CHAPTER 3 EXPERIMENTAL CONDITIONS AND PROCEDURE

Transcription

1 50 CHAPTER 3 EXPERIMENTAL CONDITIONS AND PROCEDURE This study was undertaken to perform milling operations on AISI D2, AISI D3, AISI H13 and AISI P20 steels, using different carbide cutting tool inserts under various speed-feed combinations. The experiment was performed under a constant depth of cut. The cutting environments evaluated in the milling process were: Dry machining, Conventional cooling (Wet) and Cryogenic cooling (LN 2 ). A cryogenic cooling setup was developed for supplying liquid nitrogen at the cutting zone. In the present work, the cutting temperature, cutting force, surface roughness, tool wear, chip shape and chip morphology are considered, for studying the effect of cryogenic cooling. The influence of cryogenic cooling using liquid nitrogen was compared to that of dry and wet machining. This chapter explains experimental procedure, workpiece materials, cutting tool materials and the equipment being used. 3.1 WORK MATERIALS Tool and die steels are high carbon steels, possessing high hardness, strength and wear resistance. Materials having a hardness of over 45 HRC (Rockwell hardness C scale), can be classified as difficult-to-machine materials (Becze and Elbestawi 2002, Poulachon and Moisan, 2000). The four most widely used die steel materials by the tool and mould making industry, have been chosen as work materials for this experimental work. These are High Carbon High Chromium (HC-HCr) type

2 51 AISI D2 die steel, High Carbon High Chromium (HC-HCr) type AISI D3 die steel, hot work type AISI H13 die steel and plastic mold type AISI P20 steel. The workpieces made from these four die steel materials were hardened and tempered using standard heat treatment procedures before experimentation, to bring them on par with the state in which they are actually used in the industry. In this research work, the size of the work materials used was 150mm 100mm 50mm. The work materials were tested for their composition, and the mechanical properties for homogeneity AISI D2 Steel AISI D2 steel is a high-carbon high-chromium tool steel, designated as a group D steel in the AISI classification system, and is the most highly alloyed cold-work tool steel. Chromium, at a nominal concentration of 12%, is the major alloying element, but molybdenum, vanadium, nickel, and manganese may be added in significant amounts. In view of their high carbon and alloy contents, all the D steels are deep hardening. They are hardenable by air cooling from austenitizing temperatures, and they have very low susceptibility to distortion and cracking (ASM Specialty Handbook, 1995, Roberts and Cary, 1980). The D2 type HC-HCr tool steels find wide usage as the raw material for blanking and deep drawing dies, thread rolling & forming dies, burnishing tools, shear and slitter knives. The chemical composition of the AISI D2 steel is given in Table 3.1, and its properties are given in Table 3.2.

3 52 Table 3.1 Composition of the AISI D2 steel Element Amount (weight %) Carbon 1.55 Chromium 11.8 Molybdenum 0.8 Manganese 0.4 Silicon 0.3 Vanadium 0.8 Ferrous Balance Table 3.2 Properties of the AISI D2 steel Material property Value Hardness (HRC) 62 Density ( 10 3 Kg/m 3 ) 7.7 Poisson ratio 0.30 Tensile strength (MPa) 2940 Yield strength (MPa) 2200 Modulus of elasticity (GPa) 210 Thermal conductivity (W/mK) AISI D3 Steel AISI D3 steel is an oil hardening, high-carbon, high-chromium tool steel, designated as a group D steel in the AISI classification system, and is the most highly alloyed cold-work tool steel. This D3 type steel also contains chromium, and is the major alloying element at a nominal concentration of 12%, which does not contain molybdenum, but contains an addition of

4 53 tungsten. The oil hardening D3 steel offers the advantage of a better surface condition, because a combination of lower hardening temperature and liquid quenching resulting in minimum surface decarburization and scaling. It has an excellent abrasion/wear resistance, good dimensional stability and high compressive strength. The high chromium content of these steels provides an appreciable resistance to staining, after the tools are hardened and polished. It is widely used for blanking and forming dies, forming rolls, press tools, punches and bushes. The chemical composition of the AISI D3 steel is given in Table 3.3, and its properties are given in Table 3.4. Table 3.3 Composition of the AISI D3 steel Element Amount (weight %) Carbon 2.1 Chromium 11.5 Manganese 0.4 Silicon 0.3 Nickel 0.3 Vanadium 1.0 Tungsten 1.0 Ferrous Balance Table 3.4 Properties of the AISI D3 steel Material property Value Hardness(HRC) 53 Density ( 10 3 Kg/m 3 ) 7.67 Poisson ratio 0.29 Tensile strength (MPa) 2770 Yield strength (MPa) 1900 Modulus of elasticity (GPa) 194 Thermal conductivity (W/mK) 20.5

5 AISI H13 Steel Tool steels for hot work applications, designated as group H steels in the AISI classification system, have the capacity to resist softening, during long or repeated exposures to high temperatures, needed for hot work or for die-casting other materials. These H type hot work tool steels, fall into three groups, according to the alloying element used to impart high hot hardness: chromium hot work steels, which contain nominally 5% Cr and significant amounts of other elements, including silicon, molybdenum, and vanadium; tungsten hot work steels, and molybdenum hot work steels. The outstanding characteristics of these tool steels are high toughness and shock resistance. They have air hardening capability at relatively low austenitizing temperatures, and exhibit minimum scaling tendency during air cooling. AISI H13 hot work steel is the best material for hot die works of all kinds, particularly dies for the extrusion of aluminium and magnesium, as well as die casting dies, forging dies, mandrels and hot shears, because of its ability to retain dimensional accuracy and surface characteristics at elevated temperatures. This material also possesses high resistance to erosion. The chemical composition of the AISI H13 steel is given in Table 3.5, and its properties are given in Table 3.6. Table 3.5 Composition of the AISI H13 steel Element Amount (weight %) Carbon 0.38 Chromium 5.00 Molybdenum 1.30 Manganese 0.35 Silicon 1.00 Vanadium 1.00 Fe Balance

6 55 Table 3.6 Properties of the AISI H13 steel Material property Value Hardness (HRC) 52 Density ( 10 3 Kg/m 3 ) 7.8 Poisson ratio 0.29 Tensile strength (MPa) 1990 Yield strength (MPa) 1650 Modulus of elasticity (GPa) 203 Thermal conductivity (W/mK) AISI P20 Steel Tool steels for plastic injection molding and die casting applications, designated as group P steels in the AISI classification system, are exposed to less severe wear than metal-working steels, and therefore, have low carbon content. There are three major groups of P type steels: carbon steel grades used for hubbed cavities, carbon steel grades used for machined cavities, and stainless steel grades. Plastic moulds for thermoplastics and thermosets are typically made from cold work tool steel like AISI P20 tool steels. It has properties like excellent machinability and weldability, combined with good corrosion resistance and mechanical strength. AISI P20 steel finds application in injection moulds for thermo-plastics, extrusion dies for thermo-plastics, blow moulds, forming tools, press-brake dies, (possibly flame hardened or nitrided) and structural components. The chemical composition of the AISI P20 steel is given in Table 3.7, and its properties are given in Table 3.8.

7 56 Table 3.7 Composition of the AISI P20 steel Element Amount (weight %) Carbon 0.4 Chromium 1.2 Molybdenum 0.35 Manganese 1.0 Silicon 0.4 Nickel 0.8 sulphur 0.08 Ferrous Balance Table 3.8 Properties of the AISI P20 steel Material property Value Hardness (HRC) 48 Density ( 10 3 Kg/m 3 ) 7.8 Poisson ratio 0.28 Tensile strength (MPa) 1310 Yield strength (MPa) 1172 Modulus of elasticity (GPa) 205 Thermal conductivity (W/mK) CUTTING TOOL INSERTS In this research work, indexable inserts are used as cutting tools. Three different types of carbide cutting tool inserts have been used in the research work. They are: i) CVD TiN coated carbide cutting tool insert ii) PVD TiAlN coated carbide cutting tool insert and iii) Uncoated carbide

8 57 cutting tool insert. CVD TiN coated carbide cutting tools are used for AISI D2 and AISI D3 steels; and PVD TiAlN coated carbide cutting tools are used for AISI H13 steel. The uncoated carbide cutting tool inserts are used for AISI P20 steel. Figure 3.1(a-c) shows the cutting tool inserts are used in this experiment. The specifications of the cutting tool inserts are given in Table 3.9. (a) (b) (c) Figure 3.1 Cutting tool inserts (a) CVD TiN coated carbide insert (b) PVD TiAlN coated carbide insert and (c) Uncoated carbide insert 3.3 TOOL HOLDER The tool holder used for end milling is the WIDIA M680 shoulder end mill. This tool holder is used in the research work for machining all the workpiece materials. Figure 3.2 shows the photographic view of the tool holder. The specification of the tool holder is given below:

9 58 ISO Designation : Shank diameter : 16 mm Overall length : 75 mm Head length : 27 mm Number of inserts : 2 Figure 3.2 Tool holder used in the experiment Table 3.9 Cutting tool inserts specifications Workpiece materials AISI D2 cold work tool steel AISI D3 cold work tool steel AISI H13 hot work tool steel AISI P20 plastic injection mould steel Cutting tool material CVD TiN coated tungsten carbide tool CVD TiN coated tungsten carbide tool PVD TiAlN coated tungsten carbide tool Un coated tungsten carbide tool ISO Designat ion XDHT TN 450 XDHT TN 450 XDHT HX PA 120 XDHT THM Rhombic nose angle ( C) Specifications Insert length (mm) Insert width (mm) Insert Thickness (mm) Nose radius (mm)

10 EXPERIMENTAL CONDITIONS End milling experiments were carried out on the AISI D2, AISI D3, AISI H13 and AISI P20 steels, using various tungsten carbide cutting tool inserts at different speed-feed combinations under dry, wet and cryogenic machining conditions. The experimental cutting conditions used in the milling of the AISI D2, AISI D3, AISI H13 and AISI P20 steels are given in Table Table 3.10 Experimental cutting conditions in the milling of the AISI D2, AISI D3, AISI H13 and AISI P20 steels Cutting parameters Workpiece materials Cutting velocity (m/min) Feed rate (mm/tooth) Depth of cut (mm) Cooling methods AISI D2 cold work tool steel 75, 100 and , and (i) Dry (ii) Wet (iii) LN 2 cooling AISI D3 cold work tool steel 75, 100 and , and (i) Dry (ii) Wet (iii) LN 2 cooling AISI H13 hot work tool steel 75, 100 and , and (i) Dry (ii) Wet (iii) LN 2 cooling AISI P20 plastic injection mould steel 75, 100 and , and (i) Dry (ii) Wet (iii) LN 2 cooling

11 EQUIPMENTS FOR THE EXPERIMENT CNC Vertical Machining Centre End milling experiments were carried out on an ARIX VMC 100 CNC machining centre. The machine specifications are shown in Table Table 3.11 Specifications of the ARIX VMC 100 CNC machining Centre Description 1) Working Range Longitudinal axis (X axis) Cross axis (Y axis) Vertical axis (Z axis) 2) Work spindles Tool Mounting Centre to table 3) Spindle speed and feed rates Work spindle speed, directly programmable Feed rates directly programmable along X, Y and Z axes Rapid traverse a long X, Y and Z axes 4) Electrical Equipment Voltage X,Y,Z servo motors Spindle motor 5) Weights and Space requirements Weight of machine (including vertical milling head, circular table, tool changer, cabin and switch cabinet) Machine dimension Length Width Height Technical Data 1000 mm 500 mm 500 mm ISO mm rpm 4000 mm/min 4000 mm/min 220 / 380 V 750 W 5HP 2700 kg 2700 mm 2500mm 2300 mm

12 Dynamometer During the machining tests, the feed force (F x ), radial force (F y ) and axial force (F z ) were measured using a Kistler type 9257B three component piezo-electric dynamometer, which was connected to the Kistler type 5070A charge amplifiers. The force to be measured is introduced via a top plate (Figure 3.3), and distributed between 3-component force sensors, arranged between the base and the top plates. Each of the sensors has three pairs of quartz plates, one sensitive to pressure in the z direction, and the other two to shear in the x and y directions, respectively. It has high rigidity and hence high natural frequency that enables very small dynamic changes to be measured. To obtain and record the cutting force data, data acquisition software was used. A data acquisition system consists of a personal computer equipped with an analog to digital converter card RS 232(A/D board) and also the Dynoware software (Dynoware type 2825A). Figure 3.3 A three component dynamometer assembly

13 Infrared Pyrometer The cutting temperature is measured in the tool chip interfaces, in different cutting environments, using a non-contact type Infrared pyrometer (accuracy ±1.0 C), which can measure a temperature range of -50 C to C. The cutting temperature is measured by making the Infrared ray from the Infrared pyrometer to impinge exactly on the tool chip interfaces during the machining process, and the maximum temperature attained is recorded. The Infrared pyrometer is calibrated for its measurement Surface Roughness Tester The surface roughness after end milling was measured, using a Taylor- Hobson Surtronic 3+ surface roughness tester. This surface roughness tester combines advanced technology with high precision and its use for the efficient measurement of the surface finish in the inspection room or laboratory. The instrument can be handheld on horizontal, vertical and inclined surfaces, or bench mounted with accessories for batch measurement or laboratory applications. The pick-up holder is mounted on a slide for vertical adjustment, and can also be rotated to different measuring positions, including a right-angled measurement. The surface roughness was measured at three locations on the machined workpiece. The value of the surface roughness is the average of the three points taken for each measurement. Table 3.12 shows the equipment specification. Table 3.12 Specification of the Taylor-Hobson Surface Roughness tester Model Surtronic 3+ Manufacture UK Cut-off length 0.25mm, 0.8mm and 2.5mm Accuracy 0.01 m

14 Toolmaker s Microscope A toolmaker s microscope was used to measure the tool wear after every interval, after every observation point of the run. After different intervals of machining, the conditions of the tools were monitored and average flank wear (VB) was measured. Once the VB reached 0.3 mm, the tool life was considered to be over. The machine specification is listed in Table 3.13 Table 3.13 Specification of the TM-505 Toolmaker s Microscope Model Manufacture Magnification Table travel (X-axis) Table travel (Y-axis) TM-505 Mitutoyo 30x 150 mm 150 mm Scanning Electron Microscope The Scanning Electron Microscope(S-3400N) was used to observe and capture the image of the tool wear and chip formed at high magnification. Table 3.15 Specification of the S-3400N Scanning Electron Microscope 3.0 nm (at 30 kv, secondary electron image, High Vacuum) Resolution 10 nm (at 3 kv, secondary electron image, High Vacuum) 4.0 nm (at 30 kv, backscattered electron image Low Vacuum) Magnification x 5 x 300,000 Accelerating voltage kv Variable pressure range Pa (Through GUI) Specimen size 200 mm diameter Specimen stages X: 80 mm, Y: 40 mm, Z: 5 35 mm, T: , R: 360

15 CUTTING ENVIRONMENTS As suggested by researchers, it is desirable for a number of reasons, to study the environment effects on the cutting process. In this research, the three different environments studied are, dry machining with no coolant, wet machining using soluble oil as coolant, and cryogenic machining using LN 2 as a coolant Dry Machining Dry machining is conducted at atmospheric temperature, without the aid of a coolant to dissipate the heat generated at the tool - chip interface Wet Machining In wet machining, a commercial soluble oil coolant was applied at the cutting zone, using the flood coolant application method available with the milling machine. The emulsion cutting fluid was formulated by mixing the concentrate with water, at a ratio of 1:20 soluble oil. The coolant flow rate was maintained constantly at 0.6 litres/min during the machining process Cryogenic Machining Cryogenic machining involves the application of a cryogenic fluid LN 2, as a coolant in the machining process. The cryogen could be applied as an external spray through a nozzle to perform both conductive and convective cooling of the cutting process, or it could be applied indirectly to cool the cutting tool through conduction alone. The most efficient cooling system will be the one that can apply the LN 2 to the tool chip interface, where the heat generation is large.

16 CRYOGENIC COOLING SYSTEM In this research work, a cryogenic cooling system was developed to supply liquid nitrogen to the tool chip interfaces. The available liquid nitrogen storage container (IBP-TA55, Capacity 51.5 litres) is modified for a pressurized cryogenic coolant flow. The schematic diagram of the cryogenic cooling set-up is shown Figure Figure 3.4 Schematic diagram of the cryogenic cooling set-up Constructional Features of the Cryogenic Cooling System components: The developed cryogenic cooling system consists of the following 1. TA55 Liquid nitrogen container 2. Compressor 3. Drier 4. Pressure regulator 5. Pneumatic Hose

17 66 6. Stainless steel pipes 7. Pressure relief valve 8. Braided Stainless steel hose 9. Nozzle In this cryogenic cooling system, the TA55 liquid nitrogen container of 51.5 litres capacity is used. It is made of aluminium, and the walls of the container are completely sealed, so that the atmospheric temperature does not affect the fluid inside the container. The container is closed with a stainless steel cap at the top. Inlet (Ø 6 mm) and outlet (Ø 4 mm) pipes are inserted in to the container through the cap, which is made of stainless steel. These pipes are used for taking the compressed air into the container and transferring the fluid to the cutting zone. A compressor is placed before the container, which is used to compress the air to the required pressure. A drier is placed after the compressor, which removes the moisture from the compressed air before entering the LN 2 container. This will avoid water contamination inside the container. A pressure relief valve is attached at the inlet pipe by means of a T Joint. A flexible Braided Type stainless steel hose made of very low thermal conductivity is attached to the nozzle at one end and the stainless steel pipe at the other end, in order to supply the liquid nitrogen. The hose is thermally insulated with polymer foam, in order to prevent the fluid being affected by external heat during the transfer. The nozzle is used to direct the LN 2 at the tool chip interface. The modified TA55 liquid nitrogen container cap is shown in Figures 3.5 and 3.6, respectively.

18 67 Figure 3.5Cryogenic container cap Figure 3.6 Modified TA55 cryogenic container Working Principle of the Developed Cryogenic Cooling System Liquid nitrogen is stored in the cryogenic container (IBP-TA55, Capacity 51.5 litres). The compressed is supplied by an external compressor air at a 3 bar pressure. To maintain the consistency of the compressed air pressure during an experiment, the pressure regulator was used. The compressed air was dried using a drain filter and passes to the cryogenic container through an inlet pipe. This compressed air starts to force the fluid down. The outlet pipe is placed at the bottom of the container. Due to the air

19 68 pressure inside the container, the liquid nitrogen starts rising through the outlet pipe, and it is directed to the tool chip interfaces through a nozzle of 2 mm diameter. The photographic view of the experimental set-up, and the LN2 delivery nozzle is shown in Figures 3.7 and 3.8, respectively. Figure 3.7 Experimental set-up for cryogenic cooling Figure 3.8 LN2 delivery nozzle at the cutting zone