Welding Thermal Process Research Articles

Non-destructive testing of an aluminum alloy welded joint based on a mathematical model of the thermal welding process and computer microtomography

Strength control of welded joints is used in many industries. X-ray microtomography as a method of non-destructive testing allows not only to detect the presence of defects, but also to classify them and assess the size of cracks and non-crack-like defects (pores). This article discusses the non-destructive testing of welded joints of aluminum alloy 6061 T6 using computer micrototomography and a mathematical model of the thermal welding process implemented in ANSYS Workbench. Experimental results of the X-ray microtomograph are presented, and the size of defects in this sample is estimated. The mathematical model allowed us to obtain the thermal histories at which the faults were formed.

“Anneal” softening effect of 2219-T8 aluminum alloy joint during welding and its influence on prediction of welding residual stresses

Prediction of welding residual stresses (WRS) could be unreliable if mechanical property model of the material is inaccurate in finite element model. For TIG welding of 2219-T8 aluminum alloy, mechanical properties change with welding thermal process in a complex pattern. To build complete mechanical property model, influence of temperature, working hardening and “anneal” softening during welding must be considered. Gleeble and DIC system were used in this study to acquire tensile properties in different thermal process. Working hardening and “anneal” softening were studied by tensile curves. It was found that working hardening of 2219-T8 aluminum alloy would be eliminated at 200°C–300°C, which was called “anneal” softening. Then, fined mechanical property model was built for welding simulation. Results showed that “anneal” temperature had a great influence on longitudinal residual stress (LRS) in welding zone (WZ), partly melted zone (PMZ), over aged zone (OAZ) and part of heat affected zone (HAZ in this study) where maximum temperature was higher than 380°C. With correct consideration of “anneal” softening, LRS in these regions could be 100–150 MPa lower than that without “anneal” softening. Yield strength in these regions was greatly reduced due to “anneal” softening. However, for the regions with maximum temperature below 380°C, like other part of HAZ, “anneal” softening had little effect on WRS.

Nondestructive Testing of an Aluminum Alloy Welded Joint Based on a Mathematical Model of the Thermal Welding Process and Computer Microtomography

Nondestructive Testing of an Aluminum Alloy Welded Joint Based on a Mathematical Model of the Thermal Welding Process and Computer Microtomography

Morphological characteristics and atomic evolution behavior of nanojoints in Ag nanowire interconnect network

Ag nanowires (AgNWs) have shown great application value in the field of flexible electronics due to their excellent optical and electrical properties, and the quality of its joints of AgNWs in the thin film network directly plays a key role in its performance. In order to further improve the joint quality of AgNWs under thermal excitation, the thermal welding process and atomic evolution behavior of AgNWs were investigated through a combination of in situ experimental and molecular dynamics simulations. The influence of processing time, temperature, and stress distribution due to spatial arrangement on nanojoints was systematically explored. What is more, the failure mechanisms and their atomic interface behavior of the nanojoints were also investigated.

Microstructure characteristics of Q345R-steel welded joints and their corrosion behavior in a hydrofluoric acid environment

The evolution of joint-microstructure characteristics during multipass welding of medium and heavy Q345R plates was analyzed, and the corrosion characteristics were investigated in a hydrofluoric acid environment. Weld joints were manufactured by pulsed tungsten–inert-gas (P-TIG) priming and submerged-arc-welding (SAW) filling. After SAW filling, the microstructure of the P-TIG-priming root pass changed from acicular ferrite + sorbite to polygonal ferrite + worm-like troostite. As the ferrite and cementite lamellae are very dense in the troostite, the acceleration of galvanic corrosion soon caused the ferrite to dissolve. Meanwhile, the worm-like troostite near the surface was equivalent to a single mass of cementite that formed a corrosion couple with the surrounding ferrite to accelerate corrosion dissolution and form corrosion pits. Therefore, designing the welding thermal process and realizing the control of the pearlite morphology in the weld joint root pass are effective means to improve the corrosion resistance of Q345R steel.

A MODEL OF WELDING THERMAL PROCESSES WITH A LIMITED NUMBER OF FICTITIOUS POINT SOURCES

Analytical methods of calculation of thermal processes at welding enable to estimate operatively the influence of welding modes on parameters of a temperature field and to predict structural changes in a zone of thermal influence. In the mid-twentieth century it was suggested to calculate the heat processes in welded products with parallel boundary planes by summing up the temperature fields of the main and infinite number of fictitious point sources using thereflection method. However, due to computational difficulties, this method could not be brought to a state of wide practical application. Furthermore, since the schematization of heated bodies is conditional, the choice of the body scheme (plate, flat layer, semi-infinite body) and therefore the thermal calculation model is performed intuitively based on the accumulated experience. This uncertainty inevitably leads to discrepancy between the calculated and actual parameters of the thermal welding process. The purpose of this work is to adapt a model of thermal processes with a limited number of fictitious fast moving point sources to the calculation of thermal welding processes. The proposed model, solvable in Mathcad software, contains the equations of temperature field and temperature rate of change with account of the number
 of fictitious heat sources involved in the calculation. This model allows a reasonable limitation of the number of fictitious heat sources for providing the required accuracy of calculations, since as the fictitious heat sources move away from the product and their influence on the product heating decreases. In most cases for practical calculations 2 pairs of fictitious sources are enough for welding of 2 cm thick steel products, of 0.5 cm thick products ‒ 5 pairs, of 0.2 cm thick products ‒ 12 pairs. Comparison of the calculated values of the proposed model with actual temperature field measurements and known effects of cooling rate in the plate, plane layer and semi-infinite body within the accepted limits have confirmed its adequacy. The presented comparative results demonstrate versatility of the proposed model with a limited number of fictitious fast moving point sources, which can be applied sufficiently and successfully for analysis of the thermal welding processes of real industrial products of arbitrary thickness without discriminating into plate, plane layer and semi-infinite body models.

Analysis of Temperature Field Dynamics in Electrofusion Welding of Polymer Pipes at Low Temperatures

Technique for determining technological parameters of electrofusion welding of polyethylene pipes at ambient temperatures below standard, based on analysis of temperature field dynamics in heat-affected zone, is proposed. Mathematical model of thermal welding process used in calculations is presented, considering temperature dependence of phase transformation heat and crystallinity degree of polymer material. Results of determining parameters for electrofusion welding of polyethylene pipes and welding of branch saddle topload to polyethylene pipe are presented. Effectiveness of calculated determination of welding parameters is confirmed by tests of welded joints made according to proposed welding technology at naturally low temperatures.

Molecular dynamics simulation of thermal welding process of Ag nanoparticles

The thermal welding process of Ag nanoparticles was studied by molecular dynamics using embedded atoms method (EAM), and the melting point of Ag nanoparticles was analyzed by radial distribution function (RDF) and root mean square displacement (RMSD). The results show that for Ag nanoparticles with diameter less than 10 nm, driven by surface energy, the welding temperature will be much lower than the melting point when the spacing between nanoparticles is smaller than the radius of particle, the mechanism at low temperature is different from that at high temperature. The spontaneous rotation of nanoparticles during welding at suitable temperature makes the lattice orientation parallel to each other in three-dimensional space, which may be caused by collision of nanoparticles or short-range interaction near the surface. From the thermodynamic point of view, this behavior is driven by the reduction of surface energy, with the increase of temperature, the interface between particles will become chaotic gradually.

Facile Strategy for Fabrication of Flexible, Breathable, and Washable Piezoelectric Sensors via Welding of Nanofibers with Multiwalled Carbon Nanotubes (MWCNTs).

Piezoelectric materials have been widely explored, due to their potential applications related to wearable sensors, energy harvesting, and electronics. However, the majority of previously reported flexible sensors cannot simultaneously possess the properties of conductivity, washability, and air permeability, which limits their further development in textile applications. Herein, we have processed a nanofibrous mat to successfully develop a textile, which acquired effective conductivity, while maintaining a soft and porous nature for comfortable wearing. Two steps were implemented in this strategy. First, the surface of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers was coated with multiwalled carbon nanotubes (MWCNTs) through electrospinning; second, for preventing the damage by washing, the MWCNTs were welded into the nanofibers to generate a strong connection with the nanofibers by a thermal welding process. With the incorporation of MWCNTs, the electrospun PVDF-HFP nanofibrous mat manifests good conductivity with a sheet resistance of 7.1 ± 2.8 kΩ and excellent mechanical properties (up to 35.4 ± 7.3 MPa). The as-prepared nanofibrous mat exhibits high stability, good bending stability over 10 000 bending cycles, and superior wearability in terms of washability and breathability. Besides, such a nanofibrous mat can be used in contact with the human skin or attached to textiles for body motion monitoring, displaying great potential for wearable devices.

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.

A novel high efficiency low heat input welding method: High frequency electric cooperated arc welding

High efficiency and low heat input are the pursuit of modern welding. In this paper, an innovative high frequency electric cooperated arc welding (HFAW) was developed. The process and joint characteristics of the HFAW were compared with that of traditional arc welding (TAW). In the HFAW, a part of welding heat is generated all over the surface of weld side wall (WSW) directly and rapidly due to the skin effect and proximity effect of high frequency current. The limitation of heat conduction on the TAW was broken through. Benefited from the ultra-speed welding thermal process, the HFAW reaches 3–4 times the welding speed of the TAW, while the heat input needed in the HFAW was only about 60% that in the TAW. Meanwhile, the HFAW joint presented sound weld formation, complete penetration and finer microstructure, which made the joint possess an equal tensile strength with the base metal.

Methodology to Reduce Distortion Using a Hybrid Thermal Welding Process.

Welding is a thermal process which results in high strain and stress values in the material and leads to its change of phase. This might cause significant distortions in the welded structure, which often becomes a relevant design and manufacturing issue. This study deals with a line-heating-based forming process that is applied at the moment of the welding operation, with the final objective of minimizing distortion. A FEM (finite element method) based on a thermo-elastoplastic approach is used here. The computational method is first calibrated in three stages: heatline forming calibration, flame heat source calibration, and the weld process. The final model presented in this work simulates a hybrid process called htTTT (high-temperature thermal transient tensioning) which was optimized over large T welds to minimize the final distortion of the components.

Numerical simulation of VPPA-GMAW hybrid welding of thick aluminum alloy plates considering variable heat input and droplet kinetic energy

In order to realize the high-efficiency and high-quality welding of thick aluminum alloy plates, a paraxial variable-polarity plasma arc–gas metal arc welding (VPPA-GMAW) hybrid welding method was developed in the present study. The target material was a thick plate of 7A52 aluminum alloy, which is widely used in the aerospace field because it offers high specific strength and high corrosion resistance. The thermal action characteristics of paraxial VPPA-GMAW hybrid welding during direct-current electrode-negative and direct-current electrode-positive phases were considered. A variable combined heat source model was developed to investigate the hybrid welding thermal process. In the model, the variable-polarity plasma arc (VPPA) was treated as two curve-rotated body heat sources with different distribution parameters for electrode-negative and electrode-positive durations; the two heat sources were loaded periodically to express the heat input form. The GMAW arc was described as a double-ellipsoidal heat source, the overheated droplet was described as a uniform-body heat source, and the impact effect of the overheated droplet on the weld pool was considered by accounting for its kinetic energy. This heat source model was employed to conduct a finite-element analysis of hybrid welding temperature profiles. The numerical simulation results indicated that the VPPA provided deep penetration in the hybrid welding. The subsequent GMAW increased the penetration achieved by the VPPA and filled the weld pool to achieve deep-penetration welding. Experiments were performed to measure weld geometries and thermal cycles, and the weld pool shape was captured by high-speed camera. The calculated weld bead geometry, weld pool shape and thermal cycles showed excellent agreement with the experimental results.

Application of Johnson-Mehl-Avrami-Kolmogorov type equation in non-isothermal phase process: Re-discussion

A new non-isothermal phase kinetic equation which keeps the form of Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation is established. This non-isothermal JMAK type equation is expressed asX(t)=1−exp[−(Kt)n],K=K0exp(−QRT0)T0 can be given a linear relationship with transformation temperature T in relatively large parameter fluctuation ranges. The applications of non-isothermal JMAK type equation in γ→α phase process of Fe-0.002wt%C alloy under linear cooling process and welding thermal process do show its good agreements with the experimental data.

Mathematical modelling of the thermal process in electro socket welding of polyethylene pipes at low temperatures

By the mathematical model of thermal processes in welding of polyethylene pipes for gas pipelines, it is proposed to take into account the phase transition in the temperature range. The results of comparison of the theoretical dependences of temperature on time with the experimental data are presented and the efficiency of this approach in the determination of the dynamics of the temperature field in the welded joint is shown.

Precise Absorbent ink Deposition and Optical Inspection System

ABSTRACTFoils from the ethylene-tetrafluoroethylene (ETFE) copolymer are used as transparent, humidity resistant and UV-stabile facade and roof coverings, e.g. for stadia, indoor swimming pools or greenhouses [1]. With pneumatically supported cushions, large translucent structures can be realized. Until now, they are assembled through a thermal welding process [2]. The development of welding techniques using laser irradiation is under way.Laser welding of transparent polymer foils requires an optical absorber placed in the interface between the two welding pairs. Usually, dye molecules with absorption properties adapted to the laser wavelength are used as absorbers. At a well-defined temperature, the dye molecules will be chemically modified, and transparent laser welding seams can be achieved. To get reproducible laser welding results, a homogenous layer of absorbent molecules or materials at the welding interface have to be realized, which is often very hard to achieve by wet deposition of dye molecules dispersed in a solution.In our contribution, we report on an inkjet printing system that can be mounted on a R2R manufacturing setup. The main challenge to this approach is to find the right ink that is compatible with this highly hydrophobic ETFE foil. Therefore, both the pretreatment of the substrate as well as the utilization of different inkjet technologies are dealt with in this contribution. It is demonstrated that the inkjet printing of a laser absorbent ink in a defined way onto the substrate is possible.For quality assurance, an optical inspection system has been developed to ensure a proper deposition of material. This ensures the quality control for the inkjet printing process of the special functional material. In that part the results of the comparison of the use of a dedicated 14‑bit grey level CCD line camera is compared to a high quality webcam.Laser irradiation of the foil with printed laser absorbent lines together with the untreated joining partner was performed by a continuous wave diode laser at a wavelength of 808 nm using a defocused laser spot. A nearly transparent welding seam was achieved. Mechanical tensile tests of the laser welding seams have demonstrated that their tensile strength is comparable to conventional thermal welding seams.

Temperature Field Numerical Simulation of Laser Welding Joint of Stainless Steel Automobile Vent Pipe

Ferrite stainless steel has been widely used in foreign automobile vent pipe. During the production of domestic automobile vent pipe, because the crystal grains of welding Joint heat-affected zone are not refined by heat treatment and become very big, forming properties of cold machining of ferrite stainless steel become bad. In order to optimize welding technology, the temperature field of laser welding joint of stainless steel automobile vent pipe is simulated. The temperature field model of laser welding joint of 409 L stainless steel pipe is built based on the pair heat source model. At the same time, the affecting factors are considered, such as thermal radiation, phase change latent heat, thermophysical properties of materials with temperature changes and the spread of convective heat in the liquid metal in the model. The temperature field in different welding parameters are simulated. The impact of process parameters on the temperature distribution is acquired. The experimental results can help works, such as accurately understanding laser welding thermal process, predictive control of welding stress and deformation, and improving cold forming quality of automobile vent pipe.

Development of prediction model for precipitates kinetics and austenite grain growth in the weld heat affected zone

During welding process, austenite grain growth in the weld heat affected zone occurs and this austenite grain growth is important factor for the mechanical properties and safety of welded steel structures. Therefore, many researches have been performed to control austenite grain growth by using the precipitates of micro alloying elements such as niobium and/or titanium or vanadium. The austenite grain growth pinning effect by precipitates is affected by size and fraction of precipitates. Therefore, the behaviors of precipitates during welding thermal process have been focused. Most previous researches on the particle behavior treated only continuous growth of mean sized particles at a temperature below the dissolution temperature. But, in the view point of Ostwald ripening by Gibbs-Thomson effect, coarsening kinetic of particle occurs simultaneously with dissolution of small particles. Then, it is expected that there is a critical size which does not shrink and grow. In this study, an isothermal kinetic model of TiN considering critical particle size was developed and it was transformed as a general kinetic model by using the additivity rule. Finally, from combining austenite grain growth model and particle kinetic model, generalized austenite grain growth model was established in Ti added low alloyed steel HAZ .

Using MathCAD software for studying non‐stationary thermal processes in welding

(2002). Using MathCAD software for studying non‐stationary thermal processes in welding. Welding International: Vol. 16, No. 9, pp. 735-736.

Regulation of GMA welding thermal characteristics via a hierarchical MIMO predictive control scheme assuring stability

This paper deals with the regulation of the thermal characteristics of gas metal arc welding (GMAW). A complete treatment of the welding control problem requires the regulation of both the geometrical and thermal characteristics of the weld. Both classes of characteristics are of critical importance, but the thermal ones have received less attention in the majority of previous work. The present paper proposes a hierarchical predictive control scheme for the metallurgical characteristics of GMAW. A previously developed model for the regulation of the heat-affected zone, the cooling rate and the nugget cross-sectional area is used for the open-loop predictions. At the first level of the hierarchy, a parameterized generalized predictive control (GPC) algorithm is selected, among other control techniques, due to the inherited difficulty of the welding thermal process and the robustness of this algorithm against modeling errors and parameter variations. The main drawback of GPC is that it does not guarantee stability. The second level of the control hierarchy, where a coordinator specifies a set of reliable values for the parameters of GPC, so that stability is assured, and transmits them to the controller of the first level overcomes this difficulty. This paper provides a representative set of simulation results obtained by the present hierarchical GPC scheme, including a comparison with the classical linear-quadratic optimal control scheme. These results show the superiority of the GPC scheme.