Laser welding of AA 5083 samples by high power diode laser

Laser welding of AA 5083 samples by highpower diode laser

J. M. Sanchez-Amaya*1, T. Delgado1, J. J. De Damborenea2, V. Lopez2 andF. J. Botana1

Laser welding is a very attractive technique to join different alloys at the industrial level, due to its

low heat input, high flexibility, high weld quality and high production rate. In this work, the

weldability of the aluminium alloy AA 5083 with a high power diode laser has been tested.

Concisely, samples were subjected to lineal treatments of laser radiation, with the objective of

studying the properties of the bead on plate welds generated. The main objective of the present

work has been to study the influence of both the processing rate and the superficial treatment of

the AA 5083 samples, on the morphological, microstructural and corrosion properties of the laser

weld beads. The sizes of the welds were higher as the processing rate was decreased. The weld

beads were seen to have better behaviour against corrosion than the base metal due to the

microstructural refinement. It was also verified that a blasting process before processing gave

beads with lower size but better corrosion resistance than the application of a black layer, due to

the minimisation of the magnesium evaporation in this former superficial treatment.

Keywords: Aluminium alloy, Laser welding, Bead on plate, Microstructure, Corrosion

IntroductionLaser welding (also known as laser beam welding, LBW)is at the frontier of the welding technology, beingattractive to a lot of industrial sectors because of thepossibility of processing materials of different thick-nesses.1 Nowadays, the commercially available andstable high power laser systems allow LBW to be a veryattractive and viable welding technique for joining andrepairing materials in industrial production. The advan-tages of the LBW are its low heat input, high localisationability (thus leading to processed samples with smallheat affected zones), high welding speed, high flexibility,high weld quality and high production rate.1–4

It is widely known that there are two laser weldingmechanisms, keyhole (density power higher than106 W cm22) and conduction (density power lower than106 W cm22).1,5 Between these two modes, keyholewelding is most used3,4,6,7 because it produces weldswith high aspect ratios and narrow heat affected zones.However, keyhole welding mechanism can lead to someproblems, such as instability, keyhole oscillation andintermittent closure of the keyhole that often producesporosity due to gas entrapment. Additionally, in some

alloys, the high weld speed (and hence high rates ofcooling) can lead to embrittlement in the weld or heataffected zone.1 Advancements in laser keyhole weldingand investigations into its difficulties are well documen-ted in literature.5,8 Analytical, numerical and empiricalstudies are being undertaken in order to achieve betterunderstanding of the process.1

On the other hand, conduction welding is a morestable process, since metal vaporisation occurs at a lowerlevel than at keyhole mode.1 In addition, it offers analternative way of welding traditionally difficult materi-als such as aluminium alloys.1 Different studies havebeen carried out concerning the laser welding ofaluminium alloys by means of conduction mechanism.1

Thus, the applicability of the diode laser to theconduction welding of AA 2024 and titanium alloys isanalysed and interpreted by thermal modelling.9 Zhaoand DebRoy10 developed a model to study thevaporisation rate and weld metal composition changeof automotive aluminium alloys, based on principles oftransport phenomena, kinetics and thermodynamics.

Although several studies have reported many advan-tages in using laser welding, there still exists thefundamental problem that the welding reliability ofaluminium alloys is lower than that of other industrialmetals, such as steels, due to its higher reflectivity, higherthermal conductivity and lower viscosity.4 In fact, thethermal conductivity of the aluminium alloys is aboutone order of magnitude higher than that of steels. Thus,the thermal conductivity of low carbon steel isy14 W m21 K21, being 143 W m21 K21 for the alu-minium alloy AA 5083.11 Another important point to

1Departamento de Ciencia de Materiales e Ingenierıa Metalurgica yQuımica Inorganica, Grupo de Ensayos, Corrosion y Proteccion,Universidad de Cadiz, Lab. 712, CASEM, Avda. Republica Saharaui s/n,11510 Puerto Real, Cadiz, Spain2Departamento de Corrosion y Proteccion, Centro Nacional deInvestigaciones Metalurgicas (CENIM-CSIC), Av. Gregorio del Amo 8,28040 Madrid, Spain

*Corresponding author, email [email protected]

� 2008 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 31 March 2008; accepted 1 August 2008DOI 10.1179/136217108X347629 Science and Technology of Welding and Joining 2009 VOL 14 NO 1 78

take into account is the high reflectivity of thealuminium alloys, which can be higher than 80%, beingmore reflective as the aluminium alloy is purer. Thus, thehigh reflectivity makes the aluminium alloys absorb littlefraction of the incident radiation, the high thermalconductivity provokes a rapid heat transfer, avoidingthe concentration of energy in the weld pool and finally,the lower viscosity of the welding pool limits theexpansion of the pool before the solidification.

Some investigations can be found regarding the studyof mechanical properties and formability of the basemetal or the laser welding characteristics of the bead onplate or butt welding processes on aluminium alloys.12–17

However, few studies have been published relating to theformation of welds on these alloys.4,18 Recently, theinfluence of the laser pulse modes on the properties ofbutt welds of AA 5754 and AA 6022 is analysed using a2?5 kW high power Nd:YAG laser.4 Concisely, twowelding modes are studied:4 continuous welding (CW)and pulse welding (PW). In the first one, the laser poweris kept constant at 1500 W, while in the PW, twodifferent powers (1000 and 2000 W; 1300 and 1700 W)are alternating every 5 ms. This study shows that CWmode leads to a higher penetration than the PW mode.The alternating periods in the PW mode is thought toinduce an unstable heat input, which would result in adispersion of the energy in the specimen. By contrast,the energy input remains uniform and stable whenwelding is performed in the CW mode, generatingdeeper penetration beads. These results are attributed tothe very high thermal conductivity of aluminium alloys,since the heat transfer via conduction mechanism is veryefficient in these alloys.4 Therefore, it is difficult toconcentrate the laser energy on the irradiated area, asthe heat is quickly transferred to the whole processedpiece of aluminium alloy. Taking into account theirproperties (high reflectivity, high thermal conductivityand low viscosity), it can be deduced that a relativelyhigh power density, in continuous laser welding mode, isrequired to get an effective coupling of the laser beamand aluminium, thus leading to an appropriate bead onplate or butt welding of these alloys.

During the welding process of aluminium alloys, somedefects can appear in the weld bead and in the heataffected zone, such as shallow penetration, porosities,blowholes, inappropriate microstructural changes andsolidification cracks.4,19,20 These defects usually decreasethe mechanical strength of the weld. Therefore, when thelaser welding process is used to join aluminium alloys, itis essential to control the experimental laser weldingconditions in order to minimise the appearance of thesedefects.4 The optimisation of the LBW of aluminiumalloys usually involves the generation of high penetra-tion beads and the prevention of welding defects.Additionally, a study of the microstructure evolutionand of the mechanical properties of the welded jointshelps the evaluation of the obtained joint.6

Aluminium alloys of the 5XXX21,22 and 6XXX23,24

series have been employed in the fabrication of car bodypanels because of their excellent formability character-istics and their good corrosion behaviour in comparisonwith other commercial aluminium alloys.4,20,25 Thesealloys can be joined by non-conventional weldingtechnologies such as LBW or friction stir welding. Thegreat interest of these techniques is that they allow one

to obtain high quality welds of similar or even dissimilarmaterials.20 A relatively high research has been per-formed on welding aluminium alloys with high powerCO2 or Nd:YAG lasers, but few reports on diode laserwelding of these alloys have been found.25 Among them,a high power diode laser is successfully employed toweld aluminium alloys of the series 5XXX and 6XXX.25

This work centred its attention on the butt welding of1 mm thickness samples, analysing the influence of theprocessing rate on the depths and widths of the weldsobtained.

In the present work, AA 5083 samples have beenprocessed with a high power diode laser of 2?8 kW inorder to generate bead on plate welds. The mainobjective of the present work has been to study theinfluence of both the superficial treatment and theprocessing rate on the properties of the weld beadsgenerated by the laser radiation. In order to characterisethe beads, both the widths and the depths weremeasured. In these conditions, defects are not observedin the welds. Additionally, the microstructure and themicrohardness of the different zones of the beads (fusionzone and base material) have been studied. Althoughmechanical properties of aluminium alloys welds havebeen previously reported in literature, little informationis available about other properties such as the behaviouragainst corrosion. For this reason, in the present paper,the corrosion behaviour of these zones in NaCl solutionsis also studied by means of electrochemical techniques.

ExperimentalIn this work, a high power diode laser of Rofin-Sinar,with a maximum power of 2800 W, has been employedto generate bead on plate welds on aluminium alloyAA 5083 samples, whose composition has been includedin Table 1. The size of the samples was approximately7062065 mm. According to the conclusions previouslycommented in the Introduction section, the laser treat-ments were performed in continuous mode and at arelatively high value of power, 1500 W. All laser treat-ments were performed at the beam focus spot (69?3 mmfrom the focusing lens). The laser focus spot shape isincluded in Fig. 1, being 1?772 and 2?272 mm indiameter in the X and Y axes respectively. The influenceof the processing rate on the characteristics of thegenerated weld beads has been evaluated. Two differentlaser processing rates were employed, v151?5 m min21

and v253 m min21.

Different superficial treatments were applied in orderto determine the one that most increase the radiationabsorption of the alloy AA 5083. These aluminium alloysamples were not affected by the laser radiation whenthey were simply ground or polished, due to the highreflectivity. To solve this problem, different superficialtreatments can be used in order to improve the laserradiation absorption. In this work, three differentsuperficial treatments were applied to AA 5083 samples

Table 1 Chemical composition of aluminium alloyAA 5083, wt-%

Si Fe Cu Mn Mg Zn Cr Ti Al

0?155 0?295 0?068 0?471 4?85 0?074 0?135 0?022 93?883

Sanchez-Amaya et al. Laser welding of AA 5083 samples by high power diode laser

Science and Technology of Welding and Joining 2009 VOL 14 NO 1 79

before being irradiated with laser, which are described asfollows:

(i) Treatment I. The samples were firstly ground upto 80 grits, obtaining a superficial roughness of2?2 mm. Later, the samples were covered by ablack marker layer

(ii) Treatment II. The samples were initially groundup to 80 grits and later subjected to the followingchemical cleaning treatment: the samples wereimmersed in an alkaline solution of the commer-cial product Turco 4215-NCLT, raised withdistilled water, later immersed in an acid solutionof Ardrox and finally raised again with distilledwater. The superficial roughness was 2?2 mm.Finally, as in treatment I, the samples werecovered by a black marker layer

(iii) Treatment III. The AA 5083 samples wereblasted with corindon particles, increasing theirroughness up to 50 mm.

To sum up, two laser processing rates and threesuperficial treatments were employed to generate beadon plate welds in samples of AA 5083. The codesemployed in this paper to identify each condition havebeen included in Table 2. Figure 2 shows a front view ofthe weld generated on one of the samples encoded asTIIIv2.

Microhardness measurementsThe microhardness measurements were performed onthe transversal sections of the samples, starting from theouter zone of the weld bead (Fig. 3). The profiles of 11microhardness values were obtained in the six conditionsabovementioned: three superficial treatments and twolaser processing rates. The Standard UNE-ENISO 6507-1 has been followed to perform the Vickersmicrohardness profile shown in Fig. 3. The charge

employed was 0?245 N. The procedure employedallowed the measurement of some microhardness valuesin the bead zone (between three and six values, infunction of the depth of the weld bead), the rest of thevalues were obtained in the base metal.

Determination of corrosion behaviourIn order to determine the corrosion behaviour of theweld beads generated in different conditions, polarisa-tion tests were carried out. The polarisation curves wereregistered after leaving the AA 5083 samples in3?5%NaCl at open circuit potential for 1 h, with theobjective of letting the systems corrosion potentialstabilise. After this open circuit, the samples werepolarised from 20?050 to z0?400 V with respect tothe open circuit potential, at a polarisation rate of0?1667 mV s21. These measurements were performedwith a potentiostat 1287 of Solartron, controlled withthe program CorrWare 2?0 of Scribner. From thesepolarisation curves, the polarisation resistance Rp wasestimated. This parameter is inversely proportional tothe corrosion rate strictly in the case of systems whichcorrode uniformly under activation control.26 However,Rp is always inversely proportional to the corrosionactivity, the systems with lower electrochemical activityhaving higher Rp values.

In most AA 5083 samples processed, the width of theweld beads obtained ranged from 2 to 3 mm. Thisconstitutes a limitation when the corrosion behaviourwas intended to be estimated, since the area exposed tothe NaCl solution must be small. The disadvantage ofemploying small areas in electrochemical tests is that theresults show higher scattering than when higherexposure areas are used. The area of the weld beadsamples exposed to the NaCl solution and laterpolarised was 0?12 cm2. For comparative purposes,samples with the same three treatments TI, TII and

1 Laser focus spot shape

Table 2 Codes employed in this paper to identify AA 5083 samples subjected to laser radiation

Sample code Superficial treatment Laser processing rate, m min21

TIv1 TI: ground to 80 gritszblack marker layer v151?5TIv2 TI: ground to 80 gritszblack marker layer v253TIIv1 TII: ground to 80 gritszchemical cleaning treatmentzblack marker layer v151?5TIIv2 TII: Ground to 80 gritszchemical cleaning treatmentzblack marker layer v253TIIIv1 TIII: blasted v151?5TIIIv2 TIII: blasted v253

2 Front view of weld generated on one TIIIv2 sample



TIII without being treated with laser, were alsosubjected to the polarisation tests. In these latter cases,a higher area (1 cm2) was exposed in order to decreasethe scattering in the Rp values.

Results and discussionIn continuation, the results concerning different proper-ties of the weld beads are described: depth and width,metallographic observations, microhardness measure-ments and the comparison between the corrosionbehaviours of the beads and the base metal. Alltreatments and measurements have been made intriplicate to assure the reproducibility.

Depth and width of weld beadA metallographic image of the weld bead cross-sectionof the TIv1 sample has been included in Fig. 4. In thisimage, the width and depth of the weld bead have beenmarked with arrows. Both parameters have beenmeasured in AA 5083 samples subjected to the sixtreatments described before (TIv1, T1v2, TIIv1, TIIv2,TIIIv1 and TIIIv2). From the analysis of the imagesobtained from all the samples, it can be concluded thatthe employed conditions allow one to obtain weld beadswithout signs of any defects.

The obtained values of the weld bead depth and widthhave been included in Figs. 5 and 6 respectively. It can

be seen in these figures that for the three superficialtreatments, both the width and depth values are higherwhen the processing rate is lower. It can be appreciatedin Figs. 5 and 6 that in samples with TI treatment, thedepth decreases from 800 mm at v1 to 678 mm at v2; andthe width is 2637 mm at v1 and 2443 mm at v2. Similarly,in TII treatment, the depth and width change from 829and 2777 mm at v1 to 688 and 2378 mm at v2 respectively.Finally, in samples with TIII, the depth and widthdecrease from 651 and 2251 mm at v1 to 493 and2015 mm at v2 respectively. Therefore, when the rate islower (v1), the interaction time between the laserradiation and the sample is higher and consequently,the welding melt pool is of bigger size, provoking beadon plate welds with higher values of depth and width.Concisely, when the processing rate is decreased from v2

to v1, the penetration increases by 18% for TI, 20% forTII and 32% for TIII.

On the other hand, it can be also seen in Figs. 5 and 6that at the two laser processing rates, the TIII treatmentleads to lower values of both depth and width than TIand TII. This can be due to the black marker layerapplied in treatments I and II, which encourages the

4 Metallographic image of weld bead cross-section of

one sample of AA 5083-TIv1

3 Schematic of experimental procedure followed to mea-

sure microhardness values in different points of weld

beads5 Mean and standard deviation values of weld beads

depths obtained in different superficial treatments and

laser processing rates

6 Mean and standard deviation values of weld beads

widths obtained in different superficial treatments and

laser processing rates



radiation absorption. It seems that the black layerproduces higher radiation absorption that the blastingprocess used in treatment III. Although the blastingprocess leads to less wide and deep beads, it should beunderline that it clearly improves the radiation absorp-tion in the absence of any black layer, since when thesamples were only ground, no weld beads weregenerated. Additionally, the blasting process is consid-ered as a clean treatment compared with othertreatments based on the application of films. Indeed,in these latter treatments, the composition of the weldbead could change under some conditions, because ofthe possible incorporation to the bead of chemicalcomponents of the applied coatings.

Metallographic analysis of weld beadsIn Fig. 7, it has been included a micrograph (650) ofthe cross-section of the weld bead of the sampleAA 5083-TIv1. It has been appreciated that the fusionzone is homogeneous under the six studied conditions(TIv1, T1v2, TIIv1, TIIv2, TIIIv1 and TIIIv2). It has beenalso verified that the microstructures of the beads aresimilar in the six working conditions, the values of widthand depth of the fusion zone being the only differencesfound when the superficial treatment and the laserprocessing rate are varied. In all cases, the microstruc-ture of the bead consists of a fine precipitation of thesecond phases in a solid solution matrix of Mg in Al, as

shown in the micrograph (6500) of the fusion zone ofthe sample TIv2 (Fig. 8).

Figure 9 shows an image (6200) of the interface basemetal/fusion zone of the sample AA 5083-TIv1. It can beobserved in this figure that the zones of the bead close tothe base metal (central zone of the image) arecharacterised by showing a dendritic growth, whichcorrespond to the zones with higher solidification rates.In these zones, it has been observed that intermetalliccompounds are not completely dissolved. Althoughthese types of precipitates are observed in the basemetal and in the zones with dendritic structure close tothe interface base metal/fusion zone, they do not appearin the central zones of the beads (fusion zone, Fig. 8).According to the results obtained with EDS analysis,these particles are rich in Al, Mn, Fe, Si and Cr, as canbe seen in the spectrum of Fig. 10a.27–29 For compara-tive purposes, the EDS of the aluminium matrix hasbeen included in Fig. 10b. The compositions of thealuminium matrix and of these intermetallic particleshave been included in Table 3. A SEM image of theinterface base metal/fusion zone can be seen in Fig. 11,where the Al–Mn–Fe–Si–Cr compound appears in whitecolour. In the fusion zone, these intermetallic com-pounds are dissolved during the laser processing, so thata fine microstructure is generated.

Microhardness measurements of weld beadsIn Tables 4 and 5, the Vickers microhardness values ofthe samples processed at v1 and v2 respectively, havebeen included. Note that each condition has beenrepeated in triplicate in order to assure an appropriatedreproducibility. The indentations made in the weldbeads have been italicised, while the indentations inthe base metal have not been. The metallographicobservation and the microhardness values of theprocessed samples have not shown evidence of heataffected zone between the base metal and the weldbeads. In general terms, it can be observed that for allthe systems, the HV values taken in the weld beads arehigher than those measured in the base metal. In Fig. 12,all microhardness values have been represented for abetter view of the obtained data. In this figure, it can beseen that the microhardness values of the weld beads areslightly higher than those measured in the base metal. InFig. 12, it can be seen that the difference between thesezones are y10 HV, most bead data belonging to the

7 Metallographic image (650) of cross-section of welded

bead of sample TIv1

8 Metallographic image (6500) of cross-section of fusion

zone of sample TIv2

9 Metallographic image (6200) of cross-section of inter-

face base metal/fusion zone of sample TIv1



interval between 85 and 100 HV, while most metal basedata range between 75 and 90 HV. The increase in thehardness in the weld bead is due to the microstructuralchanges, reported and discussed in the previous section,provoked by the laser heat treatment. Thus, the highervalues of hardness measured in the fusion zone could bea consequence of the microstructure refinement.

Corrosion behaviour of weld beadsThe corrosion behaviour of the weld beads formed onAA 5083 samples with the three superficial treatmentsdescribed before (TI, TII and TIII) has been studied.Since the samples processed at the rates v1 and v2 havebead on plate welds with similar microstructure and

(a)

(b)

a intermetallic compound; b aluminium matrix10 EDS spectra of intermetallic compounds found in base metal and in interface base metal/fusion zone and of alumi-

nium matrix

Table 3 EDS semiquantitative analysis of Al–Fe–Mn–Si–Cr intermetallic compound and aluminium matrix,wt%

Mg Al Si Cr Mn Fe

Intermetallic compound 1?39 66?57 3?80 1?49 8?84 17?90Aluminium matrix 4?66 94?68 0?04 0?13 0?41 0?09 11 SEM image of interface base metal/fusion zone



microhardness values, it was decided to study only thesamples at one rate (3 m min21). Initially, polarisationtests were performed over single scans of laser radiation.After these preliminary tests, the samples surfaces showsimilar characteristics to the sample included in Fig. 13.It could be seen that samples show two zones withdifferent activities (Fig. 14). In both figures, it can beobserved that the central zone, which belongs to weldbead, does not show corrosion products. Meanwhile, theouter area, which belongs to base metal, shows blackcorrosion products. Therefore, it can be deduced thatthe weld bead generally behaves as cathode, while thebase metal surrounding the bead normally behaves asanode. These results agree with those recently pub-lished,30 where the weld fusion zone was seen to behaveas cathode and the base metal as anode within a bead onplate laser weldment of AA 6061 base metal andAA 4043 filler wire. The differences in the activitiesbetween the base metal and the weld beads can be due tothe microstructure refinement produced during the laserprocessing. As a consequence, this weld bead area canbehave as a cathode with respect to the base metal.

As Fig. 14 summarised, the preliminary tests per-formed on single lineal laser treatments evaluate thecorrosion behaviour of the pair weld bead/base metal.As commented, if the samples with only one laser scanare subjected to the polarisation tests, the electrochemi-cal response is not related to the weld beads, but to the

galvanic coupling between the inner and the outer zones.In order to study the corrosion behaviour of the weldbeads, several parallel laser scans were made with theobjective of having more area of beads to perform thepolarisation tests. The separation between the consecu-tive beads was 2 mm for the three superficial treatments,obtaining a 25% of overlapping zones between beads.Subsequently, the samples with various parallel beadswere subjected to the linear polarisation tests. Afterthese tests, the samples did not show any signs ofgalvanic coupling.

The Rp values of AA 5083 samples with differentsuperficial treatments and later subjected to parallellaser scans have been plotted in Fig. 15. For eachtreatment, the corrosion behaviours of the base metaland of the parallel weld beads have been studied. Whenthe three non-processed samples are compared, it can beobserved that the chemical cleaning (treatment II) leadsto very low Rp values, revealing the high activity of thesystem. Therefore, before being processed with laser, thetreatment II provokes a higher susceptibility to corro-sion than the others. On the contrary, the higher Rp

values of the base metal are reached when the samplesare blasted (treatment III). After the samples areprocessed with laser, the weld beads are generated,changing the corrosion properties. It can be appreciatedin Fig. 15 that the corrosion behaviour is clearly

Table 4 Vickers microhardness measurements at0?025 kgf of samples processed at v1:indentations made inside weld beads have beenitalicised

Samples TIv1 Samples TIIv1 Samples TIIIv1

96?9 99?1 104?7 92?6 86?8 102?2 91?4 93?9 85?791?8 101?9 104?8 93?7 93?0 101?9 90?2 93?9 90?294?6 97?8 97?7 95?1 89?2 99?1 95?4 91?1 89?192?0 93?9 97?7 90?2 91?4 95?7 94?4 92?6 93?985?3 89?1 93?9 89?1 87?9 95?5 88?1 89?1 90?286?8 89?6 85?7 85?1 96?1 84?3 84?2 87?8 84?387?9 83?5 85?7 82?5 81?5 85?0 84?6 84?0 95?186?8 86?9 84?5 78?5 87?9 85?3 84?2 84?4 80?988?1 79?5 83?5 80?5 87?4 85?3 84?2 86?2 82?784?1 86?8 79?5 82?5 81?5 79?8 83?9 90?3 91?281?9 83?5 82?8 85?7 84?6 86?2 80?5 89?1 84?6

Table 5 Vickers microhardness measurements at0?025 kgf of samples processed at v2:indentations made inside weld beads have beenitalicised

Samples TIv2 Samples TIIv2 Samples TIIIv2

107?8 95?1 96?4 93?9 92?8 91?4 95?1 92?6 91?4100?8 101?3 97?7 94?1 98?1 91?9 95?1 96?4 94?797?6 95?1 93?9 93?2 98?0 90?2 93?9 94?8 87?297?2 97?2 91?1 91?9 95?7 92?6 95?5 85?3 85?497?4 89?4 86?8 83?9 91?6 81?4 88?4 88?1 87?785?0 87?9 80?5 79?6 80?9 84?6 85?8 89?0 86?490?7 83?1 87?8 85?7 84?9 80?5 87?0 83?2 85?886?6 87?6 77?0 82?4 87?3 91?8 90?2 89?2 88?286?4 82?5 82?8 83?6 84?3 84?3 85?0 86?4 88?187?0 83?5 82?5 87?1 88?8 85?4 87?4 85?8 85?388?2 77?6 90?2 89?4 81?9 85?7 82?4 82?1 84?1

12 Vickers microhardness values taken at 0?025 kgf of

weld beads and base metal of AA 5083 samples

13 Image of zone subjected to preliminary linear polarisa-

tion test, corresponding to sample of AA 5083 with

superficial treatment II



improved in all conditions. Thus, in the three treat-ments, the Rp values of the weld beads are higher thanthose of the metal base. This corrosion resistanceimprovement is thought to be due to the microstructuralrefinement that takes place during the laser treatment.

In the recent literature, it has been observed that theweld beads of 5XXX alloys can have lower magnesiumcontent than the metal base. Obviously, the magnesiumcomposition of the weld depends on the extent of themagnesium evaporation, which is closely related to theprocessing conditions. The magnesium depletion of weldbeads generated under different laser keyhole weldingconditions has been measured in literature for differentAl–Mg alloys. Thus, weld beads of AA 5754 samplesranged between 2?32 and 2?55 wt-%, depending on thewelding conditions, the base metal being 2?9 wt-%.4 Thismeans that the magnesium loss during the laser weldingunder keyhole regime ranged between 12 and 20%.4 InAA 6061 samples, a magnesium loss of y20% has beenalso observed in the weld fusion zone, with respect to thebase metal,31 while in AA 5083 samples, a loss of Mgbetween 13 and 22% in the middle of the keyhole isreported.32

The contents of magnesium contained in the basemetal and in the weld beads of the AA 5083 samplesstudied here have been measured by means of EDSsemiquantitative analysis. These results, given as per-centage of magnesium and percentage of magnesiumloss, have been included in Table 6. As can be observedin the table, the loss of magnesium in the samplesprocessed under conduction regime ranged between 1and 4%, values much lower than those reported inliterature for keyhole regime. These data are expectedsince the keyhole mode involves a higher input energydensity than the conduction mode. Indeed, AA 5083samples welded under keyhole regime leaded to beadswith lower Mg content (because of the higher evapora-tion during the laser processing) than those sampleswelded under conduction mode.1

In Table 6, the percentage of magnesium loss in eachlaser treatment can be compared. It can be appreciatedthat the blasting treatment (TIII) leads to lowermagnesium evaporation than the other two treatments(TI and TII). It has been recently reported that themagnesium evaporation during the laser processing cancause composition changes in the weld, modifying

consequently both the mechanical properties and thecorrosion behaviour of the joint.33 The loss of alloyingelements is normally not desirable, as the properties ofthe melted zone can be different from the base metal.34

Although very few corrosion data of weld beads areavailable in literature, some results concerning mechan-ical properties can be found. Thus, weld beads ofAA 5083 and AA 5754 with lower magnesium evapora-tion have showed better tensile strength than those withhigh evaporation.1,4,35 Additionally, it is reported thatthe main factors affecting the tensile properties andformability of aluminium joints are the microstructure,the magnesium evaporation and the porosity.4 Thus, amicrostructure refinement, a lower Mg evaporation anda porosity reduction usually improve the quality of theweld bead.4 The analysis of the weld beads obtained inthe present work for the different processing conditionsshowed no porosities and similar microstructure refine-ment. However, as can be seen in Table 6, lowermagnesium evaporation was measured for the blastingtreatment (TIII). Therefore, the higher magnesiumcontained in these weld beads seems to be the reasonwhy the TIII samples show better behaviour againstcorrosion than TI and TII (Fig. 15).

To sum up, the laser treatments on AA 5083 sampleslead to surfaces with lower corrosion activity than thebase metal for the three superficial treatments studied.The microstructural refinement provokes improvementson the corrosion behaviour of the weld beads withrespect to the base metal. Additionally, when comparingthe corrosion behaviour of the weld beads generatedunder different conditions, it can be seen that thetreatment III, the blasting process, is the treatment thatleads to samples with a higher corrosion resistance inNaCl solutions, due to the low magnesium evaporationduring the laser treatment.

14 Model of galvanic coupling behaviour between weld

bead and base metal observed during preliminary lin-

ear polarisation tests 15 Rp values of both base metal and weld bead, obtained

with different superficial treatments

Table 6 Percentages of magnesium (in base metal and inweld beads) and percentage of magnesium lossin welds with respect to base metal

Sample Mg, % Mg loss, %

Base metal 4?85Weld bead TI 4?73 2?47Weld bead TII 4?66 3?92Weld bead TIII 4?80 1?03



ConclusionsIn the present paper, a high power diode laser workingat a continuous power of 1500 W has been employed togenerate bead on plate welds on aluminium alloyAA 5083 samples. The influence of both the processingrate and the superficial treatments on the morphological,microstructural and corrosion properties of the weldbeads has been analysed.

The penetration of the weld beads were between 18and 32% higher at 1?5 m min21 than at 3 m min21. Thesuperficial treatments which incorporated a blackmarker layer leaded to larger values of both depth andwidth due to the higher radiation absorption.

The microstructural phases of the beads were similarfor the six laser processing conditions studied, notshowing porosity. The microstructure of the inner partof the bead consisted of a fine equiaxed grainsmicrostructure, while the zones of the bead close tothe base metal showed a dendritic growth. Because ofthe microstructural refinement of the fusion zone withrespect to the metal base, the microhardness values ofthe weld beads formed in all conditions were slightlyhigher than those of the metal base.

After the samples were processed with laser, thecorrosion behaviour was clearly improved for all condi-tions, due to the microstructural refinement. The blastingprocess was the superficial treatment that leaded to beadon plate welds with better corrosion behaviour, as withthis treatment, the magnesium evaporation is minimised.

The obtained results allow one to confirm thatAA 5083 samples can be welded by means of a highpower diode laser. However, further research is neces-sary to optimise the laser welding experimental condi-tions, in order to obtain high quality bead on plate andbutt welds of similar and dissimilar alloys.

Acknowledgement

The present work has been financially supported bythe Ministerio de Educacion y Ciencia (projectno. DPI2005-09244-C04-02) and by the Junta deAndalucıa.

References1. P. Okon, G. Dearden, K. Watkins, M. Sharp and P. French: Proc.

21st Int. Cong. on Applications of Lasers and Electro-Optics

(ICALEO 2002), Scottsdale, AZ, USA, October 2002, Laser

Institute of America, 233–241.

2. R. G. Ding, O. A. Ojo and M. C. Chaturvedi: Intermetallics, 2007,

15, 1504–1510.

3. R. Akhter, L. Ivanchev and H. P. Burger: Mater. Sci. Eng. A, 2007,

A447, 192–196.

4. T. Y. Kuo and H. C. Lin: Mater. Sci. Eng. A, 2006, A416, 281–289.

5. W. W. Duley: ‘Laser welding’, Chapter 4; 1998, New York, John

Wiley & Sons.

6. Y. Shi, F. Zhonga, X. Li, S. Gong and L. Chen: Mater. Sci. Eng. A,

2007, A465, 153–159.

7. P. Bassani, E. Capello, D. Colombo, B. Previtali and M. Vedani:

Composites Part A, 2007, 38A, 1089–1098.

8. J. F. Ready and D. F. Farson (eds.): ‘LIA handbook of laser

materials processing’; 2001, Orlando, FL, Magnolia Publishing.

9. S. W. Williams, G. Scott and N. J. Calder: Laser Opto, 2001, 33,

50–54.

10. H. Zhao and T. DebRoy: Metall. Trans. B, 2001, 32B, 163–172.

11. H. Ahmed, M. A. Wells, D. M. Maijer, B. J. Howes and M. R. van

der Winden: Mater. Sci. Eng. A, 2005, A390, (1–2), 278–290.

12. M. Pastor, H. Zhao, R. P. Martukanity and T. DebRoy: Weld. J.,

1999, 78, 207s–216s.

13. X. J. Wang, S. Katagama and A. Matsunawa: Weld. J., 1997, 76,

70s–73s

14. S. Venkat, C. E. Albright, S. Ramasamy and J. P. Hurley: Weld. J.,

1997, 76, 275s–283s.

15. S. Ramasamy and C. E. Albright: Sci. Technol. Weld. Join., 2001,

6, (3), 182–190.

16. K. M. Zhao and J. K. Lee: J. Eng. Mater. Technol., 2001, 123, (3),

287–292.

17. A. Punkari, D. C. Weckman and H. W. Kerr: Sci. Technol. Weld.

Join., 2003, 8, (4), 269–281.

18. B. Irving: Weld. J., 1991, 71, 39–45.

19. S. Ramasamy and C. E. Albright: J. Laser Appl., 2000, 12, (3), 101–

115.

20. P. Negre, D. Steglich and W. Brocks: Eng. Fract. Mech., 2004, 71,

2365–2383.

21. P. J. Bolt, N. A. P. M. Lamboo and R. P. J. C. Mozier: J. Mater.

Process. Technol., 2001, 115, (1), 118–121.

22. M. Jain, J. Allin and M. J. Bull: Mater. Sci. Eng. A, 1998, A256, (1–

2), 69–82.

23. M. Z. Wang and M. E. Kassner: J. Mater. Eng. Perform., 2002, 11,

(2), 166–168.

24. R. G. Kamat, J. F. Butler, Jr, S. J. Murtha and F. S. Bovard:

Mater. Sci. Forum, 2002, 396–402, 1591–1596.

25. N. Abe, M. Tsukamoto, K. Maeda, K. Namba and J. Morimoto: J.

Laser Appl., 2006, 18, (4), 289–293.

26. J. M. Sanchez-Amaya, R. A. Cottis and F. J. Botana: Corros. Sci.,

2005, 47, 3280–3299.

27. A. Aballe, M. Bethencourt, F. J. Botana, M. J. Cano and M.

Marcos: Corros. Sci., 2001, 43, 1657–1674.

28. A. Aballe, M. Bethencourt, F. J. Botana, M. Marcos and J. M.

Sanchez-Amaya: Corros. Sci., 2004, 46, 1909–1920.

29. M. Bethencourt, F. J. Botana, M. J. Cano, M. Marcos, J. M.

Sanchez-Amaya and L. Gonzalez-Rovira: Corros. Sci., 2008, 50,

(5), 1376–1384.

30. A. B. M. Mujibur Rahman, S. Kumar and A. R. Gerson: Corros.

Sci., 2007, 49, 4339–4351.

31. A. B. M. Mujibur Rahman, S. Kumar and A. R. Gerson: Corros.

Sci., 2008, 50, 1267–1273.

32. T. Sibillano, A. Ancona, V. Berardi, E. Schingaro, P. Parente and

P. M. Lugara: Opt. Lasers Eng., 2006, 44, 1324–1335.

33. A. Ancona, P. M. Lugara, D. Sorgente and L. Tricarico: J. Mater.

Process. Technol., 2007, 191, 381–384.

34. T. Sibillano, A. Ancona, V. Berardi and P. M. Lugara: Opt.

Commun., 2005, 251, 139–148.

35. L. Tricarico, R. Spina, D. Sorgente, A. Ancona, T. Sibillano and

G. Basile: Key Eng. Mater., 2007, 344, 745–750.



Laser welding of AA 5083 samples by high power diode laser

Documents

Transcript of Laser welding of AA 5083 samples by high power diode laser