焊

Left 火 is the fire radical in its full standing form (the bottom-form is 灬). Indexing radical, putting 焊 in the heat-and-flame family (烧 burn, 炒 stir-fry, 烤 roast, 炼 refine). Carries the core meaning: welding is metal joined by intense heat. The radical announces 'this action requires fire.'

Right 旱 supplies the sound - hàn straight through, no drift. The 'drought' meaning of 旱 (日 sun beating down) does carry a faint semantic flavour: both drought and welding involve searing dry heat. Same phonetic family: 悍 fierce, 捍 to defend, 汗 sweat - all carrying the hàn sound and a hint of intense heat / pressure.