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Journal of Materials Processing Technology 191 (2007) 381–384 Mechanical characterization of CO 2 laser beam butt welds of AA5083 A. Ancona a,, P.M. Lugar` a a,b , D. Sorgente c,d , L. Tricarico c,d a CNR-INFM Regional Laboratory LIT 3, Dipartimento Interateneo di Fisica, via Orabona 4, 70126 Bari, Italy b Universit` a degli Studi di Bari, Dipartimento Interateneo di Fisica, via Orabona 4, 70126 Bari, Italy c Dipartimento di Ingegneria Meccanica e Gestionale, DIMeG, Politecnico di Bari, viale Japigia 182, 70126 Bari, Italy d CNISM, Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Unit` a del Politecnico di Bari, via Orabona 4, 70126 Bari, Italy Abstract Laser beam welding experiments have been carried out on 3-mm thick aluminium–magnesium alloy 5083 specimens in butt-joint configuration. The mechanical properties of the joints have been evaluated by performing tensile tests, hardness profiles across the weld sections, porosity measurements and EDX analyses. A design of experiment technique has been used to study the effects of the welding speed (v) and the incident laser power (P) on the aforementioned response variables that are considered to be representative for the weld quality. By comparing the welds obtained by operating at constant linear energy input released onto the material (P/v ratio), the best results have been found for higher laser powers and welding speeds. A clear correlation was found between the incidence of porosity, the tensile strength and the hardness of the fused zone. Welding reliability was enhanced for selected sets of process parameters capable of producing butt-joints showing mechanical properties very competitive if compared with the performances obtained, on similar aluminium alloys, using alternative joining technologies like friction stir welding or gas tungsten arc welding. © 2007 Elsevier B.V. All rights reserved. Keywords: Laser welding; Butt-joint; Mechanical properties; Aluminium alloys 1. Introduction The use of aluminium–magnesium alloys in shipbuilding industry is rapidly growing and is projected to expand further in the next years, thanks to their high strength-to-weight ratio, high corrosion resistance, relatively good mechanical properties and high recycle potential. Conventional joining technologies have resulted to be inadequate for welding these new materials because of their high thermal conductivity, hot cracking suscep- tibility and high incidence of porosity due to the surface tension of the molten aluminium, which is lower than the one of steels. Another important issue in welding aluminium–magnesium alloys is the pronounced vaporization of alloying elements that takes place on the weld pool surface due to their lower boiling temperature compared with aluminium. The selective vapor- ization of volatile alloying elements, especially magnesium, Corresponding author. E-mail addresses: ancona@fisica.uniba.it (A. Ancona), lugara@fisica.uniba.it (P.M. Lugar` a), [email protected] (D. Sorgente), [email protected] (L. Tricarico). causes a metal composition change in the joint, thus affecting the mechanical properties and the corrosion resistance of the weld [1]. For this reason new joining technologies, characterized by lower heat inputs, are required, capable of reducing the loss of volatile elements. Friction stir welding and laser beam weld- ing are promising techniques for joining aluminium alloys [2]. Recent progresses in laser welding make this technology partic- ularly advantageous because of its low heat input and thermal distortion of the joint, high welding speed, potential for automa- tion and flexibility. However, the application of laser welding to aluminium alloys is far from being a mature technology because of the aforementioned issues and material-related problems like the high surface reflectivity at the laser wavelength. Fundamen- tal questions remain open such as the laser coupling with the material, the process reliability, and the chemical composition, structure and properties of the resulting joints. Based on the results of a previous experimental investiga- tion, where the influence of the main process parameters on the cross-sections profiles of several bead-on-plate laser welds was evaluated [3,4], this work provides a quantitative study on the laser weldability of AA5083 aluminuim magnesium alloy specimens in butt-joint configuration. 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.03.048

Mechanical characterization of CO2 laser beam butt welds of AA5083

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Journal of Materials Processing Technology 191 (2007) 381–384

Mechanical characterization of CO2 laserbeam butt welds of AA5083

A. Ancona a,∗, P.M. Lugara a,b, D. Sorgente c,d, L. Tricarico c,d

a CNR-INFM Regional Laboratory LIT 3, Dipartimento Interateneo di Fisica, via Orabona 4, 70126 Bari, Italyb Universita degli Studi di Bari, Dipartimento Interateneo di Fisica, via Orabona 4, 70126 Bari, Italy

c Dipartimento di Ingegneria Meccanica e Gestionale, DIMeG, Politecnico di Bari, viale Japigia 182, 70126 Bari, Italyd CNISM, Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia,

Unita del Politecnico di Bari, via Orabona 4, 70126 Bari, Italy

bstract

Laser beam welding experiments have been carried out on 3-mm thick aluminium–magnesium alloy 5083 specimens in butt-joint configuration.he mechanical properties of the joints have been evaluated by performing tensile tests, hardness profiles across the weld sections, porosityeasurements and EDX analyses. A design of experiment technique has been used to study the effects of the welding speed (v) and the incident

aser power (P) on the aforementioned response variables that are considered to be representative for the weld quality. By comparing the weldsbtained by operating at constant linear energy input released onto the material (P/v ratio), the best results have been found for higher laser

owers and welding speeds. A clear correlation was found between the incidence of porosity, the tensile strength and the hardness of the fusedone. Welding reliability was enhanced for selected sets of process parameters capable of producing butt-joints showing mechanical propertiesery competitive if compared with the performances obtained, on similar aluminium alloys, using alternative joining technologies like friction stirelding or gas tungsten arc welding. 2007 Elsevier B.V. All rights reserved.

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eywords: Laser welding; Butt-joint; Mechanical properties; Aluminium alloy

. Introduction

The use of aluminium–magnesium alloys in shipbuildingndustry is rapidly growing and is projected to expand furthern the next years, thanks to their high strength-to-weight ratio,igh corrosion resistance, relatively good mechanical propertiesnd high recycle potential. Conventional joining technologiesave resulted to be inadequate for welding these new materialsecause of their high thermal conductivity, hot cracking suscep-ibility and high incidence of porosity due to the surface tensionf the molten aluminium, which is lower than the one of steels.nother important issue in welding aluminium–magnesium

lloys is the pronounced vaporization of alloying elements that

akes place on the weld pool surface due to their lower boilingemperature compared with aluminium. The selective vapor-zation of volatile alloying elements, especially magnesium,

∗ Corresponding author.E-mail addresses: [email protected] (A. Ancona),

[email protected] (P.M. Lugara), [email protected] (D. Sorgente),[email protected] (L. Tricarico).

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924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.03.048

auses a metal composition change in the joint, thus affecting theechanical properties and the corrosion resistance of the weld

1]. For this reason new joining technologies, characterized byower heat inputs, are required, capable of reducing the loss ofolatile elements. Friction stir welding and laser beam weld-ng are promising techniques for joining aluminium alloys [2].ecent progresses in laser welding make this technology partic-larly advantageous because of its low heat input and thermalistortion of the joint, high welding speed, potential for automa-ion and flexibility. However, the application of laser welding toluminium alloys is far from being a mature technology becausef the aforementioned issues and material-related problems likehe high surface reflectivity at the laser wavelength. Fundamen-al questions remain open such as the laser coupling with the

aterial, the process reliability, and the chemical composition,tructure and properties of the resulting joints.

Based on the results of a previous experimental investiga-ion, where the influence of the main process parameters onhe cross-sections profiles of several bead-on-plate laser weldsas evaluated [3,4], this work provides a quantitative study on

he laser weldability of AA5083 aluminuim magnesium alloypecimens in butt-joint configuration.

3 rocessing Technology 191 (2007) 381–384

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. Experimental procedure

The aluminium alloy to be investigated was AA5083, 3-mm thick sheets.he objective of this work was to find a reliable operating window of processarameters capable of producing sound welds in butt-joints configuration.

A Rofin-Sinar DC-025 RF excited CO2 slab laser source, operating in con-inuous wave, with maximum output power of 2.5 kW and a TEM00 Gaussianransverse mode, was used in the experiments.

A preliminary experimental study carried out through bead-on-plate tests on-mm thick specimens of the same alloy, allowed to select the beam focusingystems, the shielding gas and its shroud design giving the best weld results,mong several solutions investigated [4]. Therefore, beam focusing was per-ormed using a parabolic mirror having a 200 mm focal length. Helium wassed as shielding gas with a flow rate of 80 l/min. It was fed by means of a gasozzle consisting of two pipes placed sideways the incident beam and orthog-nal to the welding direction; each pipe provided a shielding gas flow at 45◦nclination angle to the horizontal plane. The nozzle standoff distance was set atmm and the beam focal plane was positioned 1 mm below the sample surface.hese process parameters have been kept constant for all the experiments since

he preliminary tests showed that their variations produced a negligible changef the weld bead profiles [4].

The experiments were planned using a general full factorial design method.he influence of the welding speed and laser power on the response variables thatre considered to be representative for the weld quality (tensile strength, poros-ty, microhardness), have been evaluated. Two levels of incident laser power2000, 2500 W) and four levels of welding speed (60, 80, 100, and 120 mm/s)ave been investigated, with several repetitions for each set of process para-eters.

Each couple of specimens to be welded was fastened using an appositivelyeveloped mechanical clamping system in order to keep the air gap within theolerance all along the joint length. Furthermore, two laser spot welds have beenerformed at the beginning and at the end of the joint before continuous weldingo reduce distortions during the laser treatment. Every produced butt-joint haseen cut in order to eliminate the spot welds and obtain a dog-bone specimen forhe tensile tests with a gauge length of 75 mm and a width of 25 mm; the weldas orthogonal to the tensile direction. The remaining part of the butt-joint of

he same sample was sectioned, mounted and polished to obtain transverse andongitudinal sections for macrographical analyses and porosity measurements.ickers microhardness tests were performed on the etched cross-sections of the

oints with an application load of 300 gf. A universal testing machine (INSTRON485) was used for measuring the ultimate tensile stress (UTS) of the weldedaterial.

. Results and discussion

.1. Tensile tests

It was found that full penetration could be achieved for eachombination of process parameters except for the lowest energynput: P = 2000 W and v = 120 mm/s. In these cases, during theensile tests, the joints fractured before the base material yieldoint was reached (see Fig. 1), as a consequence of the reductionf the cross-sectional area due to the incomplete penetration. Fullut discontinuous penetration was rather obtained for the fol-owing sets of process parameters: P = 2500 W and v = 120 mm/sFig. 2a); P = 2000 W and v = 100 mm/s. The corresponding ten-ile strength was considerably reduced: less than 70% of the baseetal and a substantial dispersion of the experimental data was

ound because of the instability of the process. Generally it was

ound that all the butt welds performed at 2000 W laser powerhowed a large dispersion of the tensile strength data, probablyecause, for such a low incident power, keyhole process was nottable.

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ig. 1. Tensile tests of the two butt welds performed at 25 J/mm and at 16.7 J/mmf linear energy input, compared with the base metal.

Weld dropout occurred when the laser energy input exceededcertain threshold due to a higher power density or a longer inter-ction time. In both cases the low viscosity of liquid aluminiumas not able to sustain the welding pool (see Fig. 2b).All results of the tensile tests are reported in the graph of

ig. 3a, where the ratio between the UTS of the joint (UTSweld)nd the UTS of the base metal (UTSbase) has been reporteds a function of the linear energy input (P/v). The best perfor-ances have been obtained for 2500 W incident laser power and

00 mm/s welding speed, where more than the 90% of the ten-ile strength of the native material was achieved for both thepecimens (Fig. 3b).

It is important to note that the sets of process parameters:= 2500 W, v = 100 mm/s and P = 2000 W, v = 80 mm/s produce

he same linear energy input (P/v = 25 J/mm) released onto theaterial, but the tensile properties are considerably different,

s it is also shown in Fig. 1, where the corresponding tensileest results are reported. Selective vaporization of volatile con-tituents is probably responsible for this behaviour, since theower the welding speed, the more the magnesium loss fromhe fused zone [1]. In the range of process parameters explored,y increasing the linear energy input above 25 J/mm the tensiletrength was found almost unchanged for each level of incidentower.

.2. Microhardness

All the laser welded butt-joints performed in our experimentsxhibited, in the fused zone, a equiaxed grain structure alonghe weld centre and fine columnar dendrites oriented towardshe cooling direction close to the interface with the base metal,s it can be clearly seen from the macrographical sections ofigs. 2a and b and 3b.

Fig. 4 shows the microhardness transverse profiles of the twoutt-joints performed with the same value of P/v. Each pointlotted on the graph is the result of the average of several mea-urements performed at different depths from the weld surface.

n overall hardening of the welded zone, with respect to thease metal, was found for all the investigated joints, rangingrom +10% to +16% according to the set of process parameters.his is due to the very fine solidification structure caused by the

A. Ancona et al. / Journal of Materials Processing Technology 191 (2007) 381–384 383

Fig. 2. Macrographs of the sections: (a) partially penetrated joint performed at P = 2500 W and v = 120 mm/s and (b) weld dropout obtained for P = 2500 W andv = 60 mm/s.

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ig. 3. (a) Ultimate tensile strength ratio % vs. linear energy input P/v. (b) Tran

teeper temperature gradient, especially close to the interfaceith the base metal [5]. Such effect is much more evident forigher welding speeds when a faster cooling rate takes place inhe weld metal, resulting in a finer structure [6]. This hardenings less evident along the weld axis. It is worth noting that by com-aring the two hardness profiles in Fig. 4 a significant softeningf about −6% than the base metal was found along the weld axisn case of P = 2000 W and v = 80 mm/s. This behaviour could beaused by a concurrent effect of magnesium loss, responsible for

decrease of the mechanical properties of the joint. EDX anal-ses of the magnesium content along the weld axis confirmed aifference of about 4% between the two examined joints.

ig. 4. Microhardness transverse profiles of the butt welds performed with theame linear energy input but different welding parameters.

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e cross-section of the best butt-joint obtained for P = 2500 W and v = 100 mm/s.

.3. Porosity

Porosity is a very important parameter to evaluate the qualityf a butt-joint since it is recognized to be one of the major con-erns during laser welding of aluminium alloys and it has beenidely documented to be harmful to the tensile properties of theelds. Pore formation is attributed to the entrapment of gas bub-les due to imperfect collapse of the keyhole either caused bynsufficient laser beam energy or because of the turbulent flown the weld pool generated by too high a laser intensity [1].

The amount of porosity has been quantified by the imagenalysis of the longitudinal sections performed on each weldingample. Results are shown in Fig. 5 where the percentage oforosity has been sketched as a function of the tensile strengthor each joint, except for the cases of partially penetrated weldsince the experimental data were inconsistent.

All the welds performed at 2500 W exhibited porosity lesshan 3% corresponding to tensile strengths well above 80% ofhe base metal. Only a small decrease of the tensile strength haseen observed as far as the welding speed was slowed down,robably due to the aforementioned larger loss of magnesium.he tensile strength drops below 80% when porosity is higher

han about 5% because of a reduction in cross-sectional area. Theighest levels of porosity have been achieved when operating at000 W, when the incident laser power is likely insufficient to

eep the keyhole steadily open, thus determining its collapsend the consequent bubble entrapment.

384 A. Ancona et al. / Journal of Materials Proce

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ig. 5. Average weld porosity vs. ultimate tensile strength of the fully penetratedoints.

. Conclusions

Sound butt welds have been obtained on aluminium–agnesium alloy 5083 3-mm thick specimens, using a 2.5 kWO2 laser and helium as shielding gas. The influence of theariation of the welding speed and the laser incident power onhe tensile properties, hardness profiles and porosity of the buttelds have been investigated.The following conclusions can be drawn from the work:

1) Best performances have been obtained by operating at2500 W and 100 mm/s where it was found a tensile strengthof more than 90% with respect to the native material, aporosity level less than 3% and just a small hardening acrossthe weld zone. The process was demonstrated to be robust

and reliable under these conditions.

2) Partial or discontinuous penetrations were achieved forlower laser powers or higher welding speeds, while for toohigh energy inputs weld dropout occurred.

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ssing Technology 191 (2007) 381–384

3) Welding tests carried out with a lower incident laser power(2000 W) revealed a large dispersion of the experimentaldata due to the instability of keyhole process. The imper-fect collapse of the keyhole, due to the lower power density,caused a higher incidence of porosity that affected the met-allurgical properties of the welds confirmed by a suddendrop of the measured tensile strengths.

4) Being constant the linear energy input (P/v) released ontothe alloy, a decrease of the mechanical properties of thejoints was found for lower welding speeds, probably becauseof the larger selective vaporization of magnesium in thefused zone that would cause a softening along the weld axis.

Overall results are very competitive if compared to welderformances reported in literature on the same alloy with alter-ative techniques like friction stir welding or GTAW [2].

cknowledgments

This research was supported by MIUR under projectD1105. Authors would like to acknowledge Prof. R. Spina,r. G. Basile and Dr.ssa T. Sibillano for the useful suggestions.

eferences

1] H. Zhao, et al., Current issues and problems in laser welding of automotivealuminium alloys, Int. Mater. Rev. 44 (6) (1999) 238–266.

2] M. Czechowski, Low-cycle fatigue of friction stir welded Al–Mg alloys, J.Mater. Proc. Tech. 164/165 (2005) 1001–1006.

3] L.M. Galantucci, et al., A quality evaluation method for laser welding of alu-minium alloys through neural networks, Ann. CIRP 49 (1) (2000) 131–134.

4] A. Ancona, et al., Comparison of two different nozzles for laser beam weld-ing of AA5083 aluminium alloy, J. Mater. Proc. Technol. 164/165 (2005)971–977.

5] A. Haboudou, et al., Reduction of porosity content generated during Nd:YAGlaser welding of A356 and AA5083 aluminium alloys, Mater. Sci. Eng. A363 (2003) 40–52.

6] Cheng Liu, et al., Tensile fracture behaviour in CO2 laser beam welds of7075-T6 aluminum alloy, Mater. Design 25 (2004) 573–577.