Ductility Of Welds Research Articles

Unveiling Impact of Electron Beam Optics on Weld Attributes in EN30B Steel

The influence of electron optics in terms of beam oscillation (BO) has been studied on weld quality of several steels and reported to develop cosmetic weld with smooth surface and enhanced weld properties. However, effect of electron BO on the weld quality of medium carbon low alloy steel, especially EN30B, has not reported well and investigated in this study. By manipulating magnetic lens in-line with electron beam, the diameter of oscillating beam (OD) and its effect on weld quality has been studied. The OD was changed between 0.5 mm and 2 mm. Numerous tests, including X-ray diffraction, scanning electron microscopy (SEM), Electron Backscattered Diffraction (EBSD), hardness, wear, and tensile, were used to characterize the weld quality. Welds with BO significantly lowered the heat input rate than welds without oscillation. In presence of BO, retained austenite (RA) in the weld increased and it became least in welds made without BO. Tensile residual stress was found in the fusion zone (FZ) and it was higher with BO, especially with higher OD. In contrast to columnar grains predominantly found in welds made without BO, significant equiaxed grains were identified in welds made with BO. Nonmetallic inclusions (NMIs) in the weld became finer and less by use of BO. Introducing BO enhanced the hardness and reduced wear rate of weld, particularly at greater beam OD. Although weld strength was diminished in comparison to the base metal (BM), BO increased the weld's ductility and strength.

Flux-cored arc welding of 316L austenitic stainless steel: the effect of different shielding gas mixtures on microstructural features and weld performance

ABSTRACT Double-pass flux cored arc welding (FCAW) of 316 L austenitic stainless steel was performed using different shielding gas mixtures. Microstructural features and weld performance were evaluated. Based on the Creq/Nieq ratio of fusion zone and cooling rate, different morphologies of ferrite (skeletal, lathy, and dendritic) and austenite with various volume fractions were produced. An increase in Creq/Nieq ratio can increase the strength level because the presence of ferrite in the weld metal acts as a second-phase strengthening agent. On the other hand, ferrite is a suitable place for sigma phase precipitation. Highest joint efficiency resulted in the specimen welded via GTAW. The ductility of welds was ranked as CO2 +Ar shielded FCAW > GTAW > CO2 shielded FCAW > self-shielded FCAW. At room temperature, variation of delta-ferrite content (Creq/Nieq ratio) does not affect toughness. However, an increased amount of CO2 in shielding gas will result in decreased toughness values because of the enhanced formation of oxide inclusions. Increasing CO2 content of shielding gas from 0% to 18% and finally to 100% will result in decreasing toughness values from the highest value of 76J to 40 J and finally the lowest value of 38J.

Monitoring of friction stirred spot weld quality for dissimilar Al6061 to polycarbonate using tool assisted thrust-torque signatures

Friction stir processing procedures are often found to be an alternative choice for the joining of thermoplastic polymers without melting the base materials. In recent years, dissimilar metallic to thermoplastic assemblies are highly used in aerospace and automotive industries as mechanical fastener or fusion welding techniques are rather inadequate for extreme discrepancy in thermal properties. The present work addresses the feasibility of friction stirred spot welding for non-ferrous aluminium (Al 6061) to polycarbonate using taper cylindrical tool of hardened H13 steel along with machine integrated axial thrust and stirring torque signals. The real-time positional and axial velocity of the rotating tool has been acquired to distinguish between plunging and dwelling stage. The process stability that is, the deviation of thrust-torque in respective phases was processed. The interaction of plunge depth and dwell time on tool pin assisted keyhole with associated material expulsion for different tool revolving speed was established. The weld mechanical behaviour has been scanned using load versus elongation diagram with associated weld pair interface. Lastly, sensors’ based process monitoring extended to weld quality prediction have been made using various significant features of thrust-torque-velocity based approaches. The tool spinning speed followed by dwell time was found to be important on weld quality index. The joint efficiency was significantly upgraded (more than 60%) at intermediate dwell time (40 s) with low tool plunging (0.3 mm) or reversed parametric setting at high tool revolving speed (2500 rpm) due to substantial reduction in axial thrust with higher stirring torque. The local deviation in thrust-torque during plunging was equally responsible with global mean thrust-torque in dwelling to dictate the joint strength and the weld ductility. The material stirring torque was better indicator of weld shear strength while pin assisted slot size highly correlated with tool thrust though tool plunging speed was highly crucial.

Toward accurate prediction of partial-penetration laser weld performance informed by three-dimensional characterization – Part II: μCT based finite element simulations

The mechanical behavior of partial-penetration laser welds exhibits significant variability in engineering quantities such as strength and apparent ductility. Understanding the root cause of this variability is important when using such welds in engineering designs. In Part II of this work, we develop finite element simulations with geometry derived from micro-computed tomography (μCT) scans of partial-penetration 304L stainless steel laser welds that were analyzed in Part I. We use these models to study the effects of the welds’ small-scale geometry, including porosity and weld depth variability, on the structural performance metrics of weld ductility and strength under quasi-static tensile loading. We show that this small-scale geometry is the primary cause of the observed variability for these mechanical response quantities. Additionally, we explore the sensitivity of model results to the conversion of the μCT data to discretized model geometry using different segmentation algorithms, and to the effect of small-scale geometry simplifications for pore shape and weld root texture. The modeling approach outlined and results of this work may be applicable to other material systems with small-scale geometric features and defects, such as additively manufactured materials.

Thermal, Metallurgical and Mechanical Determinants of Laminar Nickel/Aluminum Dissimilar Alloys during Laser-material Interaction Part I: Nickel-based Superalloy

To develop in-depth understanding of metallurgical phenomena for completeness, mechanical heterogeneity is kinetically and thermodynamically explained by thorough microstructure characterization of engineering materials, e.g. polycrystalline nickel-based superalloy, for alleviation of weldability-related problems during laser-induced keyhole fabrication. Because of nonequilibrium solidification behavior inside weld pool, finer γ phase dendrite substructure is confined to partition-resistant keyhole bottom part, coarser dendrite is crystallography-independently circumscribed at partition-vulnerable neck transition region of full penetration weld, and thus microstructure is lack of homogeneity. Abundance of segregation-driven eutectic phase or intermetallic phase in the interdendrite area throughout weld is essentially attributable to nonequilibrium solidification conditions, and morphologically increases susceptibility to mechanical properties deterioration. As a result of γ phase instability, Niobium-aided Laves/γ eutectic reaction in the vicinity of dendrite interstices at terminal stage of solidification contributes to severe dendrite boundaries brittleness, impairs mechanical properties, which is consistent with metallography and fractography results, and is more deleterious to weldability, since solute redistribution and supersaturation adversely exacerbate segregation behavior in the residual interdendrite liquid, especially asymmetric weld pool shape. There is inverse parabolic relationship between secondary dendrite arm spacing and solute partition coefficient, when location varies from nail-shaped weld upper site to bottom site. Chemical, microstructural and mechanical heterogeneities are more geometrically favorable in the curvature-related neck transition region. In addition, the mechanism of thermal, metallurgical and mechanical inhomogeneities, which are attributed to asymmetric weld pool shape, is consequently proposed. Untoward metallurgical phenomena, such as microstructure heterogeneity and brittle Niobium-rich Laves/γ eutectic phases mitigate strength, ductility and toughness of weld. In order to macroscopically and microscopically satisfy superior mechanical properties requirement, chemistry and microstructure of high quality weld are metallurgically controlled. Fruitful metallurgical information and mechanical data further support the reasonable explanations. It is imperative to progressively advance welding metallurgy, weldability and fabricability of intricate shape for welding defects minimization, suppress segregation and further develop mechanical properties through viable design and control of laser processing, simultaneously.

Investigation on tool positioning in friction stir lap welding of AA6061 to polycarbonate sheets using force-torque signals

The lightweight high-performance thermoplastics are often replaced the metallic parts in aircraft and automobile industries due to high corrosion resistance along with upgraded strength to weight ratio. However, the joining of dissimilar non-ferrous aluminium to amorphous polymers is highly difficult caused by a drastic change in thermal properties which indicates the necessity of solid state welding processes. Therefore, the current work reports the practicality of friction stirred welding unlike AA6061 aluminium alloy and polycarbonate sheets for overlapped conditions by a cylindrical tool having a taper pin of H13 steel. The prime objective was to analyse the influence of tool tilt angle at different plunging depths and tool rotational speeds on the dissimilar weldability using a load-elongation curve. The aluminium to polycarbonate weld interface has also been studied to correlate with mechanical behaviour due to tensile load. The axial thrust force with tool stirring torque signals has been acquired for these parametric conditions for further process monitoring by segregating consecutive stages in the welding cycle. The weld bead profile along welding direction has been monitored using time domain thrust-force signals in the welding phase whose extracted significant features were compared. Finally, a sensitivity analysis was also processed to investigate the degree of parametric impact on thrust-torque as well as weld quality features. The highest joint strength efficiency was found to be 39.4% at higher tool rotational speed (1400 rpm) with reduced plunge depth (0.1 mm) without any tool tilting whereas weld ductility was significantly improved at reversed parametric settings with tool inverted position. The tool plunging force is a better indicator whereas both plunge depth as well as tilt angle were almost equally important for the prediction of lap weld quality.

Laser welding of fuselage panels from aluminum V-1579 and aluminum-lithium V-1481 alloys

The laser welding technology parameters of new V-1579 (Al-Mg-Sc) and V-1481 (Al-Cu-Li) alloys were investigated. Weldability of such alloys was studied. Laser welding conditions and filler materials were selected to provide high weld ductility and impact strength. It was found that weld strength in regard to the parent material was ca. 0.8 for V-1481 alloy and ca. 0.9 for V-1579 alloy. Structurally similar samples of welded fuselage panel were manufactured based on conducted research. Weld inspection was carried out.

Inhomogeneity of microstructure in friction stir welded TC17 alloy joint and its effects on mechanical behavior

Friction stir butt welding was carried out to assemble 6 mm-thick TC17 titanium alloy plates with a W–Re weld tool. Inhomogeneous microstructures were obtained in stir zone (SZ) and heat affected zone (HAZ) of the weldment. A series of tests were conducted to reveal the influence of inhomogeneity of microstructure on mechanical behavior. Comparative refined β phase without precipitating secondary metastable microstructure generated a soft SZ and favored on the ductility of weldment. Strain localization during tension was promoted by the interface of HAZ and SZ that formed by thermal-mechanical coupling effects. Coincidentally, fatal fatigue cracks were verified to nucleate around the same region, and presented in a stress dependent mode.

Relationship between welding conditions, abnormal grain growth and mechanical performance in friction-stir welded 6061-T6 aluminum alloy

In this work, a relationship between welding conditions, annealing behavior, and mechanical performance of friction-stir welded (FSWed) 6061-T6 aluminum alloy was examined. In the entire studied FSW range, the welded material was found to be unstable against abnormal grain growth during post-weld solutionizing treatment. However, a lowering of the FSW heat input tended to inhibit this undesirable phenomenon. In addition to the stir zone, the abnormal microstructural coarsening was also observed in the heat-affected zone, whereas the thermo-mechanically affected zone experienced static recrystallization. In all cases, the abnormal grain growth was found to result in ~25-35°<110> rotation of the original crystallographic texture. Hence, this process was suggested to be governed by the orientation-growth mechanism. Due to the relatively high strain-hardening ability intrinsic to the coarse-grained materials, the abnormal grains suppressed tensile strain in the stir zone during transverse tensile tests thus degrading weld ductility.

Finite Element Simulation of Friction Stir Welding of Inconel 718 to Nimonic 80A

This paper presents results of simulation of friction stir dissimilar welding of 5 mm thick, Nickel-based super-alloys, Inconel 718, and Nimonic 80A using Abaqus software. Four different trials were done to understand the influence of tool rotation speed on temperature distribution in weld zone while travel speed remains constant. The temperature in the weld zone was found to increase with the increase in tool rotation speed and travel speed. The temperature on the advancing side of the tool was higher than that of the retreating side. The tensile strength of weldment was found, by simulation, to be 25% more than that of base metal, Inconel 718. This may be due to grain refinement and dynamic recrystallization during FSW. The simulated bend test revealed an adequate level of ductility of weldments.

Simultaneously enhanced strength and ductility of TIG welds in Inconel 718 super-alloy via ultrasonic pulse current

Abstract This paper presents a novel self-induced ultrasonic arc welding (U-TIG) with high-efficient, which improves tensile strength and ductility of TIG welds in Inconel 718 super-alloy simultaneously. The effects of ultrasonic frequencies on arc shape, weld forming and weld penetration characteristics were systematically studied by high-speed camera and optical microscope. The characteristics of grains and precipitated phases in the weld metal were quantitatively analyzed by backscattered electron microscope, electron backscatter diffraction. The evolution models of grains refinement and reduction-dispersion with Laves phase were constructed based on molecular dynamics model and numerical simulation. The evolution diagrams of microstructures in tensile stretching revealed the mechanism of enhancing strength and ductility. Ultrasonic pulse current not only reduces and refines Laves phase, respectively by 81.8% and 52.2%, but refines grains (almost 33%) with higher orientation. The excellent distribution of the finest grains and Laves phase with dispersion is corresponding to ultrasonic frequency nearly 80 kHz, which benefits ductility enhancement of welds from 311% to 410%. Grains refinement leads to the increasing of grains boundary length and zigzag, meanwhile, the morphology of Laves phase changes from long-strip shape to refined pebble one, which is responsible for tensile strength improvement, closing to base metal of 92%.

Mechanical behaviour of longitudinal lap-welded joints of high strength steel: Experimental and numerical analysis

Mechanical behaviour of twenty-eight longitudinal lap-welded joints made of high strength steels (HSS) under tension load was investigated by experimental and numerical study. Weaknesses due to traditional deformation measurements for fillet welded joints can be perfectly solved by digital image correlation techniques (DIC). Distribution and development of strain field throughout the specimens along the whole loading process were recorded by DIC measurement. The effect of parameters (e.g. weld size, weld length and mismatch ratio) on mechanical properties (e.g. ultimate strength, failure modes, weld ductility and fracture angle) of longitudinal fillet welds and transverse fillet welds, which was introduced in detail in previous work by the authors, were compared. Generally, because of the difference on the combination of shear force and tension force, the fracture angle of longitudinal welded specimens (around 50∘) were much more divergent from transverse welded specimens (around 20∘) even though both of them failed at welded zone (welded zone only refers to weld metal in this paper), resulting that the mean strength of longitudinal welded specimens were only 0.58 time of transverse welded specimens. Conversely, the mean deformation capacity of longitudinal welded specimens was almost 4.0 times of transverse welded specimens. All longitudinal welded specimens have “platform” or “decline” feature while all transverse welded specimens have “rise” or “platform” feature load-deformation curves. Generally, the specimens with “rise” feature load-deformation curve are more likely to suffer sudden fracture, while specimens with “decline” or “platform” feature have satisfactory ductility performance. Based on the strain distribution and development features provided by DIC measurement, a new strength criterion, in which the ultimate strain obtained in relevant coupon tests was an indicator, was proposed and through the comparison between the numerical results and test results, its validation was verified by FE simulation. Moreover, it was confirmed that the predicted loads of EC3 and AISC Specification were close and slightly conservative for all specimens.

Reduction of Copper to Steel Weld Ductility for Parts in Metallurgical Equipment

Despite being challenging, the welding of the dissimilar metals copper and steel is an essential process that is required for improving quality of equipment manufacturing in the fields of metallurgy, machine construction, and chemical industry. Restricted solubility of iron in copper leads to the formation of a supersaturated solid solution of iron and other chemical elements in the weld pool. Investigations have found the possibility of enhancing the process of welding copper with steel. In the case of using a flux-cored welding wire and an improved welding technique, the number of dendritic inclusions is reduced, and the weld ductility is improved. Studying the microstructure of a copper to steel weld confirmed the ability to enhance the outcome of the welding process of the dissimilar metals. The implementation of recommended preparation techniques of parts before welding, and optimization of the welding technique will increase the strength of the welds and, increases the operational reliability of metallurgical equipment.

Structure, Mechanical Properties, and Stressed State of Weld Joints of the Tial(Nb, Cr, Zr) Intermetallic System

Structural features, mechanical properties, and stressed state of welded joints of the TiAl system intermetallic compound (Nb, Cr, Zr) after electron-beam welding and subsequent electron-beam local heat treatment are determined. It was shown that stresses generated immediately after welding contributed to the appearance of cold cracks in the weld. Residual stresses in welded joints were determined using the technology and compact equipment based on the method of electronic speckle interferometry created at the Paton Electric Welding Institute of the National Academy of Sciences of Ukraine. It was found that after electron-beam welding, due to rapid cooling of the weld metal, phase transformations occurred, which deteriorated its mechanical characteristics. A method was developed for the local heat treatment of a welded joint by an electron beam immediately after the end of the welding process. Parameters of the local heat treatment process can significantly reduce the cooling rate, maintaining the temperature of the welded joint for 5 min at the level of 900°C. This can significantly reduce the level of residual stresses in the weld and contribute to the formation of a favorable three-component structure: γ-phase, (γ + α2)-phase and β-phase, which increases the weld ductility. A study of mechanical properties during the compression test of the weld showed that after local heat treatment, the yield strength of the weld was improved, while the tensile strength and degree of deformation under compression increased significantly. The proposed local heat treatment is effective and economically justified, which allows to reduce residual welding stresses by 30%, thus preventing the formation of cracks in welds, as well an improving structural and mechanical characteristics.

Mechanical behavior of transverse fillet welded joints of high strength steel using digital image correlation techniques

Mechanical behavior of 24 lap-welded fillet joints and 20 cruciform fillet joints made of high strength steels (HSS) under tension load is investigated by experimental study. Traditional deformation measurements for transverse fillet weld joints have many weaknesses, which can be solved by digital image correlation techniques (DIC). The strain distribution development over the specimen along the whole loading process were analyzed by using DIC measurement. Test specimens were prepared using four classifications of filler metal and two weld sizes were included in each type of transverse fillet welded specimens. The effect of parameters (e.g. weld size and mismatch ratio) on mechanical properties (e.g. failure modes, fracture angle, ultimate strength and weld ductility) of those two types transverse fillet weld were compared. Due to the softening and other metallurgical effects, cruciform type specimens are more prone to fail around fusion lines than weld metal. Generally, because of the existence of friction force, mean strength of lap-welded specimens are higher than that of cruciform type specimens. The mean ductility of lap-welded specimen is similar to that of cruciform type specimen. Two load-deformation curve features, “rise” feature and “platform” feature, were identified. Most lap-welded specimens have “rise” feature load-deformation curve while most cruciform type specimens have “platform” feature load-deformation curves. Generally, the specimens with “rise” feature load-deformation curve are more likely to suffer sudden fracture. The instantaneous and simultaneous fracture was observed at fracture surface of lap-welded specimen. Joint misalignment has significant impact on the deformation patterns of cruciform type specimens. Based on load-deformation curves of cruciform type specimens, three deformation patterns were proposed. Due to the existence of three deformation patterns, the variability of strength and ductility are much larger than lap-welded joints. Moreover, it is confirmed that EC3 and AISC Specification are conservative for all mismatched filler metals.

Effect of pre-strain path on suppression of abnormal grain growth in friction-stir welded 6061 aluminum alloy

In this work, an efficiency of various pre-strain paths for suppression of abnormal grain growth in friction-stir welded 6061-T6 aluminum alloy was studied. To this end, the produced welds were cold rolled at either 0°, 45° or 90° to welding direction prior to the standard T6 tempering. A particular emphasis was given to elucidation of a relationship between microstructures and ductility of the post-processed welds.In all cases, the pre-strain rolling was found to be effective for inhibition of the abnormal grain growth. At relatively low rolling strains, this effect was sensitive to the strain path. Specifically, the pre-rolling along the welding direction resulted in the finest grain structure in the heat-treated joints. This result was attributed to relatively high Taylor factor as well as to low orientation stability of the stir zone texture for this particular rolling path, which enhanced recrystallization behavior during subsequent annealing.Tensile behavior of the heat-treated welds was shown to be dictated by strain localization in coarse-grained microstructural regions. It was established that appropriate microstructural control by the pre-strain rolling can provide a reasonably uniform microstructure distribution across the weld zone and thus essentially enhance weld ductility.

Study of MA Effect on Yield Strength and Ductility of X80 Linepipe Steels Weld

Multipass GMAW (Gas Metal Arc Welding) welding was used to join X80 linepipe materials using two weld metals of slightly different compositions. Welding wires with diameters of 0.984 and 0.909 mm were used while applying the same heat input in each pass. The slight difference in the wire diameters resulted in different HAZ microstructures. The microstructures in the doubly reheated HAZ of both welds were found to contain bainite-ferrite. However, etching also revealed a difference in martensite-austenite (MA) fraction in these reheated zones. The MA exhibited twice the hardness of ferrite when measured by nanoindentation. Tensile testing from the reheated zone of both welds revealed a difference in yield strength, tensile strength and elongation of the transverse weld specimens. In the reheated zone of weld A, (produced with a 0.984 mm wire) a higher fraction of MA was observed, which resulted in higher strength but lower elongation compared to weld B. The ductility of weld A was found severely impaired (to nearly half of weld B) due to formation of closely spaced voids around the MA, along with debonding of MA from the matrix, which occurs just above the yield stress.

Through-thickness microstructure and mechanical properties of electron beam welded 20 mm thick AISI 316L austenitic stainless steel

Through thickness microstructure and mechanical properties of defect-free electron beam welded 20mm thick AISI 316L austenitic stainless steel are investigated as a function of beam power. The weld microstructure characterised by columnar and equiaxed dendritic ferrite in an austenite matrix. The dendritic structure was finer at the bottom of the weld zone. A microstructural boundary called “Parting” was seen along the weld centreline. Tensile tests, using digital image correlation technique, demonstrated that the highest strain concentrated in the FZ. The bottom section of the weld metal showed yield strength of about 14–52MPa higher than the top section. The ultimate tensile strength in the bottom of the weld was also about 4% higher than the top. The final fracture was detected in the parting region. It was observed from the EBSD scan that the grains in the weld zone contained a weak orientation and showed high Schmid factor intensity with interception between some strong grains and soft grains at the weld centerline boundary. This explains the high weld ductility and the failure to happen in the parting region.

Research on weld cracking of TP321H stainless steel pipeline under elevated temperature

The failure of pipeline which adopted material type TP321H austenitic stainless steel and occurred cracking after servicing at elevated temperature for less than two years had been investigated. The cracks were appeared repeatedly although they had been repaired for several times. The pipeline stress analysis was conducted to determine stress levels of cracking positions by finite element analysis software ABAQUS. The mechanical properties of base metals and welds including tensile and charpy impact tests were carried out. The test results showed that ductility of welds cut from the serviced pipeline was very poor. The microstructure investigations suggested that it was intergranular crack located in the HAZ near fusion line. It could be determined that it was reheat cracking based on some other works such as metallographic inspection, SEM, X-ray diffraction, etc. Welds analysis results showed that the welding of pipeline had not been in accord with right qualification of welding procedure leading to poor welding quality. The cracking reasons and preventive measures were discussed. Several suggestions were proposed to help extend service lifetime of the stainless steel pipeline under elevated temperature condition.

Microstructures of magnetically assisted dual-phase steel resistance spot welds

Traditional spot welds and magnetically assisted spot welds were made in 2.25 mm thick galvanised dual-phase steel, and the weld microstructures were compared. The magnetically assisted weld nugget had a ‘dog bone’ shape, and it had an increased diameter, which indicates a larger load-bearing area that will improve mechanical performance. The fusion zone of the magnetically assisted welds had a finer and less-directional grain structure than the conventional welds, which would improve weld strength, plastic strains, and ductility. Both types of welds contained an unusual soft zone very close to the fusion zone that is thought to be an integral part of the fusion zone. The soft zone of the magnetically assisted welds was wider than the conventional welds.