Surface Layer Modification by Cryogenic Burnishing of Al 7050-T7451 Alloy and Validation with FEM-based Burnishing Model

Abstract

As one of the chipless machining processes, burnishing has been performed on manufactured components as the final operation to improve the surface integrity, including reduced surface roughness and increased surface and subsurface hardness. Refined surface layers with ultra-fine grains or nano-grains could be generated during the burnishing process due to imposed severe plastic deformation and the associated dynamic recrystallization (DRX). These harder layers with compressive residual stresses induced by the burnishing process also provide added benefits by enhancing wear/corrosion resistance and increasing the fatigue life of the components. The research findings presented in this paper show the effects of cryogenic burnishing with roller burnishing tool on Al 7050-T7451 alloy, using liquid nitrogen as the coolant. Burnishing forces and temperatures are measured to compare the differences between dry and cryogenic burnishing. Higher tangential forces and lower temperatures are observed from cryogenic burnishing due to the work-hardening and the rapid cooling effects introduced by cryogenic burnishing. Also, refined layers with nano-grains (grain size of approximately 40 nm) are formed in the cryogenically burnished surface, in which an average hardness increase of 9.5%, 17.5% and 24.8% within the 200 m depth are achieved in comparison with the hardness values obtained from dry burnishing, at the corresponding burnishing speeds of 25, 50 and 100 m/min. A finite element model (FEM) is developed to simulate the burnishing forces and temperatures for validation of the experimental results, based on the modified Johnson-Cook flow stress model combined with the constitutive equations concerning DRX. Good agreement is obtained between the predicted and experimental results.

Study on the Surface Integrity of a Thin-Walled Aluminum Alloy Structure after a Bilateral Slid Rolling Process

“…Figure 4 shows that, as a consequence, the profile of the microhardness distribution beneath the surface is altered. These refined surface layers with higher hardness and compressive residual stress due to the mechanism of severe plastic deformation and dislocation would provide benefits to enhance the mechanical and physical properties of the components, such as improving their corrosion/wear resistance and increasing their fatigue lives [11]. During the preparation operations of the samples, surfaces were corroded to different extents, although the corrosion time was equal.…”

“…Recently, with the development of assistive technology, a variety of new burnishing methods have been proposed, such as the ultrasonic surface rolling process (USRP) [9], laser-assisted burnishing (LAB) [10], and cryogenic burnishing (CB) [11]. In the LAB processes, the workpiece surface layer is temporarily and locally softened by a controllable laser beam and then immediately processed by a conventional burnishing tool.…”

Comparison between Dry, MQL, and Cryogenic Cooling Technique on Surface Integrity of Burnished Surface

“…Burnishing is a cleaner process and capable to eliminate slower and costlier finishing processes and secondary operations such as grinding, honing or lapping [12]. It is the post machining activities that improves dimensional accuracy, surface hardness and residual stresses [13]. Generally, burnishing process is operated under dry condition without using any lubricant.…”

Impact of Indentor Sliding Velocity and Loading Repetition Factor on Shear Strain and Structure Dispersion in Nanostructuring Burnishing

“…Qualitative surface layers containing ultra-fine grains or nano-grains can be formed in the process of cryogenic burnishing of an AI 7050-T7451 alloy with a roller tool [19]; it can be achieved by means of applied severe plastic deformation and dynamic recrystallization (DRX) inherent in it.…”