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Laser welding of ultra-high strength steels for automotive applications (April 2008)

   

Steve Shi and Steve Westgate
TWI Ltd, Cambridge, UK

Paper presented at PICALO 2008, held on 16-18 April, Beijing, P R China - Paper 306

'Laser welding of ultra-high strength steels for automotive applications,' paper 306, PICALO 2008 Proceedings. Copyright 2008, Laser Institute of America. All rights reserved. The Laser Institute of America disclaims any responsibility or liability resulting from the placement and use in the described manner.

Abstract

The work in this paper was concerned with the development of welding procedures for joining thin-sheet UHS steels, using fibre-delivered solid state lasers and establishing the strength and formability of the welds produced. Autogenous laser welding trials were conducted with three different fibre-delivered solid state lasers to achieve fully penetrating butt and lap welds in dual phase (DP), martensitic, transformation induced plasticity (TRIP), Usibor and boron steels of 0.8-1.5mm thickness and 600-1550N/mm2 tensile strength with similar and dissimilar material combinations. The hardening and softening behaviour in the fusion zone and the heat affected zone were determined. Influences of laser process parameters on welding speed, weld profile and joint performance were discussed.

Introduction

The increasing demand for improved fuel efficiency and reduced emissions has prompted the automotive industry to seek methods of reducing vehicle body weight by using materials with good strength to weight ratio, such as ultra-highstrength steels and aluminium alloys.[1-2] With 75% of vehicle fuel consumption directly related to vehicle weight, the potential benefits of weight reduction using ultra-high strength (UHS)steels have been well recognised. Laser welding of UHS steels, either for tailor-welded blanks (TWBs) or, more recently, for continuous or stitched body-in-white (BIW) applications, will form part of this strategy of reducing vehicle weight.[3] The work reported in this paper was concerned with the development of welding procedures for joining thin-sheet UHS steels, using fibre-delivered solid state lasers and establish the performance and formability of the welds produced. A range of UHS steels was studied, in butt and lap joint configuration, and in the thickness range from 0.8 to 1.5mm.

Experimental Work

Materials

Laser welding trials were carried out on the following steels of thicknesses between 0.8 and 1.5mm and minimum tensile strengths between 600 and 1500N/mm2, in butt and lap joint configurations:

  • 0.8mm thick zinc coated DP600.
  • 1.0mm thick zinc coated DP800.
  • 0.8mm thick uncoated DP1000.
  • 0.8mm thick martensitic uncoated 1200.
  • 1.5mm thick zinc coated TRIP700.
  • 1.5mm thick uncoated boron steel.
  • 1.2mm thick Usibor steel (Al/Si coated).
  • 1.5mm thick zinc coated low carbon steel.

Scope of work

Welding trials were carried out using the following approach, to establish the effects of material combinations and process parameters on hardness, formability and static mechanical properties of the laser welded joints.

  • Trials with different laser parameters to develop welding procedures for achieving fully penetrating welds in butt and lap joint configurations.
  • Trials on lap joints using 4kW laser power with different spot sizes, to establish the effect of laser spot diameter on the laser welding process and weld performance.
  • Trials on butt joints in steels using 4kW laser power with different spot sizes, and using the same spot diameter at different levels of laser power, to establish the influences of laser power and laser spot diameter on welding speed, weld profile and weld performance.

As a result of the particular need to reduce the weld hardening in TRIP and boron-alloyed steels (due to their higher carbon content compared with other UHS steels), these steels were also welded to a low carbon steel. This was to examine the influence of reduction in weld metal carbon content on the weld performance. These steels represent the available UHS steel types for automotive applications. The low carbon steel was only used in some of the trials with dissimilar material combinations. The above steels were chosen to be representative of the thickness range appropriate to automotive applications.

Laser equipment used

Laser welding was carried out using three different fibre-delivered solid state lasers: a 4kW continuous wave (CW) Nd:YAG laser, a 5kW Yb fibre laser and a 7kW Yb fibre laser.

The Nd:YAG laser used was a Trumpf HL4006D, 4kW, lamp-pumped laser, its power fed to the workpiece via an optic fibre of 0.6mm diameter. A standard Trumpf optical assembly, including a 200mm focal length lens, was used to generate alaser spot of minimum diameter 0.6mm on the surface of the workpiece.

The first Yb fibre laser used was an IPG Yb fibre laser, capable of producing 7kW of output power. The output laser power was transmitted into the processing head using a single optical fibre of 300µm diameter. Focussing lensesof 250mm and 160mm focal length were used to produce a minimum spot diameter of 0.6mm and 0.4mm, respectively.

The second fibre laser used was an IPG YLR5000 Yb fibre laser, capable of producing a maximum output power of 5kW. The output laser power was transmitted to the processing head using a single optical fibre of 100µm in diameter. Focussing lenses with 500mm and 160mm focal lengths were used to produce minimum spot diameters of approximately 0.4mm and 0.2mm, respectively.

A Kawasaki JS6 6-axis articulated robot was used to move the welding head over the workpiece. A CNC controlled X-Y table was also used to move the sample below the fixed welding head.

Laser welding trials

Laser welding trials were carried out in both butt and lap joint configurations. The lap joints had an overlap of 50mm. Uncoated steels were clamped in close contact, while the zinc coated steels were clamped with spacing shims to provide a 0.2mm pre-set gap. The butt joints were simply clamped with the edges in close contact.

The trials were carried out with the laser beam focussed on the workpiece surface for both joint configurations. Welding speed was adjusted to achieve fully penetrating welds for each laser power/spot size combination.

No shielding gas was used in this work on the top of the weld. A high-pressure air knife was used to protect the laser optics from weld spatter.

Weld quality assessment

All the welds produced were visually checked for any surface defects. Selected welds were sectioned to check the weld profile and penetration. Transverse shear tests were carried out to establish the strength of lap welds and tensile tests were conducted to determine the strength of butt welds. Erichsen cupping tests were carried out to provide a simple formability test of selected butt welds.

Experimental results and discussion

Laser welding of lap joints

Figure 1 shows the maximum welding speed at which full penetration welds could be achieved, when using the three different sizes of laser spot for five different steels, ranging in thickness from 0.8 to 1.5mm. The effect of increasing the laser power density can be easily seen, regardless of material composition or thickness. For example, fully penetrating welds could be achieved in these steels at a welding speed between 2.5 and 6.5m/min, using the 0.6mmdiameter spot and 4kW laser power, whereas fully penetrating welds were achieved at over 20m/min when the 0.2mm diameter spot was used.

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Fig.1. Influence of laser spot diameter on welding speed for achieving fully penetrating lap welds with 4kW laser power in different steels (similar thickness combinations)

Changes in laser spot size produced significant effects on the weld profile. The weld became narrower and the size of the HAZ was reduced when a smaller laser spot was used. Figure 2 show cross sections of welds produced using 4kW of laser power but with three different sizes of laser spot for Usibor steel. Although much higher welding speed was achieved with the 0.2mm spot size, the weld width was only about 0.3mm. Similar results were achieved in other steels.

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spssapr08f2b.jpg
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Fig.2. Cross sections of typical lap welds produced with 4kW laser power and different sizes of laser spot in 1.2mm thick Usibor steel. Note that this steel has a special aluminised coating:

a) 0.6mm diameter spot, 3m/min welding speed;

b) 0.4mm diameter spot, 4m/min welding speed;

c) 0.2mm diameter spot, 17m/min welding speed 

Figure 3 shows the influence of laser spot size on the hardness profile of lap welds in the Usibor steel. The maximum hardness in the weld fusion zone was slightly increased when the welding was carried out with a smaller laser spot in both steels. The weld hardness was increased from 450HV to about 550HV when the laser spot size was reduced from 0.6 to 0.2mm diameter, for example. All the welds exhibited a softened HAZ, even the weld produced at17m/min, using the 0.2mm diameter spot. However, the HAZ became narrower when the welding was carried out with a smaller laser spot and higher speed.

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Fig.3. Influence of laser spot diameter on the hardness of lap welds produced with 4kW of laser power in 1.2mm thick Usibor steel

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Fig.4. Influence of laser spot diameter and laser power on the shear load and weld width of lap welds in 1.2mm thickness Usibor steel

Changes in laser spot size and laser power exhibited limited influence on the shear load of the welds, compared with their effect on the welding speed achieved. Figures 4 and 5 show the maximum shear load the weld could bear prior to failure in two steels. The welds tested were produced with different laser power densities (achieved with different combinations of laser power and laser spot size). In these figures, the interface weld width is also included for comparison. The three data points for each welding condition were measured from specimens taken in regions near the weld start, in the middle and near the weld end. The shear load was slightly reduced and the interface weld width decreased when a smaller diameter laser spot was used.

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Fig.5. Influence of laser spot diameter and laser power on the shear load and weld width of lap welds in 0.8mm thick DP1000 steel 

Triple layer overlap

Figure 6 shows cross sections of typical triple layer overlap welds in the DP600 and Mart1200 steels, produced with different parameters. Fully penetrating welds, free from surface imperfections could be achieved at3-20m/min welding speed depending on the laser parameters.

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Fig.6. Cross sections of triple layer overlapped welds produced with 4kW laser power and different sizes of laser spot, in different steels: a) Zinc-coated 0.8mm thick DP600, 0.6mm diameter spot, 3m/min welding speed (0.2mm preset gap); b) Uncoated 0.8mm thick DP1000, 0.2mm diameter spot, 20m/min welding speed 

Laser welding of butt joints

Welding speed and weld profile: Figure 7 shows the effect of laser spot diameter on the welding speed achieved for fully penetrating butt welds in different steels. The results were achieved with 4kW of laser power and two different spot sizes.

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Fig.7. Influence of laser spot size on the welding speed achieved with 4kW of laser power in different steels (similar thickness combinations) 

The welding speed was significantly increased, due to a change of laser power density. Similar results were also achieved with a 0.6mm diameter spot and two different levels of laser power (4 and 7kW). However, increase in laser power was more effective than the use of a smaller laser spot, in terms of the maximum welding speed achieved.

spssapr08f8a.jpg
spssapr08f8b.jpg

Fig.8. Cross sections of typical butt welds produced with 4kW of laser power and a 0.6mm diameter spot: a) 1.2mm thick Usibor steel, 6m/min welding speed; b) 1.5mm thick TRIP700, 5m/min welding speed; 

Figure 8 shows the cross sections of typical butt welds in different steels. Cross sections of welds with dissimilar material combinations are shown in Fig.9. All these welds were produced using 4kW of laser power and a 0.6mm diameter spot. The welds exhibited acceptable profiles. No cracks or large pores were found in the cross sections. Welds made with dissimilar material or gauge combinations showed smooth transitions.

spssapr08f9a.jpg
spssapr08f9b.jpg

Fig.9. Cross sections of typical butt welds with dissimilar material combinations produced with 4kW of laser power and a 0.6mm diameter spot: a) 1.2mm thick Usibor steel to 1.5mm TRIP700, 5m/min welding speed;b) 1.5mm thick TRIP700 to 1.0mm DP800, 5m/min welding speed


Weld strength: Results for the tensile strength of butt welds in different steels are summarised in Fig.10. These welds were all produced with 4kW of laser power and a 0.6mm diameter laser spot. Three specimens were taken from each weld at regions near the weld start, at the weld centre and near the weld end. The tensile strength of each parent material is also included in this figure for comparison.

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Fig.10. Tensile strength of butt welds in a range of steels produced with 4kW of laser power and a 0.6mm diameter spot 

It is clear from these results that the weld strength achieved was dependent on the parent material. Failures occurred in the parent material in steels with a tensile strength up to 1000N/mm2. Weld strength was lower than that of the parent material in steels above 1000N/mm2 and the welds failed from the HAZ of the weld in these steels, such as in the Usibor and boron steel.

Weld formability and hardness: Erichsen cupping tests on butt welds were conducted to examine the effect of process parameters, parent material strength and steel type, on formability. The test results are shown inFig.11, which presents the actual displacement to failure measured during the tests, as a function of the different weld combinations.

spssapr08f11.gif

Fig.11. Formability of butt welds in different steels produced with 4kW of laser power and a 0.6mm diameter spot. Note that the material thickness is different 

There was a general trend in the results in that the formability of the weld decreased with the increase in parent material strength. Of the UHS steels tested, welds in DP600 exhibited the best formability. Similar welds in theDP1000, Usibor and boron steel showed the least level of formability in the Erichsen cupping tests. The formability of welds in TRIP700, Usibor and boron steel was improved when they were welded to the low carbon steel. For these dissimilar material combinations, most of the deformation and final failure occurred in the parent material of the weaker LC steel.

Conclusions

Welded butt joints were as strong as the parent material in steels such as DP600, TRIP700 and DP1000 with strengths up to 1000N/mm2. Welded joints were slightly weaker than the parent material in the1500N/mm2 strength boron-alloyed steels.

Fully penetrating lap welds could be achieved in UHS steels of 0.8-1.5mm thickness at 2.5-6.5m/min with 4kW of laser power and a 0.6mm diameter laser spot. The welding speed could be increased by 40% and 200-700% respectively, depending on the thickness of steels, when 0.4 and 0.2mm diameter laser spots were used.

Fully penetrating butt welds could be achieved in these steels at 5-8m/min, with 4kW of laser power and a 0.6mm diameter laser spot, depending on the thickness of the material. The welding speed could be increased to 10-17m/min for the same laser power when a 0.4mm diameter laser spot was used.

As expected, the formability of butt welds decreased with an increase in the parent material strength. Of the UHS steels tested, welds in DP600 exhibited the highest, and welds in the boron-alloyed steels showed the lowest levels of formability. The formability of welds in TRIP700, Usibor and boron steel, could be improved when these steels were welded to a lower carbon steel.

References

  1. Haglund P, Molin B and Olsson K. (1994) Extra-high strength dual phase auto steels for safety components, Steel Times 3, 101-105.
  2. Flaxa V and Shaw J (2002) Materials applications in ULSAB-AVC, in Proc. Conf. of SAE/IBEC Conference Paris 2002, Paris, 9-11 July 2002.
  3. Belforte D (2002) Tailor blank welding in Europe, Industrial Laser Solutions, 2, 6-13.
  4. Irving B (2000) Lasers continue to penetrate automotive production lines, Welding Journal, 6, 33-37.

Meet the authors

Steve Shi is a principal project leader in Laser & Sheet Processes Group of TWI. Steve Westgate is a consultant - resistance welding, in the Laser & Sheet Processes Group of TWI.

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