MACHINING CHARACTERISTICS OF A HYBRID PROCESS OF EDM IN GAS COMBINED WITH ULTRASONIC VIBRATION AND AJM SAJIN R LYANEME045

INTRODUCTION  In this paper the AJM and USV mechanisms were incorporated into the process of EDM in gas to develop a HYBRID EDM process.

ELECTRICAL DISCHARGE MACHINING (EDM)

 Electrical Discharge Machining (EDM) is a non conventional machining process.  EDM machining method are used for hard metals which are electrically conductive.  The principle of EDM is spark erosion or electro erosion.  Uses a pulsating electric charge carrying high frequency DC current.  Generates a plasma channel between the two electrodes.

 Dry EDM is a modification of the oil EDM process in which the liquid dielectric is replaced.  Molten materials during EDM process was ejected from machining zone by a high-speed air jet.. Contd….  Kerosene used as the dielectric media in EDM process induce some problems.

ABRASIVE JET MACHINING (AJM)

 Abrasive jet machining (AJM) is process of material removal by impact erosion.  A focused stream of abrasive particles 50 microns grit size, carried by high pressure air or gas at velocity of 200 m/s is made to impinge on the work surface.  Aluminum oxide, Silicon carbide, and sodium bicarbonate are some of abrasives used in AJM.

ULTRASONIC MACHINING (USM)

 In Ultrasonic machining (USM), tool vibrates at an ultrasonic frequency (19-25 kHz) with an amplitude of around 5-50 μm over the workpiece.  Tool is pressed downward with a feed force.  As the tool vibrates over the workpiece, the abrasive particles act as the indenters and indent both the work material and the tool.

HYBRID EDM PROCESS  Develop a hybrid process integrating various mechanisms to improve the machining characteristics.  The effects of dielectric media and peek current in the hybrid EDM process were examined on the machining characteristics such as material removal rate (MRR), electrode were rate (EWR), and surface roughness (SR).  To find surface morphologies and thickness of recast layer of SKD 61 tool steel in hybrid process.

EXPERIMENTAL METHOD

 A die-sinking EDM machine with transistor controlled circuit integrated is used.  SKD 61 steel was selected as the workpiece material, which dimension is Φ15 mm×5 mm.  The dielectric media were adjusted by a regulated pressure valve to fit the requirement of experimental conditions.  Gas with specific abrasives was delivered through central hole of electrode. WORKING PROCESS

 Melting and vaporizing material was ejected as fast as possible by high-speed compressed gas jet.  USV and AJM were incorporated to facilitate the materials removal efficiency.  Workpiece and electrode specimens were weighed before and after each experiment to calculate the MRR and EWR.  The SR was measured by a precision profilometer.

 Scanning Electronic microscope (SEM) was used to explore the integrities of machined surface  The dielectric media were used are air, oxygen, and argon.  Input variables such as peak current (Ip), dielectric media were varied to explore their influences on the machining characteristics.  The machining time for each experiment is 10 min.

MATERIAL REMOVAL RATE  Improved transport rate of debris and molten material.  Preventing the secondary sparks.  EFFECTS OF THE HYBRID EDM PROCESS WITH VARIOUS DIELECTRIC MEDIA RESULTS AND DISCUSSION

 Higher EWR in hybrid EDM process than EDM in gas.  AJM mechanism abrade the inner wall of the electrode ELECTRODE WEAR RATE Electrode wear rate (mg/min) Dielectric media AIR ARGONOXYGEN

 Lower SR of hybrid EDM process.  Hybrid EDM process reduce the asperities caused by USV and AJM. SURFACE ROUGHNESS

Peak current, Amp Material removal rate (mg/min )  MRR enhanced with an increase of peak current.  Oxygen: promote oxidation & sparking of EDM. MATERIAL REMOVAL RATE  EFFECTS OF THE HYBRID EDM PROCESS WITH PEAK CURRENT Oxygen Air Argon

Peak current, Amp Electrode wear rate (mg/min)  EWR increases with the increasing of peak current.  Highest EWR was gained by using oxygen.  Argon has lowest EWR. ELECTRODE WEAR RATE

Peak current, Amp Surface roughness (Ra/ μ m)  SR increased with peak current.  surface morphologies would be coarser & the SR was larger as the peak current set at high levels. SURFACE ROUGHNESS

EDM INCORPORATED WITH AJM (EDM+AJM)  Morphology caused by EDM in gas combined AJM illustrates obvious micro pores on the machined surface.  SURFACE INTEGRITY

HYBRID EDM PROCESS (EDM+AJM+USV)  Molten materials suffer from rapid cooling rate due to the high compressed dielectric media and the high frequency USV.  The surface tension stress exceeded the ultra-tensile strength. Thus, the micro cracks were easily formed.

 Thickness of the recast layer obtained by the EDM in gas combined AJM is larger than that obtained by the developed hybrid EDM process. EDM INCORPORATED WITH AJM (EDM+AJM)  RECAST LAYER

 Hybrid EDM process incorporates the USV which would inhibit the molten material to be redeposited on the machined surface.  The recast layer is thinner. HYBRID EDM PROCESS (EDM+AJM+USV)

CONCLUSION  The MRR and EWR obtained by hybrid EDM process was higher than that by EDM in gas.  SR gained by the hybrid EDM process was lower than that by EDM in gas.  The machining characteristics such as MRR, EWR, and SR increased with an increase of peak current.  The oxygen dielectric media generated the highest MRR, EWR and SR in hybrid EDM process.

 The argon dielectric media produced the lowest MRR, EWR and SR in hybrid EDM process.  EDM in gas combined AJM have micro pores on the machined surface, and that from hybrid EDM process reveals micro cracks on the machined surface.  Thickness of the recast layer obtained by the EDM in gas incorporated with AJM is thicker than that obtained by the developed hybrid EDM process.

REFERENCES  Electro Physical and Chemical Machining[2016] –Yan Cherng, Jung Chou Hungb.  Machining characteristics of hybrid EDM with ultrasonic vibration and assisted magnetic force[2014] – Chuang FP, Wang AC, Chow HM  Electrical discharge machining in Gas[1997]-Kunieda M, Yoshida  Abrasive micro-blasting to improve surface integrity of electrical discharge machining[2005]-Qu J, Shih AJ, Scattergood RO, Luo J  Machining performance on hybrid process of abrasive jet machining and electrical discharge machining[2012]- Lin YC, Chen YF, Wang AC, Sei WL  An investigation of ultrasonic assisted electrical discharge machining in gas[2006]- Zhang QH, Du R, Zhang JH, Zhang QB