High Pressure Aluminum Die Casting

[nottoobright]

Dear Experts
We are high pressure injecting Aluminum Grade ADC12. The part is 30cm x 30cm x 1.5 cm (ht). Injection pressure is 150 bar.
Once injected we rough grind the surface and then send for sand blasting. After sand blasting we are not getting a uniform appearance on the surface. Near the injection ports end of the mold we find a pattern shows up. This pattern / lines are not visible before sand blasting. We tried machining 0.5 mm off the top surface, no indication of anything, and then sand blasted again. The same pattern appeared. I'm attaching some photos to illustrate. From the photos you can't tell, but when you run your hands across the line you can't feel any height difference.
As these are to be used as top and bottom plates in a weigh scale for race cars we are concerned that this may be a point where failure could occur. Anyone ever run across this before? Any suggestions on how to rectify? We need a consistent appearance on the surface after sand blasting.

 

[Compositepro]

It appears to be a knits lines where flow fronts meet, leaving a visible anomaly. High pressure compresses voids but does not make gas molecules disappear. High pressure (hot) air in contact with hot, molten aluminum will react to form aluminum oxide that ends up at the knit lines. Flow front control and controlled mold venting while filling will prevent knit lines. Or it could be where one grain structure in the aluminum meets the grain structure on the other side of the knit line. Gravity and mold orientation will be a factor.

 

[SwinnyGG]

Yep. Compositepro has it on the head.

The line you're seeing is a microstructure changed, either due to molten flow or trapped gas. If this is truly a problem for your in-service parts, you can eliminate it by altering gate geometry, mold orientation, venting procedures during mold fill, etc.

A competent high-pressure casting house should be able to provide guidance on why this is happening with your gate configuration, if it is actually a problem.

 

[EdStainless]

If it is just cosmetic though you need to look at other surface treatment methods that won't highlight it.

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P.E. Metallurgy, consulting work welcomed