A study of the machinability of Ti-6Al-4V compacted powders

Machining is an integral shaping process for most titanium alloys based components. However, the ease of machining (machinability) of Titanium and its alloys is generally poor because the alloys have low thermal conductivity, high strength at high temperatures, which both combine to speed up the tool deformation and wear. The poor machinability of the titanium alloys contributes to their high cost, which prevents their widespread industrial application. In this study, this challenge of poor machinability is investigated for the most common titanium alloy, Ti-6Al-4V. The envisaged approach was to machine and characterise the machinability of the alloy in the green state. Green compacts of blended powders of Ti-6Al-4V were pressed at room temperature. To enable machining, a minimum green strength to handle clamping and withstand mechanical forces during machining was required. The green strength of this study was varied by changing the compaction load and the binder during pressing. The compacts were subjected to a drilling operation where the tool type, feed rate and the cutting speed were varied. The machinability was evaluated by measuring average edge breakout and the drilling forces. Results indicate that high strength steel (HSS) twist drills, low feed rate and high speeds generally reduce machinability of the Ti-6Al-4V compacted powders. The drilling forces were independent of the green strength of the compacts.