An Investigation of Dissimilar Pipe Welding of AISI 304 Stainless Steel with CP Copper

Abstract

A study on dissimilar metal welding of commercially pure copper and AISI 304 SS pipes with different thicknesses has been carried out. Three different welding processes, namely, pulsed Nd:YAG laser beam, pulsed TIG Arc and continuous CO2 laser beam welding have been compared. For studying the effects of process parameters, viz. peak power, pulse duration, frequency, beam diameter (for laser beam) / stand-off distance (for TIG), processing speed, energy density etc. Controlled set of experiments are conducted as per standard DOE (Design of Experiment) methods. Temperature buildup during welding has been measured by K-type thermocouples with a data acquisition system. Mechanical and metallurgical characterization of the welds, i.e., tensile strength, bead geometry, microhardness, etc., have been performed. Microstructure of the weld pool has been characterized by SEM, EDX and XRD. Effect of ramp down parameters on the closure bead shape has been studied in pulsed TIG arc welding. The results have been analyzed statistically with RSM (Response Surface Method). Finite element modeling of the process has been done and simulations have been run at the parameter combination levels selected during experimentation. Customized jigs and attachments have been designed and fabricated for automation of the welding process of pipes.
Results from mechanical testings have been encouraging, with weld strengths realized in between those of the base metals. In most cases, joint failure has taken place away from the fusion zone and in the weaker component (Copper). A few welds fabricated with pulsed Nd:YAG laser failed at HAZ or in the base metal. Some parametric combinations resulted in solidification cracking at the crown. Microstructural characterization shows presence of unmixed zones, partially mixed zones and carbide precipitation similar to results reported by previous researchers. Use of frequency as a tool for controlling weld pool agitation and the weld bead shape has been tried and found to be effective. Pulsed TIG arc welding and CO2 laser welding have been found to be more suitable for welding load bearing structures. Pulsed Nd:YAG laser has been found to be more suitable for welding smaller pipe thicknesses. The results and the models proposed by the present study can serve as an useful tool for further improvisations in autogenous welding of CP Copper and AISI 304 SS.