Posted by TVP on May 07, 2008 at 16:56:25:
In Reply to: Flowlines in Aluminum Forging posted by canderim onder on May 07, 2008 at 05:29:58:
It is best to use mixed-acid etchants for revealing the flowlines in 6xxx-series Al alloys. One suitable etchant has the following composition: 10mL HCl + 30 mL HNO3 + 20 mL H2O + 5 g FeCl3. It must be mixed fresh prior to each use (do not store previous mixes), with the HCl added last. It is used at room temperature, with the sample immersed in the etchant for a few seconds, rinsed in cold water, and then repeated until the desired effect is obtained. The sample can also be etched by swabbing instead of immersion. Staining the etched sample with a ink followed by light polishing can improve the contrast and hence the appearance of the flowlines. The ink fills the etched "grooves" and the polishing removes the ink from the surface "peaks", exposing bright, reflective metal that contrasts with the dark grooves.