Computational Analysis of Machining Induced Stress Distribution during Dry and Cryogenic Orthogonal Cutting of 7075 Aluminium Closed Cell Syntactic Foams

Abstract

The addition of hollow aluminium oxide bubbles to the 7075 aluminium matrix results in a lightweight syntactic foam with a reduced density and an increased peak compression strength. The presence of ceramic bubbles also aids in a reduced coefficient of thermal expansion and thermal conductivity in comparison to aluminium alloys. In spite of their enhanced material properties, the inclusion of hollow aluminium oxide bubbles presents the challenge of poor machinability. In order to elucidate the problem of poor surface machinability, an attempt has been made to develop a thermo-mechanical finite element machining model using AdvantEdge^TM software with which surface quality and machined syntactic foam material can be analyzed. If the novel model developed is combined with virtual reality technology, CNC technicians can observe the machining results to evaluate and optimize the machining program. The main novelty behind this software is that the material foam is assumed as a homogeneous material model for simplifying the material model as a complex heterogeneous material system. The input parameters used in this study are cutting speed, feed, average size and volume fraction of hollow aluminium oxide bubbles, and coolant. For the output parameters, the numerical analysis showed a 6.24% increase in peak tensile machining induced stress as well as a 51.49% increase in peak cutting temperature as cutting speed (25 m/min to 100 m/min) and uncut chip thickness (0.07 mm to 0.2 mm) were increased. The average size and volume fraction of hollow aluminium oxide bubbles showed a significant impact on the magnitude of cutting forces and the depth of tensile induced stress distribution. It was observed on the machined surface that, as the average size of hollow aluminium oxide bubbles became coarser, the peak machining induced tensile stress on the cut surface reduced by 4.47%. It was also noted that an increase in the volume fraction of hollow aluminium oxide bubbles led to an increase in both the peak machining induced tensile stress and the peak cutting temperature by 29.36% and 20.11%, respectively. This study also showed the influence of the ceramic hollow bubbles on plastic deformation behavior in 7075 aluminium matrix; the machining conditions for obtaining a favorable stress distribution in the machined surface and sub-surface of 7075 closed cell syntactic foam are also presented.

Keywords: FE model; aluminum; machining; surface integrity; syntactic foam.

Figure 1

(a) Boundary conditions and meshing employed for workpiece and machining tool used in numerical prediction model for AdvantEdge^TM.; (b) Zoomed in version of tool and workpiece.

Figure 2

(a) Different views of compression test set-up; (b) a close-up view of 7075 Aluminum sample kept between pressure plates during compression; (c) Stress-strain compression behavior of 7075 Aluminium.

Figure 3

(a) Machining experimental test setup; (b) Sample force graph taken from dynamometer; (c) Representative machined chip for 7075 syntactic foam.

Figure 4

(a–c) Variation of cutting velocity on cutting forces, machining induced stress on machined surface, peak equivalent plastic strain, and peak cutting temperature (AdvantEdge^TM simulation) under dry conditions. 7075/hollow alumina shell syntactic foams (vol% = 10%, h = 0.07 mm, b = 3 mm, Dry cut); Von Mises stress contour at (d) 100 m/min and (e) 25 m/min; peak cutting temperature contour at (f) 100 m/min and (g) 25 m/min.

Figure 5

(a–c) Effect of undeformed chip thickness 7075/hollow alumina shell syntactic foams on cutting forces, machining induced stress on machined surface, peak equivalent plastic strain, and peak cutting temperature (AdvantEdge^TM simulation) (vol% = 10%, Vc = 50 m/min, b = 3 mm) under dry condition; Von Mises stress contour at (d) h = 0.07 mm and (e) h = 0.2 mm.

Figure 6

(a–c) Effect of volume fraction of alumina bubble present in 7075 on cutting forces, machining induced stress on machined surface, peak equivalent plastic strain, and peak cutting temperature (AdvantEdge^TM simulation) (h = 0.07 mm, Vc = 50 m/min, b = 3 mm) under dry conditions; Von Mises stress contour at (d) 10% and (e) 20%.

Figure 7

(a–c) Effect of average size of alumina bubble on cutting force, machining induced stress on machined surface, peak equivalent plastic strain, and peak cutting temperature (AdvantEdge^TM simulation) under dry cutting. 7075/hollow alumina shell syntactic foams (Vc = 50 m/min, h = 0.07 mm, 10% volume fraction, b = 3 mm); Von Mises stress contour at (d) d = 0.3 mm and (e) d = 0.6 mm.

Figure 8

Numerical machining induced stress distribution under the influence of cutting speed (10% volume fraction, uncut chip thickness = 0.07 mm, dry cutting).

Figure 9

Numerical machining induced stress distribution under the influence of uncut chip thickness (10% volume fraction, cutting speed = 50 m/min, dry cutting).

Figure 10

Numerical machining induced stress distribution under the influence of average-sized hollow aluminium oxide bubbles (10% volume fraction, cutting speed = 50 m/min, uncut chip thickness = 0.07 mm, dry cutting).

Figure 11

Numerical machining induced stress distribution under the influence of volume fraction of hollow aluminium oxide bubbles (10% volume fraction, cutting speed = 50 m/min, uncut chip thickness = 0.07 mm, average size = 0.3 mm, dry cutting).

Figure 12

(a) Numerical machining induced stress distribution, and variation of (b) peak equivalent plastic strain and (c) peak cutting temperature under the influence of coolant (10% volume fraction, cutting speed = 50 m/min, uncut chip thickness = 0.07 mm, average size = 0.3 mm, volume fraction 10%); Corresponding temperature contours at (d) dry and (e) liquid nitrogen conditions respectively.