M. Govinda Raju

Bio: M. Govinda Raju is an academic researcher. The author has contributed to research in topics: Boundary layer & Physics. The author has an hindex of 1, co-authored 1 publications receiving 40 citations.

Microstructure and mechanical properties of Sc modified Al–Cu alloy (AA2219) electron beam welds

Abstract: In this paper, the formability of YAG and diode laser welded blanks of a dual phase (DP) steel with banded martensite was investigated by using limiting dome height (LDH) testing method. One high strength low alloy (HSLA) steel with equiaxed ferrite matrix was included for comparison. Both steels had a ferrite matrix and dispersed secondary phase, namely martensite and carbide. The failure mode and dome height at fracture were examined on both parent metals and welded blanks. It was found that the failure pattern of DP parent metal was determined by its rolling direction. With increasing plastic strain, voids initiated along interfaces between ferrite/martensite due to decohesion between two phases and propagated preferentially along the interface. For DP welded blanks, dome test failures occurred within the softened zone in the heat affected zone (HAZ) regardless of the orientation between weld and rolling direction. Voids formed at the interfaces between ferrite and tempered martensite. The presence of the softened HAZ zone led to dramatic LDH decrease compared to the parent metal. The diode laser welded blanks had a lower dome height than that of YAG laser welded blanks due to their more severe HAZ softening. In comparison, HSLA parent metal and welded blanks exhibited almost the same LDH values and showed insensitivity to welding process. The failure of HSLA welded blanks initiated from the weld and propagated perpendicular to the unaffected base metal.

Stagnation point flow of power-law fluid in unsteady 3D boundary layer over a moving surfaces

Abstract: Here, unsteady boundary layer flow under the action of magnetic effect over a moving stagnation surface has been investigated numerically. This study examines non-Newtonian fluid flow with respect to combined effect of magnetic field, movement of surface and time. Governing equations are dimensionless by applying nondimensional quantities. Then system of coupled ordinary differential equations are obtained by appropriate similarly transformations, in terms of the governing parameter including unsteady parameter ([Formula: see text], magnetic number ([Formula: see text], power-law index ([Formula: see text] and three-dimensional parameter ([Formula: see text]. Thus, the obtained modelled equations are solved numerically by the shooting technique. The problem’s parameters are thoroughly discussed and verified physically and graphically. The obtained results validate to literature result. The two relevant flows’ velocity profile’s numerical results have been examined.

Microstructure and mechanical properties of newly developed aluminum–lithium alloy 2A97 welded by fiber laser

Abstract: The newly developed aluminum–lithium alloy 2A97 was for the first time joined by laser beam welding in order to meet the ever-increased long-term requirements of aerospace, aviation and armament industries. The weld appearance, microstructure, solute segregation, precipitate behavior, and their relationships with mechanical properties of welded joints were investigated. Sound joints with no crack and a few small porosities are obtained under appropriate heat inputs. As a result of heterogeneous nucleation involving the effect of Zr and Li, a non-dendritic equiaxed zone forms between partially melted zone and fusion zone. The crystal morphologies in fusion zone vary from columnar dendrite to equiaxed dendrite, with the increase of constitutional supercooling. Solute segregation leads to the variations of Cu content in grain interior and boundary, as well as the weak ability of re-precipitation of fusion zone. Most precipitates in the base metal dissolve during welding, and fusion zone contains a decreased quantity of δ ′, β ′, θ ′, and T 1 . The ultimate tensile strength of laser welded joints is 83.4% of that of the base metal, and can meet the application requirements from related industries, but the ductility still needs to be improved. Welding defects and loss of solid solution/precipitation hardened structure lead to the degradation of mechanical properties. Tensile fracture occurs in weld with the brittle intergranular dominated mode and premature failure occurs and extends in the equiaxed zone.

Tensile Properties and Work Hardening Behavior of Laser-Welded Dual-Phase Steel Joints

Abstract: The aim of this investigation was to evaluate the microstructural change after laser welding and its effect on the tensile properties and strain hardening behavior of DP600 and DP980 dual-phase steels. Laser welding led to the formation of martensite and significant hardness rise in the fusion zone because of the fast cooling, but the presence of a soft zone in the heat-affected zone was caused by partial vanishing and tempering of the pre-existing martensite. The extent of softening was much larger in the DP980-welded joints than in the DP600-welded joints. Despite the reduction in ductility, the ultimate tensile strength (UTS) remained almost unchanged, and the yield strength (YS) indeed increased stemming from the appearance of yield point phenomena after welding in the DP600 steel. The DP980-welded joints showed lower YS and UTS than the base metal owing to the appearance of severe soft zone. The YS, UTS, and strain hardening exponent increased slightly with increasing strain rate. While the base metals had multi-stage strain hardening, the welded joints showed only stage III hardening. All the welded joints failed in the soft zone, and the fracture surfaces exhibited characteristic dimple fracture.