A Study on Lap Joint Welding of Thin Plate ASTM F1684 Using Fiber Laser Welding

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Article A Study on Lap Joint Welding of Thin Plate ASTM F1684 Using Fiber Laser Welding

Jaewoong Kim 1, Younghyun Kim 1, Jisun Kim 1, Yongtai Kim 2, Dongwoo Kim 2, Sungwook Kang 3 and Changmin Pyo 1,*

1 Smart Mobility Materials and Components R&D Group, Korea Institute of Industrial Technology, Gwangju 61012, Korea; [email protected] (J.K.); [email protected] (Y.K.); [email protected] (J.K.) 2 Cargo Containment System Research Dep’t, Hyundai Heavy Industries, Seoul 03058, Korea; [email protected] (Y.K.); [email protected] (D.K.) 3 Transport Machine Components R&D Group, Korea Institute of Industrial Technology, Jinju-si 52845, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-62-600-6080

Abstract: The International Maritime Organization (IMO) has developed stricter regulations on emission standards for sulfur oxides, etc., and the demand for Liquefied Natural Gas (LNG) is increasing as an alternative to satisfy these standards. This study relates to fiber laser welding, an approach which offers high-speed welding and low welding deformation for ASTM F1684, which has a low coefficient of thermal expansion (CTE) even in a cryogenic environment. In this study, through three preliminary experiments using 0.25 mm thick Invar, the conditions required to secure sufficient penetration depth and back bead were identified. Through the cross-sectional observation analysis, the welding conditions without defects were identified and the trend of penetration shape

according to increasing welding speed was identiﬁed. Following a lap joint laser welding experiment under the secured conditions, the mechanical properties were evaluated through the shear strength

Citation: Kim, J.; Kim, Y.; Kim, J.; test and the heat influence range of a fiber laser was identified through the temperature measurement Kim, Y.; Kim, D.; Kang, S.; Pyo, C. A of a welding part. As a result, it was confirmed that the shear strength of the lap joint laser welding Study on Lap Joint Welding of Thin part was 86.8% that of the base metal. Plate ASTM F1684 Using Fiber Laser Welding. Processes 2021, 9, 428. Keywords: fiber laser welding; lap joint welding; tensile strength; ASTM F1684 https://doi.org/10.3390/pr9030428

Academic Editor: Ireneusz Zbicinski 1. Introduction Received: 9 February 2021 To reduce marine environmental pollution, the International Maritime Organization Accepted: 22 February 2021 (IMO) has strictly regulated the sulfur content standard for ship fuel oil, reducing it from Published: 27 February 2021 3.5% m/m (Mass by mass) to 0.5% in January 2020 [1]. As typical offshore petroleum- based fuels cannot meet this regulation, the demand for liquefied natural gas (LNG) as Publisher’s Note: MDPI stays neutral an alternative fuel is increasing and the demand for ships that use LNG as fuel is rising with regard to jurisdictional claims in as well [2,3]. When LNG is used, the emission of sulfur oxides (SOx) is reduced by published maps and institutional affil- 60–90% and the emission of nitrogen oxides (NOx) is about 30% lower. In terms of CO iations. 2 emission, reductions of more than 20% can be achieved [2–5]. Natural Gas (NG) has a boiling point of −163 ◦C, and cryogenic conditions are required for its storage and transportation in a liquefied state. Since general metals cannot be used due to their brittleness under the above condition, care should be taken when Copyright: © 2021 by the authors. selecting materials [6]. Additionally, many researches of cryogenic metals for treating LNG Licensee MDPI, Basel, Switzerland. were performed [7–10]. This article is an open access article ASTM F1684 is known to have a low coefficient of thermal expansion (CTE) from room distributed under the terms and temperature to around Curie temperature (230 ◦C), and excellent mechanical properties conditions of the Creative Commons Attribution (CC BY) license (https:// in cryogenic environments. Due to these characteristics, it is widely used as a high- creativecommons.org/licenses/by/ reliability and high-precision material, in space equipment, precision equipment, and LNG 4.0/). tanks [11–13].

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There are quite a few welding studies on ASTM F1684. Harrison et al. confirmed that the low thermal expansion characteristic, which is the main characteristic of Invar, is maintained when using SLM (selective laser melting), a specific 3D printing technology that completely melts and fuse metal powder using a high-density laser [14]. Yakout et al. applied SLM and confirmed that laser energy density of SLM affect the density and microstructure of Invar [15]. Zhan et al. compared the residual stress and deformation amount for various parameters by applying the multi-layer Laser-MIG (Metal Inert Gas) Hybrid Welding method to 19.5 mm thick Invar. As a result, residual stress and deformation were reduced in three-layer welding compared to two-layer welding [16]. Zhan et al. compared MIG Welding with Layer Laser-MIG Hybrid Welding of Invar in terms of welding efficiency and deformation and found that the laser-MIG hybrid welding showed less deformation than MIG welding and the welding time was also reduced by eight times [17]. Bharat et al. applied friction stir welding to Invar with a thickness of 12.7 mm and per-formed a cross sectional analysis and compared mechanical properties depending on pin tools. As a result, the coefficients of thermal expansion, tensile strength, and micro hardness of the welds were essentially unchanged from the parent material, and the pin tool produced remnant-free welds with insignificant tool wear [18]. Yue et al. applied friction stir welding to 3.0 mm thick Invar and achieved defect-free welding conditions through X-ray analysis, cross-sectional analysis, and mechanical property tests depending on the parameters [19]. Corbacho et al. applied GTAW (gas tungsten arc welding) to Invar with a thickness of 4 mm to 8 mm and analyzed its microstructure. As a result, GTAW, a low heat input welding process, exhibited a narrow HAZ(Heat Affected Zone) area, and Invar requires attention because surface and grain-boundary oxidation are prone to occur [20]. Wang et al. performed lap joint welding by applying the GTAW welding method to Invar of 0.7 mm and 1.0 mm thickness and compared its microstructure, hardness, and shear tensile characteristics depending on duty cycle size. As a result, the tensile shear force is directly proportional to the duty cycle, and the maximum tensile shear force is 4425 N [11]. Kim et al. applied fiber laser welding using cold wire to a cross-shaped Invar of 1.5 mm and 3.0 mm thickness. By optimizing the welding conditions for the cross-shaped structure, which previously required 4 welding passes, the number of passes was reduced to 2 [21]. Base on the above studies, this study was conducted to confirm the welding feasibility of ultra-thin plates with a thickness of 1 mm or less. To this end, the study was conducted with fiber laser welding with easy systemization, automation, and robotization, enabling automatic welding, high-speed welding, and low welding deformation. For this study, a preliminary experiment was performed with the laser power and welding speed as variables to secure an appropriate amount of heat input and a stable back bead when welding the 0.2 mm thick ASTM F1684 lap joint through fiber laser welding. After performing lap joint welding under the welding conditions obtained through the preliminary experiment, the bead width, bead height, weld zone width, and penetration were analyzed through cross-sectional observation, and the shear strength of the weld zone was compared through a shear test.

2. Materials and Methods In this study, a lap joint ﬁber laser welding experiment was performed on ASTM F1684 with a thickness of 0.25 mm and the optimum welding conditions were derived through three series of preliminary experiments, cross-sectional analyses, and shear strength tests. The material used in this study is ASTM F1684 and its chemical composition is shown in Table1. Further mechanical properties are shown in Table2. Processes 2021, 9, 428 3 of 15 ProcessesProcesses 2021, 9, 2021x FOR, 9 ,PEER x FOR REVIEW PEER REVIEW 3 of 16 3 of 16

Table 1. Chemical composition of ASTM F1684 [21]. Table 1.Table Chemical 1. Chemical composition composition of ASTM of ASTMF1684 [21].F1684 [21]. Material Composition (wt%) MaterialMaterial CompositionComposition (wt%) (wt%) NiNi Ni MoMo Mo CC C Mn MnMn 3636 36 ~0.5~0.5 ~0.5 ~0.1~0.1 ~0.1 ~0.06 ~0.06 ~0.06 ASTMASTM F1684ASTM F1684 F1684 PP P SS S SiSi Si Cr CrCr ~0.025~0.025~0.025 ~0.025 ~0.025~0.025 ~0.35~0.35 ~0.35 ~0.5 ~0.5~0.5

Table 2. Mechanical properties of ASTM F1684 [21]. Table 2.Table Mechanical 2. Mechanical properties properties of ASTM of ASTMF1684 [21]. F1684 [21]. CoefficientCoefficient of Thermal of Thermal Expansion Expansion Young’sYoung’s Modulus Modulus DensityDensity Coefﬁcient of Thermal Expansion 3 Young’s Modulus (GPa) Density (kg/m3 )3 (mm/mmK)(mm/mmK)(mm/mmK) (GPa) (GPa) (kg/m(kg/m) ) 1.21.2× e10−6 1.2−6 e−6 148.0148.0 148.0 8100.0 8100.08100.0

In thisIn Instudy, this this study, study,a specimen a a specimen specimen with a with withsize ofa a size 100 size ofmm of 100 100 × mm200 mm mm× ×200 200× mm0.25 mm ×mm 0.25× was0.25 mm used mm was wasfor used the used for the for lap jointlapthe fiber joint lap jointlaser fiber ﬁber welding laser laser welding experiment. welding experiment. experiment. After overlappingAfte Afterr overlapping overlapping two plates two two platesby plates about by by about30 aboutmm, 30 a 30mm, mm, a weldingweldinga weldingexperiment experiment experiment was performed was was performed performed on the on overlapped onthe the overlapped overlapped central central part, central and part, part,its and schematic and its schematic its schematic di- di- agramagramdiagram is shown is shown is in shown Figure in inFigure 1. Figure 1.1 .

Figure Figure 1. ASTM 1. ASTM F1684 thinF1684 plate thin lap plate joint lap weldingweld jointing weld testing piece test schematic piece schematic diagram. diagram.

When WhenWhena thin a aplate thin thin isplate plate used, is is used,significant used, significant signiﬁcant welding welding welding deformation deformation deformation can occur, can can occur, so occur,a jig so is a so re-jig a is jig re- is quiredquiredrequired to control to tocontrol the control deformation. the the deformation. deformation. The layout The The layout of layout a welding of ofa welding a weldingsystem system with system the with withjig the installed the jig jiginstalled installed is is shownshownis in shown Figure in inFigure 2. Figure 2. 2.

6-axis6-axis articulated articulated robot robot

Laser Laser emission emission equipment equipment

WeldingWelding jig jig

Figure FigureFigure2. Fiber 2. 2. laser FiberFiber welding laser laser welding welding system system systemwith jig. with with jig. jig.

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InInIn this this this study, study, study, all all allsurfaces surfaces surfaces except except except 20 20 mm 20 mm mm around around around the the thewelding welding welding line line linewere were were tightly tightly tightly fixed fixed ﬁxed in in orderorderin order to to control tocontrol control the the welding the welding welding deformation deformation deformation as as much as much much as as possible as possible possible and and and to to tominimize minimize minimize the the the gap gap betweenbetween the the specimens specimens for for the the lap lap joint joint weldin weldin welding.g. g.Two Two Two specimens specimens were were inserted inserted and and fixed fixed ﬁxed inin inthe the the middle middle middle of of the the jig jig as as shown shown in in Figure Figure 3.3 3..

ProtectiveProtective gas gas supplysupply tube tube

WeldingWelding end end Welding start pointpoint Welding start LapLap joint joint test test positionposition piecepiece BackBack purging purging supply supply line line

Figure 3. Lap joint welding jig setting. Figure 3. Lap Figurejoint welding 3. Lap jig joint setting. welding jig setting.

ForFor laser laser laser welding, welding, welding, Miyachi Miyachi Miyachi 5 kW5 kW 5 fiber kW fiber ﬁberlaser laser welding laser welding welding equipment equipment equipment was was used. used. was The used.The equip- equip- The mentmentequipment consists consists consistsof of a lasera laser of welding welding a laser oscillator, welding oscillator, oscillator,an an optical optical ansystem, system, optical a controller,a system, controller, a and controller, and a chiller,a chiller, and as as a shownshownchiller, in in asFigure Figure shown 4. 4.The in The Figure optical optical4. Thesystem system optical used used system in in this this usedstudy study in has this has a studyspota spot diameter has diameter a spot of of diameter400 400 μ m,μm, a of a focalfocal400 lengthµ lengthm, a focalof of 148.8 148.8 length mm, mm, of and 148.8and a focala mm, focal depth and depth a of focal of 6 mm.6 depthmm. The The of 6-axis 6 6-axis mm. automatic Theautomatic 6-axis robot automaticrobot was was from robotfrom Yaskawa.Yaskawa.was from Yaskawa.

Figure 4. Fiber laser equipment. FigureFigure 4. 4.Fiber Fiber laser laser equipment. equipment. A macro test was performed to analyze weld zone, width, and penetration. The test A Amacro macro test test was was performed performed to to analyze analyze weld weld zone, zone, width, width, and and penetration. penetration. The The test test was performed based on ASTM E340 and the environmental conditions were a temperature waswas performed performed◦ based based on on ASTM ASTM E340 E340 and and the the environmental environmental conditions conditions were were a tempera-a tempera- tureof 23of 23C °C and and a humidity a humidity of 43% of 43% RH, RH, and and 3% Nital3% Nital was was used used as etching as etching solution. solution. Theetched The turecross-section of 23 °C and was a observed humidity using of 43% equipment RH, and made3% Nital by Olympuswas used and as etching is shown solution. in Figure The5. etchedetched cross-section cross-section was was observed observed using using eq equipmentuipment made made by by Olympus Olympus and and is isshown shown in in FigureFigure 5. 5.

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Figure 5. OpticalFigureFigureFigure microscope 5. 5. 5. Optical OpticalOptical microscope microscopeequipment microscope equipment andequipment equipment cross-section and and and cross-section cross-section cross-section of a sample. of of a a sample. sample. A shear tensile strength test was performed to analyze the fracture shape of a welding A shear tensileAA shear shear strength tensile tensile test strength strength was performed test test was was performed performedto analyze to theto analyze analyze fracture the the shape fracture fracture of a shape weldingshape of of a a welding welding part and measure its strength. The tensile strength was measured using the Hounsﬁeld part and measurepartpart and and itsmeasure measure strength. its its Thestrength. strength. tensile The The strength tensile tensile wasst strengthrength measured was was measured usingmeasured the using Hounsfieldusing the the Hounsfield Hounsfield H10KS universal testing machine (UTM), as shown in Figure6. To measure the shear H10KS universalH10KSH10KS universal testinguniversal machine testing testing (UTM),machine machine as (UTM), (UTM),shown asinas shown Figureshown in 6.in Figure ToFigure measure 6. 6. To To themeasure measure shear the the shear shear strength, a test piece with a size of 370 mm × 20 mm × 0.25 mm was made, as shown in strength, astrength,strength, test piece a a test withtest piece piecea size with with of 370 a a size sizemm of of × 370 20370 mmmm mm ×× × 0.2520 20 mm mmmm × × was0.25 0.25 made,mm mm was was as shownmade, made, as inas shown shown in in Figure7. The tensile speed was ﬁxed at 5 mm/min. Figure 7. TheFigureFigure tensile 7. 7. The The speed tensile tensile was speed speedfixed was atwas 5 fixedmm/min. fixed at at 5 5 mm/min. mm/min.

Figure 6. TensileFigureFigureFigure testing 6. 6. 6. Tensile TensileTensile equipment. testing testing equipment. equipment.

Figure 7. Schematic diagram of tensile test piece. Figure 7. SchematicFigureFigure 7. 7. diagramSchematic Schematic of diagram tensilediagram test of of tensilepiece. tensile test test piece. piece.

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3. Results 3. Results To select an appropriate amount of heat input, the welding speed was fixed at 1 m/min To select an appropriate amount of heat input, the welding speed was fixed at 1 and the first preliminary experiment of five cases was performed using the laser output m/min and the first preliminary experiment of fivefive casescases waswas performedperformed usingusing thethe laserlaser value as a variable. Each experiment condition is shown in Table3. output value as a variable. Each experiment condition is shown in Table 3.

Table 3. 1st Conditions of laser welding for 0.25 mm thickness of ASTM F1684. Table 3. 1st Conditions of laser welding for 0.25 mm thickness of ASTM F1684.

WeldingWelding Parameters Parameters Experimental Experimental Conditions Conditions LaserLaser power power (kW) (kW) 0.50, 0.50, 0.55, 0.55, 0.60, 0.60, 0.65, 0.65, 0.70 0.70 WeldingWelding speed speed (m/min) (m/min) 1 1 Defocus (mm) 0 Defocus (mm) 0 Shielding gas Ar (L/min) 15 Shielding gas Ar (L/min) 15 TiltingShielding angle gas & Working Ar (L/min) angle (◦) 15 0 Tilting angle & Working angle (°) 0

After the welding experiment for each case, the welding part was visually observed After the welding experiment for each case, the welding part was visually observed and its appearance is as shown in Figure8. and its appearance is as shown in Figure 8.

FigureFigure 8. Surface 8. Surface geometry geometry of theof the ﬁrst first experiment experiment of of Lap Lap joint joint welding. welding.

InInIn CasesCasesCases 111 andandand 2,2,2, theretherethere waswaswas nonono penetrationpenetrationpenetration due due to to welding welding failure.failure. failure. CaseCase Case 33 3 showedshowed showed partialpartial weldingwelding andand CasesCases 4 and 5 made a back bead through through sufficient sufficient penetration, penetration, but but theretherethere waswas alsoalso aa a humpinghumping humping bead.bead. bead. InIn Inaddition,addition, addition, throughthrough through laserlaser laser powerpower power monitoring,monitoring, monitoring, itit waswas it con-con- was confirmedfirmedfirmed thatthat that thethe the high-powerhigh-power high-power laserlaser laser oscillatedoscillated oscillated inin inthethe the initialinitial initial stagestage stage comparedcompared compared toto to thethe the inputinput input powerpower duringduring lowlow powerpower welding,welding, and it was was further further confirmed confirmed that that its its output output was was stable stable atat 11 kWkW oror more.more. TheThe results are shown in Figure 99..

Figure 9. Laser Figurepower monitoring. 9. Laser power monitoring.

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It is necessary to increase the laser output power to avoid the welding defects seen It is necessary to increase the laser output power to avoid the welding defects seen in Cases 1 to 5, and the second preliminary experiment for the ﬁve cases was performed in Cases 1 to 5, and the second preliminary experiment for the five cases was performed using the welding speed as a variable after ﬁxing the output value to 1 kW where stable using the welding speed as a variable after fixing the output value to 1 kW where stable laser power is generated. Each experimental condition is shown in Table4. laser power is generated. Each experimental condition is shown in Table 4.

TableTable 4. 4.2nd 2nd ConditionsConditions ofof laser welding fo forr 0.25 0.25 mm mm thickness thickness of of ASTM ASTM F1684. F1684.

Welding Parameters Parameters Experimental Experimental Conditions Conditions LaserLaser powerpower (kW) (kW) 1 1 WeldingWelding speed speed (m/min) (m/min) 1.7, 1.7, 1.8, 1.8, 1.9, 1.9, 2.0, 2.0, 2.1 2.1 DefocusDefocus (mm) (mm) 0 0 Shielding gas Ar (L/min) 15 Shielding gas Ar (L/min) 15 Tilting angle & Working angle (◦) 0 Tilting angle & Working angle (°) 0

AfterAfter thethe weldingwelding experiment for for each each case, case, the the welding welding part part was was visually visually observed observed andand thethe resultsresults areare asas shown in Figure 10.10.

FigureFigure 10. Surface10. Surface geometry geometry of theof the second second lap lap joint joint welding welding experiment. experiment.

CasesCases 66 toto 1010 inin thethe secondsecond preliminarypreliminary experimentexperiment hadhad sufficientsufficient penetration, leadinglead- toing the to creationthe creation of aof back a back bead, bead, but but it it was was confirmedconfirmed that burn burn through through and and humping humping beadsbeads werewere alsoalso producedproduced at the same time. time. A A burn burn through through was was created created when when molten molten metalmetal flowsflows toto thethe opposite side of of a a test test piece piece locally, locally, and and the the humping humping bead bead is isa welding a welding defectdefect thatthat occurs due due to to excessive excessive heat heat inpu inputt when when the molten the molten metal metal supplied supplied to the rear to the rearis supplied is supplied unstable. unstable. To control To control burn through burn through and humping and humping beads, it was beads, judged it was that judged the thatamount the amountof heat input of heat needs input to be needs reduced. to be Therefore, reduced. the Therefore, welding speed the welding was increased speed in was increasedthe third inpreliminary the third preliminary test. The laser test. output The laser power output was powerfixed to was 1 kW, fixed as to in 1 the kW, second as in the secondpreliminary preliminary experiment, experiment, and then and its then speed its speedwas increased was increased to 2.5 tom/min, 2.5 m/min, 3.0 m/min, 3.0 m/min, 3.5 3.5m/min, m/min, 4.0 m/min, 4.0 m/min, and 4.5 and m/min. 4.5 m/min. An experi An experimentment was performed was performed for five for cases five each cases and each andeach each experimental experimental condition condition is shown is shown in Table in Table 5. 5.

TableTable 5. 5.3rd 3rd ConditionsConditions ofof laserlaser welding fo forr 0.2 0.2 mm mm thickness thickness of of ASTM ASTM F1684. F1684. Welding Parameters Experimental Conditions Welding Parameters Experimental Conditions Laser power (kW) 1 WeldingLaser powerspeed (m/min) (kW) 2.5, 3.0, 3.5, 1 4.0, 4.5 Welding speed (m/min) 2.5, 3.0, 3.5, 4.0, 4.5 DefocusDefocus (mm) 0 0 ShieldingShielding gas ArAr (L/min)(L/min) 15 15 TiltingTilting angle angle & Working angleangle 0 0

After the welding experiment for each case, the welding part was visually observed After the welding experiment for each case, the welding part was visually observed as shown in Figure 11. as shown in Figure 11.

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FigureFigure 11. Surface 11. Surface geometry geometry of the of the third third experiment experiment of of Lap Lap joint joint welding. welding. Figure 11. Surface geometry of the third experiment of Lap joint welding. DuringDuring the the thirdthird preliminarypreliminary experiment, experiment, Case Case 11 11showed showed multiple multiple points points of burn of burn through.through.During In In Case Case the 12,third 12, there there preliminary was was burnburn experiment, throughthrough at theCase the starting starting 11 showed point point multiple and and the the centerpoints center ofof ofweld- burn welding. through. In Case 12, there was burn through at the starting point and the center of weld- Ining. Cases In Cases 13, 14, 13, and14, and 15, 15, there there was was burn burnthrough through at at the the starting starting point point of welding, of welding, but but ing. In Cases 13, 14, and 15, there was burn through at the starting point of welding, but sufﬁcientsufficient penetration penetration depthdepth and back back bead bead were were created. created. The The burn burn through through that thatoccurs occurs at at sufficientthe starting penetration point of welding depth and can backbe solved bead werethrough created. welding The output burn through pulse control. that occurs at thethe starting starting point point of of weldingwelding can be be solved solved through through welding welding output output pulse pulse control. control. 3.1.3.1. Analysis Analysis of of Cross-Section Cross-Section Observation Results Results 3.1. AnalysisA cross-sectional of Cross-Section analysis Observation was performed Results based on the third preliminary test speci- A cross-sectional analysis was performed based on the third preliminary test specimen. men.A The cross-sectional specimen was analysis taken fromwas performedthe center of based the welding on the third part andpreliminary the cross-sectional test speci- The specimen was taken from the center of the welding part and the cross-sectional men.observation The specimen results arewas shown taken infrom Figure the center12. of the welding part and the cross-sectional observationobservation results results areare shownshown in Figure Figure 12. 12 .

Figure 12. Bead geometry of Lap joint welding. Figure 12.Figure Bead 12.geometryBead geometry of Lap joint of welding. Lap joint welding. Through a visual observation, no internal crack was found in the welding part in CasesThroughThrough 11 to 15, aa but visualvisual there observation,observation, were under nofill no internaland internal root crack concavity crack was was foundin Case found in 11 the inand thewelding root welding concavity part partin in CasesCasesin Cases 11 11 to to12 15, 15,and but but 13. therethere In Cases were 14 under under and fill15, ﬁll andthere and root rootwere concavityconcavity complete in penetrationCase in Case 11 and 11 andandroot rootbackconcavity concavitybead ininwithout Cases Cases 12 any12 and and visible 13. 13. InIncracks CasesCases or 14 14porosity. and and 15, 15, Next,there there thewere were top complete bead complete width, penetration penetration top bead and height, andback back weldbead bead withoutwithoutzone width, any any visible backvisible bead crackscracks width, or porosity. back bead Next, Next, heig theht, the andtop top beadpenetration bead width, width, oftop Cases topbead bead 11 height, to height,15 weldwere weld zonezonemeasured width, width, as back backshown beadbead in Figure width,width, 13. back back Table bead bead 6 show heig height,sht, the and cross-sectional and penetration penetration observationof Cases of Cases 11 resultsto 11 15 to were and 15were measured as shown in Figure 13. Table 6 shows the cross-sectional observation results and measuredthe measured as shown values in for Figure each 13welding. Table condition.6 shows the cross-sectional observation results and the measured values for each welding condition. the measured values for each welding condition.

Table 6. Sectional observation measurement result.

Bead Bead Weld Back Bead Back Bead Penetration Case Width Height Zone Width Height (mm) (mm) (mm) (mm) (mm) (mm) 11 0.690 0.105 0.620 0.485 −0.018 0.716 12 0.812 0.030 0.559 0.423 −0.021 0.648 13 0.766 0.051 0.499 0.433 −0.020 0.654 14 0.704 0.020 0.485 0.448 0.025 0.670 15 0.698 0.004 0.474 0.435 0.027 0.663 ProcessesProcesses2021 2021,,9 9,, 428x FOR PEER REVIEW 99 ofof 1516

Figure 13. Sectional observation measurement location.location.

TableAs 6. Sectional Table6 shows, observation Cases measurement 11, 12, and result. 13 showed a negative value for the back bead height. If the back bead height value is negative, it is considered that root concavity Bead Bead Back Bead Back Bead has occurred. In other words,Weld Cases Zone 11, 12, and 13 showed welding defectsPenetration due to the Case Width Height Width Height occurrence of root concavity.(mm) Cases 14 and 15 have positive values for the(mm) back bead height, meaning(mm) back(mm) beads were successfully(mm) created. Next,(mm) the values measured from

Processes 2021, 9, x FOR PEER REVIEWcross-sectional11 0.690 observation0.105 were 0.620 compared and 0.485 analyzed for each−0.018 laser speed.100.716 The of 16 trends of top12 bead0.812 width, 0.030 top bead height, 0.559 weld zone 0.423width, back bead−0.021 width, back bead0.648 height, and13 penetration 0.766 depending0.051 on laser0.499 speed increase 0.433 are as shown−0.020 in Figure 14 below.0.654 14 0.704 0.020 0.485 0.448 0.025 0.670

0.74 15 0.698 0.004 0.4740.85 0.435 0.027 0.663 0.72 0.80 0.7 As Table 6 shows, Cases 11, 12, and 13 showed a negative value for the back bead 0.68 0.75 height. If the back bead height value is negative, it is considered that root concavity has 0.66 0.70 0.64 occurred. In other words, Cases 11, 12, and 13 showed welding defects due to the occur- 0.65 Bead width(mm) Penetration(mm) 0.62 rence of root concavity. Cases 14 and 15 have positive values for the back bead height, 0.6 meaning back beads were successfully0.60 created. Next, the values measured from cross- 2.533.544.5 2.533.544.5 sectionalLaser speed(m/min) observation were compared and analyzedLaser speed(m/min) for each laser speed. The trends of top

bead width, top bead height, weld zone width, back bead width, back bead height, and (a) (b) penetration depending on laser speed increase are as shown in Figure 14 below.

0.12 0.7

0.1 0.6 0.5 0.08 0.4 0.06 0.3 0.04 0.2

0.02 Weld zone(mm) Bead height(mm) 0.1 0 0 2.533.544.5 2.533.544.5 Laser speed(m/min) Laser speed(m/min)

0.49 0.49 0.48 0.48 0.47 0.47 0.46 0.46 0.45 0.45 0.44 0.44 0.43 0.43 0.42 0.42 0.41 0.41 Back bead width(mm) 0.4 Back bead height(mm) 0.4 0.39 0.39 2.5 3 3.5 4 4.5 2.533.544.5 Laser speed(m/min) Laser speed(m/min)

(e) (f)

Figure 14. Changes in section observationFigure 14. Changes measurement in section values.(observationa) Penetration; measurement (b values.() Beada width;) Penetration; (c) Bead (b) heightBead width; (d) Weld (c) Bead height (d) Weld zone; (e) Back bead width; (f) Back bead height. zone; (e) Back bead width; (f) Back bead height. In Figure 14, it is not easy to see the tendency related to laser speed at 2.5 m/min, 3.0 m/min, and 3.5 m/min conditions in (a) Penetration, (e) Back bead width, and (f) Back bead height. This is because the value has been affected by the occurrence of root concavity. As the laser speed increased, the (b) Bead width, (c) Bead height, and (d) Weld zone values tended to decrease. It was confirmed that the back bead was created under the conditions of laser speed 4.0 m/min and 4.5 m/min in the (f) Back bead height graph.

3.2. Shear Strength Test From the results of the cross-sectional analysis in Section 3.1, the optimum conditions Case 14 for lap joint fiber laser welding for 0.25 mm ASTM F1684 can be identified. When σ = P/A1, τ = P/A2, a fracture occurs in a base metal if the tensile stress(σ) is greater than the shear stress(τ) when the load (P) applied to the welding part is the same and the welding line is the same, as shown in Figure 15. To prevent the breakage of a welding part and induce the breakage of a base metal, Cases 14 and 15 were selected, where the WL (weld line) was 0.25 mm or more and the back bead was created.

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In Figure 14, it is not easy to see the tendency related to laser speed at 2.5 m/min, 3.0 m/min, and 3.5 m/min conditions in (a) Penetration, (e) Back bead width, and (f) Back bead height. This is because the value has been affected by the occurrence of root concavity. As the laser speed increased, the (b) Bead width, (c) Bead height, and (d) Weld zone values tended to decrease. It was conﬁrmed that the back bead was created under the conditions of laser speed 4.0 m/min and 4.5 m/min in the (f) Back bead height graph.

3.2. Shear Strength Test From the results of the cross-sectional analysis in Section 3.1, the optimum conditions Case 14 for lap joint ﬁber laser welding for 0.25 mm ASTM F1684 can be identiﬁed. When σ = P/A1, τ = P/A2, a fracture occurs in a base metal if the tensile stress(σ) is greater Processes 2021, 9, x FOR PEER REVIEW τ 11 of 16 than the shear stress( ) when the load (P) applied to the welding part is the same and the welding line is the same, as shown in Figure 15. To prevent the breakage of a welding part and induce the breakage of a base metal, Cases 14 and 15 were selected, where the WL (weld line) was 0.25 mm or more and the back bead was created.

P P

0.25mm

A1 A2 WL Figure 15. Shear strength prediction model. Figure 15. Shear strength prediction model.

AA total total of of eight eight repeated repeated experiments werewere performed, performed, i.e., i.e., four four times times each each under under CasesCases 14 14 and and 15 15 welding welding conditions, conditions, andand thethe resultsresults of of the the shear shear strength strength test test are are as as shown shown inin Tables Tables 77 andand 88..

TableTable 7. 7. CaseCase 14 14 shear shear strength strength test result.result.

SpecimenSpecimen Size Size (mm) (mm) CaseCase # # MaxMax Force Force (N) ThickThick Width Width CaseCase 14_1 14_1 0.25 0.25 20.22 20.22 2043 2043 CaseCase 14_2 14_2 0.25 0.25 19.97 19.97 2045 2045 Case 14_3 0.25 20.14 2050 CaseCase 14_3 14_4 0.25 0.25 20.14 20.15 2050 1998 Case 14_4 0.25 20.15 1998 Average 2034 Average 2034 Standard deviation 20.94 Standard deviation 20.94

Table 8. Case 15 shear strength test result. Table 8. Case 15 shear strength test result. Specimen Size (mm) Case # Specimen Size (mm) Max Force (N) Case # Thick Width Max Force (N) Thick Width CaseCase 15_1 15_1 0.25 20.00 20.00 2008 2008 Case 15_2 0.25 19.90 2048 CaseCase 15_2 15_3 0.25 19.61 19.90 1988 2048 CaseCase 15_3 15_4 0.25 20.23 19.61 2043 1988 Case 15_4 Average 0.25 20.23 2021.75 2043 StandardAverage deviation 24.842021.75 Standard deviation 24.84

In the shear tension test of the fillet joint, only the maximum load value was presented at the time of shear tension because interfacial failure occurred at the weld. As shown in Tables 7 and 8, it was confirmed that the average max force value of Case 14 was 2034 N and the average value of Case 15 was 2021.75 N. After the shear strength test, the fracture appearance of each case was observed, as shown in Figures 16 and 17.

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In the shear tension test of the ﬁllet joint, only the maximum load value was presented at the time of shear tension because interfacial failure occurred at the weld. As shown in Tables7 and8, it was conﬁrmed that the average max force value of Case 14 was 2034 N Processes 2021, 9, x FOR PEER REVIEW 12 of 16 Processes 2021, 9, x FOR PEER REVIEWand the average value of Case 15 was 2021.75 N. After the shear strength test, the fracture12 of 16 appearance of each case was observed, as shown in Figures 16 and 17.

FigureFigure 16.16. CaseCase 1414 conditioncondition appearanceappearance shape after shear fracture. fracture.

Figure 17. Case 15 condition appearance shape after shear fracture. FigureFigure 17.17. CaseCase 1515 conditioncondition appearance shape after shear fracture. fracture.

ObservingObserving thethe fracturefracture appearance, it it was was found found that that under under all all conditions conditions a a fracture fracture occurredoccurred atat thethe interfaceinterface ofof a welding part, not the base base metal. metal.

3.3.3.3. TemperatureTemperature MeasurementMeasurement ofof WeldingWelding Part TheThe temperaturetemperature in the vicinity of of the the weld weldinging part part was was measured measured in in order order to to check check thethe heat-affectedheat-affected zonezone ofof thethe ﬁberfiber laserlaser of 0.25 mm thick ASTM F1684. F1684. A A reproduction reproduction experimentexperiment waswas performedperformed by applying the we weldinglding conditions conditions of of Case Case 14, 14, which which did did not not showshow weldingwelding defectsdefects in the cross-sectional analysis, analysis, as as shown shown in in Section Section 3.1. 3.1 .To To measure measure thethe temperature,temperature, oneone thermocouplethermocouple waswas attached to each each of of the the top top and and back back surfaces. surfaces. The The temperaturetemperature waswas measuredmeasured at four locations, locations, i.e., i.e., 1 1 mm, mm, 10 10 mm, mm, 20 20 mm, mm, and and 30 30 mm, mm, as as shownshown inin FigureFigure 1818 toto checkcheck the temperature at at a a location location as as close close to to the the welding welding line line as as possible,possible, andand thethe actualactual attachmentattachment is shown in Figure 19.19.

Figure 18. Schematic diagram of thermocouple attachment location.

Processes 2021, 9, x FOR PEER REVIEW 12 of 16

Figure 16. Case 14 condition appearance shape after shear fracture.

Figure 17. Case 15 condition appearance shape after shear fracture.

Observing the fracture appearance, it was found that under all conditions a fracture occurred at the interface of a welding part, not the base metal.

3.3. Temperature Measurement of Welding Part The temperature in the vicinity of the welding part was measured in order to check the heat-affected zone of the fiber laser of 0.25 mm thick ASTM F1684. A reproduction experiment was performed by applying the welding conditions of Case 14, which did not show welding defects in the cross-sectional analysis, as shown in Section 3.1. To measure the temperature, one thermocouple was attached to each of the top and back surfaces. The temperature was measured at four locations, i.e., 1 mm, 10 mm, 20 mm, and 30 mm, as Processes 2021, 9, 428 shown in Figure 18 to check the temperature at a location as close to the welding line12 as of 15 possible, and the actual attachment is shown in Figure 19.

Processes 2021, 9, x FOR PEER REVIEW 13 of 16 Processes 2021, 9, x FOR PEER REVIEW 13 of 16

Figure 18. SchematicFigure 18. diagramSchematic of diagram thermocouple of thermocouple attachment attachment location. location.

(a)( aExperiment)Experiment top top thermocouple thermocouple location (b)Experiment Experiment back back thermocouple thermocouple location location)

FigureFigureFigure 19.19. 19. ThermocoupleThermocouple setting. setting.setting.

◦ TheTheThe highest highest highest temperature temperature temperature of 203203 °C°CC waswas was measuredmeasured measured at at aat a location alocation location 1 mm1 mm1 mm from from from the the welding the welding welding part. The temperature distribution chart obtained from the temperature measurements is part.part. The The temperature temperature distributiondistribution chartchart obtaobtainedined from from the the temperature temperature measurements measurements is is shown in Figure 20. shownshown in in Figure Figure 20. 20.

250 250 250 Top 1mm 250 Back 1mm Top 1mm Back 1mm 194.4 Top 10mm 203.5 Back 10mm 200 194.4 Top 10mm 200 203.5 Back 10mm 200 Top 20mm 200 Back 20mm Top 20mm Back 20mm Top 30mm Back 30mm Top 30mm °C) Back 30mm 150 150°C) 150 150 81 100 81 100 100 39.3 100 82.5 Temperature (°C) Temperature ( 82.526.9 39.333.3 Temperature (°C) 50 Temperature ( 50 26.927.3 50 33.3 50 27.3 0 0 0 0 50 100 150 200 250 0 0 50 100 150 200 250 0 50 100Time (Sec) 150 200 250 0 50 100Time (Sec) 150 200 250

Time (Sec) Time (Sec) (a) Top temperature (b) Back temperature (a) Top temperature (b) Back temperature FigureFigure 20. 20. TemperatureTemperature distribution graph. graph. Figure 20. Temperature distribution graph. By comparing top and back, it was confirmed that similar temperature distributions. ThicknessBy comparing is thought top to and have back, no iteffect was onconfirme the temperatured that similar because temperature the experiment distributions. was Thicknessconducted is on thought thin sheet to materialshave no effectwith high on thethermal temperature conductivity. because By comparing the experiment distance was conductedaway the weldon thin line, sheet it was materials found thatwith as high the thermaldistance conductivity.from became smaller,By comparing the tempera- distance awayture increased.the weld Inline, addition, it was the found temperature that as decrease the distance per distance from showed became a non-linearsmaller, the temperaturetendency. The increased. maximum In temperatureaddition, the and temp theerature time when decrease the maximum per distance temperature showed a oc- non- linearcurs aretendency. shown in TableThe maximum9. temperature and the time when the maximum temperature occurs are shown in Table 9. Table 9. Maximum temperature and the time when the maximum temperature.

TableDistance 9. Maximum from Welding temperature Max and Temp. the time (°C) when Time the (s) maximum at Max temperature. Temp. (°C) Time (s) at Line (mm) of Top Plate Max Temp. of Back Plate Max Temp. Distance from Welding Max Temp. (°C) Time (s) at Max Temp. (°C) Time (s) at 1 194.40 27.25 203.50 27.5 Line (mm) of Top Plate Max Temp. of Back Plate Max Temp. 10 81.00 37.50 82.50 35.00 120 194.40 39.30 27.2527.25 26.90203.50 35.7527.5 1030 33.3081.00 27.2537.50 27.3082.50 61.7535.00 20 39.30 27.25 26.90 35.75 30 33.30 27.25 27.30 61.75

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By comparing top and back, it was conﬁrmed that similar temperature distributions. Thickness is thought to have no effect on the temperature because the experiment was conducted on thin sheet materials with high thermal conductivity. By comparing distance away the weld line, it was found that as the distance from became smaller, the temperature increased. In addition, the temperature decrease per distance showed a non-linear tendency. The maximum temperature and the time when the maximum temperature occurs are shown in Table9.

Table 9. Maximum temperature and the time when the maximum temperature.

Distance from Max Temp. (◦C) Time (s) at Max Temp. (◦C) Time (s) at Welding Line (mm) of Top Plate Max Temp. of Back Plate Max Temp. 1 194.40 27.25 203.50 27.5 10 81.00 37.50 82.50 35.00 20 39.30 27.25 26.90 35.75 30 33.30 27.25 27.30 61.75

4. Discussion In this study, ﬁber laser welding was applied to the lap joint for 0.25 mm thick ASTM F1684 and an experiment was performed to derive the optimum conditions. As an additional experiment, three tensile strength tests were performed on a base metal specimen 370 mm × 20 mm × 0.25 mm in size, which is the same as the shear tensile strength test specimen from the welding part in Figure7. The base metal was cut to a size of 200 mm × 20 mm × 0.25 mm in the directions of 0◦ and 90◦, and tensile strength tests were performed three times each. Nine tensile strength tests were performed and then the results were compared to the strength of the welding part. Table 10 shows the maximum load value when the base metal breaks.

Table 10. Base material shear strength test result.

CASE Length (mm) Rolling Direction (◦) Max Force (N) Base metal 1 370 0 2395 Base metal 2 370 0 2308 Base metal 3 370 0 2325 Base metal 4 200 0 2275 Base metal 5 200 0 2318 Base metal 6 200 0 2285 Base metal 7 200 90 2243 Base metal 8 200 90 2238 Base metal 9 200 90 2280 Average 2296.33 Standard deviation 45.04

As shown in Table 10, it was conﬁrmed that there was no difference in tensile strength depending on the direction of a base metal. In addition, even if the length of a specimen was varied and the experiment was performed in UTM, it was conﬁrmed that there was no effect related to the length. The average maximum load of ASTM F1684 base metal is 2296.33 N and the tensile strength of the welding part is about 88.6% that of the base metal because 2034 N is the average maximum load of the lap joint welding part, as shown in Table6. Fiber laser welding has a volumetric speed that is 2 to 10 times faster than typical arc welding, which means that a shorter manufacturing time and excellent quality without defects can be expected when applied to thin plate ASTM F1684 welding due to its low welding deformation. Processes 2021, 9, 428 14 of 15

5. Conclusions In this study, fiber laser welding was applied to lap joint welding of 0.25 mm thick ASTM F1684 to identify welding conditions that can lead to excellent welding quality and to analyze the maximum temperature during welding. The summary of the findings is as follows. 1. The conditions required for excellent lap joint welding quality of 0.25 t ASTM F1684 were a welding power of 1 kw and a welding speed of 4.0 m/min or 4.5 m/min. When welding at the above conditions, it was confirmed that excellent quality was obtained by securing sufficient beads on the back side. 2. The tensile strength of a welding part was compared with that of a base metal after lap joint welding under the condition that sufficient beads appeared on the back side. When the width was 20 mm and the thickness was 0.25 mm, the tensile strength of the base metal was 2296.33 N and the tensile strength of the welding specimen was 2034 N, which was 88.6% that of the base metal. 3. By comparing top and back, it was confirmed that similar temperature distributions were shown. In addition, it was confirmed that the maximum temperature at a location 1 mm from the welding line was 203.5 ◦C, and the maximum temperature at a location 10 mm away was 81 ◦C.

Author Contributions: Conceptualization, J.K. (Jaewoong Kim) and Y.K. (Younghyun Kim); method- ology, J.K. (Jisun Kim); validation, D.K., and C.P.; formal analysis, S.K.; investigation, C.P.; resources, Y.K. (Yongtai Kim) and D.K.; data curation, J.K. (Jaewoong Kim); writing—original draft preparation, J.K. (Jaewoong Kim), Y.K. (Younghyun Kim); writing—review and editing, C.P.; supervision, C.P.; project administration, J.K. (Jaewoong Kim), Y.K. (Yongtai Kim); funding acquisition, J.K. (Jaewoong Kim). All authors have read and agreed to the published version of the manuscript. Funding: This research was financially supported by the Korea Institute of Industrial Technology (KITECH) through the Research and Development “The dynamic parameter control based smart welding system module development for the complete joint penetration weld” (PEH21035). Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

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