Directed energy deposition of Invar 36 alloy using cold wire pulsed gas tungsten arc welding: Effect of heat input on the microstructure and functional behaviour

Abstract

Invar alloys exhibit low thermal expansion and are useful in applications requiring dimensional stability when subject to temperature changes. Conventional production of Invar faces certain challenges that can be offset by exploiting additive manufacturing processes. This study employed pulsed gas tungsten arc welding (GTAW) to deposit Invar 36 alloy blocks at five heat inputs (HI) ranging from 200 to 550 J mm−1. The results show that the microstructure comprised of columnar grains and remained in the austenitic phase regardless of the HI. Ductility dip cracking was found to prevail in all the blocks except the block deposited at the lowest HI. The decreased susceptibility to cracking with a reduction in the HI was due to the preservation of the grain boundary area, consequently leading to an improved partitioning of strain among the grain boundaries. On lowering the HI from 550 to 200 J mm−1 the average yield strength, tensile strength and elongation improved by 16%, 23% and 38%, respectively. The HI had a negligible effect on the mean linear coefficient of thermal expansion (CTE) in different temperature ranges as the CTE values were nearly identical between the blocks deposited at 200 and 550 J mm−1. In general, the CTE in the building direction was slightly higher than the travel direction, with a maximum difference between the CTE of the two directions being 15%. In summary, this work demonstrates the application of the cold wire GTAW process as an alternative to conventional/laser based methods for realizing the functional properties of Invar.