Tool wear mechanism of difficult-to-cut materials

"Titanium alloys (Ti), Cast Graphite Iron (CGI), Carbon Fiber Reinforced Plastics (CFRP) and CFRP/Ti Stack are difficult-to-cuts materials which have been extensively used and attracted attention in aerospace, automobile, chemistry, biomedicine, sport and other industrials. These materials have distinguished properties, i.e. Ti is a light-weight metal (4.3 g/cm3 in density) which has a low thermal conductivity, 6.7 W/m.K (50.8 W/m.K for steel), CGI has superior physical and mechanical properties compared to gray cast iron which is commonly used in the automotive industry, CFRP and CFRP/Ti are light-weight materials with high corrosion resistance, and better mechanical properties. However, the common drawback of these materials is their poor machinability.The first part of this study examines the flank wear mechanism in machining Titanium alloy Ti-6Al-4V (Ti64). Titanium alloys typically do not contain hard inclusion phases typically observed in other metallic alloys. However, characteristic scoring marks, in addition to more distinctive micro- and/or macro-chipping, are ubiquitously observed on the flank faces of cutting tools when machining titanium alloys, which is the direct evidence of abrasive wear. Thus, an important question lies with the nature of the hard phases present in the titanium microstructure. In this work, we present a comprehensive study that examines the microstructural impact on flank wear attained by turning various Ti-6Al-4V bars having different microstructures with uncoated carbide tools. In particular, four samples with elongated, mill-annealed, solution treated & annealed and fully-lamellar microstructures were selected. After turning each sample, the inserts were observed with confocal laser scanning microscopy (CLSM) and analyzed to determine the flank wear behavior in relation to the four distinct microstructures. To do so, the microstructure was examined to distinguish the phases present using scanning electron microscopy (SEM) and the content and topography of each phase was examined to relate to the flank wear and its behavior. The flank wear is also affected by interface conditions such as temperature and pressure, which were estimated using finite element analysis (FEA) models. The temperature and pressure dependence of abrasion models enable the flank wear rates to be estimated for each microstructure and are compared with the experimentally measured wear data.Secondly, the tool wear mechanism of cBN inserts is examined after it is machined. In the literature, it was reported that the presence of a MnS layer at the interface between cBN inserts and the flake graphite iron (FGI) workpiece enhances the machinability of FGI. This work presents the results of our experimental investigation on the differences in the layer formation between CGI and FGI. Straight turning experiments were carried out with both FGI and CGI with various cubic boron nitride (cBN) grade inserts in dry conditions at high cutting speeds (mostly > 400m/min). The evolutions of both flank and crater wear were investigated and assessed. When cutting FGI, speckles of MnS, not as a layer, were present on the cBN inserts, mainly from the MnS inclusions in the FGI microstructure smearing on the cBN inserts. However, the total area of MnS speckles did not increase as the turning process progressed, questioning the formation of the extensive MnS layer protecting the insert. When turning CGIs, both Mn and S were present but not as MnS on the cBN inserts. Therefore, the presence of the MnS layer cannot be claimed as a main reason for the machinability difference between FGI and CGI. However, it is interesting to note that, instead of MnS layer, Al2O3 layer is formed from Al2O3 binder in cBN inserts on the rake side, when cutting at high cutting speeds, which functions as the solubility barrier on the cBN. The machinability difference comes from the fact that the Al2O3 layer formed when turning FGI is much more stable compared to that formed when turning CGI.The third part of the study investigates the effect of ply angle on tool wear when edge-trimming carbon fiber reinforced plastics (CFRP) with particular ply angles of 0, 45, 90, and 135° via up-milling operations. The edge-trimming experiments were conducted with micro-grain tungsten carbide endmills at a constant feed of 0.3 m/min under two cutting speed conditions, 1000 RPM (19.9 m/min) and 6000 RPM (119.7 m/min). A laser confocal microscope was used to measure the flank wear land, edge radius, and worn area in order to evaluate the impact of ply angle quantitatively. A qualitative analysis was also conducted using the scanning electron microscopic (SEM) images of the tool edges to delineate the wear mechanisms as well as the machined surfaces of each ply to characterize the machined surfaces at various cutting distances. The 45° plies resulted in the most extensive flank wear. The 90° plies yielded the worst edge radius rounding and largest worn tool area at both cutting speeds. The SEM images of the machined surfaces enabled determination of the number of exposed fibers and their exposed length, which directly impacts the extent of flank wear. Edge rounding may be related to more aggressive 2-body abrasion on the cutting tools by the broken ends of the carbon fibers interacting with the cutting edge. The 0° ply angle has the least amount of tool wear for all three measurements due to the minimal interactions between the carbon fibers and cutting edge with the delamination in the chip formation.The fourth part of the study investigated the effectiveness of several superhard ceramic coatings on carbide drills when drilling carbon fiber reinforced plastics (CFRP) composite/Ti-6Al-4V alloy (titanium or Ti) stacks. The drilling experiments of CFRP/Ti stack were conducted with diamond-like coating (DLC) coated, alternating layers of the nanocomposite of AlCrN & Si3N4 and TiN or (AlCrSi/Ti)N coated, and uncoated tungsten carbide drills. Tool wear evolution of each drill was measured qualitatively as well as quantitatively using the scanning electron and confocal laser scanning microscopes (CLSM) by interrupting after making a certain number of holes. Based on the drilling experiments, the performance of each coating when drilling CFRP/Ti stack are discussed. Among these coated and uncoated drills, uncoated and DLC coated drills failed before making 5 holes while (AlCrSi/Ti)N coated drills performed the best, making more than 80 holes. The DLC coating, despite of high hardness of DLC coating, does not provide any significant protection especially when drilling Ti layer.Based on the success of nanocomposite in drilling CFRP/Ti stack, the comparative study in turning Ti64 was carried out with of several coatings: BAM coated, AlTiN coated, ZrN coated, (AlCrSi/Ti)N coated, and uncoated tungsten carbide inserts. Those experiments were conducted at three speeds, 61, 91, and 122 m/min, and then tool wear was analyzed quantitatively and qualitatively using advanced SEM and confocal techniques. The results show superior performance of (AlCrSi/Ti)N coated inserts at low speed, 61 m/min; its tool life was at least 3 times longer compared to uncoated carbide tools while the tool life from other coatings did not improve considerably for any cutting the speed.Finally, the last part focused on the wear evaluation of dry, minimum quantity lubrication (MQL) and MQL with nanofluid in turning the most common titanium (Ti) alloy, Ti-6Al-4V, in a solution treated and aged (STA) microstructure. In particular, the nanofluid evaluated here was vegetable (rapeseed) oil mixed with small concentrations of exfoliated graphite nanoplatelets (xGnP). This work focuses on the turning process, which imposes a challenging condition to apply oil droplets directly onto the tribological surfaces of a cutting tool due to the uninterrupted engagement between tool and work material. A series of turning experiments was conducted with uncoated carbide inserts while measuring the cutting forces with a dynamometer under various conditions. The inserts were retrieved intermittently to measure the progress of flank and crater wear using confocal microscopy. This preliminary experimental result shows that MQL and in particular MQL with the nanofluid improve the machinability of Ti alloys even for turning process. However, to attain the best performance, the MQL conditions such as nozzle orientation and the concentration of xGnP must be optimized."--Pages ii-iii.