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Diffy housings are generally a nodular or ductile iron, definitley not run of the mill cast iron. As such they are weldable with MIG. TS, if you've welded small stuff without problem you'll have noticed this. When welding cast iron with MIG it behaves oddly (it seems to burn the cast away and leave you mig weld puddled in the middle, and is also porous) and you will not get fusion. With ductile/nodular, or cast steel the MIG welds have decent fusion and look just fine. If any body doubts this try weling up a known cast iron part (exhaust manifold, water pump housing etc), as soon as you do you'll KNOW the difference.
To clear things up a little, can a truss be migged entirely to the (nodular) housing or for larger projects such as that,or should regular cast iron welding practices be followed?
TS, don't take this as pro advise, as I don't regularly weld on rear housings and know just enough about rear axles to fix the ones I have. I am a welding contractor and have welded a few rear end pieces together though. So long as your mig is welding it without porosity then you should be able to get away with mig. If you are getting porosity then you can try going with a nickle rod. Either way you are likely to warp the housing if you are welding contiuosly across it. I am not sure if preheating the whole housing would lessen the warpage or not. If I was set on doing an axle truss from tube to tube across the center section I would likely tack the thing up as best I could, then bolt/clamp it down to my welding bench (with a dummy center section in it) with a slight extra force in the opposite direction that the welds are going to pull it out. Then skip weld (1" welds moving around the perimeter and allowing to cool completely between welds). Afterwards you will likely needs to shirnk the opposite side with welds, or the torch and quench method, to straighten it back up.
I hope I haven't muddied the water too much for you. There are likely a bunch of guys on this or other boards who do this regulary and you may want to post around a bit. My only point in the begining was that the center sections were not cast iron and could be welded with other processes.
there is a site that has some pretty unreal builds.. it is www.evolutionmachine.com any questions you have on any type of rig build ups im sure you could get alot of help there as that is what they specialize in.. they are a shop out of calgary alberta and they do some amazing work there..