Tribological principles and measures to reduce

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Tribological measures which are used to decrease the intensity of wear are listed in this paper. ..... s Interneta, http://books.google.hr/books?id=db7dEFk9qw0C&printsec=fro ... 81f2bc42aa37e5609d7e80eace4e1.pdf , 11. veljače. 2013.
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International conference “Mechanical Technologies and Structural Materials”Split,

26-27.09.2013 ISSN 1847-7917

Tribological principles and measures to reduce cutting tools wear Zvonimir Dadić1) 1) Fakultet elektrotehnike, strojarstva i brodogradnje, Sveučilišta u Splitu (Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split) Ruđera Boškovića 32, 21000 Split, Republic of Croatia

[email protected]

Keywords mechanisms of wear cutting tools tool wear tribological measures Ključne riječi mehanizmi trošenja alati za obradu odvajanjem čestica trošenje alata tribološke mjere

Review article Abstract: This article explains the basic mechanisms of tribological wear and tribological processes that occur during cutting processes in the tribosystem of cutting tool and workpiece. Given the large impact of surface wear of tool blade on processing costs but also the quality of the final product the intensity of wear should be reduced as much as possible. In order to reduce the intensity of wear it is necessary to understand the processes that occur within the tribosystem of the tool blade and the workpiece, and other factors that affect the tool life. Tribological measures which are used to decrease the intensity of wear are listed in this paper.

Tribološki principi i mjere pri obradi odvajanjem čestica Pregledni članak Sažetak: U ovom članku pojašnjeni su osnovni tribološki mehanizmi trošenja te tribološki procesi koji se javljaju prilikom obrade odvajanjem čestica u tribosustavu reznog alata i obratka. S obzirom na veliki utjecaj površinskog trošenja oštrice alata na troškove obrade ali i kvalitetu konačnog proizvoda potrebno je što više smanjiti intenzitet trošenja. Da bi se smanjio intenzitet trošenja potrebno je poznavati procese trošenja koji nastaju unutar tribosustava oštrice alata i obratka i ostale faktore koji utječu na postojanost alata. U ovom radu navedene su tribološke mjere kojima se postiže manji intenzitet trošenja tj. veća postojanost alata za obradu odvajanjem čestica.

1. Introduction

Table 1.

Expenses due to wear, [2]

Tablica 1. Troškovi uzrokovani trošenjem

Tribology is a part of science that deals with the events that occur when two surfaces are in relative motion. Friction occurs due to normal force acting between the surfaces in contact and transforms kinetic energy to heat. One third of world’s energy sources appear as friction in one form or another and most of these result in waste.[1] These events usually include removing of particles from one or both of the surfaces. This kind of wear on the surface of materials is considered normal to a certain intensity. Beyond that, wear can cause serious damage to the elements in the tribosystem. About 70% of mechanical component failures are caused by tribological aspects.[1] Undesirable wear of mechanical components often results with a decrease in precision and efficiency of the elements, larger radiance between components, vibrations, increased wear and sometimes fatigue of the material. Therefore, wear can represent a significant financial loss. In 1976. American experts in the field of tribology conducted a research to measure the costs of excessive wear. The results of the research are shown in Table 1.

Category Naval aircraft Naval ships Cutting tool wear Auto Maintenance and repair costs

Costs $243.87 per flight hour $38.92 per hour $9,000,000,000 per year $40,000,000,000 per year

These costs can be significantly reduced by understanding the cause of wear and the mechanisms of wear and taking the appropriate tribological measures.

2. Mechanisms and processes of wear Cause of surface wear of the material can be a dynamical contact with the surface of another rigid body, with a fluid or with various particles. Every process of material wear consists of two or more basic mechanisms of wear, depending on the tribosystem type. Basic mechanisms of wear are:  abrasion (micro cutting on the surface of a material with a harder material);  adhesion (adhesion forces of contact areas between two materials are stronger than the cohesion forces inside the materials so removal of particles involved in contact occurs);  fatigue (wear on the surface of material caused by long-term periodic variable load);  tribocorrosion (surface wear due to oxidation).

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2.1. Cutting tools wear During contact with another metal, cutting tools are subjected to extreme friction. High intensity forces (sometimes over 15 kN) and high temperatures develop (around 1000 ˚C). As a result cutting blade wear is inevitable. Wear is manifested with the gradual loss of tool material and change of tools shape during the cutting process which changes cutting properties of the tool. Cutting process depends on series of input values (tool material, tool and work piece geometry, parameters of cutting) which directly affect the output values (work piece dimensions, surface roughness, cutting forces, noise, temperature, tool wear). Cutting tool is subjected to mechanical, thermal and chemical stresses which cause different types of wear:  Mechanical wear; Consists of abrasive and adhesive wear. Abrasive wear occurs when the hard particles detached from the work piece cut the surface of the tool. Adhesive wear occurs due to adhesive bond between the work piece chip and rake face of the tool.  Thermo-mechanical wear; Refers to surface fatigue wear. Small cracks occur due to fluctuations of temperature and periodical variable strain. Cracks propagate until pieces of the tool separate.  Thermo-chemical wear; Exchange of atoms among cubic crystals from high concentration to low concentration areas by diffusion due to extreme contact stress and high temperatures. Rate of diffusion increases exponentially with temperature. Diffusion of atoms from the tools surface causes a brittle layer with reduced strength. The mechanism of diffusive wear is illustrated schematically in Figure 1 with a tungsten carbide tool as an example.  Electrochemical wear. Refers to tribocorrosion. Under influence of high temperatures and oxygen a brittle oxidation layer forms. Its periodical forming and destroying causes loss of material.

Figure 2. Characteristic areas of wear on a lathe tool Slika 2. Karakteristična područja trošenja na alatu za tokarenje

Processing ability of a cutting tool is diminished mostly because of wear on the flank face so the width of wear zone on the flank face is frequently used as a criterion of wear. When the zone width reaches certain dimensions it is necessary to change the tool with a new one. For example it is necessary to change a lathe tool when the dimension of the wear zone on the flank face reaches from 0,2 to 1 mm, depending on the required surface roughness. A number of factors affects the intensity of wear such as parameters of cutting, work piece material, means of cooling and lubrication, tool geometry and tool material. Wear is very intense in the initial period of cutting. After a wear zone of certain width is formed, the intensity of wear decreases. For a period of time the intensity of wear is low until the beginning of a catastrophic period when the wear intensity increases rapidly as shown in Figure 3.

Figure 3. Lathe tool flank face wear as a function of Slika 3.

Figure 1. Mechanism of diffusive wear with a tungsten carbide Slika 1.

tool as an example, [3] Mehanizam difuzijskog trošenja na primjeru alata od volframovog karbida

Characteristic areas of adhesive, abrasive, diffusion and oxidation wear on a lathe tool are shown in Figure 2.

processing time for different cutting speeds, [21] Trošenje stražnje površine alata za tokarenje u ovisnosti o vremenu obrade za različite brzine rezanja

Most of cutting processes (turning, drilling, milling etc.) can be described according to Figure 3. Wear in the initial period as a function of processing time forms a parabolic shape which depends mostly on the work piece material, tool material, cutting parameters and the quality of cooling and lubrication. After that, a nearly linear period follows, the wear intensity is constant, until the last period when the wear intensity increases and the cutting tool has to be replaced.

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3. Tribological measures cutting tools wear

for

reducing

Generally, wear intensity can be determined during the process of wear by measurable quantities such as friction, noise, vibrations and temperature or it can be measured by spectrographic analysis of oil, magnetic particle detector, radioactive methods or ferrography. General tribological measures for reducing wear are:  constructional measures;  right choice of materials in the tribosystem (choosing the materials depending on the mechanisms of wear);  protection of the material surface (changing the surface structure to increase resistance to a certain wear mechanism);  run-in period (period of adjustment of contact surfaces);  lubrication (reducing friction and wear applying lubricants). Similar tribological measures can be applied to decrease wear on a cutting tool. The most important factors for cutting tools life span are the tool material, surface treatments, cooling and lubrication. 3.1. Cutting tool material Cutting tools are be subjected to:  mechanical loads (high hardness and toughness requirements);  thermal loads (high thermal stability requirements);  chemical reactions (requirements of low tendency to diffusion and oxidation). Tools resistance to wear is proportional to the hardness of the material. High hardness is appropriate when cutting resistance is constant. When there are impact loads during cutting, high toughness of material is necessary. Hardness and toughness are inversely proportional properties of materials so there is no ideal material for any type of cutting process as can be seen in Figure 4. Material has to be chosen depending on the exploatation requirements.

Figure 4. Hardness and toughness of cutting tool materials, Slika 4.

[24] Tvrdoća i žilavost materijala alata za obradu odvajanjem čestica

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HSS steel is used when high toughness is required. It has a hardness of about 65 HRc below 600 ˚C. In the contact area of tool and work piece the temperature is usually higher so it is necessary to use cooling and lubrication. There are several types of HSS steels. They differ in the amount of alloying components. Usual alloying components are:  tungsten (increases temperature stability);  vanadium (forms hard carbides and causes fine grain structure);  molybdenum (increases toughness and temperature stability);  and cobalt (increases temperature stability). HSS steel is often used in cutting processes such as milling, drilling, broaching, hobbing and tapping threads. In processes such as turning and face milling hard metals are usually used. Hard metals consist of tungsten carbides, titanium and cobalt as matrix. Carbides have a high hardness and thus a high wear resistance. Increasing the content of matrix material increases toughness which is necessary during impact loads but decreases resistance to wear. During cutting at elevated temperatures change in hardness is negligible so the process is often possible without cooling. In processes with constant cutting resistance hard tools consists of about 95% of carbides and 5% of matrix. In operations with often impact loads hard tools have more matrix material, up to 30%. Improvement of material properties can also be achieved with modification of the surface or applying a protective coating on the surface. Surface coating is applied by CVD (chemical vapor deposition, for HSS steels) or PVD (physical vapor deposition, for hard metals) procedures. Most common coatings are:  TiN (titanium nitride); Most common coating bright-gold ceramic cover with high hardness (around 2300 HV), small friction coefficient and medium oxidation resistance. It is applied with PVD procedure on cutting tools and deformation tools.  CrN (chromium nitride); Bright silver ceramic coating applied with PVD procedure. It has high hardness (around 2000 HV), toughness and high resistance to oxidation. It is resistant to adhesive wear. It is better than TiN at cutting copper, aluminum or titanium but has a worse friction coefficient.  TiCN (titanium carbonitride); Developed for demanding conditions it has hardness of around 3500 HV (TiN has 2300 HV). It is applied with CVD procedure on cutting tools for work pieces with high tensile strengths.  TiAlN (titanium aluminum nitride); Blue and red ceramic coating applied with PVD procedure. It has high hardness (around 3500 HV) and low friction coefficient. Resistant to oxidation at temperatures under 800 ˚C. It has excellent resistance to abrasive and adhesive wear. It is possible to apply multiple coatings but the total thickness is usually about 10 µm.

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3.2. Cooling and lubrication Tool hardness decreases at higher temperatures (as shown in Figure 5) which leads to faster development of tribological processes and shorter tool life span which increases the costs of production. Main purpose of means for cooling and lubrication is to drain away heat developed in the cutting zone and decrease friction between the tool and the work piece. Lubricants can decrease friction up to 80%. They enable high cutting speed and low cutting resistance with sufficient tool life span and required surface quality.

Figure 5. Hardness of cutting tool materials as a function of Slika 5.

temperature, [22] Tvrdoća alata za obradu odvajanjem čestica u ovisnosti o temperaturi

Means for cooling and lubrication can be classified in three groups:  oil emulsions;  chemical compounds;  pure cutting oils. Oil emulsions are a mixture of water, mineral oil and emulsifier. Emulsions with a lot of emulsifier are appropriate in grinding where the oil is dispersed in many small drops. Oil emulsions often contain EP (extreme pressure) additives which include sulfur, chlorine and phosphorus based products which enable excellent lubrication properties. Various chemical compounds were created from desire to make a coolant and lubricant without the use of mineral oils. These cutting fluids contain additives which decrease surface tension of water and can also contain sulfur, chlorine and phosphorus based additives to improve lubrication properties. There are two types of chemical compounds according to the purpose:  synthetic fluids for grinding; Contain additives which protect from oxidation. Fluid remains transparent for longer period of time which facilitates monitoring of the process.  synthetic fluids for turning, milling, drilling etc. They have a wide application. Ratio between water and synthetic fluids depends on the type of process and material of the work piece (usually 20:1 or 40:1). There are also semi chemical compounds which represent a combination of oil emulsions and chemical compounds. These sort of fluids contains a smaller amount of oil (10-

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45%) and has a higher content of emulsifiers to reduce drop size during dispersion and to enhance other exploitation properties. Components which are most often used in the production of oil emulsions and chemical compounds are:  emulsifiers (enable better mixing od water and oil);  antioxidants and corrosion inhibitors (prevent oxidation and corrosion of metal surfaces);  defoamers (reduce the formation of foam during a cutting process);  EP additives (increase lubrication properties, decrease wear);  bactericides (prevent the formation of all microorganisms harmful to the health of workers). Oil emulsions and chemical compounds are used in processes where the cutting speed is high and the pressure on contact surfaces between the tool and the work piece is low. In these conditions a significant amount of heat develops and cooling properties of fluids are essential. When the contact pressure is high and less heat is developed cutting oils are used. Cutting oils are mineral oils with or without additives which never contain water. Types of cutting oils are:  mineral oils; Suitable for processing softer steel, copper and light alloys. They are anticorrosive, stable and can be used for unlimited periods of time if aren’t polluted. Recommended at workplaces where oil comes into contact with the skin of the worker.  greasy oils; Excellent lubricating properties but other features very modest. Rarely used because of high price. They have an unpleasant smell and produce significant amounts of smoke which makes them unusable in modern production.  mixture of mineral oils. Mixture of mineral oils and greasy oils increase the surface quality of machined steel, aluminum, and copper alloys, especially bronze. Mineral oils are also mixed with EP additives and in the combination with greasy oils various features can be achieved like low viscosity, good lubrication properties, long tool life span etc.

4. Conclusion After a certain period of usage a cutting tool is not economically justified. Cutting precision decreases, surface roughness increases, cutting forces and temperatures increase and vibrations occur. Factors affecting tools life span are cutting parameters, tool geometry, tools mechanical properties, work piece material and type of cooling and lubrication. Cutting parameters should be adjusted to minimize tool wear and in the same time to satisfy economical demands. Tool geometry and workpiece material are usually defined by the quality requirements of the workpiece. Mechanical properties of the tool are determined by the material. High hardness enables higher resistance to wear while high toughness enables good resistance to

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dynamical impacts. These material properties are inversely proportional so a universal cutting tool material does not exist but is chosen according to its purpose. Except hardness and toughness stability of properties at elevated temperatures is very important as elevated temperatures ussually develop in cutting processes. Tool wear also happens due to diffusion of particles on the material surface so a cutting tool material should have a chemical resistance to reduce this kind of wear.

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These properties are greatly dependant of materials chemical composition. Table 2 shows some hard metal properties according to the chemical composition of the material. This indicates that with further research, studying the wear of tools considering the chemical composition, estimation of tools life span could be produced, according to its chemical composition.

Table 2. Hard metal properties according to the chemical composition Tablica 2. Svojstva tvrdih metala s obzirom na kemijski sastav

Chemical composition

Hardness, HRa

97%WC-3%Co 94%WC-6%Co 90%WC-10%Co 84%WC-16%Co 75%WC-25%Co 71%WC12,5%TiC-12%TaC-4,5%Co 72%WC-8%TiC-11,5%TaC-8,5%Co

92,5-93,2 91,7-92,2 90,7-91,3 89 83-85 92,1-92,8 90,7-91,5

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Modulus of elasticity, GPa 641 648 620 524 483 565 558

Relative resistance to abrasion wear 100 58 22 5 3 11 13

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der/content/1/2011-2012/Predavanje%2010.2011.pdf?forcedownload=1 , 11. veljače. 2013. [21] N.N.: „Lathe master“, s Interneta, http://www.lathemaster.com/HSS%20LATHE%20TOOLS. htm , 22. veljače. 2013. [22] Branko Ivković: „Osnovi tribologije u industriji prerade metala“, Građevinska knjiga Beograd, 1983.

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[23] Hanjiang Wang: „HSS and its market position“, s Interneta, http://www.diamondbladeselect.com/knowledge/someknowledge-of-high-speed-steel-hss-and-its-market-position/ , 6. ožujka 2013. [24] N.N.: „Cutting tool materials“, s Interneta, http://www.mitsubishicarbide.net/contents/mmus/enus/html/pro duct/technical_information/information/sessaku.html , 7. ožujka 2013.