Remember that you don't necessarily need a dock repair company, anyone who does metal fabrication could fix it. The key thing to be mindful of is that aluminum is a difficult metal to work with, and welding it is not something just anyone can do, it really is almost an "art form" so to speak.
Two reason for that. Aluminum is very soft and cannot hack excessive amounts of heat. The welding solution must be able to get just enough heat to properly penetrate without blowing holes through the metal. That is a fine line to walk and easier said than done.
To do it right it needs to be TIG welded which requires a more elaborate setup and uses compressed Argon gas. Now there are those that say it can be mig (spool fed) or even arch welded with a stick, both are not as good because with aluminum heat is your enemy and very hard to control with both a mig or arch getup. Matter of fact if anyone suggests they can arch weld it they are a fool and don't let them touch your dock! I'll briefly explain each solution.
TIG welding uses argon gas as a medium to control heat around the fixed arch point. It also acts as a flux to blow out any impurities in the surface of the metal, and with aluminum the metal has to be super duper clean. The control of the heat applied when creating a welding puddle can be far better controlled and media used for welding can be better control fed into the puddle because it is NOT the source of the arch. The net result good heat control, good penetration and less likely to have a blow through.
MIG welding uses a wire feed and the welding puddle is created by the arch created at the end of the wire (welding media) as it feeds into the puddle once established. Control of the heat is directly affected by the gauge of the wire used and the feed rate of the gun. Since no flux is used, and aluminum being what it is, MIG welding is more subject to spattering and potential blow through if any oxidation is present. The heat source being the welding media must be kept steady hence the reason why this is not a preferred method.
Stick welding is well using a aluminum rod covered in flux where the welding is manually controlled by the steady hand of the welder feeding the rod as it melts into the puddle once formed. The problem with stick welding is that it requires the greatest amount of heat to keep that rod lit up, and is extremely unforgiving. Heat can vary especially as the rod is consumed and as I said the key to proper aluminum repair is controlling the heat. Doing steel it's great with a stick, unless it's sheet metal, as it takes a lot to screw it up or blow through your work material.
So my recommendation to you would be to seek out a metal fabrication place with somebody that is experienced with aluminum repair. Make sure they are going to TIG the spots that need to be repaired and one last thing, any areas that show ANY sign of stress need to be removed. Once aluminum has been stretched or stressed it's junk. It's not going to be a cheap fix so be prepared to spend a few bucks, but at least you know a little bit about the process and options out there so you can be a little more knowledgeable when questioning how the repair work is going to be done.
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