Yeah, I'll take the bait on this one, mainly because I'm not an AME, nor an expert! But at least we have a thread for the topic, whether useful or not, which is not hijacking someone else's thread!
So, "where and when"?
And did that AME who charged Strega $4k for an oil change
I have absolutely zero knowledge of this event. So I wonder if there's something else we don't know... Maybe where and when?
For my own recent experience, a client could not pass a required test for an approval to their aircraft, so I had to witness a second test attempt. The duration of the second test was 1.5 hours (and it was a pass, after their changes). But my invoice for my witnessing that retest exceeded $10,000. Oooo, Strega's getting ready to slam me for that. But, understand that to witness that test I had four whole days of travel, and $7,700 in airfare, to get to and from the test location. It turns out that they had inquired about hiring a King Air to fly me direct and wait for me, to save time, but that was quoted at more than $20,000. 'Interesting that there are no direct flights from North America to Greenland in September... But, I drift, or digress, or hijack, or whatever.... Did I rip off my client by charging my standard rate, and performing the requested task where and when they wanted it, on two days notice? They don't seem to think so, they thanked me happily, and paid my invoice promptly.
So, okay, a person who demonstrates the required skill set is issued an AME license. Does that make them an expert? Well, no more than the 16 year old, with the new driver's license should be driving a coach full of casino goers, or is expert at curling! Any expertise is gained with experience, and personal growth, and might be unrelated to the "official" qualifications. I can certainly think of Transport Canada inspectors who have audited or inspected my work over the decades, who demonstrated considerable expertise in a very useful and applicable skill, which was only peripherally related to what they were auditing. So there I am, getting audited by TC at no cost, and getting all this other excellent advice for free! How happy was I! I wrote it all down!
During a break from my training a very nice fellow in his new (to him) flying boat in BC yesterday, he asked me for my advice on a welder purchase. My first words were "I'm no expert at that", I have a stick welder, and oxy-acetelene set, but I hardly profess expertise in welding, nor selection of equipment. He hired me to train him in his aircraft - and I'm no expert at that either, I'm just good enough for the insurance company, and to grow his skills apparently much to his satisfaction. Oh, and he happily paid for my time and travel, and has invited me back to do more in the spring.
So if you have a maintenance task, and you ask an "AME", maybe you've got the right person, or maybe you have an AME who knows very little about it. Hopefully that AME is forthright about their skills, and you are paying attention to the factors which could affect your selection.
So to respect the OP's question, Being an AME does not make one a structural welding expert - being a trained welder in that realm of welding might. Being a very experienced aircraft structures welder would be even better! Choose the person most qualified for the task, even if you have to bring them in from afar for it. I know some excellent aircraft structural welders who are not AME's - so much the better! And, by the way, aircraft welding is specialized work, so it's an AMO you're looking for, rather than an AME to perform it.
So in all of that, I have presented nothing new, just the facts as they were before the question was asked. But, if you have an aircraft which has been issued a full C of A, what you have to do to keep that valid is very well described, and you will need a suitably rated AME to carry out, or at least supervise and inspect the work. They deserve to be fairly paid for their investment in their skills and qualifications, and time and overhead. If you don't agree, don't ask for their services. They did not get into the industry to provide charity aircraft maintenance services to frugal owners....