APPLICATION OF UNDERWATER FRICTION STIR STITCHING FOR CRACK/ NOTCH REPAIR OF STRUCTURAL PARTS

Abstract

Underwater friction stir stitching using non consumable tool is one the latest technologies in the area of metal joining. It is economical in nature, because it bypasses the need to pull the structure out of the sea and save much valuable time. The main difficulties in other underwater welding processes are the presence of a higher pressure due to the water head under which welding takes place, chilling action of the water on the weld metal (which might change the metallurgical structures and properties), the possibility of producing the arc mixture of hydrogen and oxygen in pockets, which might set up an explosion, and the common danger sustained by divers, of having nitrogen diffused in their blood in dangerous proportions. All these have been taken care of by this solid state method. The cost of producing a consumable tool is also eliminated. In this work, metal inert gas (MIG) welding was carried out on 3mm thick (AISI 201) stainless steel plates. Notches of 2mm were along the line of weld using 5mm drill bit. Conventional vertical milling machine, capable of withstanding the forces generated during welding and having a powerful spindle motor to generate the torque needed for friction stir stitching welding (FSSW) was selected for the purpose. Clamping fixtures were used to firmly hold the plate in place during FSSW process. Extensive preparatory operations were conducted to develop the correct welding procedure for obtaining defect-free repairs. Process parameters such as spindle speed ranging from (1200 – 1800) rpm, plunge dept of between 0.4 – 0.8mm and dwell time of 2 – 6s were used in the stitching process. The process parameters were varied for each run as specified in center surface response Box-behnken’s design of experiment in the presence of rich supply of fresh water. Tensile test was carried out on all the 45 welds and the microstructural analysis of the welds produced were also investigated using scanning electron microscope (SEM) equipped with energy disperse x-ray (EDX) to examine the grain sizes fractured surfaces. Based on the results gotten from tensile test, the highest, medium and lowest mean failure stress designated as (H+1), (M0) and (L-1) were 835.61MPa, 618.35 MPa and 598.45 MPa respectively. These optimized parameters gave a weld whose failure stress was 844.39 MPa. This produced failure stress increment of about 1.1% %. Friction stir stitching process was done at a temperature below the melting point of the work piece material which is about 0.5 to .075 of the base metal melting temperature. This makes phase transformation very difficult thereby leading to dynamic recrystallization that result in a highly refined grain structure in the weld nugget. The microstructural analysis was investigated using scanning electron microscope (SEM) equipped with energy disperse x-ray (EDX) to examine the grains sizes and fracture surfaces. The result of SEM shows that there were uneven fractures with mixed fracture mode containing both ductile and brittle results. This was so because stitching was not done on the entire thickness. Energy disperse spectrum (EDS) also show a relatively gain in oxygen in the joint which was as a result of the wet environment where the experiment was done to form the oxides of chromium, magnesium and iron. This project showed the potential to fix cracks in wet condition