the influence of laser pulse waveform on laser–tig hybrid welding of az31b magnesium alloy
TRANSCRIPT
Optics and Lasers in Engineering 49 (2011) 82–88
Contents lists available at ScienceDirect
Optics and Lasers in Engineering
0143-81
doi:10.1
n Corr
E-m
journal homepage: www.elsevier.com/locate/optlaseng
The influence of laser pulse waveform on laser–TIG hybrid weldingof AZ31B magnesium alloy
Gang Song n, Zhimin Luo
Key Laboratory of Liaoning Advanced Welding and Joining Technology, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
Article history:
Received 5 May 2010
Received in revised form
22 August 2010
Accepted 23 August 2010Available online 15 September 2010
Keywords:
HPDP
Laser
Hybrid
Welding penetration
Magnesium alloy
66/$ - see front matter & 2010 Elsevier Ltd. A
016/j.optlaseng.2010.08.011
esponding author. Tel./fax: +86 411 8470781
ail address: [email protected] (G. Song).
a b s t r a c t
By dividing laser pulse duration into two parts, three kinds of laser waveforms are designed, including a
high power density pulse (HPDP) laser in a short duration set at the beginning of the laser waveform.
This paper aims to find out the laser pulse waveform and idiographic critical values of HPDP, which can
affect the magnesium penetration in laser–tungsten inert gas (TIG) hybrid welding. Results show that
when the laser pulse duration of HPDP is not more than 0.4 ms, the welding penetration values of lasers
with HPDP are larger than otherwise. Also, the welding penetration values of laser with HPDP have
increased by up to 26.1%. It has been found that with HPDP, the laser can form the keyhole more easily
because the interaction between laser and the plate is changed, when the TIG arc preheats the plate.
Besides, the laser with high power density and short duration strikes on the plates so heavily that
the corresponding background power can penetrate into the bottom of the keyhole and maintain the
keyhole open, which facilitates the final welding penetration.
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Since the laser–arc hybrid welding technology was conductedfirst by Steen and Eboo [1] in 1979, it has received significantattention in recent years due to its unique advantages, such as itslarge welding penetration, high welding speed, less deformationand good bridging ability for relatively large gaps, etc. [2,3].The influence of welding parameters, the lowest theoretical laserpower input, arc discharge, arc stability, molten efficiency, etc.have been investigated in the laser–arc hybrid welding process[4–7]. There are some reports authenticating that the laserwaveform has a significant impact on welding defects in pulselaser welding. Pleterski et al. [8] believed that a special shape oflaser pulse with a relatively long duration (over 40 ms) can avoidthe occurrence of crack in laser repair welding of cold work toolsteels. Gower et al. [9] proved that the number of weld crackwith the ramped down pulse laser shape is the least, comparedwith the ramped up and rectangular pulse, regardless of theshape of the pulse, time or power in laser spot welding. However,investigations on the influence of laser pulse waveform onpenetration have not been reported in laser–arc hybrid welding.In our previous work [10], we have modulated the distribution oflaser energy density as a constant in laser pulse energy by addingHPDP at the beginning of laser waveform, and the weldingpenetration increased up to 30%.
ll rights reserved.
7.
In this paper, low power laser–TIG arc hybrid welding processis used to weld a magnesium alloy, assuming all the laser datahave the same pulse power and the same frequency. Setting anHPDP at the beginning of laser to modulate the distribution ofenergy density of a laser pulse, the effects of pulse laser waveformand accurate idiographic critical values of HPDP on weldingpenetration in the hybrid welding process are studied.
2. Experimental setup
2.1. Experimental procedure
Bead welds are carried out on AZ31B magnesium alloy platesof size 150 mm�120 mm�5 mm using a 500 W pulse Nd:YAGlaser with an AC-TIG, as shown in Fig. 1. The chemical composi-tions of the materials used in the experiment are listed in Table 1.Before welding, the magnesium alloys were brushed by stainlesssteel wire in order to eliminate oxides and cleaned with acetoneto remove residues. In the welding process, the welding sampleswere clamped by two pieces of armor plates shelving the magne-sium alloys. The arc length is constant in the welding process. Thetrifling bend of the magnesium plate is caused by the thermaldistortion after welding. Every weld is far enough apart fromother welds to ensure they do not affect each other.
The pulse laser with a wavelength of 1.064 mm is focusedthrough a lens of focal length 120 mm and the focus diameter on thesurface of target is 0.3 mm. The average power of the laser beam is
Fig. 1. Principle of the laser–TIG hybrid welding process.
Table 1Chemical composition of AZ31B magnesium alloy (wt%).
Alloy Al Zn Mn Si Cu Ni Fe Others
AZ31B 2.5–3.5 0.7–1.3 0.2–1.0 o0.05 o0.01 o0.01 o0.02 o0.01
Table 2Welding parameters of the welding process of magnesium alloy.
Parameters (unit) Value
Laser frequency f (Hz) 32
Laser power P (W) 390–394
Electrode height H (mm) 1.0
Arc current I (A) 120
Diameter of laser spot D (mm) 0.5
Defocused distance Z (mm) �1
Welding speed V (mm min�1) 800
Argon flow rate Q (L min�1) 15
Angle of the tungsten electrode axis to target a (deg) 45
Distance between the beam axis and the electrode Dla (mm) 1
Fig. 2. Energy distribution during the laser pulse wave: (a) without HPDP (single
pulse duration), (b) without HPDP (two pulse durations) and (c) with HPDP.
Table 3Values of X, Y and other parameters.
X (ms) 0.1 0.2 0.3 0.4 0.5 0.6
Y (A) 159 158 157 154 150 146
Power (W) 392 392 390 393 394 390
Frequency (Hz) 32 32 32 32 32 32
Defocused distance (mm) �1 �1 �1 �1 �1 �1
G. Song, Z. Luo / Optics and Lasers in Engineering 49 (2011) 82–88 83
adjusted in the range 50–450 W by adjusting the pulse current,pulse frequency (1–50 Hz) and pulse duration (0.1–4.0 ms). Argonwith a purity of 99.99% is used as the shielding gas. Samples areplaced perpendicular to the laser beam. The TIG torch is placed alongthe axis with a laser beam. Both are aligned in the welding direction,and the former is TIG torch. The experiments are performed at roomtemperature and atmospheric pressure. The main welding para-meters are shown in Table 2.
2.2. Laser pulse waveform design
In this experiment, three types of laser waveforms shown inFig. 2 are designed. All of the laser curves shown in Fig. 2 have thesame laser power of 390–394 W and the same frequency of 32 Hz.All of the laser power is measured by a power meter, and thecorresponding power density is calculated.
As shown in Fig. 2a and b, the laser current is 160 A in the wholepulse duration of 3.0 ms, which is included in single pulse durationand two pulse durations. In order to achieve the effect of HPDP laseron welding penetration, the laser waveform shown in Fig. 2c isdesigned except for the laser waveform shown in Fig. 2a and b. InFig. 2c, the pulse duration of HPDP laser is X (ms); Y is the corre-sponding background current value, and in this case all the laserdata have the same single pulse energy, although the value of X isvariable. The values of X, Y, pulse laser power, laser frequency andlaser defocused distance are listed in Table 3.
3. Results
3.1. Welding mode
Fig. 3 shows the characteristic penetration geometry ofmagnesium alloy in hybrid welding. W is bead width, measuredbetween the two weld interfaces intersecting with the workpiecesurface. M is arc penetration depth measured from the worksurface to the extent of the weld cast structure. L is laser beampenetration measured from the end of M to the end of penetra-tion. WL is penetration width of laser beam. L/WL is defined to bethe depth-to-width ratio of the laser beam.
In the hybrid welding process, the penetration induced by laserbeam is remarkably improved. It indicates that the weldingcapability of laser beam is intensified with the assistance of TIGarc. A keyhole weld is characteristic of a small width-to-depthratio (less than 1:1) [11] and a narrow and deeply penetratedvapor cavity with an aspect ratio higher than 1.2 [12]; as shown inFig. 3, the ratio L/WL is more than 1.4. So in this experiment, thelaser welding mode is keyhole mode.
3.2. Effect of different waveforms on welding penetration
In this paper, the values of hybrid welding penetrations,including the TIG and the laser penetrations, are marked as
G. Song, Z. Luo / Optics and Lasers in Engineering 49 (2011) 82–8884
summation of M and L as shown in Fig. 3. The welding length is150 mm. The welding beams are cut every 30 mm along thewelding direction, and the cross sections are smoothed withsandpapers and etched with 5% hydrochloric acid and 95%ethanol.
W
M
WL
L
1 mm
Fig. 3. Characteristic penetration geometry of hybrid welding of magnesium
alloys.
Fig. 4. Hybrid welding penetration comparison of laser with and without HPDP
(single pulse duration).
2mm
tHPDP=0ms
2mm
tHPDP=0.1ms
2mm
tHPDP=0.2ms
2
tHPDP=0.
Fig. 5. Cross sections of the penet
3.2.1. Laser without HPDP in the case of single pulse duration
YAG laser–TIG hybrid welding on magnesium alloys is carriedout with two kinds of laser waveforms shown in Fig. 2a and c. Thewelding surface of the welding penetrations, shown in Fig. 4, isfine. The corresponding cross sections are shown in Fig. 5.
In Fig. 5, the cross sections are chosen separately from the fourcross sections along the welding direction. Besides, the penetrationvalues of the figures are closest to the average shown in Fig. 4.In Fig. 4, the figures are arranged from left to right corresponding tothe values of pulse duration of HPDP from large to small.
In Fig. 4, when the laser pulse duration of HPDP is not morethan 0.4 ms, the welding penetration values with HPDP are largerthan otherwise. Furthermore, the range of the increased penetra-tion values is 10.9–26.1%. With the increase in duration values ofHPDP in the case where the pulse duration is not more than0.4 ms, the welding penetration gap between the laser with andwithout HPDP is smaller. Besides, it is obvious that the minimalvalues of welding penetrations of parameters without HPDPapproach the maximal values of welding penetrations of para-meters with HPDP of 0.5 and 0.6 ms. Meanwhile, the weldingpenetration of laser parameter with HPDP of 0.4 ms is close to thepenetration of laser parameter without HPDP.
3.2.2. Laser without HPDP in the case of two pulse durations
In the experiment, it was found that there was a gap of weldingpenetrations between single pulse duration and two pulse durationswith the same pulse duration and the same frequency. To obtain amore obvious effect of HPDP on welding penetration, besides thewaveforms shown in Fig. 2a and c, another waveform is designed asshown in Fig. 2b. The penetration values of the YAG laser–TIG hybridwelding with the waveform in Fig. 2b and c are shown in Fig. 6.
According to the curves in Fig. 6, it can be seen that all of thewelding penetrations of the parameters with HPDP and pulseduration values of 0.1, 0.2 and 0.3 ms are deeper than thosewithout HPDP. Moreover, with the increase in pulse duration ofHPDP, the difference decreases from 14.5% to 9.8%. When thepulse duration values of HPDP are 0.4, 0.5 or 0.6 ms, the weldingpenetrations of parameters with HPDP are shallower than thosewithout HPDP. The gap between the curves increases from 5.1% to16.1% with the increase in pulse duration of HPDP.
In Fig. 7, the cross sections are chosen separately from the fourcross sections along the welding direction. Furthermore, thepenetration values of the figures shown in Fig. 6 are proximal tothe average. All of the figures shown in Fig. 7 are displayedcorresponding to the penetration values in Fig. 6. Taking Fig. 7(a)for instance, the cross sections marked with ‘‘tHPDP¼0 ms’’and‘‘tHPDP¼0.1 ms’’ are penetrations of waveforms without HPDP andwith HPDP of 0.1 ms as shown in Fig. 6(a).
3.3. Influence of values of laser pulse duration on welding
penetrations
Fig. 8 shows another laser waveform except for the waveformspresented in Fig. 2.
mm
3ms
2mm
tHPDP=0.4ms
2mm
tHPDP=0.5ms
2mm
tHPDP=0.6ms
ration values shown in Fig. 4.
Fig. 6. Penetration comparison of laser with and without HPDP (two pulse durations).
G. Song, Z. Luo / Optics and Lasers in Engineering 49 (2011) 82–88 85
As shown in Fig. 8, the pulse duration of HPDP is 0.2 ms andthe pulse duration is defined as Z. Y is the corresponding back-ground current value for various pulse duration values Z. Thevalues of Y, Z, pulse laser power, laser frequency and laserdefocused distance are shown in Table 4.
Fig. 9 shows the penetration values of the laser–TIG hybridwelding with the waveform presented in Fig. 8.
As shown in Fig. 9, it can be seen that when the pulse durationswith HPDP are 3.0, 3.2 and 3.4 ms, the welding penetration of laserparameters with HPDP is deeper than that without HPDP, and the
No. ba
figu
res
No. dc
figu
res
No. fe
figu
res
2mm 2mm 2mm
2mm 2mm 2mm 2mm
2mm 2mm 2mm
2mm
2mm
2mm
tHPDP = 0ms tHPDP = 1.0ms tHPDP = 0ms tHPDP = 2.0ms
tHPDP = 0ms tHPDP = 3.0ms tHPDP = 0ms tHPDP = 4.0ms
tHPDP = 0ms tHPDP = 5.0ms tHPDP = 0ms tHPDP = 6.0ms
Fig. 7. Cross sections of the penetration values shown in Fig. 6.
Fig. 8. Laser waveform with various pulse durations.
Table 4Values of Z, Y and other parameters.
Z (ms) 3.0 3.2 3.4 3.6 3.8 4.0
Y (A) 158 153 148 142 138 134
Power (W) 392 393 392 391 393 394
Frequency (Hz) 32 32 32 32 32 32
Defocused distance (mm) �1 �1 �1 �1 �1 �1
Fig. 9. Influence of laser pulse values duration on welding penetrations.
G. Song, Z. Luo / Optics and Lasers in Engineering 49 (2011) 82–8886
increased values are up to about 15.2%. The trend of the upper threecurves is complicated. In other words, in Fig. 9, it is impossible to tellthe maximal value or the minimal value of welding penetrationsfrom the curves with the pulse durations of 3.0, 3.2 and 3.4 ms.However, the average order of welding penetration value ispenetration (3.4 ms)4penetration(3.0 ms)4penetration(3.2 ms).
When the pulse durations with HPDP are 3.6, 3.8 and 4.0 ms, thewelding penetrations of laser parameters with HPDP are almostshallower than those without HPDP. The penetration value order is
2mm
without HPDP
2mm
t=3.0ms
2mm
t=3.2ms
2mm
t=3.4ms
2mm
t=3.6ms
2mm
t=3.8ms
2mm
t=4.0ms
Fig. 10. Cross sections of the penetration values shown in Fig. 9.
G. Song, Z. Luo / Optics and Lasers in Engineering 49 (2011) 82–88 87
penetration (4.0 ms)4penetration(3.8 ms)4penetration(3.6 ms) inthe curves; however, the average welding penetration value order ispenetration(4.0 ms)4penetration(3.6 ms)4penetration(3.8 ms) invirtue of mutation of the fourth value of curve of pulse duration of4.0 ms. Besides, the curves with HPDP are more fluctuant than thecurves without HPDP, because the HPDP strikes the plates, while themagnesium alloys vaporize more intensely and the background laserpenetrates the keyhole through the magnesium plasma atmosphere.The laser energy density with HPDP is so fluctuant that the weldingpenetrations of the laser with HPDP are more fluctuant than thelaser without HPDP.
In Fig. 10, the cross sections are chosen separately from thefour cross sections along the welding direction. Moreover,the penetration values of the figures in Fig. 9 are closest to theaverage. In Fig. 10, the specimens are displayed from left to rightcorresponding to the values of the laser pulse duration in Fig. 9from large to small. In Fig. 10, the figure marked with ‘‘withoutHPDP’’ is the penetration figure of the laser without HPDPcorresponding to the curve shown in Fig. 9. Other figures inFig. 10 are the figures of the welding cross section of laser withoutHPDP of 0.2 ms and the laser pulse durations are the correspond-ing values shown in each figure.
4. Discussion
From above, it can be seen that welding penetration is changedby setting an HPDP at the beginning of laser to modulate thedistribution of energy density during laser pulse in laser–TIGhybrid welding on magnesium alloy.
In this experiment, as shown in Fig. 4, when the pulse laserdurations of HPDP are not more than 0.4 ms, the weldingpenetrations are deeper than the laser parameters without HPDPin laser–TIG hybrid welding. The reasons of the results arerevealed as follows.
First and foremost, in laser–TIG hybrid welding, the platemelts because of the interaction between the TIG arc and themagnesium alloy without laser, and on the other hand the arcplasma is attracted to the keyhole [13]. The attraction acceleratesthe melt of magnesium plates. Therefore, the keyhole can beformed more easily. In laser welding, the effect of laser–materialinteraction experiences five steps: heating, surface melting,surface vaporization, plasma formation and ablation. Dependingon the magnitude of the rise in temperature, physical effects varyin the material, including heating, melting and vaporization of thematerial [14]. However, in laser–TIG hybrid welding, the platemelts due to the preheating of TIG arc so that when laser interactswith the plates, the plates begin to vaporize without experiencingheating and surface melting. In the welding process, massivemagnesium elements vaporize and thus a larger quantity of metalplasma can be formed, compared with the laser without HPDP.
Second, it is well known that not only is the formation ofthe keyhole very sensitive to power density, but also highpulse power density of the workpiece is crucial to achieve keyholewelding and to control the formation of welds [15]. When the
laser interacts with the surface of the magnesium alloy, the highpower density and short duration strike intensely on the platesand the magnesium alloy melts and vaporizes rapidly due to thelow melting and boiling points (650 and 1090 1C) and the lowionization energy (7.6 eV) of magnesium atom [15], therebyforming the welding keyhole. Remarkably, the laser strikes theplate in a short time even if the laser current of the power densitydoes not increase sharply, as the laser waveform shown in Fig. 2band c, the welding penetration of waveform c is deeper than thatwith waveform b basically.
Last but not the least, due to the plasma atmosphere at thetop of the weld pool, the evolving vapor from the surface exertsthe recoil pressure on the surface. It is well known that the higherpower density laser with HPDP gives rise to more vapor mole-cules. The laser with HPDP causes more recoil pressure to theweld pool filled with molten metal, so that a deeper keyhole canbe formed, compared with laser without HPDP.
It is noteworthy that the oscillation of penetrations overwelding distance in Figs. 4, 6 and 9 is normal. In this paper, pulselaser with determinate duty cycle interacts with the plates one byone. When the prior pulse laser interacts with the plate, magne-sium alloy begins to melt and solidify. The subsequent pulse laserstrikes the liquid metal, which is not completely solidified. Theprocess of consecutively melting and solidifying gives rise to theoscillation of penetrations.
After the HPDP, the last segment of the laser pulse with aconstant background current of 159–154 A penetrates into thekeyhole and goes deep into the bottom of the keyhole by multiplereflections from the keyhole wall, resulting in the increase ofkeyhole depth. With the decrease in laser background currentsfrom 154 to 146 A, the corresponding power densities of thebackground currents decrease as well. In this experiment, whenthe background current decreases to 154 A with the pulse laserpower density of 1.61�105 W/cm2, the laser begins to stay at thebottom of the keyhole and cannot make more magnesium alloysmelt and vaporize; the keyhole can even be closed, which causeslower welding penetration.
5. Conclusions
The influence of HPDP to penetration in laser–TIG hybridwelding on AZ31B magnesium alloys is studied in this paper, andthe following conclusions can be drawn.
During Nd:YAG laser–TIG hybrid welding, the laser pulseduration of 3.0 ms and the frequency of 32 Hz are maintained.When the pulse duration of HPDP is not more than 0.4 ms withthe background current of 154 A, the corresponding pulse powerdensity of the background current is 1.61�105 W/cm2. It can beobserved that the welding penetration values are greater thanthose for the laser without HPDP. Furthermore, the increasedvalue is up to 26.1%.
The welding penetrations with pulse durations of 3.0, 3.2 and3.4 ms are deeper than those without HPDP. With increase inpulse duration of laser, the background current decreases, so that
G. Song, Z. Luo / Optics and Lasers in Engineering 49 (2011) 82–8888
the corresponding power density decreases, which causes lowerwelding penetration. In laser–TIG hybrid welding, because of pre-heating the magnesium alloy by TIG arc not only is the interactionbetween laser and the plate changed, but also the laser attractsTIG arc. The high power density strikes the plates heavily in ashort time, so that the HPDP forms keyhole easily. Moreover, theenergy provided by HPDP keeps the keyhole open and stable, andthe corresponding background laser penetrates the bottom of thekeyhole, which causes deeper welding penetration.
Acknowledgements
The authors gratefully acknowledge the sponsorship fromImportant National Science and Technology Specific Projects ofChina (no. 2009ZX04007-032), the Fundamental Research Funds forthe Central Universities (DUT10ZD108) and Research Fund for theDoctoral Program of Higher Education of China (no. 20070141031).
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