The image above shows the Fusion Zone, Fusion Boundary and Heat Affected zone fo

Question

The image above shows the Fusion Zone, Fusion Boundary and Heat Affected zone for Commercially Pure Titanium - Stainless Steel, CPTi-SS

The Heat Affected Zone to the right of the Fusion Boundary is for the CPTi.

The material was TIG welded with no filler metal and then cracked and failed after the weld had been completed.

We are trying to assess when the crack occurs on the Stainless Steel side of the weld.

But relative to this image: why do the black cracks occur in the fusion zone and then stop once they reach the Fusion Boundary?

Thanks

Explanation / Answer

In TIG welding there is use of nonconsumable copper electrode. While welding of CPTi-SS without filler material there is formation of stress because of longitudinal shrinkage acting on weld metal. these welding causes initial dissolution of primary austenite followed by grain growth in delta-ferrite and there is formation of austenite during Coolling. As fusion zone stopes the unaltered heat affected zone remelted fusion zone get started.

Hence cracks are only remain up-to fusion boundary. Due to presence of hardening carbon black cracks are seen.

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