Resistance spot welding of high strength steels

G Shi and S A Westgate

Paper presented at JOM - Eleventh International Conference on the Joining of Materials, 25-28 May 2003, Helsingor, Denmark.

Abstract

High strength (HS) steels, up to about 600N/mm ² tensile strength, have been used successfully for several years in the vehicle manufacturing industry. More recently, there has been interest in introducing steels of greater than 600N/mm ². Whilst these steels allow the overall weight of the vehicle to be lowered, their resistance spot weldability and joint performance have become the main concerns. This paper describes the main issues associatedwith resistance spot welding of HS steels and the necessary modifications to welding schedules. The effects of parent material strength and weld hardening on the static properties of spot welds of such HS steels are discussed.

1. Introduction

Resistance spot welding is used extensively in the automotive industry to produce lap type joints in a range of components. Recent legislative requirements on motor vehicle emission levels have resulted in an increased uptake ofadvanced steel technologies. This has led to a move towards the increased use of higher strength steels within the vehicle manufacturing industries. These steels offer greater specific strengths and good formability, and therefore,lead to an ultimate reduction in weight while improving performance without modifications to the manufacturing process. This provides automotive designers and manufacturers with the unique option of combining lightweighting with thetraditional steel advantage of low cost.

Whilst this allows the overall weight of the vehicle (and resulting emission levels) to be lowered, the weldability, particularly the resistance spot weldability, of HS steels has become one of the main concerns for theirapplication. At the same time, the increase in the quality requirements of welded joints necessitates modification of resistance spot welding procedures to produce acceptable welds with good performance for producing welded structureswith higher efficiency and reliability. For some HS steels, the resistance spot welding process certainly appears more critical than with lower carbon steels when using baseline welding conditions.

This paper firstly introduces the characteristics of HS steels that are intended for use in the automotive industry. The potential problems associated with resistance spot welding of these steels are then described, and the mainfactors affecting their weldability and weld performance discussed.

2. High strength steels for automotive applications

High strength steels obtain their strength from grain refining, solution strengthening, transformation hardening and precipitation strengthening. Transformation hardened steels are the most recently developed type. These userelatively high levels of carbon and manganese, together with heat treatment to increase their strength. The finished product has a duplex microstructure of ferrite with varying levels of martensite. A great increase in theavailability and application of HS steels that can meet the requirements of the automotive industry has occurred in the last 20 years. These include transformation induced plasticity steel (TRIP), dual phase steels (DP), complex phase(CP) and martensitic steels, with tensile strengths up to 1400N/mm ². ^[1-4] Fig.1 compares the tensile strength and elongation of these and other steel types.

Fig.2a) Hardness of weld metal and HAZ

Fig.2b) Weld hardening and HAZ softening

As the applied load on a weld concentrates the stress on the notch at the edge of the weld at the sheet interface, this can cause fracture across the interface microstructure with low ductility. If HAZ softening occurs, this mayallow the fracture to propagate through the sheet thickness, giving the preferred plug failure. This softened HAZ may, therefore, improve the overall ductility of the weld, but its response to early local necking under tensile loadingcould possibly limit the strength of the weld. Thus, the joint strength depends on the relative strength of the weld and HAZ.

3.3. High Susceptibility to Partial Plug or Interface Fracture

Plug failure is normally required on peel or chisel testing in most automotive standards, as it demonstrates weld toughness and is a reliable method of checking weld diameter. It is more difficult to achieve this mode of fracture inhigher strength steels, compared to low carbon steels. The reason lies in the predominant occurrence of interface failure and partial plug failure under static tests, which reduces the weldability lobe to an unacceptable value,according to some standards. This is particularly true for steel grades exhibiting the highest values of carbon equivalent such as TRIP steels.

The high susceptibility to interface fracture of the welds in HS steels is related to their hard and brittle microstructure, which exhibits high notch sensitivity. ^[5,9-11] In the case of sharp notches at the edge of the weld (see Fig.3a), the stress concentration in front of the notch tip is very high, leading to the preferential occurrence of the interface failure mode. On the contrary, for square morphologies ( Fig.3b), stress concentration is reduced. Moreover, the maximum stresses are located at the corners of notches, away from the interfaces. Cracks initiate principally at these corners and propagate directly within the HAZtowards the sheet interface giving a full plug failure. ^[5]

Fig.3. Schematic illustration of the notch geometry of resistance spot welds:
a) Sharp notch tip
b) Square notch tip

The notch morphology of welds for given materials mainly depends on the electrode force and welding current. Lower electrode forces will not allow the molten and heated metals to be forged sufficiently, leading to formation of sharpnotches. Misalignment of electrodes will also cause problems. For HS steel, a sharp notch is more likely to be produced due a greater resistance to deformation. The notch geometry can be possibly changed from sharp to square byincreasing both the electrode force and welding current, and hence, weld size. However, it may be difficult to control notch shape reliably under production conditions.

For most HS steel welds, the presence of hard martensite essentially allows easier propagation of cracks and more readily generates interface failures. ^[12] The joint geometry, in terms of the weld diameter to sheet thickness ratio, also affects the fracture appearance of welds. For a low ratio, stress concentrations at the edge of the weld are high and weld cracking can initiate. Ahigh ratio reduces the degree of stress concentration at the weld edge and can promote plug failure through the HAZ, ^[12] rather than interfacial failure through the weld. Consequently, larger than normal weld sizes are sometimes suggested for HS steels.

3.4. Modification of fracture mode

There has been a great effort devoted to methods of changing the fracture appearance of welds in HS steels. It was suggested that welding procedures that reduce the weld hardening effect, and decrease the notch effect sensitivity,would improve the weld metal toughness and decrease the risk of for micro-cracking in the weld nugget. ^[5,7]

It was suggested ^[5] that the notch effect sensitivity, which is the key factor affecting the fracture mode, can be reduced by an increase of electrode force and total welding time. This was achieved not by changing the notch geometry but rather byincreasing the length of the diffusion zone, which is the distance separating the less ductile phases of the spot weld (the martensitic phases located within the fusion zone). The increased diffusion zone was shown to reduce the stresslevel in the fusion zone by up to about 30%. A lower occurrence of interfacial failure would be expected, which leads to larger domains in the weldability lobe.

Longer weld times increase the size and depth of the softened HAZ, and would promote plug failure at the cost of weld strength. Welds produced with short hold times cool more slowly and add ductility to the weld metal, as thequenching effect of the electrode is removed more quickly. However, short hold times can adversely affect the weld nugget during cooling, especially in thicker sheet steels.

Controlled cooling can be applied to the baseline welding cycle by adding a low level current pulse immediately after the weld time. This reduces the cooling rate and, thus, weld hardness. This would also reduce the risk ofmicro-cracking in the weld or HAZ imposed by the thermal stress. Alternatively, a cool time and temper pulse may be required after the weld pulse to temper the hardened weld. However, this could add about one second to the sequencetime.

Weld dilution can be achieved by introducing a low carbon steel shim insert, ^[7] or naturally, in the case of welding a high strength to a low carbon steel. However, an insert would be of limited practical applicability and the dilution technique does not reduce the HAZ hardening.

4. Effect of metallic coating on the resistance spot weldability of high strength steels

The use of metallic coatings significantly affects the spot welding behaviour of sheet steels in two ways.

Firstly, the higher conductivity and lower melting point of the coating reduces the contact resistance. Consequently, an increase in welding current and weld time is required to produce the required weld size. Generally, theweldability lobes for zinc coated steels are shifted to a higher current level and have a narrower range than for uncoated steels.

Secondly, alloying of the coating with the spot welding electrodes causes accelerated electrode wear. Many of the HS steels are available with a zinc coating, although the type of coating which is suitable may depend on theprocessing route of the steel, and the sensitivity to the thermal cycle associated with HDG (hot dipped galvanised) coatings. It is expected that coating the higher strength steels would not affect the microstructure of the steelitself, but that welding conditions would need to be modified to take account of the coating. However, there is a concern that the higher strength steels are more susceptible to surface cracking in the electrode indentation (associatedwith liquid metal penetration) than low carbon steels.

5. Mechanical properties of resistance spot welds in high strength steels

It is generally agreed that the shear strength of welds in HS steels (UTS<600 N/mm ²) is higher than those in low carbon steels. Tensile-shear strength increases linearly with increasing plug diameter. ^[5] The tensile-shear strength of spot welds increased approximately linearly with the parent material strength in steels up to 600N/mm ². Increase in parent material strength above about 800N/mm ² did not give any improvement in the tensile-shear strength of the joint because of the softening in the HAZ. The effect of softening in the HAZ on the joint strength could be a potential problem for joining HSsteels. Softening in the HAZ may improve the overall ductility of the weld, but its response to early local fracture under loading possibly limits the strength of the weldment. This effect would need to be taken into account duringdesign.

The cross-tension strength of HS steel welds normally decreases with an increase in carbon content or carbon equivalent and is less related to the parent steel strength. Recent work ^[6] found that the cross-tension strength was virtually constant for all sheet steel combinations for HS steels.

Fatigue performance is often of importance in automotive applications. Preliminary work in this area found that high cycle fatigue strength of such joints under cross-tension and tensile-shear loading conditions was mainly dependanton the weld size and joint design, and less related to the strength of the parent steel. ^[6] The fracture path in fatigue depended on the mode of loading, with fracture through the higher strength steel in shear and cross-tension but fracture through the lower strength steel in plane bending. ^[6] Generally, the fatigue properties of the spot welded joints in HS steel sheets are no worse than those achieved with lower strength steels.

6. Summary

In comparison to low carbon steels, HS steels provide some opportunity for reduction in gauge and weight saving without significant impact on weldability or process requirements. For steels up to 600N/mm ² tensile strength, no additional time is normally required in the welding cycle, either in weld time itself or modifications to the welding sequence. However, slightly higher electrode forces are usuallyrecommended to maintain a good welding current range, typically a 20 to 50% increase compared to low carbon steel.

Weldability has become one of the key factors determining the application of some HS steels above 600N/mm ² tensile strength in the automotive industry. High weld hardening and susceptibility to interface fracture can be the main problems that affect joint performance in some steels. Electrode forces used onetypically around double the value used for low carbon steels and the welding sequence may require modification.

In general, the resistance spot weldability of thin sheet DP and martensitic steels was good, in terms of weld nugget size and fracture mode. TRIP steels appeared to have high weld hardening and susceptibility to interface andwelding sequences need to be modified to avoid partial or complete interface fracture. This may add one or two seconds to the sequence time, when using a quench and temper pulse. For steels above 1000N/mm ², the benefits of using HS steels could be restricted by limitations in spot weld strength caused by HAZ softening.

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