Weld Area Research Articles

Failure analysis of cracking in the thin-walled pressure vessel of electric water heater

The inner tank is the kernel component of the electric water heater. However, the stress concentration and presence of corrosive media often result in premature failure of the weld area. In this paper, the leakage failure of the circumferential weld (C-Weld) of the DC01EK inner tank was analyzed by optical microscopy, scanning electron microscopy, energy dispersive spectrometer, and microhardness test. Quantitative calculations of stress distribution of the C-Weld were performed by finite element simulations. Results show that the fracture of the C-Weld was induced by stress corrosion cracking (SCC). The corrosion and crack propagation resistance of the C-Weld are worsened by the presence of coarse grains in HAZ. The high axial stress component generated by the superposition of welding residual stress and working stress is the main driving force of crack expansion. The presence of the root gap of the C-Weld leads to stress concentration, which is more likely to cause scaling and accumulation of chloride ions, resulting in crevice corrosion. Reducing the root gap of the C-Weld, decreasing the welding heat input, and using the butt joint instead of the lap joint are suggested to eliminate such failures.

Interlaminar Microstructure and Mechanical Properties of Narrow Gap Laser Welding of 40-mm-Thick Ti-6Al-4V Alloy.

Narrow gap laser welding (NGLW) is a common solution for the welding of thick structures. NGLW was carried out on narrow-gap butt joints of 40 mm-thick Ti-6Al-4V alloy plates with a U-shaped groove. The distribution characteristics of the interlaminar microstructure in different height ranges of the joint were investigated, and the evolution behavior and formation mechanism of the interlaminar microstructure of the joint were also revealed. This showed that a large amount of short needle martensite nucleated and grew up near the fusion line and the upper boundary of the remelting zone. The "softening" phenomenon occurred in all welds except the cover layer weld. The microstructure evolution and defect migration, induced by multiple welding thermal cycles in the upper weld forming process, were the main reasons for the "softening" of the lower weld. The tensile strength of each sample changed in the range of 920~990 MPa; the fracture mode of the sample belongs to a transgranular ductile fracture. In addition, compared with the upper part of the joint, the plasticity and toughness of the weld area in the lower part of the joint was improved.

A Smart Handheld Welding Torch Device for Manual Spot Laser Welding

In producing custom-made systems, using a robotic welding line that can fine-tune welding parameters is not economical, and laser welding is usually done manually. The most common operator error in manual welding operations is the angular positioning error between the laser beam and the plate surface. This study introduces a smart handheld welding torch device that assists the welding operator with visual warnings. It measures the sheet surface angle to eliminate angular positioning errors, calculates the appropriate torch holding angles accordingly, and helps the laser welding process with the right angle. For this purpose, this study focused on micro-laser spot welding applications of stainless (inox) kitchen and hotel equipment, and the effect of angular positioning errors on the welding quality was investigated experimentally. Experiments show that when the angle between the surface normal and the welding torch is smaller than the critical welding angle, heat-induced traces or micro-deformations occur on the visible surface of the thin stainless material. In addition, there is a significant decrease in the weld quality, since a large enough weld area cannot be created at large values of this angle. The optimum torch angle range was determined using experimental results for the available laser welding parameters. With a standard welding torch and the smart torch, the welding operator was allowed to repeat the same task, and the payloads of the samples prepared in this way were measured. Test results show that using a smart welding torch with an angular positioning assist system significantly improves welding quality. Breaking force values vary in a wide range of welds made with a standard welding torch, and visual problems such as burning, puncture, and swelling are encountered on the visible surfaces of many samples with high strength values. When the developed smart torch was used, the breaking force remained within the desired reference range, and no visual defects were found in any sample.

Quantifying influence of LME inner cracks on joint strength of resistance spot weld

It is well known that liquid metal embrittlement (LME) cracking can occur during the resistance spot welding (RSW) of zinc-coated high-strength steels. These cracks can be classified into two types, based on location, namely outer cracks inside or outside the electrode indentation region, and inner cracks around the weld area. Recent research has proposed that the latter cracks can be inhibited by controlling weld schedules, though there have been a few reports concerning especially the effects of these cracks on joint properties. In this study, an X-ray CT scan and a tensile-shear test were carried out consecutively with 10 mm-wide RSW joints in order to visualize the cracks in 3 dimensions, and to clarify their effects on tensile-shear strength (TSS). The cracks in the RSW joints of the zinc-coated ultra-high strength steel could be clearly observed by the scan. The results showed that the TSS decreased to 40% by the small interface cracks even with the maximum depth of 170 µm.

Pore formation and melt pool analysis of laser welded Al-Cu joints using synchrotron radiation

Known as challenging material combination, the welding of aluminium and copper, both with strongly different thermophysical properties, causes joining failures such as pores, cracks or intermetallic phases in the solidified welding area. To investigate the mixing of the materials and the occurrence of pores, the laser welding process is observed with synchrotron radiation which visualizes the phase boundaries between solid, liquid and gaseous material phases. This allows the visualization of pore formation and density differences of the materials inside the melt pool. In this investigation, pore formation in front and bottom of the keyhole is observed. The movement of the bubbles in the melt pool can be tracked until solidification at the material transition. Regarding the intermixing of the materials, the high-speed images show a fluctuating copper flow towards the keyhole and a material mixing over the entire aluminium melt pool depth. By understanding the mechanisms, compensatory measures for an improved process can be developed to enable the usability of aluminium and copper connections, for example in electromobility applications.

Analysis of weld seam characteristic parameters identification for laser welding of dissimilar materials based on image segmentation

Welded joints of dissimilar materials increase the flexibility in design and manufacturing process greatly and, hence, have been widely used in aerospace, rail transportation, and other related industries. Due to the difference in physical and chemical properties of dissimilar materials, the formed weld during laser welding is different from that of the same material welding. The geometric morphology of the formed weld is an important factor affecting the welded joints performance. Therefore, an identification method of weld seam characteristic parameters is proposed in this paper for evaluating the welding quality by image segmentation in the laser welding of low carbon steel (Q235) and stainless steel (316L). The region of interest of the weld metallograph from experimental observation is defined, converted into grayscale image and then denoised by filter. The weld is segmented by the seeded region growing method with initial seed automatic selection. The weld seam characteristic parameters including the weld area, left weld width, right weld width, and weld penetration in the laser welding of dissimilar materials are identified based on the segmented image. The obtained results are validated by the experimental measurements of weld and good agreement between them has been found. The identified weld seam characteristic parameters are adopted for assessing weld fusion status, depth-to-width ratio, and symmetry quantitatively. The results indicate that the proposed method is reasonable and feasible for the weld quality evaluation to improve the laser welding quality of dissimilar materials in practical production.

Effect of longitudinal magnetic fields on the grain growth of hollow stud welded joints

In this study, a drawing arc stud welding method combined with a longitudinal magnetic field was used for welding a 30CrNi3MoV low-alloy super strong steel substrate and Q235 hollow steel, and the effects of a longitudinal magnetic field on the microstructure growth process and mechanical properties of stud welds were studied. The results show that an arc controlled by a longitudinal magnetic field stirred the molten pool continuously, reducing the number of coarse proeutectoid ferrite, increasing the proportion of acicular ferrite, and smoothing the transition of alloyed elements near the weld joint fusion line within a certain range of excitation current. Under the action of the magnetic field, the weld grain size was refined from 4.30 μm to 3.14 μm, and the proportion of large-angle grain boundaries in the weld area increased from 24.27% to 34.2%. After the magnetic field application, the toughness of the welded joint was significantly improved. With an increase in the excitation current, the shear strength of the joint first increased and then decreased to 312 Mpa. Similarly, the impact toughness of the joint first increased and then decreased to 31.88 J·cm−2

Topological optimization of magnetic pulse welding coils with a connectivity-constrained particle swarm optimization algorithm

A connectivity-constrained optimization methodology based on a discretized particle swarm optimization (PSO) algorithm is proposed for the continuum structural design of a magnetic pulse welding coil. To address the classical continuum topological challenges of conductors, such as simple connectivity and checkerboard phenomena in the optimal topology, the PSO algorithm is combined with connectivity constraints and filtration. The evolutionary history and parametric tests in an electromagnetic structure study are employed to demonstrate the efficiency and robustness of the novel algorithm. Physical tests show that the optimized coil can form a larger effective welding area between aluminum alloy (AA5052) and steel (HC420LA) sheets compared with the joint formed by the original coil. The maximum tensile load is also significantly improved by 19.88% with a discharge energy of 22 kJ.

Mechanical and microstructural properties of dissimilar copper and stainless-steel butt welds prepared using zigzag and circular fiber laser oscillation methods

Dissimilar copper and stainless-steel butt welds are prepared using zigzag and circular laser beam oscillation welding methods. The tensile strengths, microhardness values, and weld bead morphologies of the joints are evaluated and compared for various values of the oscillation amplitude and frequency. The optimal laser oscillation parameters are found to be a laser power of 1550 W, an oscillation frequency of 15 Hz, and an oscillation amplitude of 1 mm. Under these optimal processing conditions, the tensile strengths of the joints produced using the zigzag and circular oscillation welding modes are 232.6 MPa and 235.7 MPa, respectively. The superior strength of the weld produced using the circular oscillation method is attributed to a more uniform mixing of the alloying elements in the weld zone and a greater penetration depth. The addition of a thin Ni interlayer to the joint is found to improve the tensile strength to 240 MPa for the zigzag oscillation mode and 242 MPa for the circular oscillation mode. By contrast, the addition of a Ti interlayer reduces the tensile strengths of the joints to around 163 MPa and 198 MPa, respectively. The reduction in the tensile strength is the result mainly of a more brittle fracture mode within the heat affected zone (HAZ). Nonetheless, the circular oscillation welding mode results in a higher tensile strength than the zigzag mode due to a greater suppression of intermetallic compound (IMC) phase in the weld area and a reduction in the number of pores and cracks. Compared to conventional straight-line welding, the zigzag and circular oscillation modes reduce the grain size by around 32% and 52%, respectively, and enhance the mechanical properties accordingly.

PENGARUH VARIASI GAS PELINDUNG PADA TIG BRAZING ANTARA CEMENTED CARBIDE DAN BAJA KARBON TERHADAP KEKUATAN GESER DAN STRUKTUR MIKRO

Dissimilar metals have found wide use in electric power generation, electronics, reactors, nuclear, petrochemical, chemical industries, civil construction, vessels, and heat exchange as well as several other industries. TIG welding (Tungsten Inert Gas Welding) is a type of electric welding that uses tungsten as the electrode material that is not consumed. Brazing is a way of joining metal materials through a heating process with an adhesive or filler material that has a melting point below the material to be combined or connected. The main objective of this study was to determine the shear strength and microstructure of the weld area that was joined by the brazing method. From the results of the shear strength test, there is a comparison of the difference in the shear stress strength. The results obtained indicate that where the shear stress test at a flow rate of 5 liters/minute of protective gas gets the highest shear stress value of 108 N/mm2, compared to the shear stress test with a gas flow rate of 10 liters/minute, the shear stress value is equal to 97 N/mm2, but at the protective gas flow rate of 15 liters/minute the lowest shear stress value was 76 N/mm2. The results of the microstructure obtained the result that the protective gas flow rate of 15 liters/minute got the lowest value due to a decrease in the amount of Si elements, resulting in the lowest shear stress value.

Finite Element Simulation of the Effect of Phase Transformation on Residual Stress in a Thick Section T-Joint

It has long been known that residual stresses can profoundly affect the integrity of engineering components. Evidence has recently emerged to confirm that solid-state phase transformations in steels can significantly influence the as-welded residual stresses. A two-dimensional thermo-mechanical, generalized plane strain finite element model was created to simulate the effect of phase transformation on residual stress during the multi-pass weld process in a thick section T-joint. The effect of phase transformation and martensite start temperatures was investigated. The results showed that phase transformation generated compressive residual stress underneath the last bead to be deposited for this multi-pass weld model. However, some constraints around the bead were essential to provide that stress. Tensile residual stress was generated in the bulk of the weld area when phase transformation was considered. Therefore, phase transformation may be helpful for single pass and other groove welds but may be unhelpful in the case of the T-joint examined here. The effect of the martensite start temperature is small compared with the main difference between having a phase transformation and not having one.

Vickers Hardness Variation on the Part and Welding Cord Subjected to Plastic Deformation

By performing Vickers identifications in the area of the weld bead and the areas adjacent to it, the degree of plastic deformation can be assessed, highlighting the different hardness in the case of welding and the base material. During this work, welded specimens were made in the central area with a double V-shaped cord. One of the specimens was loading until breaking, noting that the rupture occurred in an area further away from the welding area. Another 13 specimens were required at varying degrees of deformation. Stresses above the elastic limit create differences in the plastic deformation of the areas of the weld bead and those in its vicinity. Due to the fact that the base material and the material of the weld bead have different elastic characteristics, much more pronounced plastic deformations occur in the base material compared to the weld bead. It was found that the base material suffered significantly greater plastic deformation than the weld bead. Significant differences in hardness occur in the area of the weld bead relative to the area of the base material.

Microstructure and Mechanical Properties of Laser-Welded Joint of Tantalum and Stainless Steel

Bimetallic components welded by Ta and stainless steel have great promise for engineering applications, but there is relatively little relevant research. In this study, 0.6 mm-thick Ta and 304L stainless steel plates were laser welded, and the forming characteristics, microstructure, and mechanical properties of the welded joints were analyzed. First-principles calculations were also performed to explore the structural stabilization mechanisms of the two intermetallic compounds, TaFe2 and TaCr2, in terms of mechanical properties as well as electronic structure. The results showed that the weld surface was smooth and free from any defects. Fe-based solid solutions formed grains, while TaFe2, TaCr2, and some Fe-based solid solutions formed intergranular and eutectic structures. In addition, due to the presence of the brittle phases of TaFe2 and TaFe, the microhardness of the weld area can reach 650HV, with an average hardness of 530HV, which is much higher than that of the base material. The tensile shear of the joint at room temperature was 154.77 N/mm, and the fracture occurred in the weld zone on the steel side, showing brittle fracture. TaFe2 is a brittle intermetallic compound, while TaCr2 is ductile. Both TaFe2 and TaCr2 systems have dual properties of metallic and covalent bonding within them, and metallic bonding dominates.

Prediction of weld area based on image recognition and machine learning in laser oscillation welding of aluminum alloy

Laser oscillation welding is an advanced aluminum alloy welding technology which can effectively reduce weld porosity induced by physical and chemical properties of aluminum alloy. Weld area is often as an evaluation index of geometric characteristics to evaluate the welding quality since it has great effect on the mechanical properties of welded joints. In this paper, a prediction method of weld area based on process parameters is proposed for laser oscillation welding of 6061 aluminum alloy. From the metallographic micrographs of welding experiments, the cross-sectional area of weld is calculated by image recognition technology and the error of recognized weld area is less than 8.8%. Additionally, various prediction models for weld area under different process conditions are established by machine learning algorithms including linear regression, polynomial regression and back propagation neural network (BPNN). The established prediction models are compared by the mean square error (MSE) and coefficient of determination (R2). The results show that the BPNN prediction model indicates the highest accuracy and robustnessand can be adopted to predict the cross-sectional area of weld. The proposed method can be used for selecting the optimal process condition for the ideal welded joints with the desired weld geometry to improve the quality of laser oscillation welding.

Simulation and experimental analysis of Al/Ti plate magnetic pulse welding based on multi-seams coil

Magnetic pulse welding is an effective way to achieve Al/Ti welding of light alloy materials via pulse electromagnetic forces. A new multi-seams structure coil applied to magnetic pulse welding technology was proposed to improve welding performance in this paper. The performance of the magnetic field, force filed, and displacement field applied by the new coil was also studied. Joining mechanism, mechanical properties and interface morphology of welded joints were studied by numerical simulation, tensile test and microscopic observation. The results show that the multi-seams coil has a larger welding area, and can improve the strength of welded joint by 28.8 % ~ 178.6 % under different standoff distance. The images from scanning electron microscopy showed that all welding interfaces were well connected without defects. And it was observed that the discharge energy would affect the welding interface morphology. This research can broaden the related applications of Al/Ti alloy welding.

Strategized friction stir welded AA6061-T6/SiC composite lap joint suitable for sheet metal applications

Friction Stir Welding (FSW) is one of the best choices of joining light weight metal structures involving lap joints as conventional welding methods find difficult in getting sound weld joints. Aluminium alloys are lightweight materials relatively used in space frame structures, ship buildings and panels of automotive vehicles. Main objective of this research is to investigate Al–SiC FSW lap joints and to examine the suitable pre-fill nanoparticles in the configured grooves to augment the weld strength. Three different volume additions of SiC nanoparticles and tool rotational speed were considered to fabricate lap joints for the investigations. Tool rotational speed and volume of SiC reinforcement have a significant impact on the dissemination of SiC in the weldment and weld quality. The spread of SiC nanoparticles in the weld area strongly affects the weld microstructure and mechanical characteristics. Nano SiC filled lap joint has shown a significant improvement in mechanical properties as compared to a joint without nanoparticles. The best lap joint is achieved with 20 vol% of SiC and 1500 rpm of rotational speed. The investigations imply that the tensile strength is decreased for joints with 26 vol% of SiC, due to clustering of reinforcement particles. The supplement of SiC nanoparticles restricts the growth of grains and refines the stir zone (SZ) microstructure and hence this method of weld process is recommended for lap weld joints in metal structures.

INVESTIGATION OF THE CORROSION BEHAVIOR OF ALUMINIUM 1980T1 IN SEAWATER AND DRILLING MUD BY ELECTROCHEMICAL METHODS

The main methods of corrosion protection of equipment and metal structures of offshore drilling platforms are considered. The corrosion resistance of the high-strength 1980T1 aluminium alloy used for the manufacture of the riser in seawater and drilling mud, the effectiveness of its protection with zinc-rich coatings using the methods of polarization resistance, potentiometry, as well as appearance assessment studied. In seawater at 20 and 40 °C, alloy 1980T1 refers to resistant materials - 3-4 (6 grade according to GOST 5272- 90), intense pitting formation is observed, in drilling mud the corrosion rate (Km) increases more than 10 times refer to seawater (low-resistance, 6 grade), which is due to the alkaline nature of the medium. There is intense continuous corrosion, large pitting, ulcers on the seam and its border. In seawater at 20 °C, the Km of aluminium and coatings are comparable. In the drilling mud at 20 °C, the coatings provide a 2,5…4-time reduction in Km. After 30 days of exposure in seawater and drilling mud at 20 °C, the zinc-polyurethane coating detached to40…70%, intense pitting of base observed. Bubbles all over the surface of zinc-epoxy coating observed. Zinc-silicate and zinc-polyurethanalkyde coating with a high concentration of zinc had only small delamination in the weld area. In seawater at 20…40 °С and drilling mud at 20 °C zinc-silicate coating has the greatest tread capacity, the zinc-polyurethane, zinc-polyurethanalkyde and zinc-epoxy coatings are insignificant. Additional protection of welds and pipe surfaces in seawater achieved using insulated, metallized, protective coatings and cathode protection, in drilling mud it is also necessary to study the possibility of pH reduction and the use of effective corrosion inhibitors.

Effect of fibre laser welding parameters on the microstructure and weld geometry of commercially pure titanium

The primary purpose of the study was the metallurgical characterization of laser welds. The weldability of commercial production of pure titanium and titanium alloy (CP-Ti) has also been examined. In this research, the laser fibre method was used to weld sheets of pure titanium, and then microscopy and scanning electron microscopy were used to study the changes in the microstructure, the depth of weld penetration and the width of the weld area with changing welding parameters. The results proved that increasing the laser power significantly increases the depth of weld penetration and weld width. When the heat input is increased, the shape of the weld pool changes from a V shape to an hourglass shape. It was also observed that the depth of the crater formed increases with the increase in the laser power due to the increase in the melting and evaporation of the weld metal. Increasing the welding speed also has a negative impact on the weld geometry because it reduces the heat input and absorption of laser energy by the weld metal and thus reduces the melting of the metal. The microstructure of the fusion zone consists of acicular α. Fine grains formed in the weld centre at low heat input; the granules became columnar-like. Since commercially pure titanium contains a small amount of beta-phase stabilizers, the cooling rate is extremely high for martensite to occur. In the future, it is recommended to study the effect of changing welding parameters on the mechanical properties of pure titanium because of its great importance in industrial and medical applications. Studying the effect of changing laser power and welding speed on the metallurgical properties of pure titanium, and consequently its effect on the mechanical properties of welds.

Twin Boundary Induced Grain Coarsening in Friction Stir Welding of Fine- and Ultra-Fine-Grained Commercially Pure Titanium Base Metals

The mechanical properties of commercially pure titanium can further be improved through the grain refinement processes; however, welding fine-grained materials is challenging due to the grain coarsening in the weld area and hence the weakening of the mechanical properties locally. Meanwhile, friction stir welding is a promising process in which the metallurgical bonding is established through the solid-state mechanical mixing of materials to be welded; no studies have reported friction stir welding of the ultra-fine-grained commercial purity titanium to date. In this research, friction stir welding of fine-grained and ultra-fine-grained commercially pure titanium (1.58 and 0.66 μm, respectively) was conducted. The effect of the microstructural feature of base metals on the microstructural evolution of the stir zone and the feasibility of the friction stir welding process for those materials were discussed. It was found that the fraction of twin boundaries in ultra-fine-grained material was higher than in fine-grained material. It accelerated dynamic recrystallization and recovery in the stir zone, hence inducing the grain coarsening and the loss of ultra-fine-grained structure and character after welding.

Forming and weld grain growth behavior of arc stud welding driven by rotating magnetic field

Local non-fusion is easy to occur in the process of medium and large diameter hollow stud welding, which reduces the forming quality and mechanical properties of the joint. In this study, a method of arc hollow stud welding driven by external rotating magnetic field is proposed to weld 30CrNi3MoV substrate and Q235 hollow stud. The effects of rotating magnetic field on joint forming and grain growth in weld zone are studied. It was found that under the action of the rotating magnetic field, the charged particles move under the action of Lorentz force, the arc rotates and burns uniformly at the end of the stud, and promotes the expansion of the molten pool, which avoids the local unfused phenomenon and improves the appearance of the joint. The arc rotates and stirs the molten pool under the action of a magnetic field. The undercooling of the molten pool is reduced, the grain size of the weld is refined to 3.06 μm, the proportion of large-angle grain boundaries in the weld area is increased to 41.2 %, and the shear strength and impact toughness of the magnetron stud welded joint are up to 323 MPa and 34.28 J.cm−2, and the microhardness of the weld is up to 325 HV.