Confirm material grade and electrod

I presume that this was a hand held XRF unit.
What sample prep did you do? Sand paper and solvent wipe at least?
Did you run in double precision mode?
But, if you want to weld then hardenability is the biggest issue so you will need a method that will measure C content also.
Portable OES or a LIBS unit would do that.

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P.E. Metallurgy, consulting work welcomed

2.5% Cu? Are you sure about the composition, as per EdS's question?
And are you sure about welding it?

Can you consider other options?

Treat this as low carbon steel and use E7018 2.5 mm dia electrode for weld build up.

DHURJATI SEN
Kolkata, India