cast silicon bronze cracking

by julian wiseman
(gloucestershire, uk)

I often weld cast silcon bronze sculpture and where sometimes the core has shifted to one side leaving sometimes areas of 1mm metal thickness. When I attempt to tig weld small Pitts in these areas the metal around the weld often cracks.
How can this be avoided please?
I would weld this metal thickness around 70 amps with a foot pedal set to Max 150 amps. The tungsten would be 5mm out from the ceramic sharpened to a point of about 60 degrees. I also leave torch over weld for around 5 seconds post gas flow.

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I think a little preheat of the area might help the cracking issue.
Also, using a torch and a brazing alloy might work better if the color match issue can be overcome