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Metallurgy/Shrinkage defect


QUESTION: Production: Aluminum cylinder heads for auto industry.
Process SPM gravity.
Question: Recent increase in shrinkage in casting. Review of process and all other inputs reveals reasonably consistent control. One deviation noted is the amount of chromium has increased and the timeline coincides with the increase in shrinkage. Normal Cr levels 0.020-0.30.
Current levels 0.040 - 0.055. What if any negative impacts can Cr have on the solidification process?
Feel free to request additional information that may help with this inquiry.

ANSWER: Dear Mr. John,

Hi! it is my great pleasure for offering me an opportunity which fits in my expertise.

Lets understand what chromium does in aluminium alloys:
1) It forms brittle alpha-Alx(Fe, Mn,Cr)ySiz inter-metallic phase when addition is beyond 0.35%.This retards nucleation and grain growth.
2) It has slow rate of diffusion and forms fine dispersed phase
3) It forms very coarse constituent with Mn, Fe and Ti.
4) It prevents re- crystallization of Al-Mg-Si impacting mechanical properties, during hot working and heat treatment.
5) It changes morphology of accicular Al5FeSi to more cubic  to Al5(Mn,Fe)3Si2.

Summerising, Cr beyond 0.35% (ideally 0.25-0.30%), acts as inhibitor to nucleation and growth, its brittle phase acts as crack initiator, prone to brittle fracture and accounts for loss of mechanical properties.

Now, please tell me :
1) What is the alloy you use
2) What is application of alloy
3) Chemical composition of alloy\
4) Which finished product you are making
5) What problem you are facing

If I could know these information, surely, I can be a source to resolve your issue. Meanwhile, bring down the 0.3-0.35%.

Best Regards,

---------- FOLLOW-UP ----------

QUESTION: 1 & 3. 319 alloy

Silicon        6.5 - 8.5
Copper        3.0 - 4.0
Magnesium    0.3 - 0.5
Iron        0.8 max
Manganese    0.2 - 0.5
Tin        0.10 max
Lead        0.15 max
Zinc        0.8 max
Nickel        0.3 max
Titanium     0.25 max
Strontium    90 - 130 ppm
Each other   0.050 max
Total other  0.50 max
Aluminum     balance
2. Our process is semi permanent mold process. We produce aluminum cylinder heads for the auto industry. Molds are made of H13 steel
with sand core inserts. Sand core are made with a two part binder phenolic system and we use DMPA as the catalyst.
4. End product: Aluminum cylinder heads for the auto industry.
5. Solidification issue/Directional solidification feeding during the solidification process: The castings we produce normally have a reject rate of 0.15%. This reject rate is based on pressurized verification.  .
Current reject levels are 1.00%.

ANSWER: Dear John,

Zinc and iron levels are very high.It seems you are using recycled scrap alloys in this application.I would like to suggest you few tips (as I had manufactured cylinder heads and blocks for 6-years at castmaster Ltd,somewhere in early 2000, at Faridabad, India) which are as under:

1) If possible, keep iron below 0.5%
2) Zinc is highly objectionable and prone to cavitation defects. Keep it as low as possible.
3) Cr should never exceed 0.35%
4) Cores should be baked and free from gases
5) Chlorine based degasser are the best for removing hydrogen from liquid metal.
6) Titanium more than 0.2% is not advisable, best range is 0.15-0.2%
7) Why lead is there in your composition, very dangerous.
8) Keep Mn on higher side if iron is more than 0.35%
9) With best of my practical experience, composition: {C=7.8-8.2%, Cu=3.3-3.5%,Mg=0.5-0.6,Ti=0.18 average,Mn=0.45, Cr=0.25-0.3%, Fe= or<0.5%, remaining as low as possible and Zinc and Lead are banned}, will yield best results in zirconium based coated H13 molds.Process may be GDC or LPDC.

Which foundry you are working for.By visiting its web and seeing products, better advice can be offered.

All the best,


---------- FOLLOW-UP ----------

QUESTION: Hello Mr. Sharma,
You are correct. The majority (70%) of our aluminum is from recycled.
It is delivered in the form of sows or ingots and we remelt house.
This is a bad side of trying to balance cost. Purchasing aluminum of lower quality without affecting the quality of the casting. That is why we
are currently dealing with the issue of high Cr. A difficult balance and some controversy between the aluminum purchasing groups and the quality groups.
I have added a simple graph that illustrates our shrinkage/leaker defects and the Cr levels from each production date. There seems to show
a correlation. However, I have three days that have very low rejects.
Nov 30 2015, Jan 27 2016 & February 12 2016. I have not been able to explain why these days are low. We compared all process related data
and chemistry information from the two days prior and the two days after
and have not found an explanation. See attached.

I work for Nemak. My facility is in Dickson Tennessee.
We produce ~1.7M cylinder heads p/year for Chrysler and Ford.
I believe we have a facility in Chennai.

Dear John,

Fact remains you were near to root cause and you have constraints in aluminium purchase.Had it been a pressure die casting, recycled alloys work well but in case of GDC or LPDC, it may or may not depending on the composition of the blend.

Even in India, for example in case of Continental Engine Ltd, Cast-masters Ltd, Shriram Pistons Ltd etc who make similar items like your having similar processes do use virgin metal mostly as grain size of recycled alloy is coarser than virgin metal alloy.

Anyway, to sum up, do your best to control alloying elements in permissible limits; work on lower limit of metal temperature and treat with best quality fluxes (Foseco).

You may further consult in case your issue stands unattended on experts site or :

Best Wishes,

RP Sharma


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Questions that i can answer:- 1-sand casting of cast iron,S.G. iron,steel and aluminium alloys 2-Gravity die casting of aluminium 3-Low pressure die casting of aluminium 4-High pressure die casting of aluminium 5-Failure analysis of ferrous and non-ferrous castings Questions that i can`t answer:- 1-testing of iron and steel 2-Problem related to welding 3-Problems related to corrosion 4-Amorphous metals,rubbers,nylon,plastics and plastic moulding


1-Sand casting and ferrous and non-ferrous alloys 2-Gravity die casting,low pressure die casting,high pressure die casting of aluminium alloys.With a specialisation in auto-mobile components castings. 3-Alloy steel making on arc furnace,AOD,VOD,Con-cast root of alloy steel making. I have been working as Foundry man since last 27 years.

Hindustan motors Ltd.

I am a Metallurgical Engineer graduated from National institute of Technology,warangal(India) in 1984 and in 1991 i completed my masters degree in business administration(M.B.A.) from Institute of Management Technology,Ghaziabad(India).

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