MACHINABILITY OF STEELS AND TITANIUM-ALLOYS UNDER LUBRICATION

The machinability of AISI 304 stainless steel and Ti-6Al-4V alloy was investigated using a tapping tester under lubrication. Three overbased sulfonates with/without sulfurized olefin were used as lubricants for the tests and compared with a reference lubricant to calculate the tapping efficiencies. Tapping test results showed that sodium sulfonate and calcium sulfonate gave better performance than magnesium sulfonate, which was related to the presence of carbonate and their reactivity with metals at high temperatures. It was also observed that the addition of sulfurized olefin to the overbased sulfonates enhanced tapping efficiencies. Surface analysis showed that the improved efficiency and surface finish of stainless steels tested with a combination of overbased sulfonates and sulfurized olefin is due to the presence of a binding oxide layer underneath the mixed sulfide/oxide layer and an increased total film thickness. For Ti-6Al-4V alloy, the presence of titanium transfer was observed. It is suggested from this study that the tapping efficiency is inversely proportional to the degree of titanium transfer from the Ti-6Al-4V plate to the high strength steel tap.