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Effects of operating parameters on surface quality for the pulsed laser welding of zinc-coated steel
Journal of Materials Processing Technology 100 (2000) 163±170
Effects of operating parameters on surface quality for the pulsed laser welding of zinc-coated steel Yih-Fong Tzeng* Department of Mechanical Engineering, Chang Gung University, 259 Wen-hua 1st Road, Kwei-shan, Tao-yuan 333, Taiwan Received 31 August 1998
Abstract This paper presents an analysis of the in¯uences of dominant processing parameters of pulsed laser seam welding on weld surface quality. The parameters are the average peak power density (APPD), the average output power, the welding speed, the pulse energy and the pulse duration. Only one temporal pulse type, a `rectangular' power pulse without a characteristic leading-edge power spike, is used in this study. Laser seam welds are produced in 0.7 mm thick electro-galvanised steel sheets. A surface study of acceptable welds reveals that various operating parameters are in¯uential to different degrees on the surface ®nish and the presence of slumps in the welds. It is found that proper control of the above-mentioned processing parameters can produce the desired top surface quality of lap welds in the laser seam welding process. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Pulsed Nd:YAG laser welding; Laser metrology; Lap-joint con®guration; Electro-galvanised steel
1. Introduction The pulsed laser welding process essentially has a complicated dynamic mechanism which involves close interactions between the pulsed laser beam, the shielding gas, the plasma, the keyhole, and the melt pool. Any pulsed laser welded sample inevitably has a series of characteristic surface solidi®cation ripples that de®nes the weld surface quality. As such, any variations in the process parameters will in¯uence the solidi®cation process of the welds and may signi®cantly affect the weld surface pro®le. Violent oscillations of the keyhole within the molten pool [1] and an appreciable amount of metal vapourisation are the two dominant features in the pulsed laser seam welding process. They are the likely results of periodic heating of the weld pool by a high peak power density pulsed laser beam incident normal to the workpiece. The molten pool oscillates during processing mainly due to the surface tension gradient and vapour pressure forces, and hence solidi®es to various weld surface pro®les. As the laser is operated in a pulsed mode, the oscillating molten metal will slump to the centre
* Tel.: 886-03-3283016; fax: 886-03-3283031. E-mail address: [email protected] (Y.-F. Tzeng).
of the keyhole and hardens on completion of the pulse, sometimes leaving an undercut or humping at higher travel speeds [2]. Alternatively, a strong molten pool ¯ows towards the centre of the weld as the keyhole formation is too slow to evenly redistribute the pool in time and is hence frozen as an undercut or humping in the middle of the weld [3]. The weld surface topography on zinc-coated steel under pulsed laser welding may indicate a number of weld defects, namely undercut, slumping, humping and blow-holes. These features re¯ect the weld quality and can be translated as the weld surface roughness, which is represented quantitatively by a roughness value, this value being related closely to the process variables. Additionally, the depth of weld surface defects is taken as another main assessing parameter of weld quality. This measurement can be used to qualify the weld quality for speci®c service requirements. A number of previous studies [4±10] have investigated the underlying mechanism of weld surface topography during weld solidi®cation. At present, no comprehensive theory has been developed to fully account for the ¯uid mechanics behind the weld solidi®cation ripple. In fact, the de®nition of the full mechanism with regard to absorption conditions, vapour pressure, material properties and contact angle is extremely dif®cult [11]. The present work attempts not only to qualitatively but also to quantitatively analyse the weld
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 4 7 0 - 7
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It is observed from Eq. (1) that for a given laser power PM, there are various combinations of EP and PRR and that their relationship is inversely proportional. This observation applies to the other relevant mathematical expressions and it indicates the ¯exibility and complexity in the selection of the pulsed laser parameters. The question thus arises as how to select a satisfactory combination of the parameters to enable ef®cient and effective pulsed laser welding application. Fig. 1. Schematic diagram of the input pulses in a self-designed shape with illustrations of the de®ned average peak power, the mean power, the pulse duration, and the pulse-to-pulse time.
surface pro®le. In this way, the effects of the main process parameters on weld quality can be better understood. 2. Terminology for pulsed Nd:YAG laser material processing A schematic diagram of the laser power output for a series of constant energy pulses of self-designed shape is shown in Fig. 1. A set of pulsed laser parameters is de®ned below: PP average peak power kW
pulse energy J pulse duration ms
PD average peak power density APPD kW=mm2 average peak power kW spot area mm2 PM mean laser power kW pulse energy J pulse repetition rate ms where TP is the pulse duration (ms), EP the pulse energy (J), PRR the pulse repetition rate (sÿ1), TF the pulse-to-pulse time (ms), and CDTP/TF is the duty cycle. 3. Mathematical equations for pulsed Nd:YAG laser welding The mathematical relationships used for pulsed Nd:YAG laser welding can be expressed by various mathematical equations: PM EP PRR
EP TP TP TF
(1)
PM PP CD
(2)
PM PD D CD
(3)
PM PD C D DV V
(4)
where D is the laser spot area and V is the travel speed.
4. Materials and experimental methods A Lumonics JK701 Nd:YAG pulsed laser welding machine with a maximum mean laser power of 400 W was used in the experiments. The material used is electro-galvanised sheet steel the speci®cation of which is 0.7 mm thickness with 7.5 mm pure zinc coating on both sides. The chemical compositions in weight percentage of the steel substrate are given in Table 1. The laser welding experiments were conducted using 60 mm10 mm zinc-coated steel sheet specimens in lapjoint con®guration. Before laser welding, the surface of all of the specimens are cleaned using acetone solution to remove dirt and oil. The clamping arrangement is very crucial to ensure that there is no gap between the clamped sheets and that the weld distortion does not create a gap whilst welding. The laser processing parameters set-up are as given in Table 2 and Fig. 2. In the parameters set-up for the experiments, it is noted that there are four arbitrarily de®ned levels of APPD values. These are high, upper-medium, lower-medium, and low level, their values being 2.98109, 3.73109, 5.47109, and 7.46109 W/m2, respectively. To investigate the effects of the main laser welding parameters on the welds in terms of roughness and slumping, the targeted samples are restricted to visually acceptable welds. The experimental results show that APPD between lower- and upper-medium level (3.73109APPD5.47 109 W/m2) produces good welds within the corresponding operating region of travel speed and mean power. Unacceptable welds produced with travel speeds outside the operating region, and with high and low levels of APPD are not embodied in the study as their values are beyond the measuring range of the scanning instrument. The roughness is scanned along the centreline of the top weld surface of acceptable welds to be analysed by the Proscan 1000 laser metrology instrument, a powerful PCcontrolled tool for shape analysis, object digitising and Table 1 Chemical compositions of the steel substrate Steel substrate
Chemical composition (wt.%)
Electro-galvanised steel
C: 0.01, S: 0.008, P: 0.013, Mn: 0.160, Al: 0.036, N2: 0.0027, S: 0.006, Ti: 0.054, Nb<0.01
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Table 2 Parameters set-up for the experiments Pulse shape Laser power: PM Speed Pulse duration: TP Pulse energy: EP
Rectangular without a leading-edge power spike at the front of the pulse (see Fig. 2) 396, 363, 330, 297 0.4±15 mm/s 4, 8, 12 ms 5.9, 7.5, 11, 15 J for 4 ms 11.8, 15, 22, 30 J for 8 ms 17.7, 22.5, 33, 45 J for 12 ms Focused on the top surface of the material 120 mm focusing lens from Lumonics (borosilicate crown glass) Kept constant as 0.8 mm Argon gas blowing at a constant rate of 20 l/min at 458 onto the leading edge of the focused position
Focal location Lens Spot size (diameter) Shielding gas
accurate surface measurement. Weld slumping is de®ned as the ratio of defect depth to melt depth from measurements made with a micrometer under an optical microscope. Note that the underside weld surface defect, root concavity, is ignored in the measurements because it mostly does not exist within the operating range. 5. Experimental results and discussion 5.1. Effects of acceptable welding speeds on the weld bead roughness It is observed that most of the acceptable lap welds have a good surface ®nish without the commonly seen undercutting and humping. Fig. 3(a) is a typical plot of the surface roughness for the top weld versus acceptable welding speeds. The relationship is of a polynomial form with one minimum value. The weld bead roughness at travel speeds beyond the higher bound of the acceptable conditions increases rapidly due to the presence of blow-holes or cavities that are out of the measuring range. Travel speeds below the lower-bound produce deep slumping that is usually beyond the measuring range.
5.2. Effects of laser power on weld bead roughness Examination of Fig. 3(a)±(f) reveals that as the mean laser power is increased, the sensitivity of the weld bead roughness to the acceptable welding speeds becomes less pronounced. At the same time, the distribution of the roughness values measured within the operating region of the welding speed is generally lowered. Conversely, as the mean laser power is decreased to a certain limit, the changes in the roughness values increase rapidly. This is due to a sharp shrinkage in the operating region of welding speed, which implies an increase in thermodynamic instability in the weld pool. This becomes particularly apparent for longer pulse durations due to the longer heating times. 5.3. Effects of pulse energy and APPD on weld bead roughness Comparison of the results of Fig. 3(g)±(i) suggests that as the pulse energy is increased (7.5±11 J, 15±22 J, and 22.5± 33 J), whilst keeping a constant pulse duration (4, 8, and 12 ms, respectively), there is an increase in the weld bead roughness distribution. Indeed, as the APPD is increased there would be more pronounced thermodynamic effects
Fig. 2. Examples of the rectangular laser power pulses used in the study.
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Y.-F. Tzeng / Journal of Materials Processing Technology 100 (2000) 163±170
Fig. 3. The dependency of the top weld bead roughness on the acceptable welding speed and the mean power for various parameters set-up as: (a) 4 ms and 7.5 J; (b) 4 ms and 11 J; (c) 8 ms and 15 J; (d) 8 ms and 22 J; (e) 12 ms and 22.5 J; and (f) 12 ms and 33 J. Comparison of the top weld bead roughness with varied APPDs for pulse durations of: (g) 4 ms; (h) 8 ms; and (i) 12 ms.
contributing to form an unfavourable weld bead pro®le. It can be concluded that the weld bead roughness is increased by increasing the APPD from the lower-medium to the upper-medium level. 5.4. Effects of pulse duration on weld bead roughness At lower-medium APPD level (see Fig. 3(g)±(i)), increasing the pulse duration (4±12 ms) results in a gentle rise in the
weld surface roughness distribution. On the other hand, at the upper-medium APPD level, there is an obvious trend of a rapid increase in the weld surface roughness distribution. Indeed, longer pulse duration at a higher APPD level implies the more profound effects that the pulse energy has on the weld bead roughness. The set of parameters (12 ms, 33 J) therefore gives the highest weld bead roughness distribution for the range of laser mean powers from 396 to 330 W. It is noted that there is only one welding speed available for
Y.-F. Tzeng / Journal of Materials Processing Technology 100 (2000) 163±170
Fig. 3 (Continued )
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Y.-F. Tzeng / Journal of Materials Processing Technology 100 (2000) 163±170
Table 3 Polynomial curve ®t equations for M1 Welding parameters
Roughness: Yabxcx2dx3a
4 ms, 7.5 J, 396 W 4 ms, 7.5 J, 330 W 4 ms, 11 J, 396 W 4 ms, 11 J, 330 W 8 ms, 15 J, 396 W 8 ms, 15 J, 330 W 8 ms, 22 J, 396 W 8 ms, 22 J, 330 W 12 ms, 22.5 J, 396 W 12 ms, 22.5 J, 330 W 12 ms, 33 J, 396 W 12 ms, 33 J, 330 W
Yÿ4.3165xÿ52.37x212.57x3 Y230.48ÿ328.42x149.17x2 Yÿ2734.734276.46xÿ2183.85x2367.15x3 Y193.3ÿ197.7x59x2 Yÿ97269.6xÿ181.35x237.857x3 Y53.1ÿ56.84x34.64x2ÿ6.31x3 Y821.1ÿ944.67x365x2ÿ45.83x3 Y554.23ÿ570.5x161.67x2 Y89.61ÿ49.34x2.68x23.12x3 Y1440.8ÿ1761.5x565x2 Yÿ877.51325.5xÿ622.5x2ÿ95.8x3
a b
Travel speed for minimum roughness (mm/s)
b
1.87 1.10 2.20 1.68 2.01 1.24 2.23 1.76 2.04 1.56 2.44 2.00
Energy input per unit weld (EI) (J/mm) 211 300 180 196 197 266 178 188 194 211 162 165
a, b, c>0, YRa (mm), xtravel speed (V). The curve is non-existent due to there being only one welding speed.
330 W. The extremely narrow operating region corresponds to a very high surface roughness distribution for acceptable welds. 5.5. Effects of the laser power on the welding speed for minimum weld bead roughness Using graphical software, the curve ®t as a polynomial equation for Fig. 3(a)±(f) can be found as follows. The travel speed for minimum roughness is determined by @Ra @y 0 @V @x
(5)
In addition, the energy input per unit weld for a constant spot
size (D) is equal to W Z W V V
(6)
assuming that the absorptivity Z1 for simpli®cation and convenience of data interpretation. The data in Table 3 is plotted in Fig. 4(a) and (b) showing the relationship between the welding speed for minimum roughness and its corresponding heat input per unit weld. It is seen clearly that for a given set of welding parameters, the travel speed for minimum roughness decreases as the mean laser output power reduces due to the reduction in the energy input rate, but increases with increasing pulse duration due to the longer heating effect. At the same
Fig. 4. Relationship between the welding speed for minimum surface roughness and its corresponding heat input per unit weld for: (a) lower-medium APPD; and (b) upper-medium APPD.
Y.-F. Tzeng / Journal of Materials Processing Technology 100 (2000) 163±170
169
time, the energy input per unit weld (EI) for minimum roughness rises with decreasing mean laser output power because a higher energy input rate is needed to overcome the increase in thermodynamic instability within the weld formation. It is noted that at upper-medium APPD (11 J for 4 ms, 22 J for 8 ms, and 33 J for 12 ms), the EI per unit weld for minimum roughness in general are lower than that at lowermedium level (7.5 J for 4 ms, 15 J for 8 ms, and 22.5 J for 12 ms). Indeed, the higher energy heating effect has led to higher travel speed. As the pulse duration is increased, however, the EI values are slightly decreased at constant APPD. This is a result of longer heating effect leading to a faster welding speed.
5.7. Effects of pulse energy and APPD on weld slumping
5.6. Effects of an acceptable welding speed on weld slumping
5.8. Effects of the pulse duration on weld slumping
Examination of Fig. 5(a) and (b) suggests that the weld depth decreases with increasing welding speed. As the travel speed is lowered, deeper weld defects are produced due to there being greater thermal input per unit weld. Hence, it is concluded that an inverse relationship between the depth of weld surface defects and the welding speed exists, more signi®cantly so at the lower bound of the acceptable welding speed. This is seen at both lowermedium APPD and upper-medium APPD for 4 ms, which yield around 20 and 18% of defect depth to weld depth ratios, respectively.
Examination of the results in pairs from Fig. 5(a) and (b) reveals that increasing the APPD by increasing the pulse energy (from 7.5 to 11 J for 4 ms, 15 to 22 J for 8 ms, 22.5 to 33 J for 12 ms) whilst keeping the pulse duration constant leads to slightly larger weld defects depth. At the travel speed of 2 mm/s, the increased APPD for the 4 ms pulse increases the slumping from approximately 7.6 to 11%, and for the 8 ms pulse from 7.5 to 10.5%. This could be due to the fact that at higher APPD levels, more Zn gas, metal vapour and the instant ejection of spatters are produced by more ef®cient heating effects from the higher heat energy input rate per power pulse.
Fig. 5(a) and (b) show that as the APPD is constant, there is not a signi®cant effect of pulse duration on the weld defect depth, but they also reveal that a shorter pulse duration leads to a slightly deeper weld defect depth, particularly at low welding speeds. This result obviously con¯icts with the argument that longer pulse duration creates more metal loss due to stronger heat ¯ow effects. However, it is observed that at a given laser mean power, a longer pulse duration does produce larger weld pools, and that more metal vapourisation in reality does not cause deeper weld slumping: instead, a shallower slumping results. Hence, the longest pulse duration of 12 ms yields the shal-
Fig. 5. Comparison of the top weld surface slumping produced at: (a) lower-medium APPD; and (b) upper-medium APPD.
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lowest slumping. The maximum depth is observed for 4 ms pulse duration at the higher pulse energies and lower speeds. 6. Conclusions A qualitative and quantitative analysis of acceptable welds produced in the pulsed laser seam welding of zinccoated steel using rectangular power pulses are described. The results are summarised in the following. 1. The main factors affecting the surface roughness of acceptable welds have been identi®ed as the welding speed, the laser power, the pulse duration, and the average peak power density. 2. The main factors affecting the surface defects of acceptable welds such as slumping are principally the welding speed, followed by the average peak power density and pulse duration. References [1] T. Klein, M. Vicanek, I. Decker, G. Simon, Oscillations of the keyhole in penetration laser beam welding, J. Phys. D 27 (1994) 2023±2030.
[2] C. Dawes, Laser Welding, A Practical Guide, Abington Publishing, Cambridge, UK, 1992. [3] W.M. Steen, Laser Material Processing, Springer, Berlin, 1991. [4] B.J. Bradstreet, Effects of surface tension and metal ¯ow on weld bead formation, Welding Res. (Suppl.) 47(6) (1968) 314s±322s. [5] J.W. Wealleans, B. Adams, Undercutting and weld bead turbulence in Tig welding, Welding and Metal Fabrication 39(6) (1969) 255± 257. [6] D.J. Kotecki, D.L. Cheever, D.G. Howden, Mechanism of ripple formation during weld solidi®cation, Welding Res. (Suppl.) 51(8) (1972) 386s±391s. [7] T.R. Anthony, H.E. Cline, Surface rippling induced by surface tension gradients during laser surface melting and alloying, J. Appl. Phys. 48(9) (1977) 3888±3894. [8] P.G. Moore, L.S. Weinmain, Surface alloying using high-power continuous lasers, Laser Appl. Mater. Process., SPIE 198 (1979) 120±125. [9] K. Ishizaki, A new approach to the mechanism of penetration, Proceedings of the Conference on Arc Physics and Weld Pool Behaviour, The Welding Institute, UK, 1980. [10] I.C. Hawkes, M. Lamb, W.M. Steen, Surface topography and ¯uid ¯ow in laser surface melting, Proceedings of the Conference of Third International Colloqium on Welding and Melting by Electrons and Laser Beams, 5±9 September 1983, pp. 125±131. [11] G. Shannon, Introduction to the laser welding of steel sheet, AMWL Lecture Notes, The University of Liverpool, 1996.
Effects of operating parameters on surface quality for the pulsed laser welding of zinc-coated steel Yih-Fong Tzeng* Department of Mechanical Engineering, Chang Gung University, 259 Wen-hua 1st Road, Kwei-shan, Tao-yuan 333, Taiwan Received 31 August 1998
Abstract This paper presents an analysis of the in¯uences of dominant processing parameters of pulsed laser seam welding on weld surface quality. The parameters are the average peak power density (APPD), the average output power, the welding speed, the pulse energy and the pulse duration. Only one temporal pulse type, a `rectangular' power pulse without a characteristic leading-edge power spike, is used in this study. Laser seam welds are produced in 0.7 mm thick electro-galvanised steel sheets. A surface study of acceptable welds reveals that various operating parameters are in¯uential to different degrees on the surface ®nish and the presence of slumps in the welds. It is found that proper control of the above-mentioned processing parameters can produce the desired top surface quality of lap welds in the laser seam welding process. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Pulsed Nd:YAG laser welding; Laser metrology; Lap-joint con®guration; Electro-galvanised steel
1. Introduction The pulsed laser welding process essentially has a complicated dynamic mechanism which involves close interactions between the pulsed laser beam, the shielding gas, the plasma, the keyhole, and the melt pool. Any pulsed laser welded sample inevitably has a series of characteristic surface solidi®cation ripples that de®nes the weld surface quality. As such, any variations in the process parameters will in¯uence the solidi®cation process of the welds and may signi®cantly affect the weld surface pro®le. Violent oscillations of the keyhole within the molten pool [1] and an appreciable amount of metal vapourisation are the two dominant features in the pulsed laser seam welding process. They are the likely results of periodic heating of the weld pool by a high peak power density pulsed laser beam incident normal to the workpiece. The molten pool oscillates during processing mainly due to the surface tension gradient and vapour pressure forces, and hence solidi®es to various weld surface pro®les. As the laser is operated in a pulsed mode, the oscillating molten metal will slump to the centre
* Tel.: 886-03-3283016; fax: 886-03-3283031. E-mail address: [email protected] (Y.-F. Tzeng).
of the keyhole and hardens on completion of the pulse, sometimes leaving an undercut or humping at higher travel speeds [2]. Alternatively, a strong molten pool ¯ows towards the centre of the weld as the keyhole formation is too slow to evenly redistribute the pool in time and is hence frozen as an undercut or humping in the middle of the weld [3]. The weld surface topography on zinc-coated steel under pulsed laser welding may indicate a number of weld defects, namely undercut, slumping, humping and blow-holes. These features re¯ect the weld quality and can be translated as the weld surface roughness, which is represented quantitatively by a roughness value, this value being related closely to the process variables. Additionally, the depth of weld surface defects is taken as another main assessing parameter of weld quality. This measurement can be used to qualify the weld quality for speci®c service requirements. A number of previous studies [4±10] have investigated the underlying mechanism of weld surface topography during weld solidi®cation. At present, no comprehensive theory has been developed to fully account for the ¯uid mechanics behind the weld solidi®cation ripple. In fact, the de®nition of the full mechanism with regard to absorption conditions, vapour pressure, material properties and contact angle is extremely dif®cult [11]. The present work attempts not only to qualitatively but also to quantitatively analyse the weld
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 4 7 0 - 7
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It is observed from Eq. (1) that for a given laser power PM, there are various combinations of EP and PRR and that their relationship is inversely proportional. This observation applies to the other relevant mathematical expressions and it indicates the ¯exibility and complexity in the selection of the pulsed laser parameters. The question thus arises as how to select a satisfactory combination of the parameters to enable ef®cient and effective pulsed laser welding application. Fig. 1. Schematic diagram of the input pulses in a self-designed shape with illustrations of the de®ned average peak power, the mean power, the pulse duration, and the pulse-to-pulse time.
surface pro®le. In this way, the effects of the main process parameters on weld quality can be better understood. 2. Terminology for pulsed Nd:YAG laser material processing A schematic diagram of the laser power output for a series of constant energy pulses of self-designed shape is shown in Fig. 1. A set of pulsed laser parameters is de®ned below: PP average peak power kW
pulse energy J pulse duration ms
PD average peak power density APPD kW=mm2 average peak power kW spot area mm2 PM mean laser power kW pulse energy J pulse repetition rate ms where TP is the pulse duration (ms), EP the pulse energy (J), PRR the pulse repetition rate (sÿ1), TF the pulse-to-pulse time (ms), and CDTP/TF is the duty cycle. 3. Mathematical equations for pulsed Nd:YAG laser welding The mathematical relationships used for pulsed Nd:YAG laser welding can be expressed by various mathematical equations: PM EP PRR
EP TP TP TF
(1)
PM PP CD
(2)
PM PD D CD
(3)
PM PD C D DV V
(4)
where D is the laser spot area and V is the travel speed.
4. Materials and experimental methods A Lumonics JK701 Nd:YAG pulsed laser welding machine with a maximum mean laser power of 400 W was used in the experiments. The material used is electro-galvanised sheet steel the speci®cation of which is 0.7 mm thickness with 7.5 mm pure zinc coating on both sides. The chemical compositions in weight percentage of the steel substrate are given in Table 1. The laser welding experiments were conducted using 60 mm10 mm zinc-coated steel sheet specimens in lapjoint con®guration. Before laser welding, the surface of all of the specimens are cleaned using acetone solution to remove dirt and oil. The clamping arrangement is very crucial to ensure that there is no gap between the clamped sheets and that the weld distortion does not create a gap whilst welding. The laser processing parameters set-up are as given in Table 2 and Fig. 2. In the parameters set-up for the experiments, it is noted that there are four arbitrarily de®ned levels of APPD values. These are high, upper-medium, lower-medium, and low level, their values being 2.98109, 3.73109, 5.47109, and 7.46109 W/m2, respectively. To investigate the effects of the main laser welding parameters on the welds in terms of roughness and slumping, the targeted samples are restricted to visually acceptable welds. The experimental results show that APPD between lower- and upper-medium level (3.73109APPD5.47 109 W/m2) produces good welds within the corresponding operating region of travel speed and mean power. Unacceptable welds produced with travel speeds outside the operating region, and with high and low levels of APPD are not embodied in the study as their values are beyond the measuring range of the scanning instrument. The roughness is scanned along the centreline of the top weld surface of acceptable welds to be analysed by the Proscan 1000 laser metrology instrument, a powerful PCcontrolled tool for shape analysis, object digitising and Table 1 Chemical compositions of the steel substrate Steel substrate
Chemical composition (wt.%)
Electro-galvanised steel
C: 0.01, S: 0.008, P: 0.013, Mn: 0.160, Al: 0.036, N2: 0.0027, S: 0.006, Ti: 0.054, Nb<0.01
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Table 2 Parameters set-up for the experiments Pulse shape Laser power: PM Speed Pulse duration: TP Pulse energy: EP
Rectangular without a leading-edge power spike at the front of the pulse (see Fig. 2) 396, 363, 330, 297 0.4±15 mm/s 4, 8, 12 ms 5.9, 7.5, 11, 15 J for 4 ms 11.8, 15, 22, 30 J for 8 ms 17.7, 22.5, 33, 45 J for 12 ms Focused on the top surface of the material 120 mm focusing lens from Lumonics (borosilicate crown glass) Kept constant as 0.8 mm Argon gas blowing at a constant rate of 20 l/min at 458 onto the leading edge of the focused position
Focal location Lens Spot size (diameter) Shielding gas
accurate surface measurement. Weld slumping is de®ned as the ratio of defect depth to melt depth from measurements made with a micrometer under an optical microscope. Note that the underside weld surface defect, root concavity, is ignored in the measurements because it mostly does not exist within the operating range. 5. Experimental results and discussion 5.1. Effects of acceptable welding speeds on the weld bead roughness It is observed that most of the acceptable lap welds have a good surface ®nish without the commonly seen undercutting and humping. Fig. 3(a) is a typical plot of the surface roughness for the top weld versus acceptable welding speeds. The relationship is of a polynomial form with one minimum value. The weld bead roughness at travel speeds beyond the higher bound of the acceptable conditions increases rapidly due to the presence of blow-holes or cavities that are out of the measuring range. Travel speeds below the lower-bound produce deep slumping that is usually beyond the measuring range.
5.2. Effects of laser power on weld bead roughness Examination of Fig. 3(a)±(f) reveals that as the mean laser power is increased, the sensitivity of the weld bead roughness to the acceptable welding speeds becomes less pronounced. At the same time, the distribution of the roughness values measured within the operating region of the welding speed is generally lowered. Conversely, as the mean laser power is decreased to a certain limit, the changes in the roughness values increase rapidly. This is due to a sharp shrinkage in the operating region of welding speed, which implies an increase in thermodynamic instability in the weld pool. This becomes particularly apparent for longer pulse durations due to the longer heating times. 5.3. Effects of pulse energy and APPD on weld bead roughness Comparison of the results of Fig. 3(g)±(i) suggests that as the pulse energy is increased (7.5±11 J, 15±22 J, and 22.5± 33 J), whilst keeping a constant pulse duration (4, 8, and 12 ms, respectively), there is an increase in the weld bead roughness distribution. Indeed, as the APPD is increased there would be more pronounced thermodynamic effects
Fig. 2. Examples of the rectangular laser power pulses used in the study.
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Fig. 3. The dependency of the top weld bead roughness on the acceptable welding speed and the mean power for various parameters set-up as: (a) 4 ms and 7.5 J; (b) 4 ms and 11 J; (c) 8 ms and 15 J; (d) 8 ms and 22 J; (e) 12 ms and 22.5 J; and (f) 12 ms and 33 J. Comparison of the top weld bead roughness with varied APPDs for pulse durations of: (g) 4 ms; (h) 8 ms; and (i) 12 ms.
contributing to form an unfavourable weld bead pro®le. It can be concluded that the weld bead roughness is increased by increasing the APPD from the lower-medium to the upper-medium level. 5.4. Effects of pulse duration on weld bead roughness At lower-medium APPD level (see Fig. 3(g)±(i)), increasing the pulse duration (4±12 ms) results in a gentle rise in the
weld surface roughness distribution. On the other hand, at the upper-medium APPD level, there is an obvious trend of a rapid increase in the weld surface roughness distribution. Indeed, longer pulse duration at a higher APPD level implies the more profound effects that the pulse energy has on the weld bead roughness. The set of parameters (12 ms, 33 J) therefore gives the highest weld bead roughness distribution for the range of laser mean powers from 396 to 330 W. It is noted that there is only one welding speed available for
Y.-F. Tzeng / Journal of Materials Processing Technology 100 (2000) 163±170
Fig. 3 (Continued )
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Table 3 Polynomial curve ®t equations for M1 Welding parameters
Roughness: Yabxcx2dx3a
4 ms, 7.5 J, 396 W 4 ms, 7.5 J, 330 W 4 ms, 11 J, 396 W 4 ms, 11 J, 330 W 8 ms, 15 J, 396 W 8 ms, 15 J, 330 W 8 ms, 22 J, 396 W 8 ms, 22 J, 330 W 12 ms, 22.5 J, 396 W 12 ms, 22.5 J, 330 W 12 ms, 33 J, 396 W 12 ms, 33 J, 330 W
Yÿ4.3165xÿ52.37x212.57x3 Y230.48ÿ328.42x149.17x2 Yÿ2734.734276.46xÿ2183.85x2367.15x3 Y193.3ÿ197.7x59x2 Yÿ97269.6xÿ181.35x237.857x3 Y53.1ÿ56.84x34.64x2ÿ6.31x3 Y821.1ÿ944.67x365x2ÿ45.83x3 Y554.23ÿ570.5x161.67x2 Y89.61ÿ49.34x2.68x23.12x3 Y1440.8ÿ1761.5x565x2 Yÿ877.51325.5xÿ622.5x2ÿ95.8x3
a b
Travel speed for minimum roughness (mm/s)
b
1.87 1.10 2.20 1.68 2.01 1.24 2.23 1.76 2.04 1.56 2.44 2.00
Energy input per unit weld (EI) (J/mm) 211 300 180 196 197 266 178 188 194 211 162 165
a, b, c>0, YRa (mm), xtravel speed (V). The curve is non-existent due to there being only one welding speed.
330 W. The extremely narrow operating region corresponds to a very high surface roughness distribution for acceptable welds. 5.5. Effects of the laser power on the welding speed for minimum weld bead roughness Using graphical software, the curve ®t as a polynomial equation for Fig. 3(a)±(f) can be found as follows. The travel speed for minimum roughness is determined by @Ra @y 0 @V @x
(5)
In addition, the energy input per unit weld for a constant spot
size (D) is equal to W Z W V V
(6)
assuming that the absorptivity Z1 for simpli®cation and convenience of data interpretation. The data in Table 3 is plotted in Fig. 4(a) and (b) showing the relationship between the welding speed for minimum roughness and its corresponding heat input per unit weld. It is seen clearly that for a given set of welding parameters, the travel speed for minimum roughness decreases as the mean laser output power reduces due to the reduction in the energy input rate, but increases with increasing pulse duration due to the longer heating effect. At the same
Fig. 4. Relationship between the welding speed for minimum surface roughness and its corresponding heat input per unit weld for: (a) lower-medium APPD; and (b) upper-medium APPD.
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169
time, the energy input per unit weld (EI) for minimum roughness rises with decreasing mean laser output power because a higher energy input rate is needed to overcome the increase in thermodynamic instability within the weld formation. It is noted that at upper-medium APPD (11 J for 4 ms, 22 J for 8 ms, and 33 J for 12 ms), the EI per unit weld for minimum roughness in general are lower than that at lowermedium level (7.5 J for 4 ms, 15 J for 8 ms, and 22.5 J for 12 ms). Indeed, the higher energy heating effect has led to higher travel speed. As the pulse duration is increased, however, the EI values are slightly decreased at constant APPD. This is a result of longer heating effect leading to a faster welding speed.
5.7. Effects of pulse energy and APPD on weld slumping
5.6. Effects of an acceptable welding speed on weld slumping
5.8. Effects of the pulse duration on weld slumping
Examination of Fig. 5(a) and (b) suggests that the weld depth decreases with increasing welding speed. As the travel speed is lowered, deeper weld defects are produced due to there being greater thermal input per unit weld. Hence, it is concluded that an inverse relationship between the depth of weld surface defects and the welding speed exists, more signi®cantly so at the lower bound of the acceptable welding speed. This is seen at both lowermedium APPD and upper-medium APPD for 4 ms, which yield around 20 and 18% of defect depth to weld depth ratios, respectively.
Examination of the results in pairs from Fig. 5(a) and (b) reveals that increasing the APPD by increasing the pulse energy (from 7.5 to 11 J for 4 ms, 15 to 22 J for 8 ms, 22.5 to 33 J for 12 ms) whilst keeping the pulse duration constant leads to slightly larger weld defects depth. At the travel speed of 2 mm/s, the increased APPD for the 4 ms pulse increases the slumping from approximately 7.6 to 11%, and for the 8 ms pulse from 7.5 to 10.5%. This could be due to the fact that at higher APPD levels, more Zn gas, metal vapour and the instant ejection of spatters are produced by more ef®cient heating effects from the higher heat energy input rate per power pulse.
Fig. 5(a) and (b) show that as the APPD is constant, there is not a signi®cant effect of pulse duration on the weld defect depth, but they also reveal that a shorter pulse duration leads to a slightly deeper weld defect depth, particularly at low welding speeds. This result obviously con¯icts with the argument that longer pulse duration creates more metal loss due to stronger heat ¯ow effects. However, it is observed that at a given laser mean power, a longer pulse duration does produce larger weld pools, and that more metal vapourisation in reality does not cause deeper weld slumping: instead, a shallower slumping results. Hence, the longest pulse duration of 12 ms yields the shal-
Fig. 5. Comparison of the top weld surface slumping produced at: (a) lower-medium APPD; and (b) upper-medium APPD.
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lowest slumping. The maximum depth is observed for 4 ms pulse duration at the higher pulse energies and lower speeds. 6. Conclusions A qualitative and quantitative analysis of acceptable welds produced in the pulsed laser seam welding of zinccoated steel using rectangular power pulses are described. The results are summarised in the following. 1. The main factors affecting the surface roughness of acceptable welds have been identi®ed as the welding speed, the laser power, the pulse duration, and the average peak power density. 2. The main factors affecting the surface defects of acceptable welds such as slumping are principally the welding speed, followed by the average peak power density and pulse duration. References [1] T. Klein, M. Vicanek, I. Decker, G. Simon, Oscillations of the keyhole in penetration laser beam welding, J. Phys. D 27 (1994) 2023±2030.
[2] C. Dawes, Laser Welding, A Practical Guide, Abington Publishing, Cambridge, UK, 1992. [3] W.M. Steen, Laser Material Processing, Springer, Berlin, 1991. [4] B.J. Bradstreet, Effects of surface tension and metal ¯ow on weld bead formation, Welding Res. (Suppl.) 47(6) (1968) 314s±322s. [5] J.W. Wealleans, B. Adams, Undercutting and weld bead turbulence in Tig welding, Welding and Metal Fabrication 39(6) (1969) 255± 257. [6] D.J. Kotecki, D.L. Cheever, D.G. Howden, Mechanism of ripple formation during weld solidi®cation, Welding Res. (Suppl.) 51(8) (1972) 386s±391s. [7] T.R. Anthony, H.E. Cline, Surface rippling induced by surface tension gradients during laser surface melting and alloying, J. Appl. Phys. 48(9) (1977) 3888±3894. [8] P.G. Moore, L.S. Weinmain, Surface alloying using high-power continuous lasers, Laser Appl. Mater. Process., SPIE 198 (1979) 120±125. [9] K. Ishizaki, A new approach to the mechanism of penetration, Proceedings of the Conference on Arc Physics and Weld Pool Behaviour, The Welding Institute, UK, 1980. [10] I.C. Hawkes, M. Lamb, W.M. Steen, Surface topography and ¯uid ¯ow in laser surface melting, Proceedings of the Conference of Third International Colloqium on Welding and Melting by Electrons and Laser Beams, 5±9 September 1983, pp. 125±131. [11] G. Shannon, Introduction to the laser welding of steel sheet, AMWL Lecture Notes, The University of Liverpool, 1996.