Ti6Al4V alaşımının PTA kaynağında 1800W kaynak gücünde birleştirilebilirliğin araştırılması

Abstract

Bu çalışmada, elektrik enerji endüstrisi, nükleer enerji, petro kimya, otomotiv, uzay araçları ve medikal endüstrisinde yaygın kullanım alanına sahip Ti ve alaşımlarından Ti6Al4V alaşım çifti Plazma Transfer Ark (PTA) kaynak yöntemiyle kaynak edilmiştir. Plazma transfer ark kaynağında, torçla elektrodun etrafından gelen argon gazı tungsten elektrod ve nozul arasındaki pilot ark sayesinde iyonize olmaktadır. Çalışma esnasında PTA kaynakları, argon koruyucu gaz atmosferinde, sabit 1800 W kaynak gücünde ve 0,50-0,75-1 m/dk. kaynak hızlarında yapılmıştır. Kaynak sonrası ITAB’da meydana gelen mikro yapı değişiklikleri optik mikroskop, SEM, EDS ve XRD analizleri ile incelenmiştir.Malzemenin birleşme mukavemetini belirlemek için çekme deneyi yapılmıştır. Mikrosertlik ölçümleri Vickers skalasında 50 gr’lık yük altında yapılmıştır. Yapılan incelemeler sonrasında, mikro ya da makro çatlaklara rastlanmamıştır. Numuneler kaynak dikiş karakterizasyonu açısından değerlendirildiğinde kot farkı altında oluşan kaynak çukurunun tüm numunelerde oluşmadığı, ergimiş bölge genişliğinin ve ITAB’ın kaynak hızındaki artışla ters orantılı olarak arttığı tespit edilmiştir. Mikrosertlik değerlerinin ana metalden ergimiş bölgeye ilerledikçe arttığı ve en yüksek değere ergimiş bölgede ulaştığı görülmüştür. Ayrıca mikrosertlik değerlerinin kaynak hızına bağlı olarak ters orantılı arttığı, yüksek kaynak hızında düşük değerde, düşük kaynak hızında ise yüksek değerde olduğu tespit edilmiştir. Bütün kaynaklarda, kaynak hızı arttıkça kaynağın mekanik özelliklerinin kötüleştiği tespit edilmiştir. Sabit kaynak gücünde, 0,50 m/dk. kaynak hızıyla yapılan PTA kaynağının, metalurjik açıdan kaynak kalitesi en yüksek birleştirme olduğu görülmüştür

Keywords

• Ti6Al4V, Plazma Transfer Ark (PTA) Kaynağı, Kaynak Gücü, Kaynak Hızı

The Investigation of the Joinability on 1800W Welding Power on the Plasma Transfer Arc (PTA) Welding of Alloys Ti6Al4V

Abstract

In this study; Ti6Al4V alloy couple from the Ti and its alloys, which are commonly used in electrical energy industry, nuclear energy, petrochemistry, automotive, space vehicles and medical industry, was welded by using Plasma Transferred Arc (PTA) method. In plasma transferred arc welding; argon gas coming with the torch from the surrounding of the electrode is ionized via the pilot arc between the tungsten electrode and the nozzle. Thus, the plasma gas that has become conductive passes through the narrow end of the nozzle and is transferred to the piece as a narrow column. Shielding gas is transferred from the external environment of the nozzle to the weld zone and protects the welding bath. Cooling liquid coming from the cooling system goes towards the torch by passing through special channels and cools the heat here again in the coolant as a closed circuit. The power source for plasma welding is almost exclusively DC and, as in TIG, the drooping, or constant current, output characteristic will deliver essentially constant current for a given power source setting. The power source is ideal for mechanised welding as it maintains the current setting even when arc length varies and, in manual welding, it can accommodate the natural variations of the welder.The plasma process is normally operated with electrode negative polarity to minimise heat produced in the electrode (approximately 1/3rd of the heat generated by the arc is produced at the cathode with 2/3rds at the anode). Special torches are available, however, for operating with electrode positive polarity which rely on efficient cooling to prevent melting of the electrode. The positive electrode torch is used for welding aluminium which requires the cathode to be on the material to remove the oxide film. AC is not normally used in the plasma process because it is difficult to stabilise the AC arc. Problems in reigniting the arc are associated with constriction by the nozzle, the long electrode to workpiece distance and balling of the electrode caused by the alternate periods of electrode positive polarity. The square wave AC (inverter, switched DC) power source, with an efficiently cooled torch, makes the use of the AC plasma process easier; rapid current switching promotes arc reignition and by operating with very short periods of electrode positive polarity, electrode heating is reduced so a pointed electrode can be maintained. The plasma system has a unique arc starting system in which HF is only used to ignite a pilot arc held within the body of the torch. Samples were prepared of 80x40x4 mm prior to the work. During the study, PTA weldings were performed under argon shielding gas atmosphere at constant 1800 W welding power and at 0.50-0.75-1 m/min. welding speeds. Microstructure changes, which occurred in HAZ after the welding, were examined by optical microscope, SEM, EDS, and XRD analyses. Samples were prepared by Metallographic in order to perform this analysis. Tensile test was applied in order to determine the joining strength of the material. Tensile tests were performed ın the drawing device that has 50k/N load and 1mm/min in tensile speed. Microhardness measurements were performed under 50-gr load at Vickers scale. As a result of the examinations performed, no micro or macro cracks were observed. Ti6Al4V alloy couple is combined without a problem by PTA method in Ar atmosphere. Metallographic terms in the area of the merger is complete has been found to be void or craters. When the samples were assessed in terms of weld nugget characterization; it was found that the welding cavity occurred under elevation difference was not observed in all the samples and it increased as inversely proportional to width of the molten area width and the increase in the welding speed of HAZ. According to the XRD analysis results, crystal systems such as Hexagonal Ti, Cubic Al, Hexagonal Al2O3, Tetragonal Al3V formed in weldıng seam. It was observed that microhardness values increased while progressing from the main metal to the molten area and reached the maximum value at the molten area. Furthermore, it was determined that microhardness values increased as inversely proportional to the welding speed and they were low at high welding speed and high at low welding speed. In all welds, it was observed that as the welding speed increased, mechanical properties of the weld deteriorated. It was found that PTA welding performed at 0.50 m/min. welding speed at constant welding power was the joining with the highest welding quality in terms of metallurgical aspect.

Keywords

• Ti6Al4V, Plasma Transfer Arc (PTA) Welding, Welding Power, Welding Speed