28-11-2012, 12:30 PM
HYBRID MACHINING PROCESS EVALUATION AND DEVELOPMENT
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INTRODUCTION
Today’s manufacturing industry is facing challenges from advanced difficult-to-machine materials (tough super alloys, ceramics, and composites), stringent design requirements (high precision, complex shapes, and high surface quality), and machining costs.
Advanced materials play an increasingly important role in modern manufacturing industries, especially, in aircraft, automobile, tool, die and mold making industries. The greatly-improved thermal, chemical, and mechanical properties of the material (such as improved strength, heat resistance, wear resistance, and corrosion resistance), while having yielded enormous economic benefits to manufacturing industries through improved product performance and product design, are making traditional machining processes unable to machine them or unable to machine them economically. This is because traditional machining is most often based on removing material using tools harder than the workpieces. For example, polycrystalline diamond (PCD), which is almost as hard as natural diamond, cannot be effectively machined by traditional machining process. One of the most commonly used conventional techniques is diamond grinding. In order to remove the material from a PCD blank, the diamond layer of the grinding wheel must be renewed by truing or dressing operations resulting in rapid wear of the wheel, because the G-ratio (ratio of workpiece volume removal rate to grinding wheel volume wear rate) is 0.005 to 0.02. Thus, the grinding wheel wear rate is 50 to 200 times higher than the workpiece removal rate. Hence, classical grinding is suitable only to a limited extent for production of PCD profile tools. The high costs associated with machining ceramics and composites, and damage generated during machining are major impediments to the implementation of these materials. For example, the costs of machining structural ceramics (such as silicon nitride) often exceed 50% of the total production costs in the engine industry. In some cases, current machining methods cannot be used and innovative techniques or modifications of existing methods are needed.
A METHODOLOGY FOR HYBRID MACHINING PROCESSES EVALUATION
Many processes such as plastic flow, mechanical abrasion, heating, melting, evaporation, and dissolution, and others, change both physico-chemical conditions of the above mentioned processes and workpiece material properties. The performance characteristics of hybrid machining processes must be considerably different from those that are characteristic for the “component” processes, when performed separately. For example, productivity of electro discharge electrochemical hybrid machining (EDCM), which consists of making use of electrical discharges in electrolyte for metal removal, is 5 to 50 times greater than productivity using individual processes of ECM or EDM [2, 3].
There are generally two categories of HMPS
Processes in which all constituent processes are directly involved in the material removal,
Processes in which only one of the participating processes directly removes the material while the others only assist in removal by changing the conditions of machining in a “positive” direction from the point of view of improving capabilities of machining.
In both of these categories thermal, chemical, electro-chemical and mechanical interactions occur. A brief description of these interactions is given below.
Thermal Interaction
Laser beam treatment (LBT), welding (LBW) and machining (LBM), electron beam machining(EBM), electrical discharge machining (EDM), and plasma beam machining (PBM) are thermal processes where material is removed through a phase change, either by melting or vaporization. Additionally, many secondary phenomena relating to surface quality which occur during machining, such as micro-cracking, formation of heat-affected zone and formation of striations, can also be related to the thermal effect of the laser or electron beam or electrical discharges.
If we the distribution of the power density of a heat source on a treated surface as result of electrical discharge, laser, plasma or electron beam action is known, the temperature field can be determined and finally it is possible to estimate the critical power density needed to reach a given temperature at a given point or in a given point in a given volume of the material in a given time interval tI (pulse duration). For instance, the critical power density required to reach on the surface of the melting temperature Tm or the boiling temperature Tb under normal pressure at the workpiece surface can be determined [4].
Chemical and electro-chemical interaction
Chemical milling (CM), etching (E), electrochemical machining (ECM), pulse electrochemical machining (PECM), electropolishig (EP) areshaping and finishing processes based on application of dissolution of material.
Chemical milling is the selective and controlled metal removal by chemical action. It is especially useful for removing metal from sheet components to reduce weight, and it can be employed after parts have been formed and heat-treated. Any metal that can be chemically dissolved in solution can be chemically milled.
Electrochemical shaping and finishing is based on controlled anodic electrochemical dissolution process of the workpiece (anode) with a cathode tool in an electrolytic cell. Being a non-mechanical metal removal process, ECM is capable of machining any electrically conductive material with high stock removal rates regardless of their mechanical properties, such as hardness, elasticity and brittleness.
Shaping by electrochemical dissolution is described by distribution of the dissolution velocity, Vn, on the surface of anode-workpiece,
Electrical Discharge Machining with Ultrasonic Assistance (EDMUS)
In ultrasonically assisted EDM, it is recognised that the role of the acoustic wave and cavitation phenomena is to improve the flushing and material removal from the surface craters.These process conditions are significant for micro drilling and production of slots and grooves.
The vibrating movement of the tool electrode or the workpiece, improves the slurry circulation and the pumping action, by pushing the debris away and sucking new fresh dielectric and which provides ideal condition for discharges, their efficiency and gives higher removal rate.
The second beneficial effect that has been observed concerns structure modifications. The alternate motion of the tool electrode/workpiece with a high frequency due to ultrasonic motion, creates more turbulence and cavitation, and therefore results in a better ejection of the molten metal from craters. This of course increases the removal rate, but also lets less liquid material recast on the surface. Thus, structure modifications are minimized, less micro-cracks are observed, and fatigue life is increased [26].