Thermo-mechanical Investigation of Dissimilar Welding of AISI 304 Stainless Steel with Commercially Pure Copper

Abstract

Laser welding represents a delicate balance between heating and cooling within a spatially localized volume overlapping two or more solids such that a liquid pool is formed and remains stable until solidification. The main objective of this research work is to analyses thermal, metallurgical and physical stages of AISI 304 Stainless Steel – Copper dissimilar couple during laser welding in keyhole mode by numerically and then validate it experimentally. 10.6 μm wavelength CO2 laser and 1.064 μm wavelength Nd: YAG welder machine is used for conducting this experimental analysis and ANSYS FLUENT® software is used for simulation. The difference in metallurgical, chemical, thermal and physical properties of AISI 304 Stainless Steel and Copper makes their laser welding a challenging task. When compared to steel, the thermal conductivity and diffusivity of copper is very high. This result in 90-98% reflection of total laser power impinged on the copper surface, i.e., the property of absorptivity decreases with increase in diffusivity of metal. The transport phenomena (heat transfer, fluid flow and species distribution) are numerically modelled for the case of laser welding of dissimilar metals. The model involves convection in the weld pool along with melting and mixing. The associated metallurgical phenomenon is an extremely complex one, and the present work is a preliminary attempt to model the process after making suitable assumptions. The numerical study is performed using a pressure based finite volume technique after making appropriate modifications to the algorithm to include the associated phase change processes and dissimilarity in the metal properties. The phase change process is modelled using an enthalpy-porosity technique, while the dissimilar metal properties are handled using appropriate mixture theories. As a case study, we have used dissimilar couples of copper- 304 SS. It is observed that the weld pool shape becomes asymmetric when the heat source is symmetrically applied on the two metals forming the couple. As the weld pool develops, the side melting earlier is found to experience more convection and better mixing. Corresponding experiments are performed using the same parameters as in the computations, showing a good qualitative agreement between the two results. After a calibration phase between simulation and experimental results for the same working environment, the FLUENT® model has been very good agreement with the experimental tests.