Author Topic: should metal be scuffed or as flat/polished as possible for brazing/soldering?  (Read 2473 times)

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Offline coppercone2

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So assuming you setup the metal correctly (i.e. for a silver solder joint say 5 thousanths), people just say it needs to be clean.

Do you get a better joint if you make it as flat/polished as possible, or do you want some particular surface finish?

I thought the brazing occurs on a microscopic level (where the braze enters the fissures that form in the material as its heated). So if there was a impulse I am thinking that a level polished surface would transfer it better then a scuffed one? Is it possible it would make it stronger towards beatings?

If you made both, how would mechanical wave transfer through both interfaces appear? In terms of the metrology of impulse transmission through metal (its gonna be a small difference)?

I also imagine the flow might be janky if its not polished?
« Last Edit: February 27, 2019, 11:45:15 pm by coppercone2 »
 

Online KL27x

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Yes, you should try to get more fresh air and sleep. Then ask yourself a new question, tomorrow. Do I still care?

IF you do still care, maybe you will try it instead of thinking about it. And when you post, you will have some news.
« Last Edit: February 28, 2019, 04:18:05 am by KL27x »
 

Offline coppercone2

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how am I going to try it? You know what kind of equipment would be necessary to test the difference?

I am asking a theoretical question about metallurgy. I am curious about the behavior of braze/solder joints.

I will not be able to detect something like a 0.1% difference. There is no way. Unless its big.

I want to know what happens with the mechanical wave in the say 100 microns (~200 grit finish) between the bulk braze compound and the boundary where the metal dendrites peak average is formed from finishing. Bulk thickness of braze is something liek 150 microns

This boundary gets smaller the more polished it is. I also want to know what difference flatness makes. I am going to assume a very polished finish is something like 1 micron peak (Ra).

Why are you busting my balls?
« Last Edit: February 28, 2019, 04:37:49 am by coppercone2 »
 

Offline David Hess

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My experience is with silver brazing where usually the base metal rips before the joint or filler so other than having clean surfaces, I do not think it matters.

But it would be easy enough to test by brazing some joints under different conditions and measuring the fracture strength in bending or tension.
 

Offline coppercone2

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do you think it would be significant enough to detect without very expensive equipment? I am not interested in getting more strength but understanding the mechanism.

I never really busted open a brazed joint (usually their not flat).

Also, would it behave differently with regards to flex cracking ? like from heavy vibration.
« Last Edit: February 28, 2019, 04:55:25 am by coppercone2 »
 

Offline David Hess

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do you think it would be significant enough to detect without very expensive equipment? I am not interested in getting more strength but understanding the mechanism.

If there is a significant difference, then I am sure you could detect it using a torque measurement or perhaps just some weights on a long lever arm.  Now that I think about it, the torque measurement seems particularly easy using a bolt silver soldered to a substrate and then twisted off, unless the bolt breaks first.

Quote
Also, would it behave differently with regards to flex cracking ? like from heavy vibration.

That is a completely different question which will depend more on modulus of elasticity so unless the surface preparation creates a crack, I doubt it matters.

Honestly if the surface finish matters, then I think you might be better off using a stronger attachment method like welding although that has its own problems.

« Last Edit: February 28, 2019, 05:31:27 am by David Hess »
 

Offline IanB

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I thought the brazing occurs on a microscopic level (where the braze enters the fissures that form in the material as its heated).

This thought is incorrect. In soldering and brazing there is a dissolution, dissolving, alloying of the two metals where they are in contact. The brazing or soldering alloy undergoes a physical/chemical reaction with the metal being joined and at the interface there is a diffusion of metal atoms across the boundary.

For example, if you solder to copper then there is a gradual transition between 100% copper and 100% solder across the joint, with a 50% copper/50% solder alloy at the interface. The copper dissolves into the molten solder just as various metals like gold dissolve into mercury.

Because of this, it scarcely matters whether the surface is smooth or rough. It won't make any essential difference to the joint strength.
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Offline coppercone2

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so its like semiconductor diffusion?

I thought I read somewhere that the metal 'pored up" when it got red hot and the braze flowed into the 'fissures' that don't exist outside of a expansion phase?

And that this was the reason you should not rebraze joints, because you can get excessively deep 'dendrite' formation.

It was in a white paper I think? And that after cooling this results in a braze joint having a 'weakness' similar to hydrogen imbritlement where the crystal structure is being pressured by different tempco stuff. Like kinda a velcro type effect.

I understood what you describe to be soldering. I thought brazing was a bit different. If I ever get lapping plates and a metallurgical microscope I can check.

Are you saying its 100% atomic scale diffusion? And if you do lapping samples you will just get different alloy concentrations?
« Last Edit: February 28, 2019, 06:17:10 am by coppercone2 »
 

Offline IanB

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I understood what you describe to be soldering. I thought brazing was a bit different. If I ever get lapping plates and a metallurgical microscope I can check.

There is no difference between soldering and brazing other than the typical temperatures and alloys involved. Brazing and soldering are essentially the same process.

If done properly, there is no "join" that can be separated afterwards, such as there is with gluing or adhesives. The joint is a continuous transition between base metal and brazing alloy at the atomic scale.

If you separate a brazed joint then the metal will fracture at the weakest point, usually in the filler metal since this is softer and weaker than the base metal. It will not be possible to separate or remove the filler metal from the base metal surface since it will have become one with the base metal.
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Offline SparkyFX

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The flux plays a huge role how the solder flows and really covers the maximum area.
So it might not matter at all, as the solder has a surface tension that can only bridge gaps of a certain size after the flux is gone.
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Offline flolic

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From my experience, base metals to be silver brazed needs to be only reasonably clean. There is absolutely no need for polishing.
You must use enough flux (Borax) and then the filler will flow beautifully.
 

Offline coppercone2

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So the depth of diffusion ensures that the roughness is dissolved into a even metalsolution?

I think i got confused because of the terminology. I heard people say penetrate and mix but they should be saying diffusion and dissolving/solution forming.

The terms i heard make me think of non atomically uniform phenomena.

Is this still true if carbide is brazed?

So can you extrapolate that the better the metal solubility is the less flatness and surface texture has an effect on tge concentration gradient of the cooled joined solids?

Does this have something to do with raults law ?
« Last Edit: February 28, 2019, 05:14:07 pm by coppercone2 »
 

Online eKretz

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Yes it is true.  Tungsten carbide bonds via brazing by the same mechanism as steel or any other metal. The layer at the junction is basically a mix of the two -  the braze or solder alloy and the base metal.
 

Offline coppercone2

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Hmm so how should i think about it.

1) the solid metal at the molten silver boundary dissolves into the silver and diffuses to the bulk molten silver in an attempt to make a solution (i think so)

2) the silver enters the hot metal like a sponge ?

Do both happen kinda with some grains dissolving faster then others in the silver then silver surrounds them like a salient or peninsula and acts to dissolve them with more surface area or is it a even layer that moves uniformly(if viewed as a transient process)?

Like whats the topology look like if you just graphed the interface geometry over time?
« Last Edit: February 28, 2019, 05:23:38 pm by coppercone2 »
 

Offline coppercone2

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Maybe a force diagram of sme test point atoms in a imaginary interface would help me with all concievable forces even if irrelevant (say to 100ppm of peak)

I assume gravity and bouyancy are irrelevant
 

Offline T3sl4co1l

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Yes it is true.  Tungsten carbide bonds via brazing by the same mechanism as steel or any other metal. The layer at the junction is basically a mix of the two -  the braze or solder alloy and the base metal.

I don't think this is fair -- but, well, it depends.

Cemented carbide (the most common kind) is WC in a Co matrix.  Co has a little solubility for C and W but not very much even at the melting point.  You'd have to go very high indeed to get a full melt.

I think it's more accurate to think of cemented carbide as WC particles soldered together with a Co filler.  Little diffusion or dissolution takes place, it's primarily a metal-ceramic interface.  (I don't know the mechanics of these sorts of bonds.  They certainly seem strong enough, as many cermets can attest to.  It's some kind of mixed ionic, covalent or metallic bond -- for sure, orders of magnitude stronger than just proximity (van Der Waals) forces.)

In turn, when you braze an insert to a tool, you're using a brazed joint, which will have some Co and filler (and Fe (or other base metal) and filler) interaction on each joint, and that will probably be more intimate than a sharp (non-diffusive) interface.  Typical fillers are in the Cu-Ag-Zn family (including yellow brass, "silver solder"*, and low-melting "silver solder") and Ni-Ag (or Ni-B I've heard of, too) family.  These fillers and base metals all have mutual solubility.

*By convention these are called "silver solder", but they're actually braze.  Go figure.  So I'm quoting them for clarity.

Soft soldering, is typically done at such a temperature and rate, that the diffusion layer is very thin, and this is usually a good thing as a lot of soft solders and base metals have brittle intermetallics that, if allowed to grow, would weaken the joint.  The Cu-Sn system has a lot of these; some are very strong (mostly on the Cu side, hence what's so great about bronze), but the intermediate ones tend to be weaker.  You can also soft solder metals that are very different indeed, e.g., Ti with Sn filler (if you don't mind the extremely toxic and corrosive flux required :) ).  Although this is a neat example, as Sn is standard in a number of Ti alloys, having an analogous effect as, say... Cr in Fe, maybe?

Intermetallics aren't obligatory, though.  Cu-Pb is a nearly immiscible system (no intermetallics, only a little solubility of Cu in Pb at various temperatures, and vice versa; separate liquid phases until quite high temperatures), but it's just fine for soldering.  Again, it's not that there's necessarily a diffusion reaction (reaction as in, intermetallics are formed), or necessarily dissolution even, just that the atomic bond at the interface is usefully strong.

Now, I could imagine a rough surface -- lots of "tooth", and lots of microscopic surface area -- is more suitable for soft soldering, given the tendency for intermetallics, or for weak interface bonding -- this might be interesting to test, and is pretty easy to do.  Prepare some metal surfaces, of various metals (say, Fe, Cu, Al, Ni would be interesting?), prepare joints with various fillers (Sn100, Pb100 and Sn63 might be interesting datapoints), and test the tensile and shear strength.  Also, for surfaces that are prepped with strong linear grooves (e.g., 150 grit + belt sander), it would be interesting to test them in parallel and crossed orientations; and for shear, testing groove to shear force angles as well.

Polished surfaces are tricky.  It's easy to make a smooth surface (just apply finer and finer grits), but it's a heck of a lot harder to get planar surfaces at the same time.  It might be practical to start with some gage blocks (as awful as it feels to suggest defacing their precision ground surfaces :( ), since those are lapped and polished to very good planarity.  You would be able to ensure micron-thick filler layers in that case (you'd also run the risk of fusing the blocks together -- essentially liquid phase sintering -- if the diffusion layer is allowed to grow too much!), and can modify the filler or gap or surface or clamping to test thicker layers as well.

Clamping pressure in the soldering fixture would also be relevant.

I do recall it's supposed to be that epoxy bonds best with a couple thou gap, and glass microbeads can be added to ensure this gap.  I don't know if that might vary with material being bonded, or what.  I wonder if solders do the same thing.

Definitely lots of room to test fairly simple things.  No idea if these have been evaluated before.  I'm sure there's a lot of literature out there on soldering; you'd have to peruse the ASTM or other libraries to see.  (May have free access at a school library?)

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Offline coppercone2

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unatainable experiment # 4523925
« Reply #16 on: February 28, 2019, 07:55:30 pm »
on a side note, another thing that got me confused is carburization. I think that the carbon actually migrates into the steel some how, I am pretty sure when I see a picture of something carburized cut open and polished, you can see its almost like a ink stain.

That's why a force diagram would help clarify.

You can get a cheap sample flat by sanding it on glass though, thats something I can try. Otherwise lapping plates and granite blocks, which I don't have or have a particular use for at the moment.

I do have gauge blocks i don't really give a fuck about, if other experiments prove promising you can get a data point from that maybe.

A problem with the test setup is to make some kind of micrometer adjustment clamp so you can separate the surfaces consistently. Maybe just make the test samples long and use feeler gauges that you pull out to make a gap but its kinda finicky. I would like a polished thread for adjustment.

What allows carbon to pass into steel like that? Can you do it with other light things like boron, lithium, etc?
« Last Edit: February 28, 2019, 08:01:48 pm by coppercone2 »
 

Offline IanB

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Re: unatainable experiment # 4523925
« Reply #17 on: February 28, 2019, 09:11:05 pm »
What allows carbon to pass into steel like that? Can you do it with other light things like boron, lithium, etc?

Steel is by definition an alloy between carbon and iron (and maybe other things too). Carbon dissolves in iron. When the proportion of carbon is small you have steel. If there is a lot more carbon in the alloy you have cast iron.
« Last Edit: February 28, 2019, 09:17:13 pm by IanB »
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Offline coppercone2

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Offline T3sl4co1l

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Carbon diffuses through iron, yes.  Perhaps more perplexing is getting it in there in the first place, apparently mediated by CO and nitrogen (especially in the form of cyanide?).  Contact between powdered carbon (in whatever form) is very poor indeed, in the solid state.

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Offline coppercone2

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I thought it was vapor pressure diffusion, like a gas sponge. When you carbonize a part you usually put it in a box completely filled with charcoal and heat it glowing red. Like it sublimes.

I actually wonder what would happen if you put it in a pressure cylinder and cranked up the pressure some how during carburization. Maybe it does not work because if you use a gas filler it would lessen the carbon vapor concentration? And its glowing red hot so you can't mechanically compress it.
« Last Edit: March 01, 2019, 01:36:44 am by coppercone2 »
 

Offline IanB

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I thought it was vapor pressure diffusion, like a gas sponge. When you carbonize a part you usually put it in a box completely filled with charcoal and heat it glowing red.

This is referring to a process called case hardening of iron or steel where you pack a carbon rich substance around the part and bake it in a furnace. This causes carbon to diffuse into the surface and produce a thin layer of high carbon steel around the outside.

It is not required to have "pressure" to make this happen. Diffusion of different atoms or molecules can happen when there is a concentration gradient. The concentration gradient itself is the driving force.
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Offline coppercone2

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But I thought diffusion constants change under pressure?

Is it just a sublimed carbon gas going inside? Or do you have something else going on (like teslacoil asked). Like it turning into some kinda liquid stuff?

https://en.wikipedia.org/wiki/Mass_diffusivity

this lists some pressure dependence.
« Last Edit: March 01, 2019, 01:41:15 am by coppercone2 »
 

Offline T3sl4co1l

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It is not required to have "pressure" to make this happen. Diffusion of different atoms or molecules can happen when there is a concentration gradient. The concentration gradient itself is the driving force.

Well yeah but that's the thing, the vapor pressure of carbon is SFA until well into the white-hot range.  How could it be gaseous diffusion if the vapor concentration is ~nil?  Surely it should be evaporating out of the iron, if anything!

The trick is to have something carrying a higher equivalent pressure or concentration of carbon: a catalyst.  Apparently several salts do this, and CO is implicated as well.  The reaction between C, CO, CO2, Fe (at the surface), CN-, CO3(2-) and who knows what else, is likely very complicated, as surface chemistry inevitably is.  (Maybe not so complicated that it hasn't been studied -- this is an important subject, and I bet some very illuminating research exists!)

Actually, come to think of it, carbonate may be a curious case, and not at all relevant.  A lot of anions and cations do have ranges of oxidation states, but carbonate isn't one of them, at least not at these temperatures.

At room temperature, the oxidation states of carbon (using strictly C, H and O) give methane, methanol, formaldehyde, formic acid and CO2.  The acidity (pKa) of these is something like 50, 16, 13, 4 and 6/10 (pKa above about 14 means the anion cannot form in aqueous solution; only CO2 has two ionic charges: bicarbonate and carbonate, and both are given).

At high temperature, most of these decompose, giving mixes of CH4 (or other hydrocarbons, depending on decomposition method and pressure), H2O, OH-, CO, CO2 and CO3(2-).  I'm not aware of there being any salt of reduced carbon between carbonate and carbide.

Point being, carbonate shouldn't be a catalyst, as far as I can think!  Unless it's reduced all the way to a carbide, or weird transient species are present.

Which, on that note, you can reduce, say, potassium carbonate with carbon -- the result is often explosive however, having such strange products as potassium acetylenediolate, K2C2O2 (a linear molecule), or potassium benzenehexolate K6C6O6 (a carbon ring studded with -O-K in all positions, but which is most definitely not all okay!).

Which reminds me of a joke...
A chemistry student asked out another student; it didn't go well.
Friend: "What did she say?"
Student: "Hexanitrosylbenzene. What the hell does that mean?"
Friend: "It's -NO in all positions."

Tim
« Last Edit: March 01, 2019, 03:12:17 am by T3sl4co1l »
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Online eKretz

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Oy. I don't want to quote the book length reply above,  so... Yes,  I was using the colloquial "tungsten carbide" - it is commonly referred to that way.  Cemented carbide is a more accurate description. The base metal in this case that would alloy with the braze is most commonly cobalt, although I have also heard of others being used and have also read that there are other metals added to the cobalt/TC mix in trace amounts depending on the manufacturer and grade.
 

Offline coppercone2

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i think you can do vacuum carburization though.
 

Offline IanB

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Well yeah but that's the thing, the vapor pressure of carbon is SFA until well into the white-hot range.  How could it be gaseous diffusion if the vapor concentration is ~nil?  Surely it should be evaporating out of the iron, if anything!

Yes, but diffusion doesn't have to be in the gas phase. Diffusion can happen in any phase, gas, liquid, or solid, where there is a concentration gradient. Isn't that how doping occurs in semiconductor manufacture? Aren't the dopants made to diffuse into the pure silicon to produce N- and P- type materials? The silicon is solid, but the atoms diffuse through the crystal lattice.
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Offline T3sl4co1l

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Well, the point I was making is, it's not just carbon granules and iron, in the traditional method.  Some gas pressure is apparently required, preferably a bit of CO (or air, to burn into CO all the same).

Also, that carbonate salts probably aren't a suitable catalyst, but some kind or another of salt is effective, cyanides being the best known (AFAIK).  That gives a liquid phase option to carry that concentration gradient.

In semiconductor manufacture, when a solid-phase method is used, the first step is evenly coating the surface (e.g., uh... phosphate glass, spin-coated from solution??).  It's hard to do that with elemental carbon, even if you, say, blacken the surface directly (lampblack is still very porous).  Maybe decomposed petroleum pitch would do -- that's the stuff used to stick together artificial graphite, and it can decompose to an adherent layer of amorphous carbon.  Hmm, I wonder if that would work?  Maybe it's too thick of a layer, too hard to control?  Maybe it cracks off before it can form a diffusion bond and eventually dissolve?

Vacuum carburization is a bit of a misnomer of course, as it's actually a low pressure gas (the vacuum is relative, as a "vacuum arc furnace" is as well).  Looks like they use methane and such, which makes sense -- under various conditions (including surface temperature and preparation (catalyst), gas mixture, ionization and more), methane decomposes to form various allotropes of carbon.  Any of which, when formed directly on a hot iron surface, would dissolve readily.

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Offline coppercone2

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are you sure? i think methane would foul a kiln .

Do they use the kind with sealed elements or something?

Won't that carburate the elements? Or do they have a inner shell?
 

Offline coppercone2

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I want to try to boron-diffuse metal. I have some boron. I wonder if you can just do it ghetto by smearing boron on something and putting it on a BBQ burner for a while. No kilns yet. Wrap in copper sheet. It's boron in oil, maybe you can just paint it on and heat it red. Should get carbon from the oil. 
« Last Edit: March 01, 2019, 06:32:59 am by coppercone2 »
 

Offline SparkyFX

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Would polishing lead to a work hardening of the surface or alter the grain structure of e.g. steel at the surface?
I can imagine both might influence its ability to be brazed (coming from a diffusion layer approach to the problem), so the suggestion to try brazing gauge blocks could provide usable results, otoh gauge blocks are usually hardened and therefore brittle. A test would need same hardness and tensile strength base material.
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Offline T3sl4co1l

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Depends on the polishing process.  Lapping is a very low-impact process as far as I know, so if there is any microstructure change, it would be to less depth than the diameter of the particles used for the final polishing step.  So, fractional microns most likely.

Also, as it's already hard, as you note, there wouldn't be much room left for work hardening. :)

It's not a bad observation though -- in fact it's very relevant sometimes.  I've seen copper sheet physically curl up as it's stressed from glass bead blasting -- it's a much more aggressive process than most would think!

Brazing temperature is around where steel's microstructure itself relaxes (annealing, normalizing, austenizing, whatever it's called in the case), so you wouldn't expect to have hard parts when you're done, at least in the average case with common alloys and no special treatment.

You can quench harden steel after it's been brass brazed, as it freezes at a higher temperature than heat treating requires.  Of course, this is a bit harder to do with a filler that melts below the austenite transformation temperature!

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Offline coppercone2

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I thought your not supposed to cool brazes because it can fracture?

Do you mean heat treating tbe entire part after it cools naturally, not like squirting water on it after you done?

Also to be more technical lapping and reaming are scraping prkcesses. I think grinding makes alot more heat. As far as working metal goes lapping is super cool because its slow, high thermal surface area and coolant between your work piece and the lapping plate.

I think processes that use machine rotation are inherently more heating because you get a particle with kinetic energy of wheel striking the plate like a meteorite.

I find what you say interesting because of wood. If you compare precision planed wood (which is a scraping process) its actually hydrophobic. If you sand it its hydrophillic.

How ever i am pretty sure carburized parts grow at least a surface oxide layer, so any tiny amounts of work hardening should turn to rust?

« Last Edit: March 01, 2019, 05:18:09 pm by coppercone2 »
 

Offline coppercone2

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Can someone explain work hardening with a diagram showing as few atoms as possible?
 

Offline T3sl4co1l

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Yeah, so it's not suitable for all brazes -- many alloys are indeed quite hard (silicon bronze? nickel boron?).

It can be done with brass braze, which is soft enough not to care.  After brazing, either cool it and do whatever finishing steps you need (remove slag, grind to near-net shape?), then heat to austenite temperature (coincidentally about the Curie temperature as well) and quench.  Check hardness, then temper, then grind to final shape and finish.

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Offline coppercone2

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does metal ever work harden from any metal cutting processes like milling?
 

Offline David Hess

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does metal ever work harden from any metal cutting processes like milling?

Stainless steel sure does when you drill it necessitating cobalt steel drills but maybe that is due to the temperature rise?  Softer metals might but you would never notice because the tool bit is so much harder.
 

Offline T3sl4co1l

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Yes, the surface is work-hardened under a cutter, in the shear zone where metal is pushed aside.  The depth of this layer corresponds to material properties, depth of cut, and geometry of the cutter (worse for a low rake angle).  The amount of work hardening depends on the material.  (Copper and stainless are examples that go from fairly soft to surprisingly hard with not much shear, making them difficult to machine.)

Doesn't have to do with temperature, as far as I know.  If anything, that would make things easier?  Which brings to mind, turning hardened steel with a carbide tool at high speed.  Seeing glowing hot, stringy chips flying off is an awesome sight...

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Offline coppercone2

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does it ever harden in a axis that is not the direction of material removal?

Like drilling makes sense. But if your just cutting or grinding to the side, does force get transferred underneath? I thought it goes into tearing/stretching.
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With something fast. I feel like a lathe is more likely to do it then a fast milling bit that shoots chips off?
« Last Edit: March 02, 2019, 02:35:50 am by coppercone2 »
 

Offline T3sl4co1l

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Shear at the surface.

So, the wall of a drilled hole is worked, the side and bottom of a milled slot is worked, the side and face of a turned cut is worked, and same for grinding, sandblasting and so on.

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Offline coppercone2

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so can you imagine it as 'necking' like when you put a steel sample into a yield test machine? That each 'instance' of the blade cutting through the material when a curl/chip is formed, you get the metal in the yield mode directly near the cutting edge of the tool, so when the tool cuts it, its like a spring that unyields and the metal actually recoils back downwards and hardens as it impacts? Or is it not really recoiling back down, and its just yielding and it gets hard as this happens?

Can you almost imagine it like directly above the cutting surface, its like putting a rubber band between your fingers and smacking it down every time it tears? But its a continuous time process some how (weird to think about) since its rotating)?

If you had a powerful pulsed laser you could clean it up from the mechanically deformed stuff after if you blow up the surface atoms layer? Or do you get some kind of laser peening too from the vapor explosions that happen (more like sand blasting then the yield tearing)?


Maybe you just need to dissolve it in acid or base? That should be gentle.


I have trouble because with sandblasting you get a downward impact like beating it with a hammer, With the machine process it seems like your just i dunno, like pulling it out. Does the metal get hardened the same way? or is there a counter force from spring like I said? In the yield test machines you can see the shockwave after it tears so you might imagine it hits it just like a hammer.

lasers out

« Last Edit: March 02, 2019, 03:10:59 pm by coppercone2 »
 

Online eKretz

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Any metal that will work harden with cold working (like hammering) can work harden when machined - with any type of cutting operation, whether drilling,  milling,  turning, whatever. To avoid this, it's important to keep cutting edges SHARP so that they don't rub. If a dull cutting tool is used on something like the right grade of stainless steel and it does more rubbing than cutting, work hardening can definitely occur, even to the point of a HSS cutting tool being unable to further cut the material.
 

Offline David Hess

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So can you imagine it as 'necking' like when you put a steel sample into a yield test machine?

Absolutely, it is still strain hardening where the metal is pushed past its yield point.

If a dull cutting tool is used on something like the right grade of stainless steel and it does more rubbing than cutting, work hardening can definitely occur, even to the point of a HSS cutting tool being unable to further cut the material.

I have seen this happen many times with inexperienced operators on drill presses.  The cutting tool needs to be kept cool to prevent loss of hardness which means low pressures and low cutting rates.
 
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Offline coppercone2

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but is the type of hardening different for downwards drilling and the resulting hardening from a surface pass?

With the drilling I imagine the force being heavy and downwards. With the surface pass I just imagine it stretching out.

When it 'tears' is there a downwards shock wave when it actually breaks? On the necking machines you see that it gets long and then it snaps back. Can you imagine that during milling, you get a continuous 'shock wave' happening as the metal tears? Or is the mechanism different?

Does anyone have a ULTRA slow motion video of a necking test? I can't find what I am thinking about. Maybe I saw a weird material in SEM. Does it just get super dampened?
« Last Edit: March 03, 2019, 05:04:35 pm by coppercone2 »
 

Online eKretz

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The hardening is work hardening.  It doesn't matter what operation is being performed. It isn't about keeping the cutting operation slow or cool, it's about keeping the tool sharp so it shears the material. Sometimes this requires slowing the cutting tool or keeping it cooler so it doesn't dull, but it's the dulling that causes the problem. A dull tool pushes the material around more rather than cutting it cleanly, effectively cold working it. When a milling cutter dulls, it pushes against the bottom surface as well as the side surface.
 

Offline T3sl4co1l

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No, no shock waves.  There may be some cracking of the surface as the material tears (in tension) as well as shearing of the chip from the base material.

There are micrograph videos of cutting tools in action, check those out.

Actually, there may be shock waves, in certain materials.  Zinc and tin come to mind, as materials that can exhibit supersonic slippage.  In effect, you're "cracking" a crystal along a plane, and it slips very suddenly as the tension is relieved.  The shock is limited to the width of a given crystal.  In bulk, this manifests as "tin cry", a crackling sound emitted as the material is bent.

The way that deformation works, is slippage planes in the crystal lattice.  The reason work hardening occurs, is those planes do not slip evenly, creating dislocations.  The paths of dislocations (like line faults) flow through the crystal in 3D; if two paths slip into each other, they can annihilate, or they can entangle.  The general accumulation of overlapping defects is what gives rise to work hardening.  Eventually, so many defects accumulate that the material tears apart (it is too congested to slip in an orderly manner anymore), and a crack forms.

What distinguishes ductile metals from brittle substances, is the number of coordination sites per atom, and the electron density.  Ionic crystals are brittle because they have relatively low coordination; when a slip occurs, the planes rarely line up correctly, and a crack forms almost immediately.  Metallic crystals have many sites so that, even if the planes no longer line up properly, the atoms can still share electron orbitals, and maintain a bond.

I think.

You can still have ductile motion in typically brittle substances, but it's much rarer to see, most often on the microscopic scale where more perfect motion is possible.  Example, the marks left from grinding and polishing.  Sometimes a smooth track is left, implying a fairly smooth cutting process; other times, it's a jagged track where material was effectively chipped away from the base.  I would guess the relative amount of each type depends on the angle and pressure of the cutter, and the material properties.  In a typical ground face, the cutting angles are random (angular bits of grit), so the relative mixture of both speaks to the properties of the base material.

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Offline coppercone2

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Surface roughness reduces the effective contact angle causing more spreading and scratch mark or “lines” act as capillaries causing directional spreading in one direction but also act as dams reducing the spread or flow of alloy in the perpendicular direction.

http://www.altairusa.com/wetting-braze-flow/

almost wonder if you are doing a deep lap joint, can a specific scratch pattern be used to flow solder correctly.  like if a carbide micro engrave CNC scratch thingy could be used

would be difficult to figure out the correct dimensions though.. maybe very sharp sand paper (SC based?) of a particular grit would work
« Last Edit: September 16, 2019, 03:49:44 am by coppercone2 »
 

Offline SparkyFX

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The graining of metal to increase shear resistance is quite common, some establish a rasp like structure before e.g. glueing. It is hard to tell when effects go from the micro- to the macroscopic scale, such as work hardening.

The point of brazing is mostly to connect two dissimilar metals, with the braze often the toughest spot. So it becomes the question which part develops the highest shear strength via surface before the braze fails. If a braze fails right at the surface of a part people would assume the surface was not clean enough, hence would need better preparation (=sanding).

From an economic perspective any sanding/polishing step brings new problems, like additional cleaning to get the polishing/abrasive compound off and out of the surfaces (it shouldn't contaminate the brazing, but might have been embedded in the material).

So it might be a trade off between removing impurities before brazing and just brazing. The flux does also play a significant role anyway, etching the surface a bit or at least preventing oxides from forming while heat is applied.
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Online KL27x

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Quote
almost wonder if you are doing a deep lap joint, can a specific scratch pattern be used to flow solder correctly.  like if a carbide micro engrave CNC scratch thingy could be used
I think it would be more useful to have ridges (thin lands, wide grooves) than scratches/grooves (wide lands, thin grooves). Ridges a few thous tall would prevent (larger areas of) tight spots that close up the gap between materials.

Quote
The flux does also play a significant role anyway, etching the surface a bit or at least preventing oxides from forming while heat is applied.
Agree. I think in many or most brazing operations, any fine sanding/polishing is moot, because the flux (+ heat + atmopheric oxygen) is going to etch/obliterate it by the time the joint flows. Acid etching increases the surface area on a microscopic level to a much greater degree than mechanical sanding. Etching leaves a delicate "fluffy" surface. The dimensions of the material can actually initially grow a bit rather than reduce, cuz of the fluff. This etched surface sucks the braze material into the surface of the metal.

In fluxless TIG brazing, you don't get any capillary action. The gas prevents oxidation, but there's no etching. And in TIG brazing, you just lay the bead/filet across the two materials to be joined. You don't set a gap to try to get the filler to flow between materials, cuz it's not really gonna happen. That is partially due to the focused/localized heat source, perhaps. But the TIG torch can penetrate deep on a weld, so heat isn't necessarily the limiting factor. Just imagine trying to solder without any flux.

You might think that the flux/acid etching is a significant component to capillary/wetting action. All this scratching/polishing idea is perhaps nonsense, because it can't produce the type of surface that acid etching does.
« Last Edit: September 16, 2019, 08:14:59 pm by KL27x »
 


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