I see you have not had an answer yet, so I will give my opinion although I do not have much experience in the type of application you are talking about.

Personally I think that GMAW is not the greatest process for making tack welds because it is very difficult to control the arc at initiation. This often results in "pieces of wire" sticking to the start, as well as other defects. This is generally not such a big problem when making long welds, but if the arc is not given much time to stabilize before being extinguished, you may end up with defects in these tack welds. I would suggest GTAW for this job.

Also, I do not believe that the presence of the tack welds would actually reduce crater cracks at the arc stop points, because one needs to supply additional moulten metal into the top of the solidifying crater to fill up the shrinkage voids. Metal below the solifidying surface will not do this job, even if it is moulten, because then it just contributes to the volume of solidifying metal which would mean a greater shrinkage hole. The "backtravel" that you mentioned should be good enough to fill up the craters.

In terms of the strength, the tack welds should have little or no influence because the weld that comes after would generally remove any effect that the tack weld would have had on the base metal.

I do not know what alloy you are welding, but just remember to follow the general rules for welding aluminium, based on the type of alloy you are welding.

Hope this helps.

Regards
Niekie Jooste

I suggest you go to www.weldreality.com and ask him, he can help you with very specific info on that process. From experience I have found that alum gmaw is not a viable process for tack welds, you may however be able to find a niche and have acceptable tacks. We have successfully tacked with gmaw by preheating but this is not always viable. As for using the tack to fill your end crater it has merit but the difficulty lies in tying into the tack. Trying to "remolten" a tack is not and easy task. I agree with the previous post, GTAW is a much better answer, at least for tacking, then if you need speed go to gmaw for your welds. good luck!

Hello,
I agree with the above two replies. If you go to a GTAW tack, you might consider using the DC helium process. That would eliminate the need for preheat and give you a deep penetration, strong tack. Of course, you would need to get a GTAW machine and helium.

An alternative might be to make an extra large tack with the GMAW. This would allow you to have the puddle well started for a strong tack. Then grind it down just before you weld over it. To get full penetration welds and to minimize porosity, you should grind down tacks before you weld over them anyway.

Roger