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Galvanizli çelik saçların karbonlu ve ferritik paslanmaz çelikler ile nokta kaynağında kaynak parametrelerinin kaynak davranışına ve bağlantının dayanımına etkisi

The Effect of welding parameters on welding behaviour and tensile-shear strength of joints in resistance spot welding of galvanized steel sheets with carbon steel and ferritic chromium stainless stell sheets

  1. Tez No: 21714
  2. Yazar: MURAT VURAL
  3. Danışmanlar: PROF. SELAHATTİN ANIK
  4. Tez Türü: Doktora
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1992
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 119

Özet

ÖZET Konstrüktif nedenlerle, farklı çelik saçların elektrik direnç nokta kaynağı, otomotiv, beyaz eşya, kimya, petrokimya ve uçak-uzay endüstrilerinde gittikçe daha yaygın bir durum almaktadır. Galvanizli çelik sacların gerek karbonlu gerekse ferritik kromlu paslanmaz çelik saclarla elektrik direnç nokta kaynağında, kaynak parametrelerinin kaynak bağlantısına ve bağlantıların çekme-makaslama dayanımlarına etkisinin incelenmesi, bu bakımdan güncel ve önemli bir konudur. Bu konuyu incelemek üzere yapılan bu çalışmada, önce konunun önemi, ikinci bölümde ise elektrik direnç nokta kaynağının prensipleri ve esasları açıklanmıştır. Üçüncü bölümde, galvanizli çelik sacların, karbonlu çelik sacların ve ferritik kromlu paslanmaz çelik sacların fiziksel, metalurjik özellikleri ve kaynak kabiliyetleri ele alınmıştır. Dördüncü bölümde, yapılan deneysel çalışmalar anlatılmıştır. Kaynaklı bağlantıların elde edilmesi için, 120 kVA gücünde, tek kollu, pnömatik basma tertibatlı, elektronik akım ve zaman kontrollü bir kaynak makinası kullanılmıştır. Elektrik akımı 4.5 kA'den 13.5 kA'e kadar l'er kA aralıklarla arttırıl mıştır. Kayşnak süresi, 5, 15 ve 25 periyot olarak değiştirilmiş, elektrot basıncı 5kN'da sabit tutulmuştur. Elde edilen bağlantılara, çekme-makaslama deneyi, sertlik deneyi uygulanmıştır. Elektrot alaşımlanma ve deformasyonunun tesbiti için 2400 noktalık bir seri deney yapılmıştır. Bağlantıların metalografisi, optik ve tarayıcı elektron mikroskobu ile incelenmiş ve iş parçalarının, piyasa koşullarındaki temas dirençleri ölçülmüştür. Sonuçta, galvanizli-karbonlu çelik sac ve galvanizli-ferritik kromlu paslanmaz çelik sacların elektrik direnç nokta kaynağında çeşitli koşullara uygun kaynak parametreleri önerilmiştir.

Özet (Çeviri)

THE EFFECT OF WELDING PARAMETERS ON WELDING BEHAVIOUR AND TENSILE-SHEAR STRENGTH OF JOINTS IN RESISTANCE SPOT WELDING OF GALVANIZED STEEL SHEETS WITH CARBON STEEL AND FERRITIC CHROMIUM STAINLESS STEEL SHEETS SUMMARY The choice of an engineering material involves many factors, including cost. In the majority of applications made with metallic materials, the constructions occured with different types of steels are used. The sections subjected to different stresses in the same construction are made with materials suitable to these stresses. For example, corrosion resisting materials are selected for the sections subjected to corrosion and high strength materials for those subjected to high stresses. Recently, due to the great demand to corrosion- resisting materials, many researches are being done on this matter, and many new technics are developed for these purposes. The main methods to make the steels corrosion- resistance is either the coating of the material's surface with a corrosion-resisting layer or to add some corrosion-resisting metals into the composition of them. An example for the first group is galvanized steels and for the second group the stainless steels. Zinc is an example to isolate the steel surfaces from corroding environments. The main alloying element for producting the stainless steel is chromium. At least 12 % chromium content must be in the composition of steel to gain this property. The stainless steels can be divided into three main groups being ferritic chromium stainless steels, martensitic chromium stainless steels and austenitic chromium-nickel stainless steels. The most used pressure welding method to form the constructions in the industries of automotive, durable consumption goods, chemistry, aircraft and space is the viresistance spot welding. The basis of resistance spot welding is to form a joint at the intersections of two or more sheets using the resistance of materials to the passing of electric current along two electrodes holding the work pieces together. In this thesis, in the first chapter, the importance of the subject is emphasized and why this subject is selected for the doctorate thesis is explained. In the second chapter, the principles and fundamentals of electric resistance spot welding method are explained in detail. The heat production at the welding area, the variations of the method, the electrodes used in spot welding, technological properties and the limitations for the application of this welding method are described. In the third chapter, the steel sheets used as experiment materials in this work and their weldability are held. Firstly the material properties are given under the titles of carbon steel sheets, galvanized steel sheets and ferritic stainless steel sheets. In the section of carbon steels, a general description for the carbon steels is given according to their composition. In the section of galvanized steels, firstly the corrosion behaviour of zinc is given, and then, the production of galvanized steels are explained. Later on, the properties of galvanized steels are given. In the section of ferritic chromium stainless steels, firstly the composition and general properties of ferritic stainless steels are described. Secondly, the physical properties of ferritic stainless steels, and finally the metalurgical characteristics of ferritic stainless steels are explained. After these explanations the spot weldability of these materials are given according to the general informations about latest investigations. At the last part of this chapter the importance of the subject and the planning of the experimental works are described in detail. In the fourth chapter the experimental works are explained. Firstly the chemical compositions of experiment materials and the dimensions of work pieces are given. The galvanized steel sheets used in this study are low-carbon unalloyed steels coated with zinc layer of 50 jum in thickness. The carbon steels are of low-carbon tpyes of 0.08 % C. The ferritic chromium stainless steels are of AISI 430 type (X6Crl7). The dimensions of work pieces are 30x100x1 mm. After cutting the materials in given dimensions, ther are cleaned in ethanol of 96 % and finished with a clean, cotton cloth for purifying the materials' surfaces from dirts, oils etc. viiFor the spot welding of work pieces an electric resistance spot welding machine with single arm, equipped with pneumatic pressure and electronic time and current control units is used, and electrode pressure is measured with a manometer located between upper and bottom arm of the machine. The welding currents are continuously measured and controlled with a magnetic field measuring, wide-range ampermeter. At the experiments, the CuCrZr type and spherical shaped electrodes with a tip diameter of 16 mm are used. The electrode pressure is kept constant at 5 kN. The welding times are changed as 5, 15 and 25 periods, and the sequeezing and holding times are kept constant as 25 periods during all the experiments. The welding current is changed from 4.5 kA to 13.5 kA with intervals of 1 kA. The tensile-shear tests is applied to all the series welded under these conditions. The microVickers hardness testsa are applied to those series after cutting the welded samples from their cross-sections. After these tests, the microstructure photographes of the samples are taken. The chromium carbide precipitation at the grain boundaries of ferritic chromium stainless steels are investigated by examining with the scanning electron microscope (SEM). The determining the deformations and pick-up of electrode tips from the zinc layer, an electrode life test is made by obtaining 2400 spots and by taking the photographes of electrode tips after each 400 spots. At the same conditions some welded samples are obtained and the tensile-shear tests are applied to these samples to determine the decreasing of tensiel-shear strength of joints due to the electrode tip conditions. The contact resistance of experiment materials being at as-delivered conditions are measured in line with the principles given in IIW/IIS.Doc.III-905-88, IIIG-126-88. In the fifth chapher, the experimental results are discussed. Firstly the effect of welding current, welding period and electrode tip conditions on the tensile-shear strength of joints are held, and the results are given in figures. The hardness figures are plotted at each weld condition for galvanized-carbon steel sheet pairs and galvanized-f erritic chromium stainless steel pairs. The microstructure photographes are tabulated as viiiheat affected zones of two different materials and weld metals in each welding current and period. The results of TEM examinations are given as photographes and shematic figures. Finally, the effect of welding parameters on nugget dimensions are given and some photographes about the nugget formation are added. The conclusional remarks and recommendations obtained all these experiments are listed in the last chapter as follows: 1. The maximum tensile-shear strength for galvanized-carbon steel sheet pairs are obtained at 9.5 kA for 15 and 25 perioda. For galvanized-f erritic chromium stainless steel sheet pairs, these values are 10.5 kA for 15 and 25 periods. The electrode indentation ratios for these joints are 22 % and 24 % of sheet thickness respectively. For this reason, the advisable welding parameters must be 9.5 kA and 15 periods, because the electrode indentation ratios at these conditions are 19 % and 17 % for galvanized- carbon steel sheet pairs and for galvanized-f erritic chromium stainless steel pairs respectively. 2. It is understood that the tensile-shear strengths of joints is decreased 10 % after 2000 spot welds. Therefore the electrode tips must be renewed after each 2000 spot welds. 3. In the f erritic chromium stainless steels, the chromium carbide precipitation at the coarse grained area's grain boundaries adjacent to the weld metal is determined. For this reason, a post-heat treatment must be applied at 1100°C to the joints which may work in corroding environment. If it is not possible due to constructive conditions, the stabilized types of ferritic 17 % chromium stainless steels are recommended. 4. According to the chemical analysis, the weld nugget of galvanized-f erritic chromium stainless steel sheet pair joints is lost its stainless property. 5. During the hardness tests, more high values are obtained at 5 periods. Therefore this point must be taken into considerations in applications. 6. Especially in the spot welding of galvanized- ferritic chromium stainless steel sheet pairs, it is seen that a grain coarsening is occured adjacent to the weld nugget. ix7. After the comparison of the contact resistance measurings with the tensile-shear strengths, a certain relation can not be seen between the contact resistance and tensile-shear strengths.

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