African Fusion

March 2016

AFRICAN FUSION

CMW Alloy (Class 2)

C18200

Chemical composition

Cu–99.1%; Fe–0.10%; Cr–0.60%, Si–0.10%;

Pb–0.05%

Rockwell Hardness (HRB)

Electrical Conductivity % IACS@68F

Tensile Strength (KSI)

Yield Strength (KSI)

Elongation % in 2 inch

Thermal Conductivity (W/m.K (min))

187

Thermal Expansion (/K)

9.8×10-6

Figure 2: The dimension of electrodes on the

welding materials.

Figure 5: Carbon to stainless steel weld (real vs

simulation).

Table 1: Properties of copper-chromium electrodes.

Figure 3: Carbon steel weld (real vs simulation).

Figure 4: Stainless steel weld (real vs simulation).

chemical composition of the carbon

steel sheets was: C–0.23, Mn–0.095,

Si–0.006, S–0.050 and P–0.040.

Hardness of austenitic stainless

steels wasmeasured at 86.2HRBwhere-

as as for the carbon steel, it was about

65 HRB. A pair of water cooled (4.0 ℓ/

min) truncated-cone electrodes with

5.0 mm diameters were applied to join

these base metals as shown in Figure 2.

Approximately nine hundred weld-

ing cycles were completed and the

electrode caps were sharpened once

to remove the mushrooms after com-

pleting about four hundred welds. The

electrode caps are then removed from

the holder and cut along the central

diameter using an abrasive cutter to

form flat surfaces.

Once cut, they were mounted using

resin powder on a hot press mount-

machine to show the cross sectional

viewof the electrode caps. Themounted

samples were then polished using

silicon paper, graded as 1 200/800p and

600/200p and also finish-polished using

Metadi polishing cloth. This polishing

process was conducted for about thirty

minutes to an hour on each sample until

mirror-like surfaces could be seen.

V2A etchant, consisting of 100 mℓ of

water, 100 mℓ of hydrochloric acid and

10 mℓ of nitric acid was used to etch the

polished samples. Thesewere immersed

for about 45-60 minutes. After that the

samples were well rinsed using plain

water, dried using an air blower before

ananti-corrosion liquidwas applied. The

samples were then stored in a vacuum

chamber for later SEM scanning.

These preparatory steps and the

above listed polishing materials were

deemed good enough to get reasonable

micro- and macrographs for analytical

purpose.

Results and discussion

Weld nuggets for the carbon, stainless

and dissimilar welds

Classical concerns about the spot weld-

ing of carbon and stainless steels are

based on the dissimilarity of melting

points in individual weld joints and also

the heat imbalances in the dissimilar

weld joints [7]. In this experiment,

both issues were observed for several

combinations of process parameters,

for example, the variations of welding

current levels against the variations of

welding time cycles weremonitored [7].

Figure 3, 4 and 5 show the carbon-

carbon steel, stainless-stainless steel

and carbon-stainless steel spot welds

performed using the copper-chromium

electrode caps. The right of Figures 3, 4

and 5 represent the corresponding SOR-

PAS simulations in which the maximum

temperatures are clearly shown before

the solidification processes started;

whereas the left sides shows the real

welds after completion of the solidifica-

tion processes.

Colour representations are used to

distinguish the molten zones and the

surrounding heat affected zones. Gen-

erally, the copper-chromium electrode

caps resulted in the formation of sound

welds in carbon, stainless and carbon-

stainless materials, even though the

caps themselves, deteriorate with time.

Electrode mushrooming and

chemical changes

The class-two spot welding electrode

caps are primarily made of copper and

chromium materials as major compo-

nents according to RMWA’s classifica-

tion [8]. The material is a dual-phase

mixture of chromium and alpha copper

as major chemical elements, although

it contains other minor ones. (See Table

1 for the detailed-list of chemical ele-

ments as well as other properties of the

electrode cap material).

Changes in properties happen at

elevated temperatures (Q=I

Rt) due to

the precipitation of chromium out of

the solid solution. When the electrode

is heated together with themetals being

welded, it has a high tendency to form