Thanks for the response Lawrence. Your replies are always filled with good info.
The weld assembly that we are currently discussing is pretty big and has lots of fillet welds. We are manually welding it with DCEN right now because the welders are most comfortable with that. I'd rather be using AC, but the welders need more practice to be able to use this process and produce acceptable welds. The subsurface quality always seems to vary when they manually weld with AC. We are trying to look using a robotic welding system for this assembly and have been looking at what our options are. The joints arent too complicated and the shortest weld is probably no less than 20". In terms of joint access, using the TIG process might present some challenges due to one weld joint we have. I would prefer to use MIG if we can get consistent high quality.
I've talked to both Miller and Lincoln and it's interesting that they both feel that MIG, most likely, will not produce the results we want. However, these parts will be machined and chemically etched prior to fitup to help maintain cleanliness. There is talk of possible alodining these parts so that if they sit around, they won't start to pit. But, regardless, they will still be chemically etched prior to fitup. That is one issue that I've heard mixed reviews about. Some say that when you alodine aluminum, the coating actually migrates into the material quite a bit further than people think and that the only true way to remove it is to machine the surface off. Have you heard this in your experience? I have assumed that a chemical etch that removes 0.0004"-0.0006" material would be enough, but haven't really done any testing to verify that the surrounding grain structure is free of the chemicals in alodine. Even though we chemically etch 1" on either side of the weld joint, it still would be good to know how deep this coating really goes. Is it possible that the heat from the weld would draw in some of the components from the surrounding alodine, even 1" away? If porosity is just coming from hydrogen, then the source has to be coming from the way we are cleaning the part.
Anyways, we are looking to to some weld testing with lincoln to see what sort of quality we can achieve, but I'm am somewhat curious as to why they feel that process is not capable of achieveing the higher quality results.
In regards to pulsing, my interest was to see if that had any effect of porosity formation. Our automated procedures right now just use straight DC, but I read that powerful short pulses have a better effect of boiling out contamination. I would assume that if you were to pulse, the amount of time on the low side of the pulse would have to be pretty short so that you aren't freezing the puddle too quickly. Does that sound reasonable, or is that too big of a generalization? If we can achieve better results with pusling, I'd imagine we could move faster as well.