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Techniques for improving the weldability of trip steel using resistance spot welding

G Shi and S A Westgate

Paper presented at 1st International Conference Super-High Strength Steels - Rome, Italy. 2-4 November 2005.

Abstract

High strength steels (UTS >600N/mm 2 ) 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. TRIP (Transformation Induced Plasticity)steels have become of considerable interest in recent years, because of their exceptional combination of high strength and ductility. In the work reported here, advanced resistance spot welding schedules were developed to achieveacceptable welds with improved static mechanical properties in thin sheet (1.05mm thick) 700N/mm 2 tensile strength TRIP steel. Improved resistance spot welding schedules were developed using up-slope and post heating current to reduce the cooling rate, or in-process tempering to reduce the hardness of theweld produced. Resistance spot welding of dissimilar steels was also carried out to examine the benefit of weld carbon reduction (i.e. reducing the carbon content of the weld nugget). The effects of material combination and processparameters on hardening, fracture mode and static mechanical properties of the joints (cross-tension and shear) were determined.

Introduction

TRIP (Transformation Induced Plasticity) steels have become of considerable interest in the automotive industry in recent years because of their exceptional combination of high strength and ductility. Resistance spot welding is themain joining method for sheet steels in the automotive industry. The high carbon content of TRIP steels, coupled with fast weld cooling rates associated with these joining processes, leads to high hardness levels (up to 600HV) in theweld. When such welds are submitted to shear stress, a high joint strength can be achieved. However, when welds are submitted to peel or tension stress, the interface between the sheets acts as a notch and, due to the very hard weldnugget, brittle interface fracture is normally observed. [1-3] As a direct consequence, the strength of the weld decreases.

Modified resistance spot welding schedules, such as long weld times, to control cooling rates, and post-weld tempering [2,4-6] , have been suggested to reduce weld brittleness in some high strength steels. These approaches are intended to reduce the cooling rate after welding or to temper the weld, so that a more ductile microstructure is achieved inthe weld. When spot welding dissimilar steels, the weld metal carbon level is roughly the average of that of the two materials. Hence by welding a lower carbon steel to the relatively high carbon TRIP steel, a reduction in theproportion of martensite and its hardness in the weld nugget should reduce the hardness and brittleness of the weld itself.

In the work described here, modified resistance spot welding schedules were developed to achieve improved static properties in TRIP700 steel. The effect of process parameters and material combinations on the welding range and weldproperties were examined.

1. Experimental work

1.1 Materials

Resistance spot welding trials were performed on 1.05mm thick electroplated zinc-coated (EZ) TRIP700 steel sheets. A 0.8mm thick hot dip zinc-coated (HDG) DP600 (dual phase) steel sheet was used for the dissimilar materialcombination. The chemical compositions of these steels are shown in Table 1.

Table 1 Chemical compositions (weight %) of the materials used

Steels C Si Mn S P Cr Ni Al B
TRIP700 0.31 0.29 1.54 0.004 0.015 0.023 0.021 1.100 0.0004
DP600 0.12 0.19 1.58 0.003 0.011 0.450 0.009 0.047 0.0005
TWI analysis ref. S/01/356 and S/02/50

 

1.2 Equipment and set-up

Resistance spot welding trials were conducted on a single phase AC (100kVA) British Federal projection-welding machine with a 30kA short circuit current and a 10kN electrode force capacity. Welding current was recorded with aMiyachi printer current meter, type MM336A. Cu/Cr/Zr electrodes were to the standard ISO 5821 type B with a 6mm flat tip.

1.3 Resistance spot welding schedules

Welding trials were carried out developing the following modified welding schedules to achieve the required weld nugget size, fracture mode and plug size in the TRIP700 steel:

  • Resistance spot welding with controlled cooling, using up-slope and post-heating current (immediately after the weld time), to reduce the cooling rate and reduce the amount of martensite in the weld.
  • Resistance spot welding with in-process tempering where the weld time is followed by a cool time to allow the transformation to martensite. Further resistance heating during the temper time then tempers the martensite to reduce its hardness.
  • Resistance spot welding dissimilar materials (TRIP700 and DP600), to examine the influence of weld carbon reduction, i.e. reducing the carbon content of the weld nugget.

Squeeze time was set at 60 cycles (1.2s) to ensure full achievement of the welding force. Weld time was set at 14 cycles, and hold time was set at ten cycles throughout. The applied force was 4kN for all the trials. The weldingparameters for the steels tested are shown in Table 2.

Weld growth curves were produced by pre-setting the electrode force and weld time, and then making welds at progressively increased current levels. The range of currents used was sufficient to define the limits of minimum weld sizeand weld splash. The minimum acceptable weld size was taken as 4 √t (where t is the thickness in mm of the thinnest sheet in the combination). The effect of weld size on the cross-tension and shear strength of the weld wasexamined.

Table 2 Resistance spot welding parameters for TRIP700

Spot welding schedule Electrode force, kN Up-slope time, cycles Welding time, cycles Cool time, cycles Post-heat time, cycles
Baseline schedule 4 N/A 12 N/A N/A
Baseline schedule with controlled cooling 4 10 12 N/A 5-50
Baseline schedule with in-process tempering 4 N/A 12 5-20 5-80
Welding of TRIP700 to DP600 with baseline schedule 4 N/A 12 N/A N/A
N/A - not applicable

 

1.4 Static mechanical properties and hardness of welds

Cross-tension and shear tests were conducted to examine the static properties of the spot welded joints. Hardness distributions for selected welds were examined at a load of 1kg.

2. Experimental results

2.1 Resistance spot welding with controlled cooling

Controlled cooling methods are intended to permit some transformation of the austenite before the martensite start temperature, to reduce the proportion of martensite in the weld. It was found that interface and partial plugfailures were dominant in the welds, as shown in Fig.1. The preferred full plug failures could not be achieved using the modified welding schedule with controlled cooling, although plugs of acceptable size could be produced at certain conditions (see Fig.1b).

Fig.1. Appearance of welds subjected to peel tests in 1.05mm EZ coated TRIP700, welded with 4kN electrode force, 12 cycles weld time and 6.7kA welding current (scale in mm):
a) Without controlled cooling - showing full interface failure, cracked on testing b) With controlled cooling of 10 cycles up-slope, 6.1kA post heating current and 30 cycles post heat time - showing partial plug failure
√t to be achieved. Figure 2 shows the effect of the post-heating current and post-heat time on the weld nugget diameter and plug diameter. The post-heat parameters had a more significant effect on the fracture mode (weld plug diameter) than onthe weld nugget diameter. Under constant post-heat times, the weld nugget diameter increased slightly with the increase in post-heating current, whereas a more significant increase in plug diameter was achieved ( Fig.2a), indicating improved fracture mode. However, the weld still exhibited a partial plug failure mode even when a post-heating current of the same level as the welding current was used, as shown in Fig.2a.
Fig.2. Effect of post heating current and post heating time on the weld diameter and fracture mode of welds in 1.05mm EZ coated TRIP700 welded with 4kN electrode force, 12 cycles weld time, 6.7kA welding current and 10 cycles up-slope:
a) Effect of post heating current at 20 cycles post heat time b) Effect of post heat time at 4.7kA post heating current
√t to the current resulting in the onset of splash, was adopted as the welding current range. The welds were produced using the baseline welding sequence. Thisindicated that a welding current range larger than 1kA above the minimum required size (4 √t) could be achieved with full plug fractures for the TRIP700 to DP600.
√t weld and a maximum size weld near splash are shown in Fig.11. The hardness within the weld nugget was about 500 to 550HV, which was only slightly lower than that of the weld in the TRIP steel welded using the baseline schedule (i.e. 550 to 600HV). The hardness decreased slightlywith the increase of welding current.
Fig.11. Cross sections of welds between 1.05mm EZ coated TRIP700 and HDG coated 0.8mm DP600, welded with 4kN electrode force and 12 cycles weld time (scale in mm):
a) 7.2kA welding current b) 8.2kA welding current
√t welds, the maximum size without weld splash and welds with splash. At each condition, three welds were tested and the peak failure load from each sample is shownin these figures.
Fig.12. Relationship between the weld static strength and weld nugget size in welds between EZ coated 1.05mm TRIP700 and HDG coated 0.8mm DP600:
a) Cross-tension b) Shear
√t welds but just in the DP600 (which showed greater deformation in peel than the TRIP700). This may be related to a softening of the material around the weld orgreater electrode indentation with the larger weld nugget sizes.

3. Discussion

3.1 The effect of cooling rate

The cooling rate of the weld, in the baseline resistance spot welding sequence, is very high due to the accelerated cooling effect of the water-cooled copper electrodes. It has been shown for resistance spot welding [7] that the time to cool over the temperature range 900 to 300°C could be as short as 0.2s, and only slightly longer if the hold time was short and the electrodes were quickly raised. The incubation time for the bainite nosein the calculated time-temperature-transformation (TTT) diagram of TRIP700 is longer than one minute [8] . This results in the formation of a martensite dominated microstructure in welds in TRIP steels. The results show high hardness and brittle welds are produced, which give interface failures and particularly low cross-tensionstrength.

3.1 Modified welding sequences

Longer weld times or the use of a lower current, post-heat pulse immediately following the weld pulse, are designed to reduce the cooling rate to prevent excessive martensite formation. This controlled cooling approach gave a slightbenefit, as the weld cross-tension and shear strengths could be improved by some 25-30%. However, it had little effect on the weld hardness and fracture mode using the selected parameters. Also, the improvement was probably partlyrelated to the slight increase in weld nugget size (about 10%).

The in-process tempering schedule was effective in reducing the brittleness of welds in TRIP700. Weld hardness was significantly reduced (from about 600HV to 400HV) and the cross-tension strength was increased by up to a factor offour, when using the conditions developed in this work. A cross-tension strength of up to 50% of the shear strength could be achieved, and the shear strength itself was improved by up to 25-30%. The weld fracture mode was changed fromthe brittle interface failures to the preferred full plug failures. However, it should be noted that the use of suitable cool time, tempering current and temper time was critical to achieve the best results in terms of weld strength,fracture mode and process efficiency. Insufficient cool time prevents the full transformation to martensite, allowing it to form on final cooling at the end of the temper pulse. Insufficient temper time or current gives insufficienthardness reduction of the martensite. However, too long a temper time or too high a current allows retransformation to austenite, and martensite will again form on final cooling.

It should be noted that these improvements were achieved by in-process tempering with an increase of the sequence time of each weld by up to 45 cycles (0.9s), depending on the welding parameters selected. Longer times may berequired for thicker material. Furthermore, in-process tempering schedules would need to be set up to give reliable results, depending on the material and thickness combinations being welded and the benefits achieved may be sensitiveto other production variables.

3.2 Weld carbon reduction

Weld carbon reduction, by introducing a low carbon steel interlayer, has proved effective in reducing the weld brittleness in some transformation hardened high strength steels [4] . Current work found that virtually full plug failure could be achieved over a reasonable welding range (1.2kA), when TRIP700 was resistance spot welded to DP600. This was achieved despite only a slight (10%) hardness reductionin the weld nugget, and no reduction of the HAZ hardness in the TRIP steel. The shear strength was slightly lower than that of the welds in TRIP700 to itself but similar to that of the welds in DP600 to DP600. Conversely, the crosstension strength was higher than that of the welds in TRIP700 to itself but lower than that of the welds in DP600 to DP600 [1] .

4. Conclusions

  • In-process tempering was effective in reducing the brittleness of spot welds in TRIP700. Weld hardness was reduced from about 600HV to about 400HV and the welds exhibited full plug failures. The cross-tension strength of welds was up to four times higher than that of welds produced without tempering and the shear strength could also be improved by 25-30%.
  • In-process tempering required an increase in sequence time of typically 0.9s was required for the schedule to be effective for the 1.05mm material, and the results may be sensitive to other process variables
  • Controlled cooling schedules, using up-slope and a post-weld current pulse, had limited effect on the reduction of brittleness of welds in TRIP700. Although cross-tension strength could be improved by 75% and shear strength was increased by 25%, compared to the baseline sequence, little reduction in weld hardness was achieved and full plug failures could not be reliably achieved.
  • Welds between TRIP700 and DP600 gave acceptable plug failures, despite little reduction in weld hardness. The shear strength of welds was similar to those in DP600 to itself but cross-tension strengths were substantially lower.

 

References

  1. G. Shi and S.A. Westgate, 'Resistance spot welding of high strength steels' Conference on Joining of Materials (JOM 11), Helsingor , Denmark. 25-28 May, 2003.
  2. L. Cretteur, A.L. Koruk and L. Tosal-martinez, 'Improvement of weldability of TRIP steels by use of in-process pre- and post-heat treatments', Proc. Int. Conf. on TRIP-Aided High Strength Ferrous Alloys, Ghent, Belgium, 19-21 June (2002), pp.353-358.
  3. L. Tosa-martinez, L. Cretteur, O. Moriau and A.L. Koruk, 'Characterisation of cold rolled TRIP steels for structural crash-relevant applications in the automotive industry', Proc. Conf. of SAE/IBEC Conference Paris, 9-11 July (2002).
  4. W. Peterson, 'Dilution of weld metal to eliminate interfacial fractures of spot welds in high and ultra-high strength steels', Paper presented at ICAWT 1997 International Conference, Advances in Welding Technology, Columbus, 17-19 September (1997), pp.331-346.
  5. A. Beevers and E.J. French, 'Post-weld heat treatment of spot welds in hardenable steels', BWRA Report A3/15/61.
  6. W. Chuko and J.E. Gould, 'Development of appropriate resistance spot welding practice for transformation-hardened steels', Sheet Metal Welding Conference IX, Michigan, 17-20 October, (2000).
  7. T. Nishi,'Evaluation of spot weldability of high strength sheet steels for automobile use', Nippon Steel Technical Report, No.20, December (1982).
  8. M. Eliason, L. Svensson, R. Johansson and J.K. Larsson, 'Improvement of mechanical properties in laser welded TRIP700 steel'. IIW Doc. IV-832-03 (2003).