Heat Treatment for Nonferrous Metals

The evolution of heat-treatment processes for aluminum and copper
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Manufacturer's Adapting for Aluminum Heat Treatment's Evolving Demand

By Graeme Kirkness and Tim Donofrio

Back-to-back years of record automobile sales are infrequently realized, celebrated and almost mythical in today’s modern, highly competitive world. One outstanding year is hoped for, planned for and often expected. Two consecutively across an entire industry is nothing short of spectacular and a sign that, as a group, something is being done right and well.

As is often the case, however, record breaking is quickly followed by downturn, forcing adaptation and innovation based on the consumer’s shifting wants and demands. Once again, the automotive industry finds itself preparing for this unenviable position.

One is hard-pressed to find an industry, that isn’t Internet-based, being forced to undergo monumental shifts with the frequency of the automotive industry, particularly due to reasons beyond their control. Increasing consumer awareness has already driven one shift within the automotive industry – lightweighting – and is starting to drive another – electrification.

Since the introduction of Audi’s all-aluminum body in the 1994 A8 (Fig. 1), automakers have steadily replaced ferrous castings and weldments with equivalent nonferrous components. Originally driven by weight reduction (in pursuit of fuel economy), these nonferrous components offer improvements in compositional and mechanical properties; none more so than thin-walled structural aluminum components. Despite being around for over a decade, demand and expectations of thin-walled structures have matured to where demand exceeds capability as the automotive industry is pushed toward the greater challenges of electrification by consumer demand.

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Fig. 1. Audit A4 frame

Desire for electric cars is quickly gaining consumer momentum, and automakers are rapidly shifting focus to developing affordable, efficient electric vehicles (EVs) for the mass market. This is where aluminum structural components will further serve the automotive industry. Lightweighting directly benefits electric vehicles in two crucial areas: weight and driving distance (battery life).

Until EV batteries can be lightened, the weight must be overcome elsewhere. Body panels, chassis and further structural components are the likely candidates for lightweighting via plastics, aluminum, magnesium and carbon-fiber-based components. While advances in high-pressure die-casting (HPDC) processes, production and material sciences have enabled the implementation of thin-walled castings, their ultimate success is dependent on their mechanical (and crash worthiness) properties being exactly as specified, which is dependent on capable post-casting thermal processes, or heat treatment.

Electrification: Aluminum’s Double-Edged Sword

Electrification, the apparent future of the automotive industry, is the beacon of growth for aluminum components in an increasingly penetrated and crowded market. It’s a beacon that simultaneously serves notice of technology beginning to look for what’s next. Currently, aluminum structural-component use is sufficiently prominent within the automotive industry such that certain components (shock towers, cross members, rear rails and door pillars) are produced at such high volumes they are essentially standard in luxury vehicles.

As lightweighting integration costs decrease, we will see the expansion of use across the full range of lightweight vehicles. As the industry progresses toward electrification, these high-volume production requirements could transition to additional components. “Could” is an understatement because the eventual path of electrification component requirement is unknown.

This uncertainty does not, however, prevent preparation of this industry shift. One need only look at the change in EVs to understand inevitable steps. While environmental concerns may be the marketing appeal of EVs, greatly increased driving distance is the practical appeal of EVs and provides valuable insight into aluminum components’ future in the automotive industry.

The force behind the increased driving range of EVs is increased battery densification, as demonstrated in Figure 2. However, a side effect of this increased density, and aluminum’s opportunity, is the increased battery weight. Electric vehicles need to make up this weight elsewhere, be it through balancing front and rear weight or through new, electric/hybrid vehicle-specific components, such as battery/high-voltage housings. Here is where thin-walled aluminum components have the greatest potential for growth.

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Fig. 2. Battery density

Moving forward, it is important to remember that nothing is certain or set in stone. Initially, aluminum-based lightweighting components are well positioned to support the transition to electrification. The International Energy Association (IEA) predicts that there will be 715 million electric vehicles in 2040, a drastic increase from the approximately 1 million vehicles in 2015. This rapid growth is where “problems” or decreased opportunities for aluminum materials, most notably engine blocks, will be realized. Hybrid and electric vehicles require either smaller or (more drastically) no engine, which is currently the main component for aluminum within “light vehicles” today.

Looking to the future, it seems likely that with shifting technology new materials will be developed, possibly requiring reduced aluminum content in favor of opportunities to integrate carbon fibers, magnesium and 3D-printed components. These opportunities and potential problems highlight an increasing need for modern, flexible aluminum heat-treating equipment.

Modern Aluminum Structural Component Heat-Treatment Systems

The needs and expectations of new heat-treating systems have rapidly shifted over the past decade and are shifting yet again. Increasing metallurgical, process and product knowledge combined with increasingly integrated supply chains have greatly impacted how furnace engineers develop modern aluminum-component heat-treatment systems. Heat-treatment systems no longer have dedicated buildings or wide-spanning floor space. Space is limited and, more importantly, valuable. The more flexible floor space available within a single production facility, the greater the built-in future-proofing for capacity and/or process changes.

Traditional heat-treatment systems (Fig. 3) were large, linear systems occupying (and wasting) valuable floor space. Systems like those depicted in Figure 3 would frequently occupy several thousand square feet, with substantial portions of floor space unoccupied and, therefore, underutilized. Steadily encroached upon by the increasing needs of an overall production facility, modern systems must be more tightly integrated (Fig. 4), occupying 35% less space than traditional systems – all while production requirements are increasing.

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Fig. 3. Traditional aluminum heat-treat line

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Fig. 4. Rotary heat-treat system

During the early introduction of thin-walled aluminum structural components, developers of customized heat-treatment systems were exposed to production volumes of 150,000 pieces/year. By 2005, systems were requested with production rates of 500,000 pieces/year, and by 2017, requested production rates have doubled to 1,000,000 pieces/year. While these increased production rates demonstrate market growth and production centralization, it is the accompanying detail of these requests that demonstrates the increased demands on capacity and flexibility of modern systems.

Of the keys to the success for both compact solutions and overall system flexibility, none may be more important than tightly integrated material-handling systems. Initially, introducing robots into heat-treatment cells allowed for individual part or fixture handling. Gone are the days of manual handling thin-walled structural castings 10 times before being prepared for delivery; costly, excessive work-in-process inventories (WIP); and centralized heat-treatment systems that consume thousands of square feet of floor space and bottom-line profit.

Today, engineers are challenged to develop systems that eliminate the need for large, centralized heat-treatment systems, multiple handling scenarios and costly WIP inventories. Advanced thin-walled aluminum structural casting heat-treat systems now receive castings directly from the HPDC cells, thereby eliminating the need for added labor, dunnage and storage. Traditionally, these systems have been identified as basketless heat-treatment systems (BHTS).

These modern systems receive castings directly from the casting cell (in some cases recovering the remaining heat from the casting operation), while the controls systems track their valuable critical processing-parameter history throughout the entire process via unique part serialization (2Dmatrix). This tracking feature allows each part to be historically evaluated and audited for proper processing parameters such as solution temperatures, precision air quenching (PAQ™) and artificial aging temperatures.

Today’s structural-component casting heat-treatment systems are designed for rapid heating and uniform soaking cycles, which greatly reduce the systems’ overall size. This is accomplished through the elimination of large steel component carriers and fixtures that consume tremendous amounts of heating energy and capital and maintenance costs. In addition, engineers are utilizing state-of-the-art forced-air modeling tools and advance recirculation designs to provide efficient processing technologies.

HPDC aluminum structural castings develop their final properties through thermal processes. One of the most important parts of the process is quenching. In this process, the component is rapidly discharged from the solution furnace to a precision air quench (PAQ™). Traditional processes would receive large batches of castings for quenching, producing non-uniform cooling and ultimately unpredictable mechanical properties. Today’s modern, space-efficient heat-treatment systems are harnessing the benefits of robotic handling to manage the rapid quenching transfer required while also benefiting from the PAQ of smaller, individual lots where quenching results in more predictable mechanical properties and casting dimensional accuracy (Fig. 5).

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Fig. 5. Mechanical properties and dimensional accuracy is more predictable

As previously stated, flexibility is the single greatest need of modern aluminum heat-treating equipment. With the addition of vision to robotic material-handling systems, a single heat-treatment cell is no longer limited to being a single T5, T6 or T7 process. Now cells can be multi-process along with multi-part.

Utilizing vision and interconnectivity with a PLC, material-handling systems can now determine a part’s required heat-treatment process – all based on prior programming. Additionally, these multi-process cells still occupy smaller footprints than traditional heat-treatment cells.

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Fig. 6. Aluminum heat-treat system

Graeme Kirkness is with CAN-ENG Furnaces International Ltd.; Niagara Falls, Ontario, CANADA

Tim Donofrio is with Can-Eng Furnaces International Ltd., Niagara Falls, CANADA

 

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