...welding is normally stronger than the original material they say. When repaired by professional people, they do all sorts of testing before sending it out, including pressure testing.
'They' are wrong. VERY wrong when it comes to welding aluminium...
Welds in carbon steels can be a good match for the properties of the parent material or they can be harder (stronger) and more brittle if we're talking about a hardenable flavour of steel and cooling rates were excessive or the wrong flavour of filler metal was chosen*
With stainless steels the corrosion resistance can be ruined (local to the weld) very easily
With aluminium and it's alloys welding ALWAYS softens them in the HAZ (heat affected zone) regardless of skill or knowledge
Alloy wheels are virtually always heat treated after being cast, cast and flow formed or forged. Here's an article on the production of MB wheels that describes the process
Production: the best quality from excellent suppliers - Following strict Mercedes-Benz guidelines | Daimler Global Media Site > Brands & Products > Mercedes-Benz Cars > Mercedes-Benz Passenger Cars > Special Topics
GK-AlSi7 is a partial designation that covers several flavours of aluminium, the actual alloy they're talking about is probably AlSi7Mg or a proprietary variation of it and the heat treatment process they're describing is T6 (solution annealed and artifically aged). Welding locally destroys heat treatment and, generalising, the best case scenario with something in the T6 temper is around 60% of the origional strength in the HAZ
Flow forming and forging both impart a 'grain structure' in the material that improves mechanical properties. Welding is, in effect, casting...
Pressure testing just tells you that the weld is air tight, nothing more. Dye penetrant inspection (DPI) will uncover welding flaws that are invisible to the naked eye but will only highlight flaws that break the surface. Underneath the surface there can be liquation cracks if the wrong flavour of filler wire was used or LOF (lack of fusion) defects, inclusions etc. Need something like X ray inspection or UT (ultra sonic testing) to find those. DPI is cheap and doesn't take long, X ray and UT inspection involves expensive gear and specialists i.e. puts repair costs up significantly
If the
proffesional painter with a TIG says the repair will make the wheel as good as new or will be stronger than the origional they are either ignorant or lying. Repairs to wheels can work because everything is designed with a safety factor i.e. overbuilt. FWIW i'm a welder and fabricator that specialised in aluminium and exotics (read i weld more magnesium or titanium than carbon steel) and as a general rule don't touch wheels for the above reasons... no way for me to know what sort of safety factor the wheel was designed with so while i could physically repair the wheel in the OP easily and the penalty for getting it wrong is usually a tyre that'll go flat long before a crack has grown long enough for properly bad things to happen i wouldn't because there are too many unknowns
* For example with chromoly steels if the part is gonna be quenched and tempered after fabrication you need a matching filler wire if the weld will respond to heat treatment and pre/post heat is very necessary. Especially for say a butt weld in heavy wall pipe. If the part is made from normalised chromoly and won't be heat treated (tube chassis etc) then using a matching filler wire will result in welds that are significantly harder and more brittle than the base material. Mild steel filler is the correct choice for this application, an undermatch on paper but we're talking fillet welds on thin wall tube and provided the weld size is correct compensates for the softer mild steel resulting in a strong but still ductile part
Just saying.......
