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The aim of the current research is to compare the surface roughness models based on the Box–Behnken Design and Levenberg–Marquardt Algorithm-based artificial neural networks. For prediction of the surface roughness of magnetic abrasive flow machining (MAFM) finished rare earth oxides (REOs) aluminium composites, Box–Behnken Design models were developed using three-level factorial design as magnetic flux density, number of cycles and extrusion pressure as process parameters. The artificial neural networks predictive models of surface roughness were developed using feed forward back propagation network procedures called the Levenberg-Marquardt Algorithm. Also, an attempt has been made to compare the Levenberg–Marquardt Algorithm-based artificial neural networks and Box–Behnken Design for the modeling of surface roughness results. The value of coefficient of determination for the Box–Behnken Design model is found to be high ( R2 = 0.9737), which is an indication of good fit for the model with high significance. The percentage error for the Box–Behnken Design model is observed to be more as compared to the Levenberg–Marquardt Algorithm-based artificial neural networks model. The comparison evidently indicates that the prediction capabilities of trained artificial neural networks models are far better than the Box–Behnken Design models. Further, specimens were examined using atomic force microscopy for three-dimensional surface profiles.

The primary aim of this investigation is to examine the resistance spot weldability of dissimilar interstitial-free (IF) and high-strength low-alloy (HSLA) steels. The effect of dynamic contact resistance on the nugget diameter is examined. The mechanical properties of the spot-welded specimens are investigated in both tensile shear and coach peel configurations. These experiments are supplemented by macro- and microstructural examinations, determination of microhardness profiles, and post-failure examinations, including fractography. It is observed that the nugget diameter increases as mean dynamic contact resistance decreases. The heat-affected zone of the HSLA side of the weld joint shows the maximum hardness, whereas the base metal of the IF side shows the minimum hardness. The hardness at the fusion zone of the dissimilar HSLA–IF joint lies in between that of fusion zone of similar HSLA–HSLA and IF–IF joints because of the homogenization of the chemistry of both the grades. The load carrying ability of the IF–HSLA joint is found to be closer to that of IF–IF joint but significantly lower than that of HSLA–HSLA joint; this is because the location of failure for IF–HSLA and IF–IF joint is same, i.e., the HAZ/base metal interface at IF side, while that of HSLA–HSLA joint is the base metal of HSLA steel.

This study aims to investigate the influence of nugget diameter on tensile-shear and fatigue properties of the dissimilar resistance spot-welded joints of interstitial free and high strength niobium microalloyed steel sheets. The spot-weldings are done at currents of 7 kA and 9 kA at welding time of 300 ms with electrode force of 2.6 kN. Fatigue tests are done at different load amplitudes, considering the maximum tensile-shear load bearing capacity of the joints. The tests are interrupted before the final failure to understand the crack initiation and propagation paths. The study is supplemented by the microstructural and detailed fractographic analyses. The results indicated that the fatigue strength of the spot-welds increased with a decrease in nugget diameter. This could be attributed to the higher microhardness of heat affected zone (HAZ), presence of compressive residual stress and lower variation in thickness reduction on the IF steel side, welded at 7 kA. However, the tensile-shear load bearing capacity of the spot-welds increased with increase in the nugget diameter. The failure of the spot-welded joint under static loading initiates from the HAZ/base metal interface of the IF steel side. Nevertheless, under cyclic loading crack initiates from the notch root at the interface of two sheets lying in the HAZ of IF steel side. Fractographic investigations indicate intergranular fracture features in combination with striations, secondary cracks on the IF steel side under cyclic loading. Fracture surfaces of the specimens failed under static loading shows however, shear dimples on the IF steel side.

In present scenario the automobile manufacturers are thriving to reduce the mass of passenger cars, by improving fuel efficacy and minimizing the CO2 emissions. Dual-phase steels are one of the most commonly used advanced high strength steel grades in the automotive industries. These steels have higher strength, good formability, and adequate weldability. The steel finds several applications in the fabrication of components such as chassis, suspensions, and body frame. Resistance spot welding (RSW) is one of the most widely used processes to join thin metal sheets. The current study aims to examine the microstructural and failure behaviour of spot-welded dual phase (DP) steel joints of 1.4 mm thickness. The findings show that the microstructure of fusion zone (FZ) of the joint mostly consisting of lath martensite, while the microstructure of heat affected zone (HAZ) manifests distinct characteristics viz. heat affected zone coarse grain (HAZ-CG) with coarser martensite, heat affected zone fine grain (HAZ-FG), and heat affected zone intercritical (HAZ-IC) with some ferrite and martensite packets at grain boundaries. The base metal (BM) predominately shows ferrite with martensite islands at grain boundaries. Under tensile-shear loading, crack initiated from the base metal. While the failure of coach-peel specimen takes place from HAZ-CG. The work concludes that the structural integrity of the joint is significantly controlled by microstructural variations, joint configuration, and location of the failure.

In the automotive applications, sheet metals are inevitably subjected to pre-straining during stamping operation prior to spot welding. The integrity of vehicles is governed by the static and fatigue strengths of the joints. However, the effect of pre-strain on the tensile and fatigue properties of the spot-welds on the high strength low alloy (HSLA) steel sheets has not been well studied. To abridge this gap, steel sheets of 1.2 mm thickness in as-received condition as well as after pre-straining to three different levels of 5%, 10%, and 15% were spot-welded in the form of lap-shear joints. With an increase in pre-strain, the load-bearing capacity of the joints monotonically increased, while there was no significant effect of pre-strain on the fatigue behavior and endurance limit of the joints. This difference in tensile-shear and fatigue behavior of the joints for pre-strained samples is attributed to the variation in the location of the failure, which has been explained in terms of the difference in failure mechanisms involved with the tests. The crack initiated at the base metal in case of tensile-shear tests, whereas under cyclic loading, fatigue cracks initiated between fine-grained heat affected zone (FGHAZ) and coarse grain heat affected zone (CGHAZ) located near the interfacial notch between the two welded sheets. While the location of failure in tensile-shear test is dictated by its lower strength across the joint, that in fatigue test is governed by the stress concentration. The magnitude of dislocation density has been evaluated quantitatively in the base metal, heat affected zone (HAZ), and fusion zone of weld joint using X-ray diffraction (XRD), and the dislocation substructures have been examined by transmission electron microscopy (TEM). The increase in tensile-shear load-bearing capacity is due to increased dislocation density in base metal with pre-strain. As most of these dislocations generated due to the pre-straining are annihilated at the HAZ during welding, there was no significant effect of pre-strain on the fatigue strength of the joints. These results were corroborated by microstructural examination and micro-hardness measurements across the weld joints in as-received and pre-strained conditions.