Beware of Aging

Consider a part-development process when everything proceeds smoothly, timely and thoroughly. Forming simulation evaluates the full range of potential sheet properties. The steel and aluminum mills design their products to meet OEM specifications, which typically call for properties to remain within specification limits over a 6-mo. period after mill production. (Note that this differs from properties remaining stable during that time.) When the part hits production, sheet metal arrives on time and stamping occurs within the targeted number of days, allowing for a rapid turnover of inventory while minimizing floor space and staff devoted to work-in-process.

While this scenario may be typical, it does not always occur. Many stamping companies receive their sheet metal from service centers rather than the mill, when they cannot justify the quantity required for a mill-direct purchase. The service center, however, must purchase at least one full coil. If the stamper needs only a fraction of a coil every few months, the remainder of the coil sits in the service center’s inventory.  During that time, the properties of certain grades change—strength increases and ductility decreases (see the accompanying figure). We call this change in properties over time ‘aging,’ and BH steels and aluminum grades are particularly susceptible. Even if the material properties remain within specification, the progressive property change over time may affect how the panels form at the stamping plant, and impact overall panel quality.

Paint Bake Hardening

…essentially is a controlled, accelerated aging process, where the paint-curing temperature of around 170 C allows for aging to occur in 20 min., rather than 6 mo. at room temperature.  The paint-bake thermal cycle increases the strength of stamped panels, and the resulting reduction in ductility is inconsequential, as the panels already are in their finished shape. However, if the sheet metal ages prior to stamping, the higher strength and reduced ductility may preclude split-free forming in a process originally designed to accommodate lower-strength and more ductile material.

Any elevated temperature accelerates the time until properties no longer fall within specification. For example, coils may ship in rail cars from the mill to the stamping plant. Logistics can lead to unscheduled delays, resulting in extended periods of time spent with the railcar stalled in direct sunlight. Coils of aging-sensitive material grades subjected to such conditions will exhibit degraded performance much sooner than otherwise expected.

When it comes to documenting a material’s certified properties, many service centers do not conduct a new test for every shipment. Instead, they often will replicate the strength and elongation shown on the metal certs received when the service center initially purchased the coil from the mill. Limited risks exist with this approach if the properties do not change over time, as seen with many steel and aluminum grades.  

On the other hand, for grades that do age, new shipments with certs showing results from coil production may give the stamping plant a false sense of security. If splits occur, stampers must take corrective actions requiring additional work and time, and may lead to a loss of process control. Should stampers encounter splits on BH grades, they should send a test coupon for tensile testing to ensure that properties remain in compliance against the specification before making costly changes to the tooling and process. MF

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials

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