loch

im having a recurring problem on my car, it seems for some reason or the other the exhaust downpipe running from the engine cracks and makes an awful noise

first time it happened it wud only happen wen the engine was cold and den after about 30 seconds it wud go away but then one day it never went away

so what i did under the suggestion of a mechanic was find another one and got it changed. this remedied the prob for about 2-3 months now im hearing the sound again when the engine is first started after a long time of being off

so my questions 1. what can be causing the cracking and what is a solution to this prob 2. is it possible to get the downpipe welded and if so will it last a long time?

car is a 1996 mazda lantis 1500cc

any help is appreciated