Modulated fiber laser welding of high reflective AZ31

ORIGINAL ARTICLE

Modulated fiber laser welding of high reflective AZ31

Lin-Jie Zhang & Xing-Jun Zhang & Jie Ning &

Jian-Xun Zhang

Received: 20 February 2014 /Accepted: 19 August 2014# Springer-Verlag London 2014

Abstract Power sine modulation fiber laser welding (FLW)of AZ31 magnesium alloy was conducted. When the diameterof transmission fiber and focal spot was 400 μm and 0.4 mm,respectively, the average laser powers for penetrating a 2.7-mm-thick AZ31 plate at a welding speed of 5 m/min werereduced by about 33 % by using modulated FLW. The rela-tionship between modulation parameters (i.e., amplitude, fre-quency, and average laser power) and weld depth which wasclosely related to the transfer efficiency of laser energy wasstudied through partial penetration laser welding test on 8-mm-thick AZ31 plate based on quadratic regression orthogo-nal design. It was found that influence of power modulationon laser welding of AZ31 alloy was highly dependent on laserpower density. In the low power density range, laser energycoupling efficiency could be significantly improved by com-bining low amplitude with high frequency or high amplitudewith low frequency. With the increasing of laser power den-sity, the optimum frequency corresponding to maximum welddepth decreased, and the positive effect of the favorable com-bination of high amplitude and low frequency on laser energycoupling continuously weakened. When laser power densitywas high enough, power modulation had hardly positive effecton weld depth and energy coupling efficiency. It was arguedthat improvement of energy coupling efficiency in laserwelding of AZ31 by using power modulation was due to thereduction in the portion of energy lost into surroundings.Finally, laser butt welding was conducted on 2.7-mm-thickAZ31 under the condition of high beam quality and the tensile

strength of both butt-welded joint and base metal that wastested.

Keywords AZ31magnesium alloy . Power modulation .

Laser welding . Regression orthogonal design .Weld depth

1 Introduction

Magnesium alloys have a low density, high specific strength,high elastic modulus, good vibration resistance, and strongability to withstand shock loads and also possess good thermalconductivity, electrical conductivity, corrosion resistance,electromagnetic shielding performance, and good recyclabil-ity. Thus, these alloys have been widely used in aviation,aerospace, transportation, chemicals, electronics, and otherindustrial sectors [1, 2]. Currently, tungsten inert gas (TIG)arc welding and metal inert gas (MIG) arc welding dominatethe magnesium alloy welding processes. However, arcwelding of magnesium alloy has the disadvantages of lowefficiency, large weld size, large heat-affected zone, largesolidification shrinkage, and large residual deformation. Laserwelding has merits of high efficiency, small welded joint size,and small welding residual deformation. In recent years, fiberlaser technology has rapidly developed [3, 4]. Many scholarshave applied fiber laser in magnesium alloy welding due to itsgood beam quality, small volume, and high conversionefficiency.

S. M. Chowdhury et al. compared the microstructure andmechanical properties of AZ31 magnesium alloy-weldedjoints between fiber laser welding (FLW) and semiconductorlaser welding. They found that, compared with the semicon-ductor laser-welded joint, the magnesium alloy-welded jointusing FLWwas narrower and the grains around its fusion zonewere finer [5]. S. H. Chowdhury et al. studied the effects ofwelding speed on the microstructure and properties of fiber

L.<J. Zhang :X.<J. Zhang : J. Ning (*) : J.<X. ZhangState Key Laboratory of Mechanical Behavior for Materials, Xi’anJiaotong University, Xi’an 710049, Chinae-mail: [email protected]

L.<J. Zhange-mail: [email protected]

Int J Adv Manuf TechnolDOI 10.1007/s00170-014-6303-8

laser-welded AZ31 magnesium alloy joints and found thatwhen the welding speed increased, the weld width decreased,the grain size within the welded joint and heat-affected zonedecreased, the microhardness of the welded metal increased,and the tensile breaking strength of the welded joint increased[6]. M. Harooni et al. reported that preheating the workpiecewith an arc plasma beam during laser-welding process couldeffectively suppress the porosity defects in laser lap-weldedjoints of AZ31 magnesium alloy and improve the strength oflap joints [7]. Recently, M. Harooni et al. showed that duallaser beamwelding could effectively suppress porosity defectsduring laser lap welding of AZ31 magnesium alloy [8]. Theresults of Wang et al. demonstrated that during the AZ31magnesium alloy FLW process, a 1-kW increase of laserpower could increase the weld penetration by 2–3 mm andthat the effect of welding speed onweld penetration was not assignificant as that of laser power. In addition, they also ob-served that when the welding heat input was large, the originalmicropores in the base material were the main causes of thedefects in welded joint. When the welding heat input wassmall, the instability of the keyhole was the main reason forthe formation of porosity defects in the welded joints [9].These studies demonstrate that there has been good progressin AZ31magnesium alloy FLW technology. In addition, someproblems still remain in magnesium alloy laser welding, suchas the presence of pores, undercut defects, and low laserenergy coupling efficiency caused by the high reflectance ofthe magnesium alloy to the laser.

Laser power modulation technique has been used by manyresearchers to eliminate pores, undercuts, and other defectsduring laser welding. Kuo and Jeng observed that after laserpower modulation, the number of pores in Nd:YAG laser-welded joints of SUS304 stainless steel and Inconel 690 alloydecreased and the weld penetration increased [10]. A.Matsunawa et al. found that CO2 laser power modulationreduced the number of pores in CO2 laser-welded joints ofA5083 aluminum alloy [11]. Kawaguchi et al. observed thathigher laser power modulation amplitude led to less pores inCO2 laser-welded joints [12]. J. E. Blackburn et al. applied thelaser power modulation technique to Nd:YAG laser-weldingprocess of titanium and observed that the porosity defect in thewelded joints was effectively suppressed. In addition, theyfound that an appropriate reduction of the laser power modu-lation amplitude could effectively eliminate the undercut de-fects [13]. Recently, German researchers reported that laserpower modulation welding technology was valuable inachieving high quality, efficient welding of highly reflectivematerials. P. Stritt et al. conducted power sine modulationFLWexperiments on two materials with significant differencein laser absorption rate [14]. Their research demonstrated thatfor the AlMgSi material with higher reflectivity, the criticalpower for keyhole collapsing upon decreasing the laser in-stantaneous power was significantly smaller than that upon

increasing the laser instantaneous power. For the St37materialwith lower reflectivity, the critical power for keyhole collaps-ing upon decreasing the laser instantaneous power was similarto that upon increasing the laser instantaneous power. In otherwords, materials with higher laser reflectance were associatedwith higher amplitude improvement of the power couplingefficiency during the welding process using laser power mod-ulation. Their study clearly revealed the application value oflaser power modulation technology in laser deep penetrationwelding of high reflective material and its mechanism. Fur-thermore, even if the average power was greater than thecritical power for the transition from thermal conductingwelding to deep penetration welding, the weld penetrationobtained in high reflective materials using laser power modu-lation technology welding would be deeper than that usingcontinuous laser welding. The research results of A. Heideret al. confirmed that laser power sine modulation not onlysignificantly improved the penetration of laser-welded jointsof metal with high reflectance but also reduced pore andsplash during the welding process [15]. Recently, with theaid of X-ray high-speed photography, A. Heider et al. ob-served that under an appropriate modulation frequency, thekeyhole stability during the copper FLW process was signif-icantly improved [16].

Magnesium alloy has a high reflectance to the most com-monly industrially used 1-μm wavelength laser beam. Atroom temperature, the absorption rate of magnesium alloysfor a laser with a 1-μm wavelength ranges from 8 to 20 %[17]. According to previous studies, the application of laserpower modulation welding technique in the welding of highreflective magnesium alloys will not only improve the cou-pling efficiency of the laser energy but may also reducedefects such as porosity, undercutting, and spatter by optimiz-ing the power and modulating parameters. Therefore, in thispaper, experimental studies on power modulation FLW ofAZ31 magnesium alloy were performed to better understandthe effect of modulation parameters on FLW of magnesiumalloy and to explore the efficient, high-quality, and energy-saving welding technology for magnesium alloy.

2 Research methods

2.1 Test materials

The test material used in this study was AZ31 magnesiumalloy plates with 2.7- and 8-mm thickness, respectively. Thenominal composition of AZ31 magnesium alloy was 3.34–3.63 wt.% Al, 0.45–0.53 wt.% Zn, 0.27–0.29 wt.% Mn,balance Mg. Before welding, the oxide film on the surfaceof plate was removed with a wire brush. Then, the grease wasremoved with acetone. The welding tests were completedwithin 2 h after plate cleaning.

Int J Adv Manuf Technol

2.2 Test equipments and test procedures

The welding equipment used in the present study was IPGYLS-4000 multimode fiber laser system with a maximumoutput power of 4 kW. Sinusoidal modulation of the laserpower was achieved by inputting a time-varying sinusoidalvoltage signal generated by a function generator to the lasersystem. The two kinds of transmission fiber diameter involvedin this work were 200 and 400 μm, respectively. The focusinglens diameter was 50 mm. Two kinds of focal length (i.e., 150and 250 mm) were used in this work. The focal spot diameterdepended on the transmission fiber diameter and the focallength employed. As shown in Fig. 1, the laser beamrearwardly inclined 10° in welding process to prevent thedamaging of optical elements from reflected light. A side-blowing nozzle with an inner diameter of 15 mm was locatedin front of the melt pool, which delivered argon to the areanear melt pool. Behind the melt pool, a protective cover filledwith argon was employed to protect the solidified weld metal.

The welding conditions considered in this work are listedin Tables 1 and 2. The welding speed in all the weldingexperiments was 5 m/min, and the defocusing distance was0 mm unless otherwise specified.

In the first part of welding test, modulated FLWof 2.7-mm-thick AZ31 plates was conducted with low beam quality, aslisted in Table 1. The diameter of transmission fiber and focal

spot was 400 μm and 0.4 mm, respectively. Continuous laser-welding experiments with different powers were first conduct-ed, and the relationship between laser power and weld depthwas determined. The laser power that yielded partial penetra-tion but large weld depth was selected as the average power.Half of the average power was selected as the amplitude of thesine modulation. Effect of modulation frequency on weldpenetration was studied. Then, a modulation frequency thatdid not result in full penetration of the plate was adopted toinvestigate the effect of modulation amplitude on welded jointcross section welded penetration under same modulation fre-quency. In addition, under three low modulation frequencies(i.e., 4, 8, and 12 Hz), welding experiments using variousmodulation amplitudes (i.e., 40, 60, 80, and 100 % of theaverage laser power) were performed. Because of the lowmodulation frequency adopted, the changes of weld penetra-tion along welding direction could be examined, and the effectof modulation parameters on the laser-welding process ofmagnesium alloy was analyzed.

In the second part of welding test, in order to get moreknowledge about the interaction between modulated fiber laserenergy and the AZ31 alloy during modulated laser welding,bead on plate welding test was carried out on 8-mm thicknessAZ31 plate based on a quadratic regression orthogonal designwith three factors (i.e., laser average power, frequency, andamplitude), as listed in Table 2. A regression equation between

Fig. 1 Experiment equipments

Table 1 Welding parameters used in laser-welding test of 2.7-mm-thick AZ31 alloy

Diameter offiber (mm)

Focallength (mm)

Focal spotdiameter(mm)

Joint type Amplitude (kW) Frequency (Hz) Average power (kW)

400 150 0.4 Bead on plate – – 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4

400 150 0.4 Bead on plate 1 100, 200, 300, 400, 500, 600, 700, 800, 900 2

400 150 0.4 Bead on plate 0, 0.5, 1.5 300 2

200 150 0.2 Butt welding – – 2.3


modulation condition (i.e., average power, frequency, and am-plitude) and weld depth, which was closely related to laserenergy transfer efficiency, was obtained. Since a transmissionfiber with a diameter of 200 μm was used in the second part ofwelding test, the influence of power density on modulated laserwelding of AZ31 alloy could be discussed by comparing thetest results between the first part and the second part.

Finally, butt-welded joint of 2.7-mm-thick AZ31 was pro-duced by using a transmission fiber with a diameter of 200 μmand a focal spot diameter of 0.2 mm, and the tensile strength ofbutt-welded joint was measured. Since that power modulationhad almost no positive effect on the energy coupling efficien-cy of welding process when laser power density was high,only normal continuous wave laser welding with constantpower was conducted.

3 Research results

3.1 Modulated laser welding of 2.7-mm-thick AZ31using Φ400-μm transmission fiber

3.1.1 Continuous wave fiber laser welding under differentpowers

Figure 2 shows the cross-sectional morphology of weldedjoints obtained under different laser powers. It can be seenthat the obtained welded depth increased monotonically withthe increasing of laser power until the plate was fullypenetrated.

As shown in Fig. 2a, a typical thermal conductive weldingseam was observed under a power of 0.5 kW. The weld depthwas only approximately half the width. As observed inFig. 2b, the weld depth was greater than the weld bead widthwhen the power was 1 kW, which indicates that under suchconditions, keyhole was already formed in the melt pool; thus,the laser energy coupling efficiency increased significantly.According to the results presented in Fig. 2a, b, when the laserpower increased from 0.5 to 1 kW, even though the laserpower was only doubled, the weld depth was increased morethan four times, and the cross-sectional area of the weld wasincreased more than tenfold. Based on the results presented inFig. 2b–e, it can be seen that increase in the laser power causedsignificant increase in weld bead width and weld depth. Asobserved in Fig. 2f–h, when the laser power was greater thanor equal to 3 kW, the test plate was fully penetrated. Compar-ing the results in Fig. 2e, f, it can be seen that when the testplate was fully penetrated, the weld width reducedsignificantly.

In the case of full penetration welding, after multiple re-flections, laser beam reached the lower orifice and directlyescaped from the lower orifice into the surrounding environ-ment. In the case of partial penetration welding, when the laser

Table 2 Three-factor quadratic regression orthogonal designmatrix usedin laser welding of 8-mm-thick AZ31 alloy (in terms of coded factors)

x1 (amplitude) x2 (frequency) x3 (average laser power)

1# 1 1 1

2# 1 1 −13# 1 −1 1

4# 1 −1 −15# −1 1 1

6# −1 1 −17# −1 −1 1

8# −1 −1 −19# 1.215 0 0

10# −1.215 0 0

11# 0 1.215 0

12# 0 −1.215 0

13# 0 0 1.215

14# 0 0 −1.21515# 0 0 0

Fig. 2 Weld profiles of bead on plate tests performed on 2.7-mm-thick AZ31 at various laser powers (welding speed 5 m/min, focal position 0 mm, laserincidence angle 10°)


beam reached the bottom of keyhole after multiple reflections,it could not escape into the surrounding environment directly.Thus, keyhole wall could absorb more laser energy.

3.1.2 Effect of modulation amplitude and modulationfrequency

The average power was selected as 2 kW, and the modulationamplitude was selected to be 1 kW, namely, 50 % of theaverage power. The typical results of weld cross section atvarious modulation frequencies are presented in Fig. 3. Fig-ure 4 shows the lower surface morphology of the jointsobtained at various modulation frequencies.

Results in Figs. 3 and 4 clearly show that the laser energycoupling efficiency during the welding process increased withthe increasing of modulation frequency. Under modulation

frequencies of 800 and 900 Hz, the test plates were both fullypenetrated. By comparing the full penetration weld shape inFig. 3 and the full penetration weld shape in Fig. 2, it can befound that the weld width of former, namely, with powermodulation, was larger than that of the latter. This result maybe due to the fact that instantaneous laser power at the negativesinusoidal half cycle was relatively small, which might lead tothe intermittent closure of lower orifice. When the lowerorifice was closed, the laser beam that reached the bottom ofthe hole failed to escape directly into the surrounding envi-ronment; so, keyhole wall can absorb more laser energy.

Figure 5 illustrates the effect of modulation amplitudechange on the typical cross-sectional weld morphology whenthe average power was 2 kW and the modulation frequencywas 300 Hz. Figure 6 shows the surface morphology of beadon plate-welded joints obtained at various amplitude of sine

Fig. 3 Typical weld profiles of bead on plate tests made in 2.7-mm-thickAZ31 at various frequencies of sine modulation (average laser power2 kW, welding speed 5 m/min, focal position 0 mm, sine modulation

amplitude 1 kW, laser incidence angle 10°, transmission fiber dia.0.4 mm, focal length 150 mm, focal spot dia. 0.4 mm)

Fig. 4 Lower surface of 2.7-mm-thick AZ31 welded joints at variousfrequencies of sine modulation (average laser power 2 kW, welding speed5 m/min, focal position 0 mm, sine modulation amplitude 1 kW, laser

incidence angle 10°, transmission fiber dia. 0.4 mm, focal length 150mm,focal spot dia. 0.4 mm)


modulation. As observed in Figs. 5 and 6, when the modula-tion amplitude increased to 1,500 W, the test plate was fullypenetrated. The upper weld width obtained under a modula-tion amplitude of 1,500 W (i.e., 1.8 mm) was smaller than theupper weld width obtained under a modulation amplitude of1,000 W (i.e., 2 mm) but greater than those obtained bycontinuous laser full penetration welding such as shown inFig. 2f–h (i.e., upper widths were 1.25, 1.4, and 1.3 mm,respectively).

3.1.3 Effects of modulation frequency and modulationamplitude on the penetration evolution processon the longitudinal section

To better understand the effect of modulation amplitude andmodulation frequency on the laser-welding process of mag-nesium alloys, variation of welding depth along weldingdirection was studied. The following parameters were select-ed: an average power of 2 kW, a welding speed of 5 m/min, adefocusing distance of 0 mm. Under three low modulationfrequencies (i.e., 4, 8, and 12 Hz), welding experiments withvarious modulating amplitudes (i.e., 40, 60, 80, and 100 % ofaverage laser power) were conducted, as shown in Fig. 7. Lowlaser power modulation frequencies were selected to facilitate

the observation of the fluctuation of weld penetration onlongitudinal section. In Fig. 7, Li, Lj, and Lk are the distancesthat the heat source moved during one sinusoidal period whenthe welding speed was 5 m/min and the modulation frequen-cies were 4, 8, and 12 Hz. For ease of analysis, it might beassumed that the laser energy coupling efficiency was lowwhen the weld penetration was less than h (as observed inFig. 7e, j,, h) and high when the test plate was fully penetrated.Then, when the modulation parameters changed, the changesof the weld section length LL in one sinusoidal period with aless-than-h weld depth and low energy coupling efficiencyand the changes of the weld section length LH with fullpenetration and high energy coupling efficiency were ana-lyzed. In Fig. 7, a1, a2, a3, a4, and a5 are LL within onesinusoidal period under different conditions, while b1, b2, b3,b4, and b5 are LH within one sinusoidal period under differentconditions.

As observed in Fig. 7, when the modulation frequency wasin the low range, the modulation frequency had little effect onthe maximum weld depth on longitudinal section; however,the modulation frequency had a significant effect on LL in asinusoidal cycle. Comparing Fig. 7j–l, it can be found thatwhen the other parameters remain unchanged, the value of LLobtained in one sine period decreased (i.e., a3>a4>a5) with an

Fig. 5 Typical weld cross-section profiles of bead on plate tests per-formed on 2.7-mm-thick AZ31 using various amplitudes of sine modu-lation, a without modulation, b amplitude 1 kW, and c amplitude 1.5 kW

(average laser power 2 kW, welding speed 5 m/min, focal position 0 mm,sine modulation frequency 300 Hz, laser incidence angle 10°, transmis-sion fiber dia. 0.4 mm, focal length 150 mm, focal spot dia. 0.4 mm)

Fig. 6 Surface morphology of2.7-mm-thick AZ31 weldedjoints at various frequencies ofsine modulation, a amplitude0.5 kW, b amplitude 1 kW, and camplitude 1.5 kW (average laserpower 2 kW, welding speed 5 m/min, focal position 0 mm, sinemodulation frequency 300 Hz,laser incidence angle 10°,transmission fiber dia. 0.4 mm,focal length 150 mm, focal spotdia. 0.4 mm)


increase in modulation frequency. More importantly, the ratioof LL obtained in one sinusoidal period to the total weld lengthin one sinusoidal cycle decreased with an increase of modu-lation frequency (i.e., a3/Li>a4/Lj>a5/Lk). The results inFig. 7j–l also demonstrate that the value of LH obtained inone sinusoidal period decreased (i.e., b3>b4>b5) with anincrease in the modulation frequency; however, the ratio ofLH obtained in one sine period to the total weld length in onesinusoidal cycle increased with an increase in the modulatedfrequency (i.e., b3/Li<b4/Lj<b5/Lk). This phenomenon mightbe due to the process of keyhole collapsing driven by surfacetension and gravity usually required sometime (Δt). WithinΔt, the keyhole did not disappear. Therefore, the couplingefficiency of laser energy within time interval Δt remainedrelatively high. Higher modulation frequencies would lead tohigher ratio ofΔt to one cycle time, and then higher couplingefficiencies of laser welding process. When the modulationfrequency was sufficiently high, one sinusoidal period wouldbe less than or equal to the time required for keyhole collaps-ing. It was possible that keyhole existed in the entire weldingprocess; thus, the laser energy coupling efficiency significant-ly increased. Fujinaga et al. used an X-ray high-speed

Fig. 7 Effects of modulation parameters on the weld depth changeprocess on longitudinal cross section of 2.7-mm-thick AZ31 joint (aver-age laser power 2 kW, welding speed 5 m/min, focal position 0 mm, laser

incidence angle 10°, transmission fiber dia. 0.4 mm, focal length 150mm,focal spot dia. 0.4 mm)

Table 3 Design matrix and measured results of laser welding of 8-mm-thick AZ31 alloy (in terms of actual parameters)

Amplitude(W)

Frequency(Hz)

Average laserpower (W)

Welddepth (mm)

1# 800 800 2,800 3.98

2# 800 800 1,200 1.60

3# 800 200 2,800 5.13

4# 800 200 1,200 1.90

5# 200 800 2,800 5.09

6# 200 800 1,200 1.82

7# 200 200 2,800 5.47

8# 200 200 1,200 1.40

9# 865 500 2,000 2.81

10# 136 500 2,000 3.57

11# 500 865 2,000 2.70

12# 500 136 2,000 2.72

13# 500 500 2,972 5.32

14# 500 500 1,028 1.41

15# 500 500 2,000 3.16


photography technique to record the keyhole formation andcollapse process during the pulse YAG laser-welding processand observed that the time from the end of the laser pulse tothe close of the aperture was approximately 1 ms [18]. Kaplanet al. also observed that the keyhole collapse process tookapproximately 1 ms [19]. Thus, in Fig. 3, with the increase ofmodulation frequency, the laser energy coupling efficiencycontinuously increased. It is important to point out here thatsince the formation and collapse time of keyhole could beinfluenced by factors such as material properties, level of laserpower, laser beam quality, wavelength of laser beam, and soon, the effect of modulation frequency on modulated laser-welding process should be dependent on those factors too.

As observed in Fig. 7, the modulation amplitude had asignificant effect on weld depth. As the modulation amplitudeincreased, the instantaneous maximum laser power increased,and the instantaneous minimum laser power decreased. Thus,as the modulation amplitude increased, the maximum weldeddepth increased, and the minimum welded penetration de-creased. When the modulation amplitude was 40 %, themaximum penetration depth obtained under various modula-tion frequencies was obviously less than the thickness of theplate. When the modulation amplitude was 60 %, the maxi-mum penetration depth obtained under various modulationfrequencies was similar to the thickness of the plate. Whenthe modulation amplitude was 100 %, fully penetrated weldsection could be observed under various modulation frequen-cies. FromFig. 7e, h,, k, it can be seen that while keeping otherparameters unchanged, increasing the modulation amplitude

caused the increase of LL within one sine period (i.e., a1<a2<a4). Notably, when the modulation amplitude increased, theratio of LH to LL in one sine period decreased (i.e., b2/a2>b4/a4). This result demonstrates that the effect of modulationamplitude was more significant on the LL than on the LH in asinusoidal period (i.e., a4/a2=1.38, b4/b2=1.15).

3.2 Modulated laser welding of 8-mm-thick AZ31 usingΦ200-μm transmission fiber based on quadratic regressionorthogonal design

In order to get a comprehensive understanding of the role ofpower modulation in laser welding of AZ31, bead on platelaser-welding test was carried out based on quadratic regres-sion orthogonal design. Since variation of weld depth under

Fig. 8 Cross-sectional profiles obtained by modulated laser welding of 8-mm-thick AZ31 based on a quadratic regression orthogonal design (weldingspeed=5 m/min, transmission fiber dia. 0.2 mm, focal length 250 mm, focal spot dia. 0.33 mm, focal position 0 mm, laser incidence angle 10°)

Fig. 9 Plot of actual versus predicted results of weld depth


Fig. 10 Response surface plots showing the interaction of amplitude and frequency on weld depth at different average laser power

Fig. 11 Influence of laser powerdensity on the role of modulationfrequency in modulated laserwelding of AZ31


same average laser power can reflect the change of laserenergy coupling efficiency of modulated laser-welding pro-cess, 8-mm-thick AZ31 plate was adopted to ensure thatpartial penetration laser-welding process could be achievedunder different welding conditions. All the welding conditionsconsidered and the corresponding results of weld depth arelisted in Table 3. Figure 8 shows cross sections of the jointsobtained under different welding conditions. A longitudinalsection of the weld obtained under high amplitude and lowfrequency is also given in Fig. 8 to show the variation of welddepth along welding direction.

According to the experimental results in Table 3, a three-factor quadratic regression equation about the relationshipbetween weld depth and power modulation parameters wasdeduced. The mathematical model for weld depth in terms ofcoded factors, which can be used for prediction within samedesign space, is given as follows:

weld depth ¼ 3:2047−0:1909x1−0:1314x2 þ 1:617x3

−0:1864x1x2−0:2171x1x3−0:2039x2x3þ 0:1643 x21−0:8

� �−0:1613 x22−0:8

� �

þ 0:279 x23−0:8� �

ð1Þ

where x1, x2, and x3 are the amplitude, frequency, andaverage laser power, respectively.

Figure 9 shows the relationship between the actual andpredicted values of weld depth, which indicates that the de-veloped model is adequate and predicted results are in goodagreement with measured data.

Figure 10 shows the interaction of amplitude and frequencyon weld depth at different average laser power. It can be seenfrom Fig. 10 that the influence of modulation parameters onweld depth was highly dependent on the level of average laserpower. In other words, influence of power modulation on theenergy transfer efficiency of laser-welding process of AZ31highly depends on the level of average laser power.

As shown in Fig. 10, the maximum values of weld depthincreased with average power. According to Fig. 10a, whenthe average laser power was low, laser energy coupling effi-ciency could be significantly improved by combining lowamplitude with high frequency or high amplitude with relativelow frequency. When the average laser power increased, thepositive effects of the latter combination (i.e., combination ofhigh amplitude and low frequency) on laser energy couplingefficiency continuously weakened (as shown in Fig. 10b, c)and eventually disappeared (as shown in Fig. 10d). Withregard to the former favorable combination (i.e., a low ampli-tude combined with high frequency), the optimum frequencycorresponding to maximum weld depth decreased with theincreasing of average laser power, as shown in Fig. 10a–e,which implies that weld depth obtained with a constant laserpower was more and more closing to the optimum value ofweld depth when the average laser power increased continu-ously. In other words, if the average laser power was highenough, it was impossible to improve the laser energy couplingefficiency of laser-welding process by power modulation.

In welding process with low average power, a considerableportion of laser energy was lost into surroundings; so, anotable improvement of the power coupling efficiency by

Fig. 12 Impact of the diameter of transmission fiber on the distribution of power density on focal plane (laser power 1,300 W, focal length 250 mm,length of transmission fiber 20 m)

Fig. 13 Effects of power modulation on weld cross section in laserwelding process of 2.7-mm-thick AZ31 using Φ200-μm transmissionfiber (laser power 2 kW, focal length 150 mm, welding speed 5 m/min,incident angle 10°), a without power modulation, b modulated weldingunder 300-Hz frequency and 1,000 W amplitude


using laser power modulation was possible. In welding pro-cess with high average laser power, a deeper keyhole wasformed due to high power density and the times of laser beamreflection within keyhole increased, which led to the fact thatmost of the beam energy could be absorbed by keyhole wall.Therefore, it was impossible to improve power coupling effi-ciency by power modulation furthermore when average laserpower was high enough.

3.3 Effect of power density on modulated fiber laser weldingof AZ31 alloy

Since the formation and evolution of keyhole mainly dependon laser power density other than laser power, it might bebetter to say that influence of modulation parameters on weld

depth was highly dependent on laser power density other thanon the level of average laser power. As shown in Fig. 11a, b,under the same average laser power (i.e., 2 kW) and samemodulation amplitude (i.e., 1 kW), the laser energy couplingefficiency of welding process decreased with modulation fre-quency when the diameter of transmission fiber was 200 μm(as indicated by the red curve in Fig. 11a) and increased withmodulation frequency when the diameter of transmission fiberwas 400 μm (as shown in Fig. 11b). Figure 12a shows thedistribution of laser power density on focal plan when thediameter of transmission fiber was 0.2 mm, while Fig. 12bshows the distribution of laser power density on focal planwhen the diameter of transmission fiber was 0.4 mm. It can beseen from Fig. 12 that impact of the diameter of transmissionfiber on power density was remarkable.

Fig. 14 Surface morphology of specimen after laser butt welding of 2.7-mm-thick AZ31 (laser power=2.3 kW, welding speed=5 m/min, focal length=150 mm, fiber diameter=0.2 mm, focal position=0 mm, laser incidence angle 10°)


Figure 13 compares the cross-sectional profile of weldbetween constant power laser welding and modulated laserwelding under the condition of high beam quality. Shown inFig. 13a, b are the results of laser welding with constant powerand the result of modulated laser welding, respectively. It canbe seen from Fig. 13 that power modulation had no significantinfluence on weld depth when high average laser power andhigh beam quality were employed in the modulated laser-welding process of AZ31.

Compared to CO2 laser and YAG laser, fiber laser hashigher beam quality, making FLW process more sensitive togap size variation. At the end of this work, laser butt weldingof 2.7-mm-thick AZ31 plate was carried out, and the tensilestrengths of both base metal and the obtained joint were testedand compared. In the welding process, laser power was2.3 kW, welding speed was 5 m/min, diameter of transmissionfiber was 0.2 mm, focal length was 150 mm, and focal spotdiameter was 0.2 mm. Since power modulation had no signif-icant influence on weld depth when high power density wasemployed, only laser welding with constant power was con-ducted. Figure 14 shows surface morphology of the specimenafter laser butt welding. Figure 15 shows the results of tensiletest. It can be seen from Fig. 15 that the laser butt-welded jointhad a tensile strength similar to base metal and a plasticityproperty lower than that of base metal.

4 Conclusions

1. A mathematic model describing the relationship betweenweld depth and power modulation parameters was de-duced, which helps the comprehensive understanding ofthe role of power modulation in laser-welding process of

AZ31 alloy. The estimated results are in good agreementwith the measured data, which indicate that the developedmodel can predict the responses adequately within thelimits of welding parameters being used.

2. It was found that the influence of power modulation onlaser welding of AZ31 alloy was highly dependent onlaser power density. In the low power density range, laserenergy coupling efficiency of laser-welding process canbe significantly improved by using proper laser powermodulation. When the laser power density increased, thepositive effect of power modulation on laser energy cou-pling continuously weakened and eventually diminished.

3. In the low power density range, laser energy couplingefficiency can be significantly improved by combininglow amplitude with high frequency or high amplitudewith low frequency. With the increasing of laser powerdensity, the optimum modulation frequency correspond-ing to maximum weld depth decreased continuously.

4. When the diameter of transmission fiber and focal spotwas 400 μm and 0.4 mm, respectively, the average laserpowers for penetrating a 2.7-mm-thick AZ31 plate at awelding speed of 5 m/min can be reduced by about 33 %by using modulated FLW. When the diameter of trans-mission fiber and focal spot was 200 μm and 0.2 mm,respectively, power modulation had hardly significantinfluence on weld depth in laser-welding process ofAZ31 under an average laser power of 2 kW.

5. Laser butt welding was carried out for 2.7-mm-thickAZ31 plate. Tensile test result shows that fiber laserwelded 2.7-mm-thick AZ31 joint has a tensile strengthsimilar to base metal.

Acknowledgments This work was supported by the National NaturalScience Foundation of China (Grant No. 51275391), Natural ScienceFoundation of Shaanxi Province (Grant No. 2011JM6008), and Funda-mental Research Funds for the Central University of China.

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Modulated fiber laser welding of high reflective AZ31