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Effect of Laser Welding Process Parameters and Filler Metals on the Weldability and the Mechanical Properties of AA7020 Aluminium Alloy Saheed B. Adisa 1, * 1 2

*

ID

, Irina Loginova 2 , Asmaa Khalil 2

ID

and Alexey Solonin 2

ID

Department of Chemical and Materials Engineering, University of Idaho, Moscow, ID 83844, USA 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.

J. Manuf. Mater. Process. 2018, 2, 33; doi:10.3390/jmmp2020033

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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 Table 1. Table 1. Composition of AA7020. Element, wt.% Al Balance

Zn 5

Mg 1.2

Mn 0.5

Cu 0.3

Cr 0.2

Fe 0.3

Si 0.3

Ti 0.05

Zr 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 C2 H5 OH + 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 H3 BO3 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.


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TheMater. micro-hardness the etched samples was tested with a Wolpert Wilson HVD-1000AP micro J. Manuf. Process. 2018, 2,of x FOR PEER REVIEW 3 of 10 Vickers hardness testing machine with a hardness scale of HV 0.2, at duration of 5 s, 1960 N testing force, ◦ . Tensile performed using an pyramid all-around series Zwick/Roell Z250 testing machine. The strainwas rateperformed was 4 mm with a square-based indenter with an apical angle of 136 testing per minute and the standard deviation from mean value was within MPa theper measured using an all-around series Zwick/Roell Z250the testing machine. The strain±(2–4) rate was 4 of mm minute value. and the standard deviation from the mean value was within ±(2–4) MPa of the measured value. The experiment experimentwas wasdesigned designedto to study study the the influence influenceof of each each parameter parameter on on the the weld weld quality quality when when The other parameters parameters were were kept kept constant. constant. With With these these experiments, experiments, optimized optimized parameters parameters for for high-quality high-quality other full penetration penetration and and high-strength high-strengthwelded weldedjoint jointcan canbe bedetermined. determined. full 3. 3. Results Results and and Discussion Discussion 3.1. 3.1. Effect Effect of of Peak Peak Power Power Experimental Experimental observation observation of of the the weld weld showed showed that that both both the the width width and and depth depth of of the the weld weld area area increases with increasing peak power, as can be seen in Figure 1. This change with peak power is more increases with increasing peak power, as can be seen in Figure 1. This change with peak power is clearly visible in the graph shown in Figure 2. A higher peak power raises the input energy delivered on more clearly visible in the graph shown in Figure 2. A higher peak power raises the input energy the weld seam, thus resulting a comparatively amount of large vaporized or melted metal. By delivered on the weld seam, in thus resulting in alarge comparatively amount of vaporized or careful melted observation of the observation weld seam cross-section, it is clear that near full occurred within the metal. By careful of the weld seam cross-section, it ispenetration clear that near full penetration laser peakwithin powerthe range ofpeak 0.91–0.97 kW at a of welding speed observed at a occurred laser power range 0.91–0.97 kWof at 2a mm/s; weldingsputtering speed of 2was mm/s; sputtering laser peak of 0.97 kW. was observed at a laser peak of 0.97 kW.

Figure 1.1. Influence Influence of of laser laser peak peak power power on onthe thecross-section cross-sectionshape shapeof ofthe theweld weldseam. seam.(V (V==22mm/s; mm/s; Figure −;1Δz Arff = 15 mm−1 Ar ; ∆z= =−2−mm). 2 mm).

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Width Width and and Penetration Penetration in in mm mm

1.1 1 0.9 0.8 0.7 Width Penetration

0.6 0.5

0.8

0.85

0.9 Peak Power (kW)

0.95

1

penetration. Figure 2. Influence of laser peak power on the weld width and penetration.

3.2. Effect of Laser Speed Figure 3 shows how the shape shape of of the the weld weld changes changes as as the the laser laser speed speed increases. increases. It was observed welding depth and width width decrease decrease with with increasing increasing laser speed, speed, as clearly clearly represented represented in that the welding Figure i.e., welding speed is Figure 4. 4. This This is is because becausethe thespeed speedacts actsininthe theopposite oppositemanner mannertotothe theinput inputheat, heat, i.e., welding speed is inversely proportional to the input energy per unit weld length delivered on the weld line, reducing inversely proportional to the input energy per unit weld length delivered on the weld line, reducing the width became narrower. the intermixing of the melt. this case,the thedepth depthbecame becameshallow shallowand and the the width intermixing of the melt. In In this case, narrower. at aa laser laser speed speed of of 77 mm/s. mm/s. Spatter occurs at

Figure 3. Influence of laser speed on the cross-section morphology of the weld seam. (P == 0.91 0.91 kW; −1 − 1 Ar f = 15 mm ; Δz = −2 mm). Arf = 15 mm ; ∆z = −2 mm).

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Width Penetration in mm Width andand Penetration in mm

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width Penetration width Penetration

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Figure 4. Influence of laser speed on the weld width and penetration. Figure penetration. Figure 4. 4. Influence Influence of of laser laser speed speed on on the the weld weld width width and and penetration.

3.3. Effect of Laser Pulse Shaping 3.3. Laser Pulse 3.3. Effect Effect ofthe Laser Pulse Shaping Shaping Since of reflectance of the laser beam on aluminium alloys is high, there is insufficient absorption of the laser beam tothe meltlaser the surface unless the laserisalloys power high enough. barriers Since the reflectance beam on aluminium isis high, there isThe insufficient Since the reflectance of of the laser beam on aluminium alloys high, there is insufficient absorption to thethe melting point ofmelt theunless alloy are laser low laser beam absorption at room temperature and absorption of laser to the surface unless the power high enough. barriers of reaching the laser beam to meltbeam the surface the power islaser high enough.isThe barriers to The reaching the latent heat of fusion at the melting point. A higher peak pulse power compensates for the reflectivity to reaching the melting point of the alloy are low laser beam absorption at room temperature and melting point of the alloy are low laser beam absorption at room temperature and latent heat of fusion of surface, which results inAreaching the desired surface temperature. latent heat of fusion atAthe melting point. highercompensates peak pulse power compensates for thealuminium reflectivity at the the aluminium melting point. higher peak pulse power for the reflectivity of the The influence of square and ramp-down pulse shaping on laser spot welding were investigated of the aluminium surface, which results in reaching the desired surface temperature. surface, which results in reaching the desired surface temperature. at theThe optimised peak power and andramp-down speed. Figure 5a shows a designed square pulsewere shapeinvestigated consisting influence of and ramp-down pulse shaping on welding investigated The influence of square square pulse shaping on laser laser spot spot welding were of 8 sectors with constant peak power and pulse duration. Figure 5b shows a designed ramp-down at the optimised optimised peak peak power power and and speed. speed. Figure shows aa designed designed square square pulse pulse shape shape consisting consisting at the Figure 5a 5a shows pulse shaping consisting of 8 sectors with constant pulse duration, where the peak power is initially of 8 sectors with constant peak power and pulse duration. Figure 5b shows a designed ramp-down of 8 sectors with constant peak power and pulse duration. Figure 5b shows a designed ramp-down at a level high enough to induce melting of the metal surface and subsequently decreases for the pulse shaping consisting of 8 sectors with constant pulse duration, where the peak power is pulse shaping consisting of 8 sectors with constant pulse duration, where the peak power is initially initially remaining pulses. Experimental observation of the spot weld shown in Figure 6a reveals that at a level level high high enough enough to to induce induce melting melting of of the the metal metal surface surface and and subsequently subsequently decreases decreases square for the at a for the pulse shaping induces hot cracking on the weld due to the rapid and uncontrolled cooling rate. No remaining pulses. Experimental observation of the spot weld shown in Figure 6a reveals that square remaining pulses. Experimental observation of the spot weld shown in Figure 6a reveals that square hot cracking observed on theon spot with ramp-down pulse shaping (Figure 6b), because pulse shapingwas induces hotcracking cracking onthe the weld due rapid uncontrolled cooling No pulse shaping induces hot weld due to to thethe rapid andand uncontrolled cooling rate.rate. No the hot reduced power level at the end of the pulse sequence provided for a more gradual and controlled hot cracking was observed on the spot with ramp-down pulse shaping (Figure 6b), because the cracking was observed on the spot with ramp-down pulse shaping (Figure 6b), because the reduced cooling rate. reduced power level of sequence the pulseprovided sequencefor provided for a more andcooling controlled power level at the endatofthe theend pulse a more gradual andgradual controlled rate. cooling rate. 90 100 80 90 70 80 60 70 50 60 40 50 30 40 20 30 10 20 0 10 0

100

% Power,Power, %

% Power,Power, %

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(a) (b) (a) (b) Figure 5. Laser pulse shaping: (a) square pulse shaping; and (b) ramp-down pulse shaping.

Figure 5. Laser pulse shaping: (a) square pulse shaping; and (b) ramp-down pulse shaping. Figure 5. Laser pulse shaping: (a) square pulse shaping; and (b) ramp-down pulse shaping.

8 8

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(a) (a)

(b) (b)

Figure 6. Influence Influence of pulse shaping on on laser laser spot spot weld: weld: (a) (a) square; square; and (b) (b) ramped ramped down. down. Figure Figure 6. 6. Influence of of pulse pulse shaping shaping on laser spot weld: (a) square; and and (b) ramped down.

3.4. Influence of of Filler Metal Metal on the the Microstructure and and Mechanical Properties Properties 3.4. 3.4. Influence Influence of Filler Filler Metal on on the Microstructure Microstructure and Mechanical Mechanical Properties Generally, laser beam beam welding requires requires there to to be no no gap between between the workpieces. workpieces. However, Generally, Generally, laser laser beam welding welding requires there there to be be no gap gap between the the workpieces. However, However, using a filler metal increases the weld gap bridging ability, thus reducing the amount amount of of work work using a filler metal increases the weld gap bridging ability, thus reducing the using a filler metal increases the weld gap bridging ability, thus reducing the amount of work required required to to prepare prepare the seam seam and and can canalso also be be used used to to alter the the properties properties of of the the metal metal in inspecific specific ways. ways. required to prepare the seamthe and can also be used to alter the alter properties of the metal in specific ways. Al-5Mg Al-5Mg and Al-5Ti-B were the two filler metals used. Al-5Mg is the most used filler metal for welding Al-5Mg and Al-5Ti-B were two filler metals used. Al-5Mg is the used most filler used metal filler metal for welding and Al-5Ti-B were the twothe filler metals used. Al-5Mg is the most for welding 6xxx 6xxx series series aluminium aluminium alloys. alloys. Al-5Ti-B Al-5Ti-B is is known known to to be be an an excellent excellent grain grain refiner refiner [19]. [19]. Al Al33Ti Ti is is the the 6xxx series aluminium alloys. Al-5Ti-B is known to be an excellent grain refiner [19]. Al3 Ti is the primary primary phase phase formed formed during during solidification solidification and and TiB TiB22 phase phase provides provides grain grain refinement. refinement. These These phases phases primary phase formed during solidification and TiB2 phase provides grain refinement. These phases have have similar lattice parameters (a = 0.5446 nm and a = 0.3029 nm for Al 3Ti and TiB2, respectively) as have similar lattice parameters (a = 0.5446 nm and a = 0.3029 nm for Al 3 Ti and TiB 2 , respectively) similar lattice parameters (a = 0.5446 nm and a = 0.3029 nm for Al3 Ti and TiB2 , respectively) as as compared with Al Al (a == 0.4049 0.4049 nm). The The filler materials materials were 0.25 0.25 mm thick thick and were were placed between between compared compared with with Al (a (a = 0.4049 nm). nm). Thefiller filler materialswere were 0.25 mm mm thick and and were placed placed between the base alloys. alloys. The microstructures microstructures of the the welded joints joints are presented presented in Figure Figure 7. ItIt was was observed the the base base alloys. The The microstructures of of the welded welded joints are are presented in in Figure 7. 7. It was observed observed that the joint with Al-5Ti-B has little or no porosity or hot cracking and has a fine equiaxed grain with with that the joint with Al-5Ti-B has little or no porosity or hot cracking and has a fine equiaxed grain that the joint with Al-5Ti-B has little or no porosity or hot cracking and has a fine equiaxed grain with an average size size of 2.00 2.00 ± 0.26 µm. µm. The grains grains in the the joint welded welded with Al-5Mg Al-5Mg filler are are columnar an an average average size of of 2.00 ±±0.26 0.26 µm. The The grainsin in the joint joint welded with with Al-5Mg filler filler are columnar columnar and porosity appears as a defect which might be associated with the high level of magnesium in the and porosity appears as a defect which might be associated with the high level of magnesium and porosity appears as a defect which might be associated with the high level of magnesium in in the the weld zone. weld weld zone. zone.

(a) (a)

(b) (b)

(c) (c)

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

Figure 88 shows shows the the elemental elemental distribution distribution in in the the base base metal metal and and the the weld weld zone. zone. Alloying Alloying Figure Figure 8 shows the elemental distribution in because the base of metal and the weld zone. Alloying elements elements are found depleted in the fusion zone the large amount of energy accumulated elements are found depleted in the fusion zone because of the large amount of energy accumulated are found depleted in the fusion zone because of the large amount of energy accumulated during welding. welding. The The levels levels of of Zn Zn and and Mg Mg present present in in the the weld weld are are lower lower in in the the fusion fusion zone zone and andduring higher during higher welding. The levels of Zn and Mg present in the weld are lower in the fusion zone and higher near the near the heat affected zone due to their low boiling point. Ti concentration is highest in the fusion near the heat affected zone due to their low boiling point. Ti concentration is highest in the fusion heat zone dueto toother their alloying low boiling point.because Ti concentration is highest in the fusion zone when zoneaffected when compared compared element of the the added added Al-5Ti-B to the the molten pool. Ti zone when to other alloying element because of Al-5Ti-B to molten pool. Ti and Zr Zr play play aa vital vital role role in in refining refining the the grains. grains. Mn Mn concentration concentration is is lower lower in in the the fusion fusion zone, zone, but but the the and


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compared to other alloying element because of the added Al-5Ti-B to the molten pool. Ti and Zr play J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW of 10 a vital role in refining the grains. Mn concentration is lower in the fusion zone, but the little 7that 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 Figure 9. The hardness profile shows that the joint2018, is mechanically and compositionally heterogeneous, which makes the 8fusion J. Manuf.welded Mater. Process. 2, x FOR PEER REVIEW of 10 zone the weakest part of the welded structure. It can be seen that the hardness decreases gradually fusion zone the value weakest part of the welded structure. It can be seen the weld hardness from a maximum in the zone of the base metal to a minimum valuethat in the zone.decreases The lower gradually a maximum value of the base to a minimum valuethereby in the weld zone. hardness in from the weld zone was duein tothe thezone evaporation of metal strengthening elements, preventing The lower hardness in the weld zone was due to the evaporation of strengthening elements, thereby the formation of strengthening precipitates [21]. However, the refinement of grains in the weld zone preventing formation strengthening However, refinement of grains the with Al-5Ti-Bthe explains the of higher hardnessprecipitates in the joint[21]. relative to thethe joint with Al-5Mg. Theinmean weld zone with Al-5Ti-B the higher hardnessfiller in the joint is relative to and the joint with Al-5Mg. hardness in the weld zone explains with Al-5Ti-B and Al-5Mg metal 102 HV 88 HV, respectively. The mean hardness in the weld zone with Al-5Ti-B and Al-5Mg filler metal is 102 HV and 88 HV, Table 2 presents the result of the tensile tests of the specimens. The welded joint with Al-5Ti-B has 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 , MPa σ0.2, MPa σσyy,, MPa MPa Base Metal 310 372 Base Metal 310 372 With Al-5Ti-B 273 (88%) 360 360 (97%) (97%) With Al-5Ti-B 273 (88%) With Al-5Mg 210 (68%) With Al-5Mg 210 (68%) 250 250 (67%) (67%)

115

δ, % 14.0 8.0 4.2

δ, % 14.0 8.0 4.2

HV

110

105

100

95

AlTi Filler AlMg Filler

90

85 -1.5

-1

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0.5

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Distance, mm Figure9.9.Micro Microhardness hardnessof ofthe the cross cross section section of Figure of the the weld weldjoint jointof ofAA7020. 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 These the highest aspect ratio. Therefore, from the quality point of view, these values give values the bestgive result. the best result.  give Ramp-down pulse shaping when compared to the square pulse shaping gives a spot weld that • Ramp-down when compared to the square pulse shaping gives a spot weld that is is free frompulse visibleshaping cracks and porosity. from visible cracks and porosity.  free Fine grains having an average size of 2.00 ± 0.26 µm were achieved with the joints welded with • Fine grainsfiller having an average size of 2.00 ± of 0.26 were joint achieved withofthe welded with Al-5Ti-B metal, and the yield strength theµm welded was 97% thejoints base metal yield strength. The joints with Al-5Mg have columnar grains and also show some porosity in the weld Al-5Ti-B filler metal, and the yield strength of the welded joint was 97% of the base metal yield zone. The yield strength was lower than that of the jointsand using at 67% of the base metal strength. The joints with Al-5Mg have columnar grains alsoAl-5Ti-B show some porosity in the 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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