Analysis of Porosity Defect Origin: Differentiate Gas‑porosity, Shrinkage‑porosity and Mixed‑type Pore for CPC Cast Aluminum Components
Pore defect falls into gas‑porosity, shrinkage‑porosity and mixed‑type. Pore morphology, distribution position, metallographic feature and XCT characteristics help root‑cause judgement for LPDC, gravity and CPC counter‑pressure casting EV structural‑parts quality analysis.
Gas‑porosity forming mechanism: hydrogen precipitation, release‑agent decomposition gas, trapped cavity air. Gas‑pore feature: smooth inner wall, round‑oval shape, isolated or scattered distribution; often appears near mold‑wall surface for EV structural‑part mold castings.
Shrinkage‑porosity forming mechanism: volume contraction during alloy solidification without sufficient feeding. Shrinkage pore characteristic: irregular jagged inner surface, interconnected pore cluster, concentrates at hot‑spot thick‑section junction for CPC counter‑pressure casting mold batches.
Mixed‑type pore defect: gas pore and shrinkage pore co‑exist. Initial micro‑shrinkage void provides space for hydrogen‑gas precipitation; gas further expands original shrinkage cavity, forming complex‑shape mixed pore for gravity casting mold production.
Typical gas‑porosity trigger factors: excessive melt hydrogen content, over‑thick release‑agent coating, mold surface residual water, poor cavity venting capacity for LPDC casting aluminum wheel blanks.
Shrinkage‑porosity trigger factors: insufficient riser feeding, unreasonable cooling gradient, inadequate CPC holding‑pressure, hot‑spot formed by abrupt wall‑thickness transition for aluminum casting mold mass‑production.
X‑ray image distinguishing tips: gas‑porosity presents clear smooth circular shadow; shrinkage‑porosity shows fuzzy irregular cloud‑like shadow. XCT 3D reconstruction further confirms pore inner‑surface morphology for EV structural‑part quality inspection.
Metallographic section verification: gas‑pore inner wall clean without dendritic‑crystal trace; shrinkage pore inner wall attaches clear dendritic‑crystal outline, which is the most reliable discrimination basis for CPC counter‑pressure casting failure analysis.
Troubleshooting mis‑judgement risk: treat mixed‑type pore purely as gas‑porosity. Operators focus on degassing and release‑agent adjustment, but ignore feeding‑pressure and cooling optimisation; scrap rate cannot drop for gravity casting mold trial batches.
CPC‑specific characteristic: insufficient holding‑pressure amplifies shrinkage‑porosity risk. Even good degassing and venting condition cannot compensate feeding shortage during solidification stage for LPDC casting mold.
Acceptance complexity: many drawing specifications set unified pore‑size limit, ignoring pore‑type difference. Same‑size shrinkage pore does more harm to fatigue performance than isolated round gas‑pore for safety‑critical aluminum casting components.
Cross‑border quality‑complaint note: overseas customers often classify all pore defects as gas‑defect. Metallographic evidence is required to distinguish shrinkage / gas / mixed pore before carrying out targeted process rectification.
FAQ
Q: What morphological feature distinguishes gas‑porosity from shrinkage‑porosity?
A: Gas‑porosity: smooth round‑oval inner wall; shrinkage‑porosity: irregular jagged interconnected cluster.
Q: What is the most reliable metallographic evidence for identifying shrinkage‑porosity?
A: Dendritic‑crystal outline attached on inner surface of shrinkage pore cavity.
Q: How does mixed‑type pore defect generate inside aluminum casting matrix?
A: Micro‑shrinkage void provides space for hydrogen precipitation; gas expands shrinkage cavity.
Q: What typical process factor leads to shrinkage‑porosity in CPC counter‑pressure casting?
A: Inadequate holding‑pressure, insufficient feeding and local hot‑spot region.
Q: What X‑ray / XCT image feature corresponds to gas‑porosity defect?
A: Clear circular shadow with smooth boundary on X‑ray projection.
Q: Why mis‑judging mixed‑type pore brings failure in defect rectification?
A: Only optimize gas‑related parameters while ignoring feeding‑pressure and cooling measures.
Q: What performance difference exists between equal‑size shrinkage pore and isolated gas pore?
A: Shrinkage pore causes more severe fatigue‑performance degradation than isolated round gas‑pore.
