Project 1 from jakeru: Outboard Motor Skeg Repair (TIG aluminum)

[jakeru]

I repaired the broken skeg of an 8HP Yamaha outboard motor. I advised the customer of the risk of welding near heat sensistive components (like the lower power transmission "lower unit".) I said I have some techniques to mitigate, but not eliminate the risk; and it was up to him - he was game.

This was used in salt water, so came in with plenty of oxidation on all bare aluminum surfaces that had to be removed. I used a medium (brown) scotch brite 3" circular pad and it worked well for the flat surfaces. There was a stainless steel horizontal piece that was clamped onto this through the holes.

As I got prepping it, it turns out it had been previously weld repaired (and with some porosity also.) Current owner was not aware, so must have been from a previous owner.
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I prepped two areas for tack welding by grinding the oxidized cracked surfaces away completely on the two outer areas (where I would have clean oxide-free material for tack welding), leaving the center of the fracture in place (unprepped) for positioning the two parts together.
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After getting some strong tack welds on the ends, I used an aluminum 4.5" grinding wheel to vee out the remaining crack in the middle of the joint on both sides, veeing down to leaving probably about 1/4 of the material thickness in tact when I was done. I am experimenting a bit with using the aluminum grinding wheel instead of a carbide burr I usually use. So far I find the nonferrous grinding wheel is faster than a burr and a little less precise, needs a more wider area to reach but this was perfect. It is easier to control and less "chattery" than a burr as well.
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I didn't have any problems fully penetrating through doing it this way. I used 5356 filler. I did at times make full use of my machine's 200 amps (controlled by footpedal), and used probably about 25-30% EP AC Balance. 3/32" blunted lanthanated tungsten.

I did the welding in about 4 heat cycles (first to tack weld, second and third to "rough weld" and "finish weld" one side, and the last to weld the opposite side with 100% penetration.) I took breaks and cooled down the nearby components (focusing on keeping the lower unit housing cool enough to touch) in between heat cycles.

I used a wet, clay-like product called "heat fence" smeared on the parts I needed to protect from heat. It was a good start for heat control, but but a complete answer for the amount of heat input necessary for this repair, I found additional cooling withcompressed air, wet rag, and breaking down the work into multiple heat cycles was necessary. The heat fence material could be re-wetted to some degree using the wet rag.
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Some of the red die on the wet rag I used transferred to the welded surfaces, but the owner didn't care as it'd be covered up anyway. The owner was very happy with the repair, which took me 75 minutes start to finish. I sent him home with some pieces of zinc sheetmetal to put between the bolted together aluminum and stainless skeg pieces, to "sacrifice itself" and make the otherwise corrosion-prone bare aluminum last longer. I also told him now is a good time to replace the oil/fluid in the lower transmission, (which he said he had never done before) and take note of if the fluid was burned smelling, feeling, or appearing. I said if there is no evidence of burned fluid, it would be a good sign that the heat management techniques were effective and probably everything would be fine with the repair.
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