Investigation of Fusion Weldments of Semi-Solid Aluminium A356 Alloy: Pool Geometry and Microstructure S. Sandhya a and G. Phanikumar b Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Madras 600036, India a sandhya_seshagiri@yahoo.com, b gphani@iitm.ac.in Keywords: Semi-solid, Partially melted zone, Thermal gradient, Globular microstructure, TIG welding Abstract. A fusion welding technique to join a semi-solid processed A356 cast plate is explored using Gas Tungsten Arc Welding (GTAW). Semi-solid metal (SSM) billets of non-dendritic microstructure produced by rheocasting in a mould placed inside a linear electromagnetic stirrer were used for this study. GTAW experiments were conducted to simulate different thermal gradients near the fusion zone. The geometries of the weld pool as well as the temperature gradient in the fusion boundary were measured to understand the microstructure evolution. Simulation of the welding process was performed to aid in the analysis. Quantitative metallography provided the shape factor as a measure of globularity of the primary a-Al phase. Based on the studies, a model has been proposed to explain the observation of globular microstructure in the fusion zone of the welds. Conclusions show a positive correlation of thermal gradient with globular microstructure formation in this class of alloys. Introduction Cast aluminium alloys are extensively used in aerospace and automotive industries due their higher specific strength. Semi-solid processing involves [1,2] shearing of the metal in the mushy state during solidification to alter the dendrite morphology of the solid into a spherical/globular microstructure. GTAW is usually the fusion welding technique selected for aluminium alloys. The partially melted zone (PMZ) is the region adjacent to the fusion zone (FZ), where liquation occurs due to heating above eutectic temperature [3]. It is known that the temperature gradients determine the width of the PMZ and thus play a vital role in the nucleation mechanism and microstructural evolution in the fusion zone. By modifying the convective heat transfer conditions, it is possible to change the width of the PMZ in Gas Tungsten Arc (GTA) weldments [4]. GTA weldments in aluminium alloys generally exhibit dendritic microstructures. This study aims to demonstrate that by increasing the width of the PMZ, one can obtain a near globular microstructure in weldments of SSM alloys. Experimental Details Semi-solid processed A356 billets were produced from a Linear Electromagnetic Stirring (LEMS) setup of the rheocasting type [5]. Cast plates of thickness 8 mm were used in this investigation. Bead-on-plate tungsten inert gas (TIG) welding was performed on the aluminium plates using a current of 150 A and voltage 16.5 V and argon was used as the shielding gas. TIG welding of the plates was performed under two conditions, 1) SSM with copper backing plate and 2) SSM with copper backing plate and water cooling. Type “K” thermocouples were inserted into drilled holes in the plates and thermal cycles were recorded using a computer based data acquisition system. The experimental arrangement is shown in Fig. 1. Metallographic samples of transverse cross-section were taken at the centre of the welds. The microstructures of the FZ and PMZ were observed using optical microscopy. The morphology of grains, was analysed by measuring a shape factor the (4πA)/P 2 . Image analysis was performed to Materials Science Forum Vol 765 (2013) pp 751-755 Online: 2013-07-31 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.765.751 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 203.199.213.66, Indian Institute of Technology Madras, Chennai, India-03/04/15,18:56:37)