Entire Weld Research Articles

Visual inspection of weld surface quality

Welding is an important method for modern material processing. In actual processing, due to the influence of processing accuracy and welding thermal deformation, various defects often appear in the appearance of the weld. At present, visual inspection is mainly used for the appearance inspection of welds. The detection of weld defects mainly depends on the work experience of the staff. Based on the above background, the purpose of this article is to visually inspect the weld surface quality. This article uses visually obtained fringe images of weld contours as information sources to explore a visual-based weld appearance detection algorithm, including the measurement of weld formation dimensions and the detection of weld appearance defects. This algorithm overcomes manual measurements of the misjudgments and omissions caused by eye fatigue and experience differences. It improves the efficiency and accuracy of welding appearance inspection, and meets the needs of automation and intelligence of the entire welding process. In this paper, a subpixel stripe centerline extraction algorithm based on the combination of the Hessian matrix method and the center of gravity method is used; to further improve the accuracy of the extraction of the centerline of the weld seam, this article also performs the work of removing the wrong points and the compensation of the broken seam. Obtain a fringe centerline with better connectivity. Comparing the extraction algorithms of each centerline, the centerline obtained by this method has high accuracy, less time-consuming and high stability. It laid the foundation for the subsequent inspection of weld appearance. Through the training of the model, the accurate classification and recognition of surface defects of tube and plate welds have been achieved. The experimental results show that the improved vision-based welding surface defect recognition and classification proposed in this paper has better performance and accuracy. Up to 96.34%.

Effect of the eddies formed inside a weld pool on welding defects during plasma keyhole arc welding

In this study, the formation process of welding defects in plasma keyhole arc welding was investigated based on the formation of eddies inside the weld pool. An experimental approach using an advanced X-ray visualization system was employed to clarify this relationship. The results indicated that two large eddies are produced in the weld pool. One exhibits an upward tendency (upper eddy) and the other a downward tendency (lower eddy). The relative strength of each eddy significantly changed depending on the welding current through the change in the keyhole profile (shape and size). For a suitable welding current, the lower eddy was larger and much longer than the upper eddy. Virtually no welding defects were found out in the cross-section and longitudinal sections of the weld pool and keyhole. In the case of low current, the upper eddy was larger and longer than the lower eddy. Severe undercut and humping phenomena were observed on the top side. Contrarily, for a high welding current, the upper eddy disappeared to only form a significantly large lower eddy that covered the entire weld pool. Here, a severe concave defect on the top surface and a severe burn-through defect on the bottom side of the weld bead were seen. These results indicated that when the penetration ability of an arc on the base metal is sufficient, the relative strength of each eddy should be adjusted to a suitable scale with a larger downward eddy and smaller upward eddy. This can ensure a stable and high welding quality without welding defects.

Effects of SiC and Al2O3 Reinforcement of Varied Volume Fractions on Mechanical and Micro Structure Properties of Interlock FSW Dissimilar Joints AA7075-T6-AA7475-T7

In this research work, the effect of SiC and Al2O3 reinforcements is analyzed on the interlock dissimilar joints. The joints are welded with different wt% of SiC and Al2O3 (1 to 5 wt%) placed in the groove of 0.5 mm depth between the sheets. The rotational speed of 1400 rpm, weld speed of 30 mm/s and plunge speed rate of 0.06 mm/s are selected input parameters to perform the weld. Thereafter Micro-hardness, tensile strength and microstructure properties of interlock lap joints were investigated. AA7075 T6–AA7475 T7/2wt% SiC showed maximum tensile strength and hardness of 190.51 MPa, 220 HV respectively. All the SiC reinforced FSW interlock lap joints mechanical properties showed improvement on the static properties than the joints made with Al2O3 reinforcement, because of the hard silica particles and substantial increase in grain refinement that occurred in the stir zone of the reinforced welded joints. The microstructure analysis of joints revealed that reinforcements have mixed uniformly and homogenous distribution occurred in the entire weld. The fracture analysis revealed that the joints with SiC possess ductile fracture.

Direct Finite Element Analysis of the Stress Evolution and Interaction in Resistance Spot Welding with Multiple Processes and Multiple Spots Based on Reverse Engineering Technology

The current work achieved quantitative analysis of stress evolution and interaction in continuous spot welding processes that consider the stamping dimensional deviation. The reverse component that contains the stamping dimensional deviation was first established with reverse engineering technology. Then the two spot welding processes with twenty weld spots were simulated by the direct finite element analysis method. Welding experiment was conducted, and the actual residual deformation was measured by a 3D laser scanning system to validate the finite element model. The measured deformation shows good agreement with the simulated one. After that, the stress evolution was analyzed through the transient stress at a single spot weld and the peak stress during the entire welding process. The stress interaction between the spot welds in the same process and in the different processes was also analyzed. The results show that the stress interaction depends on the distance between the spot welds, the distribution of the spot welds, and the structure of the sheet metal parts. It is difficult to generalize a universal theorem to predict stress interaction. The value of the current research lies in its achievement in quantitative analysis of the stress evolution and interaction in components containing stamping dimensional deviation with direct finite element analysis, which can be extended to other complex components.

Effective thermal elastic plastic finite element computation for welding distortion investigation of pozidriv-type welded structure with rectangular pipes and its mitigation

Welding distortion of pozidriv-type welded structure with rectangular pipes by 20 welding passes was examined with experimental and computational approaches, and mitigation techniques were also investigated for precision fabrication. Welding experiment to fabricate pozidriv type welded structure was conducted beforehand, and out-of-plane welding distortion was measured with contact type displacement sensor. Effective thermal elastic plastic finite element computation with iterative substructure method and parallel computation was developed, and then employed to examine the thermal-mechanical response during the entire welding process and predict the residual out-of-plane welding distortion. Good agreement between computed results and measurement data was observed with comparison. The influences of welding sequence and clamping constraint with tack welding on welding distortion were considered, which were also practiced for out-of-plane welding distortion mitigation. Both experiment and finite element computation show that out-of-plane welding distortion with welding sequence optimization and clamping constraint can be significantly reduced with about 38% and 56% magnitude of original welding distortion, respectively, while their mechanisms were also clarified by means of stiffness variation of solving welded structure.

Autonomous programming and adaptive filling of lap joint based on three-dimensional welding-seam model by laser scanning

Traditional teaching programing for the welding of large-scale complex structures is extremely difficult, with low efficiency and poor consistency, and it is very difficult to obtain the random change of joint geometry and achieve excellent welding quality along the entire welding path. In this paper, an integrated method of automatic generation of welding paths and adaptive filling of lap welding based on laser-structured light scanning is proposed. First, the point-cloud data of the welding joint on the entire welding path were acquired by laser scanning a workpiece, and the welding-seam model of the lap joint was accurately reconstructed using a cubic smoothing spline algorithm. The key feature points of the welding path were then extracted by Douglas-Peucker algorithm and the corresponding postures calculated and optimized. Furthermore, a neural network model that correlated the optimized process parameters with the gaps in the lap joints was established. During the welding process, the welding parameters were adaptively adjusted according to the variation of the obtained joint geometry based on the model. The results of the experiments showed that the proposed method has excellent adaptability to complex-curve welding seams with gaps randomly distributed between 0 and 2 mm, achieving satisfactory results in both path accuracy and weld appearance.

Cross-Wire Projection Welding of Aluminium Alloys in Relation to Pneumatic and Electromechanical Electrode Force Systems

The cross wire projection welding of wires (Al 5182, =4 mm) performed using the conventional (i.e. pneumatic) electrode force system was subjected to thorough numerical analysis. Calculations were performed until one of adopted boundary conditions, i.e. maximum welding time, maximum penetration of wires, the occurrence of expulsion or the exceeding of the temperature limit in the contact between the electrode and the welded material was obtained. It was observed that the ring weld was formed within the entire range of welding parameters. The process of welding was subjected to optimisation through the application of a new electromechanical electrode force system and the use of a special hybrid algorithm of electrode force and/or displacement control. Comparative numerical calculations were performed (using SORPAS software) for both electrode force systems. Technological welding tests were performed using inverter welding machines (1 kHz) provided with various electrode force systems. The research also involved the performance of metallographic and strength (peeling) tests as well as measurements of welding process characteristic parameters (welding current and voltage).The welding process optimisation involving the use of the electromechanical force system and the application of the hybrid algorithm of force control resulted in i) more favourable space distribution of welding power, ii) energy concentration in the central zone of the weld, iii) favourable (desired) melting of the material within the entire weld transcrystallisation zone and iv) obtainment of a full weld nugget.

Filler metal distribution and processing stability in laser-arc hybrid welding of thick HSLA steel

Welds made by high power laser beam have deep and narrow geometry. Addition of filler wire by the arc source, forming the laser-arc hybrid welding (LAHW) process, is very important to obtain required mechanical properties. Distribution of molten wire throughout the entire weld depth is of concern since it tends to have low transportation ability to the root. Accurate identification of filler metal distribution is very challenging. Metal-cored wires can provide high density of non-metallic inclusions (NMIs) which are important for acicular ferrite nucleation. Accurate filler distribution can be recognized based on statistical characterization of NMIs in the weld. In the present study, it was found that the amount of filler metal decreased linearly towards the root. The filler metal tends to accumulate in the upper part of the weld and has a steep decrease at 45–55 % depth which also has wavy pattern based on longitudinal cuts. Substantial hardness variation in longitudinal direction was observed, where in the root values can reach > 300 HV. Excessive porosity was generated at 75 % depth due to unstable and turbulent melt flow based on morphology of prior austenite grains. The delicate balance of process parameters is important factor for both process stability and filler metal distribution.

Mechanical behavior of the metal parts welded with extraterrestrial regolith simulant by the solar concentrator in ISRU & ISRF application

The use of in-situ resources plays an important role on future extraterrestrial human activities for the facility repair and habitat construction, especially in sustainable space exploration of Moon and Mars. A method of the metal welded with extraterrestrial regolith simulant using solar processing under ambient conditions is presented. Metal parts are made of Q235B ferroalloy and TA2 titanium alloy into standard tensile members according to the ASTM code. They are disconnected from the middle in advance, and then welded together with lunar and Martian regolith simulant under ambient conditions, respectively. The entire welding process and precautions are detailed. Additionally, the mechanical behavior of weldments is characterized regarding their tensile strength. Furthermore, the fusion zone of weldments is studied by Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) measurements. The results show that it is possible to weld metal parts together with extraterrestrial regolith simulant by the solar concentrator. The average ultimate tensile strength of ferroalloy specimens welded with lunar and Martian regolith simulant is 2.94 MPa and 1.66 MPa; The average ultimate tensile strength of titanium alloy specimens welded with lunar and Martian regolith simulant is 4.95 MPa and 2.59 MPa. Moreover, the failure mode of all weldments was brittle failure. The welding joints strength derives from the phases that the regolith as the solder fusing into ferroalloys in a homogeneous way and titanium alloys in an inhomogeneous way. The presented method may provide a new thought for astronaut assistance associating with repairing and fabricating in subsequent Moon and Mars missions.

A numerical framework for determination of stress concentration factor distributions in tubular joints

Fatigue design of welded tubular joints implies the calculation of stress concentration factors at distinct weld toe locations using parametric formulas for basic loading cases. A Python based framework for automated calculation of hot spot stress and stress concentration factor distributions along the entire weld is presented. Hot spot stresses are calculated based on extrapolation of surface stresses extracted from a finite element analysis output database. For each node of the weld toes, the extrapolation direction and location dependent distances are first determined. Next the complete stress tensor in each read-out point is extrapolated towards the weld toe. The hot spot stress is determined as the maximum value of three critical stress values following the guidelines of DNV.A validation study is carried out in which the framework results are compared to both experimental data and parametric equations from literature for three different load cases: axial loading, in-plane bending and out-of-plane bending. The results obtained from the developed framework show good agreement with the experimental results and capture the shape of the stress concentration factor distribution much better than reference parametric equations. Finally, the framework is used to demonstrate the importance of including the weld geometry in the finite element model of the tubular joint. The results show that using a model without a weld can result in overly conservative estimations of stress concentration factors.

Effect of welding speeds on microstructure and mechanical properties of dissimilar friction stir welding of AA7075 and AA5083 alloy

Effect of welding speeds is studied in Friction stir welding (FSW) using cylindrical tool for the welding of Aluminum 7075 and 5083 alloys. Sound joint of similar and dissimilar FSW were obtained by use of 20 and 45 mm/min welding speed. The temperature distribution on entire weld zone influences the grain size of FSW. The grain size at different zones, affect the properties of FSW. This study, observed that the dissimilar joining of AA7075 and AA5083 exhibits better strength at the welding speed of 20 mm/min. Similar FSW achieved better strength at 45 mm/min welding speeds. Dissimilar welding required higher heat input as compare to similar welding.

Residual life assessment of welded joints in the girth weld root

The article analyzes the influence of the structural crack-like defect in the girth weld root welding the bottom to the collector body on the crack growth resistance. A mathematical model has been developed to estimate the residual life resource of welded joints with an internal crack-like defect in the root of the weld under creep. Results of metallographic studies of bench tests results for the welded joints long-term strength formed the basis of the calculation model. In all tested models the cracks start at the top of the narrow structural gap in the weld root and develop along almost the entire weld axis, perpendicular to the wall of the pipe and have a discrete growth pattern. As a result, discrete rupture of material occurs on a length of rc, and the initial crack is incremented by this length. After that, a “plastic core” of rc length is formed in the newly formed crack tip, additional damage is accumulated, and the crack development process continues until the stress level in the section weakened by the crack reaches a critical value. The obtained dependence characterizes the crack growth time from the initial crack-like defect to its critical length.

Study on the Evaluation of Weld Soundness Based on Arc Stability and Bead Quality using the Statistical Analysis of Arc Power and Arc Dynamic Resistance

The goal of this study was to evaluate weld soundness based on arc stability and bead quality using a statistical analysis of the arc power and the arc dynamic resistance. To achieve this, 50 passes of welding experiments were carried out. The welding current and voltage for every pass were obtained at a sampling rate of 5,000 Hz. The weld beads were classified into 4 quality levels by visual inspection and X-ray tests. The measured arc current and voltage were converted to a dimensionless value that was independent of the value itself, and then mapped onto a non-dimensional current-voltage plane. The welding data in the non-dimensional current-voltage plane were grouped into 50 areas (A0 ~ A49) surrounded by the lines of constant arc power and arc dynamic resistance, which changed every ±2% of mean value. The number of welding data points (N0 ~ N49) in each group were distributed in a quasi-lognormal distribution form. When this distribution skewed more leftward, the data in the center became denser and the arc became more stable. The statistical analysis of the entire welding data distribution of 50 passes showed that the cumulative probability=0.8 at the percentile α = 0.22 was an indication of weld soundness. The cumulative probability for a good bead was higher than 0.8, and was lower for a poor bead. This tendency remained consistent in a piecewise analysis of 125 sections of each bead. The sections with irregular bead or pores showed a much lower cumulative probability than 0.8, while sound bead sections had higher than 0.8, that is, a higher arc stability. The piecewise analysis makes it possible to evaluate weld quality in real time during welding Key words: Welding current and voltage, Arc stability, Arc power, Arc dynamic resistance, Weld quality, Statistical analysis

Performances of regression model and artificial neural network in monitoring welding quality based on power signal

In this study, a systematic research was conducted to compare the performances of the regression model and artificial neural network in predicting the nugget diameter of spot-welded joints by monitoring the dynamic power signature. The TC2 titanium alloy with a thickness of 0.4mm was used as the welding material, and a high-frequency precision spot welder was used to join the titanium alloy sheets. The dynamic welding current curve was obtained using the Rogowski coil, while the voltage curve was detected via two leads clipped onto the upper and lower electrodes during the entire welding process. The variations in the welding power signal in the welding process were investigated, and the characteristics of the power signals for different welding currents and electrode forces were analyzed. The power signals of different types of welding joints varied significantly. Five characteristics were extracted from the power signal to describe the shape of the curve. The stepwise regression analysis and back propagation neural network were respectively used to classify the welding joints into three categories: bad welds, good welds, and welds with expulsion. The performances of the two established prediction models were compared, and their behavioral discrepancies were attributed to their own data-mapping capabilities.

Hardness prediction based on microstructure evolution and residual stress evaluation during high tensile thick plate butt welding

Two High Tensile Strength Steel (EH47) plates with thickness of 70 mm were butt-welded together by multi-pass Submerged Arc Welding (SAW), also the hardness and welding residual stress were investigated experimentally. Based on Thermal-Elastic-Plastic Finite Element (TEP FE) computation, the thermal cycles during entire welding process were obtained, and the HAZ hardness of multi-pass butt welded joint was computed by the hardenability algorithm with considering microstructure evolution. Good agreement of HAZ hardness between the measurement and computational result is observed. The evolution of each phase was drawn to clarify the influence mechanism of thermal cycle on HAZ hardness. Welding residual stress was predicted with considering mechanical response, which was dominantly determined by last cap welds through analyzing its formation process.

Microstructural Changes and Impact Toughness of Fill Pass in X80 Steel Weld Metal

Multi-pass welding is used in high-pressure and thick-walled pipes in natural gas and oil pipelines. When a welding layer of a welded joint is subjected to different welding thermal cycles, its microstructure and properties change, thereby affecting the overall welding performance. In this study, the temperature and microstructural variations of the fill pass 2 (FP2) in the entire welding process were investigated by combining the thermal cycle with the cascade welding method. The original FP2 and FP2 after double thermal cycles had the worse deformation ability by tensile test. The toughness of FP2 improved after a single thermal cycle, decreased after double thermal cycles, and improved again after triple thermal cycles. The content of martensite–austenite (M–A) constituents and the average grain size of FP2 in the cascade samples were inversely proportional to FP2 toughness. Massive M–A constituents and their unique distribution at the inter-critical temperature were harmful to weld metal toughness. Controlling the size and fraction of M–A constituents can improve weld metal toughness.

The evolution of polycrystalline solidification in the entire weld: A phase-field investigation

The solidification patterns are dominated by the surface tension of solid/liquid interface and the transfer process of heat and mass. The continuously changing solidification parameters in the molten pool make the differences between welding and directional solidification. It could enhance our understanding of solidification behavior to investigate the solidification evolution in the molten pool, with dynamic and nonlinear temperature field. In this paper, the evolution of polycrystalline solidification in the entire weld was performed. Specifically, the welding thermal process was simulated through the finite element method, and the solidification process was simulated by the phase-field model. The comparison between the simulation and experimental result shows good agreement in general, which validates the accuracy of current model. Based on the simulation results, the dynamical solidification evolution during welding was discussed, including planar instability, epitaxial growth and competitive growth. Then the effects of surface tension and grain size on solidification were investigated. The simulation results demonstrate the ability and accuracy of phase-field model reproducing the evolution of polycrystalline solidification in the entire weld. The investigations in this paper lay the foundation for clearly revealing solidification behavior in the molten pool.

Miniature CoCr laser welds under cyclic shear: Fatigue evolution and crack growth

Miniature laser welds with the root depth in the range of 50–300 μm represent air-tight joints between the components in medical devices, such as those in implants, growth rods, stents and various prostheses. The current work focuses on the development of a fatigue test specimen and procedure to determine fatigue lives of shear-loaded laser welds. A cobalt-chromium (CoCr) alloy is used as a benchmark case. S–N graphs, damage process, and fracture surfaces are studied by applying x-ray analysis, atomic force microscopy, and scanning electron microscopy both before and after the crack onset. A non-linear material model is fitted for the CoCr alloy to run finite element simulations of the damage and deformation. As a result, two tensile-loaded specimen designs are established and the performance is compared to that of a traditional torque-loaded specimen. The new generation specimens show less variation in the determined fatigue lives due to well-defined crack onset point and, therefore, precise weld seam load during the experiments. The fatigue damage concentrates to the welded material and the entire weld experiences fatigue prior to the final, fracture-governed failure phase. For the studied weld seams of hardened CoCr, a regression fatigue limit of 10.8–11.8 MPa, where the stress refers to the arithmetic average shear stress computed along the region dominated by shear loading, is determined.

Effects of pass arrangement on angular distortion, residual stresses and lamellar tearing tendency in thick-plate T-joints of low alloy steel

Three thick-plate T-joints with different pass arrangements were fabricated using low alloy steel. Experiments were conducted to measure the angular distortion and residual stress distribution of these T-joints. Based on ABAQUS code, a sequentially coupled thermal-mechanical finite element method considering moving heat source was developed to model the welding deformation and residual stress distribution of the multi-pass T-joints. The evolution of the angular distortion and stress distribution during welding process of the three T-joints was analyzed by means of numerical simulation. The peak stress distribution in the thickness direction of the flange during the entire welding process was examined based on the simulation results, and the lamellar tearing tendency of the T-joints was investigated. Among the three pass arrangements, the pass arrangement that welding passes are attached to the flange with surfacing welding produces the minimum angular distortion and has the lowest tendency of lamellar tearing for thick-plate T-joints.

Prediction of Melting Profile of Mild Steel Weld Metals Using Regression Analysis

The microstructure of a weldment can be maintained by ensuring a steady state homogenous melting profile of the welding operation, which includes the deposition of optimum volumes of melted filler wire and substantial part of the heat affected zones of the parent metal to form the weld pool. The melting pattern of the entire welding process should be protected from atmospheric air, so as to enhance weldment quality. In this study, the melting profile of mild steel is investigated by looking at the parent metal angular distortion bead width and penetration volumes of deposited filler and the melting efficiency was determined. Predictive models were also developed to determine the above listed melting properties by applying the regression analysis. The result obtained showed that, there is almost a perfect fit between the calculated and predicted angular distortion, as well as between the calculated and predicted volume of filler wire melted. There is also a close correlation between the calculated and predicted melting efficiency. However, for the bead width, bead penetration and volume of weld metal deposited, there were variations of values and a heterogeneous correlation between the calculated, measured and predicted values. The effects of the process parameters on the obtained properties of the melting profile were investigated and optimum process parameters were determined.