Coating effects on contact conditions in resistance spot weldability

Accepted Manuscript Title: Coating effects on contact conditions in resistance spot weldability Authors: Edouard Geslain, Philippe Rogeon, Thomas Pierre, C´edric Pouvreau, Laurent Cretteur PII: DOI: Reference:

S0924-0136(17)30512-5 https://doi.org/10.1016/j.jmatprotec.2017.11.009 PROTEC 15483

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

13-7-2017 31-10-2017 3-11-2017

Please cite this article as: Geslain, Edouard, Rogeon, Philippe, Pierre, Thomas, Pouvreau, C´edric, Cretteur, Laurent, Coating effects on contact conditions in resistance spot weldability.Journal of Materials Processing Technology https://doi.org/10.1016/j.jmatprotec.2017.11.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Coating effects on contact conditions in Resistance Spot Weldability Edouard Geslaina,b,*, Philippe Rogeona, Thomas Pierrea, Cédric Pouvreaua, Laurent

IRDL, FRE CNRS, Université Bretagne Sud, F-56100 Lorient, France

b

ArcelorMittal Global R&D, F-60761 Montataire, France

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a

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Cretteurb

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* Corresponding author: Edouard Geslain [email protected]

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ABSTRACT

An experimental apparatus was designed to reproduce a resistance spot welding

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operation and allow observation via infrared camera. A specific ex situ device was used to measure the variations of electrical contact resistances, versus pressure and

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temperature, for the electrode-sheet and sheet-sheet interfaces. The high values of the electrical contact resistances associated to the Al-Si coated Press Hardened Steel

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sheet explains the spark effect observed at these interfaces by infrared thermography. The initiation and the growth of the nugget from the Al-Si coated

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sheet to the galvanized low carbon thin sheet are also due to these high contact resistances. The different observation methods and measurements concur to explain the nugget formation.

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Keywords: resistance spot welding, dissymmetric assembly, electrical contact resistance, infrared thermography.

1. Introduction

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Resistance Spot Welding (RSW) process is largely optimized to join assemblies with

similar steel sheets; nevertheless, difficulties still exist with heterogeneous assemblies

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combining different steel sheets. The dissymmetry may be due to the differences in thicknesses, grades, and coatings between the steel sheets. This work deals with the

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understanding of the mechanisms that influence the development of the nugget in a

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specific combination of three steel sheets. The stack includes zinc coated Low Carbon

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Steel (LCS), Advanced High Strength Steel (AHSS) sheets and a Press Hardened Steel

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(PHS) sheet with specific aluminized coating. By applying standard RSW conditions defined by the AFNOR norm (AFNOR, 2005), there is a high probability that the thin

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LCS sheet will not be welded. A particular attention is paid to the role of the coatings during the development of the nugget.

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Recent works already mentioned operative weldability problems when joining three sheets assemblies combining a thin mild steel sheet and AHSS thicker sheets by RSW.

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Nielsen et al. (2011) found that the thin sheet is dominantly welded to the thicker sheet by bonding diffusion while Huda and Park (2012) were particularly interested in three

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sheets stacks with a thick Al-Si coated PHS sheet. They showed from macrographs that significant heating occurs at the interfaces of the Al-Si coated sheet, placed at the centre of the three sheets stack. This coating appears to have an important role in the heating process. With rounded tip electrodes, Raoelison et al. (2014) have shown that the nugget thickness reaches a maximum value in the early times and decreases until the end of the 2

welding time according to the progress of the electrodes indentation within the sheets. At the end of the welding time, the molten volume is quenched when the current is interrupted. Several relevant works have already highlighted, through numerical approaches, the

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importance of contact conditions in the case of dissimilar assemblies. Feulvarch et al. (2004) described precisely the contact formulation used in models frequently employed

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in numerical simulation of RSW. The nature of the coating on the sheets mainly influences the contact conditions. With galvanized steel sheets, electrical contact resistance at the faying interface is strongly reduced compared to non-coated sheets

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(Rogeon et al., 2008). In situ measurements of the specific electrical contact resistances

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(ECR) during welding tests remains a great technical challenge. The in situ techniques as

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considered by Rao et al. (2017) do not permit the measure of the most important

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parameters: contact areas and contact temperature in the interfacial area. Recently, Kaars et al. (2016a) have chosen to combine in situ measurements of electrical resistances,

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including the contact resistances, and FEM analysis to identify the evolutions of the ECR at sheet-sheet (S/S) and electrode-sheet (E/S) interfaces versus contact temperature and

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pressure. The ECR evolutions are described in their RSW model by using a mathematical function whose parameters should be identified by inverse method. However, the contact

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model should take into account the coupled thermal-electrical phenomenon to predict the correct contact temperature. The thermal contact resistance (TCR) seems to be ignored in

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their simulation despite it having a major impact on the contact temperature particularly at the S/S interface (Rogeon et al., 2009). Moreover, Kaars et al. (2016a) did not propose any comparison between calculated and experimental contact radius evolutions at E/S and S/S interfaces to validate the contact pressure predicted using the mechanical analysis. Raoelison et al. (2014) specify that an accurate assessment of the contact 3

resistances, depending on the interfacial pressure and temperature level, is required for a predictive simulation of the nugget’s thickness. Furthermore, with rounded tip electrodes and galvanized steel sheets, the high initial stress value (around 800 MPa) involves low contact resistance values, and the nugget formation and growth are mainly linked to the

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E/S contact radius evolution (Raoelison et al., 2012). Several authors have used a specific method to study the degradation of electrodes

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(Upthegrove and Key, 1972), the nugget development (Cho and Rhee, 2003) or the effect of coating (Saha et al., 2015). This method consists in squeezing and welding the border of the steel sheets stack with truncated electrodes to open the welding area and allow the

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recording by high-speed or infrared (IR) cameras. Füssel et al. (2012) have used this

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method on a specific welding device to observe by IR camera the nugget formation and

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its growth inside the stack. They compared the temperature field evolutions in the

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thickness of the sheets during the heating stage filmed by IR camera with numerical

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simulations.

To highlight mechanisms that influence the formation and growth of the nugget in the dissymmetric combination studied here, an experimental approach is used. A specific

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welding apparatus, similar to the device described by Füssel et al. (2012) allows us to reproduce RSW operation with entire or truncated electrodes. Interrupted welding tests

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are performed with entire electrodes to follow the nugget development. IR observations

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of truncated electrodes experiments allow at identifying the location of heating during the first instants of welding. The ECR are characterised ex situ by using a dedicated apparatus described in details in Rogeon et al. (2008). Even if their importance has already been mentioned previously, the characterization of the TCR have not yet been performed to complete this study. However, ECR and TCR are finally related to IR observations in

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order to explain the particular role of coatings in the operative weldability difficulties encountered with this heterogeneous assembly.

2. Material and Methods

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2.1.Material

Welding machine

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The welding tests are achieved by using a GYSPOT welding machine developed for

automotive body repair presented in Figure 1. It is a 1 kHz medium frequency direct

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current (MFDC) machine with a pneumatic pressure system. A tubular structure has been

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designed to fix the welding gun in a horizontal position. Two welding configurations can

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be used: a standard spot weld or a half-spot weld. Welding tests realized with entire

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electrodes are the standard configuration. Truncated electrodes squeezed on the border of steel sheets are the half-spot configuration (Figure 2). The Infrared camera is fixed in a

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vertical position to observe the border of the stack and the extremities of both truncated electrodes. Water-cooled electrodes are in copper-chromium-zirconium (Cu-Cr-Zr) alloy

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with a 32 mm curvature radius on the tip face and a 13 mm diameter (ISO 5821 – A0 – 13 – 18 – 32). When welding with MFDC, some authors have reported that the size of the

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nugget could be affected by the direction of the current and concluded to the polarization of the electrodes due to thermoelectric effects (Fukumoto et al., 2003; Ramasamy et al.,

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2002). In our case, several comparative welding tests have revealed no significant differences on nugget shape when changing the direction of the welding current. However, the same disposition of the sheet stack between the two electrodes was kept for all welding tests, with the thin sheet in contact with the positive electrode (Figure 2). During the process, no shielding gas is used to protect steels against oxidizing. 5

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Figure 1. GYSPOT welding clamp (left) and truncated electrodes in half-spot

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configuration (right).

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Figure 2. Scheme of welding configurations.

The dissimilar assembly considered in this study is presented in Table 1. It includes a

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LCS and an AHSS sheets coated by galvanization, and a PHS sheet with Al-Si coating. The Alusi® coating protects the PHS sheet during the hot stamping process and during the life of the final automotive part. The PHS sheets used here are in pressed state, after a heat treatment of 6 min at the austenitization temperature around 900°C. The coating is composed of a first Al-Fe diffusion layer of 17 µm thick completed by intermetallic layers

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(Kaars et al., 2016b). The hot dip galvanised coating is constituted of a sub-microscopic layer of Fe2Al5 at the steel/coating interface, and a 10 µm layer of pure Zn.

Steel type

Steel grade

Total thickness

Coating

Coating

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Position

(mm)

thickness

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(µm)

Low Carbon

Galvanized

AM54

Upper

0.57 ± 0.01

Steel Advanced High

Galvanized

1.47 ± 0.01

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DP600

Middle

10

(Zn)

Pressed

Usibor®

Hardened Steel

1500P

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Strength Steel

Aluminized 30 (AluSi®)

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A

1.2 ± 0.01

Lower

10

(Zn)

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IR camera

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Table 1 : Description of the assembly and characteristics of the different sheets used.

The IR camera is the middle-wave range model X6580sc by FLIR. Its main features are

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the full frame rate up to 350 Hz, the spectral response between 1.5 µm and 5 µm, and the

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spatial resolution (640 x 512 pixels with a 15 µm pitch). An optical microscope allows a distance of 300 mm between the IR camera and the welding area. Only blackbody temperatures are available as the surfaces emissivity are not determined yet. Filters allow measurements until 1500 °C.

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Electrical contact resistance The ex situ apparatus used to measure the ECR was developed by Rogeon et al. (2008). On this device, the experimental conditions of the tests (interface temperature and contact pressure) are well controlled. The temperature rise is slow (10 degrees Celsius per minute)

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compared to those encountered during in situ resistance welding tests (several thousand degrees Celsius per second). Consequently some mechanisms at interfaces, oxidation

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and/or creep, could progress differently between in situ and ex situ conditions.

Different stacks of three samples have been composed to measure the ECR, thus characterizing the four interfaces of the assembly: Cu-LCS, LCS-AHSS, AHSS-PHS and

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PHS-Cu (Table 2). Steels samples of square shape with two lengths, 7.0 ± 0.1 mm or 9.0

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± 0.1 mm, were cut directly from sheets while Cu-Cr-Zr alloy samples were machined

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from a bar to 8.0±0.1 mm diameter discs of 2.5 ± 0.1 mm thickness. The uncertainty on

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the dimensions (±0.1 mm) is mainly due to the accuracy of the cutting operation.

Cu-LCS

LCS-AHSS

AHSS -PHS

7 mm x 7 mm

7 mm x 7 mm

AHSS

AHSS

Cu-Cr-Zr 8 mm

Cu-Cr-Zr 8 mm

diameter and 2.5 mm

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Sample 1

diameter and 2.5 mm

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thickness disc

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Sample 2

Sample 3

PHS-Cu

thickness disc 7 mm x 7 mm

7 mm x 7 mm

LCS

PHS

9 mm x 9 mm LCS

9 mm x 9 mm PHS

Cu-Cr-Zr 8 mm

Cu-Cr-Zr 8 mm 7 mm x 7 mm

7 mm x 7 mm

diameter and 2.5 mm

diameter and 2.5 mm AHSS

AHSS

thickness disc

thickness disc

Table 2 : Description of the samples used for each measurement. 8

Figure 3 presents the ex situ ECR measurement device. The samples stack is squeezed between two punches fixed on the jaws of a force-controlled electrical press (30 kN Instron). Stainless steel punches are equipped with heaters for heating the samples stack

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indirectly by conduction. Two thermocouples are attached at the punches extremities, and two others close to the heating collars. Two copper wires are crimped in small holes at

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mid-thickness of the two samples at the extremities of the stack. These wires are

connected to a nanovoltmeter (Keithley 2002) in order to measure the potential difference at the extremities of the stack. The two punches are connected through electrical wires to

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a current supply power (Keysight E3644A), which delivers a small current (1 A) to avoid

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A

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supplementary heating by Joule effect.

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Figure 3. Picture and scheme of the ECR ex situ measurement device. AHSS-PHS contact (left) and Cu-LCS contact (right).

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2.2.Methods

Welding The GYSPOT provides a continuous current I (9 kA) during the welding time t (400 ms), while the load F is held constant (400 daN). The welding current is ramped up for 80 ms

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up to the plateau. In the half-spot configuration, the volume of matter and the contact

areas are halved. To reproduce the same heating conditions in the standard and the half-

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spot configurations, welding parameters (current, load) are adapted to these specific

welding conditions (Table 3) as explained by Cho and Rhee (2003). However, since only

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the extremities of the electrodes have been cut, conductive heat losses towards the

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electrodes remain in excess. To compensate this phenomenon, the welding current should

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A

be greater than half the value of I.

Standard welding

Half-spot welding

9.00 ± 0.27

6.00 ± 0.18

Welding time t (ms)

400

400

Load F (daN)

400 ± 2

200 ± 1

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Intensity I (kA)

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Table 3 : Welding parameters in standard and half-spot configuration.

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The nugget development is studied thanks to interrupted welding tests and macrographs like Harlin et al. (2002) in standard welding configuration. Chemical etching using picric acid reactant reveals the fusion zone (FZ) and heat affected zone (HAZ). The IR images are obtained in situ during the half-spot welding tests. They give qualitative information about the first moments of heat generation at the different interfaces (E/S and S/S), then

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on the location of the nugget initiation inside the sheets.

Measurement of ECR The total electrical resistance 𝑅𝑡 () between the two copper wires is calculated thanks to the measurement of the current I (A) and the potential difference 𝑈 (V):

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𝑈 𝐼

(1)

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𝑅𝑡 =

The total electrical resistance is also the sum of bulk resistances 𝑅𝑖 and ECR (·m²):

(2)

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1 𝐸𝐶𝑅 𝐸𝐶𝑅 1 𝑅𝑡 = 𝑅1 + + 𝑅2 + + 𝑅3 2 𝐴𝑐 𝐴𝑐 2

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where 𝑅1 , 𝑅2 and 𝑅3 are bulk resistances (m² of samples 1, 2 and 3 according to table

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2 and Ac the contact area (m²). It is important to notice that for the coated sheets, the bulk resistance integrates only the electrical resistance of the steel and does not take into

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account the resistance of the coating (3): 𝑡ℎ𝑠1 𝜎1 (𝑇)𝐴𝑐 𝑡ℎ𝑠2 𝑅2 = 𝜎2 (𝑇)𝐴𝑐 𝑡ℎ𝑠3 𝑅3 = { 𝜎3 (𝑇)𝐴𝑐

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𝑅1 =

(3)

where 𝜎𝑖 is the electrical conductivity (S·m-1) (see Figure 4 for values) and this is the

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thickness of the steel, deduced from the measurements of the total sheet thickness and the coating thickness (Table 1).

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9 8 7

5

LCS

4

AHSS

3

PHS

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si (MS.m-1)

6

2

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1 0 200

400 600 Temperature (°C)

800

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0

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A

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Figure 4. Electrical conductivity of steels measured by a four-wire sensing method.

From equations (1) to (3), ECR can be calculated by using equations (4):

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1 𝑈 𝑡ℎ𝑠1 𝑡ℎ𝑠2 𝑡ℎ𝑠3 𝐸𝐶𝑅 = (𝐴𝑐 − ( + + )) 2 𝐼 2𝜎1 (𝑇) 𝜎2 (𝑇) 2𝜎3 (𝑇)

(4)

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The ECR value can be split into a bulk resistance of the coatings 𝑅𝑐𝑜𝑎𝑡𝑖𝑛𝑔 (zinc or Al-Si

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coating) and an electrical contact resistance 𝑅𝑐𝑒 : 𝑡ℎ𝑍𝑛 𝑡ℎ𝐴𝑙−𝑆𝑖 + 𝜎𝑍𝑛 (𝑇) 𝜎𝐴𝑙−𝑆𝑖 (𝑇)

(5)

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𝐸𝐶𝑅 = 𝑅𝑐𝑒 + 𝑅𝑐𝑜𝑎𝑡𝑖𝑛𝑔 = 𝑅𝑐𝑒 +

In a first step, ECR were measured at room temperature with a varying pressure (5-200 MPa), which is quite representative of the squeezing stage of the welding process. Then, the evolution of ECR with temperature (20-500 °C) was recorded for two levels of pressure (100 MPa and 200 MPa).

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3. Results and discussions

3.1.Standard spot welds: nugget development The macrographs showing the nugget development in the standard welding configuration at different interrupted times are presented in Figure 5. The main sizes of the nugget

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considered here are the diameters (d2) and (d1) at the two faying interfaces, AHSS/PHS and LCS/AHSS respectively, the total thickness (e2) and the thickness (e1) inside the LCS

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sheet. The dimensions have been measured from macrographs by using the software

ImageJ. The precision is related to the pixel size (± 50 µm). It is usually assumed that the

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boundary of the dendritic grain zone, resulting from the abrupt solidification of the

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melting zone when the electric current is shut down, corresponds to the limit of the

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nugget, while the HAZ is characterised by a fine grain microstructure. The FZ can be

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observed since the first interrupted welding test at 80 ms, mainly in the PHS sheet and at the faying interface between the AHSS and the PHS sheets. The FZ appears very early

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before the current intensity reaches its plateau. Between 80 ms and 200 ms the thickness (e2) quickly increases and reaches the very thin LCS sheet (Figure 5). After 200 ms the thickness (e2) reaches a maximum value and decreases slightly, while the diameter (d2)

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increases with the indentation of the rounded tip of the electrodes into the sheets

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(Raoelison et al., 2014). It may be due to the enlargement of the E/S contact areas, which causes the decrease of the current density and the increase of the conductive heat losses

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towards the water-cooled electrode during the welding time.

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9 kA - 200 ms - 4 kN

9

9000

8

8000

7

7000

6

6000

5

5000

4

4000

3 2 1

9 kA - 400 ms - 4 kN

3000

0

2000

0

1000 0

100 200 300 400 Welding time (ms) e2e2

IsI

N

U

dd2 2

A

Figure 5. From the upper side: LCS-AHSS-PHS. Macrographs of interrupted spot welds

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dendritic zone (right).

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(left) and measurement of nugget diameter and thickness d2 and e2 from the limit of the

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Welding current (A)

d2

10000

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e2

10

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Nugget diameter and thickness (mm)

9 kA - 80 ms - 4 kN

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A

Figure 6. Macrographs of the LCS-AHSS interface (top left), SEM analysis and micro

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nugget in LCS sheet (bottom).

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hardness measurements (top right) and measurements diameter (d1) and thickness (e1) of

Both phenomena are responsible for an early solidification of the nugget in the thin sheet.

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Consequently, in the macrographs after 300 ms (Figure 6), the FZ presents two parts: one part slowly solidified during welding, leading to small interdendritic segregation (FZf);

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and a main part solidified rapidly by quenching at the end of the welding time and leading to larger interdendritic segregation (FZd). At higher magnification, it is confirmed that a

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part of the FZ with fine grain (FZf) is localized in the thin sheet (Figure 7). Micro hardness measurements and the addition elements quantities by SEM analysis are clearly similar in the fine grain area between LCS and AHSS sheets (FZf) and in the dendritic zone (FZd) (Figure 5). Thus, the maximum penetration of the FZ does not correspond to the limit of the dendritic zone at the end of the welding time (Figure 5). Furthermore the bonding 15

between LCS and AHSS sheets does not result here from solid state diffusion mechanisms, as stated by Nielsen et al. (2011). These analyses allow the accurate determination of the nugget penetration in LCS sheet (e1) and the diameter of the FZ inside the LCS sheet (d1) (Figure 7). During the welding stage, the thickness (e1) inside the LCS sheet begins to grow at 250 ms, reaches a maximum around 300 ms, and

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decreases slightly after, according to the evolution of the total thickness (e2) (Figures 5

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and 6).

3.2.Half-spot welds: heating initialisation

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In the half-spot configuration, the nugget is no longer confined and can flow out through

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the free surface (Figure 8). The electro-thermal heating mechanisms occurring in the half-

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spot and standard configurations can be assumed equivalent only during the first moments

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of the welding stage, until growth of the molten zone.

Figure 7. Molten metal flow through the free surface in the half-spot configuration (t = 400 ms).

Figure 8 presents IR images that have been captured at the frequency of 350 Hz during 16

the first moments of the welding stage. As the IR camera could not be triggered simultaneously with the welding machine, the time 𝑡0 corresponds to the first IR image with the smallest increase of temperature. This time t0 is assumed to coincide with the starting time of the welding stage. The colour gradient changes according to the temperature gradient until 1 500 °C considering blackbody surfaces. The initiation of the

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heating phenomena is localized at the interface between the PHS sheet and the bottom

electrode. Heat generation at the two interfaces LCS-AHSS and AHSS-PHS appears later.

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After 22.8 ms, Al-Si coating at Copper-PHS interface seems molten with a drop, which appears through the free surface. From 34.3 ms, the increase of the PHS steel bulk resistance with the rise of the temperature provides internal Joule heating. After 45.7 ms,

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the temperature in the PHS sheet is the highest, and the AHSS sheet is heated directly by

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Joule effect and indirectly by conduction from the PHS sheet. After 68.6 ms, darker areas

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A

are visible on IR images suggesting the apparition of molten steel. In fact, the emissivity

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value should probably decreases abruptly when the molten steel appears.

Figure 8. From the upper side: LCS-AHSS-PHS. Time sequence of IR images in halfspot configuration.

These observations confirm the assumption, admitted by different authors (Feulvarch et 17

al., 2004) about the precursor role of the interfaces on the heating process. Furthermore, the heat generation by Joule effect seems much greater at the level of both interfaces with the PHS sheet (Cu-PHS and PHS-AHSS) as shown by Huda and Park (2012) through macrographs analysis. As discussed later, the Al-Si coating presents a high internal

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electrical resistivity and leads to high ECR values too.

3.3.Electrical Contact Resistance measurement

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For every contact, the ECR values decrease abruptly with the pressure at room temperature (Figure 9)in agreement with different experimental results reported in the

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literature by Song et al. (2005). Two mechanisms, which promote the increase of the real

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contact area, occur simultaneously: the plastic deformation of the asperities in contact

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and the multiplication of the contact points. In the range [0-20 MPa], the abrupt decrease

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of the ECR values, divided by a factor 10, should be mainly due to the multiplication of contact points, which probably prevails at low stress. Above 20 MPa, plastic deformation

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and work hardening should become the dominant mechanisms and could explain the reason why the ECR decreases more slowly with the applied pressure. At 200 MPa, the ECR of contacts AHSS-PHS and PHS-Cu are one order of magnitude higher than the

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contact without Al-Si coating (Cu-LCS and LCS-AHSS). This might be due to two

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reasons: 1) the Al-Si coating is thicker and intermetallic layers are less conductive than the Zn coating, resulting in a higher bulk resistance of the coating, and 2) its hardness and

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roughness are also higher than that of the Zn coating, leading to a much higher contact resistance component.

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Cu-LCS

1

LCS-AHSS AHSS-PHS PHS-Cu

0.01

0.001

0.0001 50

100 Pressure (MPa)

150

200

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0

IP T

ECR (.mm²)

0.1

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Figure 9. Evolution of ECR versus pressure for the different contacts.

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The contact AHSS-PHS includes two coatings Zn and Al-Si, which play specific roles in

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the behaviour of the ECR. The roughness and the high resistivity of the Al-Si coating provide a very high level of ECR at room temperature under the pressures of 100 and 200

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MPa (Figure 9 and Figure 10). The evolutions of ECR with the temperature at the pressures of 100 MPa and 200 MPa for this contact (Figure 10) present a similar trend until 420°C. At this temperature, the zinc melts and fills the asperities of Al-Si coating,

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which causes the fall of ECR. Beyond 420 °C, the ECR increases slightly with the

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temperature. This behaviour suggests that, after 420 °C, the contribution of the contact resistance 𝑅𝑐 between the two coatings collapses. Consequently, the measured value

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contains only the contribution of the bulk resistance of the Al-Si coating after 420 °C (equation 5). According to this assumption, the hysteretic evolution of the ECR during the cooling stage provides the evolution of the bulk resistance of the Al-Si coating versus temperature, from which its resistivity evolution can be deduced. This assessment enables to confirm the high values of the resistivity of the Al-Si coating after hot stamped

19

treatment. At room temperature, the estimated value of Al-Si coating conductivity is

0

100

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IP T

5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 200 300 Température (°C)

400

500

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ECR (mΩ.mm²)

0.13 MS·m-1.

AHSS-PHS 200 MPa

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AHSS-PHS 100 MPa

A

Figure 10. Evolution of ECR versus temperature for AHSS-PHS contact at pressures of

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100 and 200 MPa.

The ECR measurements for the four contacts under a pressure of 200 MPa are shown in Figure 11. With the Zn coating, the weak hardness and the weak roughness enable to

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reach an intimate contact leading to low the ECR values even at room temperature. Consequently, under the high pressure (200 MPa), the ECR values begin to increase with

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temperature, influenced by the increase of the Zn coating resistivity (ERb-Zn), until a

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maximum value. When the temperature approaches the melting temperature, the softening and the crushing of the asperities cause the abrupt decrease of the ECR. After 420 °C, the values of ECR are very weak and can be assumed negligible compared to the bulk resistances of the LCS and AHSS sheets. These evolutions of ECR for Cu-LCS, LCS-AHSS contacts at the pressure of 200 MPa are in good agreement with experimental

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LCS

AHSS

1.2 1 0.8 0.6 0.4 0.2 0

PHS-Cu

AHSS

PHS

3.5 3 2.5 2 1.5 1 0.5 0 0

200 400 Température (°C)

200 400 Température (°C)

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0

AHSS-PHS

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LCS-AHSS

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Cu-LCS

ECR / bulk resitance (m.mm²)

ECR / bulk resitance (m.mm²)

results from literature (Rogeon et al., 2008).

𝑡ℎ𝑠𝑖 𝜎𝑖 (𝑇)

of

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Figure 11. Evolution of ECR for each contact at 200 MPa and bulk resistance

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steels versus temperature.

For the Cu-PHS and AHSS-PHS contacts, the values at room temperature of the ECR are largely higher than the bulk resistances of the sheets (Figure 11). Furthermore, the ECR

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values for both contacts are very similar until 350 °C. However, contrarily to the AHSSPHS contact (Figure 11), the ECR for the Cu-PHS contact decreases more slightly and

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continuously with the temperature until 500 °C. The ECR value remains higher than the

A

bulk resistance of the PHS sheet at 500 °C. The low ECR values at the thin LCS interface explain the IR observation (Figure 8): a very weak amount of heat is generated by Joule effect at this interface. Even once the fusion occurs on PHS side, the LCS remains relatively cold. Inversely, the huge values of ECR at both interfaces with the PHS steel sheet explain the fast heating rate occurring

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at these interfaces. However, it is observed in Figure 8 that the heating in the first 34 ms is more intense at the electrode/sheet interface. This is due to higher Joule effect (ECR.J2) at the contact E/S, probably due to the smaller E/S contact area.

4. Conclusions

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This work highlights the effect of the coating on the contact resistances and allows to explain the welding difficulties encountered during RSW of a three dissimilar steel sheets

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assembly including an Al-Si PHS sheet and a very thin LCS sheet on the top. From this research, the conclusions are the following:

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1. IR images present fast contact temperature rise at both interfaces with the Al-Si

N

PHS sheet at the first moments of the welding process.

A

2. The very high values of the ECR measured at both interfaces with the Al-Si coated

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PHS sheet on an ex situ device are related to the Al-Si coating properties and

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explain the spark effect observed at these interfaces with the infrared camera. When the Zn coating melts the ECR at the AHSS/PHS interface decreases abruptly.

EP

3. Due to the precursor effect of the ECR at both interfaces with the Al-Si coating

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PHS sheet, the nugget initiates inside the PHS sheet at the opposite of the LCS sheet. Its thickness grows quickly up to the LCS sheet and decreases slightly during

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the electrode indentation inside the sheets.

4. At the end of the welding stage, micro hardness and SEM analysis confirm that the FZ presents two parts. One part slowly solidifies during welding, leading to small interdendritic segregation (FZf), which ensures the bonding of the fine LCS sheet.

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A main part solidifies quickly by quenching at the end of the welding stage and leading to larger interdendritic segregation (FZd).

Acknowledgements

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The authors would like to thank the ArcelorMittal Global R&D for the steels supplying

and the grant of this study. The authors also would like to thank the Gys Company for its

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technical support.

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