Development of innovative TCT saw blades for high speed cutting of metallic alloys

The subject covered in this thesis concerns the development of an innovative PVD coated multipoint cutting tool with cemented carbide brazed inserts for high-speed cutting of ferrous alloys. The aim of the research was to optimize the properties of the constituent materials in order to maximize their durability and provide a constant level of surface finishing of the machined parts. Fast cutting technologies are nowadays spreading because of the high productive rate that they guarantee, but on the other side wear is more severe with high speed cutting. Tungsten Carbide Tipped (TCT) saw blades are typically used in wood cutting but since 30 years are gradually taking the place of traditional band saws due to the higher cutting speed that are possible to reach and thanks to a better surface finishing on machined surfaces. The research work that was part of an industrial project was divided into three steps: 1. characterization of cemented carbide grades for application in machining 2. optimization of the cutting geometry 3. development of a tailored CAE – PVD coating. The first part of the research work involved the study of literature in order to define the most suitable grades of cemented carbide for experimentation and to define some possible coating composition and architectures. Both plain grades and mixed grades with secondary Ti and Ta carbides were chosen, the relations among hardness, toughness, grain size and wear resistance were investigated through microstructural and mechanical characterization; finally discs made of cemented carbide were tested against pins of steel to characterize the resistance to sliding wear. From this characterization a mixed grade cemented carbide with 12% cobalt binder and micrometric grain size was chosen due to the best toughness properties shown from characterization. Saw blades work under interrupted cutting conditions so toughness was required as the most important feature. In the second part of this study the cutting geometry of the cemented carbide inserts was optimized via experimental cutting tests and CAE methods. After a set of benchmark cutting tests on an industrial sawing station, the experimental cutting forces were calculated analytically and than used to calibrate a FEM 2D numerical calculation model. Two cutting geometries were then tested among those simulated: -15° and -25° rake angles. Thanks to the use of an hard metal with increased toughness (KIC> 15 MPa), a tool with a rake angle of -15 ° has been designed to guarantee lower cutting forces (less than 90 N in the first cuts), friction and temperature on the surface of the tool’s rake face (Figure 1). By experimental validation of the simulated geometries the cutting model gained predictive power. In the second phase of the work, three CAE - PVD coatings of the Al - Ti - Cr - N system were studied. Two of them were monolayer and one multilayer. The aim of this part of the work was to investigate the mechanical and microstructural properties of the analyzed coatings using different experimental methods to describe their behavior. The coatings were characterized not only from the mechanical point of view (hardness, toughness and adhesion) but also from the morphological (defective), and microstructural point of view. From the tests carried out, a multi-layered coating with improved toughness for use in interrupted cutting was designed.