In response to a request from an Industrial Member, TWI Materials Department developed a technique for on-site metallography of welds in aluminium alloy plate. This represented an extension of the site metallographic service that the department operates for steels, stainless steels and nickel alloys.
For on-site work, replicas are normally taken from the surface of suitably prepared locations on the component for examination under laboratory conditions. The quality of replication routinely obtained enables quantitative measurements, such as grain size, or photomicrographs, to be made.
The problem requiring attention in this case was the examination of the distribution of microporosity in welded Al-Mg alloy plates. Because of the ratio of pore diameter to section thickness, the pores could not be detected radiographically and microscopic examination was needed. Electropolishing was specified for the final surface preparation to eliminate the danger of obscuring fine pores by mechanically flowing the surface.
Preliminary trials indicated that it was necessary to use a good standard of mechanical preparation to obtain satisfactory electropolishing. So the weld areas were prepared by grinding and polishing up to 3µm diamond finish. Then a Struers Movipol electropolisher was used to produce polished areas approximately 5mm diameter.
Replicas were made from these areas by wetting the polished region with solvent and pressing a clear acetate sheet on to the surface. As the solvent evaporated the replica hardened and the reverse side was coated with an anti reflectant medium to improve the contrast in the subsequent microscopic examination. The replica was stripped from the surface and mounted on a glass slide. A portable microscope was used to establish that the replica was good enough to allow detailed laboratory examination to be carried out.
For information about TWI's capabilities please contact us.