Better and better....resistance spot weldability progress made in car, truck and bus applications

TWI Bulletin, September - October 2006

This year's Richard Weck paper is authored by the award-winning resistance welding specialists Steve Westgate and Steve Shi.

Steve Westgate is a consultant in resistance welding in the Laser and Sheet Processes Group at TWI. He gained an Honours degree in industrial metallurgy at the University of Birmingham. Since joining TWI in 1972, Steve has had involvement in and managed, a wide range of research projects in all resistance welding processes, including European and group funded projects. He has produced numerous reports and papers both nationally and internationally. Steve also serves on British Standards committees and has made contributions to standard textbooks.

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.

Weight reduction is the main driving force behind material selection for transportation applications. With 75% of vehicle fuel consumption directly related to vehicle weight, lightweight material technology has been identified as critical in reducing fuel consumption.

Over the past 20 years, there has been a substantial effort to reduce vehicle body weight by the use of high strength (HS) steels (up to 600N/mm ² tensile strength) and aluminium alloys. Even higher strength steels (with tensile strengths between 600 and 1500N/mm ² ) have now been developed and are beginning to be used, particularly in components where there is a high demand on safety performance. The steels of interest, generally referred to as ultra-high strength (UHS) steels, include dual phase (DP), transformation induced plasticity (TRIP), complex phase (CP), martensitic and boron-alloyed steels. The relatively high carbon equivalent of these steels, coupled with the fast weld cooling rates observed, particularly with resistance spot welding, leads to high hardness levels and brittleness of the weld.

Resistance spot welding is the main joining process in the automotive industry. Consequently, development of suitable welding schedules, to achieve the required joint mechanical properties, has become the key issue for maximising the benefits of using these materials. Previous TWI work demonstrated good weldability of thin gauge UHS steels but indicated that enhanced resistance spot welding schedules were needed to achieve the required fracture mode and improved mechanical properties in thick sheet UHS steels.

There are a number of options for enhanced resistance spot welding schedules. Pulsed welding and longer weld times aim to influence weld cooling rates, and thus reduce weld hardening. These would be preferred approaches, with minimum increase in weld sequence duration. In-process tempering may be necessary for the more difficult to weld steels, although a much longer sequence time is required. Here, the welding pulse is followed by a cool time to allow the weld to harden fully, and a temper current pulse reheats the weld to temper the hardened structure. This proved effective for the thin (1.0mm) TRIP steel, where weld hardness was reduced from about 600HV to about 400HV, and the welds exhibited full plug failure on testing.

In thicker, stronger steels, production factors such as part fit-up, gaps and slight electrode misalignment would be more likely to affect weld growth, particularly at the start of the weld. Pulsed welding schedules, particularly with preheat or current upslope are also likely to be of benefit under these conditions, although variable fit-up was not specifically examined in this work. The high electrode forces suggested for the higher strength steels also takes account of the potentially greater fit-up problems.

Objectives

• To develop enhanced resistance spot welding schedules for UHS steel sheet in the thickness range 1.6-2.0mm.
  
  
• To test dissimilar material/gauge combinations on the weldability and joint performance of UHS steels.

Materials

Resistance spot welding trials were performed using 1.65mm thick TRIP700, 2.0mm thick DP800 and 1.8mm thick CP1000 zinc-coated steel sheets. The material designations represent their minimum UTS (N/mm ² ). A 2.0mm thick zinc-coated low carbon steel was used in some of the dissimilar material combinations. DP steels are mainly based on the simple Fe-Si-Mn-Cr-C system (a typical composition is: 0.1-0.2%C, 1.5-1.8%Mn, 0.1-0.4% Si and 0.02-0.4%Cr).

Strengthening in DP steels is achieved by a combination of grain refinement and formation of hard martensite. Complex phase (CP) steels are similar but additionally have small quantities of niobium or titanium to form fine strengthening precipitates. TRIP steels have the exceptional combination of high strength and ductility. The microstructure in TRIP steels is composed of ferrite, bainite, martensite and a large volume fraction of retained austenite. A typical composition of a TRIP700 (UTS 700N/mm ² ) steel is: 0.3%C, 1.5%Mn, 0.3%Si and 1.2%Al. All the steels examined had a carbon equivalent of about 0.57.

Experimental approach

Enhanced resistance spot welding schedules were developed for thick sheet TRIP700, DP800 and CP1000 UHS steels using pulsed currents and welding with in-process tempering. Examples of the schedules used are shown in Fig.1.

The procedure incorporating in-process tempering comprised the baseline welding schedule Fig.1a followed by a relatively long cool time to quench the nugget, and the application of the tempering current (for a pre-set temper time), as shown in Fig.1b. In this way, resistance weld tempering was accomplished without the need for additional equipment. A number of alternative pulsed schedules such as that shown in Fig.1c were examined with preheating or post-heating (controlled cooling), in which a low current pulse was introduced before or after the main welding pulses. The effect of weld carbon reduction (dilution) has also been examined when welding the TRIP700 to lower carbon grades.

Fig.1. Examples of welding schedules used in the work 

a) Baseline schedule

b) Resistance spot welding with in-process tempering

c) Pulsed resistance spot welding with preheating

Weld growth curves were produced with various welding parameters to assess the weldability of the steels used and establish modes of failure on chisel testing. Weld hardness was measured to assess the hardening behaviour in the weld and heat affected zone (HAZ). Static cross-tension and shear tests were carried out to establish the effect of weld nugget size, weld hardening behaviour and parent material strength on the static properties of welds. The effect of process parameters on weldability and weld static mechanical properties was examined.

Results and discussion

Welding trials with in-process tempering were conducted with different cool and temper times using a fixed electrode force, welding current and weld time. Tempering current was initially adjusted to give reasonable results then controlled at nominally 6.5 to 7.0kA. Figure 2 shows that a minimum cool time was required to allow transformation to martensite and then an appropriate combination of temper time and temper current provided sufficient heat to temper the hardened structure. The appearance of a selected weld section is shown in Fig.3a. The general appearance of the weld section was similar to that produced using the baseline schedule ( Fig.3b). In-process tempering was effective in reducing the weld hardening and brittleness in 1.65mm TRIP700 steel. The outer part of weld nugget and HAZ were softened (see Fig.4), allowing the weld to exhibit full or virtually full plug failures, rather than interface failures, in welds produced using the baseline schedule. Figure 5 illustrates the improvement in fracture mode that could be achieved. The weld hardness was reduced from almost 600HV to less than 400HV while the weld cross-tension strength was improved, and almost doubled at the best condition. There were no apparent cracks inside the weld nugget when examining the section in the unetched and lightly etched conditions. The increased sequence time of up to 1.4s, would however, have a bearing on productivity.The appropriate temper schedule appeared to be affected by the welding pulse used and may also be affected by the electrode cooling conditions. Furthermore, excessive heating in the temper pulse itself may lead to retransformation of the nugget microstructure, and the weld could reharden.

Fig.2. Effect of cool time between welding and tempering pulses on the weld diameter and fracture mode of welds in 1.65mm TRIP700, welded with 5kN electrode force, 20 cycles weld time, nominally 10.5kA welding current,50 cycles temper time and 6.7kA tempering current

Fig.3. Sections of spot welds in 1.65mm TRIP700, welded with 5kN electrode force, 20 cycles weld time, 10.5kA welding current (scale in mm):

a) With in-process tempering (50 cycles cool time, 50 cycles temper time and 7.2kA tempering current);

b) Baseline schedule (no temper pulse)

Fig.4. Effect of in-process tempering on the hardness of the welds in 1.65mm TRIP700 welded with 5kN electrode force, 20 cycles weld time, 10.5kA welding current. Tempering conditions - 50 cycles cool time, 50 cycles temper time and 6.7kA tempering current

Fig.5. Effect of cool time on fracture appearance for chisel tested welds in 1.65mm TRIP700, welded with 5kN electrode force, 20 cycles weld time and nominally 10.4kA welding current, 50 cycles temper time and nominally6.7kA tempering current (scale in mm):

a) 30 cycles cool time;

b) 40 cycles cool time;

c) 50 cycles cool time

As these high carbon UHS steels are most likely to be welded to other grades in production, the effect of weld nugget dilution when welding to lower carbon grades was examined. A baseline schedule with preheat gave good results,with little increase in sequence time, when welding the TRIP700 to either DP800 or a low carbon steel. Acceptable plug failures were achieved, and some improvement in cross-tension strength was made (see Fig.6) but there was inevitably no reduction in HAZ hardness in the TRIP700.

Fig.6. Comparison of the cross-tension and shear strength of welds between TRIP700 and TRIP700 (interface failure mode), and between TRIP700 and DP800 (plug failure mode), welded with 5kN electrode force, 20 cycles weldtime, 10.9kA current for TRIP700 to TRIP700 and 10.5kA for TRIP700 to DP800

Pulsed schedules with preheat, but without a specific in-process temper, were beneficial for the DP800, but full plug failure appeared difficult to achieve reliably. Weld hardness remained high with the pulsed schedules (400 to450HV) and there was little change in cross-tension strength with pulse parameters, the nugget diameter having the main influence.

The CP1000 responded best to the temper treatment, but the sequence time required to temper the weld reliably was almost three seconds longer than the baseline schedule. In addition, although the fracture mode was improved, the cross-tension strength remained at only about 30% of the shear strength. Furthermore, there appeared to be an additional risk of overheating and rehardening the weld with such schedules. This may make it unreliable for production application in this case.

As there remained a risk of partial plug failures with the UHS steels studied, it is necessary to be aware of the potential limitations in cross-tension strength when considering application of such materials. When considering the variability of production conditions, it is likely that the schedules investigated may be appropriate for improving fracture mode, but that improvement in cross-tension strength could not be guaranteed.

Conclusions

• In-process tempering was effective in reducing weld hardness and achieving plug failures in 1.65mm thick TRIP700. Cross-tension strength was improved but only over a narrow range of settings.
• Plug failure could be achieved when welding 1.65mm thick TRIP700 to lower carbon steels (DP800 and LCS) with a simple pulse schedule, using a single current pulse with preheat.
• The weld fracture mode and weld performance in 2.0mm thick DP800 could be improved by pulsed welding schedules, with an increase in weld sequence time of only about 0.2s.
• Weld hardness was reduced and plug failures achieved, in 1.8mm CP1000 steel, by using in-process tempering, but only over a narrow range of settings. Sequence time was much increased and only a small increase in cross-tension strength was achieved.

Recommendations

The work has demonstrated that enhanced welding schedules are necessary to weld thick sheet TRIP, DP and CP steels to meet standard automotive requirements. The following recommendations are made for welding schedules when using 8mmtip diameter electrodes.

TRIP700

In-process tempering of resistance spot welds is recommended as the best method of reducing weld and HAZ hardness, and obtaining plug failures. Suitable parameters for 1.65mm material were:

• 5kN electrode force.
• 25 cycles weld time, 50 cycles cool time and 20-30 cycles temper time
• 10.5kA welding current, 6.7kA tempering current.

The welding schedule will be dependent on the material thickness combination used and needs to be set up carefully to achieve reliable results.

TRIP700 to lower carbon steels

Schedules should be chosen on a case by case basis. Depending on the steels used and the thickness combination, welds may exhibit the desired plug fracture mode on testing, without the need for modified schedules.

DP800

The best results in terms of weldability, weld fracture mode and static properties, could be achieved in 2.0mm thick DP800 steel using the following pulsed welding schedules:

• 6kN electrode force.
• One pulse of 10 cycles at 6.4kA and two pulses of 20 cycles at 10kA current. Two cycles cool time between the two pulses.

CP 1000

In-process tempering is also suitable for reducing hardness and achieving virtually full plug fracture mode in 1.8mm CP1000. A suitable schedule is:

• 6kN electrode force.
• 25 cycles weld time, 70-90 cycles cool time and, 40 cycles temper time.
• 10.4kA welding current, 6.8kA tempering current.

However, this represents a sequence time of almost three seconds and little improvement in cross-tension strength is obtained.