Pulsed-Electric-Current Bonding of Oxygen-Free Copper and Austenitic StainlessSteel+1

Hayato Nakao1,+2 and Akio Nishimoto2,+3

1Graduate School of Science and Engineering, Kansai University, Suita 564-8680, Japan2Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University,Suita 564-8680, Japan

Although pure copper has good electrical and thermal conductivity, it is often used in combination with other metals due to its poorstrength. An inexpensive austenitic stainless steel that has excellent corrosion resistance is a good joining material for pure copper, and thejoining of these dissimilar metals enables the fabrication of parts for a wide range of applications. However, it is difficult to weld pure copper andstainless steel due to the large differences between their thermal properties. Therefore, in this study, the pulsed-electric-current bonding wasapplied to achieve solid state bonding of oxygen-free copper (OFC) and SUS304 austenitic stainless steel, and the bonding characteristics wereinvestigated. The results show that the joint tensile strength improves with the increase in the bonding temperature and the applied pressure, andthe sample bonded at 973K with an applied pressure of 20MPa exhibited a high strength of greater than 200MPa, which caused a fracture in theOFC base material. [doi:10.2320/matertrans.MT-M2020277]

(Received August 28, 2020; Accepted December 7, 2020; Published January 22, 2021)

Keywords: oxygen-free copper, stainless steel, diffusion bonding, pulsed-electric-current bonding, tensile strength, spark plasma sintering

1. Introduction

Although pure copper has good electrical and thermalconductivity, it is often used in combination with othermetals due to its poor strength. An inexpensive austeniticstainless steel as a material with excellent corrosionresistance has a good bonding property with pure copper,and the satisfactory bonding of these dissimilar metals isharnessed in components fabrication in a wide range ofapplications. However, it is difficult to weld pure copper andstainless steel due to the differences between their meltingpoints and thermal conductivities as well as the low solidsolution limit of copper compared to stainless steel.1,2)

Therefore, the solid-phase bonding method without meltingis considered to be a suitable technique for joining thesemetals. Solid-state bonding methods mainly include diffusionbonding, friction welding, hot pressure welding, coldpressure welding, explosion pressure welding and gaspressure welding.3) Precedents for solid-state bonding ofcopper/stainless steel include friction welding, friction stirwelding (FSW), explosive welding, and diffusion bonding,all of which do achieve good bond strength. In the cases offriction welding, FSW, and explosion welding, it has beenreported that the hardness of the samples increases due tothe effect of plastic deformation.4­6) Diffusion bonding is amethod in which a limited pressure is applied such that thereis moderate plastic deformation, and the diffusion of atomsoccurring at the bonding interface is utilized in bonding.7)

Compared with friction welding that requires a rotary drivein the welding process, diffusion bonding is suitable forjoining mechanical parts that require high precision due to thedegree of freedom of the weldable components and the largeshape of components that can be joined. Considering these

advantages, we propose that pulsed-electric-current bonding,which is a kind of solid-phase diffusion bonding, is aneffective joining technique.

The pulsed-electric-current bonding is called spark plasmasintering bonding (SPS bonding); it is a method in whichthe bonding material is sandwiched by punches of the samematerial as the graphite die, and a pulse current is applieddirectly to the bonding material. Thus, the joint is achievedby utilizing the discharge plasma arising from the minutedischarge generated at the initial stage of the energization.The important parameters to be considered in this studyinclude discharge impact pressure, the Joule heat diffusion,and the electric field diffusion effect by the electric field.Since the joining surfaces are is locally heated by directlyenergizing the components, joining with reduced energyconsumption is possible even in a shorter time compared withthe hot isostatic pressing (HIP) and hot pressing methods.Also, since a joint surface without inclusions can be achievedby cleaning the adjoining surfaces, extensive researches areongoing reports on the joining of dissimilar metals andceramics.8­19) Based this understanding, we attempted directjoining of oxygen-free copper and austenitic stainless steelusing the pulsed-electric-current bonding method andevaluated the characteristics of this method.

2. Experimental Methods

The samples used were º10 © 45mm round bars ofoxygen-free copper (OFC) and austenitic stainless steel(SUS304). The surfaces of the samples were lathe-processedand ground to #2000 using wet emery paper, then buffedusing Al2O3 powder with a particle size of 1 µm for a mirrorfinish, and finally washed with acetone ultrasonically.

A spark plasma sintering machine (model: SPS-1020)manufactured by Fuji Electronic Industrial Co., Ltd (formerSumitomo Coal Mine Co., Ltd.) was used for the pulsed-electric-current bonding. The OFC and SUS304 wereintroduced into a graphite die with an internal diameter of

+1This Paper was Originally Published in Japanese in J. Jpn. Inst. Copper59 (2020) 91­95.

+2Graduate Student, Kansai University+3Corresponding author, E-mail: akionisi@kansai-u.ac.jp

Materials Transactions, Vol. 62, No. 3 (2021) pp. 448 to 452©2021 Journal of Japan Institute of Copper

10mm and pressed vertically. The temperature of the joiningsurfaces was measured by inserting a sheath of K-typethermocouple into a º4mm hole at the side surface of themold. After the bonding, the load was removed, and theassembly was cooled in a chamber under atmosphericpressure. Then, the chamber, dies, and sample componentswere cooled, and air was charged into the assembly to allowremoving the sample from the mold. The joining conditionswere vacuum pressure of 10 Pa or less, a heating rate of50K/min, holding time of 30min, applied pressures of15 and 20MPa, and bonding temperatures of 773, 873, and973K.

Using three samples, the joint tensile strength wasanalyzed by Shimadzu Autograph (AG-IS). Then, observa-tion of the microstructure of the joint interface and thefractured surface was conducted using a scanning electronmicroscope (SEM) (model: JSM-6060LV, JEOL), and theelemental analysis was performed using and an energydispersive X-ray (EDX) attached to the SEM. The SEMwas conducted at an acceleration voltage of 15 kV and theobserved surfaces were mirror-polished prior to the character-ization. Furthermore, to identify the elemental compoundsexisting on the fractured surface after the tensile test, X-raydiffraction (XRD) analysis was performed using a diffractionX-ray tester (model: RINT-2550V) manufactured by RigakuCorporation.

3. Results and Discussion

3.1 Observation of the cross-sectional microstructureFrom the observation of the cross-sectional microstructure,

no crack occurred at an applied pressure of 15 and 20MPa,and no intermetallic compound was formed according to theXRD analysis. Since no void or crack was observed at thejoint interface, it was confirmed that a good joint wasachieved. Figure 1 shows the results of a line analysis of thebond interface by SEM-EDX at a pressure of 20MPa andbonding temperatures of 773 and 973K. The diffusion of Cucould not be confirmed at 773 and 973K, but at 973K therewas a gradation in the Cu concentration at the bond interfacewhich suggests the possibility of bonding at the interface.

3.2 Tensile testFigure 2 shows the experimental results of the tensile test.

The right vertical axis of the figure represents the tensilestrength of the OFC base metal at 100% joint efficiency. At apressure of 15MPa, the joint tensile strength was 27MPa at773K, 51MPa at 873K, and 74MPa at 973K. Thus, it canbe deduced that the tensile strength improved as the bondingtemperature increased. Achieving a bonding strength even at773K suggests that the chromium oxide film on the SUS304surface can could be dissociated also at a relatively lowtemperature. This is attributed to the fact that the pulsed-electric-current bonding method applies the current directlyto the sample. This direct application of current causes localheat generation due to the resistance of the oxide film on thebonding surfaces and the contact resistance at the bondinginterface. Furthermore, the tensile strength of the jointsbonded under the pressure of 20MPa was higher than that ofjoint at 15MPa; the strengths were 36MPa at 773K, 86MPaat 873K and 222MPa at 973K. Particularly, the joint bondedat 973K had the highest strength and fractured in the OFCmatrix. It was assumed that the surface unevenness deformedas the applied pressure increased, thereby more increasing thecontact area and ultimately results in better contact betweenthe joining interface. Specifically, the tensile strength of thejoints increased at the bonding temperature of 973K becausethe high temperature strength of OFC is extremely low andthe unevenness is easily deformed.

3.3 Structural observation of the fractured surfaceFigure 3 shows the results of the SEM observation of

the fractured surface after the tensile test. At a bondingtemperature of 773K, the fractured surface of the specimenswas relatively smooth at 15 and 20MPa pressures.Considering that the tensile strength of the joint is low atthis temperature as described in 3.2, suggests that the drivingforce at the joint was insufficient at the temperature. At thebonding temperatures of 873 and 973K, different micro-structures were observed at the center region and the outeredge of the fracture surface. Specifically, different micro-structures were evident at the fracture surface under thepressure of 15MPa at 873K and the difference was more

5 μm

Cu Cu

773 K 973 K

Fig. 1 SEM and EDX-line analysis of joint interface of OFC/SUS304bonded by SPS with pressure of 20MPa at 773 and 973K.

0

20

40

60

80

100

0

50

100

150

200

250

700 800 900 1000

aPM / htgnerts elisnet tnioJ

Bonding temperature / K

Join

t effi

cien

cy (%

)

20 MPa15 MPa

N = 3

Fig. 2 Joint tensile strength of OFC/SUS304 joint.

Pulsed-Electric-Current Bonding of Oxygen-Free Copper and Austenitic Stainless Steel 449

pronounced under 20MPa at 873K. This is because there isan interfacial resistance between the graphite mold and thesample surface, and the temperature tends to rise locally. Inaddition, it was considered that Joule’s heat was less likely tobe generated due to the high conductivity of OFC, indicatingthat the thermal diffusion from the graphite mold had a greatinfluence. The reason for the increased difference in thefractured surface morphologies observed at the outer edgeand the center region as the pressing force increases isattributed to the fact that the deformation of the surfaceirregularities at the center region caused the adherence of thebonding surfaces and decrease in the interfacial resistance.Ultimately, there is an increased difficulty in the heatingprocess.20) On the other hand, the outer edge may be incontact with the graphite mold, causing it to be easily affectedby heat.

Figures 4 and 5 show SEM images and EDX elementalmaps of the SUS304 fractured surface at bonding temper-

atures of 873 and 973K and a pressure of 15MPa. At bothtemperatures, the mating material Cu was detected on theouter edge of the SUS304, however, more Cu was detected at973K, shown in Fig. 5. It was assumed that fracture occurredin the OFC matrix. However, no Cu was detected in thecentral region of the SUS304. This suggests that in pulsed-electric-current bonding, interfacial bonding commencesinterface bonding proceeds from the outer edge.

Figure 6 shows the EDX analysis of the fractured surfaceof the sample bonded under 15MPa at 973K. The elementalmap of O did not show a corresponding Cr enrichment, thus,fails to provide the information of inclusions such as

OFC side SUS304 sideCenterCenterEdge Edge

873

K77

3 K

973

K87

3 K

773

Ka

PM 51

aP

M 02

50 μm

Fig. 3 SEI of fractured surface of the OFC/SUS304 joint bonded by SPS.

SEI Cu Fe

retneC

egdE

50 μm

Fig. 4 SEI and EDX analysis of fractured surface of SUS304 side of theOFC/SUS304 joint bonded by SPS with pressure of 15MPa at 873K.

SEI Cu Fe

retneC

egdE

50 μm

Fig. 5 SEI and EDX analysis of fractured surface of SUS304 side of theOFC/SUS304 joint bonded by SPS with pressure of 15MPa at 973K.

50 μm

ON

iC

rFe

Cu

SE

I

OFC side SUS304 sideCenterCenterEdge Edge

Fig. 6 SEI and EDX analysis of fractured surface of the OFC/SUS304joint bonded by SPS with pressure of 15MPa at 973K.

H. Nakao and A. Nishimoto450

chromium oxides formed by the aggregation of the chromiumoxide film at the fractured surface. In the diffusion bondingof OFC and SUS304, it was reported that the process ofthe chromium oxide film removal was through condensation,and the inclusion of the chromium oxide film was confirmedon the fractured surface.21) On the other hand, in the pulsed-electric-current bonding, these inclusions did not exist evenin the bonding under a low vacuum of 10 Pa or less,indicating that the oxide film was not aggregated at thehigh temperature and undestroyed by the pressure. It waspresumed that the oxide film was destroyed by directenergization on the contact surface, and a good joint withno inclusions was obtained. There are some examplesregarding the report of joining dissimilar metal by the pulsedcurrent under low vacuum lesser than below 10 Pa.20,22)

3.4 XRD analysis of fractured surfaceFigure 7 shows the XRD pattern of the OFC and SUS304

fractured surfaces. The diffraction line of Cr23C6 was detectedon the SUS304. This suggests that a chromium carbide phasewas formed near the bonding interface. This was attributedto the fact that chromium carbide was formed at the grainboundaries by due to its prolonged retainment at 873 to1073K where the solid solubility limit of carbon is low.23)

This can be avoided by increasing the bonding temperatureor using low carbon materials. On the OFC and SUS304, asshown in Fig. 4 and Fig. 5, the elements of the matingmaterial are detected at the outer edges. However, the XRDpattern in Fig. 7 did not show Cu and £-Fe diffraction lines.This is because both Cu and £-Fe have the FCC structure andthe diffraction lines exist at very close angles, making itdifficult to distinguish the fractured surfaces of Cu andSUS304. Also, it was assumed that the chromium oxide filmformed on the SUS304 bonding surface after the tensile testwas extremely thin and imperceptible.

4. Conclusion

The following findings were obtained from the character-ization and evaluation of directly welded oxygen-free copperand austenitic stainless steel via pulsed-electric-currentbonding.(1) The tensile strength of the joint increased as the

bonding temperature and pressure increased. Also, atthe bonding temperature of 973K and pressure of20MPa, a satisfactory joining state that leads to fracturein the OFC base material was obtained.

(2) The bonding interface was free of inclusions such aschromium oxide and cracks despite bonding under alow vacuum of 10 Pa or less.

(3) The bonding commences from the outer edge of thesample, this is suggested by thermal diffusion from thegraphite mold.

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