Influence of Microstructure in Machining of Nickel and Nickel-Iron Based Alloys

Doctoral thesis, 2011

Superalloys are used in applications such as turbines, steam power plants and nuclear power systems in components that require good high temperature properties and/or corrosion resistance. They are classified as difficult-to-cut materials and hence, from a sustainable production point of view, tool wear and tool life are major factors when machining these alloys. The work presented in this thesis provides insight to the influence of microstructure on the machining of the nickel and nickel-iron based superalloys Waspaloy and Alloy 718 (commonly known as Inconel 718), respectively. The influence of microstructure, with respect to grain size and hardness, on machining is investigated in a transverse turning operation through the assessment of tool wear, wear mechanisms, surface deformation in the machined material, burr formation and characteristics of the chips. Concerning flank wear, machining of Waspaloy is associated with less wear than machining of Alloy 718. Fully age hardened Waspaloy showed even less flank wear than solution annealed Alloy 718 irrespective of grain size. This observation cannot be explained by cutting resistance, since Waspaloy shows higher cutting forces. Surface analysis shows a difference in tribological conditions on the flank face of the tool, which influences the magnitude of flank wear. The wear mechanisms when machining these alloys are concluded to be a combination of adhesive, abrasive, diffusion and dissolution mechanisms, but also associated with a beneficial effect of oxidation of tungsten, leading to potential formation of low-friction surface just beneath the flank wear zone. The grain size of the work material was found to affect the deformation behaviour in the metal cutting process. Inhomogeneous deformation occurs when the size of the grains is in the same order of magnitude as the undeformed chip thickness, in both solution annealed and fully precipitation hardened condition irrespective of the alloy composition. The inhomogeneous deformation was connected with large depth of cut notch wear and burr formation, together with formation of segmented chips. Depth of cut notch wear is the tool life limiting factor when machining the work material with the large grain size. Finally, a method for analysing and identifying crucial variables in the material specification that affect the machinability of the work material is presented.