Criteria for Machinability Evaluation
of Compacted Graphite Iron Materials
Design and Production Planning Perspective on Cylinder Block Manufacturing
ANDERS BERGLUND
Doctoral thesis KTH Royal Institute of Technology Department of Production Engineering Machine and Process Technology Stockholm, Sweden 2011TRITA‐IIP‐2011‐10 ISSN 1650‐1888 ISBN 978‐91‐7501‐159‐2 KTH Industriell produktion Maskin och processteknologi SE‐100 44 Stockholm, Sverige Akademisk avhandling som med tillstånd av Kungliga Tekniska högskolan framlägges till offentlig granskning för avläggande av teknologie doktorsexamen i industriell produktion fredagen den 2 december 2011 kl 09:00 i sal F3, Kungliga Tekniska högskolan, Lindstedtsvägen 26, Stockholm.
Copyright © Anders Berglund
ABSTRACT
The Swedish truck industry is looking for new material solutions to achieve lighter engines with increased strength to meet customer demands and to fulfil the new regulations for more environmentally friendly trucks. This could be achieved by increasing the peak pressure in the cylinders. Consequently, a more efficient combustion is obtained and the exhaust lowered. This, however, exposes the engine to higher loads and material physical properties must therefore be enhanced. One material that could meet these demands is Compacted Graphite Iron (CGI). Its mechanical and physical properties make it ideal as cylinder block material, though there are drawbacks concerning its machinability as compared to other materials that are commonly used for the same purpose. Knowledge about machining of the material and its machinability is consequently inadequate.
The main goal of this thesis is to identify and investigate the effect of the major factors and their individual contributions on CGI machining process behaviour. When the relationship between the fundamental features; machinability, material microstructure, and material physical properties, are revealed, the CGI material can be optimized, both regarding the manufacturing process and design requirements. The basic understanding of this is developed mainly through experimental analysis as, e.g., machining experiments and material characterization.
The machining model presented in this thesis demonstrates the influence of material and process parameters on CGI machinability. It highlights machinability from both design and production planning perspectives. Another important objective of the thesis is an inverse thermo−mechanical FE model for intermittent machining of CGI. Here, experimental results obtained from a developed simulated milling method are used as input data, both to calibrate and validate the model. With these models, a deeper understanding is obtained regarding the way to achieve a stable process, which is the basis for future optimization procedures. The models can therefore be used as a foundation for the optimization of CGI component manufacturing.
Keywords: Metal Cutting, Compacted Graphite Iron (CGI), Machinability, Design of Experiments (DoE), Inverse Finite Element (FE) Modelling, Simulated Milling Method
PREFACE
This doctoral thesis is based on research work conducted at the department of Production Engineering at the Royal Institute of Technology in Stockholm, Sweden during 2006 to 2011. It has been supported by the VINNOVA MERA program and the VINNOVA FFI program.
This work would not have been possible without the support of several people. To start with, I would like to show gratitude and respect towards my supervisor, Professor Cornel Mihai Nicolescu, for sharing his deep knowledge in the field of metal cutting and teaching me scientific thinking. He has always supported me and made time for me in his otherwise so busy schedule. Secondly, a special thank to my roommates Dr Andreas Archenti and Tech Lic Mathias Werner. Thanks Andreas for your positive attitude. It has been very motivating to work with you and you have given me great ideas. However, most of all I would like to thank you for being such a good friend. Thanks Mathias, for your support, it has been a pleasure to work with you. My colleague and dear friend, Tech Lic Lorenzo Daghini is also acknowledged for always having his door open to me and helping out in all situations. You have also introduced me to the Italian culture.
All my other colleagues and friends at the department of Production Engineering are also recognized for giving me the opportunity to work in such a stimulating environment. Thanks for showing me great patience when spreading CGI graphite dust in the workshop during the years of machining thousands of kilos cast iron workpieces. A special acknowledgement goes to technician Mr Jan Stamer for his technological creativity and deep knowledge in all fields which has been very inspiring. Your help during preparation and execution of all machining experiments has been invaluable. Thank you, Dr Thomas Lundholm for initiating the “Fredagsrus” tradition and making us push ourselves to the limit in the running tracks of Lill‐ Jansskogen.
I also appreciate the help from all other members in the OPTIMA CGI and OPTIMA phase two project; Scania, Volvo Powertrain, Sandvik Coromant, Sintercast, Novacast, Federal‐Mogul, Chalmers University of Technology, Jönköping University and Swerea SWECAST. A special thank to Dr Henrik Svensson at Swerea SWECAST for material characterization and to Mikael Hedlind at KTH for workpiece design contribution to the development of the simulated milling method. Finally I would like to thank my family. Hope you will have a good reading. Stockholm, November 2011
”Ett materials bearbetbarhet är en synnerligen sammansatt egenskap, varför det fordras en ganska omfattande utrustning för att densamma på ett rationellt sätt skall kunna bestämmas.
Bortsett från att alla prov måste utföras av tränad, kunnig och erfaren personal, måste den tekniska utrustningen vara speciellt avpassad för försöksändamål. Försöken måste nämligen utföras laboratoriemässigt, men det oaktat i möjligaste mån i verkstadsmässig form, för att de erhållna resultaten skola bli så användbara som möjligt.” Professor Ragnar Woxén, 1944
TABLE OF CONTENTS
PUBLICATION LIST ... XI NOMENCLATURE AND ABBREVIATIONS ... XIII 1 INTRODUCTION ... 1 1.1 Project and research background ... 1 1.2 Scope and aim of the thesis ... 2 1.3 Thesis outline and relation to the appended papers ... 4 2 STATE OF THE ART, CGI MATERIAL PROCESSING ... 5 2.1 Compacted Graphite Iron (CGI) ... 5 2.2 CGI machining process behaviour ... 8 2.3 Cutting tool temperature modelling ... 12 3 A MACHINING MODEL FOR CGI MACHINABILITY STUDIES ... 13 3.1 Method to evaluate machinability ... 15 3.2 Influence of microstructure on CGI machinability in milling ... 17 3.3 Influence of carbide promoting elements on CGI machinability in milling ... 22 3.4 Influence of cutting parameters on CGI machinability in milling ... 28 3.5 Machinability of CGI from a process planning perspective ... 35 4 A NOVEL METHOD TO STUDY THE CHARACTERISTICS OF THE INTERMITTENT CUTTING PROCESS ... 45 4.1 Simulated milling in turning operation ... 45 4.2 Experimental evaluation of the technique ... 48 4.3 Conclusions ... 53 5 AN INVERSE THERMO–MECHANICAL FE MODEL FOR INTERMITTENT MACHINING OF CGI ... 55 5.1 Thermo−mechanical FE model for intermi ent machining of CGI ... 55 5.2 Conclusions ... 64 6 DISCUSSION AND CONCLUSIONS ... 65 6.1 Discussion and conclusions ... 65 6.2 Future research ... 66 REFERENCES ... 69 APPENDED PAPERS ... 75
PUBLICATION LIST
APPENDED PAPERS The following papers constitute the basis of this thesis. Paper A Berglund, A., Nicolescu, C.M., Richnau, K., “Effect of carbide promotingelements on CGI material processing”, Proceedings of CIRP 2nd International Conference on Process Machine Interactions, Vancouver, Canada, 2010, ISBN: 978‐0‐9866331‐0‐2
Paper B Berglund, A, Nicolescu, C.M., Svensson, H., “The Effect of Interlamellar
Distance in Pearlite on CGI Machining”, ICME 2009: International
Conference on Mechanical Engineering, Tokyo, Japan, 2009, ISSN: 2070‐ 3740
Paper C Berglund, A., Grenmyr, G., Nicolescu, C.M., Kaminski, J., “Analysis of
Compacted Graphite Iron Machining by Investigation of Tool Temperature and Cutting Force”, Proceedings of 1st International Conference on Process Machine Interactions, Hannover, Germany, 2008, ISBN: 978‐3‐939026‐95‐2
Paper D Berglund, A., Nicolescu, C.M., “Investigation of the Effect of
Microstructures on CGI Machining”, The Swedish Production
Symposium, Gothenburg, Sweden, 2007, TRITA‐IIP‐07‐06
Paper E Grenmyr, G., Berglund, A., Kaminski, J. and Nicolescu, C.M., “Investigation of tool wear mechanisms in CGI machining”, International Journal of Mechatronics and Manufacturing Systems, Vol. 4, No. 1, pp. 3–18, 2011, ISSN: 1753‐1039
NOT APPENDED PAPERS
The following papers contribute but are not appended to this thesis.
Paper F Berglund, A., Archenti, A., Nicolescu, C.M., “Analytical modelling of CGI
machining system dynamic behaviour”, The International 3rd Swedish production symposium, Gothenburg, Sweden, 2009
Paper G Grenmyr, G., Berglund, A., Kaminski, J., Nicolescu, C.M., “Analysis of
Machining Compacted Graphite Iron (CGI) by Join Investigation of Tool Temperature, Cutting Force and Tool Wear”, The Swedish Production
Symposium, Stockholm, Sweden, 2008, TRITA‐IIP‐08‐12
Paper H Berglund, A., Nayyar, V., Nicolescu, C.M., Kaminski, J., “Machinability of
Compacted Graphite Iron in Continuous and Intermittent Machining”, in
manuscript
Research work related to this thesis has also been presented in a licentiate thesis: Berglund, A., “Characterization of factors interacting in CGI machining:
machinability, material microstructure, material physical properties”, Licentiate
Thesis, KTH Royal Institute of Technology, Stockholm, Sweden, 2008, ISBN 978‐91‐7415‐158‐9
NOMENCLATURE AND ABBREVIATIONS
ALE ‐ arbitrary lagrangian‐eulerian ap ‐ depth of cut [mm] ae ‐ width of cut [mm] CGI ‐ compacted graphite iron cp ‐ specific heat of the material [J/(kg∙K)] Dc ‐ diameter of the milling cutter [mm] DoE ‐ design of experiments E ‐ elastic modulus [GPa] FE ‐ finite element fz ‐ feed, milling [mm/tooth] K ‐ thermal conductivity [W/(m∙K)] K0 ‐ thermal conductivity of unalloyed iron [W/(m∙K)] LOM ‐ light‐optic microscope LGI ‐ lamellar graphite iron ‐ volumetric energy addition [W/m3] ρ ‐ density [kg/m3] RCD ‐ rotating cutting force dynamometer Rp0.2 ‐ yield strength [MPa] SEM ‐ scanning electron microscope SGI ‐ spheroidal graphite iron t ‐ time [sec] T ‐ temperature [°C] UTS ‐ ultimate tensile strength [MPa] vc ‐ cutting speed [m/min] γp ‐ axial rake angle [°] γf ‐ radial rake angle [°] κr ‐ entering angle [°] Z ‐ number of inserts
1 INTRODUCTION
This chapter describes the background of the research project, in which most of the studies presented have been conducted. The structure of the thesis and the research approach are also addressed.
1.1 Project and research background
The automotive industry in Sweden is of great importance to the general welfare of the country. During 2010, 13% of the total industrial investment in Sweden was put in the automotive industry and in the first quarter of 2011 12% of the total Swedish export was linked to the automotive industry. Around 120 000 people were working directly in the automotive industry in Sweden in 2010. Furthermore, in the year 2009, during the global economic crisis 27% of all industrial investment in research was put in the transportation sector, where the automotive industry is the dominating area [1], [2]. This is one reason why Swedish automotive companies are so successful.One of the research projects that have been supported by the Swedish automotive industry together with the Swedish Governmental Agency for Innovation Systems (VINNOVA) is the OPTIMA project, which started in 2006. The goal of the project was to study the interaction between the machining process, the material and the casting process of Compacted Graphite Iron (CGI) and in the end develop a machinability and casting model for CGI. The reason why the project was initiated was the Swedish automotive industry’s great interest in the material. Its mechanical and physical properties (75% higher tensile strength [3]) make it ideal as cylinder block material, though there are drawbacks concerning its machinability as compared to other materials that are commonly used as for the same purpose, e.g., gray iron. The knowledge about machining of the material and its machinability is consequently inadequate. For a successful implementation of CGI as engine material it is necessary to obtain deeper knowledge about the material and its machinability. As CGI is a material family [4] it is also critical to investigate the effect of the variation of chemical composition on machinability. This has initiate several research studies throughout the years but there is still a lack of deeper knowledge in what factors affect the machinability of CGI.
In order to successfully implement CGI as cylinder block or cylinder head material, it is first necessary to obtain a robust machining and casting process. Then it is possible to optimize the production lines to achieve the required process accuracy and high productivity. Generally, a robust machining process is not affected by the operator handling the machine, material variations or time factors. The robustness of the machining process is more specifically described with regards to a certain quantifiable response, e.g., surface roughness, tool life or machine down time. For example, a machining process is robust, considering surface roughness, if the quality of the surface stays within the tolerances, even if, e.g., a tougher material suddenly is introduced. However, in order to achieve a robust machining process it is first essential to understand which factors affect the machining process behaviour, and thereby the machinability. It is only when these factors are found, and their
influence on the machining process behaviour is fully understood, that optimization can be performed, both from a design perspective and a process planning perspective. This will allow for a highly productive component manufacturing with high process accuracy.
Design perspective refers to the parameters that have to be addressed in the production development process of a new engine. In addition to the usual parameters related to the product functionality, in the early stage, proper material physical properties for the engine are set. The material physical properties of the engine are obviously of greatest importance for design but not sufficient. The cast cylinder block will need to be machined in order to fulfil the engineering requirements (e.g., geometrical tolerances). It is therefore, important that also the machinability of the material is considered in this early stage. In this thesis, the major focus is on machining of cylinder blocks. No consideration is therefore taken on the design requirements, regarding material physical properties of the engine, even if these cannot by any means be neglected.
1.2 Scope and aim of the thesis
The main goal of this thesis is to identify and investigate the effect of the major factors and their individual contributions on CGI machining process behaviour. When the relationship between the fundamental features; machinability, material microstructure, and material physical properties, are revealed, then the CGI material can be optimized, both regarding the manufacturing process and design requirements. A machinability model will be presented that demonstrates the most important features affecting intermittent machining of CGI. The model consists in two sub‐models, see Figure 1.
Figure 1: Illustration of the CGI machinability model with its two sub‐models.
The first sub‐model is a CGI machining model for milling, illustrating machinability both from a design perspective and from a process planning perspective. This model is mainly developed based on the results of three larger full factorial design of experiments (DoE) studies. Full factorial studies have advantages over “one factor at the time studies” as they not only show single factor effects but also illustrate the
CGI Machinability model
Sub‐model:CGI Machining model
Sub‐model:Inverse Thermo−Mechanical FE model
INTRODUCTION
factors´ relation to each other. The presented machining model in this thesis is based solely on full factorial DoE studies, in contrast to most other published work concerning machinability of CGI.
The other sub‐model is an inverse thermo−mechanical Finite Element (FE) model for intermittent machining of CGI. This model illustrates the thermal and mechanical load distributions on the chip and insert. These are important factors affecting tool wear. The model is required as it is difficult to investigate the cutting zone physics during chip removal, especially in milling, as sensor placement is difficult and often practically undoable due to the rotating cutting tool. Experimental results have been used as input data in the model. E.g., the used CGI material model in the FE representation is described by the material physical properties and by the response of the material to thermal and mechanical stresses as obtained from experimental investigations. Furthermore, IR camera measurements of cutting tool temperature have been performed in intermittent machining of CGI and used to both calibrate and validate the FE model.
A complete characterization of the machinability of CGI is not possible as there are many factors that affect the machining process behaviour of the material. However, by focusing on the most significant factors and keeping other factors constant, important trends can be found.
No attempt will be made to optimize the material or the machining process in this thesis. This should be done with respect to the specific design requirements and manufacturing process and system for the existent component manufacturing situation. However, the model presented in this thesis may be used as a foundation for optimization procedures concerning:
1. Optimization of the CGI chemical composition and thereby establishing decision rules for material design.
2. Process planning, as selecting proper process parameters for the specific CGI component manufacturing situation. From a process planning perspective, the model can be used not only when selecting the basic process parameters, as cutting speed and feed, but also when considering component configuration, milling cutter positioning with respect to the workpiece, milling cutter diameter and number of inserts.
The work, presented in this thesis, is related to face milling of CGI. However, the methodology used for acquiring deeper understanding of the machining process behaviour of the material is general, and could be extended to other types of materials or in other machining operations, e.g., turning and drilling. In this thesis, the same type of standard cemented carbide insert with K20W coating has been used for all machining experiments in the different DoE studies. The choice of the cutting insert was motivated by the fact that it is the most commonly used cutting insert and also the recommended insert for face milling of both gray iron machining and CGI machining, at least in rough machining of cylinder blocks, which is an important case study in this thesis. The choice of coated cemented carbides has also shown relatively high performance, in relation to gray iron. It is therefore not economically justified to go over to other more expensive tools such as CBN. As the same type, and number of inserts, has been used for all experiments, common conclusions for all DoE studies can be drawn. The same type of milling cutter has also been used for all machining experiments. Furthermore, all machining
experiments have been performed in the same machining centre, with the same type of clamping. However, some results have been verified in industrial environment, such as milling of real cylinder block components. Reference cutting data have been used in all DoE studies in order to validate the results from the different experiments.
1.3 Thesis outline and relation to the appended papers
Five appended papers constitute the basis of this thesis (four conference contributions and one journal paper). The thesis has six chapters. There is a short introduction to each chapter that address what will be discussed.
The second chapter of the thesis is a state‐of‐the art study of CGI material processing and modelling of cutting tool temperature, introducing the reader to the subject of CGI machining. The third chapter illustrates some important factors affecting CGI machinability. Here, the CGI machining model will be presented. These results come from several machining experiments. This chapter is based on Paper A, B, D and E. In Paper A, the effect of carbide promoting elements on CGI machinability is investigated. Paper B and D illustrate the effect of microstructure on CGI machinability. Paper E, contributes to the CGI machining model demonstrating the effect of microstructure on CGI tool wear behaviour.
In the fourth chapter the cutting tool temperature will be evaluated in intermittent machining of CGI. A novel method will be presented which enables a milling operation to be reproduced in turning application. This method opens new possibilities for refined studies of the intermittent machining process behaviour. This is achieved by the development of a novel method applied to a special designed workpiece.
The fifth chapter illustrates a developed inverse thermo−mechanical FE model in intermittent machining of CGI. Data is extracted from the cutting tool temperature studies, presented in Chapter 4, and used both to calibrate and validate the FE model. Furthermore, results obtained from material characterization are also used in the FE model. The FE model, presented in this chapter, is a developed version of the model presented in Paper C.
The last chapter concludes the work and addresses some opportunities for future research in the field.
2 STATE OF THE ART, CGI MATERIAL
PROCESSING
The first section of this chapter introduces the CGI material to the reader. Then, a state‐of‐the‐art is presented, regarding CGI machining, focused on milling. Some earlier work in the field of temperature modelling is also addressed.
2.1 Compacted Graphite Iron (CGI)
Cast iron is a family of alloys divided into several classes, defined by their graphite morphology and metallic matrix structure. There are mainly three different classes of cast irons classified by their graphite morphology; lamellar graphite iron (LGI), compacted graphite iron (CGI) and spheroidal graphite iron (SGI). LGI, commonly known as gray iron or flake graphite iron has a stable eutectic with graphite shaped as lamellas or flakes, see Figure 2a. In Figure 2c, SGI is illustrated. It is also called ductile iron or nodular cast iron and has a stable eutectic with the graphite shaped as spheroids or nodules. The third class of cast iron is CGI which has a stable eutectic with a worm‐like shaped graphite, also called compacted graphite or vermicular graphite, see Figure 2b [5].
(a) (b) (c)
Figure 2: Microstructure of (a) gray iron, (b) CGI and (c) ductile iron (source Sintercast).
According to the ISO standard 16112:2006, proper CGI should contain a minimum of 80% of the graphite particles in vermicular form and no flake graphite should be present. In other words; the nodularity should not exceed 20% [6]. The nodularity value is a measure of the roundness of the graphite particles. A cast iron material with a nodularity of 100%, solely contain graphite nodules and it is therefore ductile iron.
In order to obtain the nodularity value, first a two‐dimensional polished surface of the material needs to be prepared. The surface will be studied with image analysis. The roundness shape factor is needed according to
4
Equation 1
where A is the area of the graphite particle, lm is the maximum axis length of the
graphite particle and Am is the area of a circle with the diameter lm. Secondly, the
roundness value of each graphite particle in the polished surface is used to differentiate between the three graphite forms. A graphite particle with a roundness value of 0.625‐1.000 is considered to have nodular form (ISO form VI), a roundness value of 0.525‐0.625 intermediate form (ISO form IV and V) and if it has a roundness value of less than 0.525 compacted form (ISO form III). Flake graphite and other under modified structures are not included in the analysis, as they are not permitted in the compacted graphite iron structure. Only graphite particles having lm exceeding
10 µm are taken into account in the evaluation. The percentage of nodularity can then be calculated with
% ∑ 0.5 ∑
∑ 100
Equation 2
where Anodules is the area of graphite particles classified as having nodular form,
Aintermediates is the area of graphite particles classified as having intermediate form and
Aall particles is the total area of all graphite particles having lm exceeding 10 µm [5], [6].
In the following section, the procedure to produce CGI, its material physical properties and its characteristics as engine material, will be presented.
2.1.1 Casting of CGI
There are several commercial casting methods available on the market to produce CGI, e.g., the Sintercast method [7], Graphyte batch and the Graphyte flow process [8]. However, the basic procedure of producing CGI is to carefully monitor and control the amount Magnesium (Mg) in the melt. The amount of Mg affects the graphite form, and therefore also the material physical properties of the cast component [9]. If there is not sufficient magnesium, the graphite begins to grow with a flake morphology during solidification, which reduces the strength of the material drastically. Too high concentration of Mg, on the other hand, leads to nodular graphite which results in undesirable properties [7]. The magnesium content must be controlled simultaneously with the inoculation level in order to produce high quality CGI microstructures. Postinoculation can suppress carbide formation, practically in thin walls but it provides more sites for graphite precipitation which favours the growth of spheroidal rather than compacted graphite particles [9].
STATE OF THE ART, CGI MATERIAL PROCESSING
The chemical composition of the melt also affects the ferrite/pearlite ratio. This strongly affects the material physical properties and therefore also the machinability. The pearlite content is mainly controlled by the pearlite promoting elements Copper (Cu) and Tin (Sn). These act as diffusion barriers, making carbon diffusion from the austenite into the graphite harder; hence pearlite will preferably be formed at the solid state transformation [10].
Two other parameters that need to be considered during casting are the cooling and solidification rate which are affected by the section thickness. These parameters influence coarseness of the pearlite and the nodularity and thus the material physical properties [9]. Overall, process control while casting CGI is highly important.
2.1.2 Material physical properties of CGI
The main factors setting the mechanical properties of CGI materials both at room temperatures and at elevated temperatures are the graphite morphology and the metallic matrix. The graphite morphology and the metallic matrix are furthermore mainly affected by the chemical composition, inoculation level and the section thickness, as mention above. However since the matrix can be controlled in a similar way for the different morphologies, the main difference in properties between the cast irons will be due to the graphite morphology.
In gray iron, the graphite flakes have sharp edges which give the material its characteristic properties. It has good damping properties and heat conductivity and also excellent machinability. On the negative side, gray iron has, in some applications, unsatisfactory strength and thus alloys have to be added, leading to difficulties in machinability. Ductile iron has spheroidal shaped graphite particles; it has excellent strength to the cost of machinability, and also presents problems in casting. CGI has vermicular graphite particles, with stubby flakes and small amounts of graphite spheroids, resulting in both material properties and foundry processing characteristics that are intermediately between those of gray and ductile iron [9], [11]. It exhibits some of the castability of gray iron, but with higher strength and ductility. Compared to ductile iron, it has better thermal conductivity, machinability and damping capacity [12]. Typical mechanical properties of gray iron, CGI and ductile iron can be seen in Table 1.
Table 1: Typical material physical properties of gray iron, CGI and ductile iron [13].
Property Gray Iron CGI Ductile Iron Tensile strength [MPa] 250 450 750 Elastic modulus [GPa] 105 145 160 Elongation [%] 0 1.5 5 Thermal conductivity [W/(m∙K)] 48 37 28 Relative damping capacity 1 0.35 0.22 Hardness [BHN 10/3000] 179‐202 217‐241 217‐255 R‐B Fatigue [MPa] 110 200 250
2.1.3 CGI as engine material
CGI is used in several applications today. Exhaust manifolds, hydraulic housings and brackets, and to large castings as ingot moulds, are some examples [9]. However, CGI has also great potential to be the engine material of tomorrow, especially regarding heavy duty diesel engines. Today, there are some truck diesel engine components produced in CGI, e.g.: Scania V8 (16.4 L), cylinder block Navistar (12.4 L), cylinder block MAN (12.4 L), cylinder block Hyundai (12.3 L) , cylinder head Ford‐Otosan (9.0 L), cylinder block and cylinder head DAF (12.9 L), cylinder block and cylinder head [14]
The main motives of using CGI as engine material are the characteristics of its material physical properties, which are ideal for engine materials. Damping and heat conductivity, are though not as good as for gray iron but it has, on the other hand, superior strength [13]. When comparing the material with aluminium, there are studies that show higher power per weight ratio for the CGI engine, with the same performance. This is because, due to its greater strength, the engine can be made with lesser wall thickness [15].
2.2 CGI machining process behaviour
Cast irons are the foremost common engine material for all manufacturers of heavy trucks. As for machining of all other materials, much information about the machining process behaviour that occurs during cutting of cast irons can be extracted from studying the chip formation during material removal. The chip formation process is the result of the interaction between several factors; tool geometry, tool material, work material as well as the chosen cutting parameters. These factors all contribute to the final component´s surface generated during the chip formation process [16]. It is essential to understand this chip formation process as it is a fundamental parameter that affects the productivity in all component manufacturing [17]. This is therefore also essential when machining CGI.
There are clear differences in the chip formation process when machining gray iron, CGI and ductile iron. As gray cast iron materials contain flake graphite dispersed in a silicon–iron matrix, the sharp edges of the flakes provide a very effective stress riser for the machining loads exerted by the cutting edge. When the shear plane approaches a graphite pocket, cracks start to propagate from the edge of the flake and the iron fractures. The fracture starts at the stress riser and ends in an adjacent pocket until the shear load builds up to the fracture strength of the next stress riser. In CGI, the graphite form is vermicular. When machining CGI, it will shear, as for gray, through a graphite pocket which has the least resistance to shear forces. The round edges of the compacted graphite does not initiate cracks as easy as the sharp edges of the flake graphite in gray cast iron which leads to higher cutting forces when machining CGI. The chip formation during machining ductile iron is similar to the formation during steel machining. The nodules of graphite deforms by the
STATE OF THE ART, CGI MATERIAL PROCESSING compressive tool loads prior to the chip separation. The matrix does therefore not crack, leading to the formation of a continuous chip over the tool edge [9], [18]. In the following section, a brief introduction to the known parameters affecting CGI machinability will be presented.
2.2.1 Influence of microstructure
The machinability of CGI is strongly dependent of the microstructure of the material. It has been shown in previous research studies that CGI is a material family where combinations of various microstructures span over a wide range [4]. The microstructure affects the material physical properties which influence the machinability parameters. Therefore, it is necessary to investigate the interaction between machinability, material microstructure and material physical properties before CGI successfully can be implemented as engine material. Dawson et al, studied this interaction and showed how the pearlite content and nodularity affected the tool life [13]. It has also been shown in other studies that pearlite content is the foremost important microstructural parameter affecting both the CGI material physical properties and the CGI machinability [19], [20]. The nodularity of the CGI material is also important as it affects the tool wear mechanisms, see Paper E [21].The components being manufactured in the industry today are rarely plain homogeneous blocks, on which many studies are performed in the research labs. Usually real components have various section thicknesses, holes and slots. This strongly affect the microstructure and therefore also the machinability [19]. E.g., in a cylinder block, which has different section thicknesses, the different cooling and solidification rates during casting lead to an inhomogeneous microstructure [5], [9]. A faster solidification rate, in the thinner sections, leads to a more spheroidal graphite structure, which consequently also increases the tensile strength [22]. Further it also increases the percentage of carbides [23]. Heisser showed that the simulated values for the nodularity were between 12% and 70% in one cylinder block, which was very close to actual inspected values [24]. Regarding the effect of cooling rate on material microstructure, and therefore also machinability, it affects coarseness of the pearlite and could result in a difference of 50 MPa in tensile strength [25]. Such a difference in tensile strength is likely to affect machinability. The microstructure in a complex component could therefore not be considered as homogeneous since the thinner the section the stronger the material [23], [26]. This must be taken into consideration when selecting the proper cutting tool and cutting parameters for the machining of CGI.
2.2.2 Influence of carbides
Hard carbide inclusions can drastically reduce the tool life in machining of all types of materials. In machining of CGI, much focus has been put on the carbide promoting element Titanium (Ti). The reason for this is that Ti can increase the magnesium range over which CGI is stable. Ti effectively “poisons” the growth of graphite nodules and extends the plateau toward higher Mg contents [9]. Ti, however, increases the strength of the material, by increasing the percentage of pearlite content [22], but more importantly, it can react with carbon and/or nitrogen in the molten iron and form hard and abrasive inclusions of titanium carbon nitride
(Ti(C,N)). These inclusions reduce the machinability significantly [27], [28]. Machining experiments in turning have shown that a slight increase of the trace level of Ti from 0.01% to 0.02% is sufficient to reduce the tool life by 50%, see Figure 3a [27]. In high speed milling (400‐1000 m/min) of CGI materials with different Ti content, Sadik [28] showed that ceramic grades were the best choice for obtaining high productivity rate. However, there are no grades available that efficiently could machine a CGI material with titanium content ≥ 0.05% at this cutting speed, see Figure 3b.
(a) (b)
Figure 3: (a) Tool life in turning of CGI as a function of the trace level of Ti [27]. (b) Tool life in
milling of CGI as a function of the trace level of Ti for different tool grades [28].
Carbide promoting elements are also found in the scrap material, used for the casting of new components. The chemical composition of that scrap material is highly important and reflects on the material physical properties. Some elements are however more important than others, from a material physical properties and machinability point of view. Chromium (Cr), Manganese (Mn) and Molybdenum (Mo) have a negative effect on machinability and should therefore be monitored carefully. Scrap material with a low concentration of Cr and Mn is desirable from a machining perspective, it is however expensive to purchase this high quality scrap material to be used for the casting of new cylinder blocks [29]. Mo, on the other hand, is not commonly present in the scrap material. It can however be added in the synthesis of CGI cylinder heads in order to increase the strength of the material at higher operational temperatures, which is crucial for a cylinder head material. Mo also improves the thermal fatigue life of the material. It is therefore important to find the right balance between material strength and machinability so that a high productive manufacturing can be achieved with high quality [5], [30].
2.2.3 Tool wear behaviour, cutting parameters and cutting
tools for CGI machining
CGI is an excellent material for truck engines, as mentioned above. Machining of CGI components would however affect the manufacturing lines in a different way, compared to the commonly used gray iron, in terms of productivity and machinability. It is therefore essential to compare the two materials with each other 0 1 2 3 4 5 6 7 8 9 10 0 0,05 0,1 0,15 0,2 0,25 C utt in g lengt h [k m] to 300 μm fl an k w ear Titanium [%] Carbide turning 250 m/min 150 m/min 0 10 20 30 40 50 60 70 80 90 100 0 0,02 0,04 0,06 0,08 0,1 0,12 C utt in g ti m e [m in ] Titanium [%]
Tool life as function of Ti content in CGI for face milling, vc=700
m/min, ap=2 mm, fz=0.125 mm/tooth, ae=40 mm, Dc=80mm
Uncoated ceramic Coated ceramic Coated carbide Still very good
STATE OF THE ART, CGI MATERIAL PROCESSING
to see what a change would lead to. Furthermore it should be investigated how manufacturing of CGI components could be optimized.
One clear difference in the machining process behaviour of the two materials is the tool wear behaviour. This was carefully studied by Reuter. He showed that tool wear, when machining CGI, was highly dependent on cutting speed [31]. This is mainly because of the manganese sulphide (MnS) layer which forms, acting as a lubricant and as a diffusion barrier when machining gray iron at high speed. Such a layer is not formed when machining CGI because the MnS content is much lower [32]. The explanation to this is that in CGI, magnesium is added to obtain the desired graphite shape. Magnesium is a very strong sulphide builder which results in magnesium sulphide inclusions rather than MnS inclusions [13]. This is reflected on the tool life. Some investigations indicated that a 50% loss of tool life could be expected at high speed milling of CGI with PCBN inserts (Polycrystalline Cubic Boron Nitride) and a 90% loss of tool life at high speed turning with PCBN inserts [33]. This indicates that machining of CGI is preferably done at lower cutting speed, at least when concerning tool life. When studying the tool wear behaviour more specifically in CGI machining it has been found that abrasive wear of the insert is the dominant wear mechanism in milling at a low range of cutting speed (≤ 300 m/min) [28]. This has also been noticed in other studies [34]. At higher cutting speeds (≥ 600 m/min), Da Silva suggests that adhesive wear is the most dominating tool wear mechanism in milling of CGI [35]. However, the tool wear behaviour also differs for the specific type of CGI material that is being machined. Jönsson found that milling of high pearlitic CGI has a different tool wear behaviour compared to low pearlitic CGI. Figure 4 clearly illustrates the more even wear when machining the high pearlitic CGI. Tool wear development is also different. The high pearlitic CGI material has a more predictable tool life as it has controlled and gradually increasing tool wear, while it is more unpredictable for the low pearlitic material [36].
(a) (b)
Figure 4: Typical wear of the cemented coated carbide insert when milling (a) low pearlitic
CGI, (b) high pearlitic CGI [36].
Concerning the cutting parameters that are suited for the machining of CGI, it has been observed that it is always a balance between high productivity and acceptable tool life since the machinability is strongly dependent on the choice of cutting parameters. The machinability of CGI also varies for different types of machining operations and selected cutting tool material. In terms of high performance, when milling CGI at cutting speeds below 300 m/min, cemented carbide grades should be used, in combination with high feed rates and width of cut. Here, the ceramic grade does not provide enough abrasive wear resistance, compared to cemented carbides.
The ceramic grade does however give good diffusion wear resistance at high cutting speed (≥ 300 m/min), when the feed and/or the width of cut is small. By increasing the feed and/or the width of cut, the ceramic grade will reduce the tool life to the same level as the cemented carbides, because its ability to deform plastic is very limited, which leads to partial edge destruction [37]. In another study, it has however been shown that carbide grades are preferable to ceramic grades even at higher cutting speed (850 m/min) [38].
2.3 Cutting tool temperature modelling
During machining, the energy introduced to the process, is to a large extent converted into heat that increases the temperature near the cutting edge. The heat generation affects the momentary thermo−mechanical conditions of the cutting tool–workpiece interface. Often, high temperatures are the direct cause of tool wear and tool failure, especially in machining of cast iron and steel. With these higher melting point metals, the tool is heated to high temperatures as metal removal rate increases and, above certain critical speeds, the tools tend to collapse after a very short cutting time under the influence of mechanical and thermal stresses [39]. Since the cutting temperature distribution is of such great importance for the machining performance, it would be of great interest to predict the temperature field on the tool–chip interface. As limited data are available from experiments due to difficulties to access the tool–chip interface, an appropriate cutting tool temperature model can be used for optimizing the cutting parameters or for the development of new cutting tools. This should be considered for CGI machining.
There are many different methods to model the cutting temperature. The methods have either an analytical or numerical approach. Analytical models where early developed by for example Trigger and Chao [40], and more recently by Ståhl [41]. The Finite Element (FE) method is a type of numerical modelling approach that can be used to obtain the cutting tool temperature distribution. This method has in recent years become the main tool for simulating metal cutting processes [42]. Klocke et al, mean that these FE models have advantages compared to analytical approaches where the mathematical equations which describe the cutting process are so complicated that a solution is no longer possible [43]. There are, however, studies that show the benefit of analytical models. Such a study was performed by Grzesik. He compared one analytically predicted cutting temperature model and one numerically predicted cutting temperature model with the results obtained by thermocouple‐based measurement. It was shown that both the analytical model and the FE model had good comparison with the experimentally measured values [42]. However, some researchers state that neither experimental nor simulated results are yet able to describe the complex cutting process. It is only the combination of simulations and experiments that allows a better description of the cutting process [43]. One method to obtain a better model is by inverse FE modelling where experimental data is used to both validate and calibrate the model. Pujana et al. also mean that the use of experimental data in the FE model reduces the error value from the simulation [44]. The inverse method has been used by Lin [45]. He measured the cutting temperature on the machined surface in milling using an infrared pyrometer, and utilized the results for solving the unknown boundary at the cutting tool‐workpiece interface.
3 A MACHINING MODEL FOR CGI
MACHINABILITY STUDIES
This chapter presents a machining model which takes into consideration the CGI microstructure, presence of carbide promoting elements and cutting parameters effect on tool life and tool wear mechanisms. Then, tool life is used to evaluate the machinability. The first section of the chapter describes the methods used for evaluating the machinability. This is followed by a section that highlights the influence microstructure has on CGI machinability based on a DoE study, presented in Paper B. In the third section, the effect of carbide promoting elements on CGI machinability is presented. More details about these experiments are found in Paper A. It should be noted that some of the results presented in Section 3.2 and Section 3.3 are not to be found in Paper A and Paper B. The results from these papers have been analysed further. The fourth section demonstrates how the cutting parameters affect CGI machinability. The last section in Chapter 3 deals with machinability of CGI from a process planning perspective.
Compacted graphite iron is to some extent even today used as engine material in the heavy truck industry. But up until now, it has not been as widely used as the more commonly used material for this application; gray iron. One reason for this is its drawbacks in machinability and lack of experience in machining of the material. Knowledge about machining of the material and its machinability is consequently inadequate. For a successful implementation of CGI as engine material it is necessary to obtain deeper knowledge about the material and its machinability. A CGI machining model was for this reason developed. The machining model is a part of the machinability framework as illustrated in Figure 5.
Figure 5: Machining model as part of machinability.
Figure 5 illustrates the basic concept of the machining model. The microstructure of the CGI materials reflects on the mechanical properties and machinability of the material as studied in Section 3.2 and 3.3. Machinability is affected to a large extent by mechanical and thermal loads generated during the cutting process. Therefore, cutting forces, analysed in Section 3.5, and heat generation, treated in Section 4, are important components in the machining model as they affect tool wear mechanisms and tool life as well as surface integrity of the machined part. The interaction
Tool life Tool wear mechanisms Cutting forces Vibration Heat generation Temperature distribution
CGI material
Mechanical properties Hardness Material strength Elastic modulus M ac h in ing op er at io n Surface roughness Microstructure Nodularity Pearlite content Chemical composition Section effectbetween cutting process and the elastic structure results in vibrations. Its contribution to the machining model is considered in Section 3.5.
The CGI machining model, presented in this thesis, is based on this framework. The model considers machinability both from a design perspective and from a process planning perspective. The contribution of the CGI physical properties has to be emphasized. High strength of the material, good heat conductivity and high damping, are of course important from a design perspective of a heavy duty diesel engine in CGI. However, in order to achieve high process accuracy and a highly productive component manufacturing it is also important that other parameters are taken into account during the production development process from a design perspective. The material parameters, e.g., microstructure, chemical composition and thickness of the sections have a great effect on the machinability of the material.
Furthermore, the process parameters, e.g., tool material, tool geometry, cutting parameters, fixturing and machine tool characteristics also strongly affect the machinability. These parameters should be considered in the production development process of engines so that in the end, high process accuracy and high productivity may be achieved. Figure 6 illustrates a fundamental concept of which factors affect the machinability from a design perspective.
Figure 6: Illustration of important factors to consider from a design perspective in order to
obtain good machinability, high process accuracy and high productivity.
When optimizing the material with regards to machinability, the process parameters are strongly linked to the material parameters. Since they do not only have an individual contribution on machinability, they could also cross‐correlate with each other. One particular cutting tool can, for example, demonstrate a certain correlation with one specific type of graphite morphology, while another sort of cutting tool could behave in a different way. It is therefore challenging, if even possible, to identify the optimal process parameters for certain material parameters. However, by studying the effect different process and material parameters have on
Machinability
Process parameters Material parameters Microstructure Section thickness Chemical composition Cutting parameters Tool material Tool geometry Fixturing Machine tool characteristicsA MACHINING MODEL FOR CGI MACHINABILITY STUDIES
the machinability, deeper knowledge about this interaction is obtained. This facilitates future optimization procedures.
The process parameters are also important from a process planning perspective. In this, there are other factors that also affect the machining results and therefore the machinability. The component configuration (geometrical shape), milling cutter positioning with respect to the workpiece, milling cutter diameter and number of inserts, all strongly affect the machinability, especially in regards to tool fracture. The fundamental understanding regarding machinability of the material can be obtained by studying the influence of material and process parameters. A complete picture of the material machinability in a certain machining operation is however not obtained until it is also seen from a process planning perspective. The machining model developed in this chapter is part of the machinability framework as illustrated in Figure 5. However, there are factors that are not considered in the machining model such as workpiece fixturing, machine tool characteristics and cutting tool (Figure 6). The CGI machining model, however, illustrate the most important aspects of CGI machining.
3.1 Method to evaluate machinability
There are several factors that affect the machinability, as mentioned above. The machinability of the material can furthermore be evaluated by numerous parameters. Tool life, cutting force, surface roughness and chip formation are some examples. The experimental work that has been carried out in this thesis is driven by the needs of the heavy truck industry, and focused on intermittent machining of cylinder blocks and cylinder heads. Surface roughness is not considered in this thesis. This is because the presented experimental work is related to rough machining, where surface finish is not of greatest concern. High process accuracy is of course important in manufacturing of these components but the economical aspect is related to tool life versus material removal rate (MRR). Therefore special attention has been put on tool life. Tool life is also the most widely used machinability criterion [46].
A tool life criterion has been used to acquire the tool life end in all milling experiments, presented in this thesis. Three, evenly pitched, cutting inserts have equipped the milling cutter and the tool life criterion states that tool life end is reached whether the average of the maximum flank wear of all three inserts reaches 0.3 mm, or the maximum flank wear of any two cutting inserts reaches 0.3 mm. There are other tool wear mechanisms that can occur during machining resulting in tool failure, for example plastic deformation, chipping, oxidation or tool fracture. In the milling experiments performed in this thesis, the cutting parameters were selected such that the dominant tool wear mechanism was related to abrasive wear on the flank face of the inserts. Flank wear is also the most desirable type of wear [46] and it has therefore been used as the quantitative response in determining the tool life.
To this it should be noted, that when setting up a machining operation, a stable cutting situation is desirable. From a process planning perspective, stable cutting means controlled and gradually increasing tool wear, resulting in predictable tool life. In this respect it is important to select cutting parameters within a parametric domain where the specific tool wear criterion is enforced. If the tool wear behaviour
is predictable it will contribute to a more robust machining process, facilitating process planning in terms of tool change intervals. The goal of all machining experiments, presented in this thesis, has therefore been to obtain a controlled and a gradually increasing tool wear. This motivates the selected tool life criterion. Some problems have been seen with tool fracture during the experiments. However, tool fracture is a non‐desirable tool wear mechanism in component manufacturing as it is impossible to foresee. In these undesirable situations, the machining parameters were corrected and the tests were repeated.
The tool wear investigations have mostly been done with Light‐Optic Microscope (LOM), but also using Scanning Electron Microscope (SEM).
Even if tool life is the foremost important machinability response, some concern in this thesis is also given to cutting forces. Cutting force is one of the few quantitative responses that can be measured during the machining operation which can be used as a basis for further analysis [47]. Cutting forces in milling can be acquired in several ways. The most common method is by a fixed force plate dynamometer on which the workpiece is placed. This enables measurement of the three cutting force components in a fixed coordinate system relative the table. However, this method has its limitations, e.g., with respect to the size of the part that can be clamped. Also, the recorded cutting forces could vary during the machining experiment, since the dynamometer will be affected by the changes of the workpiece weight. Therefore, in the experiments carried out in this thesis, a rotating cutting force dynamometer (RCD) has been used, see Figure 7. Figure 7: Illustrative picture of the rotating cutting force dynamometer (RCD) used for acquiring cutting forces. KISTLER dynamometer (type 9124B1111). The dynamometer measures the three cutting force components Fx, Fy, Fz as well as of the momentum Mz and is mounted on the milling cutter. The dynamometer has
high rigidity and thus a high natural frequency. An advantage it has over the fixed dynamometer is that the cutting forces can be measured independently of the size of the workpiece and in any spatial position (four or five axis milling).
F
xF
yF
zM
zA MACHINING MODEL FOR CGI MACHINABILITY STUDIES
One limitation with both techniques is that the dynamometers will register the sum of the cutting force if more than one insert is engaged in cut simultaneously. The contribution from each insert could be studied with more advanced technique, e.g., as Andersson did [48], if that is of interest. The aim of the cutting force experiments, presented in this thesis, was to identify the differences in average cutting force characteristics. The RCD technique is well suited for this aim.
No cutting fluids have been used in the experiments. Cutting fluid is sometimes used in industry, when milling cast iron, not because it prolongs tool life but mainly because it binds the dust of graphite particles (and keeps it within the machine) and removes the chips from the cutting area. In milling of CGI, it is recommended to avoid the use of cutting fluid. The dust should be taken care of with other means, e.g., using vacuum equipment [49].
In the following sections, the influence of microstructure, carbide promoting elements and cutting parameters, on CGI machinability is presented. These are important parameters, from a design perspective, affecting the machinability. In Section 3.5, CGI machinability is considered from a production planning perspective. There, it will be shown that other parameters, as component configuration, milling cutter positioning with respect to the workpiece, milling cutter diameter and number of inserts strongly affect the CGI machinability. These sections contribute to the presented CGI machining model.
3.2 Influence of microstructure on CGI machinability in
milling
As part of the CGI machining model, studying the effect of the material microstructure on machinability will reveal, apart from the wear mechanisms and tool life, the influence on cutting force, dynamic phenomena, heat generation and surface roughness of the machined part. The latter is not studied in the thesis but the contribution of the surface roughness to machining model and further to the machinability cannot by any means be neglected. The main results presented in Section 3.2 are taken from Paper B.
Material microstructure is one of the most important material parameters (Figure 6) affecting the CGI machinability [4]. The material microstructure reflects on the material physical properties which affect the machinability, as mentioned in Section 2.2.1. It is important to fully understand the interaction between machinability, material microstructure and material physical properties in order to design a material with the required material parameters. For this reason a preliminary study of microstructure´s effect on CGI machinability was initiated, see Paper D [20]. However, for that study, a special “component like” workpiece was used “Sandvik provkropp 16” which had a complex geometry, resulting in non homogenous microstructure. This complicated the analysis. For that reason, a more careful DoE study was started using homogenous workpieces in order to better distinguish the influences of the different microstructural parameters on CGI machinability.
