Abstract
In this study, cryogenic temperature large strain extrusion machining (CT-LSEM) as a novel severe plastic deformation (SPD) method for producing ultra-fine grained (UFG) microstructure is investigated. Solution treated Al 7075 alloy was subjected to CT-LSEM, room temperature (RT) LSEM, as well as CT free machining (CT-FM) with different machining velocities to study their comparative effects. The microstructure evolution and mechanical properties were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Vickers hardness measurements. It is observed that the hardness of the sample has increased from 105 HV to 169 HV and the chip can be fully extruded under CT-LSEM at the velocity of 5.4 m/min. The chip thickness and hardness decrease with velocity except for RT-LSEM at the machining velocity of 21.6 m/min, under which the precipitation hardening exceeds the softening effect. The constraining tool and processing temperature play a significant role in chip morphology. DSC analysis suggests that the LSEM process can accelerate the aging kinetics of the alloy. A higher dislocation density, which is due to the suppression of dynamic recovery, contributes to the CT-LSEM samples, resulting in greater hardness than the RT-LSEM samples.
Highlights
Ultra-fine grained (UFG) Al alloys and its preparation have raised increasingly attention recently due to its excellent mechanical properties compared with its coarse-grained counterparts
The chip formation process, which is mainly determined by combined effects including material properties and machining parameters, is especially complicated for large strain extrusion machining (LSEM)
Unlike for conventional machining, the material undergoes extrusion and shearing simultaneously after it passes through the primary deformation zone, forming a geometrically controlled chip
Summary
Ultra-fine grained (UFG) Al alloys and its preparation have raised increasingly attention recently due to its excellent mechanical properties compared with its coarse-grained counterparts. Severe plastic deformation (SPD) techniques, including equal channel angular pressing (ECAP), accumulative roll bonding (ARB), high-pressure torsion (HPT), et al, are most commonly used methods to prepare UFG materials. An innovative SPD method called large strain extrusion machining (LSEM) was developed, which can impart large strains in a single step of deformation [1,2,3]. In order to suppress shear localization in LSEM, Rao et al [8] suitably altered the texture of Ti-6Al-4V and eventually obtained fine-grained continuous Ti-6Al-4V foils
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