Effect of Laser Welding Process Parameters and Filler Metals on the Weldability and the Mechanical Properties of AA7020 Aluminium Alloy

Journal of Manufacturing and Materials Processing

Article Effect of Laser Welding Process Parameters and Filler Metals on the Weldability and the Mechanical Properties of AA7020 Aluminium Alloy

Saheed B. Adisa 1,* ID , Irina Loginova 2, Asmaa Khalil 2 ID and Alexey Solonin 2 ID

1 Department of Chemical and Materials Engineering, University of Idaho, Moscow, ID 83844, USA 2 Department of Physical Metallurgy of Non-Ferrous Metals, National University of Science and Technology (MISIS), Moscow 119049, Russia; [email protected] (I.L.); [email protected] (A.K.); [email protected] (A.S.) * Correspondence: [email protected]; Tel.: +1-208-5965-232

Received: 7 April 2018; Accepted: 29 May 2018; Published: 1 June 2018

Abstract: This research work aims at finding the optimum process parameters for the laser welding of AA7020 aluminium alloys. The use of 7xxx series alloys is limited because of weldability problems, such as hot cracking, porosity, and softening of the fusion zone despite its higher specific strength-to-weight ratio. AA7020 aluminium alloy was welded while varying the process parameters so as to obtain optimal welding efficiency. The welded samples were analysed to reveal the microstructure, defects, and mechanical properties of the welded zone. The samples were prepared from a plate of AA7020, which was hot rolled at a temperature of 470 ◦C to a thickness of 1 mm. The welding was carried out at an overlap of 0.25 mm, duration of 14 ms and argon shield gas flow rate of 15 L/min. Process parameters, such as peak power, welding speed, and pulse shaping, were varied. The samples were welded with Al-5Ti-B and Al-5Mg as filler metals. The welding speed, peak power, and pulse shaping have a great influence on the weldability and hot cracking susceptibility of the aluminium alloy. Al-5Ti-B improves the microstructure and ultimate tensile strength of AA7020 aluminium alloy.

Keywords: laser beam welding; Aluminium alloys; ﬁller materials; mechanical properties; process parameters

1. Introduction As environmental awareness grows among consumers as well as government agencies, attempts to improve fuel economy in automobiles are accelerating [1,2]. In addition to improved powertrain efﬁciency, vehicle weight reduction is an important factor. The ability of a steel body structure to deliver weight savings is limited, so the use of aluminium in auto body structures is increasing, and is projected to expand further in the next decade [3]. Owing to their low density and good mechanical properties, aluminium alloys are increasingly being employed in many important manufacturing areas, such as the automobile industries, aeronautics and the military [4–6]. Friction-stir welding (FSW) and laser beam welding (LBW) are noteworthy for their present use in commercial aircraft manufacturing [7–11]. LBW has accounted for a 5% reduction in the weight of Airbus A300 series structures and has also been employed to join some parts of the fuselage [4]. LBW was used to join critical parts in the fuselage in order to mitigate possible materials problems. Hybrid laser arc welding is also a promising technology that has been used to achieve high performance and low welding deformation. In addition, this technology has the possibility of varying the chemical composition and mechanical properties of welds due to the melting of the electrode.

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It takes advantage of the positive properties of both laser and arc welding and produces a synergistic effect. The use of this process is fairly limited when compared with laser welding because of the large amount of process parameters involved which must be adjusted simultaneously and must be chosen carefully in order to achieve an effective process [12]. Thermal field modelling by E. Ukar et al. has helped to reduce surface roughness and identify the optimum process parameters with minimum experimentation in laser processing of 1.234 tool steel [13]. An algorithm solution for feed variation control together with optimum laser power has been applied to obtain more homogenous structure in laser applications [14,15]. Medium-strength aluminium alloys such as 5xxx (Al-Mg) and 6xxx (Al-Mg-Si) series alloys are already widely used due to their high strength-to-weight ratio, good formability, weldability, and corrosion resistance. The use of 7xxx (Al-Zn) series alloys is limited when compared to 5xxx and 6xxx despite its higher strength-to-weight ratio, because of weldability problems such as hot cracking, porosity, and softening of the fusion zone [16]. AA7020 (Al-Zn-Mg) has moderate strength and is widely used in welded joints of armoured vehicles, cryogenic pressure vessels, components of spacecraft liquid-propellant engines, and bridge beams for roads and railroad systems [17]. Process parameters such as laser power, welding speed, defocusing distance, feed rate and pulse shaping have a strong influence on the mechanical properties, weld penetration and bead quality; high speed may result in incomplete fusion, porosity, or high spattering, and high laser power can lead to collapse and instability of the weld pool. Selecting appropriate process parameters is difficult, which necessitates the need to build a relationship between the processing parameters and the quality of weld joint [18]. In this research, 1.0-mm-thick samples of AA7020 were butt welded with filler metals using a Nd:YAG laser. Laser power, welding speed, and pulse shaping were varied as well as the filler metal used to evaluate their effect on the microstructure and mechanical properties of the welded sample.

2. Experimental Work The chemical composition of the AA7020 alloy used is given in Table1.

Table 1. Composition of AA7020.

Element, wt.% Al Zn Mg Mn Cu Cr Fe Si Ti Zr Balance 5 1.2 0.5 0.3 0.2 0.3 0.3 0.05 0.2

The filler metals used were Al-5Ti-B and Al-5Mg. Samples were cut from a sheet of AA7020, which was prepared by hot rolling at 470 ◦C to a thickness of 300 mm and subsequent cold rolling down to 1 mm in a laboratory rolling mill. Before welding, the surface of each sample was cleaned using acetone and dried to remove any oxide layer and contamination present. After the oxide layer has been removed, the surfaces to be welded were electropolished using a C2H5OH + HClO4 electrolyte under 20 V and then dried in a furnace at 150 ◦C for 10 min. The welding process was carried out using a MUL-1-M200 pulse-periodic laser welding machine equipped with a Nd:YAG laser operating at a wavelength of 1064 nm, defocus amount of −2 mm, and a pulse duration of 8 ms. A shield gas flow of high-purity argon was applied during welding from the top, rear, and lateral sides of the sample at a flow rate of 15 L/min. Metallographic samples of the weld seam were prepared using standard mechanical polishing procedures and etched in a saturated solution of H3BO3 and HF in distilled water at 17 V. The microstructure of the weld seam was characterized using a TESCAN VEGA 3LMH scanning electron microscope (SEM). The effects of varying the welding speed from 0.25 to 7 mm/s, peak power from 0.81 to 0.97 kW, and square and ramp-down pulse shaping were evaluated. J. Manuf. Mater. Process. 2018, 2, 33 3 of 10

J. Manuf.The Mater. micro-hardness Process. 2018, 2 of, x FOR the etchedPEER REVIEW samples was tested with a Wolpert Wilson HVD-1000AP micro 3 of 10 Vickers hardness testing machine with a hardness scale of HV 0.2, at duration of 5 s, 1960 N testing force, withperformed a square-based using an pyramidall-around indenter series Zwick/Roell with an apical Z250 angle testing of 136 machine.◦. Tensile The testing strain wasrate performedwas 4 mm usingper minute an all-around and the standard series Zwick/Roell deviation from Z250 the testing mean machine. value was The within strain ±(2–4) rate was MPa 4 of mm the per measured minute andvalue. the standard deviation from the mean value was within ±(2–4) MPa of the measured value. TheThe experimentexperiment waswas designeddesigned toto studystudy thethe inﬂuenceinfluence ofof eacheach parameterparameter onon thethe weldweld qualityquality whenwhen otherother parametersparameters werewere keptkept constant.constant. With thesethese experiments, optimized parameters for high-quality fullfull penetrationpenetration andand high-strengthhigh-strength weldedwelded jointjoint cancan bebe determined.determined.

3.3. ResultsResults andand DiscussionDiscussion

3.1.3.1. EffectEffect ofof PeakPeak PowerPower ExperimentalExperimental observation of the weld showed that both the widthwidth andand depthdepth ofof thethe weldweld areaarea increasesincreases withwith increasing increasing peak peak power, power, as as can can be seenbe seen in Figure in Figure1. This 1. changeThis change with peakwith powerpeak power is more is clearlymore clearly visible visible in the graph in the shown graph in shown Figure 2in. AFigure higher 2. peak A higher power peak raises power the input raises energy the input delivered energy on thedelivered weld seam, on the thus weld resulting seam, inthus a comparatively resulting in a largecomparatively amount of large vaporized amount or melted of vaporized metal. Byor carefulmelted observationmetal. By careful of the observation weld seam cross-section,of the weld seam it is clear cross-section, that near fullit is penetration clear that near occurred full penetration within the laseroccurred peak within power the range laser of peak 0.91–0.97 power kW range at a of welding 0.91–0.97 speed kW ofat 2a mm/s;welding sputtering speed of 2 was mm/s; observed sputtering at a laserwas observed peak of 0.97 at a kW. laser peak of 0.97 kW.

FigureFigure 1.1. InﬂuenceInfluence ofof laserlaser peakpeak powerpower onon thethe cross-sectioncross-section shapeshape ofof the the weld weld seam. seam. (V(V = = 22mm/s; mm/s; −1 1 ArArff = 15 mm ; Δ; ∆z z= =−2− mm).2 mm).

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1.1

0.9

0.8

0.7 Width Width and Penetration in mm Width and Penetration in mm 0.6 Penetration

0.5 0.8 0.85 0.9 0.95 1 Peak Power (kW)

Figure 2. InfluenceInﬂuence of laser peak power on the weldweld widthwidth andand penetration.

3.2. Effect of Laser Speed Figure3 3 shows shows howhow thethe shapeshape of of the the weld weld changes changes as as the the laser laser speed speed increases. increases. ItIt waswas observedobserved thatthat thethe weldingwelding depthdepth andand widthwidth decrease withwith increasingincreasing laserlaser speed,speed, as clearlyclearly representedrepresented inin Figure4 4.. This This is is because because the the speed speed acts acts in in the the opposite opposite manner manner to to the the input input heat, heat, i.e., i.e., welding welding speed speed is inverselyis inversely proportional proportional to theto the input input energy energy per per unit unit weld weld length length delivered delivered on theon the weld weld line, line, reducing reducing the intermixingthe intermixing of the of the melt. melt. In In this this case, case, the the depth depth became became shallow shallow andand thethe widthwidth became narrower. narrower. Spatter occurs atat aa laserlaser speedspeed ofof 77 mm/s.mm/s.

Figure 3.3. InﬂuenceInfluence ofof laserlaser speed onon thethe cross-sectioncross-section morphology of thethe weld seam. (P = 0.91 0.91 kW; kW; Arf = 15 mm−−1;1 Δz = −2 mm). Arff == 15 15 mm mm ; Δ; z∆ z= =−2− mm).2 mm).

J.J. Manuf. Manuf. Mater. Mater. Process. Process. 20182018,, 22,, x 33 FOR PEER REVIEW 55 of of 10 10 J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 5 of 10 0.8 0.8

0.7 0.7

0.6 0.6

0.5 width Penetration 0.5 width Penetration 0.4

Width and Penetration in mm 0.4 Width and Penetration in mm 0.3 0.3

0.2 012345678 0.2 012345678Speed, mm/s

Speed, mm/s Figure 4. Influence of laser speed on the weld width and penetration. Figure 4. InfluenceInfluence of laser speed on the weld width and penetration. 3.3. Effect of Laser Pulse Shaping 3.3. Effect Since of the Laser reflectance Pulse Shaping of the laser beam on aluminium alloys is high, there is insufficient absorptionSince theofthe the reflectance reflectance laser beam of of the to the lasermelt laser beamthe surfacebeam on aluminium onun lessaluminium the alloys laser is alloyspower high, thereisis high,high is insufficient enough.there is The insufficient absorption barriers toabsorptionof reaching the laser beamofthe the melting tolaser melt beam point the surface to of melt the unless alloythe surface theare laserlow un laser powerless thebeam is laser high absorption power enough. is Thehighat room barriers enough. temperature to The reaching barriers and the latenttomelting reaching heat point of the fusion of melting the at alloy the point aremelting low of the laser point. alloy beam A arehigh absorption lower peak laser pulse at beam room power absorption temperature compensates at androom latent for temperature the heat reflectivity of fusion and oflatentat the the aluminiumheat melting of fusion point. surface, at A the higher which melting peak results point. pulse in A reaching powerhigher compensatespeak the desiredpulse power surface for the compensates reflectivitytemperature. for of thethe aluminiumreflectivity ofsurface, theThe aluminium whichinfluence results surface,of square in reaching which and ramp-downresults the desired in reaching pulse surface shaping the temperature. desired on laser surface spot temperature. welding were investigated at theThe optimised influence influence peak of squarepower andand ramp-downspeed. Figure pulse 5a shows shaping a designedon laser spot square welding pulse were shape investigated consisting ofat 8 the sectors optimised with constant peak power peak and power speed. and Figure pulse 5 5aduration.a showsshows a aFigure designeddesigned 5b shows squaresquare a pulse pulsedesigned shapeshape ramp-down consistingconsisting pulseof 8 sectors shaping with consisting constant of peak 8 sectors power with and constant pulse duration. pulse duration, Figure 55bwhereb showsshows the aa peak designeddesigned power ramp-downramp-down is initially atpulse a level shaping high consisting enough to of induce 8 sectors melting with constant of the metal pulse surface duration, and where subsequently the peak decreases power is initially for the remainingat a level highpulses. enough Experimental to induce observation melting of of the the metal spot weld surface shown and in subsequently Figure 6a reveals decreases that squarefor the pulseremaining shaping pulses. induces Experimental hot cracking observation on the weld of the due spot to theweld rapid shown and in uncontrolled Figure 66aa revealsreveals cooling thatthat rate. squaresquare No hotpulse cracking shapingshaping was inducesinduces observed hothot cracking cracking on the on spoton the the with weld weld ra due mp-downdue to to the the rapid pulserapid and andshaping uncontrolled uncontrolled (Figure cooling 6b), cooling because rate. rate. No the hotNo reducedhotcracking cracking power was was observed level observed at onthe the end on spot theof the withspot pulse ramp-downwith sequence ramp-down pulse provided shapingpulse for shaping (Figurea more 6(Figure b),gradual because 6b), and thebecause controlled reduced the coolingpowerreduced levelrate. power at the level end at of the the end pulse of sequencethe pulse provided sequence for provided a more gradualfor a more and gradual controlled and cooling controlled rate. cooling rate.

100 100 90 90 100 100 80 80 90 90 70 70 80 80 60 60 70 70 50 50 60 60 Power, % Power, % 40 40 50 50 30 30 Power, % Power, % 40 40 20 20 30 30 10 10 20 20 0 0 10 10 12345678 12345678 0 0 Time, ms Time, ms 12345678 12345678

Time, ms Time, ms (a) (b) (a) (b) FigureFigure 5. 5. LaserLaser pulse pulse shaping: shaping: ( (aa)) square square pulse pulse shaping; shaping; and and ( (bb)) ramp-down ramp-down pulse pulse shaping. shaping. Figure 5. Laser pulse shaping: (a) square pulse shaping; and (b) ramp-down pulse shaping.

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((aa)) ((bb))

FigureFigure 6. 6. InfluenceInfluenceInﬂuence ofof pulsepulse shaping shaping on on laser laser spot spot weld: weld: ( (aa)) square;square; and and ( (bb)) ramped ramped down. down.

3.4. Influence of Filler Metal on the Microstructure and Mechanical Properties 3.4.3.4. Influence Influence of of Filler Filler Metal Metal on on the the Microstructure Microstructure and Mechanical Properties Generally, laser beam welding requires there to be no gap between the workpieces. However, Generally,Generally, laser beam welding requires requires there there to to be be no gap between the the workpieces. workpieces. However, However, using a filler metal increases the weld gap bridging ability, thus reducing the amount of work usingusing aa fillerfiller metal metal increases increases the the weld weld gap gap bridging bridging ability, ability, thus reducingthus reducing the amount the amount of work of required work required to prepare the seam and can also be used to alter the properties of the metal in specific ways. requiredto prepare to theprepare seam the and seam can alsoand becan used also tobe alterused the to alter properties the properties of the metal of the in metal specific in specific ways. Al-5Mg ways. Al-5Mg and Al-5Ti-B were the two filler metals used. Al-5Mg is the most used filler metal for welding Al-5Mgand Al-5Ti-B and Al-5Ti-B were the were two the filler two metals filler metals used. Al-5Mgused. Al-5Mg is the mostis the usedmost fillerused metalfiller metal for welding for welding 6xxx 6xxx series aluminium alloys. Al-5Ti-B is known to be an excellent grain refiner [19]. Al3Ti is the 6xxxseries series aluminium aluminium alloys. alloys. Al-5Ti-B Al-5Ti-B is known is know to ben anto excellentbe an excellent grain refiner grain [refiner19]. Al 3[19].Ti is Al the3Ti primary is the primary phase formed during solidification and TiB2 phase provides grain refinement. These phases primaryphase formed phase duringformed solidificationduring solidification and TiB and2 phase TiB2 providesphase provides grain refinement.grain refinement. These These phases phases have have similar lattice parameters (a = 0.5446 nm and a = 0.3029 nm for Al3Ti and TiB2, respectively) as havesimilar similar lattice lattice parameters parameters (a = (a 0.5446 = 0.5446 nm nm and and a =a 0.3029= 0.3029 nm nm for for Al Al3Ti3Ti andand TiBTiB22,, respectively) as as compared with Al (a = 0.4049 nm). The filler materials were 0.25 mm thick and were placed between comparedcompared with with Al Al (a (a = = 0.4049 0.4049 nm). nm). The The filler filler materials materials were were 0.25 0.25 mm mm thick thick and and were were placed placed between between the base alloys. The microstructures of the welded joints are presented in Figure 7. It was observed thethe base base alloys. alloys. The The microstructures microstructures of of the the welded welded joints joints are are presented presented in in Figure Figure 77.. ItIt waswas observedobserved that the joint with Al-5Ti-B has little or no porosity or hot cracking and has a fine equiaxed grain with thatthat the the joint joint with with Al-5Ti-B Al-5Ti-B has has little little or or no no porosity porosity or or hot hot cracking cracking and and has has a a fine fine equiaxed equiaxed grain grain with with an average size of 2.00 ± 0.26 µm. The grains in the joint welded with Al-5Mg filler are columnar anan average average size size of 2.00 ± 0.260.26 µm.µm. The The grains grains in in the the joint joint welded welded with with Al-5Mg Al-5Mg filler filler are are columnar and porosity appears as a defect which might be associated with the high level of magnesium in the andand porosity porosity appears appears as as a a defect defect which which might might be be associated associated with with the the high high level level of of magnesium magnesium in in the the weld zone. weldweld zone.

((aa)) ((bb)) ((cc)) Figure 7. Microstructure of the welded joints: (a) with Al-5Ti-B; (b) with Al-5Mg; and (c) without FigureFigure 7. MicrostructureMicrostructure of of the the welded welded joints: joints: ( (aa)) with with Al-5Ti-B; Al-5Ti-B; ( (bb)) with with Al-5Mg; Al-5Mg; and and ( (cc)) without without filler metal. fillerﬁller metal metal..

FigureFigure 88 showsshows thethe elementalelemental distributiondistribution inin thethe basebase metalmetal andand thethe weldweld zone.zone. AlloyingAlloying Figure8 shows the elemental distribution in the base metal and the weld zone. Alloying elements elementselements areare foundfound depleteddepleted inin thethe fusionfusion zonezone bebecausecause ofof thethe largelarge amountamount ofof energyenergy accumulatedaccumulated are found depleted in the fusion zone because of the large amount of energy accumulated during duringduring welding.welding. TheThe levelslevels ofof ZnZn andand MgMg presentpresent inin thethe weldweld areare lowerlower inin thethe fusionfusion zonezone andand higherhigher welding. The levels of Zn and Mg present in the weld are lower in the fusion zone and higher near the nearnear thethe heatheat affectedaffected zonezone duedue toto theirtheir lowlow boilinboilingg point.point. TiTi concentrationconcentration isis highesthighest inin thethe fusionfusion heat affected zone due to their low boiling point. Ti concentration is highest in the fusion zone when zonezone whenwhen comparedcompared toto otherother alloyingalloying elementelement becabecauseuse ofof thethe addedadded Al-5Ti-BAl-5Ti-B toto thethe moltenmolten pool.pool. TiTi andand ZrZr playplay aa vitalvital rolerole inin refiningrefining thethe grains.grains. MnMn concentrationconcentration isis lowerlower inin thethe fusionfusion zone,zone, butbut thethe

J. Manuf. Mater. Process. 2018, 2, 33 7 of 10 compared to other alloying element because of the added Al-5Ti-B to the molten pool. Ti and Zr play a vitalJ. Manuf. role Mater. in reﬁning Process. 2018 the, 2 grains., x FOR PEER Mn REVIEW concentration is lower in the fusion zone, but the little 7 that of 10 is present forms dispersed second phases of MnAl6 and (Cr, Mn)Al12, which is effective in suppressing little that is present forms dispersed second phases of MnAl6 and (Cr, Mn)Al12, which is effective in grain growth [20]. suppressing grain growth [20].

Figure 8. Elemental distribution in the base metal and weld zone of AA7020 after welding with AlTiB Figure 8. Elemental distribution in the base metal and weld zone of AA7020 after welding with AlTiB filler metal. ﬁller metal. The microhardness profile of the welded joint is presented in Figure 9. The hardness profile shows that the welded joint is mechanically and compositionally heterogeneous, which makes the

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The microhardness profile of the welded joint is presented in Figure9. The hardness profile shows thatJ. Manuf. the welded Mater. Process. joint 2018 is mechanically, 2, x FOR PEER REVIEW and compositionally heterogeneous, which makes the 8 fusion of 10 zone the weakest part of the welded structure. It can be seen that the hardness decreases gradually fromfusion a maximum zone the valueweakest in thepart zone of the of thewelded base metalstructure. to a minimumIt can be seen value that in thethe weldhardness zone. decreases The lower hardnessgradually in from the weld a maximum zone was value due in to the the zone evaporation of the base of metal strengthening to a minimum elements, value thereby in the weld preventing zone. theThe formation lower hardness of strengthening in the weld precipitates zone was due [21 ].to However,the evaporation the refinement of strengthening of grains elements, in the weld thereby zone withpreventing Al-5Ti-B the explains formation the of higher strengthening hardness precipitates in the joint [21]. relative However, to the the joint refinement with Al-5Mg. of grains The in mean the hardnessweld zone in thewith weld Al-5Ti-B zone explains with Al-5Ti-B the higher and Al-5Mghardness filler in the metal joint isrelative 102 HV to andthe joint 88 HV, with respectively. Al-5Mg. TableThe2 meanpresents hardness the result in the of weld the tensile zone with tests Al-5Ti-B of the specimens. and Al-5Mg The filler welded metal joint is 102 with HV Al-5Ti-B and 88 hasHV, a respectively. Table 2 presents the result of the tensile tests of the specimens. The welded joint with yield strength and ultimate tensile strength of 88% and 97% of the value of the base metal respectively, Al-5Ti-B has a yield strength and ultimate tensile strength of 88% and 97% of the value of the base and the welded joint with Al-5Mg filler metal has yield strength and ultimate tensile strength of 68% metal respectively, and the welded joint with Al-5Mg filler metal has yield strength and ultimate and 67% of the base metal, respectively. tensile strength of 68% and 67% of the base metal, respectively.

Table 2. Mechanical properties of the welded joint of AA7020. Table 2. Mechanical properties of the welded joint of AA7020.

σ0.2, MPaσ0.2, MPa σσyy,, MPaMPa δ, % δ,% Base MetalBase Metal 310 310 372 372 14.0 14.0 With Al-5Ti-BWith Al-5Ti-B 273 (88%)273 (88%) 360 (97%)(97%) 8.0 8.0 With Al-5MgWith Al-5Mg 210 (68%) 210 (68%) 250 (67%)(67%) 4.2 4.2

HV 115

110

105

100

90 AlTi Filler AlMg Filler

85 -1.5 -1 -0.5 0 0.5 1 1.5 Distance, mm

FigureFigure 9. 9.Micro Micro hardness hardness ofof thethe crosscross sectionsection of the weld weld joint joint of of AA7020. AA7020.

4. Conclusions 4. Conclusions  In order to obtain an efficient laser welding process, the effects of process parameters such as • In order to obtain an efﬁcient laser welding process, the effects of process parameters such as peak power and welding speed on the width and penetration must be taken into consideration. peak power and welding speed on the width and penetration must be taken into consideration. The welding speed and laser peak power were optimized at 1 mm/s and at 0.91 kW respectively. The welding speed and laser peak power were optimized at 1 mm/s and at 0.91 kW respectively. These values give the highest aspect ratio. Therefore, from the quality point of view, these values Thesegive valuesthe best give result. the highest aspect ratio. Therefore, from the quality point of view, these values  giveRamp-down the best result. pulse shaping when compared to the square pulse shaping gives a spot weld that • Ramp-downis free from pulsevisible shaping cracks and when porosity. compared to the square pulse shaping gives a spot weld that is  freeFine from grains visible having cracks an average and porosity. size of 2.00 ± 0.26 µm were achieved with the joints welded with • FineAl-5Ti-B grains filler having metal, an averageand the yield size ofstrength 2.00 ± of0.26 theµ weldedm were joint achieved was 97% with of the the joints base metal welded yield with Al-5Ti-Bstrength. ﬁller The metal, joints with and theAl-5Mg yield have strength columnar of the grains welded and joint also show was 97% some of porosity the base in metal the weld yield strength.zone. The The yield joints strength with Al-5Mg was lower have than columnar that of the grains joints and using also Al-5Ti-B show some at 67% porosity of the base in the metal weld yield strength.

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zone. The yield strength was lower than that of the joints using Al-5Ti-B at 67% of the base metal yield strength.

Author Contributions: S.B.A. designed and performed the experiment and wrote the manuscript; I.L. supervised the experiment and reviewed the manuscript; A.K. helped with sample preparation; A.S. supervised the experiment, reviewed the manuscript, and helped with discussion of the results. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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