Heat treating aluminum is a widely used, albeit complex, process today. It fulfills different requirements in various industries, including automotive, aerospace, and many more. The process involves notable procedures, including solution heat treating, quenching, and aging, among others, and utilizes different types of aluminum furnaces for optimal performance.

The core purpose of solution heat treating

Solution heat treating mainly provides adequate thermal energy to dissolve alloy elements present in a solid solution. The parts are heated to a solution temperature, followed by a rapid cooldown or quenching that locks all dissolved alloy elements at the high solution heat treatment temperature. This process is managed by industry standards like CQI-9 (Automotive) and AMS (Aerospace) to ensure desired results in welding applications of aluminum alloys.

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Furnace and quench tank requirements

The furnaces are high-velocity recirculation/convection type units, using either a gas (direct or indirect) or electrical heating system. The heating media are not exposed to the load to avoid radiation that could affect surface temperature. The design would include a fast-acting door(s) system.

These furnaces' maximum operating design temperature is 650°C (1200°F) for heat treatable aluminum alloys. The typical temperature uniformity requirement would be ±6°C (±10°F). The quench tank media is water or a polymer solution (PAG) and has appropriate agitation to limit the vapor blanket formed at the surface of the parts when quenching.

Some heat treatable alloys may need to be quenched in media at a higher temperature. In these cases, the installation of a heating system in the quench tank may be required. A cooling system could also be required to reduce the quenching media to the maximum quenchant temperature at the start of the quench requirements.

Different equipment configurations

Drop Bottom Furnace: This arrangement places the furnace directly above the quench tank. Either the furnace or the quench tank can be moved to allow loading/unloading of the load inside and off the furnace. This design would include a bottom opening, a fast-acting door(s) system, and a heavy-duty lifting mechanism to pull up the load inside the furnace and quickly lower it into the quench tank.

• PROS:
  
  - Fastest quenching delay
  - Versatile production management (multiple units)
  
  
• CONS:
  
  - Height requirement
  - Lower production capacity
  
  
  

Front Loading Furnaces: This arrangement places the quench tank directly in front of the furnace with access below the level of the bottom part of the load. The load rack is moved either by a roller hearth system or a push/pull system located at the back of the furnace, with roller rails on the floor. This design would include a front opening, a fast-acting door system, and an elevator system in the quench tank to lower and lift the load in the quench media.

• PROS:
  
  - Versatile production management (multiple units)
  - Lower height requirements
  - No lifting system inside the furnace
  - Good rack support for heavier loads
  
  
• CONS:
  
  - Slower quenching delay (could be an issue for thinner material)
  - Lower production capacity
  
  

Roller Hearth Furnace: This arrangement normally offers one or multiple positions on the loading side of the furnace, with multiple rack positions inside the chamber. The design puts the quench tank at the unload side of the furnace with access below the level of the bottom part of the load and one or multiple positions on the unload side of the quench position.

The loading rack is driven by a roller hearth conveyor system designed to move each load rack through the chamber for the anticipated soak time to achieve the desired temperature before quenching. This type of furnace would include a vertical fast-acting door system on both sides and an elevator system in the quench tank to lower and lift the load in the quench media.

• PROS:
  
  - Higher volume production rate
  - Smaller footprint than multiple units
  - Consistent production and results
  
  
• CONS:
  
  - Reduced versatility compared to multiple units
  - Increased downtime risk in case of malfunction
  
  
  

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Rackless systems

Roller Hearth Furnace: This arrangement processes the parts through the furnace without racks. They may have multiple levels loaded by some type of elevator system and multiple doors. An elevator system would take care of the quenching stage. These rackless systems commonly use a highly automated conveyor system to load/unload and quench the parts.

• PROS:
  
  Higher volume production rate
  No racks needed
  No need to load/unload racks
  Consistent production and results
  
  
• CONS:
  
  Reduced versatility
  Mainly suitable for very similar parts
  
  
  

Chain/Mesh Conveyor Furnace: This arrangement processes the parts through the furnace on a chain/mesh conveyor. These furnaces typically use highly automated systems with robots to load/unload the parts and achieve the quenching process.

• PROS:
  
  - No racks needed
  - No need to load/unload racks
  - Consistent production and results

• CONS:
  
  - Reduced versatility
  - Mainly suitable for very similar parts
  - Requires sophisticated robot systems
  
  

Carousel/Rotary Hearth Furnace: This arrangement processes the parts by loading them on a rotary unit in the furnace. The parts move around the chamber to be unloaded via either the same door or another close by dedicated to unloading. Having multiple levels on the rotary rack inside is possible and allows for higher production output with the same handling system. These furnaces typically use highly automated systems with robots to load/unload the parts and achieve the quenching process.

• PROS:
  
  - No racks needed
  - No need to load/unload racks
  - Consistent production and results
  - Smaller footprint
  
  
• CONS:
  
  - Reduced versatility
  - Mainly suitable for very similar parts
  - Requires sophisticated robot systems
  
  
  

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Aging heat treatment requirements

Following solution heat treatment, the parts normally require natural or artificial aging. To be artificially aged, the parts must undergo an additional treatment in an oven. This is necessary to achieve the desired mechanical properties, such as yield strengths and elongation.

There may be a maximum required time between solution heat treatment and aging, and if this time cannot be met, it may be necessary to put the parts in a freezer to slow the natural aging process. Aging ovens are also available in different arrangements.

Conclusion

While a complex process, heat treating aluminum is crucial in numerous industries and applications. The correct understanding and use of various industrial furnaces, metal processing techniques, and alloys can lead to significantly improved durability, versatility, and cost-effectiveness.

With the above insights, players in related industries can better understand how to maximize the benefits of these processes for their specific needs.

Frequently asked questions – Aluminum heat treating

What is solution heat treating?

Solution heat treating primarily provides enough thermal energy to dissolve alloy elements present in a solid solution. The heat treating process locks all dissolved alloy elements in place.

What types of furnaces are used in heat treating aluminum?

Aluminum heat treating utilizes high-velocity recirculation/convection-type units, using either a gas or electrical heating system. These furnaces also require a fast-acting door system for efficiency.

What is the operating temperature of these furnaces?

The usual operating design temperature of these industrial furnaces is around 650°C (1200°F) with an average temperature uniformity requirement of ±6°C (±10°F).

What is the role of aging in aluminum heat treating?

Aging is integral to aluminum heat treating, contributing to achieving desired mechanical properties such as yield strengths and elongation. Aging ovens, just like other industrial furnaces, are available in different arrangements to suit varying needs.

What are some common equipment configurations in heat treating aluminum?

Heat treating aluminum encompasses different equipment configurations, such as the Drop-Bottom Furnace, Front-Loading Furnace, Roller Hearth Furnace, and rackless systems like Chain/Mesh Conveyor Furnace and Carousel/Rotary Hearth Furnace.