Green Machining for Conventional P/M Processes

Abstract

The advantages of green machining have been well recognized, including longer cutting tool life and the ability to make complex parts with sinter-hardenable powders. The high green strength of the compacted parts also prevents green cracks and damages from handling. But, green machining is yet to be widely used since the required high green strength (in excess of about 20MPa) cannot be readily obtained through conventional P/M techniques. The limitation of using green machining is being removed with a newly developed high green strength (HGS) polymeric lubricant at QMP. One distinguished characteristic of this lubricant is its response to temperature. High green strength can be obtained at a compacting temperature of 55°C, which can be easily reached throughout a green part by cold compaction. Even a higher green strength up to ∼ 48 MPa can be reached by a subsequent curing process for parts pressed to density of 6.8 g/cm3. This high green strength has made green machining possible for those parts fabricated through conventional P/M processes. This paper presents the results of green machining tests performed on several timing sprockets. The material used was a sinter-hardenable powder mix of QMP ATOMET 4601 and the HGS lubricant. The sprockets were pressed to a density of ∼6.8 g/cm3 by cold compaction. Green machining (turning a groove along the middle of the teeth) was performed on as-compacted and cured parts. Results showed that the HGS lubricant provided sufficient green strength for clamping and machining operations, especially after curing. The machined surfaces appeared to be smooth and the edges remained integrated. The green-machined sprockets made of sinter-hardenable powder eliminated heat treatment, providing much longer tool life and improved size tolerances.