Stage 2: Alloying of AISI P20 mold steel with the use of powder metallurgy electrodes of titanium and aluminium has been carried out in a hydrocarbon oil dielectric medium
This section discusses the equipment used in the present study, namely electric discharge machine, precision balance, hydraulic press, profilometer, sample molding press, optical microscope, micro-hardness tester, field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), pin- on-disc setup and reference 600 galvanostat.
3.1.1 Experimental setup for electric discharge alloying
In the present study, electric discharge alloying has been carried out by using an electric discharge machine with facilities such as changing the tool polarity, dielectric medium, and using powder metallurgy tool. For this, a separate working tank has been fabricated.
In electric discharge alloying, both the tool and workpiece are submerged in a dielectric medium, and the shape of the alloyed surface is complementary to the tool geometry.
Figure 3.1 shows the schematic diagram of the experimental setup. The typical components are tool holder, work table, DC power supply, servo control unit, working tank, flushing unit, dielectric reservoir, filter pressure gauge, and high-pressure pump.
For the present study, all the experimental works were performed by using ZNC/ENC35 die-sinking EDM (Made by: Sparkonix). It is shown in Figure 3.2. Pulsed power was supplied to the tool and workpiece electrodes from the DC power supply unit. In electric discharge alloying, the tool, workpiece, and the type of dielectric media play a very important role in the type of alloy formed over the workpiece after the EDA operation.
Figure 3.1 Schematic diagram of the experimental setup
Figure 3.2 Die sinking electric discharge machine used for EDA
A good tool electrode should have properties such as good thermal and electrical conductivity, machinability, should be readily available and inexpensive. In the present study, powder metallurgy tool electrode of Ti and Al has been used for alloying of AISI P20 mold steel. The tool electrode is fed along the Z-axis direction (vertical) into the workpiece by using a servo control feeding arrangement. The tool servo-mechanism is used to maintain a pre-determined working gap between the tool and the workpiece by sensing the gap voltage. Once the gap voltage sensor senses that the gap between the tool and the workpiece is bridged, a signal will be sent to the servo motor, and the tool will reverse its direction. This up and down motion of the tool ensures proper flushing of the unwanted debris, thereby resulting in a uniform sparking phenomenon. To ensure proper flushing, the dielectric ﬂuid is ﬂushed through the spark gap to remove gaseous and solid debris during the alloying process. The dielectric fluid should have high dielectric strength. The types of dielectric media used for the present study are hydrocarbon oil, deionized water, and urea mixed deionized water. Effective filtration of the dielectric fluid is essential.
Figure 3.3 shows the working tank used to perform the electric discharge alloying (EDA) in hydrocarbon oil. It consists of a tool electrode and the workpiece immersed in a dielectric with a provision for flushing the debris particles from the inter-electrode gap.
The flushing pressure is being monitored by a pressure gauge. The dielectric stored in the reservoir is recirculated during the operation with the help of a high-pressure pump after filtration.
Figure 3.3 Working tank for using hydrocarbon oil as dielectric
To investigate the EDA phenomenon in the water-based dielectric, i.e., deionized water or urea mixed deionized water, a separate tank has been fabricated. The EDM machine has an oil level interlock. A minimum level of oil of 10 cm has to be provided to keep the machine running, the height of the tank is maintained in such a way that the oil level interlock is open, and the hydrocarbon oil should not enter and mix with the dielectric liquid in the fabricated tank. Figure 3.4 shows the tool and workpiece arrangement with the fabricated tank. The fabricated tank is of dimension 400 × 350 mm with a height of 230 mm. It is of capacity to hold 30 L of liquid and is made from a tin sheet of 2 mm thick. To prevent the leakage, the joints were sealed at the outer cover. The tank is fitted onto the machine bed. The working fluid is poured into the tank only after ensuring that the tank is leak-proof. During the EDA operation, debris in the inter-electrode gap needs to be removed as the debris can cause arcing during the EDA operation. For this purpose, a pump is provided in the tank to ensure effective flushing of the debris.
Figure 3.4 Fabricated working tank arrangement for using water-based dielectric The controllable input machine parameters of EDA are discharge current, discharge voltage, pulse on-time and off-time, duty cycle, flushing pressure, and polarity. The available ranges of the input parameters are as tabulated in Table 3.1. These parameters have their impact on output parameters such as tool wear, material deposition, hardness of the alloyed surface, surface finish, dimensional accuracy, and overall surface integrity.
Table 3.1 Machine parameters and their available ranges Sl.
Parameter Minimum value Maximum value In steps of 1. Discharge current,
0 49 1
2. Pulse on – time, (µs) 0.5 1050 1 to 99
3. Duty cycle (%) 1 (10%) 9 (90%) 1
4. Discharge voltage, (V)
10 99 1
5. Flushing pressure (lb/inch2)
0 30 1
6. Polarity Normal (Tool as positive terminal) Reverse (Tool as negative terminal)
The main input parameters considered in the present study are discussed below.