The importance of flatness measurement for automotive manufacturing

Aluminium is just a third the density of steel, with a high strength-to-weight ratio that contributes to lighter, more energy efficient vehicles with longer range, greater fuel economy and improved handling performance. Aluminium absorbs twice the crash energy of steel, making it a critical element in the design and fabrication of safer vehicles. The self-healing characteristics of automative grade aluminium alloys also mean they are less prone to corrosion. While steel vehicle bodies rely on special treatment processes to resist rusting, aluminium naturally forms a protective barrier if it’s cracked or deformed. Aluminium vehicles can thus offer improved anti-corrosion performance, offering the potential for a longer service life without the need for costly protective treatments.

Aluminium sheet flatness and uniformity: the quest for quality

With aluminium alloys playing a critical role in modern vehicle manufacturing, metal producers are under pressure to enhance the quality of their products supplied to customers in the automotive industry.

The flatness and finish of rolled aluminium sheets and strips is of particular importance for auto manufacturers, who are reliant on a supply of material that adheres to tight physical tolerances in order to satisfy their own production needs.

Consistently flat, smooth aluminium sheets are essential to ensuring the accurate alignment and structural integrity of body panels, frames and other critical components. Reducing the need for time-consuming adjustments during assembly, material flatness is a contributor to production line efficiency – reducing the need for skilled labour to correct imperfections during assembly.

Material flatness and uniformity also play a major role in ensuring a vehicle’s strength and structural integrity. Aluminium sheet flatness ensures that mechanical stresses are uniformly distributed across structural and load-bearing components, reducing the risk of localised weak spots that may lead to potentially dangerous failures. Vehicle safety is further enhanced by the accurate assembly of crumple zones and impact beams that must perform predictably in the event of a collision.

Flatness and smoothness influence the aesthetic quality of body panels and other visible elements, contributing directly to the perceived appearance and quality of the vehicle in customers’ eyes. Aluminium parts with an extremely flat, high-quality finish also simplify the application of paint and other surface treatments, influencing the finished vehicle’s appearance and perceived value.

From an overall production perspective, consistently flat aluminium sheets are easier for vehicle manufacturers to handle and form, reducing the need for time-consuming and costly reworking. They also simplify advanced joining techniques such as adhesive bonding and laser welding, where extremely uniform surfaces are critical to achieving optimal results with less likelihood of waste or reworking.

Acceptable flatness limits for rolled aluminium sheeting can vary depend on its intended use for a particular automotive application. Dimensional tolerances may range from ±1.5mm or more for larger structural parts to as little as ±0.1mm for critical components within engines, transmissions and battery housings for EVs.

Optimising quality, reducing costly waste, and reworking

There are several flatness-related issues that can pose challenges for aluminium alloy manufacturers and their automotive customers. Overall, variations in sheet thickness can lead to uneven stress distribution within worked components, compromising their performance and potentially leading to premature failure under load conditions. 

Wave-like undulations on the sheet’s surface can cause a number of problems during vehicle assembly, including potential misalignment of body panels that affect the finished vehicle’s structural integrity and its visual appearance.

Longitudinal or lateral curvature of the aluminium sheet is referred to as bowing, manifested as an arching of the sheet. This can make precise handling, machining and fitting more difficult, with the attendant risk of spoiled components that require costly re-working or are scrapped.

Waves at the edge of the sheet can lead to poor alignment between components, compromising their structural integrity as well as their aesthetic appearance. Conversely, buckling at the centre of the sheet can cause problems in forming and stamping that may weaken the strength of the worked part.

Rolled aluminium sheets can also suffer from a range of surface imperfections including localized dents and scratches. As well as impairing the aesthetic appearance of body panels and other visible vehicle parts, these defects can compromise the strength and integrity of components when they are under stress.

Accurate measurement and control of the material’s flatness can have a profound effect on product quality and consistency during the aluminium rolling process. Mill operators must be able to rely on the accuracy of flatness measurement and control systems to ensure that rolling processes meet exacting specifications in terms of flatness and dimensional tolerances (thickness and width). And with typical cold rolling strip speeds reaching several hundred meters per minute, errors arising from incorrect thickness control can have sudden and costly consequences on production yields and profitability for mill operators.

Ensuring optimum flatness through combined accurate measurement and control

While hot and cold rolling processes are broadly similar to those for steel, aluminium production poses an additional challenge since the softer metal’s stiffness is just a third that of steel. This means that significantly higher accuracy is needed when measuring and controlling aluminium’s flatness, since its deformation under a specific pressure is correspondingly higher than steel’s.

Recognising the importance of ensuring optimised aluminium strip flatness throughout the rolling process, ABB’s Stressometer System allows mill operators to benefit from industry-leading flatness measurement and control accuracy and resolution. Exceptional stability and highly reliable operation are complemented by low maintenance needs and lowest ownership costs.

A true parallel flatness measurement solution, at the heart of the Stressometer System is a solid flatness measuring roll. Featuring a parallel arrangement of force sensors, it enables measurement of force distribution across the width of the aluminium strip. Real-time measurement of stress distribution across the sheet’s full width allows for extremely rapid detection and correction of deviations from desired flatness. The system’s parallel sensor arrangement also makes it effectively immune to disturbances caused by variations in strip tension. This contributes further to highly accurate and responsive flatness control – including strip heads and tail ends – significantly reducing reject rates with a consequent improvement in overall yields.

Typically 26 or 52mm wide, each measurement zone includes four Pressductor magneto elastic transducers, whose output translates directly into an indication of strip flatness. Unlike conventional transducers that require physical movement or displacement to produce an output signal, the Pressductor is very stiff with minimal dimensional changes when subjected to an external mechanical force. In addition, since the all-metal transducers feature the same thermal coefficient as the surrounding material, their measurement accuracy is not affected by the high temperatures or temperature variations typically experienced in rolling mills. The Stressometer System’s outstanding stability and reliability compared with other measurement solutions is demonstrated by a mean time between repairs (MTBR) exceeding 20 years proven in around 1,000 installations worldwide.

With a measurement response time of better than 5 milliseconds, the Stressometer System provides an extremely fast and accurate output to the rolling mill’s flatness control system. This ability to detect and correct flatness issues in near-real time reduces the frequency of production interruptions and downtime, leading to more efficient manufacturing processes. What’s more, the system’s ability to deliver accurate control over rolling processes leads directly to higher yields of usable material and enhanced plant profitability, since fewer sheets are rejected due to flatness defects.

The driving force for aluminium sheet quality that vehicle makers depend on

The automotive industry is increasingly reliant on a dependable supply of high-quality aluminium alloy sheeting and strips to satisfy a wide range of vehicle assembly needs.

Offering measurement and accuracy, ABB’s Stressometer System satisfies rolling mills’ stringent requirements for material flatness and finish that their auto customers demand, together with exceptional reliability, low maintenance and very appealing lifetime ownership costs.