MACHINING CHARACTERISTICS OF AUSTEMPERED DUCTILE IRON

Transcription

1 MACHINING CHARACTERISTICS OF AUSTEMPERED DUCTILE IRON Ashwin Polishetty, Sarat Singamneni, Guy Littlefair Abstract Austempered Ductile Iron (ADI) is a Spheroidal Graphite Iron (SGI) modified by a specialised heat treatment processes austenitising and austempering. The material produced is versatile and has application bases in many different areas of engineering. In the post heat-treated conditions, ADI does not exhibit good machinability because of its strain and temperature induced microstructural phase transformation. The work hardening phenomenon stems from the change in the microstructure, from a matrix of acicular ferrite to martensite. Hence, a suitable solution is required, which is robust and economically viable to machine ADI. The experimental design consists of conducting drilling trials on ADI grade900, Grade 1050 and Grade1200. Drilling trials were conducted at combinations of two different speeds (rpm) of 697 and 929; feed rates (mm/rev) of 0.1 and 0.2. To check the effect of coolant on drilling, the cutting operations were conducted under dry and wet conditions. A comparative analysis was performed based on the results obtained from microhardness, bulk hardness and surface roughness (Ra) tests, between the four grades of ADI. Cutting force analysis was conducted on the four grades of ADI to understand the effect of varying cutting speed and feed rate on the cutting force. The present research on machining ADI is an effort to bridge the gap in machining technology and popularise the material among the industrial community by exploring its machining characteristics. As the research is in its early stage, the primary factors under considerations are cutting force analysis, hardness changes and studying the phase transformations along the length of the cut. Keywords -ADI, Machinability and Phase transformations

2 1. INTRODUCTION Machining plays an important role in product realisation from the design to finish level and in delivering a product, which is defects free, has dimensional accuracy, good surface finish and appeal. Research efforts have provided machinist with innovative techniques to machine any hardest possible material with ease, safety and productivity. Some of the results focus on machining materials at micro-level. Recently, an article in ASME magazine, by Alan S. Brown at University of Illinois, Urbana-Champaign explains issues such as manufacturing submillimeter parts with micrometer-scale features. The potential in solving such issues are highly valuable and bring about drastic changes in the field of manufacturing technology [1]. With the introduction of new materials, there arises a need to machine these materials effectively and efficiently. The success of using these new materials in design application is related to the science of machining these materials. 2. WHAT IS ADI? Austempered Ductile Iron (ADI) belongs to the family of nodularised ductile cast irons. Recent trends in the heat treatment technology have given rise to this new form of lightweight iron known as ADI. ADI is a modified Spheroidal Graphite Iron (SGI) produced by a two-staged heat treatment process of austenitising and austempering. A. Composition of ADI The current research programme being undertaken at AUT comprises of a number of companies and foundries and the precise composition and heat treatment process is commercially sensitive. However, generally ADI consists of nodule forming agents such as magnesium and beryllium. Silicon is present in quantities of 1-3% and copper in the range of 0.8% increases hardenability. Manganese acts as carbide stabilizer and maximum percentage preferred is 5%. ADI contains small quantities of Sulphur (0.12%) and nickel (2%). Molybdenum, vanadium, tungsten are also added for increased stability and toughness at high temperatures during cutting.

3 3. WHY MACHINE AUSTEMPERED DUCTILE IRON (ADI)? As mentioned earlier, the versatile nature of ADI is effective unless a suitable or viable technique is developed to machine the potential material, ADI. The material looks appealing but machining ADI is problematic. ADI has a unique ausferrite microstructure, which consists of ferrite needles against austenite background and spheroidal graphite nodules. The unique microstructure is advantageous in providing better mechanical properties, when compared to Aluminium and Steel. At the same time, the microstructure is disadvantageous, in case of machining as the austenite present in it undergoes phase transformation to a hard and difficult to machine martensite. The phase transformation can be under the influence of the high cutting forces involved or due to the heat generated during machining. On analysing different theories of metal cutting, high cutting force generates heat and dissipation is done by using cutting fluids[2]. In case of ADI machining, no external coolant is required as the graphite present in it helps in heat dissipation. Generally, use of external cutting fluid is avoided in ADI as the sudden changes in temperatures during machining give rise to pearlitic structure or in worst cases even lead to formation of martensite. The question on machining ADI is under the purview of the author. The hope of finding a solution to the machining problem of ADI lies in carrying out extensive research in the area of machining ADI. The research should find out the effect of varying cutting parameters on machining outputs such as cutting forces, surface finish and tool wear. Cutting parameters include speed, feed, depth of cut and the tool profile- tool geometry and tool material. The solution also lies in establishing a structured method or mathematical formula to calculate the amount of martensite in the microstructure after machining. The research is currently underway at AUT university and the author would be back soon with some interesting results. 4. MACHINABILITY ASSESSMENT The history of machining says numerous techniques have been developed and used to assess machinability. The technique used depends on the researchers view. Some of the views or criterion used to assess machinability; number of parts machined before the tool failure, tool life, surface finish, dimensional accuracy, power consumed, chip flow and heat generation and

4 dissipation. Machinability definitions are connected to the factors effecting machining such as metallurgy, chemistry, heat treatments, additives, inclusions, surface skin, cutting edge, tool holding, operation and machining conditions [3, 4]. Machining is a production process where a harder metal cut a softer metal and the metal removed is in the form of chips (swarf) by a plastic deformation process. The deformation temperature and force will significantly contribute to the quality of the process. Temperature affects the cutting tool material and the forces effect the power and strength needed to perform the process [5]. 5. EXPERIMENTAL SETUP The experimental design allows conducting analysis on ADI Grade 900, Grade 1050, Grade 1200 and Grade 1400 by drilling a set of holes at speeds (rpm) of 697 and 929; feed rates (mm/rev) of 0.1 and 0.2. To check the effect of coolant on drilling, the cutting operations were conducted under dry and wet conditions. The equipment used for the measurement purpose consists of a Kistler cutting force dynamometer 9257B, 16 channel Kistler charged amplifier. The cutting tool used for the purpose is an insert type drill made and supplied by ISCAR (tool holder, DSM A-3D and insert IDK137). The drilling operation was carried on a CNC machine. The representation of holes drilled at combinations of different speeds (rpm) of 697 and 929; feed rates (mm/rev) of 0.1 and 0.2 are shown in Table I. Dynamometer Type 9257B Machine Tool Kistler Charge Amplifier type 5070 Computer with data acquisition card DADiSP Analysis Software Dynaware - cutting force analysis software Fig. 7 Experimental setup for cutting force measurements

5 Table I Hole representation for ADI grade 900, 1050, 1200 and 1400 Hole Material Speed (rpm) Feed (mm/rev) Coolant A1 ADI Grade Yes A9 ADI Grade Yes C1 ADI Grade No C9 ADI Grade No E1 ADI Grade Yes E9 ADI Grade Yes G1 ADI Grade No G9 ADI Grade No J1 ADI Grade Yes J9 ADI Grade Yes L1 ADI Grade No L9 ADI Grade No N1 ADI Grade Yes N9 ADI Grade Yes Q1 ADI Grade No Q9 ADI Grade No 6. RESULTS AND DISCUSSION A. Cutting force analysis The average values of thrust force (Fz) and moment (Mz) for the ADI grades machined at 693rpm and 0.1 mm/rev, is compared and shown in the Table II. A sample graph in fig.8 implies the variations in thrust force, Fz (N) and moment, Mz (Nm) for ADI Grade 900. The following inferences can be drawn from the machinability results. 1. There is no significant variation in the thrust force, Fz (N) on comparison with the ADI grade 900, 1050, 1200 and The variations almost remain constant between the different grades of ADI. 2. The moment, Mz (Nm) is almost constant between the grades 900, 1050, 1200, 1400 of ADI.

6 3. The above two inferences indicate that the thrust force is not influenced by the bulk hardness of ADI. The authors have observed sudden rise in the thrust force during drilling because of the cutting edge encountering hard spots. 4. The conclusions made by previous researchers that ADI microstructure is sensitive to changes in cutting forces and temperatures during machining can be corroborated to some extent. B. Bulk hardness The bulk hardness test conducted on ADI grade 900, grade 1050, grade 1200 and grade 1400 are shown in table III. The test was conducted using a Rockwell hardness testing machine of indentation load 150 kg. The readings were converted into Brinell hardness scale using standard hardness conversion methods. Rockwell hardness type Rc was used for the measurement purpose. Table II Comparison of thrust force and moment between ADI grades ADI 900 ADI 1050 ADI 1200 ADI 1500 Thrust Force (Fz), N Moment (Mz), N-m Fig.8 Comparison of thrust force, Fz (N) and Moment, Mz (Nm) for ADI Grade 900

7 Grade 900 Rc, BHN Table III Rockwell Hardness values for ADI grades Grade 1050 Rc, BHN Grade 1200 Rc, BHN Grade 1400 Rc, BHN Reading1 26, , , , 421 Reading2 26, , , , 432 Reading3 28, , , , 409 Reading4 26, , , , 421 Reading5 27, , , , 421 Reading6 27, , , , 421 Average Reading 26.66, , , , The measured values tally with the values mentioned in ASTM A897 standard. A slight deviation was observed on Grade 900, as it shows a lower value than the value mentioned in ASTM A897 standard. The reason may stem from the conversion from the Rockwell to Brinell scale. The other grades of ADI (1050, 1200 and 1400) are within the range of ASTM A897 standards. The hardness tests results were also confirmed using the information provided by Steele & Lincoln Foundry, Australia (ADI supplier). C. Microhardness test Micro hardness test were conducted on ADI grade 900, grade 1050, grade1200 and grade 1400 using Knoop s indenter. The test time was 15seconds and the load applied was 300grams. The curve in Fig.9 has shown the variations in Knoop s microhardness values under dry and wet machining. The graph infers the microhardness values are higher for dry machining for all the grades of ADI and optimised microstructure of grade 1200 slightly offsets from the pattern of increase from wet to dry machining. The microhardness values are following an increasing trend from grade900 to grade 1050, optimise from grade 1050 to grade 1200 and reduce from Grade 1200 to grade D. Surface roughness test The drilled holes are subjected to surface roughness test in order to derive a relation between the hardness values and the surface texture of hole. The surface roughness tests were conducted using instrument Taylor Hobson s- talysurf form50 and analysed using the taylor hobosn software - µltra. Taking into consideration, the range of surface roughness (Ra) values provided from

8 Fig.9 Knoop s Microhardness values for ADI grades 900, 1050, 1200 and 1400 manual by experiences researchers. For drilling, the Ra value lies between 1.6 to 3.3 µm [6]. The cut-off (Lc) and data length selected for the analysis are 2.5 mm and 12.5 mm. The curve shown in Fig.10 indicates the variations in Ra values for different grades of ADI. The effect of tool retraction on the Ra values has been ignored, as it remains constant for all the holes. The middle section of the curve, which is smooth, infers that the surface roughness for Grade 1050 and grade 1200 are having constant variations (optimised). The Ra value increase on holes drilled under dry conditions for all the grades of ADI. The variations observed in the Ra values tally with those observed in microhardness values. Fig.10 Surface roughness (Ra) Graph for ADI grades 900, 1050, 1200 and 1400

9 Fig.11 ADI Grade1200 after drilling Mag: 400X Fig.12 ADI Grade1200 before drilling Mag: 400X E. Metallographic analysis for martensite formation Metallographic analysis was performed on all the grades of ADI and checked for martensite formation or any microstructural changes. The microstructure observed after drilling was compared against the original microstructure of ADI before drilling. The comparison infers that the chosen speeds and feed rates were ineffective in bring about a microstructural change. The analysis was performed with special emphasis on microstructural observation on the hole boundaries by cross sectioning the hole along the hole length. A sample comparison of the metallography is shown in fig CONCLUSION The drilling trials conducted on ADI grades900, 1050, 1200 and 1400 have yielded the following results in support of machining characteristics of ADI. 1 The cutting force analysis has shown that the cutting force varied according to the hardness of the ADI grades and increased proportionately. The moments remained almost constant for all the grades of ADI. 2 The effect of using cutting fluid on all the grades of ADI can be observed as the holes machined using coolant had a good surface finish when compared to holes machined without coolant.

10 3 The variations observed in surface roughness (Ra) values tally with the microhardness variations indicating a basic principle in machining as the hardness increases, the Ra value increases. 4 The metallography analysis reveals that there was no microstructural changes due to drilling at the chosen speeds, rpm (697 and 929) and feed rates, mm/rev (0.1 and 0.2) 5 The results obtained in each of the test conducted to determine the machinability characteristics of ADI imply that the chosen speeds and federate were not effective in triggering a change in the machining characteristics of ADI. 6 The conclusions made by previous researchers that ADI microstructure is sensitive to changes in cutting forces and temperatures during machining has been true to some extent. 7 Research on a structured method to relate the microstructural change at a distance from the edge of the hole to the cutting force at that particular point using explicit mathematical models and principles of coordinates, is the next task to be taken up by the authors as a part of the long research on machining ADI. ACKNOWLEDGEMENT The authors acknowledge the support of tooling supplier -ISCAR, New Zealand, ADI Supplier Steele & Lincoln Foundry (Vic), Pty. Ltd, Australia and AUT University for the funding and technical support in procurement of equipment required to conduct the experiment such as Kistler cutting force Dynamometer, charge amplifiers and allotment of time at the machining lab for the experiments. REFERENCES 1. Brown, A.S., Big Stage {for a small idea}, in Mechanical Engineering Littlefair, G. Evaluating Cutting Fluid Machinability Performance. in 3rd International conference on Machining & Grinding Cincinatti, Ohio: Society of Manufacturing Engineers. 3. Boothroyd, G., Fundamentals of Metal Machining 1ed. 1965, London: Edward Arnold Ltd Smith, G.T. What is Machinability and how can it be assessed. in Industrial tooling conference Southampton, United Kingdom: Southampton Institute. 5. Sandvik Coromant, Modern Metal Cutting. 1994, Sandviken Sweden: A B Sandviken Coromant. 6. Smith, G.T., Industrial Metrology. 2002: Springer.