Dissimilar Metals Laser Welding Between DP1000 Steel and Aluminum Alloy 1050

Article Dissimilar Metals Laser Welding between DP1000 Steel and Aluminum Alloy 1050

António B. Pereira 1,* , Ana Cabrinha 1,Fábio Rocha 1, Pedro Marques 1,2 , Fábio A. O. Fernandes 1 and Ricardo J. Alves de Sousa 1

1 TEMA–Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal; [email protected] (A.C.); [email protected] (F.R.); [email protected] (P.M.); [email protected] (F.A.O.F.); [email protected] (R.J.A.d.S.) 2 Industrial Engineering and Management, Universidade Lusófona, Campo Grande 376, 1749 Lisbon, Portugal * Correspondence: [email protected]; Tel.: +351-234370827

Received: 16 December 2018; Accepted: 14 January 2019; Published: 18 January 2019

Abstract: The welding of dissimilar metals was carried out using a pulsed Nd: YAG laser to join DP1000 steel and an aluminum alloy 1050 H111. Two sheets of each metal, with 30 × 14 × 1 mm3, were lap welded, since butt welding proved to be nearly impossible due to the huge thermal conductivity differences and melting temperature differences of these materials. The aim of this research was to ﬁnd the optimal laser welding parameters based on the mechanical and microstructure investigations. Thus, the welded samples were then subjected to tensile testing to evaluate the quality of the joining operation. The best set of welding parameters was replicated, and the welding joint obtained using these proper parameters was carefully analyzed using optical and scanning electron microscopes. Despite the predicted difﬁculties of welding two distinct metals, good quality welded joints were achieved. Additionally, some samples performed satisfactorily well in the mechanical tests, reaching tensile strengths close to the original 1050 aluminum alloy.

Keywords: laser welding; pulsed Nd:YAG laser; DP1000 steel; 1050 aluminum alloy; dissimilar materials welding; steel/aluminum joint

1. Introduction Currently, there is a growing interest across various industries to join different metals or alloys [1,2]. This interest is justified by the flexibility that such options would provide in terms of mechanical project and design, for instance, by utilizing high-quality alloys for critical structural points and low-quality ones for less significant areas. Additionally, considering for instance the automotive industry, this solution makes it possible to manufacture lighter components [3–6]. This would also significantly reduce fuel consumption, thereby reducing CO2 emissions, which is of the utmost importance in complying with environmental demands and policies, and also to improve manufacturers/brands in their green marketing campaigns. Steel and aluminum are two materials that are extremely difficult to join by welding. They have different chemical compositions, as well as physical and mechanical properties, which translates into a homogeneity problem. These materials have completely different melting points. Aluminum can melt during the welding process, whereas steel can remain solid [2]. In other words, the laser power necessary to melt steel is excessive for aluminum. On the other hand, aluminum’s reflexivity and the high melting temperature of its surface oxides make its melting process quite difficult. Thus, specific welding strategies must be adopted in order to properly weld these materials. Moreover, the thermal conductivities and thermal expansion coefficients of steel and aluminum alloys are also completely different, which makes welding of these materials a true challenge [4–6].

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Therefore, during the welding process, thermal stresses and residual stresses develop on the welding joint [2]. Additionally, the occurrence of intermetallic compounds (IMCs) of Fe–Al on the welding joint during solidification is possible [7]. IMCs are very hard and fragile, decreasing the joint tenacity and plasticity of the joint, i.e., basically turning it brittle [2]. The IMC layers formation is highly dependent on the temperature and time [8], which must be controlled to successfully join steel and aluminum [9]. Consequently, there is a series of problems in joining these two materials, and some solutions have been proposed. Torkamany et al. [10] reported a successful welding of low-carbon steel ST14 (0.8 mm thick) and 5754 aluminum alloy (2.0 mm thick), using a pulsed Nd:YAG laser. It was found that parameters such as power and pulse duration had a significant influence on the overall welding process. Torkamany et al. [10] reported the expected formation of IMCs. An increase in both parameters meant an equal increase in the terms of the IMCs formation. Moreover, with the increase in laser power, the extent of spatter, cracks, and pores also increased. On the other hand, for low laser power values, penetration was insufficient. Sun et al. [11] studied weld butt joints between Q235 low-carbon steel and an AA6013 aluminum alloy, also using an Nd:YAG laser and adding the aluminum alloy 4043 as filler material. As in Reference [10], the formation of the IMCs: Fe2Al5 and FeAl3 occured. Failure analysis of the joints showed a typical cleavage-type fracture with crack initiation starting within the FeAl3 phase. The maximum registered tensile strength of the joint was 120 MPa. These were the most recent and relevant studies found in the literature. Other authors have used fusion-brazing welding [12] or dual laser beam fusion welding [9] or even explosion welding [13–15], which are relatively if not widely different from the investigation at hand, especially the latter. Nevertheless, despite considering these techniques, we did not find any study relating to DP1000 steel and aluminum alloy 1050 welding. In fact, even in well performed literature reviews by Sun and Ion [1], Wang et al. [2], Shah and Ishak [4], and Katayama [3], no study was found regarding the laser welding of these metals. In this work, the authors aimed to determine the optimal set of laser welding parameters, based on mechanical testing and microstructure investigations. Nowadays, laser technology is used in many applications. Concerning mechanical technology, this can be used to perform precise welding or even precise cutting of high strength steels [16,17]. Laser welding is a joining method that is still relatively recent and makes it possible to obtain better results compared to other welding technologies. This is the main reason behind the replacement of resistance spot welding in several industries. For instance, considering the automotive industry, laser welding has been extensively used, since it makes it possible to perform smaller welding joints (small heat-affected zone) with higher precisions, penetrations, and flexibility. Additionally, it only requires one side of the base metal to accomplish the joining process and it is a technique characterized by high welding speeds, making it an excellent alternative to the conventional welding processes [2,18]. Currently, laser welding is widely used in metals such as titanium alloys [19,20] and dual-phase steels [21].

2. Materials and Methods

2.1. Laser Welding Machine The machine used was a SWA300 (SISMA, Vicenza, Italy), Figure1a. The Nd:YAG laser uses, as an active medium, a crystalline solid made of Nd:YAG (neodymium-doped yttrium aluminum Garnet - Y3Al5O12). A parametric study was performed to ﬁnd a set of welding parameters that made it possible to obtain quality welding joints between DP1000 steel and 1050 aluminum alloy. To evaluate the welded samples, the same methodology adopted in a previous study was also adopted [21], as well as performing tensile and microhardness tests. Metals 2019, 9 FOR PEER REVIEW 3

Metalsnot have2019 ,an9, 102original fixation system. Thus, a simple and effective solution was designed as depicted3 of 16 in Figure 1b.

Metals 2019, 9 FOR PEER REVIEW 3 not have an original fixation system. Thus, a simple and effective solution was designed as depicted in Figure 1b.

(b)

(a)

Figure 1. (a) SISMA SWA300 SWA300 Nd:YAG Nd:YAG laser laser welding welding machine; machine; ( (bb)) Fixation/support Fixation/support system. system.

TheThe SISMAsupport SWA300 system has was a primarily hole for inserting designed the to performprotection mold gas reparationtube. From andleft maintenance,to right, Figure with 2 or without the use of a filler material. Since this machine was designed for mold reparation, it does not illustrates the procedure(a) necessary to properly fix the samples. The first step was to insert the haveprotection an original gas tube, fixation ensuring system. that Thus,the protective a simple gas and was effective properly solution covering was the designed samples. as The depicted second in Figurestep consisted1b.Figure of 1. putting(a) SISMA the SWA300 samples Nd:YAG in their laser positi weldingons. In machine; order to ( bperform) Fixation/support lap welds, system. a third metal sheetThe with support the same system thickness has a holewas placed for inserting under thethe protectiontop one, which gas tube. meant From it was left tounder right, the Figure steel2 illustratessample.The The support the third procedure systemstep was necessary has to aensure hole to for properlythat inserting both fix part the thes were samples. prot properlyection The gas first placed, tube. step Fromusing was to leftscrews insert to right, theat the protection Figure sides. 2 gasillustratesThe tube, last step ensuring the was procedure to that guarantee the protectivenecessary the complete gasto wasproperly fixat properlyion fix by coveringthe using samples. the the upper samples. The screws,first The step secondthereby was step toclosing insert consisted the the ofprotectiongap putting between thegas the samplestube, metal ensuring sheets in their asthat positions. much the protectiveas possible. In order gas to was perform properly lap welds, covering a third the samples. metal sheet The with second the samestep consisted thickness of was putting placed the under samples the topin their one, positi whichons. meant In order it was to underperform the lap steel welds, sample. a third The metal third stepsheet was with to the ensure same that thickness both parts was were placed properly under placed, the top using one, screws which at meant the sides. it was The under last step the was steel to guaranteesample. The the third complete step was fixation to ensure by using that the both upper part screws,s were properly thereby closingplaced,the using gap screws between at the sides. metal sheetsThe last as step much was as to possible. guarantee the complete fixation by using the upper screws, thereby closing the gap between the metal sheets as much as possible.

Figure 2. Illustration of the support/fixation system operation.

In order to prevent oxidation, an isolating box was designed and placed on this device. This box was made of glass, due to its transparency and its better thermal resistance (compared to acrylic). Figure 3 shows the glass parts that were glued to create the referred to protective box, for use during the joining process. TheFigureFigure protection 2.2. IllustrationIllustration gas used ofof thethe wa support/ﬁxationsupport/fixations Argon ARCAL system system TIG MIG,operation. operation. supplied by Air Liquide (Paris, France). In order to prevent oxidation, an isolating box was designed and placed on this device. This box In order to prevent oxidation, an isolating box was designed and placed on this device. This box was made of glass, due to its transparency and its better thermal resistance (compared to acrylic). was made of glass, due to its transparency and its better thermal resistance (compared to acrylic). Figure3 shows the glass parts that were glued to create the referred to protective box, for use during Figure 3 shows the glass parts that were glued to create the referred to protective box, for use during the joining process. The protection gas used was Argon ARCAL TIG MIG, supplied by Air Liquide the joining process. The protection gas used was Argon ARCAL TIG MIG, supplied by Air Liquide (Paris, France). (Paris, France).

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Figure 3. Isolating glass box. Figure 3. Isolating glass box. 2.2. Materials and Samples 2.2 Materials and Samples As described, the dissimilar metals chosen were the DP1000 steel and the AA1050 aluminum alloyAs H111. described, Both samples the dissimilar had a thickness metals chosen of 1mm were anddimensions the DP1000 of steel 30mm and× the14 mm.AA1050 The aluminum 1mm thick samplesalloy H111. were Both cut samples using a guillotinehad a thickness (GUIFIL, of 1m Portugal).m and dimensions of 30mm × 14 mm. The 1mm thick samplesThese were materials cut using had a completelyguillotine (GUIFIL, different Portugal). melting points. Table1 presents a simple comparison betweenThese iron materials and aluminum. had completely Aluminum different can melt melting during points. the welding Table process,1 presents whereas a simple steel comparison can remain solidbetween [2]. Iniron other and words, aluminum. the laser Aluminum power necessary can melt to during melt steel the waswelding overly process, excessive whereas compared steel to can the aluminum.remain solid Therefore, [2]. In other the welding words, wasthe performedlaser power only necessary on the steelto melt part, steel whilst was slightly overly penetrating excessive thecompared aluminum to the one, aluminum. without destabilizingTherefore, the it. weldin Tablesg2 wasand 3performed present the only chemical on the compositionssteel part, whilst of bothslightly materials. penetrating the aluminum one, without destabilizing it. Tables 2 and 3 present the chemical compositions of both materials. Table 1. Properties of the Fe and Al at room temperature [22]. Table 1. Properties of the Fe and Al at room temperature [22]. Thermal Speciﬁc Heat Thermal Melting Density Metal −3 ThermalConductivity SpecificCapacity heat ExpansionThermal TemperatureMelting [K] Density[kg·m ] −1 −1 −1 −1 −1 Metal conductivity[W·m ·K ]capacity[J·kg [J.kg·K−1.K] −1] Coefﬁcientexpansion [K ] temperature [K] [kg.m−3] Fe 1809 7870[W.m−1.K 78−1] 456 coefficient12.1 × 106 [K−1] 6 Fe Al1809 933 7870 2700 78 238 456 917 23.5 12.1×10× 10 6 Al 933 2700 238 917 23.5×106 Table 2. Chemical composition of the AA1050 aluminum alloy [wt.%] [23]. Table 2. Chemical composition of the AA1050 aluminum alloy [wt.%] [23]. Ti Zn Mn Fe Si Mg Cu Ti 0.05Zn 0.07Mn 0.05 0.4Fe 0.25Si 0.05Mg 0.05 Cu 0.05 0.07 0.05 0.4 0.25 0.05 0.05 Table 3. Chemical composition of DP1000 steel [wt.%] [24]. Table 3. Chemical composition of DP1000 steel [wt.%] [24]. C Si Mn Cu Al Cr Ni Nb V C Si Mn Cu Al Cr Ni Nb V 0.141 0.49 1.47 0.02 0.041 0.03 0.04 0.016 0.01 0.141 0.49 1.47 0.02 0.041 0.03 0.04 0.016 0.01

The samples were lap welded with the steel on top (Figure4 4).). ItIt waswas veriﬁedverified thatthat asas thethe weldingwelding progressed, naturally the samples would heat, and the penetration would would increase increase longitudinally. longitudinally. Thus, forfor thethe secondsecond weldingwelding joint, joint, the the sample sample was was rotated rotated 180 180°◦ and and the the welding welding process process proceeded proceeded in thein the same same direction. direction. This This way, way, less less penetration penetration achieved achieved on the on initialthe initial regions regions of the of ﬁrst the weldingfirst welding joint wouldjoint would be compensated be compensated by the by second, the second, which which would would accomplish accomplish just that. just that.

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Figure 4. Schematic of the welding performed on the samples. Figure 4. Schematic of the welding performed on the samples. 2.3. Laser Welding Parameters 2.3 Laser Welding Parameters Regarding the laser parameters that could be analyzed using the SISMA SWA300, the followingRegarding parameters the laser wereparameters considered: that could laser be power analyzed (percentage using the of SISMA peak pulse); SWA300, pulse the duration; following frequency/superposition;parameters were considered: and laser beamlaser diameter.power (p Sinceercentage there was of nopeak data inpulse); the literature pulse regardingduration; DP1000–AA1050frequency/superposition; aluminum and welding, laser beam the diameter. ideal welding Since parametersthere was no were data foundin the literature by trial and regarding error, asDP1000–AA1050 well as taking into aluminum consideration welding, the the conclusions ideal welding of other parameters studies previously were found carried by trial out and using error, the as samewell machine.as taking From into consideration these conclusions, the conclusions the following of were other considered: studies previously carried out using the • sameLaser machine. beam power:From these the penetrationconclusions, was the proportionalfollowing were to considered: the laser beam power. The ideal values • wereLaser situated beam power: between the 6 kWpenetration and 7.20 was kW ofproportion the maximumal to the peak laser power beam of power. 12 kW. The ideal values • Pulsewere duration: situated between the penetration 6 kW and was 7.20 also kW proportional of the maximum to higher peak pulse power durations. of 12 kW. Previous values • werePulse between duration: 12–16 the ms.penetration was also proportional to higher pulse durations. Previous values • Superposition:were between It 12–16 did not ms. influence penetration until values around 80%. • • LaserSuperposition: beam diameter: It did thenot penetrationinfluence pe seemednetration to until be inversely values around proportional 80%. to the welding spots • diameter.Laser beam A constant diameter: value the ofpenetration 1 mm was seemed used. to be inversely proportional to the welding spots • Weldingdiameter. speed: A constant a high weldingvalue of speed1 mm meantwas used. high penetration. The speed was then limited by the • amountWelding of speed: power/energy a high welding and pulse speed duration meant used. high Forpenetration. higher values The speed of pulse was duration then limited and/or by powerthe amount (and thus of energy),power/energy smaller and values pulse of speedduration were used. allowed For forhigher a proper values welding of pulse process. duration and/or power (and thus energy), smaller values of speed were allowed for a proper welding Similar to Torkamany et al. [10], which evaluated the influence of power and pulse duration process. on the welding process, in the first stage, only power was varied to find its ideal value. Then, other parametersSimilar were to Torkamany changed based et al. on [10], visual which analysis evaluated and tensilethe influence testing. of This power evolution and pulse regarding duration the on weldingthe welding parameter process, values in the is described first stage, thoroughly only power in thewas results varied section. to find its ideal value. Then, other parametersTable4 shows were thechanged parameter based values on visual used analysis for each and welded tensile sample. testing. A squareThis evolution wave was regarding defined as the thewelding pulse typeparameter for all values the samples. is described The welding thoroughly speed in wasthe results the lowest section. possible concerning machine restrictions,Table because4 shows forthe higher parameter values, values the penetration used for ea onch thewelded aluminum sample. sample A square would wave be excessivelywas defined high,as the destroying pulse type any for chance all the ofsamples. a properly The weldedwelding joint. speed Additionally, was the lowest a constant possible laser concerning beam diameter machine andrestrictions, superposition because of 1 for mm higher and 60%values, were the used, penetrat respectively.ion on the aluminum sample would be excessively high, destroying any chance of a properly welded joint. Additionally, a constant laser beam diameter and superposition of 1 mmTable and 4. Parametric 60% were studyused, for respectively. steel–aluminum welding.

Sample Power [kW] Pulse Duration [ms] Table 4. Parametric study for steel–aluminum welding. 1 8.40 14 Sample2 Power 7.20 [kW] Pulse 14 duration [ms] 31 7.088.40 14 14 42 6.967.20 14 14 5 6.84 14 63 6.727.08 14 14 74 6.606.96 14 14 85 6.486.84 14 14 96 6.366.72 14 14 10 6.24 14 7 6.60 14 11 6.12 14 128 6.006.48 14 14 9 6.36 14 10 6.24 14

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Table 4. Cont. 1111 6.126.12 14 14 12 6.00 14 Sample12 Power6.00 [kW] Pulse Duration 14 [ms] 13 6.48 16 1313 6.486.48 16 16 14 6.48 15 1414 6.486.48 15 15 151515 6.486.486.48 13 13 13 161616 6.486.486.48 12 12 12 17 6.48 11 1717 6.486.48 11 11 1818 6.606.60 13 13 1819 6.606.60 15 13 191920 6.606.606.60 16 15 15 2020 6.606.60 16 16 2.4. Tensile Testing 2.42.4 TensileTensile TestingTesting To determine the mechanical properties of the welded samples, mainly the tensile strength and To determine the mechanical properties of the welded samples, mainly the tensile strength and respectiveTo determine deformation the mechanical of each set properties of parameters, of the tensile welded testing samples, was mainly performed the tensile using strength an universal and respective deformation of each set of parameters, tensile testing was performed using an universal testing,respective 10 kNdeformation machine (Shimadzu,of each set Kyoto,of parameters Japan),, Figuretensile5 .testing was performed using an universal testing,testing, 1010 kNkN machinemachine (Shimadzu,(Shimadzu, Kyoto,Kyoto, Japan),Japan), FigureFigure 5.5.

FigureFigure 5.5. ShimadzuShimadzu 1010 kNkN tensiletensile testingtesting machine.machine.

ConsideringConsidering laplap welding,welding, welding, deflectiondeflection deﬂection phenomenaphenomena phenomena areare are expectedexpected expected toto occur tooccur occur duringduring during tensiletensile tensile testing.testing. testing. WeWe Wethenthen then usedused used thethe thefollowingfollowing following equationequation equation toto computecompute to compute stress:stress: stress: 𝑆F Mx × y, (1) 𝑆 , (1) S = + , (1) A0 Ix where F is the magnitude of the forces applied, A0 is the initial cross section area of the joint, Mx and where F is the magnitude of the forces applied, A0 is the initial cross section area of the joint, Mx and where F is the magnitude of the forces applied, A0 is the initial cross section area of the joint, Mx and Ix Ix are the torque and moment of inertia about the X-axis, respectively, and y is the maximum distance areIx are the the torque torque and and moment moment of of inertia inertia about about the theX X-axis,-axis, respectively, respectively, and andy yis is thethe maximummaximum distancedistance about the Y-axis considering a xOy plane as follows: about the Y-axis considering aa xOy plane as Figurefollows:6:

Figure 6. Position of the Cartesian coordinate system plane on the sample. FigureFigure 6.6. PositionPosition ofof thethe CartesianCartesian coordicoordinatenate systemsystem planeplane onon thethe sample.sample. Since the superposition of the welding spots had a value of 60%, the area of the joint was approximatedSinceSince thethe superposition tosuperposition one of a 14 ×ofof 1thethe mm weldingwelding2 rectangle. spotsspots However, hahadd aa valuevalue the deﬂectionofof 60%,60%, thethe stresses areaarea ofof were thethe negligiblejointjoint waswas 2 approximatedapproximated toto oneone ofof aa 1414 ×× 11 mmmm2 rectangle.rectangle. However,However, thethe deflectiondeflection stressesstresses werewere negligiblenegligible (<1,5(<1,5

Metals 2019, 9 FOR PEER REVIEW 7 Metals 2019, 9, 102 7 of 16 MPa on every test) due to the small dimensions of the samples. Thus, in order to determine the (<1.5nominal MPa onstress, every we test) used due the to following the small approximation: dimensions of the samples. Thus, in order to determine the nominal stress, we used the following approximation: 𝑆 , (2) F S ≈ , (2) where S is the nominal stress, F is the magnitudeA 0of the applied force, and A0 is the cross-sectional area of the sample (since fracture should occur on the aluminum part). The normal strain was where S is the nominal stress, F is the magnitude of the applied force, and A0 is the cross-sectional areadetermined of the sample through: (since fracture should occur on the aluminum part). The normal strain was determined through: ∆𝐿 𝑒∆L , (3) e = 𝐿, (3) L0 where e is the nominal deformation, ΔL is the change in length, and L0 is the original length where e is the nominal deformation, ∆L is the change in length, and L0 is the original length considered forconsidered the sample for at hand,the sample in this at case hand, it was in 30this mm case for it each was test. 30 Themm truefor each stress test. was The calculated true stress by: was calculated by: σ = S(1 + e), (4) 𝜎𝑆1𝑒 , (4) wherewhereσ isσ is the the true true stress, stress, whilst whilst the the true true strain strainε was ε was determined determined as: as:

ε =𝜀ln( 1 𝑙𝑛1+ e), 𝑒, (5)(5)

2.5. Microstructure 2.5 Microstructure For a deeper analysis of the welding, a new sample based on the optimum welding parameters foundFor in thea deeper tensile analysis testing of was the re-created welding, a to new analyze sample its based microstructure. on the optimum Firstly, welding the sample parameters was transverselyfound in the cut tensile for the testing observation was re-created of a substantial to analyze part of its its weldingmicrostructure. joint, as Firstly, well as the longitudinally, sample was totransversely have a 25 mm cut length for the to observation ﬁt the mold of (Figurea substantial7). Cutting part of was its welding precisely joint, performed as well usingas longitudinally, a Struers cuttingto have machine. a 25 mm length to fit the mold (Figure 7). Cutting was precisely performed using a Struers cutting machine.

FigureFigure 7. 7.Illustration Illustration of of the the sample sample for fo microstructuralr microstructural observation. observation.

AfterAfter this, this, the the sample sample was was inserted inserted on aon cylindrical a cylindrical mold mold of 25 of mm 25 diameter mm diameter and then and epoxy then resinepoxy wasresin injected was injected into it, as into well it,as as a well hardener. as a hardener. Once solidiﬁed Once solidified (Figure8), (Figure the injected 8), the sample injected was sample removed was µ fromremoved the mold from and the then mold polished and then for polished observation. for observation. Polishing diamond Polishing papers diamond (3 papersm) of 180, (3 μ 320,m) of 800, 180, 1200,320, 2500, 800, 1200, and 4000 2500, grit and were 4000 used. grit Thewere machine used. The used machine was a rotaryused was polishing a rotary machine, polishing the machine, RotoPol-21 the (Struers,RotoPol-21 Cleveland, (Struers, OH, Cleveland, USA). OH, USA). Then, the injected sample was subjected to chemical etching using nital (4%) (Ethanol + Nitric Acid), to help the microstructure observation. The nital etching was executed, lasting 20 seconds. At ﬁrst, it was also subjected to a Keller’s reagent (HNO3 2.5 mL; HCl 1.5 mL; H2F2 1 mL, H2O 95 mL). However, it would cause oxidation due to the iron (Fe) present in the sample, complicating the observation. Thus, at the end, only the nital etching was considered. The microstructural observations were performed using an optical microscope and a scanning electron microscope TM4000Plus (SEM, Hitachi, Japan) with an integrated back-scattered electron detector (BSE). Elemental composition mapping was performed using an energy-dispersive X-ray spectroscopy (EDS, Hitachi, Japan).

Metals 2019, 9 FOR PEER REVIEW 8

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Figure 8. Solidified sample after injection of epoxy resin + hardener.

Then, the injected sample was subjected to chemical etching using nital (4%) (Ethanol + Nitric Acid), to help the microstructure observation. The nital etching was executed, lasting 20 seconds. At first, it was also subjected to a Keller’s reagent (HNO3 2.5 mL; HCl 1.5 mL; H2F2 1 ml, H2O 95 ml). However, it would cause oxidation due to the iron (Fe) present in the sample, complicating the observation. Thus, at the end, only the nital etching was considered. The microstructural observations were performed using an optical microscope and a scanning electron microscope TM4000Plus (SEM, Hitachi, Japan) with an integrated back-scattered electron detector (BSE). Elemental composition mapping was performedFigure using 8. Solidified an energy-dispersiv sample after injectione X-ray of spectroscopy epoxy resin + hardener.(EDS, Hitachi, Japan). Figure 8. Solidified sample after injection of epoxy resin + hardener. 3. Results Results Then, the injected sample was subjected to chemical etching using nital (4%) (Ethanol + Nitric 3.1.Acid), Laser to help Welding the microstructure observation. The nital etching was executed, lasting 20 seconds. At 3.1. Laser Welding first, it was also subjected to a Keller’s reagent (HNO3 2.5 mL; HCl 1.5 mL; H2F2 1 ml, H2O 95 ml). Considering the values presented in Table4 for each welded sample, after tensile testing, it was However,Considering it would the cause values oxidation presented due in Table to the 4 foriron ea (Fe)ch welded present sample, in the aftersample, tensile complicating testing, it was the verified that considering the speed used, the ideal interval was from 6 kW to 7.20 kW. More specifically, verifiedobservation. that Thus,considering at the end, the only speed the used,nital etch theing ideal was interval considered. was The from microstructural 6 kW to 7.20 observations kW. More the best performance on tensile testing was achieved at a laser power of 6.48 kW (sample eight from specifically,were performed the best using performance an optical microscopeon tensile testing and a wasscanning achieved electron at a lasermicros powercope ofTM4000Plus 6.48 kW (sample (SEM, 12 samples, Table4). It became clear that for welding powers higher than 7.20 kW, the laser would eightHitachi, from Japan) 12 samples, with an Table integrated 4). It became back-scatter cleared that electron for welding detector powers (BSE). higher Elemental than 7.20composition kW, the fully penetrate the aluminum part, resulting in brittle joints, which were easily breakable by hand. lasermapping would was fully performed penetrate using the aluminum an energy-dispersiv part, resultinge X-ray in brittlespectroscopy joints, which (EDS, were Hitachi, easily Japan). breakable byThen, hand. the Then, duration the ofduration the pulse of wasthe pulse altered, was and altere the laserd, and power the laser fixed power (samples fixed 13 (samples to 16). On 13 the to final16). samples (17 to 20), these two parameters were slightly changed from the ideal values. Figure9 presents On3. Results the final samples (17 to 20), these two parameters were slightly changed from the ideal values. Figurethe 20 welded9 presents samples. the 20 welded samples. 3.1. Laser Welding Considering the values presented in Table 4 for each welded sample, after tensile testing, it was verified that considering the speed used, the ideal interval was from 6 kW to 7.20 kW. More specifically, the best performance on tensile testing was achieved at a laser power of 6.48 kW (sample (1) eight from 12 samples, Table 4). It became clear that for welding powers(11 higher) than 7.20 kW, the laser would fully penetrate the aluminum part, resulting in brittle joints, which were easily breakable by hand. Then, the duration of the pulse was altered, and the laser power fixed (samples 13 to 16). On the final samples (17 to 20), these two parameters were slightly changed from the ideal values. Figure 9 presents the 20( 2welded) samples. (12)

(3) (13) (1) (11) Figure 9. Cont.

(2) (12)

(3) (13)

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(18) (8) (8) (18)

(9) (19) (9) (19)

(20) (10) (20) (10) Figure 9. Steel-aluminum welded samples. FigureFigure 9. Steel-aluminum 9. Steel-aluminum welded welded samples. samples. 3.2 Tensile testing 3.2. Tensile Testing 3.2 TensileFigure testing 10 presents the samples after tensile testing, as well as a reference sample made of aluminumFigure (6010 presents× 14 mm), the since samples ideally, after the tensile fracture test testing, occursing, as as on well the aluminum as a reference part, sample due to madeits lower of aluminumtensile strength. (60 × 14 We14 mm), mm), considered since since ideally, ideally,the normalization the the fracture fracture of occurs occurs this reference on on the aluminum sample. Nevertheless, part, due to itits was lower not tensiledone for strength. a better We We similarity considered with thethe normalizationwelded samples. of Additionally, this reference it sample. is worth Nevertheless, mentioning that it was sample not done20 was for anot better better subjected similarity similarity to tensile with the the testing, welded welded since samples. samples. we noticed Additionally, Additionally, that the it laseris worth worth significantly mentioning mentioning penetrated that sample the 20aluminum, was not subjectedwith some to wide tensile penetrations testing, since in some we regi noticedons. thatTherefore, the laser the signiﬁcantlysignificantlysample would penetrated fracture easily the aluminum,while testing, with some producing wide penetrations negligible data. in some Thus regi regions., ons.this Therefore, Therefore,specific sample the sample sample was would would removed fracture fracture from easily easily the whileexperiment. testing,testing, producing producing negligible negligible data. data. Thus, Thus this, speciﬁc this specific sample wassample removed was fromremoved the experiment. from the experiment.

(11) (1) (1) (11)

(2) (12) (2) (12) Figure 10. Cont.

Metals 2019, 9, 102 10 of 16 Metals 2019, 9 FOR PEER REVIEW 10

(3) (13)

(4) (14)

(5) (15)

(6) (16)

(7) (17)

(8) (18)

(9) (19)

(10) (Reference)

FigureFigure 10. 10. SamplesSamples after after tensile tensile testing. testing.

FigureFigure 11 11aa showsshows thethe stress–strainstress–strain curvecurve forfor thethe samplessamples wherewhere fracturefracture occurredoccurred partiallypartially oror completelycompletely throughthrough thethe aluminumaluminum part part (true (true stress/strain stress/strain values).values). OnOn thethe otherother hand,hand, FigureFigure 11 11bb showsshows the the samples samples where where fracture fracture occurred occurred through through the the welding welding joint. joint. InIn both, both, the the reference reference sample sample stress–strainstress–strain curvecurve waswas included included forfor comparison.comparison. SinceSince FigureFigure 1111 plots plots thethe truetrue stressstress vs.vs. truetrue strain,strain, thethe curves curves were were only only presented presented up up to to maximum maximum stress stress value. value. An ideal welded sample would perform similarly to the original (weakest) material in terms of tensile strength and maximum elongation. Regarding tensile strength, the samples that performed better were samples 8 and 18 (Figures 11 and 12). The latter had a slightly lower value but a higher strain value, thus being considered the best of this whole batch, reaching a 20 MPa lower tensile strength and a 1% strain difference compared the reference sample. It is also worth mentioning that for sample 19, although its maximum stress was signiﬁcantly lower than samples 8 and 18, its maximum strain was closest to the reference sample. In this group, the worst sample was clearly sample 15 which had the lowest tensile strength. It could probably be justiﬁed by the excessive laser penetration on the aluminum part, causing its fragilization. Thus, an ideal laser power between 54 and 55, with a pulse duration between 13 and 15 ms can be herein stated.

Metals 2019, 9, 102 11 of 16

Metals 2019, 9 FOR PEER REVIEW 11

(a)

(b)

FigureFigure 11. True 11. stress–strainTrue stress–strain curves forcurves the samples for the where samples fracture where occurred fracture through: occurred (a) the through: aluminum (a) the part;aluminum (b) the welding part; ( joint.b) the welding joint.

For theAn samplesideal welded where sample the welding would joint perform did not similarly remain to intact the original after tensile (weakest) testing, material the maximum in terms of stresstensile recorded strength wasmuch and maximum lower than elongation. the one in the Regardin referenceg tensile sample strength, (at best, 80the MPa samples lower). that However, performed it is importantbetter were to samples highlight 8 and the sample18 (Figures 14 strain 11 and value, 12). whichThe latter came had close a slightly to the valuelower of value the reference but a higher sample.strain It value, is worth thus mentioning being considered that the the curves best ofof samplesthis whole 3 andbatch, 16 reaching are not plotted a 20 MPa in Figure lower 11tensile, sincestrength the results and were a 1% irrelevant.strain difference For an compared exact reading the re ofference the tensile sample. strength It is also values, worth as wellmentioning as their that respectivefor sample strain 19, values, although the following its maxi barmum graphs stress are was seen significantly in Figure 12 lower. than samples 8 and 18, its maximum strain was closest to the reference sample. In this group, the worst sample was clearly sample 15 which had the lowest tensile strength. It could probably be justified by the excessive laser

Metals 2019, 9 FOR PEER REVIEW 12 penetration on the aluminum part, causing its fragilization. Thus, an ideal laser power between 54 and 55, with a pulse duration between 13 and 15 ms can be herein stated. For the samples where the welding joint did not remain intact after tensile testing, the maximum stress recorded was much lower than the one in the reference sample (at best, 80 MPa lower). However, it is important to highlight the sample 14 strain value, which came close to the value of the reference sample. It is worth mentioning that the curves of samples 3 and 16 are not plotted in Figure 11,Metals since2019 the, 9, 102results were irrelevant. For an exact reading of the tensile strength values, as well12 ofas 16 their respective strain values, the following bar graphs are seen in Figure 12.

(a) (b)

(d) (c)

FigureFigure 12. 12. ResultsResults from from the the tensile tensile tests: tests: (a ()a Ultimate) Ultimate tensile tensile strength strength (samples (samples 1 1to to 9); 9); (b (b) )Strain Strain associatedassociated with with the the ultimate ultimate tens tensileile strength strength (samples (samples 1 to 1 9); to ( 9);c) Ultimate (c) Ultimate tensile tensile strength strength (samples (samples 10 to10 19); to 19);(d) (Straind) Strain associated associated with with the the ultimate ultimate tensile tensile strength strength (samples (samples 10 10 to to19). 19). (Note: (Note: RS RS means means “reference“reference sample” sample” whereas whereas S1 S1 from from S19 S19 are are the the samples samples from from 1 1 to to 19). 19).

InIn terms terms of of laser laser power power (samples (samples 2 2to to 12), 12), the the be bestst results results were were obtained obtained as as this this value value decreased. decreased. ConsideringConsidering the the pulse pulse duration duration (sam (samplesples 13 toto 17),17), thethe contrarycontrary could could be be seen seen for for the the same same value value of of the thelaser laser power. power. However, However, in in terms terms of of strain, strain, the the results results were were inconclusive inconclusive without without a a linear linear behavior behavior of ofany any sort sort of of pattern. pattern. Sample Sample 18 18 could could be be considered considered the the ideal ideal sample, sample, with with a tensile a tensile strength strength and strainand strainof only of 20only MPa 20 MPa and 1.4%, and 1.4%, respectively, respectively, which which were lowerwere lower than the than reference the reference sample. sample. This ideal This sample ideal samplewas repeated was repeated to evaluate to evaluate its microstructure. its microstructure. 3.3. Microstructure Figures 13 and 14 show the observations using the optical microscope and SEM, respectively. In Figure 13, the welded joint is easily spotted, being observed in the way in which both metals melted and mixed in a successful way. This was also conﬁrmed through the SEM observations in Figure 14, where it shows the interface between the two metals. In Figure 14b, it was possible to spot the Fe particles in the aluminum. Metals 2019, 9 FOR PEER REVIEW 13

3.2 Microstructure Figures 13 and 14 show the observations using the optical microscope and SEM, respectively. In Figure 13, the welded joint is easily spotted, being observed in the way in which both metals melted

Metalsand2019 mixed, 9, 102 in a successful way. This was also confirmed through the SEM observations in Figure 14,13 of 16 where it shows the interface between the two metals. In Figure 14b, it was possible to spot the Fe particles in the aluminum.

FigureFigure 13.13. OpticalOptical microscope observations observations of of the the welded welded zone. zone.

Figure 15 shows the BSE images collected, as well as the microstructural and compositional analysis of the welded zone. Compositions are detailed in Table5 for a total of eight different points, six of them in the welded zone and the other two in the base materials used as the reference.

Table 5. Elemental composition mapping (normalized mass concentration [%]).

Spectrum C O Al Si Fe 1 8.25 2.10 12.17 - 77.48 2 8.30 2.82 19.13 0.59 69.15 3 13.00 10.87 62.77 9.37 3.99 4 8.18 2.37 4.73 - 84.72 5 9.39 3.33 46.69 - 40.59 6 10.11 3.22 23.85 - 62.82 Metals 2019, 9, 102 14 of 16

Table 5. Cont.

Spectrum C O Al Si Fe 7 7.83 2.11 9.53 - 80.53 Metals 2019, 9 FOR PEER REVIEW8 7.70 2.13 8.86 - 81.31 14 Mean 9.10 3.62 23.47 4.98 62.58 Sigma 1.77 2.97 20.66 6.21 27.63 SigmaMean 0.63 1.05 7.31 2.20 9.77 Metals 2019, 9 FOR PEER REVIEW 14

(a) (b)

(c)

Figure 14. Scanning electron microscope (SEM) observations: (a) Steel; (b) Aluminum; (c) Steel– aluminum joint interfaces. (c)

FigureFigureFigure 14. 14.15Scanning showsScanning the electron electron BSE microscope images microscope collected, (SEM) (SEM) observations: asobservations: well as (a )the Steel; ( amicrostructural) (Steel;b) Aluminum; (b) Aluminum; ( cand) Steel–aluminum compositional (c) Steel– analysisjointaluminum of interfaces. the weldedjoint interfaces. zone. Compositions are detailed in Table 5 for a total of eight different points, six of them in the welded zone and the other two in the base materials used as the reference. Figure 15 shows the BSE images collected, as well as the microstructural and compositional analysis of the welded zone. Compositions are detailed in Table 5 for a total of eight different points, six of them in the welded zone and the other two in the base materials used as the reference.

FigureFigure 15.15. Back-scattered electron detector detector (BSE) (BSE) images images with with elemental elemental composition composition mapping mapping using using Energy-dispersiveEnergy-dispersive X-ray spectroscopy (EDS).

Figure 15. TableBack-scattered 5. Elemental electron composition detector mapping(BSE) images (nor malizedwith elemental mass concentration composition [%]).mapping using Energy-dispersive X-ray spectroscopy (EDS). Spectrum C O Al Si Fe

Table 5. Elemental1 composition8.25 mapping 2.10 (nor12.17malized mass - concentration 77.48 [%]). 2 8.30 2.82 19.13 0.59 69.15 Spectrum3 13.00C 10.87 O 62.77Al 9.37Si 3.99Fe 1 8.25 2.10 12.17 - 77.48

2 8.30 2.82 19.13 0.59 69.15 3 13.00 10.87 62.77 9.37 3.99

Metals 2019, 9, 102 15 of 16

4. Conclusions In this work, the welding of dissimilar metals was performed with success. A pulsed Nd: YAG laser welding machine, the SISMA SWA300, was used to join the DP1000 steel and the AA1050. The adopted strategy was based on the lap welding of two sheet samples of each metal, since butt welding was nearly impossible due to the great disparity in terms of the thermal properties of both materials. Even if some predicted difﬁculties in welding these dissimilar metals were found, a good choice of welding parameters was studied resulting in good-quality welded joints. The quality was conﬁrmed by the results obtained in the tensile tests and in the observations performed using the optical microscope and the SEM. It is worth highlighting the sample performance, which reached tensile strengths and maximum elongations close to the weakest base metal, the AA1050. In conclusion, it was shown that even for highly dissimilar materials, with very distinct material properties (thermal and mechanical), one can declare the success of this work. The possibility to effectively join high strength dual-phase steels with a soft, ductile 1XXX aluminum alloy opens a new range of design possibilities and attests the versatility of laser-type welding operations. The authors hope that this study can serve as a sounding base for any other future work in this area.

Author Contributions: Conceptualization, A.C. and F.R.; Methodology, A.B.P.; Software and Validation, P.M.; Formal Analysis, P.M.; Investigation, A.C. and F.R.; Resources, A.B.P.; Data Curation, F.A.O.F.; Writing-Original Draft Preparation, F.A.O.F.; Writing-Review & Editing, R.J.A.d.S.; Supervision, P.M.; Project Administration, A.B.P.; Funding Acquisition, A.B.P. and R.J.A.d.S. Funding: Programa Operacional Temático Factores de Competitividade: CENTRO-01-0145-FEDER-022083, Fundação para a Ciência e a Tecnologia, grant UID/EMS/00481/2013-FCT. Acknowledgments: Center of Mechanical Technology and Automation, Fundação para a Ciência e a Tecnologia, grant CEECIND/01192/2017. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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