An overview of the machinability of aeroengine alloys

Advanced materials such as aeroengine alloys, structural ceramics and hardened steel provide a serious challenge for cutting tool materials during machining due to their unique combinations of properties such as high temperature strength, hardness and chemical wear resistance. Although these properties are desirable design requirements, they pose a greater challenge to manufacturing engineers due to the high temperatures and stresses generated during machining. The poor thermal conductivity of these alloys result in concentration of high temperatures at the tool-workpiece interface. This is worsened at higher cutting conditions because of the significant reduction in the strength and hardness of the cutting tool. This weakens the bonding strength of the tool substrate, thereby accelerating tool wear by mechanical (abrasion and attrition) and thermally related (diffusion and plastic deformation) mechanisms. Therefore, cutting tools used for machining aerospace materials must be able to maintain their hardness and other mechanical properties at higher cutting temperatures encountered in high speed machining. Tool materials with improved hardness like cemented carbides (including coated carbides), ceramics and cubic boron nitride (CBN) are the most frequently used for machining aeroengine alloys. Despite the superior hardness and cutting performance of CBN tools, ceramic tools are generally preferred for high speed continuous machining because of their much lower cost. Improvements in machining productivity can also be achieved with the latest machining techniques such as ramping or taper turning and rotary machining. These techniques often minimise or completely eliminate the predominant notching of the cutting tools, consequently resulting in catastrophic fracture of the entire cutting edge when machining aeroengine alloys. (C) 2002 Elsevier Science B.V All rights reserved.