Weld Fusion Zone Research Articles

Characteristics of Solidification Cracking for Ni-P Coated Cu-Al5052 Single-Mode Fiber Laser Welds

This study fundamentally examines the characteristics of solidification cracking in Cu-Al5052 single-mode fiber laser welding by utilizing uncoated and Ni-P coated Cu plates for the high durability busbar welding of pouch-type lithium-ion battery packs. Weld solidification cracking was confirmed regardless of Ni-P coating for both weld materials. However, there were differences in the regions of crack occurrence. Unlike the uncoated Cu-Al5052 welds where cracks occurred randomly within the fusion zone, for the Ni-P coated Cu-Al5052 welds, solidification cracks were concentrated near the faying surface of the upper and lower materials, particularly near the fusion line. The effect of the Ni and P components on the solidification cracking susceptibility, i.e., the welding solidification temperature range, was considered. Diffusion-controlled Scheil’s solidification calculations revealed that the mushy zone range enlarged with the use of a Ni coating layer. It was confirmed that an extremely wide mushy zone range of 1060 K was calculated at the solidification cracking region. Furthermore, P was also identified as an element that expands the solidification temperature range based on the Cu-P binary system. Consequently, during the laser welding of Cu-Al5052 dissimilar materials, the behavior of solidification cracking in the welds is influenced by the composition of the coating material and the incorporated composition within the weld fusion zone. Hence, it is necessary to select the optimal coating material and layer thickness considering weld solidification cracking susceptibility.

Microstructure-Linked Mechanical Properties of P91-Incoloy 800HT Dissimilar Metal Welds

Dissimilar metal welds (DMWs) between P91 steel and Incoloy 800HT® using shielded metal arc welding (SMAW) with ENiCrCoMo-1 filler electrode were characterized in detail. The impact of multiple weld passes on microstructure in the heat-affected zone (HAZ) and weld fusion zone (WFZ) was thoroughly investigated. Postweld heat treatment (PWHT) impacts on mechanical characteristics and microstructure were also observed. The DMW was observed with heavy C, Cr, and Mo microsegregation in the WFZ, which deteriorated mechanical strength. The microstructural changes directly affected the mechanical behavior of the DMW. The ultimate tensile strength of the weld fusion zone was low at 576 MPa compared to the base metals’ strength. The tensile specimens failed in the IN 800HT base metal and WFZ. The WFZ had significant microsegregation near the solidified grain boundaries (SGBs). The metallurgical heterogeneity in the WFZ caused the formation of localized stress zones, which directly affected the mechanical behavior of the WFZ. The carbide particles influenced the microhardness in the WFZ and P91 HAZ regions. The PWHT successfully homogenized the microhardness in the WFZ and P91 HAZ regions. The impact toughness decreased to 64 J from 82 J after PWHT due to precipitation at SGBs and its coarsening. The SMAW process involves multiple thermal cycles during welding. The deposition of filler metal in multiple welding passes created uncontrolled variations in the solidification behavior of the WFZ. Microsegregation had a detrimental impact on the microstructure and mechanical properties of the P91 and IN 800HT dissimilar metal welds.

Pitting behavior of austenitic stainless-steel welded joints with dense inclusions and methods to enhance pitting resistance

PurposeThis study aims to assess the pitting resistance of austenitic stainless steel welded joints fusion zone (FZ) with high density of inclusions before and after surface treatment, including potentiostatic pulse technique (PPT) and pickling.Design/methodology/approachThe potentiodynamic polarization tests and critical pitting temperature tests were carried out for estimating pitting resistance. The PPT and pickling were performed as surface treatment. Scanning electron microscope (SEM) and energy dispersive spectrometer were used for characterize the microstructure and elemental distribution. Electron back-scattered diffraction (EBSD) was used to assess the portion of phases and morphology of grains.FindingsThe weld metal exhibits a higher degree of alloying compared to the base metal, and it contains d-phase and sulfur-containing inclusions. Sulfur-containing inclusions serve as initiation sites for pitting, and they diminish the pitting resistance of weld metal. Both PPT and pickling can remove sulfur-containing inclusions, but PPT causes localized dissolution of the weld metal matrix around the inclusions, while pickling does not. Because of the high density of inclusions, certain pits initiated by PPT are significantly deeper, which makes the formation of stable pitting easier. Because of the high density of inclusions, certain pits initiated by the PPT are deeper. This characteristic facilitates the progression of these initial defects into fully developed, stable pits.Originality/valueAnalysis of pitting initiation in shielded metal arc welding FZ with PPT and ex situ SEM tracking observation. Explanation of why the PPT surface treatment is not able to enhance the pitting resistance of stainless steel with a high inclusion density.

Effect of Swing Amplitude on Microstructure and Properties of TC4 Titanium Alloy in Laser Welding

The welding of TC4 titanium alloy sheets with a thickness of 1 mm was successfully accomplished by a swinging laser. The microstructure and mechanical properties of the welding seam under different swing amplitudes were studied. In this paper, the microstructure, phase composition, mechanical properties, and fracture morphology of the weld with swing frequency of 50 Hz and different swing amplitudes (0.2 mm, 1 mm, 2 mm, and 3 mm) were tested and analyzed. The results show that basket-weave microstructures are present in the fusion zone of welds under different oscillation amplitudes, but the morphology of martensite within the basket-weave differs. The weld microstructure is mainly composed of acicular α′ martensite, initial α phase, secondary α phase, and residual β phase. The hardness of the weld is higher than that of the base metal, and the overall hardness decreases from the weld center to the base metal. When the oscillation amplitude A = 1 mm, the weld microstructure has the smallest average grain size, the highest microhardness (388.86 HV), the largest tensile strength (1115.4 MPa), and quasi-cleavage fracture occurs. At an oscillation amplitude of A = 2 mm, the tensile specimen achieves the maximum elongation of 14%, with ductile fracture as the dominant mechanism.

Influence of CMT welding parameters on microstructural and properties investigation of dissimilar weld joint for aluminum bronze and carbon steel

Cold Metal Transfer (CMT) welding technology has improved the appearance of weld beads and revolutionized the welding of thicker materials and dissimilar metals with precise metal deposition and reduced heat input. Cold Metal Transfer welding is predominantly used in a variety of industry applications, including aerospace, shipbuilding, automotive and marine industries. In this study, aluminum bronzes and carbon steel were joined by using CMT welding, a variant of the Gas Metal Arc Welding process, with ERCuAl-A2 as the filler metal. Optical microscopy was used to examine the microstructure of the CMT weld joint between aluminum bronze and carbon steel. Results have indicated that dissimilar metals such as aluminum bronze, and carbon steel could be successfully joined by CMT under various processing parameters. The tensile and bending strengths of the dissimilar joint were 490 MPa and 822 MPa, respectively. A variety of intermetallic compounds and solid solutions were generated in the weld zone and fusion zone. The micro-hardness in the carbon steel side of the fusion zones increased sharply, which was 164 HV in the welded condition. The dissimilar joint was stronger than the carbon steel, as the ductile fracture occurred on the carbon steel during the transverse tensile test of the welded specimen. The microstructure interpretation of welded dissimilar joints was analyzed by scanning electron microscopy and transmission electron microscopy.

Investigation on weld forming, microstructure and mechanical characteristics of dissimilar steel GTAW under novel composite magnetic field

During dissimilar steel welding, differences in magnetic permeability and thermophysical parameters between materials result in significant disparity in heat distribution on both sides of weld and poor weld formability. To solve the welding quality problems caused by significant disparity in heat distribution, a novel composite magnetic field consisting of longitudinal magnetic field (LMF) and cusp magnetic field (CMF) is applied to dissimilar steel welding. In this study, the influence of novel magnetic field on the mechanical characteristics, microstructure, and weld formation of dissimilar steel gas tungsten arc welding (GTAW) welds was investigated. It was found that arc deflection is influenced by both the material properties and LMF. When longitudinal exciting current (IL) was 0.5A, the weld melting zone tended to be symmetrical. On the premise that IL remained unchanged at 0.5A, as cusp exciting current (IC) increased from 0A to 5A: the weld fusion zone always remained symmetrical; The arc compression degree, weld penetration, melting area of weld and weld penetration/width ratio significantly increased, while the weld width progressively reduced; The heat-affected zone's (HAZ) width of the base metals and the mean grain size in the HAZ are significantly reduced; The mean microhardness of the weld metal (WM) and tensile properties are also significantly improved. The study results enrich the research foundation on heat input regulation in dissimilar steel GTAW, providing experimental evidence and theoretical support for resolving welding quality issues caused by welding conditions with different material properties.

Comparison of V groove design on mechanical performance and fracture behaviour in TIG welding of 5754-H111 aluminium alloy

The aim of this study is to enhance the performance and effectiveness of welding 5mm thick plates with different groove designs and investigate their post-effects on material properties. Specifically, the study compares single V and double V butt joints of 5mm thick AA5754-H111 strain-hardened aluminium alloy using Tungsten Inert Gas (TIG) welding with ER5356 (Al-Mg) filler rod. Two types of weld grooves, single V groove and double V groove were utilized to examine the microstructure and character of the weld and to study the effect of heat distribution. The average Vickers hardness values were observed to be 78.2HV in the double V groove of weld zone compared to the 76.6HV in single V groove weld zone. The tensile strength of single V and double V groove were 210 and 207 MPA respectively. The results showed that the failure location of all weld samples was in the fusion zone (FZ) and exhibited brittle fracture. The microstructure of the weld fusion zone revealed the presence of porosity in various sizes, which was also observed in the fractography images of tensile failed samples. Moreover, the microstructure studies exhibited clear variations in the fusion, intersection, and heat-affected zones. The formation of particles was found to differ in the weld fusion zone at various locations due to changes in the V groove welding method.

Microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 overlays on 304L stainless steel weld used in nuclear fuel reprocessing plant

Weld overlays are considered the most cost-effective surface modification technique to mitigate corrosion in the nuclear industry. Considering this, an attempt to overlay the 304L weld metal at some critical weld locations of the nuclear fuel reprocessing plant with Inconel alloy was initiated. The work focuses on the microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 weld overlays on 304L stainless steel (SS) weld metal in a nitric acid and chloride medium. Scanning electron micrographs of the 304L SS weld metal revealed a duplex microstructure containing δ-ferrite as a minor phase in the austenite matrix. The microstructure of the Inconel 82 and Inconel 625 weld overlays was found to be fully austenitic, with Nb-rich laves phases at the inter-dendritic region. The hardness of the Inconel weld overlays was found to be higher than that of the 304L SS weld metal. X-ray diffraction measurements showed austenite and δ-ferrite in the 304L SS weld metal and a single austenite phase in the Inconel weld overlays. Potentiodynamic anodic polarization experiments carried out in 4M and 8M nitric acid indicated superior passive film stability in the Inconel weld overlays when compared to the 304L SS weld metal. Comparatively, Inconel 625 weld overlay was found to be much superior to Inconel 82 weld overlay in nitric acid medium. The 304L SS weld metal exhibited inferior pitting resistance in chloride medium by virtue of the high δ-ferrite present in the weld fusion zone. The pitting resistance of the Inconel 82 weld overlay marginally improved, while a substantial enhancement occurred in the Inconel 625 weld overlay.

Crack-Tip Opening Displacement of Girth Welds in a Lean X70 Pipeline Steel.

Crack-tip opening displacement (CTOD) tests were conducted on girth welds of two API 5L X70 pipeline steels (pipe A and pipe B) to investigate the influence of base metal composition on the fracture toughness of the joint. CTOD measurements across the weld showed that the weld fusion zone had the lowest CTOD values for both pipes, with pipe B having a higher CTOD value than pipe A. Detailed microstructure characterization of the multi-pass weld showed that the fusion zone in both pipes consisted of three distinct zones: the columnar zone, the coarse equiaxed zone, and the fine equiaxed zone. Both the columnar zone and coarse-grained equiaxed zone had acicular ferrite and grain boundary ferrite microstructures, whereas the fine-grained equiaxed zone had a finer ferrite microstructure compared to the other two zones. The main difference between the two pipes was the variation in ferrite grain sizes and the volume fractions of grain boundary ferrite and acicular ferrite. In comparison to pipe B, pipe A, with a higher concentration of Mo, Ni, and Cu in both the base metal and the weld fusion zones, consisted of a higher volume fraction of grain boundary ferrite and a lower volume fraction of acicular ferrite in the columnar and coarse-grained equiaxed zones. The lower concentration of Mo, Ni, and Cu in pipe B likely resulted in the formation of a predominantly acicular ferrite microstructure in the fusion zone, thereby improving the toughness of the weld joint in comparison to pipe A.

Analyzing microstructural, residual stress, and mechanical characteristics in dissimilar welds of Inconel 59 and AISI 904L through double-pulsed gas metal arc welding

In this research, dissimilar welding of the nickel-based superalloy Inconel 59 and austenitic stainless steel AISI 904L steel was successfully done with double-pulsed gas metal arc welding (DP-GMAW) using the filler ERNiCrMo-13. Macroscopic analysis showed a defect-free weld with an aspect ratio of 2.4, attributed to the efficient heat delivery (0.642 kJ/mm) in DP-GMAW. The microstructure of the weld joint was examined through both optical microscopy and scanning electron microscopy. Examination of the microstructure of the weld interface exposed grain coarsening near the heat-affected zone on the AISI 904L side. In contrast, the weld fusion zone contained fine equiaxed dendrites. Microsegregation and alloying element migration across the entire weld area were assessed through energy dispersive X-ray spectroscopy point analysis and line mapping, revealing controlled microsegregation across the region. X-ray diffraction (XRD) analysis was used to ascertain the average grain size (38.41 nm) and secondary phase composition. The XRD-cosα method was used to assess the residual stress of the weld joint, revealing tensile residual stress in the weld zone and compressive residual stress in other regions. Tensile, Charpy impact, and Vickers hardness tests were conducted in the study to evaluate the strength, toughness, and hardness of a dissimilar weld. The impact of two cryogenic treatments, namely shallow and deep, on tensile and toughness characteristics was investigated in this study. The cryogenic treatments significantly enhanced the mechanical properties of weld samples compared to their as-welded condition, leading to an 11.5% increase in tensile strength and a 10% improvement in toughness.

Microstructural evolution of laser-welded Ti-based bulk metallic glass

A fiber laser beam is used to butt-weld of identical Ti43·3Zr21·7Ni7·5Be27.5 (at.%) bulk metallic glass (BMG) specimens. Various tests, such as SEM, TEM, XRD, DSC, and Vickers hardness, are conducted on the welded specimens to analyze the crystalline phases generated at the HAZ of each specimen during welding. The heat-affected zone (HAZ) can be obviously identified in the welded specimens owing to the clustering of the crystalline phases. The HAZ is composed of a heterogeneous group of crystalline phases, such as Ti–Zr–Ni polygonal phase, Ti and Ti–Zr dendritic phase. These crystalline phases decrease the glass transition temperature (Tg) of the welding fusion zone (WFZ) and increase the hardness at the HAZ.

Effect of multipass friction stir processing on microstructure and mechanical properties of MIG welded joints of AA6082 and AA7075

Metal inert gas (MIG) welding is widely employed for aluminum alloy welding, but high residual stress, coarse grain’s structure, porosity, and micro-cracks occur during MIG welding. To avoid these defects, friction stir processing (FSP) was employed to the MIG welded joints of AA7075 and AA6082 with ER4043 filler. The grain structure of the weld fusion zone (WFZ) was categorized by eutectic precipitated Mg2Si, and primary α-Al. The elongated, coarse grain structure of the MIG weldment decreased after a one pass FSP. The Mg2Si precipitates were disseminated due to the enhancement of dispersion and dissimilar metal fraternization. The fraction area of the refined grains increases when the FSP pass increases. Furthermore, the effective enhancement of the joint efficiency was observed in the MIG + 4Pass FSP due to the grain refinement. The ultimate tensile strength (UTS) of 169.73 MPa of the MIG welded joint was observed with a grain size of 17.9 ± 0.3, while the highest UTS of 223.84 MPa was perceived at 4 passes of MIG + FSP joints with an elongation of 12.45%. The grain structure was also refined as the FSP pass increased, and the minimum grain size (2.6 ± 0.3 µm) was perceived at MIG + 4Pass FSP. Multipass FSP leads to refinement and homogenization of the microstructure, increasing the microhardness of the weldment. The average hardness value of the MIG weldments at the WFZ was 84 HV, while the microhardness of MIG + FSP joints of 1Pass, 2Pass, 3Pass, and 4Pass FSP was 88, 96, 101, and 110 HV at the stir zone (SZ).

Understanding the pitting behavior of laser welds in different austenitic stainless steels: From the perspective of pitting initiation

The distinct pitting behavior between the fusion zone of QN2109 laser welds and that of 317 L is investigated. The segregation of Mn is different in the two fusion zones due to differences in composition and cooling rate. Both welds contain residual δ-ferrite, with QN2109 displaying higher δ-ferrite content owing to its elevated Mn and N levels. The two welded joints exhibit diverse pitting initiation sites. QN2109 produces a secondary austenite prone to pitting during laser welding, whereas in the 317 L fusion zone, M23C6 is dissolved, significantly reducing the number of pitting sites.

Dramatic improvement of the strength of laser welded joints of Nb521 to GH3128 by adding pure copper as an interlayer

Weld cracks and low weld strength of Nb521 to GH3128 dissimilar metal welded joint are key factors influencing their weldability. To obtain favorable crack-free laser welded joints, this research adopted copper as an interlayer to research the microstructure and properties of laser direct welded and laser offset welded joints of Nb521 to GH3128 superalloy. Compared to previous research results, the tensile strength of laser direct welded joints is enhanced by 2.5 folds. There is a melting reaction layer at the position where the center welded joints are adjacent to the Nb521 base metal, and Ni3Nb and Laves phases are present in the layer. The copper solid solution is located in the center of fusion zone, and Ni3Nb-Laves hard particles are found. The phases near the side of GH3128 superalloy base metal are Ni solid solution, Cu solid solution, Ni3Nb/Cr2Nb hard particles and high melting point alloy mixture. Macrosegregation at the laser offset welding fusion zone is more serious. The phases near the Nb521 base metal are Nb solid solution, Cu solid solution, Ni3Nb and Cu/Cr eutectic structure. At the center of the fusion zone, Cu solid solution, Ni3Nb and Cu/Cr eutectic structure are observed. The phases near GH3128 base metal are Ni solid solution, Cr2Nb phase and Cu solid solution. Mean Vickers hardness values of the direct welded fusion zone is 271.7 ± 129.8 HV, which reduces mismatching of hardness of the joints to a certain degree. As there are Laves phase and Ni3Nb brittle phase in the melting reaction layer adjacent to Nb521 base metal, the microhardness value is up to 589.1 HV. The average microhardness of the offset welded fusion zone is 449.1 ± 217.4 HV, which is higher than the hardness of Nb521 and GH3128 base metals, due to increasing number of brittle phases in the joints. Strength values of the direct welded and offset welded joints are 418.7 ± 49.8 MPa and 364.3 ± 24.5 MPa respectively, which are separately 86.0% and 74.8% of Nb521 base metal. Fractures are all found to occur at the fusion zone on the side of Nb521. The direct welded joints exhibit mixed fracture characteristics with dimples and cleavage facets on the fracture, while the offset welded joints are characterized by brittle fracture.

A study on cracks and IMCs in laser welding of Al and Cu

Aluminum-copper laser welding has become a crucial joining technology for battery manufacturing. However, dissimilar material mixing in welding often causes the formation of intermetallic compounds (IMCs) and cracks, which reduce weld quality. In this study, the effects of laser power and welding speed on material mixing, IMCs formations, and crack formation were investigated by 3D micro-X-ray computed tomography imaging, computational fluid dynamics, and chemical/elemental analyses. Of crack formation during Al-Cu laser welding, transverse cracking was found to be the major type. The crack formation is associated with the IMC distributions generated throughout the weld fusion zone when high laser energy input caused more extensive Al/Cu material mixing.

Analysis of the Creep Failure of P92/HR3C Dissimilar Steel Welded Joint

Harsh service environments, such as high temperature and pressure, cause many unforeseen premature failures, especially for welded joint structures, whose heat-affected zones have been in service. Cases of pressure vessel damage caused by premature cracking in the process often occur. In this paper, the finite element model of dissimilar welded joints of P92/HR3C steel was established to simulate the actual service process, and the variation law of maximum principal stress, equivalent stress, and stress triaxiality on the inner and outer surfaces of the joint and both sides of the welding were analyzed. The results show that in the long-term use of welded joints, the stress concentration occurs in the weld fusion zone; with the increase in service time, the creep strength of the HR3C weld interface is much higher than the P92 weld interface, the structural shape changes and the stress concentration is transferred from the HR3C-weld to the P92-weld.

Microstructural study of keyhole TIG welded nickel-based superalloy G27

The weld fusion zone (FZ) microstructure obtained after keyhole tungsten inert gas welding and post-weld solution heat treatments (PWSHTs) of a new nickel (Ni)-based superalloy called G27 is studied, and the grain growth behavior in the base material (BM) during PWSHTs is characterized. Microsegregation-induced interdendritic microconstituents in the FZ of as-welded G27 are identified by analytical (scanning) transmission electron microscopy ((S)TEM) as niobium (Nb)-rich MC carbides, Nb-rich Laves eutectic constituents, γ’ and η phases. The Laves eutectics are generally considered brittle and can adversely affect the mechanical properties of the weldment; thus, an hour PWSHTs were performed at 954 °C–1060 °C to eliminate the γ/Laves eutectics. PWSHT up to 1010 °C results in only partial removal of Laves eutectics with an excessive formation of η phase surrounding the Laves phase. Complete dissolution of Laves eutectics with no η phase formation is achieved after a PWSHT is performed at 1060 °C. Relative to INCONEL® alloy 718, the complete elimination of the γ/Laves eutectic constituents in the FZ of G27 through a PWSHT was proven to be achieved without causing excessive grain growth in the BM, which could be due to the pinning effect of the fine molybdenum (Mo)-rich precipitates, that are formed during solution heat treatment and identified as hexagonal close-packed phase particles through extensive (S)TEM analyses.

Electron beam welding of P91 steel and incoloy 800HT and their microstructural studies for advanced ultra super critical (AUSC) power plants

This study characterizes an electron beam welded dissimilar joint between P91 ferritic/martensitic steel and Incoloy 800HT Nickel-based alloy. Such dissimilar metal welds (DMW) have applications in nuclear and thermal powerplants where steam-carrying pipes lead to header sections operating at around 600–650 °C. The study employed an advanced welding process to obtain a clean, high-quality dissimilar metal. The microstructure characterization of the DMW involving Nickel-based alloys is prone to solidification cracking in the weld fusion zone (WFZ), which affects the overall weld properties and leads to failure. In the present study, the WFZ microstructure revealed a mixed solidification mode with a homogenous presence of equiaxed dendritic and cellular-type structures. Advanced analyses of WFZ microstructure were done to account for microsegregation. Nb, Ti, Cr, and Mo microsegregation was found in the WFZ. The Ti-rich grain boundaries in the WFZ provided the pinning action during plastic deformation. The evolution of Cr, Mo-based carbide phases affected the microhardness distribution. Microsegregation under rapid solidification of the WFZ resulted in the formation of secondary phases in the interdendritic region. The effect of the evolution of precipitates deteriorated the mechanical strength. High and uneven dislocation distribution was observed in the WFZ near the IN 800HT weld interface. Under tensile loading, the DMW failed from the IN 800HT weld interface due to coarsening of precipitated in the IN 800HT HAZ region near the weld interface and grain boundary thickening due to the deposition of Ti over them. The tensile strength of WFZ was 592 MPa. The tensile strength of WFZ was 592 MPa. The post-weld heat treatment (PWHT) homogenized the microhardness across the weld. Impact toughness after PWHT slightly increased from 63 J to 69 J. The dissimilar joint obtained using the EBW process had sufficient mechanical strength to be considered for Generation IV power plants under the American Society for Testing and Materials (ASTM) standards. The study concludes that the dissimilar metal welded joint was highly prone to failure despite sufficient strength, as IN 800HT weld interface was susceptible to grain boundary liquation.

Minimizing intermetallic Laves phase evolution and enhancing precipitation strengthening of Superalloy-718 joints using InterPulse magnetic arc constriction and high frequency pulsation

The techniques of InterPulse magnetic constriction and high frequency pulsation of arc in gas tungsten arc welding (GTAW) were deployed to minimize the detrimental intermetallic laves phase evolution in fusion zone (FZ) of Superalloy-718 welds and enhance the precipitation strengthening of joints. The Superalloy-718 joints were developed by using an advanced variant of GTAW popularly known as InterPulse gas tungsten constricted arc welding (IP-GTCAW). The joints were subjected to the post weld heat treatment (PWHT) cycles of direct aging (DA), solution annealing at 980 °C and 1065 °C followed by aging (980STA and 1065STA) respectively. The tensile properties and hardness of joints were evaluated. The microstructures of joints were analyzed using optical (OM) and scanning electron microscopy (SEM). The elemental analysis of Laves phase and dendrite core of FZ of joints was performed by using energy dispersive spectroscopy (EDS). The tensile fractured surfaces were analyzed using SEM. Results showed that the Superalloy-718 joints developed using IP-GTCAW showed better response to PWHT than GTAW joints because of the greater refinement of FZ microstructure and evolution of finer and discrete Laves phases in FZ. The 980STA joints exhibited superior tensile properties than DA and 1065STA joints. It is correlated to the greater dissolution of Laves phases in nickel austenitic (γ) matrix of FZ resulting in more niobium (Nb) accessible for the precipitation strengthening of FZ. The 1065STA joints showed slightly higher hardness of FZ than 980STA joints because of the almost complete dissolution of Laves phases in FZ. However, the tensile properties of 1065STA joints are lower than 980STA joints because of severe grain growth in FZ and BM. The evolution of microvoids at the interface of Laves phase/weld matrix leading to the development and coalescence of microcracks in FZ on tensile loading is the main mechanism responsible for the premature fracture of joints.

Grain size and shape dependent crystal plasticity finite element model and its application to electron beam welded SS316L

Electron beam welding is an autogenous welding process that leads to microstructural heterogeneities; crystallographic texture, elongated and larger grain size relative to the surrounding parent material. The goal of this study is to predict the changes in mechanical response of the weld fusion zone as a function of grain morphology and texture by using the parent material properties to avoid extensive and costly experimental campaigns. For this reason, a crystal plasticity solver, “University of BRIstol cryStal plasTicity sOLver” (BRISTOL), is implemented in a finite element framework with new constitutive laws to account for the length-scale dependence considering the size and shape of the grains. It is found that the crystallographic texture governs the orientation specific elastic stiffness and yield stress having the most dominant effect on the macroscopic response of the weldment at the grain size scales considered. Crucially the length-scale dependent model allows accurate prediction of the yield strength of the weldment over a range of microstructures, also highlighting the presence of other factors such as prior strain hardening during the welding process and residual stresses.