A scanning electron microscope study of fracture phenomena associated with rolling contact surface fatigue failure

Abstract

In a manufacturing process, studying the removed material called chip, swarf or debris, is the key to understand the process. In micro cutting process, the size effect is a dominant factor for material removal mechanism and chip generation physics. There are different types of chips but a particular type, stands out from the others due to its specificities: the spheroidal chip. It is often found in grinding and tribological tests. This type of chip has no specific classification and there are no reports of it being found in milling or micro milling research. This paper makes a review of this not so well known type of chip, to classify it by its characteristics and to present some examples of spheroidal chips obtained from micro milling operation. Spheroidal chips were obtained when micro milling slots using a 381 µm diameter TiN coated WC tool. The workpiece material was Inconel 718, a high strength refractory superalloy. SEM images were obtained to analyze and classify the chips and observe the machined surface of the slots. In the proposed classification of spherical chip, there are eight major types: Fatigue Sphere, Layer-like Sphere, Thin Plate Sphere, Dendritic-like Sphere, Partially Formed Sphere, Tadpole, Melted sphere and Hollow sphere. The thin chip is submitted to high energy, which is one of the main aspects of size effect that occurs in micro milling. Inconel 718 has low thermal conductivity and high oxidation rates, which makes it an ideal environment for the formation of spherical chip. The spheres formed can be classified as Dendritic Spheres, defined by extremely high rates of oxidation in a thin chip which leads to high heat energy causing its spherical form and dendritic microstructure.