Laser welds - ending the right way

TWI Bulletin, September/October 1993

Italian born Carlo Ferlito studied chemical engineering in Padova University, Italy and specialised in materials science and laser technology. He joined TWI in December 1990 after working for the Italian technology research organisation RTM. He has now left TWI to return to Italy.

While at Abington his responsibilities lay in conducting research projects on laser processing with particular attention to laser cutting applications.

The conditions under which laser welds are terminated are as critical as those during welding. Porosity and weld concavity can occur, as Carlo Ferlito reports.

When welding tubular or circular components, there is often a requirement to terminate the weld effectively on completion of the circumference of the joint. For keyhole welding processes, such as electron beam and laser, switching the beam off suddenly at the end of the joint will leave a depression around the termination point and higher levels of porosity in the fade-out region, which may be unacceptable for certain applications. Effects of various parameters on closing welds in flat and tubular C-Mn steel components are described here with recommendations for improving weld appearance in the termination region.

Weld closure

The main use of CO ₂ lasers for welding has been in joining sheet materials in bare and coated steel and of transmission system components where the laser technique produces welds at high speeds with low distortion. When welding sheet components, both lap and butt joints are made industrially with a high degree of reliability.

Laser welding of gear components is widely used in Europe, the USA and Japan, and termination procedures are already in use to produce consistent weld quality. When welding transmission system components, the weld is normally partially penetrating with a depth of a few millimetres. Demonstration of the consistency of the process in the automotive industry has led to an interest in extending the range of applications for laser welding to structural components where welds are made in greater thicknesses (>5mm) and are normally fully penetrated.

This has been reflected in the uptake of commercially available lasers with power of 5kW which generally have a penetration capability of up to 12mm in steel. For laser welding of tubular structures fabricated in C-Mn steel in thicknesses between 5-12mm, the weld termination procedure is a more difficult operation because the welds are generally fully penetrated and often deeper than those in gear components.

Furthermore, in tubular components, there is a requirement for improved integrity as the welds may often be load bearing. This means that only low or zero levels of defects can be tolerated. Much work on weld closures in structural components has already been undertaken in arc and electron beam fusion welding techniques. ^[1]

Laser and electron beams both weld using a keyhole technique. A sudden switching off of the beam causes a collapse of the keyhole and results in creation of a defect, normally a depression, at the termination point. In many cases this is likely to be unacceptable. The solution to this problem lies in a gradual reduction in the keyhole effect at the closure of the weld, so that this area has the same quality as the rest of the weld in the component.

For laser welding, the transition from the keyhole mode of welding to closure of the weld is particularly critical as the molten area has to progress through a transition from full penetration to partial penetration welding. Partial penetration welds tend to exhibit root porosity and this effect is quite commonly seen in the 'start-stop' region of laser welds on tubular or circular components. There are a number of possibilities for adjusting the laser welding parameters to avoid problems in the weld closure zone and these include:

• Beam spinning;
• Defocusing of the beam;
• Changing welding speed;
• Power ramping;
• Suppression of plasma control gas.

Effects of these parameters, acting independently or in selected combinations, are reported here.

Trials and results

The materials investigated were:

• Low carbon steel flat plates, 6mm thick;
• Medium carbon steel flat plates, 6mm thick;
• BS 4360 Grade 50D steel flat plates, 15mm thick;
• BS 4360 Grade 50D steel tubular components, 12mm thick.

The chemical compositions of these materials are given in the Table.

Chemical analysis of materials used

Two lasers were used in the trials, a 5kW CO ₂ fast axial flow laser manufactured by Trumpf and a 10kW CO ₂ transverse flow laser manufactured by Ferranti. Helium supplied by a 'side jet' was used as plasma control gas during the welding trials. The workpiece was moved under a stationary laser beam by a CNC table in the case of flat specimens, and by a rotary jigging system for the tubular components.

The various techniques noted above were investigated to minimise defects in the termination region of the weld. The resulting samples were radiographically inspected and visually examined to detect porosity, cracks and irregularities in the weld bead such as undercut, lack of fusion and weld disruption.

Beam spinning

The 5kW CO ₂ laser was used for these trials. The equipment used to spin the laser beam consisted of a potassium chloride optical flat, 6mm thick and 50mm diameter, tilted and mounted on a hollow spindle which was rotated below a focusing lens by a variable speed electric motor. The effect of rotating the tilted optical flat is to make the focused 'spot' rotate at a given radius (which depends on tilt angle and flat thickness) around its original position.

Weld fade-out assessment trials were carried out using this device on melt runs on flat plates of two materials; 6mm thick medium carbon steel and 6mm thick low carbon steel. The conditions initially selected for the trials were 4kW laser power at 0.4 m/min traverse speed, and 5kW laser power at 0.8 m/min traverse speed. The correct spin speeds associated with these conditions were calculated following procedures developed previously, ^[2] to produce a 'spin radius' of 0.5mm. For the workpiece speed of 0.4 m/min, the spin speed was 1005 rev/min. For the workpiece speed of 0.8 m/min, the spin speed was 2010 rev/min.

The selected conditions produced fully penetrating welds when the beam spinning device was switched off. Trials were carried out to investigate beam spinning as a technique to move from full to partial penetration by switching on the unit half way through a traverse, at the spin speeds established. The melt runs in the area of the beam spinning were evaluated for defects.

All the melt runs in both the materials showed a large number of small pores in the region using the beam spinning technique. The observed porosity level decreased with an increase of laser power from 4 to 5kW and the beam spinning speed from 1005 to 2010 rev/min. Transverse macrosections of melt runs using beam spinning techniques are presented in Fig.1 and 2. No appreciable difference in porosity levels, quantity or distribution of the pores could be noticed between the two materials evaluated.

Fig.1. Macrographs of melt runs carried out spinning the laser beam on 6mm medium carbon steel:

a) Traverse speed 0.4 m/min, spin speed 1005 rev/min, laser power 4kW;(071787)

b) Traverse speed 0.8 m/min, spin speed 2012 rev/min, laser power 5kW.(071778)

Fig.2. Melt runs carried out spinning the laser beam on 6mm low carbon steel:

a) Traverse speed 0.4 m/min, spin speed 1005 rev/min, laser power 4kW;(OY613)

b) Traverse speed 0.8 m/min, spin speed 2012 rev/min, laser power 5kW.(0Y610)

Defocusing

Trials were carried out by defocusing the beam of the 5kW CO ₂ laser on flat plates of 6mm thick low carbon steel. A 300mm focal length potassium chloride lens was used as focusing system during the welding trials. An initial parameter study was made at laser powers between 3 and 5kW and traverse speeds between 0.25 and 1.0 m/min. Two sets of conditions were then selected for the trials: 3kW laser power at 0.4 m/min welding speed, and 5kW laser power at 1.0 m/min welding speed.

Melt runs were made with the plates angled at about 15° to the horizontal. The beam was either focused at the lower end of the plate, and the plate moved, thus forcing the focal spot into the material to produce the 'defocus' condition, or it was focused at the top of the plate, the resulting motion taking the focal spot away from the material surface.

Radiographic examinations showed that large scale porosity was present in all defocused conditions. No great difference was noticed in the distribution or quantity of porosity between melt runs made with the beam focus below the workpiece and those made with the focus above the workpiece. In the low carbon steel, at focus, a narrow, full penetration melt run with nail head type profile was made at both chosen conditions which showed little evidence of porosity. At the various levels of defocus, full penetration was lost and macrosections revealed porosity, mainly at the root and edges of the welds. This is illustrated in Fig.3.

Fig.3. Melt runs on 6mm low carbon steel performed at 5kW laser power and 1 m/min traverse speed: a) At focus;(OY616)

b) 5mm below focus;(OY338)

c) 10mm below focus;(OY337)

d) 15mm below focus;(OY340)

Speed change trials

Effects of accelerating the workpiece, by increasing the traverse speed of the table, were investigated on melt runs in 6mm thick plates of low carbon and medium carbon steel. The welding speed was increased linearly from a minimum to a maximum value, and the 5kW laser beam was subsequently switched off. A 300mm focal length potassium chloride lens was used to focus the beam.

Two initial conditions were selected:

Macrographs and radiographic examinations of the melt runs showed root porosity in the regions of partial penetration, while the porosity was greatly reduced in the full penetration condition. In the low carbon steel material, a high level of small scale porosity was present in all the partial penetration melt runs. In addition, a small number (less than 10) of large pores (>25% of weld width) were present in the majority of the melt runs exhibiting partial penetration, as shown in Fig.4. In the medium carbon steel, a high level of small scale porosity was present in all the partially penetrated melt runs. In the majority of partially penetrating welds, the pores were evenly spaced along the length of the melt run. Macrosections of these welds are shown in Fig.5.

Fig.4. Melt runs carried out on 6mm low carbon steel at 5kW laser power with increasing welding speed:

a) 0.8m/min;(OY1795)

b) 1.2m/min;(OY1791)

c) 1.5m/min;(OY1793)

d) 2.0m/min.(OY1794)

Fig.5. Melt run on 6mm medium carbon steel with increasing welding speed:

a) 0.8 m/min;(OY1789)

b) 1.2 m/min;(OY1782)

c) 1.5 m/min;(OY1783)

d) 2.0 m/min;(OY1784)

Control of laser output power

These trials were undertaken using the 10kW CO ₂ laser, and a power ramping unit also manufactured by Ferranti. This device will ramp the power down to 2.5kW, the minimum power which maintains stable discharges. A spherical mirror of 350mm focal length was used during the welding trials.

Initial trials were carried out on 15mm thick 4350 Grade 50D steel plate. All the melt runs were carried out at 10kW laser power and 1 m/min traverse speed. The final power at the end of the ramp was set at 2.5kW. The ramp cycle was started after 50mm of weld length. The initial ramp down time was set at 0.5sec. Longitudinal sections and radiography showed no sign of porosity in either the fully or partially penetrating welds.

The trials were then extended to tubular components. A 50mm outer diameter C-Mn steel tube, 12mm thick was welded using 10kW laser power at a welding speed of 1.1 m/min. The fade-out procedure was initiated after one rotation of the tube at ramp times of between 0.11 and 0.62sec.

The weld sections presented in Fig.6 show the ramp down effect. The time when the ramp down began and the area where the beam was switched off, can be seen clearly. The macrosections (particularly Fig.6d) show that the keyhole was very unstable during the ramp down period. Instability (spiking of the partial penetration root) occurred less dramatically during the shorter ramp period. At this welding condition, however, a small crater remained on the surface of the weld, when the laser power was switched off by closure of the shutter mechanism.

Fig.6. Transverse sections through welds on a tubular component, with preset power ramp times of:

a) 0.11sec (59033/2)

b) 0.27sec (59033/11)

c) 0.4sec (59033/7)

d) 0.62sec (59033/16)

Laser power ramping, speed change and suppression of plasma control gas

The above trials were extended by increasing the welding speed at the final stages of the ramping cycle in an attempt to eliminate the presence of the small crater found on the top bead at the end of the termination region. In addition, the plasma control gas jet was switched off in the closure region to avoid weld bead disruption by the gas flow. An increase of welding speed from 1 up to 2.5 m/min during the ramping cycle, and suppression of the plasma control gas flow one second after the beginning of the ramping cycle, eliminated the crater at the end of the fade-out region.

Welds with a satisfactory appearance in the tubular component, having low porosity and smooth bead appearance, were obtained only by using a combination of laser power ramping, speed change and suppression of plasma control gas.

Discussion

The above results show that the possible methods of laser weld termination have different effects on the weld appearance and, in particular, on porosity in the fade-out region. The advantages and limitations of each technique ( i.e. beam spinning, beam defocusing, increasing speed, power ramping and plasma control suppression) are reviewed below, with recommendations on how to avoid defects in the termination region for laser welds in tubular or circular components.

Within the experimental trials performed using the beam spinning technique, no major differences in the resulting porosity levels were noticed between low and medium carbon steel. The pores detected by radiography were small (<0.5mm), round and mainly located at the root and along the edge of the molten metal. It is believed that the observed porosity was caused by gas trapped during solidification. The beam spinning technique tended to redistribute pores rather than eliminate them, and as a result of this, coupled with the relative complexity of the device and its control, it is unlikely that beam spinning would be used for improving weld appearance in the termination area.

When using the beam defocusing technique to terminate the weld, pores were detected in areas corresponding to a defocused position of the beam. These pores were mainly at the root and along the edge of the molten material. The main contributor to the increase of defects in the weld metal is likely to be the different solidification mechanism operating for partially penetrating welds.

In this case, the motion of the keyhole in the material is likely to produce root defects in the form of round, finely distributed porosity because of perturbations during welding. These results suggest that defocusing the laser beam to improve weld appearance in a weld termination region does not produce satisfactory results in terms of reducing porosity levels. In addition, there are also significant practical considerations which would make use of this technique very difficult to achieve reproducibly in production.

When increased speeds were applied to terminate the weld, porosity was also evident in the transition region from full to partial penetration. It appears from these trials that use of speed alone will not eliminate the problem of root porosity at the transition between full and partial penetration welds.

Results achieved using laser power ramping at the weld termination were positive, even though with the equipment being used, the level of control of the laser power was somewhat limited. Very little porosity was detected. The main defect present during these tests was a slumped region at the top of the weld corresponding to the position where the beam was switched off.

This irregularity in top bead profile has two causes. The first is the very fast solidification of the molten metal using the laser process. When the laser beam is switched off, the keyhole immediately collapses but the molten material does not have enough time to close the upper region of the keyhole, solidifying behind it. The second relates to use of a plasma control gas jet. When using such a device, the small fused area on the top of the weld can be disrupted by the gas flow when the laser power is very low, causing undercut and irregularity on the top bead. In general, use of short ramping times produced better results than longer ramping times, where the keyhole was less stable (spiking in Fig.6d) and the risk of defects higher.

For the experimental work reported during these trials, use of a combination of power ramping together with an increase in welding speed is promising for reducing defects in the fade-out region of laser welds. An increase in welding speed towards the end of the power ramping cycle can allow a further decrease in the remaining size of the keyhole down to virtually zero, with elimination of undercut in the weld termination. If a plasma control gas side jet is being used, it may also be necessary to interrupt the gas flow in the weld termination to avoid weld top bead disruption.

In conclusion

For tubular components, use of power ramping at the end of the welding sequence in conjunction with an increase in welding speed has been shown to be effective in improving weld quality by reducing root porosity. With the improvement in power ramping facilities in recent commercially available lasers, it should now be possible to reduce the problem of root porosity in the weld termination region using ramping techniques only. However, under some circumstances, if a visually high quality weld termination is required, it may also be necessary to interrupt the plasma control gas at the end of the weld.

References