Inhomogeneous Microstructure Research Articles

Effect of microstructure inhomogeneity on creep behavior of friction stir welding 7B50-T7451 aluminum alloy thick plate joint

Microstructure inhomogeneity and through-thickness creep properties of friction stir welding (FSW) 7B50-T7451 aluminum alloy thick plate joint were investigated. The results show that creep properties of the FSW joint deteriorate along the thickness direction from the top to the bottom. Both the grain size and fraction of the low-angle grain boundaries (LAGBs) in the weld nugget zone (WNZ) decrease from the top to bottom. A larger volume of precipitate free zone (PFZ) and coarser precipitates in the bottom slice reduce creep resistance compared with the top. The precipitates in the heat-affected zone (HAZ) exhibit continuous coarsening with the increase of creep temperature, inducing the decrease of threshold stress. The dominant creep mechanism transforms from dislocation creep in the HAZ to grain boundary sliding (GBS) in the WNZ when creep temperature increases. The fracture mode also changes from transgranular to intergranular fracture.

Altered microstructure characteristics and enhanced corrosion resistance of UNS S32750 duplex stainless steel via ultrasonic surface rolling process

The UNS S32750 duplex stainless steel was treated by ultrasonic surface rolling process (USRP), and effects of the plastic deformation on microstructure evolution and corrosion behavior were investigated. Results demonstrated that grain refinement, a considerable increase of the fraction of low-angle grain boundaries (LAGBs, <10°), high density of dislocations and high surface compressive residual stress were produced by the USRP. In particular, the intensity of the evolution was inhomogeneous and it was higher for austenite than ferrite phase. What’s more, an evolution toward <001> of crystallographic orientation, a decrease in intensity of texture in α phase while increase in γ phase were found for the surface deformation layer. In addition, a small amount of deformation twins in γ phase were induced on account of the USRP treatment. Taking advantages of the microstructure evolution, the corrosion potential and critical pitting potential increased, while the passive current density and metastable pitting susceptibility decreased significantly for the USRP sample. The refined grains, high-density of dislocations and high surface compressive residual stress synergistically promoted the enrichment of Cr 2 O 3 in the passive film on the deformation layer. The Cr/Fe ratio increased from 0.73 to 0.94 with the help of USRP, revealing a less defective and more stable passive film. Therefore, USRP is a promising and effective surface strengthening technique for DSS to improve the corrosion resistance in aggressive environments. • The USRP caused inhomogeneous microstructure evolution for γ and α phase of UNSS32750 DSS. • The increase of surface compressive residual stress induced by USRP was higherin γ than α phase. • The USRP decreased the metastable pitting susceptibility, while improved thecorrosion resistance of the DSS. • The ratios of Cr 2 O 3 /Cr(OH) 3 and Cr/Fe in passivefilm were increased with the assistant of USRP.

Effect of inter-pass annealing on the deformation microstructure of Ti-48Al-2Cr-2Nb alloy

TiAl alloy is a potential candidate structural material for lightweight and high temperature in aerospace, and multi-pass thermo-mechanical treatment, where dynamic recrystallization (DRX), static recrystallization (SRX), and post-dynamic recrystallization (PDRX) have significant effects on microstructure evolution, is commonly applied for microstructure control due to its poor plasticity. Accordingly, the complex recrystallization behaviors of Ti-48Al-2Cr-2Nb alloy were investigated by hot simulation compression experiments. The results of metallographic statistics showed that microstructure inhomogeneity of uniaxial compression could not be improved by changing the temperature and strain rate to adjust the DRX degree. Additionally, discontinuous dynamic recrystallization (DDRX) characterized by the formation of twining boundaries was a dominant DRX mechanism and generated at grain boundaries. Meanwhile, continuous dynamic recrystallization (CDRX) was formed within the grains and at grain boundaries, accompanied by the relatively high distortion at the initial stage. Residual deformed microstructure owing to the incomplete DRX could be eliminated by SRX and PDRX during the inter-pass annealing. A method based on the grain size difference induced by various recrystallization mechanisms was proposed to reveal that grain growth resulting from PDRX played a primary role and SRX was impaired with the increasing strain. The interaction effect of DRX, SRX, and PDRX during the multi-pass deformation could achieve the uniform microstructure, which is consistent with a near-isothermal multi-pass multidirectional canned forging process designed in the paper. Eventually, room-temperature tensile tests demonstrated that the mechanical properties of wrought billet were distinctly enhanced.

Discovering the role of the defect morphology and microstructure on the deformation behavior of additive manufactured Ti–6Al–4V

The defects remain a significant issue for additive manufactured titanium alloys, which hinders their further market penetration. In this work, we proposed a new understanding of the inhomogeneous microstructure formation affected by defects with detailed EBSD characterization, and how the defects and inhomogeneous microstructures determined the strain accumulation behavior by in-situ SEM tensile testing. Smaller prior β grains and coarser α laths were identified around the defects. In-situ tensile behavior investigations revealed that irregularly-shaped and elongated defects with high stress intensity factor (ΔKI) could induce severe strain accumulation, showing that the defect morphology was the primary factor that could determine the strain accumulation behavior. In the meantime, the inhomogeneous microstructures around the defects could further contribute to the strain accumulation, with the coarser α laths more favorable for deformation and stronger microtexture for easier slip transmission. In addition, microcracks were found initiated from a spherical pore with small loading, and arrested due to the negligible strain accumulation. Further characterization showed that oxidation, which was found more probable in spherical pores, was the cause of this microcrack initiation.

The Improvement of the Wear Resistance of T15 Laser Clad Coating by the Uniformity of Microstructure

The uniformity of microstructure and wear properties exist in the T15 coating for the laser cladding on 42CrMo steel. It can be improved by a post-heat treatment process. Temperature ranges from 1100 to 1240 °C were applied on the cladding layer to investigate the effect of the heat treatment on the wear resistance and hardness gradient. The post-heat treatment can efficiently improve the inhomogeneity of microstructure. The lower wear rate is obtained after the quenching process at 1100 °C, and the wear rate is increased though the tempering process. The carbides at the grain boundary are decomposed and integrated into the matrix during the high temperature quenching process. The carbides are precipitated and dispersed in the grain during the tempering process. The content of martensite and alloy carbide is significantly increased through the heat treatment process. The microhardness of the cladding layer is 910 HV after quenching and 750 HV after tempering. The wear mechanism of the cladding layer is mainly abrasive wear and fatigue wear. The crack and falling off from cladding layers are significantly reduced for the quenching–tempering process.

(Digital Presentation) The Electrode’s Inhomogeneous Microstructure Effect on Battery Performance

Currently, rechargeable batteries are used in everyday life applications, such as in mobiles, cameras, laptops, and cars. Accordingly, the demand for rechargeable batteries, in particular Lithium-ion batteries, is increasing. Their electrodes consist of at least four different materials: active material, a conductive additive, polymer binder, and current collector. The battery performance in terms of cycling performance, energy density, mechanical integrity, and ionic and electronic mobility depends on these materials.1 The casting procedure, the multiscale packing during the evaporation process after casting, the accumulation of the conductive additive, the adhesive failure during the charging and discharging process, and the natural irregular shape of the electrode materials produce an inhomogeneous electrode microstructure. 2 This inhomogeneity causes current density variation and a non-uniform Lithium ion concentration leading to damage to the battery structure, reducing the capacity, and degrading the battery. 2 The electrode microstructure and its effect on the battery performance have great importance. As such, several techniques have been developed to investigate electrode microstructure. Each technique gives specific information. Here, we will combine scanning electron microscopy (SEM), scanning electrochemical microscopy (SECM), and battery cycling to study the effect of microstructure on the battery performance.SECM is a high resolution probe technique that scans the electrode surface using an ultra-microelectrode. Thus it can measure the electrode`s local conductivity.3 , 4 Therefore, we can quantify the inhomogeneous microstructure distribution. Thus, by combining these results with SEM and cycling test results we will be able to find the correlation between the battery performance and the distribution of the microstructure.

The Influence of Local Microstructure Inhomogeneities on Local Drying Kinetics during Freeze-Drying.

Freeze-drying is a gentle drying technique to dry high value products, such as pharmaceuticals, without impacting the quality of the product. However, this method is very time and cost intensive. It is known that larger pores reduce the duration of primary drying due to facilitated mass transport. However, next to the pore size, other structural parameters exist whose influence on drying kinetics is still unknown. Therefore, the aim of this article is to investigate the influence of the microstructure (pore size, shape and orientation) on local primary drying kinetics. In the study, freeze-drying experiments on maltodextrin and sucrose solutions (c1 = 0.05 and c2 = 0.15 w/w) were carried out in a lyomicroscope. Two-dimensional images were recorded during the whole drying process and in the dry state and analyzed on the movement of the sublimation front, pore size, orientation and shape. Different microstructures were created by using different freezing parameters, namely two different cooling rates and solid concentrations. It could be shown that for pores with a high aspect ratio, the pore orientation was more important for the drying kinetics than the pore size, while for pores with a lower aspect ratio the pore size was the decisive parameter.

Effect of heat treatment on the anisotropy in mechanical properties of selective laser melted AlSi10Mg

Anisotropy in mechanical properties is one of the critical issues in metal additive manufacturing (AM) impeding the wider application of parts fabricated using this manufacturing technique. In this work, the anisotropy in mechanical properties of an AlSi10Mg alloy fabricated by AM was investigated in terms of grain morphology, microstructure and residual stress distribution, with a focus on changes occurring during post-AM heat treatment by isochronal annealing. It was found that microstructural inhomogeneity is the major factor that weakens the melt pool boundaries, and leads also to anisotropic mechanical properties. Volumes in the vicinity of melt pool boundaries are characterized by coarse Al–Si eutectic networks and high residual stresses, where the former are the main reason for the plastic anisotropy, and the latter increase the local strength. Post-AM heat treatment can alleviate the presence of anisotropy due to homogenization of the microstructure and a redistribution of residual stresses. The overall effect of heat treatment on mechanical strength is a balance between improvement through precipitation strengthening and weakening of grain boundary strengthening, solid solution strengthening and dislocation strengthening. Through an appropriate heat treatment process (annealing at 280 °C for 2 h), the anisotropy in mechanical properties can be effectively reduced and a good strength-elongation synergy can be obtained (strength of 350 MPa and elongation of ∼9%). This study provides new ideas for understanding and mitigating anisotropy of additively manufactured AlSi10Mg, and also has implications for the study of anisotropy of other materials.

Influence of chemical segregation on bainitic microstructures in a carburized bearing steel

Bainite to austenite reversal is one of the grain refinement techniques employed in carburized steels. However, chemical segregation influences the homogeneity of the bainitic structure, which is seminal to exploit the advantages associated with austenite reversal. It is therefore important to understand the influence of chemical segregation on bainite formation, which is investigated in this work. Characterizations were performed on the microstructures obtained from the case and core regions of a carburized steel after 30 h of bainite treatment at 320 °C for two carbon compositions: 0.85 wt% C (zcase) and 0.16 wt% C (zcore). The microstructure of zcase is shown to contain bands with bainite in alloy-lean regions and martensite/austenite in alloy-rich regions. For zcore, although the chemical bands are not composed of different phases, the alloy-rich regions have a fraction of martensite-austenite (MA) islands that is twice the fraction in alloy-lean regions. Despite this difference, the austenite phase fractions in the chemical bands of zcore are low and almost similar, indicating that the MA islands are mostly martensite. From experimental results and thermodynamic and kinetic simulations, it is elucidated that a different rate of phase transformation in the chemical bands is the cause for the observed microstructural inhomogeneities.

Micro‐Raman spectroscopy and complementary techniques applied for the study of copper and iron wastes from Motya (Italy)

AbstractThis work is the first archaeometric investigation on copper and iron wastes from the Phoenician site of Motya (Sicily, Italy), dating back to the 8th to the 4th century BC. The samples were analyzed through micro‐Raman Spectroscopy (μ‐RS), Optical Microscopy (OM), Scanning Electron Microscope‐Energy‐Dispersive X‐ray Spectroscopy (SEM‐EDS), High‐Resolution Field Emission Scanning Electron Microscopy (HR‐FESEM), and Electron Micro‐Probe Analysis (EMPA). Micro‐Raman techinique permitted to identify both primary phases, for example, calchopyrite, and secondary products such as cuprite and copper thrihydroxychlorides in the Cu‐slags and goethite in the Fe‐slags. SEM and HR‐FESEM imaging showed the occurrence of inhomogeneous microstructures in the Cu‐ and Fe‐slags due to elements segregation, solidification, and corrosion. EMPA data revealed that the archaeometallurgical wastes from Motya can be differentiated on the basis of their chemical compositions. These preliminary results showed different typologies of by‐products, such as base metals speiss, copper slags from smelting sulfide ore with matte, and iron smelting and smithing slags, suggesting different stages of copper and iron productions.

Effect of Laser re-melting on the microstructure of High Entropy Alloys

• A low density equiatomic High Entropy Alloy Zr-Nb-V-Ti-Al was synthesized. • The as-cast alloy had coarse dendritic structure with compositional inhomogeneities. • Re-melting the as-cast alloy with Nd-YAG Laser resulted in formation of fine grains. • Laser Re-melting significantly reduced inhomogeneities in distribution of Zr & Al. • The microhardness of Laser melted region was found to be similar to the BCC phase. A low density equiatomic High Entropy Alloy of composition Nb-Zr-V-Ti-Al was synthesized for nuclear reactor applications. The alloy in as-cast condition showed a coarse dendritic structure with a major BCC/B2 type dendritic phase while segregation of Zr and Al in the form of intermetallics was noted in the inter-dendrtitic region. Re-melting a part of the alloy surface with an Nd-YAG Laser resulted in formation of very fine grains (∼500 nm) with a significant reduction in compositional segregations with respect to Zr and Al. The microhardness of the Laser melted region was found to be similar to the BCC phase in the as – cast microstrucure. This indicates that Laser melting could be an effective method in reducing compositional and microstructural inhomogeneities and push the multiphase structure towards single phase.

Bending-deformation-induced inhomogeneous aging behavior and accelerated precipitation kinetics of extruded AZ80 alloy

In this study, the effects of bending deformation on the precipitation behavior of an extruded AZ80 alloy are investigated by subjecting a plate of the alloy to three-point bending and subsequent aging at 200 °C. In the upper region of the bent specimen (denoted as CZ), numerous {10−12} extension twins are formed. In the lower region of the bent specimen (denoted as TZ), small quantities of {10−12} extension twins, {10−11} contraction twins, and {10−11}–{10−12} double twins are formed, and most of the bending deformation is accommodated by dislocation slip. In the central region of the bent specimen (denoted as NZ), few twins are formed, and the microstructural characteristics of this region are almost identical to those of the initial material. When the bent specimen is aged, the peak-aging time of the CZ and TZ (8 h) is 60% shorter than that of the NZ (20 h), which is attributed to the accelerated precipitation caused by the abundant twins in and high internal strain energy of the CZ and TZ. In the CZ, nucleation and growth of continuous Mg 17 Al 12 precipitates (CPs) occur rapidly, especially inside the {10−12} twins, and this accelerated CP formation inhibits the formation of discontinuous Mg 17 Al 12 precipitates (DPs). Because of the high internal strain energy of the TZ, promoted formation of CPs occurs in the early stage of aging, which, in turn, suppresses the formation of DPs. In contrast, the NZ, because of its low internal strain energy and insufficient twins, exhibits slower CP formation behavior and has a considerably larger amount of DPs than the CZ and TZ. Although the bending deformation before aging induces inhomogeneous precipitation behaviors, the prebending deformation shortens the overall peak-aging time of the material and increases its peak hardness. • An extruded AZ80 alloy plate is subjected to bending and subsequent aging. • The bent plate has an inhomogeneous microstructure and internal strain energy. • Its upper and lower regions show a faster aging rate than its central region. • Continuous and discontinuous precipitation behaviors are different in each region. • Bending deformation shortens the peak-aging time and improves the peak hardness.

Effects of microstructure coarsening and casting pores on the tensile and fatigue properties of cast A356-T6 aluminum alloy: A comparative investigation

A356 alloys processed mostly with the low pressure die casting method are always known to contain casting pores and inhomogeneous microstructure. In this study, the effects of microstructure coarsening and casting pores on the tensile and high-cycle fatigue properties of A356-T6 alloy are comprehensively investigated. The deformation behavior and fracture mechanisms are systematically analyzed by combining tensile tests and crystal plasticity finite element method (CP-FEM) simulations with distinctively initial microstructure features (fine microstructure, coarse microstructure, and coarse microstructure with pores). The results show that microstructure coarsening is presented in the notably grown α-Al dendrites, which leads to the connection and thickening of eutectic regions accompanied by the aggregative distribution of eutectic Si particles. With the coarsening microstructure, microcrack is easier to initiate in those thick eutectic regions, due to the higher stress concentration caused by the aggregated eutectic silicon. Particularly, the dominant crack propagation path changes from the trans-dendrite for fine microstructure to along eutectic regions with coarse microstructure. In addition, it confirms that the casting pores have larger detrimental effects on the tensile and fatigue properties than the microstructure coarsening in the aspects of crack initiation. Stress concentration is prone to be induced at the local edge of pore at the initial loading stage, resulting in the micro plastic deformation and lower fatigue properties. With the rapid strain accumulation in later stage, the microcracks can initiate prematurely at the local edge of pore, and thus lead to obvious decrease in tensile elongation. Moreover, a modified fatigue model considering the effects of casting pores and the microstructure difference is proposed to describe the fatigue performance of A356-T6 alloy.

Physics-driven modeling of electron beam welding of Al-Cu alloys from molten pool flow, microstructure to mechanical properties

The process-structure-property relationship has always been the research focus in welding, where computational prediction is valuable. However, some important physical phenomena are often simplified when modeling the melting and solidification processes. In this work, an integrated modeling framework is adopted to comprehensively predict the welding quality, from a computational fluid dynamics (CFD) model for molten pool dynamics, to a cellular automata (CA) model for dendrite growth, and finally to analytical and finite element method (FEM) models for mechanical properties. Using Al-Cu alloy as a model material, the complex melting and solidification processes in electron beam welding is reproduced, and various strengthening mechanisms are considered to predict the mechanical response. Notably, the material property inhomogeneities due to inhomogeneous microstructure in the welds are considered when predicting the overall mechanical properties. Based on the simulation results, the second phase strengthening proportion reaches almost 50%. The maximum relative errors of predicted Vickers hardness and tensile strength are 13.62% and 13.30%, respectively. The simulation results show quantitative agreement with experimental data, demonstrating the appealing potential of this modeling framework. This study could be helpful for optimizing the actual welding process to tailor the microstructures and mechanical properties. • A physics-driven process-microstructure-property modeling framework is adopted. • Various strengthening mechanisms based on the microstructure modeling are considered to predict the mechanical response. • The material property inhomogeneities are considered when predicting the overall mechanical properties. • The simulation results show quantitative agreement with experimental data.

Diffusion-limited Nonequilibrium Phase Transformation of Nickel/Aluminum Dissimilar Materials during Laser Welding Part I: Precipitation-strengthened Polycrystalline Nickel-based Superalloy

Polycrystalline heterogeneities of grain growth, phase transformation and tensile properties are discussed by metallography, spectroscopy and fractography when choosing laser welding with butt-joint configuration to satisfactorily minimize effect of inappropriate weld pool shape on microstructure and mechanical properties of γʹʹ precipitation hardable nickel-based superalloy in aerospace industry. There is parabolic relationship between weld penetration and diffusion-limited nucleation, growth and coarseness of secondary dendrite arm spacing alongside fusion boundary through nonequilibrium solidification process, which is dendritically susceptible to grain growth variation and metallurgical discontinuities. The amount, size, morphology and distribution of nonequilibrium intermetallic phase near the dendrite boundaries are kinetically and thermodynamically rely on weld pool shape whose crystal structure is incoherent with γ solid solution austenite phase and increases preference for crack initiation and propagation of brittle intergranular and ductile dimple fracture failures to enormously contribute to losses of strength and ductility. Unsymmetrical keyhole weld is thermometallurgically inconvenient for amelioration of microstructure and mechanical properties on either side, and adversely mitigates resistance to hypereutectic-aided dendrite embrittlement. Beneficial grain refinement and suppression of detrimental nonequilibrium intermetallic phase at the same time are challenging. This problem is an integral part of inhomogeneous microstructure and mechanical properties. The finer grain size occurs, the larger grain boundary area is available for Laves/γ eutectic-type reaction and vice versa. Contributions of coarse grain kinetics and metallurgical reaction thermodynamics to weld disintegration and fracture failure mechanism of tensile properties are explained by microstructure characterization of multicomponent and multiphase weld. Finally, it is imperative to dendritically balance these important factors to minimize inevitable interface solidification products and anomalous substructure growth, and reasonably advance superior mechanical properties of reliable weld.

On the dual-stage partial recrystallization and the corresponding mechanical response of the Cantor alloy

In this work, we systematically characterized the microstructural evolution of the Cantor alloy, FeCoNiCrMn, during recrystallization at a typical mid-temperature, 700 ℃, and carefully bridged up with the mechanical responses. Apart from the overall trade-off phenomenon, we evidenced the presence of a two-stage strength-ductility variation, where before 5 min the yield strength drops fast without apparent ductility compensation while in the following 5–20 min the ductility increases remarkably. The first stage covers the transition from cold-rolled non-recrystallized microstructure to a eutectic-like stacking of recrystallized regions and new non-recrystallized regions with a different texture. The second stage then includes the vanishing of the new non-recrystallized regions and the completing of recrystallization. This dual-stage transition is to our knowledge never reported before for HEAs, and we suppose the occurrence is associated with the initial inhomogeneity caused by cold-rolling, and thus the different pathways for such inhomogeneous microstructure to evolve, energetically, during heating.

A novel method for estimating subresolution porosity from CT images and its application to homogeneity evaluation of porous media

We propose a new method, i.e., the statistical phase fraction (SPF) method, to estimate the total porosity and spatial distribution of local porosities from subresolution pore-dominated X-ray microtomography images of porous materials. The SPF method assumes that a voxel in a CT image is composed of either a single or a maximum of three pure phases of matter (solid, liquid and air). Gaussian function (GF) fitting is conducted on the basis that the summation of the area of each GF curve is equal to the total area covered by the CT histogram. The volume fraction of each phase corresponding to each GF is calculated based on the mean value of the GF, the area of the GF, and the CT numbers for pure phases. The SPF method is verified on three different types of components containing only air and solid phases, i.e., alumina ceramic and two sintered lunar regolith simulants with relatively homogenous and inhomogeneous microstructures. The estimated porosities of a total of 15 specimens (the total porosity ranges from 0 to 51%) via the SPF method show an average error of 3.11% compared with the ground truth. Spatial distribution of local porosities in the defined representative element volume is investigated for homogeneity evaluation. Results show that the local porosity inhomogeneity in the sintered FJS-1 specimens is more prominent than that in the sintered KLS-1 specimens.

Magnetostriction and magnetocaloric properties of LaFe11.1Mn0.1Co0.7Si1.1 alloy: Direct and indirect measurements

The results of studying the magnetization, magnetostriction, specific heat, and magnetocaloric effect in LaFe11.1Mn0.1Co0.7Si1.1 polycrystalline alloy in pulsed (up to 180 kOe) and cyclic magnetic fields of 12 kOe with a frequency of up to 20 Hz are presented. Direct and indirect methods were used to evaluate the magnetocaloric properties. The dependence of ∆SM on the magnetic field near ТС is described by the power law ∆SM∼H2/3 without signs of saturation up to 180 kOe, reaching the maximum value ∆Smax= 38 J/kg K in a field of 180 kOe. The field dependence of the magnetostriction near ТС tends to saturation (the saturation field is about 120 kOe) a further increase in the field up to 180 kOe changes the magnetostriction by only ∼6%. Direct measurements of ∆Tad in a cyclic magnetic field of 12 kOe show a decrease in the amplitude of the effect by 17% as the frequency of the cyclic magnetic field increases up to 20 Hz. This is most likely due to magnetic and microstructural inhomogeneities, which act as an additional channel for thermal dissipation.

High cycle and very high cycle fatigue properties and microscopic crack growth modeling of Ti‐6.5Al‐3.5Mo‐1.5Zr‐0.3Si titanium alloy at elevated temperatures

AbstractPulsating tension fatigue tests were conducted at 25°C, 150°C, and 250°C to investigate the effect of temperature on high cycle and very high cycle fatigue behavior of titanium alloy. The fatigue strength decreases with the increase of temperature, and the surface failure is more sensitive to temperature than interior failure. Afterwards, the fatigue fracture morphology, microstructure, and texture of titanium alloy were characterized. The results show that the fracture of larger αp phase with the maximum shear stress and higher strain concentration is responsible for the facet formation. The formation of facets leads to the interior failure at different temperatures. Finally, combining with the definition of interior crack growth rate, an assessment approach was proposed to predict the crack growth rate within the inhomogeneous microstructure area (IMA), which takes crack growth acceleration caused by the increase of temperature into consideration.

Comparative study on autogenous diode laser, CO2 laser-MIG hybrid and multi-pass TIG welding of 10-mm thick Inconel 617 superalloy

The present study investigated comparative welding performance of single-pass autogenous diode-laser (DLW) and CO2+ MIG laser-hybrid welding (LHW) processes with multi-pass conventional TIG welding in 10-mm thick Inconel 617 with comprehensive correlation of bead profile, microstructure with mechanical performance of weldments employing FESEM, XRD, high-speed Nano Indentation, uniaxial tensile and impact testing. Results indicated that welds produced by LHW and DLW constituted ‘wine-cup’ and ‘Y” shaped profiles respectively with reduced fusion zone (FZ) and heat-affected-zone (HAZ), high depth-to-width aspect ratio and low distortion as compared to that of ‘bath-tub’ shaped TIG weld on account of low heat input and high-coupling efficiency of laser. Indeed, inhomogeneous microstructures were observed in both LHW and TIG welds due to dual heat sources in LHW (laser + MIG) and multiple passes in TIG as compared to that of DLW, wherein, refined microstructure with fine secondary-dendritic arm spacing and low levels of micro-segregation in interdendritic regions of FZ. XRD confirms presence of higher amounts of M23C6 and M6C carbides in TIG FZ as compared to that of LHW and DLW counterparts and attributed to enhanced micro-segregation due to reduced cooling rate and repetitive thermal cycling effect of multiple passes. Micro and Nano-Indentation hardness profiling indicated that the hardness of FZ of TIG welds is comparatively lower to that of DLW and LHW on account of enhanced segregation and grain coarsening effect. Weld joint efficiency obtained in DLW and LHW are slightly higher than TIG weldments with highest elongation observed in DLW. Overall, joint efficiency of LHW and DLW was found adequate with vast reduction in distortion and improved adoptability for applications in thermal power plant.