Surface treatment by electric discharge machining of Ti–6Al–4V alloy for potential application in orthopaedics

Functional Surface Generation by EDM-A Review

Electro-discharge machining (EDM) removes electrically conductive materials by high frequency spark discharges between the tool electrode and the workpiece in the presence of a dielectric liquid. Being an electrothermal process and with melting and evaporation being the mechanisms of material removal, EDM suffers from migration of materials between the tool and the workpiece. Although unwanted surface modification was considered a challenge in the past for many applications, this inherent nature of the EDM process has recently become of interest to the scientific community. As a result, researchers have been focusing on using the EDM process for surface modification and coating by targeted surface engineering. In order to engineer a surface or generate functional coatings using the electro-discharge process, proper knowledge of the EDM process and science of electro-discharge surface modification must be understood. This paper aims to provide an overview of the electro-discharge surface modification and coating processes, thus assisting the readers on exploring potential applications of EDM-based techniques of surface engineering and coating generation. This review starts with a brief introduction to the EDM process, the physics behind the EDM process, and the science of the surface modification process in EDM. The paper then discusses the reasons and purposes of surface modification and coating practices. The common EDM-based techniques reported in the literature for producing coatings on the surface are discussed with their process mechanisms, important parameters, and design considerations. The characterization techniques used for the analysis of modified surfaces and coating layers, as well as the tribological and surface properties of modified surfaces or coatings are discussed. Some of the important applications of EDM-based surface modification and coating processes are generating surfaces for protective coating, for aesthetic purposes, for enhancing the biocompatibility of implants, for improving corrosion resistance, for improving wear resistance, and for improving tribological performance. The current state of the research in these application areas is discussed with examples. Finally, suggestions are provided on future research directions and innovative potential new applications of the electro-discharge-based surface engineering and coating processes.

SWOT Analysis of Electrical Discharge Coatings: A Case Study of Copper Coating on Titanium Alloy

The electrical discharge machine (EDM) has been one of the most widely used non-traditional machines in recent decades, primarily used for machining hard materials into various complex shapes and different sizes and, nowadays, used for surface modifications/hard coatings. In this study, the SWOT (strengths, weaknesses, opportunities and threats) of electrical discharge coating was analyzed by conducting a case study. For the purpose of the case study, copper was deposited on the titanium alloy surface (Ti6Al4V). Three electrodes of different copper alloy materials, viz., brass, bronze and copper, were selected for coating the Ti6Al4V surface. Input parameters such as current, pulse-on, pulse-off, flushing pressure and the electrode material were optimized to develop a uniform coating. Experiments were designed according to the L18 orthogonal array, and among them, the samples that showed proper coating, as seen with the naked eye, were selected for morphological and elemental analyses by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX. Further, the output responses, viz., the material deposition rate (MDR), electrode wear rate (EWR), surface roughness (SR), elemental data (copper (Cu) and zinc (Zn)) and coating thickness (CT), were considered for the optimization of coatings. Implementing the Technique for Order Performance by Similarity to Ideal Solution, copper coating with a thickness of 20.43 µm, developed with an MDR with input parameters of 20 A current, 600 µs pulse-on, 120 µs pulse-off, 0.5 bar flushing pressure and the brass electrode, was selected as the optimum coating. The most influential parameters in this coating process were the current and pulse-on time. In this study, a SWOT table was developed to depict the strengths, weaknesses, opportunities and threats of electrical discharge coating.

Surface Modification Techniques to Produce Micro/Nano-scale Topographies on Ti-Based Implant Surfaces for Improved Osseointegration

Titanium and titanium alloys are used as artificial bone substitutes due to the good mechanical properties and biocompatibility, and are widely applied in the treatment of bone defects in clinic. However, Pure titanium has stress shielding effect on bone, and the effect of titanium-based materials on promoting bone healing is not significant. To solve this problem, several studies have proposed that the surface of titanium-based implants can be modified to generate micro or nano structures and improve mechanical properties, which will have positive effects on bone healing. This article reviews the application and characteristics of several titanium processing methods, and explores the effects of different technologies on the surface characteristics, mechanical properties, cell behavior and osseointegration. The future research prospects in this field and the characteristics of ideal titanium-based implants are proposed.

EDM of Ti-6Al-4V under Nano-Graphene Mixed Dielectric: A Detailed Investigation on Axial and Radial Dimensional Overcuts

Ti-6Al-4V is considered a challenging material in terms of accurate machining. Therefore, electric discharge machining (EDM) is commonly engaged, but its low cutting rate depreciates its use. This issue is resolved if graphene nanoparticles are mixed in the dielectric. However, the control over the sparking phenomenon reduces because of the dispersion of graphene particles. Subsequently, the machined profile’s geometric accuracy is compromised. Furthermore, the presence of nanographene induces different sparks along axial and radial cutting orientations. This aspect has not been comprehensively examined yet and dedicatedly targeted in this study to improve the quality of EDM process for Ti-6Al-4V. A total of 18 experiments were conducted under Taguchi’s L18 design considering six parameters namely, electrode type, polarity, flushing time, spark voltage, pulse time ratio, and discharge current. The aluminum electrode proved to be the best choice to reduce the errors in both the cutting orientations. Despite the other parametric settings, negative tool polarity yields lower values of axial (A_DE) and radial errors (R_DE). The developed optimal settings ensure 4.4- and 6.3-times reduction in R_DE and A_DE, respectively. In comparison to kerosene, graphene-based dielectric yields 10.2% and 19.4% reduction in R_DE and A_DE, respectively.

A comprehensive review on metallic implant biomaterials and their subtractive manufacturing

There is a tremendous increase in the demand for converting biomaterials into high-quality industrially manufactured human body parts, also known as medical implants. Drug delivery systems, bone plates, screws, cranial, and dental devices are the popular examples of these implants - the potential alternatives for human life survival. However, the processing techniques of an engineered implant largely determine its preciseness, surface characteristics, and interactive ability with the adjacent tissue(s) in a particular biological environment. Moreover, the high cost-effective manufacturing of an implant under tight tolerances remains a challenge. In this regard, several subtractive or additive manufacturing techniques are employed to manufacture patient-specific implants, depending primarily on the required biocompatibility, bioactivity, surface integrity, and fatigue strength. The present paper reviews numerous non-degradable and degradable metallic implant biomaterials such as stainless steel (SS), titanium (Ti)-based, cobalt (Co)-based, nickel-titanium (NiTi), and magnesium (Mg)-based alloys, followed by their processing via traditional turning, drilling, and milling including the high-speed multi-axis CNC machining, and non-traditional  abrasive water jet machining (AWJM), laser beam machining (LBM), ultrasonic machining (USM), and electric discharge machining (EDM) types of subtractive manufacturing techniques. However, the review further funnels down its primary focus on Mg, NiTi, and Ti-based alloys on the basis of the increasing trend of their implant applications in the last decade due to some of their outstanding properties. In the recent years, the incorporation of cryogenic coolant-assisted traditional subtraction of biomaterials has gained researchers' attention due to its sustainability, environment-friendly nature, performance, and superior biocompatible and functional outcomes fitting for medical applications. However, some of the latest studies reported that the medical implant manufacturing requirements could be more remarkably met using the non-traditional subtractive manufacturing approaches. Altogether, cryogenic  machining among the traditional routes and EDM among the non-traditional means along with their variants, were identified as some of the most effective subtractive manufacturing techniques for achieving the dimensionally accurate and biocompatible metallic medical implants with significantly modified surfaces.

Surface Characterization and Tribological Performance Analysis of Electric Discharge Machined Duplex Stainless Steel

The present article focused on the surface characterization of electric discharge machined duplex stainless steel (DSS-2205) alloy with three variants of electrode material (Graphite, Copper-Tungsten and Tungsten electrodes). Experimentation was executed as per Taguchi L18 orthogonal array to inspect the influence of electric discharge machining (EDM) parameters on the material removal rate and surface roughness. The results revealed that the discharge current (contribution: 45.10%), dielectric medium (contribution: 18.24%) majorly affects the material removal rate, whereas electrode material (contribution: 38.72%), pulse-on-time (contribution: 26.11%) were the significant parameters affecting the surface roughness. The machined surface at high spark energy in EDM oil portrayed porosity, oxides formation, and intermetallic compounds. Moreover, a pin-on-disc wear analysis was executed and the machined surface exhibits 70% superior wear resistance compared to the un-machined sample. The surface thus produced also exhibited improved surface wettability responses. The outcomes depict that EDMed DSS alloy can be considered in the different biomedical and industrial applications.

Parametric evaluation of electrical discharge coatings on nickel-titanium shape memory alloy in deionized water

Nickel-titanium shape memory alloy (NiTi) has a unique capacity to restore its initial shape after deformation, which is highly applicable to orthopaedic implantations, especially for the minimization of invasive surgeries. The high nickel content of this alloy can lead to unfavourable effects on the human body upon dissolution; thus, a reliable barrier of coatings on the NiTi surface is required to alleviate the nickel migration and increase its biocompatibility. In this paper, analyses of a titanium oxide layer development on NiTi surface using electrical discharge coating (EDC) process is presented. The recast layer thickness, crater sizes, and surface roughness were characterized based on five parameters; polarity, discharge duration, pulse interval, peak current, and gap voltage. The results show that the discharge duration is the most significant parameter to influence all responses, followed by peak current. The surface characteristics of the EDC substrate is depending on the crater formations and is highly correlated with the discharge energy intensity. As a result, appropriate parametric conditions of the electrical discharge coating process can enhance the NiTi surface for future medical applications, without compromising the shape memory effect.

Effect of Electrical Discharge Machining on Stress Concentration in Titanium Alloy Holes

Titanium alloys have several advantages, such as a high strength-to-weight ratio. However, the machinability of titanium alloys is not as good as its mechanical properties. Many machining processes have been used to fabricate titanium alloys. Among these machining processes, electrical discharge machining (EDM) has the advantage of processing efficiency. EDM is based on thermoelectric energy between a workpiece and an electrode. A pulse discharge occurs in a small gap between the workpiece and electrode. Then, the material from the workpiece is removed through melting and vaporization. However, defects such as cracks and notches are often detected at the boundary of holes fabricated using EDM and the irregular profile of EDM holes reduces product quality. In this study, an innovative method was proposed to estimate the effect of EDM parameters on the surface quality of the holes. The method combining the finite element method and image processing can rapidly evaluate the stress concentration factor of a workpiece. The stress concentration factor was assumed as an index of EDM process performance for estimating the surface quality of EDM holes. In EDM manufacturing processes, Ti-6Al-4V was used as an experimental material and, as process parameters, pulse current and pulse on-time were taken into account. The results showed that finite element simulations can effectively analyze stress concentration in EDM holes. Using high energy during EDM leads to poor hole quality, and the stress concentration factor of a workpiece is correlated to hole quality. The maximum stress concentration factor for an EDM hole was more than four times that for the same diameter of the undamaged hole.

Growth and potential damage of human bone-derived cells cultured on fresh and aged C60/Ti films

Thin films of binary C60/Ti composites, with various concentrations of Ti ranging from ~ 25% to ~ 70%, were deposited on microscopic glass coverslips and were tested for their potential use in bone tissue engineering as substrates for the adhesion and growth of bone cells. The novelty of this approach lies in the combination of Ti atoms (i.e., widely used biocompatible material for the construction of stomatological and orthopedic implants) with atoms of fullerene C60, which can act as very efficient radical scavengers. However, fullerenes and their derivatives are able to generate harmful reactive oxygen species and to have cytotoxic effects. In order to stabilize C60 molecules and to prevent their possible cytotoxic effects, deposition in the compact form of Ti/C60 composites (with various Ti concentrations) was chosen. The reactivity of C60/Ti composites may change in time due to the physicochemical changes of molecules in an air atmosphere. In this study, we therefore tested the dependence between the age of C60/Ti films (from one week to one year) and the adhesion, morphology, proliferation, viability, metabolic activity and potential DNA damage to human osteosarcoma cells (lines MG-63 and U-2 OS). After 7 days of cultivation, we did not observe any negative influence of fresh or aged C60/Ti layers on cell behavior, including the DNA damage response. The presence of Ti atoms resulted in improved properties of the C60 layers, which became more suitable for cell cultivation.

Research of biocompatibility on bioactive films fabricated using oxygen plasma immersion ion implantation

In the present study, a nanoporous, biocompatible titanium dioxide (TiO2) film was formed on the surface of titanium, using oxygen plasma immersion ion implantation (OPIII), and the influence of this film on the bio-functionalization, including the proliferation and differentiation properties of MG-63 osteoblast-like cells, was analyzed and investigated. The OPIII-treated surface was characterized by X-ray photoelectron, which showed that a TiOx layer was formed on Ti substrates. This TiOx surface exhibited nanoscale surface roughness in the form of nanoporous structures. The results also revealed that MG-63 cells expressed increased proliferation on the OPIII-treated surface as compared with the untreated Ti substrate. The Ti specimens treated with plasma energy of 1 kW revealed better expression of alkaline phosphatase (ALP) activity and showed higher average surface roughness than untreated specimens. Thus, it can be concluded that bioactivity of Ti implants can potentially be improved by OPIII.

Structural and biocompatible characterization of TiC/a:C nanocomposite thin films

In this work, sputtered TiC/amorphous C thin films have been developed in order to be applied as potential barrier coating for interfering of Ti ions from pure Ti or Ti alloy implants. Our experiments were based on magnetron sputtering method, because the vacuum deposition provides great flexibility for manipulating material chemistry and structure, leading to films and coatings with special properties. The films have been deposited on silicon (001) substrates with 300 nm thick oxidized silicon sublayer at 200 °C deposition temperature as model substrate. Transmission electron microscopy has been used for structural investigations. Thin films consisted of ~20 nm TiC columnar crystals embedded by 5 nm thin amorphous carbon matrix. MG63 osteoblast cells have been applied for in vitro study of TiC nanocomposites. The cell culture tests give strong evidence of thin films biocompatibility.