Transcript
Module-3: ADVANCED MATERIAL REMOVAL PROCESSES Lecture No-14 Electrochemical Machining (ECM) Electrochemical machining is a method of removing metal by an electrochemical process. It is a non-traditional machining process belonging to the electrochemical category. It is used for machining extremely hard materials or materials that are difficult to machine using conventional methods. Its use is limited to electrically conductive materials. The process has the capabilities of machining or cutting the intricate contours or cavities in hard steel such as titanium, Hastelloy, Kovar, Inconel, and Carbide. External as well as internal geometries can be machined with an electrochemical machine. ECM is characterized as the opposite of electrochemical or galvanic coating or deposition process. It is sometimes referred to as reverse electroplating since it removes material instead of depositing it. In the year 1833, Faraday established the laws of electrolysis (electroplating). The mechanism in ECM process is similar to electrical discharge machining (EDM) concept-wise, wherein a high current is passed between the tool (cathode) and the workpiece (anode), through a conductive fluid (electrolyte). However, in ECM there is no tool wear. In ECM, the metal removal takes place by electrochemical dissolution of an anodically polarized workpiece. By using the ECM process, very hard metals can be easily shaped electrolytically and being a chemical process, the rate of machining does not depend on the hardness of workpiece. Soft materials can be readily used as tool materials on harder work-pieces in ECM process since the tool doesn’t wear unlike in the case of conventional machining methods. ECM Fundamentals The electrolysis process being the most fundamental activity in ECM, its characteristics are to be well understood before proceeding further into its other process details.
Electrolysis, E as the namee suggests iss a chemicaal phenomennon that occcurs betweenn two co onductors diipped in a suitable s solu ution when eelectric currrent is passeed between tthem. Example E two o copper wiires, dipped in a coppeer sulphate solution aree connectedd to a so ource of dirrect current as shown in i Fig. 3.144.1. This sollution of coopper sulphaate is teermed as thee electrolyte and it has electrical e coonducting prroperty. The entire systeem of ellectrolyte an nd electrodees is called d as the eleectrolytic ceell. As per the polarityy, the ch hemical reacctions occurrring at the anode and ccathode are called as annodic or cathhodic reeactions resp pectively.
Electrolytes E nt from thee metallic cconductors that conducct electricityy. In are differen ellectrolytes, the t current is i carried by y atoms or ggroup of atoms and not by the electtrons. The T atoms have h either lost or gain ned electronns, thereby acquiring eeither positivve or negative charrges and succh atoms are called ions.. The ions thhat carry possitive chargees are atttracted by the t cathode and they move m througgh the electrrolyte in thee direction oof the positive curreent and are referred to as the ‘cat--ions’. The negatively charged ionns get
atttracted to th he positive electrode e i.e.. anode and they are refferred to as tthe ‘anions’. Due to o the potentiial difference applied, th he movemennt of ions is accompanieed by the floow of ellectrons, in the t oppositee sense to the positive cuurrent in thee electrolyte,, outside thee cell, ass shown schematically in n Fig. 3.14.2 2
n the electrop plating process, which iss a very poppular applicaation of electtrolysis, the m metal In co oatings are deposited on n the surface of a cathoodically polaarized metall. An exampple of th he anodic dissolution d operation o iss electro-poolishing. In this polishhing processs, the workpiece w wh hich has irregularities is made as thee anode in thhe electrolytiic cell. The w workpiece gets po olished and irregularities on its surrface are disssolved prefe ferentially soo that affter the process, the item m gets shinin ng effect and becomes flaat. The T ECM and d electro-polishing process are simi lar, such thaat both are aanodic dissollution prrocesses. The rate of metal remo oval obtainned in the electro-polisshing proceess is co onsiderably less than th hat required in the metaal removal pprocesses. Soome observaations reelevant to EC CM are:
At the anode, the metal dissolves electrochemically and its rate of dissolution depends upon number of factors such as the ionic charge, atomic weight, the current and the time of current passage.
The rate of dissolution is not influenced by the hardness of the workpiece material or any other metal characteristics.
At the cathode, only the hydrogen gas is evolved. The electrode shape remains unaltered during the electrolysis process. This is the most relevant feature of ECM being used as a metal shaping process.
Mechanism of Material Removal in ECM The working principle of ECM is schematically shown in Fig. 3.14.3 (a and b), the workpiece and tool are the anode and cathode respectively. In the electrolytic cell a constant potential difference, usually of about 10 V is applied across them. A suitable electrolyte, for example an aqueous sodium chloride (table salt) solution is commonly chosen. In-order to remove the products of machining, the electrolyte is pumped through the gap between the two electrodes. The rate at which metal is then removed from the anode is approximately in inverse proportion to the distance between the electrodes. As the machining proceeds there is a simultaneous movement of the cathode towards the anode. The width of the gap along the electrode length will gradually tend towards a steady-state value. Under such conditions, a shape which is roughly complementary to that of the cathode will be reproduced on the anode. The schematic of “electrochemical machine” is shown in Fig. 3.14.4
Insulation n
Advantages A of o ECM The T major ad dvantages of the ECM prrocess are:
No heeat affected zone z is form med.
Hardeer metals thaan the tool caan be machinned.
The hardness of material m doess not affect th the metal rem moval rate.
Comp plex shapes can c be machined on hardd metals,
No too ol wear occu urs.
Burr-ffree products are obtaineed in this proocess.
There is no tool to o workpiece contact.
There is no cutting forces, theerefore clampping is not rrequired exceept for contrrolled motion of the worrk piece.
The products obtaained are freee from physiical and therrmal strains.
Depen nding on thee materials, high surfacce quality leevel is attainnable (Ra < 0.02 µm))
High dimensional d l accuracies are attainablle
Limitations L of o ECM
The co ost of toolin ng is high.
Energ gy consumpttion is high: Power conssumption is more as thee ECM proccesses operattes at high cu urrent and reelatively low w voltages (55-15V).
The saline s electro olyte poses a risk of coorrosion to the tool, woorkpiece annd the equipm ment.
Since special elecctrodes need to be develooped for eacch product, hhigher produuction nomic viabillity. Dependding on the ccomplexity oof the numbers are requiired for econ mum producct numbers arre decided. material, the optim
The electrode e dessign is comp plex and haas high initiaal cost but hhowever it hhas a long life.
Sharp corners or flat bottomss are not suittable througgh the ECM process, as there is a tendency of th he electrolyte to erode aw way the sharrp profiles.
