Wear under brittle removal regime of an under-expanded cryogenic nitrogen jet machining of bio-composites

Abstract

Machining of biocomposites using traditional techniques has shown some limitations due to the multiscale complex cellulosic structure of natural fibrous reinforcement. This paper aims to demonstrate the feasibility of the cryogenic nitrogen jet, which is a novel cutting process that combines sustainable resources and cryogenic temperatures from −175 °C to 150 °C, as a machining process for biocomposites made of unidirectional flax fibers and polylactic-acid polymer (PLA). A high-velocity stream of liquid nitrogen with and without abrasives (400–700 m/s) is hence directed onto the workpiece surface. For the abrasive jet, both conventional garnet abrasives and bio-based abrasives made from walnut shells were used for the nitrogen jet process. The kerf depth, which is representative of the erosion wear mechanisms, was calculated to assess the erosion rate of the biocomposite structure after the nitrogen jet cutting operation. Then, scanning electron microscope observations are performed to characterize the induced damages on the machined biocomposite surfaces. Results show that the pressure and the traverse speed of the nitrogen jet are the relevant process parameters that control the mechanical and thermal stresses viewed by the biocomposite during the machining operation. Each impinging abrasive particle removes a small amount of biocomposite material during the jet cutting process, which depends on the brittleness of the anisotropic work structure and the abrasiveness of the incorporated particles into the jet stream.