Steve Shi is a section manager in the Laser and Sheet Processes Group at TWI, which he joined in 2001 with a PhD in materials and experience in a range of metallic materials. He has managed projects concerned with laser materials processing and joining of high strength steels, with particular involvement in laser and laser-arc hybrid welding, in-process monitoring and adaptive control of laser welding, resistance and laser welding of high strength steels.
Steve Westgate is a consultant in resistance welding in the Laser and Sheet Processes Group at TWI Ltd. He joined TWI in 1972 with metallurgical experience and has managed a wide range of projects in all resistance welding processes. He has an active industrial consultancy and training role plus experience in mechanical fastening and lasers related to sheet metal joining.
High strength steels (UTS >600N/mm2) are increasingly used to meet the severe requirements imposed by the automotive industry in terms of safety, reliability and reduction in gauge for energy saving. As Steve Westgate and Steve Shi report TRIP steels have become of considerable interest in the automotive industry in recent years because of their exceptional combination of high strength and ductility, and are beingconsidered for structural and reinforcement components.
The particular properties (a combination of high strength and ductility) of the TRIP (transformation induced plasticity) grades result from a microstructure that differs from other types of high strength steel. The microstructure comprises a ductile ferrite matrix containing small islands of hard bainite and retained austenite. Transformation of the retained austenite to martensite during deformation causes significant strain hardening and retards the onset of necking. TRIP steels are now used for complex structural parts demanding high strength or crash resistance.
Although resistance spot welding is the main joining method in the automotive industry, laser welding is being used by some manufacturers in Europe for selected applications, such as door aperture seams, as an alternative. Laserwelding can potentially provide joints with enhanced mechanical properties, reduced distortion and improved structural stiffness compared to spot welded joints. It can also provide increased design options and manufacturing flexibility. In addition, laser welding covers almost all tailor-welded blank (TWB) production. With the development of new laser technologies and laser optics, laser welding has become one of main joining processes in the automotive industry.
The high carbon content of TRIP steels, coupled with fast weld cooling rates associated with laser welding, leads to high hardness levels in the weld and the HAZ because of the martensite dominated microstructure. Therefore, the key issues for welding TRIP steels are to reduce the embrittlement in the weld metal and the HAZ.
The work described here was concerned with the development of laser welding and hybrid laser-MAG welding for joining thin-sheet TRIP steels in the butt joint configuration, using fibre-delivered solid state lasers.
Experimental work
Materials
Laser welding trials were carried out on automotive grade steels, TRIP700 (700MPa minimum tensile strength: 0.25% C, 1.53% Mn, 1.1% Al) and low carbon steel (<0.005%C). Both steels were 1.5mm thick and hot dip zinc coated with a coating thickness of nominally 10µm each side.
The edges of the 250mm by 100mm samples were machined to avoid any effects of fit-up variations on weld quality. The joint surfaces were cleaned by dry wiping and degreased with acetone prior to laser welding to remove any contaminants.
Scope of work
The work comprised autogenous laser welding trials on TRIP to TRIP steels and on TRIP steel to a low carbon steel to establish the effect of weld metal dilution. Hybrid laser-MAG welding trials were conducted on TRIP to TRIP steels.
Equipment
The trials were conducted with a Trumpf HL4006D, 4kW, continuous wave (CW) lamp-pumped Nd:YAG laser, its power fed to the workpiece via an optical fibre of 0.6mm diameter and delivered by a beam focusing system mounted on a KawasakiJS-30 six-axis robot. A standard Trumpf optical assembly, including a 200mm focal length lens, was used to generate a laser spot of minimum diameter 0.6mm on the surface of the workpiece.
No shielding gas was used in this work on the top of the weld, but 5l/min was used as a back purge for welding trials on butt joints, to improve the weld root profile.
Hybrid laser-MAG welding trials were carried out using the same Nd:YAG laser, in combination with a Fronius 9000 welding power source. The laser output housing and the MIG/MAG torch were mounted onto the robot, traversing the arrangement over a stationary jig holding the specimens in close contact during welding.
The MIG/MAG torch had an arc travel angle of 30° and a 16mm contact tip to workpiece distance. The MAG process was operated in the pulsed mode. A Bostrand LWI low carbon steel filler wire of 0.8mm diameter at 20m/min was used.An Argoshield gas (Ar-20%CO2) was provided through the MIG/MAG welding torch at a flow rate of 15l/min. Welding was carried out with a laser-leading hybrid configuration, a process separation of 1-1.5mm and the laser orientated perpendicular to the material surface.
Weld quality and performance assessment
All the welds produced were visually checked for surface defects. Selected welds were sectioned to check the weld profile and penetration. Micro-hardness testing was carried with a load of 100g.
Experimental Results and Discussion
Autogenous laser welding of TRIP to TRIP steel
Initially, bead-on-plate melting runs were carried out to develop welding parameters for achieving fully penetrating welds with welding speed being adjusted for a specific laser power used. These conditions were then refined for closely fitted butt joints to achieve the maximum welding speed for achieving full penetration. The best results, in terms of full penetration and good visual appearance, were achieved when the laser beam was focused on the workpiece surface, the maximum speed achieved was 7m/min at 4kW and 5m/min at 3kW laser power.
Figure 1 shows the appearance of a typical weld for TRIP to TRIP steel. The welds exhibited a clean and smooth topbead and underbead and the underbead showed traces of spatter in regions adjacent to the weld metal(Figure 1b). Welds produced with a lower laser power (3kW) and slower speed (5m/min) exhibited a similar visual appearance.
Fig.1. Appearance of a typical TRIP to TRIP steel weld produced with 4kW laser power
and 7m/min travel speed:
a) Weld topbead
b) Weld underbead
Cross sections of typical welds produced are shown in Figure 2. No cracking or large pores were found on the cross sections. The weld was slightly wider and exhibited a wider HAZ at the slower rate (Figure 2b).
Fig.2. Cross sections of laser welds in TRIP to TRIP steel: a) 4kW laser power at 7m/min
b) 3kW laser power at 5m/min
The weld exhibited excessive hardening in the weld metal and HAZ compared with that of the parent material. The hardness in the weld metal was about 500HV whereas the hardness in the HAZ was around 600HV (see Figure 3). Theweld hardness was not affected by welding speed.
Fig.3. Typical hardness distribution in laser welds in TRIP to TRIP steel (example shown produced with 4kW laser power at 7m/min). (Hardness measured at centre of sheet thickness, broken lines indicate width of fused zone)
Figure 4 shows that the microstructure of the weld metal was typical needle type martensite. The region of peak hardness adjacent to the fusion zone had finer martensitic structures (see Figure 4b). Similarmicrostructures were observed in the weld produced at a slower speed (5m/min).
Fig.4. Microstructure of the weld produced with 4kW laser power and 7m/min travel speed:
a) Weld metal
b) HAZ near centre of sheet (fine martensitic zone indicated)
Autogenous laser welding of TRIP to low carbon steel
The appearance of welds in TRIP to LC steel as the weld was similar to those in TRIP to TRIP steel. The cross sections of the welds made at different conditions are shown in Fig.5. Again, no large pores or cracks were foundon the cross sections. The hardness of the weld was much lower than that of the weld in TRIP to TRIP steel (see Fig.6.) due to the reduction of carbon content of the weld metal by the LC steel. However, hardness of the HAZ inthe TRIP steel side was the same as that of the weld in TRIP to TRIP steel due to the fact that change of the weld metal composition does not affect the composition of the HAZ and the thermal effects on parent material was the same.
Fig.5. Cross sections of laser welds in TRIP to LC steel produced with different laser powers and travel speeds:
a) 4kW laser power at 7m/min
b) 3kW laser power at 5m/min
Fig.6. Typical hardness distribution in laser welds in TRIP to LC steel (example shown produced with 4kW laser power at 7m/min). (Hardness measured at centre of sheet thickness, broken lines indicate width of fusedzone)
Figure 7 shows the microstructure of a weld in TRIP to LC steel. Microstructures in the weld metal were also martensite but exhibited a plate-like morphology (Fig.7a) which is typical in low C steels.Microstructures in the HAZ of the TRIP steel were also finer martensite (Fig.7b), the same as that in the weld in TRIP to TRIP.
Fig.7. Microstructures of a typical weld in TRIP to LC steel produced with 4kW laser power and 7m/min travel speed:
a) Fusion zone near LC steel side
b) HAZ near TRIP steel side
Hybrid Laser-MAG Welding of TRIP to TRIP Steel
Hybrid laser-MAG welding was carried out in TRIP steel to examine the effect of hybrid welding on the weld hardening behaviour of TRIP steel.
Process parameters (process separation, hybrid configuration, welding speed, arc parameters) were adjusted to provide fast welding speed and process stability, with good visual appearance and weld surface profile. The best resultswere achieved with 4kW laser power and arc settings of 30V, 196A and a welding speed of 6m/min.
At this condition welds did not exhibit undercut or spatter. The weld was wider than the autogenous weld (see Fig.8,.). Hardness in the fusion zone was around 400-500HV (Figure 9), lower than that in the autogenousweld (around 500HV). Hardness in the HAZ was between 500 and 550HV, similar to that in the autogenous laser weld. Microstructures of the weld were similar to the autogenous weld between TRIP and LC steel.
Fig.8. Cross section of a typical weld in TRIP to TRIP steel produced using the hybrid laser-MAG welding condition detailed in the text
Fig.9. Micro-hardness of a typical weld in TRIP to TRIP steel produced using hybrid laser-MAG welding using the condition detailed in the text. (Hardness measured at centre of sheet thickness, broken lines indicate width of fused zone)
Conclusions
- Fully penetrating butt welds with good weld profile could be achieved at 5-7m/min, with 4kW of laser power and a 0.6mm diameter laser spot, in 1.5mm thick TRIP steel.
- The autogenous laser welds exhibited significant hardening in the weld metal and the HAZ compared to the parent material. Hardness in the fusion zone was around 500HV. The maximum hardness was up to 600HV in the HAZ. The weld hardness could be reduced when TRIP steel was welded to LC steel.
- The weld hardening in TRIP steel could also be reduced using hybrid laser-MAG welding. The weld hardness in the fusion zone was reduced to around 400HV due to the carbon reduction in the weld metal. The hardness in the HAZ was similar to that for autogenous laser welding.
