Investigation of the High-Speed Finish-Turning of Ti-5Al-5V-5Mo-3Cr Using Novel Mono/Bi-layered PVD-Coated WC Tools

Abstract

The Ti-5Al-5V-5Mo-3Cr alloy is used in aerospace, biomedical, and military fields due to its outstanding thermomechanical properties. Nevertheless, its higher strength and toughness make it more challenging to machine compared to traditional Ti alloys like Ti-64. Despite increasing demand, there is a noticeable lack of research on finish turning Ti-5553 with coated carbide tools. This thesis aims to fill that void by thoroughly investigating the tool life and micromechanical performance of new mono/bi-layered PVD-coated WC tools under high-speed finish turning in wet conditions. A commercially applied AlTiN-based coating is used as the baseline reference and compared with mono-layered ta-C (diamond-like carbon, DLC), AlCrN, and TiAlSiN coatings applied over the AlTiN base on three different cutting tools. The study focuses on assessing the tool life and workpiece surface finish outcomes of these coatings and has been divided into the following studies: Study A - Pre-Machining Analysis: This study begins with a literature review, which informs the development of the experimental methodology, including the selection of coating types, cutting speeds, and other cutting parameters. It also involves characterizing the workpiece by examining its microstructure and mechanical and chemical properties before any cutting operations. Coating characterization includes micromechanical analysis (hardness, elastic modulus, plasticity index), examining the coatings’ morphology, measuring adhesion and cohesion strengths, and coating thickness. Surface roughness data are also obtained from topography maps. The primary goal of Study A is to relate the properties of the workpiece and coatings to their respective microstructures. Study B - Machining Analysis: This study focuses on evaluating tool and coating performance through a comparative analysis of flank wear and the cutting length achieved. It also involves inspecting the surface roughness of the workpiece after each machining pass. Tribological analysis includes collecting and analyzing chips at different stages of the machining cycle using an optical microscope. The main objective of this study is to correlate machining performance with process parameters and the coatings used. Study C - Post-Machining Analysis: The final stage involves two sets. The first set, tool and coating morphology, includes analyzing morphological wear and elemental composition using SEM and XPS. Chip characterization involves examining the morphological features of the chips (length, thickness, curliness) and the roughness of their back surfaces. The aim of this study is to correlate the effects of cutting conditions on the surface integrity of the workpiece with the mechanical, chemical, and morphological characteristics of the cutting tools and chips.