Bülent Ekmekci

RESIDUAL STRESS STATE AND HARDNESS DEPTH IN ELECTRIC DISCHARGE MACHINING: DE-IONIZED WATER AS DIELECTRIC LIQUID

ABSTRACT Procedures and results of experimental work to measure residual stresses and hardness depth in electric discharge machined surfaces are presented. Layer removal method is used to express the residual stress profile as a function of depth caused by a die sinking type EDM. Thin stressed layers are removed from machined samples by electrochemical machining. Corresponding deformations due to stress relaxation are recorded for each removal to determine the stress profile from elasticity theory. The relational dependence of the machining parameters with residual stresses is obtained and a semi-empirical model is proposed for plastic mold steel for de-ionized water as dielectric liquid. These stresses are found to be increasing rapidly with respect to depth, attaining to its maximum value, around the yield strength, and then fall rapidly to compressive residual stresses in the core of the material since the stresses within plastically deformed layers are equilibrated with elastic stresses.

Geometry and surface damage in micro electrical discharge machining of micro-holes

Geometry and subsurface damage of blind micro-holes produced by micro electrical discharge machining (micro-EDM) is investigated experimentally to explore the relational dependence with respect to pulse energy. For this purpose, micro-holes are machined with various pulse energies on plastic mold steel samples using a tungsten carbide tool electrode and a hydrocarbon-based dielectric liquid. Variations in the micro-hole geometry, micro-hole depth and over-cut in micro-hole diameter are measured. Then, unconventional etching agents are applied on the cross sections to examine micro structural alterations within the substrate. It is observed that the heat-damaged segment is composed of three distinctive layers, which have relatively high thicknesses and vary noticeably with respect to the drilling depth. Crack formation is identified on some sections of the micro-holes even by utilizing low pulse energies during machining. It is concluded that the cracking mechanism is different from cracks encountered on the surfaces when machining is performed by using the conventional EDM process. Moreover, an electrically conductive bridge between work material and debris particles is possible at the end tip during machining which leads to electric discharges between the piled segments of debris particles and the tool electrode during discharging.

An Experimental Study on the Geometry of Microholes in Microelectric Discharge Machining

Shape and dimensional geometry of blind microholes produced by microelectric discharge machining (micro-EDM) is investigated experimentally to explore the relational dependency with respect to machining time. For this purpose, microholes are machined with machining times ranging from 1–80 min on plastic mold steel samples using tungsten carbide tool electrode and hydro carbide dielectric liquid composed of mineral and synthetic oils. All other operational machining parameters such as pulse time, gap voltage, average current, and so forth, are kept constant during machining. Depth, diametric expansions, end-tip shape, wall side parallelisms of holes were analyzed both on surface and cross-sections of the samples. Consequently, nine different end-tip shape formations were observed, and these shapes discussed on basis of dielectric liquid circulation and sparking conditions during machining. Enlargement in hole diameter was predicted by defining a band-width of 20 µm hole expansion. Moreover, parallel wall-side length was expressed as the microhole depth where 5% reduction in entrance diameter was measured at the tapered section.

How Suspended Particles Affect Surface Morphology in Powder Mixed Electrical Discharge Machining (PMEDM)

The effect of SiC powder mixing into the dielectric liquid on surface topology and structure of interstitial free (IF) steel has been studied. Four process parameters, namely pulse duration, pulse current, concentration of powder, and type of dielectric liquid material, have been selected as varying parameters. Surface modifications due to suspended SiC particles were identified by using optical, scanning electron microscopy, and X-ray diffraction analysis. It was observed that suspended particles around discharge column accelerated and gained sufficient velocity to penetrate to the molten pool before solidification by means of electrophoresis and negative pressure induced after cessation of a discharge, which leads a surface embedded with added fine particles under that prevent formation of penetrating cracks during machining. However, surface cracks that formed due to high transformational stresses developed during solidification were found to be unaffected by means of mechanical action of the particles.

Electrical Discharge Machining of Ti6Al4V in Hydroxyapatite Powder Mixed Dielectric Liquid

Hydroxyapatite (HA) powder suspension in deionized water was used as a dielectric liquid during electrical discharge machining (EDM) of Ti6Al4V work material. The machined surfaces were evaluated by scanning electron microscopy (SEM), energy dispersive spectroscopy, and optical microscopy. The powder particles in the dielectric liquid extensively migrated and formed an HA-rich layer on the work material surface under specific machining conditions. The result was attributed to the generation of secondary discharges due to altered discharge conditions. The particles divided the primary discharge channel into several secondary ones. When the main discharge channel subdivided into several sub-discharges with comparable energy densities, the particles in the discharge region got stuck among them. Then, they moved toward the melted cavities and penetrated the surface at the end of the discharge duration. The results suggest the process as a practical alternative for producing biocompatible interfaces or coatings for medical applications.

Suspended SiC particle deposition on plastic mold steel surfaces in powder-mixed electrical discharge machining

The discharge gap phenomena in powder-mixed electrical discharge machining are examined using SiC powder mixing in water dielectric liquid. Surface modifications on machined work materials are investigated by means of optical, scanning electron microscopy and energy-dispersive spectroscopy. The experimental studies revealed that the surface morphology drastically affected the additives as means of secondary discharges and particle migration from dielectric liquid. Such mechanisms do not occur randomly and indicate a robust dependency with respect to powder suspension concentration, pulse on duration and current. The influence on discharge transitivity with respect to suspended particle concentration is noted with pock shape development due to secondary discharges followed by an intermediate stage signifying a sudden increase in particle migration from the dielectric liquid. The particles decomposed on the surface at specific operational conditions demonstrating the possibility of methodical surface alloying using the process. Finally, the mechanisms involved were elaborated with respect to operational parameters and discussed based on the experimental results.

Hydroxyapatite Deposition onto Ti-6Al-4V Surface in Powder Mixed Electrical Discharge Machining

Hydroxyapatite (HA) is bioactive and biocompatible bioceramics that is commonly used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. The need to overcome biocompatibility problems and improve the bone bonding ability enforced the researchers to develop suitable coating techniques. Feasibility of Powder Mixed Electrical Discharge Machining of Ti-6Al-4V alloy with HA additives in water dielectric liquid is studied as an alternative coating technique. Machined samples are analyzed by means of optical, scanning electron microscopy and energy dispersive spectroscopy. The experimental results have revealed that it is possible to deposit HA powders onto the work material surface by using the process. The morphology of the deposited material signified the dependency with respect to the operational parameters such as pulse on duration and current. Moderate pulse currents together with low pulse on duration were found to be the plausible choices for HA coating applications.

Ti-6Al-4V Surfaces in SiC Powder Mixed Electrical Discharge Machining

The role of suspended particles on Ti-6Al-4V surface in Powder Mixed Electrical Discharge Machining (PMEDM) is studied using SiC powder mixing in water dielectric liquid. Surface modifications due to the additives in dielectric liquid are investigated by means of optical, scanning electron microscopy and energy dispersive spectroscopy. The attachment of added powders and surface topographies interrelated with powder suspension concentration, particle size and electrical parameters such as pulse on duration and current. The influence on discharge transitivity with respect to SiC additives is noticed with pock like features on the surface. The geometry and size of these features indicated a robust dependency with respect to operational parameters and indicated the role of secondary discharges during PMEDM. SiC particles severely transferred from di-electric liquid to machined surface at critical operational parameters and implied that the process could be also used as a surface alloying technique.

A preliminary study on the use of cyanoacrylate glue for the closure of fetal membranes

Amnion leakage, infection and inflammation are the most important problems of intrauterine fetal surgery. This study aimed to determine the shear strength characteristics of fetal membranes attached with cyanoacrylate glue. Fresh amniochorionic membrane stripes (40×30 mm) were prepared after ten term elective cesarean section patients. Two stripes were attached with 5 mm overlapping and an increasing weight was applied until separation. Shear strength of the attached segment and resultant normal stresses developed in the membrane were calculated. Finally, corresponding intraluminal pressure that shear strength could resist without rupture was estimated by assuming two perfect semi-spherical membranes that are glued together with 5 mm overlapping to form a perfect sphere with a diameter of 200 mm. Mean total weight until rupture was 156 ± 72 gr. Shear strength of the glued 5 mm segment was calculated as 10199 ± 4696 Pa (104 ± 48 cmH20) that resulted in 101989 ± 46963 Pa (1040 ± 479 cmH20) mean tensile normal stress within the membrane. Assuming a perfect sphere with a diameter of 200 mm, the maximum intraluminal pressure that attached membrane stripes could resist without rupture was calculated as 1020 ± 470 Pa (10.4 ± 4.79 cmH2O). This preliminary basic study showed that the shear strength of cyanoacrylate was strong even in small tissue segments. Considering antibacterial properties, immediate action in the presence of tissue fluids, and water impermeability of the attachment sites, this agent should be considered for closure of amniochorionic membranes after fetal surgery.

Comparative study on the wear resistance of electrical discharge-machined surfaces

ABSTRACT The properties of the surface were affected by many factors such as the pulse parameters, tool electrode material, and dielectric liquid in electrical discharge machining. Austenitic, dual-phase, and ferritic steel work materials were electrical discharge machined using graphite and copper tool electrodes in hydrocarbon- based oil and water dielectric liquids. Then the surfaces were analyzed regarding sliding friction wear responses on a comparative basis. The results revealed that the surface wear responses are sensitive to the type of the tool electrode material when machining in water dielectric liquid. However, the use of hydrocarbon-based dielectric liquid substantiality suppresses the influence of tool electrode on surface wear response due to excessive carbon release from the cracked dielectric. The machined surface topographical features were also affected due to the used electrical parameters regarding crater size, globular attachments, and microcracks that led significant alterations in sliding friction response. Primarily, weakly bounded globular attachments on the machined surface were dislodged at the initial stages of the friction tests and led higher sliding distances to the steady friction conditions. Finally, the results were compared with the subsurface microstructural properties to comprehend the wear responses.