Throwing new light on materials processing...
TWI Bulletin, May - June 2006
an addition to the laser family
A new laser type has recently been developed, producing 'super-modulated' output power
Helen Graham joined TWI in July 2003 after completing a mechanical engineering degree at Loughborough University. Since then she has worked on a variety of projects involving laser welding and cutting, and laser-arc hybrid welding.
Nd:YAG lasers for materials processing generally fall into two categories; high power continuous wave (CW) lasers, in powers from around 500W to 6kW, and pulsed lasers with average powers up to about 500W. The output from a conventional pulsed laser would typically be similar to that shown in Fig.1a, with short peaks of several kilowatts, but much lower average power. As Helen Graham reports a new laser type has recently been developed, producing 'super-modulated' output power. CW lasers can often also operate in a 'modulated' mode, where the CW laser light is delivered in gated pulses of varying frequency, up to the maximum CW power produced ( Fig.1b).
Fig.1. Typical output traces produced by:
a) conventional pulsed Nd:YAG laser
b) continuous wave Nd:YAG laser with modulated mode
c) super-modulated laser
This means the light generated is essentially continuous wave, but the laser is able to produce a pulsed output with a peak pulse power of twice the average CW power ( Fig.1c). This is done by storing some energy in the power supply during the off time in the pulse cycle, which is then released during the peak power part of the cycle. These lasers are available with average powers from 400Wto 2kW. The manufacturers claim that use of super-modulated output power offers advantages in processing speeds and quality of welding and cutting compared to conventional laser types.
Objectives
The objectives of the work were:
- To evaluate the performance of a super-modulated laser for welding of stainless steel, aluminium and zinc coated steels.
- To compare the performance of super-modulated lasers with conventional CW lasers.
A JK1002 super-modulated laser ( Fig.2) manufactured by GSI Group was installed at TWI for evaluation. This laser produces a maximum average power of 1kW, with peak pulse power of 2kW. The pulsed output can be in either square or sine wave form, in frequencies from 200 to 1000Hz. This article describes trials carried out to assess the performance of the JK1002 for welding of three materials; stainless steel, aluminium, and zinc coated steel.
Fig.2. JK1002 laser and controller ( photo courtesy GSI Group)
Welding of stainless steel is very common in a range of industries, including power generation, electronics, medical, and chemical. Laser welding of stainless steel is relatively common, but these welds can be prone to porosity formation in the weld. The effects of super-modulation on maximum welding speed and weld quality were investigated.
Laser welding of aluminium can be difficult at low input energies due to problems with the light reflecting off the material surface. A minimum power density is required to achieve coupling of the light with the surface, and in thin sheet overheating of the aluminium and excessive melting can occur once coupling is established. Trials were carried out to investigate whether super-modulation could be used to improve coupling at relatively low heat inputs.
Laser welding of zinc coated steel is of considerable interest to the automotive industry for manufacture of car bodies, mainly due to the fast welding speeds which can be achieved. However, it is usually considered very difficult to produce good quality laser welded lap joints in zinc coated steel without a controlled gap between the parts, which is not easy to achieve reliably and consistently in production.
The welds are very susceptible to blowholes and spatter caused by vaporisation of the zinc coatings on the inside surfaces of the joint. It was therefore of interest to investigate whether super-modulation could be used to improve weld quality, particularly when welding without a gap between the sheets, a configuration much easier to achieve in production.
Previous work on zinc coated steel found that even welds with severe blowholes passed tensile testing ( ie failure occurred in the parent material rather than the weld), so the aim of this work was to produce welds with good external appearance and minimal spatter.
Experimental approach
Laser set up
The JK1002 laser delivers the light to the workpiece via a 0.6mm diameter optical fibre. Both 200 and 100mm focal length focusing optics were used, producing 0.6 and 0.3mm diameter focused spots respectively at the material surface.The laser was used at 1kW average power, with varying travel speed, modulation mode (including CW operation), and modulation frequency. 0.3mm focused spot diameters were used for the welds made on stainless steel and aluminium and 0.6and 0.3mm diameters used for the trials on the zinc coated steel.
Welds were also produced at various power levels for comparison, using a HL 3006D high power CW laser manufactured by Trumpf. This laser also delivers the light through a 0.6mm diameter optical fibre, and focusing optics delivering similar focused spot diameters were used.
Welding of stainless steel
Bead on plate welds were produced in 3mm thickness, 304 stainless steel, and a total of forty welds were made with the JK1002 laser. Argon shielding gas was supplied through a coaxial nozzle with a trailing shoe, and through a channel on the underside of the weld. The resulting weld quality was assessed in terms of porosity and bead profile.
Welding of aluminium
The JK1002 was used at 1kW average power to produce bead on plate welds in 2mm 1050 aluminium alloy. Argon shielding gas was supplied through a coaxial nozzle, and through a channel on the underside of the weld.
Welding of zinc coated steel
Trials were carried out using the JK1002 at 1kW average power to produce fully penetrating lap stake welds in 0.7mm thickness zinc coated steel with 8µm zinc coating thickness. The joints were clamped such that there was no gap between the plates. No shielding gas was used for these welds.
Results and discussion
Effects on welding speed and weld quality in stainless steel
Figure 3 shows a comparison of welds made with the JK1002 laser using CW mode and sine and square wave modulation. Both the welds made with modulated mode were narrower welds with smaller fused areas, as would be expected due to the reduced energy input. It was noted that the square wave weld beads were often irregular and suffered from undercut, while the sine wave and CW weld beads were much smoother.
Fig.3. Sections from welds in 3mm stainless steel, made using the JK1002 with:
a) CW mode, welding speed 0.4m/min, heat input 150J/mm
b) sine wave mode, 200Hz, 0.6m/min, 100J/mm
c) square wave mode, 200Hz, 0.9m/min, 66.7J/mm
These results showed a significant increase in welding speed with super-modulated compared to CW output, for the same average power ( Fig.4). In sine wave mode, the increase was about 50%, and more than doubled in square wave mode. The speed increase was greatest at the lowest modulation frequency, 200Hz. These speed increases would correspond to a significant reduction in heat input and offer advantages in terms of reducing distortion in laser welded components.
Fig.4. Welding speed vs. modulation type and frequency, bead on plate welds in 3mm stainless steel
The wine glass weld shape produced in CW mode ( Fig.3a) is normally associated with a plume of vaporised weld metal above the laser keyhole. This can scatter the laser energy, resulting in a wider top bead and a reduction in penetration and/or travel speed. The results from this work suggest that the pulsing action inhibits the formation of the plume and the laser is allowed to interact better with the stainless steel, thus improving the process efficiency. This would also explain the relative improvement in square wave mode at low frequency, giving maximum disruption of the plume, and allowing greater travel speed to be achieved for the same average power.
Porosity levels were very high in the welds made in CW mode, around 18% of the weld area. Using super-modulation at 200Hz frequency, it was possible to reduce the porosity to only 1% of the weld area, in both sine and square wave modes. As shown in Fig.5, porosity levels generally increased with increasing frequency, and were higher in welds made with sine wave modulation than square wave.
Fig.5. Porosity (% area) vs. modulation type and frequency, bead on plate welds in stainless steel
A comparison weld was produced with a high power CW laser, using the maximum speed achieved with the JK1002 in pulsed mode, which was 0.9m/min. A power of 1.5kW at the workpiece was required to produce the same weld penetration, and the porosity levels were again very high. Lower porosity can be achieved by using even higher powers to produce greater weld penetration, although clearly this also increases the weld heat input and associated distortion.
These results show that super-modulated lasers may be very useful for reducing porosity in stainless steel welds. Although the work did not investigate the potential for producing partially penetrating welds, these are generally considered to be more susceptible to porosity than fully penetrating welds and there may be some further advantage in using the super-modulated laser. The increase in welding speed for a given average power, and hence reduction in heat input, also demonstrates the potential of these lasers for welding of structures susceptible to distortion during welding.
Welding of aluminium
It was found that reproducible coupling of the laser beam with the aluminium surface could only be achieved with one condition at low heat input; square wave modulation mode at 200Hz frequency. Other pulsed conditions produced some coupling, while 1kW CW laser light did not couple to the aluminium at all. However, even using the 200Hz square wave condition, the slightest variation in focus position and resulting power density caused a loss of coupling, which suggests that the power density is at the minimum limit of that required.
The use of super-modulated laser power improves coupling for welding of aluminium, and should allow welding with lower heat input than for a CW laser, which would be beneficial for thin sheet. However, a slightly higher average power than 1kW may be required to achieve consistent results for production applications.
Welding of zinc coated steel
Good quality lap welds in zinc coated steel were achieved using various pulsed conditions with a 0.6mm diameter focused spot. A typical section is shown in Fig.6. Some internal porosity was visible from inspection by radiography, but the external quality was excellent, with no blowholes or surface breaking porosity. As with stainless steel, the fastest welding speed, 1.3m/min,was achieved using square wave modulation at 200Hz. Welds made with 1kW CW laser power again needed to be made at a lower speed of 0.4m/min and contained a number of surface breaking blowholes. Fig.7 shows the difference in external quality achieved with square wave and CW modes. Use of a 0.3mm spot size allowed faster welding speeds, but the welds were very poor quality in either pulsed or CW mode.
Fig.6. A typical section from a lap weld in zinc coated steel, made using square wave mode at 200Hz
Fig.7. Top beads of lap welds in zinc coated steel, made with the JK1002 using:
a) square wave mode, 200Hz, 1.3m/min
Fig.7b) CW mode, 0.4m/min
Some comparison welds were made at 1.3m/min using a high power CW laser, with which 1.2kW was required to achieve the same penetration. These were also of good external quality, and the weld width was similar to those produced with the JK1002 in square wave mode.
This suggests that certain process windows exist in terms of power, speed, and spot size, in which good quality welds can be produced in zinc coated steel using either CW or super-modulated laser power. However, the capital cost of a JK1002 (£107k) is lower than that of the high power CW Nd:YAG laser (around £170k for 2kW) required to achieve the same results.
Conclusions
Use of super-modulated laser power for materials processing can offer the following advantages compared to continuous wave lasers:
- Improvements in weld quality specifically in the reduction of porosity.
- The ability to produce good quality lap welds with zero gap in zinc coated steel.
- Increased welding speeds, and hence reduced heat input, for the same average power, which will benefit distortion critical components in thin sheet.
- Lower capital cost, £107k for a JK1002, compared to around £170k for a 2kW CW laser which would be required to match the performance of the JK1002.
- Improved coupling of the laser light to the surface for welding reflective materials such as aluminium.
Acknowledgements
The author would like to thank the GSI Group for the loan of the JK1002 laser used in this work, and Mohammed Naeem for his assistance during the project.
