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Organosilane-based thin coating on top of galvannealed (GA) coating, known as secondary coating, provides extra protection from corrosion and thus is used for manufacturing automotive fuel tanks. In the present investigation, resistance seam weldability of 0.8-mm-thick secondary-coated GA interstitial free steel has been evaluated and compared with conventional GA sheets of identical thickness, chemistry, and mechanical properties. It has been found that the 1-3-µm-thick layer of organosilane-based secondary coating increases the electrical contact resistance by up to 50%. Consequently, the heat input increases by 34% when welding secondary coated steel as compared to conventional GA steel. This higher heat-input ensures suitable nugget formation over a wide range of parameters viz. a wider weldability lobe. This paper also addresses the test results on the service life of the welding electrodes. It has been found that the electrodes can withstand up to 30 m of continuous welding of secondary-coated steel without affecting the weld quality in terms of the joint strength. However, for conventional GA sheets, the same electrodes lay up to 20 m of welding length prior to dressing. Higher electrode face widening takes place during welding of plain GA sheets which lowers the current density and thereby affects formation of high-quality weld nugget.

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 wa...

An iron sand concentrate was mixed with coal as the reductant and binder to produce a composite pellet by a mini rotating disc. The pellet was inserted into an alumina crucible and coal was added t...

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.

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.

This paper researches the effect of zinc coating of galvanized DP600 steel on the dynamic resistance and the delayed nugget formation of dissimilar DP600 - AISI304 welded joints, obtained with resistance spot welding process (RSW). The RSW evaluations consisted of determining, from the dynamic resistance curves, the time involved in the different stages of the process, particularly the beginning of nugget formation. The experimental results showed that, from the dynamic resistance curves, it is possible to identify 8 distinct stages during the welding of galvanized DP600 steel and AISI304 stainless steel. In the case of the welding of uncoated DP600 steel with AISI304, only 6 stages are identified (except for stages 2 and 3), which are directly related to the heating, softening and melting of the galvanic coating. The energy used in stages 2 and 3, causes a delay in the beginning of nugget formation for welded joints obtained with galvanized DP600 steel compared to uncoated DP600 - AISI304 welded joints, reaching values between 37.28 ms and 52.29 ms for the welding conditions analyzed. Monitoring the time duration of stages 2 and 3, as defined from the analysis of the dynamic resistance curves, could be used as a tool to predict the beginning of nugget formation in the welding of galvanized steels, to avoid undesirable phenomena such as expulsion and to guarantee the quality of the welded joints.

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.

Resistance spot welding (RSW) of Al-Si-coated PHS was undertaken, and the effect of Al-Si coating on nugget formation and mechanical properties was investigated. Press-hardened steel (PHS) has long been applied to automotive body structure construction to support mass and corresponding greenhouse gas emission reductions. PHS materials are often combined with an Al-Si coating applied as an oxidation barrier though unfortunately, the Al-Si coating poses a challenge to the resistance spot welding (RSW) of PHS containing stack-ups. As a newly developed coating for the hot stamping process, the property of Al-Si coating is different from base metal and traditional coatings, and the influence mechanism of Al-Si coating on the welding process is not clear. It would remain at nugget edge and might cause severe stress concentration. To investigate this problem, RSW of 1.5-mm Al-Si-coated PHS was undertaken, and the results indicate that a large portion of the Al-Si coating is extruded and forms a sharp notch close to the nugget edge during the welding process. During the post-weld cooling stage, a thin layer of residual coating is formed between the nugget and notch root. The mechanical performance of the welded joints is limited by the thin residual Al-Si layer which acts as a preexisting crack and supports the interfacial fracture. The presence of the Al-Si coating at the faying interface also significantly delays nugget formation, though it contributes to a larger nugget size by inhibiting expulsion events at the faying interface.

The resistance spot welding (RSW) stands different over other welding techniques as the actual weld nugget is shaped internally relative to the surface of the base metal. The weld formed is fundamentally a mix of heat, pressure and time. The welding conditions are precisely controlled to produce sound nuggets. The application of interface pressure allows accurate prediction of interfacial heat and electrical resistance, which plays a dominant role in Joule’s heat generation. In this work, lap welding using resistance spot welding has been numerically investigated for Mild steel (G-3302) and Stainless steel (SS-202) sheets. Spot welding has been completed by utilizing a 35 kVA DC type resistance spot welding machine, constrained by a Programmable Logic Controller unit. For both materials, two spots, three spots and four spots weld were done in order to obtain various comparative results. The weld nugget formation has been simulated using SIMULIA. The 3D model has been created by using GUI of commercial FEA package ABAQUS 6.14. The model employs coupled thermal-electrical-mechanical analysis. Simulation has been performed for pertinent parameters in ABAQUS 6.14. The obtained plots from numerical model are found to be in strong consistency with experimental data.

Press-hardened boron steels with ultrahigh strength (above 1500 MPa) are widely used in crash-sensitive safety components in automobiles. Joining such steels to aluminum alloys is challenging due to various factors including the steel’s tenacious Al-Si coating. A novel application of ultrasonic plus resistance spot welding was developed for such dissimilar metal joining. The nugget formation and the interface microstructure especially intermetallics formed were correlated to the joint strength, ductility and failure behavior.

In this paper, an invented electrode named NewTon Ring (NTR) and the other two Flat Top (FT) electrodes of having 5 mm and 8 mm outer diameter were employed to weld 2 mm AA 5182-O aluminum alloy sheets. The appearance and mechanical performance of the Resistance Spot Welding (RSW) welds obtained using the selected electrodes were investigated. The results showed that the tensile shear and cross-tension strength of the joints obtained by NTR electrode were higher than those obtained by FT electrodes, which have the same outer diameter and contact area compared with NTR respectively. The improvement of the weld strength was attributed to the formation of a bigger weld nugget and spatter-free process achieved by NTR electrode compared to FT electrodes. A new nucleation mechanism was proposed for the weld nugget formation, which was different from the traditional RSW method.

Metallic coatings have been widely used for hot-stamped boron steel in automotive applications. Surface coating in hot-stamped boron steels has improved corrosion resistance. However, a thick coating layer degrades resistance spot weldability. In the case of a Zn-coated hot-stamped boron steel, the high contact resistance and material stiffness with low vaporization temperature of the alloyed layers result in a narrow current passage at the faying surface. Expulsion occurs at low current due to rapid heat development, resulting in a narrow weld current range. The pulsed current can control the heat input by applying a fast cool time during welding to improve the mechanical properties of the joint and acceptable current range. In this study, Zn-coated hot-stamped steels were resistance spot welded using various pulse current procedures, and welding condition was optimized via three pulsed current steps. Step 1: Contact area was controlled without nugget formation. Step 2: Nugget growth was minimized and contact area was maximized. Step 3: The maximum acceptable current range and nugget diameter were achieved. These welding conditions improved the acceptable current range and mechanical properties of spot welds due to increased spot weld size.

Moxa Arch is a potential site for carbon sequestration in the state of Wyoming, recognized by the US Department of Energy. In this paper, we primarily focus on improving our understanding of the geology, including lithofacies and depositional environment, of Nugget Sandstone-a potential carbon storage reservoir, by integrating results from three different techniques including seismic attributes, seismic inversion, and petrophysical modeling. The Nugget Sandstone formation is primarily an eolian sandstone, deposited in the early Jurassic and is present throughout southwestern Wyoming. Seismic attribute analysis indicates the presence of NW-SE trending elongated geological features in the Nugget Sandstone interval. Based on our seismic and well log analyses, we interpret these features to be eolian sand dunes, which is consistent with the previous publications indicating a general NE-SW paleo-wind direction at the time of the deposition of Nugget Sandstone and other equivalent formations in Wyoming and Utah. The petrophysical analysis indicates that the Nugget Formation is mostly composed of quartz; however, clay and evaporites such as anhydrite and halite are also present. The acoustic impedance, derived from well logs, indicates that high porosity dunal sandstones correspond to low impedance values whereas interdunal evaporites are characterized by high impedance values. Combined analysis of seismic attribute coherence and inverted P-impedance discriminates the dunal and interdunal deposits in 3D seismic data volume; the low coherence defines the extent of low impedance dunal deposits. Detailed analysis of the curvature attribute from the seismic data indicates a dominant paleo-wind direction of approximately N225°.

Highly non-uniform temperature gradients experienced by workpieces during resistance spot welding (RSW) can lead to the formation of a subcritical heat affected zone (SCHAZ), a region prone to “premature” failure for ultra-high strength steels. Accurate prediction of spot weld properties such as surface electrode indentation and local hardness is essential for computer-aided engineering (CAE) based design of light-weight and impact-resistant structures. In this study, a 3D fully coupled electro-thermo-mechanical model incorporating an improved electrical contact resistance formula from the literature is developed for resistance spot welding of aluminium‑silicon coated hot-stamped boron steel. The temperature profiles, contact pressure distribution, nugget formation, and electrode indentation during RSW are numerically investigated. Tempering kinetics of base metal martensite is experimentally measured by isothermal tempering tests and used to extract kinetics parameters for a Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The non-isothermal JMAK equation coupled with the process model is shown to accurately predict local SCHAZ softening. Keywords: Resistance spot welding, Hot-stamped boron steel, Electro-thermo-mechanical simulation, Martensite tempering, Johnson-Mehl-Avrami-Kolmogorov, Automotive light-weighting

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.

This study investigates the effects of superimposed ultrasonic vibration on resistance spot welding (RSW) of aluminum alloy AA6061-T6. A novel ultrasonically assisted resistance spot welding process, referred to as ultrasonic resistance welding (URW), is applied to weld AA6061 at various conditions and compared with the results from RSW. Lap shear tensile tests of URW welds show significantly improved mechanical properties, including higher strength, better ductility and higher amount of energy absorbed prior to failure compared with RSW welds. Optical micrographs of the weld cross section show that the ultrasonic vibration enhances nugget formation and minimizes porosities defects. Besides, equiaxed crystal structure is generally observed in the nugget center of URW welds whereas the majority of RSW fusion zone is dominated with columnar grain structure. In situ thermal imaging of the two processes shows a lower temperature distribution, smaller heating and cooling rates during URW. This is attributed to the ultrasonically assisted fracture and breakdown of surface oxides and contaminates, which reduces the contact resistance and accordingly heat generation rate.

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&rsquo, 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.

During a Geologic Carbon Storage process, supercritical CO2 (scCO2) is subjected to a series of dynamic and static conditions where the relationship between pore geometry and the trapped scCO2 curvature remains to be established. To mimic the dynamic process, two sandstones, Bentheimer and Nugget, were subjected to two successive drainage and imbibition (D-I) cycles and X-ray computed tomography scanned at each residual state to capture the wettability evolution at static conditions in the same pore geometry. Both sandstones contain similar grain size distributions, pore size distributions, and pore interconnectivity but differ in that the Nugget formation contains approximately half the porosity of the Bentheimer sandstone, and the pore network contains dead-end pores. scCO2 size distributions, strain calculations, and geometric contact angle measurements were used to characterize the curvature of scCO2 in different pore types between cycles. An increase in geometric contact angle was the greatest when advancement along the pore network of the same ganglion occurred between cycles while strain increased the most with pore-filling trapping. Moreover, Nugget sandstone results in a greater aggregated residual saturation and shows a clear increase in scCO2 sizes with an additional D-I cycle while scCO2 in the Bentheimer core shows a more complex response with some ganglion increasing and some decreasing in size with an additional D-I cycle. From this work, we suspect the pore geometry is playing a role in scCO2 size distributions and use this information to suggest using water pulses to enhance trapping capacity in lower porosity sandstones.

Recently, weight reduction and improvement of crashworthiness of auto bodies have become important issues. At the same time, stiffness of auto bodies is also needed to ensure a smooth ride. Using h...

Resistance Spot Welding (RSW) is one of the most efficient fusion welding processes which known as a thermo – electric process used in joining metal by a combination of adjustable parameters (heat, pressure and time), it is high speed, flexible and highly productive process. It is extensively used for many industrial purposes in particular in the automotive industry. The aim of the present work is to study the effect of welding nugget area on the tensile strength of the resistance spot welded joints, the present discussion deals with effect of the parameters of resistance spot welding process (current, time, electrode force) on nugget formation and on failure load of the spot weld joints (the relation between nugget size and strength with input parameters). The test material of the specimens which used in this study is low carbon steel. The mechanical performance (peak load, failure mode) of the welded joints was determined by tensile testing, the testing results were analyzed which were obvious the effectiveness and the necessaries of the suitable or proper welding parameters for the strength of the spot weld joints

Resistance spot welding process (RSW) is one of remarkable manufacturing processes in automotive industry. There are several process parameters; quantum of weld current, weld time, and electrode force which affect the weld nugget formation and its strength. Therefore, it is necessary to optimize the process parameters of RSW. In this paper spot welded joints of similar sheets thickness have been welded. The tensile specimens have been made according to AWS standard. The optimal parameters for welding machine are evaluated by using Design Expert Program. The tensile shear tests presented three failure modes, interfacial, partial interfacial and pullout failure. These modes related to diversity metallic composition of each steel type and the variance of the sheet thickness.

The selection and application of the type coating to steel sheets in automotive industries depend on factors like corrosion protection, manufacturing compatibility and cost. The ‘first generation’ type primer coating improved the corrosion protection of chromate free pretreatments, however it deteriorated the resistance spot weldability of the sheets. The development of 2nd generation primer coating with additives such as CNT showed dominance in corrosion protection properties along with zinc. The present paper investigates the resistance spot weldability of the CNT added primer coating. The weldability was evaluated by comparing the CNT added primer with 1st generation primer for nugget size and electrode life. At lower welding currents, the nugget size of the CNT added primer was significantly lower than the primer coated steels. The increase in the dynamic resistance of the CNT coated steel after nugget formation led to higher nugget sizes at higher currents. The decrease in aluminum and increment in the Fe concentration at the electrode surface during welding of CNT based primer coated steels would improve the electrode life.

A double-pin tool is adopted in the submerged friction stir lap welding of high density polyethylene sheets in the present investigation. Effects of welding parameters (rotational speed, welding speed, plunge depth) on weld morphology and tensile strength are investigated. The experiment is arranged by taguchi’s L9 orthogonal array. Macromorphology and microstructure of three typical welds are observed and compared. Analysis of variance is adopted to assess the importance of each welding parameter to the joint strength. The results show that joint with low tensile strength presents poor weld formation. Voids and cracks can be observed in the nugget by scanning electron microscope. Joint with high strength shows excellent nugget formation and inverted bowl-shaped weld cross section. Joint strength increases with the increase of rotational speed or the decrease of welding speed. Small or large plunge depth is not beneficial to the joint strength. The joint with maximum tensile strength of 15.3 MPa is obtained at rotational speed of 1300 r/min, welding speed of 20 mm/min and plunge depth of 0.1 mm. Welding speed has the most significant effect on the joint strength, about 75.37%, while plunge depth has the least influence.

Variation in dynamic contact resistance with the change in welding process parameters such as weld current, weld time and electrode force were taken into account for establishing the range of adequate nugget formation parameters. Influence of the welding process parameters on the shear – tensile strength, nugget diameter and the observed failure mode was analysed. The adequate resistance spot welding process parameters for galvannealed high strength interstitial free steel sheets of 1.6 mm thickness were estimated as 8 kA weld current, 250 ms weld time and 3.5 kN electrode force. Increase in the mean dynamic contact resistance led to a significant reduction in nugget diameter. A critical nugget diameter distinguishing between the IF and PF mode was experimentally determined by failure mode analysis. Different numerical models for estimation of critical nugget diameter were evaluated.

Two types of dissimilar AA6061/H70 thermo-compensated resistance-spot-welding joint configuration are designed. Thermal-electric coupled finite element models are built to reveal the temperature field of these two types of joint. An AISI 201 stainless-steel thermo-compensated sheet assists resistance spot welding through heat conduction. The nugget formation is mainly due to conductive heat from this cover sheet when the thermo-compensated sheet is set adjacent to the AA6061 workpiece (type A joint) and is driven by both Joule heat and conducted heat when the cover sheet is set adjacent to the H70 workpiece (type B joint). To ensure the nugget formation, the conducted heat portion requires a long welding time while the Joule heat portion requires a high welding current. The type B joint configuration has higher Cu and Zn contents in the fusion zone, which leads to a high intermetallic content in the Al-Cu-Zn ternary system existing in the nugget. Most type A joints thus have a higher peak load than type B joints in tensile shear tests.

Resistance spot welded joints in different configurations of DP600 and DC54D were investigated to elucidate the nugget formation process and mechanical properties of the resultant joints. Results show that, when the welding time was less than 4 cycles, the fusion zone (FZ) was not formed, but the heat-affected zone (HAZ) occurred with a “butterfly” shape. In 4 cycles, the FZ in dissimilar sheets occurred with an “abnormal butterfly” shape because of nugget shift. When the welding time increased to 14 cycles, the FZ exhibited a “bread loaf” shape and the weld shifted to “dog bones.” The nugget can be divided into three regions, namely, FZ, HAZ1, and HAZ2, and the FZ consisted of lath martensite. The micro hardness of DP600 FZ was lower than that of HAZ because of the dilution of DC54D. The failure mode of B changed from interfacial failure to plug failure during the nugget formation process. The tensile-shear load of sound weld is 6.375, 6.016, and 19.131 kN.

In this study, an effective numerical simulation on resistance spot welding of dual-phase steel was conducted. The electrical, thermal and mechanical behaviours during squeezing, welding and holding stages were analyzed, respectively. Large lath martensite was formed in the fusion zone during the experiment. A good agreement on nugget development was obtained between numerical and experimental results. The size of contact region and pattern of contact pressure distribution at the squeezing stage were found to be force-independent. The variation of contact pressure distribution during the welding process was interpreted combining the temperature and deformation distribution of the workpiece. The contact pressure at both edges of contact interfaces was reconcentrated after nugget formation. The compressive state of axial and radial stress at the weld nugget was explained through the thermal expansion mismatch and the electrode force effect. The plastic annulus and notch between workpieces were also successfully simulated.

Production processes like continuous annealing and continuous galvanizing/galvannealing rely on welding of two strips across their width, for continuous operation. The operational stability of these lines depends highly on the consistency of the welding process. Any failure of weld joint during processing is detrimental to production as the recovery times are very high (~1–3 days). Two instances of similar weld failures having identical chemical composition and dimensions of the strips were investigated to identify the root cause and establish adequate countermeasures. A systematic approach to evaluate weld performance using metallographic examination was carried out. The examinations revealed that poor nugget formation was the root cause of failure. The nugget formation, its orientation and size were also evaluated after carrying out a series of trials with different welding process parameters, and an optimum condition was arrived at. In summary, the weld failure occurred due to insufficient and unbalanced nugget formation which could be rectified by modifying the welding parameters such as current, pressure and speed.

Our study aims at developing an efficient quality monitoring system in small scale resistance spot welding based on dynamic resistance. The dynamic resistance variation was related to weld nugget formation process. An initial resistance peak caused by asperity heating was detected. The second peak in dynamic resistance and single peak in dynamic voltage could be attributed to bulk material heating. An obvious interrelationship could be found between end resistance and weld quality. The features extracted from dynamic resistance curve were mainly influenced by welding current. The overall resistance level was dropped as welding current enlarged. The multiple linear regression analysis and back propagation neural network were then used to estimate the weld quality in combination with extracted features. Result of the regression analysis for quality prediction was basically satisfactory. The proposed neural network model showed a better performance than regression analysis regarding the maximum estimation error and root mean square error. Accuracy of the neural network based quality estimation could be further improved combining quality level classification strategy. Combination of the dynamic resistance measurement with neural network model was supposed effective to achieve the quality monitoring purpose in small scale resistance spot welding.

During the resistance spot welding (RSW), the thermal process plays a crucial role on nugget formation, especially the temperature field at the workpiece–workpiece interface, since it dominates the nugget diameter which is acknowledged generally as the quality criterion for welds, whereas it could hardly be measured experimentally. This work developed a solution for the RSW process of aluminum alloy according to inverse heat conduction problems. A direct transient heat conduction model was first established considering the variation of contact resistance with temperature and electrode force, the temperature-dependent thermophysical material properties, etc. A developed inverse model was then solved via the conjugate gradient method combined with the direct model based on the experimental temperature measurements at the workpiece surface by infrared thermometry. The calculated temperature distributions at the interface of workpieces were examined by the resulting development of the nugget diameter,...

The present paper deals with the joining of 22MnMoB hot-stamping annealed steel carried out by the spot welding process. Microstructural characterization, microhardness testing and tensile-shear testing were conducted. The effects of the welding parameters, including the electrode tip diameter, welding current, welding time and electrode force upon the tensile-shear properties of the welded joints, were investigated. The results showed that a weld size of 9.6 mm was required to ensure pullout failure for the 1.8-mm-thick hot-stamping annealed steel sheet. The welding current had the largest influence upon the tensile-shear properties of the 22MnMoB steel welded joint. The bulk resistance should play an important role in the nugget formation. In pullout failure mode, failure was initiated at the heat-affected zone, where softening occurs owing to the tempering of martensite.

Ni-laterites from the Dominican Republic host rare but extremely platinum-group element (PGE)-rich chromitites (up to 17.5 ppm) without economic significance. These chromitites occur either included in saprolite (beneath the Mg discontinuity) or as 'floating chromitites' within limonite (above the Mg discontinuity). Both chromitite types have similar iridium-group PGE (IPGE)-enriched chondrite normalized patterns; however, chromitites included in limonite show a pronounced positive Pt anomaly. Investigation of heavy mineral concentrates, obtained via hydroseparation techniques, led to the discovery of multistage PGE grains: (i) Os-Ru-Fe-(Ir) grains of porous appearance are overgrown by (ii) Ni-Fe-Ir and Ir-Fe-Ni-(Pt) phases which are overgrown by (iii) Pt-Ir-Fe-Ni mineral phases. Whereas Ir-dominated overgrowths prevail in chromitites from the saprolite, Pt-dominated overgrowths are observed within floating chromitites. The following formation model for multistage PGE grains is discussed: (i) hypogene platinum-group minerals (PGM) (e.g. laurite) are transformed to secondary PGM by desulphurization during serpentinization; (ii) at the stages of serpentinization and/or at the early stages of lateritization, Ir is mobilized and recrystallizes on porous surfaces of secondary PGM (serving as a natural catalyst) and (iii) at the late stages of lateritization, biogenic mediated neoformation (and accumulation) of Pt-Ir-Fe-Ni nanoparticles occurs. The evidence presented in this work demonstrates that in situ growth of Pt-Ir-Fe-Ni alloy nuggets of isometric symmetry is possible within Ni-laterites from the Dominican Republic.

In this paper, an experimental and analytical study of the weld nugget development in resistance spot welding process of 304L stainless steel sheets having equal and unequal thickness is presented. Experimentally, the metallographic technique has been used to determine the size and location of different weld nuggets realized under a given welding parameters which varies from no nugget formation to expulsion. Two stainless steel sheet thicknesses (1 mm and 1.5 mm) were investigated. The effects of welding current intensity and time on nugget growth are studied. Moreover, the influence of the electrode applied load in weld nugget displacement has been also investigated. Analytically, a two dimensional thermal model was developed with ability to predict the electrical contact resistance and nugget size during spot welding of equal sheet thicknesses. The maximum nugget thicknesses are reached when the electrical contact resistance becomes maximal, contrary to nugget diameter which is less influenced by the electrical contact resistance. For a given sheet thickness ratio (1/1.5), the nugget displacement induced during spot welding of unequal sheet thicknesses remains not influenced by the variation of the welding parameters.

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.

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.