Gazaltı (MIG) kaynak yöntemiyle birleştirilen 6061 serisi alüminyum plakalarına yapılan kaynak tamirinin mikroyapı ve mekanik özelliklere etkisi = Effect of weld repair on the microstructural and mechanical properties of 6061 series aluminum plates joined by gas (MIG) welding method

Abstract

6xxx serisi alüminyum alaşımları hafiflikleri, termal iletkenlikleri, işlenebilirlikleri, kaynağa elverişliliği ve korozyon direçlerinden dolayı otomotiv, ambalaj, yapı sektörleri başta olmak üzere birçok alanda kullanılmaktadır. Kaynaklı imalat da bu alanlarda önemli ve kritik bir rol oynamaktadır. Bilindiği üzere kaynak işlemi malzemeyi birleştirirken aynı zamanda malzemenin mekanik ve yapısal özelliklerini de değiştirmektedir. Bu değişimler malzemenin kullanım alanını ve ömrünü kısıtlamaktadır. Kaynak işleminden sonra kaynak dikişinde yüzeysel ve hacimsel kusurlar meydana gelebilmektedir ve bu kusurlar servis şartlarını sağlamaya engel olduğundan kaynağın tamir edilmesi gerekmektedir. Isı girdisinden ötürü alüminyum kaynak tamiri malzemenin fiziksel ve mekanik özelliklerini olumsuz yönde değiştirmektedir. Bu yüzden alüminyum kaynak tamiri tavsiye edilen bir proses olmamakla birlikte kaynak kabul kriterlerini sağlamak için kaçınılmaz bir durum olarak karşımıza çıkmaktadır. Burada amaç; minimum kaynak tamiriyle kaynağın sürdürülebilirliğini sağlamaktır. Bu düşüncelerden yola çıkarak tekrarlanan kaynak tamirlerinden sonra kaynağın mekanik özellikleri yüzeysel ve hacimsel kusurları değerlendirilmiştir. Alüminyum alaşımının kaynak edilebilirliğinden yola çıkarak 4mm kalınlığında 450 kaynak ağzı açılmış 6061 serisi Al alaşım plakalar MIG kaynak yöntemiyle birleştirilmiştir. Dolgu malzemesi olarak ER 5356 dolgu teli kullanılmıştır. Kaynaklı numuneye; gözle muayene, sıvı penetrant testi, radyografik test, çekme testi, eğme testi, makro inceleme, mikroyapı inceleme, sertlik testi, EDS ve XRD analizleri uygulanmıştır. Daha sonra aynı parçalardan tekrar kaynatılarak, kaynaklı bölgeler taşlanmış, temizlenmiş ve MIG kaynağıyla tamirat kaynağı yapılmıştır. Aynı tamirat işlemi TIG kaynağıyla da yapılmıştır. Tamirat işlemi üç kez tekrarlanmış ve her tamirattan sonra kaynaklı bölgeler aynı testlere tabi tutulmuştur. Gözle muayene ve sıvı penetrant test sonuçlarına göre TIG kaynağı daha düzgün bir kaynak yüzeyi sergilemiştir. Kaynak kep ve kök yükseklikleri kabul seviyesindedir. Artan tamirat sayısıyla birlikte ısı girdisi de arttığından malzemede burulmalar meydana gelmiştir. Makroyapı sonuçlarına baktığımızda ise tamirat işleminin gözenek oluşumuna gözle görülür bir etkisinin olmadığı görülmüştür. Sertlik testi sonuçlarına baktığımızda, en yüksek sertlik değerleri sırasıyla, ana metal, kaynak bölgesi ve ITAB şeklindedir. Çekme deneyi sonuçlarına baktığımızda MIG kaynağı TIG kaynağına göre daha yüksek mukavemetli sonuçlar sergilemiştir. Eğme deneyi sonuçlarında herhangi bir hataya rastlanmamıştır. EDS sonuçlarına göre tamirat sayısı arttıkça Mg2Si oranının azaldığı görülmüştür. XRD sonuçlarında da alüminyum pikleri ile birlikte, Mg2Si'nin ise düşük şiddette bir pik verdiği gözlemlenmiştir. XRD analizinde yapıda herhangi oksitli bir bileşiğe rastlanmamıştır. Alınan sonuçlardan yola çıkarak alaşımın kaynak tamiri işlemi nedeniyle sürdürülebilirliği değerlendirilmiştir. Kaçıncı tamirattan sonra malzemenin servis şartlarında kullanılıp kullanılamayacağı tartışılmıştır.

Welding is the joining of many similar and different metals, such as metals, alloys or plastics, using heat welding and with or without pressure. During the joining process, the parts to be joined are melted by a heat source and solidified to form a permanent joint with strength equal to or greater than the base metal. Depending on the thickness of the sheet to be welded, it can be welded with or without the addition of filler material, providing a strong connection between the welded joints. The weldability of any material to be joined depends on the metallurgical factors that occur during the welding process and cause a change in the underlying material property. Aluminum alloys are widely used and preferred materials in the aerospace, spacecraft, structural and military industries. Aluminum alloys have certain properties such as high elastic modulus, high specific strength, good fracture toughness and excellent corrosion resistance. 6061 Aluminum alloys are alloys that can be heat treated, whose strength can be increased by precipitation hardening, and phase transformation occurs during heat input in welding. It is a precipitation hardened Al alloy containing major alloying elements such as magnesium and silicon. It is one of the most common alloys used for good mechanical and weldability properties and also for general applications. At the beginning of the reasons why aluminum is an economical and widely used material and its usage area is wide; in addition to its suitable mechanical properties, it has features such as low weight, easy processing, easy formability, recyclability, high thermal conductivity and electrical conductivity and non-magnetic. 6xxx series aluminum alloys are used in many areas, especially in the automotive, packaging and construction sectors, due to their lightness, thermal conductivity, machinability, weldability and corrosion resistance. Welded manufacturing also plays an important and critical role in these areas. Magnesium and silicon are the main alloying elements in 6061 T6 aluminum alloys. They are alloys that are the mostly used in the manufacture of materials produced by extrusion and have a high shaping ability. Industrial demand for 6xxx series alloys is high. Two-thirds of extruded products are aluminum materials, and most of them consist of 6xxx series aluminum alloys. These series are the most commonly used alloys. These materials can be heat treated to provide aging at various stages. T6 heat treatment is applied, which is taken into solution and artificial aging is used as a heat treatment. As it is known, while the welding process joins the material, it also changes the mechanical and structural properties of the material. These changes limit the usage area and life of the material. After the welding process, superficial and volumetric defects may occur in the weld seam and since these defects prevent the service conditions, the weld must be repaired. Due to heat input, aluminum welding repair negatively changes the physical and mechanical properties of the material. Therefore, although aluminum welding repair is not a recommended process, it is an inevitable situation to meet the welding acceptance criteria. The purpose here is; to ensure the sustainability of the welding with minimum welding repair. Based on these considerations, after repeated weld repairs, the mechanical properties of the weld, its superficial and volumetric defects were evaluated. Based on the weldability of the aluminum alloy, 6061 series Al alloy plates with 4mm thickness, 450 weld bevel, were joined by MIG welding method. ER 5356 filler wire was used as filling material. Welded sample; visual inspection, liquid penetrant test, tensile test, bending test, macro examination, microstructure examination and hardness test were applied. Then, the same parts were welded again, the welded areas were grinded, cleaned and repair welding was performed with MIG welding. The same repair was done with TIG welding. The repair process was repeated three times, and after each repair, the welded areas were subjected to the same tests. According to the results of visual inspection, no problem was found in the weld. Weld cap and root heights are at acceptance level. TIG welding is one step ahead in places where visuality rather than strength is desired according to the usage area of the welding process. When we look at the penetrant inspection results, it is seen that as the number of repairs in MIG welding increases, the porosity on the weld surface also increases. When we look at the results of the radiographic examination, no discontinuities were found in the weld seams. In the results of the macrostructure examination, it was observed that the porosity was mostly in the root region. The reason for this is that the pre-welding cleaning process is insufficient. The pore distribution and sizes were evaluated within the acceptance criteria. It is thought that the microcracks in the weld root as a result of the third repairs are due to the effect of increased heat input and thermal expansion. The dark black particles seen in the base metal in the microstructure images represent Mg2Si particles. Since the filler metal is ER5356, there are Mg2Si particles in the weld structure. As the heat input increased, the grain size also increased. As the repair number increases, the grains in the HAZ region become coarser. In accordance with the applied repair processes and mechanical strength tests, the tensile strengths decreased as the repair process was applied to the material, and it was observed that the yield and tensile strength values decreased with the welding repair. In addition, when the data in the tables are examined, it is seen that the tensile strength of MIG welding is higher than the tensile strength of TIG welding in the 1st, 2nd, and 3rd repairs. The lowest tensile strength was measured in MIG welding as 180 MPa (sample 3.1) and 175 MPa (sample 3.2) in TIG welding. As a result of TIG welding repair, the tensile strength value is out of the acceptance for the artificial aged material. In the tensile test, all fractures occurred in the HAZ region. It is thought that the dissolution of strength enhancing (Mg2Si) precipitates in the heat affected zones in the welding of 6061 T6 aluminum alloy leads to a decrease in mechanical properties. This situation is similar to the studies in the literature. When we look at the results of the bending test, no fracture or crack was found in the material. The heat inputs are calculated as 214 J/mm in MIG welding and around 285 J/mm in TIG welding. Another conclusion based on the results obtained is that TIG welding causes more physical damage to the material. Due to the higher heat input, more distortion was observed in the material after repairs made with TIG welding. When we look at the hardness test results, it is seen that the highest hardness value is in the base material, then in the weld zone and the lowest value is in the HAZ zone. In XRD results, it was observed that Mg2Si gave low intensity peaks together with aluminum peaks. No oxide compounds were found in the structure. According to the results of the EDS analysis and mapping taken from the HAZ regions, a decrease in the amount of Mg was observed in parallel with the increasing number of repairs and heat input. It is thought that the tensile strength and hardness values also decrease in relation to this situation. Based on the results obtained, the sustainability of the alloy due to the welding repair process was evaluated. It has been discussed whether the material can be used in service conditions after the number of repairs.