Characterizing roping

Besides other advantages, aluminum alloys permit lightweight structural designs and innovative solutions to weight reduction in automotive applications. Consequently, the demand for aluminum alloys is growing rapidly for applications where strength-to-weight ratios are critical. Aluminum producers supply the market with unfinished aluminum alloys that must be rolled, stretched, and pressed into the appropriate shape. As a result of forming operations, the grain structure of the alloys is re-ordered, leading to an increase in surface roughness and an “orange peel” appearance.

Roping grade determination using imprints

As a second approach to determine the roping grade, the German Association of the Automotive Industry (GAAI) recommendation VDA 239-400, which uses an imprint of the surface, can be applied. The imprint method has many similarities to making a copy of a credit card with raised font structures by placing a paper over the structures and rubbing with a pencil to make the raised font surfaces visible: the deep areas remain bright, and the high ones are darkened. Using specially prepared adhesive tape, an imprint is taken and scanned at 300 dpi to form a gray image. At this resolution, the small structures (roughness) that are not relevant are filtered out and are no longer recognizable. The grayscale values are indicators of the heights when the imprint process is done correctly. The gray image is evaluated by software, which calculates the roping degree based on the standard from 0 (no roping) to 9 (very strong roping). While this approach seems simple, it is labor-intensive, time-consuming, lacks precision and error-prone. These disadvantages are eliminated when using TopMap whitelight interferometers from Polytec.

To ensure that the measurement results are comparable to the GAAI standard, TopMap surface profilers feature an algorithm to prepare and export data for further processing for roping grade determination. In addition, the integrated surface metrology software calculates the RK parameter according to the VDA 239-400 GAAI.