Influence of Material Variations on Machinability - Machining Difficult-to-Machine Alloys

Doctoral thesis, 2015

The aim of this study is to investigate the effect of work material variations on machinability. Improved knowledge in this respect supports the development of sustainable production both from resource and economical points of view. The properties of work materials vary greatly depending on how they were manufactured. Castings generally have coarse microstructure compared to wrought material, where the added thermo-mechanical processing steps in the latter produce finer microstructure. Local within-part variations in microstructure are found in both castings and wrought material. In cast material this varies due to factors such as undercooling and cooling rate. In wrought material local variations in process parameters, such as deformation and temperature produce different microstructures. Within-process variations during machining also exist, where surfaces deformed during cutting are left to subsequent cuts. The same occurs when machining sheet metal, where degree of deformation also may vary due to local variations in strain. In this work differences in both microstructure and deformation has been investigated for three work materials; nickel-iron based Alloy 718, titanium alloy Ti-6Al-4V and the stainless steel 316L. It was found that coarse microstructures result in anisotropic chip formation, where chips had a non-periodic serrated appearance in both Alloy 718 and Ti-6Al-4V. Both alloys are known to produce continuous chips at low feed rates and cutting speeds, with a transition to serrated chips as these parameters are increased, due to instability in the cutting process. The serrations due to this instability show a more periodic appearance, clearly distinguishable to the serrations due to anisotropy. In Alloy 718 coarse microstructure was also found to produce large burrs, that were continuously built-up during machining, as well as increase notch wear at the depth-of-cut on the cutting tool. In Ti-6Al-4V the anisotropic chip formation behavior found in coarse microstructure was connected to orientations of alpha colonies, i.e. where some orientations produce highly localized deformation and others were homogeneously deformed at all feed rates and speeds investigated. Chip breakability was found to increase with size of alpha colonies, aided by the colonies that had highly localized deformation. The influence of the less periodic serrations found in coarse microstructure was also found to produce vibrations over a wider cutting speed range compared to the periodic serrations in finer microstructures. In both Alloy 718 and stainless steel 316L cutting forces were reduced when the work material had been deformed prior to machining, compared to annealed material.