Machinability of Natural Fiber Composites

Abstract

NFRCs are finding great demand and wide applications mainly in the building, construction and automobile industries, packaging and consumer products because of their advantages that include sustainability, environmental friendliness, moderately good mechanical properties, renewability, availability, and low cost. Although the manufacturing with NFRCs can result in near-net-shape components, industrial applications of NFRCs still demand machining to achieve finished parts with tight dimensional tolerances to complete the product’s final assembly. While natural fibers have lower hardness and less abrasive effects than those of synthetic fibers, their anisotropic and heterogeneous nature do cause difficulties in machining NFRCs, limiting their largescale use. Recently, an appreciable amount of research in machining NFRCs has evolved w.r.t. drilling operations; however, a very limited number of research studies cover end milling, which is also an important process to consider. Producing NFRC parts with milling can also encounter general machining challenges, such as poor surface finish, fluctuating forces, temperature issues and wear of the tool. Moreover, the focus on machinability studies thus far has been primarily on optimizing the process parameters and identifying the effects of machinability parameters (namely, cutting speed and feed rate) on machining output responses (delamination and surface roughness). As a result, there is a need for thorough investigations to understand precisely the kinematics of cutting that cause surface quality problems, tool wear and material removal mechanism (chip formation) for different cutting parameters. The current research focuses on a comprehensive machinability study of three different types of polypropylene (PP) based NFRCs (fiber reinforcement:30 wt.%) - rice husk (RH)/PP, jute/PP and kenaf/PP composites by end milling utilizing high-speed steel (HSS) cutting tool. For the first time, detailed investigations of tool life/wear, establishing of modified Taylor’s tool life equation, wear-force (cutting force and thrust force) and weartemperature empirical models for predicting the tool life/wear along with chip forming mechanisms have been undertaken. Furthermore, different cutting tool materials such as HSS-Co.8 and K20 uncoated tungsten carbide (WC) tools are employed for examining the machined surface and tool performance. In addition, the research also highlights a comparative machinability study of NFRCs (kenaf/PP composites) vs synthetic fiber- reinforced composites (glass fiber reinforced composites, GFRCs, i.e., GF/PP), which has not yet been reported in the existing literature. It is interesting to note that the identified significant differences in the silicon contents (kenaf: 0.57%, jute: 4.16% and RH: 22.2%), hardness values (kenaf: 0.15, jute: 0.20 and RH:0.59 GPa) and aspect ratios (kenaf: 7.20, jute: 5.35 and RH: 1.55) of the three diverse natural fibers, have drastically influenced the mechanical properties and machinability characteristics of NFRCs. Overall, most likely due to kenaf's fiber composition and morphology, kenaf/PP composites prove to have superior mechanical and machinability characteristics, i.e., lowest delamination damages (Fd), surface roughness (Ra) and extended tool life in comparison to those of RH/PP and jute/PP composites. In contrast, RH/PP composites proved to be the most demanding to machine showing adverse surface quality and shortest tool life with rigorous abrasive wear. The outcomes of machining experimental and statistical analysis concluded that the machined surface quality and tool life was more influenced by spindle (cutting) speed than the feed rate. The microscopic analysis of worn cutting tools has revealed the distinctive tool lives and flank wear mechanisms with 2 and 3 body abrasive wear phenomena for the machined NFRCs, demonstrating: (i) rigorous mechanical abrasion with strip chipping, cracks, galling and large nose deformation for RH/PP; (ii) less severe abrasion combined with adhesion and attrition wears for jute/PP; and (iii) mild abrasion/scratch marks with attrition wear and formation of small transfer protective film (TPF) patches for kenaf/PP composites. The machined surface textures generated by the worn tools reveal a plowing mechanism for RH/PP and jute/PP composites while shearing type for kenaf/PP composites. The modified Taylor’s tool life equation, including cutting speed, feed rate, silicon content and reinforcement aspect ratio, showed tool life prediction with an error <18%. It is expected that this equation can be successfully implemented for predicting the tool lives for various natural fibre composites. Among the wear-force and weartemperature relationships attained from regression techniques, wear-temperature seemed to be better suited for monitoring the tool wear. Besides HSS and HSS-Co.8 cutting tools, the K20 WC tool demonstrated superior surface finishes and minimal tool wear with wear mechanism by plastic deformation (grooves absent), suggesting carbide tool might be best suited for machining NFRCs. The study of fundamental in-situ material removal mechanisms by quick stop method and high-speed camera for two remarkably distinct RH/PP and kenaf/PP composites showed different cutting and cracking patterns along with different kinds of chips formed at various cutting speeds. At lower speeds, RH ruptured and got dislodged, identifying a combination of shearing with a semi-brittle mechanism, while kenaf/PP showed shear cutting without any clear dislodgement of kenaf fibers. Compared to kenaf/PP, RH/PP being less ductile with harder, more abrasive and larger width RH, revealed decreased shear band-widths, chip reduction coefficients and smaller friction coefficients at chip/tool interfaces; however, increased shear angles, tool-chip temperatures and machined slot side-wall surface roughnesses. A comparative machinability investigation between kenaf/PP and GF/PP composites revealed machining GF/PP composites comprising quite hard and abrasive GF inclusions, unexpectedly resulting in reduced wear of the tool; nonetheless, higher Ra and machining forces. Thus, the surface finish might be the governing criterion for deciding the tool life. Machining both composites demonstrated differing tool wear mechanisms through varied transfer protective film formations on the tool's nose and counterface cutting edge. Diverse chip morphology confirms chip removal by shearing and predominant melt-extrusion while machining kenaf/PP and GF/PP composites, respectively. From the overall tool wear analysis conducted for different NFRCs along with GF/PP, it might be inferred that fiber dimensions (smaller dia. or width), higher aspect ratios and superior mechanical properties of the composites result in lower wear of the cutting tool.