Weld Root Research Articles

Influence of the geometry on the fatigue strength of welded joints using the effective notch stress approach

Using the example of a cruciform joint, this study enquires into the relationship between the geometry of weld (the wall thickness of base metal plates, thickness of weld and weld penetration depth) and the fatigue strength of weld joint using the finite element method. It is assumed that the effective notch stress approach can be used to determine a FAT class for dimensioning the connection with the nominal stress approach. Filled throat thickness and weld penetration depth or the geometry of weld root were found to have a strong influence on the fatigue strength of a welded joint. When using the effective notch stress approach, the exact geometry of the calculated weld seam must be known and should be modelled accordingly. It was also found that with thin-walled welds, the effect of wall thickness produces a significant deviation between the load capacity calculated with the effective notch stress approach (effect of wall thickness up to 100%), and with the nominal stress approach (effect of wall thickness max. 10%). At the end, a proposal will be made on how thin-walled welds can be dimensioned to facilitate the use of the same thickness factor for both the notch and nominal stress approaches.

Residual Stresses in a High- and a Medium-Entropy Alloy due to TIG and Friction Stir Welding

The new alloying concept of multi-element systems with defined entropy (HEA—high-entropy alloy; MEA—medium-entropy alloy) is gaining increasing importance in materials research. Significantly improved properties or combinations of properties are shown by some HEA/MEA systems. Thus, primarily the production and resulting microstructures of HEA, as well as its properties, have been investigated so far. Furthermore, processing is a main issue in transferring HEA systems from the laboratory to real components. Since welding is the most important joining process for metals, it is crucial to investigate the influence of welding to guarantee component integrity. Welding leads to residual stresses, which significantly affect the component integrity. Hence, the focus of this study is the residual stress formation and distribution in a CoCrFeMnNi HEA and ternary CoCrNi MEA using two different welding processes: tungsten inert gas (TIG) welding and solid-state friction stir welding (FSW). As a pathway for the application of HEA in this investigation, for the first time, residual stress analyses in realistic near-component specimens were performed. The residual stresses were determined by X-ray diffraction (XRD) on the surfaces of top and root weld side. The results were correlated with the local welding microstructures. The results show that both FSW and TIG generate significant tensile residual stresses on the weld surfaces in, and transverse to, the welding direction. In the case of FSW of the CoCrFeMnNi HEA, the longitudinal residual stresses are in the range of the yield strength of approx. 260 MPa in the weld zone.

Failure analysis of the leakage in girth weld of bimetal composite pipe

A bimetal mechanically-composite pipe leaked at the entrance of processing station. The failure location was the 6o'clock direction of the girth weld connecting the gathering pipe and pipe in the station. In order to figure out the root cause of the failure, a series of experiments were carried out. The macroscopic analysis and metallographic test results show that the leakage point is located at the 6o'clock direction of the girth weld, the root welding and the base pipe of the bimetallic composite pipe are corroded, resulting in perforation. The end of the bimetallic composite pipe is not seal-welded, the girth weld has misalignment and at 6o'clock, the root weld was concave, and the wall thickness of the root weld and the lining is<1.5 mm. The FeCl3 pitting corrosion test results of the girth weld show that the pitting corrosion resistance of the root weld is lower than that of the liner and pure stainless-steel pipe. The galvanic corrosion test shows that the galvanic corrosion grades of weld-pure material, weld-liner, carbon steel-weld, carbon steel-liner are all severe corrosion, and the galvanic corrosion of carbon steel-weld is the most serious. The root cause of the failure of girth weld puncture leakage: the end of the bimetallic composite pipe is not seal-welded. At 6o’clock, the girth weld has misalignment, and the root weld is concave, resulting in that the thickness connecting the root weld and the liner being small. In the medium environment containing Cl- and CO2, local corrosion occurs, and then the medium enters the base pipe and the liner interlayer, and the galvanic corrosion aggravates the corrosion of the base pipe. To avoid the reoccurrence of such failure, the mitigation measures were proposed.

Investigation of fatigue strength of root of lap fillet welds of thin sheet and improvement method

This study investigates the dominant factors of fatigue strength at the root of thin-sheet lap fillet arc welds, which are frequently used in automobile chassis components. A method to improve the root fatigue strength was investigated. The direction of fatigue crack propagation at the root coincided with the direction of maximum circumferential stress range. The circumferential stress was reduced by increasing the distance from the toe to the root (penetration width). Fatigue strength originating from the root was improved in welded joints where the penetration width was increased by adjusting the welding aim point and heat input.

Low-Energy Pulsed-Laser Welding as a Root Pass in a GMAW Joint: An Investigation on the Microstructure and Mechanical Properties.

Root pass is a fundamental step in multi-pass welding. In gas metal arc welding (GMAW), the weld bead qualities depend on the process parameters, filler materials, and welder abilities. This work investigates the effect of a Nd: YAG pulsed laser as a first pass to reduce the welders' reliance on the AH36 low-alloy steel with 5.5 mm thickness. This autogenous automatable process delivers reduced thermal impact due to the concentrated high-energy source, pulse overlap, and higher penetration depth-to-power ratio than continuous lasers. The outcomes indicate that the PL as a root welding generated a small HAZ compared to the GMAW condition. In addition, the subsequent arc passes positively affected the microstructure, reducing the hardness from around 500 to 230 HV. The PL + GMAW achieved similar strength results to the GMAW, although its Charpy impact values at -50 °C were around 15% lower than the arc condition.

Microstructure, Fatigue Properties and Stress Concentration Analysis of 6005 Aluminum Alloy MIG Welded Lap Joint.

This paper studies the microstructure and mechanical properties of MIG (Melt Inert Gas) lap welded 6005 aluminum alloy plates. Microstructure analysis (OM) of the joint showed that 15~30 μm small grains were observed at the fusion line. Mechanical analysis shows that the small grains are broken by shielding gas and molten pool flow force. Hardness test shows that there is a softening zone (41~43 HV) in HAZ much lower than BM and WZ. The low cycle fatigue test showed that the performance of lap joint decreased sharply, and the fatigue strength of weld decreased significantly, which was only 27.34% of the base metal. The fatigue fracture (SEM) of the weld observed slip band cracking and a large number of brittle fracture characteristics. Using the stress concentration factor Kt for analysis, it was found that the cause of brittle fracture was mostly stress concentration. Lap joint stress concentration model appears in two ways: firstly, at the weld toe, the weld is subjected to eccentric force, secondly, there is a small gap between the two plates at the weld root, which cracks along the direction of 45° of the maximum shear stress.

Microstructures and mechanical properties of friction stir welded butt joints of 3003-H112 aluminum alloy to 304 stainless steel used in plate-fin heat exchanger

The valid joining of aluminum and steel plays an important role in support structure of heat exchanger, which was widely used in chemical, automobile and aircraft industries, etc. In this work, in order to improve the connection of aluminum to steel joints, the friction stir welding (FSW) method was applied to weld a 3003 aluminum alloy to a 304 stainless steel. The microstructures, mechanical properties and fracture mechanism of FSWed joints were studied. The results showed that an optimal joint was obtained at the rotational speed of 200 rpm, the traverse speed of 30 mm/min and the offset of 3.0 mm. The distance of root incomplete penetration increased with the raise of offset and the rotational speed. The offset had a more significant effect on the microstructure of the welded joints compared with the rotational speed. Both the tensile property and the microhardness of the stir zone improved with the decrease of rotational speed at the offset of 3.0 mm. The FSWed joints firstly generated stress concentration at the weld root during loading. With the further increase of the load, the weld root cracked and finally broke in the stir zone.

A new orthotropic steel deck system incorporating two novel structural details

The fatigue and fracture of orthotropic steel decks (OSDs) have compromised the toughness and life of steel bridge. The low fatigue resistance of structural detail design is the primary factor to result in the fatigue fracture of OSDs. To develop OSDs with a high fatigue resistance, two novel structural details for OSDs including rib-to-deck welded joint and rib-to-floorbeam welded joint were proposed in this study. Novel rib-to-deck welded joint was formed with a double-side fillet weld seam, which improves the fatigue performance of rib-to-deck welded joint by eliminating the crack-like defects at the weld root of traditional rib-to-deck welded joint. Novel rib-to-floorbeam welded joint is proposed with an uninterruptible weld seam and a smiley cutout, which relieves the cyclic stress from out-of-plane bending of floorbeam and transfer the potential large stress ranges from the weld seam to the smiley cutout. The comparisons of traditional and novel structural details were carried out by the finite element analysis and fatigue tests. The nominal stress method was used in this study to evaluate the fatigue performance. The results indicate that fatigue resistance corresponding to 2 million cycles (FAT) of novel rib-to-deck welded joint is higher than the traditional rib-to-deck welded joint (FAT = 95 MPa), and FAT of novel rib-to-deck welded joint is over 116 MPa. The fatigue strength of the novel rib-to-floorbeam welded joint with a smiley cutout (FAT > 140 MPa) is higher than that of traditional rib-to-floorbeam welded joint (FAT = 98 MPa). The proposed novel structural details can be effective to improve the fatigue resistance of OSDs.

Fatigue strength assessment of complex welded structures with severe force concentrations along a weld seam

This paper investigates weld root fatigue strength of complex structures including severe force concentrations along a weld seam. Fatigue tests were carried out for a pillar assembly with various weld configurations. A comparative analysis was carried out with local nominal weld stress (LNWS), structural weld stress, and effective notch stress (ENS) approaches. The study shows that the LNWS approach gives the best prediction, with a scatter range index of 1.26 to 1.68 depending on the weld type. The accuracy of the structural weld stress approach and ENS was significantly lower, with a scatter range index of 4.98 and 5.40, respectively.

Fatigue failure modes of rib‐to‐deck joints under the multiaxial stress states caused by various wheel loading characteristics

AbstractOrthotropic steel bridge decks (OSBDs) are characterized by overall structural asymmetries and severe local stress concentrations; meanwhile, complicated service loadings always cause relevant rib‐to‐3deck (RD) welded joints in multiaxial stress states. Under such circumstances, a full‐scale OSBD model was established. Six loading cases were applied, and the multiaxial fatigue deviation was calculated to represent the multiaxial stress state of the bridge deck. Based on the effective traction structural stress method, the most unfavorable loading case for the failure mode of the RD joint under a multiaxial stress state was analyzed, and the failure mode transition in the process of wheel load movement was discussed. The results indicated that the fatigue failure mode of the RD joint was determined by the transverse loading locations. In‐between‐rib loading can cause the initiation of toe‐rib cracks. Cracks are prone to occur under over‐rib loading and riding‐rib loading, and the arrangement of diaphragms can increase the risk that the fatigue crack originates from the weld root.

Mechanical and Microstructural Behaviors of Gas Tungsten Arc Dissimilar Metal Welds of High-Mn Steel and Stainless Steel 316L Using Various Fillers for Cryogenic Fuel Tanks

The gas tungsten arc (GTA) weldability of high-Mn (HMn) steel and austenitic stainless steel (STS) 316L using dissimilar metal welds (DMWs) was investigated, to fabricate a fuel tank for an LNG-fueled ship. Three types of welding fillers, HMn steel, STS 309LMo and Inconel 625, were applied to the DMWs. The weldability of the DMWs was examined by investigating mechanical properties at 25 and -196 o C, followed by microstructural evolution and weld deformation. The GTA welding process was performed with a heat input range of 0.72–1.89 kJ/mm. All of the fillers employed in the study produced reasonable mechanical properties and low levels of precipitation, such as M&lt;sub&gt;7&lt;/sub&gt;C&lt;sub&gt;3&lt;/sub&gt; and Fe&lt;sub&gt;3&lt;/sub&gt;C. However, the DN specimen using the Inconel 625 filler exhibited reduced ductility in the bending test. This was attributed to Nb-carbide precipitations at dendrite boundaries at the root weld. The DM specimen using HMn filler experienced severe thermal deformation because it had a relatively higher coefficient of thermal expansion than the other fillers during heating by arc and solidification. Therefore, STS 309LMo and HMn wires were determined to be the appropriate fillers for the DMW of HMn steel and STS 316L for cryogenic applications, and the HMn wire requires attention to weld deformation.

Indentation creep deformation behavior of local zones for X70 girth weld

Creep failure of in-service pipes is a fundamental engineering issue. Since the girth weld joint is a weak link in the pipeline, the creep deformation behavior of the girth weld must be carefully researched. This research sheds light on determining the local creep properties of girth welds. The creep deformation behavior of local zones for the X70 girth weld was studied using nanoindentation creep experiments with maximum nano-indenter loads of 10 mN, 50 mN, and 100 mN. The load dependency and creep mechanism of local zones for the X70 girth weld were investigated through creep properties such as creep strain rate, creep stress, creep stress index, and strain rate sensitivity index. In addition, the microstructures of base material (BM), root welding zone (RWZ), and heat-affected zone (HAZ) were studied to determine the differences in creep properties for the girth weld's local zones. The creep displacement of BM, RWZ, and HAZ follow the RWZ > BM > HAZ sequence as the maximum load increases. The creep stress, on the other hand, exhibits HAZ > RWZ > BM sequence. Furthermore, RWZ and HAZ have a higher load dependency of creep stress index and strain rate sensitivity than BM.

A data‐driven analysis for fatigue failure mode identification in load‐carrying fillet welded joint with mechanical data augmentation

AbstractWeld toe and weld root failures dominate the fatigue failures of the load‐carrying cruciform fillet welded joints. Compared to weld toe failure, the weld root failure is strongly affected by its inherent weld quality and throat size, which is more unpredictable and should be eliminated. This work proposes a weld sizing criterion in terms of two weld leg sizes and weld penetration to avoid root failure. The criterion is obtained by a data‐driven approach with the help of the mechanical‐directed data augmentation technique. The traction‐stress‐based method is applied for the data augmentation to identify the fatigue failure mode, which is statistically tested against 372 experimentally obtained fatigue data.

Fatigue reliability of single-sided girth welds in offshore pipelines and risers accounting for non-destructive inspection

The study aims to assess the fatigue reliability of single-sided girth welds in offshore pipelines and risers based on crack propagation analyses accounting for Non-destructive Inspection (NDI). The initial crack size distribution at the weld root is determined through the calibration of the crack propagation analyses to the S–N data. The effect of NDI, which is carried out immediately after welding, on the initial crack size distribution is modelled. The initial crack size distribution can be significantly changed by NDI and possible repair, and should be updated. The effects of the misalignment and NDI on the fatigue strength and reliability of single-sided girth welds are investigated based on the probabilistic crack propagation analyses. It has been shown that based on existing stress intensity factor solutions, the effect of the misalignment on the fatigue strength is not as significant as that recommended by the fatigue design code. The effect of the acceptance criterion of the initial crack depth on the fatigue strength and reliability can be quantified based on the updated initial crack depth distribution.

Microstructures and mechanical properties of non-thinning and penetrating friction stir welded 2219-T6 aluminum alloy

The aim of this paper is to offer a novel friction stir welding technique to achieve non-thinning and lack of kissing bond welds with excellent mechanical properties. In the conventional friction stir welding (CFSW) process, a certain size spindle tilting angle and a shoulder plunge depth were applied in order to obtain high-quality welds. However, what was the reason for the thinning phenomenon of weld and thus the weakening of weld bearing capacity? The reduction of spindle tilting angle and shoulder plunge depth was likely to cause the welding tool not being able to reach the weld root, and thus, it was engendered for the kissing bond or the absence of complete penetration. In the present study, a novel welding tool with large size shoulder and Archimedes spiral groove was designed in order to achieve non-thinning and lack of kissing bond welds. The microstructures and mechanical properties of the joints obtained by the non-thinning and penetrating friction stir welding (NTPFSW) and the penetrating friction stir welding (PFSW) had been analyzed in detail. Compared with PFSW, NTPFSW had a narrow shoulder-affected zone. The thickness of NTPFSW joint is 0.1 mm thicker than that of BM and 0.22 mm thicker than that of PFSW joint. The stir zones (SZ) obtained by the PFSW and NTPFSW were both composed of fine equiaxed grains. The grains in the upper part of NTPFSW joint were larger in size, and the number of precipitates (θ’ phase) was less. NTPFSW completely eliminated the weld thinning phenomenon, and the tensile properties were 365 Mpa, which is 2% higher than that of PFSW.

Fatigue behavior of X65 pipeline steel welded joints prepared by CMT/GMAW backing process

The fatigue behavior of X65 welded joints with cold metal transfer (CMT) backing process and gas metal arc welding (GMAW) backing process has been studied in comparison. The fatigue life of CMT-joints is improved by about 67% due to the combined effect of the low stress concentration in weld root and CMT-produced microstructure. Furthermore, the homogeneously distributed acicular ferrite and grain boundary ferrite arrested crack propagation and increased the fatigue propagation life of CMT-joints, but the necklace-type martensite/austenite constituents and lath bainite would reduce fatigue crack initiation and propagation life of GMAW-joints.

Impact behavior of beam-column joint with geometric imperfections at weld root

This study numerically investigates dynamic response of welded beam-column joint with different weld imperfections at weld root. The imperfections at weld root, such as incomplete root penetration, shrinkage groove and root concavity, are considered in this study. The size and shape of these imperfections are determined in accordance with ISO 5817 code. Numerical model of welded beam-column joint with imperfections is developed and calibrated by the test results of welded beam-column joint without imperfection under impact load as well as the numerical results of butt-welded joint with grinding profile under static load. The factors of the imperfections, such as the distribution and length of imperfections and the weld quality levels are considered to investigate their effects on the damage mode, dynamic response, and energy dissipation of welded joints subjected to impact loads, as well as the post impact behavior of the joint. Parametric study is carried out to investigate the effect of impact velocity and impact mass on welded joint with different root imperfections. The effect of different weld imperfections on the energy dissipation of joint is also investigated. The most vulnerable form of root imperfection is identified by comparing the joints with different imperfections in terms of damage mode, dynamic response and energy dissipation.

Ductile Cast Iron 350/4 using SMAW

Abstract: This article examines the weldability of ductile cast iron when the root weld is applied with a tungsten inert gas (TIG) welding process employing an Inconel 625 source rod, and when thefiller welds are applied with electrodes coated with 97.6% Ni. The welds were performed on ductile cast iron specimen test plates sized 300 mm × 90 mm × 10 mm with edges tapered at angles of 60◦ . The plates were subjected to two heat treatments. This article analyzes the influence on weldabilityof the various types of electrodes and the effect of preheat treatments. Finally, a microstructure analysis is made of the material next to the weld in the metal-weld interface and in the weld itself. The microstructure produced is correlated with the strength of the welds. We treat an alloy with 97.6% Ni, which prevents the formation of carbides. With a heat treatment at 900 ◦C and 97.6% Ni,there is a dissolution of all carbides, forming nodules in ferritic matrix graphite.

On the impact of undercuts and lack of penetration of cruciform joints from fatigue tests

The assumption of the detail categories in EN1993-1-9 is the presence of a certain weld quality level. In order to find out the sensitivity of the resulting fatigue strength for cases where a worse or better quality level compared to the selected level of EN ISO 5817 is achieved, in this paper, special fatigue experiments are presented. The experiments refer to the load-carrying cruciform joint with lack of penetration and root fusion as well as undercuts. The performed fatigue investigations on cruciform joints are based on and accompanied by further fatigue test results of literature, documented in a detailed fatigue database containing 147 test series. Detailed information on mechanical properties, fabrication and metallurgical characteristics of the cruciform specimens are given. Extensive 3D laser scans of the specimens and ultrasonic testing as well as magnetic particle testing have been performed in order to analyse the imperfection sizes and scatters. The fatigue test results of the cruciform joints show, that undercuts lead to lower fatigue strength at the weld toe, as was to be expected. When reducing the fatigue resistance, undercuts could however be tolerated at secondary structural elements or marginally fatigue stressed components. A lack of penetration and/or root fusion can be tolerated, if the fatigue verification at the weld root is performed. A verification at the weld root can only be neglected in case of lack of penetration and/or root fusion, if the weld toe is decisive due to undercuts or large enough weld angels, here shown for 45°.

Experimental and numerical investigation of laser beam-welded Al–Cu–Li joints using micro-mechanical characteristics

The local tensile mechanical properties of laser beam-welded joints of AA2198 alloy with Al–Si filler wire were experimentally investigated. For this purpose, micro-flat tensile specimens were extracted from the fusion zone and the heat-affected zone. The chemical composition of the filler wire affects the local mechanical properties in the fusion zone, showing an approximately 26% decrease in yield strength from the radiation exposure side to the weld root side. The effect of post–weld heat treatment on the tensile mechanical behavior was additionally investigated for different heat treatment artificial ageing conditions. The maximum yield strength increase was noticed for 48 h of artificial ageing for the weld root side of the fusion zone. Several approximations were proposed to correlate the hardness measurements with the local tensile mechanical properties of the welded joint that allow for a fast assessment of the global tensile mechanical behaviour of the welded joint. To evaluate the effect of (i) artificial ageing and (ii) geometrical imperfections of the weld on the mechanical behavior of the welded joint, finite element analyses were performed, using the local mechanical properties as input to the model. It is shown that the local mechanical properties of the fusion zone play a pivotal role on the strain localization and fracture of the welded joint.