356 aluminium sand casting elongation values 10

[Nadimuthu]

We have cast equivalent alloy to 356 Aluminium in sand casting ie AlSi7Mg.Its specified elongation value is 2%in fully heat treated condition(T6).But the value of elongation required for 356 is 3% which we are not getting.
The test bars are as per Foseco foundryman handbook specification chemical melt treatment was done using Foseco chemicals as per their recommendation.Heat treatment cycle was 535-540c with quench in hot water(65-100c)both inclusive in 12 hours.Cooled to Room temperature and then Precipitation hardened at 140c for 6 hours.
Please offer your advice on the procedure to be followed to achieve this elongation in 356.

 

[TVP]

I have a couple of suggestions for you. First, the aging treatment could be modified somewhat-- instead of 140 C for 6 hours, you should try 155 C for 3 to 5 hours. This is a standard T6 heat treatment for alloy 356.

The other suggestions center around microstructure. Are you using strontium for eutectic modification? If not, this would be another way to improve elongation.

Do you know what the average grain size of the cast bars is? Large grain size causes lower elongation, so refining the grain size is very important. Increasing the cooling rate and decreasing the pouring temperature can increase the rate of solidification, and therefore reduce grain size. Secondary dendrite arm spacing is another consideration, but more for fatigue strength than static strength.

Porosity, both microshrinkage due to solidication, and large pores due to entrapped gas (usually hydrogen) can greatly reduce elongation. Do the fracture surfaces show evidence of excessive porosity? If so, this would be a key area to investigate.

The last area is that of inclusions. Excessive oxidation of the melt can lead to entrapped oxide particles, which reduce mechanical properties like tensile strength and elongation.

Standard metallographic testing can reveal the presence of all of the microstructural elements-- grain size, porosity, inclusions, etc. I hope this was helpful, and best of luck with your project.

 

[TVP]

I have a couple of suggestions for you. First, the aging treatment could be modified somewhat-- instead of 140 C for 6 hours, you should try 155 C for 3 to 5 hours. This is a standard T6 heat treatment for alloy 356.

The other suggestions center around microstructure. Are you using strontium for eutectic modification? If not, this would be another way to improve elongation.

Do you know what the average grain size of the cast bars is? Large grain size causes lower elongation, so refining the grain size is very important. Increasing the cooling rate and decreasing the pouring temperature can increase the rate of solidification, and therefore reduce grain size. Secondary dendrite arm spacing is another consideration, but more for fatigue strength than static strength.

Porosity, both microshrinkage due to solidication, and large pores due to entrapped gas (usually hydrogen) can greatly reduce elongation. Do the fracture surfaces show evidence of excessive porosity? If so, this would be a key area to investigate.

The last area is that of inclusions. Excessive oxidation of the melt can lead to entrapped oxide particles, which reduce mechanical properties like tensile strength and elongation.

Standard metallographic testing can reveal the presence of all of the microstructural elements-- grain size, porosity, inclusions, etc. I hope this was helpful, and best of luck with your project.

 

[Nadimuthu]

Thank you Mr.TVP.At present we had given a rest of 12 hrs at room temperature in between solutinising and precipitation hardening.At this type of treatment we are able to get 3.2% elongation.But this practice has not been endorsed in any technical literature.
A recent webdownload has given the heat treatment cycle as 505C for 18hrs and a quenching at 65-150c of water that is circulated or sprayed within 7 seconds followed by precipiation at 160C as rightly mentioned by you.
We have discussed with our sub contracted heat treatment source to conduct experiments with this cycle.The broken test bar specimens are being prepared for micro structure analysis.
I am greatly thankful to you for your suggestions and when we get conclusive results from the micro study, I will post the same.

 

[TVP]

N,

The suggestion for solutionizing at a lower temperature (505 C vs. 535-540 C) for a longer time (18 hours vs. 12 hours) may be an attempt to reduce grain growth. Or possibly this was a suggestion to minimize local fluctuations in the furnace-- some areas may be higher than 540 C and some lower, with too high temperatures causing problems with eutectic melting. Either way, casting soundness (microstructure, porosity, etc.) generally determines elongation behavior, not heat treating practice.
I look forward to hearing hoe your trial progresses.

Best regards,

Toby Padfield

 

[etch]

we currently make and castings out of very similar material, and have found that we had same problems.

You could try 535 c for 10 hours then quenching at 30C, then followed by 6 hours at 160c.

or

Try a polymer quench this will give higher elongation

or

Try reducing the magnesium content in the alloy.

 

[Nadimuthu]

Thanks Mr.Padfield for your suggestions.
With your advice and other information I could gather from experience, with friends, academics I could achieve the desired elongation.
The steps taken were
1/Modification was done at 2% of melt weight instead of the earlier 0.75% as microstructure studies showed long Silicon needles.
2.The test bar methoding(rigging) was changed as some porosity was observed in the test specimens on micro study.
3.A ceramic foam filter was introduced to avoid any inclusions.
4.A intermediate rest period of 10 hours between solutionsing (535-540c 12 hours) and precipitation hardening(160c 4 hours) was given as suggested by one of our customers.
The test bar was tested in as cast condition(No machining)
The test bar was poured out of various moulds(Nature sand, Sod silicate bonded sand ,both types with Graphite coating, with Zircon coating etc) to find out if grain size alteration due to difference in heat abstraction amongthe various types would have an impact.
I am glad to say that in all cases the elongation values exceeded 4% (Tensile and Yield st were also very much on the higher side)
The micro structure studies revealed a well modified eutectic and silicon phase with some porosities.
I look forward to your comments and views especially on the various points .
Which of the points would have contributed to the results?
Is there any values for modifications%?.We have used Sodium modifier.The latest Foseco handbook says for hypoeutectic alloys modification is not essential.
Is the intermediate rest period of any significance.
I came across a reference to' Effect of preaging before aging treatment of AlSi7Mg 0.3' TMS JOM Feb 1997.
Have you perchance come across this article?
Is this rest period valid?
I look forward to your valued feedback.

 

[TVP]

Nadimuthu,

I am glad to hear that your testing has resulted in some positive improvements. It appears as though you have performed a thorough analysis, which I am sure will continue to be of benefit for you.

My thoughts on your results are as follows:

1. Eutectic modification is ABSOLUTELY ESSENTIAL for any aluminum casting alloy that contains an appreciable amount of silicon (from ~ 5% to the eutectic point) if you require good mechanical properties (strength, elongation, fracture toughness, fatigue strength). I cannot imagine how Foseco can say that it is not essential-- innumerable studies have proven that hypoeutectic alloys like 356 have substantially better properties after modification.

2. Porosity is always a problem for obtaining good mechanical properties. Proper metal feeding (gates, risers, overflows, etc,) and good design (thermally balanced sections, etc.) will minimize these problems. Mold filling analysis is an extremely valuable tool for predicting problem areas on complex castings.

3. It is difficult for me to comment on which variable had the largest effect. If you saw long silicon needles prior to modification, then the modification definitely improved the elongation, due to the better microstructure. Reducing porosity will definitely improve elongation. The addition of the ceramic filter probably also had a significant effect, as it will remove inclusions, both intermetallics and oxides. I would recommend you keep the filter, and re-evaluate all of the filling and solidification parameters to ensure optimal microstructure (low porosity, small grain size, etc.)

4. Since I do not work at a foundry, I cannot directly answer your question about the concentration of modifier used. One thing to consider-- phosphorus reacts with sodium (and probably with calcium and strontium as well) to form phosphides, which reduces or eliminates the modification effect. Low phosphorus metal require less modification. Also, increasing amounts of iron, nickel, and copper cause decreases in elongation. If you are using secondary metal (recycled) then you should be aware of the impurity limits before and after melting. You may be able to reduce the modifier concentration if you have cleaner metal initially.

5. Did you see any major differences in elongation based on the various mold materials (silca, zircon, etc.)? The improved strength was a definite indication of a finer grain size, but I don't know about the elongation, since you haven't separated all of the other variables (effect of modification level only, effect of filter only, effect of mold material only, etc.)

6. I will have to look through my files for the TMS paper. I should have a copy of this, so I will reply once I have a chance to review it. Artificial aging is a complex process, and I don't know the exact effects of the "rest period". I will look into this some more, and reply on the TMS article and the "rest period".

 

[1391]

mr.nadimuthu,
a very excellent exchange of technical info as regards to heat-treatment of aluminium alloys. we have a peculiar problem- we face an excess hardness ranging from 117-121 BHN in LM25WP alloy. we follow solutionizing of 520+/- 5C for 5hrs and preciptation 170C for 8hrs with which we have been achieving good results but of late this higher hardness problem has cropped up . any suggestions.

 

[Nadimuthu]

Dear 1391,
You have stated in your query that you were getting good results previously.In that case, with the same heat treatment procedure, please check whether
1.the hardness is being checked at the same spot or has any new person testing shifted the check area to a different spot?
2.Has your hardness tester been changed/become due for calibration/been calibrated recently?
3.Is the Mg % in the casting in the higher range?It is not uncommon to keep Mg level in a slightly higher range in the ingot to make up for Mg depletion during melting for casting.
4.Is your ratio for ingots/foundry returns during charging kept constant or has someone added more ingots?
Hope these could help you to reach the solution.
All the best and look forward to your results.
Regards,
Nadimuthu.

 

[1391]

mr.nadimuthu
many thanks for your suggestions,
1.hardness is being checked by the same person at the same place -no change

2.hardness tester has been calibrated only recently, we have this problem only recently and only in a few items.

3.mg is around 0.4% only
4.even if ingot/return ratio varies the mg content is within limits
5. the castings after solutionising are balckish in colour in the higher hardness casting, we have tried to reduce temperature from 530 to 520c, post slutionising hadness is
80 bhn.

6. we are conducting one more experiment of reducing precipitation from 8hrs to 6 hrs.

regards
raghuvir

 

[1391]

nadumuthu,
we would like to know if any standards for callibration of brinell hardness testers are available, we have a few queries

1.Most of the test blocks supplied along with the testers are in steel and do not fall in the range used by us say 85-110BHN.Must the test block should also be in aluminium alloy and if so where could these test blocks be procured.

2.Is it mandatory that even if the BHN tester is callibrated one should start off with checking with the test block and then check castings on a daily basis.
3.we have been using 1000kg load and 10mm ball for quite some time to check hardness is it defined in any standard, ASTM says 500 kg with 10mm ball. which is appropriate for aluminium castings?
4.kindly advice how tensile of 19kg/sqmm and 7% elongation in as cast LM-6 test bars can be achieved, we find difficulty in getting both.
regrds
raghuvir

 

[TVP]

1391,

Most of your questions can be answered by reading one of the consensus standards for Brinell testing. There are three ISO standards and one ASTM standard that specify the test method, calibration requirements, etc.:

ISO 6506-1 METALLIC MATERIALS - BRINELL HARDNESS TEST - PART 1. TEST METHOD

ISO 6506-2 METALLIC MATERIALS - BRINELL HARDNESS TEST - PART 2. VERIFICATION AND CALIBRATION OF TESTING MACHINES

ISO 6506-3 METALLIC MATERIALS - BRINELL HARDNESS TEST - PART 3. CALIBRATION OF REFERENCE BLOCKS

ASTM E 10 STANDARD TEST METHOD FOR BRINELL HARDNESS OF METALLIC MATERIALS


ISO & ASTM standards can be directly obtained from their respective organizations (

http://www.iso.ch

or

http://www.astm.org)

or from Global Engineering Documents at

http://global.ihs.com.

Hardness test blocks are available in the ranges that you are using. For example, Instron offers the following aluminum test blocks for use with their Wolpert test machine:

80 HBW5/125
60 HBW5/250
80 HBW5/250
60 HBW10/500
80 HBW10/500
60 HBW10/1000
80 HBW10/1000

All of these blocks are available with DKD certificates, which is the German institution certifying Standard Reference Materials similar to NIST in the U.S.A. I recommend you contact Instron for more information. Their website is

http://www.instron.com.

 

[TVP]

The formatting on my previous post caused a problem with the email address for Instron. The proper address is as follows:

http://www.instron.com

 

[etch]

We continually cast a356, and get fairly high test pieces. i would be intrested to know where the test pieces are being taken from, is it integral of the casting or seperately cast test bars.

Very important points to remember and i cannot state this highly enough, is the Mg content, and the quench temperature and medium.

I would check your mg content again, or that your heat treatment temps are correct. as it seems as if your mg is either too high or temp in heat treatment too high.

Quench temperature is also very important, we quench parts in both water at 30 and polymer at 70oC, if your thorughput is high and you do not allow the water to suffucently cool, you will notice that elongation values start to creep up, care has to be taken that the water is not too hot, as if it reaches boiling point the steam evolved acts as a shroud around the casting impeading quenching.

 

[alucast]

We are also having difficulties to reach high elongation in our test bars.We cast 18mm dia. test bar according to EN1706 and are reaching 3,5% elongation with AlSi7Mg T6.
I think an important issue is what kind of test bar(shape,size) and how exactly it is cast(gating,chills,...).
So,if anyone with high elongation results could post some info regarding this.

The second problem we have is the elongation in the casting.
The particular casting is a 2m long pipe and has a dia. of 400mm.
In areas with walthicknesses of 22mm+ our elongation is 3%,but in areas with 16mm(large areas)our elongation is max. 1%.

 

[etch]

I am not sure, but here in the uk we have a standard laid down for test bar size and dimensions for sand, die and chill casting, do you not all have the same?

Alucast, i would suspect that you shpuld be getting higher elongation with the thinner section. we typically get approx 7 -8 % on 19mm test bars heat treated in t6 conditions. we heat treat at 535 for 10 hours followed by a quench at 30 degrees, then age for 6 hours at 160. Quenching at 30 degrees probably would not help your distortion. VERY important is the surface finish on a test bar, I would nto belive the difference the surface finish could make on the results, and im talking between a smooth and polished finish.

What part of the world are you in, i can give you a couple of test houses that are good at prepping and testing and very cheap.

another thought would be to x-ray the test bars, this would ensure that there are no metallurgical problems.

 

[alucast]

Hi etch,
we cast our test bars according to EN 1706 in sand casting,but we add some material and machine them.I had
some bars pulled at another source and get the same results.They do their on bars and are successful but they have low pressure die casting.
Do u know any source on the web where a test bar with runner and riser is shown or drawn up.What fluxes do u use and which supplier?Do u cast at a certain density level?

 

[etch]

Alucast,

Id love to know where you are, as i can imagine we are probably in competition lol

If you are in the UK try to get hold of a book by ALARS that details all the info on alloys test bar dimensions etc.,

Your friend would get higher results as his sample would be rapidly chilled hence giving smaller grain size, typically i would say for a sand castig your elongation would be 4% lower that low pressure diecasting.

We use various fluxes usually from foseco. For Aerospace work we cast an 0.86% density index. I take it we use similar equip measure sample in and out of water etc.

Try taking a tensile sample out of the casting, raher than test bar, that also should give you good results.

 

[etch]

Found the name of the book some swine stole it off my desk,

its called

British and European Aluminium Casting Alloys

get it from

ALARS
0121 456 1103