Elsevier

Precision Engineering

Volume 30, Issue 3, July 2006, Pages 288-298
Precision Engineering

Precision and efficiency of laser assisted jet electrochemical machining

https://doi.org/10.1016/j.precisioneng.2005.09.006Get rights and content

Abstract

Laser assisted jet electrochemical machining (LAJECM) is a hybrid process, that combines a laser beam with an electrolyte jet thereby giving a non-contact tool electrode that removes metal by electrochemical dissolution. The laser beam effectively improves the precision of LAJECM as it is able to direct the dissolution to specifically targeted areas. This prevents the machining from unwanted areas due to stray current. This parallel application of a laser beam with the electrolyte jet enables an improvement of machining accuracy, also productivity. LAJECM has shown that machining with laser assistance can effectively facilitate material removal of 20, 25, 33, and 54% for Hasteloy, titanium alloy, stainless steel and aluminium alloy, respectively. There is also a noticeable improvement in the shape accuracy and slight decrease in surface roughness of the holes and cavities produced due to more focused machining (the order of 20%). The measured reduction in taper is of the order of 38, 40, 41, 65% for aluminium alloy, stainless steel, Hasteloy and titanium alloy, respectively.

Introduction

Electrochemical machining (ECM) is well-established in certain niche areas as aerospace, defence and medical industries to produce complex parts. ECM has several advantages compared to other conventional or unconventional processes: the ability to machine majority of conductive materials regardless of their properties; does not impart mechanical or thermal stress on the workpiece; the ability to produce mirror-effect surface finish; tool does not wear. However, the main drawbacks that prevent the use of ECM in a wider field are low precision and difficulty in tool design [1].

Laser assisted jet electrochemical machining (LAJECM) is designed to overcome both these deficiencies. Firstly, the tool is an electrolyte jet therefore no complex tool design is necessary. Secondly, the incorporation of the laser beam in parallel with the electrolyte jet has the potential to improve precision of ECM process [2], [3]

The main reason for low precision in ECM is the stray electric field which removes material from unwanted areas. For example for machining holes taper will result due to side machining. Much research have been done to improve ECM precision by localising ECM dissolution. The most typical way to minimise stray machining is to insulate parts of the tool electrode. However this method involves costly tool coating which sometimes does not give desired machining accuracy [1], [4]. Zhu et al have proposed a method of improving accuracy in ECM by using a dual pole tool electrode with a bimetallic bush outside the insulated coating of a cathode tool. The bush is anode, so the electric field at the side gap is significantly weakened [5]. The use of pulsed current has enabled better accuracy as it provide better gap conditions distribution—lower gap temperatures, smaller void fractions, cleaner and more equal electrolyte concentration, lower polarisation potentials, which in overall enable better electrolyte flow [1], [6]. Ahn et al. have used ultra short pulses of tens of nanosecond duration to localise dissolution area to achieve micro-holes in stainless steel [7]. Another approach for ECM localisation has been proposed by De Silva et al. based on using passivating electrolytes of low concentrations. Passivating electrolytes enable the machining gap to be targeted into high efficiency and low efficiency metal removal zones [8], [9]. Preliminary tests on steel have indicated that the use of laser energy may significantly limit stray machining [2], [3]. In this research this idea is further investigated by experimental and theoretical analysis.

Section snippets

Process principles

LAJECM combines two different sources of energy simultaneously: energy of ions (ECM) and energy of photons (a laser beam). The main aim of combining a laser with a jet of electrolyte (giving a laser-jet) is to assist electrochemical dissolution from a specific workpiece surface area. The laser beam is aligned coaxially with the jet of electrolyte creating a non-contact tool-electrode. Electrochemical dissolution is the main material removal mechanism supported by the parallel action of the low

Apparatus set-up

The experimental apparatus set-up consists of four systems: electrolyte supply system, laser delivery system, power supply system and data acquisition system (Fig. 3(a)). Electrolyte is passed through a filter and entered to a jetcell under a pressure of up to 0.4 bar. The focused laser beam is introduced through the heat resistive glass located on the top of the jetcell (Fig. 3(b)). The laser beam–electrolyte absorption coefficient for the laser wavelength used 532 nm is 0.045 m−1 [13] assuming

Material removal efficiency and distribution

The most evident result of laser assistance in LAJECM is the increase in volumetric removal rate (VRR) compared to Jet-ECM (Fig. 5). VRR can be defined as follows (Eq. (1)):VRR=mρtwhere: m is the dissolved mass, ρ the material density, t is the machining time.

VRR is higher for LAJECM regardless of the process variables used as shown in Fig. 5. VRR, according to ECM principles, increases with voltage (which also increases current), electrolyte concentration (Cp), electrolyte jet speed (V) and

Current, current density and current efficiency analysis

Current monitoring for LAJECM and JECM verifies the material removal measurements as well as providing information regarding possible phenomena that occurs in the IEG. Generally, for LAJECM, registered current was usually higher than for JECM. That shows that electrolyte conductivity increases with temperature giving higher current. During the process, while material is removed and a cavity created, the IEG distance rises, thus decreasing current gradually. The range of current depends on the

Process energy distribution—theoretical and experimental approach

An enhanced explanation of LAJECM features can be found on the base of laser energy influence and interaction. The LAJECM can be explained assuming that energy absorbed by the workpiece is utilised to carry out electrochemical reactions of dissolution. Briefly, the laser heat energy causes the temperature to rise and accelerates the reaction, but it also increases the standard enthalpy, therefore influencing the change of thermodynamic potential [2], [12]. Finally it lowers the reaction

LAJECM process temperature

Local increase in temperature in the localised machining zone is one of the main drivers of efficiency and precision improvement in LAJECM. In order to prove the phenomenon of enhanced dissolution in the localised zone experiments on a rod-shaped specimen were carried out. Temperatures were measured as described in Section 3.3 using the special set-up as shown in Fig. 4. Experiments on different materials revealed that temperature in the localised machining zone is most commonly up to 3.5 times

Shape precision

Shape precision in LAJECM depends on the effectiveness of machining localisation. The faster dissolution in axial direction (as discussed in Section 2) results in reduction in through-hole taper for LAJECM when compared to JECM. The narrower taper can be produced by LAJECM than by JECM. For the same machining time LAJECM can also produce holes of larger diameters due to the higher material removal rate. Fig. 16 shows the measurement method of the taper and overcut; Fig. 17 shows the percentage

Possible applications

The LAJECM process is in the investigation and development stage. At present tests are done for holes and cavities. The next step will demand re-design of the existing apparatus giving the possibility to machine 3D shapes such as a micro-grove. This technology, in contrast to some others used, would preserve the workpiece structure unchanged as well as its surface without any residual stresses. It is anticipated that the groove or path width can be minimised to 100 μm without any stray effects.

Conclusions

Laser assistance can effectively enhance Jet-Electrochemical Machining. In LAJECM dissolution is initiated easier and proceeds faster due to local temperature increase in the laser-jet spot on the workpiece, therefore higher material removal rates are achieved. Experiments carried out on a range of materials have revealed that LAJECM can improve volumetric removal rate of 20, 25, 33, and 54% for Hasteloy, titanium alloy, stainless steel and aluminium alloy, respectively. It has been found that

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