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Heats effect on heat-treated aluminum
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Posted by: KXaggerator---------------------
Am I correct in my thinking that high heat will weaken T6 or T7 aluminum? Will the heating process associated with powder coating effect the strength of an aluminum sub-frame to any degree? I know pistons are subjected to about 1200 deg. F. so I doubt that 400 degrees will weaken my aluminum parts, but I just want to double check. At what point does the strength properties become affected? :think
Posted by: Jaybird---------------------
Shouldn't effect it one iota. Motopuffs can confirm this I bet.
Posted by: Norm---------------------
Yes, you should worry about decreasing the strength of heat treated aluminum alloys by exposing them to high temperatures. The T designations you mentioned describe the age hardening heat treatment, where the alloy is heated to a specific temperature for a specific time to achieve desired strength. If you heat it too high or hold it too long you will overage the alloy and the strength will drop.
About the temperatures you mentioned: A piston may be exposed to 1200¡F during combustion, but that doesn't mean it really gets that hot (the piston is cooled by the intake charge during each cycle). Aluminum melts at ~1220¡F, and if it really got as hot as the combustion gases, it wouldn't last very long :scream: 400¡F is in the range of temperatures where wrought aluminums are heat treated, so it may weaken the alloy if held at that temperature for much time (~several hours). But that temperature is still relatively low, so the strength won't drop quickly. If the temperature was higher, the strength would drop at a faster rate. The actual temperatures and times for aging will vary with the alloy system, so this is only a rough guideline.
At the temperature you mentioned the strength will probably not drop much, but I would still be uncomfortable about heating a subframe the way you described.
Norm
Posted by: Jaybird---------------------
A powder coated part will cure in less than one hour at 400-450f. Probably closer to 15 mins.
Posted by: Norm---------------------
Jaybird,
Thanks for the info about how much time the cure takes for powdercoating. For times that short, the change in properties should be negligible.
Norm
Posted by: biglou---------------------
We do some powdercoating of aluminum (furniture industry), and depending on the type of coating, we cure for 12-15 minutes at around 375 degrees. Granted, our few Al parts won't see the stresses a subframe or swingarm will, but it gives a general idea as to how long the part will be heated and to what temp.
Posted by: orion163---------------------
If aluminum is aged at too high of a temperature(over about 400f), the strength will be reduced, but other physical properties like corrosion resistance can be improved. Higher properties are achieved by aging at lower temperatures for longer times(about 250f).
Very high temperatures well anneal the aluminum, and very low temperatures will retard the aging process.
Maximum strength occurs at an optimum combination of temperature and time.
Posted by: motometal---------------------
Jaybird, thanks for your confidence in my knowledge. I'm actually much more knowledgeable on steels, pretty weak in the aluminum area...just know enough to be dangerous.
Time at temperature was mentioned...you have to be careful with that. Sounds like you are probably working with thin sections, and aluminum has a high rate of thermal conductivity, so in some cases this won't be much of an issue.
My point is, many people assume that when the furnace is up to temp, the workpiece is as well. For heavier sections, if time is critical, for the best accuracy temperature and time wise, I suggest using a scrap part that is a simulation of the section size of the workpiece, drilling it in the heaviest section, and inserting a separate thermcouple.
With any precipitation, temperature can be critical, so the above method also eliminates any questions on the temperature uniformity of the furnace or oven.
I would venture a guess that combustion temperatures are way above 1200 F, but the piston doesn't actually get very hot. Another way of looking at this, if the piston reached, say 500 F, wouldn't the oil to start breaking down, and leaving depostits on the bottom side of the piston?
Posted by: KXaggerator---------------------
Thanks for all the information.
When aluminum is tempered what is an average heat/cool time to temper T6? I know that there may be several methods, but what would the primary numbers be. Is there a time tensile curve readily available?
Posted by: Norm---------------------
The T6 is only a temper designation, which has nothing to do with the alloy composition. Before we can begin to talk heat treatments, you first need to know the alloy system.
For aluminum, the alloy designation is given by a three or four digit number, which tells you the amounts of alloying elements. Typical age hardened aluminum alloy systems include the 2XXX (Al-Cu), 6XXX (Al-Mg-Si), and 7XXX (Al-Zn) series. 6061 and 7075 are especially common.
The T6 designation after the number means the alloy has been solution treated and artificially aged (at an elevated temperature) to obtain desired properties. If you look in a handbook on heat treating metals, you can find guidelines for tempering different alloys. For the two alloys mentioned above, I found the following numbers.
6061 T6: 350¡F for 8 hours after solution treating
7075 T6: 250¡F for 24 hours after solution treating
The times are very sensitive to variations in temperature. The change in structure during heat treating is diffusion limited, which means it will be exponentially dependent on temperature. Here's an example using 6061
400¡F: ~1hr 15 min for a T6 temper
300¡F: ~200 hrs for a T6 temper
250¡F (same temperature given for 7075): >1000 hrs for a T6 temper
If you want something more than just a heat treatment in a table, you can find curves of strength (yield or tensile) versus time for different temperatures. The T6 designation is the point on the curve where the strength maxes out. If you look in a basic materials science or metallurgy text you should be able to find an overview of age hardened aluminums. It may include this type of plot for some aluminum alloy. Handbooks (try looking under American Society of Metals) may have these plots for several alloy systems in addition to tabulated heat treatments.
Norm
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