Aluminium Casting ( Solidification Defect or not?) 1

[taila]

I am working in an aluminium pressure die casting using reclaimed silica sand for coremaking.We manufacture cylinder heads.Recently we've been getting cracks on the inner edge of the boss area.Visual examination revealed no foreign particles and microscopic evaluation revealed large shrinkage defects close to the cracked region.Also large domains of eutectic network were present in the direction of the crack.The structure is the normal AlSiCu eutectic with no inclusions present.

My problem is that this occurs even though we are still following the same process as usual,I suspected that it might be hot tearing resulting from thermal stresses or hot spots.We manipulated the pre-heat temperature to avoid thermal shock but still the problem persisted.What could be the problem, could the chemical composition or the diecoat be a contributing factor to this defect? The alloy is AlSi9Cu2.

 

[Modey3]

I'm currently studying Al/Si casting defects. You said you found large shrinkage defects near the cracked region,and also large eutectic networks were in the direction of the crack. My question before I can help you is what is the nature shinkage defects? Is it microporosity? If so, current research suggests that eutectic networks of Sr modified (Im assuming modification)have a tendency to enhance interdendritic feeding difficulties. Thus less solid fration is needed before interdendritc feeding before microporosity becomes a problem. What is the hydrogen content of the melt? Did you use the SEM to check for folded oxide film defects around to cracked region? I can help you more, but I need more info. I hope this helps.

Modey3

 

[Modey3]

I just want to follow up on what I just posted. I was driving on my way home from the reasearch lab I work at UCONN and I had some thoughts about your situation.
Those cracks that you mentioned could very well be from thermal shock, but I would need to know more info about the mold design and the direction of heat flow. Raising the mold temperature would make the temperature gradients within solidified casting smaller and reduce the resultant stresses from thermal shock. You were right to do this and thats what I would have done initially.
Shear stresses from the mold wall upon casting removal would be my next guess. It could be the die-coating.
The shrinkage casting defects (micro-porosity) that you mentioned would provide stress concentrations enough to crack a material from thermal shock or hot tearing.
I cannot make a comment on the chemical composition because I dont know what kind of Si-Eutectic modification you are using.
I hope this helps.

Modey3