Since the 1970s, governments have enacted legislation, which varies from country to country, with some focusing on emission levels and others on fuel economy. In the U.S., CO₂ emissions and the Corporate Average Fuel Economy (CAFE) standards for mileage are the key drivers of weight reduction. Figure 1 illustrates these objectives and the pressure automotive manufacturers face today.

Weight-reduction initiatives have taken many forms over the last 35 years and include redesigning of suspension systems, designing smaller and more powerful engines, and integrating high-strength steel and aluminum over other heavier materials. This article will review the recent advancements made integrating aluminum-intensive structural castings into automobiles, developments in casting and will focus on the new heat-treatment technologies available to meet the increasing demands of productivity and quality.

History

The use of aluminum in automobiles dates back to the 1940s and has steadily increased since then.

The Aluminum Association’s Drucker Report (Fig. 2) recently reported that aluminum-intensive automobile components have grown four times from an average of 84 pounds per car in 1975 to 343 pounds in 2012. Furthermore, it is projected that there will be 550 pounds of aluminum used in North American light vehicles by 2025, which represents a 63% increase.

Historically, we have seen aluminum automotive use exploited in the manufacture of engine blocks, cylinder heads, wheels and suspension components. Progressively over the years, we have seen premium auto manufacturers integrate aluminum into vehicle structures and body. Today, it is commonplace to see development engineers focus on the next wave of suspension and body-in-white structure transformations that integrate lightweight structural aluminum components. Specifically, these components include aluminum extrusions, castings and sheet products that are transformed for use as automotive strut towers, engine cradles, torque boxes, B-pillars, frames, battery boxes, doors, roofs and front-end structures. Figure 3 illustrates how these components have been introduced in various forms of a new vehicle structure.

Figure 4 provides a good example of a lightweight structural casting that replaced a number of steel fabrication and assembly components, resulting in 40% weight reduction. This new structural casting was made possible as a result of developments in high-pressure die casting and heat-treatment technology that provide manufacturers the flexibility to weld, rivet and apply adhesives for attachment of other vehicle components.

Casting Technology

Structural castings are of particular interest today as new efficient casting and heat-treatment methods can be used to produce high-integrity automotive structural castings, which are complex and integrate thin-wall sections that create metal-flow challenges. Historically, thin-walled casting manufacturing was isolated to non-critical covers and housings due to conventional casting limitations and alloy development. As a result, developers were limited to as-cast mechanical properties and could not apply any benefits in strengthening through heat treatment, which are available to other aluminum casting methods.

    Today, the landscape has changed considerably with the advancement of high-pressure die-casting (HPDC) processes that produce thin-walled, high-integrity castings that are porosity free. Furthermore, suppliers of aluminum alloys have developed special alloys that are specifically tuned for the HPDC process and manufacture of high-integrity structural automotive components. Through the combination of casting process and alloy development, casters now have options to develop new, more complex structural automotive castings whose properties can be further enhanced by heat treatment.

Heat-Treatment Process

The purpose of introducing the heat-treatment process to aluminum structural components is to alter the mechanical properties and arrive at an optimal combination of strength and ductility required for the vehicle’s structure. Depending on the requirements of the structure, these processes may include T5 artificially aged only; T6 solution treated, quenched and artificially aged; and T7 solution treated, quenched and artificially over-aged.

Heat-treatment processes include multiple heating and holding steps with intermediate cooling or quenching operations. The soak time at temperature required to achieve the desired properties is a function of the alloy’s ability to develop a homogeneous solid solution and the castings’ microstructure prior to heat treatment. Process uniformity is the key processing driver and dictates how uniform properties are achieved across the various casting sections with elimination of part distortion and reduction of residual-stress levels.

Quenching is a precarious and necessary step in the process, particularly so when processing thin-walled structural components. As a result, extreme care is taken to ensure processes are validated using a combination of CFD (computational fluid dynamic) modeling and physical-testing tools. It is imperative that these tools be used in development of heat-treatment facilities in combination with integrating system flexibility through constant feedback and variable control parameters. An example of these CFD tools used to simulate processing is shown in Fig. 5.

Part Carriers

Significant design efforts go into optimizing the component carrier design due to the thin-walled characteristics of the castings. Careful consideration of the following design inputs, modeling and physical-testing studies must be undertaken when designing a structural casting carrier and heat-treating facility:

•   Casting loading method to and from the carrier

•   Service temperature ranging from 450-510?C (842-950?F) in combination with rapid quenching

•   Maximize loading density while meeting uniformity requirements

•   Carrier transfer system within the casting facility – fork trucks or other methods

•   Casting orientation and support methods required for the control of distortion and uniform properties

•   Automatic furnace conveying systems and rapid quench-transfer capability

•   Flexibility for processing families of part geometries

Solution Treatment

Soak times for structural castings can vary from 10-150 minutes at processing temperatures of 450-510?C (842-950?F). System configurations include stand-alone batch systems, modular continuous systems and conventional continuous systems. The solution-treatment system can integrate multiple heating zones and roller hearth or overhead-rail conveying systems. In cases where floor space is limited, the overhead rail conveyors can offer advantages in plant space utilization. Carriers are charged automatically into the solution furnace and rapidly heated via natural gas or electric forced-convection recirculating systems. Following the programmed frequency, the logic controller automatically discharges the solution-treated carrier of castings to the pre-conditioned quench system.

Precision Air Quench (PAQ™)

Conventional water quenching has proven to be incompatible for thin-walled structural-casting production. This aggressive form of quenching and resultant distortion can be seen in Fig. 6, which compares a distortion-free, properly air-quenched, thin-walled casting with a water-quenched casting.

Significant developments in high-density air-quench processing have been achieved. With over 10 years of development experience, a comprehensive database has been assembled that provides predictable methods for the precision air quenching (PAQ™) of various components, alloys and applications.

PAQ systems can be integrated in-line or off-line for flexible modular processing. They integrate a unique combination of recirculating air chambers, distribution nozzles, dampers and directional ductwork that uniformly delivers conditioned quench media, which leads to repeatable and uniform property and dimensional results. Quench rates are predetermined based on the desired mechanical properties and alloy selection. Manual controls have been eliminated and replaced with automatic valves and dampers. This allows quench rates to be controlled automatically for the specific part being processed. Quench parameters are developed for each component and, once validated, can be integrated as part of the product recipe. The system includes features for verifying in real time that each component’s quench-rate parameters have been met, and confirmation of results are automatically archived to the system’s supervisory control and data-acquisition system.

Artificial Aging Treatment

Following PAQ, loaded carriers are transferred to the artificial-aging oven, where they are heated and held at temperatures from 150-250?C (300-482?F). System configurations include stand-alone batch systems, modular continuous systems and conventional continuous systems. Artificial-aging systems integrate similar features and benefits provided with the solution-treatment furnace system.

Conclusions

Cast aluminum structural components require increased heat-treatment processing demands over conventional aluminum casting requirements. Because of these demands, structural-component manufacturers will require systems that meet the following guidelines:

•   Structural heat-treatment systems are designed using modern modeling tools and integrate sophisticated and unique features for structural castings.

•   The relationship between structural component, carrier and automated furnace handling are carefully considered and CFD modeled to achieve the required heating and quenching uniformity expectations of a robust automotive process.

•   Casting dimensional stability, reduced residual stress levels and enhanced mechanical property levels are optimized through proven Precision Air Quench technology.

•   Solution and aging furnace designs integrate flexibility for future modular capacity expansions.

•   Quench-rate parameters are automatically controlled for variations in ambient and/or local conditions to provide reproducible part-to-part properties.

•   Quench-system optimization can be provided through integration with multiple solution systems.

•   Equipment floor space is optimized through high-density quenching systems.

•   Level-2 automation systems provide automatic recipe selection of quench rates for various components being produced.

•   Quench systems are available for heavy part sections and castings containing core sand.

•   Various system layouts can be considered for optimal facility material flow.

•   Quench-system sound power levels are minimized via acoustical design features.

•   Structural-component heat-treatment systems are designed to be CQI-9 compliant.

•   Systems are made available globally, integrating local codes and specification requirements.

 Aluminum structural-component heat-treatment systems, along with a wide variety of other ferrous and nonferrous heat-treatment systems, are made available by Can-Eng Furnaces International. Can-Eng Furnaces International is a global provider of state-of-the-art thermal-processing systems and is a significant supplier to the automotive industry through direct and tier supply. IH

For more information: Contact Tim Donofrio, vice president, Standard and Aluminum Equipment; CAN-ENG Furnaces International Limited, 6800 Montrose Rd., Niagara Falls, Ontario CANADA L2E 6V5; tel: +1-289-292-2027; fax: +1-905-356-1817; e-mail: tdonofrio@can-eng.com; web: www.can-eng.com