Experimental cutting model of metal matrix composites (MMCs)

Abstract

Metal matrix composites (MMCs) are important engineering materials with good performances for industry. This article presents an experimental physical model on the cutting process for MMCs (aluminium alloy reinforced with 20% of particulate silicon carbide-SiC). The turning experiments were carried out on MMCs extruded workpieces using polycrystalline diamond (PCD) cutting tools. The objective of this study is to evaluate the chip compression ratio (R_c), chip deformation (ɛ), friction angle (ρ), shear angle (ϕ), normal stress (σ) and shear stress (τ), under prefixed cutting parameters (cutting velocity and feed rate). The experimental physical model was compared with the Merchant equation.

Introduction

Recently, the use of metal matrix composites (MMCs) has increased in various areas of science and technology due to their special mechanical and physical properties. MMCs are characterized by having a combination of light weight, very high strength and a high stiffness. Therefore, MMCs tend to replace conventional materials in various fields of application such as aeronautical, aerospace, automotive, mechanical engineering, as well as in other industries because of its own properties. As result of these properties and potential applications, a great necessity exits to understand the questions associated with the production and machining of such materials. Machining MMCs is a rather complex task owing to its heterogeneity and to the fact that reinforcements are extremely abrasive [1], [2], [3], [4], [5], [6].

The machinability of MMCs reinforced with particles of SiC (20%) using PCD tools was studied having in consideration the evolution of the cutting time of the tool wear, the cutting forces and the surface roughness workpieces [1], [2], [3]. Later, was evaluated the performance of the CVD diamond tools in machining of these composites [4]. A machinability study of MMCs reinforced with particles of SiC (5%) using cemented carbide tools were studied having in consideration the tool wear and the surface roughness workpieces [5]. The optimization of cutting parameters in this type of composites was makes using FEM models [6].

The orthogonal model, presented by Merchant (Fig. 1) can be used to approximate turning, and certain other single-point machining operations, as long as the feed in these operations is small relative to depth of cut [7], [8].

The undeformed chip thickness (e) can be calculated from the cutting edge angle (χ) and the feed rate (f) with the following equation:

$$e=f\sin \chi$$

The chip compression ratio, as defined by Merchant, is [7], [8]:

$$R_{\text{c}}=\frac{e^{\prime }}{e}$$

being e′ the chip thickness after cutting.

The shear plane angle (ϕ) can be calculated from the chip compression ratio by the following equation [7], [9], [10]:

$$\varphi =\text{arctan}\frac{\cos \gamma }{R_{\text{c}}-\sin \gamma }$$

being γ the rake angle and R_c obtained by Eq. (2).

From the cutting forces and the shear plane angle, the shear and normal stresses (N/mm²) along shear plane can be calculated [7], [9], [10]:

$$\tau =\left(\frac{F_{\text{c}}\cos \varphi -F_{\text{t}}\sin \varphi }{ce}\right)\sin \varphi$$

$$\sigma =\left(\frac{F_{\text{c}}\sin \varphi +F_{\text{t}}\cos \varphi }{ce}\right)\sin \varphi$$

being F_c the cutting force (N), F_t the thrust force (N), c the width of cut (mm) and e the undeformed chip thickness (mm).

The chip deformation (ɛ) can be obtained from the values of R_c and γ [9]:

$$\epsilon =\frac{1+R_{\text{c}}^2-2R_{\text{c}}\sin \gamma }{R_{\text{c}}\cos \gamma }$$

The shear plane angle (ϕ) is the angle at which the shear stress equals the shear strength of the work material, and so the shear deformation occurs preferably at this angle. The shear plane angle can be determined by taking the derivative of the shear stress (Eq. (4)) with respect to ϕ and setting the derivative to zero (relative maximum of the function τ), according to Merchant [7], [9], [10]:

$${\varphi }_{\text{m}}=\frac{\pi }{4}-\frac{1}{2}(\rho -\gamma )$$

The mean friction angle (ρ) can be estimated from the cutting forces (F_c and F_t) and the rake angle (γ) by the following equation [7], [9], [10]:

$$\rho =\arctan \mu =\frac{F_{\text{c}}\sin \gamma +F_{\text{t}}\cos \gamma }{F_{\text{c}}\cos \gamma -F_{\text{t}}\sin \gamma }$$

The objective of this experimental work is to compare the results of the Merchant equation with the experimental cutting model during the turning MMCs with PCD cutting tools.