Kesici takımlar üzerine yapılan Tin sert seramik film kaplamanın kesici takım ömrüne etkisi

Abstract

Bu çalışma, ARÇELÎK A.Ş. Çayırova tesislerinde çamaşır makinalarında kullanılan rulman yuvalarının talaşlı imalatının yapıldığı takım tezgahında farklı özelliklerdeki kesici takımların kaplamasız ve TİN sert seramik film ile kaplı olarak kullanılması sonucunda, kesici takımların ömürlerinde meydana gelen değişimlerin belirlenmesi amacı ile yapılmıştır. Rulman yuvalarının talaşlı imalatında kullanılan kesici takımlar, DİN 1.3343 ve DİN 1.3243 malzemelerden imal edilmiştir. DİN 1.3243 malzemeden imal edilen kesici takımlar, Türkiye' de sert seramik film kaplama yapabilen iki yerli kaplayıcı firma tarafından TİN ile kaplanmıştır. TİN kaplanmış kesici takımlar üzerinde, kaplama kalınlıklarının yüzey pürüzlülüklerinin, sertliklerinin ve alt malzemeye olan yapışmalarının belirlenmesi için çeşitli karakterizasyon çalışmaları yapılmıştır. Performans deneylerinde, kesici takımların serbest yüzeylerinde meydana gelen serbest yüzey aşınması kriter olarak seçilmiştir. Kesici takımların serbest yüzeylerinde meydana gelen serbest yüzey aşınma şerit genişliği 0.4 mm1 ye ulaşan takım, aşınmış olarak kabul edilmiştir ve kesici takıların aşıncaya kadar işlemiş oldukları parça sayısı, takım ömrü olarak alınmıştır. Performans deneyleri sonucunda DİN 1.3243 malzemeden imal edilmiş kesici takımlar, DİN 1.3343 malzemeden imal edilmiş kesici takımlara göre, kesici takım türüne bağlı olarak % 20 ile 75 arasında daha yüksek takım ömürleri göstermişlerdir. DİN 1.3243 malzemeden yapılmış kesici takımların 1. Firma tarafından kaplanması ile DİN 1.3343 malzemeden imal edilmiş kesici takımlara göre kesici takım türüne bağlı olarak takım ömürlerinde, % 233 ile 450 arasında artış sağlanmıştır. 2. Firma tararından kaplanan kesici takımlarda ise % 166 ile 350 arsında takım ömürlerinde artış sağlanmıştır. 1. Firma tarafından kaplanan kesici takımların performans deneylerinde daha uzun takım ömürleri göstermeleri, 1. Firma tarafından yapılan kaplamaların 2. Firma tarafından yapılan kaplamalara göre; 1- Daha düşük yüzey pürüzlülüğü, 2- Daha yüksek sertlik, 3- Alt malzemeye daha iyi yapışma göstermelerinden kaynaklanmaktadır. 1. Firma tarafından kaplanan kesici takımların, tribolojik açıdan yukarıda sayılan olumlu özellikleri içermeleri, bu takımların performans deneylerinde daha üstün performans göstermelerine neden olmuştur.

Machining can be described as getting metals desired shapes and dimensions by means of cutting. Metal cutting is achieved by cutting tools which are specially designed for any kind of machining such as turning, milling, drilling, tapping ete. During machining cutting tool is forced into metal and cuts off öne layer of metal (cutting depth) which is called chip. The surface of the tool which rubs against över fireshly fonned surface of the workspace is called as clearance ör flank face and the cutting edge is formed by the intersection of the rake face with the clearance face of the tool. The relative motion between tool and work piece during cutting, compresses the work piece near the cutting tool and results in shear deformation which is defined as primary deformation. in addition to primary deformation, another deformation takes place in the chip during the chip posses över the rake face of the tool and is called secondary deformation. There are three components of the force acting on the tool. The component of the force acting on the rake face of the tool, normal to the cutting edge, is called cutting force and this is the largest force acting on the tool, parallel to the feed direction, is defined as feed force. The last component of the force in the system is tendency to push the tool away from the workpiece in the radial direction and this is the smallest force acting on the tools. The analysis of the forces in the cutting process are so complex. As far as heat generation in metal cutting is concerned, mechanically given energy to the system converts into heat. Metal cutting is an energy consuming procedure. More than 90% of mechanical energy is transformed into thermal energy on the cutting edge. The chip formation area in the workpiece and the contact zone between the chip and the rake face of the tools are to be considered as the main heat sources. Temperature on the rake face of the tool during cutting process can be as high as 900 to 1300 ° C. 90% of the heat produced in the contact has been estimated to originate from heat generated by the mechanical deformation of the chip, 18% of by friction between tiıe chip and the rake face of tiıe metal and only 2% is created on tiıe tool tip. The temperature on the tool changes, depending on cutting speed and feed rate. xviii There are three kinds of main tool failure mechanisms which are fracture, plastic deformation and wear. Tool failure due to the fracture and plastic deformation are usually sudden and catastrophic. Therefore, the requirement for fracture and plastic deformation resistances are prerequisites for an acceptable cutting tool materials. However, tool failure because of wear develops gradually but eventually shows its harmful effects. Such as cutting edge breaking, increasing in cutting and feed force and more heat generated during machining. Cutting tools are subjected to a combination high thermal and mechanical load when being used for the machining of metals. Friction processes betvveen the chip and the rake face and workpiece on flank faces respectively lead to wear of the tool materials. As it is well known, four mean mechanisms are made responsible for the tool's material removal. These four mean wear mechanisms can be ordered as; -Adhesive wear -Abrasive wear -Diffusion wear -Oxidation Adhesive wear is the most common type of wear. Adhesive wear mechanism can be explained as follows; When öne metal slides ör rolls över another metals, junctions between two materials take place if metals dissolve into each other. Junctions between two metals are broken due to continuity of sliding and rolling motions between metals and material transfer takes place from öne metal surface to the another. in the metal cutting processes, adhesive wear is dominant wear mechanism between flank face of tool and freshly formed surface of workpiece at low cutting speeds. Abrasive wear occurs when hard particles slide ör roll under pressure across surface, ör when a hard surface rubs another surface. in abrasive wear, material is removed ör displaced from a surface, forced against and sliding along the surface. in the metal cutting processes, abrasive wear is dominant wear mechanism betvveen flank face of the tool and workpiece at ali cutting speeds. Harder particles from tool material in tiıe work material, ör broken hard particles from the built up edge plaugh the rake face of tool and abrasive wear can also take place an the rake face of tool. During metal cutting, temperature on the rake face of tool can be so high that cutting tool's atoms diffuse through chip and material loss from tiıe rake face of tool takes place. Wear, takes place on the rake face of tool in this way is called diffusion wear. Crater forms region of taking place of diffusion wear on the rake face. When dimensions (length and deptiı of crater) of crater reach a critical value, cutting edge of the tool is broken by acting force över the tool. Machining of metals at high cutting speeds, diffusion wear occurs commonly. Oxide layer forms on the rake face of tool, during machining of metals at high cutting speeds. Then, formed oxide layer on the rake face is broken by mechanical effects and carried away by chip. in this way material removal from the rake face of tool is called as oxidation wear ör tribooxidation. xix A thin hard coating on a softer, tough, substrate has proved to be a tribologically veıy beneficial material combination. Many new coatings have been investigated and developed in the 70's and 80's and also taken successfully into commercial use. A hard layer on a softer substrate will give improved protection against scratching from hard counter parts ör debris. Hard coathıg have thus been especially useful in applications involving abrasive ör erosive wear. The most successful application so far is ceramic coating an cutting tools where they give good protection against a combination of diffusion and abrasive wear at high temperatures. Hard, wear resistant coatings which are deposited by Physical Vapor Deposition (PVD) techniques are generally applied to provide a surface having olher beneficial properties such as wear resistance, corrosion protection, optical and electrical properties and decorative appearance. These coatings are mainly based on carbides, nitrides and borides of the transition metal elements. PVD processes can be divided into two groups according to generation of vapor: evaporation and sputtering. Evaporation is the oldest and the simplest PVD method. Material to be deposited is placed in a boat ör crucible and then heated resistively ör by the high current electron beam ör laser beam ör are. in ali cases the material evaporates and form a vapor flux in the vacuum chamber. Condensation of this vapor onto the substrate produces the desired fihn. When the surface of a material is bombarded with high energetic particles, generally ions, the physical erosion of the material from the surface is occurred. This effect is known as sputtering. Sputtering is widely used as a source of vapor for thin film deposition. in ali sputtering PVD processes the ions for sputtering is produced by glow discharge plasma. Introducing of reactive gas/gases into the chamber while depositing thin fihns causes compound fihn formation onto the substrate which is called as reactive PVD. lonizing the vapor to be deposited and applying negative potential (BIAS) to 1he substrate relative to the vacuum chamber walls is named as ion plating. lon plating improves the fihn properties - such as adhesion, the increase of fihn density - and deposition rate. Ali thin film processes contain four sequential steps. A source of fihn material is provided, the material is transported to the substrate, deposition takes place and finally it is analyzed to evaluate the process. The results of the analysis are then used to adjust the conditions of Ihe other steps for the fihn property modification. Properties of coatings can be determined by coating characterization techniques. The main coating characterization parameters for triboelements are; XX - Coating thickness, - Coating surface roughness, - Coating hardness, - Coating adhesion to substrate. The application of PVD coatings for machine tools was restricted to a single coating material, TiN for a long time. This binary metallic hard material is easily produced in PVD processes and is characterized though a well balanced property profile, making TiN rather universal for metal cutting operations. Other binary nitridic hard materials such as ZrN, HfN or NbN showed no significant advantages in metal cutting applications. Recently only CrN has proved successful for machining of non-ferrous metals. Binary carbidic hard materials such as TiC or ZrC can not be easily deposited in PVD processes and showed no favorable properties for practical use. However, various research studies had already shown in the mid 1980s that with ternary hard materials, e.g. (Ti,C)N or (Ti,Al)N, PVD coating for specific applications with a superior cutting performance can be produced. (Ti,C)N can be characterized by a higher hardness and a better abrasion resistance compared to TiN. The optimal machining condition for (Ti,C)N are in the area of low application temperature, e.g. low cutting speed, or interrupted cutting. The outstanding qualification of (Ti,C)N for interrupted cutting is based on a very favorable friction behavior at low temperatures and a high heat conductivity, resulting in a chip flow which is smooth and easy on the cutting edge. (Ti,Al)N also posses a higher hardness compared to TiN. However, (Ti,Al)N has a lower heat conductivity and friction coefficient in contrast with (Ti,C)N. (Ti,Al)N shows a higher strength at elevated temperature, a higher oxidation resistance and better thermal barrier properties compared to TiN. Therefore (Ti,Al)N is ideally qualified for abrasive cuttings with high cutting temperatures. Ceramic film coated cutting tools show larger tool life compared with uncoated tools due to their own higher mechanical, physical and chemical properties. Higher mechanical properties of ceramic coatings such as hardness and young modules increase the abrasion resistance and load carrying capacity of tools. Lower thermal conductivity of ceramic films decreases heat flow to substrate (tool) and prevent softening of tool materials at high cutting speed. Chemical stability of ceramic films increase the solution and oxidation wear resistance of cutting tools. In addition to these, contact condition between tool, chip and work piece is changed in a way to reducing cutting force by using ceramic film coated tools. The lower forces obtained for coated tools result in less heat generation and the temperature dependent tool wear is slowed down. In this study, the effects of TiN coatings on the tool lifes of several cutting tools are investigated. The TiN coatings are produced by Arc PVD technique at two commerical coating companies with different parameters. The results of the present investigation are summerized below. xxi 1-) To determine the effects of cutting tool materials on tool life, cutting tools (drills and taps) made of DIN 1.3343 and DIN 1.3243 steels are examined in field tests with the same cutting parameters. Drills and taps produced from DIN 1.3243 steel performed 25-50 % better. 2-) TiN coating of DIN 1.3243 steel tools inreased the tool life 250-450 %. 3-) The coatings with lower surface roughness, higher hardness and good adhesion performed better in the field tests. 4-) The resuls of the field tests indicated a defSnite effects of cutting speed on the performance of TiN coated tools when machining GG20 cast iron. With tool opereting at higher cutting speeds better performances is obtained. 5-) From scanning electron microscope studies of used uncoated and coated cutting tools following results are obtained. i - Dominant flank wear mechanism of cutting tools in the machining of GG 20 cast iron work piece is adhesive wear. ii- At mechanically loaded parts of drills such as chisel edge and rake face coating failure mechanism is micro fracture. 6-) It is determined that low tool lifes of cutting tools at low cutting speeds are due to adhesive wear more severe at low cutting speeds in machining of GG 20 cast iron work piece.