Análise de defeitos de dobramento e inclusão em 5052 Círculos de alumínio laminado a quente
This document provides a systematic analysis of two typical defects in 5052 hot-rolled aluminum circles—Folding/Lape Inclusion—covering characteristics, causes, detecção, and prevention. These defects directly impact stamping, desenho profundo, qualidade da superfície, e propriedades mecânicas, making them key focuses in production and quality control.
1. Folding Defect (Folding/Lap)
1️⃣ Characteristics
- Macroscopic: Tongue-shaped, linear, or layered overlapping patterns on the surface, aligned with the rolling direction. Edges are often rounded, and severe cases may show cracking.
- Microscopic: Oxidation, decarburization/dealloying at the fold. Material flow lines are cut, local microstructure is disturbed, creating stress concentration points.
- Typical Locations: Edges, corners, areas of severe deformation, or regions extending from original ingot defects.
2️⃣ Root Causes
- Ingot/Billet Defects: Surface cracks, subsurface blowholes, segregation bands, unfilled flash/burrs from casting, which are pressed into the material during rolling.
- Improper Hot Rolling Process:
- Non-uniform heating and large temperature gradients cause uneven metal flow and local metal accumulation.
- Excessive reduction per pass or deformation ratios cause edge metal backflow and folding.
- Poor roll shape/guide setup, or roll surface spalling leads to local over-compression and metal entrapment.
- Circle Forming/Trimming Issues: Shearing burrs or tears during cutting, which develop into folds during subsequent rolling/annealing; excessively sharp corners hindering metal flow.
- Material Influence (5052): Higher Mg content results in slightly lower plasticity compared to pure aluminum, increasing the tendency for folding and micro-cracking under uneven deformation.
3️⃣ Detrimental Effects
- Destroys metallic continuity, reducing tensile strength, força de rendimento, e alongamento.
- Causes cracking or wrinkling during stamping/deep drawing; leads to black lines or color differences after surface treatment.
- Acts as a fatigue crack initiation site, significantly reducing service life.
2. Inclusion Defect (Inclusion)
1️⃣ Characteristics
- Macroscopic: Spot, stringer, or block-shaped dark/bright foreign objects on/near the surface, often elongated into lines or bands along the rolling direction.
- Microscopic: Primarily non-metallic inclusions (por exemplo, Al₂O₃, MgO, refractory materials, refining slag, external contaminants). They disrupt matrix continuity and often initiate micro-cracks and stress concentration in surrounding areas.
- Types: Endogenous(formed during melting/casting) e exogenous(introduced from furnace lining, tools, environment).
2️⃣ Root Causes
- Fusão & Processo de fundição (Primary Source):
- Dirty raw materials (aluminum/magnesium ingots) with excessive oxide skins.
- Inadequate melt purification, insufficient degassing and slag removal.
- Damaged or eroded furnace lining, troughs, or filtration systems contaminating the melt.
- Casting-level fluctuations, slag entrapment, or failed filtration.
- Laminação a Quente & Downstream Processes:
- Incomplete removal of furnace scale, leading to rolled-in oxide inclusions.
- Wear debris from rolls, guides, or cutting tools contaminating the metal.
- Environmental dust, graxa, or other foreign matter being pressed into the surface.
- Material Influence (5052): Mg is prone to oxidation, forming hard-to-remove MgO·Al₂O₃ spinel inclusions, easily forming banded defects.
3️⃣ Detrimental Effects
- Severely reduces plasticity, toughness, and fatigue strength, with transverse properties more significantly affected.
- Causes cracking or delamination during stamping; leads to black spots, furos, or pitting after surface treatment.
- Increases scrap rates and rework costs.
3. Detection Methods
- Visual & Macroscopic Examination: For surface folds and obvious inclusions; clearer after acid/alkali etching.
- Teste ultrassônico (EUA): Detects internal inclusions, delamination, and folds.
- Eddy Current Testing (ET): Rapid screening for surface/near-surface defects.
- Análise Metalográfica: Determines inclusion type, tamanho, distribution, and fold characteristics (oxide layer, microestrutura).
- Teste Mecânico: Tensile and bend tests to verify the impact of defects on properties.
4. Prevention & Control Measures (Full Process)
1️⃣ Melting & Fundição (Source Control)
- Strictly control raw material purity; use high-quality remelt ingots; store Mg ingots under protective atmosphere or sealed.
- Optimize melting practice: Melt at lower temperatures, stir adequately, empregar online degassing (por exemplo, SNIF/Alpur) + multi-stage filtration (por exemplo, 30ppi + 50ppi).
- Stabilize casting parameters: Low metal level, slow casting speed, strong cooling to reduce slag entrapment and porosity.
- Apply ingot homogenization and scalping to thoroughly remove oxide layers, segregation zones, and subsurface defects.
2️⃣ Hot Rolling & Circle Forming
- Ensure ingots are heated uniformly and thoroughly, maintaining consistent temperature without overheating.
- Design合理的道次压下 rational pass reduction schedules, control total deformation, and avoid edge backflow and metal accumulation.
- Optimize roll profiles and guide systems; promptly repair roll surface damage.
- Employ burr-free shearing or laser cutting for blanking; control corner radii to prevent tearing.
- Maintain clean production throughout to prevent contamination by scale, pó, or tool debris.
3️⃣ Specific Optimizations for 5052 Liga
- Appropriately increase hot rolling temperature (por exemplo, 420–480ºC) to improve plasticity and flow uniformity.
- Control Mg content towards the mid-lower limit of the specification to reduce oxide inclusion formation.
- Introduce intermediate annealing when necessary to relieve work hardening and internal stress, reducing the risk of folding and cracking.
5. Quick Assessment & Disposition Guidelines
- Folding: Visible overlapping patterns with oxidation tint and cracking tendency → Typically classified as non-conforming. Requer scalping/grinding or scrapping.
- Inclusões: Foreign object spots/strings on surface or internally → Rate according to relevant padrões. Material exceeding limits should be scrapped or downgraded.
Observação: The above is based on the characteristics of 5052 aluminum alloy and common defects in hot-rolled circles. Actual production should apply these guidelines flexibly in conjunction with specific processes and standards.