Development of a new additive for improving machinability of PM steels

Development of a new additive for improving

machinability of PM steels

Thèse

Amin Molavi Kakhki

Doctorat en génie des matériaux et de la métallurgie

Philosophiae doctor (Ph. D.)

Québec, Canada

Development of a new additive for improving

machinability of PM steels

Thèse

Amin Molavi Kakhki

Sous la direction de :

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Résumé

Bien que la métallurgie des poudres (MP) soit connue comme un méthode de fabrication aux cotes presque finales, un volume ignificatif de pièces MP nécessitent que l’on ait recours à une opération d’usinage. Les opérations d'usinage secondaires sont habituellement requises pour la conformité dimensionnelle ou la production de caractéristiques géométriques compliquées qui ne peuvent pas être obtenues par le procédé de pressage. Cependant, en raison de la présence de porosité, l'usinabilité des pièces en aciers MP est difficile en comparasion aux aciers corroyés et peut ajouter 20% ou plus au coût total de fabrication de ces pièces. Parmi les diverses mesures connues pour améliorer l'usinabilité des aciers MP, l'utilisation d'additifs d'usinage, soit mélangés, soit pré-alliés, est de loint la méthode la plus utilisée. Il existe des dizaines d'éléments et de composés différents qui peuvent améliorer l'usinabilité à différents niveaux. Néanmoins, leurs effets négatifs sur d'autres propriétés des aciers MP tels que les propriétés mécaniques et la résistance à la corrosion rendent leur utilisation peu intéressante.

Dans cette étude, des particules de graphite libre sont présentées comme un nouvel additif qui non seulement améliore significativement l'usinabilité des aciers MP, mais le fait sans affecter de façon notable les propriétés mécaniques et la résistance à la corrosion. Il a été démontré qu'il est possible d'obtenir des particules de graphite libres dans un acier MP par enrobage de celles-ci. Cet enrobage permet d’empêcher le graphite de diffuser dans la matrice de fer pendant le frittage. Dans cette étude, des particules de graphite enrobées de nickel ont été recouvertes de cuivre par le procédé de cémentation. Un traitement thermique a ensuite été réalisé sur ce nouveau matériau afin d’obtenir un revêtement plus uniforme. Les résultats des essais de caractérisation mécanique sur des échantillons frittés (FC-0208) contenant des particules prémélangées de graphite enrobées de cuivre / nickel, soit traitées thermiquement ou non, montrent que ce nouvel additif ne détériore pas les propriétés mécaniques statiques et dynamiques des aciers MP. De plus, la résistance à la corrosion des échantillons contenant cet additif se révèle être la même que celle des échantillons sans additif. Les effets de l’additif non traité thermiquement et traité thermiquement sur l'usinabilité des aciers MP ont également été caractérisés en utilisant un opration de

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perçage. Les résultats obtenus montrent que ce nouvel additif peut améliorer significativement l'usinabilité en réduisant la force de coupe requise.

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Abstract

Although powder metallurgy (PM) is known as a near-net-shape fabrication method, noticeable amount of PM parts need some sort of machining. Secondary machining operations are usually required for dimensional conformance or producing complicated geometrical features that cannot be achieved at the compaction stage. However, due the presence of porosity, machinability of PM steels is difficult compared to wrought steels and can add 20% or more to the overall fabrication cost of PM parts. Thus, improving machinability of PM steels can definitely reduce their production costs. Among the various measures known to improve machinability of PM steels, addition of machining aids, either as admixed or pre-alloyed, is the most popular one. There are tens of different elements and compounds that can improve machinability at different levels. Nevertheless, their negative effects on other properties of PM steel components, such as mechanical properties and corrosion resistance, make their utilization somewhat limited.

In this study, free graphite particles are introduced as a new additive that not only significantly improve machinability of PM steels, but also does not affect the mechanical and corrosion properties. It was found that it is possible to have free graphite particles in a PM steel, sintered using conventional sintering conditions, by coating the graphite particles. This coating can prevent graphite from diffusing into the iron matrix during sintering. In this research, nickel coated graphite particles were coated with copper through cementation process. A heat treatment was then performed on this newly developed material to have a more uniform single layer coating. The results of mechanical characterization tests on the copper steel sintered samples containing admixed copper/nickel coated graphite particles, either in the form of non-heat-treated or heat-treated, showed that this new additive does not deteriorate static and dynamic mechanical properties of PM steels. Moreover, the corrosion resistance of the samples containing copper/nickel coated graphite was found to be the same as samples without additive. The effects of non-heat-treated and heat-treated copper/nickel coated graphite on machinability of PM steels were also characterized using drilling test. It was seen that this new additive can significantly improve machinability through reducing the required cutting force.

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Table of content

1 INTRODUCTION ... 1

2 LITERATURE REVIEW ... 3

2.1 MACHINING OF METALS ... 3

2.1.1 Different machining operations ... 3

2.1.1.1 Drilling ... 3 2.1.1.2 Turning ... 4 2.1.1.3 Milling ... 5 2.1.1.4 Grinding ... 6 2.1.1.5 Broaching ... 7 2.1.2 Principles of machining ... 7

2.1.3 Chip formation mechanism ... 8

2.1.3.1 Orthogonal cutting ... 9

2.1.3.2 Forces in orthogonal cutting ... 9

2.1.3.3 Stresses ... 11

2.1.3.4 Shear angle ... 12

2.1.3.5 Speeds ... 13

2.1.4 Tool/work-piece and tool/chip interactions ... 13

2.1.5 Tool wear ... 15 2.1.5.1 Abrasion wear ... 15 2.1.5.2 Adhesion wear ... 15 2.1.5.3 Diffusion wear ... 16 2.1.5.4 Fatigue wear ... 16 2.1.5.5 Deformation wear ... 17

2.1.6 Heat generation in machining ... 17

2.2 MACHINABILITY ... 19

2.2.1 Machinability characterization techniques ... 20

2.2.1.1 Tool wear or tool life ... 20

2.2.1.2 Temperature during machining ... 21

2.2.1.3 Cutting forces or energy consumption ... 21

2.2.1.4 Chip morphology ... 21

2.2.1.5 Quality of surface finish ... 22

2.2.1.6 Physical and mechanical properties ... 22

2.2.2 Machinability of powder metallurgy steels ... 23

2.2.3 Factors affecting machinability of PM steels ... 24

2.2.4 Effect of fabrication processing on machinability of PM steels ... 25

2.2.4.1 compaction ... 25

2.2.4.2 sintering ... 25

2.2.5 Effect of material characteristics on machinability ... 26

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2.2.5.2 Chemical composition ... 29

2.2.5.3 Microstructure ... 32

2.3 IMPROVING MACHINABILITY OF PM STEELS ... 33

2.3.1 Using machining aids to improve the machinability of PM steels ... 35

2.3.2 Different kinds of machining aids ... 36

2.3.2.1 Metals ... 36

2.3.2.2 Sulfides ... 37

2.3.2.3 Oxides ... 38

2.3.2.4 Fluoride ... 38

2.3.2.5 Boron nitride ... 39

2.4 GRAPHITE AS A MACHINING AID ... 41

2.4.1 Reported studies on the effect of free graphite on machinability ... 41

2.4.2 Solubility of carbon in iron ... 42

2.4.3 Retained graphite in a sintered part ... 44

2.4.3.1 Graphitization ... 44

2.4.3.2 Preventing diffusion of carbon into iron particles ... 44

2.5 COATED GRAPHITE ... 45

2.5.1 Oxidation protection of carbon: ... 45

2.5.2 Increasing the wettability and decreasing the reactivity of carbon: ... 46

2.5.3 Liquid state of a coating layer on the surface of graphite ... 50

2.5.4 Proper coating material and process for the purpose of diffusion prevention of graphite ... 51

3 METHODOLOGY ... 54

3.1 MATERIAL PREPARATION ... 54

3.1.1 Coating copper on nickel coated graphite ... 54

3.1.2 Heat treatment of CNCG particles ... 55

3.1.3 Preparation of powder mixture ... 55

3.1.4 Compaction and sintering ... 56

3.2 CHARACTERIZATION METHODS ... 57

3.2.1 Determination of size distribution of Nickel-coated graphite and CNCG powders .... 57

3.2.2 Measuring density of CNCG particles ... 58

3.2.3 Measuring density of the sintered and green samples ... 58

3.2.4 Compressibility of the powder mixtures containing CNCGs ... 58

3.2.5 Mechanical properties measurement ... 59

3.2.6 Measurement of the three-point fatigue endurance limit ... 59

3.2.7 Measuring the humidity adsorption of CNCG particles ... 60

3.2.8 Evaluation of corrosion resistance ... 60

3.2.9 Machinability characterization using drilling ... 61

3.2.10 Measurement of the diameter of the drilled holes ... 62

3.2.11 Microstructural characterization methods ... 63

4 RESULTS AND DISCUSSIONS ... 64

4.1 COPPER/NICKEL COATED GRAPHITE PARTICLES ... 64

4.1.1 Choosing copper/nickel combination as the coating material ... 64

4.1.2 Optimization of copper coating on nickel coated graphite particle ... 67

4.1.3 Characterization of copper/nickel coating layer ... 69

4.1.4 Heat treatment of CNCG particles ... 73

4.1.5 Measurement of the density of CNCG particle ... 77

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4.1.7 Studying the concentration distribution of nickel, copper and iron in the coating layer 79

4.1.8 Conclusion ... 81

4.2 MICROSTRUCTURAL AND MECHANICAL PROPERTIES CHARACTERIZATION OF THE COPPER STEEL CONTAINING CNCG PARTICLES ... 83

4.2.1 Compressibility characterization of powder mixture containing CNCG ... 83

4.2.2 Microstructural study of the sintered parts containing coated graphite particles ... 84

4.2.3 Concentration distribution of Fe, Ni and Cu in the coating layer ... 88

4.2.4 Effect of CNCG particles on density variation of FC-0208 sintered samples ... 92

4.2.5 Characterization of Static Mechanical Properties ... 94

4.2.5.1 Transverse rupture strength (TRS) ... 94

4.2.5.2 Tensile properties ... 96

4.2.6 Characterization of fatigue properties ... 97

4.2.7 Humidity adsorption measurement of CNCG powder ... 100

4.2.8 Corrosion test ... 101

4.3 MACHINABILITY CHARACTERIZATION OF SAMPLES CONTAINING CNCG PARTICLES ... 104

4.3.1 Tool wear measurement ... 105

4.3.1.1 Proposed mechanisms for the positive effect of CNCGs on machinability ... 108

4.3.2 Chip analysis ... 112

4.3.2.1 Cutting force analysis based on chip microhardness ... 112

4.3.2.2 Microstructural analysis of chips ... 115

4.3.2.3 SEM analysis of the chips ... 117

4.3.3 Analysis of the densified layer ... 119

4.3.4 Analysis of the surface of the tool ... 120

4.3.5 Analysis of hole diameter and circularity variation ... 124

5 GENERAL DISCUSSION ... 128

6 CONCLUSIONS ... 133

6.1 KEY FINDINGS ... 133

6.2 SUGGESTED FUTURE WORKS ... 134

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List of figures

Figure 2-8- Schematic presentation of the forces involved in orthogonal cutting, a=cutting angle and f=shear angle [2]. ... 10 Figure 2-9- Simplified presentation of the forces involved in cutting, b=friction angle [2]. ... 10 Figure 2-10- Schematic presentation showing the variation of the thickness before and after cutting [4]. .... 12 Figure 2-11- Schematic presentation of three vectors of the speeds V, VC and VS [4] ... 13 Figure 2-12- Built up edge, flank wear and crater wear of the cutting tool [5]. ... 14 Figure 2-13- Schematic presentation of the tool wear by diffusion mechanism, a) diffusion of workpiece components into the cutting tool and b) diffusion of the tool component into the chip [1]. ... 16 Figure 2-14- Temperature profile at the interface of chip and tool [8]. ... 18 Figure 2-15- Temperature contours of the tool used for cutting low carbon steel at different cutting speed, for a cutting time of 30 s [2]. ... 19 Figure 2-16. Classification of parameters affecting the machinability of PM materials [23]. ... 25 Figure 2-17. Schematic demonstration of cutting the surface of a porous material [25]. ... 27 Figure 2-18. Deformation cutting theory. (a) pore closure caused by deformation of work-piece material at tool/work-piece interface, (b) formation of densified layer at the machined surface [28]. ... 28 Figure 2-19. the effect of carbon content on tool wear in machining of a sintered iron [1]. ... 30 Figure 2-20. The effect of carbon content on the surface finish of a machined PM steel [1]. ... 31 Figure 2-21. Solubility of carbon in alpha iron [58]. ... 43 Figure 2-22. Combined and free carbon content for Fe-0.8%C sintered at different sintering temperature for 60 minutes under N2 atmosphere [57]. ... 43 Figure 2-23. Wetting angle between liquid and solid substrate [78]. ... 51 Figure 3-1- Schematic presentation of TRS bar. ... 57 Figure 3-2- Schematic presentation of dog-bone (dimensions shown in mm). ... 57 Figure 3-3- Photograph used as the reference for rakning the level of corrosion [82] ... 61 Figure 3-4- CNC machine is drilling a cylinder. ... 62 Figure 4-1- Optical micrographs of cross sections of nickel coated graphite particles coated with copper at three diferent weight ratios of copper sulfate to nickel coated graphite: a) 1.3, b) 2.6 and c) 4. ... 68 Figure 4-2- Size distribution of CNCG particles. ... 69 Figure 4-3- SEM micrographs of CNCG particles after copper coating process. ... 70 Figure 4-4- Optical micrographs of the cross section of CNCG particles after coating. ... 70 Figure 4-5- Distribution of the thickness of the a) nickel coating and b) copper coating. ... 71 Figure 4-6- Backscattered SEM micrographs of the cross-section of the coated graphite particle at different magnifications; a: lower magnification and b: higher magnification. ... 72 Figure 4-7- EDS patterns of the coating layers; top: outer layer and bottom: inner layer ... 72 Figure 4-8- Cu-Ni binary phase diagram ... 73

x Figure 4-9- SEM micrographs of a coated graphite particles after heat treatment at 700 °C, a) Secondary electron imaging of the particle and b) back scatered electron imaging of the cross section of the particle .. 74 Figure 4-10- EDS patterns of the cross section of the coating layer of CNCG particle after heat treatment at 700 °C; a) outer layer and b) inner layer. ... 75 Figure 4-11- a) cross section of a non-heat-treated CNCG showing the scan line (red dotted line) and b) concentration profile of copper and nickel through the scan line obtained by EPMA. ... 76 Figure 4-12- a) cross section of a heat-treated CNCG showing the scan line (red dotted line) and b) concentration profile of copper and nickel through the scan line obtained by EPMA. ... 76 Figure 4-13- Optical micrographs of the sintered part containing iron powder and 1.75 wt. % CNCG particles, etched in Nital 2% for 20 seconds, a) lower magnification and b) higher magnification. ... 80 Figure 4-14- Concentration distribution of a) Fe, b) Ni and c) Cu in the coating layer of a non-heat-treated CNCG as well as d) Fe, e) Ni and f) Cu in the coating layer of a het-treated CNCG. ... 81 Figure 4-15- Compressibility curve of the powder mixtures without additive and with different amounts of non-heat-treated and heat-treated CNCGs. ... 84 Figure 4-16- Optical micrographs of sintered parts containing 1.75 wt.% of non-heat-treated CNCG particles. a) as polished, b), c and d) etched in Nital 2% for 10 seconds,. ... 85 Figure 4-17- Optical micrographs of sintered parts containing 1.75 wt.% of heat-treated CNCG particles. a) as polished, b), c and d) etched in Nital 2% for 10 seconds,. ... 86 Figure 4-18- X-ray mapping of the sintered FC-0208 samples containing CNCG. ... 89 Figure 4-19- a,c) cross section of a non-heat-treated and heat-treated CNCG showing the scaned point (gray dots) and b,d) concentration profiles of iron copper and nickel through the scan line obtained by EPMA. ... 91 Figure 4-20- Variation of density of the FC-0208 samples containing different amount of non-heat-treated or heat-treated CNCG particles. ... 93 Figure 4-21- Mean TRS values and mean densities of the minimum of 5 specimens for FC-0208 sintered samples without additive, with 0.5 wt.% MnS and containing 0.77, 1.2, 1.75 and 3.5 wt.% of non-heat-treated and heat-treated CNCG particles. ... 95 Figure 4-22- S-N fatigue curves of the samples without additve, containing 1.75 wt.% heat-treated CNCG and containing 0.5 wt% MnS ... 98 Figure 4-23- Relative mass loss of non-heat-treated and heat-treated CNCG and MnS powders by increasing temperature ... 101 Figure 4-24- Percentage of the samples that were corroded at different levels, at different intervals, a) FC-0208, b) FC-0208+1.75 wt.% HT-CNCG and c) FC-0208+0.5 wt.% MnS. ... 103 Figure 4-25-Machinability of samples containing non-heat-treated and heat-trated CNCG particles and samples containing MnS, in terms of variation of flank wear as a function of voume of material removed . 106 Figure 4-26- Graphite lamellae in cast iron matrix, a) before and b) after indentation test [107]. ... 110 Figure 4-27- The cutting surface of a FC-0208 sample containing 1.2 wt.% of heat-treated CNCG particle, a) formation of microcracks initiated from the free space left by graphite break-out of CNCG particle and b) a highly compacted graphite in the middle of a CNCG particle, both are shown by arrows. ... 111 Figure 4-28- Microhardness isocotours over the cross section of the chip segments collected from samples containing a, b) 1.2 wt.% heat-treated CNCG, c, d) 1.75 wt.% heat-treated CNCG and e, f) 0.5 wt.% MnS. Left column (a, c and e): after drilling 48 holes and right column (b, d and f): after drilling the last sample. ... 113 Figure 4-29- Schematic illustration of forces acting on a) new and b) worn tool [110]. ... 115 Figure 4-30- Optical micrographs of the etched cross section of the chips collected after drilling 48 holes of samples a) containing 1.2 wt.% and b) 1.75 wt.% heat-treated CNCGs ... 116 Figure 4-31- High magnification optical micrographs of the etched cross section of the chips collected after drilling 48 holes in the samples containing CNCGs. ... 116 Figure 4-32- SEM micrographs of the chips collected from drilling samples containing a) 1.75 wt.% heat-treated CNCG and b) 0.5 wt.% MnS, after making 48 holes. Higher magnification of the marked chips c) I and d) II of image a. ... 118

xi Figure 4-33- SEM micrograph of the cross section of a chip collected from drilling samples containing CNCG. ... 118 Figure 4-34- Optical micrographs of the as-polished cross sections of the holes for samples a) containing 1.75 wt.% HT-CNCG and 0.5 wt.% MnS. densified layers are shown with the red rectangles. ... 119 Figure 4-35- a) SEM micrograph of the flank surface of the tool used for cutting all the samples containing 1.2 wt.% non-heat-treated CNCG, b, c and d) EDS spectrums of the regions 1, 2 and 3 respectively. ... 122 Figure 4-36- a) SEM micrograph of the flank surface of the tool used for cutting all the samples containing 1.2 wt.% heat-treated CNCG, b, c and d) EDS spectrums of the regions 1, 2 and 3 respectively. ... 122 Figure 4-37- a) SEM micrograph of the flank surface of the tool used for cutting all the samples containing 1.75 wt.% non-heat-treated CNCG, b, c and d) EDS spectrums of the regions 1, 2 and 3 respectively. ... 123 Figure 4-38- a) SEM micrograph of the flank surface of the tool used for cutting all the samples containing 1.75 wt.% heat-treated CNCG, b, c and d) EDS spectrums of the regions 1, 2 and 3 respectively. ... 123 Figure 4-39- a) SEM micrograph of the flank surface of the tool used for cutting all the samples containing 0.5 wt.% MnS, b and c) EDS spectrums of the regions 1 and 2 respectively. ... 124 Figure 4-40- Variation of the diameter error as a function of number of holes drilled on samples a) containing 1.2 wt.% NHT and HT CNCG and b) containing 1.75 wt.% NHT and HT CNCG and 0.5 wt.% MnS ... 126 Figure 4-41- Schematic presentation of the cirularity ... 127 Figure 4-42- Frequency of the holes with the circularity value in each of the defined ranges. ... 127

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List of tables

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Acknoledgement

First of all, thanks to Prof. Carl Blais. I cannot tell you how much I have appreciated your support, your faith in me, your kindness, and the freedom you provided me in order to finish this dissertation. You have been much more than a supervisor. You are an inspiring role model. You are the best supervisor ever.

Many thanks to all my colleagues and staffs of Mining, Metallurgical and Materials Engineering Department of Laval University for their invaluable support and helps.

Thanks to my Mom and dad. I have no words to acknowledge the sacrifices you made and the dreams you had to let go, just to give me a shot at achieving mine. One of the most beautiful thing in this world is to see your parents smiling, and knowing that you are the reason behind that smile.

Finally, thanks to my beloved soulmate, Saba. In the past thirteen years of our life, you have always helped me to regain hope after despair, resume life after obstructions, restart journeys after detours, revive strength after defeat and revive dreams after rejection. If there was a number higher than zillion, bazillion or gazillion, I would thank you that many number of times for everything you have done for me. I love you more than Canadian love hockey.

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1 Introduction

Although the PM industry prides itself on being a near net-shape process, almost 40% of parts produced by this technology are submitted to at least one of the various machining operations. Specific geometrical features that cannot be fabricated by the compaction process is the main reason for the necessity of the secondary operations on PM parts. Furthermore, the tight tolerances required for PM components in high-performance applications make machining inevitable. Nevertheless, due to the presence of residual porosity in typical PM steel components, their machinability is noticeably lower compared to that of the wrought counterparts. Thus, in the case of parts having a relatively complex shape, the cost of machining PM steel components can add up to 25% of the total production cost.

Various strategies have been considered to find a solution to improve the response of PM steel parts to machining. The vast majority of these strategies rely on the addition of a chemical compound, called machining aid, to the base powder mixture. This machining aid enhances machinability by reducing the cutting forces required to remove material with a cutting tool. Tens of different compounds such as MnS, BN-h, MgSiO3, etc. have been used

and have shown to significantly improve machining. However, this improvement was obtained at the expense of lower static and dynamic mechanical properties. Moreover, some of these machining enhancers, especially MnS, are hygroscopic and may degrade while reacting with constituents of the sintering atmosphere. Thus, the problem to be solved is the lack of a cost-effective additive that can improve machinability of PM steel without deteriorating its mechanical properties and corrosion resistance.

In this study, graphite is going to be introduced as a machining aid for PM steels. As it is known, gray cast iron presents a wonderful machinability due to the presence of free graphite in its microstructure. Graphite is a well-known lubricant that can reduce the cutting force by reducing the friction coefficient of the sliding surfaces in the machining processes and also by accelerating the fracture of the chips. Moreover, it is the most frequently used alloying element in PM steels thus, can be used in the fabrication of all PM steels without

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causing any contamination to the production equipment. However, graphite in its native form cannot be used directly in ferrous PM since it easily diffuses within the iron matrix of the base powder to form steel and possibly pro-eutectoide iron carbides. Consequently, in order to have free graphite in a PM steel component, it is required to prevent its diffusion into the iron matrix. The hypothesis of this research is that it is possible to have free graphite in a PM steel by coating graphite particles with copper and nickel to form a barrier that prevents graphite from diffusing within the iron matrix. This new additive will be able to solve two problems. First, it will rely on graphite to significantly improve machinability of sintered PM steel components; secondly, due to the presence of a metallic coating at the surface of the graphite particles, the overall strength of the PM steel component will be improved, thus increasing the static and dynamic mechanical properties over parts relying on other machinability enhancers.

Based on the aforementioned hypothesis, the objectives of this study can be summarized as follows:

• Finding a cost-effective and simple process to coat nickel-coated graphite particles with copper. The coating layer obtained by this process should cover the surface of the graphite particles thoroughly and uniformly.

• Assessing the ability of the coating layers to prevent graphite diffusion into the iron matrix.

• Studying the effect of coated graphite particles on the mechanical properties and corrosion resistance of PM steel components.

• Investigation of the effect of coated graphite particles on the machinability of PM steel components.

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2 Literature review

2.1 Machining of metals

Machining includes all the secondary operations in which thin layers of materials are removed in the form of chips. Nowadays, these secondary finishing operations are required for the majority of metal products fabricated through different techniques (powder metallurgy, casting, etc.) to change their geometry or to improve their surface finish. Several machining methods may be used depending on the type of modification to be made to a part. However, the generic principles of machining are common to most machining processes, such as the formation of chips, cutting parameters and wear mechanisms.

In this section important machining operations, principles of machining and mechanisms of tool wear will be explained. The more specific field of machining of parts produced by the powder metallurgy process, which has several similarities and some differences with the machining of parts produced by conventional metallurgy, is also addressed.

2.1.1 Different machining operations

Machining operations can be classified based on different characteristics such as the relative motion between cutting tool and the work-piece, continuity of the cutting process or type of cutting tool. For instance Salak et al. [1] categorized cutting processes into two major groups based on the geometry of the cutting edge of the machining tool, namely defined or undefined geometry of the cutting edge. In this section, five of the most important machining operations are discussed.

2.1.1.1 Drilling

Drilling is one of the most common machining operations in the industry. This cutting operation is known as a process for making a cylindrical hole by means of a rotary tool (drill or drill bit). Drilling is usually done on a press drill or on a lathe and it is a combination of two movements: a main rotating motion as well as linear feed motion as shown in Figure 2-1-b. The chips generated in drilling are evacuated by the flutes present

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on the stump. Drilling with different tools allows generating holes of different depths and different surface finishes. For machining operations requiring a surface finish of superior quality, boring operations are generally carried out as an additional operation.

2.1.1.2 Turning

Turning is one of the most common machining operations in the industry. As it can be seen in Figure 2-1-a, turning is carried out on a rotating piece in which the material is removed in the form of chips by a tool with a defined cutting edge. The tool moves in a working plane passing through the axis of rotation of the piece. Parts made in turning may be finished parts, which can be directly assembled or used, and/or parts that need to be subjected to further machining operations. The main machining variables of the turning process are cutting speed, feed rate and depth of cut. Turning is generally considered as a non-interrupted machining process. An important advantage of turning is its flexibility in performing different kinds of operations. Figure 1-2 shows different shaping operations related to turning.

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Figure 2-2- Different operations that can be performed by turning, a) facing, b) taper turning, c) contour turning, d) form turning, e) chamfering, f) cutoff and g) Threading.

2.1.1.3 Milling

Milling is a machining operation for which the tool has a circular cutting movement and the work-piece is clamped on a table and the feed action is obtained by moving the latter under the cutter. In this process, material removal is performed by a rotating tool composed of several cutting edges (milling cutter). Since during the cutting operation, parts of these edges are in contact with the work-piece while the others move freely, milling is a typical machining operation with interrupted cut [1]. Examples of the milling process are shown in Figure 2-3.

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Figure 2-3- Examples of milling opertation

2.1.1.4 Grinding

Grinding is a machining process that is used to increase the surface quality and the accuracy of shape and dimensions of a part. Grinding is typically a finishing operation. In this process, the grinding wheel spindle rotates at high speeds and metal chips from the work-piece are detached by the sharp abrasive grains of this grinding wheel. The surface quality and the amount of metal to be removed are related to the grain size of the grinding wheel. The finer the grains of the grinding wheel, the better the surface finish [1]. Figure 2-4 shows two different grinding processes based on the relative motion of tool and work-piece.

Figure 2-4- Schematic presentation speed motion and feed motion in a) transverse grinding and b) infeed grinding.

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2.1.1.5 Broaching

The cutting tool used in broaching operation has multiple transverse cutting edges as shown in Figure 2-5. In this machining process, the broach is pulled or pushed over the surface to be cut and the consecutive cutting edges of the tool remove thin layers of metal. The broaching is used to produce flat surfaces and different forms of hole. Low cutting speeds and feeds are typically required for this operation [2].

Figure 2-5- Schematic presentation of broaching operation [2]

2.1.2 Principles of machining

Machining is the precise removal of materials from a work-piece in the form of chips by the action of a cutting tool to obtain a desired geometric shape and size. Material removal is accomplished due to the presence of a shear force along the primary shear plane. The material present in this region is initially compressed to the point where a crack is initiated. This crack propagates in a direction parallel to the primary shear plane from the cutting edge to the upper surface of the removed material. At this point, a separate chip is formed and evacuated from the machined area. During this process, there is also considerable friction between the chip and the surface of the tool. This friction is caused by the compressive stress exerted by the tool on the chip as well as the shear stresses resulting from the sliding of the chip on the tool. The region where this friction occurs is defined as the secondary shear plane (Figure 2-6).

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Figure 2-6- Primary and secondary shear zone in machining [3]

In the above-explained process of machining, all the metal to be removed is plastically deformed in the primary shear zone. Moreover, while the formed chip is moving away from the work-piece sliding on the rake face of the tool, it is diverted through an angle of at least 60 degree, depending on the angle of the tool edge. All of these steps including chip formation and movement of the chip require noticeable amount of energy. Furthermore, in the chip formation process, two new surfaces are created by the action of the tool. These two surfaces are the upper surface of the piece and the lower surface of the chip formed. Formation of these new surfaces also requires energy, which is insignificant compared to the required energy for the plastic deformation of metals [2].

2.1.3 Chip formation mechanism

A good understanding of the mechanisms involved in metal cutting is necessary to improve machinability. By having a better knowledge of the behavior of the tools during machining, the selection of the cutting tools in a given cutting operation can be optimized and the important microstructural parameter changes to achieve a satisfying machinability can be determined. Mechanisms related to orthogonal (2-D) cutting operations on perfectly dense and homogeneous materials will be discussed in this section.

Chip Secondary shear zone Primary shear zone Cut surface V V

9 2.1.3.1 Orthogonal cutting

An orthogonal cut refers to an ideal operation that assumes that the material is completely homogeneous. Other hypotheses related to this two-dimensional model are the following [4]:

• A perfectly sharpened and straight cutting edge perpendicular to the displacement; • No contact between the clearance face and the work-piece;

• Continuous chip formation without accumulation flowing on the flat face; • Constant cutting depth and cutting speed;

• The width of the tool is larger than the work-piece;

• Uniform shear stresses along the shear plane and the normal stresses along the tool;

In the majority of practical cases, the orthogonal section model can present a good approximation of the performance of the cutting edge of the tool. Figure 2-7 shows the schematic presentations of the orthogonal cutting and oblique cutting. Figure 2-8 shows the results of the forces involved in an orthogonal cut.

Figure 2-7- Schematic presentation of a) orthogonal cutting and b) oblique cutting.

2.1.3.2 Forces in orthogonal cutting

Figure 2-8 shows the forces applied on a chip in orthogonal cutting. As it can be seen, the resultant forces R and R' are the force between the tool and the chip and the force between the work-piece and the chip, respectively. At equilibrium condition, these two forces should be equal and can be calculated based on three different orthogonal systems, namely:

10

• According to the horizontal and vertical axes FP and FQ;

• According to axes parallel and perpendicular to the surface of the tool FC and NC;

• According to axes parallel and perpendicular to the shear plane FS and N5;

All the forces involved in orthogonal cutting shown in Figure 2-8 can be transferred to the tip of the cutting tool to simplify the calculations (Figure 2-9).

Figure 2-8- Schematic presentation of the forces involved in orthogonal cutting, a=cutting angle and f=shear angle [2].

Figure 2-9- Simplified presentation of the forces involved in cutting, b=friction angle [2].

According to Figure 2-9, the involved forces can be resolved to their components as follows:

11

𝐹_" = 𝐹_$cos 𝜙 − 𝐹_*sin 𝜙 𝑁_" = 𝐹_*cos 𝜙 + 𝐹_$sin 𝜙 𝐹_/ = 𝐹_$sin 𝛼 + 𝐹_*cos ∅ 𝑁_/ = 𝐹_$cos 𝛼 − 𝐹_*sin 𝛼

Knowing that friction coefficient is equal to the tangent of the friction angle and using the last two equations, the friction coefficient on the surface of the tool can be calculated:

𝜇 = 𝐹/ 𝑁_/ = (𝐹_$sin 𝛼 + 𝐹_*cos 𝛼) (𝐹_$cos 𝛼 − 𝐹_*sin 𝛼)= 𝐹_*+ 𝐹_$tan 𝛼 𝐹_$ − 𝐹_*tan 𝛼 2.1.3.3 Stresses

Using the first two equations of the previous section, shear stress and stress normal to the shear plane can be calculated. It is known that:

𝜏 = 𝐹8

𝐴"

𝜎 = 𝑁8

𝐴"

where AS is the surface of the shear plane and can be calculated as:

𝐴_{8 = (𝑏𝑡) sin 𝜙}

where b and t are the cutting width and depth of cut, respectively.

Thus, shear stress (t) and normal stress (s) will be:

𝜏 =(𝐹$cos 𝜙 − 𝐹*sin 𝜙) sin 𝜙 (𝑏𝑡)

𝜎 =((𝐹$sin 𝜙 + 𝐹*cos 𝜙) sin 𝜙 (𝑏𝑡)

12 2.1.3.4 Shear angle

Since according to experimental findings, density of the chip is the same as the density of the workpiece (assuming a perfectly dense metal), it can be said that the volume of the material before and after cutting are identical [4]. Thus,

𝑡𝑏𝑙 = 𝑡_/𝑏_/𝑙_/

where l is the cutting length. C index shows the parameter related to the chip. It has been found that if 𝑏 𝑡 ≥ 5, the width of the material before and after cutting are the same. So, the cutting ratio, r, which is the ratio of the undeformed chip thickness to the deformed chip thickness is:

𝑟 = 𝑡 𝑡_A = 𝑙 𝑙_A

In practice the chip is never thinner than the feed, which makes r<<1. According to Figure 2-10 the shear angle (F) can be calculated as:

𝑟 = 𝑡 𝑡_/ = 𝐴𝐵 sin 𝜙 𝐴𝐵 cos(𝜙 − 𝛼) 𝜙 = tanCD 𝑟 cos 𝛼 1 − 𝑟 sin 𝛼

13 2.1.3.5 Speeds

There are three different speeds, V, VC and VS in machining. Cutting speed, V, which is the

relative speed of the tool with respect to the work-piece in the direction of FP, chip speed,

VC, which is the relative speed of the chip movement with respect to the tool in the

direction parallel to the surface of the tool and shear speed, VS, which is the relative speed

of the chip with respect to the work-piece and in the direction of the shear plane. Based on the kinematic principles, the three vectors of speeds can make a closed diagram as shown in Figure 2-11 according to this diagram, chip speed and shear speed can be calculated according to the cutting speed and the angles of the triangle [4].

𝑉_/ = 𝑉 sin 𝜙

cos 𝜙 − 𝛼 = 𝑟𝑉 𝑉_" = 𝑉 cos 𝛼

cos 𝛼 − 𝜙

Figure 2-11- Schematic presentation of three vectors of the speeds V, VC and VS [4]

2.1.4 Tool/work-piece and tool/chip interactions

Longer tool life or less tool wear is the most important machinability criteria. In order to study tool wear as well as the different measures to decrease it, it is necessary to know the interactions taking place at the interface between tool and work-piece as well as tool and chip.

The ideal condition of machining occurs when a thin layer of the work-piece is easily removed by a cutting tool and then moves freely on its surface without significant

14

interactions. Unfortunately, this is not the reality of what is happening during machining. For example, welding of chips to the surface of the tool is an unfavorable interaction that may occur.

At lower cutting speed or in the case of cutting softer metals the welded chips might be deposited on the tool and become part of it. This phenomenon, which is called built up edge (BUE), changes the geometry of cutting tool and thus deteriorates drastically the surface quality of the work-piece [5].

In the case of higher cutting speeds or machining a medium to high strength metal, the problem caused by the welded chips is different. In these cases, the welded chips are pushed by the subsequent chips and broken off resulting in wear on the rake surface of the tool that is in contact with the chips. This type of tool wear is called crater wear[5].

There is also another kind of tool wear, called flank wear that occurs at the flank surface of the tool, the surface that is in direct contact with the work-piece. This type of wear is caused by the gradual removal of sections of the tool by the abrasive constituent of the work-piece. Work-pieces may contain abrasive particles such as oxides that increase this type of wear[5]. In Figure 2-12 tool/work-piece as well as tool/chips interactions are illustrated.

15 2.1.5 Tool wear

The premature wear of a cutting tool is one of the problems encountered during machining. The criterion for determining the end of life of a cutting tool depends on the type of application. For instance, the tool should be replaced when wear prevents it from cutting or the temperature in the cutting zone increases. Vibration or noise generated by wear as well as unsatisfying surface finish are also signs of the end of tool life. In machinability tests, the cutting tool is usually changed whenever wear reaches a predetermined value [2].The main wear mechanisms of the cutting tools are wear by abrasion, adhesion, diffusion, fatigue, and deformation of the tool.

2.1.5.1 Abrasion wear

Abrasion wear occurs when hard particles in the material are dragged mainly on the face of the cutting tool. These hard particles may be carbides or oxide inclusions originating from the microstructure of the material or, more rarely, broken particles of the cutting edge of the tool. A chemical reaction between the chip and the cutting fluid during machining can also lead to the formation of hard particles. This is particularly happening during high-speed machining of steels containing chromium. Oxygen contained in the air or in the lubricant/coolant emulsion can react with the chromium contained in the chip at high temperature and therefore, hard chromium oxides are formed [6, 7].

2.1.5.2 Adhesion wear

Adhesion is the result of a cold weld (solid state) that may occur between the machined material and the irregularities present on the surface of the tool. If this weld is torn off, small volumes of the cutting tools are pulled out and adhesion wear occurs. This wear mechanism usually takes place at low surface speed and low temperature of the cutting zone. Adhesion wear is more severe when cutting a high strength material due to the higher tangential stress on the cutting edge and therefore, the higher chance of pressure (cold) welding [1, 7].

16 2.1.5.3 Diffusion wear

This mechanism can occur at the interface between the tool and the work-piece at high temperature. A minimum temperature is required for this type of wear to occur. For instance, in the case of machining steel parts, wear by diffusion is possible when the temperature of the cutting zone is between 700 ° C to 900 ° C. However, in addition to temperature, the level of diffusion depends mainly on the solubility and the affinity between the different phases in contact as well as on their physical properties. The tool may be worn away by the diffusion of carbon or metal atoms from the surface of the work-piece or from the chips sliding on the face of the tool. There may also be diffusion of chemical elements from the protective coating of the tool that may react with the metal in contact with it. This can weaken the tool surface and increase its vulnerability to wear. The two-aforementioned diffusion mechanisms are shown in Figure 2-13. As it can be seen in Figure 2-13-a, a new surface with lower hardness compared to the cutting tool, which is originally made of WC, is formed on the face of the tool whereas in Figure 2-13-b diffusion of cutting tool component into the formed chip is the reason of tool wear [1].

Figure 2-13- Schematic presentation of the tool wear by diffusion mechanism, a) diffusion of workpiece components into the cutting tool and b) diffusion of the tool component into the chip [1].

2.1.5.4 Fatigue wear

Fatigue wear can be caused by two different mechanisms including vibrations applied to the tool and thermal fatigue. Machining porous microstructure such as that of powder metallurgy parts can cause accelerated wear of the cutting tool due the vibrations. In fact,

g-Fe(C) g-Fe(W,C), Fe2W2C WC+COxWyCz+ Fe2W2C g-Fe(C) g-Fe(C) g-Fe(W,C) g-Fe (C ) WC+COxWyCz+ Fe2W2C a _b

17

the presence of pores can lead to the periodic loading and unloading of the cutting tool resulting in the formation of microcracks on the surface of the tool. Thermal fatigue is also the result of the sequential expansion and contraction of the surface layers of the cutting tool due to heating and cooling during intervals of machining [7].

2.1.5.5 Deformation wear

If the cutting tool has a low resistance to deformation, especially at high temperature, deformation wear can occur, which results in the blunting of tip of the tool. This wear mechanism is generally related to the creep properties of the material of the cutting tool and can be explained by the combination of stresses and high temperatures localized near the tip of the tool causing a plastic deformation of the tool [7].

2.1.6 Heat generation in machining

As it was explained before, significant amount of energy is required for cutting metals. Almost all of the energy used for machining (up to 95%) is converted into heat at the cutting edge and transferred to the sections that are present in the cutting zone. For example, at the cutting speed of 150 m/min, the generated heat during metal machining is dissipated in the chip (75-80%), in the cutting tool (10-15%) and in the machined part (5-10%) [7]. The three main sources of heat are primary plastic deformation (at the primary shear zone), secondary plastic deformation (at the secondary shear zone) and the friction between the lower surface of chip and the rake face of the tool. Due to these three sources of heat, the distribution of temperature in the tool and chip is not uniform [1]. Figure 2-14 shows an example of temperature profiles at the interface of chip and tool during machining. As it can be seen the maximum temperature is obtained at the contact zone of the chip and tool.

18

Figure 2-14- Temperature profile at the interface of chip and tool [8].

Cutting speed has the most important effect on heat generation, while the effect of the thickness and the width of the machined layer on heat formation are less significant. Increasing the cutting speed increases the speed of sliding of the chips on the tool and therefore the friction in contact zone between the tool and the chip will be higher, which results in the augmentation of the temperature in this region [1]. Figure 2-15 shows different temperature contours while drilling a low carbon steel at different cutting speeds. It can be seen that increasing the cutting speed increases the temperature at the cutting edge and also increases the depth of the region affected by high temperature [2].

Chip

Wokpiece

19

Figure 2-15- Temperature contours of the tool used for cutting low carbon steel at different cutting speed, for a cutting time of 30 s [2].

2.2 Machinability

In order to investigate machinability, it is important to first define what it is. While Mills [9] and Zheng [10] introduced machinability as a property of a material that shows the ease or difficulty of machining a given material, Salak [1] and Stephenson [11] believe that machinability is not a material property but a machining system property. The latter definition seems to be more comprehensive than the first one since machinability is affected not only by material properties but also by a variety of factors including machining conditions. However, in as much as this research is mainly focused on the metallurgical aspects of the machining system, machinability in this document refers to a behavior of the

Cutting speed: 91 m/min

Cutting speed: 122 m/min

Cutting speed: 152 m/min

Cutting speed: 183 m/min

20

material to be machined and thus the best machinability mainly means fast chip removal, relatively long tool life and acceptable surface finish [1]. Apart from these three main parameters, the other criteria that can be used to characterize machinability are as follows [1, 12, 13]:

• lower temperature generated at the cutting interface • shorter time to perform a given machining operation • minimum overall production cost

• lower power consumption for removing a constant amount of material in equal condition

• similar dimensional precision in a consecutive process

It should be kept in mind that these various criteria are not comparable. In other words, while a material might have good machinability according to one of them, it could be characterized as having poor machinability by another one [1, 14]. Thus, in order to describe machinability of a material, it is mandatory that the characterization criteria be clearly stated.

2.2.1 Machinability characterization techniques

Evaluating the machinability of a material with an index considering all of the above criteria at the same time is complicated and confusing. Although characterizing machinability via a single index on the basis of just one criterion might be one-sided and not sufficiently comprehensive, it is more practical and useful [14].

2.2.1.1 Tool wear or tool life

The most relevant and popular criterion to characterize machinability is the wear of the cutting tool. It can be said that for the same machining operation, while all the machining parameters such as cutting speed and feed rate are similar, machinability of a material is better than another if the wear is lower for a given quantity of material removed.

In order to evaluate machinability using this technique, tool wear should be measured at various intervals of machining until it reaches a fixed value (for instance 380 microns in

21

case of measuring flank wear) or the failure of the tool. The disadvantage of using this criterion is that the tests are time consuming especially in the case of materials having excellent machinability [1]. At the end of this test, the weight or volume of removed material in a machining test before tool failure is reported as the machinability index.

2.2.1.2 Temperature during machining

Measurement of the temperature at the interface of the chip and work-piece is another criterion for machinability evaluation. For the same machining conditions, the material that generates lower heat and therefore causes lower temperature in the cutting zone will be ranked as the one with better machinability [12]. As it was explained in section 2.1.5, some of the wear mechanisms such as diffusion are related to temperature and an increase in temperature at the tool/chip interface can activate or intensify those mechanisms, and therefore increase tool wear. However, complexity of measuring the temperature at the chip/tool interface is the disadvantage of this technique [1].

2.2.1.3 Cutting forces or energy consumption

The cutting forces required to remove material in the form of chips are generally an indication of tool wear. The greater the tool wear, the greater the forces required to machine the material. Usually a load cell is required to measure the cutting force during a machining process. This technique can give a quick and precise indication of the relative machinability of two materials. The variation of required cutting force by increasing the amount of material removed can be reported as the result of this machinability characterization method. On this machinability curve, energy consumption can also be assessed by comparing the area under the curve, which is a function of the force and slope of the curve.

2.2.1.4 Chip morphology

Chips are the byproducts of the machining process and can reveal some information on the machinability characteristics of the material. Generally, a material that generates short chips during machining operations is considered to have better machinability than a material that generates long chips. Short chips are easier to evacuate, which can lead to a better removal of the material near the chip/tool interface and thus, lower cutting force. In

22

addition, long and continuous chips can impair the quality of the machining by settling at the cutting site. Shape of the chips can also be an indicator of the friction at the chip/tool interface. The smaller radius of the chips can indicate the lower contact between the chips and the rake face of the tool and therefore, lower shear stress at the secondary shear zone, which means lower tool wear [15].

A quick and inexpensive way of characterizing machinability is to measure the time required to drill one hole (using fixed drilling parameters), although the discrepancies of the results of different tests show that it is not a reliable index [13].

2.2.1.5 Quality of surface finish

Surface finish is another criterion to compare the machinability of materials. It can be evaluated either visually, via optical microscopy or quantitatively via surface roughness measurement methods [16, 17]. However, in the case of visual measurements, the result may not be sufficiently reliable [13]. High surface roughness is the sign of a worn tool and means low machinability. Measuring the quality of surface finish might not be precise in the case of PM parts due to their porosity and probable difference in the morphology of the porosity.

2.2.1.6 Physical and mechanical properties

Machinability of a material can also be estimated by its physical or mechanical properties. For instance, Henkin & Datsko [18] related machinability of metals to Brinell hardness (HB), thermal conductivity (B), cutting length (L) and reduction of the cross section of the

sample during tensile test (Ar) through the following equation. This relationship can be

used for metals within the same alloy family and thus cannot be used to compare machinability of steels and other metals.

𝑉₆₀ ∝ 𝐵 𝐿𝐻_𝐵 1 − 𝐴_𝑟 100 1 2

According to this equation, the material with lower hardness is expected to show higher machinability. However, it should be reminded that materials with similar macrohardness

23

do not necessarily have microstructure with similar microhardness, which can have significant effect on machinability.

Thermal conductivity as a physical property of a material can also affect machinability. Higher thermal conductivity can improve machinability by evacuating heat from the cutting zone more rapidly, which leads to lower temperatures at the chip/tool interface and thus can reduce the wear mechanism that are activated at high temperatures. It should be noted that the physical and mechanical properties of the materials can be an indication of machinability by presenting the effect of microstructure on machinability indirectly. In other words, the microstructure is primarily responsible for the determination of machinability and not the physical and mechanical properties.

As it can be seen, each machinability characterization technique has pros and cons that should be considered and perfectly understood before selecting one. Measuring tool life directly through measuring the weight of removed material before tool failure is a reliable method since it is easy to perform and sufficiently precise. The popularity of this method also makes it easier to compare machinability between different materials.

2.2.2 Machinability of powder metallurgy steels

The powder metallurgy (PM) process is known as a near net shape manufacturing route. However, almost 50% of its products require secondary shaping operations [19]. Complexity in shape such as undercuts and holes perpendicular to the pressing direction, which might be impossible or inefficient to form through conventional compaction, is one of the main reasons why machining operations are required. Discrepancies as a result of non-uniform dimensional change during sintering could be also another cause necessiting machining of a sintered part. According to an article by Benner and Beiss [20] machining costs can add up to 20% or more of the overall fabrication cost of a PM part, showing the importance of considering some measures to reduce the cost of machining. In order to minimize machining expenses, the machinability of PM parts as well as the machining technology should be studied and optimized.

Comparing the machinability of wrought and PM steels, lots of researchers believe that the latter has poor machinability compared to the first one due to two unique characteristics of

24

PM steels, porosity and heterogeneous microstructures. On the contrary, some others are of the opinion that this conclusion is a general and unclear statement in terms of machinability of PM steels since comparing a wrought and a PM steel is not straightforward. For instance, PM steels having the same mechanical properties as wrought steels usually require higher amounts of alloying elements, which means different composition and microstructure. Consequently, it is hard to find a PM steel and a wrought steel with the same microstructure and mechanical properties to make a precise comparison of machinability. Moreover, the exceptions refuting the above statement are that high density PM materials such as forged parts as well as those compacted via hot isotactic pressure show the same machinability as wrought materials as a result of their high density [21]. Talking about the machinability of PM compared to wrought parts, it should be kept in mind that while the maximum amount of removed material in machining of a PM part is 10-15%, it can be 40-60% in case of machining of a wrought part [22].

Despite the difficulties in comparing the machinability of PM and wrought steels, the opinions that support the poor machinability of PM parts seem more acceptable because of detrimental effects of porosity and heterogeneous microstructure, two inherent characteristics of a PM part, on machinability. The theories supporting this fact are discussed in detail in the following sections. Moreover, the effects of powder metallurgy parameters on machinability of a PM part are also discussed through studying their effect on porosity and microstructure.

2.2.3 Factors affecting machinability of PM steels

A machining system is affected by various parameters that can be divided generally into three categories including: work-piece, tool and cutting condition regardless of wrought and PM materials [23]. In the case of machining of a PM part, machinability is influenced by additional parameters related to the characteristics of a powder metallurgy product that are considered in the work-piece category. This classification is shown in Figure 2-16 as a fishbone diagram for the machining of PM steels. Since the main concern of this document is about the characteristics of a PM steel work-piece, the other two categories are not going to be considered in more details.

25

Figure 2-16. Classification of parameters affecting the machinability of PM materials [23].

From a metallurgical point of view, effects of a PM work-piece on machinability can be classified into two categories [1]:

• Fabrication processes; including compaction into green part and sintering

• Material characteristics; including base powder (which is iron in this report), alloying elements, microstructure, mechanical properties and porosity.

2.2.4 Effect of fabrication processing on machinability of PM steels 2.2.4.1 compaction

Compaction has an indirect effect on machinability. Compaction pressure as well as compaction technique have a significant influence on density. Increasing the compacion pressure decreases porosity and thus decreases the adverse effects of pores on machinability as it will be discussed later in section 2.2.5.1. Specific compaction techniques such as warm compaction, hot isostatic pressure and hot forging, also decrease the porosity and by this improve the machinability of PM steels [24].

2.2.4.2 sintering

Sintering of a green part can change the machinability by affecting the final microstructure, the amount and morphology of porosity, distribution of alloying elements and mechanical properties. For instance, the type of sintering atmosphere, carburizing or decarburizing,

26

determines the carbon content of the sintered product through which machinability could be affected. Moreover, it can affect oxidation of the iron particles and thus change machinability. On the other hand, sintering temperature can affect the volume fraction of porosity as well as its shape and indirectly change machinability. Increasing the sintering temperature decreases the proportion of porosity while making it rounder and thus, improves machinability [1, 22].

2.2.5 Effect of material characteristics on machinability 2.2.5.1 Porosity

Although porosity is known as the main factor affecting negatively the machinability of PM parts, it can be said that there is still no verified and decisive theory explaining this detrimental effect. Various machinability/porosity relationships considered in literature are as follows:

• Interrupted cutting theory: during machining, either turning or drilling, the tool edge moving in the metallic phase enters into the pore and immediately re-enters into the metallic phase. Cutting the surface of a porous material with the cutting tool is demonstrated schematically in Figure 2-17. This discontinuity in cutting causes alternative loading-unloading cycles on the tool and consequently imposes mechanical and thermal fatigue that leads to accelerated failure of the cutting tool [1]. Moreover chatter and undesired vibrations is increased as a result of the discontinuity in the contact between tool edge and work-piece. This also results in poorer machinability [25]. With respect to this theory there are lots of pores, most likely interconnected, at the surface to be machined that provides a path for cutting fluid to escape from the cutting zone thus resulting in increased tool wear [26]. Although interrupted cutting theory is widely used to describe the detrimental effect of porosity on machinability, it could be refuted by two other points of view. Agapiou and Devries [12] believed that this theory is impossible practically since the size of tool edges and consequently the contact area of tool and work-piece is at least 20 times larger than the size of each pore. Thus, considering a fully engaged

27

cutting edge with a pore is not a precise assumption in this theory. The other refuting opinion of this theory is related to the deformation cutting theory.

Figure 2-17. Schematic demonstration of cutting the surface of a porous material [25].

• Deformation cutting theory: machining of a wrought metal causes formation of a deformed and work-hardened layer due to high stress and local pressure caused by the tool edge. Microstructural investigations showed that deformation is also occurring in machining of a porous part and leads to the formation of a densified layer i.e. a layer without or with less amount of porosity [27]. According to the deformation cutting theory the cutting edge imposes permanent pressure on the machined surface and pushes the material into the adjacent pores leading to the formation of an almost dense layer on the machined surface. This means that during machining of a PM part, the cutting edge rarely comes into contact with a pore interrupting the cutting process [25], which refutes the interrupted cutting theory explaining machinability/porosity relationship. Pore closure as well as formation of the densified layer at the surface that has been already machined are shown in Figure 2-18.

In the case of machining of PM parts, work hardening of the subsurface is lower compared to wrought metals since a significant portion of the imposed pressure by the tool is consumed for pushing the material into the pores therefore a lower cutting force is required to machine a PM part, which equals to a longer tool life [1, 22, 25]. In summary, according to the deformation cutting theory the higher amount of porosity leads to less work hardening which itself causes lower cutting forces and thus a longer tool life. This conclusion is in cotradiction with emperical results. As it can be understood, this theory is not able to describe the detrimental effect of

Chip Pores Continuous material Tool Machined surface

28

porosity on the machinability of PM parts. However, it seems to be much closer to reality than the interrupted cutting theory [1]. With respect to the deformation cutting theory, the adverse effect of porosity on machinability via letting the cutting fluid escape from the cutting area is also no longer valid.

Figure 2-18. Deformation cutting theory. (a) pore closure caused by deformation of work-piece material at tool/work-piece interface, (b) formation of densified layer at the machined surface [28].

• Reducing heat conductivity: the thermal conductivity of a porous metal is decreased by increasing the porosity as shown in the following equation:

in which and are the thermal conductivity of a porous and a solid material respectively, P is the pore volume fraction and B is a constant depending on the shape and distribution of the pores [29]. With respect to this fact, it can be concluded that the temperature at the cutting edge increases rapidly thus accelerating tool wear as well as tool edge deformation [26, 28]. Furthermore, increasing temperature at the cutting zone could increase the affinity of the surface

(1

. )

p s

K

=

K

-

B P

p