Poor machined surface on weld

[AiR_GuNNeR]

Hi guys,
I need a little advice from the experts out there.
I put together a TIG outfit consisting of a DX200, a home made cooler and a WP20 torch.
I make a 6" diameter sidewheel for rifle scopes that mount to the existing focus wheel out of 6061. I start out with a 5/8" x 6" round chunk, and most of the material is machined away. In order to decrease my machining time, I thought I would experiment with welding up components and performing a final clean-up on it afterwards. For a trial, I took a piece of 1" round stock, machined the surface off and machined a close fitting hole in a 1/8" thick plate of 6061.
I tig welded a fillet around it expecting to machine off a minimal amount of material to put a nice finish on it. The weld machined horribly. I couldn't get a nice smooth surface on it. I looked like instead of machining off smooth, it tore like it was gummy. I suspect that I might need a hard welding rod? Is there a special filler rod that should be used for something like this? Does welding the 6061 soften it up to the point that it is hard to get a smooth cut on it?
Thanks in advance guys!
Eric

[Rocky D]

I'm not a machinist, but that's what happened, you softened it by welding. Other machinists here will tell you how to machine it. Freeze it, maybe?

[HT1]

WAIT!

No seriously, 6061 is age hardened, so it may machine better after a couple of weeks. though a heat treatment process, which I cannot recommend,( I was never taught how to heat treat aluminum) would help.

personnaly, (and I hope someone corrects me, so I can use their advice) "as welded" 6061 probably is not machinable

[Rocky D]

I don't believe "age hardening" means that after a period of time, it gets harder. To age harden aluminum, one process takes the part out of a freezer, then heats it up to a certain temp for a specified amount of time, then drops it into a cold water bath in a split second. Another process is the salt bath where the part is suspended in a 975° liquid salt for a certain length of time then plucked out and dropped in cold water. Not something you can do in your garage.

[HT1]

Age hardening: precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment).


Both are correct, your method would of course be much faster, and beyond most peoples means


Back when the earth was young when I worked in a foundry, we had ALOT of thumb rules

"Cool it as slow as possible, to keep it from getting hard and being un machinable( refering to Steel)"

"Hide it as long as possible before delivering it to the machine shop So it wont be "gummy"( refering to Aluminum) A bit of copper 1-3% if memory serves Helpped with this, more then once I have sent cast aluminum parts to the machine shop, that where like trying to machine clay"

Belay that, my technique will only work on castings!

A weldement would require a uniform heating to dissolve the CuAl2 into the grain structure

[Goat]

This is what happens to 6061-T6 when welded. It pretty much turns to dead soft. (-O temper).

The softer it is the worse the machinabilty gets because the material just spalls.

You didn't mention what tooling you used so here is are some general suggestions. Cut wet or with a air nozzle at the tool, this will aid in chip ejection and reduce edge build up on the cutter face. WD40 is good, unless you plan to anodize. Kerosene is the best...

Your tool must be sharp! I usually pull a brand new end mill for dead soft. You need a keen edge so you cut the material instead of mushing it around. This is why I tend to use coated HSS cutters instead of carbide. Buy some cutters that are TiCN coated, 40 degree plus helix and have polished flutes, dedicate them for the soft stuff.

[B_C]

Just curious.....What filler did you weld your part with?

[vicegrip]

This is what happens to 6061-T6 when welded. It pretty much turns to dead soft. (-O temper).

The softer it is the worse the machinabilty gets because the material just spalls.

You didn't mention what tooling you used so here is are some general suggestions. Cut wet or with a air nozzle at the tool, this will aid in chip ejection and reduce edge build up on the cutter face. WD40 is good, unless you plan to anodize. Kerosene is the best...

Your tool must be sharp! I usually pull a brand new end mill for dead soft. You need a keen edge so you cut the material instead of mushing it around. This is why I tend to use coated HSS cutters instead of carbide. Buy some cutters that are TiCN coated, 40 degree plus helix and have polished flutes, dedicate them for the soft stuff.

I am a machinist, but Mr. Goat has pretty well nailed it.
I could add and fine tune a tad but I'm seeing double and shadow-people:eek::D
Lack of sleep will do that.

Cheers
VG

[seattle smitty]

Weld some test pieces with 4043 and 5356 filler, and see which machines better. Your problem, as you describe it, is not with the 6061 base metal, whether softened (it is) or not, but with the weld deposit. Try different feeds and speeds, tool angles, etc.. And make sure you aren't introducing problems such as porosity when you weld. Do all the surface prep, and try different type tungstens and other controllable parameters. Experiment.

Vicegrip, I've never understood the mechanics of chip formation when machining aluminum. A tiny pile of aluminum immediately builds up on the raked edge of the cutter . . . does that build-up itself become the cutting edge, or is it constantly removed and renewed?

[vicegrip]

Vicegrip, I've never understood the mechanics of chip formation when machining aluminum. A tiny pile of aluminum immediately builds up on the raked edge of the cutter . . . does that build-up itself become the cutting edge, or is it constantly removed and renewed?

You nailed it:
There's aluminum and there's aluminum.
AKA
2024 & 7075 are metal (alloys) shall we say.
You do get "chip formation".
That is to say a clean fracture has occured.
Elvis has left the building.
In all chip formation, the chip had indeed suffered some
compaction / distortion / elongation, or some combination
of the above. But it has yeilded and been torn free of the parent metal.

6061, 5052, and dia-cast, and even pourcast as in cast jig-plate,
Are not purely metals "alloys" so to speak.

Sillicon / and other ingrediants are deliberately added, just for the purpose
of making these metals elastic / plyable / draw-able.

Think of the your aluminum storm-door / lawn-chair arms etc.
These are extrusions, intended to be shot thru an opening
like a kids fun-factory with Play-Doh. Die-cut to lenght and shape.
maybe some holes punched and story's over.
Similarly carburator's, old wash-machine agitators, lawn-mower engine-cases
etc. etc. are "die-cast" and the metal is forced under tremenous presure
into the mold. Now even the pressure is not enough for the demanding
cavitation geometry of today's needs. Now we use vacume to pull the leading
metal all the way into the over-flows, so-as to insure that the mold-cavity
has completely molten aluminum in it at the end of "the shot".

You only get adequit "chip formation" at grossly exagerated rake-angles
and or helix angles. High-polish in the flutes helps as well.
And this is the machining where coolant, as much and as filmy as you
can get is required.
The material is so ply-able you don't really acheive "fracture".
Elvis wasn't even in the building.
More like try-ing to carve a wax figuring with a table-spoon.
The material just sort of mooshes ahead untill enough of it has been
"drawn" too far to hang-together. The wrong type of heat generation doesn't help either.
When "real metal" molecules are torn appart, heat is released.
Ideally you use feeds speeds and cutting edge geometry to try to get as much of
this heat in the chip as possible. The heat caused by machining
putty-metals is a product of abrasion on the cutter.
Further agrivating the mix is those same non-metal components
cause a bond during this friction heat-generation as the removed material
is smudged against the flutes.

A momentary loss of cutting fluid can mean a completely bonded end-mill
or saw blade. In seconds if feed is not stopped the tool will be lost.

Remarkable some machining is done dry like cutting a three inch thick
plate of cast-jig-plate in half on a 7 H.P. table saw. With a carbide tipped blade.
Feed not too heavy, not sure why this works but after being
reassured I actually did this in a pattern-shop back in the 80's.
It was every-day buisness.

Cheers
VG

[seattle smitty]

Well thanks, VG. I do this at the local library, so I can't make immediate responses, thank yous, etc.. I don't understand the Elvis reference, but the rest is informative.

I imagine that the big aluminum producers have worked out best machining practices for as-deposited 4043 and 5356. If I find out something, I'll pass it on. I remember seeing something like what Eric refers to, when I milled a piece of 6061 after welding something to it with 5356. The surface finishes of the weld metal and the base metal were markedly different, and the 5356 looked like I'd tried to mill solder with a dull endmill, all ragged and ripped up, even though that alloy is said to machine fairly well. It didn't matter at the time, so I never pursued it, but now I'll see what I can find out.

[flamin]

Seattle Smitty-


Fisrt off, some good pointers so far. I'll just a couple.
If your're getting material build up on the cutting edge of your tool, there could be a few things or a combination of things happening. Running too slow of RPM many times is the root cause. If you're using a HSS cutter, 600-800 SF is ideal, if you're using carbide, you could run it up to about 1200 SF. Also, slower feed rates could cause the build up as well, especially if the cutting tool is dull or if the wrong tool geometry is used. A sharp cutting edge with a positive rake can over come that. And of course using some sort of coolant helps. Honing a .002 or more radius on a pointy cutting tool helps also. Too pointy of a cutting can tear up the surface.

[dynasim]

Also realize that if it machines like that, it is likely weak, and if the welded joints have stress concentrations, you'll have a different part than what you get from a fully machined block. If you heat treat, though, they can be the same.

From my understanding(I only have cursory knowledge of aluminum), the heat treatment steps are relatively "cookbook", if only one had the cookbook, particularly if the part is relatively small.

Good Luck

Chris