Mechanical and Metallurgical Properties of CO2 Laser Beam INCONEL 625 Welded Joints

applied sciences

Article Mechanical and Metallurgical Properties of CO2 Laser Beam INCONEL 625 Welded Joints

Harinadh Vemanaboina 1,* , Edison Gundabattini 2 , Suresh Akella 3, A. C. Uma Maheshwer Rao 3, Ramesh Kumar Buddu 4, Paolo Ferro 5,* and Filippo Berto 6

1 Department of Mechanical Engineering, Sri Venkateswara College of Engineering and Technology (Autonomous), Chittoor 517127, India 2 Department of Thermal and Energy Engineering, School of Mechanical Engineering, Vellore Institute of Technology (VIT), Vellore 632014, India; [email protected] 3 Department of Mechanical Engineering, Sreyas Institute of Engineering & Technology, Hyderabad 500068, India; [email protected] (S.A.); [email protected] (A.C.U.M.R.) 4 Institute for Plasma Research, Gandhinagar 382428, India; [email protected] 5 Department of Engineering and Management, University of Padua, Stradella San Nicola, 36100 Vicenza, Italy 6 Department of Engineering Design and Materials, Norwegian University of Science and Technology, 7491 Trondheim, Norway; ﬁ[email protected] * Correspondence: [email protected] (H.V.); [email protected] (P.F.)

Abstract: In the frame of the circular economy, welding of Ni-based superalloys has gained increasing importance when applied, for instance, to repairing highly expensive components widely used in strategical sectors, such as the defense and aerospace industries. However, correct process parameters avoiding metallurgical defects and premature failures need to be known. To reach this goal, Inconel

625 butt-welded joints were produced by CO2 laser beam welding and different combinations of Citation: Vemanaboina, H.; process parameters. The experimental investigation was carried out with three parameters in two Gundabattini, E.; Akella, S.; Rao, A.C.U.M.; Buddu, R.K.; Ferro, P.; levels with an L4 orthogonal array. Laser power, welding speed, and shielding gas ﬂow rate were Berto, F. Mechanical and varied, and the results were reported in terms of mechanical properties, such as microhardness,

Metallurgical Properties of CO2 Laser tensile strength, distortion, residual stress, and weld bead geometry, and metallurgy. At a lower Beam INCONEL 625 Welded Joints. welding speed of 1 m/min, the full penetration was observed for 3.0 kW and 3.3 kW laser powers. Appl. Sci. 2021, 11, 7002. https:// However, sound welds (porosity-free) were produced with a laser power of 3.3 kW. Overall, the doi.org/10.3390/app11157002 obtained full-penetration specimens showed a tensile strength comparable with that of the parent material with residual stresses and distortions increasing with the increase in heat input. Academic Editors: Ana M. Camacho and Theodore E. Matikas Keywords: laser beam welding; radiography; tensile strength; residual stress; microstructure

Received: 25 May 2021 Accepted: 26 July 2021 Published: 29 July 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Welding is a highly complex process of permanently joining metals that involves heat with regard to jurisdictional claims in source movement, mass exchange, and phase and microstructure transformations that published maps and institutional afﬁl- in turn affect the joint’s mechanical properties [1]. In response to the growing demand iations. for high-performance materials by the chemical, nuclear, fuel, aerospace, marine, and petroleum-based industries that are extremely resistant to a corrosive atmosphere, high- strength nickel-based alloys are mainly used. They combine excellent corrosion resistance with high strength at high temperatures. Nickel-based materials are austenitic superalloys containing about 50% of Ni and different percentages of Fe, Cr, Mo, and Ti (present as trace Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. elements). Nickel offers good thermal and corrosive resistance at elevated temperatures. This article is an open access article The grain size at the fusion zone depends on the alloy’s chemical composition, as well distributed under the terms and as welding parameters such as interaction time, welding speed, and current. The fusion conditions of the Creative Commons welding process promotes a thermo-mechanical variation of the parent metal properties; Attribution (CC BY) license (https:// the high heat input and uncontrolled cooling rates negatively affect the microstructure of creativecommons.org/licenses/by/ the fusion zone (FZ) and heat-affected zone (HAZ), inducing precipitation of undesirable 4.0/). phases, grain coarsening (in HAZ) and tensile residual stresses, among others. Moreover,

Appl. Sci. 2021, 11, 7002. https://doi.org/10.3390/app11157002 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7002 2 of 14

these mentioned changes in the welded joint also have a negative impact on components’ performances when working at elevated temperatures. The gas tungsten arc welding (GTAW) process is widely employed to join high- strength alloys with suitable filler materials with both constant and pulsed current processes [2–4]. As per the earlier research, the microstructures were observed to have satisfac- tory solid solubility with parent and filler metals but a wide fusion zone. Hot cracking is one of the major problems observed during single-pass welding, resulting in unacceptable permanent damage to the structure. As an alternative, this was overcome by employing a multipass welding procedure with the foresight to maintain appropriate inter-pass temperature [5–8]. However, the mechanical and metallurgical properties can be improved by employing advanced welding techniques such as electron beam welding (EBW) and laser beam welding (LBW). Their high-power density allows for reducing the heat input and therefore the FZ and HAZ dimensions together with a higher welding speed. This results in improved metallurgical and mechanical properties and productivity. Moreover, these processes can be easily automated compared to conventional welding techniques. The full-penetration welding is carried out in a key-hole regime, which results in quality welds with a narrow heat-affected zone (HAZ), as mentioned above. Excess penetrations are eliminated, and no further machining process for finishing in LBW welds compared to the arc welding process is needed. Microstructural changes are limited because of the low heat input with faster cooling rates [9–15]. In references [16,17], authors carried out a comparative study using both arc and electron beam welding process applied to 316L stainless steels; they found that metastable phases formed during solidification of fusion zone in EBW joints compared to that obtained with the GTAW process. Roshith et al. [18] focused on the joining of SMO 254 stainless steel by means of autogenous CO2 laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW). They found that for a 5 mm thick plate, the LBW heat input was only 120 J/mm, compared to a value of 3032 J/mm required by the PCGTAW process. Surprisingly, tensile residual stress was observed in the weld region and heat-affected zone of PCGTAW joints, while pure compressive residual stresses were induced in the case of laser-welded joints due to lower heat input value. Full penetration was observed at 120 and 180 J/mm for Inconel 825 and Inconel 625 of 5 mm-thick plates, respectively, with faster cooling rates and reduced grain growth at the fusion zone, compared to that induced by the GTAW process [9,18,19]. Finally, it is worth mentioning that process parameter optimization can also be achieved by numerical simulation [20–29] that, however, cannot disregard experimental tests to be validated. In recent years, Ni-based alloys are growing in their strategical importance given their use in manufacturing and machining industry applications where mechanical and corrosion resistance at high temperatures are needed. However, the effect of welding process parameters on the weldability of Inconel 625 and its structural properties are not yet completely well studied in the literature, in particular when considering the recognized benefits of high-power density welding techniques. This work is aimed at contributing to fill this gap. L4 orthogonal array was used for the CO2 laser beam welding trials of Inconel 625 plates. The microstructural changes and thereby the corresponding mechanical properties of the final welds are analyzed in detail.

2. Materials and Methods 2.1. Parent Material and Edge Preparation In this work, 5 mm-thick Inconel 625 plates were butt-welded. Each plate was 110 mm long and 80 mm wide. The chemical composition of the parent material is summarized in Table1. Care was taken to prepare the samples in order to avoid any type of defect due to contamination of the material. Sample edges were therefore cleaned by a wire electrical discharge machine (WEDM). In particular, the machining was carried out on the lateral and top surfaces to remove oxide layers. Before welding, the perfect positioning of the plates, having a 90◦ square groove without any gap in the welding line, was accurately checked. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 14

wire electrical discharge machine (WEDM). In particular, the machining was carried out Appl. Sci. 2021, 11, 7002 on the lateral and top surfaces to remove oxide layers. Before welding, the perfect3 of 14 positioning of the plates, having a 90° square groove without any gap in the welding line, was accurately checked.

Table 1. Nominal composition of Inconel 625 (wt.%). Table 1. Nominal composition of Inconel 625 (wt.%).

Ni C CMn Mn S S Cu Si Cu Cr Si P Cr P Others Others Fe 5, FeAl 5,0.40, Al 0.40, Bal. 0.1 0.1 0.5 0.5 0.015 0.015 0.5 0.5 0.5 20–23 0.5 0.015 20–23 0.015 Mo 8-10,Mo 8-10,Ti 0.1 Ti 0.1

2.2.2.2. MeltMelt RunRun TrialsTrials andand Butt-WeldingButt-Welding

MeltMelt runrun trialstrials werewere carriedcarried outout onon 55 mm-thickmm-thick InconelInconel plates plates using using a a CO CO22laser laser sourcesource inin orderorder toto analyzeanalyze thethe effectseffects ofof welding welding parameters parameters onon geometrical geometrical characteristicscharacteristics andand microstructuremicrostructure of of thethe bead.bead. ParticularParticular attentionattention waswas paidpaid toto thosethose parametersparameters assuringassuring fullfull penetrationpenetration inin thethe subsequentsubsequent butt-welding op operations.erations. The The previous previous experiment experiment using using a alaser laser power power of of 2.5 2.5 kW kW did did not not show show good good result resultss (Figure (Figure 1a),1 a),and and so it so was it was decided decided to start to startwith witha minimum a minimum power power of 3 of kW, 3 kW, as asshown shown in in Figure Figure 11b.b. L44 orthogonalorthogonal array approachapproach waswas selected for the the experimentation. experimentation. Laser Laser power, power, welding welding speed, speed, and and shielding shielding gas flow gas ﬂowrate were rate were chosen chosen as critical as critical process process para parametersmeters for the for present the present work. work. Laser Laser power power and andwelding welding speed speed are arethe the main main parameters parameters regulating regulating the the heat heat input input during thethe weldingwelding operation.operation. On the other hand,hand, weldmentsweldments responses areare affectedaffected aboveabove allall byby thethe heatheat input.input. FourFour combinationscombinations ofof thethe threethree selectedselected processprocess parametersparameters werewere chosenchosen accordingaccording toto TableTable2 (design2 (design of experimentationof experimentation (DOE)). (DOE)). Argon Argon was employed was employed as shielding as shielding gas (purity gas equal(purity to equal 99.9% to) during 99.9%) welding.during welding. During weldingDuring welding operations, operations, plates were plates clamped were clamped in jaws inin orderjaws in to order assure to the assure absence the ofabsence gaps, asof showngaps, as in shown Figure 2ina. Figure The CO 2a.2 laserThe CO beam2 laser welding beam setupwelding is shown setup is in shown Figure 2inb. Figure 2b.

FigureFigure 1.1. ImagesImages illustratingillustrating InconelInconel 625625 meltmelt runrun trials:trials: ((aa)) 2.52.5 kWkW laserlaser power,power, showingshowing poorpoor penetration,penetration, ((bb)) 33 kWkW laserlaser power,power, inducing inducing good good penetration. penetration.

Table 2. CO2 laser beam welding experimental process parameters. Table 2. CO2 laser beam welding experimental process parameters. Laser Power Welding Speed Shield Gas Flow Rate Heat Input Trial No Laser Power Welding Speed Shield Gas Flow Rate Heat Input Trial No. [kW][kW] [m/min][m/min] [lpm] [J/mm][J/mm] A B C HI A B C HI 1. 3.0 3.0 1.0 1.0 10 180 180 2. 3.0 3.0 1.5 1.5 15 120 120 3. 3.3 3.3 1.0 1.0 15 198 198 4. 3.3 3.3 1.5 1.5 10 132 132 Appl. Sci. 2021, 11, 7002 4 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 14

Figure 2. (a) Clamping conditions; (b) CO2 laser beam welding machine. Figure 2. (a) Clamping conditions; (b) CO2 laser beam welding machine.

TheThe heatheat inputinput ((HIHI,, J/mm),J/mm), i.e., the ratioratio betweenbetween thethe laserlaser powerpower andand thethe weldingwelding speed,speed, isis calculatedcalculated forfor eacheach experimentalexperimental trialtrial byby usingusing EquationEquation (1)(1)

6060𝑊W HI𝐻𝐼= = (1)(1) V 𝑉 wherewhere WW is laser laser power power (kW), (kW), and and VV isis the the welding welding speed speed (m/min). (m/min). In general, In general, it is itwell is wellknown known that the that lower the lower the heat the heatinput, input, the bette the betterr the welded the welded joint jointquality quality in terms in terms of bead of beadmicrostructure microstructure and andmechanical mechanical properties. properties. The The heat heat input input values values resulting resulting from thethe combinationscombinations ofof thethe chosenchosen valuesvalues ofof processprocess parametersparameters areare collectedcollected inin TableTable2 .2.

2.3.2.3. Defects andand MicrostructureMicrostructure InvestigationsInvestigations WeldmentsWeldments werewere carefullycarefully analyzedanalyzed usingusing thethe X-rayX-ray radiographicradiographic techniquetechnique toto checkcheck thethe absenceabsence of of defects defects such such as macroporosity as macroporos andity lack and of penetration.lack of penetration. After non-destructive After non- analysis,destructive the analysis, as-welded the samplesas-welded were samples cut to were produce cut to samples produce for samples metallurgical for metallurgical and me- chanicaland mechanical tests. Optical tests. Optical and scanning and scanning electron electron microscopy microscopy (with EDS)(with were EDS) used were to used carry to outcarry metallurgical out metallurgical investigations investigations on the weldon the bead weld cross-section. bead cross-section. Samples wereSamples prepared were followingprepared thefollowing standard the metallurgical standard metallurgical procedure with procedure polishing with using polishing different using emery different sheets andemery ﬁnal sheets polishing and final using polishing a double-disc using a polisherdouble-disc with polisher Al2O3 slurry.with Al The2O3 slurry. weld zone The weld was investigatedzone was investigated by XRD analysis, by XRD as well.analysis, In particular, as well. theIn particular, X-ray diffraction the X-ray technique diffraction was ◦ ◦ operatedtechnique at was 30 mAoperated and 40 at kV 30 withmA and a 2θ 40angle kV with between a 2θ 20angleand between 90 . 20° and 90°.

2.4.2.4. Welding Distortions,Distortions, ResidualResidual Stresses,Stresses, andand MechanicalMechanical TestsTests AnyAny unwantedunwanted geometricalgeometrical changechange oror departuredeparture fromfrom speciﬁcationsspecifications inin aa fabricatedfabricated structure or component, as a consequence of welding, is called welding distortion. Tem- structure or component, as a consequence of welding, is called welding distortion. perature gradients induced by welding cause dimensional changes in the weldments that Temperature gradients induced by welding cause dimensional changes in the weldments adversely affect the structure assembly. In particular, welding distortions are due to the non- that adversely affect the structure assembly. In particular, welding distortions are due to homogeneous plastic deformation occurring during welding because of the constrained the non-homogeneous plastic deformation occurring during welding because of the alloy thermal expansion and contraction. Before welding, the plates were clamped and constrained alloy thermal expansion and contraction. Before welding, the plates were the constraints were removed after process completion, as shown in Figure2a. The weld clamped and the constraints were removed after process completion, as shown in Figure distortion was measured using the facility schematized in Figure3 (Vernier height gauge). 2a. The weld distortion was measured using the facility schematized in Figure 3 (Vernier In more detail, the distortion is quantiﬁed by the angle α(◦) given by Equation (2): height gauge). In more detail, the distortion is quantified by the angle α(°) given by Equation (2): h − h α = sin−1 1 2 (2) b

where α = distorted angle, h1 = total height at Vernier height, h2 = total height of the workpiece from the surface, and b = length of the plate. Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 14

Appl. Sci. 2021, 11, 7002 5 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 14

Figure 3. Schematic showing the welded joint disposition for the measurement of the distortion via the Vernier height gauge and definition of the distortion angle α(°).

ℎ −ℎ 𝛼 = 𝑠𝑖𝑛 (2) Figure 3. Schematic showing the welded joint𝑏 disposition for the measurement of the distortion via Figure 3. Schematic showing the welded joint disposition for the measurement of the distortion via the Vernier height gauge and definition of the distortion angle α(°). where α = distortedthe Vernier angle, height h1 gauge= total and height deﬁnition at Vernier of the distortion height, angleh2 = totalα(◦). height of the workpiece from the surface, and b = length of the plate. During the joining process, the weld area experiences heating and cooling cycles so During the joining process, the weld area experiencesℎ −ℎ heating and cooling cycles so that differential thermal expansion and contraction𝛼 of the = 𝑠𝑖𝑛 weld metal and parent material (2) that differential thermal expansion and contraction of𝑏 the weld metal and parent material cause weldingcause residual welding stresses residual (RS). In stresses this wo (RS).rk, RS In is this measured work, RS in is a measuredtransverse in direction a transverse direction (perpendicular(perpendicularwhere to the αweld = distorted line) to theat variousangle, weld line)h distances1 = attotal various heightfrom distances the at weldVernier from line height, on the both weld h2 the= line total top on height both the of topthe and bottom surfacesandworkpiece bottom of the from surfaces weldment. the surface, of the Before weldment. and measuring, b = length Before theof measuring,the sample plate. surface the sample was cleaned surface was cleaned with emery sheetswith ofDuring emery 20 mm sheetsthe × 20 joining mm. of 20 Theprocess, mm Pulstec× 20the μ mm.weld-X360n Thearea Portable Pulstecexperiences X-rayµ-X360n Residualheating Portable and Stress cooling X-ray Residualcycles so analyzer, usedStressthat for differentialmeasurements, analyzer, thermal used is for shown expansion measurements, in Figure and contra 4a. is The shownction cos( of inα the) Figuremeasuring weld4a. metal The method and cos( parent α) measuring material with a spot sizemethodcause of 2 weldingmm with is adopted; a residual spot size the stresses of camera 2 mm (RS). is image adopted; In this is wodisplayed therk, camera RS is in measured Figure image is4b. in displayed Samplea transverse in Figure direction4b. ◦ measuring parametersSample(perpendicular measuring are the todiffraction the parameters weld angleline) are at(=150.876 thevarious diffractiono ),distances interplanar angle from (=150.876 spacing the weld (d), = interplanar line1.077 on both spacing the top Å), X-ray wavelength(andd = 1.077bottom (Cr) Å), surfacesK-alpha X-ray wavelength (=2.29093of the weldment. Å) (Cr) and K-alpha K-BetaBefore (=2.29093 (=2.08480measuring, Å)Å). the and sample K-Beta surface (=2.08480 was Å). cleaned with emery sheets of 20 mm × 20 mm. The Pulstec μ-X360n Portable X-ray Residual Stress analyzer, used for measurements, is shown in Figure 4a. The cos(α) measuring method with a spot size of 2 mm is adopted; the camera image is displayed in Figure 4b. Sample measuring parameters are the diffraction angle (=150.876o), interplanar spacing (d = 1.077 Å), X-ray wavelength (Cr) K-alpha (=2.29093 Å) and K-Beta (=2.08480 Å).

FigureFigure 4. Residual 4. Residual stress stress measurement: measurement: (a) residual (a) residual stress stress equipment, equipment, (b) camera (b) camera image image for sample for sample location. location. The Vickers microhardness test was performed on the weldments following the ASTM The VickersE-384-16 microhardness Standards. test In particular, was performed a 1 kg loadon the was weldments employed, following while the indentationthe dwell ASTM E-384-16time Standards. was about In particular, 8 s. Microhardness a 1 kg load proﬁles was employed, were obtained while for the the indentation weld bead cross-section dwell time wasnearFigure about the 4. 8 root,Residual s. Microhardness middle, stress andmeasurement: cap profiles bead were region.(a) residual obtained In morestress for detail,equipment, the weld the proﬁles (beadb) camera cross- were image obtained for sample at a section near thedistancelocation. root, ofmiddle, 0.8, 2.5, and and cap 4.2 mmbead from region. the beadIn more root. detail, Tensile the tests profiles were performed were according obtained at a todistance ASTM E8-15aof 0.8, 2.5, Standards. and 4.2 Cutting mm from of samplesthe bead from root. the Tensile welded tests plates were was carried out usingThe the wire-cutVickers microhardness electrical discharge test machinewas performed (EDM). on the weldments following the ASTM E-384-16 Standards. In particular, a 1 kg load was employed, while the indentation dwell time was about 8 s. Microhardness profiles were obtained for the weld bead cross- section near the root, middle, and cap bead region. In more detail, the profiles were obtained at a distance of 0.8, 2.5, and 4.2 mm from the bead root. Tensile tests were Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 14

Appl. Sci. 2021, 11, 7002 6 of 14 performed according to ASTM E8-15a Standards. Cutting of samples from the welded plates was carried out using the wire-cut electrical discharge machine (EDM).

3.3. ResultsResults andand DiscussionDiscussion 3.1.3.1. DefectsDefects andand MetallurgyMetallurgy TheThe weldmentsweldments werewere foundfound byby visualvisual inspectioninspection freefree fromfrom surfacesurface defects.defects. NDTNDT X-X- rayray radiographyradiography teststests detecteddetected defectsdefects suchsuch asas macroporositymacroporosity inin thethe fusionfusion zonezone (FZ)(FZ) oror incompleteincomplete penetrationpenetration (Figure(Figure5 5).). ASME ASME SEC SEC IX-2017 IX-2017 standard standard was was used used for for the the quality quality testingtesting ofof thethe weldments.weldments. ItIt waswas observedobserved thatthat allall weldmentsweldments hadhad straightstraight whitewhite lineslines indicatingindicating acceptableacceptable weldments.weldments. UnderﬁllUnderfill andand somesome porositiesporosities werewere identiﬁedidentified inin trialtrial 1,1, whilewhile incompleteincomplete penetrationpenetration waswas observedobserved inin trialtrial 22 andand trialtrial 4;4; moreover,moreover, toto betterbetter verifyverify thethe weld’sweld’s internal internal quality, quality, macrographs macrographs of theof the weld weld bead bead cross-section cross-section were were obtained obtained from allfrom trials all andtrials deeply and deeply exanimated. exanimated.

FigureFigure 5.5. WeldmentsWeldments radiographicradiographic imagesimages (A(A––DD).).

WeldingWelding ofof high-strengthhigh-strength alloysalloys isis alwaysalways challengingchallenging inin weldingwelding researchresearch aimedaimed atat achievingachieving goodgood metallurgicalmetallurgical andand mechanicalmechanical propertiesproperties byby reducingreducing thethe heatheat input.input. TheThe observedobserved macrostructuremacrostructure asas aa functionfunction ofof processprocess parametersparameters isis shownshown inin FigureFigure6 .6. Weld Weld poolpool dimensionsdimensions werewere carefullycarefully measured.measured. FullFull penetrationpenetration atat thethe highesthighest heatheat inputsinputs valuesvalues of 180 180 and and 198 198 J/mm J/mm was was observed. observed. In trial In trial1, in-line 1, in-line gas porosity gas porosity was found was (Figure found (Figure7), whereas7), whereas trial 3 trialreached 3 reached full penetration full penetration without without detectable detectable porosity. porosity. The Theweld weld pool poolwidth width at the at thecap capis wider is wider and and decreases decreases at atthe the root. root. Table Table 33 summarizessummarizes thethe main main geometricalgeometrical parameters of of the the FZ. FZ. It It is is observed observed that that the the weld weld pool’s pool’s Y-shape Y-shape is due is due to the to thelaser laser beam beam diffraction diffraction promoted promoted by by the the plasma plasma formed formed at at the the top top of of thethe plate,plate, calledcalled ‘plume’.‘plume’. It It is is worth worth noting noting that that the the depth depth to width to width ratio ratio is less is lessthan than2 (trials 2 (trials 1 and 13), and which 3), whichis much is muchlower lowerthan that than reached that reached by arc by welding arc welding processes processes (often (often greater greater than than2). A 2). very A verynarrow narrow heat-affected heat-affected zone zone characterizes characterizes all alljoints joints (Figures (Figures 7c7 cand and 88c),c), whilewhile the the FZ FZ microstructuremicrostructure showsshows thethe typicaltypical dendriticdendritic microstructuremicrostructure growngrown inin thethe heatheat dissipationdissipation directiondirection (Figures(Figures7 7bb and and8 b).8b). Moreover, Moreover, the the typical typical epitaxial epitaxial grain grain growth growth is is observed observed at at thethe FZ/HAZFZ/HAZ interface, as illustrated in Figures 7 7cc andand8 c.8c.

Table 3. Weld pool geometry as a function of heat input.

Weld Pool Weld Pool Width Heat Input Weld Pool Width (Cap) Trial No. Height (Root) Penetration [J/mm] [mm] [mm] [mm] 1 180 3.3 5.5 1.1 Full 2 120 2.6 4.3 1.2 Partial 3 198 3.5 5.4 1.4 Full 4 132 2.3 3.9 0.9 Partial Appl.Appl. Sci.Sci.2021 2021,,11 11,, 7002x FOR PEER REVIEW 77 of 1414 Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 14

FigureFigure 6.6. MacrographsMacrographs ofof InconelInconel 625625 laserlaser beambeam weldedwelded jointsjoints as a functionfunction of process parametersparameters Figure 6. Macrographs of Inconel 625 laser beam welded joints as a function of process parameters (laser(laser powerpower andand weldingwelding speed).speed). (laser power and welding speed).

Figure 7. (a) Macrograph of Inconel 625 laser beam welded joint (weld 1) showing gas porosity on Figure 7. (a) Macrograph of Inconel 625 laser beam welded joint (weld 1) showing gas porosity on FigureFZ and 7. optical(a) Macrograph microscope of images Inconel of 625 the laser (b) weld beam cap welded and (c joint) HAZ. (weld 1) showing gas porosity on FZ and optical microscope images of the (b) weld cap and (c) HAZ. FZ and optical microscope images of the (b) weld cap and (c) HAZ. No solidification or liquation cracking at the FZ/HAZ interface occurred in the No solidification or liquation cracking at the FZ/HAZ interface occurred in the present Inconel 625Table butt-welded 3. Weld pool joints. geometry This is as a functionconsequence of heat of input. the lower heat input of present Inconel 625 butt-welded joints. This is a consequence of the lower heat input of the CO2 laser beam process compared to arc welding processes where, in contrast, such the CO2 laser beam processWeld compared Pool to arcWeld welding Pool processesWeld where, Pool in contrast, such defectsTrial wereHeat detected, Input as reported by Harinadh [3], Ramkumar [26,30], Venkata Ramana defects were detected, asWidth reported (Cap) by HarinaHeightdh [3], RamkumarWidth [26,30], (Root) VenkataPenetration Ramana [31]No. and Manikandan[J/mm] [32]. The microstructure is refined at the FZ/HAZ interface and [31] and Manikandan [32]. [mm]The microstructure[mm] is refined at the[mm] FZ/HAZ interface and became coarse, moving toward the weld centerline as a consequence of the competitive became1 coarse, 180 moving toward 3.3 the weld centerline 5.5 as a consequence 1.1 of the competitive Full grain growth phenomenon, so that favorably oriented (FO) grains, whose preferred grain2 growth 120 phenomenon, 2.6so that favorably 4.3 oriented (FO) grains, 1.2 whose Partialpreferred growth angle relative to the temperature gradient direction is small, grow preferentially growth3 angle 198relative to the temperature 3.5 gradie 5.4nt direction is small, 1.4 grow preferentially Full relative4 to unfavorably 132 oriented 2.3 (UO) grains, which 3.9 are characterized 0.9 by a larger Partialpreferred relative to unfavorably oriented (UO) grains, which are characterized by a larger preferred growth angle. The XRD analysis as shown in Figures 9 and 10 revealed the precipitate of growth angle. The XRD analysis as shown in Figures 9 and 10 revealed the precipitate of Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 14

secondary phases in the fusion zone. The precipitates observed are Cr2Ni3, Mn3Si, Al2FeSi, NbNi3 and Cr2Ni3, Fe7NI3, Al2FeSi, NbNi3 in weld 1 and weld 3, respectively. It is evident from the XRD analysis that Ni was enriched in the weld zone [12,24]. The major element in Inconel 625 is the weld bead, indicating intense peaks at 44.78°, 52.7°, and 76.35°. Cr Appl. Sci. 2021, 11, 7002 and Ni are major constituents, and the precipitates of Cr and Ni were8 of observed 14 both in weld 1 and weld 3.

FigureFigure 8. 8.(a )( Macrographa) Macrograph of Inconel of Inconel 625 laser 625 beam laser welded beam joint welded (weld 3) joint showing (weld a defect-free 3) showing weld a defect-free weld withwith full full penetration penetration and optical and optical microscope microscope images of images the (b) weld of the cap, ( (bc) HAZ,weld andcap, (d ()c weld) HAZ, root. and (d) weld root. No solidiﬁcation or liquation cracking at the FZ/HAZ interface occurred in the present Inconel 625 butt-welded joints. This is a consequence of the lower heat input of the CO2 laser beam process compared to arc welding processes where, in contrast, such defects were detected, as reported by Harinadh [3], Ramkumar [26,30], Venkata Ramana [31] and Manikandan [32]. The microstructure is reﬁned at the FZ/HAZ interface and became coarse, moving toward the weld centerline as a consequence of the competitive grain growth phenomenon, so that favorably oriented (FO) grains, whose preferred growth angle relative to the temperature gradient direction is small, grow preferentially relative to unfavorably oriented (UO) grains, which are characterized by a larger preferred growth angle. The XRD analysis as shown in Figures9 and 10 revealed the precipitate of secondary phases in the fusion zone. The precipitates observed are Cr2Ni3, Mn3Si, Al2FeSi, NbNi3 and Cr2Ni3, Fe7NI3, Al2FeSi, NbNi3 in weld 1 and weld 3, respectively. It is evident from the XRD analysis that Ni was enriched in the weld zone [12,24]. The major element in Inconel 625 is the weld bead, indicating intense peaks at 44.78◦, 52.7◦, and 76.35◦. Cr and Ni are major constituents, and the precipitates of Cr and Ni were observed both in weld 1 and weld 3.

Figure 9. Back scattered SEM micrograph and EDS spectrum of the particles marked by arrows on sample 3. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 14

secondary phases in the fusion zone. The precipitates observed are Cr2Ni3, Mn3Si, Al2FeSi, NbNi3 and Cr2Ni3, Fe7NI3, Al2FeSi, NbNi3 in weld 1 and weld 3, respectively. It is evident from the XRD analysis that Ni was enriched in the weld zone [12,24]. The major element in Inconel 625 is the weld bead, indicating intense peaks at 44.78°, 52.7°, and 76.35°. Cr and Ni are major constituents, and the precipitates of Cr and Ni were observed both in weld 1 and weld 3.

Appl. Sci. 2021, 11, 7002 9 of 14 Figure 8. (a) Macrograph of Inconel 625 laser beam welded joint (weld 3) showing a defect-free weld with full penetration and optical microscope images of the (b) weld cap, (c) HAZ, and (d) weld root.

Appl. Sci. 2021, 11, x FOR PEER REVIEWFigure 9. Back scattered SEM micrograph and EDS spectrum of the particles marked by arrows9 of on14 Figure 9. Back scattered SEM micrograph and EDS spectrum of the particles marked by arrows on samplesample 3. 3.

Figure 10. XRD analysis on FZ of CO2 laser welded joints as a function of heat input (due to the use ofFigure Cr radiation, 10. XRD peaksanalysis of theon FZ Ni-based of CO2 solidlaser solutionwelded joints are not aseasily a function detectable). of heat input (due to the use of Cr radiation, peaks of the Ni-based solid solution are not easily detectable). 3.2. Microhardness Profiles 3.2. MicrohardnessMicrohardness Profiles profiles covering the FZ, HAZ, and parent metal in cross-sections of weldsMicrohardness 1 and 3 are shown profiles in Figure covering 11. Firstly, the FZ, we HAZ, observed and aparent slight increasemetal in in cross-sections microhardness of bywelds approaching 1 and 3 the are fusion shown zone in attributed Figure to11. the Firstly, finer microstructurewe observed ina thatslight zone increase compared in tomicrohardness parent metal by [17 approaching,25,26]. It was the not fusion possible zone to attributed highlight to the the HAZ finer via microstructure microhardness in profilesthat zone because compared of its veryto parent narrow metal size. [17,25,26]. In more detail, It was for not weld possible 1, the microhardnessto highlight the values HAZ atvia the microhardness fusion zone in profiles the cap, becaus middle,e of and its very root zonenarrow are size. 255, In 263, more and detail, 270 HV, for respectively, weld 1, the whereas,microhardness in weld values 3, the at values the fusion are 255 zone HV atin thethe capcap, and middle, 270 HV and at root middle zone and are root 255, zone. 263, Finally,and 270 the HV, average respectively, microhardness whereas, value in weld at the 3, fusionthe values zone are was 255 262 HV HV. at the cap and 270 HV at middle and root zone. Finally, the average microhardness value at the fusion zone was 262 HV.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 14

Figure 10. XRD analysis on FZ of CO2 laser welded joints as a function of heat input (due to the use of Cr radiation, peaks of the Ni-based solid solution are not easily detectable).

3.2. Microhardness Profiles Microhardness profiles covering the FZ, HAZ, and parent metal in cross-sections of welds 1 and 3 are shown in Figure 11. Firstly, we observed a slight increase in microhardness by approaching the fusion zone attributed to the finer microstructure in that zone compared to parent metal [17,25,26]. It was not possible to highlight the HAZ via microhardness profiles because of its very narrow size. In more detail, for weld 1, the microhardness values at the fusion zone in the cap, middle, and root zone are 255, 263,

Appl. Sci. 2021, 11, 7002 and 270 HV, respectively, whereas, in weld 3, the values are 255 HV at the cap and10 of270 14 HV at middle and root zone. Finally, the average microhardness value at the fusion zone was 262 HV.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14

Figure 11. Microhardness profiles proﬁles on joint cross-section; (a)) trialtrial 1,1, ((b)) trialtrial 3.3. 3.3. Tensile Strength 3.3. Tensile Strength Tensile tests were carried out at room temperature using specimens taken from weld 1 Tensile tests were carried out at room temperature using specimens taken from weld and weld 3. The ultimate tensile strength (UTS) is found to be 811 and 853 MPa for weld 1 1 and weld 3. The ultimate tensile strength (UTS) is found to be 811 and 853 MPa for weld and weld 3, respectively, while the yield stresses (YS) are 551 and 559 MPa, respectively 1 and weld 3, respectively, while the yield stresses (YS) are 551 and 559 MPa, respectively (Figure 12a). Failures are located close to the weld line, as shown in Figure 12b. Details (Figure 12a). Failures are located close to the weld line, as shown in Fig. 12b. Details about about crack initiation and propagation will be presented in a future work. Since the UTS crack initiation and propagation will be presented in a future work. Since the UTS and YS and YS of the parent material are about 827 and 414 MPa, respectively [27], good mechan- of the parent material are about 827 and 414 MPa, respectively [27], good mechanical ical properties were found, proving the excellent laser beam weldability of Inconel 625, propertiesprovided that were optimal found, process proving parameters the excellent are used. laser Thebeam obtained weldability results of are Inconel supported 625, providedby Janicki’s that work optimal [33], proces who founds parameters that a properare used. selection The obtained of laser results welding are parameters supported byallows Janicki’s for obtaining work [33], the who yield found strength that and a ultimateproper selection tensile strength of laser of welding the joints parameters essentially allowsequivalent for toobtaining those of thethe baseyield material. strength and ultimate tensile strength of the joints essentially equivalent to those of the base material.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14

Figure 11. Microhardness profiles on joint cross-section; (a) trial 1, (b) trial 3.

3.3. Tensile Strength Tensile tests were carried out at room temperature using specimens taken from weld 1 and weld 3. The ultimate tensile strength (UTS) is found to be 811 and 853 MPa for weld 1 and weld 3, respectively, while the yield stresses (YS) are 551 and 559 MPa, respectively (Figure 12a). Failures are located close to the weld line, as shown in Fig. 12b. Details about crack initiation and propagation will be presented in a future work. Since the UTS and YS of the parent material are about 827 and 414 MPa, respectively [27], good mechanical properties were found, proving the excellent laser beam weldability of Inconel 625, provided that optimal process parameters are used. The obtained results are supported Appl. Sci. 2021, 11, 7002 by Janicki’s work [33], who found that a proper selection of laser welding parameters11 of 14 allows for obtaining the yield strength and ultimate tensile strength of the joints essentially equivalent to those of the base material.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 14

Figure 12. Tensile tests results: (a) UTS and YS, (b) failure location. Figure 12. Tensile tests results: (a) UTS and YS, (b) failure location. 3.4. Distortion and Residual Stresses 3.4. DistortionThe Vernier and heightResidual gauge Stresses was used to measure the distortion in the laser weldments, definedThe asVernier the difference height gauge between was the used heights to measur h1 ande the h2, distortion as schematized in the in laserFigure weldments,3 . Distor- definedtion was as measured the difference over the between top surface the afterheights the h weldment1 and h2, attainedas schematized the ambient in Figure tempera- 3. Distortionture. As expected, was measured the higher over the the heat top input, surfac thee after higher the the weldment weld distortion. attained Weld the ambient 1, with a ◦ temperature.heat input of As 180 expected, J/mm, shows the higher a distortion the heat of aboutinput,3.57 the higher, whereas the weldweld 3,distortion. obtained withWeld a ◦ 1,heat with input a heat of 198input J/mm, of 180 is foundJ/mm, toshows have aa distortiondistortion ofof 4.45about. Therefore, 3.57°, whereas an increase weld 3, of ◦ obtainedabout 1 withis observed a heat input when of moving 198 J/mm, from is 180 foun tod 198 to J/mmhave a heat distortion input. of 4.45°. Therefore, an increaseThe residual of about stresses 1° is observed were measured when moving on the topfrom and 180 bottom to 198 surfaces J/mm heat of theinput. weldment usingThe XRD residual techniques. stresses The were measurements measured on th weree top carried and bottom out in surfaces the transverse of the weldment direction usingand were XRD focused techniques. on the The stress measurements component perpendicularwere carried out to thein the weld transverse line, since direction this is of andgreater were interest focused when on the considering stress component the fatigue perpendicular strength of weldedto the weld joints line, [32 since,33]. Figurethis is of13 greatershows interest the residual when stress considering distribution the fatigue measured strength on the of welded top and joints bottom [32,33]. surfaces Figure of the13 showsfinal weldments. the residual Tensile stress distribution residual stresses meas [ured34–40 on] were the top found and throughout bottom surfaces the plate. of the M- finalshaped weldments. stress distribution Tensile residual profiles stresses are observed [34–40] for were weld found 1 and throughout weld 3 according the plate. to M- the shapedliterature stress [33]. distribution The highest residualprofiles stressare obse (158rved MPa) for was weld measured 1 and weld on joint 3 according 3 that, compared to the literatureto weld 1, [33]. was characterizedThe highest byresidual higher heatstress input. (158 Moreover,MPa) was for measured each sample, on thejoint maximum 3 that, comparedresidual stresses to weld were1, was measured characterized on the by bottom higher surface.heat input. While Moreover, the maximum for each sample, residual thestresses maximum were measuredresidual instresses the HAZ were of jointmeas 1ured (116 on MPa), the inbottom joint 3 thesurface. highest While residual the stress arose in FZ, showing a reverse M-shaped pattern. This result is quite surprising maximum residual stresses were measured in the HAZ of joint 1 (116 MPa), in joint 3 the and suggests that residuals stresses are highly sensitive to welding process parameters highest residual stress arose in FZ, showing a reverse M-shaped pattern. This result is quite surprising and suggests that residuals stresses are highly sensitive to welding process parameters and boundary conditions. If the residual stresses on the top surface of joint 1 and joint 2 are compared, tensile (57 MPa) versus compressive (−41 MPa) stresses are observed in FZ [31,34]. Appl. Sci. 2021, 11, 7002 12 of 14

and boundary conditions. If the residual stresses on the top surface of joint 1 and joint 2 Appl. Sci. 2021, 11, x FOR PEER REVIEWare compared, tensile (57 MPa) versus compressive (−41 MPa) stresses are observed12 of 14 in

FZ [31,34].

FigureFigure 13. Residual 13. Residual stress stress distribution distribution of the of top the andtop bottomand bottom surface surface of the of ﬁnal the final welds: welds: (a) joint (a) joint 1 and 1 (andb) joint (b) joint 3. 3.

4.4. Conclusions Conclusions LaserLaser beam beam welding welding of of Inconel Inconel 625 625 was was investigated. investigated. Melt Melt run run trials trials were were first ﬁrst carried carried outout to to optimize optimize the the process process parameters. parameters. Almo Almostst defect-free defect-free specimens specimens were were then then deeply deeply analyzedanalyzed via via metallurgical metallurgical investigation, investigation, residual residual stress stress and and deformation deformation measurements, measurements, andand mechanical mechanical tests. tests. The The main main outcomes outcomes can can be be summarized summarized as as follows follows 1.1. FullFull penetration penetration of 5 of mm 5 mm thick thick Inconel Inconel 625 plates 625 plates butt joint butt was joint obtained was obtained using a using laser a powerlaser powerof both of 3 both and 33.3 and kW 3.3 kWand anda welding a welding speed speed of of1.0 1.0 m/min. m/min. These These values values correspondcorrespond to toa heat a heat input input of 180 of and 180 198 and J/mm, 198 J/mm, respectively, respectively, which whichare much are lower much comparedlower compared to those to used those in used conventional in conventional fusion fusion welding welding processes processes for for the the same same materialmaterial and and geometry. geometry. 2.2. MetallurgicalMetallurgical investigation investigation showed showed a atypical typical epitaxial epitaxial growth growth in in the the fusion fusion zone zone with with eventually some porosity and an extremely narrow heat-affected zone. eventually some porosity and an extremely narrow heat-affected zone. 3. The microhardness values in the weld bead of all specimens were found slightly 3. The microhardness values in the weld bead of all specimens were found slightly greater than those measured on parent metal; this effect was attributed to the fusion greater than those measured on parent metal; this effect was attributed to the fusion zone ﬁner microstructure compared to that of the parent metal. zone finer microstructure compared to that of the parent metal. 4. Despite the presence of some porosity in the fusion zone, the tensile strength of the 4. Despite the presence of some porosity in the fusion zone, the tensile strength of the joints was comparable to that of the parent metal, indicating excellent laser weldability joints was comparable to that of the parent metal, indicating excellent laser of such kind of alloy, weldability of such kind of alloy, 5. The higher the heat input, the higher the residual stress and distortion. 5. The higher the heat input, the higher the residual stress and distortion. 6. The results obtained in this paper will be used, in future work, for calibrating a laser 6. The results obtained in this paper will be used, in future work, for calibrating a laser beam welding process numerical model. beam welding process numerical model.

Author Contributions: Conceptualization, H.V., R.K.B. and E.G.; methodology, H.V., S.A. and Author Contributions: Conceptualization, H.V., R.K.B. and E.G.; methodology, H.V., S.A. and R.K.B.;R.K.B.; investigation, investigation, H.V., H.V., E.G. E.G. and and A.C.U.M.R.; A.C.U.M.R.; resources, resources, H.V. H.V. and and E.G.; E.G.; writing—original writing—original draft draft preparation,preparation, H.V., H.V., A.C.U.M.R. A.C.U.M.R. and and P.F.; P.F.; writing—review writing—review and and editing, editing, H.V., H.V., P.F. P.F. and and F.B. F.B. All All authors authors havehave read read and and agreed agreed to to the the pub publishedlished version version of of the the manuscript. manuscript. Funding:Funding: ThisThis research research received received no no external external funding. funding. InstitutionalInstitutional Review Review Board Board Statement: Statement: NotNot applicable. applicable. InformedInformed Consent Consent Statement: Statement: NotNot applicable. applicable. DataData Availability Availability Statement: Statement: DataData isis contained contained within within the the article. article. Acknowledgments:Acknowledgments: TheThe authors authors are are gratefully gratefully acknowledged acknowledged for for the the provision provision of of equipment equipment from from thethe Science Science and and Engineering Engineering Resear Researchch Board, Board, Department Department of ofScie Sciencence and and Technology, Technology, New New Delhi, Delhi, under the Young Scientist Scheme (Ref. No. SB/FTP/ETA-0190/2014). The authors are also thankful to the management of the Vellore Institute of Technology (VIT), Vellore for enabling us to conduct Residual stress measurements using Pulstec Residual stress analyzer test in Advanced Material Processing and Testing Lab. Appl. Sci. 2021, 11, 7002 13 of 14

under the Young Scientist Scheme (Ref. No. SB/FTP/ETA-0190/2014). The authors are also thankful to the management of the Vellore Institute of Technology (VIT), Vellore for enabling us to conduct Residual stress measurements using Pulstec Residual stress analyzer test in Advanced Material Processing and Testing Lab. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Kuo, T.Y.; Jeng, S.L. Porosity reduction in Nd–YAG laser welding of stainless steel and inconel alloy by using a pulsed wave. J. Phys. D Appl. Phys. 2005, 38, 722–728. [CrossRef] 2. Yelamasetti, B.; Rajyalakshmi, G.; Venkat Ramana, G.; Sridhar Babu, B.; Vemanaboina, H. Comparison of metallurgical and mechanical properties of dissimilar joint of AISI 316 and Monel 400 developed by pulsed and constant current gas tungsten arc welding processes. Int. J. Adv. Manuf. Technol. 2020, 108, 2633–2644. [CrossRef] 3. Vemanaboina, H.; Edison, G.; Akella, S. Weld bead temperature and residual stresses evaluations in multipass dissimilar INCONEL625 and SS316L by GTAW using IR thermography and x-ray diffraction techniques. Mater. Res. Express 2019, 6, 0965a9. [CrossRef] 4. Sharma, S.; Taiwade, R.V.; Vashishtha, H. Effect of Continuous and Pulsed Current Gas Tungsten Arc Welding on Dissimilar Weldments Between Hastelloy C-276/AISI 321 Austenitic Stainless Steel. J. Mater. Eng. Perform. 2017, 26, 1146–1157. [CrossRef] 5. Korrapati, P.K.; Avasarala, V.K.; Bhushan, M.; Ramkumar, K.D.; Arivazhagan, N.; Narayanan, S. Assessment of Mechanical Properties of PCGTA Weldments of Inconel 625. Procedia Eng. 2014, 75, 9–13. [CrossRef] 6. Buddu, R.K.; Chauhan, N.S.; Raole, P. Mechanical properties and microstructural investigations of TIG welded 40 mm and 60 mm thick SS 316L samples for fusion reactor vacuum vessel applications. Fusion Eng. Des. 2014, 89, 3149–3158. [CrossRef] 7. Vemanaboina, H.; Chandh, R.G.; SivaKrishna, P.; Kumar, A.K.; Karjuna, K.M.; Reddy, Y.S. A X-ray Diffraction Study of Residual Stresses Due to Multipass Welding of INCONEL600. Adv. Lightweight Mater. Struct. 2020, 561–567. [CrossRef] 8. Prabu, S.S.; Ramkumar, K.D.; Arivazhagan, N. Microstructural evolution and precipitation behavior in heat affected zone of Inconel 625 and AISI 904L dissimilar welds. IOP Conf. Ser. Mater. Sci. Eng. 2017, 263, 062073. [CrossRef] 9. Arivarasu, M.; Roshith, P.; Padmanaban, R.; Thirumalini, S.; Prabhakar, K.V.P.; Padmanabham, G. Investigations on metallurgical and mechanical properties of CO2 laser beam welded Alloy 825. Can. Met. Q. 2017, 56, 232–244. [CrossRef] 10. Sokolov, M.; Salminen, A. Improving Laser Beam Welding Efﬁciency. Engineering 2014, 06, 559–571. [CrossRef] 11. Oyyaravelu, R.; Kuppan, P.; Arivazhagan, N. Metallurgical and mechanical properties of laser welded high strength low alloy steel. J. Adv. Res. 2016, 7, 463–472. [CrossRef] 12. Manikandan, M.; Hari, P.; Vishnu, G.; Arivarasu, M.; Ramkumar, K.D.; Arivazhagan, N.; Rao, M.N.; Reddy, G. Investigation of Microstructure and Mechanical Properties of Super Alloy C-276 by Continuous Nd: YAG Laser Welding. Procedia Mater. Sci. 2014, 5, 2233–2241. [CrossRef] 13. Vemanaboina, H.; Naidu, G.G.; Kumar, G.V.; Reddy, D.R. Welding characteristics of butt-welded Inconel625 plate using CO2 laser beam. Mater. Today Proc. 2019, 19, 859–863. [CrossRef] 14. Gordine, J. Some problems in welding Inconel 718. Weld. J. 1971, 50, 480. 15. Pan, Y.-M.; Dunn, D.S.; Cragnolino, G.A.; Sridhar, N. Grain-boundary chemistry and intergranular corrosion in alloy 825. Met. Mater. Trans. A 2000, 31, 1163–1173. [CrossRef] 16. Joseph, B.; Katherasan, D.; Sathiya, P.; Murthy, C.V.S. Weld metal characterization of 316L(N) austenitic stainless steel by electron beam welding process. Int. J. Eng. Sci. Technol. 2018, 4, 169–176. [CrossRef] 17. Soltani, H.M.; Tayebi, M. Comparative study of AISI 304L to AISI 316L stainless steels joints by TIG and Nd:YAG laser welding. J. Alloys Compd. 2018, 767, 112–121. [CrossRef] 18. Roshith, P.; Arivarasu, M.; Arivazhagan, N.; Srinivasan, A.; Phani Prabhakar, K.V. Investigations on induced residual stresses, mechanical and metallurgical properties of CO2 laser beam and pulse current gas tungsten arc welded SMO 254. J. Manuf. Process. 2019, 44, 81–90. 19. Buddu, R.K.; Chauhan, N.L.; Raole, P.M. Investigations of Microstructure and Mechanical Properties of 60-mm-Thick Type 316L Stainless Steel Welded Plates by Multipass Tungsten Inert Gas Welding and Electron Beam Welding for Fusion Reactor Applications. Fusion Sci. Technol. 2014, 65, 248–254. [CrossRef] 20. Campagnolo, A.; Ferro, P.; Romanin, L.; Meneghetti, G. Residual Notch Stress Intensity Factors in Welded Joints Evaluated by 3D Numerical Simulations of Arc Welding Processes. Materials 2021, 14, 812. [CrossRef][PubMed] 21. Vemanaboina, H.; Edison, G.; Akella, S.; Buddu, R.K. Thermal Analysis Simulation for Laser Butt Welding of Inconel625 Using FEA. Int. J. Eng. Technol. 2018, 7, 85–89. [CrossRef] 22. Romanin, L.; Ferro, P.; Bonollo, F.; Berto, F. A Numerical and Experimental Analysis of Inconel 625 Electron-Beam Welding— Thermal Aspects. Procedia Struct. Integr. 2019, 18, 63–74. [CrossRef] 23. Ahmad, G.N.; Raza, M.S.; Singh, N.; Kumar, H. Experimental investigation on Ytterbium ﬁber laser butt welding of Inconel 625 and Duplex stainless steel 2205 thin sheets. Opt. Laser Technol. 2020, 126, 106117. [CrossRef] 24. Devendranath Ramkumar, K.; Sridhar, R.; Periwal, S.; Oza, S.; Saxena, V.; Hidad, P.; Arivazhagan, N. Investigations on the structure—Property relationships of electron beam welded Inconel 625 and UNS 32205. Mater. Des. 2015, 68, 158–166. [CrossRef] Appl. Sci. 2021, 11, 7002 14 of 14

25. Katayama, S.; Kawahito, Y.; Mizutani, M. Elucidation of laser welding phenomena and factors affecting weld penetration and welding defects. Phys. Procedia 2010, 5, 9–17. [CrossRef] 26. Arivarasu, M.; Devendranath Ramkumar, K.; Arivazhagan, N. Effect of continuous and pulsed current on the metallurgical and mechanical properties of gas tungsten arc welded AISI 4340 aeronautical and AISI 304 L austenitic stainless steel dissimilar joints. Mater. Res. 2015, 18, 59–77. [CrossRef] 27. Inconel 625. Available online: https://static.markforged.com/downloads/Inconel-625.pdf (accessed on 20 May 2021). 28. Ferro, P.; Berto, F. Quantification of the Influence of Residual Stresses on Fatigue Strength of Al-Alloy Welded Joints by Means of the Local Strain Energy Density Approach. Strength Mater. 2016, 48, 426–436. [CrossRef] 29. Ferro, P. The local strain energy density approach applied to pre-stressed components subjected to cyclic load. Fatigue Fract. Eng. Mater. Struct. 2014, 37, 1268–1280. [CrossRef] 30. Arivarasu, M.; Ramkumar, K.D.; Arivazhagan, N. Characterization of tensile strength and impact toughness of autogenous PCGTA weldments of aeronautical steel and austenitic stainless steel. Met. Mater. 2016, 54, 279–288. [CrossRef] 31. Venkata Ramana, P.; Madhusudhan Reddy, G.; Mohandas, T.; Gupta, A.V. Microstructure and residual stress distribution of similar and dissimilar electron beam welds—Maraging steel to medium alloy medium carbon steel. Mater. Des. 2010, 31, 749–760. [CrossRef] 32. Arulmurugan, B.; Manikandan, M. Development of welding technology for improving the metallurgical and mechanical properties of 21st century nickel based superalloy 686. Mater. Sci. Eng. A 2017, 691, 126–140. [CrossRef] 33. Janicki, D.M. Fiber Laser Welding of Nickel Based Superalloy Inconel 625. Available online: https://www.spiedigitallibrary.org/ conference-proceedings-of-spie/8703/87030R/Fiber-laser-welding-of-nickel-based-superalloy-Inconel-625/10.1117/12.20134 30.short?SSO=1 (accessed on 30 June 2021). 34. Ramana, P.V.; Reddy, G.M.; Mohandas, T. Microstructure, hardness and residual stress distribution in maraging steel gas tungsten arc weldments. Sci. Technol. Weld. Join. 2008, 13, 388–394. [CrossRef] 35. Ferro, P.; Petrone, N. Asymptotic thermal and residual stress distributions due to transient thermal loads. Fatigue Fract. Eng. Mater. Struct. 2009, 32, 936–948. [CrossRef] 36. Ferro, P.; Bonollo, F.; Tiziani, A. Laser welding of copper–nickel alloys: A numerical and experimental analysis. Sci. Technol. Weld. Join. 2005, 10, 299–310. [CrossRef] 37. Ferro, P. Influence of phase transformations on the asymptotic residual stress distribution arising near a sharp V-notch tip. Model. Simul. Mater. Sci. Eng. 2012, 20, 085003. [CrossRef] 38. Ferro, P.; Bonollo, F.; Tiziani, A. Methodologies and experimental validations of welding process numerical simulation. Int. J. Comput. Mater. Sci. Surf. Eng. 2010, 3, 114. [CrossRef] 39. Vemanaboina, H.; Akella, S.; Rao, A.U.M.; Gundabattini, E.; Buddu, R.K. Analysis of thermal stresses and its effect in the multipass welding process of SS316L. Proc. Inst. Mech. Eng. Part E J. Process. Mech. Eng. 2020, 235, 384–391. [CrossRef] 40. Costa, E.S.; Assunção, P.D.C.; Dos Santos, E.B.F.; Feio, L.G.; Bittencourt, M.S.Q.; Braga, E.M. Residual stresses in cold-wire gas metal arc welding. Sci. Technol. Weld. Join. 2017, 22, 706–713. [CrossRef]