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Aluminum-silicon-based and zinc-based metallic coatings have been widely used for hot-stamped boron steel in automotive applications. In this study, resistance spot weldability was explored by investigating the effects of the properties of metallic coating layers on heat development and nugget growth during resistance spot welding. In the case of t...
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... 11,13 However, there exists a lack of understanding about the optimised condition for H-RSW. 11,[14][15][16] The existing studies neither consider parameter optimisation for H-RSW, nor the effect of welding parameters on thermal cycle, which is critical to LME cracking during RSW. Leading to the impact of process parameter combinations on the thermal cycle remaining unexplored. ...
Half-section welding has been proposed as a method to observe nugget growth throughout the resistance spot welding process. Some past studies used this technique as an in-situ monitoring approach to analyse the cracking behaviour during welding. However, there is a lack of understanding regarding the half-sectioned welding parameters, which critically impacts the liquid metal embrittlement cracking index. This study optimised half-sectioned welding parameters for liquid metal embrittlement crack analysis by developing process maps that compare the half-sectioned and full-section processes. It was found that the liquid metal embrittlement crack located at the weld shoulder can correlate to the temperature gradient in the heat-affected zone. This study proposes half-sectioned welding as a viable technique for liquid metal embrittlement analysis.
... Besides, the Aluminum Silicon (Al-Si) coating, utilized to prevent surface oxidation during the hot stamping process, would remain at the nugget edge and cause stress concentration after the welding process. It would diminish the mechanical performance of PHS welded joints [8][9][10]. ...
Press-hardened steel (PHS) with extremely high strength has wide applications in vehicle body manufacturing as an innovative lightweight material. However, the poor weldability of PHS results in poor weld toughness and a high risk of interfacial fracture, posing challenges to the resistance spot welding (RSW) process. Introducing an external magnetic field in the welding process to perform electromagnetic stirring (EMS), magnetically assisted RSW (MA-RSW) process has been proven an effective method to improve the weld toughness of high-strength steel, but it may increase the risk of expulsion. In this study, a new process called SPMA-RSW is developed to improve the weldability of PHS by combining MA-RSW and the stepped-current pulses (SP) technique, which can enlarge the weld lobe. Nugget appearance, microstructure, microhardness, and mechanical properties were systematically investigated by comparing traditional RSW, MA-RSW, SP-RSW, and SPMA-RSW. The result showed that the SPMA-RSW process would significantly increase the nugget size, inhibit the shrinkage voids, finer the grain, and harden the nugget. This increased the lap-shear strength, energy absorption, and changed the fracture mode from brittle interfacial (IF) mode to ductile plug fracture (PF) mode at the same heat input. Then, a simple model was developed to reveal the mechanism of the effect of EMS on the fracture mode transition and was verified by experiment. This work can help improve the weld quality and thermal efficiency of the RSW process for PHS.
... Besides, the Aluminum Silicon (Al-Si) coating, used to prevent surface oxidation in the hot stamping process, would remain at the nugget edge and cause stress concentration after the welding process. It would lower the mechanical performance of PHS welded joints [7][8][9]. ...
Press-hardened steel (PHS) with extremely high strength has wide applications in vehicle body manufacturing as an innovative lightweight material. However, the poor weldability of PHS results in low weld toughness and a high risk of interfacial fracture, posing significant challenges to the resistance spot welding (RSW) process. Magnetically assisted RSW (MA-RSW) process, which introduces an external magnetic field in the welding process to perform electromagnetic stirring, has been shown to effectively improve the weld toughness for high-strength steel, although it may increase the risk of expulsion. To further improve the weldability of PHS, this study proposes a new process called stepped-current pulses magnetically assisted resistance spot welding (SPMA-RSW), which combines the MA-RSW with the stepped-current pulses (SP) method to enlarge the weld lobe. The study systematically investigates nugget appearance, microstructure, microhardness, and mechanical properties by comparing traditional RSW, MA-RSW and SPMA-RSW. The result showed that the SPMA-RSW process would significantly increase the nugget size, inhibit the shrinkage voids, refine the grain size and harden the nugget region of PHS welds, resulting in increased lap-shear strength and a transition from a brittle interfacial (IF) mode to a ductile button pullout fracture (BPF) mode at the same heat input. Specifically, the peak load and energy absorption were increased by 32.3% and 84.2%, respectively. This work can help improve the weld quality and thermal efficiency of the RSW process for PHS.
... The existence of zinc coating further worsens the weldability of UHSSs. For example, Ji et al. [2] found that there was no weldable current range for the zinc-coated hot-stamped boron steel, but a 2 kA weldable current range was observed for the aluminum-silicon-coated hot-stamped boron steel. ...
The resistance spot weldability of galvanized ultra-high-strength steels is not satisfied, the joints are prone to interfacial fracture and the weldable current range is narrow. To solve the problems, a novel method called resistance spot welding with double-sided cover sheets was introduced to weld a galvanized Q&P980 steel with the thickness of 1.2 mm. Two thin SPCC mild steel sheets were chosen as cover sheets and were placed symmetrically at both sides between the Q&P980 steels and the electrodes, then the RSW process was carried out. Compared with the traditional RSW method, the joints obtained by using the novel method achieved larger tensile shear strength and energy absorption, which increased by 26.9% and 52.6%, respectively. With increasing the welding current, the failure mode transferred from interfacial fracture to nugget pull-out fracture or base metal tearing fracture. By contrast, the joints always showed interfacial fracture without cover sheets. The improvement of the joint performance was mainly attributed to the enlargement of the nugget. With the help of finite element simulation, it was found that the cover sheets helped increase the contact area and reduced the current density during welding, which postponed the expulsion, and a larger area could be evenly heated. The application of the novel method can be easily extended to the resistance spot welding of other ultra-high-strength steels with various thicknesses.
... The weld process window, also known as suitable current range, was between the current at which minimum FZ diameter are equal to 5 t (where t is specimen thickness) and expulsion. 11) The TS strength and CT strength were measured using a CMT5105-SANS machine at speed of 2 mm/min at room temperature. ...
The quality of spot welds between galvanized dual phase steels of 590 MPa (DP590Z) and Al–Si coated press-hardening steels (PHS) of 22MnB5 (PHS1500AS) are determined by welding metallurgy of both base metal and the coating. In this study, extending the dwelling time between two pulses is proposed to suppress splash in the broad process window of welding current and improve mechanical properties of spot welds. The increased welding current enlarges the fusion zone (FZ) size and consequently enhances the strength of welds in both shear and cross tensile tests. Furthermore, martensite with high carbon content and retained austenite near the fusion line was found for the first time in the spot welds in these kinds of steels. The high carbon zone alters the location of broken button and deteriorates the mechanical properties of spot welds. Down-slope pulse is proposed in this study to eliminate carbon enrichment, which improved the mechanical properties of the welds.
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... Scholars have done more studies on the weldability of these two steels [31][32][33][34][35][36][37][38][39]. For example, Jianjiang et al. [31] studied the microstructure and mechanical properties of laser-welded joints of DP980 steel with three heat inputs. ...
... Wu et al. [33] improved the strength of DP980 steel pulse arc welding joints and reduced the softening degree of heat-affected zone by using the method of post-weld heat treatment. Lin et al. [34] and Gerhards et al. [35] studied the effect of coating on laser welding of 22MnB5 steel before and after hot forming. After hot forming, the ferrite content in the weld was increased, and the lath martensite became smaller. ...
Hot-formed performances of laser tailor-welded blanks fabricated with 22MnB5 (one hot-formed steel) and DP980 (one dual-phase steel) are investigated. Tensile tests, microscopic analysis, and hardness tests are used to evaluate the thermoforming properties after laser stitching. The results show that the tensile strength of the thermoformed samples was increased by 75%, and the elongation at break was decreased by 50%. The sample without thermoforming fractured on the 22MnB5 side, while the thermoformed sample fractured on the DP980 side. Moreover, the thermoformed DP980 side showed the disappearance of the sub-critical HAZ (heat-affected zone) with an average hardness increase of 42%, and the inter-critical HAZ still presents. The microstructure of the subcritical HAZ and the critical HAZ on the DP980 side transformed into martensite, bainite, ferrite, and less residual austenite. Therefore, the tissue is still softened. In contrast, the microstructure of the fine-grained HAZ and coarse-grained HAZ on the DP980 side consists of hardened zones of slate-martensite. Thanks to the gradual transformation of the microstructure of 22MnB5 from pearlite and ferrite to slate-martensite, the microhardness of the hot-formed steel reach 2.6 times that of the unhot-formed sample. This study provides a way to make layered composites suitable for crash-resistant and energy-absorbing components for lightweight automotive.
Graphical abstract
Laser tailor-welded hot forming of DP980 and 22MnB5 can improve the properties of the weld zone and heat-affected zone.
... Though the studies by Milititsky et al. [5] Eliott and Jiang [6], Ma et al. [7] address the voids in RSW, none addressed the behaviour of voids in RSW, or depict a conclusive mechanism of its formation. On the other hand, Pouranvari and Marashi [8], Wan et al. [9] and Ji et al. [10] addressed the microstructural characteristic features of voids in dual phase (DP) steels and observed that voids mainly comprised of dendritic structure attributing to the liquid-solid contraction during solidification for its formation in nugget. ...
Among various defects formed in resistance spot welds of advanced high strength steels (AHSS), shrinkage voids are seldomly studied. Apparently, the voids are thought to be formed towards the end of solidification due to the material volumetric shrinkage. Although, the voids are reported to be influenced by rich material chemistry of AHSS, the exact mechanism of its formation in RSW is yet to be understood. Hence, this paper attempts to study the influence of various RSW parameters on the void size and its formation mechanism. Computed tomography (CT) technique was utilized to visualize and quantify the void formed in a nugget and to study the influence of resistance welding parameters such as weld current, squeeze force and non-welding factors such as sheet thickness material strength on the void volume. A comprehensive analysis reveal that the parameters such as weld current (nugget size), sheet thickness and sheet strength had direct influence on the void volume, but the influence of electrode squeeze force was dependent on the nugget size. Besides, an attempt was also made to develop an empirical relation for the prediction of void volume formed in resistance spot welds of AHSS utilizing the results of present study.
... However, the presence of the Al-Si at the faying interface presents challenges to the development of robust weld strength. Ji et al. [4] found the liquid Al-Si coating in the welding process to act as an additional current flow path and thus increased the nugget size. However, Choi et al. [5] found that the sharp notch filled with residual Al-Si coating at the weld boundary increased the risk of interfacial fracture (IF). ...
For the automotive industry, it is important to predict the fracture mode of resistance spot-welded joints. Traditional 4t0.5 (where t is sheet thickness) criterion for the transition from interfacial fracture (IF) to button pullout fracture (BPF) mode is not applicable for 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.
... They proposed an optimized pulse current to improve the appropriate welding current range [8]. Ji et al. investigated the effects of Al-Si and Zn coating on the nugget geometry of hot-stamped boron steel and showed that in the presence of Al-Si coating, the acceptable welding current was wide enough (while there was no weldable current fund for Zn coating) [9]. Ighodaro et al. compared the effects of galvannealed and Al-Si coatings on the weldability of hot-stamped steels. ...
... On the other hand, after hot-stamping, the Al-Si coating diffused to the base metal, transformed into an alloyed intermetallic layer, and the average thickness of the layer increased from 27 µm to 37 µm. This coating layer structure consists of Al-Fe-Si and different intermetallic compounds of α-Fe with Al and Si in a solid solution (e.g., FeAl 2 and Fe 2 SiAl 2 ) [9,22,23]. ...
... On the other hand, after hot-stamping, the Al-Si coating diffused to the base metal, transformed into an alloyed intermetallic layer, and the average thickness of the layer increased from 27 μm to 37 μm. This coating layer structure consists of Al-Fe-Si and different intermetallic compounds of α-Fe with Al and Si in a solid solution (e.g., FeAl2 and Fe2SiAl2) [9,22,23]. ...
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.
... However, the thick oxide layer influences the contact resistance during resistance spot welding in the automotive industry. Moreover, the presence of oxides at the surface can lead to inhomogeneous current flow and violent heat generation [11,38]. In contrast to the smooth and uniform features characteristic of the hot-dipped condition (Figure 1), the occurrence of voids and microcracks accompanies phase transformations in the coating and steel during austenitization and die-quenching (Figure 4). ...
The chemical and phase composition of the coating and the coating/substrate interface of an Al-Si-coated 22MnB5 hot stamped steel was investigated by means of SEM-EDS, XRD, micro-XRD and electron diffraction. Moreover, the surface profile was analyzed by XPS and roughness measurements. The XPS measurements showed that the thickness of the Si and Al oxide layers increased from 14 to 76 nm after die-quenching, and that the surface roughness increased as well as a result of volume changes caused by phase transformations. In addition to the FeAl(Si) and Fe2Al5 phases and the interdiffusion layer forming complex structures in the coating, electron diffraction confirmed the presence of an Fe2Al5 phase, and also revealed very thin layers of Fe3(Al,Si)C, Fe2(Al,Si)5 and Al-bearing rod-shaped particles in the immediate vicinity of the steel interface. Moreover, the scattered nonuniform layer of the Fe2Al8Si phase was identified in the outermost layer of the coating. Despite numerous studies devoted to researching the phase composition of the Al-Si coating applied to hot stamped steel, electron diffraction revealed very thin layers and particles on the coating/substrate interface and outermost layer, which have not been analyzed in detail.
