Interfacial evolution of WC-Co/AISI 304L diffusion bonded joint

obtained by flash SPS technique

BILLEL CHENITI1, BRAHIM BELKESSA1, BOUZID MAAMACHE1, NAIMA OUALI1, VIKTOR

PUCHÝ2, PAVOL HVIZDOŠ2,

1Research Center in Industrial Technologies CRTI,

P.O. BOX 64, Cheraga 16014 Algiers

ALGERIA.

2Institute of Materials Research, Slovak Academy of Sciences,

Watsonova 47, 04353 Kosice,

SLOVAK REPUBLIC.

Abstract: - In this work, WC-Co cermet was successfully joined to AISI 304L stainless steel using flash spark

plasma sintering (FSPS) technique under a constant load of 5 MPa with ultra-rapid holding times. The results

revealed that increasing the holding time to 12s resulted in massive interfacial deformations accompanied with

an important diffusion activity of Co, Ni and Fe across the interface. Toughness measurement of WC-Co cermet

at the vicinity of the bonding interface was assessed using Vickers indentation fracture (VIF) method. The results

revealed that the mechanical properties of the bonded joints deteriorated with increasing holding time, leading to

increased brittleness. This outcome was observed despite the significant inter-diffusion that occurred between

the WC-Co cermet and the constituents of the 304L steel

Key-Words: - WC-Co cermet, inter-diffusion, Flash SPS, Microstructure, Interface, Fracture toughness.

Received: June 21, 2022. Revised: July 20, 2023. Accepted: August 29, 2023. Published: September 18, 2023.

1 Introduction

In the last decades, the challenge of the WC-Co

cermet/steel joining raises much interest, in order to

perform a material with highly distinguished

properties [1,2]. Due to the great difference in their

physical properties and their chemical nature, joining

WC-Co cermet to steel using conventional welding

methods is so difficult [3,4]. Therefore, an important

research works has been performed to achieve an

acceptable dissimilar joint, with high strength using

various advanced techniques such as: diffusion

bonding [5,6], laser welding [7], friction welding

[1,2] and hybrid and TIG welding [8,9]. In the

dissimilar joining, the key to obtain a high quality

joint, is to avoid the formation of brittle phases and/or

intermetallic at the weld interface which can cause

the premature failure of the weld joint. Many

research studies have shown that the WC-Co/steel

interface is the weakest area of the join where the

fracture occurs frequently. As reported by Ma et al.

[10] in WC-Co/invar laser weld joint, the cracks

initiate at the interface and propagate through the WC

facet, which makes the WC-Co at the vicinity of the

interface susceptible to cracking. This behavior is

attributed to several factors, among them, the

mismatch between the steel and the WC-Co cermet

thermal properties during cooling process. Most

often, the thermal expansion coefficient of the steel

is larger than that of the WC-Co cermet [11,12].

Consequently, detrimental residual stresses are

generated along the joint interface especially in the

WC-Co cermet can be expected, and their level

depend on one hand, how close they are to the

interface and, on the other hand, the joining

parameters and/or conditions.

In this work, WC-Co cermet has successfully been

welded to AISI 304L stainless steel using rapid

diffusion bonding process. The weld joints have been

performed using flash SPS machine with different

holding times (4s to 12s) at 5MPa constant load. We

aim to investigate through this work, the

interdependence between the manufacturing

conditions (holding time), residual stress and the

microstructure features of WC-Co/steel joints issued

from different times.

International Journal of Chemical Engineering and Materials

DOI: 10.37394/232031.2023.2.7

Billel Cheniti, Brahim Belkessa,

Bouzid Maamache, Naima Ouali,

Viktor Puchý, Pavol Hvizdoš

E-ISSN: 2945-0519

Volume 2, 2023

2 II. Materials and experimental

procedures

In this work, WC-10%Co was joined to AISI

304L austenitic stainless steel (8 wt. % Ni and 18

wt.% Cr) with cylindrical shape of 5mm of length and

8mm of diameter. The dissimilar assembly was

obtained by using flash spark plasma sintering

(FSPS) machine as illustrated in Fig. 1. The WC-Co

cermet was placed onto the AISI 304L steel, put into

a graphite die and inserted in the FSPS machine under

a constant load of 5 MPa. The bonding operation was

carried out at different times (4s, 6s, 8s, 10s and 12s)

under shielded atmosphere of argon using DC mode.

The cooling process was conducted in the FSPS

machine and the cooling rate was about (100°C/s). A

Zeiss optical microscope and a Zeiss AURIGA

scanning electron microscope with an energy

dispersive spectroscope (EDS) were utilized for

microstructural examination and semi-quantitative

chemical analyses.

Fig. 1: Schematic illustration of the flash SPS

bonding process.

3 Results and discussion

Fig. 2(a-d) illustrates the optical macrographs of

the samples joined by FSPS using different holding

times. The weld joints exhibit a good bending where

the line interface can easily be distinguished. As

expected, increasing the holding time results in

important plastic deformation of the AISI 304 steel

pointed by the red dashed lines in Fig. 2(b-d). For 4s

to 8s weld joint, no micro-defect can be observed

along the weld interface, however, in 10s weld joint,

remarkable cracks are identified at WC-Co/AISI

304L steel interface (Fig. 2d).

Fig. 2: Macrographic images of the FSPS weld

joints obtained using different holding times: (a):4s,

(b): 6s, (c): 8s and (d): 10s.

The effect of holding time with respect to the quality

of the weld joint can be appreciated in Fig. 3(a-b). At

low holding time (4s), the WC-Co cermet/AISI 304L

steel interface has a linear aspect which can be

attributed to the low diffusion of elements across the

weld interface. As observed in Fig. 3a, with

increasing the holding time to 6s, the Fe and Ni

elements diffuse into the cermet and vice versa.

Another point should be noted is the significant

dissolution of W and Co at the interface and the

substitution of this latest with Ni due to the high

mutual solubility between the W and Ni compared to

that with Co [13] as shown in region 1 (Fig. 3a). The

weld joint obtained using 10s presents an important

plastic deformation of the steel with high

concentration of cracks in the WC-Co cermet along

the line interface. This behavior results in a consistent

inter-diffusion of the cermet and the steel

constituents and enhances the metallurgical bending

of the joint. However, due to the low thermal

conductivity of the WC-Co compared to the steel

one, high stresses are generated at the interface which

induce cracks appearance in the WC-Co cermet along

the interface (region 2 and 3). In order to better

understand the inter-diffusion and elements activity

occurred at the WC-Co/AISI304 steel interface,

EDS profile and point analyses were conducted as

shown in Fig. 4(a-b). EDS analysis approve that the

Ni and Fe elements dissolve into the WC-Co cermet

and the WC particles undergo high plastic

International Journal of Chemical Engineering and Materials

DOI: 10.37394/232031.2023.2.7

Billel Cheniti, Brahim Belkessa,

Bouzid Maamache, Naima Ouali,

Viktor Puchý, Pavol Hvizdoš

E-ISSN: 2945-0519

Volume 2, 2023

deformation and form with the steel elements a new

component rich in W, Ni and Fe (see Fig. 4a and 4b).

Fig. 3: SEM micrographs of the WC-Co/AISI 304

L interfaces for (a): 6s and (b): 10s holding time

Fig. 4: EDS analysis conducted in region 1

highlighted in Fig. 3a, (a): EDS SEM micrograph of

the interface. (b): EDS profile analysis,

The effect of holding time on the fracture

toughness (KIC) of the WC-Co cermet/AISI 304L

steel joints calculated at the vicinity of the weld

interface using Shetty formula (1) [14] is illustrated

in Fig. 5

KIC= 0.0028 √𝐻 𝑃

∑𝐿 (1)

Where H is the hardness (N/mm2), P is the applied

load (N) and ΣL is the sum of crack lengths (mm).

It is observed that the VIF toughness decreases

with holding time and achieves 14.8 MPa.m-1/2 for

10s. This behavior of WC-Co cermet is strongly

related to the compression stresses induced from the

plastic deformation of the materials during FSPS

welding process and that increase with holding time.

Hence, increasing the bonding time results in the

brittleness of the WC-Co cermet which can causes

the premature failure of the weld joint. Besides, these

cracks start from the corners of the indents and

develop and propagate much more in the parallel

direction to the line interface than the perpendicular

one which is more likely due to the difference in

thermal expansion between the different zones in

WC-Co cermet.

Fig. 5: Variation of fracture toughness (KIC) as

function of holding time

4 Conclusion

The key issues of this work are summarized as

follows:

 Flash SPS process is believed to be an

effective technique to perform an acceptable

WC-Co/steel joints with an appropriate

holding time considering the other

parameters.

 At the joint interface, the inter-diffusion of

Ni, Fe and W elements is activated by the

plastic deformation which became more

prominent with holding time.

 Increasing the bonding time results in the

brittleness of the WC-Co cermet which can

causes the premature failure of the weld

joint.

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Billel Cheniti, Brahim Belkessa,

Bouzid Maamache, Naima Ouali,

Viktor Puchý, Pavol Hvizdoš

E-ISSN: 2945-0519

Volume 2, 2023

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Contribution of individual authors to

the creation of a scientific article

(ghostwriting policy)

Bille Cheniti: designed and conducted the

experiments, analyzed the data, and wrote the

manuscript

Brahim Belkessa and Bouzid Maamache : performed

data analysis, and provided critical revisions to the

manuscript

Naima Ouali: contributed to the data collection and

analysi

Viktor Puchy has organized and executed the FSPS

experiments

Pavol hvizdos, supervised the project, provided

guidance throughout the research, and reviewed the

manuscript

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

No funding was received for conducting this study.

Conflict of Interest

The authors have no conflicts of interest to declare

that are relevant to the content of this article.

Creative Commons Attribution License 4.0

(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en

_US

International Journal of Chemical Engineering and Materials

DOI: 10.37394/232031.2023.2.7

Billel Cheniti, Brahim Belkessa,

Bouzid Maamache, Naima Ouali,

Viktor Puchý, Pavol Hvizdoš

E-ISSN: 2945-0519

Volume 2, 2023