Advanced Materials Research Vols. 690-693 (2013) pp 2437-2441 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.690-693.2437
Effect of Microstructure on Cutting Force and Chip Formation during Machining of Ti-6Al-4V Alloy Shoujin Sun1,a, Milan Brandt1,b, John P.T. Mo1,c 1
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, Victoria 3083, Australia
[email protected],
[email protected],
[email protected]
a
Keywords: Machining, microstructure, Ti-6Al-4V, cutting force, chip formation.
Abstract. Dry machining was conducted on Ti-6Al-4V alloy with three different types of microstructure: globular, bi-modal and fully lamellar microstructures. The effects of cutting speed on the cutting force and chip formation were investigated. The differences in cutting force and chip morphology are found only at cutting speed lower than 100m/min. The main cutting force and chip thickness when machining Ti-6Al-4V alloy with globular microstructure are lower than these when cutting Ti-6Al-4V alloy with bi-modal and fully lamellar microstructures at cutting speed lower than 100m/min. The tendency of segmented chip formation is the highest for cutting Ti-6Al-4V alloy with fully lamellar microstructure and the lowest for machining Ti-6Al-4V alloy with bi-modal microstructure at cutting speed lower than 100m/min because of their differences in increase of shear strength with strain rate. Introduction Titanium alloys, possessing the high strength-to-weight ratio due to the low density, the ability to retain high strength at high temperature up to 500oC, high heat resistance due to low thermal conductivity and high corrosion resistance, have wide applications in aerospace industry because of these unique properties. However, the poor machinability of titanium alloys makes the titanium parts expensive because of the high cost of machining process. Two crystal structures exist in titanium alloys after the equilibrium cooling condition, α-phase (hexagonal close-packed structure, HCP) and β-phase (body-centered cubic structure, BCC), depending on aluminum (alpha stabilizer) equivalent and molybdenum (beta stabilizer) equivalent in an alloy. The Al equivalent value indicates the capacity of the alloy to obtain a given hardness, the Mo equivalent value indicates the capacity to obtain an ultimate tensile strength and hardness in the aged condition [1]. Both α and β phases are relatively soft by themselves, but an α – β interface is an effective hindrance to dislocation and crack propagation. Hence, β alloys use the α phase as a primary strengthening mechanism and vice versa [2]. It has been reported that the machinability of titanium alloy is strongly affected by the microstructure, coarse grain microstructure alloys are more difficult to cut than the finer grain structure alloys [3]. Volume fraction of primary alpha phase is also an important factor to determine the machinability of a titanium alloy because of its correlation with the strength [3]. Ti-6Al-4V alloy is the most popular alloy among the titanium alloys, it contains alpha and beta phases. The volume fraction and morphology of these phases which depend on the heat treatment temperature determine the deformation behavior and mechanical properties of this alloy under both quasi-static and dynamic loading conditions [4-7]. Machinability of an alpha+beta titanium alloy (Ti54M) is affected by heat treatment conditions by which the hardness and phase morphology of the workpiece are altered [8]. Dry machining was conducted on Ti-6Al-4V workpieces with the same volume fraction of alpha phase but different phase morphologies at a wide range of cutting speeds and feed rates. The machinability of Ti-6Al-4V alloy with different phase morphologies is evaluated by the cutting forces and chip formation. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 122.107.143.183, Defence Materials Technology Centre; RMIT University, Bundoora, Australia-29/03/13,01:02:21)
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Materials Design, Processing and Applications
Workpiece materials and Experimental procedures Workpiece materials. The workpiece material used in this study is Ti-6Al-4V alloy, its chemical composition is listed in Table 1. The microstructures obtained after different heat treatment routs are shown in Fig. 1. The final heat treatment was carried out at temperature of 780oC for 2hrs on the worpieces which have undergone different annealing processes to achieve the same volume fraction of alpha phase. The microstructures show a globular morphology in which a small amount of intergranular beta phase disperses around the equiaxed alpha grains in (a), bi-modal morphology which contains globular primary alpha grains and alpha/beta colonies in (b) and fully lamellar morphology in (c). The microhardness measured at applied load of 200g for loading time of 15s is 310 ± 21 Hv0.2 for (a), 354 ± 35 Hv0.2 for (b) and 325 ± 17Hv0.2 for (c). Table 1. Chemical composition of workpiece alloy (mass percentage) C Al O Fe V H N Ti Y 0.010 5.86 0.120 0.200 4.02 0.0023 0.007 Bal.
