Tel çekme işleminde bakırın ve bakır oksit inklüzyonlarının deformasyon davranışı
Deformation behaviour during drawing of copper wires and copper oxide inclusions
Yazar: MUSTAFA M. ARIKAN
Danışman: PROF.DR. E. SABRİ KAYALI
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1991
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Bakır = Copper ; Bakır oksit = Copper oxide ; Deformasyon = Deformation ; Tel çekme = Wire drawing ; İnklüzyon = Inclusion

Özet:
Bu çalışmada tel çekme işlemi sırasında oksijeni}, bakırın deformasyon davranışı ile bakır oksit inklüz- yonlarmın deformasyon özellikleri incelenmiştir. Bu amaçla SCR prosesinde sürekli döküm ve haddeleme metodu ile üretilmiş 8 mm çapındaki oksijenli bir bakır filma- şinden soğuk tel çekme ile elde edilmiş çeşitli çaplar daki bakır teller kullanılmıştır. Bu tellerde bulunan bakır oksit inklüzyonlarınm deformasyon Özelliklerini belirlemek için görüntü analizi metodu ile tellerin enine ve uzunlamasına kesitlerinde düzlemsel inklüzyon yoğunluğu, miktarı ve boyutları ölçülmüştür. Ayrıca tel çekmedeki soğuk işlemin malzemenin me kanik özelliklerine etkisini tesbit etmek için; sertlik, çekme ve burma deneyleri ile çeşitli mekanik özellikle ri de belirlenmiştir. Farklı çaplardaki tellerin çekme deneyiyle belirle nen akma mukavemetinin (a), tel çekmedeki deformasyon oranına (e) göre çizilen akma diyagramının analizinden, bakır telin tel çekme sırasındaki deformasyon davranışı nın, soğuk işlem için uygulanan en genel denklemlerden biri olan Holloman denklemi ile karakterize edilemeyece ği tesbit edilmiştir. Deformasyon sertleşmesi hızının (6) artan tel çekme deformasyonu ile bir minimum ve bir maksimum yaptığı be lirlenmiştir. Literatürde e'nın maksimuma ulaştığı de formasyon oranının tel çekme sırasında meydana gelen di namik toparlanma olayı ile ilgili olduğu belirtilmekte dir. Bakır oksit inklüzyonlarınm deformasyon özellikle ri ile ilgili olarak, artan tel çekme deformasyonu ile bu inklüzyonlarm boyutlarının hem enine hem de uzunla masına kesitlerde azaldığı, birim alana düşen ortalama sayılarının ise uzunlamasına kesitte arttığı gözlenmiş tir. Bu ise bakır oksit inklüzyonlarınm tel çekme^iş- lemi sırasında kırılarak küçüldüğünü ve sayılarının art tığını göstermektedir.

Abstract:
Deformation behaviour during drawing of copper wires and copper oxide inclusions were investigated in this work. Most copper rods are now produced using continuous casting and hot working processes. Then copper wires have traditionally been manufactured by wire drawing which is a cold working process. Hot working is generally applied to the metals in the first steps of the metal working. Recovery pro cesses occur in the hot working and large strains can be reached without strain hardening. Cold working is applied with plastic deformation methods to the materials. Plastic deformation both provides dislocation movement and causes new dislocation occurence. The cause of the increase in the strength after the cold working is strain hardening. Strain hardening occurs as a result of interactions of the dislocations eachother and various obstacles which prevent their movement. Metal's strength and hardness increases and ductility decreases after the cold working. Because inclusions can significantly influence material properties and behaviour, they have been studied extensively. Inclusions are usually categorized according to origin, i.e., exogenous or indigenous. Exogenous inclusions come from external sources such as slag or refractories. Indigenous inclusions arise from naturel processes, such as deoxidation or precipitation of sulfides. Most inclusion studies concentrate on the indigenous types because they can be controlled by the melting practice and they are more numerous and more predictable in distribution. Indigenous inclusions are usually classified by composition, such as oxides or sulfides. Inclusions are also categorized by size, i.e., microscopic or macroscopic. In general, indigenous inclusions are small. Macroscopic inclusions are usually exogenous in origin, but small exogenous inclusions are also observed. A variety of methods have been used to measure or describe inclusion content. These methods involve the following: -Chart comparison -Counting -Volume fraction determination. Inclusion analysis requires quantification of the amount, size, shape and distribution of inclusion types. Most standart charts used in inclusion analysis depict fields at 100X magnification, thus providing a large field of view for rapid assessment of a large surface area. For steels with low inclusion content, higher magnifications may be required for detection and classification. It is very difficult to translate ratings at higher magnifications to 100X standarts. In examining a sample, the entire polished surface, or a specific area, is usually examined starting at one corner. Contiguous field selection's employed. Ratings can be qualitative, i.e., the operator rates only the worst conditions, or quantitative, i.e., every field is rated. Chart methods are generally used in two ways. The simplest procedure is to find the worst fields of each type that are depicted in the chart and rate them. Then the averages of these ratings on each sample are calcu lated to represent the heat. The second method requires rating every field within a given area on each sample. Numerous nonchart methods have been proposed for rating inclusion content. In one of these methods based on the stereological principles, a point-counting proce dure is used. In this standart, an eyepiece reticle with 20 horizontal and vertical lines is used. Field selec tion is performed randomly, and at least 30, but pre ferably 60 fields are measured. The number of grid points occupied by inclusions is counted and volume fraction is calculated by means of a formula. 400X magnification is recommended. Although stereological methods have the potential for rigorously describing inclusions, manual determina tion of all the required parameters with adequate accu racy is quite tedious. Therefore, automation answers a long-recognized need in microstructural analysis for more precise data for quality control and structure- property studies. Because use of automated devices eliminates operator fatigue and reduces analysis time, more measurements can be conducted more accurately. This is important because materials are not homogenous. To obtain better-quality data, a larger sample area and more samples must be analyzed. Thus, image analysis can provide better statistical accuracy and more meaningful results. Image analysis consists of sample selection and preparation, image processing, measurement, and data analysis and output. Images can be provided from diverse sources; however, the most common input peripheral is the optical microscope. All analyzers use a scanner to display the image on a TV screen. Each system uses a central elec tronic processor for image detection and measurement of stereological and nonstereological parameters. Most systems incorporate data-handling devices, which range from desktop calculators to minicomputers. Finally, each system has a device to produce a hard copy of the data. The primary mode of feature detection is gray-level thresholding. Setting the threshold to detect only gray levels within specific ranges enables selective detec tion of constituents. More sophisticated image analy zers can perform feature-specific measurements; that is, each distinct feature in the field is measured individu ally. The amount, size, distribution, type and shapes of the inclusions are important while their effects on the mechanical properties are detected. Inclusions cause the occurence of internal cracks in the structure of steel. During the plastic deformation of the steels, sulfide inclusions fracture or creates voids separating at the matrix-sulf ide interfaces. The fracture of steel takes place by means of the joining of these interface voids. The major uses of copper wires studied in this work are based principally on its high electrical conducti vity. Since this property is adversely affected by almost all other elements which go into solid solution in copper, metal purity becomes of major concern. When the electroref ined copper is melted, it picks up oxygen from gases in contact with the liquid metal. Oxygen is too small an atom to go into substitutional solid solu tion and too large an atom to be dissolved interstiti- ally to any degree. Thus it exists in solidified cop per as CU2O, cuprous oxide. The major differentiation between the several grades of unalloyed copper is the amount of oxygen present and/or the amount of resi dual deoxidizer element. The grades of unalloyed cop per can be categorized as electrolitic tough pitch cop per (ETP), oxygen-free high conductivity copper (OFHC), deoxidized high-phosphorus copper (DHP), deoxidized low phosphorus copper (DLP), silver bearing copper. Copper wire rods have traditionally been manufac tured mainly by hot rolling wire bars or extruding billets, but in the recent years, the development of new continuous casting techniques to produce copper wire rods has displayed conventional procedures so the hot rolling process of copper wire bars and continuous cast billets could be exchanged for a combination of contin uous casting and rolling in the same heat. The close connection between continuous casting and direct hot rolling of the cast strand takes advantage of the high temperature of the strand for the rolling process, thereby reducing energy consumption. In addition, smaller cast sections close to the dimensions of the final products can be produced and these cast products can be further processed without hot rolling. Most copper wire rods are now produced using continuous casting and rolling, for example, Southwire Continuous Rod (SCR), Dip Forming, Contirod, Outokumpu and Properzi processes. Most of the deformation in copper between e=0.25 to e=l occurs by microband formation during the drawing of the wire rod. Macro shear bands frequently were found in plain strain deformation processes, such as rolling, after moderate strains (e>l). At large strains (E>2), shear bands assume dominant role in deformation of cop per. Dynamic recovery occurs at large strains and evidence of dynamic recrystallization has been observed at very large strains (e>4). The aim of this study is to investigate the defor mation behaviour during drawing of copper wires and copper oxide inclusions. For this purpose, the copper wires drawn to different diameters between 8 - 1.8 om at 10 passes were used. These wires were drawn from a copper wire rod (8 mm in diameter) produced by SCR process which contains 393 ppm oxygen. At the wire drawing process, mean area reduction was about 30 % at each pass. Total reduction of area was 94.9 %. To determine the mechanical properties of cold drawn wires, samples were taken after each drawing operation for hardness, tensile and torsion tests. Hardness tests were performed on a microhardness testing machine. Tensile and torsion tests were per formed on a hydraulic tensile testing and a torsion testing machines respectively. Three tests were carried out after each drawing operation and mechanical proper ties were determined as the average of the three results. Samples for metallographic examinations were taken from drawn wires in various diameters. Longitudinal and cross sections were prepared from these samples. All specimens used for optical metallography- were prepared using standart polishing techniques. Inclusion analyses were performed on polished cross and longitudinal sections using an Optomax V image analyzer. Wires at each strain were subjected to micro- hardness test and observed that hardness first increased then stayed constant after a certain strain value (6=1.2) with the increasing drawing strain. According to the data obtained from the tensile test of the copper wires; it was detected that the yield strength increased and percent elongation decreased with the increasing drawing strain up to a certain strain value (E=l. 4 ), then did not change after this value.The flow curves of the materials were obtained as the envelope of tensile yield stress versus wire drawing true strain cuxves. The averages of the yield strengths of three tensile tests after each drawing operation were plotted as flow stress against true strain which is the amount of deformation introduced by wire drawing. It was observed that the material had two different strain hardening exponent values when the flow curve was plotted in Log-Log scale, and it was also understood that the flow curve could not been characterized by the Holloman equation. Also it was detected that the strain hardening rate curve had a minimum at e=1.8 and a maxi mum at e=2.5. The maximum at £=2.5 is related to the dynamic recovery process which occurs during the wire drawing reported in the literature. Number of twist to failure of the copper wires stayed constant up to a certain strain value (8=0.8) then increased with the increasing wire drawing strain, It was observed that the longest dimension and breadth values of copper oxide inclusions decreased at both cross and longitudional sections with the increa sing drawing strain. Average number of these inclusions per unit field did not change at the cross section, but increased at longitudinal section of the copper wires with the increasing drawing strain. It can be concluded from these results that the copper oxide inclusions in the tough-pitch copper break and become smaller at both sections during the wire drawing operation and their numbers increase at the longitudinal section with the increasing drawing strain.