Sheet aluminium is uniform, predictable and, once you know the alloy, reasonably well behaved. A casting is none of those things. Its thickness varies, its grain structure varies, and its surface has been in contact with a mould rather than a roll. Welding one is a different job from welding sheet, even when the alloy designation is similar.
The practical differences show up immediately. Castings often have a skin that is contaminated, sections that are much thicker than the surrounding wall, and internal voids that were there from the day it was poured. The arc reveals all three, usually at the worst possible moment.
Castings commonly contain gas porosity and shrinkage voids. Some of it is deep inside the section and irrelevant. Some of it sits just under the surface and opens up as soon as the arc melts the skin, which produces a puddle that keeps collapsing into a hole no matter how much filler you add.
This is why castings are inspected before welding rather than after. A quick check with a light and, where the geometry allows, a look at the machined faces tells you whether the area you are about to weld is sound. Finding porosity at that stage is a chance to reposition the joint. Finding it mid-bead is a repair inside a repair.
Pre-heating a casting does two useful things. It drives off moisture that would otherwise turn into gas in the weld, and it narrows the temperature difference between the arc and the surrounding mass, which reduces the quenching effect that makes cast aluminium welds crack.
It also makes the whole part hotter to handle and slower to cool, which matters on a large casting because heat spreads unevenly. The aim is an even, moderate warmth across the region being welded, not a hot spot under the torch. Uneven pre-heat is worse than none, because it adds a new distortion pattern to a part that already has one.
Cast surfaces carry mould residue, oxidation and, on used parts, oil that has soaked into the porosity. All of it has to go before the arc arrives. Solvent cleaning removes the oil; a stainless brush removes the surface oxide; and on a used casting, grinding back to bright metal in the weld zone is usually the only honest way to start.
The brush matters. A carbon steel brush leaves particles that contaminate the aluminium and produce black specks and porosity in the bead. A dedicated stainless brush, used only on aluminium, is the standard answer. It is a small discipline that pays for itself on the first part.
The filler for a casting is chosen for crack resistance more than for matching strength. Castings tend to be sensitive to hot cracking, and the filler that welds most comfortably is one that pushes the pool composition away from the crack-sensitive range, even though the resulting weld is softer than the casting around it.
Dilution is unavoidable on a repair, because the arc has to melt into the casting to fuse. So the weld metal is always a mixture of filler and parent, and the properties sit somewhere between the two. That is acceptable on a repair, and it is one more reason not to over-penetrate: more dilution means more of the casting's weaknesses in the weld.
Welding over a crack or a porous patch without removing it produces a weld that looks repaired and is not. The defect is still underneath, and the heat of welding usually makes it worse by opening it slightly and filling it with gas. The groove is the repair; the bead only closes the groove.
Grooving means cutting back to sound metal on both faces, with a profile wide enough for the torch to reach the bottom and for the bead to be cleaned between passes. A narrow, deep groove looks economical and produces a weld with slag traps and lack of fusion at the root. Width is not waste on a casting repair.
Cast aluminium cracks when it cools too fast, because the section is stiff and the weld metal is weak while it is still hot. Slow cooling gives the weld time to develop strength before the shrinking casting starts pulling on it.
The usual methods are simple: wrap the part, cover it, keep it out of a draught, or leave it in the warmth of the pre-heat instead of quenching it. What must not happen is a part that is welded warm and then dropped on a cold bench or, worse, cooled with water to speed up the next operation.
Not every casting is repairable, and the honest answer is sometimes that it should be replaced. A casting that is cracked through a highly stressed boss, one that has multiple cracks radiating from the same point, or one where the porosity extends across the whole joint face is usually beyond economic repair.
The judgement is about what the part does rather than what it looks like. A cosmetic cover with a crack is a straightforward repair. A load-bearing housing with a crack at a mounting point is a redesign question disguised as a welding question, and welding it will only move the failure somewhere less convenient.
Cast aluminium work is quoted on the assumption that the casting is sound and the repair is local. Both assumptions are worth testing before the order is placed.
Cast repairs are a small but steady part of our aluminium work. The wider aluminium side of the shop is on our aluminium TIG welding pages, and the fabricated parts that come off the same benches are under fabricated components. Pipe and tube sections are listed under exhaust pipe work and exhaust pipe sections, stainless systems under stainless steel exhaust systems, and the stainless welding discipline we apply to them under TIG stainless steel welding.
