Resistance spot welding of high strength steels (May 2003)

TWI Ltd, Granta Park, Great Abington, Cambridge, CB1 6AL, UK

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 associated with 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 of advanced 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 light weighting with the traditional 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 their application. 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 structures with 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 main factors 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 use relatively 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 the availability 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.

DP steel is a class of HS steel having a microstructure of soft ferrite and, depending on strength, between 20 and 70% volume fraction of hard phases, normally martensite. The soft ferrite phase is generally continuous, giving these steels good formability and ductility. When these steels are deformed, strain is concentrated in the lower strength ferrite phase, creating the unique high work hardening rate exhibited by these steels. DP steels are designed to provide a tensile strength up to 1000N/mm ². Strengthening in DP steels is achieved by a combination of grain refinement and the formation of hard martensite, and they can be produced by either hot or cold rolling. The strength level is controlled by the amount of martensite in the steel, as well as the heat treatment during processing.

TRIP (Transformation Induced Plasticity) steels have become of considerable interest in recent years because of their 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. TRIP steels have a higher carbon content than DP steels, together with silicon and/or aluminium to lower the martensite finish temperature to below ambient temperature, in order to retain the austenite phase. The retained austenite transforms into martensite during subsequent deformation, and provides excellent formability for manufacturing complex parts. In addition, high work hardening during crash deformation, for example, provides excellent energy absorption. TRIP steels are suitable for complex structural parts demanding high strength and automotive parts with good crash resistance.

Complex phase (CP) steels consist of a very fine microstructure of ferrite, bainite, martensite, carbide precipitates and retained austenite. These steels use many of the same alloy elements found in DP and TRIP steels, but additionally have small quantities of niobium or titanium to form fine strengthening precipitates. CP steels have tensile strengths of 800N/mm ² and above, and are characterised by high deformability, high energy absorption, and high residual deformation capacity. Typical candidate applications for CP steels are those that require high energy absorption capacity in the elastic and low plastic range, such as bumper and B-pillar reinforcements.

In martensitic steels, the austenite that exists during hot rolling is transformed almost entirely to martensite during fast cooling on the run-out table. Martensitic steels are based on a low carbon with several alloying element additions. Martensitic steels can provide very high strength (up to 1550N/mm ²) and relatively good formability due to high strain hardening. Martensitic steels are often subjected to post-quench tempering to improve ductility, and can provide reasonable formability, even at extremely high strength. Typical candidate applications for martensitic steels are wheel discs, chassis parts and body reinforcements.

3. Resistance spot welding of high strength steels

3.1. Resistance spot weldability of high strength steels

In general, HS steels are readily weldable. However, resistance spot welding of HS steels is different from that of low carbon steels in terms of welding parameters and weld quality. They also appear to be more susceptible to expulsion because of their high electrical resistivity, hence higher electrode forces and lower welding currents should be used to minimise expulsion. Typically, 20 to 50% increase in electrode force is recommended for HS steels up to about 600N/mm ² tensile strength compared to low carbon steels, but around 100% increase is suggested for the higher strength steels. Quality problems generally arise when the metal in the vicinity of the weld nugget undergoes the fast cooling rate associated with spot welding. This can cause unacceptable failure modes (interface fracture) in testing. In addition, cracking may occur in the brittle microstructure of the weld metal or heat affected zone (HAZ). Softening may also be encountered in the HAZ of certain steel types. The welding process may need to be modified to suit the variety of material compositions, dissimilar steel type and thickness combinations and any zinc coatings used. The main aspects of the resistance spot weldability of HS steels are briefly summarised below.

3.2. Weld Hardening and HAZ Softening

High strength steels usually have a higher hardenability than for low carbon steels because of the increased level of carbon and alloying elements. When these hardenable steels are resistance spot welded using conventional settings, the subsequent rapid cooling from the quenching effect of the electrodes in contact with the sheet surface produces a weld with a brittle martensitic structure. ^[5-8] Such a weld fails more easily across the interface between the sheets, in a brittle manner, when tested in peel or tension. ^[7] This may lead to a reduction in tensile strength and impact performance of the joint, and possible cracking of the weld.

The weld and HAZ hardening is mainly dependent on the composition of the parent steel. Carbon strongly affects the hardness of martensite, so increasing carbon content leads to an increasing susceptibility to cleavage fracture. TRIP steels have a higher carbon content than the other types of HS steel, giving a higher weld hardness (see Fig.2a). The alloying elements that increase the hardenability of steels, such as manganese, also significantly increase the weld hardness of HS steels.

The sheet thickness also has an influence on the hardening behaviour. Whilst the thicker sheet promotes greater quenching and hardening in arc welds, the opposite is the case for spot welds, where the cooling is provided principally by the water cooled electrodes, and thinner sheets can be more sensitive to hardening.

In general, the weld hardness level for most of the HS steels of ≥ 600N/mm ² is above 400HV as shown in Fig.2a, and this is much higher than low carbon and lower strength HS steels.

The hardness profile of HS steel welds, which is related to the metallurgical characteristics of the steel and welding conditions used, can have a significant influence on the weld properties. Generally, a fully hardened weld and HAZ is obtained in HS steels having a tensile strength below 1000N/mm ². However, a fully hardened weld but softened HAZ is produced in HS steels with a tensile strength above 1000 N/mm ², as shown in Fig.2b for the high strength DP and martensitic steels.

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 may allow 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 loading could 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 in higher 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 HAZ towards the sheet interface giving a full plug failure. ^[5]

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 sharp notches. 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 by increasing 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. A high 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 by increasing 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 stress level 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 the quenching 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 of micro-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 sequence time.

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, the weldability 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 the processing 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 steel itself, 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 (associated with 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 HS steels. 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 during design.

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 dependant on 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 usually recommended 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 one typically 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 and welding 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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