Process Optimization in Non-Conventional Processes: Experimentation With Plasma Arc Cutting

Introduction

The quality and quantity requirements of end products have become increasingly complex as customers expectation requires enhanced performance across a variety of diverse and changing system operating conditions. Systems are now being reconfigured to have capacity and capability in order to meet complex objectives and function effectively in changing operating conditions with adaptability to deliver value in dynamic market conditions. Reconfigurable systems are designed so as to maintain a high level of performance by altering their configuration to cater to the multiple function requirements or a change in operating conditions within acceptable reconfiguration time, cost and quality.

Plasma Arc Cutting (PAC) is an important non-conventional machining process employing thermal cutting technique. It has been successfully used in the cutting of stainless steel, high hardness metals, high melting point metals, and other difficult to machine alloys.

The plasma arc cutting process has always been seen as an alternative to the oxy-fuel process. It grew out of plasma welding in the 1960s, and emerged as a very productive way to cut sheet metal and plate in the 1980s. Early plasma cutters were large, slow and expensive, therefore, tended to be dedicated to repeating cutting patterns in a mass manufacturing situation. CNC (computer numerical control) technology was integrated to plasma cutting machines in the late 1980s into the 1990s, giving plasma cutting machines greater flexibility to cut diverse shapes based on tailor made programs programmed into the machine's numerical control. These CNC plasma cutting machines were generally limited to cutting patterns and parts in flat sheets of work-piece using only two axes of motion, i.e. X and Y.

As illustrated in Figure 1, the basic principle is that the arc formed between the electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases the temperature and velocity of the plasma emanating from the nozzle. The temperature of the plasma is in excess of 20000°C and the velocity can approach the speed of sound. When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed in the efflux plasma through the material and molten material is removed in the efflux plasma. Inside the nozzle, primarily a pilot arc occurs between the negative electrode (cathode) and the workpiece. The energy of this arc leads to ionization of the cutting gas. Then the gas’s enthalpy rises abruptly and it turns into plasma state. A mixture of plasma and heated gas is then forced to flow from the internal geometry of the torch to a narrow bore, where it gains momentum and is superheated due to ohmic heating. Outside the nozzle, the supersonic jet plume strikes to the work-piece, quickly melting the metal and expelling the molten material through the kerf. A secondary (shield) gas assists the process, covering the cutting zone, helping the cooling of the torch, and constricting the plasma jet (Bini et al. 2008).

Figure 1.

Schematic representation of plasma arc cutting process

Literature Review

Some of the important areas of research in the field of PAC are heat affected zone (HAZ) (Yang 2001, Vasil’ev 2003, Gullu and Atici 2006, Zajac and Pfeifer 2006, Kadirgama et al. 2010, Ismail and Taha 2011), depth of cut (Gane et al. 1994, Xu et al. 2002, Gariboldi and Previtali 2004, Yang 2007, Bahram. 2009) and kerf (Ramakrishnan et al. 2000, Teulet et al. 2006, Bini et al. 2007, Wang et al. 2011). Some researchers have also studied surface roughness characteristics, MRR, conicity etc. in PAC.