Welded Elements Research Articles

Numerical Study of Temperature Evolution During Friction Element Welding

Abstract Welding of dissimilar materials is critical in industries where mixed materials with high strength-to-weight ratios are urgently needed. Friction element welding (FEW) is a promising solution, with the ability to join high strength materials for a wide range of thicknesses with low input energy and a short processing time. However, the temperature evolution and the influence of different processing parameters remain unclear. To bridge this knowledge gap, this work develops a coupled thermal–mechanical finite element model to study the FEW process. The simulation results agree well with the experimental measurements of material deformation and transient temperature evolution. It is found that the friction element’s rotational speed has the greatest impact on friction heat generation, followed by the processing times for different steps. The aluminum layer is heated during the penetration and cleaning steps, thus a lower rotational speed during the penetration step can help prevent the aluminum layer from undesired overheating. The steel layer and the friction element are mainly heated during the cleaning and welding steps. The strong heating, potentially melting, will be beneficial to the friction element’s plastic deformation and bond formation. To enhance the heating of the steel layer and the friction element, faster rotational speeds or longer processing periods could be employed during the cleaning and welding steps. The results by this study establish the relationship between processing conditions and the temperature evolution of different parts, which will guide the design and optimization of the FEW technique for various applications.

Relationship between cross tension strength and carbon content of lower sheet in friction element welded steel joints

High-strength steels (HSSs) have been increasingly used in car bodies to simultaneously achieve the weight reduction and high collision safety of vehicles. In resistance spot welding, which is widely used for joining car bodies, low cross tension strength (CTS) of joints using HSSs is a problem. In this study, we focused on friction element welding (FEW) to enhance the CTS. To investigate the joint strength of steel sheets jointed by FEW, a pre-hole was provided in the upper sheets, and the element was passed through the pre-hole to joint with a lower sheet, and then cross tension tests of the joints were conducted. In addition, the microstructures of the joints were observed to investigate the fracture positions. In the cross tension test, three types of fracture modes were observed. When the carbon content of the lower sheet was as low as 0.07–0.15 mass%, the joint broke at the head of the element. However, when the carbon content was increased to 0.20 mass%, the joints were fractured at the softened area of heat affected zone in the thickness direction of lower sheets. While the carbon content was further increased to 0.30 mass% (C30), the cracks propagated inside the area quenched from the two-phase temperature region (inter-critically quenched area) of the lower sheets and then fractured in the lower sheet thickness direction. Consequently, the CTS was decreased. To clarify the mechanism of the decrease in CTS, the fracture surface was observed together with the hardness difference measurement between ferrite and martensite by the nanoindentation method. As a result, it was clarified that the hardness difference between the two phases was significantly large, leading to the ductile fracture. Based on these findings, the low CTS of C30 was inferred to be caused by the poor local ductility in the inter-critically quenched area.

Особенности наложения ультразвуковых колебаний в процессе сварки

Introduction. The main problem in obtaining welded joints is the nonuniform heating of the joint zone, which leads to differences in the structure and properties of the weld metal and the base metal. One of the ways to intensify the welding process is the use of ultrasonic vibrations. As a result of the analysis of methods for introducing ultrasonic vibrations into the melting zone, a method of superimposing vibrations on the elements to be welded was chosen for experimental studies. This method makes it possible to influence the welded elements throughout the entire welding cycle from the melt bath to complete crystallization of the metal. Methods. Experimental studies were carried out on plates made of carbon structural steel St3 (ASTM A568M, AISI 1017, DIN 17100) and aluminum deformable non-hardened alloy AMg4 (EN AW-5086, AW-AL Mg4, 5086). As a source of oscillations, a rod magnetostrictive oscillatory system was used, the end of which was rigidly fixed on one of the welded plates. To determine the places of application of the oscillation source and the welding zone, a calculation method is proposed based on the equality of the resonant frequencies of the used oscillatory system and the natural frequency of bending vibrations of the welding component. It is shown that the optimal places for the application of vibrations and welding will be the antinodes of oscillations, which have the maximum amplitude. Welds were obtained by the method of semi-automatic gas metal arc welding. Results and Discussion. Microstructural study of obtained samples showed a significant decrease in the proportion of dendritic segregation. The changes in the structure are the result of the effects that occur in the liquid melt when ultrasonic vibrations are introduced. The main effects are sound pressure, cavitation and acoustical streaming. The structure change mechanism consists in the dispersion of growing dendrites and crystallization nuclei under the action of shock waves and cumulative jets that occur when cavitation bubbles collapse. The formed fragments of dendrites are new crystallization nuclei that propagate through the melt pool under the action of acoustic currents. Then the process is repeated. The resulting effects affect the kinetics of the crystallization process – the degree of supercooling increases, the number of crystallization nuclei formed per unit time increases, and the rate of its growth decreases. Changes in the structure of the weld metal lead to an increase in the quality of the welded joint, which reduces welding deformations, increases the tensile strength and significantly increases ductility.

Enhancement of resistance element welding of AA6061 to DP600 steel by using external magnetic field

The application of aluminum (Al)/steel hybrid structure has a broad prospect in automobile weight reduction. In this work, AA6061/DP600 steel was joined using a resistance element welding (REW) method to achieve high strength and plasticity. To modify the deviation of weld nugget and increase the diameter of nugget, annular permanent magnet was designed and used. The effect of external magnetic field on microstructure evolution and mechanical properties was investigated. An electromagnetic stirring effect was generated inside weld nugget, resulting in a larger bonding area between Q235 rivet and DP600 steel. Convection in weld nugget contributed to restraint of dendrite growth and changes in heat distribution, leading to grain refinement at weld nugget. The microstructure of weld nugget and heat affected zone (HAZ) was bainite in magnetically assisted resistance element welding (MA-REW) specimens. The microhardness distribution presented a symmetrical distribution in both horizontal and vertical direction of cross section while the highest microhardness value occurred in weld nugget. Compared to REW specimens, MA-REW specimens achieved improvement of 10.2 % in peak load and 26.8 % in displacement during the tensile test. When the welding current reached 11 kA, the fracture mode of MA-REW specimen transformed from interfacial fracture to button fracture. Enlarged weld nugget size and refined grains were responsible for the improved performance.

Optimizing the mean and variance of bead geometry in the wire + arc additive manufacturing using a desirability function method

Wire + arc additive manufacturing is an arc welding process that uses non-consumable tungsten electrodes to produce the weld. The material used in this study is a titanium, carbon, zirconium, and molybdenum alloy that is physically and chemically stable and has good performance for use as a welding and high-temperature heating element. In this study, welding experiments are designed based on a central composite design, and single-layer wire + arc additive manufacturing is performed using the titanium, carbon, zirconium, and molybdenum alloy. Consequently, 17 beads are obtained and the height, width, left and right toe angles, which represent the geometry of the beads are measured. Based on the measured geometry, response surface models for mean and standard deviation of the four geometries are fitted. Mean absolute percentage error of the four response surface models is 16.6% on average which implies that the models are reasonably well fitted. Based on the response surface models, the optimal settings for the Wire + arc additive manufacturing parameters are obtained by using a desirability function method. At the optimal setting, the desirability function value shows 0.85 on average which is close to ideal value of 1.00. This result indicates that valid optimal settings for the process parameters can be obtained via the proposed method.

Selected properties of RAMOR 500 steel welded joints by hybrid PTA-MAG

The paper describes the selected properties of RAMOR 500 anti-ballistic (martensitic structure in the initial state) high yield strength steel (1450 MPa) welded joints produced by the hybrid PTA-MAG (Plasma Transferred Arc - Metal Active Gas) method. The welded elements were metal sheets 6.7 mm thick in a rectangular shape with dimensions of 200 mm x 350 mm. The tested butt weld joints have been made with process parameters selected according to criterion of lowest level of material weakening in the heat-affected zone (HAZ). The results of metallographic research of welds heat affected zone and base material, hardness distribution and XRD patterns of specific areas have been presented. Depth-sensing indentation (DSI) is used in this work to determine the distribution of mechanical properties affected by annealing/tempering by thermal cycle of hybrid PTA-MAG welding process. The investigation results show that the use of hybrid PTA-MAG heat source for welding of martensitic structure steel makes it possible to use high strength steel filler material (yield strength 890 MPa) without of cold cracks high risk. The hybrid plasma based welding method has a potential to become a beneficial alternative to other welding processes for ballistic protection steel due to its high efficiency, reduced amount of weld metal content or limited requirements for a preparation of edges of welded joints. The weakest area of welded joints is part of HAZ located close to the base material which has been secondarily tempered by heat of welding thermal cycle. The heat input about 0,57 kJ/mm required to achieve full penetration butt welded joints with no defects and with wide enough capillary channel to cover the welding gap during welding process. Hardness decrease in that area is about 25% in relation of base material. The width of the softened zone was approx. 4.5 mm.

Microstructure and Fatigue Properties of Resistance Element Welded Joints of DP500 Steel and AW 5754 H22 Aluminum Alloy

The modern concept of lightweight design (LW) requires the application of different materials in one structure (multi-material structures). The structure of different materials has a good perspective for application in the automotive and aerospace industries but only if it is possible to achieve a quality joint between different materials. The most used technology for joining different materials in the automotive industry is Resistance spot welding (RSW). Due to different mechanical, physical, and chemical properties, the joining of different materials by RSW technology does not provide a quality joint, and accordingly, alternative technologies for joining different materials have emerged. Resistance element welding (REW) was developed to enable joint of different materials. This paper presents the welding of AW 5754 H22 Al alloy (1.0 mm-thick) and DP500 steel (1.5 mm-thick) using novel REW. The peak load, absorption energy, microstructure, microhardness and fatigue strength of the REW joint has been investigated. The joint of the same materials has been done also using conventional RSW to compare some results. The results that will be presented in this paper show that that REW can achieve reliable joining of the two materials at relatively low welding currents compared to RSW. Using REW process with a significantly lower welding current, satisfactory mechanical characteristics of the weld joint can be achieved, so peak load is between 2300–2500 N, displacement is between 2.5–3 mm and the absorption energy is between 3.3–5.7 J. REW joints showed fatigue strength with the fatigue limit of 882 N.

Microstructural and interface geometrical influence on the mechanical fatigue property of aluminum/high-strength steel lap joints using resistance element welding for lightweight vehicles: experimental and computational investigation

The present study investigates a fatigue property and its microstructural and interface geometrical effect on the resistance element welded (REWed) aluminum (Al)/high-strength steel (Fe) lap joint. The assembled Al/Fe joints were subjected under the fatigue strength up to 80% of tensile-shear loads with 0.1 of the load ratio. As a result, an exceptional fatigue strength has been secured inducing the base material fracture because there are no noticeable interfacial defects at the 10.5 kA welding condition compared to assemblies joined with 3.5–4.5 kA of welding currents. Even, maximum tensile-shear loads exceed 9 kN at the 10.5 kA welding condition. Microstructural developments of base materials and welding interfaces were thoroughly analyzed by an optical microscopy (OM) and electron backscatter diffraction (EBSD), the development of prior-austenite-grain-boundaries (PAGBs) and α'-martensite phases were observed in the welding interfaces at all conditions. Heat affected zone (HAZ) developments of AA5052 were quantitatively analyzed in terms of recrystallization and grain growth, providing significant differences in the fatigue performances and striation, crack propagation developments. In addition, with the help of a finite element (FE) computational modeling, a mechanical stress distribution and strain behavior by geometries of the joining interface are discussed in detail.

Improving the Quality of Manufacturing of Locomotive Body Frames

The paper describes the process of manufacturing the frame of locomotive bodies, which is a complex step-by-step process of assembling (welding) elements. It has been established that internal stresses cause welding deformations, while the deviation from the straightness of the sidewalls of the electric locomotives body frames of 2ES6, 2ES10, 2ES7 series (manufactured by Ural Locomotives LLC) can reach up to 25 mm with a tolerance of 5 mm. The results of internal stress analysis for linear displacements of 25 mm sidewalls showed that the maximum stress values can reach up to 45 MPa. To improve the accuracy of parameters, the operation of straightening by local heating is introduced into the body frame manufacturing technology. The paper describes the main characteristics of the existing straightening technology in production, shows the main disadvantages. Studies of technical solutions for straightening methods have been carried out. The results show that there are ample opportunities to replace thermal straightening with mechanized vibrational processing, which reduces residual stresses in the product material. The efficiency of vibration treatment largely depends on the selected mode parameters: frequency, amplitude, duration of vibration exposure, as well as the optimal frame vibration treatment scheme. The paper proposes an original scheme for straightening the body frame of electric locomotives in production environment. It is indicated that in order to minimize the forces of static bending, it is necessary to suspend the frame under constraints and move the vibrator(-s) along the length of the frame, dividing it into approximately 3 equal parts. To work out the straightening technology, it is proposed to use PXI Platform equipment with the LabVIEW application. The cost of the straightening operation can be reduced by at least 20%, which will affect the cost of the electric locomotive as a whole.

Nature of metal drop displacement during deposit welding elements of railway transport

The transformation of the microstructure of the heat-strengthened steel 65G section deposited with an electrode with a low-carbon rod is complex. Increased hardness values of individual areas contribute to an increase in the abrasive wear resistance of the part. The presence of a fusion zone ensures the resistance of the deposited area of an element on railway transport to cracking. The use of electrodes with a low-carbon rod is advisable when carrying out surfacing reinforcement of heat-hardened steels. The authors obtained mathematical expressions for the speed of displacement of a metal drop falling from the electrode material and the path it traverses when deposit welding elements on railway transport. They show that the displacement of this drop is accompanied by acceleration followed by deceleration. In electromagnetic deposit welding, the experimental assessment of the level of circumferential and radial residual thermal stresses during deposit welding of a coating on a hollow cylinder is of considerable practical interest. As an example, the calculation and experimental analysis of the stress state during deposit welding of the cylinder on the inner surface is made. He showed that residual compressive stresses, significant in magnitude, are formed on the outer surface. The method, which is limited to cutting the cylinder along the generatrix, without subsequent layer-by-layer removal of metal from its inner surface, is a simple and effective method for determining the circumferential residual stresses that provides the required accuracy.

Microstructural and mechanical characterization of steel-DP780/Al-5052 joints formed using resistance element welding with concealed rivet cover

Resistance element welding (REW) with a concealed rivet cover (Q235) was used to join steel-DP780 and Al-5052. The macroscopic morphology and the microstructure were observed using optical microscopy and scanning electron microscopy. The mechanical properties of the joint were also tested. The results show that the intermetallic compound (IMC) formed at the Q235/Al interface is FeAl3, while the IMCs formed at the DP780/Al interface are Fe2Al5 and FeAl3. The welding current has a significant influence on the nugget size, peak load, average hardness of aluminum heat-affected zone, and the bearing area of the aluminum plate. There are three fracture models of the countersunk rivet REW joint under a tensile-shear force: the interface failure fracture model, the fusion pullout failure fracture model, and the heat-affected zone failure fracture model. At a welding current of 18 kA, the nugget pullout failure mode is observed, and the best mechanical properties of the joint are obtained. The shear resistance of the joint can reach up to 5712 N.

Features of damage to welded joints of steam pipes made of steel 12X1M1F and 15X1MF

A significant number of heat and power equipment, including welded connections of steam pipelines made of 15x1m1f and 12x1mf Steels, have now reached the design service life. Their practical experience is about 200,000 hours at an estimated temperature of 545°C, and the park resource according to regulatory documentation is 250,000 hours. Note that aging welded joints are operated in conditions of start-ups and stops, which have become more frequent, which contributes to an increase in the intensity of their damage. In this connection, the question arose about the possibility of their operation longer than the estimated period in the new conditions. Goal. The goal is study of the features of damage to welded elements of steam pipelines operating in conditions of creep and low cycle fatigue. Methodology. The study of samples from existing damaged steam pipelines was carried out by the method of structural analysis, based on the consideration of the processes of structural and phase changes occurring in the areas of welded joints and conclusions were drawn regarding their resistance to destruction during operation. Results. The mechanisms of crack formation, their causes and the main provisions of metallographic analysis on the study of damaged welded joints operating under creep conditions and representing a certain number of types of cracks are presented. Such cracks are cold-breaking cracks, which are brittle cracks at moderate temperatures (up to 100–150 0C) during hydraulic tests and start-ups; long-term ductility failure cracks, which are brittle cracks caused by the failure of long-term ductility creep temperatures and rated stresses below permissible; cracks on exhausted areas, which are cracks when operating in the range of creep temperatures and rated stresses above permissible and fatigue cracks, which are cracks caused by the action of variable stresses with an amplitude greater than permissible values. Scientific novelty and practical significance. It is established that the mechanism of aging of long-term operated welded joints of steam pipelines is associated with structural transformations that cause the appearance of cracks in the welded joints.

FORMING PROCESS SIMULATION OF BIMETALLIC BILLET BY EXTRUSION FOR REW METHOD

The bimetallic joining elements were designed for lap joints of thin metallic (Fe-Fe, Fe-Al) as well as metallic – nonmetallic (Fe-PMMA, Al-PMMA) sheets by Resistance Element Welding (REW). The Cu tubes with an outer diameter of 4 mm, wall thickness of 0.5 mm, and a length of 11 mm filled with a solder Sn60Pb40 were used for the bimetallic joining elements producing. The required shape of joining elements is obtained by cold forming. Simulation by ANSYS software was chosen for the optimization of the forming process and geometry of functional parts of the forming tool allowing to use only one extrusion forming operation. The simulation results are stresses, strains, and modification of cross-section geometry of elements for the three proposed forming modes. The geometry of functional parts of the forming tool was compared with the results of cross-section macroanalysis of joining elements.

Robust bonding and microstructure behavior of aluminum/high-strength steel lap joints using resistance element welding process for lightweight vehicles: Experimental and numerical investigation

Here we present the resistance element welding (REW) process as a novel alternative to aluminum (Al)/steel (Fe) joints for the automotive structural assembly process. Before the REW process on the SPFC980 steels and AA5052 alloys, the low-carbon steel, S20C elements were riveted to AA5052 sheets to resolve the inconsistency of properties between Al and Fe. The REW processes were conducted with the 6 input parameters from 3.5 kA to 12 kA with a welding current under 200 ms of an energization time. As a result, robust bonding was successfully achieved exceeding 9 kN of tensile-shear strengths with excellent failure energies from the welding currents in 6.5 kA–10.5 kA without any common weld defects, e.g., crack, lack of fusion, lack of penetration, undercut, etc. These results present exceptional characteristics compared with the strength of previous reported Al/Fe joining technologies. The crystallographic characteristics of the welded metals in this process were quantitatively investigated using electron backscatter diffraction (EBSD) analysis. Due to the cooling effect the phase transformation was verified by theoretical values and finite element method (FEM) computational calculation. Furthermore, the fracture behavior analysis was thoroughly performed using the in-situ digital image correlation (DIC) method. This study provides a substantial alternative to achieve robust bonding of the automotive structural Al alloy/high tensile steel through the REW process and can be extended to mass production of lightweight structural assembly applications as well.

Laser Projection Positioning Improving at a Machine- Building Manufacturing Enterprise Method

The goal of this work is to research a possibility of laser projection system adaptation to a machine-building manufacturing enterprise. A model example of the use of a computer vision system in production is the task of reducing the labor intensity of installing welded elements on a product, reducing the production time, and reducing the cost of work. A formal statement of the problem is given - to use a laser projection system in the conditions of assembly and welding production for positioning welded elements on the main product with respect to dimensional accuracy. For experimental research, a section of a machine-building production enterprise with a pre-defective product and welded elements was used. The research took place in two stages. As a result of the research, the results were obtained, confirming the efficiency of the method.

REW Application Possibilities for the Production of Combined Metal - Plastic Joints

In this paper, innovative resistance element welding (REW) technology for joining galvanized steel sheets to thermoplastics (PMMA) is introduced. The essence of the innovation is in the use of a special bimetallic joining element consisting of the core made of a Sn60Pb solder, and the sleeve made of a Cu tube. During resistance heating, the solder melts, thus allowing the formation of a metallurgical joint with galvanized steel sheet. Since Sn60Pb solder melting occurs at temperatures (from 183 to 190 °C) be-low the thermal decomposition temperature of most thermoplastics (for PMMA above 300 °C), there is no thermal destruction of the PMMA material around the joint. The mechanical fixation of the thermo-plastic material at the overlap joint is provided by the sleeve made of Cu tube which has a substantially higher strength than a Sn60Pb solder.

Finite Element Welding Residual Stress Analysis of CRDM Penetration Nozzles

Abstract This article investigates numerically welding residual stress distributions of a tube with J-groove weld in control rod drive mechanisms of a pressurized nuclear reactor vessel. Parametric study is performed for the effect of the tube location, tube dimensions, and material's yield strength. It is found that residual stresses increase with increasing the inclination angle of the tube, and the up-hill side is the most critical. For thicker tube, residual stresses decrease. For material's yield strength, both axial and hoop residual stresses tend to increase with increasing the yield strength of Alloy 600. Furthermore, axial stresses tend to increase with increasing yield strength of Alloys 82/182.

Resistance element welding of 7075 aluminum alloy to Ti6Al4V titanium alloy

In this paper, Al/Ti joints are produced by resistance element welding (REW). A hole is made in the Al sheet and a welding element (Ti6Al4V rivet) is inserted into the hole followed by resistance welding to create a metallurgical bond between the Ti rivet and Ti sheet. The mechanical properties, fracture mode, microstructure, and hardness distribution of the joints are analyzed. The REW joints show higher tensile-shear load and energy absorption than present riveted joints. The maximum peak load and energy absorption of the REW joints improved with the increasing of rivet diameter. The failure mode for Al/Ti REW joints transits from interfacial to pullout mode, and then to fully pullout mode with increasing welding current. The nugget microstructure mainly consists of acicular α′ phase martensite. A discontinuous intermetallic compound layer is formed at the Ti/Al interface, and it consists of TiAl2, TiAl, or a mixture thereof.

Resistance element welding of sandwich laminates with hidden inserts

Resistance element welding of sandwich laminates with hidden inserts

Preventing nugget shifting in joining of dissimilar steels via resistance element welding: a numerical simulation

Joining the advanced high-strength steels and conventional steels is a critical issue for the manufacturing of lightweight vehicles. Resistance element welding (REW) is an emerging joining method for dissimilar metals and alloys by applying an auxiliary rivet-like resistance element in resistance spot welding (RSW). In this study, an electrical-thermal-mechanical coupled REW model for high-strength dual-phase (DP) steel and Q235 steel was developed by considering contact resistances as functions of temperature and surface contacting area. The results show that the welding element in REW serves to concentrate the current flow and thus Joule heat generation at the faying interface between the element and workpiece. For welding DP600 and Q235 workpieces with a small thickness ratio (≤0.4) or a high electrical resistivity ratio (≥3), REW could effectively mitigate nugget shifting between workpieces and reducing the thermal excursion to the electrode compared to RSW. Adding well-designed insulation layers in REW could further concentrate the current within the welding element, and enables a large-sized nugget at a lower current. This study is significant because it provides a better understanding of the nugget development under the electrical-thermal-mechanical interaction at interfacial contacts in REW and contributes to its further advance.