Spot Welding Technique Research Articles

The High-Cycle Tensile-Shear Fatigue Properties and Failure Mechanism of Resistance Spot-Welded Advanced High-Strength Steel with a Zn Coating.

Advanced high-strength steels (AHSSs) with Zn coatings are commonly joined by the resistance spot welding (RSW) technique. However, Zn coatings could possibly cause the formation of liquid metal embrittlement (LME) cracks during the RSW process. The role of a Zn coating in the tensile-shear fatigue properties of a welding joint has not been systematically explored. In this study, the fatigue properties of tensile-shear RSW joints for bare and Zn-coated advanced high-strength steel (AHSS) specimens were comparatively studied. In particular, more severe LME cracks were triggered by employing a tilted welding electrode because much more stress was caused in the joint. LME cracks had clearly occurred in the Zn-coated steel RSW joints, as observed via optical microscopy. On the contrary, no LME cracks could be found in the RSW joints prepared with the bare steel sheets. The fatigue test results showed that the tensile-shear fatigue properties remained nearly unchanged, regardless of whether bare or Zn-coated steel was used for the RSW joints. Furthermore, Zn mapping adjacent to the crack initiation source was obtained by an electron probe micro-analyzer (EPMA), and it showed no segregation of the Zn element. Thus, the failure of the RSW joints with the Zn coating had not initiated from the LME cracks. It was concluded that the fatigue cracks were initiated by the stress concentration in the notch position between the two bonded steel sheets.

Friction stir spot welding of recycled scrap thermoplastics

There is a real demand for sustainable lightweight structures because of the growing environmental concerns. One important solution is developing structures through recycled scrap/waste thermoplastic materials. The current work studies the friction stir spot weldability of recycled thermoplastics, which will help to analyze the potential of friction stir-based welding techniques towards developing these sustainable structures. The combined behavior of recycling-welding procedures is investigated, as they may cause degradations; to ensure that the base thermoplastic polymer's chemical, thermal, and mechanical properties are retained. Scrapped milk bottles made from HDPE material are used as a case study. The highest lap-shear load of 1528 N was achieved at the optimum welding conditions of 1600 rpm rotational speed, 1 mm plunge depth, and 60 s dwell time. Fractographic studies (macroscopic and SEM-based) suggested four types of fracture morphologies depending on welding conditions used. The DSC results showed no significant differences in melting temperature and crystalline content of the polymeric material. The TGA tests showed no significant thermal degradations. The FTIR analysis of all the samples (bottle, recycled sheet, weld material) exhibited characteristic HDPE peaks. All these results suggest combined-welding recycling had a minimal impact on the polymeric structure. Thus, friction stir spot welding (FSSW) technique joins recycled thermoplastic scrap/waste materials with high lap-shear load and without any significant polymer degradations.

Prediction of peel strength of sandwich sheets made of aluminium alloys fabricated by friction stir spot welding based hybrid process using cohesive zone modeling and finite element simulations

The present work examines the fabrication and numerical modelling of honeycomb core sandwich sheets utilizing Friction Stir Spot Welding (FSSW) techniques as prospective substitutes for adhesive-bonded structures. Here, the sandwich sheet contains AA5052-H32 skins and AA3003 honeycomb core. Two strategies, FSSW with disc insert (FSSW_D) and FSSW with disc insert and adhesive bonding (FSSW_D_AB), were evaluated against traditional adhesive-bonded sandwiches using peel test performance and finite element (FE) simulation with cohesive zone modelling (CZM). In the CZM, a homogenized core with an equivalent cohesive layer was substituted for the honeycomb core and cohesive layer. The utilization of FSSW_D and FSSW_D_AB methods resulted in substantial improvements in maximum load capacity, with enhancements in the hybrid sandwich specimen ranging from 254 % to 399 % compared to adhesive-bonded connections. The maximum load of hybrid joints (FSSW_D_AB) was 15 %–23 % higher than that of ordinary spot-welded joints (FSSW_D). An approach to model the hybrid sandwich specimen using both the homogenized core and equivalent cohesive zone model accurately was proposed. FE analysis, including peel tests of adhesive-bonded, spot-welded, and hybrid joints, was performed, and the numerical predictions agreed satisfactorily with the experimental values for all the joint types.

A new resistance insert spot welding method for injection-molded FRP–steel component

For weight reduction, multi-material designs comprising metal and fiber-reinforced plastic (FRP) components in vehicle body structures have been increasingly used. However, the commonly used resistance spot welding (RSW) technology for car body assembly cannot be employed to join sheet metal and FRPs, limiting the use of FRPs. To solve this problem, a novel resistance insert spot welding (RISW) technique was developed in this work for RSW of steel parts and FRP structure parts made by injection molding. Small inserts were developed by using finite element method and experiments that may be welded to different micro-alloyed and dual-phase sheet steels using the projection welding method. The usual flange width of original equipment manufacturers could be kept unchanged. Using the developed insert and welding parameters, the maximum temperature in the FRPs surrounding the inserts was limited to 255 °C, minimizing the damage to polyamide 6 (PA6) material (with 40 wt% glass fiber). A weldability range between 2.5 and 7 kA could be achieved. The joining strength of RISW between a micro-alloyed HC340 steel in 0.75 mm and 1.5 mm thickness and a 2.5 mm/3.0 mm PA6-GF40 material is 20 to 80% higher than self-piercing riveting (SPR). For high-speed loading, RISW strength increases by 39 to 56% further. Finally, RISW was successfully applied to an FRP–steel roof-frame sub-assembly that consists of 19 simultaneously integrated inserts, achieving 10% weight reduction.

Exploring the boundaries of refill friction stir spot welding: influence of short welding times on joint performance

Refill friction stir spot welding is a solid-state spot-welding technique suited to lap joining of thin aluminium sheets, including difficult-to-weld 2xxx series alloys that are prone to hot cracking during fusion welding processes. Long welding time is an ongoing challenge that hinders industrial adoption of the process. To address this, the present study explores much shorter welding times than those previously reported in the literature and assesses the impact on joint quality. Joints of 1.8 mm thick AA2024-T3 sheet were produced with welding times from 3 s, down to 0.75 s and rotational speeds of 1000 rpm to 2500 rpm. Defect formations within the welds were studied with the aid of optical microscopy. The mechanical properties were evaluated using tensile lap shear testing and microhardness mapping, and failure modes were characterised using scanning electron microscopy. Various weld defects were found at all welding times and rotational speeds, and the defects enlarged with decreasing welding time and increasing RS. The highest lap shear strength of 9.21 kN was achieved with a welding time of 3 s and rotational speed of 2000 rpm; lap shear strengths of 7.02 kN and 6.37 kN were achieved for 1.5 s and 0.75 s welds, respectively.

Microstructure and failure behavior of ultrasonic-assisted resistance spot welded AA6061/Ti6Al4V joints

The ultrasonic-assisted resistance spot welding (UaRSW) technique was applied to join AA6061 aluminum alloy to Ti6Al4V titanium alloy, the microstructure and mechanical performance of the joints with and without ultrasonic were investigated. Ultrasonic increased the maximum load by 43%, doubled the displacement at fracture and significantly enhanced energy absorption capacity. The solid phase transformation zone formed in Ti6Al4V, and its microstructure was lath martensite. Due to the welding defects and coarse columnar dendrite, conventional spot welded joints fractured by interfacial failure mode with poor performance. Ultrasonic promoted the diffusion of AA6061, removed the welding defects, generated consistent equiaxed grains in weld zone, and the joints fractured in AA6061 base metal by button pull-out mode result from the higher bonding strength.

Competing fracture modes in Al-steel resistance spot welded structures: Experimental evaluation and numerical prediction

Multi-material design has been increasingly used in the automotive industry for structural lightweighting. A novel resistance spot welding technique has been developed to join aluminium to steel with acceptable joint strength. However, aluminium to steel resistance spot welds can fail in either interfacial (IF) or pull-out (PO) fracture modes. In the present study, coupon-level tensile shear and coach peel samples are tested under quasi-static condition, and component-level T-joint structures are tested under impact load. Experimental results show that in the IF fracture mode, weld failure occurs due to the facture of the intermetallic layer, while in the PO fracture mode, weld failure occurs due to a combination of different mechanisms including through-thickness tearing of the weld nugget and possibly partial fracture of the intermetallic layer. In the finite element analysis, two force-based fracture criteria are implemented via mesh-independent connector element considering a general loading condition, and whichever fracture potential reaches unity first determines the fracture mode. The proposed method is validated by experiments and has the following features: firstly, it can capture the behaviour of fracture mode transitioning from IF to PO as weld diameter increases; secondly, it can simulate the maximum load and deformation at fracture; thirdly, it can indicate the global response and absorbed energy in impact analysis with reasonable accuracy. Furthermore, a distinct advantage of the model is that it is free of non-physical calibration. The parameters in the model are obtained from geometric and material properties of the weld directly. The developed method shows desirable performance and is potentially suitable for applications in large-scale automotive structural analysis and design.

Convection-effect-enhanced micro thermoelectric module directly heated by catalytic combustion

A novel micro thermoelectric power generation module directly heated with localized catalytic combustion has been proposed. Arrays of high-temperature plate-type thermoelectric generator (TEG) elements, of which the hot junction is directly heated by exothermic catalytic oxidation of hydrocarbon fuel, are designed. Heat losses caused by heat conduction through the TEG element are recovered by the convection effect with unburned mixture flows. Catalyst layers were prepared by impregnating Pd or Pt catalyst on porous Al2O3 supports coated by using the suspension plasma spray. A series of one-dimensional numerical analyses considering the catalytic combustion in addition to the conductive and convective heat transfer were made to examine the temperature field of the TEG module and to investigate how various parameters affect the convection effect. A model combustor using quartz plates was developed, which mimics the temperature fields of the SiGe TEG module. It is found that, with the aid of the convection effect, temperature differences over 700 °C between the hot/cold junctions can be achieved, while the unreacted fuel concentration is less than 10%. Finally, 40 mm-long Ni-based alloy TEG elements have been fabricated using laser cutting and spot welding techniques, which ensure high operating temperatures and reliability. In a power generation demonstration using butane as a fuel, an open circuit voltage of 0.64 V has been obtained from a single TEG module with a peak output power and a power generation density as high as 10.1 mW and 10.8 kW/m3, respectively.

Investigation of welding defects in friction stir spot welding of plastics

In friction stir spot welding applications, there are some factors that provide welding strength, welding performance and the formation of defects that may occur in welding. Among these factors, machine parameters and welding tool design should be compatible with each other. If these factors are not chosen appropriately, the welding will not be of the desired quality. In this study, high density polyethylene sheets were joined using friction stir spot welding technique. It has been determined that some of the joints made have low weld strength, defects on the weld surface and around the stir zone. By investigating the causes of these defects, suggestions were made to prevent these defecs It has been observed that these defects are not repeated in the welds made with the suggestions made and it has been determined that high weld strength can be obtained. The highest weld strength and performance were obtained by 0.2–0.4 mm plunge depth, 45–60 s stirring time, 700–900 tool rotation speed and 30 mm shoulder diameters. In order to obtain a perfect weld in friction stir spot welding, it was determined that a homogeneous mixture should be formed between the stirring zone and the thermo-mechanically affected zone. Low weld strength was obtained in all joints with defects detected in these regions. Low weld strengths and weld defects were detected at dwell times of less than 30 s, at all tool rotational speeds with a stirring time of less than 45 s, at a plunge depth greater than 0.4 mm, and at low tool rotation speeds.

Improvements in the thermal, mechanical and electrical conductivity properties of a structural adhesive: A preliminary study

Polymeric nanocomposites composed of an organic matrix and inorganic nanoparticles makes a useful union of the polymer functionalities, such as low weight and moldability, with the good mechanical, thermal and electrical properties of the nanofillers. The possibilities to improve mechanical or thermal properties, developing a new material, by nanoparticles, have found applications in both academia and industry. To increase the application of adhesive bonding, many researchers have developed special hybrid welding technologies to improve bonding properties. Although epoxy resins have presented the disadvantage of not being electrically conductive – as a matrix for adhesives – electrical conductivity is a necessary property for hybrid joints of adhesive and resistance spot welding techniques. In the way to promote the weldability in adhesive joints, the objective of this paper is to develop a conductive structural adhesive by adding nanoparticles in a well-established structural adhesive. The results show that multiwalled carbon nanotubes can be the desirable filler to improve the electrical conductivity of epoxy adhesives with the addition of low concentrations of nanoparticles with a formulation comprising 1.5% CNT added to the studied adhesive presenting the best results.

Microstructure and tensile-shear behavior of dissimilar resistance spot welds between 2304 duplex stainless steel and Inconel 718 superalloy

Dissimilar joints between duplex stainless steels and nickel-based superalloys are of high significance in some industries. In this research, 2304 duplex stainless steel and Inconel 718 superalloy were welded by the resistance spot welding technique. The studies showed that the fusion zone microstructure was composed of a dendritic structure related to the grains of nickel-based solid solution ( γ-phase) and γ′ islands. The heat-affected zone microstructure of 2304 duplex stainless steel includes the austenite phase with a slight volume fraction and the ferrite phase with a high volume fraction. The austenite phase was formed with two morphologies of Widmanstätten and grain boundary allotriomorphs. The heat-affected zone microstructure of Inconel 718 superalloy also contained γ phase (nickel-based solid solution), annealing twins, and some precipitates. The results of the tensile-shear test revealed that tensile-shear strength (i.e. peak load) and fracture energy were increased by increasing the welding current from 1 to 2 kA and the welding time from 1 to 2 s. However, they were reduced by increasing the welding current from 2 to 4 kA and the welding time from 2 to 4 s. This was caused by the surface splash between the electrodes and the sheets. In the above-mentioned range, an increase in the welding current and time may also intensify the surface splash. It was also found that when the welding time was constant (i.e. 2 s), the spot weld obtained at the welding current of 1 kA failed by interfacial fracture mode. All spot welds obtained at the welding currents of 2, 3, and 4 kA failed by pull-out fracture mode. On the other hand, all spot welds associated with the welding times of 1, 2, 3, and 4 s failed by PF mode when the welding current was constant (i.e. 3 kA).

Microstructure and mechanical properties of refill friction stir spot welded joints: Effects of tool size and welding parameters

A novel refill friction stir spot welding (RFSSW) technique employing large-sized tools is proposed. The microstructure and mechanical properties of joints produced with a large-sized welding tool and a conventional tool are compared. The results show that the exit line resulting from the sleeve becomes longer with increasing the plunge depth, and the diameter of nugget increases with higher rotational speed for the joints produced by both conventional and novel tools. The plunge depth increases from 2.0 mm to 2.2 mm, then to 2.4 mm, which affects the hook defect to bend upwards, almost parallel to the lap interface, to bend downwards, respectively. The joints produced with the novel tool have a flat hook compared to the conventional tool. The microstructure evolution of the conventional and novel joints is similar. The tensile-shear and tearing forces measured of the novel joints are higher than those of the conventional joints for the same welding parameters. For conventional joints, the maximum tensile-shear and tearing forces are 8.6 ± 0.1 kN and 4.4 ± 0.2 kN, respectively. The maximum tensile-shear and tearing forces for novel joints are 10.9 ± 0.1 kN and 5.6 ± 0.1 kN, respectively. After the tensile-shear test, there are three modes of fracture, the upper-mixed, the lower-mixed, and the shear fracture one. The plunge depth has a pronounced effect on the fracture mode of joints.

Control of Static and Dynamic Parameters by Fuzzy Controller to Optimize Friction Stir Spot Welding Strength

Solid-state welding is a derivative of the friction stir spot welding (FSSW) technique, which has been developed as a new method for joining aluminum alloys. FSSW is a variant of linear friction stir welding intended to deal with lightweight alloy resistance spot welding (RSW) and riveting. Tensile strength refers to a material’s ability to withstand excessive stress when being stretched or pulled before necking; it is expressed in terms of force per unit area. The tensile strength in stir spot welding is affected by dynamic and static parameters. The control of dynamic parameters and static parameters is studied in this paper to optimize the friction stir spot welding strength. A fuzzy logic control system is used to optimize the process as a new approach that can be used in this field. The obtained results prove that the fuzzy logic control system is an easy and inexpensive technology that can be used in prediction and optimization for the strength of FSSW. Furthermore, the results show the efficacy and adequacy of the proposed fuzzy logic control system.

Insights into the structural design strategies of multi-spot ultrasonic welded joints in thermoplastic composites: A finite element analysis

Ultrasonic welding is a typical spot-welding technique for thermoplastic composites (TPCs) with high weld strength and short welding time. Therefore, the most straightforward way to apply ultrasonic welding to large-scale TPC structures is adopting multi-spot welded configurations fabricated by a sequential welded method. In order to gain an insight into the structural design optimization strategies of multi-spot welded joints, a finite element (FE) model was proposed in this paper to analyse the mechanical behaviour of a single-lap TPC joints assembled by ultrasonic spot welding. The welded areas were simulated by using cohesive zone elements. The presented FE model was firstly validated by a comparative study between the numerical results and the experimental data. On the basis of that, a comprehensive study was conducted on the effects of typical geometrical parameters, i.e. spot diameters and spot spacing, on the overall mechanical behaviour of the multi-spot welded TPC joints. The numerical results provide a potential guidance for the structural design of ultrasonic welded TPC joints in aircraft structures.

A numerical study on contact conditions, dynamic resistance, and nugget size of resistance spot weld joints of AISI 1008 steel sheets

Current and heat flow at the contact surfaces of electrode and sheet materials play a vital role in joining of sheet metals by the Resistance Spot Welding (RSW) technique. Further, the contact resistances primarily depend on contact pressure and temperature during the RSW process. In this article, a 2-D axisymmetric, fully coupled electro-thermomechanical model developed in COMSOL software is used to study the effects of weld current and time on the evolution of temperatures, dynamic resistance and nugget size. Contact conductance method for contact resistances and latent heat capacity method for phase change (melting/solidification) process are used. The predicted nugget sizes are compared with experimental data.

Characterizations of AA5083-H116 produced by friction stir spot welding technique

ABSTRACT This research investigates the influence of friction stir spot welding (FSSW) process parameters on friction stir spot welded aluminium 5083-H116 alloy properties. Various characterisation techniques were applied such as optical microscope and scanning electron microscope, the microhardness profile was used to determine the hardness profile, while X-ray diffraction (XRD) was used to determine the phase formation. The electrochemical potentiodynamic polarisation technique was utilised to determine the corrosion behaviour. Based on the findings obtained, the process parameters were found to play a significant role in the microstructural evolution. It was established that the structural, mechanical, corrosion and metallurgical properties were significantly improved compared to the base metal in this research work. It was revealed that the lowest corrosion rate was obtained at a rotational speed of 900 rpm and dwell time of 15 s, respectively. It was also revealed that the welded samples crystallinity was improved from the XRD analysis compared to the base metal. The highest hardness profile was obtained at the stir zone of the welded area. The highest hardness value was achieved at the stir zone of the sample produced at a rotational speed of 900 rpm with an average hardness value of 112.62 HV.

Friction stir welding and friction stir spot welding of polymethyl methacrylate (PMMA) to other materials: A review

Polymers are widely used in packaging and automobile industries because of their desirable properties such as lightweight, transparency, strength to weight ratio, good hardness and stiffness, low water absorption, ultraviolet rays' protection. Keeping Polymethyl methacrylate (PMMA) polymer in focus, this review research is being done here to collect the literature available best of our knowledge on the joining of PMMA to PMMA/Polymers/ Metal alloys by Friction stir welding/ Friction stir spot welding (FSW/FSSW). Friction stir welding is a solid-state joining process in which the temperature remains below the material's melting temperature. This friction stir welding technique is widely applied on materials that are difficult to weld by fusion welding, such as aluminium alloys, titanium alloys, and polymers. Friction stir welding is an ecological weld method and environmentally friendly as it does not produce harmful radiation toxic gases, requires less energy, produces less waste than conventional fusion welding processes. It was found out that by applying Friction stir welding/ Friction stir spot welding technique, the Polymethyl methacrylate can be welded to itself, to polymers, and metal alloys. In this review, efforts are made to point out the optimum level of process parameters of FSW concluded by researchers in their studies of PMMA joining. For each study, its parameter range and the optimum value of process parameters are presented in the form of tables and graphs, and explanations and discussions are made to better understand welding Polymethyl methacrylate polymer by Friction stir welding process.

Effects of interlayer on the friction stir spot welding of stainless steel

Comparison of Interlayer Friction Stir Spot Welding (IL-FSSW) and conventional Friction Stir Spot Welding (FSSW) of 304 stainless steel is presented in this study. IL-FSSW is introduced as a novel spot-welding technique to avoid the keyhole formation at the weld spot. This process is employed to fabricate a joint between two stainless steel sheets. The utilization of an interlayer material prevents the occurrence of keyhole without any additional post-process treatment. In addition, the interlayer material also enhances ductility and provides a joint with a maximum lap shear load of 7 kN. The micrographs of the weld zone cross-section were recorded and it was observed that the interlayer material played a significant role in the development of the nugget zone by supporting the formation of a hook structure.

A Novel System for Growth of Single Crystals from the Melt with an Innovative New Pulling Mechanism

This paper describes a crystal growth system employing novel concepts in fabrication of a high temperature furnace, thermocouples and a novel crystal pulling mechanism. This has reduced the costs substantially, without compromising the quality of grown crystals. The core of the furnace, a wooden cylindrical dummy had been prepared with equi-spaced helicalgroves with widths equaling the diameter of the heating wire on its outer surface machined by a lathe machine. The Kanthal heating wire was wound in the groves. It was covered with a thick layer of natural clay available locally. After the clay had dried up, an electric current was passed through the heating wire and the wooden frame was burnt out. A thick layer of theclay was applied on the inner and the outer surfaces. The furnace can operate at temperatures up to about 1000 °C. The temperature was measured with a chromel-alumel thermocouple prepared by an ingenious spot-welding technique established in the laboratory. The seed holder was hanged above the melt kept in the crucible with help of a float kept in awater container, which has a small tap at the bottom. When the tap is opened the float goes down and the seed assembly goes up. In this manner a quality pulling system, without any motor has been developed and reported here. It has been possible to grow good quality crystals of potassium chloride with excellent diameter control.

Microstructural Evaluation and Influence of Welding Parameters on Electrode Plunge Depth in Resistance Spot Welded Dissimilar DP800HF/1200M Steel Joints

Advanced high strength steels (AHSS) are newly developed steels that has versatile mechanical properties. These steels enables to design low weight cars with high safety standards. Also, weight reduction in vehicles plays a significant role for saving fossil fuels which is limited and causes carbon emissions. Dual phase (DP) and Martensitic steels are prominent in AHSS family because they are inexpensive and has vast application areas. DP steels are used for general purpose applications and Martensitic steels are used for reinforcement parts in vehicles. In this study, high formable grade Dual phase steel with 800 MPa tensile strength and Martensitic steel with 1200 MPa tensile strength were welded with resistance spot welding technique which is the most widely practiced joining method in the industry. Electrode indentation depths, its effect on tensile-shear loads and microstructural characterizations were investigated. According to the results, the lowest tensile shear loads were acquired between both between 0-0.2 mm and between 0.85-1mm electrode plunge depths. Medial electrode plunge depths showed high tensile shear loads. Some welding defects were encountered including secondary phase formations, shrinkage voids, intergranular shrinkage gaps and vertical cracks in the weld nugget. It is find out that the weld defects were formed due to cooling gradient while solidifying, electrode force, and improper weld parameters.