Nugget Formation Research Articles

Resistance Spot Welding of Additively Manufactured Maraging Steels—Part I: Nugget Formation

Abstract Application of additively manufactured steels is unavoidably involved in the resistance spot welding with conventionally manufactured steels. However, the microstructural evolution of an additive manufactured steel at high temperatures is still unknown, especially for the rapid solidification process. This paper investigated the microstructural evolution of a selective laser melted maraging steel during the rapid solidification process via resistance spot welding. Asymmetrical fusion zone with boat shape was found in the spot weld due to the rougher surface and larger electrical resistance of maraging steel via selective laser melting (SLM) process. The rapid expansion of fusion zone at end of welding process was caused by the carbide formation at the heat-affected zone of maraging steel via selective laser melting process. Besides, printing orientation affected the surface roughness of a selective laser melted maraging steel and subsequently significantly influenced the early stage of formation of fusion zone of additively manufactured maraging steel. We expect that our findings will pave the way to the future application of additively manufactured steels in the industries.

Metal recycling tracked by mercury and helium isotopes in platinum–palladium nuggets from Córrego Bom Sucesso, Brazil

The enigmatic botryoidal nuggets of platinum (Pt) and palladium (Pd) from Córrego Bom Sucesso in the southern Serra do Espinhaço, Minas Gerais, Brazil, are considered to have formed during supergene alteration of placer deposits. This requires hitherto unidentified precious-metal enrichment processes and is inconsistent with Pt–Os age of 180 Ma that entails formation at depth, as recently proposed. Here we report the first mercury (Hg) and helium (He) isotopic determinations of Pt–Pd nuggets. Mercury isotopic compositions have a mass-independent fractionation (MIF) signature with an odd-mass deficit (Δ199Hg = − 0.22 ± 0.04; 1SD, n = 15), which requires aqueous photochemical reduction of Hg (II). Extremely low 3He/4He (<0.001 Ra) and extremely high concentrations of He (up to 1.9 × 1017 at/g) are indicative of nugget formation from He-enriched fluids within the quartzite sequence of the Espinhaço basin, not from meteoric surface water. The data are consistent with a nugget-forming setting in the deep biosphere, as a result of groundwater interaction with Pt–Pd–Hg minerals in Pan-African-Brasiliano post-orogenic veins. We propose that the negative Hg-MIF signature was inherited from the vein minerals that originally acquired their Hg from Earth's surface during the intracratonic sedimentation of the Proterozoic Espinhaço basin.

Machine Learning Modeling and Hyper-Parameter Optimization for Weld Nugget Formation and Failure Behavior of Resistance Spot Welds

The welding process parameters of resistance spot welding are determined by quality indicators, such as nugget diameter, and tensile shear test behavior, such as failure load and location. In this study, deep-learning models were investigated to predict the quality indicators from base materials and process parameter information. For each model, hyperparameters, such as the number of hidden layers, number of nodes in the hidden layer, learning rate of the optimizer, and number of epochs, were optimized based on the model performance. The regression models for nugget diameter and failure load showed coefficients of determination of 0.90 and 0.95, respectively. Two models were developed to classify failure location: a 1-step model that estimates the failure location from the base material information and process parameters, and a 2-step model that estimates the failure location from the base material information and the nugget diameter as predicted by the developed regression model. The classification models for failure location showed similar accuracies of approximately 90%.

Early Mass-varying Neutrino Dark Energy: Nugget Formation and Hubble Anomaly

Abstract We present a novel scenario in which light (∼few eV) dark fermions (sterile neutrinos) interact with a scalar field as in mass-varying neutrino dark energy theories. As the eV sterile states naturally become nonrelativistic before the matter–radiation equality (MRE), we show that the neutrino–scalar fluid develops strong perturbative instability followed by the formation of neutrino nuggets, and the early dark energy (EDE) behavior disappears around MRE. The stability of the nugget is achieved when the Fermi pressure balances the attractive scalar force, and we numerically find the mass and radius of heavy cold nuggets by solving for the static configuration for the scalar field. We find that for the case when dark matter nugget density is subdominant and most of the EDE go into scalar field dynamics, it can in principle relax the Hubble anomaly. Especially when a kinetic-energy-dominated phase appears after the phase transition, the DE density dilutes faster than radiation and satisfies the requirements for solving the H 0 anomaly. In our scenario, unlike in originally proposed early DE theory, the DE density is controlled by (eV) neutrino mass and it does not require a fine-tuned EDE scale. We perform a Markov Chain Monte Carlo analysis and confront our model with Planck + SHOES and baryon acoustic oscillation data and find evidence for a nonzero neutrino–scalar EDE density during MRE. Our analysis shows that this model is in agreement to nearly 1.3σ with SHOES measurement, which is H 0 = 74.03 ± 1.42 km s−1 Mpc−1.

Nugget formation analysis of Al/Steel clinch-resistance hybrid spot welding

A clinch-resistance hybrid spot welding method was presented to join Al and steel. The nugget formation was analysed via microstructure observation and numerical simulation. A cup section nugget was formed, and the melting zone was mainly formed at the nugget edge with a thin layer of interfacial intermetallic compounds. Numerical simulation results showed that peak contact stress on the steel/Al contact surface was located at the connection between the punch tip and die tips. Aa remarkable amount of heat was first generated at the nugget edge and then transferred to the centre, but the temperature at the edge was relatively low. Compared with traditional spot welding, a unique distribution of heat occurred during the welding process.

Numerical and Experimental Study of AlSi Coating Effect on Nugget Size Growth in Resistance Spot Welding of Hot-Stamped Boron Steels

In recent years, increasing automotive safety by improving crashworthiness has been a focal point in the automotive industry, employing high-strength steel such as press hardenable steel (PHS). In addition to the improved strength of individual parts in the body of the vehicle, the strength of the resistance-spot-welded joints of these parts is highly important to obtain a safe structure. In general, dimensions of weld nuggets are regarded as one of the criteria for the quality of spot-welded joints. In the presented research, a three-dimensional axisymmetric finite element model is developed to predict the nugget formation in resistance spot welding (RSW) of two types of PHS: the uncoated and AlSi-coated 1.8 mm boron steel after hot stamping. A fully coupled electro-thermo-mechanical analysis was conducted using the commercial software package Abaqus. The FE predicted weld nugget development is compared with experimental results. The computed weld nugget sizes show good agreement with experimental values.

Particle simulation of nugget formation process during steel/aluminum alloy dissimilar resistance spot welding and thickness estimation of intermetallic compounds

Particle simulation of nugget formation process during steel/aluminum alloy dissimilar resistance spot welding and thickness estimation of intermetallic compounds

Effect of different lapping orders on mechanical performance and nugget forming process for three-sheet dissimilar resistance spot welding joints between DC06 and unequal-thickness hot-stamped B1500HS steel sheets

In this study, the mechanical properties of two welded joints under different lapping orders (B1500HS-1.4 mm/B1500HS-1.6 mm/DC06, denoted as type I; B1500HS-1.4 mm/DC06/B1500HS-1.6 mm, denoted type II) were compared. The nugget formation mechanism was analysed by a coupled electrical-thermal finite element model (FEM). It is found that different lapping orders significantly affect the mechanical properties of three-sheet RSW joints. All RSW joints tend to fail in the pull-out or tearing failure mode. The peak load of the two interfaces of type II RSW joint is more balanced, and the failure load of which is much higher than that of the type I RSW joint; The warpage was observed at type I welded joint. Considered the load-carrying capacity, type II was excellent. The simulation results indicate that the diameter of the weld nugget at the upper interface of the type I RSW joint was larger than that on the other interfaces, which agree well with the experimental results. The nugget formation mechanism of dissimilar high strength steel three-sheet RSW joint was obtained that forming the nugget firstly from two interfaces and final formed nuggets were asymmetric.

Short-Pulse Resistance Spot Welding of Aluminum Alloy 6016-T4 - Part 1

Short-pulse welding parameters for resistance spot welding (RSW) of aluminum alloy AA6016-T4 using mediumfrequency direct current (MFDC) systems were developed to reduce the heat input required for nugget formation. Optimization of current and time parameters is critical during RSW of aluminum alloys for reducing energy requirements and avoiding weld imperfections, such as solidification cracking and expulsion, while maintaining weld quality, particularly given the high electrical and thermal conductivities of the materials. The welding time and the applied current level of the current pulse were varied systematically for thin sheets (1 mm or 0.04 in.) of AA6016-T4. The quality of the welds was evaluated by pull-out testing, ultrasound testing, and metallography techniques. Simulations of the same welding processes were performed with the finite element-based SORPAS® software. The results showed short-pulse MFDC RSW can reduce the energy required for sound welds in this alloy without requiring an increase in welding current. The simulations and experiments also showed the welding process had distinct weld nugget nucleation and growth phases.

Role of Welding Current on Mechanical Properties and Microstructural Characteristics of Resistance Spot Welded Dual Phase Steel Joints

Electrical Resistance Spot Welding (ERSW), an ideal sheet metal welding process for automotive applications due to the excellent reproducibility and weld integrity of the process. Dual-phase steels, the next-generation advanced high strength steel is utilized in white body applications. This paper discusses the effect of current on mechanical properties and microstructural features of the weld nugget. Macrostructure reveals the optimal welding current as 5.5 kA for decent nugget formation with maximum penetration and less heat-affected zone. The nugget microstructure consists of lath and needle-like martensite structure in the iron matrix. The shear fracture test revealed nugget pull-out failure at the periphery. The results concluded the role of current in the penetration and nugget formation along with the softening effect, which corresponds to the nugget failure.

Heat evolution and nugget formation of resistance spot welding under multi-pulsed current waveforms

The pulsed current waveform is essential for the precise control of nugget size in resistance spot welding (RSW). Understanding this link may enable high-quality RSW for the emerging materials. To shed light on this issue, a 3D electrical-thermal-mechanical coupled model is developed to simulate transient multi-physical fields and nugget shapes in RSW. The effects of pulsed current waveforms on the heat evolution and nugget formation are elaborated. The model is validated by calculating nugget size and weld indentation well consistent with the experimental measurements. The results show that with the same energy input, although the continuous and pulsed currents can provide acceptable nugget sizes, the “ramp-up” pulsed current with a high initial current can well match the variations of temperature-dependent electric resistance and lead to an enlarged nugget size. The electrode temperature maintains at a low level with an extended electrode life. This work will provide a better understanding of the heat evolution and nugget formation during RSW process under different multi-pulsed current waveforms, and thus can help to select a proper current waveform.

Experimental investigation on resistance spot welding of dissimilar weld joints

In the present study, galvanized High Strength Interstitial Free (HIF) steel sheets, and Dual Phase (DP780) steel sheets were used for the investigations. Resistance spot weld joints were fabricated between dissimilar steel sheets. The variation in dynamic resistance (DR) with the change in welding process parameters such as weld current, weld time and electrode force were used for establishing the range of adequate weld nugget formation parameters. Effect of these parameters over tensile strength, nugget diameter and the observed failure mode was studied using one factor at a time (OFAT) approach. Microstructure and hardness of parent metal, fusion &amp; HAZ region has also been studied.

Numerical and experimental analysis of heat transfer in resistance spot welding process of aluminum alloy AA5182

In this work, a numerical model and experiments are used to investigate heat transfer processes during resistance spot welding process of aluminum. For this purpose, calibrated heat transfer conditions and thermal contact conductance are transferred from a previous work to a coupled thermal-electrical-mechanical finite element model. First, all domains of the numerical model are validated by an experimental study. The experimental setup includes the measurement of current, voltage drops, electrode force, electrode displacement, and temperatures while two sheets of aluminum alloy AA5182 are joined. Computational results show that most of the generated Joule heat (78%) is stored in the electrodes or transferred to cooling water until the end of weld time. Heat transfer by natural convection and thermal radiation is very small and can in general be neglected for complete process. Afterwards, the influence of electrode water-cooling on welding process is investigated numerically. The results indicate that the generation of Joule heat and thermal energy of the sheets during weld time is only slightly affected by electrode water-cooling. As a consequence, water-cooling conditions do not affect nugget formation. In contrast, electrode water-cooling highly influences cooling conditions during hold time.

Experimental Investigation of Contact Welding Characteristics of Electromechanical Relays

Contact dynamic welding failure problems seriously limit the electrical lifetime of electromechanical relay. However, the welding behavior occurred in contact making and breaking process and relevant influencing factors are still unknown. In this article, a series of experiments are carried out in make only, break only, and make & break (M&B) operation modes individually to differentiate the effects of making and breaking processes on contact dynamic welding. The typical dynamic force and electrical waveforms of contact making and breaking processes are obtained and the statistical results of welding force are presented. The roles of bounce arc and molten bridge in welding force are interpreted explicitly by using the variations in electrical parameters such as bounce energy, contact resistance, molten bridge energy, break arc energy, and surface substructure morphologies. It is determined that the bounce arc plays a crucial role in the formation of weld nugget and strength, and the strongest welding occurs in the M&B operation mode.

Comparison of mechanical properties and the nugget formation of composite ceramic-centered annular welding and traditional resistance spot welding

A resistance welding method that uses a composite ceramic-centered annular electrode (RAW) was adopted to improve the weldability of high-strength steel. The design idea was to increase the strength of welded joints by the formation of a large nugget with low indentation depth. The strength and the nugget formation of joints produced by the RAW, and the traditional spot welding (RSW) were comparatively investigated by the microstructural, artificial neural network, and finite element model analyses. The highest failure load reached 22.3 kN in the RAW, and the strongest influence on the strength was welding time, followed by welding current, and electrode force at the end. The interaction between these welding parameters significantly influences the strength of joints in the RAW but not in the RSW. Based on the ANN models, a detailed relationship between the parameters and the strength in RAW were obtained, which also determined that the optimal window of welding parameters in the RAW was larger than that in the RSW. The FEM model analysis showed that the annular nugget was generated first, then expanded outward and inward in RAW, and finally completely fused in the nugget center. The mode of heat transfer and the nugget formation could contribute to better dissipation of the resistance heat generated in the RSW. This “soft” heat distribution yields a large nugget with low indentation depth.

Joint formation mechanism and performance of resistance rivet welding (RRW) for aluminum alloy and press hardened steel

The options for joining aluminum (Al) alloy to ultra-high strength steels are extremely limited due to the great differences in their thermophysical properties. In this work, a novel process, e.g., resistance rivet welding (RRW), was proposed to join Al alloys to press hardened steels (PHS) through combining traditional self-piercing riveting and resistance spot welding. The materials plastic flow, element diffusion and nugget growth characteristics during RRW were studied in depth to reveal the nugget formation mechanism. It was found that in RRW process, the intercepted Al alloy, internal wall of rivet shank and PHS near the faying surface were melted together and produced a unique Al/steel mixed nugget. The microstructure of the mixed nugget was equiaxed ferrite with a large number of B2-structured intermetallic compounds (IMCs) FeAl dispersed within it. The competition between the nugget and Al sheet determined the joint mechanical performance. The nugget dominated joint strength when the heat input is comparatively low while the Al sheet played a significant role with the increase of heat input.

Nugget Formation and Mechanical Behaviour of Friction Stir Welds of Three Dissimilar Aluminum Alloys.

The aim of this research was to investigate the influence of the properties of the base materials and welding speed on the morphology and mechanical behavior of the friction stir welds of three dissimilar aluminum alloys in a T-joint configuration. The base materials were the AA2017-T4, AA5083-H111, and AA6082-T6 alloys in 3 mm-thick sheets. The AA6082-T6 alloy was the stringer, and the other alloys were located either on the advancing or retreating sides of the skin. All the T-joint welds were produced with a constant tool rotation speed but with different welding speeds. The microstructures of the welds were analyzed using optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, and the electron backscatter diffraction technique. The mechanical properties were assessed according to micro-hardness, tensile, and fatigue testing. Good quality welds of the three dissimilar aluminum alloys could be achieved with friction stir welding, but a high ratio between the tool’s rotational and traverse speeds was required. The welding speed influenced the weld morphology and fatigue strength. The positioning of the skin materials influenced the nugget morphology and the mechanical behavior of the joints. The joints in which the AA2017 alloy was positioned on the advancing side presented the best tensile properties and fatigue strength.

Effect of a slightly concave electrode on resistance spot welding of Q&amp;P1180 steel

Abstract The resistance spot welding (RSW) of advanced high strength steel exhibits a high tendency to fail in unfavorable interfacial fracture. To solve this problem, a novel slightly concave electrode was designed. The RSW joints of Q&P steel obtained with the conventional dome-radius electrode and the novel slightly concave electrode in terms of microstructure, micro-hardness and tensile-shear properties were investigated and compared. Numerical simulations were conducted to reveal the thermo-electric field evolution and explain reasons for the improvement of weldability with the slightly concave electrode. The results show that by aid of slightly concave electrode, the welding current range meeting the requirement of button pullout failure was doubled and upper limits on tensile-shear load reached 28.5 kN and achieved 16 % improvement. By the employment of slightly concave electrode, a ring-shaped distribution of current density formed at the initial stage due to the ring-shaped contact interface of electrode/steel, which fabricated an annular nugget. Because of the lower current density and the stronger corona bond zone after the formation of nugget, the slightly concave electrode decreased the possibility of early expulsion, resulting in larger upper limits on nugget and wider welding current range.

Mechanical properties of dissimilar Ti-Al resistance spot welds

Abstract In this study, the mechanical behavior of titanium (ASTM Grade 2) alloy sheet, resistance spot welded with 5754 aluminum alloy, was investigated with respect to nugget formation and mechanical properties. The welding operations were performed using resistance spot welding at various welding currents (10, 12.5 and 15 kA), welding times (10, 15 and 20 cycles) and electrode forces (5, 10 and 15 kN). The weld nugget diameter and nugget center height, hardness, and tensile-shear test results of the welded samples were examined, and the mechanical properties were compared. As a result of the tests, it was seen that the welding parameters have a significant effect on the mechanical properties of the titaniumaluminum alloy welding joints. The tensile-shear load of the resistance spot welded joints increased as the welding current increased, and welding time increased for 15 cycles. On the other hand, when the electrode force increased, the tensile-shear load decreased markedly.

Mechanical properties of dissimilar Ti-Al resistance spot welds

AbstractIn this study, the mechanical behavior of titanium (ASTM Grade 2) alloy sheet, resistance spot welded with 5754 aluminum alloy, was investigated with respect to nugget formation and mechanical properties. The welding operations were performed using resistance spot welding at various welding currents (10, 12.5 and 15 kA), welding times (10, 15 and 20 cycles) and electrode forces (5, 10 and 15 kN). The weld nugget diameter and nugget center height, hardness, and tensile-shear test results of the welded samples were examined, and the mechanical properties were compared. As a result of the tests, it was seen that the welding parameters have a significant effect on the mechanical properties of the titaniumaluminum alloy welding joints. The tensile-shear load of the resistance spot welded joints increased as the welding current increased, and welding time increased for 15 cycles. On the other hand, when the electrode force increased, the tensile-shear load decreased markedly.