Shrinkage‑porosity and gas‑porosity are frequently confused in aluminum casting quality work. Pore morphology, distribution location, CAE prediction and metallographic observation support root‑cause judgement for LPDC, gravity and CPC counter‑pressure casting troubleshooting.
Shrinkage‑porosity originates from volume contraction during alloy solidification. It mostly appears at casting hot‑spot zones predicted by Procast solidification simulation, where last‑to‑solidify metal cannot receive sufficient feeding from adjacent molten aluminum for EV structural‑part mold.
Typical shrinkage‑porosity morphology: irregular dendritic‑texture pore, rough inner wall. Pore clusters concentrate at casting thick‑wall intersection position; isolated spherical‑shape pore rarely belongs to pure shrinkage‑origin defect for gravity casting mold castings.
Multiple triggering factors for shrinkage‑porosity: insufficient feeding‑path, premature solidification of runner‑system, local mold hot‑spot and improper counter‑pressure holding‑time for CPC counter‑pressure casting mold mass‑production.
Gas‑porosity mainly comes from dissolved hydrogen, release‑agent decomposition gas or infiltrated suction‑air. Gas‑porosity mostly presents round or near‑round pore shape; inner‑wall condition varies according to different gas‑source for LPDC casting mold aluminum wheel blanks.
Hydrogen‑origin gas‑porosity shows smooth bright pore inner‑wall; suction‑air‑origin pore carries oxide‑film on inner‑surface; release‑agent‑decomposition pore contains carbon‑rich residue inside pore cavity for aluminum alloy foundry quality analysis.
Mixed‑type porosity exists widely in real workshop batches: gas gathers inside pre‑existing shrinkage cavity, forms shrinkage‑gas compound defect. Simple visual inspection often mis‑classifies compound‑defect as pure gas‑porosity for EV structural‑part castings.
Procast simulation distinction: solidification‑time gradient forecast shrinkage risk; hydrogen‑diffusion or filling negative‑pressure simulation predicts gas‑porosity risk. Two kinds of defect require totally different optimisation directions for gravity casting mold projects.
Process‑test discrimination method: increase mold cooling intensity. If defect severity decreases, it points to shrinkage‑origin; if pore quantity remains unchanged or even worsens, gas‑source problem is highly suspected for CPC counter‑pressure casting mold.
Improper external‑spray cooling may induce both defects: insufficient spray leaves hot‑spot shrinkage‑porosity; over‑intensive spray causes rapid surface‑shell solidification and blocks feeding channel, aggravating internal shrinkage despite surface temperature drop for LPDC casting mold.
Troubleshooting priority sequence: confirm defect metallographic feature first. Distinguish shrinkage, gas or mixed‑type, then target mold‑structure, cooling, pressure‑curve or melt‑quality; avoid blind parameter adjustment for aluminum casting mold batches.
Cross‑border after‑sales pitfall: overseas customers frequently attribute all pore‑defects to hydrogen. Many shrinkage‑defect cases waste production capacity by repeatedly increasing melt degassing time without solving real mold‑side root‑cause.
FAQ
Q: Where does pure shrinkage‑porosity preferentially distribute on aluminum castings?
A: Concentrates on hot‑spot thick‑wall intersection zones predicted by solidification simulation.
Q: What typical morphological feature distinguishes shrinkage‑porosity pore appearance?
A: Irregular dendritic‑texture pore with rough inner wall, clustered distribution characteristic.
Q: What pore‑inner‑wall feature belongs to hydrogen‑dissolution‑origin gas‑porosity?
A: Smooth and bright pore inner wall without oxide or carbon residue contamination.
Q: What defect type is formed when gas accumulates inside pre‑existing shrinkage cavity?
A: Shrinkage‑gas mixed compound porosity, easy to be misjudged as pure gas‑porosity.
Q: What simple process trial helps differentiate shrinkage versus gas‑origin porosity?
A: Raise mold cooling intensity; defect reduction indicates shrinkage‑dominated defect.
Q: How can over‑strong external‑spray cooling aggravate internal shrinkage‑porosity?
A: Fast surface shell blocks feeding channel and traps shrinkage cavity inside casting part.
Q: Why is degassing‑process ineffective for pure shrinkage‑porosity defect?
A: Shrinkage originates from solidification volume contraction rather than dissolved hydrogen gas.
