metals

Article Effects of Pulsed Nd:YAG Laser Welding Parameters on Penetration and Microstructure Characterization of a DP1000 Steel Butt Joint

Xin Xue 1,2, António B. Pereira 2 ID , José Amorim 2 and Juan Liao 1,*

1 School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350116, China; [email protected] or [email protected] 2 Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal; [email protected] (A.B.P.); [email protected] (J.A.) * Correspondence: [email protected] or [email protected]; Tel.: +86-0591-2286-6793

Received: 13 June 2017; Accepted: 27 July 2017; Published: 1 August 2017

Abstract: Of particular importance and interest are the effects of pulsed Nd:YAG laser beam welding parameters on penetration and microstructure characterization of DP1000 butt joint, which is widely used in the automotive industry nowadays. Some key experimental technologies including pre-welding sample preparation and optimization design of sample fixture for a sufficient shielding gas flow are performed to ensure consistent and stable testing. The weld quality can be influenced by several process factors, such as laser beam power, pulse duration, overlap, spot diameter, pulse type, and welding velocity. The results indicate that these key process parameters have a significant effect on the weld penetration. Meanwhile, the fusion zone of butt joints exhibits obviously greater hardness than the base metal and heat affected zone of butt joints. Additionally, the volume fraction of martensite of dual-phase steel plays a considerable effect on the hardness and the change of microstructure characterization of the weld joint.

Keywords: pulsed Nd:YAG laser beam welding; DP1000 steel; penetration; hardness; phase transformation

1. Introduction The need to reduce fuel consumption and greenhouse gas emissions motivates vehicle manufacturers to utilize lightweight metals with better ductility and strength [1,2]. Nowadays, dual-phase (DP) have already built a good reputation for performance and safety [3,4]. Meanwhile, the application of DP steels unavoidably involves welding in the manufacturing process and the properties of welded joints due to the integrity and safety requirements. Although such traditional welding methods as resistance spot welding (RSW) [5], tungsten inert gas welding (TIG) [6], metal inert-gas welding (MIG) [7], and gas arc [8,9] have been widely used, the advantages of laser welding are also well known: energy efficiency, slight heat-affected zone, little heat input per unit volume, and deep penetration [10]. The chief limitation, i.e., cost per watt, is swiftly falling with new advances in laser technology [11]. Although in some cases a proper filler material may be demanded when an interfacial gap attends to be filled [12], autogenous welding is still one of the most common forms of laser welding. Recently, many efforts have been performed to understand the sensitivity of the laser welding process in advanced high-strength steels. Pulsed Nd:YAG laser welding has especially attracted attention recently in many academic research and industry applications [13–19]. Tzeng [20] used zinc-coated steel and attempted to make a laser welding joint without gas-formed porosity. Xia et al. [21] used laser welding of DP steels and investigated the sensitivity of heat input and

Metals 2017, 7, 292; doi:10.3390/met7080292 www.mdpi.com/journal/metals Metals 2017, 7, 292 2 of 18

Metals 2017, 7, 292 2 of 18 martensite on adjacent heat-affected zone (HAZ) softening. Several Nd:YAG and diode laser welds wereDP450, made DP600, on DP450,and DP980 DP600, steels and over DP980 a wide steels range over of aheat wide input. range Dong of heat et al. input. [22] studied Dong et the al. rate- [22] studieddependent the rate-dependentproperties by means properties of a wide by means strain of rate a wide range, strain deformation, rate range, deformation,and fracture andbehavior fracture of behaviorDP600 steel of and DP600 its welded steel and joint its welded(WJ) subjected joint (WJ) to the subjected Nd:YAG to laser the Nd:YAGwelding. laserTheir welding.results showed Their resultsthat the showed DP600 thatWJ exhibits the DP600 continuous WJ exhibits yielding continuous at quasi-static yielding atstrain quasi-static rates and strain develops rates anda yield develops point aat yield higher point strain at higher rates. strainBaghjari rates. et al. Baghjari [23] paid et al. attention [23] paid to attention the Nd:YAG to the Nd:YAGlaser welding laser weldingof AISI420 of AISI420martensitic martensitic stainless stainlesssteel as well steel as as the well sensitivitie as the sensitivitiess of voltage, of laser voltage, beam laser diameter, beam energy diameter, frequency, energy frequency,pulse duration, pulse and duration, welding and velocity welding to velocitythe size toand the deformation size and deformation of the welded of the joint. welded However, joint. However,previous studies previous [24–28] studies indicated [24–28] indicatedthat the mechanic that the mechanicalal properties properties of DP steel of DP butt steel joint butt have joint effects have effectsevident evident in the process in the process operation operation parameters. parameters. This may This be due may to be the due soft to zone the soft formed zone in formed the adjacent in the adjacentheat-affected heat-affected zone (HAZ). zone (HAZ).Then the Then question the question arises ariseshow the how welding the welding parameter parameter manipulates manipulates the themechanical mechanical properties properties of DP of DP steel steel butt butt joints joints by by using using laser laser welding. welding. Although Although many many efforts have been made to improve the tensile properties of DP steel butt joints, investigations into the penetration and microstructure propertiesproperties ofof thisthis kindkind ofof weld are limited. Therefore, it is essential to characterize the weldweld penetrationpenetration andand microstructuremicrostructure inin termsterms ofof multiplemultiple weldingwelding parameters.parameters. The purposepurpose of of this this article article is to is experimentally to experimentally investigate investigate the influences the influences of key process of key parameter process onparameter weld penetration on weld penetration of the fusion of zonethe fusion during zone pulsed during Nd:YAG pulsed laser Nd:YAG beam laser welding, beam and welding, to analyze and theto analyze hardness the and hardness microstructure and microstructure evolution ofevolution DP steel of butt DP joint.steel butt In Section joint. In2, Section the base 2, materialthe base used,material DP1000, used, DP1000, was characterized was characterized by some by general some general mechanical mechanical tests. Section tests. Section3 introduces 3 introduces the main the experimentalmain experimental procedures procedures including including cutting cutting surface surface preparation preparation for for pre-welding pre-welding sample, sample, pulsedpulsed Nd:YAG laserlaser beambeam welding, optimization design of welding sample fixture fixture and the microstructure, and hardness testing method. Section4 4 provides provides somesome correspondingcorresponding processprocess parameterparameter dependentdependent theory for a a better better understanding understanding of of the the interactio interactionn between between weld weld quality quality and and process process parameters. parameters. In InSection Section 5, 5the, the influences influences of oflaser laser welding welding parameters parameters on ondepth depth penetration penetration and and micro-hardness micro-hardness are areanalyzed analyzed and and discussed, discussed, as well as well as the as themicrostructure microstructure evolution evolution before before and and after after welding welding process. process.

2. Base Material Characterization: DP1000 Steel The studied dual-phase dual-phase steel steel (DP1000) (DP1000) sheet, sheet, a type a type of advanced of advanced high-strength high-strength steel steel (AHSS), (AHSS), has hasbeen been widely widely utilized utilized in automotive in automotive or shipping or shipping structures structures such such as frames, as frames, wheels, wheels, and andbumpers. bumpers. DP DPsteel steel contains contains a soft a soft ferrite ferrite matrix matrix with with hard hard ma martensiticrtensitic “islands”. “islands”. It It ex exhibitshibits good performanceperformance during thethe forming forming processes processes [29 [29].]. Figure Figure1 shows 1 show the microstructures the microstructure of the experimentalof the experimental base material base bymaterial means by of means a Scanning of a ElectronScanning Microscope Electron Microscope (SEM) (Hitachi, (SEM) Tokyo, (Hitachi, Japan). Tokyo, The selectedJapan). DP1000The selected steel withDP1000 sheet steel thickness with sheet of 1.0 thicknes mm wass of supplied 1.0 mm was by the supplied SSAB (Stockholm, by the SSAB Sweden) (Stockholm, in as-received Sweden) in state. as- Tworeceived identical state. sheetsTwo identical with dimensions sheets with of dimensio 40 mm ×ns14 of mm40 mm (length × 14 mm× width) (length were × width) in one were butt in joint one configurationbutt joint configuration after welding. after Thewelding. chemical The compositionchemical composition is presented is presented in Table1. in Table 1.

Figure 1. Microstructure observation of dual-phase DP1000 steel used. Figure 1. Microstructure observation of dual-phase DP1000 steel used.

Metals 2017, 7, 292 3 of 18

Table 1. Chemical composition of the studied DP 1000 steel in wt %.

MetalsC 2017 Si, 7, 292Mn P S N Cr Ni Cu Al V B Nb3 of Cekv 18 0.14 0.20 1.46 0.013 0.003 0.002 0.03 0.03 0.01 0.051 0.01 0.0004 0.014 0.39 Table 1. Chemical composition of the studied DP 1000 steel in wt %. Cekv = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 C Si Mn P S N Cr Ni Cu Al V B Nb Cekv 0.14 0.20 1.46 0.013 0.003 0.002 0.03 0.03 0.01 0.051 0.01 0.0004 0.014 0.39 In order to obtain the basicCekv mechanical = C + Mn/6properties, + (Cr + Mo + asV)/5 listed + (Ni in + Cu)/15 Table2 , uniaxial tension tests along the rolling direction (RD) were carried out with a nominal strain rate of 10−4 s−1. A Shimadzu universal tensile testingIn order machine to obtain (Schimadzur, the basic mechanical Kyoto, Japan) properties, was used as listed with in a topTable capacity 2, uniaxial 100 kN.tension All tensiontests samplesalong werethe rolling cut by direction a water-jet (RD) and were the dimensionscarried out with complied a nominal with thestrain ASTM rate E8of 10 standard−4 s−1. A [S30himadzu]. The test wasuniversal repeated tensile four times testing to machine verify the (Schimadzu reproducibilityr, Kyoto of the, Japan tested) was results. used Inwith order a top to accuratelycapacity 10 measure0 kN. theAll strain, tension a digital samples image were correlationcut by a water (DIC)-jet and system the dimensions was adopted complied to capture with the ASTM displacement E8 standard fields. The[30]. ARAMIS-5M The test was commercial repeated four code times [31] wasto verify utilized theto reproducibility obtain the strain of the from tested numerous results. optical In order images. to Moreaccurately detailed measure descriptions the strain, on this a digital measurement image correlation technology (DIC) can besystem found was in previousadopted to works capture [9,32 the–34 ]. Thedisplacement macroscopic fields. hardness The ARAMIS of the base-5M material commercial was obtainedcode [31] usingwas utilized a Vickers to obtain hardness the tester,strain namelyfrom Shimadzunumerous HMV-2000 optical images. (Schimadzu, More detailed Kyoto, descriptions Japan). on this measurement technology can be found in previous works [9,32–34]. The macroscopic hardness of the base material was obtained using a Vickers hardnessTable 2. tester,Basic namely mechanical Shimadzu properties HMV of the-2000 studied (Schimadzu DP1000, (RD:Kyoto Rolling, Japan direction).).

Table 2. Basic mechanical properties of the studied DP1000 (RD: Rolling direction). Elastic Modulus, Poisson’ Yield Stress Ultimate Stress Elongation Hardness Direction ElasticE (GPa)Modulus, Poisson’Ratio υ Yieldσy Stress(MPa) Ultimateσu (MPa)Stress Elongatione (%) HardnessHV0.5 Direction RDE ( 210GPa) Ratio 0.3 υ σy (MPa 779) σu (MPa 1125) e (% 9.35) HV0.5 382 RD 210 0.3 779 1125 9.35 382 3. Experimental Procedures 3. Experimental Procedures 3.1. Cutting Surface Preparation for Pre-Welding Sample 3.1. Cutting Surface Preparation for Pre-Welding Sample The sheet cutting processes usually involve heating and melting of a material. This can lead The sheet cutting processes usually involve heating and melting of a material. This can lead to to inhomogeneous microstructure changes and influence the performance of the materials used inhomogeneous microstructure changes and influence the performance of the materials used in the in the subsequent welding process. For instance, Figure2 illustrates a typical cross-sectional surface subsequent welding process. For instance, Figure 2 illustrates a typical cross-sectional surface cut by cuta byguillotine, a guillotine, mainly mainly includi includingng a cutting a cutting area, area,deformation deformation area, rough area, rougharea, and area, butt. and This butt. leads This to leads a tosignificant a significant anomalistic anomalistic gap gap at the at the pre pre-welding-welding sample sample inspection. inspection. In fact, In fact,laser laser beam beam welding welding is is seriouslyseriously dependent on on the the position position and and surface surface quality quality of ofthe the prepared prepared samples, samples, especially especially when when a a narrownarrow laser beam beam is is adopted. adopted. Thus, Thus, it is it vital is vital to cut to cutprecise precise edges edges for almost for almost no gap no between gap between the thesamples samples to to be be welded. welded.

Deformation Cutting area area

Rough area

Anomalistic gap Burr

Figure 2. Illustration of a typical cross-sectional surface cut by guillotine and the anomalistic gap at Figure 2. Illustration of a typical cross-sectional surface cut by guillotine and the anomalistic gap at the the pre-welding inspection. pre-welding inspection. In this work, to ensure all the samples are welded under the same configuration, the inhomogeneousIn this work, cross to section ensure of the all cut the sample samples was firstly are welded machined under flat using the computer same configuration, numerical thecontrol inhomogeneous (CNC) and cross then section the burr of thewas cut carefully sample removed was firstly with machined sandpaper. flat usingTo remove computer oil or numerical other impurities, the machined samples were cleaned with acetone. Figure 3 illustrates this additional and important experimental preparation work, namely cutting surface preparation. It can be observed that there is almost no gap, which leads to consistency in pre-welding inspection.

Metals 2017, 7, 292 4 of 18 control (CNC) and then the burr was carefully removed with sandpaper. To remove oil or other impurities, the machined samples were cleaned with acetone. Figure3 illustrates this additional and important experimental preparation work, namely cutting surface preparation. It can be observed that thereMetals is almost2017, 7, 29 no2 gap, which leads to consistency in pre-welding inspection. 4 of 18

Metals 2017, 7, 292 4 of 18 Sample 1 Sample 2

CNC Acetone Clean Removed No gap burr

FigureFigure 3. 3Cutting. Cutting surface surface preparationpreparation forfor pre-weldingpre-welding inspection inspection by by using using computer computer numerical numerical control (CNC). controlFigure (CNC). 3. Cutting surface preparation for pre-welding inspection by using computer numerical 3.2. Pulsedcontrol Nd: (CNC).YAG Laser Welding Set-Up 3.2. Pulsed Nd:YAG Laser Welding Set-Up 3.2. PulsedIn this Nd:YAG work, pulsed Laser Welding Nd:YAG Set-Up laser tested system, i.e., SISMA SWA300, was utilized to perform experiments.In this work, Figure pulsed 4 shows Nd:YAG the experimental laser tested system,apparatus i.e., used. SISMA The SWA300, main parameters was utilized of the to control perform In this work, pulsed Nd:YAG laser tested system, i.e., SISMA SWA300, was utilized to perform experiments.system are as Figure follows:4 shows maximum the experimentalmean power of apparatus 300 W, wave used. length The of main 1064 parametersnm, maximum of peak the control laser experiments. Figure 4 shows the experimental apparatus used. The main parameters of the control systempower are of as12 follows:kW, maximum maximum pulse meanenergy power of 100 J of, and 300 the W, range wave of length pulse ofrange 1064 from nm, 0.2 maximum ms to 25 ms. peak system are as follows: maximum mean power of 300 W, wave length of 1064 nm, maximum peak laser laserTo powerensure the of 12pool kW, of maximumthe butt joint pulse against energy the oxidation, of 100 J, argon and the shielding range ofgas pulse with rangehigh purity from (99.99%) 0.2 ms to power of 12 kW, maximum pulse energy of 100 J, and the range of pulse range from 0.2 ms to 25 ms. 25was ms. supplied To ensure to the both pool bottom of the and butt top joint surfaces against of the the specimens oxidation, at argon a gas shieldingflow rate of gas 10 withL/min. high A s purityolid To ensure the pool of the butt joint against the oxidation, argon shielding gas with high purity (99.99%) (99.99%)state pulsed was supplied Nd:YAG to laser both bottomwith the and above top surfacesparameters of theis appropriate specimens atfor a gasfibe flowr passing rate of from 10 L/min. the was supplied to both bottom and top surfaces of the specimens at a gas flow rate of 10 L/min. A solid A solidresonator state to pulsed the laser Nd:YAG beam head. laser This with ensur the abovees that parameterslittle welding is occurs appropriate far from for the fiber laser passing source. from It state pulsed Nd:YAG laser with the above parameters is appropriate for fiber passing from the theis resonator particularly to theuseful laser for beam large head.or complex This ensuresstructures. that In littlethis work, welding the occursdistance far between from the the laser laser source. exit resonator to the laser beam head. This ensures that little welding occurs far from the laser source. It It isand particularly the sample useful was set for at largea value or of complex 105 mm structures.with optimal In resolution. this work, the distance between the laser is particularly useful for large or complex structures. In this work, the distance between the laser exit exit and the sample was set at a value of 105 mm with optimal resolution. and the sample was set at a value of 105 mm with optimal resolution.

Figure 4. Experimental apparatus of pulsed Nd:YAG laser welding. Figure 4. Experimental apparatus of pulsed Nd:YAG laser welding. 3.3. Design OptimizationFigure of 4. WeldingExperimental Sample apparatus Fixture of pulsed Nd:YAG laser welding. 3.3. DesignIn order Optimization to avoid accidental of Welding misalignment Sample Fixture or movement during laser welding, a characteristic 3.3.welding Design sample Optimization fixture of has Welding been Sampledesigned Fixture to ensure stable and consistent experimental tests. It is a In order to avoid accidental misalignment or movement during laser welding, a characteristic relatively simple but good enough sheet metal sample fixture. It includes three steps to assemble the weldingIn order sample to avoid fixture accidental has been misalignment designed to ensure or movement stable and during consistent laser experimental welding, a characteristictests. It is a welding samples, as shown in Figure 5. The samples snap into place and clamp with screws along weldingrelatively sample simple fixture but good has enough been sheet designed metal tosample ensure fixture. stable It includes and consistent three steps experimental to assemble the tests. the longitudinal and normal directions. It iswelding a relatively samples, simple as butshown good in enoughFigure 5. sheet The metalsamples sample snap fixture.into place It includesand clamp three with steps screws to assemble along thethe welding longitudinal samples, and as normal shown directions. in Figure 5. The samples snap into place and clamp with screws along the longitudinal and normal directions.

Metals 2017, 7, 292 5 of 18 Metals 2017, 7, 292 5 of 18

MetalsMetals 2017 2017, ,7 7, ,292 292 55 of of 18 18

FigureFigure 5. 5.Design Design of welding sample sample fixture. fixture. Figure 5. Design of welding sample fixture. For laser welding, the flowFigure ways 5. of Design assisted of welding gas inlet sample are usually fixture. ignored or at casual placement duringFor laser the pre-welding welding, the preparation. flow ways ofIn assistedthis work, gas a two-hole inlet are inlet usually structure ignored design or at (see casual Figure placement 6) of ForFor laser laser welding, welding, the the flow flow ways ways of of assisted assisted gas gas inlet inlet are are usually usually ignored ignored or or at at casual casual placement placement duringassisted the pre-weldinggas flow was preparation.developed to Inensure this work, that both a two-hole the top inletand bottom structure surfaces design of (see the Figurewelding6) of duringduring the the pre-welding pre-welding preparation. preparation. In In this this work, work, a a two-hole two-hole inlet inlet structure structure design design (see (see Figure Figure 6) 6) of of assistedsample gas have flow entire was developed shielding togas ensure during that the both welding the top process. and bottom Figure surfaces 7 compares of the three welding types sample of assistedassisted gasgas flowflow waswas developeddeveloped toto ensureensure thatthat bothboth thethe toptop andand bottombottom surfacessurfaces ofof thethe weldingwelding haveassisted entire gas shielding solutions, gas i.e., during without the assisted welding gas, process. one-hole Figure inlet structure,7 compares and three two-hole types inlet of assistedstructure. gas samplesample havehave entireentire shieldingshielding gasgas duringduring thethe weldingwelding process.process. FigureFigure 77 comparescompares threethree typestypes ofof solutions,It indicates i.e., withoutthat the assisteddesign with gas, a one-hole two-hole inlet inlet structure, structure andallows two-hole the possibility inlet structure. of reducing It indicates the assistedassisted gas gas solutions, solutions, i.e., i.e., without without assisted assisted gas, gas, one-hole one-hole inlet inlet structure, structure, and and two-hole two-hole inlet inlet structure. structure. thatoxidation the design on withboth athe two-hole top and inletbottom structure weld surfaces. allows the possibility of reducing the oxidation on both ItIt indicatesindicates thatthat thethe designdesign withwith aa two-holetwo-hole inletinlet structurestructure allowsallows thethe possibilitypossibility ofof reducingreducing thethe theoxidationoxidation top and bottomon on both both weldthe the top top surfaces. and and bottom bottom weld weld surfaces. surfaces.

Figure 6. Optimization design of assisted gas flow solution with two-hole inlet structure.

FigureFigureFigure 6. 6. Optimization6. Optimization Optimizationdesign design design of of assisted assisted gas gas flow flow flow solution solution solution with with with two-hole two-hole two-hole inlet inlet inlet structure. structure. structure.

Figure 7. Comparison of welding top surface quality with different assisted gas inlet structures. FigureFigure 7. 7. Comparison Comparison of of welding welding top top surface surface quality quality with with different different assisted assisted gas gas inlet inlet structures. structures. 3.4. MicrostructureFigure 7. Comparison and Micro-Hardness of welding top Testing surface quality with different assisted gas inlet structures. 3.4.3.4. Microstructure MicrostructureThe microstructure and and Micro-Hardness Micro-Hardness examination Testing Testingwas conducted by means of scanning electron microscope 3.4.(SEM). Microstructure The etching and condition Micro-Hardness was carried Testing out through the immersion of the samples with 5% nital TheThe microstructuremicrostructure examinationexamination waswas conductedconducted byby meansmeans ofof scanningscanning electronelectron microscopemicroscope solution. The first macro-hardness tests were performed with a holding time 10 s using a Vickers (SEM).(SEM).The microstructure TheThe etchingetching conditioncondition examination waswas carriedcarried was conducted outout throughthrough by the meansthe immersionimmersion of scanning ofof thethe electron samplessamples microscope withwith 5%5% nitalnital (SEM). hardness tester, namely Shimadzu HMV-2000 (Shimadzu, Kyoto, Japan). The test force of 20 mN can Thesolution.solution. etching The conditionThe firstfirst macro-hardnessmacro-hardness was carried out teststests through werewere performedperformed the immersion withwith a ofa holdingholding the samples timetime 1010 with ss usingusing 5% nital aa VickersVickers solution. be used as well as the application of 115° triangular pyramid indenter. The distance between the test Thehardnesshardness first macro-hardness tester, tester, namely namely tests Shimadzu Shimadzu were performedHMV-2000 HMV-2000 with(Shimadzu, (Shimadzu, a holding Kyoto, Kyoto, time Japan). Japan). 10 s using The The test test a Vickers force force of of hardness 20 20 mN mN can can tester, positions is 0.25 mm. The second is a CSM Nano-indenter (TTX-NHT, CSM Instruments, Neuchatel, bebe used used as as well well as as the the application application of of 115° 115° triangular triangular pyramid pyramid indenter. indenter. The The distance distance between between the the test test namelySwitzerland) Shimadzu used HMV-2000 for the (Shimadzu,small-scale Kyoto,micro-ha Japan).rdness The measurement. test force of 20The mN tested can besamples used aswere well as thepositions applicationpositions is is 0.25 0.25 of 115 mm. mm.◦ triangular The The second second pyramid is is a a CSM CSM indenter. Nano-indenter Nano-indenter The distance (TTX-NHT, (TTX-NHT, between CSM CSM the Instruments, Instruments, test positions Neuchatel, Neuchatel, is 0.25 mm. Switzerland) used for the small-scale micro-hardness measurement. The tested samples were TheSwitzerland) second is a CSMused Nano-indenterfor the small-scale (TTX-NHT, micro-ha CSMrdness Instruments, measurement. Neuchatel, The tested Switzerland) samples usedwere for

Metals 2017, 7, 292 6 of 18 the small-scale micro-hardness measurement. The tested samples were polished by emery particle paper TTP2000 (Zebra Technologies, Lincolnshire, IL, USA). For the measurement of weld penetration, a microscope, namely MitutoyoTM (Kanagawa-ken, Tokyo, Japan), was used. All the corresponding experimental tests were sufficiently spaced to prevent any underlying obstruction of strain fields giving rise to contiguous indentations.

4. Process Parameter Dependent Theory

4.1. Process Parameters Relationship of General Laser Welding To obtain exhaustive penetrated high-quality joint after laser beam welding, it is essential to study the optimal process conditions. Generally, they consist of three main groups: laser beam parameters (diameter, wavelength, power, and divergence), sample capabilities (material density, thermal conductivity, thickness, potential heat of melting and cooling), and key welding parameters (velocity, shielding gas direction, surface absorptivity, and focusing element length). Compared to absorptivity about 10–20% at room temperature for most steels, it is able to increase to 80–90% during heating and leaps when the material’s melting point is reached. The basic relationship of necessary heat Q and mass m can be explained by [35]:

Q = m[c(Tm − T0) + Lm], (1) where c, Tm, T0, and Lm are the particular heat, melting and initial temperatures, and potential heat of melting, respectively. For continuous laser beam welding, the above relationship can be transformed as follows: Q/t = ρDhυ[c(Tm − T0) + Lm], (2) where Q/t denotes requested power for melting. ρ, D, h, and υ are the density, laser diameter, penetration, and welding velocity, respectively. The depth of penetration is obtained by:

Q/t h = . (3) ρDυ[c(Tm − T0) + Lm]

Including the absorption of weld surface and the losses of heat conduction, the calculation of penetration is estimated as: P h = κ , (4) Dυ where P and κ are the power and a constant associated with surface absorption and latent energy losses. P/υ defines the ratio of heat input to a unit length. The penetration depth is related to the adopted power and is proportionally opposite to laser spot diameter and welding velocity. More detailed descriptions of pulsed laser parameters can be seen in the next section.

4.2. Pulse Welding Parameters For a pulsed laser, the main conditions, i.e., flash lamp charging voltage U (V), frequency f (Hz), and pulse duration t (ms) are three key process parameters in terms of laser source. The practical pulse energy E (J) can be defined by the above parameters. Peak power Ppeak (kw) is given by a portion of laser beam energy and pulse duration as follows:

Ppeak = E/t. (5)

For a given spot size, the laser beam intensity is dependent on peak power. Average laser power P (W) is defined as a product of practical energy and frequency:

P = E f , (6) Metals 2017, 7, 292 7 of 18 Metals 2017, 7, 292 7 of 18

PEf= , (6) and gives the laser welding velocity. Nowadays, the first general pulsed laser welding method is spot and gives the laser welding velocity. Nowadays, the first general pulsed laser welding method is spot welding. It is also generally applied for rude fixing of structures to be subsequently seam process. welding. It is also generally applied for rude fixing of structures to be subsequently seam process. One typicalOne advantage typical advantage of this procedure of this procedure is to mitigate is to dimensional mitigate dimensional distortions distor causedtions by caused high thermalby high gradients withthermal respect gradients to power with respect density. to Topower obtain density. successive To obtain strict successive welds, strict pulse welds, overlap pulse (P Ooverlap) must (P beO) adopted asmust well be as adopted the appropriate as well as the combination appropriate of combination welding parameters. of welding parameters. Thus, pulse Thus, overlap pulse overlap can be given by: can be given by: υ PO = 1 − .υ (7) D=− f PO 1 . (7) Compared to conventional and continuous laser beamDf welding, higher pulse densities result in higher thermalCompared transformation to conventional rates. and continuous This could laser induce beam welding, weld defects higher andpulse inhomogeneous densities result in microstructure.higher Thus,thermal another transformation important rates. parameter, This could namely induce effective weld peakdefects power and densityinhomogeneous (Depp), should be consideredmicrostructure. for Thus, the important another important contribution parame of weldter, namely quality. effective It can bepeak defined power by density a product ( Depp of), peak powershould density be considered (Dpp) and for overlapping the important factor contributionO f : of weld quality. It can be defined by a product of peak power density ( D ) and overlapping factor O : pp f 1 D = O × D , with O = . (8) epp f pp f 1 − P O 1 D=× O D, with O = epp f pp f − . (8) 4.3. Design of Experiments for Laser Welding 1 PO In order to explore the effects of each single process factor on penetration and microstructure properties4.3. of DP Design steel, of Experiments experimental for Laser design Welding is performed. First, determine the parameters that ensure fairly good laserIn quality.order to explore In other the words, effects the of each full weldsingle penetration process factor should on penetration be obtained. and microstructure Then, based on the fullproperties weld penetration, of DP steel, further experimental process design parameters is performed. optimization First, determine can be the performed parameters to that analyze ensure the effect offairly microstructure good laser quality. properties, In other e.g., words, micro-hardness. the full weld penetration Additionally, should the be optimal obtained. weld Then, should based ensure no cracks,on the full high weld resistance, penetration, a suitable further sizeprocess and para shapemeters of meltingoptimization fusion, can and be performed so on. to analyze Table3the presents effect of the microstructure selected welding properties, parameters e.g., micro-hardness. of the experimental Additionally, plan. the More optimal detailed weld lists should of ensure no cracks, high resistance, a suitable size and shape of melting fusion, and so on. each analyzed welding parameter are given in the AppendixA, Tables A1–A6. In each set of one-factor Table 3 presents the selected welding parameters of the experimental plan. More detailed lists experiments, only one parameter was varied, keeping all the other parameters fixed to identify of each analyzed welding parameter are given in the Appendix A, Tables A1–A6. In each set of one- the sensitivityfactor of experiments, the one tested only parameter.one parameter Seven was varied, different keeping types all of the pulse, other i.e.,parameters a simple fixed rectangular to identify pulse, a modularthe sensitivity pulse withof the three one tested sections, parameter. a rectangular Seven d pulseifferent with types a rampof pulse, start, i.e., aa rectangularsimple rectangular pulse with a ramppulse, end, a amodular rectangular pulse pulse with three with sections, a ramp starta rectangular and end, pulse the emission with a ramp of laserstart, beama rectangular pulses withpulse three consecutivewith a ramp equal end, intervals a rectangular of 1 ms, pulse and awith pulse a ramp with start simple and power end, the divided emission to beof betterlaser beam resolution pulses at low power,with were three analyzed consecutive and equal discussed. intervals It of should 1 ms, an bed noteda pulse that with the simple first simplepower divided rectangular to be pulsebetter is the bestresolution and most widelyat low power, used in were real analyzed or experimental and discus tests.sed. It should be noted that the first simple rectangular pulse is the best and most widely used in real or experimental tests.

Table 3. DesignTable 3. ofDesign experiments of experiments for welding for welding parameters. parameters.

SeriesSeries Parameters Parameters Variation Variation of Single Factor A Power percent (%) 20, 40, 60, 80, 86 A Power percent (%) 20, 40, 60, 80, 86 B Duration (ms) 0.3, 3, 6, 9, 12, 15, 18, 21, 23 B Duration (ms) 0.3, 3, 6, 9, 12, 15, 18, 21, 23 C Overlap (%) 0, 20, 40, 60, 80, 95 C Overlap (%) 0, 20, 40, 60, 80, 95 D D Laser Laser beam beam diameter diameter (mm) (mm) 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0

EE PulsePulse type type , , , , , , FF Velocity Velocity (mm/s) (mm/s) 0.1, 1.0, 1.5, 2.0, 2.7

5. Results and Discussions

5.1. Effects of Laser Beam Parameters on Weld Penetration

5.1.1. Effects of Laser Beam Power on Weld Penetration Laser beam power was changed in the interval from 20% to 80%. Figure8 shows the weld surfaces on the top and bottom and at the beginning (left side) and end (right side) of the perpendicular Metals 2017, 7, 292 8 of 18 Metals 2017, 7, 292 8 of 18 5. Results and Discussions 5. Results and Discussions 5.1. Effects of Laser Beam Parameters on Weld Penetration 5.1. Effects of Laser Beam Parameters on Weld Penetration 5.1.1. Effects of Laser Beam Power on Weld Penetration 5.1.1.Metals Effects2017, 7, 292of Laser Beam Power on Weld Penetration 8 of 18 Laser beam power was changed in the interval from 20% to 80%. Figure 8 shows the weld Laser beam power was changed in the interval from 20% to 80%. Figure 8 shows the weld surfaces on the top and bottom and at the beginning (left side) and end (right side) of the surfaces on the top and bottom and at the beginning (left side) and end (right side) of the perpendicularlongitudinal section. longitudinal It can besection. observed It can that be the observed weld penetrations that the weld reach penetrations almost full reach when almost the power full perpendicular longitudinal section. It can be observed that the weld penetrations reach almost full percentageswhen the power are higher percent thanage 60%.s are However, higher than the40% 60%. laser However, power isthe obviously 40% laser insufficient. power is In obvious addition,ly when the power percentages are higher than 60%. However, the 40% laser power is obviously insufficient.there are some In addition, spatters there on the are top some weld spatters surface on when the top the weld laser surface power when percent the laser is larger power than percent 60%. insufficient. In addition, there are some spatters on the top weld surface when the laser power percent Thisis larger might than be 60%. due This to the might clamping be due force to the on clamping the samples. force The on potentialthe samples. question The potential of thermodynamic question of is larger than 60%. This might be due to the clamping force on the samples. The potential question of thermodynamicinstability in the instability welding pool in the arises. welding pool arises. thermodynamic instability in the welding pool arises. As shown in Fig Figureure9 9,, itit isis possiblepossible toto estimateestimate thatthat thethe idealideal powerpower forfor thethe weldingwelding ofof thethe studiedstudied As shown in Figure 9, it is possible to estimate that the ideal power for the welding of the studied case should be between 40% and and 60%. 60%. Weld Weld penetration penetration has has a a sharp sharp rise rise in in this this range range of of laser laser power. power. case should be between 40% and 60%. Weld penetration has a sharp rise in this range of laser power. However, the sensitivity of the weld penetrationpenetration to laser beam power greater than 60% becomes less However, the sensitivity of the weld penetration to laser beam power greater than 60% becomes less pronounced. In In a a word, the effects of power on penetration are significant significant and must be taken into pronounced. In a word, the effects of power on penetration are significant and must be taken into account during laser welding applications. account during laser welding applications.

Top (15×) Bottom (15×) Perpendicular section(15×)

20%

40%

60%

80%

Figure 8. Comparison of weld surfaces under different laser beam powers. FigureFigure 8. 8. ComparisonComparison of of weld weld surfaces surfaces un underder different different laser laser beam beam powers. powers. Laser beam power vs. Penetration Laser beam power vs. Penetration 1 1 0.8 0.8 0.6 0.6 0.4 Penetration (mm) Penetration 0.4 Penetration (mm) Penetration 0.2 0.2 0 0 20% 40% 60% 80% 86% 20% 40% 60% 80% 86% Percent of lase beam power Percent of lase beam power Figure 9. The effects of laser beam power on penetration. Figure 9. The effects of laser beam power on penetration. Figure 9. The effects of laser beam power on penetration.

5.1.2. Effects of Pulse Duration on Weld Penetration Figure 10 reveals that the greater the pulse duration the deeper the weld penetration. The longest pulse duration of 21 ms and/or 23 ms induces full penetration. By varying the pulse duration and MetalsMetals 2017 2017, ,7 7, ,292 292 99 of of 18 18

5.1.2.5.1.2 .Effects Effects of of Pulse Pulse Duration Duration on on WeldWeld Penetration Penetration Figure 10 reveals that the greater the pulse duration the deeper the weld penetration. The longest MetalsFig2017ure, 7, 29210 reveals that the greater the pulse duration the deeper the weld penetration. The longest9 of 18 pulsepulse durationduration ofof 2121 msms and/orand/or 2323 msms inducesinduces fullfull penetration.penetration. ByBy varyingvarying thethe pulsepulse durationduration andand keepingkeeping the the energy energy constant, constant, the the longer longer the the pulse pulse duration, duration, the the lower lower the the energy energy received received by by the the part, part , leadingkeepingleading thetoto energyaa decreasedecrease constant, inin penetration.p theenetration. longer the However,However, pulse duration, inin reality,reality, the lower thethe thehighhigh energy pulsepulse received durationduration by the isis part,notnot recommended.leadingrecommended. to a decrease ThisThis is inis because penetration.because tootoo However,highhigh aa pulsepulse in reality, dudurationration the highcancan cause pulsecause durationthethe pupulselse is overlap notoverlap recommended. toto notnot bebe constant,Thisconstant, is because especiallyespecially too highforfor continuouscontinuous a pulse duration laserlaser beambeam can cause weldwelding.ing. the Therefore, pulseTherefore, overlap thethe toexperimentalexperimental not be constant, resultsresults especially ofof thesethese samplesforsamples continuous overover aa laserlonglong beamtimetime areare welding. notnot reliable,reliable, Therefore, butbut only theonly experimental provideprovide aa trendtrend results oror sensitivitysensitivity of these samples reference.reference. over ForFor a long thethe analyzedtimeanaly arezed notDP1000 DP1000 reliable, steel, steel, but the the only optimal optimal provide du durationration a trend of of or the the sensitivity pulse pulse should should reference. be be in in the the For middle middle the analyzed range, range, DP1000from from 9 9 ms ms steel, to to 15the15 ms.ms. optimal duration of the pulse should be in the middle range, from 9 ms to 15 ms.

PulsePulse duration duration vs.vs. PenetrationPenetration 1.01.0

0.80.8

0.60.6

0.4

Penetration (mm) Penetration 0.4 Penetration (mm) Penetration

0.20.2

0.00.0 0.30.3 33 66 99 1212 1515 1818 2121 2323 PulsePulse duration duration (ms) (ms) FigureFigureFigure 10. 10.10 .The The effects effects of ofof pulse pulsepulse duration durationduration on onon weld weldweld penetration. penetration.penetration.

5.1.3.5.1.3.5.1.3 .Effects Effects of ofof Overlap OverlapOverlap on onon Weld WeldWeld PenetrationPenetration ItItIt can can bebebe understood understoodunderstood from fromfrom Equations EquationsEquation (7)s (7)(7) and andand (8) (8) that(8) thatthat overlapping overlappingoverlapping is related isis relatedrelated to the toto selection thethe selectionselect ofion pulse ofof pulseduration,pulse duration,duration, spot size spotspot (diameter), sizesize (diameter),(diameter), and traverse andand traversetraverse velocity vevelocity forlocity a specific forfor aa specificspecific mean power. meanmean Inpower.power. this work, InIn thisthis a serieswork,work, of aa seriesoverlapseries of of valuesoverlapoverlap were values values tested were were experimentally tested tested experimentally experimentally to assess to to the assess assess effects the the oneffects effects weld on on penetration. weld weld penetration. penetration. Figure Figure11Fig showsure 1111 showsthatshows the that that overlap the the overlapoverlap seems not seemsseems to affect notnot toto the affectaffect weld thethe penetration, weldweld penetration,penetration but only ,but but up only only to the up up value to to the the of value value 80%. of Forof 80%. 80%. the For lastFor thesamplethe lastlast sample appliedsample appliedapplied to an overlap toto anan overlapoverlap of 95%, ofof the 95%,95%, laser thethe pulse laserlaser is pulsepulse similar isis similar tosimilar the continuous toto thethe continuouscontinuous laser as laserlaser the pulses asas thethe pulsesarepulses almost are are almost almost constant. constant. constant. For this For For value this this value value of overlap, of of over overlap, thelap, samplesthe the samplessamples are completelyareare completely completely melted melted melt anded and and crossed crossed crossed by bytheby the the beam. beam. beam. The The The suitable suitable suitable overlap overlap overlap should should should be be lessbe less less than than than 80%, 80%, 80%, although although although it hardly it it hardly hardly affects affects affects weld weld weld penetration penetration penetration for forpulsedfor pulsed pulsed Nd:YAG Nd:YAG Nd:YAG laser laserlaser beam beam beam welding. welding.welding.

Overlap vs. Penetration 1.0

0.8

0.6

0.4 Penetration (mm) Penetration

0.2

0.0 0% 20% 40% 60% 80% 95% Overlap Figure 11. Effects of overlap on weld penetration. FigureFigure 11.11. Effects ofof overlapoverlap onon weldweld penetration.penetration. 5.1.4. Effects of Spot Laser Diameter on Weld Penetration 5.1.4.5.1.4. Effects ofof SpotSpot LaserLaser DiameterDiameter onon WeldWeld PenetrationPenetration Figure 12 shows that an increase of diameter decreases the weld penetration. This experimental FigureFigure 1212 showsshows thatthat anan increaseincrease ofof diameterdiameter decreasesdecreases thethe weldweld penetration.penetration. ThisThis experimentalexperimental result is consistent with the theoretical relationship in Equation (4). It is known that a greater diameter resultresult isis consistentconsistent withwith thethe theoreticaltheoretical relationshiprelationship inin EquationEquation (4).(4). ItIt isis known thatthat a greater diameter

leads to smaller power density due to the larger area in the same laser beam power. The small power density induces a decrease in the laser impact in the piece. In other words, when the diameter decreases Metals 2017, 7, 292 10 of 18 Metals 2017, 7, 292 10 of 18 Metals 2017, 7, 292 10 of 18 leads to smaller power density due to the larger area in the same laser beam power. The small power leads to smaller power density due to the larger area in the same laser beam power. The small power density induces a decrease in the laser impact in the piece. In other words, when the diameter density induces a decrease in the laser impact in the piece. In other words, when the diameter decreasesor the power or the is channeled power is channeled to a smaller to point, a smaller the laserpoint, beam the laser should beam be moreshould concentrated be more concentrated and potent, decreases or the power is channeled to a smaller point, the laser beam should be more concentrated andcausing potent, an increasecausing inan weldincrease penetration. in weld penetration. and potent, causing an increase in weld penetration.

Spot laser diameter vs. Penetration Spot laser diameter vs. Penetration 1.0 1.0

0.8 0.8

0.6 0.6

0.4

Penetration (mm) Penetration 0.4 Penetration (mm) Penetration 0.2 0.2

0.0 0.0 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Spot laser diameter (mm) Spot laser diameter (mm) Figure 12. Effects of spot laser diameter on weld penetration. FigFigureure 12 12.. EffectsEffects of of spot spot laser laser diameter diameter on on weld weld penetration. penetration.

Figure 12 indicates that the penetration decreases sharply when the applied spot diameter FigFigureure 1212 indicatesindicates that that the the penetration penetration decreases decreases sharp sharplyly when when the the applied applied spot spot diameter diameter increases from 1.2 mm to 1.4 mm. To further observe the distinction, the weld surfaces under different increasesincreases from from 1.2 1.2 mm mm to to 1.4 1.4 mm. mm. To To further further observe observe the the distinction, distinction, the the weld weld surfaces surfaces under under different different spot diameters are compared in Figure 13. Meanwhile, there are some significant fusion spatters on spotspot diameters diameters are are compared compared in in Fig Figureure 13.13. Meanwhile, Meanwhile, there there are are some some significant significant fusion fusion spatters spatters on on the top and bottom weld surfaces when the spot diameter is smaller than 1.0 mm. This might be thethe top top and and bottom bottom weld weld surfaces surfaces when when the the spot spot diameter is is smal smallerler than 1.0 mm. This This might might be be because the thermodynamic behavior in the welding pool manifests shrinkage when a small laser becausebecause the the thermodynamic thermodynamic behavior behavior in in the the welding welding pool pool manifest manifestss shrinkage shrinkage when when a a small small laser laser beam spot size is applied. These additional spatters imply that the weld penetrations are sufficient. beambeam spot spot size size is is applied. applied. These These additional additional spatters spatters imply imply that that the the weld weld penetrations penetrations are sufficient. sufficient.

0.6 mm 1.0 mm 1.2 mm 1.6 mm 2.0 mm

Top （15×)

Bottom (15×)

Figure 13. Comparison of weld surfaces under different spot diameters. FigFigureure 13 13.. ComparisonComparison of of weld weld surfaces surfaces under under different different spot spot diameters. diameters. 5.1.5. Effects of Pulse Type on Penetration 5.1.55.1.5.. Effects Effects of of Pulse Pulse Type Type on Penetration Figure 14 compares the effects of seven different pulse types on weld penetration. It indicates Figure 14 compares the effects of seven different pulse types on weld penetration. It indicates that theFigure pulse 14 type compares has no the significant effects of effect seven on different penetration, pulse with types the on exception weld penetration. of the last It type, indicates namely that that the pulse type has no significant effect on penetration, with the exception of the last type, namely scalethe pulse expanded type has pulse. no significant This special effect pulse on penetration, type results with in thea low exception penetration of the last(Height type, h namely = 0.3 mm) scale scale expanded pulse. This special pulse type results in a low penetration (Height h = 0.3 mm) comparedexpanded pulse.to the others This special (between pulse 0.5 type mm results and 1.0 in amm low). penetrationThis is because (Height the pulse h = 0.3 with mm) simple compared power to compared to the others (between 0.5 mm and 1.0 mm). This is because the pulse with simple power isthe divided others (betweento be better 0.5 resolution mm and 1.0 at mm).low power. This is However, because the this pulse kind with of pulse simple maintains power is a divided penetration to be is divided to be better resolution at low power. However, this kind of pulse maintains a penetration thatbetter is resolutionvirtually constant at low power. throughout However, the cord. this kindIt should of pulse be noted maintains that the a penetration “scale expanded” that is virtuallypulse is that is virtually constant throughout the cord. It should be noted that the “scale expanded” pulse is usedconstant mainly throughout as heat treatment. the cord. It should be noted that the “scale expanded” pulse is used mainly as usedheat treatment.mainly as heat treatment.

Metals 2017, 7, 292 11 of 18 Metals 2017, 7, 292 11 of 18

Metals 2017, 7, 292 11 of 18 Pulse type vs. Penetration 1.0

0.8 h = 0.3mm 0.6

0.4 Penetration (mm) Penetration

0.2

0.0 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Pulse type FigureFigure 14. 14Effects. Effects ofof pulsepulse type on weld weld penetration. penetration. Figure 14. Effects of pulse type on weld penetration. 5.1.6.5.1.6 Effects. Effects of Weldingof Welding Velocity Velocity on on Weld Weld Penetration Penetration 5.1.6. AsEffects shown of Weldingin Figure Velocity 15, the penetration on Weld Penetration seems to be directly influenced by the welding velocity: As shown in Figure 15, the penetration seems to be directly influenced by the welding velocity: The higher the welding velocity, the deeper the weld penetration. In the case of DP1000 laser beam The higherAs shown the welding in Figure velocity, 15, the penetration the deeper seems the weld to be penetration. directly influenced In the caseby the of welding DP1000 velocity: laser beam Thewelding, higher weld the weldingpenetration velocity, has anthe obvious deeper theincrease weld forpenetration. the low boundIn the caseof the of DP1000acceptable laser welding beam welding, weld penetration has an obvious increase for the low bound of the acceptable welding velocity welding,velocity thatweld the penetration welding velocities has an areobvious less than increase 1.5 mm/s. for the Th elow relationship bound of between the acceptable welding welding velocity that the welding velocities are less than 1.5 mm/s. The relationship between welding velocity and velocityand penetration that the welding may be velocities explained are as less follows: than 1.5 The mm/s. greater The relationshipthe welding between speed, thewelding less timevelocity the penetration may be explained as follows: The greater the welding speed, the less time the material andmaterial penetration has to cool may, so be the explained temperature as follows:of the material The greater increase thes until welding the end speed, of the the weld. less The time fusion the hasmaterialmaterial to cool, has becomes so to the cool, temperature“soft so theer” temperatureand facilitates of the of material the the penetration mate increasesrial increases of the until laser until the beam, the end end especially ofof the weld. weld. for a The thin The fusion sheet. fusion materialmaterialIn addition, becomes becomes the “softer”cycle “softer” of heating and facilitates facilitates and cooling the the penetration make penetrations a different of the of laser thechange beam, laser in beam,especially the microstructure especially for a thin for sheet.of the a thin sheet.Inmaterial, addition, In addition, e.g., the causing cycle the cycle of an heating increase of heating and in thecooling and hardness cooling makes of makes thea different material a different change, hindering change in the further microstructure in the penetration. microstructure of the of thematerial, material, e.g., e.g., causing causing an an increase increase in the in thehardne hardnessss of the of material, the material, hindering hindering further further penetration. penetration. Welding velocity vs. Penetration 1.0 Welding velocity vs. Penetration 1.0 0.8 0.8 0.6 0.6 0.4 Penetration (mm) Penetration 0.4 Penetration (mm) Penetration 0.2 0.2 0.0 0.0 0.1 1.0 1.5 2.0 2.7 0.1 Welding1.0 velocity1.5 (mm/s)2.0 2.7 Figure 15. EffectsWelding of welding velocity(mm/s) velocity on penetration. Figure 15. Effects of welding velocity on penetration. 5.1.7. ANOVA for QuadraticFigure Model 15. Effects of Weld of weldingPenetration velocity on penetration. 5.1.7. AccordingANOVA for to Quadratic the design Model of experiments of Weld Penetration for welding parameters listed in Table 3, the second- 5.1.7. ANOVA for Quadratic Model of Weld Penetration orderAccording polynomial to (regression)the design of equation experiments used tofor represent welding theparameters response listed surface in TableRxˆ() is3, giventhe second- by: orderAccording polynomial to the (regression) design of experimentsequation used for to weldingrepresent parameters the response listed surface in TableRxˆ()3 , is the given second-order by: polynomial (regression) equation used to representnn the response surface Rˆ (x) is given by: Rˆ() x =+++ββx β x2 β xx + η, 0 ∑nnii ∑ iii ∑∑ ijij (9) i=11 i= ij> 1 Rxˆ()=+ββn x +n β x2 + β xx + η, ˆ 0 ii iii2  ijij (9) where β is the polynomialR(x) coefficient, = β0 + ∑ βηii==i11x isi + the∑ minorβiixi error,+ ij∑ and > 1∑ nβ ijisx theixj +numberη, of design variables (9) i=1 i=1 i j>1 where(six factors β is thein thispolynomial work). coefficient,Statistical testingη is the of minor the empirical error, and models n is the hasnumber been of done design by variables Fisher’s where(sixstatistical βfactorsis the te stin polynomial forthis Analysis work). coefficient, ofStatistical Variance ηtesting—isANOVA. the of minor the Table empirical error, A7 shows and modelsn thatis the thehas number ANOVAbeen done of tests design by applied Fisher’s variables to (sixstatistical factors intest this for Analysis work). of Statistical Variance—ANOVA. testing of theTable empirical A7 shows models that the has ANOVA been tests done applied by Fisher’s to statistical test for Analysis of Variance—ANOVA. Table A7 shows that the ANOVA tests applied Metals 2017, 7, 292 12 of 18

Metals 2017, 7, 292 12 of 18 to the individual coefficients in the model are significant. The F-value is the ratio of the mean square duethe individual to regression coefficients to the mean in squarethe model due are to residual. significant. The The Model F-valueF-value is the of weld ratio penetration of the mean response square isdue 12.94, to regression which implies to the mean that sq theuare models due to are residual. significant. The Model There F-value is only of 0.36%weld penetration chance in theresponse weld penetrationis 12.94, which response implies model that thatthe themodels “Model are Fsignif-Value”icant. could There occur is dueonly to 0.36% noise. chance The determination in the weld coefficientpenetration of response the regression model that analysis the “Model results isF-Value”R2 = 0.966, could indicating occur due a to high noise. degree The determination of correlation betweencoefficient the of experimental the regression values analysis and empiricalresults is calculated R2 = 0.966, values indicating obtained a high from degree the response of correlation models. between the experimental values and empirical calculated values obtained from the response models. 5.2. Effects of Laser Parameters on Hardness and Its Microstructure Observation 5.2. EffectsIn order of Laser to evaluate Parameters the on microstructure Hardness and Its evolution Microstructure of the materialObservation before and after laser beam welding,In order one of to the evaluate optimal the welding microstructure parameters, evolutio as listedn of in Tablethe material4, can be before obtained and through after laser the abovebeam experimentalwelding, one sensitivityof the optimal analyses. welding The parameters, tested sample as listed had full in penetrationTable 4, can withbe obtained almost nothrough oxidation the andabove spatter experimental on both top sensitivity and bottom analyses. surfaces. The Figure tested 16 sample shows thehad weld full surfacespenetration of fusion with zonealmost using no theseoxidation optimal and parameters.spatter on both top and bottom surfaces. Figure 16 shows the weld surfaces of fusion zone using these optimal parameters. Table 4. Optimal welding parameters of DP 1000 steel used. Table 4. Optimal welding parameters of DP 1000 steel used. Power Pulse Overlap Diameter Velocity Energy Pulse Material Power Pulse Duration Overlap Diameter Velocity Energy Pulse Material (%) Duration (ms) (%) (mm) (mm/s) (J) Type (%) (ms) (%) (mm) (mm/s) (J) Type DP1000 57 9.0 60 0.6 0.3 45 Rec. DP1000 57 9.0 60 0.6 0.3 45 Rec.

Figure 16.16. WeldWeld surfaces surfaces of of fusion fusion zone zone using using the optimalthe optimal parameters: parameters: (a) Top (a surface) Top surface (15×); ( b(15×);) bottom (b) surfacebottom (15surface×); ( c(15×);) perpendicular (c) perpendicular longitudinal longitudinal section. section.

5.2.1. Microscopic and Macroscopic Hardness The hardness is consideredconsidered an importantimportant indicatorindicator of welded joint properties and performance. Too muchmuch hardnesshardness insideinside thethe fusion and the heat-affe heat-affectedcted zones require higher pre-heat temperatures, temperatures, slower cooling,cooling, and moremore complicatedcomplicated hydrogenhydrogen controlledcontrolled weldingwelding proceduresprocedures [[16].16]. The laser Nd:YAG processprocess gives gives higher higher hardness hardness values values compared compared to other to processes other processes [20]. Thus, [20]. the mechanismThus, the ofmechanism formation of of formation softening of zones softening HAZ zones and fusion HAZ zoneand fusion (FZ) of zone the welded(FZ) of jointthe welded should joint account should for theaccount high capabilityfor the high of capability the welds. of In the this welds. work, theIn this metallurgical work, the propertiesmetallurgical of the properties welded joint of the of DP1000,welded includingjoint of DP1000, the variations including of the microscopic variations and of mi macroscopiccroscopic and hardness macroscopic of FZ hardness and HAZ of under FZ and optimal HAZ weldingunder optimal parameters, welding are parameters, observed experimentally. are observed experi Accordingmentally. to Taylor According et al. [31 to], Taylor the microscopic et al. [31], andthe macroscopicmicroscopic hardnessand macroscopic can be transformed hardness can into be the tran samesformed unit forinto comparison, the same unit as listed for comparison, in Table5. as listed in Table 5. Table 5. Comparison of average measured hardness. Table 5. Comparison of average measured hardness.

Material TestedMaterial Tested Micro-Indentation Micro-Indentation Vickers Vickers Base DP 1000Base steel DP 1000 steel 425 425 HV HV (10.7 (10.7 GPa) GPa) 382 HV 382 HV Heat-affectedHeat-affected zone zone 372 372 HV HV (9.36 (9.36 GPa) GPa) 316 HV 316 HV Fusion zone ofFusion weld zone of weld 504 504 HV HV (14.5 (14.5 GPa) GPa) 449 HV 449 HV

Figure 17 shows the micro-indentation hardness distribution and measured macroscopic Figure 17 shows the micro-indentation hardness distribution and measured macroscopic hardness hardness of laser welded DP1000 steel butt joints. The fusion zone (FZ) of welded joints shows higher of laser welded DP1000 steel butt joints. The fusion zone (FZ) of welded joints shows higher hardness than the base metal and HAZ of butt joints. Table 5 shows that the hardness in the HAZ is lower compared to the base metal. The previous study indicates that the soft zone in the HAZ of the dual-phase butt joints is caused by the tempering of pre-existing martensite [20].

Metals 2017, 7, 292 13 of 18 hardness than the base metal and HAZ of butt joints. Table5 shows that the hardness in the HAZ is lower compared to the base metal. The previous study indicates that the soft zone in the HAZ of theMetals dual-phase 2017, 7, 29 butt2 joints is caused by the tempering of pre-existing martensite [20]. 13 of 18 Metals 2017, 7, 292 13 of 18 18 (a) 600 (b) 600 15 500 (b) 500

, (HV) 400 , (GPa) 12 400 9 300 300 6 200 HAZ FZ HAZ HAZ FZ HAZ 200 3 Macrohardness 100 HAZFZ HAZ Microhardness

Macrohardness, (HV) Macrohardness, 100 0 0 1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 0 1 3 5 7 9 11 13 15 17 19 21 23 Indentation number 1357911131517192123Vickers testpoint

Vickers testpoint FigureFigure 17. 17(a. )(a Micro-indentation) Micro-indentation hardness; hardness; ( (bb)) measuredmeasured macroscopic hardness hardness (HAZ: (HAZ: Heat Heat-affected-affected zone;Figurezone FZ:; FZ: 17. Fusion Fusion(a) Micro-indentation zone). zone). hardness; (b) measured macroscopic hardness (HAZ: Heat-affected zone; FZ: Fusion zone). 5.2.2.5.2.2 Microstructure. Microstructure Characterization Characterization of of Nd:YAG Nd:YAG LaserLaser WeldedWelded JointJoint 5.2.2. Microstructure Characterization of Nd:YAG Laser Welded Joint TheThe SEM SEM examinations examinations ofof HAZ and and FZ FZ regions, regions, as asshown shown in Fig inure Figure 18, indicate18, indicate that the that themicrostructure microstructureThe SEM examinationsof ofDP1000 DP1000 steelof steel HAZ butt butt andjoints joints FZ inregions, inFZ FZ region regionas shown is ismartensitic martensiticin Figure together18, together indicate with with that a afewthe few microstructure of DP1000 steel butt joints in FZ region is martensitic together with a few encompassingencompassing ferrite ferrite and and bainitebainite molecules molecules.. The The changeable changeable hardness hardness in the in FZ the of DP1000 FZ of DP1000 butt joints butt encompassingreveals the existence ferrite andof a bainiteso-called molecules. multiple -Theingredient changeable microstructure. hardness in The the quickFZ of -DP1000cooling buttprocess joints of joints reveals the existence of a so-called multiple-ingredient microstructure. The quick-cooling process revealsthe weld the butt existence pool during of a so-called the laser multiple-ingredient welding should lead microstructure. to the production The quick-cooling of martensite process in the FZof of the weld butt pool during the laser welding should lead to the production of martensite in the FZ theregion weld. Thus, butt thepool martensite during the volume laser welding fraction shouldof dual lead-phase to steelsthe production should play of martensitea considerable in the effec FZt region. Thus, the martensite volume fraction of dual-phase steels should play a considerable effect on region.on the changeThus, the of microstructuremartensite volume characterization fraction of dual-phase during the steelslaser beam should welding. play a considerable effect theon change the change of microstructure of microstructure characterization characterization during during the the laser laser beam beam welding. welding.

(a) (b) Figure 18. SEM micrographs showing the microstructure change of laser welded DP1000 steel butt FigureFigurejoints: 18. (18.a)SEM HAZ SEM micrographs; micrographs(b) FZ. showing showing thethe microstructuremicrostructure change change of of laser laser welded welded DP1000 DP1000 steel steel butt butt joints:joints: (a )(a HAZ;) HAZ; (b ()b FZ.) FZ. This may be explained as follows: Firstly, the peak temperature experienced in the soft zone duringThis laser may beam be explained welding seems as follows: to result Firstly, in a partialthe peak disappearance temperature of experienced pre-existent in martensite. the soft zone It is This may be explained as follows: Firstly, the peak temperature experienced in the soft zone during duringlikely that laser the beam peak welding temperature seems in to the result soft inzone a part canial be disappearance high enough toof stimulatepre-existent partial martensite. martensite It is- laserlikelyto- beamaustenite that welding the solid peak seemsstate temperature transformation to result in in the a partial. softMeanw zone disappearancehile ca,n the be subsequenthigh ofenough pre-existent cooling to stimulate phase martensite. partialmight Itnotmartensite- is likelyprovide that theto-austenitea peak trigger temperature of solidthe backward state in thetransformation. softaustenite zone- canto -Meanwhilemartensite be high enough, solidthe subsequent state to stimulate transformation, cooling partial phase martensite-to-austeniteor mightfull martensite not provide-to- solidaaustenite trigger state transformation. oftransformation the backward in Meanwhile,austenite-to-martensite the heating thephase subsequent and solidpartial state cooling austenite transformation, phase-to martensite might or not full transformation provide martensite-to- a trigger in ofaustenite the backwardcooling transformation phase, austenite-to-martensite or both in thepartial heating transformations phase solid and state pa transformation,inrtial the austenite-to heating and ormarten cooling full martensite-to-austenitesite phases transformation during the in transformationthewelding cooling thermal phase, in the cycle. or heating both Actually, partial phase the andtransformation occurrence partial austenite-toof thes in microstructure the heating martensite and change transformationcooling depend phasess on intheduring the location cooling the phase,weldingwithin or boththe thermal welde partial cycle.d joint transformations Actually,, in terms the of occurrence inthe the complexity heating of theand microstructureof the cooling temperature phases change duringfield depends changes. the weldingon Secondthe location thermally, if cycle.withinthere Actually, was the nowelded theoccurrence occurrence joint, ofin theterms of theabovementioned of microstructure the complexi solidty change of-state the depends phasetemperature transformations, on the field location changes. the within presence Secondly, the welded of ifa joint,therehigher in termswas amount no of occurrence the of complexity martensite of the of abovementionedin the the temperature DP steels solid-s fieldmighttate changes. cause phase greater Secondly,transformation tempering if theres, the was of presence nopre occurrence-existing of a highermartensite amount in the of HAZ, martensite giving risein tothe more DP severesteels softeningmight cause in this greater material. tempering of pre-existing martensite in the HAZ, giving rise to more severe softening in this material.

Metals 2017, 7, 292 14 of 18 of the abovementioned solid-state phase transformations, the presence of a higher amount of martensite in the DP steels might cause greater tempering of pre-existing martensite in the HAZ, giving rise to more severe softening in this material.

6. Conclusions The pulsed Nd:YAG laser beam welding of dual-phase DP1000 steel is addressed using theoretical and experimental methods. Some key experimental technologies, including cutting surface preparation for pre-welding sample and optimization design of welding sample fixture for a sufficiently shielding gas flow, are performed to ensure consistent and stable testing. For a better understanding of the interaction between weld quality and process parameters, a parameter-dependent theory is introduced. The main conclusions are as follows.

1. The ideal laser beam power for welding in the studied case should be between 40% and 60% with a sharp rise. However, the sensitivity of the weld penetration to the greater than 60% laser beam power becomes less pronounced. 2. The greater the pulse duration, the deeper the weld penetration. 3. The overlap seems not to affect the weld penetration, but only up to the value of 80%. 4. The weld penetration has a sharp decrease when the applied spot diameter increases from 1.2 mm to 1.4 mm. 5. The pulse type has no significant effect on penetration, with the exception of the scale expanded pulse. 6. The penetration seems to be directly influenced by the welding velocity, i.e., the higher the welding velocity, the deeper the weld penetration. 7. The ANOVA tests applied to the individual coefficients in the regression model show that the Model F-value of weld penetration response is 12.94, which implies that the models are significant. The determination coefficient of the regression analysis results indicates that a high degree of correlation between the experimental values and empirical calculated values is obtained from the models.

Acknowledgments: The authors would like to thank Engineer António Festos for support concerning the experimental tool machining. This work was funded by the Portuguese Foundation of Science and Technology (UID/EMS/00481/2013 and SFRH/BPD/114823/2016 and CENTRO-01-0145-FEDER-022083) and the Scientific Program of Fuzhou University (0020-650421 and 0020-650425). Author Contributions: António B. Pereira and José Amorim conceived and designed the experiments; José Amorim performed the experiments; Xin Xue and José Amorim analyzed the data; Juan Liao and António B. Pereira contributed reagents/materials/analysis tools; Xin Xue, José Amorim, and Juan Liao wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A The detailed design of experiments on the effects of each analyzed welding parameter on weld penetration are presented in this section.

Table A1. Experimental plan for the effects of laser beam power on weld penetration.

Power Duration Overlap Diameter Pulse Energy Velocity Max. Velocity Series (%) (ms) (%) (mm) Type (J) (mm/s) (mm/s) A1 20 24.0 6.3 A2 40 43.5 3.5 A3 60---- 66.1- 2.2 A4 80 91.6 1.6 A5 86 100.0 1.4 Metals 2017, 7, 292 15 of 18 Metals 2017, 7, 292 15 of 18

Metals 2017, 7Table, 292 A2. Experimental plan for the effects of pulse duration on weld penetration. 15 of 18 Metals 2017, 7, 292 15 of 18 Power Duration Overlap Diameter Pulse Energy () Velocity Max. Velocity Series Table A2. Experimental plan for the effects of pulse duration on weld penetration. (%)Table A2.(ms) Experimental (%) plan for (mm)the effects ofType pulse duration(J) on weld(mm/s) penetration. (mm/s) B1 Power Duration0.3 Overlap Diameter Pulse Energy1.3 () Velocity Max. Velocity10 Series PowerPower Duration OverlapOverlap DiameteDiameterr PulsePulse EnergyEnergy () VelocityVelocity Max.Max. Velocity Velocity SeriesB2Series 3 13.0 10 (%)(%) (ms) (%)(%) (mm)(mm) TypeType (J) (J) (mm/s)(mm/s) (mm/s)(mm/s) B3 B1 60.3 26.11.3 10 5.8 B1 B1 0.3 1.3 1.3 10 10 B4 B2 9 3 Simple 13.0 39.1 10 3.9 B2B2 3 13.0 13.0 10 10 B3B5 B3 50 12 6 50 1.3 rectang 26.1 52.2 26.11.4 5.8 2.9 5.8 B4B6 B4 15 9 Simpleular 39.1 65.2 39.1 3.9 2.3 3.9 B4 9 SimpleSimple 39.1 3.9 B5B7 B5 5050 1812 5050 1.3 1.3 rectang 52.2 78.3 52.21.4 2.9 1.9 2.9 B5 50 12 50 1.3 rectangrectangular 52.2 1.4 1.4 2.9 B6B8 B6 2115 ular 65.2 91.3 65.2 2.3 1.6 2.3 B7B9 B7 2318 100.078.3 78.3 1.9 1.4 1.9 B8B8 21 91.3 91.3 1.6 1.6 B9B9 23 100.0 100.0 1.4 1.4 B9 Table A3.23 Experimental plan for the effects of overlap 100.0 on weld penetration. 1.4

Table A3. Experimental plan for the effects of overlap on weld penetration. Max. Power TableTableDuration A3. A3.Experimental Experimental Overlap plan planDiameter for the effect effects s of of overlap overlap on on weldEnergy weld penetration. penetration.Velocity Series Pulse Type Velocity (%) (ms) (%) (mm) (J) (mm/s) Max. Power Duration Overlap Diameter Energy Velocity Max.(mm/s) Series PowerPower DurationDuration OverlapOverlap DiameterDiameter PulsePulse Type EnergyEnergy VelocityVelocity Max.Velocity Velocity SeriesSeries Pulse Type Velocity C1 (%)(%) (ms)(ms) 0(%) (%) (mm)(mm) Type (J)(J) (mm/s)(mm/s) (mm/s)5.5 (mm/s) C2 20 (mm/s) 4.4 C1C1 0 0 5.55.5 C3 C1 400 Simple 5.5 3.3 C2C2 50 12.5 20 20 1.3 54.3 0.2 4.4 4.4 C4 60 rectangular 2.2 C3C3 40 40 SimpleSimple 3.3 3.3 C5 5050 12.5 12.5 80 1.31.3 54.354.3 0.2 0.2 1.1 C4C4 60 60 rectangularrectangular 2.2 2.2 C6C5 95 80 0.2 1.1 C5 80 1.1 C6 95 0.2 C6 95 0.2 Table A4. Experimental plan for the effects of spot laser diameter on weld penetration. TableTable A4. A4.Experimental Experimental plan planfor for thethe effects of of spot spot laser laser diameter diameter on on weld weld penetration. penetration. Table A4. Experimental plan for the effects of spot laser diameter on weld penetration. Max. Power Duration Overlap Diameter Energy Velocity Series Pulse Type Max.Velocity (%)PowerPower Duration(ms)Duration Overlap(%)Overlap Diameter(mm)Diameter Pulse EnergyEnergy(J) VelocityVelocity(mm/s) Max. Velocity SeriesSeries Pulse Type Velocity(mm/s) (%)(%) (ms)(ms) (%)(%) (mm)(mm) Type (J)(J) (mm/s)(mm/s) (mm/s) D1 0.6 (mm/s)1.2 D2D1 D1 0.80.60.6 1.2 1.71.2 D3D2 D2 1.00.8 0.8 1.7 2.1 1.7 D3D3 1.0 1.0 2.1 2.1 D4 D3 1.21.0 Simple 2.1 2.5 D4D4 50 12.5 50 1.2 1.2 SimpleSimple 54.3 1.2 2.5 2.5 D5 D4 50 12.5 50 1.41.2 rectangularSimple 54.3 1.2 2.5 3.0 D550 12.5 50 1.4rectangular 54.3 1.2 3.0 D5 1.4 rectangular 3.0 D6D6 1.6 1.6 3.4 3.4 D6 1.6 3.4 D7D7 1.8 1.8 3.8 3.8 D7 1.8 3.8 D8D8 D7 2.01.8 2.0 3.8 4.3 4.3 D8 2.0 4.3 Table A5. Experimental plan for the effects of pulse type on weld penetration. TableTable A5. A5.Experimental Experimentalplan plan forfor the effects of of pulse pulse type type on on weld weld penetration. penetration. Max. Max. PowerPowerPower DurationDurationDuration OverlapOverlapOverlap DiameterDiameter Pulse EnergyEnergyEnergy VelocityVelocityVelocity Max.Max. Velocity SeriesSeries Power Duration Overlap Diameter Pulse Energy Velocity VelocityVelocity SeriesSeries (%)(%) (ms)(ms) (%)(%) (mm)(mm) Type (J)(J) (mm/s)(mm/s) Velocity (%)(%) (ms)(ms) (%)(%) (mm)(mm) Type (J)(J) (mm/s)(mm/s) (mm/s)(mm/s) (mm/s) E1 E1 54.354.354.3 2.72.7 2.7 E1 54.3 2.7 E2 E2 54.554.554.5 2.72.7 2.7 E2 54.5 2.7 50 12.5 50 1.3 2.6 E3 E3 59.059.059.0 2.62.6 2.6 E3 59.0 2.6 E4 E4 50 50 12.512.5 50 50 1.3 1.3 59.059.059.0 2.62.6 2.62.6 2.6 E4 50 12.5 50 1.3 59.0 2.6 2.6 E5 E5 56.956.9 2.6 2.6 E5 E5 56.956.9 2.62.6 E6 E6 53.453.4 2.8 2.8 E6 E6 53.453.4 2.82.8 E7 28.5 5.3 E7 E7 28.528.5 5.3 5.3 E7 28.5 5.3

Metals 2017, 7, 292 16 of 18

Table A6. Experimental plan for the effects of velocity on weld penetration.

Power Duration Overlap Diameter Pulse Energy Velocity Max. Velocity Series (%) (ms) (%) (mm) Type (J) (mm/s) (mm/s) F1 0.1 F2 1.0 Simple F3 50 12.5 50 1.3 54.3 1.5 rectangular 2.7 F4 2.0 F5 2.7

Table A7. ANOVA for quadratic model of weld penetration.

Source SS DF MS F-Value p-Value Evaluation Model 1.446 14 0.13 12.94 0.0036 Significant A 0.115 1 0.12 10.37 0.0181 B 0.010 1 0.010 0.88 0.3839 C 0.379 1 0.379 34.13 0.0011 D 0.014 1 0.014 1.30 0.2976 E 0.003 1 0.003 1.02 0.3505 F 0.008 1 0.008 2.59 0.1587 AB 0.074 1 0.074 6.65 0.0419 AC 0.138 1 0.138 12.40 0.0125 AD 0.003 1 0.003 0.28 0.6188 AE 0.060 1 0.060 5.36 0.0599 AF 0.045 1 0.045 4.02 0.0919 BC 0.099 1 0.099 8.91 0.0245 BD 0.073 1 0.073 23.42 0.0029 BE 0.020 1 0.020 6.48 0.0437 BF 0.065 1 0.065 20.66 0.0039 - CD 0.079 1 0.079 25.35 0.0024 CE 0.040 1 0.040 12.73 0.0118 CF 0.009 1 0.009 2.81 0.1447 DE 0.007 1 0.007 2.40 0.1724 DF 0.019 1 0.019 5.93 0.0508 EF 0.154 1 0.154 49.40 0.0004 A2 0.001 1 0.001 0.09 0.7744 B2 0.001 1 0.001 0.05 0.8341 C2 0.013 1 0.013 1.13 0.3293 D2 0.002 1 0.002 0.17 0.6938 E2 0.080 1 0.080 25.61 0.0023 F2 0.007 1 0.007 2.30 0.1798 Residual 0.067 6 0.011 - - Total 1.512 20 - - - R2 = 0.966; SS—sum of square; DF—degree of freedom; MS—mean square.

References

1. Rossinia, M.; Spena, P.R.; Cortese, L.; Matteis, P.; Firrao, D. Investigation on dissimilar laser welding of advanced high strength steel sheets for the automotive industry. Mater. Sci. Eng. A 2014, 628, 288–296. [CrossRef] 2. Blacha, S.; Weglowski, M.S.T.; Dymek, S.; Kopuscianski, M. Microstructural characterization and mechanical properties of electron beam welded joint of high strength grade S690QL. Arch. Metall. Mater. 2016, 61, 1193–1200. [CrossRef] 3. Grouve, W.J.B.; Warnet, L.; Akkerman, R.; Wijskamp, S.; Kok, J.S.M. Weld strength assessment for tape placement. Int. J. Mater. Form. 2010, 3, 707–710. [CrossRef] 4. Hernandez, V.H.B.; Panda, S.K.; Kuntz, M.L.; Zhou, Y. Nanoindentation and microstructure analysis of resistance spot welded dual phase steel. Mater. Lett. 2010, 64, 207–210. [CrossRef] Metals 2017, 7, 292 17 of 18

5. Ramazani, A.; Mukherjee, K.; Abdurakhmanov, A.; Abbasi, M.; Prahl, U. Characterization of microstructure and mechanical properties of resistance spot welded DP600 Steel. Metals 2015, 5, 1704–1716. [CrossRef] 6. Huang, Y.M.; Wu, D.; Zhang, Z.F.; Chen, H.B.; Chen, S.B. EMD-based pulsed TIG welding process porosity defect detection and defect diagnosis using GA-SVM. J. Mater. Process. Technol. 2017, 239, 92–102. [CrossRef] 7. Narwadkar, A.; Bhosle, S. Optimization of MIG welding parameters to control the angular distortion in Fe410WA Steel. Mater. Manuf. Process. 2015, 31, 2158–2164. [CrossRef] 8. Ahmadzadeh, M.; Farshi, B.; Salimi, H.R.; Fard, A.H. Residual stresses due to gas arc welding of aluminum alloy joints by numerical simulations. Int. J. Mater. Form. 2013, 6, 233–247. [CrossRef] 9. Ramazani, A.; Mukherjee, K.; Abdurakhmanov, A.; Prahl, U.; Schleser, M.; Reisgen, U.; Bleck, M. Micro-macro-characterisation and modelling of mechanical properties of gas metal arc welded (GMAW) DP600 steel. Mater. Sci. Eng. A 2014, 589, 1–14. [CrossRef] 10. Mei, L.F.; Chen, G.Y.; Jin, X.Z.; Zhang, Y.; Wu, Q. Research on laser welding of high-strength galvanized automobile steel sheets. Opt. Lasers Eng. 2009, 47, 1117–1124. [CrossRef] 11. Sharma, R.S.; Molian, P. Yb:YAG laser welding of TRIP780 steel with dual phase and mild steels for use in tailor welded blanks. Mater. Des. 2009, 30, 4146–4155. [CrossRef] 12. Xu, W.; Westerbaan, D.; Nayak, S.S.; Chen, D.L.; Goodwin, F.; Zhou, Y. Tensile and fatigue properties of fiber laser welded high strength low alloy and DP980 dual-phase steel joints. Mater. Des. 2013, 43, 373–383. [CrossRef] 13. Ghaini, F.M.; Hamedi, M.J.; Torkamany, M.J.; Sabbaghzadeh, J. Weld metal microstructural characteristics in pulsed Nd:YAG laser welding. Scr. Mater. 2007, 56, 955–958. 14. Mahamood, R.M.; Akinlabi, E.T. Scanning speed and powder flow rate influence on the properties of laser metal deposition of titanium alloy. Int. J. Adv. Manuf. Technol. 2017, in press. [CrossRef] 15. Hazratinezhad, M.; Arab, N.B.M.; Sufizadeh, A.R.; Torkamany, M.J. Mechanical and metallurgical properties of pulsed neodymium-doped yttrium aluminum garnet laser welding of dual phase steels. Mater. Des. 2012, 33, 83–87. [CrossRef] 16. Mirakhorli, F.; Malek Ghaini, F.; Torkamany, M.J. Development of weld metal microstructures in pulsed laser welding of duplex stainless steel. J. Mater. Eng. Perform. 2012, 21, 2173–2176. [CrossRef] 17. Seang, C.; David, A.K.; Ragneau, E. Effect of Nd:YAG laser welding parameters on the hardness of lap joint: Experimental and numerical approach. Phys. Proc. 2013, 41, 38–40. [CrossRef] 18. Sun, Q.; Di, H.S.; Li, J.C.; Wang, X.N. Effect of pulse frequency on microstructure and properties of welded joints for dual phase steel by pulsed laser welding. Mater. Des. 2016, 105, 201–211. [CrossRef] 19. Wang, J.F.; Yang, L.J.; Sun, M.S.; Liu, T.; Li, H. A study of the softening mechanisms of laser-welded DP1000 steel butt joints. Mater. Des. 2016, 97, 118–125. [CrossRef] 20. Tzeng, Y.F. Effects of operating parameters on surface quality for the pulsed laser welding of zinc-coated steel. J. Mater. Process. Technol. 2000, 100, 163–170. [CrossRef] 21. Xia, M.S.; Biro, E. Effects of heat input and martensite on HAZ softening in laser welding of dual phase steels. ISIJ Int. 2008, 48, 809–814. [CrossRef] 22. Dong, D.Y.; Liu, Y.; Yang, Y.L.; Li, J.F.; Ma, M.; Jiang, T. Microstructure and dynamic tensile behavior of DP600 dual phase steel joint by laser welding. Mater. Sci. Eng. A 2014, 594, 17–25. [CrossRef] 23. Baghjari, S.H.; Akbari Mousavi, S.A.A. Effects of pulsed Nd:YAG laser welding parameters and subsequent post-weld heat treatment on microstructure and hardness of AISI 420 stainless steel. Mater. Des. 2013, 43, 1–9. [CrossRef] 24. Farabia, N.; Chen, D.L.; Zhou, Y. Microstructure and mechanical properties of laser welded dissimilar DP600/DP980 dual-phase steel joints. J. Alloys Compd. 2011, 509, 982–989. [CrossRef] 25. Lee, J.H.; Park, S.H.; Kwon, H.S.; Kim, G.S.; Lee, C.S. Laser, tungsten inert gas, and metal active gas welding of DP780 steel: Comparison of hardness, tensile properties and fatigue resistance. Mater. Des. 2014, 64, 559–565. [CrossRef] 26. Reisgen, U.; Schleser, M.; Mokrov, O.; Ahmed, E. Shielding gas influences on laser weldability of tailored blanks of advanced automotive steels. Appl. Surf. Sci. 2010, 257, 1401–1406. [CrossRef] 27. Schemmann, L.; Zaefferer, S.; Raabe, D.; Friedel, F.; Mattissen, D. Alloying effects on microstructure formation of dual phase steels. Acta Mater. 2015, 95, 386–398. [CrossRef] Metals 2017, 7, 292 18 of 18

28. Wang, X.N.; Chen, C.J.; Wang, H.S.; Zhang, S.H.; Zhang, M.; Luo, X. Microstructure formation and precipitation in laser welding of microalloyed C-Mn steel. J. Mater. Process. Technol. 2015, 226, 106–114. [CrossRef] 29. Liao, J.; Sousa, J.A.; Lopes, A.B.; Xue, X.; Barlat, F.; Pereira, A.B. Mechanical, microstructural behaviour and modelling of dual phase steels under complex deformation paths. Int. J. Plast. 2017, 93, 269–290. [CrossRef] 30. Xue, X.; Liao, J.; Vincze, G.; Pereira, A.B.; Barlat, F. Experimental assessment of nonlinear elastic behaviour of dual-phase steels and application to springback prediction. Int. J. Mech. Sci. 2016, 117, 1–15. [CrossRef] 31. ARAMIS—3D Motion and Deformation Sensor. Available online: http://www.gom.com/metrology- systems/aramis.html (accessed on 30 July 2017). 32. Tasan, C.C.; Diehl, M.; Yan, D.; Zambaldi, C.; Shanthraj, P.; Roters, F.; Raabe, D. Integrated experimental-simulation analysis of stress and strain partitioning in multiphase alloys. Acta Mater. 2014, 81, 386–400. [CrossRef] 33. Ghadbeigi, H.; Pinna, C.; Celotto, S. Quantitative strain analysis of the large deformation at the scale of microstructure: Comparison between digital image correlation and micro grid techniques. Exp. Mech. 2012, 52, 1483–1492. [CrossRef] 34. Alharbi, K.; Ghadbeigi, H.; Efthymiadis, P.; Zanganeh, M.; Celotto, S.; Dashwood, R.; Pinna, C. Damage in dual phase steel DP1000 investigated using digital image correlation and microstructure simulation. Model. Simul. Mater. Sci. 2015, 23, 085005. [CrossRef] 35. Duley, W.W. Laser Welding; John Wiley & Sons: Hoboken, NJ, USA, 1998; ISBN 978-0-471-24679-4.

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