Processing–Microstructure Correlations Group

Fusion Welding of Dissimilar Metals

Every material system has its own percuiliarities when it comes to joining. However, fusion welding of components made of different materials raises the bar a notch higher by bringing in the aspect of thermal and metallurgical dissimilarities of the base metals into picture.

First, materials differ, sometimes wildy (e.g., say, Cu and steel), in their thermophysical properties. The most relevant properties in this respect can be melting point, thermal diffusivity (or conductivity), latent heat of fusion etc. These dissimilarities can have quite unexpected effects on the heat transfer during welding. This, coupled with geometrical features of the joint, can result in an asymmetric fusion zone even if the welding heat source is placed symmetrically with respect to the parts to be joined.

Second, weld composition in the fusion zone, which varies only to a limited extent in the case of joining similar metals, can have a much wider variation while joining dissimilar metals, going all the way to hundred atom percent in the case of binary dissimilar couples. This opens up a whole lot of new possibilities which were simply absent earlier. Many material systems have intermediate phases and intermediate invariant transformations taking place at different temperatures. So the variation of weld composition, both spatial (as one moves from one base metal end to the other) and temporal (as every points goes through the weld thermal cycle), and the concomittant variation of temperature, assume critial importance. The spatio-temporal coupling between these two variables dtermine the phase selection and microstructure formation in an inhomogeneous melt (both thermally and compositionally) that is a dissimilar metal weld fusion zone.

Our analyses of laser and electron beam welds of Ti/Ni binary couples bring out these aspects of dissimilar metal welding very convincingly. This system forms the basis for two key classes of alloys used in aerospace applications: the Ti-base and Ni-base alloys. The critical influence of the thermal dissimilarity of Ti and Ni (the later conducts heat much faster than the former) on the heat transfer, and consequently on the size and shape of the fusion zone, is most strikingly demonstrated for thick butt joints made by laser beam welding. In the left image below, we can see that Ti has melted significantly more compared to Ni, because the local rise of temperature in Ti is more in this case due to inefficient heat transfer through it. On the other hand, the thin-plate electron beam weld (right) illustrates the importance of beam – material interactions and sample geometry on the heat transfer.


Cross-setional views of binary Ti/Ni butt joints made by laser (left) and electron (right) beam welding

Geometry of the fusion zone (primarily its size and shape) is important even in similar metal welding, for example, due to its role in determining the interface orientation and cooling rate, and thereby the microstructure (which in turn controls the mechanical properties). In addition to this, it has another crucial significance in dissimilar welding: it determines the composition of the weld. Since usually the base metals melt in different propotions in this case, the melting ratio can be directly correlated with the average melt composition using the density and atomic weights of the base metals. To a good approximation, we have found this to hold good for the Ti/Ni welds. We also found that the phases in the weld are primarily those in the phase diagram corresponding to this composition. The melt ratio can serve as an useful target in engineering the weld microstructure and hence the properties. Images below illustrate this effect for laser and electron beam welds.


Phases and micrsotructure in dissimilar welds are determined by the melt ration of the base metals. Left: Laser welded Ti/Ni with average composition Ti–40at%Ni and a microstructure made up of NiTi dendrites and Ti2Ni in between the dendrites. Right: Ti/Ni EBW having average composition Ti–60at%Ni with NiTi–Ni3Ti eutectic forming the microstructure.

Dissimilar welds also exhibit unique microstructural features near the fusion interfaces in the base metals. The steepest gradients of composition occur in these regions, and often its interplay with the thermal fields result in things like discontinuous movement of the solid-liquid front (which is no longer defined by a unique liquidus temperature as in similar welds) and dendrites growing into the base metal from the fusion zone. Detailed mechanisms behind these, including the coupling between macroscopic transport and thermodynamic and kinetic processese at the microscopic space, is yet to be worked out fully; however, we have analyzed the latter partially using a phase field model.


Wire-based Additive Manufacturing

Knowledge and information gained from weld solidification can be used to understand solidification of metallic materials in additive manufacturing (AM) technologies. Typically, AM of metallic components involves laser or electron beam based processes. Although they offer good dimensional accuracy and tolerance, these suffer from low productivity and higher capital costs. Wire based AM is a promising alternative which can increase the deposition rates, albeit sacrificing some of the accuracy of the aforementioned processes. In collaboration with Dr. Suryakumar of the Mechanical and Aerospace Engineering Department at IITH, we have started looking at the materials issues involved in the twin-wire AM technique.

Joining of High Temperature Materials

High temperature materials such as advanced steels (martensitic and austenitic grades such as P91/92, S347H etc.) and Ni-base superalloys derive their required properties from carefully designed alloy chemistry and thermomechanical treatments. Components made of these alloys invariably need to be joined to others in any application, and the process of joining, which is very often one of the fusion welding techniques, almost always destroys or damages the very microstructure which gives rise to their exceptional properties (strength, toughness etc.). Hence to minimize the damage and recover properties, it is crucial to investigate the welding-microstructure-properties correlations in these alloys. My primary interest in this topic lies in studying the microstructural changes taking place during welding and post-weld treatments.

A new class of precipitation strengthened Co-base alloys intended for high temperature applications has recently been developed by Prof. K. Chattopadhyay and co-workers. For these new materials with very promising properties to be adopted for actual applications, it is absolutely essential to investigate their weldability and effect of welding process on the fine tuned micrstrocture and properties. In collaboration with IISc Bangalore and ARCI Hyderabad, I have just started working on their laser welding characteristics.

Concentrated Multicomponent Alloys

The high entropy alloys are multi-component alloys with significant concentrations of the majority constituents. They are a promising class of materials which exhibit very simple crystallographic and thermodynamic features despite having many elements in significant amounts. We have started looking at the structural aspects of this class of alloys in collaboration with Dr. Pinaki P. Bhattacharjee, who has made significant contributions towards understanding their thermomechanical processing and deformation behavior.

Phase Field Models of Microstrutural Evolution

Solidification

Although fundamentals of the phase field models were established by Cahn-Hilliard and Allen-Cahn long time back, it was only since the late 1990's that the method really took off for studying solidification problems. We used it analyze a situation that is usually present near the fusion interfaces in dissimilar welds, namely, to understand what happens when a melt rich in solute is in contact with a pure solid, and heat is being extracted through the solid. Starting with a temperature profile where the initial solid-liquid interface is at the melting point of the pure solid, our simulations using a fully non-isothermal phase field model for binary alloys demonstrated a solute induced melt-back period of the substrate before it could re-grow into the melt. We also observed that this melt back is strongly correlated with the effective slope of the liquidus temperature profile in the melt. We wish to extend this simple model to study that more complex phenomena not only in dissimilar welds but solidification processing in general.

Phase Transformations in Constrained Systems

Another interest of ours (I and my collaborator Dr. Saswata Bhattacharya) is microstructural evolution in constrained systems. This include phase transformations taking place, for example, within Li-ion battery materials where the presence of free surface may control the several physico-electro-chemical processes in these materials.