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How do I harden K&S soft aluminum

Old 09-10-2002, 08:36 PM
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Default How do I harden K&S soft aluminum

I am in the process of making the Aluminum front panels for a Fokker D VII. The panels have many louvers in them which must be put in while the metal is still soft. Once the louvers are in the material needs to be hardened. I put the panels in the oven @ 350 for a few minutes and then quenched it in cold water upon removal. they stiffened up some,but not enough. Any suggestions?
Old 09-11-2002, 05:52 AM
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Default How do I harden K&S soft aluminum

I could be wrong but I always thought quenching aluminum actually anneals (softens) it.

As far as I know the best hardening is work hardening and natural age hardening.

Only specific types of aluminum can be tempered with heat.
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Old 09-11-2002, 02:50 PM
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Default How do I harden K&S soft aluminum

BQuartucy,

My dusty old metallurgy book is at home, but I did a quick browse of the web and turned up about what Merlin65 said. It seems that only specific alloys of aluminum can be hardened enough for high-strength applications. The process that was briefly described involved heating to 500 deg. C, and quenching to room temp. in water. Following this, a process called precipitation occurs over a fairly long period, or can be accelerated by another trip to the oven, at 175 deg. C. This is called precipitation hardening, oddly enough. If I find anything else in my metallurgy book, I will post it.

You certainly could try a poor man's version of work-hardening. I don't know about alumnum for sure, but most steels are signicantly hardened by large deformations, which is why a paperclip gets all brittle when you try to twist it apart. You could take a scrap piece of you aluminum, and try pre-bending it one or more times along the line you need to form you louvres. You might get lucky and find that after some number of bends, the final louvre is much stiffer. Allow me to be Cliff Claven for a moment, and mention that some of the perforated steel drums used in clothes dryers are initially formed inside out, and then drawn right-side-out, specifically so that the large deformation will work-harden the steel. Let me know whether it works for aluminum.

Good luck,

banktoturn
Old 09-11-2002, 03:42 PM
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Default How do I harden K&S soft aluminum

Okay, I see some clarification is in order here. Allow me...

Quenching, by definition, is hardening. Annealing is where you heat the alloy to past its eutectic temp and then allow it to cool gradually. And yes, it tends to relieve any work-hardening or residual stress in the alloy - thus making it "softer". When you cool it rapidly, either by water quenching or oil quenching, different grain structures occur which may contribute to hardening of the metal.

"Natural age hardening" - okay, this a a misnomer. Age hardening is a term used for heat treatment on a metal alloy. There is nothing natural about it. Alloys certainly do not harden with age, as "natural age hardening" seems to suggest. Another term for age hardening is precipitation hardening.

Now to answer the original question. Most of the aluminum we used in RC are probably T6 Aluminum., which are a class of Al alloys in the 6xxx and 7xxx groups - mostly Al-Mg-Si alloys. And yes, most of them are heat treatable. However, you'd need to know the exact alloy in order to know what the solvus temp is, as it varies greatly depending on alloy content. For example, the solvus temp of Al-Mg alloys varies up to 451deg F for 15% Mg.

Age hardening involves 3 steps:

1. Solution treatment: where the alloy is heated up to above the solvus temp. This step dissolves the "theta" precipitate and reduces any segregation present in the alloy, until a homogeneous solid solution "alpha" is produced.

2. Quench: the alloy is cooled rapidly such that the "theta" phase has no time to form, but the still homogeneous "alpha" phase is in a supersaturated solid solution.

4. Age: Finally the supersaturated "alpha" is heated to a temp below the solvus temp. THis allows the atoms to diffuse short distances to allow "theta" structures to form at nucleation sites. If we allow sufficient time at the aging temp, the equilibrium "alpha" and "theta" structure is produced.

I guess what I'm trying to say is that, without knowing what the exact alloy is, it's near impossible to design a heat treatment schedule for it to achieve optimum results. My blind guess would be to heat it to 425deg F, quench it, then heat it to 175deg. for an hour. Result is by no means guaranteed. And please, don't nobody take these numbers as gospel.
Old 09-11-2002, 05:18 PM
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Default How do I harden K&S soft aluminum

Volfy,

A couple comments. Quenching, by definition, is rapid cooling. For many ( most?? ) steels, this is in itself a hardening operation, if the initial temperature was appropriate. For aluminum, the quenching itself does not result in hardening. The ensuing precipitation of the relevant alloy component is actually the process which results in hardening.

The lowest temperature I found mentioned for hardening 6XXX and 7XXX alloys of aluminum was 990 F. A reasonable suggestion for trial and error heat treating would be to heat to 990F and quench to room temperature in water. As you say, though, without knowing the alloy, it is guesswork. There seems to be much more variation in the temperatures and times used to finish the precipitation process. Even with no heating after quenching though, the precipitation process will proceed at room temperature, to some point. It takes days rather than hours though.

BQuartucy, I did also find reference to work-hardening in the 6XXX and 7XXX alloy families, so the bending-back-and-forth experiment is probably worthwhile.

Sounds like a great science experiment!

banktoturn
Old 09-11-2002, 09:07 PM
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Default How do I harden K&S soft aluminum

BQuartucy,

The K&S web site says that their sheet aluminum is 6061. Another vendor gives this procedure for hardening 6061 alloy aluminum:

Heat to 950 F until heated through
quench to room temperature in water
heat to 320 F for 18 hours
air cool to room temp.
heat to 350 F for 8 hours
air cool

The lower temp. heating phases accelerate the precipitation process that could take a long time ( years, according to one web site ) at room temperature. If you have a way to get your aluminum to 950 F, this process sounds feasible.

Good luck,

banktoturn
Old 09-12-2002, 06:23 AM
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Default How do I harden K&S soft aluminum

You could annodize it. But it requires stuff like a bucket, battery acid, and a battery charger. Basically it just oxidises the snot out of it until it won't conduct electricity anymore. Then you can put it in a pan of RIT dye and by repeated dipping you can get the color you want. Plus the color won't come off and it's real scratch resistant. Then after it's a little darker than you want you put it in hot water and it seals itself up. Some of the color bleeds out though. If you don't color it it looks like a ruff silver color. But it won't rust too.
Old 09-12-2002, 10:10 PM
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Default How do I harden K&S soft aluminum

banktoturn, I got my temp scale mixed up. I was looking at the Al-Mg phase diagram in one of my materials textbooks and forgot it was in Celcius. Sorry.

This got me curious so I did a little webbrowsing and found this pdf on age hardening of 6061.

http://www.me.uh.edu/ceramics/3445La...EHARDENING.pdf

Apparently, the second phase of 6061 is magnesium silicide (Mg2Si), not magnesium & silicon as I'd assumed. In any case the article explains it very well so there no need for me to repeat. The best hardness seems to be achieved by age-hardening at 160degC for 30 minutes, any longer than that the hardness actually decreases.
Old 09-17-2002, 03:34 AM
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Default How do I harden K&S soft aluminum

Volfy, are you a metallurgy student or an industrial heat treater. I was once a bright faced engineering student and may have given the answer you did. Now, I just call my local heat treater and quiz their process experts. 30 minutes talking with those guys yields more useful information than my entire engineering education. The simple thing to do is follow the K&S process as previously posted.

There are many acceptable process by which 6061 can be hardened to various hardnesses. And yes, if given enough time, annealed 6061 will harden at room temperature. When I say enough time, I mean eons.

The bending paper clip experiment is not necessarily a work hardening process. Virtually all metallic materials will fail with repeated cyclic stress. Cyclic hardening is just one of several failure mechanisms.

Quenching has different effects on different materials. It has little effect on low carbon steel, it hardens carbon and alloy steel, it softens aluminum and copper alloys. Getting the desired final hardness is called different things for different materials. For steel, its tempering. For aluminum its age hardening or precipitation hardening.

My recommendation is use the K&S process. It's an anneal followed by double age hardening process. It should yield excellent properties for this alloy.
Old 09-17-2002, 10:06 AM
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Default How do I harden K&S soft aluminum

I used to do this, using almost the same numbers as K&S and it will work good enough for our purposes.
Old 09-17-2002, 01:31 PM
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Default How do I harden K&S soft aluminum

ilikeplanes, nope I'm neither. At least not right now anyways. I did get my undergrad in mechanical engineering, which is mostly where all this crap I spout comes from. I'm currently a practicing engineer in the oil field service industry (big surprise, given my location), and even though I don't work in the mechanical design dept., I do work with them closely and deal with heat treatment occasionally when I get involved in the heavy iron part of my projects - mostly spec reading and code compliance verification and rarely actually doing the heat treat. That's what subs are for.
Old 09-17-2002, 05:47 PM
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Default How do I harden K&S soft aluminum

I worked at a Nuke power plant as a student. That's where I came to appreciate the term "Paperwork Engineer". I hope spec reading is different from paperwork engineering (unless you enjoy it). I have been lucky to be in R&D since I graduated in '93. It suits me very well and I get a lot of exposure to many facets of the industry (automotive now and helicopter logging in a previous life). Oh sure, I do my share of pushing paper. I just try to avoid it when possible. Our lab supervisor says I'm the highest paid mechanic in the company. I wonder how long I can get away wit it.

Glad to hear from a fellow ME.
Old 09-18-2002, 03:32 AM
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Default How do I harden K&S soft aluminum

Hey, whaddayaknow! I worked as a "Junior Technician" the summer after my college freshman year at Indian Point Nuclear Power Plant, Buccanan NY. I shuffled paperwork on replacement parts to avoid reactor shutdown. Made me realize paperwork wasn't for me.

Thanksfully, nowadays I'm in mostly R&D.
Old 09-18-2002, 02:04 PM
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Default How do I harden K&S soft aluminum

ilikeplanes,

Just to make sure that no one goes away and uses the wrong process to try to harden their aluminum, the prescribed process for hardening 6061 is quenching, not annealing, followed by age hardening. No process or amount of time will result in age hardening of aluminum that has been annealed.

I believe that the failure mode we see in the paper clip experiment is clearly a result of work hardening. The steel is able to undergo plastic deformation without failing until, as a result of work hardening, it can no longer withstand the large deformation without failing. I don't know of any other failure mechanism that fits the bill.

banktoturn
Old 09-18-2002, 06:14 PM
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Default How do I harden K&S soft aluminum

banktoturn-
My understanding (and I just consulted my handbook) is that quenching of aluminum alloys is synonymous with annealing. That is to say, the alloy elements are in solid solution. If I'm getting this wrong, maybe an explanation of the difference between O temper and T1 temper would shed some light.

Also, according to Machinery's Handbook "...alloys are not stable after quenching at room temperature and precipitation of the constituents begin." And, "After a period of several days, ...the allays are considerably stronger".

On the mechanisms of metal fatigue, I respectfully comment that it is much more complex that work hardening. Please consult the many volumes of reference material from ASM, SAE, ASTM and others if you wish.

Volfy- I once worked two week to get the required pedigree on a replacement diaphragm for a valve of a non critical cooling system. It was something like a 1/2 valve for a tank wash-out. Not my idea of fun.
Old 09-18-2002, 06:56 PM
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Default How do I harden K&S soft aluminum

ilikeplanes,

Even for aluminum, quenching and annealing are virtually opposites. Quenching is the very rapid cooling of the alloy from a temperature which was high enough to assure that the relevant components are completely in solution. This is done to get a supersaturated solution at room temperature, because the precipitation that occurs subsequently results in hardening. Annealing, which is a gradual cooling from high temperature, results in a much different precipitation process, which results in almost minimum hardness.

I agree that fatigue is much more complicated than work hardening. It also takes place in conditions of cyclic loading over many more cycles than we go through with the paper clip, in which there is no macroscopically plastic deformation, whereas the deformation to which we subject the paper clip is clearly plastic.

banktoturn
Old 09-18-2002, 07:21 PM
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Default Hard Coat

You can Hard Coat at your local annodizer. This process leaves a hard shell (.001-.002) on the surface. It is very hard and resistant to scratches, but will chip if hit hard enough.

wolf 152
Old 09-18-2002, 10:12 PM
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Default How do I harden K&S soft aluminum

I guess I'll have to look at my TTT diagram. That should clear it up for me.
Old 09-18-2002, 10:34 PM
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Default How do I harden K&S soft aluminum

I'll just add that, from what I remember from my materials classes, the main difference between annealing and quenching - besides the obvious rate of cooling and the desired end goal - is the temp to which the alloy is heated.

As I stated above for quenching, the alloy is heated to just above the solvus temp, above which only the alpha phase is present. Alpha is where the percipitate (Mg2Si for 6061) is in solid solution of the base metal (aluminum).

For annealing, depending on the desired effect, the alloy is heated to either one of three temps:

1. Recovery
2. Recrylstalization
3. grain growth

All of which are substantially lower than the solvus temp of the alloy. The Recrystalization temp, for example, is typically around 0.4 of the absolute meting temp of the metal. For aluminum this is around 150degC or 300degF.

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