Effect of lubrication and cutting parameters on ultrasonically assisted turning of Inconel 718

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Abstract

The paper further develops the finite element (FE) model of ultrasonically assisted turning (UAT) discussed in Mitrofanov et al. [A.V. Mitrofanov, V.I. Babitsky, V.V. Silberschmidt, Finite element analysis of ultrasonically assisted turning of Inconel 718, J. Mater. Process. Technol., in press]. The advanced FE model (based on the general FE code MSC.MARC) allows transient, coupled thermomechanical simulations of both UAT and conventional turning of elasto-plastic materials. This model is used to study the effect of cutting parameters (such as the cutting speed, depth of cut and feed rate) and influence of lubrication on various features of two turning techniques, including cutting forces and chip shapes. The recently obtained results on three-dimensional FE modelling of UAT are also presented. This 3D model allows a study of chip formation in oblique turning.

Introduction

Ultrasonically assisted turning (UAT) is an advanced machining technique, where high-frequency vibration (frequency f  20 kHz, amplitude a  15 mm) is superimposed on the movement of the cutting tool. Compared to conventional turning (CT), this technique allows significant improvements in processing intractable materials, such as high-strength aerospace alloys, composites and ceramics. Superimposed ultrasonic vibration yields for Inconel 718 (a nickel-base alloy widely used in aerospace industry) a multi-fold decrease in cutting forces, as well as an improvement in surface finish by up to 50% compared to CT [2]. The prototype of the UAT system has been designed at Loughborough University, UK, and a number of experimental tests have been performed confirming advantages of UAT in comparison to CT [3].

This dynamics analysis has been used in Ref. [4] to study and to analyse UAT as a nonlinear vibro-impact process and the amplitude response of the cutting tool under load for this cutting technique. However, thermomechanics of the tool–workpiece interaction, which is especially important for the regime with multiple microimpacts in the process zone, and other specific features of the cutting process in UAT has not been fully understood, and the first finite element (FE) model of the UAT has been proposed only recently [5]. That initial, purely mechanical finite element model was further developed into a transient, fully thermomechanically coupled one for both UAT and CT. Some computational results obtained with the latter model were discussed in Ref. [1]. The current paper offers further results obtained with this improved model with regard to the influence of lubrication and cutting parameters on the turning process, as well as with its 3D version.

Finite element modelling is a main computational tool for simulation of the process zone and of tool–workpiece interaction in metal cutting. A detailed review of such FE models can be found in the monographs [6], [7], with a short overview being given in our previous paper [1].

Since this present paper presents a transition from the two-dimensional to a three-dimensional one that is applied to ultrasonically assisted turning, a brief review of 3D FE models for conventional cutting processes is given below. The majority of the suggested schemes employ the method of chip separation along a predefined line, separating finite elements in the initial discretisation of the area, hence reducing the flexibility of the analysis. Only a few schemes use other techniques, such as elements deletion based upon penetration [8], adaptive remeshing of the workpiece elements [9] and combination of both the manual deletion and remeshing [10]. Adaptive remeshing that is employed in the current paper maps calculated fields of parameters onto the new mesh to eliminate distorted in the shape of elements, which could otherwise cause termination of simulations. The method has an advantage of a relatively easy adjustment in the cutting direction and angles, as well as other cutting parameters, such as feed rate, without a necessity to reformulate the boundary value problem as in the case of separation along a predefined.

An FEA analysis of heat generation in machining of isotropic materials was conducted in Ref. [11] in order to study the effects of the convective heat transfer. Another approach, using an orthogonal FE model coupled with an analytical 3D model of cutting, was developed in Ref. [12] to predict a chip flow angle and three-dimensional forces in the tool. Another 3D model was developed in Ref. [9] that took into account dynamic effects, thermomechanical coupling, constitutive damage law and contact with friction in order to study the cutting forces and plastic deformation.

With 3D modelling of CT being used for the study of tool forces and chip flow in conventional turning in last two decades, this paper presents the first three-dimensional FE model of UAT. It has been recently developed and some computational results, emerging from this 3D formulation, are discussed.

Section snippets

FE approach

The detailed description of the suggested numerical model for a 2D formulation is given in Refs. [1], [13]. Its main features of the computational scheme are described below. Both the two-dimensional and three-dimensional thermomechanically coupled FE models are based on the MSC.Marc general FE code [14].

In the plain–strain 2D model, an orthogonal turning process, i.e. the cutting process where the tool edge is normal to both cutting and feed directions, is considered. Fig. 1 shows a scheme of

Results of simulations and discussion

All variants of numerical (finite element) simulations below are performed for two cutting techniques (CT and UAT) for identical parameters so that results for CT could serve as a reference for ultrasonically assisted turning. Two contact conditions are studied at the tool–chip interface (a) a frictionless contact and (b) a contact with friction (μ = 0.5). The former case corresponds to the well-lubricated cutting process, with heat generation occurring only due to plastic deformation processes.

Conclusion

A 2D thermomechanically coupled FE model of ultrasonically assisted turning (UAT) is used to study the influence of lubrication and cutting parameters on the process of UAT. A 3D model is presented as an extension of this model and allows us to study three-dimensional chip formation and to predict distributions of stresses, strains, cutting forces and temperatures in the workpiece and cutting tool.

The effect of lubrication was studied by comparison of simulations with and without friction

Acknowledgements

The authors would like to acknowledge the help of Dr. Alan Meadows and Mr. Peter Thomas in conducting experiments on the UAT prototype.

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