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•Equimolar CrMnFeCoNi high entropy alloy coating are prepared by laser cladding.•The cladding layer forms a simple FCC phase solid solution with identical dendritic structure.•The ...cladding layer exhibits a noble corrosion resistance in both 3.5wt.% NaCl and 0.5M sulfuric acid.•Element segregation makes Cr-depleted interdendrites the starting point of corrosion reaction.
Equimolar CrMnFeCoNi high entropy alloy (HEA) is one of the most notable single phase multi-component alloys up-to-date with promising mechanical properties at cryogenic temperatures. However, the study on the corrosion behavior of CrMnFeCoNi HEA coating has still been lacking. In this paper, HEA coating with a nominal composition of CrMnFeCoNi is fabricated by laser surface alloying and studied in detail. Microstructure and chemical composition are determined by X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are used to investigate the corrosion behavior. The coating forms a simple FCC phase with an identical dendritic structure composed of Fe/Co/Ni-rich dendrites and Mn/Ni-rich interdendrites. Both in 3.5wt.% NaCl solution and 0.5M sulfuric acid the coating exhibits nobler corrosion resistance than A36 steel substrate and even lower icorr than 304 stainless steel (304SS). EIS plots coupled with fitted parameters reveal that a spontaneous protective film is formed and developed during immersion in 0.5M sulfuric acid. The fitted Rt value reaches its maximum at 24h during a 48h’ immersion test, indicating the passive film starts to break down after that. EDS analysis conducted on a corroded surface immersed in 0.5M H2SO4 reveals that corrosion starts from Cr-depleted interdendrites.
Keyhole-induced porosity in remote laser welded Al joints leads to weakened joint strength. In the study, the remote laser welding processes are numerically simulated to reveal mechanism of keyhole ...and keyhole-induced porosity formation. It is found that porosity formation takes three steps: bubble formation, bubble floating to the back of molten pool and bubble being captured by solidification front. The porosity prevention can be achieved by interrupting one of these three steps. The process simulation shows that violent melt flow behind the keyhole is the root cause of pore formation. It leads to keyhole collapsing and resulting in large fluctuation of keyhole depth and bubble formation. The vortex-type melt flow behind the keyhole is also the main cause of the bubbles floating from the keyhole’s bottom into the molten pool; and Al’s high thermal conductivity and strong melt flow make the bubbles difficult to escape. Various porosity prevention approaches are simulated in the study to check their effectiveness in terms of interrupting the three steps. Also, the corresponding experimental test are carried out as verification. The amounts of porosity predicted by the simulations agree very well with what being observed in the experimental test. The study suggests that high welding speed is helpful in keeping the keyhole open and not creating strong melt flow; large forward inclination angle also creates quiescent molten pool flow and hence makes the bubbles difficult to float into the rear molten pool. The findings from the study provides fundamental insights into the mechanism of porosity formation during laser welding of Al alloys and guidance in keyhole-induced porosity prevention.
•A 3D multiple-reflection model was developed to simulate full penetration laser welding.•Extreme spreading of bottom molten pool and followed hump formation were analyzed.•Periodic behavior of ...keyhole in quasi-steady stage was identified.•Keyhole-induced porosity formation mechanism in full penetration was proposed.
To study molten pool dynamics, root hump formation, periodic keyhole behavior in full penetration laser welding (FPLW) of aluminum alloy, a numerical simulation was carried out, in which volume of fluid (VOF) method and ray-tracing algorithm were adopted. A varied metallic vapor shear stress model was considered. Meanwhile a series of welding experiments on specimens being composed of aluminum alloy and quartz glass were conducted to analyze the porosity formation process. It was found that the over-heated sagging with low surface tension stagnated at the bottom side and solidified to form root hump. The dominant backward heat convection at the lower part can well explain the extreme spreading of molten pool at bottom surface. Periodic behavior of the keyhole was identified during quasi-steady stage. The porosity can be effectively suppressed in FPLW. Both the porosity ratio and average porosity number were reduced obviously. The main mechanism of porosity formation in FPLW is the collapse of the rear keyhole wall, mainly caused by bulges on the front keyhole wall. Bubble coalescence is responsible for large porosity size and coalescence efficiency depends on bubble size difference.
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The base cooling effect was improved by imposing the continuous water flow on the back of the substrate during the laser additive manufacturing of Inconel 718 (IN718). The dendritic microstructure, ...crystal orientation and hot cracking behavior were studied by using optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. The results showed that the crystal orientation was increased by increasing the base cooling effect during the deposition. Also, highly ordered columnar dendrites were established, and mono-crystalline texture was constructed in the final clad. It was fund that the effect of solidification cracking on the properties of final clad was negligible since it was only generated at the top region of the deposit, while liquation cracking was produced and remained in the heat affected zone (HAZ) and needed to be carefully controlled. The susceptibility to the liquation cracking showed a high dependence on the grain boundary misorientation, which was considered to be attributed to the stability of interdendritic liquation films, as well as the magnitude of local stress concentration in the last stage of solidification.
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•The base cooling effect was increased during laser additive manufacturing.•Highly ordered dendrites were established under improved base cooling.•The crystal orientation was increased by improving the base cooling effect.•Liquation cracking tendency was reduced due to the increase of base cooling.•Liquation cracking increased with the increase of grain boundary misorientation.
The migration behavior of intermetallic compound (IMC) layer with some distance to the interface was observed in twin-spot laser welding-brazing of AA7075-T5 aluminum to DP590 galvanized steel. The ...microstructure characterization of different types of IMCs was systematically discussed using scanning electron microscopy, focused ion beam and transmission electron microscopy methods. It was found that the dense plate-like IMC layer formed at the interface was η-Fe2Al5 phase and the acicular IMC was θ-FeAl3 phase, while the sparse plate-like IMC layer that migrated into the weld metal was the mixture of the θ-FeAl3 phase and molten Al. A 3D thermo-fluid numerical model was developed to predict the melt flow behavior and analyze the mechanism of the migration behavior of the IMC layer. The simulation results revealed that the downward violent flow under the twin-spot center impacted the sparse IMC layer and entered into the gap between the steel and IMC layer, forming a broken migrated IMC layer. The joint strength increased with maximum migration distance and the maximum joint strength of 133.14 MPa was obtained. Thicker and sparse IMC was formed at the interface when the migration distance increased, which was adverse to the joint strength.
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•IMC layer migration with some distance to the interface was observed in Al/steel joints.•A thermo-fluid model was developed to simulate the twin-spot laser welding-brazing process.•The migration behavior was caused by the downward flow under the twin-spot center.•Maximum migration distance increased with power and maximum joint strength of 133.14 MPa was obtained.
Finite element analysis is commonly used to investigate the thermal-mechanical phenomena during welding. To improve the computing efficiency of finite element analysis for welding thermal conduction, ...a novel Newton
–
Raphson method (NRM) without the computation of inverse matrix and a hybrid method combing the NRM and conventional implicit method (IMP) were developed. Comparison of computing time between the hybrid method implemented in an in-house software JWRIAN and the IMP used in a commercial software ABAQUS indicated that the computing speed of the former was about 4.5 times faster than that of the latter. Additionally, compared to the conventional IMP, the NRM exhibited higher computing efficiency in the analysis of transient thermal conduction during the welding heating process. Meanwhile, a combined hybrid method of the NRM and IMP was verified to be more efficient in analyzing the welding thermal conduction throughout the heating and cooling processes. Moreover, the thermal cycles computed by the hybrid method were consistent with those from experimental measurement, indicating the high accuracy of the hybrid method. Furthermore, the hybrid method was used to predict the temperature field of the corner boxing fillet joint welded by a low transformation temperature weld metal for generation of compressive residual stress.
The tandem narrow-gap gas metal arc welding was adopted in this study. The effects of interwire angle on the welding process stability and arc behavior were investigated. With the interwire angle ...increasing, the welding process stability gets improved. The arc interruptions which frequently occur in the leading arc are eliminated when the interwire angle is >0°. The mutual electromagnetic attraction between the two arcs decreases with the interwire angle increasing due to the increased arc distance. The two arcs move from the groove bottom to the sidewalls, which transfer more heat to the sidewalls resulting in increased weld width. This leads to serious undercutting defects especially when the interwire angle is 20°. The molten metal under the arcs flows to the rear of welding pool under the action of plasma drag force, arc pressure and droplet impingement. As a result, there is no adequate molten metal to fill the sidewalls, giving rise to undercutting defects. The undercutting at the groove sidewall close to the trailing arc is smaller. This is because the liquid metal under the trailing arc is more massive, which means more molten metal flows to the sidewall, alleviating the undercutting defect.
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•A self-designed tandem narrow-gap GMAW torch was used to perform the experiments.•The welding process stability was improved and the arc interruption was eliminated by increasing interwire angle.•Increasing the interwire angle decreases the mutual electromagnetic attraction and transferred more heat to the sidewalls.•The thin liquid metal under the leading arc obstructed the molten metal to fill the sidewall, causing a severer undercut.
We investigated whether low transformation temperature (LTT) welding materials are beneficial to the generation of compressive residual stress around a weld zone, thus enhancing the fatigue ...performance of the welded joint. An experimental and numerical study were conducted in order to analyze the residual stress in multi-pass T-welded joints using LTT welding wire. It was found that, compared to the conventional welded joint, greater tensile residual stress was induced in the flange plate of the LTT welded joints. This was attributed to the reheat temperature of the LTT weld pass during the multi-pass welding. The formerly-formed LTT weld pass with a reheat temperature lower than the austenite finish temperature converted the compressive residual stress into tensile stress. The compressive residual stress was generated in the regions with a reheat temperature higher than the austenite finish temperature, indicating that LTT welding materials are more suitable for single-pass welding.
The wettability and spreadability of the molten Al drop on the surface of bare steel and galvanized (GI) steel was studied. It was expected that the role of Zn coating during the laser ...welding–brazing process of Al/steel could be understood through this study. The bare steel without coating and the GI steel with different Zn coating thicknesses of 10 and 20
μ
m were used. Different laser power was considered. The welding time was 1000 ms. Deposited filler metal ER4043 was applied, and the feeding rate was 5 m/min. The transferring, wetting, and spreading process of the molten drop on the steel surface was observed and recorded by a high-speed video camera. The temperature field was measured by an infrared thermometer. The microstructure of the joint was observed and analyzed after welding. The GI steel with a 10-
μ
m coating improved the wettability of Al/steel compared with the bare steel, but the GI steel with a 20-
μ
m coating deteriorated the wettability and the weld appearance compared with the GI steel with a 10-
μ
m coating. The evaporation of Zn coating could protect the steel surface from oxidation and absorb the excessive heat input. The formation of the brazing interface and Zn-rich zones was revealed based on the adsorption values of Al-Si/Fe and Al-Zn/Fe systems.
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•Crack growth rate in 3.5% NaCl solution was slight slower than in pure water due to crack branching and passivation.•Under the invading of Cl- ions in multi-directions and hindering ...of nanocrystals, crack bifurcated even at lower stress intensity.•Amorphous film was discovered to form in crack due to disorder of atom arrangement generated from selective corrosion.•Driving force for crack propagation was weakened by compact amorphous film in absence of transporting channel for corrosive ions.
Multi-layer weld metal of NiCrMoV steels used in nuclear rotor was adopted to perform stress corrosion cracking (SCC) test at 180 ℃, which indicated the crack growth rate in 3.5% NaCl solution was lower than that in pure water. The results obtained from high resolution transmission electron microscopy (HRTEM) analysis revealed that the combined effect of crack branching behavior and amorphous film made contributions to the slower crack growth rate. The aggressive Cl− ions invaded grain in multi-directions that was promoted by dislocation motion, facilitating the main crack to bifurcate. Besides, nanocrystals also enhanced branching behavior by deflecting the crack growth path. An amorphous film forming in crack evolved from the disorder of atom arrangement produced by dislocation accumulation and grew stably with the supplement of carbon atom generated in selective corrosion. The complete amorphous film was absent connected path for migration of aggressive anions, thus weakened the driving force for crack propagation and improved resistance to localized corrosion attack.
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