CHAPTER 5 WORKPIECE MATERIALS AND PARAMETERS FOR EXPERIMENT

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1 71 CHAPTER 5 WORKPIECE MATERIALS AND PARAMETERS FOR EXPERIMENT 5.1 INTRODUCTION The workpiece materials for the experimental study are selected on the basis such that mathematical equations arrived at as a result of this study can bring out a common equation for all the levels of hardness of the workpiece material. Hence we have selected the workpiece material SKD11 for the experimental study which is a die or tool steel with medium hardness. Next, we have considered titanium alloy which is considered as a high hardness material and then copper material which is a low hardness material. For the case study mild steel is used as the workpiece material since it is mostly machined material on the WEDM. 5.2 WORKPIECE MATERIALS In WEDM, materials with high strength, high bending, high stiffness and low thermal expansion properties can be easily machined compared with conventional machines. Materials like SKD11, copper, mild steel and titanium alloy have these properties, and these are the material highly machined in this WEDM. For this reason we have considered the workpiece materials for the experimentation are SKD11, copper, mild steel and titanium alloy.

2 SKD 11 SKD11 is a die steel also known as DC11 or SLD high wear resistant chromium cold work tool steel. This is obtained by adding Cr, Mo and V to SK materials. SKD11 is used very frequently in press moulds. These materials are said to be the mainstream materials for punches and dies. These materials are used in the moulds for medium to large production volumes. The deformation due to heat treatment is smaller during wire cutting electric discharge machining and ease of machining is also a reason why these materials have become the mainstream materials for moulds. Since the deformation during wire cut electric discharge machining is still smaller when high temperature tempering is done at about 500 to 550 o C rather than the normal tempering about 180 to 200 o C, high temperature tempering has come to be used more often. The material properties and the mechanical properties of the workpiece SKD11 is as listed in the Tables 5.1 and Table 5.2 respectively provided from Table 5.1 Material properties of SKD11 Mo: 0.40 ~ 1.00 V: 0.15 ~ 0.35 Cr: ~ C: 1.45 ~ 1.70 Si: 0.4 Mn: 0.45 S: 0.01 P: Ni: To allow the residual content of 0.25 Cu: To allow the residual content of 0.30

3 73 Table 5.2 Mechanical properties of SKD11 Hardness :Soft annealing to about HB210 Preheating Temperature: 788 o C Tempering Temperature: 522 o C Hardening Medium : Air Cooling Working Condition: Cold working The advantages of this SKD11 material are it has very good wear resistance; the most suitable for stainless steel or high die hard materials, good toughness, heat treatment deformation is very small, having little small quenching deviation and hence the most suitable for precision mould. Cracks due to hardening by quenching is neglected due to the air cooling. Good machining properties and was vacuum degassing refining, so the quality is very clean inside. Since, this steel is easy, turning it is applied in the manufacturing of steel forming rollers, special moulding roller, precision rules and shape complexity of the cold tools, tin for mould, plastic mould etc Copper Copper is widely used where high electrical or thermal conductivity is required. Pure copper is defined as having a minimum copper content of 99.3%. Copper with oxygen content below 10 ppm is called oxygen free. The use of unalloyed copper is often limited by its low strength. Copper can be strengthened by various processes, for example, cold working, grain refinement, solid solution hardening, precipitation hardening, dispersion strengthening, etc. While these approaches can significantly increase the strength, they can also lead to a pronounced reduction in conductivity. The

4 74 challenge is to design a material with the best combination of strength and conductivity. Cold work can significantly increase the strength of pure copper and has a relatively moderate effect on conductivity. However, cold-worked copper can be softened at relatively low temperatures (-200 o C) because of its low crystallisation temperature. The physical properties of copper is given in the Table 5.3. A recent study has shown that ultra high-strength and highconductivity copper can be produced by introducing a high density of nano scale twin boundaries. The tensile strength of the nano-grained copper can be increased by a factor of 10 compared to conventional coarse-grained copper, while retaining a comparable conductivity. The potential of high strength, high-conductivity bulk nano-grained copper in nuclear energy systems, however, has not been widely explored. Alloying with copper can significantly improve mechanical strength and raise the softening temperatures. However, additions of alloying elements also reduce electrical and thermal conductivity. Among the three alloying strengthening mechanisms, namely, solid solution hardening, precipitation hardening, and dispersion strengthening, solid solution hardening have the most detrimental effects on the conductivity and is the least favoured mechanism to obtain high conductivity, high-strength copper alloys. Table 5.3 Physical properties of copper Melting point ( o C) 1083 Density (g /cm 3 ) 8.95 Thermal conductivity (W m-k -1 ) 391 Elastic modulus (GPa) 117

5 Mild Steel Mild steel, also known as plain-carbon steel, is the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications, more so than iron. The chemical composition of mild steel is given in the Table 5.4. Low-carbon steel contains approximately % carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburising. It is often used when large quantities of steel are needed, for example as structural steel. The density of mild steel is approximately 7.85 g/cm 3 (7850 kg/m 3 or lb/in 3 ) and the Young's modulus is 210 GPa (30,000,000 psi). The mechanical properties of the mild steel are given in the Table 5.5. Low-carbon steels suffer from yield-point run out where the material has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. If low-carbon steel is only stressed to some point between the upper and lower yield point, then the surface may develop slip bands. Low-carbon steels contain less carbon than other steels and are easier to cold-form, making them easier to handle. General purpose steel bars are suitable for machining, lightly stressed components, including studs, bolts, gears and shafts and they are often specified where weldability is a requirement. Wear resistance can be improved by casehardening. These materials are available in bright rounds, squares, flats and hot rolled rounds.

6 76 Table 5.4 Chemical composition of mild steel Carbon 0.16 to 0.18 % Silicon 0.40% max Manganese 0.70 to 0.90 % Sulphur 0.04% max. Phosphorus 0.04% max. Table 5.5 Mechanical Properties of mild steel in cold drawn condition Max Stress 400 to 560 N/mm 2 Yield Stress % Proof Stress Elongation 10 14% Min Black mild steel is produced by a hot rolling process, and may have a scaly, rough surface. It is not precise in its dimensions, straightness or flatness. Suitable machining allowances should therefore be added when ordering. It does not contain any additions for enhancing mechanical or machining properties. Bright drawn mild steel is an improved quality material, free of scale, and has been cold worked (drawn or rolled) to size. It is produced to close dimensional tolerances. Straightness and flatness are better than black steel. It is more suitable for repetition precision machining. Bright drawn steel has more consistent hardness, and increased tensile strength. Bright steel can also be obtained in precision turned or ground form if desired. Bright steel is the most common form of steel, it is not brittle and it is cheap. It is often used when large amounts of steel are needed. Mild steel is

7 77 a carbon steel having its composition with a maximum of 0.25% carbon and 0.4%-0.7% manganese, 0.1%-0.5% silicon and some + traces of other elements such as phosphorous, it may also contain lead (free cutting mild steel) or sulphur (again free cutting steel called re-sulphurised mild steel). Many everyday objects are made of mild steel, even some of common utensils are in mild steel (a so-called carbon steel). Mild steel is a general term for a range of low carbon (a maximum of about 0.3%) steels that have good strength and can be bent, worked or can be welded into an endless variety of shapes for uses from vehicles (like cars and ships) to building materials Titanium Alloy Ti-6Al-4V titanium alloy is a very strong and light metal which is stronger than aluminium but as strong as steel. It is 45% lighter than steel and 60% heavier than aluminium. The chemical composition and mechanical properties of the titanium alloy is given in the Tables 5.6 and 5.7 respectively. Their applications to automobile industry have been limited except for racing cars and special purpose cars because of their high cost despite the strong interest shown in titanium materials by industry in the terms of light weight, fuel efficiency and performances. Titanium alloy also has increasing use in many industrial and commercial applications due to its outstanding corrosion resistance, fatigue resistance and sufficient corrosion resistance in many environments especially in high strength applications. Titanium alloys are nowadays increasingly being used in aerospace, nuclear, chemical machine building, ship building, oil and gas industry, food industry, medicine and automobile industries because of its attractive mechanical properties and corrosive resistance. Density is 4.43 g/cc.

8 78 Table 5.6 Chemical composition of Ti-6Al-4V Al 6 % Fe 0.25 % Max. O 0.2 % Max. Ti 90% V 4% Table 5.7 Mechanical Properties of Ti-6Al-4V Brinell Hardness Ultimate tensile strength 950 MPa psi Yield tensile strength 880 MPa psi Modulus of Elasticity GPa ksi Shear strength 550 MPa psi 5.3 WORKPIECE SETTING PARAMETERS Workpiece or job setting on the WEDM is a time consuming process, and it is done by trial and error only, even by a skilled operator, and based on the skill and experience, the job setting time may vary. An operator has to set various process parameters like pulse on time, pulse off time, peak current, servo voltage, wire feed, wire tension, cutting speed, spark gap, spark cycle, spark energy, open voltage and flow rate as given in the Table 5.8.

9 79 Table 5.8 Input process parameters S.No Process Parameters Symbol Steps 1. Pulse ON Time ONT 5 ~ 99: Pulse OFF Time OFT 5 ~ 99: Power Energy Selection VS 1 ~ 3: 3 4. Working Current Range IP 0 ~ Working Gap Range VG 0 ~ Feed Rate F 0 ~ 99: Rapid Capacitor C 0 ~ 6: Spindle Feed S 0 ~ 6: Electrode Tube Diameter ED 0 ~ Flushing Pressure P 1 ~ 3: Stable SB 0 ~ 6: 16 Since the setup work of the workpiece is done on the trial and error basis, there may be wastage of wire electrode and workpiece material is addition to the high setting time of the workpiece. Hence, in this research work, the thickness of the workpiece, wire electrode wear and time taken for the machining are considered as job setting parameters and is shown in the Table 5.9. Table 5.9 Workpiece setting parameters S.No Workpiece setting parameters Symbol 1 Electrode Wear W 2 Time taken for machining Ti 3 Workpiece Thickness T

10 Workpiece Thickness Most researchers have also communicated that specific physical, metallurgical, and electrical properties of the workpiece material also influence the process. These properties include how well the metal is polished, its magnetic condition, and how the metal was removed from the heat treatment process when it was produced. One must also consider the phenomenon of expansion and contraction, according to the temperature of the material. For material processed by WEDM, the initial surface condition affects the results. A low melting point in the material increases the material removal rate and improper heat treatment of the metal results in distortion, breakage of the die and punches while machining by WEDM. As, for example, Kim and Jeong (2012) carried out WEDM tests on various cemented carbides with different percentage of cobalt present in WC [GT 10 (6% Co), GT 20 (12% Co), GT 30 (15% Co)] and found out that the percentage of cobalt has an influence on the speed of erosion. A high co-content worsens the final surface quality as a greater quantity of solidified metal deposits on the eroded surface. In the WEDM process, cutting speed decreases as the thickness of the workpiece increases. Normally, WEDM uses a transistor controlled capacitor circuit in which the cutting speed is controlled by a capacitor value. When using a fixed capacitor to machine a thicker workpiece, the cutting speed is decreased. Hatchek (2010) reported that the thicker the workpiece, the faster is the cut, all other factors being equal. In any electric discharge machining operation, every pulse does not produce a spark. However, the longer length of wire electrode in a thicker workpiece provides more opportunities for the spark to occur. This makes the process more efficient for a thicker workpiece.

11 81 Moreover, the thickness of the workpiece is the prime and the main factor for setting the job in the wire cut electric discharge machine for machining. In a practical scenario, first the operator measures the thickness of the workpiece and based on the thickness he sets the profile cutting axis in the wire cut electric discharge machine. The first important point in setting the job or the workpiece is the thickness of the workpiece. Hence, we have considered the thickness of the workpiece as the first and main input parameter Electrode Wear In WEDM, a specific wire run-off speed is applied to compensate wear and avoid wire breakage Jennes et al (1984). Since the workpiece generally stays stationary and short discharge durations are applied, the relative displacement between wire and workpiece during one single discharge is very small. Among different process parameters, voltage and flush rate were kept constant. Parameters such as bed speed, current, pulse-on and pulse-off was varied. By measuring the diameter of wire at various positions along the length of the wire, wear was calculated. Brass wire having a diameter of 0.25 mm was used as a wire electrode. Electrode wear can be measured in many ways. First, electrode wear can be measured using the universal measuring machine. Second, electrode wear can be measured by the difference between the weight of the wire electrode before and after machining. Third, electrode wear can be measured by measuring the wire thickness randomly by using vernier caliper. The selection of wire thickness, setting of the wire feed and wire tension and a selection of wire material plays a vital role in this process, which also decides the machining time and the quality of the machining. Hence it is considered as a job setting factor in our research.

12 Time Taken for Machining Time taken for machining in the WEDM, which includes the tooling time is high comparatively. But the performance and surface finish of the job was superior when compared to the other types of production like milling cutter and slotting saw. Thus the machining results attract the manufacturers to use WEDM for production of precision jobs, even though the time taken for machining is high. Hence, this is an important factor for our research and this is considered as one of the input parameter next to workpiece thickness and wire electrode wear. 5.4 SUMMARY In this chapter, consideration of the workpiece materials with its properties and various process parameters considered for the research work is provided.