S. R. Koteswara Rao

Mechanism of Weld Formation during Friction Stir Welding of Aluminum Alloy

The present work aims at identification of process parameters for sound weld and to understand the mechanism of material matrix movement and weld formation in friction stir welding (FSW) of aluminum alloy. FSW experiments were conducted on 6 mm thick AA6061-T6 plates using various rotational and welding speeds. The welded plates were cut along the transverse direction to examine the macro defects. The defect size as a function of welding and rotational speed has been studied, and three parameter zones were identified. They are insufficient material flow zone, defects free zone, and excessive material flow zone. The sound zone is identified in the moderate rotational speed range for each welding velocity that occurs between the insufficient and excessive material flow zone. In the insufficient material flow zone, the defect size is found to be inversely proportional to rotational speed, whereas in the case of excessive material flow zone the defect size is directly proportional to rotational speed. As welding velocity increases, rotational speed is to be increased proportionally to form a sound weld. The relation between weld parameters, temperature, matrix movement, and weld formation has been established.

Improving mechanical properties of 2219 aluminium alloy GTA welds by scandium addition

Abstract The effect of scandium additions to 2219 aluminium alloy weld metal has been investigated. At low levels (0·16%) of scandium, in spite of grain refinement in the weld metal, improvement in the mechanical properties has been nominal. At higher levels of scandium (0·37%), a substantial improvement in the tensile properties has been obtained. Further improvement in mechanical properties has been achieved by adding small amounts of magnesium. Transmission electron microscopy revealed the presence of fine precipitate particles in the scandium containing weld metals. Electron probe microanalysis (EPMA) and SEM–EDX revealed extensive copper segregation to grain and subgrain boundaries. The presence of scandium reduces the severity of segregation by producing fine equiaxed grains in the weld metals and also by refining the grain substructure. The morphology and size of the high copper eutectic phase at grain boundaries and sub-boundaries have been found to be finer and well distributed in the case of scandium containing weld metals. EPMA linescans and quantitative analyses proved that the depletion of copper in the matrix is minimised as a result of the fine grained structure.

Thermal model for friction stir welding of mild steel

Purpose – The purpose of this paper is to propose a three‐dimensional thermal model for friction stir welding of AISI 1018 mild steel to predict the thermal cycle, temperature distribution, the effect of welding parameters on power required, heat generation and peak temperature during the friction stir welding process.Design/methodology/approach – The mathematical expressions for heat generation during the friction stir welding process were derived. The simulations for various welding and rotational speeds were carried out on ANSYS software employing temperature and radius dependent moving heat source and applying the boundary conditions.Findings – The predicted thermal cycle, torque required and temperatures were found to be in good agreement with the experimental results. The heat generation and peak temperatures were found to be directly proportional to rotational speed and inversely proportional to welding speed. The rate of increase in heat generation and peak temperature were found to be higher at l...

Studies on salt fog corrosion behavior of friction stir welded AA7075–T651 aluminum alloy: Paper presented at “International Conference on Engineering Materials and Processes 2013” (ICEMAP 2013), 23 – 24 May 2013, Tagore Engineering College, Chennai, Tamilnadu, India

Abstract High strength aluminum alloy 7075 is used in aerospace applications. This alloy is considered unweldable using fusion welding processes. Friction stir welding being a solid state process has been proved to be a suitable process for obtaining sound welds of these materials. Although no solid–liquid phase transformation occurs during friction stir welding, relatively high temperatures, up to 475 °C for AA7075 aluminum alloy, are generated by friction between the tool and the workpiece. This produces modified microstructures that may be more susceptible to corrosion than the parent alloy. In the current study, 16 mm thick rolled AA7075–T651 aluminum alloy plates were joined using friction stir welding. The corrosion behavior of base material and friction stir welds was investigated using the Salt fog test (ASTM B117). It was observed that the corrosion resistance of the welds in the basic solution is better than that in the acidic and neutral solutions. In contrast, no significant difference has been found in the corrosion resistance of the base material with change in pH value. It was also found that the corrosion rate decreases with increase in time of exposure. It has been observed that corrosion attack is greater in the weld region than in the parent material and within the weld, the heat-affected zone has been found to be more susceptible to corrosion compared to the weld nugget and thermo-mechanically-affected zone regions. Transmission electron microscopy studies showed coarser precipitates and wider precipitate-free zones in the heat-affected zone which are concluded to be reasons for more susceptibility to corrosion.

Mechanical and corrosion properties of friction stir welded joints of Al-Cu alloy 2219-T87

Abstract In the present study, 8.1 mm thick AA 2219–T87 plates were joined by friction stir welding (FSW) process. Welds were characterized by using micro hardness survey, tensile testing, face bend, root bend tests, optical microscopy (OM), transmission electron microscopy (TEM) and salt fog test (ASTM B117) at different pH values and spraying times. Hardness survey across the joint revealed that weld nugget is the softest region and it is also found that tensile failure occurs in the nugget. Tensile testing of the transverse welded joints showed that high efficiency (> 75 %) joints could be produced. The friction stir welds subjected to the surface bend showed 1800 bend ductility, whereas in the critical root bend the cracks were initiated at a bend angle in the range of 30–400. Transmission electron micrographs obtained from various regions of the weld indicated that almost all strengthening precipitates dissolved in the nugget region while partial dissolution of precipitates occurred in the thermomechanically affected zone and coarsening occurred in heat affected zone. However, the losses in hardness in the nugget due to dissolution of precipitates and the negation of work hardening by recrystallization were compensated to a large extent by the strengthening due to the grain refinement in the nugget, which explains the high joint efficiencies exhibited by these welds. It was observed that the welds possessed much better corrosion resistance in basic and neutral solution than in acidic solution. It was found that corrosion attack was greater in the base material than in weld metal at all pH values and spraying times. It has been concluded that friction stir welding has a significant effect on mechanical and corrosion properties of the welds.

Effect of beam oscillation on borated stainless steel electron beam welds

Abstract Borated stainless steels are used in nuclear power plants to control neutron criticality in reactors as control rods, shielding material, spent fuel storage racks and transportation casks. In this study, bead on plate welds were made using gas tungsten arc welding (GTAW) and electron beam welding (EBW) processes. Electron beam welds made using beam oscillation technique exhibited higher tensile strength values compared to that of GTA welds. Electron beam welds were found to show fine dendritic microstructure while GTA welds exhibited larger dendrites. While both processes produced defect free welds, GTA welds are marked by partially melted zone (PMZ) where the hardness is low. EBW obviate the PMZ failure due to low heat input and in case of high heat input GTA welding process failure occurs in the PMZ.

Microstructure and Mechanical Behaviour of Friction Stir Welded Copper

The objective of this experimental study was analyzing the joining behavior of 6 mm thick cold rolled copper plate using Friction Stir Welding (FSW) technique. The role of tool rotational speed, travel speed and tool pin profiles on the weld quality, tensile and bend behaviors, hardness, and microstrucuture was investigated. Defect free weld was obtained at tool rotational speeds ranging between 1300 - 1600rpm and welding travel speeds ranging between 30-45mm/min. Tensile strength interms of joint efficiency was found to be 73% compare to the base material (BM). The average hardness of the nugget zone was lesser than the base material because of annealing of the cold rolled copper plates during welding. Different microstructure zones were revealed by optical microscopy (OM). The nugget zone (NZ) and heat affected zone (HAZ) were found to have fine equiaxed grains and fine elongated grains respectively.