Application of impact tensile testing to spot welded sheets

Journal of Materials Processing Technology 153–154 (2004) 80–86

Application of impact tensile testing to spot welded sheets Emin Bayraktar a,∗ , Dominique Kaplan b , Marc Grumbach b a

SUPMECA-LISMMA/PARIS, School of Mechanical and Manufacturing Engineering, 3 Rue Fernand Hainaut, Saint-Ouen, France b ARCELOR GROUP, Paris, France

Abstract Since the last decade, there is an increasing demand for applications of resistance spot welding process in automotive industry. Today, an ordinary car may contain about 3000–4000 spot weld points. It is necessary to evaluate spot welded thin sheets in the dynamic loading conditions for determining optimal welding conditions and composition of metallic materials. This paper entails the characterisation and understanding the behaviour of thin welds of different grades of steels in dynamic loading conditions such as those experienced in automotive crash test. Evaluation of resistance to dynamic failure will be studied through impact tensile test (ITT). This research will contribute to the selection of optimal welding conditions and to the development of new grade steels for automotive applications. © 2004 Elsevier B.V. All rights reserved. Keywords: Spot welding; Impact tensile testing; Heat affected zone

1. Introduction Among the guidelines for car developments, passive safety (defined as the resistance of the vehicle in the case of collision) and fuel consumption are of paramount importance. These two criteria may appear somewhat antagonist, since low consumption may be obtained by reducing the vehicle mass, which in turn can be detrimental to the crash resistance. The use of high strength steel is an effective solution for satisfying these needs. Substituting thin gauge high strength steels for thicker hot rolled mild steels has achieved significant weight reduction on suspension arms, engine mounting brackets, chassis sections, closures, supports [1]. In particular, the behaviour of welded structures in the case of dynamic loading reproducing the conditions of collision, is of special importance. In this type of event, energy absorption as a function of time is essential: the deceleration should not be too fast (“hard” shock, dangerous for the vital functions) or too slow (excessive plastic collapse) [1–12]. So, the desired behaviour can be obtained through an optimisation of the design of the part, and through the intrinsic quality of the materials and their welds. In fact, different kinds of tests are available for assessing weld quality by using a continuous welding (LASER, gas metal arc welding, etc.) or a discontinuous processes ∗ Corresponding author. Tel.: +33-1-494-52-954; fax: +33-1-494-52-959. E-mail address: [email protected] (E. Bayraktar).

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.020

(resistance spot welding). Firstly, some very simple static tests (chisel test, peeling, shear or cross tensile tests for spot welds, Erichsen tests for LASER welds) are useful for materials and joining conditions screening. But, these tests do not give real indication on the behaviour of the welds in the case of dynamic loading. Secondly, some crash tests, performed on almost “full scale” specimens are of course more representative. However, they are more complicated, and in some cases not fully adapted to evaluate rapidly the influence of welding conditions on the mechanical properties. And finally, some dynamic compression test of pillars (hat square column) seems more satisfactory. But this kind of test seems more interesting for testing overall geometry, weld disposition, base metal properties, rather than for evaluating intrinsic weld quality. This is why it is felt that another “intermediate” dynamic testing, adapted to the characterisation of resistance of continuous welding LASER and discontinuous processes (resisting spot), is needed. This technique has been applied recently with success for testing of continuous welding (LASER, TIG, etc.) of thick and thin plates and presented these results [2,3,5,6,9,13]. As known, spot weld application in automotive industry is also a widest used process, mainly by using of new grades of steels such as high strength interstitial free (IF), transformation induced plasticity (TRIP), dual phase (DP) or bake hardening (BH) steels, etc. many parts of the car body (in white) are joined by this process. However, these modern steels show many mechanical and metallurgical problems during the welding operations. For example, some certain

E. Bayraktar et al. / Journal of Materials Processing Technology 153–154 (2004) 80–86

grades of IF steels show grain growth problem in the HAZ and display intergranular fracture in that zone. Some of themes show a decreasing fatigue behaviour at the level of 20% corresponding to this metallurgical phenomenon. If it is considered that an ordinary car requires about 3000–4000 spot weld points, this situation is a prohibitive effect on the manufacturing price. Additionally, the other high strength steels also display a decrease in the stiffness values due to the spot welds. So, this illustrates well the undertaking of the study which contains the mechanical and microstructural evaluation of the spot welds of thin sheets by using the new impact tensile test particularly, for measuring the crash resistance of the welded parts or the absorbed energy levels.

2. Experimental conditions 2.1. Procedure This test is based on the use of a special tensile specimen (Fig. 1a) which includes a smooth part and a welded part. In the case of weld testing, this latter part recreates the presence of strain concentrations, which may result from weld defects (cracks, porosity) or misalignment, imperfect geometry. First of all, this specimen is mounted by using the high resistant pins in a special device (Fig. 1b) and the whole setting is brought to the desired testing temperature by means of a cooling system. Immersion in liquid nitrogen necessitates for a long enough time in order to allow thermal balance. Then this setting is removed from the nitrogen bath

81

and rapidly placed in the impact testing machine which has a quick locking system (less then 3 s) where a special housing has been designed, and then fractured (Fig. 1c). The test temperature is then determined by the time elapsed during the test specimen temperature adjustment according to the warm-up kinetics on the whole setting determined by means of temperature versus time curves realised before on each grade of base or welded specimens. By definition, initial time (t = 0) refers to the moment when the whole setting is removed from liquid nitrogen. It has been verified that there is no change in temperature within 5 s as the specimen is only in contact with the support cooled at the same time which has a very large and favourable thermal inertia in the temperature stability. All impact tensile test in different temperatures are performed at a constant speed of 5.52 m/s (strain rate about 250 s−1 ). The double hammer (“U”-shaped double blade) may be instrumented with strain gages to derive force–time or force–displacement diagrams following the same method of familiar instrumented Charpy test. Experience shows that there is no bending effect during the test on the thin sheet specimens which are in small dimensions. They are fixed on the support and impact piece. It is necessary to place the whole setting in the impact testing machine vertically and is simply necessary to make sure to be in well contact conditions of the double blade on the impact piece. Tensile shear tests on the standard spot welded specimens were performed on a computer controlled Instron Testwell machine with a cross head speed of 2 mm/min and with a maximum applied force varying from 3000 to 3500 N according to the specimen.

Fig. 1. (a) Spot welded tensile specimen; (b) the pendulum device; (c) ITT special device.

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2.2. Materials and welding conditions More then 100 case studies were performed in this study on the two different industrial grade of steel sheets as received conditions such as interstitial free, high resistance steels, etc. with carbon content varying from 0.003 to 0.082, and with manganese content varying from 0.160 to 1.44. Resistance spot welding was performed using a 30 kV, pedestal SBF44 type machine and welding parameters are chosen in conformity with the IIW and also NF A87-001. Electrode: truncated cone, 6 mm Ø, electrode force: 210 Da, weld time: 7 and 16 cycles, hold time: 7 and 50 cycles and welding current: adjusted (5.5 and 8.8 kA) for obtaining different nugget diameter. Finally, a correlation was given between transition temperature obtained by impact tensile testing realised on these different case studies and Charpy criterion.

3. Results and discussion 3.1. Impact tensile testing The principle of the impact tensile test (“ITT”) has been explained formerly [2]. It has also been applied recently with success for testing of continuos (LASER, TIG, etc.) welds of thick and thin plates [2,6,13]. In accordance with the test principle, the specimens are submitted to impact tensile testing at different temperatures. As known, the transition temperature depends of course on various parameters such as temperature, specimen thickness, deformation rate, notch geometry, metallurgical factors, etc. [2,6]. In fact, the problem is not the base metal toughness but to be able to make a good estimate of the toughness of welded parts in the conditions of car crash. In the high strain-rate conditions, low toughness of welded zones for some sensitive steel grades favours localised fracture even at moderate temperatures. For that reason, evaluation of ductile-brittle transition temperatures of welded sheet is a sensitive parameter and can be a very useful tool to qualify new welding processes specially for new steel grades. On the contrary to base metal testing, specimens including welds have heterogeneous structure because of local modification during welding. Fracture competition between these two sections during impact loading is more complex because of different mechanical properties [13]. In the case of spot welding, the interface between the two sheets acts as a “natural” stress concentration factor. Thus, according to testing temperature, fracture mode varies. Fracture may occur either at this interface (plug failure) or in the base metal section, which is, by some extent, comparable to the testing on continuous welds defining also a transition temperature characteristic of weld quality. However, the situation is a bit more complex, but can be reasonably simplified in the following manner: the maximal static resistance F of a spot weld can be described as [10] F = ktdσbm

(1)

where t is the plate thickness (mm), d the nugget diameter (mm), σ bm the base metal resistance (MPa) and k is a constant (being equal to 2.5–3, with F in N). Thus, with the same definition of the matching factor [13] is equal to α=

tensile resistance of a notched specimen tensile resistance of a smooth specimen

(2)

α=

ktdσbm twσbm

(3)

The specimen width in the base metal part is designated as w. By simplifying α=

kd w

(4)

This supposes implicitly that fracture mode does not occur by low energy fracture mechanism (poor weld quality, too small nugget, etc.). This expression indicates that overmatching factor in spot weld testing is critically dependent over the ratio nugget diameter/specimen width. The spot welded tensile specimen (a) and the ITT special device (b) and also a pendulum device (c) with a double (U-shaped) instrumented blade are given in Fig. 1. Fig. 2 displays an example of the impact tensile test result obtained on the spot welded samples of two grades of IF steel. The fracture energy level is given depending on the testing temperature. Experience shows that transition between the two fracture modes in the ITT occurs within a very narrow temperature range. Thus, transition temperatures may be defined with very great precision, better than 5–10 ◦ C. Few testing specimens are needed for transition temperature determination. 3.2. Evaluation of welded structures in dynamic and static loading conditions As known, the mechanical behaviour of a resistance spot weld point in conventional tensile shear test depend on the geometrical factors such as morphology of weld point, sheet thickness, etc. mechanical factors such as base metal behaviour, and also welding operations such as welding conditions, test, etc. but depend very little on metallurgical factors such as structure of weld point and the chemical composition, etc. However, in dynamic loading conditions, for example in impact tensile test, all of these factors should be taken into account in fracture strength of weld point. It is known also that the weldability diagram defined by lower and upper boundaries allows to characterise different zones in a spot weld point. Sparking phenomenon of weld bead due to the excessive heat quantity arising during the welding may induce damage of the spot welded point and electrodes. It is difficult to determine the limits of the sticking and the sparking of weld bead [9]. This is why it is worth to evaluate spot welded structures in different loading test conditions. Tensile shear (static) and impact tensile test

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Fig. 2. Evolution of fracture energy as a function of temperature obtained in the impact tensile test on two grades of IF steel.

(dynamic) results for the spot welded IF (steel A, C) samples are given in Fig. 3a and b in which tensile shear load and transition temperature are plotted against mean button diameter for different welding conditions. It may be seen that, for example, the sparking limit of the weld metal can not be determined easily from the conventional tensile shear test (Fig. 3a) because fracture load increases as the button diameter grows even after the sparking [12,14]. However impact tensile test (Fig. 3b) shows an extreme value for the satisfying welding conditions (medium time) and so allows to verify the sensibility of this test to the welding conditions which permit the evaluation of spot welded samples easily. For example, it seems any improvement in fracture strength after a certain value of the button diameter (∼5 mm Ø) in the experimental conditions of this study. It can be observed the same results obtained for the small button diameters with respect to the IIW standard [8]. Here, hold time sensitivity plays an important role on the transition temperature. In fact, there are several factors contributing to hold-time sensitivity. Reason for actual unacceptable weld failure is a combination of some, or all, of the factors involved [9,12]. Microstructural examination of these spot welded specimens was conducted with particular attention to the heat affected zone. An interest area in these samples was the failure mode. The most common type of failure observed in tensile shear spot welded samples of IF steels occurred by cracks which propagated circumferentially around the weld nugget. Shear failure through the nugget was never observed, nor was there any evidence of shear crack growth. Since the crack propagates through the heat affected zone, it seemed possible that this unique failure mode was a result of microstructural changes brought about by the action of spot welding. However, the fracture in impact tensile testing at lower temperature contains a brittle initiation in the HAZ and

then propagated by ductile tearing to cause either an interfacial failure or a failure around the nugget. It was displayed that the crack always occurred in the HAZ in both of the conventional tensile test and the ITT but the propagation is always is different in the ITT. This situation displays that HAZ is consequently the centre of the fracture initiation for IF steels and thus the relationship between the microstructure in HAZ and mechanical properties of spot welded structure can be easily explained by means of the ITT because it is difficult to make it by means of a conventional tensile shear test owing to the excessive plastic deformation at just near of the welded point. In fact, the presence of preferential cracks paths, such as porosity or solidification cracks, could allow a crack to initiate at the faying surface notch and propagate from one porosity or crack location to another along the faying surface of the weld. A combined effect of the button diameter and sample width on the transition temperature is given in Fig. 4 in which transition temperature is plotted against mean ratio (d/w) for different welding conditions. It seems here influence of welding parameters on the transition temperature clearly. For example, transition temperature varies very little after a certain value (
0.45) corresponding to the overmatching situation [13]. It means that the fracture meanly occurs in base metal not in the welded point. So transition temperature is always found at low temperatures (∼−100 ◦ C). Additionally, transition temperature increases under a certain ratio (0.45) corresponding to the fracture of the welded point. As known, there is also a threshold value (d/w) for static tensile shear testing [15]. Fracture load decreases after a certain value (∼0.25). Then, welding conditions are easily evaluated in the ITT because the ratio of d/w is higher regarding to the static tensile shear testing.

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E. Bayraktar et al. / Journal of Materials Processing Technology 153–154 (2004) 80–86 320 fracture load in tensile sear test (daN)

IF steel

Steel C e=0.8mm

280

240 Steel A e=0.7mm

200 3

3,5

4

4,5 5 button mean diameter (mm)

short time

5,5

medium time

(a)

6

long time Welding conditions

100

Transition temperature TK28J(˚C)

IF steel 70

40 Steel C e=0.8mm

10

-20

-50 Steel A e=0.7mm -80

-110 3

3,5

4

short time

(b)

4,5

5 medium time

5,5

6

long time Welding conditions

Fig. 3. (a) Variation of the fracture load in tensile shear test depending on the welding conditions. (b) Variation of the transition temperature in impact tensile test depending on the welding conditions.

A relationship between the fracture energy and cleavage surface measured in different welding and test conditions is given in Fig. 5 in which fracture energy is plotted against mean cleavage surface in order to evaluate influence of weld-

ing and test conditions on the microstructure and properties of the welded structure of HR steel. Cleavage surface area was measured by planimetry from the photographs of fractured samples taken in scanning electron microscopy (SEM).

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Fig. 4. Evolution of transition temperature in impact tensile test depending on the matching value (IF steel).

It seems that the low fracture energies are associated to the large size cleavage zones. So the lowest energy levels are obtained beyond a threshold value (
8 mm2 ) in experimental conditions of this study [6]. In fact, Charpy V testing is a common technique for ranking materials on the basis of transition temperature. So, it would be useful to correlate the transition temperature in Charpy and impact tensile testing and also compare these results obtained here for the spot welded samples to the earlier works in literature in which a relationship between the transition temperature and mean ferrite grain size (Cottrel–Petch).

Thus knowing the transition temperature in the ITT on the spot welded specimen, Charpy transition temperature can be estimated by Tk28J =

Ti − 115 + 0.56(10 − t)2 1.24

(5)

So, the evolution of Charpy transition temperature as a function of the ferrite grain size for spot welded samples is given in Fig. 6 in which the correlation between the earlier results [16] and the results of this study was displayed. It may be seen that the Cottrel–Petch law can be applied correctly.

Fig. 5. Evolution of fracture energy obtained in impact tensile test at different temperature as a function of surface cleavage (HR steel).

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Transition temperature (˚C)

20

80

40

20

10

5

Acknowledgements

3

Ferrite grain size dαt (µm)

The authors thank Mr. J.P. Fouquet and Mr. J. Claire from IRSID for the technical support.

-10 Steel D Steel C

Former results (K.J.Irvine)

References

-40

-70

This study

Steel A

Steel B

-100

-130 -3

-2,5

-2

-1,5

-1

-0,5 ln(dαt-1/2)

Fig. 6. Evolution of transition temperature (TK28J) as a function of ferrite grain size in spot welded samples (Four different grades of IF steel).

4. Conclusions Main results obtained in this study are as follows: • Impact tensile testing is a useful tool for the characterisation of base metal and or welded thin sheets with respect to their fracture resistance in the presence of a defect. • This test displays importance of specimen geometry (d/w, point diameter/width of sample) and welding conditions (current level, hold time sensitivity, etc.). • This test offer more complete information than the conventional tensile test and permit a clear relation with metallurgy. • Transition temperature obtained for spot welded samples in this study can be correlated with the more familiar TK28J criterion, microstructure and also grain size.

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