Shear Dimples Research Articles

Achieving superior aluminum-steel dissimilar joining via ultrasonic spot welding: Microstructure and fracture behavior

High-strength dissimilar welded joints of AA2024-T3 alloy with an AA1230 coating and galvanized high-strength low-alloy (HSLA) steel were attained through ultrasonic spot welding (USW) in this study. The tensile lap shear failure loads over a welding energy range between 2000 J and 3000 J were observed to meet or surpass the values specified in the AWS D17.2 standard. The highest average tensile lap shear strength was obtained to be ∼169 MPa due to the formation of a ∼30–70 μm thick Al-Zn diffusion layer without the presence of intermetallic compounds. The substrate failure from the softer AA1230 coating on the AA2024 side under tensile loading and cyclic loading at higher cyclic loads was observed, which reflected a robust sticking capability. The partial failure from AA1230 coating consisted of characteristic shear dimples, allowing high plasticity upon shear deformation. While the 1000 J welds failed mainly via substrate failure from AA1230 and transverse through-thickness (TTT) cracking in AA2024, the cohesive failure across the Al-Zn diffusion layer and adhesive failure from the Al-Fe-intermetallic/Al-Zn-diffusion layer interface were observed in the welds made at 3000 J, corresponding to the superior bonding strength and longer fatigue life.

Ultrasonic-induced amorphization and strengthening mechanisms of ultrasonic vibrations assisted friction stir Al/Ti welds

Integration of aluminum (Al) and titanium (Ti) alloys offers an effective approach to sustainable and high-quality development of the automotive and aviation industries. However, challenges arise when welding Al alloys to Ti alloys due to the formation of detrimental intermetallic compounds at the interface. With the implementation of ultrasonic vibrations, the temperature and strain rate were both increased during Al/Ti friction stir welding (FSW), leading to the formation of an amorphous interlayer, instead of intermetallic compound (IMC), at the interface. The interfacial microcracks were eliminated in the ultrasonic vibrations assisted friction stir welding (UVFSW). There were Ti particles separated from the Ti substrate and dispersed in the Al alloy, thereby resulting in a more gradual and moderate evolution of the interfacial microstructure. Due to the improved interfacial microstructure with ultrasonic vibrations, the lap shear strength was almost twice that of the conventional FSW welds within same welding conditions. Meanwhile, ultrasonic vibrations also improved the fabrication efficiency with a higher optimal traversing speed. The failure mode shifted from interface separation of FSW welds to a shear dimple fracture of the UVFSW welds, demonstrating the better plasticity and reliability of the UVFSW Al/Ti dissimilar joints.

The effect of ratcheting strain on post-ratcheting tensile test of metal matrix composites (MMCs) reinforced by Fe3O4 nanoparticles manufactured by the accumulative roll bonding (ARB) process

This article concerns the effect of strain accumulation during low-cycle fatigue on the post-fatigue tensile behavior of metal matrix composites (MMCs) with identical layers of aluminum reinforced by Fe3O4 nanoparticles produced via the accumulative roll bonding (ARB) technique. Work-hardening behavior, deduced from ratcheting curves, was evaluated by the post-fatigue tensile test. The key finding in the mechanical properties of homogeneous nanocomposite in terms of reinforcement distribution, evolving due to microstructural evolution during the ARB process, is that accumulation of ratcheting strain decreases, and fatigue and post-fatigue tensile behavior significantly increases. The rupture mechanism, analyzed by scanning electron microscope, revealed that deep hemispherical-shaped dimples, a well-known feature of ductile rupture, were changed to elongated shallow shear dimples by the impact of microstructural evolution due to imposed 1000 cycles of ratcheting test.

Comparison of a copper processed by asymmetric unidirectional rolling (AUR) and cross rolling (ACR): Strain distribution, microstructure, texture, and mechanical properties

In this work, the effect of strain path on the strain distribution, microstructural evolution, crystallographic texture, and mechanical behavior of electrolytic tough-pitch copper was investigated by simulation and experimental procedures. The asymmetric unidirectional and cross rolling processes with a thickness reduction of 90% were performed at room temperature. The asymmetric cross-rolling (ACR) gives much better through-thickness uniformity of shear strain distribution compared to the asymmetric unidirectional-rolling (AUR). The average equivalent strain of AUR and ACR after 90% rolling reduction was 3.99 ± 1.22 and 3.66 ± 0.26, respectively. Both AUR and ACR processes improved the particle distribution in the matrix. However, the particle distribution of the AUR sample was slightly more uniform compared to the ACR sample. The shape of primary grains in the AUR copper was straight while that in the ACR sample was zigzag. The average dislocation density of the AUR sheet was larger than that of the ACR sample (2.41 × 1016 m−2 vs. 2.31 × 1016 m−2). The major recrystallization mechanism in the AUR and ACR samples was discontinuous and continuous dynamic recrystallization, respectively. The main deformation textures (Copper, S, and Brass components, and α and β fibers) were weak in both samples due to the domination of recrystallization and shear textures in the cold-rolled copper. The recrystallization texture components in the upper region were stronger than in the lower region. The uniformity of the microhardness profile of the ACR sample was more than that of the AUR sample. The AUR sample had a larger strength and a smaller ductility compared to the ACR sample. The results showed that the strain path of asymmetric rolling was effective on the strain-hardening behavior of copper during the tensile test. The fractograph of the ACR sheet exhibited ductile dimples. However, the fractograph of the AUR sample revealed many ductile shear dimples and flat surfaces.

Inter-void shearing effect on damage evolution under plane strain deformation in high-strength aluminum alloy sheet

In this study, the ductile damage responses of high-strength 7000 series aluminum alloy (AA), AA 7075-T6 sheet samples, subjected to the plane strain deformation mode were investigated using finite element (FE) simulations. In the experiments, uniaxial tension (UT) and plane strain tension (PST) tests were conducted to characterize the plasticity and ductile damage behavior of the AA 7075-T6 sheet samples. The limiting dome height (LDH) and V-die air bending tests were conducted to evaluate the ductility of the material subjected to plastic deformation and friction between the tools, and the corresponding fractured samples were qualitatively analyzed in terms of dimples using fractography. FE simulations were performed to predict the ductility of the AA 7075-T6 sheet samples under plane strain deformation using an enhanced Gurson−Tvergaard−Needleman (GTN) model, namely the GTN-shear model. The model was improved by adding the shear dimple effect to the original GTN model. The predicted results in terms of the load–displacement curves and displacements at the onset of failure were in good agreement with experimental data from the aforementioned tests. Furthermore, virtual roll forming simulations were conducted using the GTN-shear model to determine the effect of the prediction on ductile behavior for industrial applications.

Effect of Al2O3 particle content on microstructure and mechanical properties of 1060Al/Al–Al2O3 composites fabricated by cold spraying and accumulative roll bonding

1060Al/Al–Al2O3 laminated particle-reinforced aluminum matrix composites (LPRAMCs) were successfully prepared using a combination of cold spraying (CS) and accumulative roll bonding (ARB) techniques, exhibiting a synergistic improvement in strength and plasticity. The effects of Al2O3 particle content on the microstructure, tensile properties and fracture morphology of LPRAMCs were studied. The results showed that compared with the 1# composites with higher Al2O3 particle content (20.1 vol%), the 2# composites with lower Al2O3 particle content (8.4 vol%) exhibited delayed necking fracture during the ARB process, with greater elongation (El) but lower ultimate tensile strength (UTS). After 5 passes of ARB, the UTS and El of the 1# and 2# composites were 345 MPa, 16.1%, and 293 MPa, 22.2%, respectively. This suggests that the mechanical properties of the Al–Al2O3 deposited layer have a significant impact on the mechanical properties of LPRAMCs. With an increase in ARB passes, the fracture mode of the LPRAMCs shifted from brittle to ductile fracture, displayed by equiaxed and shear dimples. These findings can provide novel insights and theoretical foundations for optimizing the mechanical properties of Al matrix composites.

Ballistic response of a high-strength steel

Quasi-static compression/tension, dynamic compression/shear mechanical property tests, fracture morphology characterization, dynamic response experiment, numerical simulation, and calculation of the ultimate penetration velocity of the high-strength steel under the impact of large mass low-velocity fragments and small mass high-velocity fragments were carried out to study the ballistic performance of high-strength steel used for armor vehicle protection and clarify its dynamic response under Ballistic impact. The fracture mode of the high-strength steel under dynamic impact compression and shear was shear failure. A large number of fine parabolic shear dimples were formed on the fracture surface. The high-strength steel had an obvious strain rate strengthening effect and adiabatic shear sensitivity. The plug formed under the low-velocity impact of a large mass fragment was a cone shape, and the impact area of the target plate underwent slight bending plastic deformation. The damage of the fragment to the target plate includes both adiabatic shear failure and ordinary shear failure. Under the high-velocity impact of small mass fragments, the fragments were seriously deformed and destroyed, and the damage model of the target plate is adiabatic shear failure.

Investigation of the microstructure and mechanical behaviour of resistance spot-welded CR210 steel joints using graphene as an interlayer

The present work investigated the resistance spot weldability of 0.85 mm thick galvanised CR210 steel sheets employing graphene nanoplatelets (GNPs) as an interlayer. The GNPs were drop-casted on the steel surface and resistance spot welding (RSW) was carried out at optimum welding current range of 6–9 kA with a constant weld time of 0.7 s. The lap-shear and cross-tensile tests were conducted to assess the mechanical behaviour of the weldments obtained with GNPs as an interlayer. An enhancement of ∼124% in the lap shear strength was observed in the specimen welded at 9 kA. However, the incorporation of GNPs led to the deterioration of the cross-tensile strength of the welded joints. The analysis of the different zones formed after welding was done using SEM and EBSD techniques which revealed the martensitic structure and finer grains obtained in the fusion zone. The presence of dislocation pileups, nano-precipitates, and interfacial shear stress transfer at the GNP-Fe interface was observed by TEM. Raman spectroscopy was carried out to get an insight into the stresses generated in the GNPs owing to RSW at different welding currents. Fracture surface analysis of lap shear specimens revealed the presence of shear dimples at the nugget zone welded at 7 kA and a mixed mode of fracture was observed in the specimen welded at 8 kA. However, a complete nugget pull-out was consistently found during the lap shear test of the specimen welded at 9 kA. Microhardness study revealed an increase in the fusion zone hardness at different welding parameters owing to the existence of GNPs and martensitic structure.

Mechanical properties and nugget evolution in resistance spot welding of Zn–Al–Mg galvanized DC51D steel

Abstract Zn–Al–Mg coating galvanized steel in resistance spot welded (RSW) in different configurations of DC51D was investigated to illustrate the nugget evolution process and mechanical properties of the joints. Results show that the microstructure of welded joints can be divided into nugget zone (FZ), heat-affected zone (HAZ), and base metal zone (BM). FZ was composed of lath martensite. The average hardness value of the weld joint was 110 HV0.2 while the FZ was up to 300 HV0.2 due to the formation of lath martensite. The failure modes can be divided into interface fracture (IF) and pull-out fracture occurred (PF) under different welding parameters, in which shear dimples showed had a typical plastic fracture morphology. The best range for welding parameters was found to be 12–18 cycles in which the nugget diameter reached 5.5 mm. The process of nugget evolution in HAZ and FZ was discussed.

Influence of alumina and zinc oxide nanoparticles on microstructure and electro-mechanical properties of aluminum metal matrix composites fabricated by customized two-step stir casting method

The present paper reports the influence of Alumina (Al2O3) and Zinc Oxide (ZnO) nanoparticles on the microstructure and electro-mechanical properties of aluminum metal matrix composites (Al MMCs) fabricated by a customized two-step stir casting method. Two types of Al MMCs were developed, (i) Al MMC-01 having 97.5 wt. % of Al and 2.5 wt. % of Al2O3 and (ii) Al MMC-02 having 95 wt. % of Al, 2.5 wt. % of Al2O3 and 2.5 wt. % of ZnO. The microstructures of Al MMC-1 and Al MMC-2 observed in a Scanning electron microscope (SEM) have affirmed the uniform distribution of Al2O3 and ZnO in the metal matrix composites. It has been observed that the addition of 2.5% Al2O3 has improved the hardness, flexural strength, and impact toughness of Al composite significantly compared with pure Al. Furthermore, both bulk and micro hardness of Al MMC-02 have also increased significantly. However, impact strength, flexural strength, modulus of elasticity, and electrical conductivity of Al MMC-02 have been reduced by 12%, 52.5%, 60.8%, and 9.81% respectively in comparison with Al MMC-01 as it becomes brittle in nature for the presence of cleavage cracks, deep shear dimples, and crystallographic planes than that of Al MMC-01 as revealed by fractured surface analysis through SEM.

Analysis of Failure Causes of 0Cr19Ni9 Blade Crack

In this paper, fracture morphology observation, microstructure analysis, standard corrosion test, and other experimental methods are used to analyze the causes of corrosion and cracking of steam turbine blades (0Cr19Ni9) produced by a certain company. The results showed that cracks appeared during the operation of the blade. A large number of dimples were observed under the scanning electron microscope, almost all of which were shear dimples; at the same time, fatigue bands were also distributed in them. The internal cause of the cracks is the long-term service, which causes the carbon and the alloy to undergo a diffusion reaction to form carbon-rich areas and carbides. They are thermally corroded in the service environment, and the carbon-rich zone is reduced, which reduces the strength of the zone; carbides accumulate at the grain boundary, causing groove-shaped defects at the grain boundary to form a cutting effect. It is easy to form a source of fatigue cracks. The external cause is that the material is subjected to transverse stress for a long time during operation, which leads to the generation of cracks. This article analyzes the failure causes of the blade and proposes related solutions, which have important reference value for production practice.

Temperature-induced enhancement of tensile strength of perforated Ti–6Al–4V sheet revealed through mechanical-thermal coupling quasi-static tension

In order to investigate the effect of temperature on the tensile strength of perforated Ti–6Al–4V (TC4) alloy, the tensile mechanical properties of TC4 alloy with various micro-hole array distributions were obtained at a temperature in a range from 20 °C to 500 °C by using a novel mechanical-thermal coupling tensile test instrument developed by ourselves. The experimental results showed that the tensile strength of all types of TC4 alloy specimens decreased with the increase in temperature, and the elastic modulus of most types of TC4 alloy specimens decreased with the increase in temperature. The tensile strength of the perforated specimen iii was 14.8% lower than that of the solid specimen and the elastic modulus of perforated specimen ii was 5.5% lower than that of the solid specimen at room temperature (RT), and all types of the perforated specimens continued to exhibit relatively lower tensile strengths and elastic modulus at 180 °C and 340 °C. Abnormally, the tensile strength of the perforated specimen iii was 2.1% higher than that of the solid specimen and the elastic modulus of perforated specimen ii was 23% higher than that of the solid specimen at 500 °C. With the increase of temperature from 20 °C to 500 °C, the fracture mode of the solid specimen gradually changed from equiaxed dimple to shear dimple. Regarding the perforated specimens, the cracks and dimples adjacent to the holes gradually decreased as a function of temperature. The existence of prefabricated holes in the specimens dissipated part of the thermal stress, maintained the integrity of the micro-hole structure, and made the nearby section relatively flat, which induced a relatively higher tensile strength and structural stiffness at an elevated temperature. The relatively lower stress in the gauge section of the perforated specimens obtained through finite element analysis also indicated the temperature-induced enhancement of tensile strength.

Study on shear buckling failure of laser-welded dissimilar aluminum alloy (Al-Li-2099/Al-Li-S4) stiffened panel

Laser welding of L-shaped aluminum alloy joints is of great significance in lightweight and efficient manufacturing of thin-wall reinforced aerospace structures. Shear performance is an important reference index for the structural design of aluminum alloy-reinforced panels. In this study, in-plane shear experiments of a continuous double-sided laser welded dissimilar Al-Li alloy (Al-Li-2099/Al-Li-S4) stiffened thin-walled panels were carried out. The deformation and buckling mode evolution of the stiffened panel were measured by the combination of fringe projection profilometry and strain gauge measurement. The results show that in the macroaspect, the stiffened panel first exhibits local skin buckling and then develops into overall buckling along the diagonal tensile direction. Finally, the stiffened panel shows weld desoldering failure. Metallographic examination and scanning electron microscopy were performed on the L-shaped joints at different positions of the stiffened panel. The microstructure observation shows that the weld was composed of a nondendritic equiaxed zone, a columnar dendritic zone, and an equiaxed dendritic zone. The failure mode of the weld is mainly brittle fracture with a small amount of shear dimples, indicating that the welded seam of the stiffened panel under the shear load is tensile shear coupling. After the force reaches material strength, the crack nucleates in the weld and extends along the weld until it penetrates the whole weld. This shows that the mechanical properties of the weld microstructure are closely related to the macroshear properties of the stiffened panel.

Friction surfacing assisted refilled friction stir spot welding of AA6061 alloy and Q235 steel

The AA6061 alloy and Q235 steel were well joined by refilled friction stir spot welding without stirring steel, as a 1 mm thick coating was deposited on the steel by friction surfacing before welding. The coating was composed of highly refined grains. The materials experienced shear deformation in the stir zones and the extrusion in hook and thermo-mechanical affected zones, resulting different predominant textures in the weld. A Fe-rich layer was depicted from the interface due to the enhanced atomic diffusion, and the layer's thickness increased as the sleeve plunged deeper during welding. Al-Fe intermetallics, including Al13Fe4, Al5Fe2, formed in the Fe-rich layer, since the solubility of Fe in Al was far lower than the measured content. The optimal shear strength reached 7.31 kN in this study. The welds fractured along the weld boundary and the Al/steel interface. The fracture initiated at the edge of the gap between AA6061 plate and coating, and propagated along the weld boundaries, resulting in the shear dimples and the faceted appearance. However, the spot weld with 2.9 mm penetration was fractured along the Al/steel interface mainly, which was attributed to the over formed intermetallics at the interface.

Resistance spot welding of high-strength DP steel and nano/ultrafine-grained IF steel sheets

In the current study, microstructural characteristics, mechanical behavior, and failure analysis of resistance spot welded joints for high-strength dual-phase (DP) steel and nano/ultrafine-grained interstitial free (IF) steel was investigated. The DP steel side indicated three different heat-affected zones including intercritical, fine-grained, and coarse-grained. The volume fraction of martensite in the coarse-grained heat-affected zone was higher than in other regions due to more growth of γ grains. The IF steel side exhibited three different subregions in the heat-affected zone, namely, ultrafine-grained, fine-grained, and coarse-grained heat-affected zone. In the coarse-grained heat-affected zone, the iron oxide was effective on the microstructure by suppressing the grain growth. The uniformity of microhardness in the fusion zone of 10 kA welding current was more than other samples due to faster homogenization of melt. All welded samples indicate hardening at fine-grained heat-affected zone (on both sides) and ultrafine-grained heat-affected zone (on the interstitial free steel side). With increasing the current of welding from 6 kA to 10 kA, the peak load increased from 7.38 kN to 11.23 kN and the energy absorption improved from 6.01 kJ to 11.72 kJ due to increasing the diameter of the fusion zone. All samples failed through partial thickness-partial pullout mode. Cleavage facets and shear dimples were observed in all samples, which indicated a mixture of brittle and ductile fractures.

Dynamic mechanical properties and microstructure evolution of AlCoCrFeNi2.1 eutectic high-entropy alloy at different temperatures

The dynamic mechanical properties of the AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) fabricated by drop-casting (DC) and suction-assisted casting (SC) are investigated under different strain rates and temperatures using Split Hopkinson Pressure Bar (SHPB) system. Microstructure analysis is conducted through SEM, EBSD and TEM to reveal the deformation and fracture mechanisms of the present EHEA. The yield strength and flow stresses for both the DC alloy and SC alloy increase with increasing strain rate, but decrease with increasing temperature. A mixture of ductile and brittle fracture morphologies exists in both the DC and SC EHEAs. Moreover, the SC alloy displays more shear dimples on the fracture surface, while brittle fracture features become dominant for the DC alloy. Abundant accumulation of dislocations in the quasi-static test produces slip traces in the FCC lamellar phase which further enhances the plasticity of the SC alloy. Meanwhile, the dislocation cells formed by high-density dislocation tangles at high strain rates, leading to apparent work hardening and strain rate sensitivity of the SC alloy. The adiabatic shear band (ASB) forms in the SC alloy before shear failure and the adjacent twisted and curved lamellar structures play a significant role in preventing shear localization. Overall, the SC alloy exhibits a stronger strain rate effect and better ductility than the DC alloy.

Diffusion Bonding of FGH 98 and CoCrNi-Based Medium-Entropy Alloy: Microstructure Evolution and Mechanical Tests

The superalloy FGH98 was successfully diffusion bonded (DB) with medium-entropy alloy (MEA) Al3Ti3(CrCoNi)94 using pure Ni as the interlayer at a temperature range of 1050–1170 °C for 1 h under 5 MPa. The microstructure and mechanical properties of joints were investigated. The diffusion bonding seam was composed of an interlayer zone (IZ) and two diffusion-affected zones (DAZ). The IZ and DAZ beside the FGH98 consisted of cubic Ni3(TiAl)-type γ′ phases due to the diffusion of Ti and Al atoms. Meanwhile, the DAZ adjacent to the MEA consisted of spherical γ′ phases. Both of the γ′ phases with different morphology kept the coherent relationship with the matrix. Moreover, increase of bonding temperature led to the morphology of interlayer γ′ phase to transform from sphere to cube. Due to the strengthening effect of a mass of γ′ phase distributed evenly in IZ and the DAZ beside the FGH98, the microhardness and Young’s modulus of these two zones were higher than that of DAZ near the MEA. The maximum shear strength of DB joint, 592 MPa, was achieved in the joint bonded by 1150 °C, which was the typical ductile fracture feature confirmed by the shear dimples.

Прочность и механизм разрушения при кручении технически чистого титана с ультрамелкозернистой структурой

The mechanical behavior and the mechanism of torsional fracture of medical grade titanium Grade4 with an ultrafine-grained (UFG) structure in comparison with a coarse-grained (CG) structure were investigated. CG titanium (dav = 25 μm) was investigated in the hot-rolled state. To obtain the UFG structure, two processing regimes were used. In the first regime, homogenization annealing was carried out at a temperature of 680°C for 1 hour, then equal-channel angular pressing (6 passes) according to the “Conform” scheme (ECAP-С) at a workpiece and tool temperature of 250°C (route Bc, φ =120°). The average grain size is equal to 0.4 μm. In the second regime, after homogenization annealing and six ECAP-C passes, the workpieces were drawn at a temperature of 200°C (ECAP-C+D). The average grain size is equal to 0.2 μm. The torsion test of samples with a diameter of 3 mm (close to the diameter of screws for fixing plates in traumatology) was carried out on a KTS 403‑20‑0.5 installation in accordance with GOST 3565-80. The torque, number of revolutions and angle of twisting of the samples made from CG and UFG titanium, as well as the mechanical properties of titanium during torsion were evaluated. Torsion tests have shown that the torsional strength and the torsional yield strength of UFG titanium increase, and the relative shear decreases in comparison with similar properties of CG titanium. In addition, the mode of nanostructuring (ECAP-C+D) provides higher values of the torsional strength properties of titanium compared to ECAP-C, which is favorable from the standpoint of resistance to fracture of titanium screws by torsion. On the surface of the fractures, there are three regions differing in microrelief: the central part of the fracture, the middle (transitional) and the peripheral parts. In the central part of the fractures, the microrelief consists of equiaxed dimples and structureless areas. In the middle part, shear dimples dominate, and in the peripheral part of fractures, areas of shear dimples alternate with areas of the rubbed surface.

Effects of Ni coating thickness on the microstructure and mechanical properties of resistance-welded WC-Co/B318 steel joints

WC-Co pellets electroplated with different Ni coating thicknesses and B318 steel were resistance welded for carbide-tipped saw blade applications. The effects of the Ni coating thickness on the welding process, interfacial microstructures, and mechanical properties of the joints were investigated. The results showed that the shear force increased before decreasing as the Ni coating thickness increased. When the coating thickness was 40 μm, expulsions began to appear during welding, while a thicker coating resulted in longer expulsion times. Voids and macro cracks were generated at the joints because of the expulsions, which reduced the shear force of the joint. The XRD results of the welding slag and the EDS results of the interface indicate that the microstructures can be divided into five phase categories: light gray (Fe3W3C), gray (Co3W3C), white (WC), fishbone eutectic structure (Fe6W6C), and dark [(γ Fe, Ni) solid solutions]. As a thicker Ni coating hinders reactions between molten Fe and dissolved WC, the joint interface contained more Fe6W6C at 20 μm and 30 μm but had more bulk Fe3W3C at 40 μm and 50 μm. The upper part of the joint showed brittle fractures on the WC-Co side, while most of the lower part fractured at the interface. Only the joints at 20 μm and 30 μm had a small fracture area on the steel side. Moreover, the fracture on the steel side at 30 μm showed elongated shear dimples, which explains the maximum shear force of the joint at 30 μm.

Microstructure and Mechanical Properties of Resistance-Spot-Welded AISI-1008 Steel Lap Joints Using Multiwalled Carbon Nanotubes as an Interlayer

The present study deals with the microstructural analysis and mechanical properties of resistance-spot-welded AISI-1008 steel joints utilizing multi-walled carbon nanotubes (MWCNTs) as an interlayer. A thin layer of approximately 50 µm thickness of MWCNTs coating is used as an interlayer between the mating surfaces of the lap joints. The microstructural investigation of the weld nugget was accomplished by light microscopy, scanning electron microscopy, and transmission electron microscopy. An enhancement of ~45% in weld strength at one of the welding parameters is obtained in lap-shear tensile tests due to the MWCNT interlayer. The shear dimple fracture is the prime mode of failure as has been confirmed by fractography. At lower and higher welding energies, interfacial and button pull-out modes of failure were observed to occur, respectively. Microhardness studies along the nugget cross section revealed a marginal increase in hardness owing to the presence of MWCNTs and also the interplay of various strengthening mechanisms.