Gentlemen and ladies, I have a s 3

[etch]

Gentlemen and ladies,

I have a small challange, we are in discussion in a good natured way with a customer. We supply a casting where at its thinest section is 4 mm and at its thickest is prob 1cm.

Now we have come to a sticking point over heat treatment times. We argue that for the alloy type of (and ill give the various equiv.) lm25 (uk) en2726 (actual spec) or a357 (us) (its typically a 7% silicon and 0.55 mg with titanium addition to grain refine.

We heat treat the component as per standard of 10 hours solution at 535 oC into a 30 degree quench followed by a precip at 160 for 6 hours.

Now the customer suggested that the solution time was too long and at their request we cut it to 5 hours. Again they came back saying that they only solution for 1 hour. I tired to explain that i would find it very hard to belive that the casting could go into solid solution in 1 hour and be totally homoginised. THey went away and spoke to their expert at austrian institute of foundrymen and came back saying he recommends 15 minutes at solution temperature.

i would like some independent respones to tell the customer we are not over reacting and what the likelihood would be of a under solution treated casting

 

[TVP]

15 minutes for the solution treatment of an aluminum casting is ABSOLUTE MADNESS!!! It may be possible for high pressure die castings to respond favorably to a reduced solution treatment time, on the order of hours, due to the extremely fast solidification that takes place. However, I have never seen anything about solutionizing a casting for 15 minutes - 1 hour. That is just absolute folly.

 

[trytrue]

There is a technical report about "Influence of Mg content on the microstructure and solid solution chemistry of Al7SiMg casting alloy during solution heat treatment" by J.A. Taylor, D.H. St John. In this report they state:
The microstructure of 356 and 357 alloys in the as cast state consists of primary of alfa Al solid solution with interdendritic regions of Al-Si eutectic, in which various intermetallic phases such as Mg2Si, Al5FeSi and Al8Mg3FeSi6 are present. The solution treatment stage of the T6 performs several important functions: dissolution of Mg2Si,; homogenisation of the solid solution; and fragmentation, spheroidisation and coarsening of the eutectic silicon. Dissolution of Mg2Si and homogenization of the matrix occurs within 15 minutes in 356 alloy at 540 C, and within 50 minutes in 357 alloy. The changes to eutectic silicon morphology are generally slower, taking up to several hours, and depend on paramenters such as solution temperature and original particle size/shape which in turn are determined by solidification conditions, grain zise and eutectic modification. The....etc.
According with this statement and with your small wall thickness parts (probably highest cooling rate and very fine grain size) you will be able to reduce the solution time between 15 to 50 minutes.
For your information, we made solution (fluidized bed) for 20 minutes at 1000 F in A356 castings; the mechanical properties in LPPM and GPM parts were close to the specifications; the mechanical properties in squeeze castings parts passed the specifications.

I hope this can help

 

[TVP]

trytrue,

Thanks for the excerpt from the paper by St. John and Taylor. Can you tell me where this was published?

Everyone,

It is clear that dissolution of intermetallic phases like Mg₂Si only takes on the order of tens of minutes, during the normally 4-12 hour solution treatment. However, homogenization and re-distribution of all of these elements continues to take place even after the first 15 minutes.

Also, quasi-static mechanical properties will not necessarily reveal the limitations of a shorter solution treatment. Fatigue strength is the key parameter, and any deficit in material homogeneity will be a likely initiation site for a fatigue crack. Fracture toughness is also a concern. However, if larger defects like gas porosity are present, then these will be the fatigue initiation sites, and perhaps you can get by with the lesser homogenization given by the reduced solution treatment time.

 

[trytrue]

tvp,
I do not remember very well, (it was 2000) how I got this but the paper has this information and I am sure it will help.

Header:
Materials Science Forum Vols. 331-337 (2000) pp. 277-282

Footer:
Published in ‘Aluminum Alloys- Their Physical and Mechanical properties’ Year:2000, pp. 277-282.
Published on

http://www.scientific.net,

email: office@scientific.net
© 2000 by Trans Tech Publications Ltd., Switzerland,

http://www.ttp.net

About fatigue test and short solution time. We made an experiment with 2.0 hr solution at 1004 F and 3.5 hr age at 320 F. All tensile and fatigue test passed, the test bars were cut from a 1/2 inch of casting wall thickness.
The minimum acceptable cycle life is 10,000,000 cycles at 70 Mpa and most of the 8 fatigue tests passed 13,000,000 cycles at 98 MPa the last million cycles. After reached the 10,000,000 they upload 7 MPa every time it passed 1,000,000 cycles.
It happens as J.A. Taylor and D.H. St John stated that the changes to eutectic silicon morphology are determined by solidification conditions (in 1/2" thickness the cooling rate is really faster), grain size and eutectic modification(we add Sr for these castings).

I hope this information will help

 

[TVP]

There are several technical papers on this topic that are currently available free-of-charge from the Science Direct (Elsevier technical journals) website. I don't know how long they will remain freely available, so I would recommend downloading them if you are interested.

1. "Alternative heat treatments for A357-T6 aluminum alloy", Engineering Failure Analysis 9 (2002) 99-107

http://www.sciencedirect.com/scienc...md5=a0b7fbcc59e83e239b2a7806d92c1c09&ie=f.pdf

2. "Effect of short solution treatment time on the microstructure and mechanical properties of a modified Al-7wt.%Si-0.3wt.%Mg alloy", Journal of Light Metals 2 (2002) 27-36

http://www.sciencedirect.com/scienc...md5=b220e8f1f7a08f5b8d37ee1b6300a176&ie=f.pdf

3. "Microstructure-based fatigue modeling of cast A356-T6 alloy", Engineering Fracture Mechanics 70 (2003) 49-80

http://www.sciencedirect.com/scienc...md5=9db3c1ee11251782c11017325f3b0c2f&ie=f.pdf

 

[trytrue]

tvp,
Thanks, those are excellent technical reports.

 

[etch]

Sorry i never answered earlier TVP, those reports were jsut what i was looking for, in fact the second one i think it was, i have been looking for. I even went so far as to email the author in Austrailia, asking for a copy, so thunmps up for these and thumbs up for web site.