Large Heat Input Research Articles

RESEARCH STATUS AND PROSPECTS FOR TIG-MIG HYBRID-ARC-WELDING TECHNOLOGY

TIG-MIG hybrid welding integrates the merits of tungsten insert-gas welding (TIG) and metal inert-gas welding (MIG), and achieves a new material-joining process with high quality, high efficiency and low cost. However, TIG-MIG hybrid welding also had some shortcomings, such as a complex process, unstable arc, large heat input, limiting its application. To further improve and promote TIG-MIG hybrid welding, numerous universities and research institutes have put forward a series of improvement programs that achieve relatively good results. In this study TIG-MIG hybrid welding is discussed in three aspects: the welding-process improvement, welding-parameters optimization and numerical simulation of the welding process. It was found that the stability of the hybrid arc welding process and the quality of the weld bead can be improved by improving the current polarity, wire type and arc swing. The TIG current, the distance between the wire and tungsten, and the heat input had an important impact on the weld quality and the formation of defects. A numerical simulation of the welding process analyzed the effects of torch angle, the distance between the wire and tungsten, the welding speed and the temperature fields on the arc morphology, molten-pool behavior, and droplet transfer. According to the above analysis, it summarized the current research status of TIG-MIG hybrid welding, and then proposed future research directions based on the existing shortcomings and deficiencies.

Magnetic fabric data on partially molten granitic rocks from the Florianópolis Batholith, Southern Brazil

Contact thermal aureoles around mafic intrusions provide information about the emplacement and segregation of melts. The Garopaba-Silveira region is characterised by intrusions with large heat input responsible for the formation of a thermal aureole in the granitoids of the Paulo Lopes Suite (PLS), which extends along the coast to more distant locations in this region. Such melting processes are observed from meso-to microscale, along the granitoid-mafic dike interface and in areas as distant as 500 m from the mafic dikes. It is suggested that these melting processes have been aided by the breakdown of hydrated minerals, such as biotite, through dehydration-melting processes, as commonly found in migmatites and partially molten rocks. On the other hand, the contacts between the granitoids and the mafic dikes are predominantly sharp and straight, as well as, in some dikes, they are sinuous, diffuse, and present back veining, attesting to mutual intrusion. Given the complexity of these melting processes along the granitoid-mafic dike interface, this study applies magnetic susceptibility anisotropy (AMS), anisotropy of anhysteretic remanent magnetisation (AARM) and rock magnetism techniques to melting processes in a thermal aureole context. The magnetic mineralogy of the mafic dikes comprises ferrimagnetic minerals, magnetite, and Ti-poor titanomagnetite. In the granitoids, the ferrimagnetic contribution is dominant, although there are paramagnetic contributions, allowing for direct lateral variations of size of the magnetic grains at different distances (profiles), which may indicate crystallisation time as well as thermal effects in these rocks due to the mafic dike intrusions.

Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nano- and submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (δ) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15° was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and low-temperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.

Toughening Mechanism of Large Heat Input Weld Metal for Marine Engineering Extra-Thick Plate

Toughening Mechanism of Large Heat Input Weld Metal for Marine Engineering Extra-Thick Plate

Elucidating the heat input on CGHAZ microstructure and its irregular effect on impact toughness for a novel V–N microalloying weathering steel

This study investigated the effect of heat inputs on coarse-grained heat affected zone (CGHAZ) microstructure and impact properties of a novel vanadium (V) and nitrogen (N) microalloyed weathering steel via a Gleeble™ system, revealing the metallurgical essence of the steel suitable for large heat input (Ej) welding. The results indicate that in CGHAZ, the dominant microstructure was composed of lath bainitic ferrite (LBF) at Ej = 15 kJ/cm, granular bainitic ferrite (GBF) at Ej = 35 kJ/cm, and a mixture of intragranular acicular ferrite (IGAF), intragranular polygon ferrite (IGPF) and grain boundary polygonal ferrite (GBPF) at Ej = 55 and 75 kJ/cm, respectively, apart from martensite/austenite (M/A) constituents in each one. The area fraction, fM/A, and mean size, dM/A, of M/A constituents increased monotonously with the Ej. In addition, with increasing Ej from 15 to 35 kJ/cm, the fraction, fMTAs＞15°, of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of grains with HAGBs increased. However, as Ej further increased to 55 and 75 kJ/cm as the two large heat inputs, the precipitation of (Ti, V) (C, N) and thereby the heterogeneous nucleation of polygonal and acicular ferrite were promoted by a lowered degree of undercooling, resulting in the decreased MEDMTA≥15° and the increased fraction fMTAs＞15°. Accordingly, the impact toughness of CGHAZ was degraded with the increase of Ej from 15 to 35 kJ/cm, but enhanced with the increase of Ej from 35 to 75 kJ/cm.

Study of Laser Welding Process Parameter on the Microstructure and Mechanical Properties of Dissimilar Joining of Dual-Phase DP780 and Cold-Rolled CR340 Steel

Abstract Laser welding was performed in dual-phase steel (DP780) and cold-rolled steel (CR340) at a series of weld parameters (series I up to series VII): power input, heat input, weld velocity, and focus offset. The study was aimed at improvement of joint performance on the laser welding process parameter settings on the weld bead morphology and thereby evaluating their microstructure and mechanical properties. Optical and scanning electron microscopy and uniaxial tensile tests were performed to observe the microstructural changes, phase formation, and evaluate the mechanical properties, respectively. Tensile strength, microhardness, and strain localization using a digital image correlation technique were investigated for acquainting the local strain mapping associated with the loading. Microstructure characterization revealed that the fusion zone consisted of martensitic structure, and the heat affected contained newly formed martensite in both steels. Vickers hardness revealed that during welding, there was a softening phenomenon at the CR340 end. The DP/CR welds show a mechanical behavior that is driven by both the DP780 and CR340 steel. The joints showed strength up to ∼410 MPa and fracture strain up to ∼16 %, however the weld join failed from CR340 steel end weld during tensile tests. The larger heat input at series VI guarantees joints with an adequate strength but wider weld seams and a lower energy efficiency.

Weld depth dynamics measured with optical coherence tomography during remote laser beam oscillation welding of battery system

This paper represents nondestructive quality monitoring technique using optical coherence tomography (OCT). It addresses online monitoring of weld depth during laser beam oscillation welding and aims at the application in joining cells in large battery assemblies. The weld depth was continuously detected with OCT while the OCT beam position was adjusted highly dynamically in accordance with the scanning optics position. By displacing the OCT measurement beam according to the current machining direction, the correlation between the position of the laser beam in an oscillating circular pattern along the circular feed direction and the periodic fluctuations of the measured weld depth was explored. It was found that the deepest part of the keyhole is located at the trailing position of the laser beam. This effect can be attributed to the large heat input due to the overlapping circular movements. The results confirm once again that instant weld depth monitoring with OCT ensures superior weld quality.

Orbital friction welding of steel bars – heat generation and process modelling

Welding of rails in the field is usually associated with a large heat input, which results in a large heat-affected zone (HAZ), which in turn may impair the welded rail head and decrease its service life. An innovative orbital friction welding (OFW) process with an intermediate eccentrically oscillating disk is proposed, and a demonstrator has been constructed. This enables welding of rails, which have a non-symmetric cross-sectional area and must be stationary during welding. The process is characterized by low heat input, creating a narrow HAZ, and low welding deformations. A thermo-mechanical finite element model is developed to determine suitable process parameters to create a narrow HAZ. A phenomenological model for heat generation during friction welding is developed for pearlitic rail steel with parameters calibrated from rotary friction welding experiments on pipes. The temperature dependence of the friction coefficient in the interface is established. Pilot tests with the demonstrator OFW machine on bars with a quadratic cross section showed that preheating will be required to guarantee a fully pearlitic weld zone. This was verified by the simulations of the thermo-mechanical finite element model.

Effect of Boron Content on Microstructure and Impact Toughness in CGHAZ of Shipbuilding Steel Plates with Ca Deoxidation

Herein, the effect of boron (B) content on the microstructure and impact toughness in the coarse‐grained heat‐affected zone (CGHAZ) after large heat input welding of 400 kJ cm−1 for the EH40 shipbuilding steel plate with Ca deoxidation is studied. As the B content is increased from 0.0002 to 0.0024 wt%, the average width of grain boundary ferrites (GBFs) is decreased from 14.6 to 11.3 μm, the formation of acicular ferrite (AF) is promoted, and the formations of GBF and Widmanstatten ferrite (WF) are hindered in CGHAZ. The heat‐affected zone (HAZ) impact toughness is increased from 39 to 97 J at −20 °C. The fraction of low‐angle grain boundaries (LAGBs) is decreased from 40 to 29%, and the high‐angle grain boundaries (HAGBs) is increased from 60 to 71%. When the B content is 0.0024 wt%, the B atom segregates on the grain boundaries, and the widths of the segregation lines range from 2.3 to 3.6 μm. GBF is found to be the weakest zone in the CGHAZ. B addition can effectively reduce the formation of GBF, enhancing the critical crack stress and improving the HAZ toughness.

Microstructures, heat treatments and mechanical properties of AerMet100 steel fabricated by hybrid directed energy deposition

As a kind of directed energy deposition (DED) method, wire and arc additive manufacturing (WAAM) is a solution for the short-process manufacturing of medium complex and integral parts. Ultrahigh-strength steel AerMet100 has been widely applied in large-scale aerospace structural parts like landing gear and the increasing demand of integrated design promotes the application of WAAM. The microstructure and mechanical properties of as-deposited AerMet100 steel are greatly affected by post-heat treatments, and thus researches on heat treatments are essential to the wider application of AerMet100 in the aerospace industry. In this paper, an AerMet100 steel sample was fabricated by WAAM with in situ microrolling (a hybrid DED method). Two heat treatment designs, based on standard and homogenization, were applied to the sample. The microstructures of the as-deposited and heat-treated specimens were examined with optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the mechanical properties were examined using an electronic universal testing machine. The results indicate that austenitization is the main grain refinement mechanism of the as-deposited specimens, and a smaller layer height and an appropriately large heat input are beneficial for the refinement. Cellular segregation formed during solidification has severe heredity and strong effects on the microstructures of the nonhomogenized heat-treated group, causing lower mechanical properties and anisotropy. A special fracture mechanism related to segregation causes fracture morphology featuring parallel structures in both the non-homogenized X-direction tensile specimens and X-Y direction KIc specimens. Compared with the nonhomogenized heat-treated group, homogenization heat treatment basically eliminated cellular segregation and achieved a better combination of the tensile and KIc properties.

Study on prevention method of hot cracking under butt welding

For the automation of butt welding, it has been considered to introduce tack welds instead of out-of-plane restraint jigs to perform welding with large heat input. However, the occurrence of hot cracking has been a significant issue in large heat input welding. In the previous report, the authors have developed a prediction method of hot cracking that can investigate the influence of not only the high-temperature strain generated in the weld metal but also the phenomena related to solidification, such as solidification shrinkage, latent heat of solidification, and crystals growth direction. In this study, the conditions for the prevention of hot cracking were investigated by numerical analysis and experiments by controlling the temperature gradient at solidification temperature using a trailing torch in tandem welding. As a result, it was confirmed that the large strain existing position predicted using the proposed hot cracking analysis has a good agreement with the cracking position observed experimentally in tandem butt weld. It was also found that with increasing the electrode distance, both the columnar crystals' associate angle became larger and the local strain became smaller. It can be concluded that the hot cracking can be successfully prevented in the practical experiments using the proposed large electrode distance.

Improving mechanical strength and isotropy for wire-arc additive manufactured 304L stainless steels via controlling arc heat input

Wire arc additive manufacturing (WAAM) has the advantage of fabricating large-scale parts with high deposition rate and low costs. But large heat input of WAAM leads to a larger molten pool during deposition, causing coarse grains of formed parts, which tend to have poor mechanical properties of the formed parts. In this study, for reducing heat input, the hot-wire method was adopted to assist WAAM in fabricating 304L austenitic stainless steel (304L SS). Four thin walled square 304L SS components were fabricated by wire arc additive manufacturing with different hot-wire currents. It is found that with the increase of hot-wire current, the arc heat input was reduced obviously. When the hot-wire current is 100 A, the arc heat input was reduced by 70% compared to without hot wire. With the arc heat input reduced, the temperature gradient of the molten pool was decreased, which refines the grains and makes the columnar grains gradually equiaxed, and reduces the anisotropy. The components manufactured in this work have high strength plasticity which the tensile strength up to 720 MPa in a transverse direction with the elongation at 51%.

Experimental study on fracture toughness of quenched and tempered and TMCP high strength steels

In this study, the fracture toughness of five kinds of high strength steel (HSS) with the nominal yield strength of 550 MPa, 690 MPa and 890 MPa were experimentally studied. Two steel delivery conditions, i.e., quenched and tempered HSS and thermo-mechanical control process HSS, were included and compared. The effect of heat input on the fracture toughness of HSS was investigated. A comparative study was organized for the fracture toughness of base material (BM), heat-affected zone (HAZ) and weld metal (WM). Results indict that the fracture toughness of the heat-affected zone of QT steels (QT550 and QT690) first increases and then decreases slightly with the increase of heat input. There is an optimal heat input for quenched and tempered HSS welding to achieve satisfactory fracture toughness. Either too small or large heat input is not beneficial to the resistance to crack propagation. With increase of steel strength, it seems the optimal heat input for quenched and tempered HSS becomes smaller. The effect of heat input on fracture toughness of TMCP HSS is different with quenched and tempered HSS: higher heat input would normally produce higher fracture toughness for HSS TMCP550. With the increase of yield strength, the fracture resistance increases for both quenched and tempered HSS and TMCP HSS.

Effect of heat input on microstructure and corrosion resistance in heat affected zone of 304 stainless steel joint by laser welding

Laser welding of 5 mm thick 304 austenitic stainless steels without grooves was carried out, and the effect of heat input and cooling time on grain boundary character distribution (GBCD), texture evolution, corrosion behavior in the heat affected zone was investigated in detail. The experimental results revealed that the fractions of low Σ coincidence site lattice (CSL) grain boundary increase and random high angle boundaries decrease by prolonging the cooling time and heat input. In addition, the dynamic recrystallization of the heat affected zone (HAZ) is influenced significantly by heat input, which resulted in the evolution of the textures. The main texture components were concentrated around {210}< 112 > and {111}< 112 > when subjected to large heat input. Moreover, the corrosion resistance was improved by increasing the cooling time, which was tested by the potentiodynamic polarization test.

Characterization of ultrafine-grained copper joints acquired by rotary friction welding

Copper rods with ultrafine-grained microstructure, obtained by multi-turn ECAP processing, were subjected to Direct Drive Rotary Friction Welding using various processing parameters, such as rotational speed and pressure, which resulted in different energy and heat input. Even though friction welding is a high energy process, by a proper selection of processing parameters it was possible to maintain grain size at around 0.7 µm in the weld zone and preserve the UFG microstructure. These microstructural features translated into mechanical properties: the YS for those specimens was around 330 MPa. Processing parameters that resulted in a larger heat input caused an increase in grain size to around 2 µm; this, however, increased ductility and led to a uniform elongation exceeding 5%. Corrosion resistance in the stir zone increased, as was evident in the higher open circuit potential and higher corrosion potential in comparison with base material; the observed differences were about 50 mV. These changes can be explained by the higher fraction of HAGBs in the SZ.

Effect of beam current on the microstructure, crystallographic texture and mechanical properties of electron beam welded high purity niobium

Fabrication of superconductive radio frequency (SRF) cavity by electron beam welding (EBW) of pure Niobium (Nb) has been a topic of discussion for quite some time. The influence of beam current on the weld properties of Nb plates has been investigated in light of mechanical properties and microstructure. The widths of the fusion zone (FZ) and heat-affected zone (HAZ) of the welded samples increase with increasing the beam current. The hardness profile across the base metal (BM) to FZ followed a typical symmetrical pattern with a plateau across the FZ. The maximum drop of hardness at FZ compared to BM is only about 15%, making the EBW process a suitable one for the joining of Nb plates for the application of SRF cavity. The development of mechanical anisotropy of the welded joints due to differential heat transfer and the associated change in microstructure in FZ and HAZ has been evaluated by conducting tensile tests along different directions to the line of welding. Elongation of the welded joint is the maximum (~94%) for samples with the tensile direction parallel to the welding direction for a beam current of 70 mA. The Lankford constant (r-value) at different angles (0, 45, and 90°) to the tensile axes has been measured and correlated with crystallographic texture measured by Electron Backscattered Diffraction (EBSD). The dominance of the {554} <225> component for samples with larger heat input is found to be responsible for higher r-values, maximum formability, and lower Kernal average misorientation (KAM) profile. A heat transfer simulation has been used to simulate the dimensions of the weld pool and subsequently correlate its impact on various properties. • Heat input controls the metallurgical phenomena like grain size, hardness and stress-strain response. • The hardness profile decreases from the BM through HAZ until it attains a minimum at the FZ. • Smaller average grain size makes the material stronger and lesser ductile. • Tensile specimens at 0° exhibits deeper and uniformly distributed dimples, an evidence of ductile fracture. • Determine the depth and length of weld pool from the heat-transfer simulation.

A path generation method for wire and arc additive remanufacturing of complex hot forging dies

Wire and arc additive remanufacturing (WAAR) technology has become a new solution for hot forging dies repair and remanufacturing. In this study, a path generation method is proposed for WAAR of hot forging dies. At first, a WAAR process of the hot forging die is presented, and considering the characteristics of large welding heat input and complex 3D digital model, the hybrid path planning strategy is confirmed as an appropriate strategy for WAAR. The developed hybrid path generation method for WAAR consists of three main steps: determine the direction of the scan line; divide and fill the internal area; and connect the sub-paths. The relatively optimal scanning direction is determined by calculating the length and inclination angle of each line segment in the contour lines, which reduces the possibility of sharp angles. The internal region is divided according to the position of the selected extreme points, and the path space is adjusted to avoid the occurrence of the unfilled phenomenon. At the stage of sub-path connection, some criteria are proposed to reduce the number of sub-paths. At last, the effectiveness and robustness of the proposed method are validated through the planar deposition experiment and the WAAR process of four damaged hot forging dies.

Tailoring the Microstructure of Coarse-Grained HAZ in Steel for Large Heat Input Welding: Effect of Ti–Mg–Ce–V Inclusion/Precipitation Particles

Tailoring the Microstructure of Coarse-Grained HAZ in Steel for Large Heat Input Welding: Effect of Ti–Mg–Ce–V Inclusion/Precipitation Particles

Fifty Year Trends in Global Ocean Heat Content Traced to Surface Heat Fluxes in the Sub‐Polar Ocean

AbstractThe ocean has absorbed approximately 90% of the accumulated heat in the climate system since 1970. As global warming accelerates, understanding ocean heat content changes and tracing these to surface heat input is increasingly important. We introduce a novel framework by organizing the ocean into temperature‐percentiles from warmest to coldest, allowing us to trace ocean temperature changes to changes in surface fluxes and mixing. Applying this framework to observations and historical CMIP6 simulations, we find that 50 ± 6% of surface heat uptake between 1970 and 2014 is confined to isotherms in the coldest 90% of the ocean volume. These isotherms outcrop over only 23% of the ocean's surface area in the sub‐polar regions, implying a disproportionately large heat input per unit area. Additionally, a cooling bias in the CMIP6 models is traced to inaccurate sea surface temperatures and surface heat fluxes into the warmest 5%–20% of the ocean volume.

Influence of Duty Ratio and Current Mode on Robot 316L Stainless Steel Arc Additive Manufacturing

Wire and arc additive manufacturing (WAAM) is usually for fabricating components due to its low equipment cost, high material utilization rate and cladding efficiency. However, its applications are limited by the large heat input decided by process parameters. Here, four 50-layer stainless steel parts with double-pulse and single-pulse metal inert gas (MIG) welding modes were deposited, and the effect of different duty ratios and current modes on morphology, microstructure, and performance was analyzed. The results demonstrate that the low frequency of the double-pulse had the effect of stirring the molten pool; therefore, the double-pulse mode parts presented a bigger width and smaller height, finer microstructure and better properties than the single-pulse mode. Furthermore, increasing the duty ratio from 35% to 65% enlarged the heat input, which then decreased the specimen height, increased the width, and decreased the hardness and the tensile strength.