Effect Of Nd-YAG Laser Pulse shaping on Weld Bead Characteristics of Commercial Materials

Laser Pulse shaping was found to effect the weld bead characteristics. Versatility of laser welding is further increased with pulse shaping. Pulse shaping can be effectively used to reduce defect generation, cracking susceptibility and heat input. Pulsed Nd-YAG laser welding is associated with many variables such as peak power, pulse duration, pulse energy, defocusing distance, spot overlapping, frequency and pulse shape. Shaping of laser pulse to get the desired weld bead geometry is getting importance. Shaping of laser pulse includes ramping up, ramping down, initial peak etc. In this paper, selection of pulses has also been discussed. Absorption of laser depends upon type of material and its surface roughness. Weld bead geometry changes with material and surface finish at a constant pulse energy. In this paper effect of pulse shaping on different materials was discussed. Study has been carried out using optical microscopy. Optical microscopy is used to analyze the changes in metallurgical structure. Distinct differences in weld bead geometry have been observed with temporal pulse shaping technique. Effect of shaping on reduction of defects has also been discussed.

Introduction
Laser welding has become an important industrial process because of its advantages over the other widely used welding techniques. The laser welding technique requires the management of many parameters depending on the thermo physical properties of the material, the environment, laser and its process parameters (power, pulse duration, pulse repetition rate, focal and pulse shaping), the operator’s skill and the welding procedure.
Laser welds have parallel-sided fusion zone, narrow weld width and high penetration. These advantages of laser welding come from its high power density and low heat input. Nd-YAG lasers continue to replace other welding techniques like GTAW and resistance spot welding. Principal advantages of laser welding over the other processes are its low heat input, low distortion, low heat affected zone and non-contact nature of the process [1, 2]. Complications associated with laser welding of end plug are narrow weld zone susceptible to solidification cracking, concavity on the weld bead, and weld spatter. Most of these defects can be eliminated by proper selection of laser welding parameters. Laser pulse shaping is one among these parameters with which one can control the bead geometry, reduce the defect generation and improve the penetration.
Generally, laser welding devices use square pulse as a default setting for which the maximum power is released within the material for all the set time. The improvement of a few recent pulsed Nd:YAG laser devices has proposed the possibility of using pulse shaping to get a better control on the thermal cycle during welding[3]. With the use of laser pulse shaping, the energy density delivered into the material can be released in different ways during the pulse period: a slow or a fast preheating process, a fast or a slow cooling process or a combination of a slow preheating and a slow cooling, are possible. Those different options are known to be effective to prevent weld defects such as porosities and hot cracking occurring in sensitive materials.
A pulsed laser emits packets of energy that consist of a fixed amount of energy for a specified duration. Pulse shaping is a technique used to temporally distribute energy with in a single laser pulse [4]. It can also be defined as a variation in power supplied to a laser to change the shape of the output pulse and subsequently the heat distribution with in the pulse. Changing the energy distribution with in a pulse can completely change the melting behavior, solidification mode, phases of final microstructure and morphology of phases [5]. Pulse shaping can be attained by dividing the pulsed current used to excite a laser flash lamp into number of small individual sectors and specifying the peak power and pulse duration of each sector. That means clubbing of many square pulses of different peak power with certain duration as a single pulse.
This provides the user an added degree of control over the heat delivered to the laser material interaction zone. Pulses that induce a gradual heating or a prolonged cooling effect can be generated with peak power or pulse energy combinations specifically tailored to control melt pool properties and defect reduction [6] and eventual weld bead formation.
The input current causes the flash lamps to emit light that is absorbed and amplified by the Nd:YAG Crystal (only for Nd:YAG laser). This amplified light is emitted in short bursts /pulses through various focusing lenses and eventually on to a work piece. The electric current supplied is therefore one of the main factors that dictate the characteristics of the laser pulse. This allows the user to specifically tailor the energy distribution to the nearest 0.5ms with in a single laser pulse.
Table 1: Absorption of materials for Nd YAG laser (wave length 1064 nm)

Different pulse shapes (Fig 1) have been studied in this paper to analyse the weld bead characteristics of metals such as Stainless steel (ASTM SS304), commercially pure aluminium, copper and brass (65%Cu-35%Zn). Square pulse was used to compare with other pulse shapes which include ramping up, ramping down, combination and initial high peak of power. Surface roughness has been kept constant for all those materials with selective polishing with different abrasives. Approximate surface absorption of the materials under study have been tabulated in table 1.

Fig: 1(a) Pulse shape 1                                Fig: 1 (b) Pulse shape 2

Fig: 1 (c) Pulse shape 3                                 Fig: 1(d) Pulse shape 4

Fig: 1(e) Pulse shape 5                                   Fig: 1 (f) Pulse shape 6

Fig 1: Pulse shapes

Experimental procedure:
Welding experiments have been carried out on four different metal sheets i.e. Stainless steel (ASTM SS304), commercially pure aluminum, copper and brass (65%Cu-35%Zn). Dimensions of the sheets employed were 0.1 × 0.05 × 0.008 meters. Pulsed Nd-YAG laser (wave length 1064 microns) spot welds were produced using laser device with fixed processing optics beam delivery (Fig 2). Average power of the laser device employed was 500watts. The laser beam was focused by a 200mm focal length convex lens with focal plane located at work piece surface. Initial focusing of the machine was verified by stainless steel metal sheet (Fig 3). Average power measured from the Optical Engineering power probe in order to make sure of energy input. Incident angle of the beam was maintained at 15 degrees to the work piece location so as to avoid any interaction of the reflected beam with the incident beam.
Focused beam was used for experimental welding with a constant energy input of 20 joules for all pulse shapes. Frequency was calculated based on spot size, which was 800 microns. Frequency was calculated using equation 1. Motor was attached with servo control for the linier motion of the work piece. Speed of welding was .0012 meters per second. Overlapping kept constant at 70% and overlapping factor is calculated using equation 2.

Figure 2: Experimental setup
V = F X (1 – O) X D     (1) 
Overlapping factor
O_f  = [1- ( V / f) / (D +VT ) ]×100   (2)
Where,
V- Speed of welding
D- Spot diameter
T- Pulse duration
f- Focal length of lens
F- Frequency
O- Overlap
Surface roughness of the sample was measured with HANDYSURF E-35B. During the experiments, Argon was used as a shielding gas. The samples were sectioned using abrasive wheel cutter. Polished samples were etched using reagents given in Table 2_. Microstructure of the samples was examined using Optical microscopy at different magnifications.
Results and Discussion:
Surface roughness of all the samples kept constant to evaluate the actual response of the material to pulse shaping. Surface roughness of the material was kept at 0.4 micro meters.

Figure 3: Defocusing plot.
Table 2: Etching reagents used for evaluation

• Stainless Steel:    

Stainless steels have got good absorption for Nd YAG lasers with a wavelength of 1064 nano meters. Fig 4 ( a to f) shows the penetration obtained using pulse shapes shown in Fig 1(a to f) respectively. Porosity was observed in microstructure obtained by using square pulse. The reason could be sudden heating mode of the pulse shape. This argument strengthened by Fig 4c which was obtained by using pulse shape 3. For pulse shape 1c, sudden heating has taken place. Gradual cooling in pulse shape 3 did not have much effect on defect reduction. Fig 4b is a smooth weld with some concavity. Gradual heating could be the reason for a good weld. Pulse shape 4 is providing both gradual heating and cooling, which resulted in a smooth weld with very good penetration. Figure 4e was obtained using pulse shape 5 with initial high peak. Absorption of laser for stainless steel is better compared to other metals so initial high is not required. Initial high peak power of the pulse shape resulted in evaporation of material at the surface and the same repeated with pulse shape 6 also.

4(a)                                                       4(b)                                      4( c)

4(d)                                                     4(e)                                              4(f)
Fig 4: Microstructures of stainless steel welds
Weld penetrations obtained with different pulse shapes is shown in Fig 5. Weld penetration for pulse shapes 1 to 4 are in the range of 1600 to 1800 microns. Penetration is maximum for pulse shape 5 because of the initial high peak power, which increased the absorption further. Pulse shape 6 also generated good penetration but less compared to pulse shape 5.

Fig 5: Weld penetration for stainless steel
2) Aluminum:       
Aluminum is one of the difficult metals to weld because of its high susceptibility for cracking and affinity for oxidation. Microstructures obtained by different pulse shapes are shown in Fig 6.

6(a)                                                       6(b)                                                         6(c)

6(d)                                                     6(e)                                                   6(f)

Fig 6: Microstructures of Aluminum welds

Fig 6a shows the structure obtained by pulse shape 1. Few cracks were noticed in Fig 6a and in Fig 6b. Both shapes are having sudden decrease in power. Solidification crack is mainly due to sudden cooling of the melt. This crack is clearly shown in Fig 7. Concavity was observed in figure 6c with pulse shape 3. Structure obtained from pulse shape 4 and pulse shape 5 are free from defects. Spatter was observed during welding in case of pulse shape 5.

Fig 7: Solidification crack (Mag: 200 X)
Initial high peak in pulse shape 5 increased the absorption of  the  material and followed by sufficient high power resulted in good weld. Interestingly no crack was found in pulse shape 5 even though sudden heating was adopted. Pulse shape 6 is similar to pulse shape 5 with step wise decrement of power. Pulse shape 6 resulted in large scattered porosity. Multiple changes in power level is also not desirable for aluminum as it ends up in porosity during solidification.

Fig 8: Weld penetration for Aluminum.
Absorption of Nd YAG laser for aluminum is lower to stainless steel and hence low weld penetrations in the range of 400 to 550 microns. Weld penetration is higher for pulse shapes 5 and 6 and is shown in Fig 8. With the initial high peak power, absorption increases and resulted in maximum penetration of 550 microns for pulse shape 5. Pulse shape 6 is having initial high peak, but step wise decrease in power resulted in higher pulse duration with less peak power for the same pulse energy. Lower peak power during actual welding regime resulted in less penetration compared to pulse shape 5.
3) Copper:
Absorption co-efficient of copper for Nd YAG laser is very low. Thermal conductivity of copper is also very high. Heat buildup will not takes place for materials having high conductivity. In order to improve the absorption of laser light to produce sufficient heat required for melting, argon gas is mixed with 30 volume % of oxygen. Absorption for copper oxide is greater than copper. Microstructures obtained by pulse shapes 1 to 6 are shown in Fig 9a to 9f respectively.

9(a)                                                          9(b)                                              9(c)

9(d)                                                        9(e)                                              9(f)

Fig 9: Microstructures of copper welds

Fig 9a reveals that weld cross section contains concavity. Square pulse is unable to produce sufficient oxide layer to improve penetration. Figure 9c obtained with pulse shape 3 is showing same behavior as that of pulse shape 1. Gradual increase of power in case of pulse shape 2 and 4 are showing good results. This can be attributed to enough oxide layer formation with slow heating followed by sufficient peak power. Weld bead geometry is good in case of pulse shape 4 compared to the bead formed by using pulse shape 3. In case of pulse shapes 5 and 6, evaporation of material taken place and resulted in large concavity.
Cracks and porosity have not found in any of the pulse shapes used for the present study. Increase in the Oxygen content may show good results in terms of penetration but associated with more chances of evaporation.

Fig 10: Weld penetration for Copper

Depth of penetration of welding is plot against pulse shapes and is shown in Fig 10.  Maximum penetration was noticed for pulse shape 4 is 450 microns. In case of pulse shape 3, minimum penetration was obtained. Generation of oxide layer was not followed by sufficient peak power to induce required heat input.
4) Brass:
Absorption for brass stands in between absorption of aluminum and copper. Spatter was found during welding. De focusing could be a good option to reduce spatter. To maintain the consistency all through out the experiments defocusing was not done. Percentage of zinc (Zn) in brass also affects the penetration depth. Thermal conductivity for brass is also very high and hence resulted in low penetration depth. Cracks and porosity was not observed in any of the welds for all pulse shapes. Microstructure of brass is shown in Fig 11.

Fig 11: Microstructure of brass for square pulse
Clean welds were produced with all pulse shapes. Penetration variation with pulse shapes is shown in Fig 12.