Heat treating cast aluminium

Andrew

Sorry but nope, no shortcut!

From my metallurgy of 20 years ago: It's a precipitation hardening alloy. The solution treatment creates a solid solution which is then held by the quench. The subsequent treatment at a lower temperature allows the hard precipitates to grow to the right size to provide the hardening mechanism. Both time and temperature are important here to get the right properties. Just watch that meter whizz round.......

Regards

Charles

relieving -

Hopefully the pivots on the meter will weld up at the speed it's turning !

The REAL problem is that some years back I installed a load measuring system that's plotting a nice bar graph minute by minute and logging it, and today sticks out like a sore thumb !

AWEM

What about suspending the casting over the gimbal of the electric meter ?

-- Regards,

John Stevenson Nottingham, England.

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This is similar to the first stage of what I've been doing here (due to limited equipment), but looking at the reply from Charles and also my own musings, it's less than ideal...

I've been removing the casting from the sand very shortly after pouring and throwing into a bucket of water.

It then goes into the kitchen cooker at 175C for 12hrs. Without doing this, it's like machining chocolate, endmills get clogged etc. etc. A good overnight 'soak' makes it easy to machine with a fine finish. For info, it's A356 I'm using (7% Si).

Where I know it's less than ideal is the first stage where the casting is pulled out of the sand at some unknown temperature, which will vary at different points in the casting - hardly a repeatable process!

pouring and

casting is

Duncan,

I'm very interested to hear that what you are doing is making the casting easier to machine - that is entirely my objective. The LM26 ("pistonium")was chosen for availability rather than mechanical properties - it's not a highly stressed component - I'm fortunate in having an aluminium alloy analyser that tells me just what I have in the pot and that composition quoted was a reading taken from one of the sprues after fettling.

Your A356 only really differs in the lack of copper (2% in my pistonium) which I think gives work-hardening if I remember my Landrover 'Birmabright' characteristics.

I got excellent results on the machining front, but I don't know how strong the resulting castings are in terms of tensile strength etc.

A word of warning - one casting I did had a wedge of oil-bonded sand stuck to it, which exploded in my face on contact the the water, so take real care with what goes in the bucket.

The real solution for me is to get an electric oven with PID controller, although I'm loathed to pay £500 for one of those pottery kiln jobs.

controller,

Duncan,

I bought a three phase kiln (ex School) from one of the machinery dealers advertising in ME - he couldn't shift it as none of his customers had three phase at that power - no use using a rotary converter.

I did a bit of lateral thinking - the load was entirely resistive, and wired star across the three phases. I just re-wired the three coil banks in parallel and run it off 240v single phase using a pid controller to control it. Works a treat.

AWEM

Duncan

In general the solution heating temperature is fairly critical to ensure that all the solutes are in solid solution. However if the end requirement is to make it machineable and the result is sucessful then your hit-and-miss approach sounds fine and the text book can be ignored. Whilst a large heat treatment furnace is a useful accessory think of the money it's costing Andrew to be scientific!

Regards

Charles

Charles, hope you can clue me in - you're the only qualified metallurgist I know :-)

In terms of the solid solution, can you go over temperature and achieve the same goal? Or is it based on a set range of temperatures?

My understanding of the T6 temper (which from the numbers, I believe Andrew's recipe is based on), is that a free state is reached where where there is no defined crystalline structure. Dropping it in water maintains this situation of small grains which become suspended (i.e. the solid solution).

After quenching, the cooker doing overtime grows the grain structure at a controlled rate to achieve the desired level of hardness.

If any of this sounds clueless, it's probably because it is! The info above is my understanding of things, altough I really am out of my depth on this one. Spending hours with a jewellers screwdriver cleaning soft ali out of endmills drove me to try and emulate the T6 process with very limited equipment.

That part has worked surprisingly well, although I cannot generally recommend it due to some of the safety issues involved. Aside from the 'big bang' when dropping the fresh casting in water, there is the side issue of picking up a freshly frozen casting. Aluminium at this temperature is weak, the core is molten, and extreme care needs to be exercised.

I've found a spare bag of castable refractory in the workshop to make a small HTF, all I need now is some element material. Suggestions for suppliers are most welcome ;-)

In these circumstances, I've seen a large magnet used to good effect..

Tom

Local electricity board have replaced ours with new fangled digital jobs - no more rotating discs, just a box of electronics, an LCD screen and a few flashing LEDs. I don't think they are susceptible to large magnets anymore...

Regards, Tony

We have one at the factory, the LED flashes once per 1/10 of a kWH. When we were testing the big R-R charger recently, the LED was on continuously! We were pulling 105A per phase against a max supply box rating of 100A.

I think the new meters are more accurate and have no errors introduced by the mechanism of the eddy current disc used on the old meters.

Peter