Weld Centerline Research Articles

Calibration of the Elasto-Plastic Properties of Friction Stir Welded Blanks in Aluminum Alloy AA6082

In recent years remarkable efforts have been devoted to the study of the formability of tailored blanks processed by friction stir welding (FSW) by means of numerical and experimental approaches. This study aims to perform an inverse analysis to calibrate the constitutive law of FSW blanks produced with different parameters. The evaluated mechanical behavior will be employed in future studies to investigate the formability of such elements by single point incremental forming. This study analyzes friction stir welded blanks in aluminum alloy AA6082-T6 produced with different advancing velocities and tool rotational speeds. Coupons for tensile testing have been cut and collected from the FSW blanks. The longitudinal dimension of the coupons was perpendicular to the tool advancing direction, with the welding centerline located at the middle point of the coupon. From the same welded blanks further coupons have been collected to conduct microhardness testing of the cross-section in the welding zone. Data from digital image correlation (DIC) have been adopted to detect the zones of local variation of the mechanical properties related to the peculiar microstructure determined by friction stir welding. In particular, the data were used as input to feed an iterative numerical routine to calibrate the constitutive law variable as a function of the distance from the welding centerline.

Effect of beam oscillation on the mechanical properties of fiber-laser-welded thin-plate X2CrTiNb18 ferritic stainless-steel joints

This study adopted fiber laser welding (FLW) and oscillating fiber laser welding (OFLW) to weld 1.2 mm-thick X2CrTiNb18 ferritic stainless steel (FSS) in the form of square butt joint. The microstructure and mechanical properties of two types of joints were characterized and compared. The results showed that beam oscillation had a marginal effect on the unidirectional tension properties of the joints. However, the plasticity and fracture toughness of the joint were markedly increased. The beam oscillation changed the morphology and microstructure of the weld. It transformed the outer profile of the weld cross-section into an inverted trapezoid, and formed an equiaxed grain zone in the weld. Beam oscillation decreased the angle between the growth direction of columnar crystals around the equiaxed grains and the weld centerline, which improved the deformation coordination of the grains. The plasticity and fracture toughness of X2CrTiNb18-FSS joints were also improved accordingly.

Effect of Residual Stress on Hydrogen Diffusion in Thick Butt-Welded High-Strength Steel Plates

Thick high-strength steel plates are increasingly being used for ship structures. Moreover, hydrogen enters the process of manufacturing and service, and large residual tensile stress occurs near the weld. Such stress can facilitate the diffusion and accumulation of hydrogen in the material, leading to hydrogen embrittlement fracture of the shell. Therefore, residual-stress-induced diffusion and accumulation of hydrogen in the stress concentration region of thick butt-welded high-strength steel plate structures need to be studied. In this study, manual metal arc welding was realized by numerical simulation of residual stress in a thick butt-welded high-strength steel plate model using the thermoelastic–plastic theory and a double ellipsoidal heat source model. To analyze residual stress, a set of numerical simulation methods was obtained through comparative analysis of the test results of relevant literature. Residual and hydrostatic stress distributions were determined based on these methods. Then, hydrogen diffusion parameters in each region of the model were obtained through experimental tests. Finally, the results of the residual stress field were used as the predefined field of hydrogen diffusion to conduct a numerical simulation analysis. The distribution of hydrogen diffusion under the influence of residual stress was obtained based on the theory of stress-induced hydrogen diffusion. The weak area of the welding joint was found to be near the weld toe, which exhibited high hydrostatic stress and hydrogen concentration. Further, the maximum hydrogen concentration value of the vertical weld path was approximately 6.1 ppm, and the maximum value of the path parallel to the weld centerline and 31 mm away from the weld centerline was approximately 6.22 ppm. Finally, the hydrostatic tensile stress in the vertical weld path was maximized (~345 MPa), degrading the material properties and causing hydrogen-related cracking. Hence, a reliable method for the analysis of hydrogen diffusion according to residual stress in thick high-strength steel plates was obtained. This work could provide a research basis for controlling and eliminating the adverse effects of hydrogen on the mechanical properties of ship structures and ensuring the safe service of marine equipment.

The experimental and numerical studies on load bearing capacity in lamellar tearing of friction stir lap weld

The load bearing capacity in lamellar tearing is one of the key mechanical properties for the friction stir lap welding which has not been investigated before. Both numerical model and experiments were used to study the load bearing capacity in lamellar tearing of friction stir lap weld. The constitutive model of the weld zone in the crack propagation model was obtained by the combination of the proposed precipitate evolution model and the moving heat source model, and the effectiveness of the model was verified by comparing the numerical data with the experimental data. Results indicate that the crack initiated from the keyhole and propagated on the centerline of weld along the weld direction. The rotation of the joint region in the tearing process is a key factor for the increase of the load capacity in the tearing test and the simulation. The joint region can be rotated in the tearing process and the rotation angle can be increased with the increasing of weld length. The load bearing capacity along the weld direction is linearly increased with the increasing of the weld length.

Control of post-weld fracture toughness in friction stir processed X80 HSLA steel

The present study investigates the fracture toughness of FSW X80 HSLA steel welds. Weld cooling rate and peak temperature were varied among welds; indirectly manipulated through FSW travel speed, rpm, and weld preheat. Fracture toughness was tested according to ASTM 1820 standard along the weld centerline using surface-notched SEB specimen cooled to −40 °C. Results demonstrate that CTOD of FSW X80 has a strong inverse linear correlation with post-weld cooling rate. Low cooling rate welds exhibited CTOD values consistently above those of base metal. Microstructure similar to the base metal can be recreated throughout the weld. Weld CTOD can be selected by controlling weld CR.

Numerical and experimental investigation on distribution of residual stress and the influence of heat treatment in multi-pass dissimilar welded rotor joint of alloy 617/10Cr steel

In the present study, numerical and experimental investigation of stress distribution pattern has been performed for thick walled nuclear rotor pipe joint of Alloy 617 and 10Cr steel. Hot wire Tungsten Inert Gas (HW-TIG) welding process has been adopted for multipass narrow groove welding since it yields better mechanical properties at low heat input than manual TIG welding and decreases the accumulation of residual stress significantly. The residual stress generated at the surface of the welded joint was evaluated experimentally using the Blind hole drilling technique (BHD), and consequently, the distribution of stresses along the thickness of the welded joint was masured using the Deep hole drilling (DHD) method. According to the findings, shrinkage at the HAZ area of the weld influences the stresses generated at the cylinder outer surface. The distribution of residual stresses in the weld region is driven mostly by the bending effect produced during the welding process. The influence of post weld heat treatment (PWHT) on residual stress distribution has also been investigated using both experimental and numerical approaches. Numerical investigations have been performed using ABAQUS finite element method. A 2D finite element model has been developed to simulate the welding process and the subsequent heat treatment, using coupled thermo-mechanical analysis. The residual stress results obtained from the numerical approach were validated with the experimentally obtained results and a good correlation has been found. The lumped pass model along with an equal density heat source has been implemented to minimize the computation complexity without affecting the accuracy of the simulated results. The obtained findings revealed that apart from weld centerline, hoop tensile residual stresses were higher around the buttered layer of Alloy 617 and 10Cr steel in an as-welded state. It has been found that nearly 80% of the stresses generated along the weld section have been released after PWHT.

An investigation of the fatigue properties of the aluminium-copper alloy AA2014-T6

ABSTRACT An approach to determine the fatigue behaviour in the heat-affected zone (HAZ) of AA 2014 has not been investigated exhaustively since the HAZ has been treated as one region. In this study, fatigue crack growth (FCG), fatigue strength and low cycle fatigue (LCF) properties of the base metal (BM) and two regions of the HAZ were studied. The two regions of the HAZ were thermally simulated at 590°C and 650°C representing regions of the weldment 4 mm and 5 mm from the weld centreline respectively. The hardness tests and microstructure characterisation of the base metal and the two regions of the HAZ were also carried out. The HAZ region thermally simulated at 590°C was found to offer the least resistance to FCG, had shorter LCF life and lowest value of fatigue strength compared to both the BM and the HAZ region thermally simulated at 650°C. The cause of weakness in this region was associated with the dissolution of strengthening phases. The degradation of the fatigue properties of AA2014 occurred severely at 590°C due to the dissolution of CuAl2 precipitates in the aluminium matrix. The HAZ region thermally simulated at 590°C was confirmed to be the weakest link in the HAZ.

Effects of material properties and mismatch on unified constraint parameter

In order to develop unified constraint-based fracture assessment method, it needs to investigate the effects of material properties and mismatches on the unified constraint parameter and develop the parameter solutions for cracked structures. In this work, the effects of material properties (yield strength and hardening exponent) and strength mismatch on the in-plane and out-of-plane unified constraint parameter Ad* of semi-elliptical surface cracks in homogeneous and welded plates have been investigated. The results show that the parameter Ad* does not essentially change with yield strength and hardening exponent of materials for cracks in homogeneous plates under bending and tension loads. The parameter Ad* of weld centerline cracks in welded plates under bending and tension loads is essentially independent on the strength mismatch. These advantages of Ad* are conducive to the development of its solutions for homogeneous and welded structures containing cracks. The effect of weld width on Ad* is related to loading mode, mismatch (undermatch or overmatch) and crack size. The mechanism of the influence of material properties, strength mismatch and weld width on Ad* has been explained by its definition and the crack-tip plastic strain distribution. The development and application of Ad* solutions for homogeneous and welded structures also have been discussed.

EFFECT OF WELD HEAT INPUT ON THE MECHANICAL CHARACTERISTICS OF Cr-Mo STEEL BARS USING THE MIG WELDING PROCEDURE

The impact of weld heat input on the mechanical characteristics of this steel was investigated in this study. The weld sample was an ASTM A304 grade A Flat Bar with a thickness of 5.00 millimetres. With the use of a data logger, a K-type thermocouple was installed in the drilled holes to measure the temperature fields at each of the places throughout the welding operation. The welding sample was created by varying three process parameters in the MIG welding process. Welding current, weld passes, and point of interest thermocouple position away from the weld centreline are the parameters. The experiment was designed using Taguchi's L9 orthogonal array to limit the number of experimental runs (DOE). To study the influence of weld heat input, the samples were subjected to mechanical and microstructure testing. The hardness test revealed that when the weld pass on the weld zone, HAZ, and base metal increases, the hardness value falls. The tensile test finding indicated that the tensile strength of samples 80A and 100A rises as the weld pass increases, however sample 60A decreases as the weld pass increases. The impact resistance increased when the weld pass was increased, according to the findings of the impact tests. The distance away from the weld centreline of the region subjected to a certain peak temperature increases as weld heat input increases, according to the weld thermal cycle. The value of the temperature fields increases dramatically as the heat source passes through the specific cross-section, where thermocouples are positioned. Later on, the temperature begins to drop at a slower rate. It's also obvious that when the distance between the measurement point and the weld centreline grows, the temperature drops.

An active visual monitoring method for GMAW weld surface defects based on random forest model

In the automatic manufacturing of robotic welding, real-time monitoring of weld quality is a difficult problem. Meanwhile, due to volatilization of zinc vapor in galvanized steel and complexity of welding process, the existence of welding defects greatly affects industrial production process. And real-time detection of welding defects is a key step in development of intelligent welding. To realize real-time monitoring of weld surface defects, an active visual monitoring method for weld surface defects is proposed. Firstly, after applying Gabor filter to remove interference signals such as arc and noise, obtain weld centerline image; then employ the slope analysis method to extract peak valley coefficient of weld defects, extract five feature points of weld surface quality by Douglas-Puke algorithm, and analyse geometric and spatial distribution features of different types of defects in weld laser stripe images. Finally, using eight feature vectors extracted from weld features, design a weld defect recognition random forest model based on decision tree. After optimizing the decision tree depth and number of model evaluators, compared with the traditional decision tree ID3 and CART algorithm model, this model has better performance than traditional machine learning algorithms on five weld surface defect data sets. The experimental results show that accuracy of weld defect identification in the training set is 99.26%, accuracy of weld defect recognition in the test set is 96%, and processing time of a single image is only 5.3 ms, which overcomes difficulty of real-time weld defect detection in intelligent welding and ensures real-time and accuracy.

Effects of Fixture Configurations and Weld Strength Mismatch on J-Integral Calculation Procedure for SE(B) Specimens.

This work presents the development of a J-integral estimation procedure for deep and shallow cracked bend specimens based upon plastic ηpl factors for a butt weld made in an S690 QL high strength low alloyed steel. Experimental procedures include the characterization of average material properties by tensile testing and evaluation of base and weld metal resistance to stable tearing by fracture testing of square SE(B) specimens containing a weld centerline notch. J-integral has been estimated from plastic work using a single specimen approach and the normalization data reduction technique. A comprehensive parametric finite element study has been conducted to calibrate plastic factor ηpl and geometry factor λ for various fixture and weld configurations, while a corresponding plastic factor γpl was computed on the basis of the former two. The modified ηpl and γpl factors were then incorporated in the J computation procedure given by the ASTM E1820 standard, for evaluation of the plastic component of J and its corresponding correction due to crack growth, respectively. Two kinds of J-R curves were computed on the basis of modified and standard ηpl and γpl factors, where the latter are given by ASTM E1820. A comparison of produced J-R curves for the base material revealed that variations in specimen fixtures can lead to ≈10% overestimation of computed fracture toughness JIc. Furthermore, a comparison of J-R curves for overmatched single-material idealized welds revealed that the application of standard ηpl and γpl factors can lead to the overestimation of computed fracture toughness JIc by more than 10%. Similar observations are made for undermatched single material idealized welds, where fracture toughness JIc is overestimated by ≈5%.

Parametric features of an UIC60 rail at weld bottom crack in transverse direction under weld-wheel contact forces

In this study, the stress fields of a welded beam track under the influence of a wheel-rail weld force are explored using a finite element model assembly of a weld wheel contact system which has been built on ANSYS space-claim. Practical observations have been shown that semi-elliptical type of cracks are most of the reason to failure of a weld thus a 3-D semi-elliptical type of crack has been induced on the bottom of the rail having transverse direction propagation. Simulation was performed using singular elemental method with ANSYS mechanical workbench for stress field singularity and transverse pattern of crack propagation on the bottom of the rail weld has been observed. The crack analysis and simulation tool in ANSYS workbench provides three modes of stress intensity factors where Mode-I factor KI shows increase in its value with increase in wheel force on its rail-weld contact under each type of forces i.e. vertical, lateral and longitudinal, and if there is an increase in the lateral force the KI took its maximum value. Now if we increase the vertical force the rest of two intensity factors KII and KIII shows a decreasing tendency. It has been observed also that if the longitudinal and lateral forces raised simultaneously both KII and KIII increases in magnitude. The amplitude of KI at the tip side of crack on welds outer edge is greatest within the Stress Intensity Factor of the crack tip, whereas the peak of KII and KIII on the crack-tip of weld centerline seems the highest in the range of Stress Intensity Factor.

CHARACTERIZATION OF WELD HEAT INPUT EFFECTS ON MECHANICAL PROPERTIES OF Cr-Mo STEEL BAR USING TIG WELDING PROCESS

Welding as an energy-consuming process is inevitable as-welded joint can be subject to various loads without failure. Therefore, this paper presents the welding thermal cycle of Cr-Mo steel bar (ASTM A304) of dimensions 100 by 50 and various thicknesses of 5, 10- and 15-mm. Pure tungsten with 2% thoriated TIG electrodes sizes 1.6 mm X 175 mm, 2.4 mm X 175 mm, and 3.2 mm X 175 were used without filler materials for the welding process. A double V-groove weld joint, with a moving heat source, was employed to determine the temperature fields and transformation in the single-pass butt-welded joint. A calibration process was attached at each point of interest using a datalogger type K-type thermocouple 3-channel–LU-MTM-380SD during the welding process on the Cr-Mo steel bar. The results showed that the welding temperature became higher at the welding centreline and decreased towards the edges of the bar. An indication that a weld thermal cycle is a veritable tool, a function of heat input to access likely consequence of the welding process at both welded and parent metal portions of steel bar. Design of Experiment using Taguchi L9 orthogonal array matrix L9(3^4), Factors:4 and Runs:18 in Minitab 17 Taguchi Design Method that suited the experimental method used. Taguchi’s L9 orthogonal array to restrict the number of experimental runs was used for the design of the experiment (DOE). Mechanical and Microstructure tests were carried out on the samples to investigate the effect of weld heat input. The hardness test result showed that samples C15 and D15 have the highest hardness values 165.0HV and 164.0HV respectively at Base metal (BM) 20mm away from weld centreline and it was also observed that samples C15 and D15 have the highest impact values 48.53J and 48.7J respectively. The microstructure of the C15 at the weld zone WZ, which consists majorly of pearlite and less ferrite, BM shows the appearance of alpha ferrite and pearlite and heat-affected zone HAZ consists majorly of pearlite and a very small proportion of ferrite resulted in increased hardness and impact values at the HAZ.

Evaluating Temperature Control in Friction Stir Welding for Industrial Applications

Reports in the literature indicate that temperature control in Friction Stir Welding (FSW) enables better weld properties and easier weld process development. However, although methods of temperature control have existed for almost two decades, industry adoption remains limited. This work examines single-loop Proportional-Integral-Derivative (PID) control on spindle speed as a comparatively simple and cost-effective method of adding temperature control to existing FSW machines. Implementation of PID-based temperature control compared to uncontrolled FSW in AA6111 at linear weld speeds of 1–2 m per minute showed improved mechanical properties and greater consistency in properties along the length of the weld under temperature control. Additionally, results indicate that a minimum spindle rpm may exist, above which tensile specimens do not fracture within the weld centerline, regardless of temperature. This work demonstrates that a straightforward, PID-based implementation of temperature control at high weld rates can produce high quality welds.

Finite Element and Experimental Analysis of Residual Stresses in Electron Beam Welded Nickel-Based Superalloys

In this paper, two different nickel-based superalloys, namely Inconel 718 and Nimonic 80A were joined using electron beam welding techniques with three different welding parameters. A finite element analysis (FEA) using Abaqus software was carried out to calculate the residual stresses due to welding. Both transverse and longitudinal residual stresses were determined. Also, an X-ray residual stress measurement system, μ-X360 Ver. 2.5.6.2 was used for measuring transverse residual stress along and across the weld centerline. The transverse residual stress found by FEA and that measured experimentally was nearly the same thus validating the FEA. Also, the peak values of longitudinal residual stress found using the FEA were close to the yield strengths of the base metals as found elsewhere.

Microstructure and Mechanical Properties of GTAW Welded Joint of Invar Alloy

Experiment of automatic gas tungsten arc welding of liquefied natural gas carrier Invar alloy with a thickness of .7 mm was completed, and the welding parameters were optimized, as well as microstructure and mechanical properties of the welded joint were measured and analyzed. The grain size of the area near the weld centerline was small, mainly cellular dendrites, and the grain size on both sides of the weld centerline increased gradually, mainly dendrites, whereas the grain size near fusion line was larger, and there were more columnar crystals. The heat-affected zone was composed of coarse austenite grains. Transgranular cracks were the main cracks in the welding seam. When welding current was 40 A, frequency was 120 Hz, and welding speed was 350 mm/min, tensile strength of the welded joint was 446.9 MPa, which 88.1% of the base metal’s tensile strength and 10.3% of the fracture elongation. The fracture surface of tensile specimens showed typical plastic fracture characteristics, with no obvious crack characteristics, and no eutectic liquid films were observed. Introduction Invar alloy is a Fe–36%Ni alloy. Its thermal expansion coefficient at room temperature is less than 1.6X10-6/k, which is about one-tenth of that of low carbon steel, and changes little in a larger temperature range. Therefore, Invar alloy is widely used in making precision measuring instruments and low temperature-resistant materials (Corbacho et al. 1998; Park et al. 2011; Qiu et al. 2016; Yakout et al. 2018). In recent years, with the increasing demand for clean energy, liquefied natural gas (LNG) carriers are developing rapidly (Zhao et al. 2015; Oh et al. 2018). LNG liquefies at temperatures below -161.5°C. The volume of LNG is about 1/ 625 of the volume of gaseous natural gas of the same mass; thus, it can transport large quantities of natural gas over long distances.

Optimizing process parameters for joint strength efficiency improvement in friction stir welding of polycarbonate sheets

AbstractThe present work addresses optimization of these friction stir welding process variables to maximize joint strength efficiency of welded polycarbonate sheets by using particle swarm optimization algorithm over response surface method based regression model. Initially, parametric influence on weld quality characteristics namely weld bead profile, bead geometry with associated microstructure along with micro‐hardness deviation through the weld centerline and stress‐strain behavior of the weld have been studied in detail as per full factorial design of experiments by using three different tool pin profiles such as cylindrical, square and triangular. The center point experiment i. e. tool rotational speed of 1800 min−1 and welding speed of 20 mm/min, and square tool pin profile were found to be the optimum combination with a maximum joint strength efficiency of 60.06 %. The regression model of weld ultimate tensile strength was developed by using response surface methodology which was found to be significant. Therefore, this model was further used for parametric optimization by using both response surface methodology and intelligent particle swarm optimization approaches. A slight improvement in joint strength efficiency with better optimization capability was found by using particle swarm optimization technique as compared to response surface methodology.

Energy reconstruction and metallurgical characteristics in 316L NG-LWFW with assisted wire wobbling by numerical and experimental analysis

A novel, new approach of narrow gap laser wire feeding welding (NG-LWFW) with electromagnetic assisted wire wobbling is proposed to solve incomplete sidewall fusion and improve solidification morphology. The energy distribution and fusion zone formation during multi-pass NG-LWFW process are clarified. The melt pool is driven into sidewall position and the melted metal accumulates towards that side groove, leading to the increase of sidewall penetration with concave bead profile. The simulation results show that the wire wobbling obviously reduces peak temperature of melt pool and increases sidewall temperature. The heat affected zone (HAZ) width reduced, and a finer and diversity weld microstructure is presented. As multi-pass welding proceeds, the former weld is heat-treated by the latter weld, leading to the re-melting of columnar grains and increasing diversity of interlayer grains. The columnar crystal zone is refining and a narrower dendrite spacing of δ-ferrite phase is found with deposited filling layers. The micro-hardness of weld centerline shows an overall increase trend and the impact energy increases with denser dimples from the bottom layer to upper layer.

Characterisation of short-length scale residual stress variations within an electron beam welded P91 Ferritic–Martensitic steel plate

In this research, an electron beam (EB) butt-welded P91 steel plate (~9 mm thick) is selected to measure longitudinal residual stresses (acting in the welding direction) using a promising residual stress measurement technique - the contour method. The EB weld causes short length scale residual stresses within the narrow width of the fusion zone with steep stress gradients lateral to the weld line, which presents a new challenge for the residual stress measurements. The special measures were taken for all main steps of the technique to improve its the spatial resolution, thereby the short length scale residual stresses were captured. The success of the measurement is assessed by comparing the contour method result to the neutron diffraction result, and a good agreement were found in the results of both the techniques. These results also shows that the contour measurement results successfully captured the steep stress gradient on both sides of the weld centre-line (0 mm) with tensile stress peaks situated just 2.8 mm apart. It is one of the best resolutions in residual stress length-scale in this component achieved in contour measurements.

Influence of Filler on the Microstructure, Mechanical Properties and Residual Stresses in TIG Weldments of Dissimilar Titanium Alloys

The influence of titanium alloy (Ti–5Al–2.5Sn) and commercially pure titanium (cpTi) as fillers on dissimilar pulsed tungsten inert gas weldments of Ti–5Al–2.5Sn/cpTi was investigated in terms of microstructure, mechanical/nano-mechanical properties, and residual stresses. A partial martensitic transformation was observed in the weldments for all the welding conditions due to high heat input. The microstructure evolved in the FZ/cpTi interfacial region was observed to be the most sensitive to the proportion of α stabilizer in the filler alloy. Furthermore, the addition of filler alloy improved the tensile properties and nano-mechanical response of the weld joint owing to the increased volume of metal in the weld joint. As compared to the Ti–5Al–2.5Sn wire, the use of cpTi filler wire proved to be better in terms of energy absorbed during tensile and impact tests, tensile strength and ductility of the dissimilar welds. An asymmetrical residual stresses profile was observed close to the weld centerline, with high compressive stresses on the Ti–5Al–2.5Sn side for both the weldments obtained with and without filler wires. This was attributed to mainly the low thermal conductivity of Ti–5Al–2.5Sn. The presence of residual stresses also influenced the nano-hardness profile across the weldments.