The words weld, welled sound the same but have different meanings and spellings. Why do weld, welled sound the same even though they are completely different words?
The answer is simple: weld, welled are homophones of the English language.
To join (metals) by applying heat, sometimes with pressure and sometimes with an intermediate or filler metal having a high melting point.
To bring into close association or union.
To be capable of being welded.
The union of two metal parts by welding.
Simple past tense and past participle of well.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").