Approximately one-third of all sheet steel is coated to enhance corrosion resistance. Although as-processed coated steels are highly durable and corrosion resistant, subsequent processing steps such as bending, temper rolling, roll forming, and stamping can strain the coating, causing it to crack and/or debond. The consequences of coating cracking can include changes in die/sheet friction conditions during stamping operations and/or the loss of the corrosion resistance. The fracture mechanics research presented in this talk is applied to the GALVALUME system, where the coating is an aluminum-zinc alloy that includes brittle silicon particles and a brittle interlayer between the coating and the steel substrate. Coatings are typically 10 to 20 microns thick and particles and interlayer structures can range from less than a micron to a few microns in size. Microscopic observations of strained specimens will be presented showing that the particles and interlayer experience cracking, and cracks through silicon particles can propagate to create a crack through the entire coating. Based on these observations, the problem of particle cracking is studied as a function of particle size and shape. Numerical results are presented which quantify the average energy release rate of a crack forming in a single particle. Results as a function of particle size, shape and orientation are used to suggest optimal particle geometries for increasing particle and coating fracture resistance. Guidance is being provided to processing engineers on the benefits of processing steps meant to control particle geometry.
Back to Metal Forming I
Back to SES Abstracts
Back to The 41st Annual SES Technical Meeting