Mikro freezeleme işleminde yüzey oluşumunun ve takım aşınmasının deneysel incelenmesi

Abstract

Teknolojinin gelişmesi ile birlikte, mikro imalat yöntemlerine duyulan gereksinim de artmıştır. Minyatür cihaz ve makine elemanlarının kullanımının yaygınlaşması, bu ekipmaları üretiminde kullanılacak mikro imalat tekniklerinin gelişmesine ve kullanımın alanlarının artmasına sebep olmuştur. Uzay ve havacılık, biomedikal, elektronik ve savunma gibi ileri teknoloji uygulama alanları mikro imalat yöntemlemlerine en çok ihtiyac duyulan sektörler olarak göze çarpmaktadır. Mikro talaşlı imalat, geniş malzeme yelpazesinde, hızlı, ekonomik, yüksek boyutsal ve geometrik toleranslarda, 3 boyutlu imalata imkan vermesi gibi özellikleri bakımından büyük önem taşımaktadır. Mikro talaşlı imalat yöntemlerinin getirdiği avantajlar, mikro imalat yöntemleri arasında çok önemli bir yer tutmasına neden olmaktadır. Boyutların önemli ölçüde küçülmesi, mikro talaşlı imalat yöntemlerinin geleneksel talaşlı imalat yöntemlerine kıyasla farklı karakteristik özellikler taşımasına neden olmaktadır. Bu farklı karakteristik özelliklerin belirlenmesi ve incelenmesi hususunda, mikro talaşlı imalat işlemleri ile ilgili çalışmalar yapılmakta, süreç temelleri incelenmekte ve süreç modelleme çalışmaları gerçekleştirilmektedir. Bu tez çalışmasının amacı doğrultusunda, mikro frezeleme işleminde, yüzey oluşumunun ve takım aşınmasının deneysel incelemesi gerçekleştirilmiş, temellerinin anlaşılması ve geleneksel frezeleme işlemlerinden farklılıklarının belirlenmesi doğrultusunda, prosese etkiyen parametreler belirlenerek süreçteki etkilerinin incelenmesi yönünde çalışmalar yapılmıştır. Bu kapsamda, tez çalışmasının ilk bölümlerinde, mikro frezeleme işleminde, farklı malzeme, takım, kesme şartları ve kesme parametrelerinde deneyler gerçekleştirilerek, bahsedilen değişkenler ile proses çıktıları arasında ilişki kurularak incelenmiş, mikro frezeleme işleminde yüzey oluşumuna etkiyen temel mekanizmalar araştırılmıştır. Bu çalışmalar ile elde edlilen kazanımlar ile yüzey oluşum modeli geliştirilmiş ve geliştirilen model deneysel sonuçlar ile karşılaştırılmıştır. Yapılan karşılaştırmalar sonucunda, geliştirilen modelin yüzey oluşumunu yakın bir şekilde öngörebildiği görülmüştür. Sonrasında, mikro frezelemede takım aşınmasının incelenmesi için deneysel çalışmalar gerçekleştirilerek mikro frezeleme işleminde takım aşınmasına etkiyen temel mekanizmalar ve faktörler incelenmiştir. Bu amaçla çeşitli takım tipleri ve farklı kesme parametrelerinde, belirlenen kesme şartlarında operasyonlar gerçekleştirilerek, takım aşınması, kesme kuvvetlerinin değişimi ve yüzey oluşumu karşılaştırmalı olarak incelenerek, takım aşınması incelemeleri gerçekleştirilmiştir. Tez çalışması kapsamında yapılan çalışmalarla, mikro frezeleme işleminde yüzey oluşumunun ve takım aşınmasının temelleri ve karakteristik özellikleri belirlenmiş ve elde edilen bulgular ve çıkarımlar son bölümde derlenerek özetlenmiştir.

With the effect of technological developments, demand for the micro manufacturing processes increased substantially. The popularization of the miniature devices and components gave rise to the improvement and widespread use of micro manufacturing methods that could be utilized in the process of manufacturing these equipments. Leading-edge technology industries such as aerospace, biomedical, electronics and defence systems stand out as the fields of application that need micro components produced by micro manufacturing processes to a considerable extent. Micro milling is one of the most important manufacturing processes which enable fast and cost-effective manufacturing of 3D shaped micro parts with high geometrical and dimensional accuracy and wide range of materials. The advantages provided by micromachining processes enable it to occupy an important position among micro manufacturing processes. Due to the scaled down processes geometry, micro milling processes show major characteristic differences from conventional milling processes. Production of miniature parts requires small diameter cutting tools. When the tool diameter is reduced, uncut chip thickness values decrease. As a result of this, uncut chip thickness values become comparable with cutting edge radius value. This leads to high negative rake angle effect on cutting process. When the uncut chip thickness is smaller than cutting edge radius, chip formation may not occur due to elastic deformation of the work piece material. This phenomenon is called ploughing. When the uncut chip thickness is increased to a certain value, chip formation starts. This limit uncut chip thickness value is defined as minimum chip thickness. Ploughing and minimum chip thickness are fundamental principles of the micro milling process and have a serious effect on chip formation, surface generation and cutting forces. With regard to the definition and inspection of these characteristic differences and process mechanism of micro milling, micro milling process fundamentals are investigated and process models are developed by researchers. The aim of this study is to investigate the surface generation and tool wear in micro milling. In line with this objective, the parameters that act on processes and their effects are identified in order to understand the fundamentals and specify the differences compared to traditional milling operations. Within this scope, in the initial phases of the thesis study, experiments at various cutting parameters, workpiece materials, cutting conditions and process geometries are performed in micro milling. Basic mechanisms affecting surface generation in micro milling are investigated conducting a relative analysis of process outputs and aforementioned variables. First stage cutting experiments are conducted with 1 mm diameter micro end mill on various materials which are 6061 Aluminium alloy, Ti6Al4V Titanium alloy and 15-5 PH Stainless steel. Investigation of the cutting parameter effects on surface generation in consideration of workpiece material properties is the main objective of the first stage cutting experiments. Evaluation of the first stage cutting experiments shows that feed rate has the greatest effect on surface generation when compared to the other cutting parameters in micro milling. The reason for this is the fact that the effect of the ploughing and minimum chip thickness on micro cutting process is associated with feed rate. Variation of cutting speed does not have as much effect on surface rougness as feed rate; however, chip formation is distinctly affected by the change in cutting speed. As the cutting speed increases, burr formation diminishes. On the other hand, cutting depth does not have a significant effect on surface roughness; however, increase of cutting depth causes excessive burr formation. Based on the obtained results from first stage cutting experiments, second stage cutting experiments are conducted in order to make further investigation of the relation between feed rate and surface generation. Besides, cutting process geometry is investigated in detail. For this purpose, cutting experiments are conducted with a greater number of feed per tooth values and various cutting depth and cutting speed levels on Ti6Al4V titanium alloy and 15-5 PH stainless steel. In spite of having similar mechanical strength characteristics, choosen workpiece materials have different characteristics in terms of modulus of elasticity and elongation at break. Thus, the effect of workpiece material ductility on surface generation is investigated. Futhermore, cutting tool geometry is examined with microscope and associated with cutting process. Evaluation of the experiments shows that feed rate is the significative parameter of the surface generation. Due to minimum chip thickness effect and ploughing, surface roughness and burr formation increase in low feed rates. With the increased level of feed rate, cutting process becomes normalized and surface quality improves. On the other hand, ductility of workpiece material has a significant effect on surface generation. Comparision of the obtained results with both workpiece materials shows that increased surface roughness and chip formation are obtained with Ti6Al4V titanium alloy. This is caused by the fact that minimum chip thickness and ploughing are more effective in micro cutting process of ductile materials. Moreover, alteration of the feed rate induces significant change in surface roughness in ductile materials. This is also related to the increased effect of minimum chip thickness and ploughing in ductile materials.With scaled up cutting speeds, better surface roughness values are obtained and burr formation is reduced. Higher cutting speed provides better chip formation process; therefore, better surface quality is obtained. On the other hand, increase of cutting depth causes excessive burr formation as in first cutting experiments. Futhermore, investigation of machined surface geometry shows that surface profile is highly influenced by tool geometry, minimum chip thickness and ploughing principles. Cutter geometry and minimum chip thickness effect defines the residual material geometry on machined surface As it is observed in microscope images, surface profile has excessive peaks as a result of residual material due to plouging effect. Besides, deep scratch marks are detected. Subsequently, the results of the surface generation experiments are utilized in the development of a surface generation model with regards to micro cutting process fundamentals and process geometry and the model is paralleled with experimental results. For this purpose, detailed investigations are carried out on tool geometry and process geometry. Different tool and workpiece material interaction conditions in cutting process are investigated with regards to feed rate, minimum chip thickness and ploughing effect. Based on investigation results, a mathematical model is developed. The developed model is intended to predict surface roughness on the centerline of the floor surface of machined slot via micro end mill. Subsequently, simulated surface roughness values with proposed model are compared with cutting experiment results which are conducted at various feed rates on Ti6Al4V workpiece material. Comparison of results shows that simulated surface roughness values are consistent with experimental results. In this regard, calibration of the minimum chip thickness coefficient is crucial for model accuracy. Additionally, with the aim of comprehending the tool wear mechanism in micro milling, basic mechanisms and factors affecting tool wear are experimentally investigated. For this purpose, operations under certain cutting conditions with ZrN and AlTiN coated micro end mills and cutting parameters are performed investigating tool wear, cutting force alterations and surface generation comparatively. Tool wear experiments are conducted in order to define the effect of cutting parameters and tool coatings on tool wear rate. Cutting forces, machined surface roughness and reduction of the tool diameter are measured in order to investigate tool wear rate. Obtained results for both surface generation and tool wear behavior are associated with micro milling process fundamentals and evaluated comparatively. Experimental results show that tool wear rate is significantly influenced by feed rate. Higher feed rates lead to increased tool wear. As a result of increased chip loads in high feed rates, cutting zone temperature rises and this leads to crater wear. As a result of crater wear, tool wedge weakens which leads to chipping of tool wedge. In low feed rates, ploughing dominant cutting regime does not cause excessive tool wear. On the other hand, greater tool wear rates are observed in ZrN coated tools since ZrN coating has a lower maximum opeating temperature compared to the AlTiN coating. As a result of progressing tool wear, surface roughness and cutting forces increase.