The resistance spot welding of high and ultra-high strength steels

TWI Ltd, UK

Paper presented at the 3rd International Seminar on Advances in Resistance Welding, Berlin 16-17 November 2004.

Abstract

Over the past 20 years, there has been a substantial effort to reduce vehicle body weight by the use of high strength, and more recently ultra-high strength steels, with tensile strengths up to 1500N/mm ² . Some of these materials present challenges to resistance spot welding, still the primary joining process for sheet steels.

This paper covers the main issues of resistance spot welding these materials, including weldability, hardenability and joint properties. The changes in welding procedures needed to weld the materials successfully will be discussed, together with the implications for the equipment used.

1. Introduction

Weight reduction and safety improvements have been the main driving forces behind material selection for transportation applications. 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. However, total vehicle weight has gradually increased as a result of greater demand for safety, comfort and accessories. Consequently, even higher strength steels (with tensile strengths between 600 and 1500N/mm ² ) have been developed and are beginning to be used, particularly in components where there is a high demand on safety performance. ^[1-3] Examples include bumper components, and fabricated posts and rails in the body structure. The steels used have high strength, coupled with reasonable formability (sometimes achieved by hot forming) and allow safe design with thinner gauges compared to the lower strength grades. The steels of interest include dual phase (DP), transformation induced plasticity (TRIP), martensitic and boron alloyed steels, and may be referred to generally as ultra high strength steels (UHS).

Resistance spot welding is the main joining method currently used in the automotive industry. Laser welding is also used, as is MIG/MAG for welding and brazing. The main area of concern when welding HS and UHS steels stems from their relatively high carbon equivalent, coupled with the fast weld cooling rates observed, particularly with resistance spot and laser welding. This can cause high hardness levels and brittleness of the weld, leading to unfavourable fracture modes (partial or complete interface failures) and low cross-tension strength. For resistance spot welding, plug failure is normally a quality requirement in routine destructive tests, and a minimum plug diameter is specified (normally 4 √t, where t is the sheet thickness in mm).

This paper covers the main issues of resistance spot welding these materials, including weldability, hardenability and joint properties. The changes in welding procedures needed to weld the materials successfully will be discussed, together with the implications for the equipment used.

2. Key issues

The main issues with the HS and UHS steels are as follows:

• Weldability - the ease of achieving welds of the required size and quality in production.
• Hardenability - the hardness levels reached in the weld and heat affected zone (HAZ) and their influence on fracture behaviour on testing.
• Joint properties - the static and fatigue behaviour of welded joints
• Weld imperfections - the occurrence and effect of various types of cracking and porosity.
• Production considerations - the stronger steels can have different spring-back properties in pressed components and make component fit-up errors more difficult to accommodate in the welding process. Changes in welding parameters can have implications on the equipment capacity requirements.

3. Material weldability

Weld growth curves and weldability lobes form the basis of weldability studies. These give a means of comparing the welding current range capable of producing acceptable welds for a particular welding schedule (force/time combination) for different materials. The width of the weldability lobe gives an indication of the anticipated tolerance of a particular welding schedule in production, the aim being to maximise the welding range to achieve the greatest safety margin on weld quality. The welding range is generally narrower for HS steels than for low carbon (LC) steels, when using a schedule suitable for the LC steel. Slightly lower welding current is required for the HS steel, because of higher electrical resistance, but weld splash occurs earlier. The early work on HS steels indicated that, by simply by increasing the electrode force, ^[4] the welding range could be opened up to give a similar performance to LC steel.

Material suppliers often recommend the force levels required for different steel types and thicknesses. For HS steels up to about 600MPa tensile strength, this can be typically 20 to 50% higher than for LC steel. Even greater increases are often suggested for some of the UHS steels. It is difficult to be precise about electrode force levels to be used, as it also depends on the weld time. Higher forces are required particularly at shorter weld times, if short sequence times are required for high production rates. However, longer weld times can also be beneficial in expanding the available welding range.

Higher electrode force can enable larger weld sizes to be achieved before splash and help to reduce internal porosity or shrinkage imperfections. The disadvantages are the need for higher capacity guns, to prevent damage to the gun, and potentially faster electrode wear.

4. Hardenability and fracture mode

One of the most significant problems with HS and UHS steels is the potential high hardenability of welds as a result of the material chemical composition and the fast cooling rate associated with HS steels. The cooling time for thin sheet spot welds over the temperature range 800 to 500°C (through the transformation range of the steel), can be in the region of 0.06s see Fig.1. ^[5] This can lead to high hardness levels and brittleness of the weld. This gives unfavourable fracture modes (partial or complete interface failures) and low cross-tension strength. In terms of routine destructive tests, plug failure is normally a quality requirement and a minimum plug diameter is specified (normally 4 √t, where t is the sheet thickness in mm).

Fig.1. Temperature curve for the spot weld zone in 0.8mm steel sheet (after Takeshi Nishi ^[5] )

Examples of the hardness levels experienced in UHS steels are given in Fig.2, and a typical interface failure in chisel testing is shown in Fig.3. This shows the characteristic transgranular fracture with a crystalline appearance. The fracture has low energy absorption and the cross tension failure load is low.

Fig.2. Hardness and macrosections of spot welds in 1.05mm TRIP 700 and 0.8mm Martensitic 1000 steels. Section scale in mm. Hardness location represents indent number

Fig.3. Interface fracture due to weld hardening and detail of the fracture surface showing crystalline brittle fracture

a) Plan view of interface fracture

b) Scanning electron microscope picture of area indicated

There are a number of options for modifying resistance spot welding schedules to reduce the quench hardening effect. ^[6-9] In some cases, the fracture mode can be changed from interface to plug failure simply by a reduction in hold time. This allows sufficient heat to be retained within the weld before the quench effect of the electrodes is removed. Steels that respond to the change in hold time are sometimes referred to as hold time sensitive. This approach is, however, subject to some risk. Releasing force on a weld with insufficient strength at temperature, especially on thicker materials and where fit-up is poor, can lead to hot tearing in the weld nugget.

Pulsed welding and longer weld times also aim to control cooling rates by introducing more heat into the surrounding metal, and thus reduce weld hardening. These would be preferred approaches, with minimum increase in weld sequence duration. Such approaches would be the first choice when welding steels with borderline hardenability. The control of cooling rate has also been achieved by adding a lower current level pulse immediately after the main welding pulse(s). The particular conditions chosen will depend on the steel type and thickness. Some steels do not respond well to controlled cooling because of very high hardenability and high resultant weld hardness, such as TRIP steels. In this case in-process tempering may be effective, although a much longer sequence time is required. Here, the welding pulse is followed by a cool time to allow the weld to harden, and a temper current pulse reheats the weld to temper the hardened structure. ^[6]

The fracture mode is not wholly dependent on the weld chemistry (such as carbon equivalent) or the weld hardness achieved. Many studies on HS steels in the 1980s attempted to derive a modified carbon equivalent formula to define weldability, in particular the borderline of potential interface failures, for resistance spot welds. While reasonable correlation was achieved, no universal relationship was found. In addition, there is the question whether a maximum weld hardness value could be specified to define the limit of suitable weldability. Although hardness levels around 400HV and above are certainly more likely to give interface failure, there appears to no ideal answer, as material thickness and material type can also have an effect.

Other factors that affect the fracture mode for a particular steel type can be material thickness, weld size and the shape of the notch at the edge of the nugget. The likelihood of interface failure is greater with thicker steels and this is probably due to the difference in stress distribution at the notch. The weld size itself has a significant effect and some sources recommend making larger welds as standard in some steels. In some borderline cases, where interface failures predominate at the smaller acceptable sizes (e.g. 4 to 5 √t), welds in the region of 6 to 7 √t can give more reliable plug failures. In practice, though, it is likely to be more difficult to maintain such proportionally large weld sizes in production. Larger weld sizes may also result in a change in notch morphology at the edge of the nugget. Certainly the stress distribution at the notch has been recognised as having an effect, a blunt notch encouraging plug failure while the sharp notch tends towards interface failures. ^[10] Again, this is only likely to be significant in borderline cases. Figure 4 illustrates the appearance of the notch profile, with one side of the weld sharp and the other blunt. A specific notch form would be virtually impossible to guarantee in production, as it is influenced particularly by any slight misalignment in the electrodes and would be impossible to check.

Fig.4. Spot weld in 2mm DP800 showing variation in notch profile at the edge of the nugget

In production, the UHS steels are more likely to be welded to other steel grades than to themselves. Consequently, the effect of dissimilar material/gauge combinations on weld hardening behaviour may be improved by dilution in the weld nugget. The results will be dependent on the particular material and thickness combination in question, but weld dilution does not affect high hardness in the HAZ, or possibly the cross tension strength. Improved performance has also been demonstrated by the insertion of a low carbon interlayer for the purpose of weld dilution. [8]

 

5. Mechanical properties

In general, weld strength increases with material strength, but shear and cross tension strength behave in different ways. In low carbon steels, the shear strength for 5mm diameter welds in 1mm sheet is around 6kN and cross tension strength about 70 to 80% of that value. For material strengths up to about 600MPa, shear strength increases gradually to almost double that for the low carbon steel. However, the cross tension strength increases less rapidly, so that around the 600MPa level, cross tension strength may be nearer 50% of the shear strength. In the event of interface failures during cross-tension testing, cross tension strength would be expected to be even lower, perhaps around 30% of the shear strength.

In the materials above about 600MPa tensile, the shear strength of welds continues to rise slowly but is limited in some of the highest strength steels. Here, slight softening occurs in the outer part of the HAZ due to tempering of the martensitic parent material. An example of joint strength in a selection of thin (0.8 to 1.05mm) UHS steels is shown in Fig.5. It can be seen that the cross tension strength of these steels can be as low as 30% of the shear strength, as partial plug failure occurred in the TRIP steel. ^[11]

Fig.5. Weld and HAZ hardness for spot welds (5 √t diameter) in different high strength steels welded using 4kN electrode force, 10-12cycles welding time and 10cycles hold time ^[11]

The greatest benefit of spot welds in HS and UHS steels is in shear and design should normally avoid peel or tension loading. Under impact conditions, a variety of modes of loading occur and account would need to be taken of the reduced strengths in peel and tension. However, the parent material and geometry of a structure can often dominate crash performance and lower welds strength may not necessarily be detrimental.

Fatigue properties of HS and UHS steels have been studied and, in general, the steel type has relatively little effect on fatigue performance. At high cycle fatigue conditions, the fatigue strength is fairly independent of steel strength despite substantial benefits in static strength. ^[9,12,13]

6. Weld imperfections

There are two main types of imperfection that occur in spot welds. These are nugget shrinkage defects and surface cracks. Shrinkage cracks and porosity occur mainly in the centre of the weld nugget as a result of the incomplete forging of the nugget during solidification, see Fig.6. The extent of such imperfections is related to the strength of the material, the electrode force and the susceptibility of the material itself. Loss of metal from the nugget due to weld splash can also increase the porosity and cracks observed. While porosity and cracking does not normally cause concern, provided plug fracture occurs, weld strength may be affected on face failure. Studies are being undertaken to establish the importance of these imperfections and to consider acceptable limits. There are no specific restrictions in place for automotive standards although limits are set in aerospace standards, where radiography and weld sections are required.

Fig.6. Shrinkage defects in the centre of the nugget (such as in Fig.4)

Surface cracks are usually associated with liquid metal penetration under conditions of stress in the surface and in the presence of the melted zinc coating, see Fig.7. The risk of cracking can be greater under hotter conditions where 'brassing' of the electrodes occurs (copper alloyed with the zinc coating on the surface), adding to the source of molten metal on the surface. Where distortion of the surface can occur, such as with electrode misalignment, the weakened grain boundaries can open into cracks. Again, the real effect on weld properties is not clear but there is a concern that fatigue may be affected. As load is concentrated at the notch at the weld interface, such cracks are unlikely to initiate fatigue, but life may be reduced slightly if a fatigue crack front links with pre-existing outer surface cracks at the edge of the nugget indentation.

Fig.7. Zinc filled crack in the outer surface of the spot weld (electrode indentation) due to liquid metal penetration. Crack depth 0.2mm

7. Production considerations

In the stronger steels, particularly in thicker, structural gauges, 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. The high electrode forces suggested for the higher strength steels also takes account of the potentially greater fit-up problems.

The impact of welding the UHS steels in particular is that the substantially higher electrode forces can affect the equipment itself. Welding guns need to be stronger and have a higher force capacity. Electrodes, adaptors and holders may need to be larger diameter to avoid excessive flexure, or problems with taper connections. This may also affect access to components where small diameter and forward angle electrodes would otherwise be used.

Some of the longer welding schedules would influence production rate but it might be possible to tolerate special procedures where a limited number of welds were to be made on the more difficult steels.

8. Summary

There is a wide range of HS and UHS sheet steels available, and this presents a tremendous choice for designers, particularly in the automotive industry. Joining is a critical aspect of manufacturing, and it is important to be aware of the capabilities and limitations with these steels. Resistance spot welding remains the main joining process for sheet assembly, and successful results can be obtained with some attention to equipment and special welding procedures. Details of the procedures required would need to be set up according to the particular material and thickness combination required.

9. References

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