Date of Award

31-8-2024

Document Type

Thesis

School

School of Mechanical Engineering

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

Dr.P.Bala Srinivasan

Keywords

AZ31B Mg Alloy, 304HCU Stainless Steel, Gas Tungsten Arc Welding, GTAW, DCEN, Pulse Welding, Metallurgy, Mechanical Properties, Repair Welding

Abstract

The automotive and aero sectors seek materials with high specific strength & specific stiffness for enhanced efficiency / emission control. Mg alloys, known for their low density, are used in the as-cast, and wrought forms, depending on the service requirements, and are predominantly used for non-loaded structural parts in varied engineering applications. Welding Mg alloys is of great interest to industry, not only for the fabrication of components but for the repair welding of casting as well. There has been considerable research in the fusion & solid-state welding of Mg alloys. Gas tungsten arc welding (GTAW) has often been preferred for Mg alloys because of adaptability, process stability, and economy. A thorough review of the literature on the fusion welding magnesium alloys by GTA welding suggests that the potential of pulse welding processes & the capabilities in the modern power sources have not yet been fully explored. This research is an attempt to address some of the identified research gaps.

The effects of pulse frequency & the pulse duration ratios on the weldability & structure-property relationships in AZ31B Mg and the use of AC/DC mix pulse welding with different pulse duration ratios were attempted. Pulse GTA welding in Direct Current Electrode Negative (DCEN) mode at lower frequencies yielded deeper penetration welds at different pulse duration ratios. However, the full penetration welds were possible only with the pulse duration ratio of 1:1 for a given heat input. All the welds produced under pulse GTA welding were free from any defects in contrast to the conventional GTA DCEN welds that had microcracks. The grain size of the pulse GTA weld metals were much finer than that obtained with the conventional GTA welding. Further, the pulse GTA weld metals had registered a higher hardness compared to the conventional GTA weld metal & also the parent alloy.

AC/DC mix pulse welding, with the peak current being AC and the base current as DC, gave rise to a much-controlled heat input, yielded shallow penetration welds with no defects. The microstructures were much finer compared to the DC pulse GTA welds. The outcome of the work reveals that the AC/DC mix pulse welding is very promising to be exploited for repair welding of magnesium alloy castings that might have near-surface defects.

High frequency pulse welding with a short / stable arc in ActivArc® mode (AA-HF) in a modern power source was explored to understand the effect of frequency in the range of 100 Hz to 1500 Hz on the resultant microstructures & properties of AZ31B Mg alloy. The arc constriction by the high frequency pulsing in the AA-HF mode could produce deeper penetration welds at frequencies above 800 Hz for a given heat input. The full penetration ActivArc®-high frequency GTA welds had good microstructural features / mechanical properties and allowed 50% enhanced productivity when the welding was done at a frequency of 1500 Hz. This welding process / technique has been found to be very effective for thicker section welding without the issues of cracking in the weldment.

The learnings from the welding of AZ31B Mg alloy using the pulse GTA welding & AA-HF welding were leveraged for the welding alloy 304HCu, a material for the superheater & reheater sections in the supercritical power plants. The AA-HF welding in the autogenous mode resulted in 304HCu welds with a good form factor and a fine dendritic structure with a mix of vermicular & lacy ferrite. Both the pulse GTA welds and the AA-HF welds had overmatched mechanical properties compared to the welds produced by conventional GTA welding. The AA-HF welding looks to be a good choice for welding stainless steels and can be explored further for welding of much thicker sections either in the autogenous mode or with use of fillers based on the need.

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