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

Steve Westgate CEng, SenMWeldI, TWI Ltd

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 theuse 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 (withtensile 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 withthinner 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 highstrength 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 fromtheir 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 unfavourablefracture 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/timecombination) 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 thegreatest 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 HSsteel, 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 greaterincreases 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, ifshort 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 thinsheet 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, plugfailure is normally a quality requirement and a minimum plug diameter is specified (normally 4 √t, where t is the sheet thickness in mm).

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 thereduced 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 steelstrength 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 incompleteforging 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 theporosity 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 theseimperfections 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.

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. Wheredistortion 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 nuggetindentation.

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 potentiallygreater 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 andholders 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 awareof 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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