus they are produced by hot stamping. The selected steels for this application have addition of boron, such as 22 MnB5. In parallel, researches related to the development of metallic coatings capable of withstanding mechanical stresses and heat treatment have been carried out. The Al-Si coating composed of Al-Si-Fe multiphases has been strongly used, because during the austenitization occurs to the formation of new phases by the inter-diffusion of Fe and solidification reactions defining the final microstructure. Samples of coated 22MnB5 steel (Al-Si) were treated between temperatures of 500°C to 900 °C. Through the analysis of microscopy (SEM) and GDOES it was possible to identify the diffusion of Fe along the coating layer. Based on the ternary phase diagram and the simulations, it was possible to propose a way of understanding the transformation of the Al-Si coating by the diffusion of the iron. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • δ-Ferrite in welded seam and fusion line was successfully suppressed via inserting Ni interlayer. • Full martensite was obtained in welded seam and fusion line of Al-Si coated PHS with Ni interlayer. • Al content had a similar content in welded seam and fusion line without segregation. • Tensile strength of welded joint with Ni interlayer was similar to that of removing Al-Si coating. Abstract Mechanical properties of Al-Si coated press-hardened steel laser welded joint can not meet requirement due to the presence of δ -ferrite. This study investigated the suppression mechanism of δ -ferrite using a nickel interlayer during fiber laser welding. The fraction of primary solidification δ -ferrite was reduced to 57% with a 5.85% Ni in welding pool, and completely transformed into austenite during peritectic reaction (L + δ → γ) and solid phase transformation (δ → γ). Full martensite microstructure was obtained in welded seam and fusion line without Al-segregation. Tensile strength of welded joint had a ∼17.1% improvement compared to as-received welded, similar to that of removing Al-Si coating. [ABSTRACT FROM AUTHOR]

Resistance nut projection weldability of Al-Si coated hot stamped steel (HSS) was investigated under the viewpoint of weldable current range and joint strength (pull-out load). The microstructural inhomogeneities in the welds were also studied in order to elucidate the factors affecting the joint strength of the welds. The weldability of the given Al-Si coated HSS was compared with the weldability of an identical HSS without the Al-Si coating (Al-Si coating was polished out) and Zn coated dual phase steel. The weldable current range of Al-Si coated HSS was found to be narrower than that of the other materials. Furthermore, the average pull-out load within the weldable current range of the Al-Si coated HSS was the lowest among the three materials. The reason for poor weld mechanical property of the Al-Si coated hot-stamped steel was attributed to the microstructural inhomogeneities such as unmixed Al-Si coating layer at the edge of the nugget and the second phase Fe3(Al, Si) intermetallic compound. The formation of Fe3(Al, Si) phase was attributed to the solidification segregation of Al and Si during the weld solidification and was confirmed with the numerical analysis of solidification segregation. [ABSTRACT FROM AUTHOR]

The effect of furnace heating rate on the intermetallic growth of Al Si coatings for 22MnB5 hot stamping steel was characterized and quantified for two different commercial coating weights, AS80 and AS150. Characterizing and quantifying the formation of the Al Fe and Al-Fe-Si intermetallic layers is crucial to the optimization of the austenitization process, which continually strives to reduce furnace time while maintaining acceptable intermetallic development. Initial nominal heating rates up to the Al Si melting temperature (577 °C) varied from Low (7 °C/s) to Very High (23 °C/s) and then proceed to reduce which resulted in the nominal heating rates to vary from Low (1 °C/s) to Very High (7 °C/s) within a temperature range of 600 to 900 °C. The coating composition was similar at 600 °C for all the heating rate tests and suggests that there is potential to save furnace time by imposing high initial heating rates. SEM and EDS analysis was conducted on the coating at 50 °C intervals. The overall intermetallic transformation occurred significantly faster at the higher heating rates due to the accelerated diffusion of Fe into the coating. The intermetallic transformation also occurred faster for the thinner AS80 coating due to the shorter diffusion path of Fe. A simple empirical model was developed to predict the coating transformation percentage for any temperature and time from 600 °C. The formation of Fe Al and Fe-Al-Si intermetallics was characterized. For each coating weight, the evolution of each intermetallic species was consistent for each heating rate, but at a delayed rate for each subsequently higher heating rate. Additionally, the tests conducted in this work showed that full austenitization of the base metal can be achieved during the heating phase (72 s for Very High heating rate). • The coating composition was similar at 600 °C for all the heating rate tests. • The overall intermetallic transformation occurred faster at the higher heating rates. • The intermetallic transformation occurred faster for the thinner AS80 coating. • The evolution of each phase was consistent for each heating rate, but at a delayed rate for higher heating rate. • Full austenitization of the base metal can be achieved during the heating phase. [ABSTRACT FROM AUTHOR]