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Article Metallurgical and Mechanical Characterization of Low Carbon Steel—Stainless Steel Dissimilar Joints Made by Laser Autogenous

Elena Scutelnicu 1 , Mihaela Iordachescu 2 , Carmen Catalina Rusu 1,* , Danut Mihailescu 1 and José Luis Ocaña 3

1 Manufacturing Engineering Department, Faculty of Engineering, “Dunarea de Jos” University of Galati, 800008 Galati, Romania; [email protected] (E.S.); [email protected] (D.M.) 2 Materials Science Department, E.T.S. de Ingenieros de Caminos, Canales y Puertos, Universidad Politécnica de Madrid, 28040 Madrid, Spain; [email protected] 3 Laser Centre, Universidad Politécnica de Madrid, 28031 Madrid, Spain; [email protected] * Correspondence: [email protected]; Tel.: +40-336-130-208

Abstract: This paper addresses the metallurgical and mechanical characterization of dissimilar joints made by laser autogenous welding between thin sheets of low-carbon steel (CS) and austenitic stain- less steel (SS). The welding technology applied, previously optimized to produce sound dissimilar joints, is based on the heat source displacement from the weld gap centerline towards CS, in order to reduce the SS overheating. The research includes optical microscopy observations, energy dispersive X-ray analysis (EDX) to assess the wt% of Cr, Ni, and Fe in all regions of the dissimilar welded joint,  hardness measurements, and tensile tests of transverse-welded flat specimens. In comparison with  classical determination of the joint overall mechanical characteristics, the novelty of this research Citation: Scutelnicu, E.; Iordachescu, consists of experimental assessment of the local mechanical behavior of the fusion and heat affected M.; Rusu, C.C.; Mihailescu, D.; Ocaña, zones by using a digital image correlation technique (VIC-2D). This is an efficient tool for determining J.L. Metallurgical and Mechanical the constitutive properties of the joint, useful for modelling the mechanical behavior of materials and Characterization of Low Carbon for verifying the engineering predictions. The results show that the positive difference in yielding Steel—Stainless Steel Dissimilar between the weld metal and the base materials protects the joint from being plastically deformed. As Joints Made by Laser Autogenous a consequence, the tensile loading of flat transverse specimens generates the strain localization and Welding. Metals 2021, 11, 810. https://doi.org/10.3390/met11050810 failure in CS, far away from the weld.

Academic Editor: Aleksander Lisiecki Keywords: laser autogenous welding; bi-metallic joint; microstructure; hardness; mechanical properties

Received: 11 April 2021 Accepted: 14 May 2021 Published: 16 May 2021 1. Introduction Joining of dissimilar metals is generally more challenging than joining similar ones Publisher’s Note: MDPI stays neutral because of the differences in the physical, mechanical, and metallurgical properties of the with regard to jurisdictional claims in base materials [1–4]. From such a point of view, welding of low-carbon steels to austenitic published maps and institutional affil- stainless grades is still considered a major issue, despite continuous development of weld- iations. ing technologies, especially those related to laser usage. The microstructure transformation in the solid-state phase, such as to or ferrite during cooling and the distinct thermal expansion coefficients of base metals, produces a significant residual stress field in the joint area. This, together with the expected formation of hard zones close to Copyright: © 2021 by the authors. the weld interface adjacent to relatively softer ones, may lead to in-service cracking, with Licensee MDPI, Basel, Switzerland. consequences such as leakage and even structural failure [5]. However, dissimilar welding This article is an open access article of low-carbon steel (CS) and stainless steel (SS) is still highly demanded in many industries, distributed under the terms and including construction, the automotive sector, oil and gas, nuclear power plants, and conditions of the Creative Commons shipbuilding [6–15]. This demand, together with the need to increase welding productivity, Attribution (CC BY) license (https:// leads to dissimilar metals joining with special techniques like (LBW) creativecommons.org/licenses/by/ and electron beam welding (EBM), which, in addition to the productivity advantages, allow 4.0/).

Metals 2021, 11, 810. https://doi.org/10.3390/met11050810 https://www.mdpi.com/journal/metals Metals 2021, 11, 810 2 of 11

joining with narrower heat-affected zones (HAZs), deep penetrations, and low distortions by controlling the beam focus and location with respect to the joint [16–24]. In recent decades, many researchers have studied dissimilar metal joining by LBW [25–28]. Published research sought to determine the effects of processing param- eters (laser power and, among others, welding speed) on mechanical and metallurgi- cal properties of the welded joint, thermal history, and welding-induced deformations. Pankaj et al. in [25] conducted experimental research for determining the influence of laser power and welding speed on mechanical properties, temperature distribution, and defor- mation in laser-welded joints made between thin sheets of mild-carbon steel and stainless steel. The results showed that a faster cooling rate is achieved in the mild-carbon steel side of the joint when compared with the stainless steel side due to distinct thermophysical properties. The inherent phase transformations that occurred during welding were also ex- plained through hardness measurements of maximum values located in the fusion zone of the dissimilar weld. The research carried out by Wu et al. in [26] addressed the microstruc- ture, mechanical properties, hardness distribution, and corrosion behavior of dissimilar welded joints made by LBW with ferritic stainless steel and carbon steel base metals. It was found that the increase of welding speed leads to the decrease in width of the weld bead and of the base metals’ HAZs. The tensile properties of the joint were similar to those of the carbon steel, and the increase in welding speed positively influences the joint elongation. Prabakaran et al. [27] addressed the dissimilar welding between austenitic stainless steel (AISI316) and low-carbon steel (AISI1018) by CO2 4 kW laser welding. The results showed that the mechanical properties of the joints improve by applying a post-weld heat treatment at 960 ◦C for 2 h, followed by air cooling at room temperature. Cao et al. [28] conducted research into fiber laser welding (FLW) of dissimilar austenitic stainless steel and carbon steels for shipbuilding applications, in order to determine the effects of welding speed on microstructure and mechanical proprieties of the welded joint. It showed that welding speed, together with the heat input and cooling rate, substantially influence the microstruc- ture of the welded joint and play an important role in determining the productivity of the process. Another important conclusion of [28] examines the mechanical properties of the weld, which exceeds those of the carbon steel employed as a base metal in the absence of welding-induced defects. The present paper addresses the metallurgical and mechanical characterization of dissimilar joints made by laser autogenous welding between thin sheets of low-carbon steel (CS) and austenitic stainless steel (SS). The applied welding technology, previously optimized to produce sound joints [10], is based on the heat source displacement from the weld gap centerline towards CS in order to reduce SS overheating. The research includes optical microscopy observations, hardness measurements, energy dispersive X- ray analysis (EDX) to assess the wt% of Cr, Ni, and Fe in all regions of the dissimilar welded joint, and tensile tests of transverse-welded flat specimens. In comparison with classical determination of the joint overall mechanical characteristics, the novelty of this research consists of experimental assessment of the local mechanical behavior of the fusion and heat-affected zones by using a digital image correlation technique (VIC-2D). This is an efficient tool for determining the constitutive properties of the joint, useful for modeling the mechanical behavior of materials and for verifying the engineering predictions.

2. Materials and Methods The dissimilar joints analyzed in this study were made by laser autogenous welding (LW) between thin low-carbon steel (AISI 1010) and austenitic stainless steel (AISI 304L) plates of 200 × 300 mm, 3 mm thick, with the nominal chemical composition (in weight percent) given in Table1. Figure1a illustrates the plate welding in a continuous wave, keyhole mode with the laser equipment (Nd: YAG DY033–3300 W) of Laser Centre, Poly- technic University of Madrid. The process was performed with the parameters given in Table2, in a single pass, by shifting the laser beam 0.25 mm from the center line of the weld gap towards CS. The was helium, with a constant flow rate of 10 l/min. Prior Metals 2021, 11, x FOR PEER REVIEW 3 of 11

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Table 2, in a single pass, by shifting the laser beam 0.25 mm from the center line of the toweld welding, gap towards the plates, CS. The cleaned shielding and ground gas was down helium, along with the a constant weld line, flow were rate fixed of 10 into l/min. the Metals 2021, 11, x FOR PEER REVIEWclampingPrior to welding, device shownthe plates, in Figure cleaned1b. and One ground of the laser-welded down along dissimilar-jointthe weld line, were panels fixed3 of is 11 illustratedinto the clamping in Figure device1c. shown in Figure 1b. One of the laser-welded dissimilar-joint panels is illustrated in Figure 1c. Table 1. Base metals chemical composition (wt%). Table 2, in a single pass, by shifting the laser beam 0.25 mm from the center line of the weld gap towards CCS. The Mn shielding Si gas was Cr helium, Cu with a Ni constant S flow rate P of 10 Fel/min. PriorAISI to 1020 welding, 0.155 the plates, 0.6 cleaned 0.25 and 0.17ground 0.04 down along 0.02 the 0.035 weld line, 0.029 were bal. fixed intoAISI the 304L clamping 0.022 device 1.81 shown 0.41in Figure 18.1 1b. One 0.33 of the 9.2 laser-welded 0.08 0.025dissimilar-joint bal. panels is illustrated in Figure 1c.

Figure 1. System setup. (a) General view acquired during autogenous laser welding; (b) clamping device used for the plates fixing; (c) dissimilar but welded joint.

Table 1. Base metals chemical composition (wt%).

C Mn Si Cr Cu Ni S P Fe FigureFigureAISI 1.1. 1020 SystemSystem setup.setup. 0.155 ( a) General0.6 view0.25 acquired 0.17 during 0.04 autogenous autogenous 0.02 laser laser0.035 welding; welding; 0.029 ( (bb )) clamping clampingbal. devicedeviceAISI usedused 304L forfor thethe 0.022 platesplates fixing;fixing; 1.81 ( (cc)) dissimilardissimilar 0.41 but 18.1but weldedwelded 0.33 joint.joint. 9.2 0.08 0.025 bal.

Table 1. Base metals chemical composition (wt%). Table 2.2. AutogenousAutogenous laserlaser weldingwelding parameters.parameters. C Mn Si Cr Cu Ni S P Fe WeldingWelding Parameters Parameters AISI 1020 0.155 0.6 0.25 0.17 0.04 0.02 0.035 0.029 bal. Laser SpotLaser Diameter, Spot Diameter, mm mm 0.5 0.5 AISI 304L 0.022Laser Power,Laser 1.81 WPower, 0.41 W 18.1 0.33 9.2 0.08 3300 0.025 3300 bal. WeldingWelding Speed, mm/s Speed, mm/s 8 8 2 Table 2. AutogenousPowerPower Density laser Density (Irradiance),welding (Irradiance), parameters. W/cm W/cm2 1680.7 1680.7 Heat Input, kJ/cm 2.063 WeldingHeat Input, Parameters kJ/cm 2.063 Laser Spot Diameter, mm 0.5 As illustrated in the sketch given in Figure2a, transverse-welded flat tensile specimens, As illustrated in the sketchLaser Power,given in W Figu re 2a, transverse-welded flat tensile 3300 speci- with a gauge length of 60 mm and a gauge width of 12.5 mm, were machined according mens, with a gauge lengthWelding of 60 Speed, mm and mm/s a gauge width of 12.5 mm, were machined 8 ac- tocording ASTM to E8 ASTM [29] from E8 [29] the from LW panelsthe LW and, panels for and, comparison for comparison purposes, purposes, from the from CS andthe CS SS Power Density (Irradiance), W/cm2 1680.7 platesand SS (Figure plates 2(Figureb). 2b). Heat Input, kJ/cm 2.063

As illustrated in the sketch given in Figure 2a, transverse-welded flat tensile speci- mens, with a gauge length of 60 mm and a gauge width of 12.5 mm, were machined ac- cording to ASTM E8 [29] from the LW panels and, for comparison purposes, from the CS and SS plates (Figure 2b).

Figure 2. TensileTensile testing testing specimen. specimen. (a ()a Sketch) Sketch of ofthe the transverse-welded transverse-welded flat flat tensile tensile specimen; specimen; (b) (b) image of tensile specimens (from(from toptop to bottom: containing the dissimilardissimilar joint, from SS,SS, andand CS).CS).

The specimens prepared were tensile tested at room temperature on 200 kN servo- hydraulic testing equipment at aa constantconstant crossheadcrosshead speedspeed ofof 11 mm/min.mm/min. In addition toto

the conventional, resistive clip-on extensometer,extensometer, VIC-2DVIC-2D waswas usedused toto assessassess thethe strainstrain andand displacementFigure 2. Tensile fields testing during specimen. the tensile (a) Sketch loading of the of transverse-welded specimens. VIC-2D flat istensile a displacement specimen; (b and) image of tensile specimens (from top to bottom: containing the dissimilar joint, from SS, and CS).

The specimens prepared were tensile tested at room temperature on 200 kN servo- hydraulic testing equipment at a constant crosshead speed of 1 mm/min. In addition to the conventional, resistive clip-on extensometer, VIC-2D was used to assess the strain and Metals 2021, 11, x FOR PEER REVIEW 4 of 11

Metals 2021, 11, 810 4 of 11 displacement fields during the tensile loading of specimens. VIC-2D is a displacement and strain measurement technique able to analyze the digital image sequence acquired during testing at every 0.5 s. VIC-2D practically tracks a speckle pattern movement, previously strain measurement technique able to analyze the digital image sequence acquired during applied on the specimen surface, through an iterative spatial cross-correlation algorithm. testing at every 0.5 s. VIC-2D practically tracks a speckle pattern movement, previously Its error is typically less than 1%. Thus, average strains obtained during tensile testing on applied on the specimen surface, through an iterative spatial cross-correlation algorithm. gauge lengths capturing distinct weld regions could be correlated with the applied, over- Its error is typically less than 1%. Thus, average strains obtained during tensile testing on all stresses, assuming that the transversely loaded specimens were in iso-stress configu- gauge lengths capturing distinct weld regions could be correlated with the applied, overall ration. stresses, assuming that the transversely loaded specimens were in iso-stress configuration. The optical and differential interference contrast microscopy (DIC), as well as scan- The optical and differential interference contrast microscopy (DIC), as well as scanning ning electron microscopy (SEM) observations were made on samples containing the dis- electron microscopy (SEM) observations were made on samples containing the dissimilar similar joint cross-section, prepared by grinding and polishing as per ASTM E3 and selec- joint cross-section, prepared by grinding and polishing as per ASTM E3 and selective tive etched under ASTM E407. Aqua regia (HCl and HNO3 in a mixing ratio of 1:3) or etched under ASTM E407. Aqua regia (HCl and HNO3 in a mixing ratio of 1:3) or Beraha IIBeraha were usedII were as used etchants as etchants in the SS in side the ofSS theside joint of the and joint the and weld, the and weld, Nital and 2% Nital in the 2% CS in sidethe CS [30 side,31]. [30,31]. TheThe energyenergy dispersivedispersive X-rayX-ray analysisanalysis (EDX)(EDX) waswas usedused forfor thethe assessmentassessment ofof chemicalchemical elements,elements, suchsuch asas Cr,Cr, Ni,Ni, andand Fe,Fe, thatthat determinedetermine thethe weldweld microstructure. microstructure. MicrohardnessMicrohardness determinations were were used used as as an an alternative alternative and and efficient efficient means means of as- of assessingsessing the the changes changes in in mechanical mechanical properties properties of of the the dissimilar dissimilar joint. In this view, VickersVickers HV0.5 hardness measurements were made according to EN ISO 6507, the distance between HV0.5 hardness measurements were made according to EN ISO 6507, the distance between twotwo consecutiveconsecutive indentationsindentations beingbeing setset toto 0.20.2 mmmm [[32].32].

3.3. ResultsResults andand DiscussionsDiscussions 3.1.3.1. DissimilarDissimilar WeldedWelded JointJoint MicrostructureMicrostructure AsAs revealedrevealed inin FigureFigure3 3a,a, the the macroscopic macroscopic appearance appearance of of the the analyzed analyzed dissimilar dissimilar joint joint isis typicaltypical forfor aa keyholekeyhole modemode welding,welding, withwith thethe onlyonly peculiaritypeculiarity visiblevisible atat thisthis scalescale beingbeing relatedrelated toto the the HAZ HAZ on on the the CS CS side, side, much much larger larger than than that that formed formed on theon SSthe side. SS side. This This is due is todue the to higher the higher thermal thermal conductivity conductivity of CS of when CS when compared compared with thatwith of that SS, of and SS, to and the to laser the beamlaser beam shifting shifting toward toward the CS the side CS by side 0.25 by mm 0.25 during mm during welding. welding.

Figure 3. Typical features of the analyzed CS–SS welded joint: (a) macrostructure containing indica- tions regarding the positions of the acquired microstructural details; (b) as-received CS; (c) refined grain region of HAZ on CS side; (d) fusion line and coarse grains of HAZ on CS side; (e) weld center; (f) martensitic structure of the weld; (g) fusion line and HAZ on SS side; (h) as-received SS. Metals 2021, 11, x FOR PEER REVIEW 5 of 11

Figure 3. Typical features of the analyzed CS–SS welded joint: (a) macrostructure containing indi- cations regarding the positions of the acquired microstructural details; (b) as-received CS; (c) re- fined grain region of HAZ on CS side; (d) fusion line and coarse grains of HAZ on CS side; (e) Metals 2021, 11, 810 weld center; (f) martensitic structure of the weld; (g) fusion line and HAZ on SS side; (h) as-re-5 of 11 ceived SS.

Figure 3b illustrates the typical ferrite pearlite microstructure of CS. The HAZ formed in theFigure CS side3b illustrates of the joint the istypical a gradual ferrite transi pearlitetion from microstructure the grain refi of CS.ned Theregion HAZ (Figure formed 3c) into the the CS coarse side ofgrain the jointmicrostructure is a gradual region transition of Widmanstätten from the grain refinedferrite, regionbainite, (Figure and acicular3c) to theferrite coarse (Figure grain 3d) microstructure resulted from region the partial of Widmanstätten solid-state transformation ferrite, bainite, of and ferrite. acicular This ferrite ends (Figureat the weld3d) resulted bead solidification from the partial front, solid-state clearly delimited transformation by the ofdecarburized ferrite. This narrow ends at band the weldshown bead in solidificationFigure 3d. Then, front, as clearly revealed delimited by the bySEM the image decarburized (Figure narrow4a) on the band CS shown side, the in Figuresolidification3d. Then, structure as revealed of the by weld the SEM is fully image martensitic, (Figure4a) Widmanstä on the CS side,tten theaustenite solidification platelets structurenucleate ofand the grow weld from is fully the martensitic,fusion line limits. Widmanstätten Figure 3e austenite illustrates platelets the central nucleate region and of growthe weld from in the which fusion the line V-shape limits. joint Figure of the3e illustratestwo lateral the solidification central region fronts of thetook weld place. in It whichcomprises the V-shapecolumnar joint grains, of the consisting two lateral of acicular solidification martensite fronts and, took together place. with It comprises its inter- columnarlaced arrays, grains, form consisting the basket-weave of acicular structure martensite of martensite and, together shown with Figure its interlaced 3f and the arrays, SEM formdetail the from basket-weave Figure 4b. Its structure formation of martensite by rapid cooling shown is Figure caused3f andby the the high SEM speed detail of from laser Figurewelding4b. and Its formation the small by thickness rapid cooling of the is plates. caused According by the high to speed [5], this of lasermartensitic welding micro- and thestructure small thicknessmight reduce of the the plates. overall According weld-induced to [5 ],distortions this martensitic in large, microstructure but thin welded might pan- reduceels. The the SEM overall image, weld-induced given in Figure distortions 4c, reve inals large, the martensitic but thin welded structure panels. solidified The SEMfrom image,the fusion given line in boundaries Figure4c, reveals of SS, less the martensiticvisible in the structure optical micrograph solidified from illustrated the fusion in Figure line boundaries3g. Through of this, SS, lessits etching visible allowed in the optical the detection micrograph of vermicular illustrated ferrite in Figure dendrites3g. Through and lacy this,ferrite its into etching the coarse allowed grains the detectionof austenite of from vermicular the SS ferriteside HAZ. dendrites Lastly, and Figure lacy 3h ferrite illustrates into thethe coarse typical grains SS microstructure of austenite from which the consists SS side HAZ.of equiaxed Lastly, Figuregrains 3withh illustrates disperse the annealing typical SStwins. microstructure which consists of equiaxed grains with disperse annealing twins.

FigureFigure 4.4.SEM SEM imagesimages illustratingillustrating the:the: ((aa)) CSCS sideside HAZ,HAZ, the the fusion fusion line line and and the the adjacent adjacent martensitic marten- structuresitic structure of the of weld; the weld; (b) weld (b) centralweld central region; region; (c) martensite (c) martensite weld structure, weld structure, fusion linefusion and line the and HAZ the HAZ on the SS side. on the SS side.

InIn viewview ofof thesethese results,results, itit cancanbe be concluded concluded thatthat thethe jointjoint overall overall microstructuremicrostructure isis stronglystrongly heterogenous,heterogenous, mainly due to to the the laser laser welding welding characteristics, characteristics, i.e., i.e., fast fast welding weld- ingspeed speed and and low lowheat heatinput, input, which which propitiate propitiate distinct distinct dilution dilution degrees degrees of CS and of CS SS andpartici- SS participantpant metals, metals, elsewhere elsewhere discussed discussed [10,11]. [10 ,11]. TheThe EDXEDX maps, the central central line line analysis, analysis, and and the the SEM SEM image image of ofthe the welded welded joint joint are areshown shown in Figure in Figure 5. The5. Theoverall overall mapping mapping of Kα ofpeaks K α ofpeaks Cr, Ni, of and Cr, Fe, Ni, given andFe, in Figure given in5a, Figureillustrates5a, illustrates a homogeneous a homogeneous distribution distribution into the weld. into This the weld.is better This revealed is better in revealedthe corre- insponding the corresponding single-element single-element maps presented maps in presented Figure 5c–e. in The Figure results5c–e. of The the line results scan of quan- the linetitative scan analysis quantitative of Cr K, analysis Ni K, and of Cr Fe K, K, Ni given K, andin Figure Fe K, 5f, given show in each Figure element5f, show variation each elementin wt%, variationalong the injoint wt%, central along line the plotted joint central in Figure line 5a. plotted Accordingly, in Figure Cr5 a.and Accordingly, Ni decrease Crin andbothNi HAZs, decrease and inFe bothincreases HAZs, from and SS Fe to increases CS, but the from quasi-stable SS to CS, butwt% the corresponds quasi-stable to wt%the weld corresponds region (Figure to the weld 5f). In region the fusion (Figure zone,5f). In the the wt% fusion averages zone, theof Cr wt% K, averagesNi K, and of Fe Cr K K,were Ni K,respectively and Fe K were12.0, 3.6, respectively and 77.5, 12.0,and substantially 3.6, and 77.5, differ and substantiallyfrom the corresponding differ from wt% the correspondingdetected into the wt% 304L detected SS, of into18.1, the9.2, 304L and SS,72.2. of These 18.1, 9.2,data and confirm 72.2. Thesethe fully data martensitic confirm thestainless-steel fully martensitic structure stainless-steel of the weld. structureAs a conseq ofuence, the weld. it can As be aanticipated consequence, that, it if canloaded be anticipatedin tension, the that, weld if loaded will remain in tension, in an the elasti weldc regime will remain during in the an elasticbase metals regime yielding, during theand basethe weakest, metals yielding, in this case and CS, the will weakest, determine in this the case breaking CS, will load determine of the joint. the breaking load of the joint. Metals 2021, 11, x FOR PEER REVIEW 6 of 11

Metals 2021, 11, 810 6 of 11 Metals 2021, 11, x FOR PEER REVIEW 6 of 11

Figure 5. EDX mapping and line analysis of the CS–SS laser-welded joint: (a) overall mapping of Cr, Ni, and Fe; (b) SEM image; (c) Cr map; (d) Ni map; (e) Fe map; (f) central line scan analysis of Cr, Ni, and Fe. FigureFigure 5.5.EDX EDX mappingmapping and and line line analysis analysis of of the the CS–SS CS–SS laser-welded laser-welded joint: joint: (a ()a overall) overall mapping mapping of of Cr, Ni,Cr, andNi, Fe;and ( bFe;) SEM (b) SEM image; image; (c) Cr (c map;) Cr map; (d) Ni (d map;) Ni map; (e) Fe (e map;) Fe map; (f) central (f) central line scan line analysis scan analysis of Cr, Ni,of 3.2. Hardness Analysis andCr, Ni, Fe. and Fe. The two analyzed hardness field profiles of the dissimilar CS–SS joint are presented 3.2.in3.2. Figure HardnessHardness 6. These AnalysisAnalysis were determined along the horizontal line L1 located at 1.5 mm from the topTheThe of twotwo the analyzedplates.analyzed The hardnesshardness hardness fieldfield average profilesprofiles of 500 ofof the theHV dissimilar dissimilar0.5 in the weld CS–SS CS–SS zonejoint joint (Figure are are presented presented 6b) is 2.4 inandin FigureFigure 2 times6 .6. These higherThese were werethan determined thatdetermined of CS and along along SS base the the horizontal metalshorizontal because line line L1 ofL1 locatedformation located at at 1.5by 1.5 rapid mm mm from cool-from theingthe topoftop the ofof considerable thethe plates.plates. The The harder hardness hardness martensitic average average stainless of of 500 500 HV HVsteel0.50.5 in(Figurein the the weld weld4b). zone In zone the (Figure HAZ (Figure from6b)6b) is the is2.4 2.4CSand andside 2 times 2of times the higher joint, higher than the than hardnessthat that of CS of gradua and CS and SSlly base SS decreases base metals metals becausefrom because the of fusion formation of formation line (230by rapid HV by rapid0.5 cool-) up coolingtoing the of refinedthe of theconsiderable considerablegrain region harder harder(200 martensitic HV martensitic0.5). On stainlessthe stainless other steel side steel (Figureof (Figurethe weld 4b).4b). Inzone, Inthe the HAZnamely HAZ from fromin thethe theHAZCS CSside from side of thethe of the joint,SS joint,side the of the hardness the hardness joint, gradua the gradually microhardnesslly decreases decreases decreasesfrom from the the fusionfrom fusion 460 line lineHV (230 (2300.5 inHV HVthe0.5) 0.5 fu-up) upsionto the to line the refined up refined 240 grain HV grain 0.5region into region the (200 (200unaffected HV HV0.5).0.5 On). SS. Onthe theother other side side of the of the weld weld zone, zone, namely namely in the in theHAZ HAZAccording from from the the SSto SStheside side results of the of the presentedjoint, joint, the the microhardness in microhardnessFigure 6b, distinct decreases decreases mechanical from from 460 460behavior HV HV0.5 0.5in of thein each the fu- fusionofsion the line joint line up upregions 240 240 HV HV are0.50.5 into expectedinto the the unaffected in unaffected tension. SS. SS. According to the results presented in Figure 6b, distinct mechanical behavior of each of the joint regions are expected in tension.

FigureFigure 6.6. HardnessHardness analysis.analysis. (a) Location of the microhardness impr imprintint line line L1 L1 on on the the joint joint section; section; ((bb)) microhardnessmicrohardness profileprofile along along the the analyzed analyzed line. line. Figure 6. Hardness analysis. (a) Location of the microhardness imprint line L1 on the joint section; According to the results presented in Figure6b, distinct mechanical behavior of each 3.3.(b) microhardnessLocal and Overall profile Mechanical along the Performance analyzed line. of Dissimilar Welded Joints of theFigure joint regions 7a illustrates are expected the first in image tension. of the sequence acquired every 0.5 s during the 3.3. Local and Overall Mechanical Performance of Dissimilar Welded Joints 3.3.tensile Local test and of Overall the flat Mechanical transverse-welded Performance specimen. of Dissimilar This Weldedwas used Joints as a benchmark in the VIC-2DFigure postprocessing 7a illustrates analysis, the first which image entailed of the sequence the following acquired stages: every the first,0.5 s theduring config- the Figure7a illustrates the first image of the sequence acquired every 0.5 s during the urationtensile testof the of areathe flat of interesttransverse-welded (in light violet) specimen. for determining This was theused strain as a field;benchmark and the in sec- the tensile test of the flat transverse-welded specimen. This was used as a benchmark in ond, the set-up of the gauge lengths for six virtual extensometers (E1–E6) capturing dis- theVIC-2D VIC-2D postprocessing postprocessing analysis, analysis, which which entailed entailed the following the following stages: stages: the first, the the first, config- the tinct joint areas. For each one of the extensometers, the strains were obtained by averaging configurationuration of the ofarea the of area interest of interest (in light (in violet) light violet)for determining for determining the strain the field; strain and field; the and sec- the displacement field on the corresponding virtual gauge lengths, while the stresses were theond, second, the set-up the set-up of the of gauge the gauge lengths lengths for si forx virtual six virtual extensometers extensometers (E1–E6) (E1–E6) capturing capturing dis- conventionally computed from the load data registered by the acquisition system of the distincttinct joint joint areas. areas. For each For one each of one the ofextensom the extensometers,eters, the strains the were strains obtained were obtainedby averaging by averagingthe displacement the displacement field on the field corresponding on the corresponding virtual gauge virtual lengths, gauge while lengths, the stresses while were the conventionally computed from the load data registered by the acquisition system of the Metals 2021, 11, x FOR PEER REVIEW 7 of 11

servo-hydraulic testing equipment, by assuming that the transversely loaded specimens were in iso-stress configuration. Figure 7b presents the overall stress–percentage elongation curve of the analyzed LW joint; the elongation data correspond to the virtual extensometer E1, of 6.5 mm gage length, which captured the joint as a whole. In addition, Figure 7b contains some of the Metals 2021, 11, 810 strain maps as a result of postprocessing the image sequence data in 7the of 11area of interest (Figure 7a). These are related to A1–A5 points and show the strain distribution maps at distinct specimen elongations: during yielding (A1), within the yielding plateau (A2), dur- ing the plastic flow (A3 and A4), and at maximum load (A5). The strain map correspond- stresses were conventionallying to A6 shows computed that the maximum from the load strains data are registered located in by CS, the while acquisition the other joint areas system of the servo-hydraulicare almost free testingof deformation. equipment, This by explains assuming the that failure the transversely of the specimen loaded in CS, occurring specimens werefar in away iso-stress from configuration. the weld at an overall elongation of 23.3%.

Figure 7. Overall stress–percentage elongation curve of the LW joint. (a) Flat specimen detail on which are indicated the Figure 7. Overall stress–percentage elongation curve of the LW joint. (a) Flat specimen detail on speckle-painted zone and the generic positions of the six virtual extensometers (E1–E6) with respect to the LW-joint zones, and the strainwhich field are analysis indicated area; the speckle-painted(b) overall stress zone vs. andpercentage the generic elongation positions of of the the LW six virtualjoint; the extensometers images are the strain maps corresponding(E1–E6) with to the respect points to A1–A6. the LW-joint zones, and the strain field analysis area; (b) overall stress vs. percentage elongation of the LW joint; the images are the strain maps corresponding to the points A1–A6. Figure 8 shows the plots of the strain–stress curves obtained by tensile testing of a full-CS flat specimen with the aid of a conventional clip-on extensometer (1) and from the Figure7b presentsvirtual extensometer the overall stress–percentageE2 (2) mounted on elongationthe CS region curve of the of flat, the transverse-welded analyzed spec- LW joint; the elongationimen. Figure data 8 correspondalso contains, to theas a virtual benchmark, extensometer the overall E1, strain–stress of 6.5 mm gage curve of the LW length, which capturedjoint (3), previously the joint aspresented a whole. in Figure In addition, 7b. From Figure the comparison7b contains of some the curves of (1) and (2), the strain mapsthe as results a result show of that postprocessing that VIC-2D theprovide image accurate sequence information data in thefrom area the ofplastic instabili- interest (Figureties7a). regionThese of are CS, related with the to A1–A5virtual extensometer points and show being the able strain to capture distribution the strain field until maps at distinctthe specimen specimen elongations: failure. The duringoverall yieldingstrain–stress (A1), curve within (3) theprovides yielding only plateau limited information (A2), during theregarding plastic flow the local (A3 andproperties A4), and of atthe maximum joint, because load its (A5). corresponding The strain mapgauge length cap- correspondingtures to A6 the shows jointthat as a thewhole. maximum The presence strains of are a small located yielding in CS, plateau while the in otherthe (3) curve is due joint areas are almostto CS participation free of deformation. at joint formation. This explains In addition, the failure the of specimen the specimen failure in CS,in the CS region occurring far awayindicates, from theas tensile weld atstrength an overall for the elongation dissimilar of joint, 23.3%. the CS tensile strength. Figure8 showsFigure the plots 9 illustrates of the strain–stress the strain–stress curves curves obtained obtained by tensile by tensile testing testing of a of a full-SS flat full-CS flat specimenspecimen with through the aid a conventional of a conventional clip-on clip-on extensometer extensometer (1) and (1)from and the from virtual extensom- the virtual extensometereter E3 (2) E2mounted (2) mounted on the on SS the region CS region of the offlat the specimen flat, transverse-welded containing the dissimilar LW specimen. Figurejoint.8 also For contains,comparison as reasons, a benchmark, Figure the 9 also overall contains strain–stress the overall curve strain–stress of the curve of the LW joint (3), previouslyLW joint (3), presented provided in Figurein Figure7b. 7b. From Accordin the comparisong to the curve of the (2), curves at joint (1) failure, the SS and (2), the results show that that VIC-2D provide accurate information from the plastic instabilities region of CS, with the virtual extensometer being able to capture the strain field until the specimen failure. The overall strain–stress curve (3) provides only limited information regarding the local properties of the joint, because its corresponding gauge length captures the joint as a whole. The presence of a small yielding plateau in the (3) curve is due to CS participation at joint formation. In addition, the specimen failure in the CS region indicates, as tensile strength for the dissimilar joint, the CS tensile strength. Metals 2021, 11, x FOR PEER REVIEW 8 of 11

region experiences some degree of deformation that is considerably smaller than the de-

Metals 2021, 11, 810 formation at maximum load determined and registered with a conventional extensometer8 of 11 by testing a full-SS flat specimen. In this case, VIC-2D could not provide information be- yond the dissimilar joint failure in the CS region.

Metals 2021, 11, x FOR PEER REVIEW 8 of 11

region experiences some degree of deformation that is considerably smaller than the de- formation at maximum load determined and registered with a conventional extensometer by testing a full-SS flat specimen. In this case, VIC-2D could not provide information be- yond the dissimilar joint failure in the CS region.

Figure 8. Engineering stress–strain curves of: 1, full CS flat flat specimen (clip-on(clip-on extensometer data); 2, 2,CS CS region region of theof the LW LW joint joint (E2 (E2 virtual virtual extensometer extensomet data);er data); 3, CS-SS 3, CS-SS joint joint (E1 virtual (E1 virtual extensometer extensometer data). data). Figure9 illustrates the strain–stress curves obtained by tensile testing of a full-SS flat specimen through a conventional clip-on extensometer (1) and from the virtual extensome- ter E3 (2) mounted on the SS region of the flat specimen containing the dissimilar LW joint. For comparison reasons, Figure9 also contains the overall strain–stress curve of the LW joint (3), provided in Figure7b. According to the curve (2), at joint failure, the SS region experiences some degree of deformation that is considerably smaller than the deformation Figureat maximum 8. Engineering load determined stress–strain and curves registered of: 1, full with CS flat a conventional specimen (clip-o extensometern extensometer by data); testing 2,a CS full-SS region flat of specimen.the LW joint In(E2 this virtual case, extensomet VIC-2Der could data); not 3, CS-SS provide joint information (E1 virtual extensometer beyond the data).dissimilar joint failure in the CS region.

Figure 9. Engineering stress–strain curves of: 1, full SS flat specimen (clip-on extensometer data); 2, SS region of the LW joint (E3 virtual extensometer data); 3, CS–SS joint (E1 virtual extensometer data).

The local strain–stress curves acquired with the virtual extensometers E4–E6 from the weld zone (2), the HAZ from the SS side (3), and the HAZ from the CS side (4) of the joint are presented in Figure 10, together with the overall one of the dissimilar LW joint (1). The complex, heterogeneous microstructure of the joint explains the distinct tensile behavior of these regions. Thus, the maximum strain of 0.005 registered in the weld at specimen failure was attributed to the drastic decrease in the plastic flow capacity of this FigureFigure 9. 9. EngineeringEngineering stress–strain stress–strain curves curves of: of: 1, 1, full full SS SS flat flat specimen specimen (clip-o (clip-onn extensometer extensometer data); data); 2, zone, propitiated by the Widmanstätten ferrite formation as a consequence of high laser- 2,SS SS region region of of the the LW LW joint joint (E3 (E3 virtual virtual extensometer extensometer data); data); 3, CS–SS 3, CS–SS joint joint (E1 virtual(E1 virtual extensometer extensometer data). data).welding speed, and of plate thickness. The strain–stress curve (2) of the weld zone indi- cates Thea positive local strain–stress difference in curves yield, acquired which gradually with the virtualdecreases extensometers into both HAZs, E4–E6 relatively from the toweld theThe zonecorrespondent local (2), strain–stress the HAZ yield from of curves SS the and SS acquired sideCS base (3), me andwitals.th the the Figure HAZ virtual from 10 showsextensometers the CS that side the (4) higherE4–E6 of the strainfrom joint thevaluesare weld presented characterize zone in(2), Figure the the HAZ 10HAZ, together from on thethe with CSSS side the overall(3),(0.06) and when one the of HAZcomparing the dissimilarfrom the it with CS LW side that joint (4) found (1). of Thethe in jointthecomplex, HAZ are presented from heterogeneous the SS in side Figure microstructure (0.045) 10, attogether joint offailure with the joint (0.24).the explainsoverall Thus, one based the of distinct theon VIC-2D dissimilar tensile information, behavior LW joint of (1).these The regions. complex, Thus, heterogeneou the maximums microstructure strain of 0.005 registeredof the joint in explains the weld the at specimen distinct tensile failure behaviorwas attributed of these to theregions. drastic Th decreaseus, the inmaximum the plastic strain flow capacityof 0.005 ofregistered this zone, in propitiated the weld byat specimenthe Widmanstätten failure was ferrite attributed formation to the as drastic a consequence decrease of in high the plastic laser-welding flow capacity speed, of and this of zone,plate thickness.propitiated The by strain–stressthe Widmanstätten curve (2) ferrite of the formation weld zone as indicatesa consequence a positive of high difference laser- weldingin yield, speed, which graduallyand of plate decreases thickness. into The both strain–stress HAZs, relatively curve to(2) the of correspondentthe weld zone indi- yield cates a positive difference in yield, which gradually decreases into both HAZs, relatively to the correspondent yield of SS and CS base metals. Figure 10 shows that the higher strain values characterize the HAZ on the CS side (0.06) when comparing it with that found in the HAZ from the SS side (0.045) at joint failure (0.24). Thus, based on VIC-2D information, Metals 2021, 11, 810 9 of 11

Metals 2021, 11, x FOR PEER REVIEW 9 of 11 of SS and CS base metals. Figure 10 shows that the higher strain values characterize the HAZ on the CS side (0.06) when comparing it with that found in the HAZ from the SS side (0.045) at joint failure (0.24). Thus, based on VIC-2D information, it may be concluded that theit may positive be concluded difference that in yieldthe positive between difference the weld metalin yield and between the base the materials weld metal protects and thethe jointbase frommaterials being protects plastically the joint deformed. from being plastically deformed.

Figure 10. Engineering stress–strain curves curves of: of: 1, 1, dissim dissimilarilar joint joint (E1 (E1 virtual virtual extensometer extensometer data); data); 2, 2, weld zone (E4 virtual extensometer extensometer data); data); 3, 3, HAZ HAZ on on SS SS side side of of the the joint joint (E6 (E6 virtual virtual extensometer extensometer data); 4, HAZ on CS side of the joint (E5 virtualvirtual extensometerextensometer data).data).

The local and overalloverall mechanicalmechanical propertiesproperties ofof thethe dissimilardissimilar laser-weldedlaser-welded joint,joint, asas well as the the strength strength overmatch overmatch coefficient coefficient M M (M (M = R =p0.2 Rp0.2-weld − weld zones zones /Rp0.2/R − CSp0.2-CS), are ),given are given in Table in Table3. It shows3. It shows that the that weld the zone, weld the zone, HAZ the on HAZ the CS on side, the CSand side, the HAZ and theon the HAZ SS onside the of SSthe sidejoint ofovermatches the joint overmatches the yield strength the yield of strengthCS for, respectively, of CS for, respectively, >24%, 12%, >24%,and 19%. 12%, These and 19%.data agree These with data the agree hardness with the field hardness profile fieldshow profilen in Figure shown 6b in and Figure the detected6b and the microstruc- detected microstructureture heterogeneities heterogeneities of the joint. of the joint.

Table 3.3. LocalLocal andand overalloverall mechanicalmechanical performanceperformance ofof thethe analyzedanalyzed dissimilardissimilar laser-weldedlaser-welded joint.joint.

Yield Tensile Elastic Elongation StrengthStrength Yield Tensile Elastic Elongation Strength, Strength, Modulus, at max. Load Overmatch M, M, Strength,Rp0.2 [MPa] RStrength,m [MPa] EModulus, [GPa] at [%]max. LoadRp0.2-Weld Zones/Rp0.2-CS Rp0.2-Weld Zones CS—AISI 1020 *Rp0.2 [MPa] 350 Rm428 [MPa] 205E [GPa] 22[%] - CS—AISI 1020 ** 353 438 205 22/R 1p0.2–CS CS—AISISS—AISI 1020 304L ** 350 395 725 428 202 205 68 22 1.12 - Overall CS-SS Joint ** 361 438 206 12.3 n.a. CS—AISIHAZ CS 1020 Side ** ** 353 399 > 438 438 216 205 n.a. 22 1.12 1 SS—AISIWeld 304L ** * 395 > 438 > 725 438 n.a. 202 n.a. 68 >1.24 1.12 OverallHAZ SS CS-SS Side ** 419 > 438 196 n.a. 1.19 361 438 206 12.3 n.a. * tensileJoint test made ** using a clip-on extensometer. ** tensile test made using a virtual extensometer. HAZ CS Side ** 399 > 438 216 n.a. 1.12 4. Conclusions Weld ** > 438 > 438 n.a. n.a. >1.24 The CS–SS welded joint produced by shifting the laser beam towards the CS side HAZ SS Side ** 419 > 438 196 n.a. 1.19 showed a complex microstructure that induced distinct mechanical performance in the weld * tensile test made using a clip-on extensometer. ** tensile test made using a virtual extensometer. regions. The VIC technique was proved as being efficient for assessing the constitutive properties of the joint, which is useful for mechanical modeling, and for verifying the 4. Conclusions engineering predictions. Based on the analysis of the research results, several significant conclusionsThe CS–SS have welded been drawn, joint produced as follows: by shifting the laser beam towards the CS side showed a complex microstructure that induced distinct mechanical performance in the • The assessed local tensile behavior in the weld zone, the HAZ on the CS side, and the weld regions. The VIC technique was proved as being efficient for assessing the constitu- HAZ on the SS side of the joint overmatches the yield strength of CS for, respectively, tive properties of the joint, which is useful for mechanical modeling, and for verifying the >24%, 12%, and 19%. These data agree with the hardness profile and the detected engineering predictions. Based on the analysis of the research results, several significant microstructure heterogeneities of the joint. conclusions have been drawn, as follows: • The assessed local tensile behavior in the weld zone, the HAZ on the CS side, and the HAZ on the SS side of the joint overmatches the yield strength of CS for, respectively, Metals 2021, 11, 810 10 of 11

• The EDX analysis, employed to assess the wt% of Cr, Ni, and Fe in all regions of the dissimilar welded joint, explicitly demonstrates the fully martensitic stainless steel structure of the weld. As a consequence, it could be anticipated and then demonstrated by tensile testing of welded specimens that the weld remains in an elastic regime during the base metals yielding, and the weakest, in this case CS, determine the breaking load of the joint. • The detected positive difference in yield between the generated martensitic stainless steel into the weld and the base materials protects the joint from being plastically deformed. As a consequence, the tensile loading of flat transverse specimens produces the strain localization and failure in CS, far away from the weld, which is favorable for dissimilar welded structures subjected to transverse tensile loads.

Author Contributions: Conceptualization, E.S.; methodology, E.S. and C.C.R.; software, M.I.; valida- tion, M.I.; formal analysis, D.M.; investigation, E.S., M.I., J.L.O. and D.M.; resources, M.I. and J.L.O.; writing—original draft preparation, C.C.R.; writing—review and editing, C.C.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors gratefully acknowledge the support received from “Dunarea de Jos” University of Galati, Romania and from Ministry of Science and Innovation in Spain (RTI2018- 097221-B-I00). Conflicts of Interest: The authors declare no conflict of interest.

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