Ali MIG/GMAW 1st-ever macro-etch

What's special?

Done it! Managed to macro-etch an aluminium (aluminum) weld cross-section. Revealing the inner form of the weld.
Using "household" and "builder's merchant" supplies.

"Macro" is the next-level test in refining your weld conditions after visual inspection of the surface appearance of the weld. Macro reveals the inner cross-sectional structure of your weld - including weld penetration always and the layering in a multi-run weld.
Doing welds, you want "macro's" whenever you need to diagnose weld quality.
So you really want this which has been achieved as a generally available method.

What has been achieved - this is it...

Why is it special?

Aluminium is very corrosion-resistant. 5000-series Al-Mg alloys, which this is, notably so. Can be used in seawater without paint or any other protection.
Hence must inevitably present a challenge to etch, given etching is a selective corrosion reaction.

What is this etchant used here?

The etchant is hot "drain unblocker" solution, taken to be mainly/entirely sodium hydroxide (NaOH)
(North American recommendation is use spray-on oven-cleaner, the brand "Easy-Off" being mentioned - also with NaOH as the active substance)

That the etchant works comes with issues?

Yes. NaOH = sodium hydroxide = "caustic soda". It is very caustic. Horrible things happen if you get it on you.

Sample preparation

Very crude:

• saw - bandsaw - to present cross-section
• linish with flap-disk in 115mm / 4~1/2inch angle-grinder
• finer-grind with wet 120-grit emergy paper - rubbed with finger pressure, no flat backing

ie this is a "robust" technique.

About the weld - noting it has defects and other flaws

This is an Ali-MIG (Ali-GMAW) T-fillet weld done in the horizontal-vertical position ("PB"/"2F") on 20mm plate thickness 5083 aluminium with a suitably matching 1.2mm wire in pure Argon shielding gas.

Single-run weld to the right-hand-side; 3-run weld to the left-hand-side.

20mm thickness of aluminium plate, with the high thermal conductivity of aluminium, presents some welding challenge.
Consequently a common defect is lack-of-fusion (no weld penetration).

An explanation...
Weld penetration on a macro can be evaluated by tracing the continuation of the plate surface over the weld surface and seeing how far into the depth of the plate the weld penetrates beyond that projected line.
On a T-fillet there are three plate-surface lines you can project: the one line projecting the "continuing" plate surface; and the two lines projecting the "terminating" plate surfaces.

Here, one defect seen in the macros is excessive "finger" penetration.
ie excessive penetration.
That being so, there is leeway to adjust welding conditions in ways which will both avoid "finger penetration" and increase weld pool fluidity to eliminate other flaws/defects.

(R. Smith, 23Nov2022)