Rates of metal removal

Many factors other than current influence the rate of machining. These involve electrolyte type, rate of electrolyte flow, and other process conditions. For example, nickel machines at 100% current efficiency, defined as the percentage ratio of the experimental to theoretical rates of metal removal, at low current densities, eg, 25 A/cm. If the current density is increased to 250 A/cm the efficiency is reduced typically to 85—90%, by the onset of other reactions at the anode. Oxygen gas evolution becomes increasingly preferred as the current density is increased. 

Surface Finish. As well as influencing the rate of metal removal, electrolytes also affect the quality of surface finish obtained in ECM. Depending on the metal being machined, some electrolytes leave an etched finish. This finish results from the nonspecular reflection of light from crystal faces electrochemicaHy dissolved at different rates. Sodium chloride electrolyte tends to produce a kind of etched, matte finish when used for steels and nickel aHoys. A typical surface roughness average, Ra is about 1 ]lni. 

An attraction of the ECAM technique is the very fast rates of metal removal attainable by the combined effects of sparking and ECM. Eor example, in comparison to hole drilling rates for EDM and ECM, respectively 0.1 and 5.0 mm/min, rates of 15-40 mm/min may be achieved by ECAM. The ECAM technique can be appHed in all the ways discussed for ECM, thus surfaces can be smoothed and ddUed. Turning is also possible, as is wire machining (17). 

Eor drilling, the discharge action occurs at the leading edge of the tool, whereas ECM takes place on the side walls between the tool and the workpiece. The combined spark erosion and ECM action yields fast rates of metal removal. Because ECM is stiU possible, any metallurgical damage to the components caused by the sparking action can be removed by a short period (eg, 15 s) of ECM after the main ECAM action. Currents of 250 A at 30 V are typically used in the process. 

So far, we have been talking in our case study about the advantage of an oxide layer in reducing the rate of metal removal by oxidation. Oxide films do, however, have some disadvantages. 

A third important factor in the economies of machining is the material of the cutting tool. This largely determines the rates of metal removal, the standards of surface finish and the frequency at which the tool needs to be reground - all of which are interrelated. These can be broadly grouped in three categories, each separated by a factor of 10 in terms of performance. 

A reference vanadium deposition experiment is carried out in order to assess the influence of quinoline and HjS. Quinoline showed to decrease the rate of metal removal, the amount of vanadium deposited is lower as compared to the reference experiment. The shape of the vanadium deposition profiles is similar in both cases. A deposition maximum is observed in the centre of the pellet, indicating that the vanadium deposition process is not diffusion limited and that a sequential reaction mechanism applies for VO-TPP HDM. Low H2S partial pressure resulted in different vanadium deposition profiles as a function of the axial position in the reactor. At the inlet of the reactor, similar shaped profiles as the reference experiment were found, however, at the outlet of the reactor a shift towards M-shaped profiles was found indicating a diffusion limited vanadium deposition proeess. This shift in vanadium deposition profiles is explained by the build-up of the last intermediate resulting in a higher metal deposition rate. 

The results of experiment II A and B show an identical course in the vanadium deposition profiles as the reference experiment I. The amounts of vanadium deposited are of course lower which is due to the competitive adsorption of quinoline and the higher liquid feed rate. The decrease in the rate of metal removal caused by quinoline was already found in previous work (8). 

The vanadium deposition process showed profiles with deposition maxima in the centre of the pellet indicating the absence of diffusion limitations and supporting a sequential reaction mechanism for VO-TPP HDM. Quinoline addition showed to have an decreasing effect on the rate of metal removal and showed similar shaped deposition profiles. The low HjS partial pressure caused a change of the vanadium deposition profiles into M-shaped profiles due to the build-up of the last intermediate and an increasing metal deposition rate. 

The equivalent electrical circuit model is perfectly valid at medium range of frequency of pulsed power supply. At high frequencies, the experimental rate of metal removal is less than the theoretical... 

For a fixed gap and applied voltage, the current density does not change much with the diamond concentration particles (Chen and Li I, 2000). Hence, to maintain a constant rate of metal removal, the applied electric field should be lower for a higher diamond concentration tool and vice versa. This electric field concentration effect is greatly reduced when the diamond particle is half exposed (Chen and Li II, 2000). This effect sharply decreases from its highest value near the diamond-metal boundary to a... 

Electrochemical machining is a recent innovation, the practice dating back less than twenty-five years. Much of the driving force for its development has come from the aerospace industry with its requirement to machine very hard alloys (e.g. those based on Ti and Fe/Co/Ni/Cr) to produce components able to perform a function reUably but also having minimum weight this specification often leads to components of very complex shape. The alloys which must be employed cause problems in conventional machining because of the low rate of metal removal and the short tool life. Electrochemical machining is, however, based on different properties of the metal independent of its hardness and it is only necessary to find an electrolyte where the alloy will dissolve anodically without passivation when the rate of metal removal may be estimated from Faraday s law. 

An acceptable rate of metal removal is obtained by designing the cell with a high... 

The rate of metal removal in the porous oxide sorbents can be described with a film transfer process and either surface and/or pore diffusion models. To simplify the mass transfer of adsorbate from bulk solution to the adsorbent surface, some studies assume a linear concentration gradient existing in a hypothetical film surrounding the adsorbent particle [25]. When film transfer limits the rate, which, for example, is likely with nonporous particles, the following equation can be used to simulate the film transfer in a batch reactor ... 

Observations relevant to ECM can now be made. Firstly, the rate of metal removal from the anode is not affected by the hardness or other mechanical properties of that electrode. Secondly, the shape of the cathode remains unchanged during the electrolysis, since gas evolution is the only reaction that occurs there. 

The optimization of metal electrodeposition in terms of reaction rate of metal removal is obtained under conditions of limiting current (iL). Hence, according to Eq. 1, the best values of 1l are obtained maximizing the mass transfer coefficient (km) and the electrode area (A) [16, 24]. These are the main reasons why three-dimensional electrodes are recommended for cathodic reduction of metals from dilute solutions, since electrode area is enhanced due to the use of the electrode volume kn, is improved by the hydrodynamic turbulence provided by the three-dimensional matrix [25]. A number of cell designs were developed in order to obtain high km, many of them mentioned in Fig. 1. 

Cu builds up, which results in a declining rate of metal removal and a variable etching profile. 

The effectiveness for metal removal of the catalyst decreases with increasing age owing to the plugging of the pore mouths. The effectiveness factor is defined as the ratio of the rate of metal removal for the aged catalyst to that for fresh catalyst ... 

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