Crack-free Welds Research Articles

Microstructure and mechanical properties of welds of AZ31B magnesium alloy produced by different gas tungsten arc welding variants

This work aimed to (i) understand conventional and pulse gas tungsten arc welding (GTAW) of AZ31B, and (ii) explore high frequency welding (100 Hz–1500 Hz). GTA welding with alternating current (AC) and direct current electrode positive (DCEP) polarities yielded crack-free partial penetration welds for 6 mm thick AZ31B alloy sheet. Welding under direct current electrode negative (DCEN) polarity with identical parameters as that for AC and DCEP resulted in full penetration welds that had microcracks. Defect-free full-penetration welds could be accomplished with pulse GTA welding using DCEN polarity at a pulse frequency of 1 Hz with a pulse duration ratio of 1:1. The resultant DCEN P 1:1 weld metal had a microstructure finer than the conventional DCEN weld. Welds produced with pulse duration ratios of 1:2 and 1:4 lacked penetration but had a much finer microstructures because of the lower heat input. The arc constriction by the high frequency pulsing in the ActivArc®-High frequency (AA-HF) mode welding was responsible for deeper penetration. Welds produced under DCEN pulsing and AA-HF conditions had hardness higher than conventional DCEN, DCEP and AC GTA welds, attributed to the finer microstructure. AA-HF GTA welding produced defect free deeper penetration welds with good microstructural features/mechanical properties and also gave an advantage of 50% enhanced productivity when welded at 1500 Hz.

High-strength and crack-free welding of 2024 aluminium alloy via Zr-core-Al-shell wire

The 2000 series aluminium alloys are qualified for widespread use in lightweight structures, but solidification cracking during fusion welding has been a long-standing issue. Here, we create a zirconium (Zr)-core-aluminium (Al)-shell wire (ZCASW) and employ the oscillating laser-arc hybrid welding technique to control solidification during welding, and ultimately achieve reliable and crack-free welding of 2024 aluminium alloy. We select Zr wires with an ideal lattice match to Al based on crystallographic information and wind them by the Al wires with similar chemical components to the parent material. Crack-free, equiaxed (where the length, width and height of the grains are roughly equal), fine-grained microstructures are acquired, thereby considerably increasing the tensile strength over that of conventional fusion welding joints, and even comparable to that of friction stir welding joints. This work has important engineering application value in welding of high-strength aluminum alloys.

Controlling solidification cracks during remote laser welding of AA6005 alloy using Al2O3 alumina nanoparticles dispersed coating

Solidification cracks are among the most common defects observed in the remote laser welding (RLW) of 6xxx aluminium alloys. This study assesses a novel route to mitigating solidification cracking during RLW of the AA6005 alloy using alumina nanoparticles (Al2O3) within the fusion zone. The AA6005 sheets were electrodeposited using a CuSO4 bath containing 40 nm sized Al2O3 at different concentrations and deposition time. Characterisation of the welds was conducted using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), energy dispersive X-ray spectroscopy (EDS) and mechanical lap shear testing. The results show that the incorporation of Al2O3 nanoparticles can be effective in producing centreline crack-free welds. Furthermore, the microstructural analysis also reveals grain refinement by 65% with increasing of Al2O3 concentration from 1 to 5 g/L, this corresponds to a 10% improvement in shear strength of the welds, compared to a standard un-coated AA6005 alloy.

High-power fiber-coupled diode laser welding of 10-mm thick Inconel 617 superalloy

In the present study, a high beam quality fiber-coupled diode laser was effectively utilized to weld 10-mm thick Inconel 617 superalloy in single pass. Influence of critical parameters of focusing distance and welding speed on weld characteristics was systematically investigated and optimized. At optimum process conditions with the power density of ≈106 W/cm2, crack-free full-penetration weld with minimal distortion, porosity, and no underfill/undercut/root-hump defects were obtained with 97%–99% joint efficiency. The weld joint quality produced was on par with multipass employing conventional lasers and advanced laser-hybrid welding techniques and sufficient enough to apply in various applications of thermal power plants, ship building, and heavy industries.

Stress state analysis of root notch for X80 girth welds with variable wall thickness and misalignment geometric features

Notches formed due to weld geometric features seriously affect the structural safety of the pipeline. In this paper, the stress states of girth welds under various forming conditions and internal pressures are thoroughly evaluated via numerical simulation analysis. Controlled studies are carried out between crack-free and cracked welds. The susceptibilities of stress state to variations in internal pressure under different forming conditions are elucidated. Also, newly modified single-edge notch tensile specimens (MSENT) are manufactured, and the strain characteristic and fracture behavior analysis are performed. The calculation results reveal that the stress states of either the root notch or the crack tip are significantly intensified by welds' geometric features, and the increased brittle fracture sensitivity can be qualitatively demonstrated by MSENT analysis. Simultaneously, the misalignment continuously enhances the sensitivity of stress states to internal pressure variation. However, opposite patterns are observed between crack-free and cracked welds characterizing variable-wall-thickness features. The research results can provide a strong reference for the structural design, service evaluation, and related research of pipelines.

Achieving superplastic TWIP steel welded joint via vacuum electron beam welding

A crack-free high-Mn TWIP steel welding joint with comparable strength and superplastic was obtained via vacuum electron beam welding (VEBW) where the microstructural evolution, mechanical behaviors and strengthening mechanism of welding joint were investigated. The results indicated that the welding joint with a typical nail-shaped welding cross-section was divided into three zones including the base metal (BM), heat-affected zone (HAZ) and fusion zone (FZ). Massive fine AlN-type precipitates with the hexagonal crystal structure were observed in welding join. The orientation relationship between the γ-austenite matrix and AlN precipitates was determined as [001]γ//[2110]AlN. In FZ, small-sized cellular grains or columnar grains with consistent growth directions were combined into large-sized columnar grains with obvious <001> preferential orientation. Compared with the base mantal, the yield strength, ultimate strength and elongation of the VEBWed specimen were slightly reduced by 6.03%, 4.7% and 13.23%, respectively. However, the impact toughness was obviously decreased from the 87.90 J–68.97 J after the VEBW process, which was mainly attributed to the brittle AlN precipitates, inhomogeneous microstructure and back stress induced by the plastic mismatch between gradient grains. The strength contributions of different regions to overall strength including of BM, HAZ and FZ in welding joint were calculated. Here, the dislocation strengthening and precipitation strengthening were the dominant strengthening mechanisms of FZ, which was attributed to the high level of density and massive fine AlN precipitates induced by micro-elements segregations during the VEBW process.

Solidification Crack Free Welding Strategy via Ultra High Precision Laser Scanning at a Single Grain of Directionally Solidified and Single Crystal Superalloys

In this study, possibility of solidification crack-free welding of directionally solidified CM247LC and single crystal CMSX-4 superalloys was metallurgically investigated using single-mode fiber laser scanning. The key metallurgical factors of solidification cracking have been identified as the formation of highly misoriented solidification grain boundaries (low level of epitaxial growth behavior) at the fusion zone that affects the repulsive force between two adjacent dendrites and dendrite coalescence undercooling at the terminal stage of weld solidification. In particular, under extremely low heat input and ultra-high-speed laser beam scan conditions (welding speed: 1,000 mm/s, heat input: 2 J/mm, and energy density: 65 J/mm&lt;sup&gt;2&lt;/sup&gt;), an effective fusion zone could be achieved within a single directionally solidified grain of CM247LC alloy owing to the characteristics of single-mode fiber laser. The fusion zone successfully showed a 99.9% of epitaxial growth achievement ratio (that is, low fraction of highly misoriented solidification grain boundaries at the fusion zone) without solidification cracking. Based on these results on CM247LC alloy, solidification crack-free welds could be successfully achieved using quadruple-pass and square-type weld geometries.

Tailoring the weld microstructure to prevent solidification cracking in remote laser welding of AA6005 aluminium alloys using adjustable ringmode beam

Solidification cracking is a prevalent defect in welding aluminium alloys which can be effectively mitigated by employing grain refinement strategies. This paper investigates the feasibility of tailoring the weld zone grain structure of 6xxx series aluminium alloys using an adjustable ring mode (ARM) laser beam to reduce the centreline cracking susceptibility. An in-process monitoring approach, comprising a high-speed camera and an ad-hoc Digital Image Correlation, was adopted to examine the strain development during laser welding on a self-restraint test rig. Results revealed that transverse strain in the weld centre exhibits a parabola-like relationship with the core/ring power ratio, with the minimum strain determined at the power ratio of ∼1.5, regardless of the total power employed. The microstructural analysis demonstrated that increasing the core/ring power ratio can lead to the refinement of columnar and equiaxed grains. Furthermore, the formation of secondary equiaxed grains is promoted when the power ratio increases, and this is proved to be related to the characteristic of the ARM laser by the like-for-like comparison. Overall, a fine-tuned power ratio of 1.5 is determined in the studied material and welding configuration for achieving a centreline crack free weld, benefiting from the improved microstructure and reduced thermal strain.

Liquation crack-free welding strategy for 247LC DS superalloy by control of pipeline diffusion via ultra-high-speed laser beam scanning

Under completely suppressed constitutional liquation of MC carbide, unexplored 247LC superalloy single-mode fibre laser welds experience unexpected liquation cracking. These cracks continuously propagate from a solidification crack, particularly at highly misoriented and epitaxially grown solidification grain boundaries, and dominated by pipeline diffusion of impurities. Two novel metallurgical preconditions were identified which, if satisfied, ensure complete suppression of liquation cracking of 247LC superalloy welds. These preconditions are (i) the constitutional liquation of MC carbide at the heat-affected zone and (ii) pipeline diffusion of impurities at the fusion zone. It is also highly recommended that the welding is carried out in a single directionally solidified grain under scanning conditions, avoiding the formation of highly misoriented solidification grain boundaries.

Crack-Free 30% Chromium-Nickel Alloy Welding Products for Nuclear Service

Prior research in the development of 30% chromium-nickel alloy nuclear welding wires has resulted in the resolution of primary water stress corrosion cracking (PWSCC) and ductility dip cracking (DDC) as well as improvement in solidification cracking (SC) resistance. The resolution of DDC exhibits some Laves phase, which has a negative effect on SC resistance. In this study, the use of an alternate carbide former, tantalum (Ta), in combination with niobium (Nb) was researched. Three heats of recently designed Filler Metal 52MSS-Ta (i.e., HV1648, HV1673A, and VX131WXW) were melted, fabricated, and systematically studied. DDC and SC were evaluated with thermodynamic modeling using the Scheil solidification simulation model, two types of varestraint tests, and strain-to-fracture (STF) testing. The varestraint and STF test results showed an improved SC resistance with reduced Laves phase and concurrent excellent DDC resistance. Optimized compositions with low Laves phase also exhibited high threshold strain values (TSVs) in the STF test. VX131WXW — which contains 2.81 wt-% Ta, 0.6 wt-% Nb, and 6 wt-% iron (Fe) - exhibited a TSV of 24%. Thermo-Calc computed the Laves phase to be 0.24% for VX131WXW compared to 0.06% in HV1673A. This difference in Laves phase resulted in the lower SC resistance of VX131WXW compared to HV1673A when measured with longitudinal varestraint testing. The maximum crack distance for HV1673A was about 0.6 mm while that of heat VX131WXW was about 1.0 mm. The typical diluted weld deposit made with VX131WXW was also resistant to PWSCC due to the chromium content exceeding 24%. These simultaneous results mark progress toward crack-free welds and provide direction for further optimization of Ta-containing filler metals.

Application of adjustable ring mode laser in remote laser welding of additive manufactured AlSi10Mg alloy

Additive manufacturing (AM) is an innovative manufacturing technology that offers the ability to build parts with both geometric and material complexities. However, limitations, including low build volume capability and production rate, yield its rapid application in high volume production. This paper presents the potential of remote laser welding (RLW) as a post-AM joining approach to scale up the AM components. The AM AlSi10Mg alloy was fabricated by direct metal laser sintering and subsequently joined by RLW without filler wire or shielding gas. A novel adjustable ring mode (ARM) laser beam was employed during the RLW process where the ring beam is designed to stabilize the keyhole by providing the preheating and postheating while the core beam guarantees a sufficient weld penetration. The impact of the ARM laser on weld porosity was evaluated in both fillet lap and bead-on-plate welding configurations, accompanied by the variation of core/ring beam power ratios. Crack-free welds with promising weld appearance were obtained among all welding trials, indicating that the ARM-RLW process can be employed for the robust connection of AM AlSi10Mg alloys. Optimizing the power ratio can substantially reduce the weld porosity area ratio from 24.3% to 13.5% in the fillet lap configuration and from 24.2% to 14.4% in the bead-on-plate configuration. Analysis of variance tests statistically confirmed the significant impact of the power ratio on the porosity area ratio. Future work has been suggested for the process maturation of RLW as a post-AM joining approach in industrial application.

A method for producing ceramic/stainless steel composites by laser welding with calculation optimisation

In this work, SS/ceramic composite material was produced by laser welding with active filler (AgCuTi). The presence of the AgCuTi filler was determined by ensuring that crack-free welds were obtained, and no brittle intermetallic compounds (IMCs) were observed. Unmelted SS was retained by laser offset to the SS side to achieve heat transfer to the filler. The AgCuTi filler was not melted but the eutectic reaction with low melting point still occurred by atomic diffusion. In accordance with the characteristics of laser brazing, a mathematical model for calculating the transient heat transfer of laser brazing was established via finite element method. The simulation results showed that the interface temperature could be directly controlled by controlling the welding power.

Relationship between interface heat input and microstructure of titanium/steel composite material was prepared by laser processing

Joining of titanium (Ti) alloy and stainless steel (SS) was of great interest for applications in the marine and aerospace fields. Despite the importance, no joining techniques have been developed that avoid the formation of brittle intermetallic compounds (IMCs) to produce high strength joints and ensure good corrosion resistance. Successful joining of Ti alloy and SS was performed by inserting Monel 400 interlayer in this work. Ni and Cu elements in Monel improved corrosion resistance of joints. When the laser focused on the SS side to form an independent molten pool, the diffusion bonding between the Ti alloy and the SS was realized through the heat conduction of the unmelted SS. The presence of the Monel 400 interlayer was performed ensured that crack free welds were obtained and no brittle IMCs were observed. Unmelted SS was retained by laser offset to the SS side to achieve heat transfer to the Monel 400 interlayer. The Monle 400 interlayerwasnot melted, but the eutectic reaction with low melting point still occurredby atomic diffusion. The simulation results showedthat the interface temperature couldbe directly controlled by controlling the welding power, and the simulation results weresimilar to the measured temperature. The diffusion of Ti, Cu and Ni at the Ti/Monel interface formed an obvious reaction layer composed of NiTi and CuTi. It was found that no Ti-Fe IMCs were found. The joint fractured at the Ti-SS interface with the maximum tensile strength of 132 MPa.

Preparation of high-strength TC4/304 dissimilar joint with TA2/304 composite interlayer by a dual-pass laser

Joining titanium (Ti) alloy to stainless steel (SS) is of great interest for applications in the marine and aerospace fields. Despite the importance, no joining techniques have been developed that avoid the formation of brittle intermetallic compounds (IMCs) to produce high strength joints. In this work, TA2/304 explosive welding composite plates was used as an interlayer to prevent the formation of these brittle phases when joining TC4 to 304 SS. The presence of this TA2/304 composite interlayer (CI) was performed ensured that crack free welds were obtained and no brittle IMCs were observed. The TA2/304 CI was produced by explosive welding, the length of explosive welding interface (EWI) in TA2/304 CI raged 1–10 mm. In addition, TA2 and 304 was remained in CI as enlarged ends to weld with TC4 and 304 base materials by laser welding, respectively. TA2/304 CI did not melt during the joining process, acting as a barrier between the TC4 and 304 base materials. Mechanical testing of the joints revealed that the strength of the TC4/304 joint increased with the increasing of EWI in TA2/304 CI. The tensile strength of joint could reach 545 MPa and the fracture at TA2 side of CI when the length of EWI was above 8 mm.

Cracking susceptibility control of 7075 aluminum alloy in pulsed laser welding with filler strip

Aiming to improve the serious cracks in laser welding of 7075 aluminum alloy, a laser welding method with filler strip is proposed, and crack-free welded joints are obtained. With the increase of strip height, the cracks in the weld are changed from Y-shape cracks to I-shape cracks, and no crack occurs when the strip height is equal to 1.9 mm. Microstructure shows that cracks with smooth fracture surfaces occur along grain boundaries, which are classified into solidification cracks. Eutectic phases are observed along grain boundaries, which prove the presence of liquid films. A non-steady state cracking model is developed to illustrate the cracking mechanism and calculate the cracking susceptibility threshold, considering the change of the liquid backfilling ability in both time and space dimensions. It is found that with the increase of strip height, the couple of cooling rate, grain size and backfilling distance enhances the ability of liquid backfilling to suppress cracks. This is because the couple effect greatly reduces the local strain rate but has less impact on the liquid backfilling rate, which leads the cracking susceptibility lower than the threshold. Therefore, the cracks in the weld are well suppressed, which is in good agreement with the experiment results. This study provides a feasible method for crack-free welding of 7075 aluminum alloy.

Effect of process parameters on pulsed laser welding of AA5083 alloy using response surface methodology and pulse shape variation

Aluminium alloys exhibit eco-friendly aspects related to global environmental issues, such as almost unlimited recyclability. Nevertheless, some intrinsic characteristics are challenges to explore all their benefits. In the welding process, the high thermal conductivity and low melt point require high control of heat input. Alternatively, the pulsed laser mode provides a sharp beam focus with precise control enabling regulating the energy delivered. In this sense, this present work analysed the effect of pulsed laser welding parameters on 3-mm-thick AA5083 aluminium alloy sheets. Trials targeted to develop sound welds with minimum defect and high penetration depth adopting statistical methods. The optimum parameter arrangement was achieved by varying peak power, spot diameter, and pulse duration. Finally, the best parameter combination was applied using different pulse shapes to mitigate crack formation and pores. As a result, the pulse shape with step-down at the end of each pulse generated crack-free spot welds.

Crack generation mechanism and control method of electron beam welded Nb/GH3128 joint

Electron beam welding of Nb/GH3128 dissimilar alloys was conducted in this research. Microstructure characterization and finite element simulation were utilized to uncover the cracking mechanism of the joint. The obtained results revealed that a brittle reaction layer, composed of Ni6Nb7/Laves phase beside the Nb substrate, was a critical cracking factor triggering the cracking along the reaction layer. The generation of cracks was affected by a certain thermal-mechanical effect. The longitudinal residual stress fluctuated dramatically in the Nb/FZ interface region, resulting in different welding deformation. High transverse residual stress was also detected in the vicinity of Nb/FZ interface, increasing the cracking propensity of Nb/GH3128 joint. This high residual stress combined with the brittle reaction layer illustrated the intrinsic cause of welding crack. Crack-free welding of Nb/GH3128 dissimilar alloys was realized via adopting the beam offset process. The thickness of the brittle reaction layer was dramatically decreased and the brittle Laves phase within FZ was replaced by the γ phase.

Evaluation and Control of Liquation Cracking Susceptibility for CM247LC Superalloy Weld Heat-Affected Zone via Visualization-Based Varestraint Test

The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored in existing research. The present study performed comprehensive metallurgical and manufactural investigations into the weldability of an Ni-based superalloy, CM247LC, from the viewpoint of the liquation cracking behavior and its susceptibility. Metallurgical solutions to suppress the liquation-cracking susceptibility were derived via the visualization-based Varestraint test, and the possibility of liquation crack-free welding was explored by employing pre-weld heat treatments and laser beam welding. The alloy that was subjected to aging treatment exhibited the lowest liquation-cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest liquation-cracking susceptibility (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC weld were elucidated via microstructural analyses and thermodynamic calculations. The suppressed liquation cracking susceptibility of the aged CM247LC can be attributed to the MC-type carbide fraction and homogenized matrix phase, as compared with those of as-cast CM247LC. The aged CM247LC specimen was subjected to gas tungsten arc welding to validate its minimal liquation-cracking susceptibility. The results confirmed the suppression of liquation cracking, due to the low susceptibility of the specimen. However, crackfree welds could not be obtained. Finally, metallurgically sound welds without liquation cracks were successfully obtained via laser beam welding. The outcomes of the present study will facilitate the generation of electric power from fossil fuels via a clean and efficient gas turbine-based power generation cycle.

Evidence of solidification crack propagation in pulsed laser welding of aluminum alloy.

It is difficult to collect the crack propagation signal under general continuous welding condition due to other signal interference of molten pool. In order to study the effect of residual stress on crack propagation, acoustic emission technology was successfully applied to monitor welding process according to the characteristics of pulsed laser welding. Crack free welding is achieved by reducing the pulse interval to limited the crack size of single pulse welding spot. The welding process was monitored synchronously by high speed photography and acoustic emission, the evidence of crack propagation after solidification of weld is successfully captured.

Effects of Al-Mg wire replacing Al-Cu wire on the properties of 2219 aluminum alloy TIG-welded joint

The effects of Al–Mg wire replacing Al–Cu wire on the microstructure, microhardness and tensile properties of 2219 aluminum alloy tungsten inert gas (TIG)-welded joints were studied. Comparing joints with Al–Cu wire, the capping welds of joints with Al–Mg wire can be strengthened by the introduction of Mg-containing strengthening phase and the hardness can be significantly improved. However, for joints with Al–Mg wire, both the solidification cracks caused by inappropriate control of alloying element content and the continuous brittle phases at grain boundaries around the weld zone (WZ) can result in the reduced tensile properties. The crack-free weld can be obtained by adjusting the alloying system of WZ. Furthermore, the geometry of WZ also affected the tensile properties of joints with Al–Mg wire.