Organic monolayers disrupt plastic flow in metals

A cutting tool architecture designed to address the parasitic mechanisms consuming excess power during machining and manufacturing operations-A review-based study towards sustainable manufacturing

Metal cutting has been extensively studied over the years for improving its efficacy, yet, parasitic mechanisms like chatter and tool wear continue to generate higher forces and energy consumption with poor surface integrity. To address these parasitic mechanisms, a single-point turning cutter design is proposed based on the physics-of-machining including chatter theory to achieve reduced power consumption during the cutting of various metallic alloys like Al-6061, Ti-6Al-4V and others used by critical sectors such as aerospace and automotive. The current work focuses on aspects of machining that effectively reduce parasitic forces feeding into cutting power. The proposed cutter amalgamates features such as optimum side and end cutting edge angles, smaller nose radius and textured rake face into the cutter-body. Such a design is further proposed for use with a mechanochemical effect on a recently discovered plastic flow mode called sinuous flow, which has been reported to bring down cutting forces significantly. Experimental and analytical tests on the cutter design features validate reduction of cutting forces and through that alleviate the tendency to chatter as well as bring about energy savings for cutting of Al 6061. The potential for reduced real-time power consumption makes this design-framework significant for multipoint milling cutters too. It will greatly facilitate frugal manufacturing to account for sustainability in manufacturing operations.

Surface stress can initiate environment-assisted fracture in metals

Controlling environmental effects in surface plasticity/fracture of metals is of interest for areas as diverse as manufacturing processes, product performance, and structural safety. The key to controlling these effects is understanding the effect of adsorbates on surface energy (γ) and surface stress (f). While γ has been well studied, the role of surface stress has received much less attention. We characterize surface stress induced in metals by adsorption of organic monolayers. Linear alkanoic acids of varying chain length (3-18) are deposited by molecular self-assembly onto one side of an aluminum cantilever, several centimeters in length. The surface stress is estimated from in situ measurement of the cantilever deflection. We find that the organic adsorbates induce large surface stress of -4 to +30N/m. Furthermore, we show that f may be tuned by varying adsorbate-molecule chain length. The stress data explain beneficial embrittlement of metal surfaces by organic adsorbates in cutting and comminution processes, and point to a critical role, hitherto ignored, for f in environment assisted cracking (EAC) phenomena. Our results suggest opportunities for utilizing controlled environment-assisted fracture as an aid-fracture as a friend-to enhance material removal processes, apart from using surface stress itself as an experimental probe to explore various manifestations of EAC.

Cutting Depth Dictates the Transition from Continuous to Segmented Chip Formation

The process of material cutting emerges from a series of nonlinear phenomena including frictional contact, plastic deformation, and fracture. While cutting dominated by shear deformation is of interest to achieve a smooth material removal and a high-quality surface finish, the fracture-induced chip breaking is of equal importance to prevent the formation of long chips. Here we show that discrepant observations and predictions of these two distinct cutting mechanisms can be reconciled into a unified framework. A simple analytical model is developed to predict the mechanism of chip formation in a homogeneous medium as a function of work piece intrinsic material properties, tool geometry, and the process parameters. The model reveals the existence of a critical depth of cut, below which the chip formation is gradually progressed by plastic deformation in the shear plane, and above which chips break off by abrupt crack propagation. The models' prediction is validated by systematic in situ orthogonal cutting experiments and literature data for a wide range of materials over multiple length scales.

Surface-Stress Induced Embrittlement of Metals

Environment-assisted fracture phenomena in metals are usually associated with surface energy reduction due to an adsorbed film. Here we demonstrate a unique embrittlement effect in Al that is instead mediated by surface stress, induced by an adsorbed organic monolayer. Atomistic simulations show that the adsorbate carbon-chain length l_c controls the surface stress via van der Waals forces, being compressive for l_c < 8 and tensile otherwise. For l_c > 8, we demonstrate experimentally that the nanoscale film causes a ductile-to-brittle transition on the macroscale. Concomitant with this transition is a nearly 85% reduction in deformation forces. Additional simulations reveal that the microscopic mechanism for the embrittlement is via suppression of dislocation emission at incipient crack-tips. In addition to challenging long-held views on environment-assisted fracture, our findings pertaining to surface-stress induced embrittlement suggest profitable utility in manufacturing processes such as machining and comminution.