I think you're 100% correct that they are all cast. I don't generally get into "Lawns & Gardens" type stuff so it's all a bit foreign to me. I see that the equivalent structures on backhoes are fabricated and therefore are weldable.
TBH I am of the opinion that Case are covering their a$$es by putting the "Do Not Weld" indications on there. Maybe the type of customer who would own a backhoe might not be very skilled at welding. I guess there is something in that.
To give you an example a Cat 992G/K loader boom is cast and is weldable. I am not sure of the material, probably not ductile. There are very lengthy and detailed instructions available giving the exact procedure that must be followed in order to carry out weld repairs on it, and also detailing certain areas of the boom where it is absolutely forbidden to weld. But then again the average 992 loader owner would likely have a fully-equipped maintenance shop plus probably one or more specialist welding companies that they could call on to do welding work on cast material.
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