Metallographic preparation of soft materials: Lead alloys

Abstract

Metallographic preparation of lead and its alloys is notoriously difficult due to the low hardness of the material and its tendency to pick up and embed abrasive particles readily. Working with component parts for lead acid batteries at Lucas Advanced Vehicle Systems Development, the odd shapes and sizes, such as connector posts and intercell welds, do not lend themselves to microtoming or electropolishing. Vibratory polishing had been used in the past but this method could take up to 6 days to produce an acceptable microsection. Using semiautomatic polishing equipment had proved more successful, but again long periods of time were taken to produce satisfatory results with all the problems of lubricant and abrasive dosing. Tests on newer, commercially available pads and abrasives provided by various manufacturers have been carried out, but it was found that a combination of machining, polishing with diamond, and final polishing with alumina gave the best results. The use of automatic polishing equipment made the preparation easier with both contrarotation and complementary rotation directions. Lubricants and abrasives recommended by the metallographic consumable suppliers for soft materials were not suitable for the easily stained and oxidized lead alloys. This report lays out the steps taken to produce the stated optimized procedure and shows some unusual structures that have been revealed in components taken from batteries undergoing testing, one of which had suffered explosive damage.