It can not be welded because the type of aluminum alloy (7075) used in fire arm frames is not weldable. It is also needs a heat treatment. It will fuse, but the weld effected zone on each side of the weld will have to much heat induce stress and will fail. No way to weld and heat treat the weld effected zone. No way I would even try it and I know guys that can weld 2 beer cans together and never blow a hole. Some gun parts are 6061 which is a bit more forgivable, but usually not use in the frame itself.

Read this: Aluminum Workshop: What’s so bad about welding 7075, 2024? - The Fabricator

and from another source

"material can become susceptible to stress corrosion cracking after welding. This phenomenon is particularly dangerous because it is not detectable immediately after welding, and usually develops at a later date when the component is in service. The completed weld joint can appear to be of excellent quality immediately after welding. However, changes which occur within the base material adjacent to the weld during the welding process, can produce a metallurgical condition within these materials which can result in intergranular micro cracking, which may be susceptible to propagation and eventual failure of the welded component. The probability of failure can be high, and the time to failure is generally unpredictable and dependent on variables such as tensile stress applied to the joint, environmental conditions, and the period of time which the component is subjected to these variables."

I looked into it. Brother is a certified welding inspector with lots of reference material and he agrees.

On a steel gun (4140-4150 alloys) you could probably make a brass insert, bevel the crack, heat the frame up to about 450f and tig weld with 70 series filler. Remove the brass insert, put the frame in an HT oven (I have one) heat it up to about 850f hold it for 2 hours and then drop the temp about 50f an hour till it was around 350 f. This is a stress relief and as every S&W frame I have checked with a hardness tester is relatively soft it would not ruin its temper. Then retap the threads and machine that part of the yoke cut. Still your doing a repair in a thin section. I might try it in a K38 or something and see what happened. If it failed again it wouldn't be catastrophic, just recrack.