Varmebehandling for støping av aluminium

1. Introduksjon

Heat treatment transforms aluminum castings from as‐cast, komponenter med variable egenskaper til nøyaktig konstruerte deler som oppfyller krevende brukskrav.

Ved å kontrollere temperaturen nøye, suge tider, og kjølehastigheter, foundries and metallurgists can tailor mechanical properties,

for eksempel strekkfasthet, hardhet, duktilitet, og utmattelsesmotstand, while also improving wear characteristics, maskinbarhet, og dimensjonell stabilitet.

This article delves into the fundamentals, prosesser, and best practices of heat treating aluminum castings.

We aim to provide a professional, autoritativ, and comprehensive guide to engineers, metallurgister, and quality professionals seeking to optimize aluminum cast components for performance and cost.

2. Why Heat Treat Aluminum Castings?

The purpose of heat treatment is to:

• Increased Tensile Strength and Hardness
• Improved Ductility and Fatigue Resistance
• Enhanced Machinability and Wear Resistance
• Dimensional Stability and Residual‐Stress Relief
• Tailored Properties for Service Conditions
• Consistency and Quality Assurance

3. Common Aluminum Casting Alloys

Aluminum casting alloys are typically divided into two main categories:

• Sandstøping / Permanent form (gravity cast) legeringer
• Die casting legeringer (trykkstøpte)

They are designated by a four-digit number (F.eks., A356, A319, A380) and fall into either the 2xx, 3xx, 4xx, or 7xx series depending on the primary alloying elements.

Bord: Overview of Common Aluminum Casting Alloys

4. What Types of Heat Treatment Are Available for Aluminum Castings?

The heat treatment process for aluminum castings varies based on the alloy composition, casting type, and desired mechanical properties.

Specialized furnaces and carefully controlled quenching methods are employed to ensure dimensional stability and prevent cracking during treatment. Below are common heat treatment types applied to aluminum castings:

TF (Fully Heat Treated)

The purpose of the TF treatment is to significantly increase the hardness and strength of aluminum castings.

The process involves heating the casting to around 515–535°C for 4 til 12 hours to dissolve alloying elements into a solid solution.

It is then rapidly quenched in warm water to prevent cracking, followed by aging at 150–160°C for 4 til 16 timer.

This treatment almost doubles the hardness of the original casting. TF is commonly used when high strength and durability are required, such as in structural components.

Its advantage lies in the substantial improvement in mechanical properties while maintaining casting integrity.

TB Condition (T4)

This heat treatment aims to improve ductility and moderate strength.

Castings are heated just below their melting point until alloy elements enter a solid solution, then quenched in water, boiling water, or polymer solution.

The quenching medium is selected to balance mechanical properties, reduce distortion, and minimize internal stress.

TB is suitable for parts that require good formability and weldability.

The advantage is the preservation of ductility and reasonable strength, which facilitates further manufacturing processes.

TB7 (Solution Treated and Stabilized)

Designed to produce castings with enhanced malleability, this treatment is similar to TF but with aging conducted at a higher temperature of 240–270°C for 2 til 4 timer.

This results in slightly softer castings compared to TF, making them easier to work with in applications where some flexibility is needed.

It’s used in components requiring better thermal stability and toughness.

TE (Alder herding)

TE heat treatment accelerates the natural aging process by heating castings to 150–170°C for 4 til 12 hours without any quenching.

This is particularly useful for intricate or finely featured castings that could be damaged by rapid cooling.

The process improves hardness and stability without risking distortion. TE is preferred for delicate parts where shape retention is critical.

T5 (Precipitation Aging)

This artificial aging process stabilizes castings by heating them at relatively low temperatures (150–200 ° C.) til 2 til 24 timer.

T5 improves machinability and dimensional stability and is typically applied to die castings where controlled hardness and surface finish are important.

The advantage is improved mechanical properties with minimal thermal impact on the casting.

T6 temperament

T6 treatment is used to achieve high strength and hardness.

The casting is solution treated at around 538°C for about 12 timer, rapidly quenched in water or glycol at 66–100°C, then artificially aged at 154°C for 3 til 5 timer.

Ofte, a straightening step follows quenching to ensure dimensional accuracy.

T6 is widely applied in aerospace, bil, and defense industries for structural parts needing excellent mechanical performance.

Its main advantage is maximizing strength while minimizing deformation under load.

TF7 (T7 or T71 – Solution Treated and Stabilized)

This treatment enhances high-temperature mechanical stability by solution treating castings and stabilizing them at 200–250°C.

While it offers slightly lower tensile and yield strength than T6, TF7 improves thermal resistance and dimensional stability.

It’s ideal for components exposed to elevated temperatures or long-term stress.

Stress Relief and Annealing (TS Condition)

Stress relief heat treatment, performed at 200–250°C, reduces residual stresses that can cause warping or cracking.

Annealing, done at 300–400°C, softens castings for easier machining or forming.

These treatments are typically used for thick or complex castings requiring further mechanical operations. Their advantage is improved dimensional stability and enhanced workability.

Polymer Quenching

Instead of water, polymer solutions are used to quench castings at a slower rate.

This reduces internal stresses and distortion, making it suitable for complex or thin-walled castings that require less hardness but high dimensional accuracy.

Polymer quenching offers a gentler cooling method to protect delicate geometries.

Common Heat Treatment Types for Aluminum Castings Table

5. Alloy-Specific Heat Treatment Recipes

A356/356.0: Standard T6 Process

• Solutionizing: 540–560 °C, 6 h (25 mm section).
• Slukk: Vann (~20 °C) with mild agitation.
• Aldring (T6): 160–165 °C, 6 h; air cool to ambient.
• Optional T7: 180 ° C., 10 h; air cool.

A380/A383: T4 and T5 Applications

• T4 (Naturlig aldring): Quench from 505–525 °C; hold 18–24 h; limited strength (~UTS 200 MPA) with good ductility (4–6%).
• T5: Direct artificial aging at 160 °C for 4–6 h; results ~UTS 210–230 MPa, elongation 3–4%.

319/319.0: SHT and Aging for HPDC

• Sht: 505–525 °C for 4–6 h (10–20 mm sections).
• Slukk: Polymer (10% PAG) to reduce distortion.
• Alder (T6): 160–170 °C for 8–10 h; yields UTS ~260 MPa, elongation ~4–5%.

A413: High-Strength Castings

• Sht: 540–560 °C for 8–10 h (thick sections 50–100 mm).
• Slukk: Vann + corrosion inhibitor; aim for 400 °C/s cooling.
• Alder (T6): 160–170 °C, 10 h; UTS ~270–310 MPa, elongation ~3–4%.
• Overage (T7): 180–200 ° C., 10–12 h; UTS ~260–290 MPa, elongation ~5–6%.

6061 (Cast Variants) and Specialty Alloys

• 6061‐Cast SHT: 530–550 °C for 4–6 h (12–25 mm sections).
• Slukk: Water or polymer (both acceptable for moderate distortion).
• Alder (T6): 160 ° C., 8 h; yields ~UTS 240–270 MPa, elongation ~8–10%.
• 6063‐Cast: Similar SHT, T5 often sufficient for UTS 165–200 MPa but T6 yields UTS ~210 MPa.

6. Mechanical Property Correlations

Strekkfasthet, Avkastningsstyrke, and Elongation Post‐Treatment

• A356 T6: UTS 240–280 MPa; YS 200–240 MPa; Elongation 6–8%.
• A380 T5: UTS 210–230 MPa; YS 160–180 MPa; Elongation 3–4%.
• 319 T6: UTS 260–280 MPa; YS 210–230 MPa; Elongation 4–5%.
• A413 T6: UTS 270–310 MPa; YS 220–260 MPa; Elongation 3–4%.

Hardness Changes Through Heat Treatment Stages

• A356: As‐cast ~70 HB; after SHT ~60 HB; T6 ~80–85 HB; T7 ~75–80 HB.
• 319: As‐cast ~75 HB; T5 ~85 HB; T6 ~90–95 HB.
• A413: As‐cast ~80 HB; T6 ~95–105 HB; T7 ~90–100 HB.

Fatigue Performance and Crack Growth Rates

• A356 T6: Endurance limit ~70 MPa; T0 ~50 MPa.
• 319 T6: ~ 75 MPa; better high‐temp fatigue resistance due to finer Cu‐rich precipitates.
• Residual Stress Impact: Proper stress relief can boost fatigue life by 20–30%.

Creep Resistance in High‐Temperature Casting Applications

• Overaged A356 T7: Maintains ~85% of room‐temperature strength at 150 ° C.; acceptable for engine brackets.
• A413: T7 retains ~80% at 200 ° C.; recommended for transmission housings under sustained loads.

7. Applications of Aluminum Castings

Bilindustri

• Motorblokker (A356 T6): Demonstrated 20% weight reduction vs. støpejern; heat treatment yields UTS ~260 MPa, enabling higher cylinder pressures.
• Sylinderhoder (319 T6): T6 treatment eliminates porosity‐related fatigue failures; repeated runs across line yield consistent performance with <1% scrap due to quench cracking.

Luftfartskomponenter

• Turbine Impellers (6061 T6): Through rigorous SHT and aging, achieve fatigue life >10⁷ cycles under 200 MPa stress; CMM post‐treatment confirms run‐out <0.01 mm.
• Landing Gear Blocks (A356 T7): Overaged for stability, beholde 75% of strength at 120 ° C.; no in‐service cracking over 15,000 cycles in evaluation.

Industrielle maskiner

• Pumpehus (A413 T6): T6 ensures UTS >280 MPA, reducing wall thickness by 20% vs. as‐cast designs; lubrication passages remain within ±0.05 mm after quench.
• Ventillegemer (A380 T5): Achieve UTS ~220 MPa, elongation ~4%; stress relief at 300 °C eliminates 80% of as‐cast distortion, reducing machining time by 30%.

Consumer Electronics and Heat Sinks

• Varmevasker (6061 T6): Yield UTS ~250 MPa and thermal conductivity ~180 W/m·K; extruded and then heat‐treated for optimal performance in high‐power LED modules.
• Laptop chassis (A356 T6): T6 ensures structural stiffness under mechanical loads; minimal warpage (<0.2 mm across 200 mm span) preserves panel fit and finish.

8. Konklusjon

Heat treatment of aluminium castings is not a “one‐size‐fits‐all” proposition.

By understanding the metallurgical fundamentals—solutionizing, slukking, and aging—metallurgists can design cycles that optimize properties for specific alloys (6061, 7075, 356, etc.) og delgeometrier.

Through careful control of furnace temperatures, quench media, and aging profiles, castings transform into high‐performance components suitable for aerospace spars, Marin maskinvare, automotive assemblies, and precision electronic enclosures.

Til slutt, successful heat treatment depends on:

• Alloy selection and chemistry
• Precise process control (temperatur, tid, quench rate)
• Post‐treatment inspection (Ndt, Mekanisk testing, dimensional checks)
• Application‐driven temper choices (T6 for strength, T7 for stability, TS for stress relief)

By adhering to these principles and leveraging advanced furnace technology and metrologies, fabricators ensure that aluminum castings not only meet but exceed the mechanical, varighet, and reliability standards of modern industries.