Fusion Boundary Research Articles

Microstructural Evolution During Welding of High Si Solution-Strengthened Ferritic Ductile Cast Iron Using Different Filler Metals

The paper deals in depth with characterizing bead-on-plate welds on EN-GJS-500-14 base metal, utilizing two filler metals: a pure Ni wire and a NiFe wire containing 45 wt pct Ni. The welds were conducted using the same heat input to ensure comparability in microstructure analysis. The microstructural observations were carried out using optical and scanning electron microscopies, X-ray tomography, X-ray diffraction analysis, and microhardness testing. Thermodynamic simulations using the non-equilibrium Scheil solidification model provide insights into the solidification process and the underlying metallurgical factors associated with the observed microstructural evolution. The observations revealed that the pure Ni wire deposited a softer fusion zone with graphite precipitation, while cementite precipitated in the fusion zone of NiFe alloy. The formation of martensite structures with different morphologies was the predominant microstructural evolution in the heat-affected zone of both welds. The partially melted zone of the pure Ni weld is narrower than the NiFe weld because more diffusion of Ni avoids widening the brittle structures in the partially melted zone. An unmixed zone in the form of a peninsula was exclusively observed in the fusion zone of the NiFe weld because of inadequate diffusion of Ni into the liquified materials along the fusion boundary.

Achieving a Columnar-to-Equiaxed Transition Through Dendrite Twinning in High Deposition Rate Additively Manufactured Titanium Alloys

AbstractThe coarse β-grain structures typically found in titanium alloys like Ti–6Al–4V (wt pct, Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), produced by high deposition rate additive manufacturing (AM) processes, are detrimental to mechanical performance. Certain modified processing conditions have been shown to lead to a more refined grain structure, which has generally been attributed to a change in the solidification conditions with respect to the experimental Hunt diagram proposed by Semiatin and Kobryn. It is shown that with Wire Arc AM (WAAM) increasing the wire feed speed (WFS) is effective in promoting a columnar-equiaxed transition (CET). Conversely, estimates of the dendrite-tip undercooling using the KGT model suggest that this will be too small for free nucleation without the addition of artificial nucleants, due to the very low solute partitioning in Ti alloys. It is also shown that it is difficult to promote a CET with plasma transferred arc WAAM as computational fluid dynamics (CFD) melt-pool simulations indicate that the solidification parameters remain within the columnar region on the Semiatin-Kobryn Hunt map, within the constraints of a stable process. However, a high fraction of twin boundaries was observed in the refined β-grain structures seen at high WFS. This has been attributed to departure of $$\left\langle {001} \right\rangle _{\beta }$$ 001 β alignment from the direction of maximum thermal gradient, caused by the curvature of the fusion boundary, stimulating dendrite twinning during solidification. In addition, it is shown that increasing the WFS leads to a change in melt-pool geometry and a reduction of remelt depth, which promoted dendrite twinning and grain refinement.

Effect of preheating temperature on the microscopic structure and mechanical performance of pulsed laser welding in sheet AZ31 alloy

To achieve high quality welding of an AZ31 magnesium alloy sheet, butt experiments are conducted using an Nd:YAG pulsed laser welder to weld the AZ31 magnesium alloy (1 mm). The influence of preheating temperature on the microscopic structure and mechanical performance of AZ31 magnesium alloy pulsed laser welding is studied. Experimental results show that with the rising of preheating temperature, the grain size of an equiaxed crystal in the weld center and columnar crystal at the fusion boundary (FB) first decreases and then increases and reaches the minimum values of 18.89 ± 0.45 and 32.09 ± 1.52 μm, respectively, when preheated at 200 °C. The maximum force and tensile strength increase first and decrease and reach the maximum of 2.32 kN and 193.3 MPa, respectively, when the preheating temperature is 300 °C. Above all, the welds with excellent mechanical performance can be obtained at the preheating temperature of 300 °C.

Softening mechanism and failure behavior of 9Cr oxide dispersion strengthened steel resistance spot welded joint

Oxide dispersion strengthened (ODS) steel is one of the most promising materials for nuclear applications. However, there are fewer studies on the spot welding of ODS steel, and the softening mechanism of ODS steel resistance spot welded joint is unclear. The microstructural features of the ODS resistance spot welded joint was characterized to analyze the softening mechanism. The failure behavior and tensile shear performance were studied combined with finite element simulation. The results showed that the fusion boundary has the lowest hardness and the softening zone is divided into fusion zone(FZ) softening and heat affected zone(HAZ) softening according to different location in the joint. In FZ softening, the softening is mainly caused by the presence of δ-ferrite and fewer oxides as well as lower dislocation density. In HAZ softening, the softening is mainly caused by larger grains, less oxides. There are two types of failure, the fracture initiated at HAZ softening shows a brittle feature, while initiated at FZ softening shows a ductile feature. The softening leads to a reduction in tensile-shear strength. Thus, the control of softening zone is critical to improve the reliability of welded joints.

Quantification of the local mechanical behavior in dissimilar metal welds using digital image correlation instrumented cross-weld tensile testing

The local yielding behavior in base metal, heat-affected zone, fusion boundary region, and weld metal of low-alloy steel/Alloy 625 filler metal welds was quantified using digital image correlation instrumented cross-weld tensile test. The tested welds exhibited undermatching, matching, or overmatching weld metal yield strength with significant gradients in the local yielding behavior. An undermatching weld yielded at 69 MPa below the base metal yield stress, accumulating to 0.72% total strain. The base metal in an overmatching weld had 110 MPa lower yield strength than the weld metal. The strong strain hardening response in the Alloy 625 weld metal, within the uniform elongation range, and its constraining effect on the fusion boundary region and heat affected zone, led to extensive strain accumulation, necking, and final failure in the base metal of all tested welds. The yielding behavior of the tested welds was compared to stress-based criteria, utilizing minimum specified and as-delivered yield and ultimate tensile strength, and to strain-based criteria. The capability of digital image correlation instrumented cross-weld tensile testing to quantify local yielding and strain accumulation demonstrates potential application in proving conformity to stress-based and strain-based design criteria of dissimilar and matching filler metal welds.

Predictive tools for the cooling rate-dependent microstructure evolution of AISI 316L stainless steel in additive manufacturing

This study presents a thorough evaluation of advanced predictive tools for the solidification microstructure of AISI 316L stainless steel, taking into account both chemical composition and cooling rate. In order to achieve a broad range of cooling rates comparable to prominent additive manufacturing processes, autogenous laser welds were carried out on the steel plate by means of powderless directed energy deposition (DED) and selective laser melting (SLM) machines. Computational analysis of cooling rates and metallographic investigations revealed that DED welds solidify at moderate rates, exhibiting a primary ferrite (FA) solidification mode. Conversely, SLM welds solidify at significantly higher rates, showing dual solidification modes: initial austenite (A) at the fusion boundary and subsequent ferrite (F) at the interior. The WRC-1992 constitution diagram predicts the FA mode in accordance with the findings in the DED welds. An implicated Schaeffler diagram, proposed for a comparable cooling rate, also predicts the same mode. An artificial neural network model, referred to as ORFN, consistently predicts the variation of ferrite content within the DED welds based on the cooling rate variation. The dual mode observed in the SLM welds aligns with established knowledge regarding the alteration of stainless steel solidification under extremely high cooling rates. While the occurrence of the A mode at the fusion boundary is anticipated according to implicated Schaeffler diagrams for extremely high cooling rates, the transition to the F mode is not addressed explicitly.

Microstructural study of microwave welded austenitic stainless steel

The joining of austenitic stainless steels through microwave hybrid heating has gained much attention due to its advantages, like uniform and volumetric heating. However, to establish a microwave joining process for promising applications, a microstructural study of the welded joint is still an area to explore. In the present study, microstructural analysis of austenitic stainless steel (SS304) joints is explained through optical microscopy, scanning electron microscopy (SEM), elemental mapping, X-ray diffraction (XRD) and Vickers' micro-hardness tester. Further, detailed microstructural analysis of interlayer and heat-affected zone (HAZ) is studied through electron back-scattered diffraction (EBSD). XRD results reveal the development of various Ni, Cr and the M23C6 type carbides at the joint interlayer region. SEM results point toward non-epitaxial grain evolution along the fusion zone. The mean microhardness of 350 ± 20 HV was determined at the joint interlayer region. A relatively fine grain structure was observed along the fusion boundary. However, moving away from the joint, two different forms of coarse grain heat-affected zone (CGHAZ) were observed. The presence of two different forms of CGHAZ is confirmed through inverse pole figure maps and grain size plots. The presence of high-angle boundaries (HABs) at 60° rotation describes the twin boundaries characterized along the <111> axis. The existence of improved fractions of HABs in the microwave-joined specimen of SS will favor the microstructure to retard bulk deformation. The joint specimen endures a maximum force of 6.98 kN, and the ultimate tensile strength of 613 MPa with 4.75% of elongation. It is observed that microhardness observations and the microstructural studies in the research help establish a solid microstructure-property correlation for the joined material.

Characterization of fusion boundary between low alloy steel and stainless steel cladding

The microstructure and hardness of fusion boundary (FB) and near FB of a low alloy steel (LAS) 16MND5 welding cladded with stainless steel 308L for nuclear reactor pressure vessel were investigated by using modern electron microscopes and nano-indenter. The wavy morphology of fusion line results in changes of nearby microstructures of both cladding 308L and substrate 16MND5 along the FB. The microstructure of the cladding 308L is composed of columnar grains adjacent most of the FB, but it is equiaxed grains at the valley of the wavy fusion line with large curvature. The microstructures of 16MND5 near the FB are ferrite at the wavy fusion line with large curvature, martensite at the wavy fusion line with small curvature, and transition mixed structure between them. A carbide-depleted zone adjacent to the FB was observed in the 16MND5 side, of which the hardness is lower than both FB and 16MND5 base metal. Three types of FB can be observed according to the width, being narrow or sharp FB, needle-like martensite FB, and partially-mixed zone FB. The FB includes high density of carbides of M7C3 and M3C, leading to a hardness peak at FB.

Melt pool electromagnetic force model extended to account for free surface deformation – Application to gas metal arc

Computational fluid dynamics models with free surface tracking intended to simulate the melt pool produced by an electric arc usually model the electromagnetic force ignoring the deformation of the free surface. However, with an arc heat source, the electromagnetic force is known to be among the leading-order forces, especially at high currents. In addition, the free surface can undergo significant deformations, especially in the presence of metal transfer. In the present study, a generalization of the electromagnetic force model that accounts for the deformation of the free surface is therefore proposed. Test cases with a pulsed gas-metal arc that transfers one metal drop per pulse were investigated experimentally at three different travel speeds to provide validation data. The cases were simulated with both the proposed and the earlier model to assess the influence of the new developments. The results showed that, in the regions where both models determine the force, the discrepancy between the models' results can reach up to an order of magnitude. Especially, the earlier model overestimates the electromagnetic force deep into the melt pool. On the other hand, it neglects it in the liquid metal that is located at an elevation above the original upper surface of the workpiece, while the proposed model showed that in this area the intensity of the electromagnetic force is the largest. These significant discrepancies result in non-negligible differences in the predicted melt pool thermal flow and geometry. Especially, the proposed extended model provides an improved prediction of the fingertip-shaped fusion boundary.

Mitigating Stress Corrosion Cracking of 304L and 316L Laser Welds in a Salt Spray through Micro-Shot Peening

Two austenitic stainless steel (ASS) plates, 304L and 316L, were cold-rolled (304R and 316R) with a 10% reduction in thickness and then subjected to laser welding. Cold rolling caused slight surface hardening and introduced residual tensile stress into the ASS plates. The susceptibility to stress corrosion cracking (SCC) of the welds (304RW and 316RW) was determined using the U-bend test pieces in a salt spray. To highlight the stress concentration at the weld’s fusion boundary (FB), the top weld reinforcement was not ground off before bending. Moreover, micro-shot peening (MSP) was performed to mitigate the SCC of the welds by imposing high residual compressive stress and forming a fine-grained structure. Cold rolling increased the susceptibility of the 304R specimen to pitting corrosion and intergranular (IG) microcracking. Moreover, pitting corrosion and SCC were found more often at the FBs of the 304RW. The corrosion pits of the peened 304RW (304RWSP) were finer but greater in amount than the those of the un-peened one. The results also indicated that the 316L ASS welds with MSP were resistant to the incidence of pitting corrosion and SCC in a salt spray. The better reliability and longer service life of dry storage canisters could be achieved by using 316L ASS for the construction and application of MSP on it.

Effect of placement configuration on the microstructure, porosity and mechanical performance of dissimilar remote laser welding of additive manufactured AlSi10Mg alloy and conventionally manufactured 1050 aluminium sheet

Additive manufacturing (AM) has gained significant attention in automotive applications, while the low weldability of AM material poses challenges when assembling AM materials with conventionally manufactured materials. This study investigated the laser welding of AM AlSi10Mg and AA1050 sheet metal in an overlap configuration, specifically focusing on the impact of placement configuration of two materials on weld properties. This aims to provide valuable insights for optimizing the localized geometry of AM components to enhance the joint performance. Results showed that positioning AM AlSi10Mg on top leads to a higher weld penetration and higher Si content within the molten pool, especially in the upper sheet. Additionally, weld zone grain structure was greatly influenced by alternating the placement configuration, where welds with AM AlSi10Mg on top exhibiting more equiaxed grains in the weld centre and refined columnar grains near the fusion boundary of upper sheet. Furthermore, significantly reduced weld porosity was determined by positioning AM AlSi10Mg at the bottom (3.2 ± 0.4 %∼7.1 ± 0.9 %) compared to placing it on top (16.9 ± 0.9 %∼20.1 ± 0.9 %) in all scenarios ranging from full penetration to partial penetration. This can be related to the fact that the hydrogen bubbles originating from AM AlSi10Mg experienced a shorter exposure in the molten pool, limiting the merging and growth of small bubbles. However, a slightly lower joint strength was determined in welds with AM AlSi10Mg at the bottom which may be due to the plastic deformation incompatibility between large columnar grains in the fusion zone and the fine grains in the as-printed material.

Atomic scale melting/solidification behavior and structural evolutions of AlCoCrFeNi high-entropy alloy in selective laser melting process

Comprehension of the evolutions and phenomena that occur on the atomic scale can become a bridge toward a greater understanding of the properties of materials on the macro scale. In this study, molecular dynamics simulations were performed to investigate nano-structural evolutions in the SLM process of the near-equiatomic AlCoCrFeNi high-entropy alloy. Fifteen simulation systems were designed by altering laser power (400–800 eV/ps), scanning speed (0.2–2.0 Å/ps), and hatch spacing (5 and 10 nm) as the process variables for addressing their effects on the structure at the atomic scale. The results showed that the effect of parameters on the atomic displacement is decreased from laser power to scanning speed and hatch spacing, respectively. It was also indicated that the HCP stripes form due to the extreme cooling rates experienced in the process. Furthermore, the reduction of the hatch space can improve the surface quality by spreading the atoms accumulated in the fusion boundary of the previous laser track. These results indicate that controlling volumetric energy density (VED) by lowering laser power and scanning speed as much as possible, is beneficial to obtain the optimum crystal structure of the manufactured parts in this process. Although decreasing the hatch space has no considerable effect on the crystal structure, it can help to achieve a good surface finish which is essential in the SLM industry.

CHARACTERIZATION OF CORROSION BEHAVIOR OF INCONEL 617 WELDED JOINTS USING CMT-GMAW MULTI-CONTROL WELDING

This work addressed the effect of CMT-GMAW multi-control welding of Inconel 617 alloy with different wire feed rates (8.9, 9.4, and 9.9 m/min) on the microstructure, hardness, and electrochemical properties. The weld joints are composed of columnar dendritic structure with cellular crystals. Electron backscattered diffraction (EBSD) study showed equiaxed dendrite at the center of weld metal with growth direction perpendicular to the fusion boundary. The weldments showed diffraction peaks at 43.53[Formula: see text], 50.12[Formula: see text], and 74.82[Formula: see text], and these peaks mainly represent gamma ([Formula: see text]) and gamma prime ([Formula: see text]) phases along with the carbide peaks of Ti (C, N), M[Formula: see text]C6 and M6C. The Base metal (BM) had a lower hardness (232 ± 10 HV[Formula: see text]) and lower corrosion rate (0.212 mpy) than the weld joints. The increase in wire feed rate (WFR) results in the decrease of microhardness (267 ± 5 − 251 ± 6 HV[Formula: see text]) and increase in corrosion rate (1.833-28.140 mpy). The base metal exhibited higher potential ([Formula: see text]) and lower current density ([Formula: see text]) than the weld joints. As wire feed rate (WFR) increases, heat input increases; solidification time increases, grain boundaries coarsen, resulting in a lower grain boundary (GB) density, and hence increased carbide precipitation and segregation in weld zone leading to higher stable anodic current density, which caused corrosion resistance to deteriorating. The BM was more corrosion resistant than the weld joints. The metallurgical and physical changes caused by the welding process affect the corrosion resistance of the weld joints. This leads to the weld metal corroding faster than the base metal.

Optimizing solidification dendrites and process parameters for laser powder bed fusion additive manufacturing of GH3536 superalloy by finite volume and phase-field method

Investigating the surface morphology and microstructure of laser powder bed fusion (L-PBF) in solidification and optimizing process parameters is crucial for improving the accuracy and performance of cladding parts of GH3536 superalloy. In this study, a computational model incorporating Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) was utilized to simulate the mesoscale evolution of the molten pool in L-PBF for GH3536 superalloy. Furthermore, the growth of columnar dendrites along the fusion boundary under transient conditions was simulated utilizing a quantitative phase-field model. The impact of scanning speed and laser power on temperature distribution, molten pool size, surface morphology of cladding layer, and columnar dendrite spacing during solidification of single-track L-PBF were examined. The findings suggest that the molten pool undergoes a fast process characterized by significant temperature gradients. Increasing the laser power or reducing the scanning speed results in higher maximum molten pool temperature, as well as increased molten pool depth and width. Overlarge or undersized scanning speed and laser power can result in poor surface quality of the cladding layer. When the laser power was 120 W and the scan speed was 0.4 m/s, the cladding layer exhibited improved surface morphology, with finer columnar dendrites observed at the molten pool's base. This study delved into understanding the evolution characteristics of the molten pool and the growth process of columnar dendrites at the molten pool's base, providing valuable insights for determining appropriate experimental process parameters for L-PBF of GH3536 superalloy.

Effect of paint baking on the fusion boundary softening and fracture behavior of Q&P 980 steel resistance spot welds

During automotive assembly, vehicles undergo low-temperature heat treatment (paint baking) to harden the paint. Although paint baking occurs at a relatively low temperature, it can remarkably affect the weld's mechanical and fracture behavior. This work studies how paint baking improving the strength and fracture behavior of Q&P 980 spot welds exhibiting a halo ring; a low carbon enriched zone in the weld nugget. The mechanical behavior of the paint-baked welds reveals an increase in cross-tensile strength and absorbed energy when baked at 180 °C for 27 min. Microstructural observation showed that the martensite present in the as-welded conditions started to decompose into tempered martensite with ε-carbide in the martensitic matrix. Gleeble thermo-mechanical simulations of the upper-critical heat-affected zone (UCHAZ) were produced to understand the mechanical and fracture micro-and macro-mechanisms, before and after the paint baking process, by widening the regions of UCHAZ. The transmission electron microscopic analysis of the Gleeble simulated sample reveals the segregation of C, Mn, Al, and Cr along the prior austenitic grain boundary which will change the nature of bonding at these boundaries. Nevertheless, paint baking treatment helps to redistribute the segregated elements from the grain boundary to the grain interior and to eliminate the solidified liquation formed at the grain boundaries during welding. The transformation of martensite to decomposed martensite, elimination of solidified liquation due to the enhanced atomic mobility and growth of surrounding grains, and the redistribution of C, Mn, Al, and Cr from the grain boundary to interior regions of grains, are the main reasons for the improvement of mechanical properties and fracture behavior of the spot welds.

Improving the Microstructure and Mechanical Properties of Laser-Welded Al–Si-Coated 22MnB5/Galvanized Steel Joints Added by Nickel

To weaken the harm of Al–Si coating and increase the strength of welded joints, variable thicknesses of Ni foil (Ni, an austenitic formation element) were added to the lap laser welding Al–Si-coated 22MnB5 hot stamping steel/galvanized steel joints. The joints’ weld appearance, microstructure, and mechanical properties were explored. The weld altered from an X shape to a Y shape with an increased thickness of Ni foil. During welding, Al–Si coating was melted and diluted into the welding pool, forming δ-ferrite (a rich-Al phase with low toughness and strength) in the fusion zone (FZ) and fusion boundary (FB). This phase deteriorated the strength of the joints. After adding Ni, the amount and size of the δ-ferrite phase decreased. With a significant thickness of Ni foil, δ-ferrite disappeared. However, a new phase (fresh martensite (FM), which formed at low temperature and contained rich Ni) probably formed, except PM (previous martensite (PM), which formed at high temperature and contained little Ni or no Ni). The heat-affected zone (HAZ) on the side of 22MnB5 comprised a coarse martensite zone, refined martensite zone, martensite + ferrite zone, and tempered martensite zone from the FZ to the basic material. HAZ on the side of galvanized steel mainly contained ferrite and pearlite. After adding the Ni foil, the strength of the joint was greater than that without Ni. The maximum strength of the joint can be up to 679 MPa because of the disappearance of δ-ferrite. Meanwhile, the toughness of the joint increased. The fracture mode was from three mixed fractures (cleavage, quasi-cleavage, and dimple) to one fracture (dimple).

Application of Digital Image Correlation in Cross Weld Tensile Testing: Test Method Validation

This study addresses the limitations of cross weld tensile testing (CWTT) in quantifying local mechanical properties across microstructural and compositional gradients in dissimilar– and matching–filler metal welds. A digital image correlation (DIC) methodology was validated for application in CWTT by direct comparison of stress-strain curves generated using conventional and virtual DIC extensometers in tensile testing of homogeneous steel samples. DIC-instrumented CWTT of dissimilar weld metal Alloy 625 filler metal on F65 steel demonstrated capability in quantifying the local yield strength, strain-hardening kinetics, and strain at failure in the base metal, heat-affected zone (HAZ), fusion boundary (FB) region, and weld metal in dissimilar and matching filler metal welds. It was shown that the high strain-hardening capacity in Alloy 625 weld metal led to base metal failure in CWTT despite the lower Alloy 625 weld metal yield strength. It was also shown that DIC-instrumented CWTT can be used for determining weld metal undermatching and overmatching conditions in compositionally matching- and dissimilar-metal welds. Furthermore, by quantifying local strain distribution (both elastic and plastic) in the HAZ, FB region, and weld metal, DIC-instrumented CWTT provides an additional method for evaluating hydrogen-assisted cracking susceptibility in dissimilar-metal welds.

Effect of filling materials on microstructure and properties of CMT-laser beam oscillation hybrid welding behavior of dissimilar Al–Mg–Si alloys

Laser oscillation-assisted cold metal transfer hybrid welding was conducted on 2.0 mm-thick 6082 and a new 6xxx aluminum alloys. The influence of wire composition on microstructure and mechanical properties of the 6082 and new 6xxx alloy joints is studied. Our results show that, due to the difference in surface tension, the cross section of a joint filled with ER4047 wire typically presents a wider U-shape profile, while the ER5183 filled joint shows a relatively narrower V-shape profile. The average grain size of the ER5183 joint is smaller than that of the ER4047 joint. The ER4047 joint exhibits better fluidity, resulting in a higher proportion of high-angle boundaries compared to the ER5183 joint, which we attribute to the higher Si content. Equiaxed grains are also observed near the fusion boundary on the new 6xxx side, while no equiaxed grain formation is observed on the 6082 side. EDS data demonstrates severe elemental segregation in the center of weld and the partially melted zone, suggesting that multiple secondary phases form unevenly within inter-dendritic regions and grain boundaries of α-Al during the welding process. The microhardness of the weld zone and the heat-affected zone on both sides in the ER5183 joint is higher than that of the ER4047 joint. The corrosion test evidences that the ER5183 joint has a slightly better corrosion resistance than the ER4047 joint. Our findings provide a new understanding on joining of dissimilar Al–Mg–Si alloys and guide the selection of appropriate filling materials and welding processes.

Unexpectedly high corrosion susceptibility near fusion boundaries of Inconel 625 manufactured by laser powder bed fusion

The intergranular corrosion (IGC) resistance of Inconel 625 produced by laser powder bed fusion was investigated according to ASTM-G28 method A. Unexpected severe corrosion near fusion boundaries was found. The initiation and expansion of corrosion along fusion boundaries and the relationship between microstructure and corrosion behavior were analyzed. This unusual corrosion behavior was related to the Volta potential variation near fusion boundaries. This Volta potential difference was caused by the Al2O3 nanoparticles and Mo, Nb-rich precipitates near the fusion boundaries. This finding may enhance the understanding of corrosion behavior of LPBFed nickel alloys.

Microstructural characterization and mechanical properties in resistance spot welding of Q&P980 steel involving “effective softening” at the fusion boundary

This study proposes a welding process for Q&P980 steel that involves prolonged post-weld pulses. The process effectively softens the nuggets and results in welded joints with excellent comprehensive mechanical properties. The relationship between microstructural characteristics, element distribution and mechanical properties of weldments was elucidated for the samples subjected to different heat input histories. The microstructure evolution was analyzed using electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), and the mechanical properties of the spot welds were evaluated at room temperature by tensile shear (TS) and cross tension (CT) tests. The findings indicate that the softened area expands from the fusion boundary towards the opposite direction of the nugget as the heat input increases. A mixed microstructure of martensite and retained austenite, characterized by a larger size of prior austenite grains and lower kernel average misorientation, is formed in the “effective softening” region. This region provides serves as the primary location for crack nucleation and governs the propagation path of cracks within the halo ring-like softening zone, which has a significant impact on the fracture mode of the weld. The energy absorption in the TS test was increased by 95.40% compared to the single pulse weld (SPW), while the best combination of strength and energy absorption of the weldment in CT test was 7.2 kN and 64.24 J, respectively.