African Fusion

AFRICAN FUSION

March 2016

Spot welding: Cu-Cr electrode caps

his research presents an ex-

perimental investigation of the

Resistance Welder Manufactur-

ers Association’s (RMWA’s) class two

(copper-chromium) electrode caps

using carbon and stainless steels in the

resistance spot welding process. A pair

of circular electrode caps of equal size

(5.0 mm diameter) was used for up to

nine hundred spot welding cycles. The

electrode caps were sharpened, when

necessary, between welding processes

using an electrode dresser, typically

after approximately four hundredcycles.

The degrading factors of chromium-

copper electrodes influence the weld

geometries of carbon and stainless

steels weld nuggets, directly impacting

on the bonding strength.

When the electrode caps used were

subjected to a micro-structural analy-

sis, several cracks were found in their

internal structures. The internal cracks

only appeared in the movable-upper

electrode cap due to the continuous

heating and contacting effects from

the pneumatic pressures, as compared

with the static-lower electrode in the

75 kVA spot welder. Mushroom growth

of the electrode cap tips is another

problem affecting weld quality, as it

reduces the resistance during the weld-

ing process.

In this experiment, the mushroom

growth seemed to be higher on the

upper side electrode than the lower

side. With the increased diameter of the

electrode tip due to the mushrooming

effect, the weld geometries become ir-

regular, which leads to inconsistency in

its appearances and to weld rejections.

Introduction

In joining carbon and stainless steels

using the spot welding process, class

two alloys are recommended for the

electrodes by the Resistance Welder

Manufacturers Association (RMWA) [1].

The grounds for this recommendation

is that these alloys have superior resis-

tance, heat tolerance and higher corro-

sion resistance [2]. Without the addition

of alloying elements, pure copper is

intrinsically soft and fails prematurely in

demanding applications [3]. Amixture of

substances is, therefore, a good choice

for themanufacturing of electrode caps

so as to produce superior quality, spe-

cificallywith respect to theirmechanical

and electrical properties.

With this consideration in mind,

copper-chromium-based electrode

caps were practically tested to weld ap-

proximately nine hundred weld joints

of carbon and stainless steel sheets.

Figure 1 shows the copper and chro-

mium phase diagram

for copper-based al-

loys [4]. It shows that

the chromium is easily

soluble in the liquidus

of copperwhenheated

above the 1 076 ºC and

below 1 860 ºC. Once

the compound is solid-

ified, it requires equal

amounts of heat to re-

melt it again [5]. This

is significant in the

welding of the carbon

and stainless steels

because the carbon

Investigating the electrodes

under the welding process of

similar and dissimilar materials

in resistance spot welding

Nachimani Charde, University of Malaya, Malaysia

Figure 1: Copper and chromium phase diagram (Chakrabarti DJ, 1984).

This paper presents an experimental investigation into copper-chromiumelectrode caps in

the resistance spotwelding process. Apair of circular electrode caps of equal sizewas used to

produce up to 900 spot welding nuggets between carbon steel sheets, stainless steel sheets

and between carbon and stainless steel. The electrode caps were examined throughout the

process for deterioration and mushrooming.

steelmeltingpoint falls between 1 426 to

1 540 ºC and the stainless steel melting

point falls between 1 400 to 1 450 ºC. The

copper and chromiumsolubility phases

areof the eutectic type. The face-centred

cubic (FCC) structure will be formed in

the copper, while body-centred cubic

(BCC) crystals form in the chromium

on solidification of copper-chromium

alloys.

Fundamentally, thewelding process

is governed by its process parameters –

welding current, weld time, electrode tip

diameters and electrode force [6]. These

parameter variations establish the corre-

sponding heat growth for anymaterials,

which in turn give the bonding strength.

While welding, the heat produced

in the enclosed areas of the electrode

tip will cause the tips’ deterioration.

Another factor that obviously affects this

deterioration is the electrode pressing

force, which was primarily supplied via

pneumatic pressure in this research.

Every time the electrodes are pressed

to hold theweldablematerials together,

the impact effects of the electrode tips

on the basemetal subject the electrodes

to fatigue.

In this experiment the mushroom

growth, degradation as well as the de-

terioration is examined for the copper-

chromiumelectrode caps using a 75 kVA

spot welder. Part of this research has

been previously published for the simu-

lation, tensile shear strength, hardness

distribution and themetallurgical analy-

sis and, therefore, such information

is excluded in this paper but relevant

references are given in [7].

Experimental procedure

Base metal plates with a thickness of

2.0 mm were prepared in rectangular

shapes to a size of 200×25 mm. The

chemical composition of the stainless

steel sheets was found to be: C–0.046,

Cr–18.14, Ni–8.13, Mn–1.205, Si–0.506,

S–0.004, N–0.051 and P–0.030. The