Experimental and computational study of laser processing of dissimilar metals.

Recent advances in the manufacturing processes using laser have led to increased use of advanced and dissimilar materials. Fusion weldments of dissimilar metallic joints are not well understood. Physical properties of the two metals being very different from each other lead to complexities in weld pool shape, solidification microstructure and segregation patterns. In this work, three binary systems, iron-copper, copper-nickel and aluminium-bismuth have been chosen to study the microstructural evolution during laser processing. Iron-copper system has solid state immiscibility and liquid miscibility. Copper and nickel have different physical properties but have complete miscibility in solid and liquid states and provide an ideal system for a detailed analysis. Aluminium-bismuth has liquid state immiscibilty and is a candidate material for bearing materials that require a uniform dispersion of soft material in the matrix. The studies comprise of both experimental and computational modelling efforts.

After a brief introduction in the first chapter, a review of literature concerning laser processing of dissimilar metals is presented in the second chapter. Issues that require further studies for understanding are highlighted to put the present work in perspective. In the third chapter, brief studies on the welding of iron and copper are presented. Microstructural features that are common to most dissimilar metal welds are described for low and high laser scan speeds. The weld pool shape is found to be asymmetric with more melting of iron, though the heat source is placed symmetrically on the butt joint. The weld interface also shows asymmetry with a smooth interface on the iron side and a rough interface on the copper side. A transition from planar to cellular growth from iron side in to the weld was also observed. Detailed composition analysis shows a gradual increase in the composition of weldment from the iron side in to the weld pool, and an abrupt increase from the copper side. Microstructural banding was characterised by a change of length scale and composition.

The fourth chapter presents a detailed analysis of laser welding of copper and nickel. Welds have been done at different scanning speeds such that the welding mode changes from conduction mode at high scan speed to keyhole mode at low scan speed. A small number of spot welds were also performed for comparison with computational studies. Detailed microstructural analysis using optical microscopy, scanning electron microscopy, transmission electron microscopy and quantitative composition analysis are presented. The weld pool shape was found to be asymmetric and the microstructural features described for the iron-copper system were found to be similar in copper-nickel system as well, at all the welding speeds. Cellular microstructure was observed in the weld pool at all welding speeds. Composition across the weld indicates good mixing on the nickel side in the weld pool. TEM study shows that the weld pool is highly strained.

A computational modelling of the transport phenomena that take place during laser welding of dissimilar metals of was developed using control volume formulation. Since the process is inherently three dimensional in nature, a 3D transient model was developed to calculate conservation of mass, momentum, enthalpy and composition. Nickel was observed to melt first and the heat is transported to the copper side by convection in the molten nickel due to marangoni and buoyancy forces. The final shape of the weld pool was observed to be asymmetric. The composition profile across the weld pool shows good qualitative agreement with the experimentally observed one. An attempt was also made to explain the interface microstructures using thermodynamic arguments.

The fifth chapter deals with the microstructure development in aluminium-bismuth system under laser processing. The aim of the work is to determine the size distribution of bismuth particles experimentally and to develop a computational model the same through computation to gain further insights in to the factors that govern the distribution. The geometry used here is that of laser surface melting. The alloy was made using laser cladding of elemental powders. Solidification at different speeds was simulated using laser remelting at three speeds namely, 5mm/s, 13 mm/s and 20 mm/s. The microstructures in transverse and longitudinal sections were observed in SEM to observe the shape of remelt pool and microstructure. Bismuth particles were uniformly distributed in the matrix of aluminium. The orientation of the grains with the laser scan direction was used to obtain the growth rate as a function of height from the bottom of the remelt pool. Size distribution of bismuth particles for alloys remelt at the three different scanning speeds, was determined using image analysis. The longitudinal section shows bending of aluminium grains towards the top with alignment of bismuth particles along the growth direction for higher growth rates.

A computational model was developed to calculate the size distribution of bismuth particles during laser remelting. The model, developed for welding to obtain temperature and velocity profiles, was modified to determine the remelt pool shape and cooling rates during laser surface remelting. Phase separation of Al-Bi takes place in the liquid state and could be understood using homogeneous nucleation and growth. The driving force for nucleation was calculated and a map of nucleation rate was constructed as a function of composition and temperature. Size distribution of bismuth was calculated at different cooling rates taking in to consideration, simultaneous nucleation and growth by diffusion in the melt, till the super saturation is exhausted. Cooling rates used were obtained from the laser remelting program. Collision of particles due to convection in the laser melt pool is taken in to account by a particle tracking algorithm. Final size distribution of bismuth particles was calculated to correlate with the actual distribution obtained for the laser remelted alloys. The thesis concludes with a summary of results and suggestions for future work.