The effect of free-machining elements on dry machining of B319 aluminum alloy

The ability to machine aluminum dry would have enormous benefits in reduced infrastructure, lower costs and a cleaner environment compared to today's practice of wet machining. However, that goal has been impractical due to the build-up of hot aluminum on tools in the absence of the lubrication and cooling supplied by metal removal fluids. While experimenting with tool coatings to enable dry machining, it was discovered that trace amounts of free-machining elements in the aluminum alloy greatly improved dry machinability. In the present study, small quantities (<1 wt.%) of the additives lead, bismuth, tin, and indium were intentionally added to B319 aluminum, a hypoeutectic Al-Si alloy. The improvement in dry machinability was striking: as little as 0.15 wt.% tin led to 1000-fold improvements in tool life. The mechanism responsible for this was determined through machine and tribological testing. High temperatures were found to enhance the effect, such that high-speed drilling worked better than low-speed drilling. Alloys with the additives exhibited a lower coefficient of friction (COF) compared to unamended B319 at elevated temperatures in friction tests against a tungsten carbide surface. Microscopy showed that a nanolayer of aluminum enriched in the additive coated the drill, thus creating a solid lubricant interface that aided in chip evacuation and temperature reduction. Of the four additives tested, tin showed the most potential for production applications. Future work will investigate the effect of additives on the casting and material properties of B319.