Heat-Treatment Advancements for Aluminum Lightweighting
Heat-Treatment Advancements for Aluminum Lightweighting
Aluminum was first discovered in the early 1800s and was produced in quantity during the turn of the
20th century using the Hall–Héroult smelting process. Interestingly enough, Carl Benz was patenting
his first version of a self-propelled internal-combustion-engine automobile during that same time.
During these times, automobiles or horseless carriages were a luxury and difficult for common people to
Henry Ford soon followed with the development of his mass-produced Model T (Fig. 1), which put America on wheels. We know Ford was consumed with integrating manufacturing efficiency anywhere it could be realized. The result of these cost-saving measures was realized in early October 1908, when the first production-line Model T Ford was assembled at the Piquette Avenue plant in Detroit.
Ford realized that the effect of simplification, standardization, scale and moving assembly lines resulted in vehicle cost reduction from $825/unit in 1908 to $260/unit by 1925, allowing vehicles to be accessible to the masses everywhere.
Automotive Pressures – CO₂ and Fuel Economy
The integration of lightweight materials and components into automobiles has been a driving force of the global automotive industry since the oil embargo of the early 1970s. Since then, manufacturers have integrated new lightweight technologies that have resulted in improved fuel-efficiency platforms and reduced powertrain size and capacity designs. Many of these technologies were derived from laws that varied from country to country, some focusing on emission levels and others on fuel economy.
In the U.S., the focus was on CO₂ emissions and Corporate Average Fuel Economy (CAFE) standards. Recently, U.S. limits have been amended. These amendments apply to 2021 through 2026 vehicle models where limits were dropped to 1.5% annual reduction, compared to the earlier requirements for 5% annual reductions. It is clear that as governments change so do polices, and arguments on both sides will surely spill out into the courts and maintain pressures on automotive producers.
Lightweighting and Safety Initiatives
Automobile weight-reduction initiatives have taken many forms over the last 35 years. They include redesigning of suspension systems, designing smaller more-powerful engines and integrating alternate lightweight materials. Furthermore, automotive and e-mobility manufactures are challenged with increased safety expectations.
These pressures are addressed by implementing active safety measures such as belt pre-tensioning, air bags, collision alarm, lane-change assist and rear vision to mention a few. Passive safety measures built into vehicle designs include structural features that maximize the impact-energy absorption capability, local dynamic stiffness and high-strength sections that minimize safety cell penetrations.
Today, most manufacturers utilize a hybrid approach for selecting structural materials and product forms for each individual platform price class. This hybrid approach can include the integration of aluminum, high-strength steels, thermoplastics and carbon-fiber materials to satisfy design requirements.
Product forms can include formed, stamped, forged and cast components of both aluminum and steel. This article will focus on aluminum lightweight structural components that can be cast via gravity, low-pressure, semi-solid metal or high-pressure die-casting(HPDC) processes. These processes are currently producing vehicle shock towers; A-B-C pillars; torque boxes; battery boxes; rear-hatch frames; electric drive-unit housings; rear nodes and longitudinal members; cross members; engine cradles; door structures; brackets; steering columns; and console panel components.
In the early 2000s, we saw premium-vehicle manufacturers integrate aluminum structural components. Today, it is commonplace as engineers focus on the next wave of vehicle and e-mobility platforms. The Aluminum Association’s Drucker Frontier Report (Fig. 3)recently informed readers that automobile platform aluminum usage is predicted to grow from 30% in 2020 to up to 45% by 2030, a50% increase over the next 10 years. More important to manufacturers is that aluminum components (excluding sheet and extruded products) are predicted to grow more than 200% from 2018 levels by 2030.
Aluminum Lightweighting – Flexible Heat-Treating System Advancements
The purpose of introducing heat treating to the manufacturing process is to alter the components’ mechanical properties and develop an optimal combination of strength and ductility performance levels. Depending on the requirements of the component, these processes may include T-5 artificially aged only; T-6 solution treated; quenched and artificially aged; T-7 solution treated; and quenched and artificially overaged.
An example of a typical three-stage T-6 thermal-process profile is shown in Figure 4 and integrates a solution-treatment stage followed by a rapid transfer to a Precision Air Quench (PAQ™) system. This is where the product temperature is dropped rapidly to the third stage, where product is artificially aged for a period of time based on the aluminum alloy’s composition and peak-performance properties.
The processing temperatures and soaking times are selected to achieve desired properties based on the aluminum alloy’s ability to develop a homogeneous solid solution and the microstructure prior to heat treatment. Heat-treat parameter uniformity is critical to the success of high-quality manufacturing processes. Highly accurate control contributes to the uniformity of the final component’s mechanical properties, elimination of part distortion and reduction of residual-stress levels.
Processing temperature-uniformity demands are constantly being pushed. As such, it is commonplace for heat-treatment systems to achieve ±5°F or better processing-temperature uniformity. In addition to temperature uniformity, the quenching process is one oft he most critical and precarious steps in the heat-treatment process – particularly so when processing thin-walled, low-mass structural components.
To address these concerns, heat-treatment system designers ensure processes are validated using a combination of computational fluid dynamic (CFD) modelling tools and R&D development testing equipment available.
Heat-treatment system process parameter and capacity flexibility is top priority when evaluating the integration of heat-treatment capacity to the manufacturing processes. In an environment where decisions are based on future requirements for electric vehicle(EV), battery electric vehicle (BEV) and internal combustion engine (ICE) platforms, great unknowns exist.
These decisions are difficult because each platform will have different heat-treatment processing needs based on the products produced, the parameters required and the anticipated volumes. The old saying “one size fits all” no longer applies since manufacturers require heat-treating equipment designs that are ultra-flexible and adaptable for the processing requirements of varied heat-treatment parameters and product types. These designs must also provide operating cost efficiency as product demands fluctuate.
Flexible heat-treatment system considerations include the following:
- Scalable operation for high efficiency regardless of product demand
- Loading configuration for adapting different product geometry and mass
- Flexible temperature set point, heating rates, soaking times and varying temperature process steps
- Flexible product transportation, delivery and product carrier systems
- Quenching flexibility via options for hybrid water and PAQ™, quench-media flow, direction and temperature settings
- Carrier tracking, product identification and critical processing parameters acquisition systems
Quench stations are integrated and can include hot-water quenching and PAQ™ systems. A common carrier transfer system is used to transfer carriers loaded with products from the entry area to an awaiting solution furnace for T-6 and T-7 processes or an aging furnace for T-5 processes. Following the appropriate soaking time at temperature, the transfer system moves the carrier to the appropriate quench station or to discharge for T-5 processing. Following quenching, carriers are transferred to the artificial aging furnace for appropriate soaking times and temperatures.
The system’s ability to recognize the required heat-treatment recipe is managed through an engineered processor-based control system that works in cooperation with a unique carrier vision system that identifies the product and recipe sequence for each loaded carrier. A level 2 supervisory control and data acquisition (SCADA) system is employed to track the products through the various stages of the process and collect process-critical parameter and event history data for each carrier or lot processed.
Manufacturers of structural automotive components place increased heat-treatment processing flexibility over other considerations today. These demands force manufacturers to seek out systems that can provide processing and capacity flexibility benefits. These include:
- Systems that are capable of processing a wide variety of product thermal recipes simultaneously.
- Flexible heat-treatment systems that allow for customized thermal profiles for a wide array of products and geometries.
- Regardless of production demand, heat-treatment costs per batch are fixed and not variable based on demand.
- Systems that can be integrated and expanded in a modular format, which provide the manufacturer benefits in matching equipment capital costs with product launch-curve demands and timing.
- Systems that allow for custom heating and soaking profiles tailored for variations in part geometry, which minimizes dimensional instability and residual-stress levels.
- Flexible product delivery and carrier systems that allow for a wide range of product throughput without equipment reconfiguration costs.
- Systems that integrate hybrid-quenching flexibility for water quenching and Precision Air Quenching (PAQ™) into one heat-treating system.
- Systems that allow quench-rate parameters to be automatically controlled for variations in product being processed and ambient conditions, ensuring reproducible part-to-part properties.
These heat-treatment system features will become increasingly important to manufacturers of aluminum structural components today and into future as they navigate how to best satisfy the demands for these products in a rapidly evolving environment.