Lazer kaynak ile işlenmiş ferritik paslanmaz çeliklerin mikroyapı ve mekanik özelliklerinin incelenmesi
Investigation of microstructure and mechanical properties of ferritic stainless steels processed with laser welding
Yazar: GÜNSELİ GÜÇ
Danışman: PROF. DR. ONURALP YÜCEL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Bilim ve Teknoloji = Science and Technology ; Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Katıhal lazerleri = Solid state lasers ; Paslanmaz çelik = Stainless steel

Özet:
Bu tez çalışmasının amacı çamaşır makinesi tambur saclarında kullanılmakta olan ve mevcutta mekanik kenet bağlantısı ile birleştirilen ferritik paslanmaz çelik sacların lazer kaynak yöntemi ile birleştirilmeye olan uygunluğunun araştırılmasıdır. 
Tez kapsamında, farklı kalitelerde ferritik paslanmaz çelik sacların katı hal disk lazer kaynağı ile sürekli dalga modunda alın alına kaynak işlemi gerçekleştirilmiştir. Kaynak parametreleri 1300 Watt güç, 6 m/dakika hız olarak belirlenmiş ve koruyucu gaz olarak argon gazı kullanılmıştır. Ferritik paslanmaz çelik sacların karşılaştırılabilmesi amacıyla kaynak parametreleri sabit tutulmuştur. Deneysel çalışmalarda farklı sac üreticilerinden temin edilen 0,5 mm kalınlıkta 430, 430Ti ve 441 kalite ferritik paslanmaz çelik saclar kullanılmıştır. Her ferritik paslanmaz çelik sacın kimyasal analizleri yapılarak elemental içeriği belirlenmiştir. Lazer kaynak uygulanan sac plakalar ile metalografik incelemeler, mikrosertlik ölçümü, SEM-EDS analizleri, çekme testleri, eğme testleri ve çökertme deneyleri gerçekleştirilmiştir. Lazer kaynak işlemi sonrası mekanik özelliklerinin değerlendirilmesi ve karşılaştırılması amacıyla lazer kaynak işlemi uygulanmamış 430, 430Ti ve 441 kalite ferritik paslanmaz çelik saclar da çekme ve çökertme deneylerine tabi tutulmuştur. Çekme deneyleri sonucunda 430 kalite ferritik paslanmaz çeliğin kaynaklı numunelerinde kopma, kaynak bölgesinden gerçekleşmiş ve kaynak kalitesinin yeterli olmadığı tespit edilmiştir. 430Ti ve 441 numunelerinde ise kopma ana metal bölgesinden gerçekleşmiştir. Kaynaklı 430, 430Ti ve 441 numunelerinin akma mukavemetlerinde, kaynaksız orjinal numuneleri ile kıyaslandığında sırasıyla %6 artış, %1 artış ve %4 azalma; çekme mukavemetlerinde ise %1 artış, %3 ve %2 oranlarında azalma tespit edilmiştir. Mikroyapı incelemelerinde 430 paslanmaz çelik sac numunesi dışında kaynak bölgesi ana metal arayüzeyi 430Ti ve 441 numunelerinde kaynak hattı boyunca düz olup, belirgin bir ısı tesiri altında bölge gözlenmemiştir.
Lazer kaynak ile birleştirilen ferritik paslanmaz sacların yapı ve özellik ilişkileri, mikroyapı incelemeleri ve mekanik özelliklerinin ölçümü ile incelenmiş ve sonuçlar karşılaştırılarak yorumlanmıştır. 430 kalite ferritik paslanmaz çelik sac numunelerinde yeterli kaynak kalitesi sağlanamamış, kaynak kabiliyetinin 430Ti ve 441 ferritik paslanmaz çelik sac numunelerine göre yetersiz olduğu görülmüştür.

Abstract:
The aim of this thesis study is to investigate the suitability of joining ferritic stainless steels used in washing machine drum sheets that currently joined by mechanical clamping, by laser welding.
Recently, rapid changes and developments in production methods and material technologies cause changes in machine designs and materials used in many sectors. A similar situation is observed in the white goods sector, where competition is intense.
The use of laser welding as a joining method in drum manufacturing, which is one of the critical elements of currently in use washing machines, is increasing in the world gradually. It is possible to reduce waste rates in production high welding quality in the joint area and fully automatic drum production in drum sheet joints by using laser welding instead of electrical resistance welding and mechanical clamping connections.
Higher capacity products can be produced by increasing the drum volume by combining and then expanding the drum sheets. When the mechanical clamping method is used, the expanding rate to increase the drum volume is limited. Mechanical clamping methods have been replaced by laser welding applications due to the needs such as providing high capacity product production with the drum with the necessary fatigue and corrosion resistance and toughness values and cost advantage in the long term, and thus, the determination and development of suitable materials with changing manufacturing methods has become a necessity. 
The drum, which is a critical part of the washing machine, is located in the tub. In front-loaded machines, it is the main part where the laundry is put and washed, and there is a perforated structure in the drum circumference sheet to remove the water after washing. While the tub does not move, the drum rotates in the horizontal axis. The power coming from the motor to the drum is transferred through the shaft and the movement of the drum is provided. This shaft is mounted on the bearings fixed on the back part of the plastic tub.
The drum material in which the washing process takes place should be selected in accordance with the operating conditions. In addition to material selection and production, 430 stainless steel sheets are used as the drum sheet material due to the high strength and corrosion resistance requirement. Depending on the production method, weldability is also considered as a separate factor in material selection.
Within the scope of the thesis, butt welding of ferritic stainless steel sheets of different grades was performed by solid state disk laser welding in continuous wave mode. Welding parameters were determined as 1300 Watt power, 6 m / min speed and argon gas was used as shielding gas. Welding parameters were kept constant in order to compare ferritic stainless steel sheets.
The structure and property relationships of ferritic stainless sheets joined by laser welding were examined by microstructure studies and measurement of their mechanical properties, and the results were compared and interpreted. Adequate welding quality could not be achieved in 430 grade ferritic stainless steel sheet samples, and it was observed that the welding ability was insufficient compared to 430Ti and 441 ferritic stainless steel sheet samples. 
In the experimental studies, 0.5 mm thickness of 430, 430Ti and 441 grade ferritic stainless steel sheets were used from different sheet manufacturers. The elemental content of each ferritic stainless steel sheet was determined by chemical analysis. Metallographic examinations, microhardness measurement, SEM-EDS analysis, tensile tests, bending tests and cupping tests were carried out with laser welded sheet samples. 
In order to evaluate and compare the mechanical properties after laser welding, original 430, 430Ti and 441 grade ferritic stainless steel sheets were also subjected to tensile and cupping tests.
In the microstructure studies, except for the 430 stainless steel sheet, the weld zone is flat at the base metal interface in 430Ti and 441 samples, and no region (heat-affected zone) was observed under a significant heat effect. In the 430 stainless steel sample, on both sides of the weld zone consisting of elongated grains, a heat-affected zone (HAZ) of approximately 100 µm size consisting of coaxial grains was detected.
As a result of the tensile tests, rupture occurred in the welded samples of 430 grade ferritic stainless steel and it was determined that the welding quality was not sufficient. In 430Ti and 441 samples, rupture occurred in the base metal zone.
After the cupping tests of the samples prepared from welded and unwelded plates in Erichsen device, the tear regions and the amount of deformation of the samples were checked. As a result of the collapse tests, the least change was detected in 430Ti welded samples with a decrease rate of 4% compared to the original samples without welding. Post-weld formability is the highest in 430Ti samples with an average deformation value of 9.4 mm. The deformation values decreased by 19% and 10% in the 430 and 441 samples, respectively.
SEM and EDS analyzes of the samples were carried out on the base metal, ITAB and weld zones. Various EDS scans, including point, line and mapping in the source region and in the regions where the deposits are seen, were performed on the samples prepared for SEM examination. 
Thin and long precipitate structures were observed in the weld zone of 430 ferritic stainless steels. In EDS line and mapping analyzes, it is seen that the C ratio increased in these structures and the precipitates were interpreted as carbides. Multi-point analysis was made with EDS in the welding area and precipitates predicted to be chromium carbide in the structure were observed. In the welding zone of 430Ti ferritic stainless steel, precipitation was observed in the grains. 
As a result of the EDS line and point analysis on the precipitate, the precipitate was interpreted as a titanium carbonitride precipitate. Similarly, in the weld zone of 441 ferritic stainless steel, precipitation was observed in the grains. As a result of the line and mapping analysis made on the precipitate, the residue (Ti, Nb) (N, C) whose main composition consists of Ti, N, C and Nb elements was interpreted as a composite precipitate. Al, Mg and O elements were also detected in the core of the precipitate.
Considering all the investigations, it was concluded that the welding quality of 430 ferritic stainless steel without stabilized elements is not suitable, and considering its formability properties, 430Ti ferritic stainless steel is a more suitable and optimum option for laser welding in drum production.