by Steve Shi
Steve Shi is a principal project leader 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.
by Steve Westgate
Steve Westgate is a consultant in resistance welding in the Laser and Sheet Processes Group at TWI. Steve joined TWI in 1972 with a degree in industrial metallurgy and since then has had involvement in, and managed, a wide range of research projects in all resistance welding processes. Close contact is maintained with industry through an active consultancy, trouble shooting and training role. His experience also includes mechanical fastening systems, adhesives and hybrid techniques, largely related to sheet metal joining.
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 Part One of this paper, the award winning duet of Steve Westgate and Steve Shi report advanced resistance spot welding schedules were developed to achieve acceptable welds with improved static mechanical properties in 1.05mm 700N/mm 2 tensile strength TRIP steel sheet.
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 the weld produced. Resistance spot welding of dissimilar steels was also carried out to examine the benefit of weld carbon reduction ( ie reducing the carbon content of the weld nugget). The effects of material combination and process parameters on hardening, fracture mode and static mechanical properties of the joints (cross-tension and shear) were determined.
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. The high carbon content of TRIP steels, coupled with fast weld cooling rates associated with resistance spot welding, leads to high hardness levels (up to 600HV) in the weld. When such welds are subjected to shear stress, a high joint strength can be achieved. However, when welds are subjected to peel or tension stress, the interface between the sheets acts as a notch and brittle interface fracture is normally observed through the weld nugget, reducing weld strength.
Modified resistance spot welding schedules, such as long weld times, to control cooling rates, and post-weld tempering, 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 in the 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 the proportion 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 weld properties were examined.
Experimental work
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 material combination. 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 |
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 a Miyachi printer current meter, type MM336A. Cu/Cr/Zr electrodes were to the standard ISO 5821 type B with a 6mm flat tip.
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, ie 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 10 cycles throughout. The applied force was 4kN for all the trials. The welding parameters for the steels tested are shown in Table 2.
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 |
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 size and 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 was examined.
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.
Experimental results
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 plug failures 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).
a) Effect of post heating current at 20 cycles post heat time
b) Effect of post heat time at 4.7kA post heating current
In general, post-heating current and post-heat time had limited effect on the weld hardness as shown in Fig.3a and 3b. Slightly higher hardness values were noted at the longest and shortest post-heat times.
a) Effect of post heating current at 20 cycles post host heat time
The cross-tension and shear strengths of spot welds, produced with the same electrode force, welding current and weld time but with various post-heating current levels and post-heat times, are shown in Fig.4a and b. The data points in these figures show the individual failure loads from three samples produced at each welding condition.
Figure 4a shows the effect of the post-heating current on the cross-tension and shear failure loads for welds produced using the same parameters in the baseline welding sequence, plus ten cycles up-slope and 20 cycles post-heat time. The weld shear strength was about 35% higher and the weld cross-tension strength was about 75% higher than those of welds produced without post-heat, when a post-heating current of 5.3kA (80% of the welding current) was used.
Figure 4b shows the effect of the post-heat time on the cross-tension and shear failure load of the weld. The joint strength increased with increase in post-heat time up to 20 cycles, with a post-heating current of 6.1kA. Further increase in post-heat time gave no additional benefit.
a) Effect of post heating current at 20 cycles post host heat time
b) Effect of post heat time at 6.10kA post heating current
End of part one
Part two of Shi's and Westgate's paper examines resistance spot welding with in-process tempering.
