Machinability of Green Powder Metallurgy Components: Part II. Sintered Properties of Components Machined in Green State
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INTRODUCTION
POWDER metallurgy (PM) is a shaping process that allows the fabrication of relatively complex components at high production rates.[1] The PM process competes with other shaping processes, such as precision casting, precision forging, and the machining of wrought metals. However, incorporating features such as threads, holes perpendicular to the pressing axis, and undercuts is very difficult to do and must be added to the sintered components via machining operations. Moreover, the machining of PM components is also done to maintain the geometrical conformance of sintered products.[2] According to a survey of the PM industry, approximately 30 pct of all PM components produced in North America require some machining operation after sintering.[3] Moreover, it is estimated that about 40 to 50 pct of all ferrous and steel PM parts made in Europe undergo some machining.[2] Unfortunately, the machining performances of PM components differ from those of wrought steel components that present the same microstructure. The reasons for the inferior machinability behavior of PM components come from the substantial volume of residual porosities (10 to 15 pct) that affect the interaction between the cutting tool and the work material.[4,5,6] The decreased tool life and the higher cost associated with the machining of PM components could counterbalance the advantages offered by the PM process. Thus, ETIENNE ROBERT-PERRON, Postdoctoral Candidate, and CARL BLAIS, Professor, are with the Department of Mining, Metallurgical and Materials Engineering, Universite´ Laval, Quebec City, QC, Canada G1K 7P4. Contact e-mail: [email protected] SYLVAIN PELLETIER, Powder Metallurgy Group Leader, and YANNIG THOMAS, Research Officer, are with the Powder Forming Research Group, Industrial Materials Institute–National Research Council, Boucherville, QC, Canada J4B 6Y4. Manuscript submitted on October 11, 2006. Article published online June 26, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
to remain competitive with other shaping processes, the PM industry has to solve the machinability issue inherent to PM components, to enhance precision and extend the shape complexity of the components it produces.[1] As mentioned in Part I of this study, successful efforts have been undertaken to include machinability enhancers (MnS, MoS2) in PM steels, for improved machining behaviors.[7,8,9] Unfortunately, these additives tend to decrease the sintered properties, especially toughness and fatigue resistance.[1,10] In recent years, a new approach to circumventing the mediocre machining performances of PM steels has been considered: green machining. According to Salak,[2] ‘‘Green machining is virtually a must for PM components that are sinter hardened.’’ In green machining, the cutting forces and the temperature at the tool/chip interface are kept to a minimum, due to the lack of strong interparticles bonding. Moreover, the hard phases that lower the cutting tool life have not been formed at this stage of the process, because phase transform
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