Cast aluminum welding repairs can be tricky.
For the most part, the level of difficulty depends on the service environment the part was subjected to.
For example, aluminum can be exposed to fresh water, salt water, exhaust gas carbon, oil, and other nasty things.
The worst cast aluminum weld repairs are usually due to oil or exhaust gas carbon....or a blend of the 2.
Cast aluminum welding is hard because castings have impurities.
Oil soaked castings like crankcases from motorcycles, machinery parts, or even cookware, can give you fits. As soon as you strike an arc, you can see crap boiling to the surface.
When heat is applied, small voids become like little pressure cookers and the gas will try to escape somewhere. Mostly in the molten aluminum puddle and the gas gets trapped as bubbles as the aluminum solidifies....and becomes porosity.
Porosity is gas bubbles....plain and simple..but what is not so simple is what causes those gas bubbles.
The gas can come from lubricants, abrasive grit, moisture, hydrogen, oxides, etc.
Sometimes it is very hard to know exactly what causes porosity.
Sometimes its something as simple as the level of humidity.
Sometimes its a thick layer of oxide...or a rough sheared edge. Determining the exact cause of porosity can be like chasing a ghost.
here is a little known resource for selection of tig welding rods for cast aluminum
5356 is stronger than 4043 but tends to get porosity when used to weld aluminum castings.
4043 is fine and usually a pretty good choice for castings because the silicon content makes it a close match for most aluminum castings.
Maxals 4943 is a fairly new alloy that is touted as a great replacement for 4043 on aluminum castings. it works pretty good.
But in my opinion, 4047 usually is better for preventing porosity.
Because of the higher silicon content.
Silicon is a deoxidizer but also lowers the melting point of aluminum alloys.
A lower melting point means the filler wire will flow at lower amperage...and that means it will be less likely to draw impurities out of the cast aluminum base metal.
4047 aluminum filler wire contains around 12 % silicon and although has a slightly lower tensile strength than 4043, tends to make welds that have considerably less porosity than welds made using 4043 filler metal.
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